研究者データベース
| 大島 慶一郎(オオシマ ケイイチロウ) |
| 低温科学研究所 水・物質循環部門 |
| 特任教授 |
Last Updated :2024/05/09
基本情報
所属
職名
学位
ホームページURL
科研費研究者番号
- 30185251
J-Global ID
プロフィール
- 専門分野は海洋物理学、極域海洋学。
極域・海氷域の海洋循環と変動、海氷の気候に対する役割などが研究テーマ。
理論・モデル研究から出発したが、現在は現場観測が主な研究手法。
研究キーワード
研究分野
担当教育組織
職歴
- 2018年04月 - 現在 日本学術振興会 学術システム研究センター 研究員
- 2016年04月 - 現在 北海道大学 北極域研究センター 教授（兼任）
- 2008年04月 - 現在 北海道大学 低温科学研究所 教授
- 1994年04月 - 2008年03月 北海道大学 低温科学研究所 助教授（准教授）
- 2003年03月 - 2004年01月 米国ワシントン 大学 海洋学部 滞在研究員
- 1990年11月 - 1992年03月 文部省 第32次日本南極地域観測隊 昭和基地越冬隊員
所属学協会
研究活動情報
論文
- Alessandro Silvano, Sarah Purkey, Arnold L. Gordon, Pasquale Castagno, Andrew L. Stewart, Stephen R. Rintoul, Annie Foppert, Kathryn L. Gunn, Laura Herraiz-Borreguero, Shigeru Aoki, Yoshihiro Nakayama, Alberto C. Naveira Garabato, Carl Spingys, Camille Hayatte Akhoudas, Jean-Baptiste Sallée, Casimir de Lavergne, E. Povl Abrahamsen, Andrew J. S. Meijers, Michael P. Meredith, Shenjie Zhou, Takeshi Tamura, Kaihe Yamazaki, Kay I. Ohshima, Pierpaolo Falco, Giorgio Budillon, Tore Hattermann, Markus A. Janout, Pedro Llanillo, Melissa M. Bowen, Elin Darelius, Svein Østerhus, Keith W. Nicholls, Craig Stevens, Denise Fernandez, Laura Cimoli, Stanley S. Jacobs, Adele K. Morrison, Andrew McC. Hogg, F. Alexander Haumann, Ali Mashayek, Zhaomin Wang, Rodrigo Kerr, Guy D. Williams, Won Sang LeeFrontiers in Marine Science 10 2023年12月08日Dense, cold waters formed on Antarctic continental shelves descend along the Antarctic continental margin, where they mix with other Southern Ocean waters to form Antarctic Bottom Water (AABW). AABW then spreads into the deepest parts of all major ocean basins, isolating heat and carbon from the atmosphere for centuries. Despite AABW’s key role in regulating Earth’s climate on long time scales and in recording Southern Ocean conditions, AABW remains poorly observed. This lack of observational data is mostly due to two factors. First, AABW originates on the Antarctic continental shelf and slope where in situ measurements are limited and ocean observations by satellites are hampered by persistent sea ice cover and long periods of darkness in winter. Second, north of the Antarctic continental slope, AABW is found below approximately 2 km depth, where in situ observations are also scarce and satellites cannot provide direct measurements. Here, we review progress made during the past decades in observing AABW. We describe 1) long-term monitoring obtained by moorings, by ship-based surveys, and beneath ice shelves through bore holes; 2) the recent development of autonomous observing tools in coastal Antarctic and deep ocean systems; and 3) alternative approaches including data assimilation models and satellite-derived proxies. The variety of approaches is beginning to transform our understanding of AABW, including its formation processes, temporal variability, and contribution to the lower limb of the global ocean meridional overturning circulation. In particular, these observations highlight the key role played by winds, sea ice, and the Antarctic Ice Sheet in AABW-related processes. We conclude by discussing future avenues for observing and understanding AABW, impressing the need for a sustained and coordinated observing system.
- Vigan Mensah, Koji Fujita, Stephen Howell, Miho Ikeda, Mizuki Komatsu, Kay I. Ohshima2023年12月07日Abstract. The Labrador Sea and contiguous Baffin Bay play an important role in the formation of the upper layer of the North Atlantic Deep Water, an essential component of the Atlantic Meridional Ocean Circulation. The hydrography of these two seas is strongly influenced by the melting of sea-ice and glacier-ice, which has likely been affected by long-term climate changes. In this study, we use historical data of ocean temperature and salinity from 1950 to 2022 to estimate the summer freshwater volume (SFV) in Baffin Bay and the Labrador Sea, establish climatologies, and assess the impact of multi-decadal climate change. The SFV climatology (1956 km3) and the summer freshwater budget (2286 km3) estimated from various components are in good agreement. Sea ice and glacial melt account for 37 % and 26 % of the freshwater budget, respectively. SFV climatologies before and after 1995 reveal an increase in Baffin Bay (+226 km3) because of enhanced glacier melting, and a decline (-112 km3) in the Labrador Sea because of recent sea ice volume decreases. The time series of Labrador Sea SFV and total freshwater content are uncorrelated at the multi-decadal scale possibly because the influx of freshwater from the Beaufort Sea dominates the long-term variability.
- Yoshihiko Ohashi, Michiyo Yamamoto-Kawai, Kazuya Kusahara, Ken'ichi Sasaki, Kay I. OhshimaProgress in Oceanography 219 103153 - 103153 2023年12月
- Mizuki Kuga, Kay I. Ohshima, Sachiko Kishi, Noriaki Kimura, Takenobu Toyota, Jun NishiokaJournal of Oceanography 2023年10月17日
- Vigan Mensah, Kay I. OhshimaJournal of Atmospheric and Oceanic Technology 40 10 1241 - 1261 2023年10月Abstract Polar and subpolar oceans play a particularly important role in the global climate and its temporal changes, yet these regions are less well sampled than the rest of the global ocean. To better understand the physical or biogeochemical properties and their variabilities in these regions, accurate data mapping is crucial. In this paper, we introduce a mapping methodology that includes a water column shrinking and stretching constraint (SSC) based on the principle of conservation of potential vorticity. To demonstrate the mapping scheme efficiency, we map the ocean temperature in the southern Sea of Okhotsk, where the bottom topography comprises a broad and shallow shelf, a sharp continental slope, and a deep oceanic basin. Such topographic features are typical of polar and subpolar marginal seas. Results reveal that the SSC integrated (SSCI) mapping strongly reduces the mapping error in the broad and shallow shelf compared with a recently introduced topographic constraint integrated (TCI) mapping procedure. We also tested our mapping scheme in the Southern Ocean, which has a comparatively slanted shelf, a wider and gentler slope, and a deep and broad oceanic basin. We found that the SSCI and TCI methods are practically equivalent there. The SSCI mapping is thus an effective method to map the ocean’s properties in various topographic environments and should be adequate in all polar and subpolar regions. Importantly, we introduced a standardized procedure for determining the decorrelation length scales—a necessary step prior to implementing any mapping scheme—in any topographic conditions.
- Daisuke Hirano, Takeshi Tamura, Kazuya Kusahara, Masakazu Fujii, Kaihe Yamazaki, Yoshihiro Nakayama, Kazuya Ono, Takuya Itaki, Yuichi Aoyama, Daisuke Simizu, Kohei Mizobata, Kay I. Ohshima, Yoshifumi Nogi, Stephen R. Rintoul, Esmee van Wijk, Jamin S. Greenbaum, Donald D. Blankenship, Koji Saito, Shigeru AokiNature Communications 14 1 2023年08月17日 [査読有り][通常論文]
Abstract The Totten Glacier in East Antarctica, with an ice volume equivalent to >3.5 m of global sea-level rise, is grounded below sea level and, therefore, vulnerable to ocean forcing. Here, we use bathymetric and oceanographic observations from previously unsampled parts of the Totten continental shelf to reveal on-shelf warm water pathways defined by deep topographic features. Access of warm water to the Totten Ice Shelf (TIS) cavity is facilitated by a deep shelf break, a broad and deep depression on the shelf, a cyclonic circulation that carries warm water to the inner shelf, and deep troughs that provide direct access to the TIS cavity. The temperature of the warmest water reaching the TIS cavity varies by ~0.8 °C on an interannual timescale. Numerical simulations constrained by the updated bathymetry demonstrate that the deep troughs play a critical role in regulating ocean heat transport to the TIS cavity and the subsequent basal melt of the ice shelf. - Sohey Nihashi, Kay I. Ohshima, Takeshi TamuraPolar Science 100978 - 100978 2023年08月
- Kay I. Ohshima, Mizuki KugaJournal of Oceanography 79 5 499 - 515 2023年06月22日Abstract This study provides formulae for monthly estimates of the Soya Warm Current (SYC) transport from the sea-level difference (SLD) across the Soya Strait and along the current, and creates a 50-yr data set of the SYC transport. The formulae are based on the dynamical balance both across the strait and along the current by considering the barotropic and baroclinic components. It is suggested that the barotropic and baroclinic components in transport variability are comparable in summer, whereas the barotropic component is dominant in winter. We compared the 50-year time series of the SYC transport with those of the Tsushima (TSC) and Tsugaru Warm Currents estimated from the SLDs from the viewpoint of the Japan Sea Throughflow (JSTF) system. The variabilities of the SYC and TSC transports are mostly shared for any season, showing relatively high coherence at periods of 2–5 years as well as 1 year. The winter variability of the JSTF transport originates partly from that of the SYC transport caused by the winter monsoon variability in the Sea of Okhotsk. The SYC transport is significantly correlated with the transport through the eastern channel of the TSC, but not that through the western channel. We found difficulty in extracting variability and relationships at periods longer than 5 years among the three transports estimated from the SLDs, probably due to incomplete correction of the ground movements for the sea-level data.
- Mizuki Kuga, Kay I. Ohshima, Noriaki Kimura, Kazuki Nakata, Yasushi FukamachiJournal of Oceanography 79 2 145 - 159 2022年11月28日 [査読有り]
- Kay I. Ohshima, Yasushi Fukamachi, Masato Ito, Kazuki Nakata, Daisuke Simizu, Kazuya Ono, Daiki Nomura, Gen Hashida, Takeshi TamuraScience Advances 8 42 2022年10月21日 [査読有り]
Antarctic Bottom Water (AABW) occupies the abyssal layer of the world ocean and contributes to the global overturning circulation. It originates from dense shelf water, which forms from brine rejection during sea ice production. An important region of AABW formation has been identified off the Cape Darnley polynya. However, it remains unclear why and how high ice production leads to AABW formation. Using moored acoustic measurements and a satellite microwave algorithm, we reveal that underwater frazil ice dominates in the polynya. This underwater ice formation prevents heat-insulating surface-cover ice forming, thereby enabling efficient ice production. The high ice production in the nearshore and longer residence times create high-salinity source water for the AABW. Underwater frazil ice occurs as long as strong winds continue and occasionally penetrates depths of at least 80 m. Deep-penetrating frazil ice is particularly prominent in this polynya, while it also occurs in other Antarctic coastal polynyas. - Yoshihiko Ohashi, Michiyo Yamamoto-Kawai, Kazuya Kusahara, Ken'ichi Sasaki, Kay OhshimaSCIENTIFIC REPORTS 12 1 2022年05月Chlorofluorocarbon (CFC) and sulfur hexafluoride (SF6) were used to investigate the timescale of Antarctic Bottom Water (AABW) that spreads off Cape Darnley (CD) in East Antarctica. The age of the AABW was estimated based on the observed SF6/CFC-12 ratio while taking into account tracer dilution by Lower Circumpolar Deep Water. Along the western canyons off CD and the similar to 3000 to 3500 m isobaths, the bottom water age was <5 years, reflecting the spread of newly formed CD Bottom Water. Higher ages of similar to 8 years obtained for areas east of CD and > 20 years in the northwestern offshore region indicate inflows of AABW through the Princess Elizabeth Trough and Weddell Sea Deep Water, respectively. This study determined the age distribution in the region off CD, where three different types of AABW spread.
- Daisuke Hirano, Yasushi Fukamachi, Kay I. Ohshima, Masato Ito, Takeshi Tamura, Daisuke Simizu, Toru Takatsuka, Andrew R. Mahoney, Joshua Jones, Craig George, Billy Adams, Hajo EickenProgress in Oceanography 102781 - 102781 2022年03月
- Vigan Mensah, Yoshihiro Nakayama, Masakazu Fujii, Yoshifumi Nogi, Kay I. OhshimaOCEAN MODELLING 165 2021年09月The formation of Dense Shelf Water (DSW) and Antarctic Bottom Water (AABW) in the Southern Ocean is an essential part of the thermohaline circulation, and understanding this phenomenon is crucial for studying the global climate. AABW is formed as DSW flows down the continental slope and mixes with the surrounding waters. However, DSW formation and its descent remains a poorly resolved issue in many ocean models. We, therefore, simulated the formation and descent of DSW and investigated the model sensitivities to horizontal and vertical grid spacings. The Massachusetts Institute of Technology general circulation model (MITgcm) was used for the region off Cape Darnley in East Antarctica, one of the main AABW production areas, where historical and mooring data are available for comparison. Simulations with coarse horizontal grid resolutions of order (10 km) yielded high volumes of DSW on the shelf. However, the largest part of this DSW was transformed into intermediate water and advected westward. Horizontal model resolutions equal to or higher than 2 km were required to simulate the descent of DSW and a realistic AABW production. Simulated time series at a mooring located at a depth of 2,600 m showed periodic fluctuations in velocity and temperature of 0.3 m.s(-1) and 0.5 degrees C, respectively, consistent with observations. We also found that high-resolution bathymetry datasets are crucial because the newly formed AABW volume was reduced by 20% when a smoother bathymetry was used on a 2-km resolution grid. Vertical resolution had little influence on model performance because the plume was much thicker (> 170 m) than the grids width. Therefore, reproducing the downslope flow of DSW and AABW formation in the Cape Darnley region can be achieved with a high horizontal resolution (<= 2 km) and a relatively coarse vertical resolution (similar to 100 m on the continental slope).
- Masato Ito, Kay I. Ohshima, Yasushi Fukamachi, Genta Mizuta, Yoshimu Kusumoto, Takashi KikuchiCold Regions Science and Technology 103382 - 103382 2021年08月
- Genta Mizuta, Yasushi Fukamachi, Daisuke Simizu, Yoshimasa Matsumura, Yujiro Kitade, Daisuke Hirano, Masakazu Fujii, Yoshifumi Nogi, Kay OhshimaFRONTIERS IN MARINE SCIENCE 8 2021年08月This study examines the seasonal evolution of Cape Darnley Bottom Water (CDBW), using the results of mooring and hydrographic measurements in the slope region off Cape Darnley in 2008-2009 and 2013-2014. Newly formed CDBW began reaching the western and nearshore part of the slope region off Cape Darnley in April, spread to the offshore and eastern part in May, and reached the easternmost part in September. The potential temperature and salinity decreased and the neutral density increased when newly formed CDBW reached mooring sites. Potential temperature-salinity properties of CDBW changed over time and location. The salinity of the source water of CDBW estimated from potential temperature-salinity diagrams started to increase at a nearshore mooring in late April, which is about 2 months after the onset of sea-ice production, and continued to increase during the ice production season. It is most probable that the accumulation of brine in the Cape Darnley polynya produces the seasonal variation of potential temperature-salinity properties of CDBW. Two types of CDBW were identified. Cold and less saline CDBW and warm and saline CDBW were present in Wild and Daly Canyons, respectively. This indicates that the salinity of the source water of CDBW increased in the westward direction. CDBW exhibited short-term variability induced by baroclinic instability.
- Vigan Mensah, Kay. I. OhshimaSCIENTIFIC REPORTS 11 1 2021年07月The western subarctic Pacific exhibits major biological productivity fed by the Oyashio Current and its two source waters: Western Subarctic Water, which supplies nutrients from the subarctic Pacific, and cold Okhotsk Sea Intermediate Water (OSIW), which supplies iron from the Sea of Okhotsk. We created seasonal climatologies of water properties to understand how the long-term trend (similar to 50 years) and 18.6-year tidal cycle affect the Oyashio Intermediate Water (OYW). We found that over the trend, decreased OSIW outflow due to weakening of North Pacific overturning modifies OYW in winter. Meanwhile, OSIW outflow increases (decreases) in strong (weak) tide years. We predict that the opposite effects of the trend and strong tide will lead to stagnation of OYW properties until the mid-2020s, followed by accelerated warming until the mid-2030s (weak tide). A predicted 1 degrees C increase in OYW temperature and 50% decrease in OSIW content between 1960 and 2040 potentially have significant impact on biological productivity and carbon drawdown in the North Pacific.
- Haruhiko Kashiwase, Kay Ohshima, Kazuki Nakata, Takeshi TamuraJOURNAL OF ATMOSPHERIC AND OCEANIC TECHNOLOGY 38 4 823 - 835 2021年04月Long-term quantification of sea ice production in coastal polynyas (thin sea ice areas) is an important issue to understand the global overturning circulation and its changes. The Special Sensor Microwave Imager (SSM/I), which has nearly 30 years of observation, is a powerful tool for that purpose owing to its ability to detect thin ice areas. However, previous SSM/I thin ice thickness algorithms differ between regions, probably due to the difference in dominant type of thin sea ice in each region. In this study, we developed an SSM/I thin ice thickness algorithm that accounts for three types of thin sea ice (active frazil, thin solid ice, and a mixture of two types), using the polarization and gradient ratios. The algorithm is based on comparison with the ice thickness derived from the Moderate Resolution Imaging Spectroradiometer (MODIS) for 22 polynya events off the Ross Ice Shelf, off Cape Darnley, and off the Ronne Ice Shelf in the Southern Ocean. The algorithm can properly discriminate the ice type in coastal polynyas and estimate the thickness of thin sea ice (<= 20 cm) with an error range of less than 6 cm. We also confirmed that the algorithm can be applied to other passive microwave radiometers with higher spatial resolution to obtain more accurate and detailed distributions of ice type and thickness. The validation of this algorithm in the Arctic Ocean suggests its applicability to the global oceans.
- S. Kishi, K. I. Ohshima, J. Nishioka, N. Isshiki, S. Nihashi, S. C. RiserGEOPHYSICAL RESEARCH LETTERS 48 6 2021年03月Seven profiling floats equipped with oxygen sensors deployed in the Sea of Okhotsk provide time series data for 33 cases of spring phytoplankton bloom period, including 9 cases in which sea ice existed just before the bloom (prior-ice case). As an index of biological productivity, we calculated net community production (NCP) based on the increasing oxygen rate using the Redfield ratio. The total NCP in the euphotic layer averaged for prior-ice cases is 31.3 mmolC m(-2) day(-1)), similar to 3 times higher than that of non-ice cases. In addition to intensification of surface stratification, other factors of sea-ice melt likely enhance the bloom. The influence of sea-ice melt is particularly large in the southwestern region, where the iron availability likely limits phytoplankton growth. A suggested scenario is that when the sea ice containing sediment/iron is transported from the northern shelves, a prominent bloom is induced via the iron supply by sea-ice melt.
- K. Nakata, K. I. Ohshima, S. NihashiGEOPHYSICAL RESEARCH LETTERS 48 6 2021年03月The importance of the area of active frazil and associated high sea-ice production in Antarctic coastal polynyas has been increasingly recognized in recent in situ and high-resolution satellite observations. However, the occurrence and spatial distribution of active-frazil area are not well understood. We present the first mapping of active-frazil area for Antarctic coastal polynyas, based on the thin ice algorithm of AMSR-E that discriminates active-frazil area. Mapping of sea-ice production is presented by taking account of active-frazil area. Active-frazil area is predominant along East Antarctica, particularly very close to the coast, locally leading to very high ice-production rates exceeding 20 m/yr. For frazil-dominant polynyas, estimated ice production is similar to 20%-50% higher compared with previous studies. Analyses of all the major polynyas suggest that active-frazil extent depends on offshore wind and air temperature, while ice production is determined by offshore wind only.
- Daisuke Hirano, Takeshi Tamura, Kazuya Kusahara, Kay I. Ohshima, Keith W. Nicholls, Shuki Ushio, Daisuke Simizu, Kazuya Ono, Masakazu Fujii, Yoshifumi Nogi, Shigeru AokiNature Communications 11 1 2020年12月 [査読有り][通常論文]
- Alexander D. Fraser, Robert A. Massom, Kay Ohshima, Sascha Willmes, Peter J. Kappes, Jessica Cartwright, Richard Porter-SmithEARTH SYSTEM SCIENCE DATA 12 4 2987 - 2999 2020年11月Landfast sea ice (fast ice) is an important component of the Antarctic nearshore marine environment, where it strongly modulates ice sheet-ocean-atmosphere interactions and biological and biogeochemical processes, forms a key habitat, and affects logistical operations. Given the wide-ranging importance of Antarctic fast ice and its sensitivity to climate change, improved knowledge of its change and variability in its distribution is a high priority. Antarctic fast-ice mapping to date has been limited to regional studies and a time series covering East Antarctica from 2000 to 2008. Here, we present the first continuous, high-spatio-temporal resolution (1 km, 15 d) time series of circum-Antarctic fast-ice extent; this covers the period March 2000 to March 2018, with future updates planned. This dataset was derived by compositing cloud-free satellite visible and thermal infrared imagery using an existing methodology, modified to enhance automation and reduce subjectivity in defining the fast-ice edge. This new dataset (Fraser et al., 2020) has wide applicability and is available at https://doi.org/10.26179/5d267d1ceb60c. The new algorithm presented here will enable continuous large-scale fast-ice mapping and monitoring into the future.
- Kan Murakami, Daiki Nomura, Gen Hashida, Shin ichiro Nakaoka, Yujiro Kitade, Daisuke Hirano, Toru Hirawake, Kay I. OhshimaMarine Chemistry 225 2020年09月10日 [査読有り][通常論文]
© 2020 Elsevier B.V. Formation of dense shelf water (DSW) in coastal polynyas (open water or thin sea-ice cover) in the sea-ice zone around Antarctica supplies Antarctic Bottom Water (AABW) through overflow down the continental slope. In coastal polynyas, atmospheric carbon dioxide (CO2) is absorbed by the ocean in the early spring because of active primary production, and DSW formation is an important process for transporting this carbon from the sea surface to the deep ocean. However, there have been few quantitative evaluations of carbon consumption by active primary production and transport in coastal polynyas. Here, we examined the carbon dynamics in the Cape Darnley polynya (CDP), East Antarctica during austral summer 2009, by using carbonate system parameters combined with oceanographic mooring data. The partial pressure of CO2 in the CDP surface water was lower than that of the atmosphere and the mean and standard deviation of sea−air CO2 flux was estimated as −6.5 ± 6.9 mmol C m−2 d−1 (a negative value indicates absorption of atmospheric CO2 by the ocean). Vertical profiles of dissolved inorganic carbon (DIC) concentration showed that concentrations in the bottom layer near the ocean floor were lower (by about 20 μmol kg−1) than those in the ambient water (e.g., modified Circumpolar Deep Water, mCDW). The low-DIC in the shelf water was maintained by the strong biological uptake of carbon imported from high-DIC mCDW within the water column. Therefore, low-DIC DSW overflowed down the continental slope, and low-DIC concentrations were maintained through an export pathway to the continental shelf. The annual production of dissolved organic carbon and particulate organic carbon on the shelf was estimated as 0.7 × 1011–1.5 × 1011 mol C using the data for the DIC of DSW and current velocity data from a mooring in the CDP. Our results provide quantitative estimates for the potential role of carbon consumption by the active primary production and carbon transport by dense water formation in Antarctic coastal polynyas. - Vigan Mensah, Kay OhshimaJOURNAL OF PHYSICAL OCEANOGRAPHY 50 8 2289 - 2313 2020年08月A time series analysis of the sea surface height anomaly (SSHa) was conducted in the Kuril Basin of the Sea of Okhotsk. The mapping of the satellite-derived SSHa data was optimized to mitigate the effects of sea ice on the SSHa field during winter and early spring. Complex empirical orthogonal functions (CEOFs) were then used to analyze the SSHa field, revealing that the first three modes account for 55% of the signal variance. Mode 1 mainly represents the coherent variability trapped over the shelves all along the coastal regions and the Kuril Islands. Both seasonal and interannual variations are strongly correlated with the alongshore wind stress and can be well explained by an arrested topographic wave. Mode 3 is a propagating mode that exhibits trains of southwestward-propagating, high-amplitude anomalies. One possible mechanism for this is first-mode baroclinic Rossby waves, whose energy propagates from the Kuril Straits toward the Kuril Basin. However, mode 3 can be better interpreted as barotropic Rossby normal modes generated in the deep Kuril Basin. Mode 2 is a standing mode that may encompass the baroclinic variability in the basin. The monthly mean of the SSHa in the Kuril Basin is primarily governed by variability in mode 1, with mode 2 contributing to a lesser extent, and mode 3 being insignificant.
- Kay I. Ohshima, Naoya Tamaru, Haruhiko Kashiwase, Sohey Nihashi, Kazuki Nakata, Katsushi IwamotoJournal of Geophysical Research: Oceans 125 7 2020年07月
- Sea ice observation with ocean HF radar.Zhang, W., N. Ebuchi, Y. Fukamachi, F. Cheng, K. I. Ohshima, B. Emery, T. Toyota, H. Abe, K. ShirasawaIEEE Transactions of Geoscience and Remote Sensing 58 1 378 - 390 2020年01月 [査読有り][通常論文]
- Masato Ito, Kay I. Ohshima, Yasushi Fukamachi, Daisuke Hirano, Andrew R. Mahoney, Joshua Jones, Toru Takatsuka, Hajo EickenJournal of Geophysical Research: Oceans 124 12 8701 - 8719 2019年12月01日 [査読有り][通常論文]
©2019. American Geophysical Union. All Rights Reserved. Arctic sea ice incorporates and transports sediment, releasing it back into the water column during the melting season. This process constitutes an important aspect of marine sediment transport and biogeochemical cycling. Sediment incorporation into sea ice is considered to occur mainly through underwater interaction between frazil ice and resuspended sediment, referred to as suspension freezing. However, harsh environmental conditions have greatly limited field observations of this phenomenon. Analysis of mooring data from a coastal polynya in the northeastern Chukchi Sea, in conjunction with coastal ice radar and meteorological data, indicates that suspension freezing is a key mechanism for sediment entrainment into sea ice. During polynya episodes, acoustic backscatter data obtained by an Acoustic Doppler Current Profiler showed the presence of frazil ice from the surface down to 20- to 25-m depth, coinciding with in situ and potential supercooling. Underwater frazil ice persisted over 1 week under windy, turbulent water column conditions. A combination of the turbidity and Acoustic Doppler Current Profiler backscatter data revealed upward sediment dispersion associated with strong currents during the polynya episodes. The fact that frazil ice and resuspended sediment were detected at the same depth and time strongly suggests the interaction between ice crystals and sediment particles, that is, suspension freezing. - Fraser Alexander D, Ohshima Kay I., Nihashi Sohey, Massoma Robert A, Tamura Takeshi, Nakata Kazuki, Williams Guy D, Carpentier Scott, Willmes SaschaREMOTE SENSING OF ENVIRONMENT 233 111315 - 111315 2019年11月 [査読有り][通常論文]
- Zhang, W, N. Ebuchi, Y. Fukamachi, F. Cheng, K. I. Ohshima, B. Emery, T. Toyota, H. Abe, K. ShirasawaIEEE Transactions on Geoscience and Remote Sensing 1 - 13 2019年09月 [査読有り][通常論文]
- Kashiwase Haruhiko, Ohshima Kay I., Fukamachi Yasushi, Nihashi Sohey, Tamura TakeshiJOURNAL OF ATMOSPHERIC AND OCEANIC TECHNOLOGY 36 8 1623 - 1641 2019年08月 [査読有り][通常論文]
Abstract The quantification of sea ice production in coastal polynyas is a key issue to understand the global climate system. In this study, we directly compared Advanced Microwave Scanning Radiometer-EOS (AMSR-E) data with the sea ice thickness distribution obtained from a mooring observation during the winter of 2003 off Sakhalin in the Sea of Okhotsk to evaluate the algorithm for estimation of sea ice thickness in coastal polynyas. By using thermal ice thickness as a target physical quantity, we found that the obtained relationship between the polarization ratio (PR) and ice thickness can provide an appropriate AMSR-E algorithm to estimate thin ice thickness, irrespective of the uniform or nonuniform ice thickness field. The relationship between the PR value and thermal ice thickness is likewise consistent with the local PR–thickness relationship that is observed at individual ice floes. This is because both the PR value and thermal ice thickness are more sensitive to thinner ice. Furthermore, we evaluated the method for detection of active frazil in a coastal polynya by comparing with the mooring data, and subsequently modified it to classify the coastal polynya into three thin ice types, namely, active frazil, thin solid ice, and mixed ice (mixture of active frazil and thin solid ice). The improved algorithm successfully represents the thermal ice thickness even for a relatively small-scale polynya off Sakhalin and is expected to be useful for better quantification of sea ice production in the global ocean owing to its high versatility. - Preusser Andreas, Ohshima Kay I., Iwamoto Katsushi, Willmes Sascha, Heinemann GuentherJOURNAL OF GEOPHYSICAL RESEARCH-OCEANS 124 8 5503 - 5528 2019年08月 [査読有り][通常論文]
- Yujiro Kitade, Keishi Shimada, Shigeru Aoki, Kay I. OhshimaOceanography Challenges to Future Earth: Human and Natural Impacts on our Seas 123 - 135 2019年02月15日© Springer Nature Switzerland AG 2019. All rights reserved. Training research vessel Umitaka-Maru belonging to the Tokyo University of Marine Science and Technology has implemented the research cruises in the Southern Ocean, counting the 19th times in the 2015 cruise. Particularly in recent years, Umitaka-Maru has greatly contributed to various findings in oceanography in the Antarctic Ocean, such as clarification of the bottom water formation and monitoring in Lutzow Holm Bay, cooperating with the Japanese Antarctic Research Expedition. This article reviews such scientific progresses implemented by Umitaka-Maru, and further describes the recent trend of Antarctic Bottom Water from the newly obtained data. Most important findings by Umitaka-Maru are the discovery of the Antarctic bottom water (AABW) formation off the Cape Darnley with a high sea ice production polynya and the subsequent discovery of AABW formation off the Vincennes Bay with a medium sized polynya. The base oceanographic observation for physical and chemical properties has been carried out continuously along 110°E line in Australian Antarctic Basin, providing the monitoring of water mass transformation. Freshening of AABW in Australia-Antarctica Basin has been caught in a series of the observation in recent years, it is suggested that the AABW originating from the Vincennes Bay Polynya gives significant effects on the AABW in the basin. To grasp an effect of freshening of AABW on the global ocean circulation, intense hydrographic observations and deployment of a huge mooring system are now in consideration.
- Arctic Technology Research Forum to consolidate opinions form industryKikuchi, T, H. Yamaguchi, H. Yoshinaga, S. Fukuba, M. Miwa, T. Kashiwagi, H. Yoshida, K. I. Ohshima, Y. Fukamachi, H. Yabuki, Y. KodamaProceedings of the 34th International Symposium on the Okhotsk Sea & Polar Oceans 2019 171 - 175 2019年02月 [査読無し][通常論文]
- Study on Oil Spill Response Technology in Cold Water Condition - Lessons learned from Exxon Valdes and Deepwater Horizon -Nakazawa, N, H. Yamaguchi, A. Kurokawa, J. Ono, T. Terashima, G. Sagawa, S. Yano, Y. Fukamachi, K. I. OhshimaProceedings of the 34th International Symposium on the Okhotsk Sea & Polar Oceans 2019 190 - 192 2019年02月 [査読無し][通常論文]
- Mensah Vigan, Ohshima Kay I., Nakanowatari Takuya, Riser StephenDEEP-SEA RESEARCH PART I-OCEANOGRAPHIC RESEARCH PAPERS 144 115 - 131 2019年02月 [査読有り][通常論文]
The seasonal variability of the hydrography and anticyclonic circulation in the Kuril Basin of the Sea of Okhotsk, from which the North Pacific ventilation originates, is studied from bi-monthly climatologies of temperature, salinity, dynamic height and current velocity. The subsurface hydrography of the Kuril Basin is characterized by the dominant role of fresh and cold waters transported by the East Sakhalin Current during winter and spring, and relatively warm and saline waters flowing with the Soya Warm Current. At the intermediate layer, the influence of dense shelf water is maximum from May to August, and this water mixes with surrounding waters in around half a year to form a well homogenized Okhotsk Sea Intermediate Water, the source of North Pacific ventilation. The anticyclonic circulation typical of the Kuril Basin exhibits a strong seasonality, being absent in winter and showing a maximum amplitude in summer. Its formation is mainly related to the dynamic response of isopycnals to wind forcing. A dynamic height maximum appears along the coastal regions from Sakhalin Island to the southern Kuril Straits in winter, likely migrates towards the Kuril Basin in March April and forms the anticyclonic system between May and October. The dynamic height and thus the flow field is mainly determined by isopycnal displacement in and around the Kuril Basin, whereas advection of water masses has a negligible effect. Surface intensification and decay of the anticyclonic circulation seems to be related to other processes such as heat exchange with the atmosphere. - Nakata Kazuki, Ohshima Kay I., Nihashi SoheyIEEE TRANSACTIONS ON GEOSCIENCE AND REMOTE SENSING 57 1 263 - 276 2019年01月 [査読有り][通常論文]
- Daisuke Hirano, Yasushi Fukamachi, Kay I. Ohshima, Eiji Watanabe, Andrew R. Mahoney, Hajo Eicken, Motoyo Itoh, Daisuke Simizu, Katsushi Iwamoto, Joshua Jones, Toru Takatsuka, Takashi Kikuchi, Takeshi TamuraJournal of Geophysical Research: Oceans 123 8 5688 - 5705 2018年08月 [査読有り][通常論文]
©2018. American Geophysical Union. All Rights Reserved. Water properties and formation processes of Alaskan Coastal Winter Water (ACWW) over the eastern Chukchi shelf along the Alaska coast, the so-called Barrow Canyon pathway, are examined using data from moorings, atmospheric reanalysis, satellite-derived sea-ice production (SIP), and a numerical tracer experiment. Along this pathway, Pacific Winter Water (PWW) can be modified to produce ACWW through SIP accompanied by production of cold, saline polynya water in the coastal polynyas, upwelling of warm Atlantic Water (AW), and mixing processes on the shelf. Three different types of ACWW are formed: (i) a mixture of AW and PWW, (ii) a mixture of AW and polynya water, and (iii) hypersaline polynya water. The northeasterly winds, correlated with the north-south atmospheric pressure gradient between Beaufort High and Aleutian Low, are common triggers of polynya SIP episodes and AW upwelling in the Barrow Coastal Polynya (BCP). Due to the dual impact of northeasterly winds, PWW modification processes in the BCP are more complicated than what occurs elsewhere in the Chukchi Polynya. The impact of AW upwelling on the ACWW formation is most prominent in the BCP, usually centered along the coast. All types of ACWW are thought to be basically transported westward or northwestward with the Chukchi slope current and/or Beaufort Gyre and finally contribute to maintenance of the lower halocline layer especially over the Chukchi Borderland, Northwind Ridge, and southern Canada Basin. Even in the Pacific sector of the Arctic Ocean, ACWW properties are strongly influenced by both Atlantic-origin and Pacific-origin waters. - Nihashi Sohey, Kurtz Nathan T, Markus Thorsten, Ohshima Kay I., Tateyama Kazutaka, Toyota TakenobuANNALS OF GLACIOLOGY 59 76 101 - 111 2018年07月 [査読有り][通常論文]
- Daiki Nomura, Mats A. Granskog, Agneta Fransson, Melissa Chierici, Anna Silyakova, Kay I. Ohshima, Lana Cohen, Bruno Delille, Stephen R. Hudson, Gerhard S. DieckmannBiogeosciences 15 11 3331 - 3343 2018年06月05日 [査読有り][通常論文]
Abstract. Rare CO2 flux measurements from Arctic pack ice show that two types of ice contribute to the release of CO2 from the ice to the atmosphere during winter and spring: young, thin ice with a thin layer of snow and older (several weeks), thicker ice with thick snow cover. Young, thin sea ice is characterized by high salinity and high porosity, and snow-covered thick ice remains relatively warm ( > −7.5 °C) due to the insulating snow cover despite air temperatures as low as −40 °C. Therefore, brine volume fractions of these two ice types are high enough to provide favorable conditions for gas exchange between sea ice and the atmosphere even in mid-winter. Although the potential CO2 flux from sea ice decreased due to the presence of the snow, the snow surface is still a CO2 source to the atmosphere for low snow density and thin snow conditions. We found that young sea ice that is formed in leads without snow cover produces CO2 fluxes an order of magnitude higher than those in snow-covered older ice (+1.0 ± 0.6 mmol C m−2 day−1 for young ice and +0.2 ± 0.2 mmol C m−2 day−1 for older ice). - Monitoring of the Soya Warm Current by HF ocean radars since 2003.Ebuchi, N, Y. Fukamachi, K. I. Ohshima, T. Takatsuka, M. Ishikawa, K. Shirasawa, M. WakatsuchiProceedings of the 4th ORCA 2018年06月 [査読無し][通常論文]
- Yasushi Fukamachi, Daisuke Simizu, Kay I. Ohshima, Hajo Eicken, Andrew R. Mahoney, Katsushi Iwamoto, Erika Moriya, Sohey NihashiJOURNAL OF GLACIOLOGY 63 241 888 - 898 2017年10月 [査読有り][通常論文]
Time series ice-draft data were obtained from moored ice-profiling sonar (IPS), in the coastal northeastern Chukchi Sea during 2009/10. Time series data show seasonal growth of sea-ice draft, occasionally interrupted by coastal polynya. The sea-ice draft distribution indicates a slightly lower abundance of thick, deformed ice compared with the eastern Beaufort Sea. In January, a rapid increase in the abundance of thick ice coincided with a period of minimal drift indicating compaction again the coast and dynamical thickening. The overall mean draft and corresponding derived thickness are 1.27 and 1.38 m, respectively. The evolution of modal ice thickness observed can be explained mostly by thermodynamic growth. The derived ice thicknesses are used to estimate heat losses based on ERA-interim data. Heat losses from the raw, 1 s IPS data are similar to 50 and 100% greater than those calculated using IPS data averaged over spatial scales of similar to 20 and 100 km, respectively. This finding demonstrates the importance of subgrid-scale ice-thickness distribution for heat-loss calculation. The heat-loss estimate based on thin ice data derived from AMSR-E data corresponds well with that from the 1 s observed ice-thickness data, validating heat-loss estimates from the AMSR-E thin ice-thickness algorithm. - Sohey Nihashi, Kay I. Ohshima, Takeshi TamuraIEEE JOURNAL OF SELECTED TOPICS IN APPLIED EARTH OBSERVATIONS AND REMOTE SENSING 10 9 3912 - 3922 2017年09月 [査読有り][通常論文]
Antarctic coastal polynyas are very high sea-ice production areas. The resultant large amount of brine rejection leads to the formation of dense water. The dense water forms Antarctic bottom water, which is the densest water in the global overturning circulation and a key player in climate change as a significant sink for heat and carbon dioxide. In this study, an algorithm was developed that uses Advanced Microwave Scanning Radiometer 2 (AMSR2) data (2012-present) to detect polynya area and estimates thin ice thickness by a method similar to that used to develop the algorithm for Advanced Microwave Scanning Radiometer for EOS (AMSR-E) data. Landfast sea-ice areas were also detected using AMSR2 data. Ice production in the polynyas was estimated by a heat flux calculation using AMSR2 sea-ice data. In four major polynyas, AMSR2 ice production was compared with AMSR-E (2003-2011) ice production through comparison of them with Special Sensor Microwave Imager (SSM/I) and Special Sensor Microwave Imager/Sounder (SSMIS) ice production. The comparison confirmed that the ice production from AMSR-E/2 data, which have higher spatial resolution than SSM/I-SSMIS data, can be used to analyze time series covering more than 10 years. For example, maps of annual ice production based on AMSR-E/2 data revealed detailed changes of the Mertz Polynya, where the ice production decreased significantly after the Mertz Glacier Tongue calving in 2010. Continuous monitoring of the coastal polynyas by the AMSR series sensors is essential for climate-change-related analyses in the Antarctic Ocean. - Sohey Nihashi, Kay I. Ohshima, Sei-Ichi SaitohDEEP-SEA RESEARCH PART I-OCEANOGRAPHIC RESEARCH PAPERS 127 65 - 76 2017年09月 [査読有り][通常論文]
Sinking of the dense water plays a significant role in the global climate system by driving thermohaline (overturning) circulation and biogeochemical cycles. Deep water convection occurs also in the Japan Sea, and the convection has been considered to be mainly caused by intense cooling of the sea surface. Another possible cause of the convection is brine rejection associated with high sea-ice production in a coastal polynya (thin sea ice) area in the northern Japan Sea. We have developed an algorithm which detects the thin ice area and estimates the thickness using passive microwave satellite data. Based on a heat flux calculation with the satellite derived ice thickness, the total sea-ice production in winter (December March) averaged over 2002/03-2010/11 is estimated to be 4.27 x 10(10) m(3). This indicates that the ice production was underestimated by about half in a previous study in which the polynya was unrealistically treated as a low ice concentration area. The main determinant factor for the total ice production is the surface air temperature in early winter (December January), which shows a rapid warming trend of 0.7 degrees C/decade for this 40-years. Based on a linear regression approach, the total ice production is estimated to have decreased by similar to 5%/decade due to air temperature warming. If brine rejection due to the ice production contributes to the deep water formation in the Japan Sea, this is consistent with the fact that the deep water formation has been decreasing for the last 50-100 years. - Takuya Nakanowatari, Kay I. Ohshima, Vigan Mensah, Yoko Mitani, Kaoru Hattori, Mari Kobayashi, Fabien Roquet, Yasunori Sakurai, Humio Mitsudera, Masaaki WakatsuchiPOLAR SCIENCE 13 56 - 65 2017年09月 [査読有り][通常論文]
The Sea of Okhotsk is a challenging environment for obtaining in situ data and satellite observation in winter due to sea ice cover. In this study, we evaluated the validity of hydrographic observations by marine mammals (e.g., seals and sea lions) equipped with oceanographic conductivity-temperaturedepth (CTD) sensors. During 4-yr operations from 2011 to 2014, we obtained total of 997 temperature-salinity profiles in and around the Soya Strait, Iony Island, and Urup Strait. The hydrographic data were mainly obtained from May to August and the maximum profile depth in shelf regions almost reaches to the seafloor, while valuable hydrographic data under sea ice cover were also obtained. In strong thermoclines, the seal-derived data sometimes showed positive biases in salinity with spikelike signal. For these salinity biases, we applied a new thermal mass inertia correction scheme, effectively reducing spurious salinity biases in the seasonal thermocline. In the Soya Strait and the adjacent region, the detailed structure of the Soya Warm Current including the cold-water belt was well identified. Dense water up to 27.0 sigma(theta), which can be a potential source of Okhotsk Sea Intermediate Water, has flowed from the Soya Strait into the Sea of Okhotsk in mid-winter (February). In summer, around the Iony Island and Urup Strait, remarkable cold and saline waters are localized in the surface layers. These regions are also characterized by weak stratification, suggesting the occurrence of tidally induced vertical mixing. Thus, CTD-tag observations have a great potential in monitoring data-sparse regions in the Sea of Okhotsk. (C) 2017 The Authors. Published by Elsevier B.V. - Haruhiko Kashiwase, Kay I. Ohshima, Sohey Nihashi, Hajo EickenSCIENTIFIC REPORTS 7 8170 2017年08月 [査読有り][通常論文]
Ice-albedo feedback due to the albedo contrast between water and ice is a major factor in seasonal sea ice retreat, and has received increasing attention with the Arctic Ocean shifting to a seasonal ice cover. However, quantitative evaluation of such feedbacks is still insufficient. Here we provide quantitative evidence that heat input through the open water fraction is the primary driver of seasonal and interannual variations in Arctic sea ice retreat. Analyses of satellite data (1979-2014) and a simplified ice-upper ocean coupled model reveal that divergent ice motion in the early melt season triggers large-scale feedback which subsequently amplifies summer sea ice anomalies. The magnitude of divergence controlling the feedback has doubled since 2000 due to a more mobile ice cover, which can partly explain the recent drastic ice reduction in the Arctic Ocean. - Kay I. Ohshima, Daisuke Simizu, Naoto Ebuchi, Shuta Morishima, Haruhiko KashiwaseJOURNAL OF PHYSICAL OCEANOGRAPHY 47 5 999 - 1019 2017年05月 [査読有り][通常論文]
Volume, heat, and salt transports through the Soya Strait are estimated based on measurements from high-frequency ocean radars during 2003-15 and all available hydrographic data. The baroclinic velocity structure derived from the climatological geopotential anomaly is combined with the sea surface gradient obtained from radar-derived surface velocities to estimate the absolute velocity structure. The annual-mean volume, heat, and salt transports are 0.91 Sv (1 Sv = 10(6) m(3) s(-1)), 25.5 TW, and 31.15 x 10(6) kg s(-1), respectively. The volume transport exhibits strong seasonal variations, with a maximum of 1.41 Sv in August and a minimum of 0.23 Sv in January. The seasonal amplitude and phase roughly correspond to those of the Tsushima-Korea Strait. Time series of the monthly transport is presented for the 12 yr, assuming that the baroclinic components are the monthly climatological values. In cold seasons (November to April), the monthly volume transport is strongly correlated with the sea level difference between the Japan and Okhotsk Seas, and an empirical formula to estimate the transport from the sea level difference is introduced. It is likely that the sea level setup by the wind stress along the east coast of Sakhalin determines the sea level difference, which explains the seasonal and interannual wintertime variations of transport through the strait. The annual flux of water through the Soya Strait with a density greater than 26.8 sigma(theta), a potential source of Okhotsk Sea Intermediate Waters, is estimated to be 0.18 Sv. - Keishi Shimada, Shigeru Aoki, Kay I. OhshimaJOURNAL OF ATMOSPHERIC AND OCEANIC TECHNOLOGY 34 3 511 - 532 2017年03月 [査読有り][通常論文]
This study investigated a method for creating a climatological dataset with improved reproducibility and reliability for the Southern Ocean. Despite sparse observational sampling, the Southern Ocean has a dominant physical characteristic of a strong topographic constraint formed under weak stratification and strong Coriolis effect. To increase the fidelity of gridded data, the topographic constraint is incorporated into the interpolation method, the weighting function of which includes a contribution from bottom depth differences and horizontal distances. Spatial variability of physical properties was also analyzed to estimate a realistic decorrelation scale for horizontal distance and bottom depth differences using hydrographic datasets. A new gridded dataset, the topographic constraint incorporated (TCI), was then developed for temperature, salinity, and dissolved oxygen, using the newly derived weighting function and decorrelation scales. The root-meansquare (RMS) of the difference between the interpolated values and the neighboring observed values (RMS difference) was compared among available gridded datasets. That the RMS differences are smaller for the TCI than for the previous datasets by 12%-21% and 8%-20% for potential temperature and salinity, respectively, demonstrates the effectiveness of incorporating the topographic constraint and realistic decorrelation scales. Furthermore, a comparison of decorrelation scales and an analysis of interpolation error suggests that the decorrelation scales adopted in previous gridded datasets are 2 times or more larger than realistic scales and that the overestimation would increase the interpolation error. The interpolation method proposed in this study can be applied to other high-latitude oceans, which are weakly stratified but undersampled. - Masato Ito, Kay I. Ohshima, Yasushi Fukamachi, Genta Mizuta, Yoshimu Kusumoto, Jun NishiokaJOURNAL OF GEOPHYSICAL RESEARCH-OCEANS 122 2 788 - 802 2017年02月 [査読有り][通常論文]
In the Sea of Okhotsk, sediment incorporation, transport and release by sea ice potentially plays important roles in the bio-related material (such as iron) cycle and ecosystem. The backscatter strength data of bottom-mounted Acoustic Doppler Current Profilers have suggested signals of frazil ice down to 30 m depth, and signals of upward sediment transport throughout the water column simultaneously in the region northeast of Sakhalin, with a water depth of similar to 100 m. Such events occurred under turbulent conditions with strong winds of 10-20 m s(-1). During such events, newly formed ice was present near the observational sites, shown by satellite microwave imagery. Sediment dispersion from the bottom occurred in association with strong currents of 1.0-1.5 m s(-1). During these events, the mixed layer reaches near the bottom due to wind-induced stirring, inferred from the high frequency component of vertical velocity. Thus the winter time turbulent mixing brings re-suspended sediment up to near the ocean surface. This study provides the first observational evidence of a series of processes on the incorporation of sedimentary materials into sea ice: sedimentary particles are dispersed by the strong bottom current, subsequently brought up to near the surface by winter time mixing, and finally incorporated into sea ice through underwater interaction with frazil ice and/ or flooding of sea ice floes. This wintertime incorporation of bottom sediment into sea ice is a possible mechanism of iron supply to sea ice which melts in spring, and releases bio-reactive iron into the ocean. - Kay I. Ohshima, Sohey Nihashi, Katsushi IwamotoGeoscience Letters 3 1 2016年12月01日 [査読有り][通常論文]
Global overturning circulation is driven by density differences. Saline water rejected during sea-ice formation in polynyas is the main source of dense water, and thus sea-ice production is a key factor in the overturning circulation. Due to difficulties associated with in situ observation, sea-ice production and its interannual variability have not been well understood until recently. Methods to estimate sea-ice production on large scales have been developed using heat flux calculations based on satellite microwave radiometer data. Using these methods, we present the mapping of sea-ice production with the same definition and scale globally, and review the polynya ice production and its relationship with dense/bottom water. The mapping demonstrates that ice production rate is high in Antarctic coastal polynyas, in contrast to Arctic coastal polynyas. This is consistent with the formation of Antarctic Bottom Water (AABW), the densest water mass which occupies the abyssal layer of the global ocean. The Ross Ice Shelf polynya has by far the highest ice production in the Southern Hemisphere. The Cape Darnley polynya (65°E–69°E) is found to be the second highest production area and recent observations revealed that this is the missing (fourth) source of AABW. In the region off the Mertz Glacier Tongue (MGT), the third source of AABW, sea-ice production decreased by as much as 40 %, due to the MGT calving in early 2010, resulting in a significant decrease in AABW production. The Okhotsk Northwestern polynya exhibits the highest ice production in the Northern Hemisphere, and the resultant dense water formation leads to overturning in the North Pacific, extending to the intermediate layer. Estimates of its ice production show a significant decrease over the past 30–50 years, likely causing the weakening of the North Pacific overturning. These regions demonstrate the strong linkage between variabilities of sea-ice production and bottom/intermediate water formation. The mapping has also provided surface boundary conditions and validation data of heat- and salt-flux associated with sea-ice formation/melting for various ocean and coupled models. - Joshua Jones, Hajo Eicken, Andrew Mahoney, M. Rohith, Chandra Kambhamettu, Yasushi Fukamachi, Kay I. Ohshima, J. Craig GeorgeCONTINENTAL SHELF RESEARCH 126 50 - 63 2016年09月 [査読有り][通常論文]
Landfast sea ice is an important seasonal feature along most Arctic coastlines, such as that of the Chukchi Sea near Barrow, Alaska. Its stability throughout the ice season is determined by many factors but grounded pressure ridges are the primary stabilizing component. Landfast ice breakouts occur when these grounded ridges fail or unground, and previously stationary ice detaches from the coast and drifts away. Using ground-based radar imagery from a coastal ice and ocean observatory at Barrow, we have developed a method to estimate the extent of grounded ridges by tracking ice motion and deformation over the course of winter and have derived ice keel depth and potential for grounding from cumulative convergent ice motion. Estimates of landfast ice grounding strength have been compared to the atmospheric and oceanic stresses acting on the landfast ice before and during breakout events to determine prevailing causes for the failure of stabilizing features. Applying this approach to two case studies in 2008 and 2010, we conclude that a combination of atmospheric and oceanic stresses may have caused the breakouts analyzed in this study, with the latter as the dominant force. Preconditioning (as weakening) of grounded ridges by sea level variations may facilitate failure of the ice sheet leading to breakout events. (C) 2016 Elsevier Ltd. All rights reserved. - G. D. Williams, L. Herraiz-Borreguero, F. Roquet, T. Tamura, K. I. Ohshima, Y. Fukamachi, A. D. Fraser, L. Gao, H. Chen, C. R. McMahon, R. Harcourt, M. HindellNATURE COMMUNICATIONS 7 2016年08月 [査読有り][通常論文]
A fourth production region for the globally important Antarctic bottom water has been attributed to dense shelf water formation in the Cape Darnley Polynya, adjoining Prydz Bay in East Antarctica. Here we show new observations from CTD-instrumented elephant seals in 2011-2013 that provide the first complete assessment of dense shelf water formation in Prydz Bay. After a complex evolution involving opposing contributions from three polynyas (positive) and two ice shelves (negative), dense shelf water (salinity 34.65-34.7) is exported through Prydz Channel. This provides a distinct, relatively fresh contribution to Cape Darnley bottom water. Elsewhere, dense water formation is hindered by the freshwater input from the Amery and West Ice Shelves into the Prydz Bay Gyre. This study highlights the susceptibility of Antarctic bottom water to increased freshwater input from the enhanced melting of ice shelves, and ultimately the potential collapse of Antarctic bottom water formation in a warming climate. - Takeshi Tamura, Kay I. Ohshima, Alexander D. Fraser, Guy D. WilliamsJOURNAL OF GEOPHYSICAL RESEARCH-OCEANS 121 5 2967 - 2979 2016年05月 [査読有り][通常論文]
Enhanced sea ice production (SIP) in Antarctic coastal polynyas forms dense shelf water (DSW), leading to Antarctic Bottom Water (AABW) formation that ultimately drives the lower limb of the meridional overturning circulation. Some studies suggest that the variability of SIP in Antarctic coastal polynyas is driven by the influence of atmospheric forcing, i.e., surface winds and air temperature. Our previous mapping of SIP in 13 major Antarctic coastal polynyas from 1992 to 2007, using a heat flux calculation with ice thickness data derived from satellite data, is extended here to examine the interannual and seasonal variability of SIP from 1992 to 2013. The interannual variability of total ice production correlates more strongly with polynya extent than with atmospheric forcing, with the exception of the Shackleton Polynya, which correlates well with wind. There is no coherent signal in the interannual variability between the major Antarctic coastal polynyas. We find that stochastic changes to the coastal "icescape," i.e., ice shelves, floating glaciers, fast ice, together with offshore first-year ice, are also important factors driving SIP variability on multiyear time scales. Both the Ross Ice Shelf Polynya and Mertz Glacier Polynya experienced a significant reduction in SIP due to calving events and the repositioning of icebergs and fast ice. Our results also show opposing trends between polynya-based SIP and sea ice extent in key regions of Antarctic sea ice change. Close monitoring of coastal icescape dynamics and change is essential to better understand the long-term impact of coastal polynya variability and its influence on regional AABW production. - Fuko Sugimoto, Takeshi Tamura, Haruhito Shimoda, Shotaro Uto, Daisuke Simizu, Kazutaka Tateyama, Seita Hoshino, Toshihiro Ozeki, Yasushi Fukamachi, Shuki Ushio, Kay I. OhshimaPOLAR SCIENCE 10 1 43 - 51 2016年03月 [査読有り][通常論文]
Under the Japanese Antarctic Research Expedition (JARE) program, sea-ice thickness has been routinely monitored off Lutzow-Holm Bay (East Antarctica) during the summer (mid-December to early January) since 2000/01, using an electromagnetic induction (EM) instrument onboard the icebreaker Shirase. Analysis of these data over a 10-year period, combined with visual observations using a simplified form of the ASPeCt (Antarctic Sea ice Processes and Climate) protocol, suggests a strong interannual variability in sea-ice thickness in this region. For the repeat pack-ice observation area, where the sea-ice thickness averaged over the nine seasons is similar to 1.9 m, mean thicknesses of observed sea-ice in 2010/11 and 2011/12 are exceptionally large, at similar to 3.3 and similar to 5.8 m, respectively. This result is strongly related to regional patterns of sea ice dynamics. Ice convergence caused by anomalous northerly winds was particularly high in 2011/12, suggesting that the extremely thick ice observed in that season resulted largely from sea-ice deformation processes (including pressure ridging). Longer-term analysis of data from the past 34 years confirms that sea-ice conditions and thickness off Lutzow-Holm Bay in summer are determined mainly by the large-scale pattern of atmospheric pressure in December. (C) 2015 Elsevier B.V. and NIPR. All rights reserved. - Observations of frazil ice formation and sediment upward transport off the northeast coast of SakhalinIto, M, K. I. Ohshima, J. Nishioka, Y. Kusumoto, Y. FukamachiProceedings of the 31th International Symposium on Okhotsk Sea & Sea Ice 124 - 127 2016年02月 [査読無し][通常論文]
- Daisuke Hirano, Yasushi Fukamachi, Eiji Watanabe, Kay I. Ohshima, Katsushi Iwamoto, Andrew R. Mahoney, Hajo Eicken, Daisuke Simizu, Takeshi TamuraJOURNAL OF GEOPHYSICAL RESEARCH-OCEANS 121 1 980 - 997 2016年01月 [査読有り][通常論文]
The nature of the Barrow Coastal Polynya (BCP), which forms episodically off the Alaska coast in winter, is examined using mooring data, atmospheric reanalysis data, and satellite-derived sea-ice concentration and production data. We focus on oceanographic conditions such as water mass distribution and ocean current structure beneath the BCP. Two moorings were deployed off Barrow, Alaska in the northeastern Chukchi Sea from August 2009 to July 2010. For sea-ice season from December to May, a characteristic sequence of five events associated with the BCP has been identified; (1) dominant northeasterly wind parallel to the Barrow Canyon, with an offshore component off Barrow, (2) high sea-ice production, (3) upwelling of warm and saline Atlantic Water beneath the BCP, (4) strong up-canyon shear flow associated with displaced density surfaces due to the upwelling, and (5) sudden suppression of ice growth. A baroclinic current structure, established after the upwelling, caused enhanced vertical shear and corresponding vertical mixing. The mixing event and open water formation occurred simultaneously, once sea-ice production had stopped. Thus, mixing events accompanied by ocean heat flux from the upwelled warm water into the surface layer played an important role in formation/maintenance of the open water area (i.e., sensible heat polynya). The transition from a latent to a sensible heat polynya is well reproduced by a high-resolution pan-Arctic ice-ocean model. We propose that the BCP, previously considered to be a latent heat polynya, is a wind-driven hybrid latent and sensible heat polynya, with both features caused by the same northeasterly wind. - Kazuki Nakata, Kay I. Ohshima, Sohey Nihashi, Noriaki Kimura, Takeshi TamuraJOURNAL OF GEOPHYSICAL RESEARCH-OCEANS 120 9 6234 - 6252 2015年09月 [査読有り][通常論文]
We examined to what degree a simplified polynya model can explain a real polynya based on satellite-derived sea-ice data. In the model, the polynya area, defined as the frazil ice production region, is determined by a balance between the offshore consolidated ice drift and frazil ice production. We used daily polynya area, ice production, and ice drift data derived from AMSR-E. The study area is the Ross Ice Shelf Polynya (RISP), which has the highest sea-ice production in the Southern Ocean. As a modification of the original model to apply the available satellite data set, we introduced the lag time by which produced frazil ice is transported and accumulated at the polynya edge. The model represents a half (48-60%) of the polynya variability when using a lag time of 1.5 days. The frazil ice collection depth at the polynya edge, a key parameter in the model, is estimated to be approximate to 16 cm. The expansion of the RISP is achieved by ice divergence, and the contraction is achieved mostly by ice production. Both the wind and the remaining components (mainly regarded as the ocean current component) in the ice divergence are larger in the western part of the RISP, which explains the larger extent there. In the one-dimensional frame, assuming that the frazil ice produced within the RISP transforms into consolidated ice with a thickness of 16 cm, the frazil ice production (approximate to 1.7 x 10(3) m(2) d(-1)) within the RISP approximately balances the export (approximate to 1.6 x 10(3) m(2) d(-1)) of consolidated thin ice from the RISP edge. - Motoyo Itoh, Robert S. Pickart, Takashi Kikuchi, Yasushi Fukamachi, Kay I. Ohshima, Daisuke Simizu, Kevin R. Arrigo, Svein Vagle, Jianfeng He, Carin Ashjian, Jeremy T. Mathis, Shigeto Nishino, Carolina NobreDEEP-SEA RESEARCH PART I-OCEANOGRAPHIC RESEARCH PAPERS 102 43 - 54 2015年08月 [査読有り][通常論文]
Over the past few decades, sea ice retreat during summer has been enhanced in the Pacific sector of the Arctic basin, likely due in part to increasing summertime heat flux of Pacific-origin water from the Bering Strait. Barrow Canyon, in the northeast Chukchi Sea, is a major conduit through which the Pacific-origin water enters the Arctic basin. This paper presents results from 6 repeat high-resolution shipboard hydrographic/velocity sections occupied across Barrow Canyon in summer 2010. The different Pacific water masses feeding the canyon - Alaskan coastal water (ACW), summer Bering Sea water (BSW), and Pacific winter water (PWW) - all displayed significant intra-seasonal variability. Net volume transports through the canyon were between 0.96 and 1.70 Sv poleward, consisting of 0.41-0.98 Sv of warm Pacific water (ACW and BSW) and 0.28-0.65 Sv of PWW. The poleward heat flux also varied strongly, ranging from 8.56 TW to 24.56 TW, mainly due to the change in temperature of the warm Pacific water. Using supplemental mooring data from the core of the warm water, along with wind data from the Pt. Barrow weather station, we derive and assess a proxy for estimating heat flux in the canyon for the summer time period, which is when most of the heat passes northward towards the basin. The average heat flux for 2010 was estimated to be 3.34 TW, which is as large as the previous record maximum in 2007. This amount of heat could melt 315,000 km(2) of 1-meter thick ice, which likely contributed to significant summer sea ice retreat in the Pacific sector of the Arctic Ocean. (C) 2015 The Authors. Published by Elsevier Ltd. - Circumpolar Mapping of Antarctic Coastal Polynyas and Landfast Sea Ice: Relationship and VariabilitySohey Nihashi, Kay I. OhshimaJOURNAL OF CLIMATE 28 9 3650 - 3670 2015年05月 [査読有り][通常論文]
Sinking of dense water from Antarctic coastal polynyas produces Antarctic Bottom Water (AABW), which is the densest water in the global overturning circulation and is a key player in climate change as a significant sink for heat and carbon dioxide. Very recent studies have suggested that landfast sea ice (fast ice) plays an important role in the formation and variability of the polynyas and possibly AABW. However, they have been limited to regional and case investigations only. This study provides the first coincident circumpolar mapping of Antarctic coastal polynyas and fast ice. The map reveals that most of the polynyas are formed on the western side of fast ice, indicating an important role of fast ice in the polynya formation. Winds diverging from a boundary comprising both coastline and fast ice are the primary determinant of polynya formation. The blocking effect of fast ice on westward sea ice advection by the coastal current would be another key factor. These effects on the variability in sea ice production for 13 major polynyas are evaluated quantitatively. Furthermore, it is demonstrated that a drastic change in fast ice extent, which is particularly vulnerable to climate change, causes dramatic changes in the polynyas and possibly AABW formation that can potentially contribute to further climate change. These results suggest that fast ice and precise polynya processes should be addressed by next-generation models to produce more accurate climate projections. This study provides the boundary and validation data of fast ice and sea ice production for such models. - Takuya Nakanowatari, Humio Mitsudera, Tatsuo Motoi, Ichiro Ishikawa, Kay I. Ohshima, Masaaki WakatsuchiJOURNAL OF PHYSICAL OCEANOGRAPHY 45 4 988 - 1008 2015年04月 [査読有り][通常論文]
Using oceanographic observations and an eddy-resolving ice-ocean coupled model simulation from 1955 to 2004, the effects of the wind-driven ocean circulation change that occurred in the late 1970s during multidecadal-scale freshening of the North Pacific Intermediate Water (NPIW) at salinity minimum density (similar to 26.8 sigma(theta)) were investigated. An analysis of the observations revealed that salinity decreased significantly at the density range of 26.6-26.8 sigma(theta) in the western subtropical gyre, including the mixed water region (MWR). The temporal variability of the salinity is dominated by the marked change in the late 1970s. With results similar to the observations, the model, selectively forced by the interannual variability of the wind-driven ocean circulation, simulated significant freshening of the intermediate layer over the subtropical gyre. The significant freshening is related to the increase in southward transport of the Oyashio associated with the intensification of the Aleutian low. Accompanying these changes, the intrusion of fresh and low potential vorticity water, originating in the Okhotsk Sea, to the MWR increased, and the freshening signal propagated farther southward in the western subtropical gyre during the subsequent 6 yr, crossing the Kuroshio Extension. These results indicate that the multidecadal-scale freshening of the NPIW is partly caused by intensification of the wind-driven cross-gyre transport of the subarctic water to the subtropical gyre. - Daisuke Hirano, Yujiro Kitade, Kay I. Ohshima, Yasushi FukamachiJOURNAL OF GEOPHYSICAL RESEARCH-OCEANS 120 2 910 - 922 2015年02月 [査読有り][通常論文]
The mixing process associated with modified Shelf Water (mSW) overflows that eventually mix to form Cape Darnley Bottom Water (CDBW) was investigated by hydrographic and microstructure observations off the Cape Darnley Polynya (CDP), East Antarctica, in January 2009. Closely spaced microstructure observations revealed that mSW properties varied considerably within a distance of similar to 4 km across the shelf edge. Near the bottom, the rate of turbulent kinetic energy dissipation was enhanced to values greater than 1027 W kg(-1), and the vertical scale of the bottom boundary layer (BBL) was on the order of 10 m. The observed BBL around the shelf edge was characterized by strong vertical mixing with turbulent eddy diffusivities of similar to O(102321022) m(2) s(-1). A geostrophically balanced density current, which resulted from the presence of mSW over the continental shelf, is considered the primary energy source for the turbulent mixing in the BBL. This turbulent mixing transforms the overflowing mSW through mixing with ambient water masses, specifically with the overlying modified Circumpolar Deep Water. The BBL is also thought to partly contribute to the gradual descent of mSW down the continental slope through bottom Ekman transport. We conclude that turbulent mixing, primarily caused by a density current, plays an important role in CDBW formation, by modifying the mSW overflowing from the CDP. - Takeshi Tamura, Kay I. Ohshima, Jan L. Lieser, Takenobu Toyota, Kazutaka Tateyama, Daiki Nomura, Kazuki Nakata, Alexander D. Fraser, Peter W. Jansen, Kym B. Newbery, Robert A. Massom, Shuki UshioANNALS OF GLACIOLOGY 56 69 436 - 444 2015年 [査読有り][通常論文]
Accurately measuring and monitoring the thickness distribution of thin ice is crucial for accurate estimation of ocean atmosphere heat fluxes and rates of ice production and salt flux in ice-affected oceans. Here we present results from helicopter-borne brightness temperature (TB) measurements in the Southern Ocean in October 2012 and in the Sea of Okhotsk in February 2009 carried out with a portable passive microwave (PMW) radiometer operating at a frequency of 36 GHz. The goal of these measurements is to aid evaluation of a satellite thin-ice thickness algorithm which uses data from the spaceborne Advanced Microwave Scanning Radiometer Earth Observing System AMSR-E) or the Advanced Microwave Scanning Radiometer-II (AMSR-II). AMSR-E and AMSR-II TB agree with the spatially collocated mean TB from the helicopter-borne measurements within the radiometers' precision. In the Sea of Okhotsk in February 2009, the AMSR-E 36 GHz TB values are closer to the mean than the modal TB values measured by the helicopter-borne radiometer. In an Antarctic coastal polynya in October 2012, the polarization ratio of 36 GHz vertical and horizontal TB is estimated to be 0.137 on average. Our measurements of the TB at 36 GHz over an iceberg tongue suggest a way to discriminate it from sea ice by its unique PMW signature. - Andrew R. Mahoney, Hajo Eicken, Yasushi Fukamachi, Kay I. Ohshima, Daisuke Simizu, Chandra Kambhamettu, M. V. Rohith, Stefan Hendricks, Joshua JonesANNALS OF GLACIOLOGY 56 69 363 - 372 2015年 [査読有り][通常論文]
Data from the Seasonal Ice Zone Observing Network (SIZONet) acquired near Barrow, Alaska, during the 2009/10 ice season allow novel comparisons between measurements of ice thickness and velocity. An airborne electromagnetic survey that passed over a moored Ice Profiling Sonar (IPS) provided coincident independent measurements of total ice and snow thickness and ice draft at a scale of 10 km. Once differences in sampling footprint size are accounted for, we reconcile the respective probability distributions and estimate the thickness of level sea ice at 1.48 +/- 0.1 m, with a snow depth of 0.12 +/- 0.07 m. We also complete what we believe is the first independent validation of radar-derived ice velocities by comparing measurements from a coastal radar with those from an under-ice acoustic Doppler current profiler (ADCP). After applying a median filter to reduce high-frequency scatter in the radar-derived data, we find good agreement with the ADCP bottom-tracked ice velocities. With increasing regulatory and operational needs for sea-ice data, including the number and thickness of pressure ridges, coordinated observing networks such as SIZONet can provide the means of reducing uncertainties inherent in individual datasets. - Masato Ito, Kay I. Ohshima, Yasushi Fukamachi, Daisuke Simizu, Katsushi Iwamoto, Yoshimasa Matsumura, Andrew R. Mahoney, Hajo EickenANNALS OF GLACIOLOGY 56 69 307 - 314 2015年 [査読有り][通常論文]
Formation of supercooled water and frazil ice was studied in the Chukchi Sea coastal polynya off Barrow, Alaska, USA, in winter 2009/10, using moored salinity/temperature sensors and Ice Profiling Sonar (IPS) data along with satellite data. Oceanographic data from two moorings revealed episodic events of potential supercooling at 30-40 m depth, including the possibility of in situ supercooling, while the polynya was open. We identified frazil ice-like signals in the IPS data down to 5-15 m depth, associated with large heat loss and windy, turbulent conditions in an active polynya. This likely represents the first IPS observation of frazil ice in the marine environment. On the day of the maximum signal of frazil ice, spaceborne synthetic aperture radar shows streaks of high backscatter within the polynya, indicating active frazil ice formation just downwind of the mooring sites. In addition, the longer-term potential supercooling that persisted for 1-3 weeks occurred twice despite the absence of polynya activity at the mooring sites. These events occurred during periods dominated by the northeastward current. A series of coastal polynyas had formed southwest of the mooring sites prior to these events. Thus, the water masses with potential supercooling were likely advected from these polynyas. - Yoshimasa Matsumura, Kay I. OhshimaANNALS OF GLACIOLOGY 56 69 373 - 382 2015年 [査読有り][通常論文]
A new modelling framework using Lagrangian particle tracking has been developed to assess dynamic and thermodynamic effects of underwater frazil ice. This frazil-ice model treats a Lagrangian particle as a bulk cluster of many frazil crystals, and calculates the thermodynamic growth of each particle and the corresponding budget of latent heat and fresh water. The effective density and viscosity of sea water depend on the mass fraction of underwater frazil ice, and hence affect ocean convection. An idealized experiment using our model successfully reproduces the formation of underwater frazil ice and its transition to grease ice at the surface. Because underwater frazil ice does not reduce the atmosphere/ocean heat exchange, surface heat flux and net sea-ice production in the experiment with frazil ice are relatively high compared with the experiment where surface cooling directly leads to columnar growth of a solid ice cover which effectively insulates the heat flux. These results suggest that large-scale sea-ice models which do not take account of the effects of frazil ice might underestimate atmosphere/ocean heat exchange, particularly at times of active new ice formation. - Takuya Nakanowatari, Tomohiro Nakamura, Keisuke Uchimoto, Hiroki Uehara, Humio Mitsudera, Kay I. Ohshima, Hiroyasu Hasumi, Masaaki WakatsuchiJOURNAL OF CLIMATE 28 2 714 - 736 2015年01月 [査読有り][通常論文]
Causes of the multidecadal-scale warming of the intermediate water in the Okhotsk Sea and the western subarctic North Pacific during 1980-2008 are investigated using an ice-ocean coupled model with interannually varying atmospheric forcing. A hindcast experiment qualitatively reproduces the warming and decadal fluctuations of the intermediate water that are similar to those of observations: the warming is significant along the western part of the Okhotsk Sea and subarctic frontal region. The effects of the thermohaline- and wind-driven ocean circulation on the warming are evaluated from perturbation experiments on thermohaline (turbulent heat and freshwater fluxes) and wind causes, respectively. The thermohaline causes are shown to contribute positively to warming in the Okhotsk Sea Intermediate Water (OSIW). The heat budget analysis for the OSIW indicates that the warming is related to a decrease in cold and dense shelf water (DSW) flux, which is caused by a decrease in sea ice and surface water freshening. In contrast, the wind cause has a cooling effect in the OSIW through an increase in DSW. In the subarctic frontal region, the warming is mainly caused by the wind stress change. The heat budget analysis indicates that the warming is related to an increase in the northward advection of the subtropical warm water. These results imply that both thermohaline- and wind-driven ocean circulation changes are essential components of the warming in the intermediate water. The atmospheric conditions responsible for the warming are related to a weakened Aleutian low and Siberian high in early and late winter. - Yoshihiro Nakayama, Kay I. Ohshima, Yoshimasa Matsumura, Yasushi Fukamachi, Hiroyasu HasumiJOURNAL OF PHYSICAL OCEANOGRAPHY 44 11 2921 - 2937 2014年11月 [査読有り][通常論文]
At several locations around Antarctica, dense water is formed as a result of intense sea ice formation. When this dense water becomes sufficiently denser than the surrounding water, it descends the continental slope and forms Antarctic Bottom Water (AABW). This study presents the AABW formation off the coast of Cape Darnley [Cape Darnley Bottom Water (CDBW)] in East Antarctica, using a nonhydrostatic model. The model is forced for 8 months by a temporally uniform surface salt flux (because of sea ice formation) estimated from Advanced Microwave Scanning Radiometer for Earth Observing System (EOS; AMSR-E) data and a heat budget calculation. The authors reproduce AABW formation and associated periodic downslope flows of dense water. Descending pathways of dense water are largely determined by the topography; most dense water flows into depressions on the continental shelf, advects onto the continental slope, and is steered downslope to greater depths by the canyons. Intense sea ice formation is the most important factor in the formation of AABW off Cape Darnley, and the existence of depressions is of only minor importance for the flux of CDBW. The mechanism responsible for the periodic downslope flow of dense water is further analyzed using an idealized model setup. The period of dense water outflow is regulated primarily by the topographic beta effect. - Kazuya Kusahara, Kay I. OhshimaJOURNAL OF PHYSICAL OCEANOGRAPHY 44 11 2909 - 2920 2014年11月 [査読有り][通常論文]
The Southern Ocean allows circumpolar structure and the Antarctic coastline plays a role as a waveguide for oceanic Kelvin waves. Under the cyclic conditions, the horizontal wavenumbers and frequencies for circum-polarly propagating waves are quantized, with horizontal wavenumbers 1, 2, and 3, corresponding to periods of about 32, 16, and 11 h, respectively. At these frequencies, westward-propagating signals are detected in sea level variation observed at Antarctic coastal stations. The occurrence frequency of westward-propagating signals far exceeds the statistical significance, and the phase speed of the observed signal agrees well with the theoretical phase speed of external Kelvin waves. Therefore, this study concludes that the observed, westward-propagating sea level variability is a signal of the external Kelvin waves of wavenumbers 1, 2, and 3 around Antarctica. A series of numerical model experiments confirms that Kelvin waves around Antarctica are driven by surface air pressure and that these waves are excited not only by local forcing over the Southern Ocean, but also by remote forcing over the Pacific Ocean. Sea level variations generated over the Pacific Ocean can travel to the western side of the South American coast and cross over Drake Passage to the Antarctic continent, constituting a part of the Kelvin waves around Antarctica. - Kay I. Ohshima, Takuya Nakanowatari, Stephen Riser, Yuri Volkov, Masaaki WakatsuchiPROGRESS IN OCEANOGRAPHY 126 71 - 79 2014年08月 [査読有り][通常論文]
A recently constructed hydrographic dataset from the Okhotsk Sea reveals a prominent freshening to depths of similar to 500 m during the past four decades, with the maximum in the northwestern part of the sea. Averaged over the sea, this freshening corresponds to an input of 0.55 m of freshwater. This leads to a decrease in density of the intermediate water and deepening of the isopycnals, with the maximum deepening at 26.8 sigma(theta) of similar to 60 m averaged over the sea. The intermediate water is significantly warmed along the pathway of dense shelf water (DSW). A simple box model shows that DSW production has decreased by similar to 30% during the past four decades. We propose that the freshening and DSW reduction are caused by the weakening of salt/freshwater redistribution through sea ice decline as well as by the increase of excess precipitation over evaporation. Since the overturning in the North Pacific originates from the Okhotsk Sea through the DSW, these changes possibly weaken the shallow overturning of the North Pacific. (C) 2014 Elsevier Ltd. All rights reserved. - Haruhiko Kashiwase, Kay I. Ohshima, Sohey NihashiPROGRESS IN OCEANOGRAPHY 126 21 - 32 2014年08月 [査読有り][通常論文]
Overturning in the North Pacific extending to the intermediate layer (about 200-800 m depth) originates from the sinking of dense shelf water (DSW) formed by sea ice production in the Okhotsk coastal polynyas. It has been suggested that this overturning has weakened during the past 50 years. The purpose of this study is to clarify the long-term variability of sea ice production in the polynyas and to discuss its linkage with DSW formation and the overturning. First, we have developed a thin ice thickness retrieval algorithm using Special Sensor Microwave/Imager (SSM/I) data for the Sea of Okhotsk, and have estimated the ice production for 21 years (1988-2008) by calculating heat flux from SSM/I-derived ice thickness. From a comparison with atmospheric variables, it is suggested that interannual variability of sea ice production in the polynyas can be explained mainly by three atmospheric parameters: autumn air temperature northwest of the sea, winter air temperature north of the sea, and late winter offshoreward wind speed north of the sea. By using these parameters from atmospheric reanalysis data, the annual ice production for the 34 years period from 1974 to 2008 is reconstructed from a multiple regression coefficient analysis. The reconstructed ice production shows a significant decreasing trend of similar to 11.4% over 34 years, which is mainly explained by the warming of autumn air temperature. It is also found that the variation in the annual total ice production corresponds well with the potential temperature variation in Okhotsk Sea Intermediate Water (OSIW). This first observational evidence of a linkage between the annual total ice production and OSIW supports a hypothesis that decreasing ice production in the Okhotsk coastal polynyas, at least in part, has led to weakening of the overturning in the North Pacific. (C) 2014 Elsevier Ltd. All rights reserved. - Masahiro Yagi, Ichiro Yasuda, Takahiro Tanaka, Yuki Tanaka, Kazuya Ono, Kay I. Ohshima, Katsurou KatsumataPROGRESS IN OCEANOGRAPHY 126 121 - 134 2014年08月 [査読有り][通常論文]
Distribution of the turbulent kinetic energy dissipation rate a across the Bussol' Strait was re-evaluated by applying an improved method using density inversions to tide-resolving density observations. The improved estimates without excessive rejection of density inversions provide about three-times higher mean values of epsilon (1.45 x 10(-7) W/kg), vertical diffusivity K-rho, (153 x 10(-4) m(2)/s) and depth-integrated epsilon 187 mW/m(2)) in comparison with previous estimates. One-day mean vertical profiles of epsilon in the potential density co-ordinate allows us to estimate diapycnal velocity at 3 x 10(-5) m/s downwards through the 26.85 sigma(theta) isopycnal surface, and was almost zero through 26.7 sigma(theta), suggesting the thinning of Okhotsk Sea Mode Water or of the core of North Pacific Intermediate Water as the water flows through the Bussol' Strait from the Sea of Okhotsk to the North Pacific. The re-evaluated turbulence and current distributions confirmed following previous knowledge but with quantitative improvements and/or expansion of coverage: (1) All-averaged e and Kp show distinctive mid-depth maxima at 800-1600 m, 26.9-27.6 sigma(theta) and at a height of 300-400 m above the bottom, and enhanced 8 within 500 m of the bottom corresponding to the enhanced shear and low-Richardson number at vertical scales of less than 50 m. (2) A significant amount of the epsilon variance (14%) is explained by the 50 m-scale vertical shear composed of mean and diurnal tidal currents. (3) The diurnal tidal currents are well approximated by six low-mode topographically trapped waves. (C) 2014 Elsevier Ltd. All rights reserved. - Daisuke Simizu, Kay I. Ohshima, Jun Ono, Yasushi Fukamachi, Genta MizutaPROGRESS IN OCEANOGRAPHY 126 33 - 43 2014年08月 [査読有り][通常論文]
The Sea of Okhotsk is the southernmost sea-ice zone with sizable ice. It is widely believed that the prevailing northwesterly wind and the southward East Sakhalin Current (ESC) are the two main factors that drive the southward drift of sea ice. However, the relative contributions of these factors have not been understood. In this paper, by using the current and ice-drift data measured with the moored Acoustic Doppler Current Profiler, a 3-D ocean model simulation, objective analysis data of the wind, and satellite sea-ice data, we examine to what degree and how the ice drift is determined by the wind and ocean current. From a linear regression of the observed ice drift, ocean current, and wind, the wind-forced component of the ice drift was best fitted when sea ice is assumed to move with a speed of 1.6% of the geostrophic wind with a turning angle of 17.6 degrees to the left of the wind. Such a relationship was adopted as the wind-drift component for all sea-ice pixels detected from Special Sensor Microwave Imager data. For the ocean-forced component of the ice drift, we adopted the current at 20 m depth from a numerical model simulation that reproduces well the ESC and its variability. We then evaluated the sea-ice drift over 46-54 degrees N during 1998-2005. For the southward drift of sea ice, the contribution of the wind component is found to be larger than the oceanic component, although the ocean contribution becomes larger, typically comparable to the wind contribution, near the coast and in the northern region where the ESC is stronger. We estimated the average annual cumulative southward ice transport to be 3.0 +/- 0.9 x 10(11) m(3) at 53 degrees N. This ice transport is comparable to the annual discharge of the Amur River. The ratio of wind to oceanic components in the transport is estimated to be similar to 1.2-1.8. We also conducted ice-drift simulations based on the modeled current velocity and the assumed wind drift of 1.5% geostrophic wind with a turning angle of 15 degrees to the left. The simulations reproduce well the ice drift north of 47 degrees N but not south of 47 degrees N, likely due to the poor representation of the current system at the latter, underevaluation of the wind factor near the ice edges, and the neglect of ice formation and melt. (C) 2014 Elsevier Ltd. All rights reserved. - Takuya Nakanowatari, Kay I. OhshimaPROGRESS IN OCEANOGRAPHY 126 58 - 70 2014年08月 [査読有り][通常論文]
We investigated the seasonal and interannual variations of the sea level in and around the Sea of Okhotsk and their causes, based on tide gauge and satellite altimeter data. The sea level all along the coastal region of the Sea of Okhotsk is found to be dominated by the seasonal variation with a maximum in winter and a minimum in summer, which cannot be explained by the annual cycle of atmospheric heat flux and pressure. This sea level variation appears to reflect ocean current variations. Both the Arrested Topographic Waves (ATWs) caused by alongshore wind stress and the Sverdrup transport by wind stress curl show corresponding seasonal variations. Seasonal amplitude of the sea level is relatively large along Sakhalin Island with a tendency of a larger amplitude toward the south. This meridional dependence is consistent with the ATWs, but not with the Sverdrup transport in the Sea of Okhotsk. Seasonal variation of the geostrophic current velocity expected from the sea level variation is comparable to that of the observed nearshore current and is consistent with the theoretical ATW transport. It is also revealed that, on an interannual timescale, the wintertime sea level fluctuates quite coherently all around the Sea of Okhotsk and further along the East Kamchatka and Oyashio coasts in the North Pacific. The altimeter data clearly show that this coherent sea level variation is trapped over the coastal and continental shelf regions with depths shallower than 1000 m. The wintertime sea levels have a higher correlation with the ATW transport than with the Sverdrup transport in the Sea of Okhotsk and the upstream East Kamchatka coast. All these suggest that the interannual sea level variation along the coastal and shelf regions in winter, as well as the seasonal variation, is mainly caused by the ATWs (coastal trapped current forced by the alongshore wind stress). The wintertime Sverdrup transport, raised by the previous studies, is the secondary contributor to these variations. (C) 2014 Elsevier Ltd. All rights reserved. - Improved mapping of sea ice production in the Arctic Ocean using AMSR-E thin ice thickness algorithmKatsushi Iwamoto, Kay I. Ohshima, Takeshi TamuraJOURNAL OF GEOPHYSICAL RESEARCH-OCEANS 119 6 3574 - 3594 2014年06月 [査読有り][通常論文]
New and improved estimates of sea ice production in the Arctic Ocean are derived from AMSRE satellite and atmospheric reanalysis data for the period 2002-2011, at a spatial resolution of 6.25 km and using a newly developed fast-ice mask. High ice production in the major coastal polynyas is well demonstrated. The total annual cumulative ice production in the major 10 polynya regions is about 1180 +/- 70km(3). The interannual variability of the ice production for each polynya is presented during 2002-2011. No obvious relationship is noted between the ice production and the recent drastic reduction in the preceding summer Arctic sea ice extent. Most polynya regions exhibit maximum ice production in autumn (October November), before areas offshore have been covered with consolidated pack ice. Sea ice production from October to November in the marginal ice zone of the Pacific Ocean sector is negatively correlated with summer ice extent there. The ice production from October to November of 2007 (a record minimum summer ice extent) was about twice as large as that in other years. The high ice production area shifted to higher latitudes i.e., toward the deep Canada Basin, due to the retreat of the summer ice edge. We speculate that the resultant increase in brine input could change the oceanic structure in the basin, specifically deepening the winter mixed layer. - Yujiro Kitade, Keishi Shimada, Takeshi Tamura, Guy D. Williams, Shigeru Aoki, Yasushi Fukamachi, Fabien Roquet, Mark Hindell, Shuki Ushio, Kay I. OhshimaGEOPHYSICAL RESEARCH LETTERS 41 10 3528 - 3534 2014年05月 [査読有り][通常論文]
One year moorings at depths greater than 3000m on the continental slope off Vincennes Bay, East Antarctica, reveal the cold (<-0.5 degrees C) and fresh (<34.64) signals of newly formed Antarctic Bottom Water (AABW). The signal appeared in June, 3 months after the onset of active sea-ice production in the nearby Vincennes Bay Polynya (VBP). The AABW signal continued for about 5 months at two moorings, with 1 month delay at the western site further downstream. Ship-based hydrographic data are in agreement, detecting the westward spread of new AABW over the continental slope from VBP. On the continental shelf, Dense Shelf Water (DSW) formation is observed by instrumented seals, in and around the VBP during autumn, and we estimate its transport to be 0.16 +/- 0.07 (x 106m3s-1). We conclude that the DSW formed in this region, albeit from a modest amount of sea-ice production, nonetheless contributes to the upper layer of AABW in Australian-Antarctic Basin. - Improved mapping of sea ice production in the Arctic Ocean using AMSR-E thin ice thickness algorithmKatsushi Iwamoto, Kay I. Ohshima, Takeshi TamuraJournal of Geophysical Research: Oceans 119 6 3574 - 3594 2014年 [査読有り][通常論文]
New and improved estimates of sea ice production in the Arctic Ocean are derived from AMSR-E satellite and atmospheric reanalysis data for the period 2002-2011, at a spatial resolution of 6.25 km and using a newly developed fast-ice mask. High ice production in the major coastal polynyas is well demonstrated. The total annual cumulative ice production in the major 10 polynya regions is about 1180±70km 3. The interannual variability of the ice production for each polynya is presented during 2002-2011. No obvious relationship is noted between the ice production and the recent drastic reduction in the preceding summer Arctic sea ice extent. Most polynya regions exhibit maximum ice production in autumn (October - November), before areas offshore have been covered with consolidated pack ice. Sea ice production from October to November in the marginal ice zone of the Pacific Ocean sector is negatively correlated with summer ice extent there. The ice production from October to November of 2007 (a record minimum summer ice extent) was about twice as large as that in other years. The high ice production area shifted to higher latitudes i.e., toward the deep Canada Basin, due to the retreat of the summer ice edge. We speculate that the resultant increase in brine input could change the oceanic structure in the basin, specifically deepening the winter mixed layer. © 2014. American Geophysical Union. All Rights Reserved. - Aoki S, Kitade Y, Shimada K, Ohshima K. I., Tamura T, Bajish C. C, Moteki M, Rintoul S. RJOURNAL OF GEOPHYSICAL RESEARCH-OCEANS 118 11 6046 - 6063 2013年11月 [査読有り][通常論文]
- Kazuya Ono, Kay I. Ohshima, Tokihiro Kono, Katsuro Katsumata, Ichiro Yasuda, Masaaki WakatsuchiDEEP-SEA RESEARCH PART I-OCEANOGRAPHIC RESEARCH PAPERS 79 62 - 73 2013年09月 [査読有り][通常論文]
Strong vertical mixing in the Kuril Straits is believed to be an important contribution to the ventilation of the intermediate layer and water mass transformation in the North Pacific, especially for density greater than 27.1 sigma(theta). Furthermore, a recent modeling study has suggested that the vertical profile of vertical diffusivity is a key factor in the determination of thermohaline circulation in the North Pacific. Here we report the distribution of vertical diffusivity in the Bussol Strait, the main conduit of water exchange and a possible central site of strong mixing in the Kuril Straits. Our analysis is based on a set of highly densed CTD observations, with a total of 127 casts across the strait in 2001. Vigorous density inversions occurred in the strait with the largest vertical displacement being over 250 m. We estimated the vertical diffusivity coefficient K-rho from the Thorpe scale for all the CTD data. The vertical average of K-rho estimated from all the casts is 60 x 10(-4) m(2) s(-1). Overall, K-rho is relatively small in the upper 300 m (density range approximately 26.5-26.7 sigma(theta)), whereas it is relatively large below a depth of 500 m (density range of >26.8 sigma(theta)), with a maximum at the depths of 1100-1700 m. The distributions of K-rho and the amplitude of the diurnal tidal current are similar, suggesting that the mixing is caused by the strong diurnal tidal current. The amplification of the diurnal (tidal) current over slopes near the bottom causes the K-rho maximum at depths of similar to 1100-1700 m. We also introduce an empirical relationship between K-rho and the amplitude of the diurnal tidal current. The vertical diffusivity is one order of magnitude larger at the spring tide than at the neap tide, suggesting that there is extremely large variability of tidal mixing with the fortnightly modulation. In the intermediate layer at densities of 27.3-27.6 sigma(theta), large K-rho values (> 60 x 10(-4) m(2) s(-1)) corresponds well to the colder and less-saline water mass characterized in the Bussol' Strait, confirming that water mass transformation occurs locally in the strait through strong diapycnal mixing. (C) 2013 Elsevier Ltd. All rights reserved. - Jun Ono, Kay I. Ohshima, Keisuke Uchimoto, Naoto Ebuchi, Humio Mitsudera, Hajime YamaguchiJOURNAL OF OCEANOGRAPHY 69 4 413 - 428 2013年08月 [査読有り][通常論文]
To conduct the simulation of oil spills in the Sea of Okhotsk, we developed a three-dimensional, high-resolution ocean circulation model. The model particularly improved the reproducibility of velocity field during the strong stratification period. Particle-tracking experiments with the effects of evaporation and biodegradation were performed using the combined data of daily ocean currents from the present model and the hourly diurnal tidal currents from the tidal model. The results are shown by the relative concentration of the particles averaged over the 8 years of 1998-2005 based on the ensemble forecast idea. For the case of particles released from the Sakhalin II oil field, the particles deployed in September-January are carried southward by the East Sakhalin Current, finally arriving at the Hokkaido coast, after 60-90 days. The particles deployed in March-August are diffused offshore by the synoptic wind drift, and hardly transported to regions south of Sakhalin. For the case of particles released from the region off Prigorodnoye, the oil export terminal, after the diffusion by the synoptic wind drift, a part of them are carried offshore of Hokkaido by the Soya Warm Current. The particles released in November-April flow out to the Japan Sea through the Soya Strait, mainly by the synoptic wind drift and secondly by the diffusion due to strong tidal currents around the Soya Strait. By considering the effects of evaporation and biodegradation, the relative concentration of the particles is considerably decreased before arriving at the Hokkaido coast, particularly in the case of drift from the Sakhalin II oil field. - Kay I. Ohshima, Yasushi Fukamachi, Guy D. Williams, Sohey Nihashi, Fabien Roquet, Yujiro Kitade, Takeshi Tamura, Daisuke Hirano, Laura Herraiz-Borreguero, Iain Field, Mark Hindell, Shigeru Aoki, Masaaki WakatsuchiNature Geoscience 6 3 235 - 240 2013年03月 [査読有り][通常論文]
The formation of Antarctic Bottom Water-the cold, dense water that occupies the abyssal layer of the global ocean-is a key process in global ocean circulation. This water mass is formed as dense shelf water sinks to depth. Three regions around Antarctica where this process takes place have been previously documented. The presence of another source has been identified in hydrographic and tracer data, although the site of formation is not well constrained. Here we document the formation of dense shelf water in the Cape Darnley polynya (65°-69°E) and its subsequent transformation into bottom water using data from moorings and instrumented elephant seals (Mirounga leonina). Unlike the previously identified sources of Antarctic Bottom Water, which require the presence of an ice shelf or a large storage volume, bottom water production at the Cape Darnley polynya is driven primarily by the flux of salt released by sea-ice formation. We estimate that about 0.3-0.7 × 106 m3 s-1 of dense shelf water produced by the Cape Darnley polynya is transformed into Antarctic Bottom Water. The transformation of this water mass, which we term Cape Darnley Bottom Water, accounts for 6-13% of the circumpolar total. Copyright © 2013 Macmillan Publishers Limited. - K. Iwamoto, K. I. Ohshima, T. Tamura, S. NihashiINTERNATIONAL JOURNAL OF REMOTE SENSING 34 2 468 - 489 2013年 [査読無し][通常論文]
In this study, we have developed an algorithm for estimating thin ice thickness in the Chukchi Sea of the Arctic Ocean using Advanced Microwave Scanning Radiometer Earth Observing System (AMSR-E) data. The algorithm is based on comparisons between the polarization ratio (PR) of AMSR-E brightness temperatures from the 89 and 36 GHz channels (PR89 and PR36) and the thermal ice thickness. The thermal ice thickness is estimated from a heat budget calculation using the ice surface temperature from clear-sky Moderate-Resolution Imaging Spectroradiometer (MODIS) infrared data. Whereas coastal polynyas have been the main target of previous algorithms, this algorithm is also applicable for marginal ice zones. AMSR-E has twice the spatial resolution of Special Sensor Microwave/Imager (SSM/I) data and can therefore resolve polynyas at a smaller scale. Although the spatial resolution of the 89 GHz data (6.25 km) is twice that of the 36 GHz data (12.5 km), the 89 GHz data can be contaminated by atmospheric water vapour. We propose an exclusion method of data affected by water vapour to resolve this issue. A combined algorithm of thin ice and ice concentration is also discussed, in which the ice thickness can be estimated independently from the open water fraction in grid cells with less than 100% ice concentration. The PR-thickness relationship in this study is somewhat different from previous studies, which is likely due to the difference in prevailing ice types caused by background environmental conditions. - S. Aoki, Y. Kitade, K. Shimada, K. I. Ohshima, T. Tamura, C. C. Bajish, M. Moteki, S. R. RintoulJournal of Geophysical Research: Oceans 118 11 6046 - 6063 2013年 [査読有り][通常論文]
Long-term water mass changes during 1994-2012 are examined from nine repeat hydrographic sections in the Seasonal Ice Zone along 140°E, off Antarctica. Significant freshening trends are detected within most of the water masses from the bottom to surface. Bottom Water freshened by 0.008-0.009 decade -1 below isopycnal surfaces and its layer thickness decreased by 120-160 dbar decade-1 throughout the study period. In addition to general thinning, the layer thickness was anomalously thin in 2012, suggesting a possible link with the sudden calving of the Mertz Glacier Tongue and subsequent reduction in sea-ice production. Winter Water freshened by 0.03 decade-1 throughout the study period, with significant interannual variability. In the offshore region, a long-term increase in precipitation can explain a substantial portion of the freshening trend. The Lower Circumpolar Deep Water on the continental slope underwent freshening at the same rate as the Bottom Water during the last two decades. Modified Shelf Water also shows robust freshening at a rate of 0.03 decade-1. Combined with the freshening of near-surface and Bottom Water masses in this region, these data indicate freshening of the entire water column over the continental slope. This widespread freshening is broadly consistent with the enhancement of the global hydrological cycle, together with a possible acceleration of land ice melting. Key Points Antarctic Bottom Water freshened off Adelie Coast, Antarctica, during 1994-2012 Precipitation increase can cause a significant part of Winter Water freshening The freshening is consistent with the enhancement of global hydrological cycle ©2013. American Geophysical Union. All Rights Reserved. - Mapping of sea ice production in the Arctic Ocean using AMSR-E thin ice thickness algorithmIwamoto, K, K. I. Ohshima, T. TamuraProceedings of the 28th International Symposium on Okhotsk Sea & Sea Ice 2013 208 - 211 2013年 [査読無し][通常論文]
- T. Tamura, G. D. Williams, A. D. Fraser, K. I. OhshimaNATURE COMMUNICATIONS 3 2012年05月 [査読有り][通常論文]
Variability in dense shelf water formation can potentially impact Antarctic Bottom Water (AABW) production, a vital component of the global climate system. In East Antarctica, the George V Land polynya system (142-150 degrees E) is structured by the local 'icescape', promoting sea ice formation that is driven by the offshore wind regime. Here we present the first observations of this region after the repositioning of a large iceberg (B9B) precipitated the calving of the Mertz Glacier Tongue in 2010. Using satellite data, we find that the total sea ice production for the region in 2010 and 2011 was 144 and 134 km(3), respectively, representing a 14-20% decrease from a value of 168 km(3) averaged from 2000-2009. This abrupt change to the regional icescape could result in decreased polynya activity, sea ice production, and ultimately the dense shelf water export and AABW production from this region for the coming decades. - Continuous observation of the Soya Warm Current by HF ocean radar since 2003.Ebuchi, N, Y. Fukamachi, K. I. Ohshima, M. WakatsuchiProceedings of First Ocean Radar Conference for Asia 82 - 86 2012年05月 [査読無し][通常論文]
- Sohey Nihashi, Kay I. Ohshima, Noriaki KimuraJOURNAL OF CLIMATE 25 7 2261 - 2278 2012年04月 [査読有り][通常論文]
Sea ice formation, its transport, and its melting cause the redistribution of heat and salt, which plays an important role in the climate and biogeochemical systems. In the Sea of Okhotsk, a heat and salt flux dataset is created in which such sea ice processes are included, with a spatial resolution of similar to 12.5 km. The dataset is based on a heat budget analysis using ice concentration, thickness, and drift speed from satellite observations and the ECMWF Interim Re-Analysis (ERA-Interim) data. The salt flux calculation considers both salt supplied to the ocean from sea ice production and freshwater supplied when the ice melts. This dataset will be useful for the validation and boundary conditions of modeling studies. The spatial distribution of the annual fluxes shows a distinct contrast between north and south: significant ocean cooling with salt supply is shown in the northern coastal polynya region, while ocean heating with freshwater supply is shown in the south. This contrast suggests a transport of freshwater and negative heat by ice advection. The annual fluxes also show ocean cooling with freshwater supply in the Kashevarov Bank (KB) region and the central and eastern Sea of Okhotsk, suggesting the effect of warm water advection. In the ice melt season, relatively prominent ice melting is shown in the coastal polynya region, probably due to large solar heating of the upper ocean. This indicates that the polynya works as a "meltwater factory'' in spring, contrasting with its role as an "ice factory'' in winter. In the coastal polynya region, the spatial distribution of phytoplankton bloom roughly corresponds with the ice melt region. - Coastal landfast sea ice decay and breakup in northern Alaska: Key processes and seasonal predictionChris Petrich, Hajo Eicken, Jing Zhang, Jeremy Krieger, Yasushi Fukamachi, Kay I. OhshimaJOURNAL OF GEOPHYSICAL RESEARCH-OCEANS 117 C02003, doi:10.1029/2011JC007339 2012年02月 [査読無し][通常論文]
Seasonal breakup of landfast sea ice consists of movement and irreversible ice detachment in response to winds or oceanic forces in the late stages of ice decay. The breakup process of landfast sea ice in the Chukchi Sea at Barrow, Alaska, was analyzed for the years 2000 through 2010 on the basis of local observations of snow and ice conditions, weather records, image sequences obtained from cameras, coastal X band marine radar, and satellite imagery. We investigated the relation of breakup to winds, tides, and nearshore current measurements from a moored acoustic Doppler current profiler. Two breakup modes are distinguished at Barrow on the basis of the degree of ice decay. Mechanical breakup due to wind and oceanic forces follows ablation and weakening of the ice. Thermal breakup is the result of ice disintegration under melt ponds, requiring little force to induce dispersion. Grounded pressure ridges are pivotal in determining the breakup mode. The timing of thermal breakup of the nearshore ice cover was found to correlate with the measured downwelling solar radiation in June and July. This linkage allows for the development of an operational forecast of landfast ice breakup. Results from forecasts during 2 years demonstrate that thermal breakup can be predicted to within a couple of days 2 weeks in advance. The cumulative shortwave energy absorbed by the ice cover provides for a measure of the state of ice decay and potential for disintegration. Discriminating between the two modes of breakup bears the potential to greatly increase forecasting skill. - Yoshihiro Nakayama, Kay I. Ohshima, Yasushi FukamachiJOURNAL OF PHYSICAL OCEANOGRAPHY 42 1 179 - 192 2012年01月 [査読無し][通常論文]
Wind factor, the ratio of sea ice drift speed to surface wind speed, is a key factor for the dynamics of sea ice and is generally about 2%. In some coastal oceans, however, the wind factor tends to be larger near the coast. This study proposes the enhancement mechanism of the sea ice drift caused by the dynamical coupling between sea ice and a coastal ocean. In a coastal ocean covered with sea ice, wind-forced sea ice drift excites coastal trapped waves (shelf waves) and generates fluctuating ocean current. This ocean current can enhance sea ice drift when the current direction is the same as that of the wind-driven drift. The authors consider a simplified setting where spatially uniform oscillating wind drifts sea ice parallel to the coast. When a barotropic long shelf wave is assumed for the ocean response, sea ice drifts driven by wind and ocean are obtained analytically. The ratio of ocean-driven to wind-driven sea ice drifts is used for the evaluation of the oceanic contribution to the enhancement of sea ice drift. The enhancement is mostly determined by the characteristics of the shelf waves, and sea ice drift is significantly enhanced close to the coast with lower-frequency wind forcing. Comparison with the observation off the Sakhalin coast shows that the degree of enhancement of sea ice drift and its characteristic such that larger enhancement occurs near the coast are mostly consistent with our theoretical solution, suggesting that this mechanism is present in the real ocean. - Coastal landfast sea ice decay and breakup in northern Alaska: Key processes and seasonal predictionChris Petrich, Hajo Eicken, Jing Zhang, Jeremy Krieger, Yasushi Fukamachi, Kay I. OhshimaJournal of Geophysical Research: Oceans 117 2 2012年 [査読有り][通常論文]
Seasonal breakup of landfast sea ice consists of movement and irreversible ice detachment in response to winds or oceanic forces in the late stages of ice decay. The breakup process of landfast sea ice in the Chukchi Sea at Barrow, Alaska, was analyzed for the years 2000 through 2010 on the basis of local observations of snow and ice conditions, weather records, image sequences obtained from cameras, coastal X band marine radar, and satellite imagery. We investigated the relation of breakup to winds, tides, and nearshore current measurements from a moored acoustic Doppler current profiler. Two breakup modes are distinguished at Barrow on the basis of the degree of ice decay. Mechanical breakup due to wind and oceanic forces follows ablation and weakening of the ice. Thermal breakup is the result of ice disintegration under melt ponds, requiring little force to induce dispersion. Grounded pressure ridges are pivotal in determining the breakup mode. The timing of thermal breakup of the nearshore ice cover was found to correlate with the measured downwelling solar radiation in June and July. This linkage allows for the development of an operational forecast of landfast ice breakup. Results from forecasts during 2 years demonstrate that thermal breakup can be predicted to within a couple of days 2 weeks in advance. The cumulative shortwave energy absorbed by the ice cover provides for a measure of the state of ice decay and potential for disintegration. Discriminating between the two modes of breakup bears the potential to greatly increase forecasting skill. Copyright 2012 by the American Geophysical Union. - K. Shimada, S. Aoki, K. I. Ohshima, S. R. RintoulOCEAN SCIENCE 8 4 419 - 432 2012年 [査読有り][通常論文]
Changes to the properties of Antarctic Bottom Water in the Australian-Antarctic Basin (AA-AABW) between the 1990s and 2000s are documented using data from the WOCE Hydrographic Program (WHP) and repeated hydrographic surveys. Strong cooling and freshening are observed on isopycnal layers denser than gamma(n) = 28.30 kg m(-3). Changes in the average salinity and potential temperature below this isopycnal correspond to a basin-wide warming of 1300 +/- 200 GW and freshening of 24 +/- 3 Gt year(-1). Recent changes to dense shelf water in the source regions in the Ross Sea and George V Land can explain the freshening of AA-AABW but not its extensive warming. An alternative mechanism for this warming is a decrease in the supply of AABW from the Ross Sea (RSBW). Hydrographic profiles between the western Ross Sea and George V Land (171-158 degrees E) were analyzed with a simple advective-diffusive model to assess the causes of the observed changes. The model suggests that the warming of RSBW observed between the 1970s and 2000s can be explained by a 21 +/- 23% reduction in RSBW transport and the enhancement of the vertical diffusion of heat resulting from a 30 +/- 7 % weakening of the abyssal stratification. The documented freshening of Ross Sea dense shelf water leads to a reduction in both salinity and density stratification. Therefore the direct freshening of RSBW at its source also produces an indirect warming of the RSBW. A simple box model suggests that the changes in RSBW properties and volume transport (a decrease of 6.7 To is assumed between the year 1995 and 2005) can explain 51 +/- 6% of the warming and 84 +/- 10% of the freshening observed in AA-AABW. - Sohey Nihashi, Kay I. Ohshima, Haruka NakasatoJOURNAL OF OCEANOGRAPHY 67 5 551 - 562 2011年10月 [査読無し][通常論文]
Sea-ice retreat processes are examined in the Sea of Okhotsk. A heat budget analysis in the sea-ice zone shows that net heat flux from the atmosphere at the water surface is about 77 W m(-2) on average in the active ice melt season (April) due to large solar heating, while that at the ice surface is about 12 W m(-2) because of the difference in surface albedo. The temporal variation of the heat input into the upper ocean through the open water fraction corresponds well to that of the latent heat required for ice retreat. These results suggest that heat input into the ice-upper ocean system from the atmosphere mainly occurs at the open water fraction, and this heat input into the upper ocean is an important heat source for ice melting. The decrease in ice area in the active melt season (April) and the geostrophic wind just before the melt season (March) show a correlation: the decrease is large when the offshoreward wind is strong. This relationship can be explained by the following process. Once ice concentration is decreased (increased) by the offshoreward (onshoreward) wind just before the melt season, solar heating of the upper ocean through the increased (decreased) open water fraction is enhanced (reduced), leading to (suppressing) a further decrease in ice concentration. This positive feedback is regarded as the ice-ocean albedo feedback, and explains in part the large interannual variability of the ice cover in the ice melt season. - Tamura Takeshi, Ohshima Kay I.JOURNAL OF GEOPHYSICAL RESEARCH-OCEANS 116 2011年07月29日 [査読有り][通常論文]
- Takeshi Tamura, Kay I. OhshimaJOURNAL OF GEOPHYSICAL RESEARCH-OCEANS 116 2011年07月 [査読有り][通常論文]
High ice production in coastal polynyas over the continental shelves in the Arctic Ocean is responsible for the formation of cold saline water, which contributes to the maintenance of the Arctic Ocean halocline. The accurate detection of coastal polynyas, including an estimate of thin ice thickness, is essential for the estimation of sea ice production. This paper presents an algorithm that estimates thin ice thickness using Special Sensor Microwave/Imager (SSM/I) data in the Arctic Ocean. Detection and estimation of sea ice thicknesses of <0.15 m are based on the SSM/I 85 and 37 GHz polarization ratios (PR85 and PR37) through a comparison with sea ice thicknesses estimated from the advanced very high resolution radiometer (AVHRR) data in the three different Arctic coastal polynyas. Thus, for the entire Arctic Ocean, the algorithm can be used for the detection of coastal polynyas and for the estimation of sea ice production through combination with heat-flux calculation. This study provides the first circumpolar mapping of sea ice production in coastal polynyas over the entire Arctic Ocean. High ice production is confined to the major Arctic coastal polynyas, with the highest ice production rate being in the North Water Polynya. This study also presents the interannual variability of sea ice production in the 10 major coastal polynyas from 1992 to 2007. In general, interannual variability in sea ice production has good correlation with polynya extent rather than surface air temperature. The mapping also provides surface heat-and salt-flux conditions in the ice-covered region, which have not been well understood to date. - Takenobu Toyota, Shuji Ono, Kohei Cho, Kay I. OhshimaANNALS OF GLACIOLOGY 52 57 177 - 184 2011年 [査読無し][通常論文]
Although satellite data are useful for obtaining ice-thickness distribution for perennial sea ice or in stable thin-sea-ice areas, they are still largely an unresolved issue for the seasonal ice zone (SIZ). We address this problem using L-band synthetic aperture radar (SAR). In the SIZ, ice-thickness growth is closely related to deformation, so surface roughness is expected to correlate with ice thickness. L-band SAR, suitable for detecting such surface roughness, is a promising tool for obtaining thickness distribution. This idea was supported by an airborne polarimetric and interferometric SAR (Pi-SAR) validation. To extend this result to spaceborne L-band SAR with coarser resolution, we conducted in situ measurements of ice thickness and surface roughness in February 2008 in the southern Sea of Okhotsk with an icebreaker in coordination with the Advanced Land Observing Satellite (ALOS)/Phased Array-type L-band SAR (PALSAR) orbit. A helicopter-borne laser profiler was used to improve the estimation of surface. roughness. It was found that backscatter coefficients (HH) correlated well with ice thickness (R=0.86) and surface roughness (R=0.70), which confirm.; the possibility of determining ice-thickness distribution in the SIZ. The interannual variation of PALSAR-derived ice-thickness distribution in the southern Sea of Okhotsk is also discussed. - Yasushi Fukamachi, Kay I. Ohshima, Yuji Mukai, Genta Mizuta, Masaaki WakatsuchiAnnals of Glaciology 52 57 1 - 8 2011年 [査読無し][通常論文]
In the southwestern part of the Sea of Okhotsk off Hokkaido, sea-ice drift characteristics are investigated using the ice and water velocities obtained from a moored upward-looking acoustic Doppler current profiler (ADCP) during the winters of 1999-2001. Using hourly-mean values of these data along with the wind data measured at a nearby coastal station, the wind factor and turning angle of the relative velocity between the ice and water velocities with respect to the wind are calculated assuming free drift under various conditions. Since the simultaneous sea-ice draft data are also available from a moored ice-profiling sonar (IPS), we examine the dependence of drift characteristics on ice thickness for the first time. As ice thickness increases and wind decreases, the wind factor decreases and the turning angle increases, as predicted by the theory of free drift. This study clearly shows the utility of the moored ADCP measurement for studying sea-ice drift, especially with the simultaneous IPS measurement for ice thickness, which cannot be obtained by other methods. - Takeshi Tamura, Kay I. OhshimaJournal of Geophysical Research: Oceans 116 7 2011年 [査読有り][通常論文]
High ice production in coastal polynyas over the continental shelves in the Arctic Ocean is responsible for the formation of cold saline water, which contributes to the maintenance of the Arctic Ocean halocline. The accurate detection of coastal polynyas, including an estimate of thin ice thickness, is essential for the estimation of sea ice production. This paper presents an algorithm that estimates thin ice thickness using Special Sensor Microwave/Imager (SSM/I) data in the Arctic Ocean. Detection and estimation of sea ice thicknesses of < 0.15 m are based on the SSM/I 85 and 37 GHz polarization ratios (PR85 and PR37) through a comparison with sea ice thicknesses estimated from the advanced very high resolution radiometer (AVHRR) data in the three different Arctic coastal polynyas. Thus, for the entire Arctic Ocean, the algorithm can be used for the detection of coastal polynyas and for the estimation of sea ice production through combination with heat-flux calculation. This study provides the first circumpolar mapping of sea ice production in coastal polynyas over the entire Arctic Ocean. High ice production is confined to the major Arctic coastal polynyas, with the highest ice production rate being in the North Water Polynya. This study also presents the interannual variability of sea ice production in the 10 major coastal polynyas from 1992 to 2007. In general, interannual variability in sea ice production has good correlation with polynya extent rather than surface air temperature. The mapping also provides surface heat- and salt- flux conditions in the ice-covered region, which have not been well understood to date. Copyright 2011 by the American Geophysical Union. - Tamura, T, K.I. Ohshima, H. Enomoto, K. Tateyama, A. Muto, S. Ushio, R. A. MassomAnnals of Glaciology 44 44, 269-274 - + 2011年 [査読有り][通常論文]
- Takeshi Tamura, Kay I. Ohshima, Sohey Nihashi, Hiroyasu HasumiSOLA 7 17 - 20 2011年 [査読有り][通常論文]
The sinking of dense water in the polar oceans plays a key role in global thermohaline circulation, leading to heat and material exchange between the atmosphere and deep ocean. This study provides the first surface heat and salt flux dataset for the Southern Ocean (including a treatment of sea ice growth and melt), based on heat flux calculations and satellite-derived sea ice data. The geographical distribution of annual net heat (salt) flux shows a distinct contrast: significant cooling of (salt release into) the ocean occurs in the coastal region, and net heating of (freshwater release into) the ocean occurs in the offshore region. The work tries a quantitative representation of heat and freshwater transport by sea ice formed in the coastal region to offshore. Since hemispheric-scale heat and salt fluxes associated with sea ice growth and melt have not been estimated from observations to date, the present dataset will provide new information with which to validate coupled ice-ocean models while providing important boundary conditions for the various models. - Takuya Nakanowatari, Kay I. Ohshima, Sachiko NagaiJOURNAL OF GEOPHYSICAL RESEARCH-OCEANS 115 C12031, doi:10.1029/2009JC006070 2010年12月 [査読無し][通常論文]
Previous studies suggested that the interannual variability of the maximum sea ice extent (MSIE) in the Sea of Okhotsk is not explained only by atmospheric conditions. In this study, we examined the effect of the ocean thermal condition on the determination of the MSIE based on observational data. We found that the MSIE is highly correlated with the sea surface temperature (SST) and ocean temperature around the East Kamchatska Current (EKC) in the Pacific in late autumn (November-December). The significant relationship between the MSIE and the SST cannot be fully explained by prevailing atmospheric variabilities. Considering that the inflow of EKC water to the Okhotsk Sea is strengthened in winter, advection of anomalous ocean temperature likely influences the MSIE. A multiple regression model constructed as a function of SST in the EKC and the air temperature at 850 hPa over the upwind region of the Okhotsk Sea in late autumn, which is another determinant factor for the MSIE, significantly improves the prediction skill for the MSIE: 70% of the variance of MSIE can be predicted at the stage of 2 to 3 months before the MSIE season. The SST in the EKC and the upwind air temperature are likely to control the sea ice extent in the northeastern and center to southern parts of the Okhotsk Sea, respectively. - Yusuke Kawaguchi, Sohey Nihashi, Humio Mitsudera, Kay I. OhshimaJOURNAL OF PHYSICAL OCEANOGRAPHY 40 11 2451 - 2465 2010年11月 [査読無し][通常論文]
This paper investigates the formation mechanism of broad coastal polynyas beyond 100 km in offshore width. It is known that two regimes for wind-driven polynya opening exist: one is a convergent regime at the polynya edge in which inner frazil ice catches up with outer consolidated ice, whereas the other is a divergent regime in which the consolidated ice drifts offshore faster than the frazil ice at the edge. In this study, the authors focus on the latter, divergent polynya-edge regime. Because in the divergent regime the polynya possibly evolves without bound, they consider a thermal growth for inner frazil ice to find a finite solution of offshore width. Then, the authors investigate responses of the polynya opening for various wind angles phi from the offshore direction from the viewpoint of the polynya-edge regimes. At first, the authors estimate the deviation angle and wind factor for the frazil and consolidated ice based on each momentum balance, because sea ice motion driven by wind varies depending on the ice thickness due to relative effect of the Coriolis force. It was found that, when the surface wind deviates leftward about 10 degrees or greater from offshore, the divergent regime at the polynya edge generates a great polynya evolution. Otherwise, the convergent regime takes place yielding small offshore extent. These theoretical results were verified for the Okhotsk northwestern polynya using thin-ice-thickness data derived from Advanced Microwave Scanning Radiometer for Earth Observing System (AMSR-E) and the Japanese 25-year Reanalysis (JRA-25) surface wind. The data show that the polynya development distinctively changes at phi similar to 0 degrees, and the largest width is shown in the leftward wind angle of phi = 0 degrees-30 degrees. This clear relationship between the surface wind angle and the offshore extent of polynya nicely supports the theory. - Yasushi Fukamachi, Kay I. Ohshima, Naoto Ebuchi, Tadao Bando, Kazuya Ono, Minoru SanoJOURNAL OF OCEANOGRAPHY 66 5 685 - 696 2010年10月 [査読無し][通常論文]
Time-series data of the vertical structure of the Soya Warm Current (SWC) were obtained by a bottom-mounted acoustic Doppler current profiler (ADCP) in the middle of the Soya Strait from September 2006 to July 2008. The site of the ADCP measurement was within the coverage of the ocean-radar measurement around the strait. The volume transport of the SWC through the strait is estimated on the basis of both the vertical structure observed by the ADCP and the horizontal structure observed by the radars for the first time. The annual transport estimates are 0.62-0.67 Sv (1 Sv = 10(6) m(3)s(-1)). They are somewhat smaller than the difference between the previous estimates of the inflow and outflow through other straits in the Sea of Japan, and smaller than those obtained in the region downstream of the strait during 2004-05 (0.94-1.04 Sv). The difference in the two periods may be attributed to interannual variability of the SWC and/or the different measurement locations. - Kay I. Ohshima, Takuya Nakanowatari, Stephen Riser, Masaaki WakatsuchiDEEP-SEA RESEARCH PART II-TOPICAL STUDIES IN OCEANOGRAPHY 57 13-14 1247 - 1256 2010年07月 [査読無し][通常論文]
This paper examines the seasonal variation in the in- and outflow of the Okhotsk Sea with the Pacific, mainly based on profiling float and satellite altimeter data. The motions of the floats at depths of 500-750 m show that the waters of the Okhotsk tend to flow out to the Pacific through the southern Kuril Straits, mostly Bussol' Strait, in winter seasons (November-March). Based on data from the floats exiting the Sea, the mean residence time of the intermediate water in the Kuril Basin is estimated as about one year. Isopycnal analysis from the profiling data shows that, in the mid-eastern area of the Okhotsk Sea, potential temperature of the intermediate water with sigma(theta)=26.6-27.2 increases by 0.2-0.5 degrees C in every spring, suggesting the inflow from the Pacific in the preceding season. These suggest that the in- and outflow predominantly occurs during winter, with the inflow at the northern straits and outflow at the southern straits. The surface current field inferred from satellite altimeter data also shows that the inflow to the Sea of Okhotsk through the northern straits is strengthened in winter, linked with strengthening of the East Kamchatka Current (EKC). From yearly anomalies of the altimeter data, the winter inflow to the Okhotsk through the northern straits is significantly correlated with the EKC and Sverdrup transport over the North Pacific. Further, interannual variations in property of Okhotsk Sea Intermediate Water appear to be related with the EKC and wind stress over the North Pacific in winter, possibly through the change in inflowing Pacific water transport. These suggest that the seasonal and interannual variations of the in- and outflow are, to some extent, controlled by the large scale wind stress over the North Pacific, being consistent with Island Rule qualitatively. (C) 2010 Elsevier Ltd. All rights reserved. - Jun Ono, Kay I. OhshimaCONTINENTAL SHELF RESEARCH 30 6 588 - 597 2010年04月 [査読無し][通常論文]
To clarify the generation and dissipation mechanisms of diurnal coastal-trapped waves (CTWs) over the Sakhalin shelf, a series of numerical experiments were conducted using a three-dimensional tidal model of the Okhotsk Sea with density stratification. The tidal model used has good reproduction owing to the careful fitting to the recent observations. The numerical experiments suggested that diurnal CTWs are primarily (similar to 60%) generated by the conversion of tidal energy at the northern corner of the Sakhalin shelf, and further amplified by vorticity generation due to the water column oscillation from Sakhalin Bay and the influence of Kashevarov Bank. From the observations, it was found that diurnal CTWs are effectively dissipated by the strong spin-down due to bottom friction. The conventional turbulent closure model cannot reproduce the observed damping of diurnal CTWs, which raises a caution in modeling the tidal fields in high-latitude regions where diurnal CTWs exist. To resolve this underestimation of the damping, the vertical eddy viscosity was parameterized using its dependence on the observed major axis length of the diurnal tidal current ellipses, which improves the model reproduction on the damping of diurnal CTWs. The model also suggests that the spin-down effects due to friction associated with the sea-ice cover play an important role in the tidal current reduction in the region where diurnal CTWs exist, as the observations suggested. (C) 2009 Elsevier Ltd. All rights reserved. - Takuya Nakanowatari, Kay I. Ohshima, Sachiko NagaiJournal of Geophysical Research: Oceans 115 12 2010年 [査読有り][通常論文]
Previous studies suggested that the interannual variability of the maximum sea ice extent (MSIE) in the Sea of Okhotsk is not explained only by atmospheric conditions. In this study, we examined the effect of the ocean thermal condition on the determination of the MSIE based on observational data. We found that the MSIE is highly correlated with the sea surface temperature (SST) and ocean temperature around the East Kamchatska Current (EKC) in the Pacific in late autumn (November-December). The significant relationship between the MSIE and the SST cannot be fully explained by prevailing atmospheric variabilities. Considering that the inflow of EKC water to the Okhotsk Sea is strengthened in winter, advection of anomalous ocean temperature likely influences the MSIE. A multiple regression model constructed as a function of SST in the EKC and the air temperature at 850 hPa over the upwind region of the Okhotsk Sea in late autumn, which is another determinant factor for the MSIE, significantly improves the prediction skill for the MSIE: 70% of the variance of MSIE can be predicted at the stage of 2 to 3 months before the MSIE season. The SST in the EKC and the upwind air temperature are likely to control the sea ice extent in the northeastern and center to southern parts of the Okhotsk Sea, respectively. © 2010 by the American Geophysical Union. - SSM/Iを用いた海氷生産量・熱塩フラックスのグローバルマッピング田村 岳史, 大島 慶一郎月刊海洋 号外54 37 - 42 2010年 [査読無し][通常論文]
- Naoto Ebuchi, Yasushi Fukamachi, Kay I. Ohshima2010 IEEE INTERNATIONAL GEOSCIENCE AND REMOTE SENSING SYMPOSIUM 3051 - 3054 2010年 [査読有り][通常論文]
Propagation of subinertial variations in the Soya Warm Current (SWC), which flows through the Soya Strait located between Hokkaido, Japan and Sakhalin Island, Russia, is investigated using data from HF ocean radars together with in situ observations, such as bottom-mounted acoustic Doppler current profilers (ADCPs) and coastal tide gauges. The subinertial variations with periods from 5 to 20 days were captured by the HF radars. The subinertial variations were significantly correlated with the meridional wind stress component over the region, suggesting that the sea level difference through the strait caused by wind-generated coastally-trapped waves on the east coast of Sakhalin and west coast of Hokkaido are considered to be a possible mechanism casing the subinertial variations in the SWC. Propagation of the subinertial variations was also clearly captured by the HF radars. The estimated phase velocity suggests that the subinertial variations propagate downstream along the coast as the 3rd-mode barotropic continental shelf waves. - Sohey Nihashi, Kay I. Ohshima, Takeshi Tamura, Yasushi Fukamachi, Sei-Ichi SaitohJournal of Geophysical Research: Oceans 114 10 2009年10月08日 [査読有り][通常論文]
From comparisons with thickness of sea ice from Advanced Very High Resolution Radiometer (AVHRR) and ice-profiling sonar data we have developed an Advanced Microwave Scanning Radiometer-EOS (AMSR-E) thin ice thickness algorithm for the Sea of Okhotsk. This algorithm can estimate ice thickness of < 0.2 m without snow using the polarization ratio of AMSR-E brightness temperature at a 36.5 GHz channel from a linear relationship with AVHRR ice thickness. When a snow cover exists on the thin ice surface, as occurred a few times in each winter, it is shown that the algorithm cannot detect the thin ice. Sea ice and dense shelf water (DSW) production in coastal polynya are estimated on the basis of heat flux calculation with the daily AMSR-E ice thickness for three winters (December-March) of 2002-2003 to 2004-2005. The ice production is largest in the northwest shelf (NWS) polynya which accounts for ~45% of the sum of ice production in major coastal polynyas. The ice production in major coastal polynyas would cover the maximum ice area of the Okhotsk Sea if the average ice thickness is assumed to be 1 m. Variability of the ice production is mainly modulated by air temperature. In the NWS polynya, which is the main DSW production area, the annual DSW formation rate is estimated to be ∼0.36 Sv. Copyright 2009 by the American Geophysical Union. - Sohey Nihashi, Kay I. Ohshima, Takeshi Tamura, Yasushi Fukamachi, Sei-ichi SaitohJOURNAL OF GEOPHYSICAL RESEARCH-OCEANS 114 2009年10月 [査読無し][通常論文]
From comparisons with thickness of sea ice from Advanced Very High Resolution Radiometer (AVHRR) and ice-profiling sonar data we have developed an Advanced Microwave Scanning Radiometer-EOS (AMSR-E) thin ice thickness algorithm for the Sea of Okhotsk. This algorithm can estimate ice thickness of <= 0.2 m without snow using the polarization ratio of AMSR-E brightness temperature at a 36.5 GHz channel from a linear relationship with AVHRR ice thickness. When a snow cover exists on the thin ice surface, as occurred a few times in each winter, it is shown that the algorithm cannot detect the thin ice. Sea ice and dense shelf water (DSW) production in coastal polynya are estimated on the basis of heat flux calculation with the daily AMSR-E ice thickness for three winters (December-March) of 2002-2003 to 2004-2005. The ice production is largest in the northwest shelf (NWS) polynya which accounts for similar to 45% of the sum of ice production in major coastal polynyas. The ice production in major coastal polynyas would cover the maximum ice area of the Okhotsk Sea if the average ice thickness is assumed to be 1 m. Variability of the ice production is mainly modulated by air temperature. In the NWS polynya, which is the main DSW production area, the annual DSW formation rate is estimated to be similar to 0.36 Sv. - Kunimitsu Ishida, Kay I. OhshimaATMOSPHERE-OCEAN 47 3 169 - 183 2009年09月 [査読無し][通常論文]
Ice-band characteristics for the region off East Queen Maud Land in Antarctica were examined and their relationship with the wind conditions was assessed using a large number of Marine Observation Satellite (MOS) Multispectral Electronic Self Scanning Radiometer (MESSR) images received at Syowa Station during the period 1989-93. Analyses from 43 examples of bands captured from August to December suggest that ice-band formation and band scale are affected by both wind speed and direction over approximately the preceding four days (defined as the effective wind). Ice-band width and spacing are significantly correlated with the effective wind speed and the maximum wind speed during that period. The long axis of ice bands tends to be oriented at 70-90 (mean of 75) to the right of the effective wind direction. The band scales decrease from winter ( August) to summer (December) with typical band spacing of 4-6 km in winter and 1-2 km in summer. This seems to be primarily due to a decrease in ice floe size and partly due to a decrease in the effective wind speed from winter to summer. Band scale decreases from the ice interior to the ice edge under conditions of off-ice winds. - HF radar technology in the Sea of OkhotskEbuchi, N, Y. Fukamachi, K.I. Ohshima, M. WakatsuchiPICES Science Report 36 197 - 202 2009年09月 [査読有り][招待有り]
- Yasushi Fukamachi, Kunio Shirasawa, Anatoliy M. Polomoshnov, Kay I. Ohshima, Ervin Kalinin, Sohey Nihashi, Humfrey Melling, Genta Mizuta, Masaaki WakatsuchiCONTINENTAL SHELF RESEARCH 29 11-12 1541 - 1548 2009年06月 [査読無し][通常論文]
From December to June 2002-2003, sea-ice and oceanic data were obtained from moorings near Sakhalin in the west central Okhotsk Sea. Ice draft measured by sonar reveals distinct periods of thin and thick ice. Thin-ice periods in January-March corresponded to offshore ice movement and increasing seawater salinity. The measured change in salinity corresponds well with that derived from heat-flux calculations using the observed ice thickness. Brine rejection from ice growing in a coastal polynya off northern Sakhalin is responsible for much of the observed salinity increase. The simultaneous observation of dense shelf water (> 26.7 sigma(0)) suggests that this region is one possible source. The periods of thick-ice incursion are likely indicative of heavily deformed pack formed further north and drifting south with the current. The mean draft (1.95 m), thick-ice ratio, and keel frequency during these periods are close to values observed in the Beaufort Sea. Freshwater transport estimated from the observed ice thickness and velocity is larger than that of the Amur River discharge. (C) 2009 Elsevier Ltd. All rights reserved. - Kazuya Kusahara, Kay I. OhshimaJOURNAL OF PHYSICAL OCEANOGRAPHY 39 3 658 - 674 2009年03月 [査読無し][通常論文]
Coastal sea level variation around Antarctica is characterized by a coherent (circumpolarly in-phase) fluctuation, correlated with the Antarctic Oscillation (AAO). This study addresses the dynamics of the wind-driven sea level variation around Antarctica. A realistic barotropic numerical model reproduced well the observed sea level around Antarctica. From numerical model experiments, the authors demonstrate that the forcing responsible for the coastal sea level is the wind stress at the coastal boundary. Both the dominant coherent signal and westward propagating signals are identified in the model, and these signals are trapped over the shelf and slope around Antarctica. As a mechanism of these trapped signals, the authors consider analytical solutions of the oceanic response to alongshore wind stress over the shelf and slope in the circumpolar domain. In these solutions, besides the shelf wave mode, a wavenumber-zero mode appears and characterizes the coastal dynamics around Antarctica. At periods from 10 to 200 days, the coherent sea level can be explained quantitatively by the solution of this wavenumber-zero mode with a 5-10-day damping time scale. The spectral peaks of the westward propagating signals can be explained by the resonance of the shelf wave mode. The wavenumber-zero mode can respond to the wavenumber-zero forcing at any frequency and the degree of response increases with decreasing frequency. In addition, the wavenumber-zero component of wind stress, corresponding to the AAO variation, is a dominant forcing. Therefore, the coherent sea level variation around Antarctica is preferably generated and becomes a dominant feature in the circumpolar domain, particularly at lower frequencies. - Naoto Ebuchi, Yasushi Fukamachi, Kay I. Ohshima, Masaaki WakatsuchiJOURNAL OF OCEANOGRAPHY 65 1 31 - 43 2009年02月 [査読無し][通常論文]
Subinertial and seasonal variations in the Soya Warm Current (SWC) are investigated using data obtained by high frequency (HF) ocean radars, coastal tide gauges, and a bottom-mounted acoustic Doppler current profiler (ADCP). The HF radars clearly captured the seasonal variations in the surface current fields of the SWC. Almost the same seasonal cycle was repeated in the period from August 2003 to March 2007, although interannual variations were also discernible. In addition to the annual and interannual variations, the SWC exhibited subinertial variations with a period of 5-20 days. The surface transport by the SWC was significantly correlated with the sea level difference between the Sea of Japan and Sea of Okhotsk for both the seasonal and subinertial variations, indicating that the SWC is driven by the sea level difference between the two seas. The generation mechanism of the subinertial variation is discussed using wind data from the European Centre for Medium-range Weather Forecasts (ECMWF) analyses. The subinertial variations in the SWC were significantly correlated with the meridional wind stress component over the region. The subinertial variations in the sea level difference and surface current delay from the meridional wind stress variations by one or two days. Sea level difference through the strait caused by wind-generated coastally trapped waves (CTWs) along the east coast of Sakhalin and west coast of Hokkaido is considered to be a possible mechanism causing the subinertial variations in the SWC. - Atomosphere-Ocean 47 169-183, doi:10.3137/OC300.2009 2009年 [査読無し][通常論文]
- T. Toyota, K. Nakamura, S. Uto, K. I. Ohshima, N. EbuchiINTERNATIONAL JOURNAL OF REMOTE SENSING 30 12 3171 - 3189 2009年 [査読無し][通常論文]
Although satellite data are known to be useful for obtaining ice thickness distribution for perennial sea ice or in stable thin sea ice areas, their use in the seasonal sea ice zone (SIZ) is still unresolved. In this study, we approached the problem of ice thickness retrieval by using L-band Synthetic Aperture Radar (SAR). In the SIZ, ice thickness growth is closely related to ridging activity and therefore we expected surface roughness to be correlated to ice thickness. L-band SAR is suitable for detecting such surface roughness and should be a useful tool for obtaining ice thickness distribution. To verify this correlation, we conducted shipborne electromagnetic (EM) inductive sounding and supersonic profiling observations with an icebreaker, coordinated with airborne L-band SAR observations in the southern Sea of Okhotsk in February 2005. The surface elevation was estimated by representing the ship's motion with a low-pass filter. Backscattering coefficients correlated well with ice thickness and surface roughness, defined by the standard deviation of the surface elevation. This result sheds light on the possibility of determining ice thickness distribution in the SIZ. - Sohey Nihashi, Kay I. OhshimaJournal of Geophysical Research: Oceans 113 6 2008年06月08日 [査読有り][通常論文]
The bulk heat transfer coefficient in the ice-upper ocean system (Kb) in the ice melt season is estimated by a new method at 18 areas that cover much of the Antarctic seasonal ice zone. The method is based on a model in which ice melting is caused only by heat input through open water and is treated in a bulk fashion in the ice-upper ocean system. Kb is estimated by fitting a convergent curve derived from the model to an observed ice concentration-temperature plot (CT-plot). Estimated Kb is 1.15 ± 0.72 × 10-4 m s-1 on average. If Kb can be expressed by the product of the heat transfer coefficient (ch) and the friction velocity (uτ), ch is 0.0113 ± 0.0055. This value is about two times larger than that estimated at the ice bottom. The relationship between Kb and the geostrophic wind speed (Uw), which is roughly proportional to uτ, shows a significant positive correlation, as expected. Further, Kb seems more likely to be proportional to the square or cube of Uw rather than a linear relationship. Since Kb estimated from our method is associated (with) ice melting in a bulk fashion in the ice-upper ocean system, this relationship likely indicates both the mixing process of heat in the upper ocean (proportional to uτ 3) and the local heat transfer process at the ice-ocean interface (proportional to uτ). Copyright 2008 by the American Geophysical Union. - Sohey Nihashi, Kay I. OhshimaJOURNAL OF GEOPHYSICAL RESEARCH-OCEANS 113 C6 2008年06月 [査読無し][通常論文]
The bulk heat transfer coefficient in the ice-upper ocean system (K-b) in the ice melt season is estimated by a new method at 18 areas that cover much of the Antarctic seasonal ice zone. The method is based on a model in which ice melting is caused only by heat input through open water and is treated in a bulk fashion in the ice-upper ocean system. K-b is estimated by fitting a convergent curve derived from the model to an observed ice concentration-temperature plot (CT-plot). Estimated K-b is 1.15 +/- 0.72 x 10(-4) ms(-1) on average. If K-b can be expressed by the product of the heat transfer coefficient (c(h)) and the friction velocity (u(tau)), c(h) is 0.0113 +/- 0.0055. This value is about two times larger than that estimated at the ice bottom. The relationship between K-b and the geostrophic wind speed (U-w), which is roughly proportional to u(tau), shows a significant positive correlation, as expected. Further, K-b seems more likely to be proportional to the square or cube of U-w rather than a linear relationship. Since K-b estimated from our method is associated with ice melting in a bulk fashion in the ice-upper ocean system, this relationship likely indicates both the mixing process of heat in the upper ocean (proportional to u(tau)(3)) and the local heat transfer process at the ice-ocean interface (proportional to u(tau)). - Yasushi Fukamachi, Iori Tanaka, Kay I. Ohshima, Naoto Ebuchi, Genta Mizuta, Hideo Yoshida, Shiro Takayanagi, Masaaki WakatsuchiJOURNAL OF OCEANOGRAPHY 64 3 385 - 392 2008年06月 [査読有り][通常論文]
The vertical structure of the Soya Warm Current (SWC) was observed by a bottom-mounted acoustic Doppler current profiler (ADCP) in the region of the SWC axis near the Soya Strait during a I-year period from May 2004. The ADCP data revealed a marked seasonal variability in the vertical structure, with positive (negative) vertical shear in summer and fall (winter and spring). The volume transport of the SWC is estimated on the basis of both the vertical structure observed by the ADCP and horizontal structure observed by the ocean radars near the strait. The transport estimates have a minimum in winter and a maximum in fall, with the yearly-averaged values in the range of 0.94-1.04 Sv (1 Sv = 10(6) m(3) s(-1)). These lie within a reasonable range in comparison to those through other straits in the Japan Sea. - Jun Ono, Kay I. Ohshima, Genta Mizuta, Yasushi Fukainachi, Masaaki WakatsuchiCONTINENTAL SHELF RESEARCH 28 6 697 - 709 2008年04月 [査読無し][通常論文]
From July 1998 to June 2000, the first long-term mooring measurements were carried out off the east coast of Sakhalin. Using these data, we examined the characteristies of the tidal heights and currents. The tidal heights and currents are dominated by the diurnal variability with fortnightly modulation over the northern part of the shelf. The K, and 01 tidal current ellipses are clockwise with their major axes along the isobaths and their signal propagates with the coast on the right with phase speeds of 3.4 and 3.8 m s(-1), respectively. The diurnal tidal Currents are almost uniform in the vertical direction except for the bottom Ekman layer. The thickness of the bottom Ekman layer caused by the diurnal tidal currents is larger in the region of stronger tidal currents, reaching 20-30 in over the northern part of the shelf. The diurnal tidal Currents over the northern part of the shelf can be explained by the first-mode diurnal coastal-trapped waves (CTWs). The diurnal CTWs are almost independent of the seasonal variability of density stratification and contribute significantly to diurnal currents, but only slightly to sea-surface heights. The diurnal tidal Currents over the southern part of the shelf are significantly smaller than those over the northern part. This is because the diurnal CTWs cannot exist south of similar to 52 degrees N from the dispersion relation. The diurnal tidal currents are significantly reduced over the northern shelf, where the diurnal CTWs exist, during the high sea-ice concentration periods. From this result, we propose the scenario that the CTWs are damped by the spin-down effect due to the Ekman layer that would occur underneath the sea ice. (C) 2007 Elsevier Ltd. All rights reserved. - Takeshi Tamura, Kay I. Ohshima, Sohey NihashiGEOPHYSICAL RESEARCH LETTERS 35 7 2008年04月 [査読有り][通常論文]
Active sea-ice production in Antarctic coastal polynyas causes dense water formation, finally leading to Antarctic Bottom Water (AABW) formation. This study gives the first mapping of sea ice production in the Antarctic Ocean, based on heat-flux calculation with ice thickness data derived from satellite data. The highest ice production occurs in the Ross Ice Shelf Polynya region. The ice production there decreased by similar to 30% from the 1990s to the 2000s, which can be one candidate for causing the recent freshening of AABW. The Cape Darnley polynya in East Antarctica is found to be the second highest production area, suggesting a possible AABW formation area. According to our estimation, around 10% of Southern Ocean sea ice is produced in the major Antarctic coastal polynyas. The mapping provides surface heat- and salt-flux conditions in the ice-covered region, which have not been well understood. - Kay I. Ohshima, Daisuke SimizuJOURNAL OF OCEANOGRAPHY 64 1 103 - 114 2008年02月 [査読有り][通常論文]
Particle tracking experiments were conducted for the Sea of Okhotsk using a three-dimensional ocean circulation model, as a step toward the simulation of oil spills. The model's reproducibility is first examined in detail. Comparison with surface drifter and moored ADCP data shows that the model successfully reproduces the velocity field over the shelves, particularly in the weak stratification period. This is because the current variability is simply determined by integration of the alongshore component of the wind stress over the coast from which arrested topographic waves propagate. Good agreement even in the ice-covered period implies that the neglect of sea ice in the model is not a problem for reproduction of the current over the shelves. Good agreement also supports the correction of ECMWF wind speed by a factor of 1.25. A series of particle tracking experiments was carried out to examine the case of particles released from the Sakhalin oil field at depths of 0 m and 15 m. Regardless of the deployment month and year, most particles at depth 15 m are transported southward along the Sakhalin coast, in accordance with the abrupt intensification of the East Sakhalin Current in October, finally arriving offshore of Hokkaido in November-january. Particles at the surface, which are affected by wind drift in addition to the ocean current, show larger yearly variability. In years when the offshoreward-wind dominates, the particles would be advected out of the mainstream of the current and would not be transported offshore of Hokkaido. - Sea-ice production in the Okhotsk coastal polynyasNihashi, S, K. I. Ohshima, T. Tamura, S. SaitohProceedings of the 23rd International Symposium on Okhotsk Sea & Sea Ice 2008 39 - 42 2008年 [査読無し][通常論文]
- Studies on sea ice and polynya off the Cape Darnley in East Antarctica, using the ALOS PALSAR and passive microwave dataOhshima K. I., T. TamuraProceedings of the ALOS PI 2008 Symposium 2008年 [査読無し][通常論文]
- Subinertial variations in the Soya Warm Current revealed by HF radars, coastal tide gauges, and a bottom-mounted ADCP.Ebuchi, N, Y. Fukamachi, K.I. Ohshima, M. WakatsuchiProceedings of IGARSS 2008 IV 53 - 56 2008年 [査読無し][通常論文]
- Development of sea ice thickness algorithm from PALSAR data, combined with in-situ observations and other satellite dataToyota, T, K. Nakamura, T. Tamura, K. Cho, K.I. OhshimaProceedings of The First Joint PI symposium of ALOS Data Nodes for ALOS Science Program in Kyoto 1 - 4 2007年11月 [査読無し][通常論文]
- Takeshi Tamura, Kay I. Ohshima, Thorsten Markus, Donald J. Cavalieri, Sohey Nihashi, Naohiko HirasawaJOURNAL OF ATMOSPHERIC AND OCEANIC TECHNOLOGY 24 10 1757 - 1772 2007年10月 [査読有り][通常論文]
Antarctic coastal polynyas are important areas of high sea ice production and dense water formation, and thus their detection including an estimate of thin ice thickness is essential. In this paper, the authors propose an algorithm that estimates thin ice thickness and detects fast ice using Defense Meteorological Satellite Program (DMSP) Special Sensor Microwave Imager (SSM/I) data in the Antarctic Ocean. Detection and estimation of sea ice thicknesses of <0.2 m are based on the SSM/I 85- and 37-GHz polarization ratios (PR85 and PR37) through a comparison with sea ice thicknesses estimated from the Advanced Very High Resolution Radiometer (AVHRR) data. The exclusion of data affected by atmospheric water vapor is discussed. Because thin ice and fast ice (specifically ice shelves, glacier tongues, icebergs, and landfast ice) have similar PR signatures, a scheme was developed to separate these two surface types before the application of the thin ice algorithm to coastal polynyas. The probability that the algorithm correctly distinguishes thin ice from thick ice and from fast ice is similar to 95%, relative to the ice thicknesses estimated from AVHRR. Although the standard deviation of the difference between the thin ice thicknesses estimated from the SSM/I algorithm and AVHRR is similar to 0.05 m and thus not small, the estimated ice thicknesses from the microwave algorithm appear to have small biases and the accuracies are independent of region and season. A distribution map of thin ice occurrences derived from the SSM/I algorithm represents the Ross Sea coastal polynya being by far the largest among the Antarctic coastal polynyas; the Weddell Sea coastal polynyas are much smaller. Along the coast of East Antarctica, coastal polynyas frequently form on the western side of peninsulas and glacier tongues, downstream of the Antarctic Coastal Current. - Takenobu Toyota, Shinya Takatsuji, Kazutaka Tateyama, Kazuhiro Naoki, Kay I. OhshimaJOURNAL OF OCEANOGRAPHY 63 3 393 - 411 2007年06月 [査読有り][通常論文]
The general properties of sea ice and overlying snow in the southern Sea of Okhotsk were examined during early February of 2003 to 2005 with the P/V "Soya". Thin section analysis of crystal structure revealed that frazil ice (48% of total core length) was more prevalent than columnar ice (39%) and that stratigraphic layering was prominent with a mean layer thickness of 12 cm, indicating that dynamic processes are essential to ice growth. The mean thickness of ice blocks and visual observations suggest that ridging dominates the deformation process above thicknesses of 30 to 40 cm. As for snow, it was found that faceted crystals and depth hoar are dominant (78%), as which is also common in the Antarctic sea ice, and is indicative of the strong vertical temperature gradients within the snow. Stable isotope measurements ( 6180) indicate that snow ice occupies 9% of total core length and that the mass fraction of meteoric ice accounts for 1 to 2% of total ice volume, which is lower than the Antarctic sea ice. Associated with this, the effective fractionation coefficient during the freezing of seawater was also derived. Snow ice was characterized by lower density higher salinity, and nearly twice the gas content of ice of seawater origin. In addition, it is shown that the surface brine volume fraction and freeboard are well correlated with ice thickness, indicating some promise for remote sensing approaches to the estimation of ice thickness. - Kazuya Ono, Kay I. Ohshima, Tokihiro Kono, Motoyo Itoh, Katsuro Katsumata, Yuri N. Volkov, Masaaki WakatsuchiJOURNAL OF OCEANOGRAPHY 63 2 281 - 291 2007年04月 [査読有り][通常論文]
Intensive CTD observations that resolve the mean and tidal components were done with a total of 129 casts in summer of 2001 at Bussol' Strait. Based on these data and all the available historical data, we have revealed the outflow from Bussol' Strait to the Pacific and the significant diapycnal mixing in the strait. In the range 27.0-27.3 sigma(theta), the water property in Bussol' Strait is almost identical to that of the Kuril Basin Water (KBW). The KBW out of Bussol' Strait forms a water mass front with the East Kamchatka Current Water (EKCW). This front also corresponds to the front of the Oyashio Current. In the lower part of the intermediate layer (27.3-27.6 sigma(theta)), part of the water in the strait is characterized by lower temperature, lower salinity, and higher dissolved oxygen than that of KBW and EKCW, which can be explained only by the diapycnal mixing. The strong diapycnal mixing in the strait can also be shown by the density inversion, occurrence frequency of which corresponds well to the amplitude distribution of the diurnal current. In the density range 26.7-26.84 sigma(theta), the water in Bussol' Strait has the lowest potential vorticity, suggesting that it is a source region of the low potential vorticity water. Seasonal change of the water can reach up to a density of 26.8 sigma(theta) around Bussol'Strait. This leads us to propose that the combination of winter convection and local tidal mixing leads to effective ventilation of the intermediate layer. - Takuya Nakanowatari, Kay I. Ohshima, Masaaki WakatsuchiGEOPHYSICAL RESEARCH LETTERS 34 4 2007年02月 [査読有り][通常論文]
On the basis of all available data, it is found that intermediate water temperature on the 26.8 - 27.4 sigma(theta) isopycnals in the northwestern North Pacific has significantly increased during the past 50 years. The largest warming area exists in the western part of the Sea of Okhotsk with a 0.68 degrees C/50-yr temperature increase observed at 27.0 sigma(theta). The warming in the Pacific is found over the Oyashio and Subarctic Current regions, where the Okhotsk water extends along the subarctic gyre. This suggests that the warming originates from the Sea of Okhotsk. The warming trend is also accompanied by the significant decreasing trend of dissolved oxygen content, suggesting the weakening of overturning in the northwestern North Pacific. We propose that these trends of the water mass property are caused by a decrease in dense shelf water production in the northwestern shelf of the Sea of Okhotsk, which is a sensitive area to the current global warming. - Takenobu Toyota, Kay I. Ohshima, Naoto Ebuchi, Kazuki Nakamura, Shotaro UtoIGARSS: 2007 IEEE INTERNATIONAL GEOSCIENCE AND REMOTE SENSING SYMPOSIUM, VOLS 1-12 3997 - + 2007年 [査読有り][通常論文]
Airborne Polarimetric and Interferometric Synthetic Aperture Radar (Pi-SAR) observation, conducted in the southern Sea of Okhotsk in February 2005, provided the opportunity to validate the retrieval of ice thickness distribution. In conjunction with the airborne SAR observation, in-situ ice thickness and ice-surface roughness measurements were carried out in the same area with ship-borne electromagnetic (EM) inductive sounding and supersonic profiling, respectively. Based on the analyzed results of data acquired in this experiment, this paper examine the possibility of ice thickness retrieval from the L-band SAR backscattering data in the seasonal ice zone (SIZ). - Variations in the Soya Warm Current observed by HF ocean radar, coastal tide gauges and satellite altimetry.Ebuchi, N, Y. Fukamachi, K.I. Ohshima, K. Shirasawa, M. WakatsuchiProceedings of ISRS2008PORSEC (CD-ROM) TA1001 2006年11月 [査読無し][通常論文]
- Kay I. Ohshima, Sohey Nihashi, Eisuke Hashiya, Tomohiro WatanabeJOURNAL OF THE METEOROLOGICAL SOCIETY OF JAPAN 84 5 907 - 919 2006年10月 [査読有り][通常論文]
We examine the interannual variability of the sea ice area in the Sea of Okhotsk in terms of surface heat flux, using highly resolved flux data with the ice concentration taken into account. On the northwest and east Sakhalin shelves, where the initial ice formation occurs, the onset of ice formation is simply determined by the local heat flux in fall (October-November); the degree to which the ocean is cooled by the atmosphere. Consequently, this heat flux mostly determines the interannual variability of sea ice area in the Okhotsk Sea in the initial stage. The air temperature anomaly is the main cause of the heat flux anomaly in fall. The fall heat flux has persistently affected the ice area anomaly, particularly until mid-January. In the later sea ice season, the relationship between the ice area and heat flux is obscured, particularly after mid-January, by the heat insulating effect of sea ice. The ice area variabilities in the southern and northeast regions are correlated with the local heat flux in the months preceding ice appearance, but not strongly. Our analysis suggests that the heat flux does not determine to what extent the ice advances finally in the Okhotsk Sea. Typical heat flux distributions in heavy and light ice years are also presented from our heat flux data: the heat loss center is located in the central Okhotsk Sea in light ice years, and moves to the eastern Okhotsk Sea near the Kuril Straits in heavy ice years. - Fukamachi Yasushi, Mizuta Genta, Ohshima Kay I., Toyota Takenobu, Kimura Noriaki, Wakatsuchi MasaakiJournal of Geophysical Research 111, C09018/,doi:10.1029/2005JC003327 C9 2006年09月14日 [査読有り][通常論文]
- Yasushi Fukamachi, Genta Mizuta, Kay I. Ohshima, Takenobu Toyota, Noriaki Kimura, Masaaki WakatsuchiJournal of Geophysical Research: Oceans 111 9 2006年09月08日 [査読有り][通常論文]
Using a moored ice-profiling sonar along with a moored acoustic Doppler current profiler, a total spatial section of draft across 3334 km of sea ice was obtained in the southwestern Sea of Okhotsk near Hokkaido in the winters of 1999-2001. Using this draft data set, the average draft and keel statistics are discussed in this sea for the first time. The mean draft was 0.60 m, which corresponds to the thickness of 0.71 m, over the three winters with the range of 0.49-0.72 m for each winter. The classification of level and deformed ice reveals a small range of the monthly mean level ice draft (0.18-0.27 m) and the dominance of the deformed ice in terms of volume (80%). The mean draft varied with the areal ratio of the deformed ice fairly well. These results suggest that dynamic processes such as ridging and rafting are important for the evolution of draft in the region of observation. The observed draft probability density distribution and keel statistics show that the thick ice ratio and keel frequency are lower than the similar data in polar regions and closer to those observed in Davis Strait west of Greenland. Along with the ice concentration and speed derived from the satellite data the southward ice transport to the southwestern Sea of Okhotsk is estimated on the basis of the observed sea ice thickness. The estimated ice transport ranged from 15 to 70 km3 in each winter. The heat and freshwater transport associated with the ice transport ranged from -3.9 × 1017 to -1.8 × 1018 J and from 12 to 57 km3, respectively. Copyright 2006 by the American Geophysical Union. - Yasushi Fukamachi, Genta Mizuta, Kay I. Ohshima, Takenobu Toyota, Noriaki Kimura, Masaaki WakatsuchiJOURNAL OF GEOPHYSICAL RESEARCH-OCEANS 111 C9 2006年09月 [査読有り][通常論文]
[ 1] Using a moored ice-profiling sonar along with a moored acoustic Doppler current profiler, a total spatial section of draft across 3334 km of sea ice was obtained in the southwestern Sea of Okhotsk near Hokkaido in the winters of 1999 - 2001. Using this draft data set, the average draft and keel statistics are discussed in this sea for the first time. The mean draft was 0.60 m, which corresponds to the thickness of 0.71 m, over the three winters with the range of 0.49 - 0.72 m for each winter. The classification of level and deformed ice reveals a small range of the monthly mean level ice draft (0.18 - 0.27 m) and the dominance of the deformed ice in terms of volume (80%). The mean draft varied with the areal ratio of the deformed ice fairly well. These results suggest that dynamic processes such as ridging and rafting are important for the evolution of draft in the region of observation. The observed draft probability density distribution and keel statistics show that the thick ice ratio and keel frequency are lower than the similar data in polar regions and closer to those observed in Davis Strait west of Greenland. Along with the ice concentration and speed derived from the satellite data the southward ice transport to the southwestern Sea of Okhotsk is estimated on the basis of the observed sea ice thickness. The estimated ice transport ranged from 15 to 70 km 3 in each winter. The heat and freshwater transport associated with the ice transport ranged from - 3.9 x 10(17) to - 1.8 x 10(18) J and from 12 to 57 km(3), respectively. - Simizu Daisuke, Ohshima Kay I.Journal of Geophysical Research 111, C05016/,doi:10.1029/2005JC002980 C5 2006年05月23日 [査読有り][通常論文]
- Daisuke Simizu, Kay I. OhshimaJournal of Geophysical Research: Oceans 111 5 2006年05月08日 [査読有り][通常論文]
Numerical model experiments have been conducted to clarify the circulation in the Sea of Okhotsk. The Princeton Ocean Model, which uses a vertical sigma coordinate, is adopted with the realistic topography. The East Sakhalin Current (ESC) is reproduced in the experiment forced by the annual mean wind stress. The ESC in the model consists of two components: a nearshore component on the shelf and an offshore one over the shelf slope, which is consistent with the drifter observations. The offshore component is interpreted as the western boundary current of the cyclonic circulation driven by the positive wind stress curl. The nearshore component is driven by the alongshore wind, regarded as an "arrested topographic wave" (ATW). The seasonal and interannual variations of the ESC transport are examined with the experiments forced by the daily wind stress for the period of 1985-1999. The model well reproduces the variabilities of the transport and structure of the ESC observed with the mooring array. Although a precise Sverdrup balance does not hold in the interior region, the amplitude of the monthly mean ESC transport corresponds roughly to one third of the Sverdrup transport with a phase lag of about a month. The transport variation of the nearshore component is explained well by integration of the onshore Ekman flux, which is consistent with the ATW theory. The circulation of the Okhotsk Sea and the transport of the ESC are affected only slightly by surface heat flux. Copyright 2006 by the American Geophysical Union. - D Simizu, KI OhshimaJOURNAL OF GEOPHYSICAL RESEARCH-OCEANS 111 C5 2006年05月 [査読有り][通常論文]
Numerical model experiments have been conducted to clarify the circulation in the Sea of Okhotsk. The Princeton Ocean Model, which uses a vertical sigma coordinate, is adopted with the realistic topography. The East Sakhalin Current (ESC) is reproduced in the experiment forced by the annual mean wind stress. The ESC in the model consists of two components: a nearshore component on the shelf and an offshore one over the shelf slope, which is consistent with the drifter observations. The offshore component is interpreted as the western boundary current of the cyclonic circulation driven by the positive wind stress curl. The nearshore component is driven by the alongshore wind, regarded as an "arrested topographic wave'' (ATW). The seasonal and interannual variations of the ESC transport are examined with the experiments forced by the daily wind stress for the period of 1985-1999. The model well reproduces the variabilities of the transport and structure of the ESC observed with the mooring array. Although a precise Sverdrup balance does not hold in the interior region, the amplitude of the monthly mean ESC transport corresponds roughly to one third of the Sverdrup transport with a phase lag of about a month. The transport variation of the nearshore component is explained well by integration of the onshore Ekman flux, which is consistent with the ATW theory. The circulation of the Okhotsk Sea and the transport of the ESC are affected only slightly by surface heat flux. - J Ono, KI Ohshima, G Mizuta, F Yasushi, M WakatsuchiDEEP-SEA RESEARCH PART I-OCEANOGRAPHIC RESEARCH PAPERS 53 3 409 - 424 2006年03月 [査読無し][通常論文]
To clarify the characteristics of tidal currents and heights over Kashevarov Bank (KB) in the Sea of Okhotsk, continuous mooring observation with ADCP and CT sensors was carried out near the summit of KB from 20 September 1999 to 20 June 2000. The velocity, sea-surface height, and bottom-water temperature were dominated by the diurnal signal with fortnightly modulation. The velocity amplitude reached similar to 1.4 in s(-1) during the spring tide. The K-1 and O-1 tidal current ellipses are clockwise with a nearly circular shape and nearly barotropic. To interpret the physical mechanism of the amplified diurnal currents over KB, a seamount-trapped wave (SMTW) model was applied to KB. The frequency of SMTW approximated to KB is close to the diurnal tidal frequencies for the first-gravest mode of first azimuthal wavenumber. The structure and properties of the SMTW mode are consistent with the observations. It is proposed that the amplified diurnal currents over KB are caused by the resonance with SMTW. The observed fortnightly variability in tidal currents and bottom- and surface-temperatures suggests that both the cold spot in summer and the low sea-ice concentration in winter over KB are due to the horizontal and vertical mixing with the surrounding subsurface water, caused by strong tidal currents. (c) 2005 Elsevier Ltd. All rights reserved. - N Ebuchi, Y Fukamachi, KI Ohshima, K Shirasawa, M Ishikawa, T Takatsuka, T Daibo, M WakatsuchiJOURNAL OF OCEANOGRAPHY 62 1 47 - 61 2006年02月 [査読無し][通常論文]
Three High Frequency (HF) ocean radar stations were installed around the Soya/La Perouse Strait in the Sea of Okhotsk in order to monitor the Soya Warm Current (SWC). The frequency of the HF radar is 13.9 MHz, and the range and azimuth resolutions are 3 km and 5 deg., respectively. The radar covers a range of approximately 70 km from the coast. The surface current velocity observed by the HF radars was compared with data from drifting buoys and shipboard Acoustic Doppler Current Profilers (ADCPs). The current velocity derived from the HF radars shows good agreement with that observed using the drifting buoys. The root-mean-square (rms) differences were found to be less than 20 cm s(-1) for the zonal and meridional components in the buoy comparison. The observed current velocity was also found to exhibit reasonable agreement with the shipboard ADCP data. It was shown that the HF radars clearly capture seasonal and short-term variations of the SWC. The velocity of the Soya Warm Current reaches its maximum, approximately 1 m s(-1), in summer and weakens in winter. The velocity core is located 20 to 30 km from the coast, and its width is approximately 40 km. The surface transport by the SWC shows a significant correlation with the sea level difference along the strait, as derived from coastal tide gauge records at Wakkanai and Abashiri. - Takeshi Tamura, Kay I. Ohshima, Hiroyuki Enomoto, Kazutaka Tateyama, Atsuhiro Muto, Shuki Ushio, Robert A. MassomANNALS OF GLACIOLOGY, VOL 44, 2006 44 269 - + 2006年 [査読有り][通常論文]
Antarctic coastal polynyas are major areas of intense ocean-atmosphere heat and moisture flux, and associated high sea-ice production and dense-water formation. Their accurate detection, including an estimate of thin ice thickness, is therefore very important. In this paper, we apply a technique originally developed in the Arctic to an estimation of sea-ice thickness using US National Oceanic and Atmospheric Administration (NOAA) Advanced Very High Resolution Radiometer (AVHRR) data and meteorological data in the Vincennes Bay polynya off Wilkes Land, East Antarctica. The method is based upon the heat-flux calculation using sea-ice surface temperature estimates from the satellite thermal-infrared data combined with global objective analysis (European Centre for Medium-Range Weather Forecasts (ECMWF)) data. The validity of this method is assessed by comparing results with independent ice-surface temperature and ice-thickness data obtained during an Australian-led research cruise to the region in 2003. In thin-ice (polynya) regions, ice thicknesses estimated by the heat-flux calculation using AVHRR and ECMWF data show reasonable agreement with those estimated by (a) applying the heat-flux calculation to in situ radiation thermometer and meteorological data and (b) in situ observations. The standard deviation of the difference between the AVHRR-derived and in situ data is similar to 0.02 m. Comparison of the AVHRR ice-thickness retrievals with coincident satellite passive-microwave polarization ratio data confirms the potential of the latter as a means of deriving maps of thin sea-ice thickness on the wider scale, uninterrupted by darkness and cloud cover. - Naoto Ebuchi, Yasushi Fukamachi, Kay I. Ohshima, Kunio Shirasawa, Masaaki Wakatsuchi2006 IEEE INTERNATIONAL GEOSCIENCE AND REMOTE SENSING SYMPOSIUM, VOLS 1-8 1860 - 1863 2006年 [査読有り][通常論文]
Three HF ocean radar stations were installed at the Soya/La Perouse Strait in the Sea of Okhotsk in order to monitor the Soya Warm Current. The frequency of the HF radar is 13.9 MHz, and the range and azimuth resolutions are 3 km and 5 degrees, respectively. The radar covers a range of approximately 70 km from the coast. It is shown that the HF radars clearly capture seasonal and short-term variations of the Soya Warm Current. The velocity of the Soya Warm Current reaches its maximum, approximately 1 m s(-1), in summer, and weakens in winter. The velocity core is located 20 to 30 km from the coast, and its width is approximately 50 km. The surface transport by the Soya Warm Current shows a significant correlation with the sea level difference along the strait, as derived from coastal tide gauge records. The cross-current sea level difference, which is estimated from the sea level anomalies observed by the Jason-1 altimeter and a coastal tide gauge, also exhibits variation in concert with the surface transport and along-current sea level difference. - Genta Mizuta, Kay I. Ohshima, Yasushi Fukamachi, Masaaki WakatsuchiJournal of Marine Research 63 6 1017 - 1039 2005年11月 [査読無し][通常論文]
Long-term current measurements of the East Sakhalin Current (ESC) in the Sea of Okhotsk are analyzed using the technique of empirical orthogonal functions (EOFs) in the frequency domain. The first and second EOFs at subtidal frequencies represent motions over the continental shelf and slope, respectively, corresponding to the variability of the two cores of the intense ESC. The first EOF can be explained by the first-mode coastal trapped wave (CTW). The structure of the second EOF is similar to that of the second-mode CTW to the first approximation. According to the distribution of the cross-spectra between EOFs and the wind stress over the whole area of the Sea of Okhotsk, the first EOF is correlated with the alongshore component of the wind stress over the northern and western shelves. The distribution of the phase of the wind stress, which is correlated with the first EOF, indicates that a resonance between the CTW and wind stress drives the motion represented by the first EOF at lower frequencies. At higher frequencies the phase of the wind stress correlated with the first EOF is almost uniform in space, being consistent with the greater speed of phase propagation of the EOF compared with that for the free CTW at these frequencies. The second EOF is correlated with the wind stress curl in the central part of the Sea of Okhotsk. The motion by the second EOF is confined over the slope at lower frequencies and becomes large over the shelf at higher frequencies. This change in the structure of the second EOF is consistent with the results of the numerical experiment of the flow induced by the offshore forcing by Chapman and Brink (1987). The phase of the wind stress curl which is correlated with the second EOF changes clearly in space at some frequencies, suggesting that the motion represented by the second EOF propagates along the isobath with the coast to the right. The wind stress curl contains the wavenumber resonant with the lowest two or three modes of CTWs. - G Mizuta, KI Ohshima, Y Fukamachi, M WakatsuchiJOURNAL OF MARINE RESEARCH 63 6 1017 - 1039 2005年11月 [査読無し][通常論文]
Long-term current measurements of the East Sakhalin Current (ESC) in the Sea of Okhotsk are analyzed using the technique of empirical orthogonal functions (EOFs) in the frequency domain. The first and second EOFs at subtidal frequencies represent motions over the continental shelf and slope, respectively, corresponding to the variability of the two cores of the intense ESC. The first EOF can be explained by the first-mode coastal trapped wave (CTW). The structure of the second EOF is similar to that of the second-mode CTW to the first approximation. According to the distribution of the cross-spectra between EOFs and the wind stress over the whole area of the Sea of Okhotsk, the first EOF is correlated with the alongshore component of the wind stress over the northern and western shelves. The distribution of the phase of the wind stress, which is correlated with the first EOF, indicates that a resonance between the CTW and wind stress drives the motion represented by the first EOF at lower frequencies. At higher frequencies the phase of the wind stress correlated with the first EOF is almost uniform in space, being consistent with the greater speed of phase propagation of the EOF compared with that for the free CTW at these frequencies. The second EOF is correlated with the wind stress curl in the central part of the Sea of Okhotsk. The motion by the second EOF is confined over the slope at lower frequencies and becomes large over the shelf at higher frequencies. This change in the structure of the second EOF is consistent with the results of the numerical experiment of the flow induced by the offshore forcing by Chapman and Brink (1987). The phase of the wind stress curl which is correlated with the second EOF changes clearly in space at some frequencies, suggesting that the motion represented by the second EOF propagates along the isobath with the coast to the fight. The wind stress curl contains the wavenumber resonant with the lowest two or three modes of CTWs. - KI Ohshima, M Wakatsuchi, SI SaitohJOURNAL OF OCEANOGRAPHY 61 5 845 - 855 2005年10月 [査読無し][通常論文]
Based on the surface drifters that moved out from the Sea of Okhotsk to the Pacific, the surface velocity fields of mean, eddy, and tidal components in the Oyashio region are examined for the period September 1999 to August 2000. Along the southern Kuril Island Chain, the Oyashio Current, having a width of similar to 100 km, exists with velocities of 0.2-0.4 m s(-1). From 40 degrees N to 43 degrees N, the Subarctic Current flows east- or northeastward with velocities of 0.1-0.3 m s(-1), accompanied by a meandering Oyashio or Subarctic front. Between the Oyashio and Subarctic current regions, an eddy-dominant region exists with both cyclonic and anticyclonic eddies. The existence of an eastward flow just south of Bussol' Strait is suggested. The 2000 anticyclonic warm-core ring located south of Hokkaido was found to have a nearly symmetric velocity structure with a maximum velocity of similar to 0.7 m s(-1) at 70 km from the eddy center. Diurnal tidal currents with a clockwise tidal ellipse are amplified over the shelf and slope off Urup and Iturup Islands, suggesting the presence of diurnal shelf waves. From Lagrangian statistics, the single-particle diffusivity is estimated to be similar to 10 x 10(7) cm(2)s(-1). - Y Shimada, A Kubokawa, KI OhshimaJOURNAL OF OCEANOGRAPHY 61 5 913 - 920 2005年10月 [査読無し][通常論文]
Recent observations suggest that the annual mean southward transport of the East Sakhalin Current (ESC) is significantly larger than the annual mean Sverdrup transport. Motivated by this observational result, transport of a western boundary current has been investigated using a simple numerical model with a western slope. This transport is defined as the instantaneous barotropic transport integrated from the western boundary to the offshore point where the barotropic velocity vanishes. The model, forced by seasonally varying wind stress, exhibits an annual mean of the western boundary current transport that is larger than that of the Sverdrup transport, as observed. The southward transport from October to March in the model nearly equals the instantaneous Sverdrup transport; while the southward transport from April to September decreases slowly. Although the Sverdrup transport in July vanishes, the southward transport in summer nearly maintains the annual mean Sverdrup transport, because the barotropic Rossby wave cannot intrude on the western slope. This summer transport causes the larger annual mean. Although there are some uncertainties in the estimation of the Sverdrup transport in the Sea of Okhotsk, the seasonal variation of the southward transport in the model is qualitatively similar to the observations. - KI Ohshima, Y Fukamachi, T Mutoh, M WakatsuchiJOURNAL OF OCEANOGRAPHY 61 2 247 - 260 2005年04月 [査読無し][通常論文]
Mesoscale eddies, particularly anticyclonic ones, are dominant features in the Kuril Basin of the Okhotsk Sea. In 1999, both surface drifter and hydrographic observations caught the same anticyclonic eddy northwest of Bussol' Strait, which has a diameter of similar to100 km, typical surface velocity of 0.2-0.3 m s(-1), and less dense core extending to a depth of similar to1200 m. Based on an idea that the generation of mesoscale eddies is caused by strong tidal mixing in and around Kuril Straits, we have conducted a series of three-dimensional numerical model experiments, in which strong tidal mixing is simply parameterized by increasing coefficients of vertical eddy viscosity and diffusivity along the eastern boundary. Initially, a regular series of disturbances with a wavelength of similar to70 km starts to develop. The disturbances can be clearly explained by a linear instability theory and regarded as the baroclinic instability associated with the near-surface front formed in the region between the enhanced mixing and offshore regions. In the mature phase, the disturbances grow large enough that some eddies pinch off and advect offshore (westward), with the scale of disturbances increasing gradually. Typical eddy scale and its westward propagation speed are similar to100 krn and similar to0.6 km day(-1), respectively, which are consistent with the observations by satellites. The westward propagation can be explained partly due to nonlinear effect of self-offshore advection and partly due to the beta-effect. With the inclusion of the upper ocean restoring, the dominance of anticyclonic eddy, extending from surface to a depth of similar to1200 m, can be reproduced. - KI Ohshima, SC Riser, M WakatsuchiGEOPHYSICAL RESEARCH LETTERS 32 6 2005年03月 [査読無し][通常論文]
The time evolution of the ocean mixed layer in the seasonal sea ice zone of the Okhotsk Sea has been observed with profiling floats. The heat storage rate in the mixed layer estimated from the float data well coincides with the surface heat flux. Mixed layer deepening and the onset of ice formation can be well reproduced by a bulk mixed layer model, suggesting that onset of ice formation can be predicted by local atmospheric conditions. This explains the remarkably high correlation of the onset day of sea ice formation with the surface heat loss in the preceding fall. The anomalously large heat loss in the fall of 2000 led to the anomalously early ice formation. Strong stratification due to the Amur River fresh water flux is indispensable for ice formation through suppression of deep convection. - Nihashi S, Ohshima K. I., Jeffries MO, Kawamura TJOURNAL OF GEOPHYSICAL RESEARCH-OCEANS 110 C2 2005年02月05日 [査読有り][通常論文]
- KI Ohshima, S NihashiJOURNAL OF PHYSICAL OCEANOGRAPHY 35 2 188 - 201 2005年02月 [査読無し][通常論文]
In the Antarctic Ocean, sea ice melts mostly by warming of the ocean mixed layer through heat input (mainly solar radiation) in open water areas. A simplified ice-upper ocean coupled model is proposed in which sea ice melts only by the ocean heat supplied from the air. The model shows that the relationship between ice concentration (i.e., fraction, C) and mixed layer temperature (T) converges asymptotically with time (C-T relationship), which agrees with observed C-T plots during summer in the sector 25 degrees-45 degrees E. This relationship can be used for estimating the bulk heat transfer coefficient between ice and ocean by fitting to observations, and a value of 1.2 X 10(-4) m s(-1) is obtained. The model shows that the ratio of the heat used for melting to the heat input through open water is inclined to be determined as a function of ice concentration. For typical conditions in the Antarctic ice melt season, the ratio ranges mostly between 0.7 and 0.9. When the model is extended to two dimensions in the meridional direction, with the inclusion of wind forcing, it approximately reproduces the meridional retreat of the Antarctic sea ice. This two-dimensional model can describe the open water-albedo feedback effect, which partly explains the year-to-year variation of the sea-ice retreat in the Antarctic Ocean. - S Nihashi, KI Ohshima, MO Jeffries, T KawamuraJOURNAL OF GEOPHYSICAL RESEARCH-OCEANS 110 C2 2005年02月 [査読有り][通常論文]
Sea-ice melting processes are inferred from various summer sea-ice and upper ocean data obtained in the Ross Sea in January 1999. Using spatially (30 km) averaged continuous data, an ice concentration-water temperature plot (CT-plot) shows that the temperature at a depth of similar to7 m increases as ice concentration decreases in the ice interior region. The CT-plot is explained by a simple ice-upper ocean coupled model in which sea-ice melting is caused only by heat input through open water. The bulk heat transfer coefficient between ice and ocean (K-b) is estimated to be 1.2 x 10(-4) m s(-1). These findings are supported by the relationship between ice concentration and heat content in the mixed layer estimated from conductivity-temperature-depth (CTD) data. Salinity at a depth of similar to7 m, salt deficit in the mixed layer associated with sea-ice melt estimated from CTD data, and their relationships with ice concentration also suggest that melting is mainly caused by atmospheric heat input through open water and that local balance of salinity nearly hold. The time evolution of ice concentration calculated from the ice-upper ocean coupled model with the estimated K-b corresponds with that derived from the Special Sensor Microwave Imager (SSM/I), confirming the model's applicability and the value of K-b. - Ohshima K. I., S. Nihashi, T. TamuraProceedings of IEEE International Geoscience and Remote Sensing Symposium 2005 4 2652 - 2655 2005年 [査読有り][通常論文]
- Seasonal variation of the Soya Warm Current observed by HF ocean radar.Ebuchi, N, Y. Fukamachi, K.I. Ohshima, K. Shirasawa, M. Ishikawa, T. Takatsuka, T. Daibo, M. WakatsuchiProceedings of ISRS 2004 183 - 187 2004年10月 [査読無し][通常論文]
- Yasushi Fukamachi, Genta Mizuta, Kay I. Ohshima, Lynne D. Talley, Stephen C. Riser, Masaaki WakatsuchiJournal of Geophysical Research C: Oceans 109 9 1 - 16 2004年09月15日 [査読有り][通常論文]
The region off the east coast of Sakhalin is thought of as an important pathway of dense shelf water (DSW) from its production region in the northwestern Okhotsk Sea to the southern Okhotsk Sea. From July 1998 to June 2000, the first long-term mooring experiment was carried out in this region to observe the southward flowing East Sakhalin Current (ESC) and DSW. Moored and associated hydrographic data show considerable modification of cold dense water via mixing with warm offshore water in the slope region off northern Sakhalin. Significant onshore eddy heat flux was observed at the northernmost mooring (54.9°N), which suggests the occurrence of baroclinic instability. The eddy heat flux was not significant farther south. At moorings along 53°N, cold anticyclonic eddies were identified that were consistent with isolated eddies seen in the hydrographic data. The three years of hydrographic data also showed large differences in extent and properties of DSW. Furthermore, the mooring data show that seasonal variability of DSW was quite different in the two years. The average DSW transport for σ θ > 26.7 evaluated using the moored data at 53°N for 1 year (1998-1999) was ∼0.21 Sv (= 106 m3 s-1). This value is at the lower end of the previous indirect estimates. Along with the DSW modification, this transport estimate indicates that DSW was not only carried southward by the ESC but was spread offshore by eddies off northern Sakhalin. Copyright 2004 by the American Geophysical Union. - Water exchange and tidal currents through the Bussol' Strait revealed by direct current measurementsKatsurou Katsumata, Kay I. Ohshima, Tokihiro Kono, Motoyo Itoh, Ichiro Yasuda, Yury N. Volkov, Masaaki WakatsuchiJournal of Geophysical Research C: Oceans 109 9 1 - 11 2004年09月15日 [査読有り][通常論文]
An outflow from the Sea of Okhotsk to the North Pacific was measured at the Bussol' Strait, the largest strait connecting these basins, from 31 August to 11 September 2001. Yo-yo casts of a lowered acoustic Doppler current profiler were performed at 13 stations across the narrowest part of the strait. Time series covering more than approximately 24 hours were obtained for each station, and the semidiurnal and diurnal tides were separated. The diurnal tide at the spring tide shows a remarkable peak of the amplitude of 1.1 m s-1 at the depth below 1000 m in the western channel of the strait. The upper part of the mean component flows toward the Pacific, and the lower part flows in the opposite direction. The outflow is in excess of the inflow, and net transport through the strait is 8.2 to 8.8 Sv (1 Sv = 106 m3 s-1). The outflow is strong in two density ranges. The upper layer peak around 26.8 σθ corresponds to the density of North Pacific Intermediate Water and the temperature-salinity characteristics of this outflow were actually observed in this density range across downstream the Oyashio off the southeastern coast of Hokkaido. Net heat and salt exchanges between the Sea of Okhotsk and the North Pacific are estimated net heat flux of -34 TW and net salt flux of - 1.9 × 106 kg s-1 are exchanged from the Sea of Okhotsk to the North Pacific. Copyright 2004 by the American Geophysical Union. - KI Ohshima, S MartinJOURNAL OF GEOPHYSICAL RESEARCH-OCEANS 109 C9 doi:10.1029/2004JC002604 2004年09月 [査読無し][通常論文]
- Observation of the Soya Warm Current using HF ocean radar.Ebuchi, N, Y. Fukamachi, K.I. Ohshima, K. Shirasawa, M. Ishikawa, T. Takatsuka, T. Daibo, M. WakatsuchiProceedings of IGARSS 2004 1175 - 1178 2004年09月 [査読無し][通常論文]
- Water exchange and tidal currents through the Bussol' Strait revealed by direct current measurementsK Katsumata, KI Ohshima, T Kono, M Itoh, Yasuda, I, YN Volkov, M WakatsuchiJOURNAL OF GEOPHYSICAL RESEARCH-OCEANS 109 C9 1 - 11 2004年08月 [査読無し][通常論文]
An outflow from the Sea of Okhotsk to the North Pacific was measured at the Bussol' Strait, the largest strait connecting these basins, from 31 August to 11 September 2001. Yo-yo casts of a lowered acoustic Doppler current profiler were performed at 13 stations across the narrowest part of the strait. Time series covering more than approximately 24 hours were obtained for each station, and the semidiurnal and diurnal tides were separated. The diurnal tide at the spring tide shows a remarkable peak of the amplitude of 1.1 m s(-1) at the depth below 1000 m in the western channel of the strait. The upper part of the mean component flows toward the Pacific, and the lower part flows in the opposite direction. The outflow is in excess of the inflow, and net transport through the strait is 8.2 to 8.8 Sv (1 Sv = 10(6) m(3) s(-1)). The outflow is strong in two density ranges. The upper layer peak around 26.8 sigma(theta) corresponds to the density of North Pacific Intermediate Water and the temperature-salinity characteristics of this outflow were actually observed in this density range across downstream the Oyashio off the southeastern coast of Hokkaido. Net heat and salt exchanges between the Sea of Okhotsk and the North Pacific are estimated; net heat flux of -34 TW and net salt flux of -1.9 x 10(6) kg s(-1) are exchanged from the Sea of Okhotsk to the North Pacific. - Y Fukamachi, G Mizuta, KI Ohshima, LD Talley, SC Riser, M WakatsuchiJOURNAL OF GEOPHYSICAL RESEARCH-OCEANS 109 C9 doi:10.1029/2003JC001906 2004年07月 [査読無し][通常論文]
The region off the east coast of Sakhalin is thought of as an important pathway of dense shelf water (DSW) from its production region in the northwestern Okhotsk Sea to the southern Okhotsk Sea. From July 1998 to June 2000, the first long-term mooring experiment was carried out in this region to observe the southward flowing East Sakhalin Current (ESC) and DSW. Moored and associated hydrographic data show considerable modification of cold dense water via mixing with warm offshore water in the slope region off northern Sakhalin. Significant onshore eddy heat flux was observed at the northernmost mooring (54.9degreesN), which suggests the occurrence of baroclinic instability. The eddy heat flux was not significant farther south. At moorings along 53degreesN, cold anticyclonic eddies were identified that were consistent with isolated eddies seen in the hydrographic data. The three years of hydrographic data also showed large differences in extent and properties of DSW. Furthermore, the mooring data show that seasonal variability of DSW was quite different in the two years. The average DSW transport for sigma(theta) > 26.7 evaluated using the moored data at 53degreesN for 1 year (1998-1999) was similar to0.21 Sv (= 10(6) m(3) s(-1)). This value is at the lower end of the previous indirect estimates. Along with the DSW modification, this transport estimate indicates that DSW was not only carried southward by the ESC but was spread offshore by eddies off northern Sakhalin. - Takenobu Toyota, Toshiyuki Kawamura, Kay Ohshima, Haruhito ShimodaJournal of Geophysical Research C: Oceans 109 6 C06001 - 18 2004年06月15日 [査読有り][通常論文]
Sea ice thickness data and sea ice samples were analyzed to examine the characteristics of the ice thickness distribution and ice texture, and to understand ice growth processes in the southern Sea of Okhotsk. Ice thickness data and samples were obtained aboard the icebreaker Soya in early February, the ice growth season. Ice thickness data, which were obtained with a video monitoring system installed on the side deck of the ship each winter from 1991 to 2000 except 1995, show that the average thickness ranges from 19 ± 7 to 55 ± 23 cm and that it matches the characteristics of a Poisson distribution. Ice structure analysis reveals that granular texture occupies about three quarters of the total ice thickness and that the ice exhibits a layered structure with unit thickness averaging 5 to 10 cm. Stratigraphy and stable isotopic composition of the ice indicate that snow ice accounts for 10% and frazil ice accounts for 64% of the total ice thickness. This suggests that dynamic ice thickening processes such as frazil ice growth and piling up are more significant than congelation. growth. On the basis of these characteristics, which resemble more those of Antarctic than Arctic sea ice, we propose a conceptual model for the ice thickening process in this region. It is shown that this model can explain the shape of the ice thickness distribution well, and is analogous with the concept of the "pancake cycle" and multiple rafting of Antarctic sea ice growth and thickening. Copyright 2004 by the American Geophysical Union. - T Toyota, T Kawamura, KI Ohshima, H Shimoda, M WakatsuchiJOURNAL OF GEOPHYSICAL RESEARCH-OCEANS 109 C6 doi:10.1029/2003JC002090 2004年06月 [査読有り][通常論文]
Sea ice thickness data and sea ice samples were analyzed to examine the characteristics of the ice thickness distribution and ice texture, and to understand ice growth processes in the southern Sea of Okhotsk. Ice thickness data and samples were obtained aboard the icebreaker Soya in early February, the ice growth season. Ice thickness data, which were obtained with a video monitoring system installed on the side deck of the ship each winter from 1991 to 2000 except 1995, show that the average thickness ranges from 19 +/- 7 to 55 +/- 23 cm and that it matches the characteristics of a Poisson distribution. Ice structure analysis reveals that granular texture occupies about three quarters of the total ice thickness and that the ice exhibits a layered structure with unit thickness averaging 5 to 10 cm. Stratigraphy and stable isotopic composition of the ice indicate that snow ice accounts for 10% and frazil ice accounts for 64% of the total ice thickness. This suggests that dynamic ice thickening processes such as frazil ice growth and piling up are more significant than congelation growth. On the basis of these characteristics, which resemble more those of Antarctic than Arctic sea ice, we propose a conceptual model for the ice thickening process in this region. It is shown that this model can explain the shape of the ice thickness distribution well, and is analogous with the concept of the "pancake cycle'' and multiple rafting of Antarctic sea ice growth and thickening. - G Mizuta, KI Ohshima, Y Fukamachi, M Itoh, M WakatsuchiCONTINENTAL SHELF RESEARCH 24 6 643 - 657 2004年04月 [査読無し][通常論文]
Hydrographic observations under sea ice were conducted in the southwestern part of the Sea of Okhotsk for 4 years from 1996 to 1999. Every year a cold mixed layer with near-freezing temperature was distributed from the sea surface to a depth of 150-300 m near the shelf break under sea ice. The thicknesses of the mixed layer and sea ice were largest in 1997. While the depth of the mixed layer was considerably deeper than that of dichothermal water, which is identified as a temperature minimum from spring to fall in this region, the density of water in the mixed layer was equal to or less than that of dichothermal water. It is shown that deepening of isopycnals due to the alongshore component of the wind stress is essential for thickening of the mixed layer. In 1997 the nearly northerly winds, which are usually directed offshore, were more parallel to the coast than those in the other years. Thickening of the mixed layer in 1997 is attributed to this wind condition. The air temperature and wind indicate that the sea-ice production rate was low in 1997, whereas the wind direction was favorable for rafting and ridging. Thus the change in wind direction is proposed As an important factor in determining the thickness of both the mixed layer and the sea ice in this region. (C) 2004 Elsevier Ltd. All rights reserved. - KI Ohshima, D Simizu, M Itoh, G Mizuta, Y Fukamachi, SC Riser, M WakatsuchiJOURNAL OF PHYSICAL OCEANOGRAPHY 34 2 513 - 525 2004年02月 [査読無し][通常論文]
It is proposed that the cyclonic gyre over the northern half-basin of the Okhotsk Sea is driven by the wind stress curl and that a major part of the East Sakhalin Current (ESC) can be regarded as its western boundary current. Both from the high-resolution ECMWF and Comprehensive Ocean-Atmosphere Dataset (COADS) data, the annual mean wind stress curl is positive over the sea. When the Sverdrup streamfunction is calculated by excluding the shallow shelves, the streamfunction shows a cyclonic pattern over the central basin, which is roughly consistent with the geopotential anomaly distribution from all the available hydrographic data. Profiling floats suggest that the cyclonic gyre extends to at least a depth of 500 m: a relatively intense southward flow (ESC) with an average speed of approximately 10 cm s(-1) near the western boundary and slow northward flow with an average speed of approximately 2 cm s(-1) in the east. Climatological data show that along zonal sections at 50degrees-53degreesN isopycnal surfaces gradually rise from the east to west and sharply drop near the western boundary, suggesting the Sverdrup balance. This feature persists throughout the year. The integrated northward baroclinic transport of 3.5 Sv along 53degreesN is comparable to the Sverdrup transport of 3.7 Sv, calculated from the annual mean wind stress. Sverdrup balance appears to hold roughly in the baroclinic field in 50degrees-53degreesN. A flat-bottom numerical model forced by realistic wind stress reproduces well the cyclonic gyre, with the observed baroclinic features. In the south, the anticyclonic circulation in the Kuril Basin cannot be explained by the wind stress curl inside the Okhotsk Sea in this simplified model. - G Mizuta, Y Fukamachi, KI Ohshima, M WakatsuchiJOURNAL OF PHYSICAL OCEANOGRAPHY 33 11 2430 - 2445 2003年11月 [査読無し][通常論文]
In order to clarify the structure and seasonal variability of the flow field near the western boundary of the Sea of Okhotsk, long-term mooring measurements were carried out from 1998 to 2000 in this region. In most of the mooring period a persistent southward flow (the East Sakhalin Current) was observed, which extends from the surface to a depth around 1000 m. The speed of this southward flow clearly changed seasonally. The peak monthly mean speed along 53degreesN at a depth of 200 m attained a maximum of 37 6 9 cm s(-1) in January and a minimum of 10 +/- 8 cm s(-1) in July. Three different cores of intense flow were identified in the southward flow. The first core was centered over the continental slope and had rather large vertical extent, reaching the bottom on the slope. The second core was trapped over the shelf near the surface and was observed from October to November. This core was associated with less saline surface water affected by the Amur River discharge. The third core was intensified toward the bottom on the slope. The spatial and temporal distribution of this bottom-intensified core coincided with that of dense shelf water, which is formed over the broad shelf in the north. The intensity of this core damped within a few hundred kilometers from the northern end of Sakhalin probably because of strong mixing of dense shelf water with surrounding waters. The total transport of the southward flow at 53degreesN was 6.7 x 10(6) m(3) s(-1) in the annual average, varying from a maximum of 12.3 x 10(6) m(3) s(-1) in February and a minimum of 1.2 x 10(6) m(3) s(-1) in October. Most of the transport was maintained by the first core of the southward flow. - KI Ohshima, T Watanabe, S NihashiJOURNAL OF THE METEOROLOGICAL SOCIETY OF JAPAN 81 4 653 - 677 2003年08月 [査読無し][通常論文]
The surface heat flux over the Sea of Okhotsk has been calculated from 1987 to 2001 by bulk parameterizations using ECMWF, ISCCP, and GISST data, corrected by LOADS data. Sea ice concentration and ice type are incorporated by using an SSM/I open water and thin-ice algorithm to better represent the sea ice conditions. The mean seasonal variation of the net heat flux, averaged over the entire Okhotsk Sea, ranges from a summer maximum of 158 W m(-2) in June (a positive value indicates that the sea or sea ice gains heat form the air), to a winter minimum of -219 W m(-2) in December. The seasonal geographic distribution of the net heat flux is determined mostly by turbulent (sensible and latent) heat flux. Because of the heat insulating effect of sea ice, the turbulent heat flux and accordingly the net heat flux has the largest variation in winter, particularly in the north region, reflecting the yearly difference of sea ice extent. The geographic distribution of the annual net heat flux shows a distinct contrast, significant cooling of the ocean in the north and net heating of the ocean in the south. This contrast is a result of heat transport by both sea ice and the southward East Sakhalin Current; sea ice formed by obtaining negative latent heat in the north is transported to the south, then it releases the negative latent heat by melting. The annual net heat budget is estimated to be -22 W m(-2) averaged over the entire sea. The Okhotsk Sea loses heat to the atmosphere on an annual basis, although at least the order of 20 W m(-2) error exists in the estimation. A trial was performed to estimate sea ice production using the heat budget calculation. The results suggest that most of the production occurs in open water and thin ice areas. The highest production area is located over the northwest shelf, within 100 km of the coast. - Y Fukamachi, G Mizuta, KI Ohshima, H Melling, D Fissel, M WakatsuchiGEOPHYSICAL RESEARCH LETTERS 30 7 doi: 10.1029/2002GL016197 2003年04月 [査読無し][通常論文]
[1] Using a moored ice-profiling sonar along with an ADCP, a spatial section of draft across 855 km of sea ice was obtained for the first time in the southwestern part of the Okhotsk Sea near Hokkaido in winter of 1999. The draft evolved from 0.20 m in mid-February to 1.45 m in late March with the overall value of 0.72 m. The draft characteristics were quite different between February and March before and after the period of strong winds. The increase of the mean draft from February to March was associated with the increase of deformed ice. The maximum draft observed was similar to17 m. Ice volume was dominated by the contribution from portions of deformed ice especially in March. These results suggest that deformed ice dominated in the region of observation and dynamical processes were mainly responsible for the evolution of draft. - Kusahara Kazuya, Ohshima Kay I., Katsumata KatsurouPolar Meteorol. Glaciol. 17 36 - 47 国立極地研究所 2003年 [査読無し][通常論文]
In this study, we investigate the wind-driven circulation in the Antarctic Ocean using a primitive two-layer model with realistic topography. A prominent feature of steady circulation driven by the annual mean wind stress is a clockwise(cyclonic) circulation in the lower layer at the Weddell Basin and the Australia Antarctic Basin. In particular, the circulation pattern in the Australia Antarctic Basin agrees with the observations. In these basins, negative vorticity input from the wind stress is transmitted to the lower layer through the diffusion term(Gent and McWilliams term) and causes prominent cyclonic gyres within closed geostrophic contours of f/H(f: Coriolis parameter, H: water depth). The model result forced by the seasonal wind stress shows that variations of the Antarctic Coastal Current are explained by wind stress variations along the coast. The transport of this current is determined by the integration of onshore Ekman transport along the coast. It is also shown that this Antarctic Coastal Current can be a part of the western boundary current in the Weddell Sea. On a time scale of 10 to 100 days, the variation of the upper layer thickness coincides with the sea level variation at Syowa Station. This variation might be attributed to coastal trapped waves driven by the alongshore wind stress. - Distribution and formation of Okhotsk Sea Intermediate Water: An analysis of isopycnal climatology dataJournal of Geophysical Research 108: 3258 doi: 10.1029/2002JC001590 2003年 [査読無し][通常論文]
- Motoyo Itoh, Kay I. Ohshima, Masaaki WakatsuchiJournal of Geophysical Research 108 C8 2003年 [査読有り]
- D Simizu, KI OhshimaJOURNAL OF OCEANOGRAPHY 58 6 851 - 860 2002年12月 [査読無し][通常論文]
We have examined wind-induced circulation in the Sea of Okhotsk using a barotropic model that contains realistic topography with a resolution of 9.25 km. The monthly wind stress field calculated from daily European Centre for Medium-Range Weather Forecasting (ECMWF) Re-Analysis data is used as the forcing, and the integration is carried out for 20 days until the circulation attains an almost steady state. In the case of November (a representative for the winter season from October to March), southward currents of velocity 0.1-0.3 in s(-1) occur along the bottom contours off the east of Sakhalin Island. The currents are mostly confined to the shelf (shallower than 200 in) and extend as far south as the Hokkaido coast. In the July case (a representative for the summer season from April to September), significant currents do not occur, even in the shallow shelves. The simulated southward current over the east Sakhalin shelf appears to correspond to the near-shore branch of the East Sakhalin Current (ESC), which was observed with the surface drifters. These seasonal variations simulated in our experiments are consistent with the observations of the ESC. Dynamically, the simulated ESC is interpreted as the arrested topographic wave (ATW), which is the coastally trapped flow driven by steady alongshore wind stress. The volume transport of the simulated ESC over the shelf reaches about 1.0 SV (1 SV = 10(6) m(3) s(-1)) in the winter season, which is determined by the integrated onshore Ekman transport in the direction from which shelf waves propagate. - KI Ohshima, M Wakatsuchi, Y Fukamachi, G MizutaJOURNAL OF GEOPHYSICAL RESEARCH-OCEANS 107 C11 doi:10.1029/2001JC001005 2002年11月 [査読無し][通常論文]
[1] Near-surface circulation of the Okhotsk Sea was observed with ARGOS drifters from August 1999 to February 2000. The drifters clearly revealed the existence of the southward boundary current (the East Sakhalin Current) off the east Sakhalin. This current is strongly controlled by bottom topography and confined to the region shallower than 1000 m. The volume transport of this current is estimated to be in the range of 4-9 Sv (1 Sv = 10(6) m(3) s(-1)), with the increase from summer to winter. The current appears to consist of two cores: one exists near the coast (50-150 m depths) with typical speeds of 0.3-0.4 m s(-1) and the other exists over the shelf slope (300-900 m depths) with typical speeds of 0.2-0.3 m s(-1). A part of this southward current continues as far as the southern tip of Sakhalin Island, while another part turns to the east around Terpeniya Bay (similar to48degreesN) flowing eastward as far as Bussol' Strait. Most of the drifters that survived in the Okhotsk Sea went out to the Pacific through Bussol' Strait by less than a half year, suggesting that Bussol' Strait is the main pathway for the surface water to flow out from the Okhotsk Sea. Mesoscale anticyclonic eddies with diameters of 100-200 km are dominant in the Kuril Basin. Diurnal shelf waves were identified over the shelf in the northern part of the east Sakhalin shelf. Large amplification of diurnal tidal currents (order of 1.0 m s(-1)) was observed over Kashevarov Bank along with the residual anticyclonic circulation. Both diurnal and semidiurnal tidal currents were found to be enhanced in and around Bussol' Strait. - In-situ ice andmeteorological observations in the southern Sea of Okhotsk in 2001 winter:ice structure, snow on ice, surface temperature, and optical environmentsPolar Meteorol. Glaciol. 16 116 - 132 2002年 [査読無し][通常論文]
- Sohey Nihashi, Kay I. OhshimaGeophysical Research Letters 28 19 3677 - 3680 2001年10月01日 [査読無し][通常論文]
Relationship between ice concentrations in the retreat (December) and advance (April) seasons is investigated in the Antarctic Ocean using SMMR and SSM/I data. For most years, the negative (positive) anomalies in ice concentration in the retreat season lead to the negative (positive) anomalies in the next advance season with strong correlation. This positive feedback can be regarded as ice-albedo feedback in a coupled ice-ocean system. In the retreat season, net heat input into the upper ocean from the atmosphere becomes maximum and is mostly determined by ice concentration. Because of large interannual variation of ice concentration in the retreat season, the anomaly in the heat input becomes tremendous. It is inferred that this anomaly is memorized in the ocean, and then affects the next advance of sea ice, skipping over the open ocean period (February-March). This process can partly explain the interannual variation of the sea ice cover. - S Nihashi, KI OhshimaJOURNAL OF GEOPHYSICAL RESEARCH-OCEANS 106 C8 16767 - 16782 2001年08月 [査読無し][通常論文]
We demonstrate the importance of heat entering the open water area from the atmosphere on sea ice decay in the Antarctic Ocean. The heat budget analyses, both from the European Centre for Medium-Range Weather Forecasts and the in situ data, show that the net heat input at the water surface reaches 100-150 W m(-2) in the active ice melting season due to large solar heating, while that at the ice surface is nearly zero because of the difference in surface albedo. Thus heat input to the ice-upper ocean system can be approximated as the product of the net heat at the water surface and the fraction of open water. Climatology data show that the total heat input to the upper ocean in the active melting season is comparable to the total latent heat required for sea ice melting in the whole Antarctic sea ice zone. The temporal variation of the heat input to the upper ocean corresponds well to the melting rate of sea ice, which is calculated from the Special Sensor Microwave Imager (SSM/I) data, in large ice extent sectors where the effect of advection is relatively small. These results suggest that melting of sea ice in the Antarctic Ocean is mostly accomplished by the heat input to the upper ocean through the open water area. On seasonal timescales the amount of heat supplied to the upper ocean is determined by the seasonal cycle of net heat input at the water surface, whereas the variability on shorter timescales and interannual differences are determined by the variation of the open water fraction. - Kay I. Ohshima, Genta Mizuta, Motoyo Itoh, Yasushi Fukamachi, Tatsuro Watanabe, Yasushi Nabae, Koukichi Suehiro, Masaaki WakatsuchiJournal of Oceanography 57 4 451 - 460 2001年 [査読無し][通常論文]
In the southwestern part of the Okhotsk Sea, oceanographic and sea-ice observations on board the icebreaker Soya were carried out in February 1997. A mixed layer of uniform temperature nearly at the freezing point extending down to a depth of about 300 m was observed. This is much deeper than has previously been reported. It is suggested that this deep mixed layer originated from the north (off East Sakhalin), being advected along the shelf slope via the East Sakhalin Current, accompanied with the thick first-year ice (average thickness 0.6 m). This vertically uniform winter water, through mixing with the surrounding water, makes the surface water more saline (losing a characteristic of East Sakhalin Current Water) and the water in the 100-300 m depth zone less saline, colder, and richer in oxygen (a characteristic of the intermediate Okhotsk Sea water). The oceanographic structure and a heat budget analysis suggest that new ice zone, which often appears at ice edges, can be formed through preconditioning of thick ice advection and subsequent cooling by the latent heat release due to its melting. - KI Ohshima, T Kawamura, T Takizawa, S Ushio, T MiyakanaJOURNAL OF GEOPHYSICAL RESEARCH-OCEANS 105 C7 17121 - 17132 2000年07月 [査読無し][通常論文]
Current measurements were made under landfast sea ice in Lutzow-Holm Bay, Antarctica, in 1991. In spite of no direct wind forcing and negligible thermohaline forcing, currents with a magnitude of 0.1-0.3 m s(-1) were observed. It is suggested that current variability strongly depends on the extent of the landfast ice on the eastward continental shelf. Before the fast ice edge extends to the shelf break the current at Ongul Strait, located near the continental coast, is highly coherent with the wind with a time lag of 1-1.5 days and somewhat coherent with the adjusted sea level with no time lag at periods of 6-8 days. After the fast ice edge is stably located along the shelf break the current is not at all coherent with the wind or sea level. We infer that the variability at periods of 6-8 days is mainly due to the first-mode shelf wave excited by the wind stress over the eastward continental shelf. The direction of the prevailing current at Ongul Strait changed drastically from southward (the same direction of the wind) to northward in May, although the prevailing wind direction did not change. This change in timing coincides with the rapid development period of the fast ice extent. - KI OhshimaJOURNAL OF GEOPHYSICAL RESEARCH-OCEANS 105 C7 17133 - 17141 2000年07月 [査読無し][通常論文]
The present study examines the effects of landfast sea ice on the response of a homogeneous coastal ocean to wind forcings under the longwave approximation. For a fluctuating wind, continental shelf waves are induced by an infinite stress curl due to the existense of the ice edge as well as by the torque due to the topographic change. The amplitude and sign of each shelf wave mode excited by the forcing depend strongly on the ice edge location. The alongshore velocity field, which can be represented as the sum of all the shelf wave modes, has a maximum in amplitude at the ice edge with a strong lateral shear, which is a result of the infinite stress curl. The amplitudes of alongshore velocity and sea level near the coast are greatly reduced as the ice edge advances. The response to a steady wind is also examined, and it is found that even for the same wind, the velocity distributions depend strongly on the ice edge location. For instance, the current direction near the coast is the same as that of the wind stress in the case of no fast ice, whereas it is opposite in the case where the ice edge is located at the shelf break. The present theory is applied to the continental shelf off Enderby Land, Antarctica, and it gives the possible explanation for the observed features that the current variability its coherence with the wind, and the prevailing current direction depend much on the extent of landfast sea ice. - KI Ohshima, T Kawamura, T Takizawa, S Ushio, T MiyakanaJOURNAL OF GEOPHYSICAL RESEARCH-OCEANS 105 C7 17121 - 17132 2000年07月 [査読無し][通常論文]
Current measurements were made under landfast sea ice in Lutzow-Holm Bay, Antarctica, in 1991. In spite of no direct wind forcing and negligible thermohaline forcing, currents with a magnitude of 0.1-0.3 m s(-1) were observed. It is suggested that current variability strongly depends on the extent of the landfast ice on the eastward continental shelf. Before the fast ice edge extends to the shelf break the current at Ongul Strait, located near the continental coast, is highly coherent with the wind with a time lag of 1-1.5 days and somewhat coherent with the adjusted sea level with no time lag at periods of 6-8 days. After the fast ice edge is stably located along the shelf break the current is not at all coherent with the wind or sea level. We infer that the variability at periods of 6-8 days is mainly due to the first-mode shelf wave excited by the wind stress over the eastward continental shelf. The direction of the prevailing current at Ongul Strait changed drastically from southward (the same direction of the wind) to northward in May, although the prevailing wind direction did not change. This change in timing coincides with the rapid development period of the fast ice extent. - Jun Inoue, Masayuki Kawashima, Kay I. Ohshima, Yasushi Fujiyoshi, Ken-Ichi MaruyamaJournal of Oceanography 56 5 507 - 515 2000年 [査読無し][通常論文]
Horizontal wind fields over Funka Bay during cold air outbreaks were simulated using a 3-D meso-scale atmospheric model. The simulated wind fields over the bay have a positive curl in the north and a negative curl in the south. These wind fields were used to simulate the current in Funka Bay using a barotropic ocean model. The simulated current pattern was composed of two vortices-one with anti-clockwise vorticity in the north and the other with clockwise vorticity in the south-and was in the opposite direction to that simulated by the uniform wind fields. This is because the wind stress curl effect on the vorticity production in Funka Bay opposes and overwhelms the bathymetry torque effect during cold air outbreaks. These results show that the non-uniformity of the wind fields caused by the land topography around a shallow lake or bay cannot be neglected in simulating its currents. - Motoyo Itoh, Kay I. OhshimaJournal of Oceanography 56 6 643 - 654 2000年 [査読無し][通常論文]
A new grid data set for the southwestern part of the Okhotsk Sea was compiled by using all the available hydrographic data from the Japan Oceanographic Data Center, World Ocean Atlas 1994 and the other additional data sources with the resolution of about 10 km. We examine the seasonal variations of areas and volumes of Soya Warm Current Water (SWCW) and East Sakhalin Current Water (ESCW) and show that the exchanges of these water masses drastically occur in April and November. The peculiar variation of sea level in this region is also related with the water mass exchange. Sea level at the Hokkaido coast of the Okhotsk Sea reaches its minimum in April about two months later than in the case of ordinary mid-latitude ocean, and its maximum in December besides the summer peak. The winter peak of sea level in December is caused by the advent of fresh and cold ESCW which is accumulated at the subsurface layers (20-150 m) through the Ekman convergence by the prevailing northerly wind. Sea level minimum in April is caused by the release of the convergence and the recovery of dense SWCW that is saline and much colder than that in summer. - S Ushio, T Takizawa, KI Ohshima, T KawamuraJOURNAL OF GEOPHYSICAL RESEARCH-OCEANS 104 C12 29771 - 29780 1999年12月 [査読無し][通常論文]
Aircraft observations off Enderby Land, Antarctica, verified that a coastal polynya forms even in severe winters and that new ice production continuously occurs there. In the polynya, water temperature profiles were measured for the first time using the aircraft-launched expendable bathythermograph (AXBT). The AXBT data show that the thickness of the winter mixed layer is 350 m or more and that the layer temperatures are near the freezing point. According to the water mass analysis of the winter mixed layer the active haline convection by the high ice production in the winter polynya contributes to the formation of more homogeneous and saline mixed layer than that under the fast ice cover, where the ice grows slowly and convective mixing is calm. Furthermore, the active convection leads to entrainment of the oxygen-poor deep water under Winter Water. Consequently, in the polynya located over the continental shelf break, the oxygen content of the mixed layer is somewhat lower than that in a coastal polynya that forms on the continental shelf, where the haline convection reaches to the sea bottom. The oxygen undersaturation of the thick mixed layer in the shelf break polynya suggests that the deep-water entrainment rate is more intense than that under the pack ice region in the Weddell Sea with a relatively thin mixed layer. It is, therefore, important to consider such polynya processes which may contribute to water mass modification in the Antarctic coastal region. - Kunimitsu Ishida, Kay I. Ohshima, Takashi Yamanouchi, Hiroshi KanzawaJournal of Oceanography 55 3 417 - 426 1999年06月 [査読無し][通常論文]
Meso- or submeso-scale features of the Antarctic sea ice are investigated using the MOS-1/1b MESSR images (spatial resolution of approximately 50 m) received at Syowa Station. Particular attention is paid to the ice bands and ice streamers in coastal polynyas. In the Antarctic Ocean, ice bands can be often seen not only at the ice edge but also in the ice interior zone throughout the year and they extend for hundreds of kilometers in the latitudinal direction. It is found that the width and spacing of ice bands tend to decrease from winter to summer. The width of ice band is about 2-6 km in August and September, and 0.1-0.7 km in December. The spacing of ice bands is about 3-10 km in August and September, and 0.1-2 km in December. In coastal polynyas, ice streamers, which are composed of new ice, are sometimes observed. In general, the row of the streamers is spaced at 0.5-2 km with a width of 0.1-1.0 km. - T Toyota, J Ukita, KI Ohshima, M Wakatsuchi, K MuramotoJOURNAL OF THE METEOROLOGICAL SOCIETY OF JAPAN 77 1 117 - 133 1999年02月 [査読無し][通常論文]
In order to estimate sea ice albedo around the marginal sea ice zone of the southwestern Okhotsk Sea, we conducted the measurement of albedo aboard the ice breaker Soya in early February of 1996 and 1997. Using upward and downward looking pyranometers mounted at the bow of the ship, we obtained albedo data. We also measured ice concentration and thickness quantitatively by a video analysis. The observations show a good correlation between albedo and ice concentration. From a linear regression, sea ice albedo (ice concentration = 100 %) is estimated to be 0.64 +/- 0.03 at the 95 % confidence level. The developed snow grains on sea ice due to sea water and/or solar radiation may be responsible for this somewhat lower value, compared with that over the snow-covered land fast ice in the polar region. Deviations of the observed values from this regression have a statistically significant correlation with solar zenith cosine at the 99 % level, and with ice thickness at the 95 % level. The linear regression formula which predicts albedo is also derived as the variables of ice concentration and solar zenith cosine. Although the regression coefficients are both statistically significant, the coefficient of ice concentration is much more significant in this formula than that of solar zenith cosine. The deviation of the observed albedo from this regression seems to be mainly caused by ice surface conditions rather than by ice thickness or cloud amount. All these results suggest that snow cover on sea ice plays an important role in determining the surface albedo. We also did albedo observations of dark nilas with snow-free surface, they were estimated as 0.10 and 0.12 for ice thickness of 1 to 1.5 cm and 2 to 3 cm, respectively. - KI Ohshima, K Yoshida, H Shimoda, M Wakatsuchi, T Endoh, M FukuchiJOURNAL OF GEOPHYSICAL RESEARCH-OCEANS 103 C4 7601 - 7615 1998年04月 [査読無し][通常論文]
During the Antarctic ice-melting season, high-resolution sea ice data were collected with the video monitoring system aboard the icebreaker Shirase along with the monitoring of temperature and salinity in the upper ocean. On the basis of these data, relationships among sea ice concentration, temperature, and salinity are investigated. In the ice interior region away from the ice-free ocean, ice concentration is negatively correlated with temperature and positively correlated with salinity for the spatially averaged data, which suggests chat the local balances of heat and salt nearly hold in a bulk area. At the ice margin, ice concentration is negatively correlated with both temperature and salinity, suggesting that the local balances are overwhelmed by the effects of ice advection. The expendable bathythermograph profiles at the ice margin also show that considerable amount of sea ice was advected into the ice-free ocean and subsequently melted there. It is pointed out that a polynya works as an "ice-melting factory" in summer; it absorbs solar radiation during the period of opening, and then melts the ice advected there. From a heat budget analysis and ocean structure in the melting season, we propose a simple ice-upper ocean coupled model in which sea ice melts on the bottom and lateral faces with the heat source supplied to the open water area by solar radiation. The relations among ice concentration, temperature, and salinity derived from the model are consistent with the observed relations. The analytic solution for the no lateral melting case shows that the concentration-temperature relation converges to a certain asymptotic curve with time, which explains that the temperature-concentration plot shows a similar relation for any region. Dependence of the relations among ice concentration, temperature, and salinity on the spatial scale is also discussed. - Mesoscale ice features in the summer Marginal Ice Zone off East Queen Maud Land observed from NOAA AVHRR imagery, Antarct. Res. Ser. 74, Antarctic Physical Processes, Interactions and Variability, edited by M. O. JeffriesAGU, Washington, D.C. 317 - 324 1998年 [査読無し][通常論文]
- Fukamachi, Y, K. I. Ohshima, T. IshikawaAntarctic Research Series 74 317 - 324 1998年 [査読有り][通常論文]
- T Kawamura, KI Ohshima, T Takizawa, S UshioJOURNAL OF GEOPHYSICAL RESEARCH-OCEANS 102 C2 3345 - 3355 1997年02月 [査読無し][通常論文]
A sea-ice/ocean study was conducted off Queen Maud Land and Enderby Land, Antarctica, from 1990 to 1991 by the Japanese Antarctic Research Expedition. Observations of multiyear land fast sea ice were made in Lutzow-Holm Bay over a period of 2 years to determine the snow and ice characteristics and ice growth processes. The snow depth in the bay reached large values of 1.0 to 1.5 m during the winter season at offshore locations. From the analysis of ice thickness measurements, it is confirmed that the fast ice with deep snow cover grew little in winter but substantially thickened during the summer months. On the basis of ice core structure, salinity, and stable isotopic composition, we conclude that the summer growth was caused by upward growth at the top of the ice to which snow ice and superimposed ice formation contribute. These processes were the primary contributors to sea-ice growth and characteristics only where the snow accumulation was large. In areas of low snow accumulation, there was no surface growth. Superimposed ice formation on sea ice in Antarctica has not been reported previously. Evidence for snow cover melting, which is a prerequisite for superimposed ice formation, was also found. - KI Ohshima, T Takizawa, S Ushio, T KawamuraJOURNAL OF GEOPHYSICAL RESEARCH-OCEANS 101 C9 20617 - 20628 1996年09月 [査読無し][通常論文]
During the period of 1990-1992, year-round oceanographic observations were conducted in the vicinity of Lutzow-Holm Bay, East Antarctica. It-was found that the thickness of the Winter Water (WW) layer, characterized by a cold fresh oxygen-rich water, exhibits its maximum in the austral fall (typically 500 m) and its minimum in the austral summer (typically 350-400 m). The associated density variation of the water column explains only about one third of the seasonal variations in sea level at the coast, which suggests a large seasonal variation in barotropic coastal flow. Prominent freshening occurs in the WW layer during fall. This appears to be caused by the accumulation of WW, whose upper portion is freshened in the preceding summer. These seasonal variations appear to occur every year. We propose that the seasonal variation in the WW layer is mainly caused by the seasonal variation in the wind over the coastal ocean. In fall the prevailing easterly wind intensifies, which increases the Ekman convergence of WW in the coastal ocean, while in summer the opposite occurs. - Takayuki Ishikawa, Jinro Ukita, Kay I. Ohshima, Masaaki Wakatsuchi, Takashi Yamanouchi, Nobuo OnoJournal of Oceanography 52 3 389 - 398 1996年 [査読無し][通常論文]
Coastal polynyas off East Queen Maud Land in Antarctica are examined using NOAA AVHRR infrared data. From image analyses, two locations of coastal polynyas in this region are identified one in Breid Bay and the other along the shelf break. The areal coverage of the Breid Bay polynya is significantly related to the strength of katabatic winds, which maintain their strength over the coastal sea due to land topography favoring for their confluence, thereby being capable of removing newly formed ice. Land fast ice in the eastern part of the bay also plays an additional role in the formation mechanism. It Is also found that the areal coverage of coastal polynyas in this region fluctuate coherently. Moreover, these fluctuations correspond to the synoptic index, which measures the strength of the offshore wind, with their peaks closely associated with the areal peaks. These facts strongly suggest the influence of synoptic scale weather on the formation and maintenance of polynyas in this region. - M WAKATSUCHI, KI OHSHIMA, M HISHIDA, M NAGANOBUJOURNAL OF GEOPHYSICAL RESEARCH-OCEANS 99 C10 20417 - 20426 1994年10月 [査読無し][通常論文]
Hydrographic and drifting buoy data from Japanese cruises show that the Antarctic Divergence in the Indian Ocean sector is composed of a street of cyclonic eddies. These eddies measure about 500 km in the zonal direction and 200 km in the meridional. Part of the eastward flowing Antarctic Circumpolar Current (ACC) meanders southward in the regions between the eddies. In the eddy regions, warm, saline Circumpolar Deep Water is upwelled into the shallow layers, while cold, dense coastal water advects into the deep layers; the advection occurs along the isobaths of ridges which extend north from the coast. The combination of the advection with the upwelling produces a water column denser than the surrounding water and leads to the formation and maintenance of the cyclonic eddies. Presence of the northward extending ridges approximately governs the location of eddy formation. The eddy formation recurs year after year, although eddy locations can vary somewhat. A polynya was observed to persistently occur and corresponded with one of the eddies in location, size, and form. The oceanographic observations also suggest that the primary meridional exchanges of heat and salt in the Antarctic are caused through the eddies and ACC meanders within the Antarctic Divergence. - Kay I. Ohshima, Toshiyuki Kawamura, Takatoshi Takizawa, Shuki UshioJournal of Oceanography 50 3 365 - 372 1994年05月 [査読無し][通常論文]
Step-like thermohaline structure was observed near icebergs trapped by fast ice in Lützow-Holm Bay, Antarctica. The structure was developed within the draft depth of the icebergs. The typical vertical distance between the succeeding step surfaces is 20-30 m, and the temperature and salinity differences across the step surface are about 0.05-0.06 deg and 0.06-0.10 psu, respectively. The structure appears to be generated by a sea-water/iceberg interaction, and is explainable by a simple model in which an ice wall is placed in the ocean linearly stratified by vertical salinity gradient. © 1994 Oceanographic Society of Japan. - KI OHSHIMAJOURNAL OF GEOPHYSICAL RESEARCH-OCEANS 99 C5 9925 - 9940 1994年05月 [査読無し][通常論文]
We investigate how the current system through the Japan Sea is driven and what determines the volume transport. We suppose that a part of the difference in geopotential anomaly between the subtropical and subpolar gyre is converted into a barotropic sea level difference across the three shallow straits which connect the Japan Sea with the Pacific and that this difference is the primary driving force of the current system. Then, we examine the flow under the condition that there is a constant sea level difference between two oceans connected through a shallow strait. We found that the strait acts as a source of arrested shelf waves or steady coastal flows for the timescale beyond the inertial period; in the northern hemisphere, steady flows axe established along the shelves with the coast to the right (left) in the ocean of low (high) sea level. We apply this notion to the current system through the Japan Sea. The Tsushima nearshore branch, the Tsugaru coastal mode, and the Soya Current can all be interpreted as a coastally trapped flow whose source is the upstream strait. Further, a series of northeastward flows along the South and East China Seas shelves should be interpreted as coastally trapped flows whose source is the downstream Tsushima Strait. Numerical model experiments incorporating the realistic topography also simulate the observed flow fields. The volume flux through each strait being limited geostrophically, relations between the sea level difference and volume transport can be represented by simple formulas. - TEMPERATURE STRUCTURE AND CHARACTERISTICS APPEARING ON SSM/I IMAGES OF THE COSMONAUT SEA, ANTARCTICAT TAKIZAWA, KI OHSHIMA, S USHIO, T KAWAMURA, H ENOMOTOANNALS OF GLACIOLOGY, VOL 20, 1994 20 298 - 306 1994年 [査読無し][通常論文]
- Ocean-atmosphere interaction through sea ice processes in the AntarcticUshio, S, Takizawa, T, Nakagawa, K, Kawamura, T, Ohshima K. I., Ono, NFinal Report of WCRP in Japan 55-61 1994年 [査読無し][通常論文]
- Ohshima Kay I., Kawamura Toshiyuki, Takizawa Takatoshi, Ushio Shuki, Ono Nobuo, Kawaguchi SadaoProceedings of the NIPR Symposium on Polar Meteorology and Glaciology 7 51 - 59 1993年 [査読無し][通常論文]
Ocean structure and current were observed below fast ice in Ongul Strait, Antarctica, over a nearly full annual cycle in 1991. In the austral fall, fresh, cold and oxygen-rich water accumulates in the upper layer. This water is diffused or mixed with the lower-layer water gradually in winter. From spring to summer, warm, saline and oxygen-poor water appeared in the mid-depth and deep layers. These features seem to be common in every year. Horizontal oceanic advection is dominant in the heat and salt budget. In spite of no direct wind forcing and negligible thermohaline forcing, the current in Ongul Strait is found to be strong, with a typical velocity being 0.3m/s. The strong current is confined only to the upper 100-300m from the surface. The direction of the mean current changes drastically from southward to northward in May. - T KAWAMURA, KI OHSHIMA, S USHIO, T TAKIZAWAANNALS OF GLACIOLOGY, VOL 18, 1993 18 97 - 101 1993年 [査読無し][通常論文]
- Preliminary results of hydrography under fast ice in Lutzow-Holm BayProceedings of the NIPR Symposium on Polar Meteorology and Glaciology 6 106 - 125 1992年 [査読無し][通常論文]
- OHSHIMA Kay I., ONO Nobuo, TAKIZAWA Takatoshi, USHIO ShukiProceedings of the NIPR Symposium on Polar Meteorology and Glaciology 4 74 - 80 国立極地研究所 1991年 [査読無し][通常論文]
Under coastal fast ice in the Ongul Strait near Syowa Station, Antarctica, warmer, more saline, and oxygen-poorer water appears in mid-depth and deep layers from winter to spring every year. This water is explained as a result of mixing between Circumpolar Deep Water (CDW) and Winter Water (WW). This water becomes more like CDW and more homogenized with time from winter to spring, and in December a mixing ratio of 1 : 3-1 : 4 for CDW to WW is required to explain the properties of the homogenized water. It is inferred that the CDW comes along the glacial troughs. Transport of CDW results in a significant amount of heat supply into the water under fast ice. - KI OHSHIMA, M WAKATSUCHIJOURNAL OF PHYSICAL OCEANOGRAPHY 20 4 570 - 584 1990年04月 [査読無し][通常論文]
- M WAKATSUCHI, KI OHSHIMAJOURNAL OF PHYSICAL OCEANOGRAPHY 20 4 585 - 594 1990年04月 [査読無し][通常論文]
- K. I. OhshimaJournal of the Oceanographical Society of Japan 43 1 49 - 60 1987年02月 [査読無し][通常論文]
Linear stability of a barotropic jet on a sloping bottom with and without a side boundary is examined. When a sloping bottom and a side boundary are absent, a symmetric jet generally has two unstable modes: a symmetric mode and an antisymmetric mode. In the presence of a sloping bottom or a side boundary, they are modified and lose their symmetry. The presence of a side boundary does not produce substantial change in the stability characteristics, except that it stabilizes the flow to some degree. In the presence of a sloping bottom, the following features are noted 1) when the direction of the jet is opposite to the propagation direction of topographic Rossby waves, the change of a preferred mode occurs at a certain slope, 2) when the direction of the jet is opposite to 1), with a side boundary, the dispersion relations change from unstable mode type to shelf wave type at a certain slope, accompanied by 'kissing'. © 1986 Oceanographical Society of Japan. - K. I. OhshimaJournal of the Oceanographical Society of Japan 43 1 61 - 67 1987年02月 [査読無し][通常論文]
The stability of the Soya Warm Current is examined, in an attempt to explain the mechanism of the formation of the wave-like pattern seen in satellite infrared imagery in summer. A linear stability theory is applied to barotropic shear flows over a realistic bottom topography. Effects of bottom friction are also taken into consideration. For this current in summer, when volume transport is greatest, the possibility of barotropic instability is suggested. The most unstable waves obtained in this study have wavelengths of 60-80 km, periods of about 1. 5 days, and phase velocities of 45-55 cm sec-1, which is in good agreement with observations. © 1986 Oceanographical Society of Japan.
書籍
- 「巨大リザーバ：南大洋・南極氷床｣ 低温科学 Vol.76大島 慶一郎 (担当:編者(編著者))
北海道大学 低温科学研究所 2018年03月 - 「低温環境の科学事典」河村公隆, 大島 慶一郎他 (担当:共編者(共編著者))
朝倉書店 2016年 - 「オホーツクの生態系とその保全」大泰司紀之, 桜井泰憲, 大島慶一郎 (担当:共編者(共編著者))
北道大学出版会 2013年 - 「地球惑星科学入門」北海道大学出版社 2010年
- 「南大洋の深層水形成と海氷過程」 月刊海洋, 号外54大島 慶一郎 (担当:共編者(共編著者))
海洋出版 2010年
作品等
- Y. Fukamachi, K. I. Ohshima, D. Simizu, T. Tamura, Y. Kitade, S. Aoki, S. Ushio, G. Hashida, and M. Wakatsuchi: Mooring measurement of Antarctic Bottom Water export from the Cape Darnley Polynya and sea-ice thickness within this polynya, WCRP Open Scie・・・2011年 Y. Fukamachi, K. I. Ohshima, D. Simizu, T. Tamura, Y. Kitade, S. Aoki, S. Ushio, G. Hashida, and M. Wakatsuchi: Mooring measurement of Antarctic Bottom Water export from the Cape Darnley Polynya and sea-ice thickness within this polynya, WCRP Open Science Conference, Denver (USA), October 27, 2011.
- Tamura, T., K. I. Ohshima, S. Nihashi, and H. Hasumi: Estimation of surface heat/salt fluxes associated with sea ice growth/melt in the Southern Ocean, IUGG 2011, Melbourne (Australia), July 3, 2011.2011年
- Tamura, T., and K. I. Ohshima: Mapping of sea ice production in the Arctic coastal polynyas, IUGG 2011, Melbourne (Australia), July 1, 2011.2011年
- Ohshima, K. I., Y. Fukamachi, Y. Nakayama, T. Tamura, S. Nihashi, Y. Kitade, Y. Matsumura, H. Hasumi, S. Aoki, and M. Wakatsuchi: Formation of Antarctic Bottom Water off Cape Darnley with huge sea ice production, IUGG 2011, Melbourne (Australia), July ・・・2011年 Ohshima, K. I., Y. Fukamachi, Y. Nakayama, T. Tamura, S. Nihashi, Y. Kitade, Y. Matsumura, H. Hasumi, S. Aoki, and M. Wakatsuchi: Formation of Antarctic Bottom Water off Cape Darnley with huge sea ice production, IUGG 2011, Melbourne (Australia), July 2, 2011.
- Ohshima, K. I., S. C. Riser: Ocean salinity profiles teach us thinning and volume decrease of the Okhotsk sea-ice cover, IUGG 2011, Melbourne (Australia), June 30, 2011.2011年
- Ohshima, K. I., S. Nihashi, K. Iwamoto, and T. Tamura: Development of sea ice thickness and concentration algorithm in thin ice regions, Joint PI Workshop of Global Environmental Observation Mission, Tokyo (Japan), December 7, 2010.2010年
- Ohshima, K. I., S. Nihashi, T. Tamura, and Y. Fukamachi: Polynya formation and sea ice production off Cape Darnley Antarctica, revealed by ALOS PALSAR and AMSR-E data, 4th ALOS Joint PI Symposium, Tokyo (Japan), November 15, 2010.2010年
- Ohshima, K. I.: Japanese SASSI activities, SASSI Workshop, Oslo (Norway), June 7, 2010.2010年
- Ohshima, K. I. and S. C. Riser: Mapping and interannual variations of sea-ice thickness in the Okhotsk Sea inferred from ocean salinity profile in spring, IGS International Symposium on Sea Ice in the Physical and Biogeochemical System, Tromso (Norway)・・・2010年 Ohshima, K. I. and S. C. Riser: Mapping and interannual variations of sea-ice thickness in the Okhotsk Sea inferred from ocean salinity profile in spring, IGS International Symposium on Sea Ice in the Physical and Biogeochemical System, Tromso (Norway), June 1, 2010.
- Tamura, T., K. I. Ohshima, and S. Nihashi: Global mapping of sea ice production and surface heat/salt-flux in the sea ice region, IGS International Symposium on Sea Ice in the Physical and Biogeochemical System, Tromso (Norway), June 2, 2010.2010年
- Ohshima, K. I., and T. Tamura: Studies on sea ice and polynya off the Cape Darnley Antarctica, using ALOS-PALSAR and other satellite data, 3rd ALOS Joint PI Symposium, Hawaii (USA), November 9-13, 2009.2009年
- Tamura, T., K. I. Ohshima, and S. Nihashi: Mapping of sea ice production and surface heat/salt-flux in the polar Oceans, IAMAS・IAPSO・IACS JOINT ASSEMBRY MOCA-09, Montreal, Canada, July 19-29 (21), 2009.2009年
- Ohshima, K. I., T. Tamura, Y. Fukamachi, and S. Aoki: Sea ice production in the polynya and the associated bottom water formation off the Cape Darnley, East Antarctica. 9th International Conference on Southern Hemisphere Meteorology and Oceanography, F・・・2009年 Ohshima, K. I., T. Tamura, Y. Fukamachi, and S. Aoki: Sea ice production in the polynya and the associated bottom water formation off the Cape Darnley, East Antarctica. 9th International Conference on Southern Hemisphere Meteorology and Oceanography, February 9-13, 2009, Melbourne, Australia.
- Ohshima, K.I., and T. Tamura: Studies on sea ice and polynya off the Cape Darnley in East Antarctica, using the ALOS PALSAR and passive microwave data. ALOS PI Symposium 2008 Rhodes, Greece 3 - 7 November 20082008年
- Nakanowatari, T., H. Mitsudera, T. Motoi, K. I. Ohshima, and I. Ishikawa: 50-yr scale change in the intermediate water temperature in the western North Pacific simulated by an eddy resolving sea-ice coupled OGCM. 4th PICES Workshop on "The Okhotsk Sea ・・・2008年 Nakanowatari, T., H. Mitsudera, T. Motoi, K. I. Ohshima, and I. Ishikawa: 50-yr scale change in the intermediate water temperature in the western North Pacific simulated by an eddy resolving sea-ice coupled OGCM. 4th PICES Workshop on "The Okhotsk Sea and adjacent areas", August 27-29, 2008
- Ohshima. K. I., T. Nakanowatari, T. Nakatsuka, J. Nishioka, and M. Wakatsuchi: Changes in the Sea of Okhotsk due to global warming ? Weakening pump function to the North Pacific ? 4th PICES Workshop on "The Okhotsk Sea and adjacent areas" August 27-29・・・2008年 Ohshima. K. I., T. Nakanowatari, T. Nakatsuka, J. Nishioka, and M. Wakatsuchi: Changes in the Sea of Okhotsk due to global warming ? Weakening pump function to the North Pacific ? 4th PICES Workshop on "The Okhotsk Sea and adjacent areas" August 27-29, 2008
- Ebuchi, N., Y. Fukamachi, K. I. Ohshima, K. Shirasawa, M. Ishikawa, T. Takatsuka, T. Daibo, and M. Wakatsuchi, 2004: Observations of the Soya Warm Current using HF ocean radar. Proceedings of IEEE International Geoscience and Remote Sensing Symposium 2・・・2004年 Ebuchi, N., Y. Fukamachi, K. I. Ohshima, K. Shirasawa, M. Ishikawa, T. Takatsuka, T. Daibo, and M. Wakatsuchi, 2004: Observations of the Soya Warm Current using HF ocean radar. Proceedings of IEEE International Geoscience and Remote Sensing Symposium 2004, Anchorage, Alaska, U.S.A
- JSTサイエンスチャンネル番組2004年
その他活動・業績
- 北出裕二郎, 溝端浩平, 渡部和帆, CHEN Linquiao, GUO Gaopin, 青木茂, 牛尾収輝, 嶋田啓資, 大島慶一郎 日本地球惑星科学連合大会予稿集(Web) 2019 2019年
- 青木茂, 市川雅明, 市川雅明, 小野数也, 深町康, 深町康, 深町康, 大島慶一郎, 大島慶一郎, 中川敏彦, 小林研吾, 小竹正人, 小竹正人, 小澤知史 海洋理工学会誌(Web) 25 (1) 29‐34(J‐STAGE) 2019年 [査読無し][通常論文]
- 北出裕二郎, 嶋田啓資, 溝端浩平, 青木茂, 田村岳史, 千手智晴, 深町康, 大島慶一郎 日本海洋学会大会講演要旨集 2017 2017年
- 嶋田啓資, 北出裕二郎, 溝端浩平, 青木茂, 大島慶一郎, 田村岳史, 小達恒夫 日本地球惑星科学連合大会予稿集(Web) 2016 2016年
- 岡英太郎, 磯辺篤彦, 市川香, 升本順夫, 須賀利雄, 川合義美, 大島慶一郎, 島田浩二, 羽角博康, 見延庄士郎, 早稲田卓爾, 岩坂直人, 河宮未知生, 伊藤幸彦, 久保田雅久, 中野俊也, 日比谷紀之, 寄高博行 海の研究 22 (6) 191 -218 2013年11月15日 [査読無し][通常論文]
- 三谷 曜子, 大島 慶一郎, 桜井 泰憲 海洋と生物 35 (1) 12 -15 2013年02月15日 [査読無し][通常論文]
- 小野数也, 大島慶一郎, 河野時廣, 安田一郎, 勝又勝郎, 若土正曉 日本海洋学会大会講演要旨集 2013 2013年
- 中野渡拓也, 中村知裕, 内本圭亮, 三寺史夫, 大島慶一郎, 羽角博康 月刊海洋 44 (8) 466 -472 2012年08月01日 [査読無し][通常論文]
- 柏瀬陽彦, 大島慶一郎, 二橋創平 月刊海洋 44 (7) 410 -415 2012年07月01日 [査読無し][通常論文]
- 田村岳史, 清水大輔, 牛尾収輝, 大島慶一郎 南極資料 55 (3) 261 -269 2011年11月30日 [査読無し][通常論文]
最新の高分解能のAdvanced Microwave Scanning Radiometer-EOS (AMSR-E) 海氷密接度データを南極地域観測隊に提供するシステムを開発した.このシステムによって,デジタルデータとしての海氷分布情報が1-2日程度の遅れで第51次及び第52次日本南極地域観測隊に送られ,係留系設置作業に関わる現地判断に役立てることができた.さらに,このデータは東京海洋大学「海鷹丸」や海洋研究開発機構「白鳳丸」等の南極海氷域周辺を航行する他の日本の観測船にも提供され,それぞれの船の運航上の参考情報として役立てられた. - Long-term variations in sea ice production in the Okhotsk SeaKashiwase, H, K. I. Ohshima, S. Nihashi Proc. 26th International Symposium on Okhotsk Sea and Sea Ice, Monbetsu, Japan 171 -174 2011年 [査読無し][通常論文]
- Numerical prediction of spilled oil behavior in the Sea of Okhotsk under sea ice conditionsOffshore Technology Conference OTC22123, doi:10.4043/22123-MS 2011年 [査読無し][通常論文]
- 大島 慶一郎, 中野渡 拓也, Riser Stephen C. 海洋 42 (11) 643 -653 2010年11月
- 二橋創平, 大島慶一郎 月刊海洋 43 -51 2010年06月01日 [査読無し][通常論文]
- 大島慶一郎, 深町康, 青木茂, 清水大輔, 田村岳史, 牛尾収輝, 橋田元, 北出裕二郎, 若土正曉 月刊海洋 12 -20 2010年06月01日 [査読無し][通常論文]
- 北出裕二郎, 平野大輔, 大島慶一郎, 深町康 月刊海洋 29 -36 2010年06月01日 [査読無し][通常論文]
- 中野渡拓也, 三寺史夫, 本井達夫, 大島慶一郎, 石川一郎 海と空 85 (4) 141 -150 2010年03月25日 [査読無し][通常論文]
- 二橋 創平, 大島 慶一郎 海洋 号外 (54) 43 -51 2010年
- 大島 慶一郎, 羽角 博康 海洋 号外 (54) 5 -10 2010年
- 大島 慶一郎, 深町 康, 青木 茂 海洋 号外 (54) 12 -20 2010年
- 岩本 勉之, 大島 慶一郎, 二橋 創平, 田村 岳史 大会講演予講集 97 228 -228 2010年
- 北出 裕二郎, 平野 大輔, 大島 慶一郎 海洋 号外 (54) 29 -36 2010年 [査読無し][通常論文]
- 渦解像海洋大循環モデルで再現される北太平洋中層水の50年規模の低塩化海と空 85 141 -150 2010年 [査読無し][通常論文]
- 岩本 勉之, 田村 岳史, 大島 慶一郎, 二橋 創平 大会講演予講集 96 138 -138 2009年
- Formation of broad opening of coastal polynyaKawaguchi, Y, S. Nihashi, F. Mitsudera, K. I. Ohshima Proc. 24th International Symposium on Okhotsk Sea and Sea Ice, Monbetsu, Japan 88 (91) 2009年 [査読無し][通常論文]
- Impact of sea ice production and its recent reduction on overturning and material circulation in the Okhotsk Sea and North PacificOhshima K. I., T. Nakanowatari, S. Nihashi, J. Nishioka, T. Nakatsuka, M. Wakatsuchi Report on Amur-Okhotsk Project, No. 6, Research Institute for Humanity and Nature 21 -29 2009年 [査読無し][通常論文]
- Changes in the Sea of Okhotsk due to global warming -Weakening pump function to the North PacificPICES Scientific Rep. 36 16 -20 2009年 [査読無し][通常論文]
- 1.2. オホーツク海と海氷 －その特殊性と重要性－ 「朝倉世界地理講座－大地と人間の物語－2巻：東北アジア」朝倉世界地理講座 2009年 [査読無し][通常論文]
- 大島慶一郎, 小野純, 小野数也, 勝又勝郎 月刊海洋 28 -33 2008年11月01日 [査読無し][通常論文]
- 大島慶一郎, 中野渡拓也, 若土正曉 月刊地球 30 (3) 127 -133 2008年05月01日 [査読無し][通常論文]
- 大島慶一郎, 小野純, 清水大輔 沿岸海洋研究 45 (2) 115 -124 2008年02月29日 [査読無し][通常論文]
オホーツク海の陸棚上の流速場をよく再現している3次元海洋循環モデルを用いて,粒子追跡実験を行った.モデルは日々の風応力と月平均の海面熱フラックスで駆動されている.アムール川からの汚染物質等の漂流・拡散を想定して,アムール河口に起源を持つ海水の0m層と15m層における粒子追跡実験を行った.15m層では,粒子を投下する月・年に拘らず,10月一気に東樺太海流が強まるのに伴って粒子はサハリン東海岸沖を南下し始め,12～1月に北海道沖に到達する.表層0mでは,海流だけでなく風によるドリフトの効果が加わり,粒子の挙動は年によって異なる.沖向きの風が強い年ほど,粒子は東樺太海流の主流からはずれてしまい,北海道沖までは到達しない傾向が強くなる.2006年2～3月に知床に漂着した油まみれの海鳥の死骸の起源を探るため,後方粒子追跡実験を行った.その結果,海鳥は北方から,おそらくはサハリン東岸のどこかから東樺太海流に乗って知床に漂着したであろうことが示唆された. - 大島 慶一郎, 小野 純, 小野 数也 海洋 号外 (50) 28 -33 2008年
- オホーツク海における漂流物の粒子追跡モデル実験沿岸海洋研究 45 115 -124 2008年 [査読無し][通常論文]
- 大島慶一郎 気象研究ノート (214) 9 -18 2007年08月03日 [査読無し][通常論文]
- 大島慶一郎, 中野渡拓也, 若土正曉 低温科学 65 67 -75 2007年03月23日 [査読無し][通常論文]
オホーツク海の北西陸棚域は海氷生産量が非常に大きく、北太平洋表層では一番重い水が作られる。この低温の高密度陸棚氷は北太平洋の中層に潜り込み、北太平洋スケールでのオーバーターン(大きな鉛直循環)の源となっている。この50年で、オホーツク海を起源に北太平洋北西部の中層水が昇温化し、オーバーターンが弱化していることが示唆された。この原因は、オホーツク海風上の大陸部(北半球の寒極)の温暖化により海氷生産量が減少し、高密度陸棚水の生産量が減少したことによると推測する。オーバーターンの弱化は物質循環・生物生産にも多大な影響を及ぼす可能性がある。 - 二橋 創平, 大島 慶一郎, 田村 岳史, 齊藤 誠一 大会講演予講集 92 470 -470 2007年
- 中野渡 拓也, 永井 祥子, 大島 慶一郎 大会講演予講集 92 243 -243 2007年
- 「オホーツク海の気象 －大気と海洋の双方向作用－： 第2章 大気海洋間熱フラックスから見たオホーツク海の海氷」気象研究ノート 2007年 [査読無し][通常論文]
- 大島慶一郎, 牛尾収輝, 大槻晃久 南極資料 50 (3) 304 -316 2006年11月29日 [査読無し][通常論文]
日本南極地域観測隊(JARE)用の船不海氷目視観測のプロトコルを提案した.ASPeCt(Antarctic Sea ice Processes and Climate)のプロトコルを簡略化したもので,海氷の定量化に必要なものに絞ったものである.このプロトコルに従って,第46次隊から目視観測を開始した.流氷域での観測では,沿岸に近づくにつれて氷厚・変形氷の割合とも大きくなり,海氷の体積が著しく大きくなることが示された.継続した目視観測データの蓄積は,南極沿岸流氷域の海氷(特にその厚さ)の気候値・年々変動を知る不で貴重なものとなる.A protocol for ship-based visual observation of sea ice is proposed for the Japanese Antarctic Research Expedition (JARE). The protocol is a simplified version of the ASPeCt protocol, used for extracting quantitative information on sea ice. The ship-based visual observations started from JARE-46. In the pack ice region, ice thickness, ratio of deformed ice, and total ice volume increased toward the coast. Continuous monitoring of sea ice, particularly its thickness, by ship-based observation is effective for obtaining information on climatology and interannual variation of sea ice in the coastal pack ice region. - Sea ice production in the Okhotsk Sea and its relation to interannual variability of Okhotsk Sea and North Pacific Intermediate WaterOhshima K. I., T. Nakanowatari, S. Nihashi, M. Wakatsuchi, M. Itoh, S. C. Riser Proc. 21st International Symposium on Okhotsk Sea and Sea Ice, Monbetsu, Japan 29 -32 2006年 [査読無し][通常論文]
- 石田 邦光, 大島 慶一郎 雪氷 69 (2) 249 -262 2005年 [査読無し][通常論文]
衛星リモートセンシングデータの中で,多くのデータ蓄積があるにもかかわらず,利用されずにいるデータの一つがMOSデータである.3つの異なったセンサによる同時観測という優れた特性を有しながら,取得されたデータが広く研究に利用されることはなかった.しかしながら,雪氷研究においては,観測対象域が極域に多くあることから,少ないながらも利用されてきたといえる.特に,南極域においては,昭和基地において精力的に受信されたことから,貴重なデータを数多く得ることができた.MOSの一つのセンサであるMESSRによって観測された範囲は昭和基地沖周辺に限られるが,これらの画像からは,南極季節海氷域のさまざまな特徴を知ることができる.a)定着氷から氷縁に向かって氷野内の海氷分布構造がいくつかの特徴的な領域に分類できること.b)沿岸ポリニヤの定常的な出現やポリニヤ内における海氷移動の特徴を知ること.c)氷縁だけでなく氷縁内部まで形成されるアイス・バンド構造を理解できることなどである.ここでは,これらの研究成果を紹介することで,MOS MESSRデータの海氷観測における有効性を示した. - Properties of thick sea ice and overlying snow in the southern Sea of OkhotskProceedings of the 17th IAHR International Symposium on Ice, St. Petersburg, Russia 406 -412 2004年 [査読無し][通常論文]
- 大島慶一郎 天気 50 (8) 604 -609 2003年08月31日 [査読無し][通常論文]
- OHSHIMA K. I. J. Phys. Oceanogr. 2003年
- 豊田 威信, 馬場 賢治, 橋谷 英介, 大島 慶一郎 大会講演予講集 82 377 -377 2002年10月09日
- 橋谷 英介, 大島 慶一郎, 渡辺 知弘, 二橋 創平 大会講演予講集 81 145 -145 2002年05月
- 大島 慶一郎, 水田 元太, 伊東 素代, 深町 康, 渡辺 達郎, 難波 江靖, 末広 孝吉, 若土 正暁, Kay I. Ohshima, Genta Mizuta, Motoyo Itoh, Yasushi Fukamachi, Tatsuro Watanabe, Yasushi Nabae, Koukichi Suehiro, Masaaki Wakatsuchi 海の研究 = Umi no Kenkyu (Oceanography in Japan) 11 (2) 368 -368 2002年03月05日オホーツク海南西部において,1997年2月砕氷船「そうや」により,海洋と海氷の観測を行った。この観測によって,ほぼ結氷温度にある海洋混合層が300mの深さにまで達しているのが見つかった。これは,今まで考えられていたよりもずっと深い。この深い海洋混合層はこの海域で形成されたのではなく,サハリン東岸沖で形成され,それが東サハリン海流によって陸棚斜面に沿って移流されたものであると考えられる。また,これらと一緒に平均0.6mの厚い一年氷も運ばれていた。この鉛直に一様な冬季水は,周りの水と混合し,この海域の表層水をより高塩化し東サハリン海流水の性質を失わせる。一方,100～300m深の水は逆に,オホーツク海中層に特徴的な低塩,低温,高酸素の海水に変質される。海洋構造と熱収支解析から,氷縁域によく見られる新生氷帯は,まず厚い海氷が移流され,それが融解することによって潜熱を奪って表面水が冷却される,という事前の条件があった上で作られることが示唆された。
- 記載なし, 神田 穣太, 秋友 和典, 石坂 丞二, 酒井 敏, 江淵 直人, 長沼 毅, 渡辺 豊, 大島 慶一郎, 須賀 利雄, 中塚 武, 齊藤 宏明, 山中 康裕, 鈴木 利一, 藤尾 伸三, 市川 香 海の研究 = Umi no Kenkyu (Oceanography in Japan) 11 (2) 225 -233 2002年03月05日 [査読無し][通常論文]
- 大島 慶一郎, 若土 正暁, 深町 康, 水田 元太 Otsuchi marine science = 東京大学海洋研究所大槌臨海研究センター研究報告 26 (26) 92 -92 2001年03月29日 [査読無し][通常論文]
平成12年度共同利用研究集会「北太平洋北西部とその縁辺海の水塊変動と循環」（2000年8月24日～25日, 研究代表者：岩坂直人）講演要旨 - 伊東 素代, 大島 慶一郎, Motoyo Itoh, Kay I. Ohshima 海の研究 = Umi no Kenkyu (Oceanography in Japan) 10 (1) 84 -84 2001年01月05日
- 猪上 淳, 川島 正行, 大島 慶一郎, 藤吉 康志, 円山 憲一, Jun Inoue, Masayuki Kawashima, Kay I Ohshima, Yasushi Fujiyoshi, Ken-ichi Maruyama 海の研究 = Umi no Kenkyu (Oceanography in Japan) 10 (1) 81 -81 2001年01月05日
- Heat budget in the Sea of Okhotsk and the role of sea ice on the budgetOhshima K. I., T. Watanabe, S. Nihashi Proc. Second Wadati Conference on Global Change and Polar Climate, Tsukuba, Japan 216 -219 2001年 [査読無し][通常論文]
- Ice albedo feedback effect on sea ice/ocean coupled system in the AntarcticOhshima K. I., S. Nihashi Proc. Second Wadati Conference on Global Change and Polar Climate, Tsukuba, Japan 114 (117) 2001年 [査読無し][通常論文]
- 渡辺 知弘, 大島 慶一郎, 二橋 創平 大会講演予講集 77 102 -102 2000年05月24日
- 猪上 淳, 川島 正行, 大島 慶一郎, 藤吉 康志 東京大学海洋研究所大槌臨海研究センター研究報告 25 70 -70 2000年03月29日平成11年度共同利用研究集会「北日本の気象と海象」（1999年8月18日～19日, 研究代表者：児玉安正）の講演要旨Atmospheric and oceanographic phenomena around the northern part of Japan(Abstracts of scientific symposia held at Otsuchi Marine Research Center in 1999)
- Surface heat budget in the Sea of Okhotsk and its relation to the production and transport of the sea iceOhshima K. I., T. Watanabe, S. Nihashi International Symposium on Atmosphere-Ocean-Cryosphere Interaction in the Sea of Okhotsk and the Surrounding Environment, Sapporo, Japan 48 -49 2000年 [査読無し][通常論文]
- OHSHIMA Kay I., YOSHIDA Kazumasa, SHIMODA Haruhito, WAKATSUCHI Masaaki, ENDOH Tatsuo, FUKUCHI Mitsuo Proceedings of the NIPR Symposium on Polar Meteorology and Glaciology 11 267 -267 1997年11月During the Antarctic ice-melting season, sea ice concentration and thickness data were collected with the video monitoring system aboard the icebreaker SHIRASE, along with the monitoring of temperature and salinity in the upper ocean. Spatial resolution of the data is about 200m for the ice concentration and about 2km for the temperature and salinity. On the basis of these data, relationships among sea ice concentration, temperature, and salinity are investigated. In the region away from the ice-free ocean (ice interior region), ice concentration is negatively correlated with temperature and positively correlated with salinity for the spatially averaged data along the cruise track, which suggests that the local balances of energy and salt nearly hold in a bulk area. At the ice margin, ice concentration is negatively correlated with both temperature and salinity, suggesting that the local balances are overwhelmed by the effects of ice advection. The XBT profiles at the ice margin also show that a considerable amount of sea ice was advected into the ice-free ocean and subsequently melted there. It is pointed out that a polynya works as an "ice-melting factory" in summer; it absorbs solar radiation during the period of opening and then melts the ice advected there. From a heat budget analysis and the ocean structure in the melting season, we propose a simple ice/upper ocean coupled model in which sea ice melts on the bottom and lateral faces. The relations among ice concentration, temperature, and salinity derived from the model are consistent with the observed relations in the ice interior region. The negative feedback effect on the upper ocean temperature explains that the temperature-concentration plot shows a similar relation for any region. The salinity-concentration relation depends on the spatial scale more strongly than the temperature-concentration relation.
- FUKAMACHI Yasushi, OHSHIMA Kay I., ISHIKAWA Takayuki Proceedings of the NIPR Symposium on Polar Meteorology and Glaciology 11 266 -266 1997年11月Mesoscale ice-ocean features off Syowa Station are examined using NOAA advanced very high resolution radiometer (AVHRR) data during the summers of 1987 and 1988. Visible-band (channel 2) and infrared-band (channel 4) images are used to examine ice and sea surface temperature distributions, respectively. As the melting season progresses, mesoscale ice tongues and eddies are found in the marginal ice zone. The growth of these features is not correlated with the wind field in this area. Comparison between visible and infrared images reveals that the mesoscale ice features are accompanied by corresponding patterns of cold water off the ice edge. Namely, regions of cold water extend off the ice tongues and eddies. This suggests that advection of ice by ocean currents generates the mesoscale features.
- 石田 邦光, 大島 慶一郎, 山内 恭, 神沢 博 学術講演会論文集 = Proceedings of the ... Japanese Conference on Remote Sensing 22 241 -244 1997年05月01日
- 大島 慶一郎, Kay I. Ohshima, Institute of Low Temperature Science Hokkaido University 海の研究 = Umi no Kenkyu (Oceanography in Japan) 6 (2) 111 -120 1997年04月05日This article reviews the author's study on variability of coastal currents in polar and subpolar regions, for which the Okada Prize for 1996 was awarded. Cited topics are (i) instability of the Soya Warm Current, (ii) inflow-outflow system of the Japan Sea, and (iii) seasonal cycle of the Antarctic coastal ocean.
- 大島慶一郎 海の研究 6 (2) 111 -120 1997年04月 [査読無し][通常論文]
This article reviews the author's study on variability of coastal currents in polar and subpolar regions, for which the Okada Prize for 1996 was awarded. Cited topics are (i) instability of the Soya Warm Current, (ii) inflow-outflow system of the Japan Sea, and (iii) seasonal cycle of the Antarctic coastal ocean. - 河村俊行, 滝沢隆俊, 大島慶一郎, 牛尾収輝 南極資料 41 (1) 395 -414 1997年03月 [査読無し][通常論文]
1987年から5カ年計画で始まった「南極域における気候変動に関する総合研究計画(ACR)」では, 「海洋-海氷-大気相互作用」の研究が中心課題の一つであった。その目的のために, 1990年から1992年までオングル海峡およびリュツォ・ホルム湾において海洋・海氷観測が勢力的に実施された。海洋観測では定着氷下の海流と海洋構造を解明するために, CTD・電磁流速計・XBTによる観測, 採水観測, および流速計・サーミスタチェインの係留を行った。また航空機による海洋観測も実施した。海氷観測では海氷の構造と成長過程を理解するため, 海氷コアの採取とその解析を行った。この報告では, まず海洋・海氷観測の概要を述べ, 次にオングル海峡を横断する観測点での海氷成長について記し, 最後にオングル海峡に人工的に開けたプールでの海氷の成長について述べる。A five-year program of Antarctic Climate Research (ACR) started in 1987. Air-ice-sea interaction study was one of the main subjects of the program. Intensive observations for this purpose were conducted in Ongul Strait and Lutzow-Holm Bay in 1990-1992. Measurements by means of CTD, XBT, AXBT and electromagnetic current meter, and seawater sampling was carried out in order to reveal the flow and oceanic structure under fast ice. Current meters and thermistor chains were also moored to collect long-term data. Sea ice cores were sampled and analyzed to determine the structure and growth processes. In this report, we describe first ocean and sea-ice observations in detail, second, structure and development of sea ice at points across Ongul Strait, and last, growth processes of sea ice in an artificial pool in Ongul Strait. - 下田春人, 遠藤辰雄, 村本健一郎, 小野延雄, 滝沢隆俊, 牛尾収輝, 河村俊行, 大島慶一郎 南極資料 41 (1) 355 -366 1997年03月 [査読無し][通常論文]
第30,31,32次日本南極地域観測隊が「しらせ」艦上で撮影した氷況観測ビデオ映像を用いて, 海氷の密接度, 氷厚を定量化した。解析を行った区間は, 30,31次ではブライド湾進入時, ブライド湾～昭和基地の2区間, 32次ではプリンスオラフ海岸沖進入時, プリンスオラフ海岸沖～ブライド湾, ブライド湾～昭和基地の3区間についてである。その結果, 各年による「しらせ」航路上の海氷状況の違い, 緯度による氷厚の変化, 30次と32次のリュツォ・ホルム湾内氷厚の年較差を調べることができた。また, 同時に計測された短波放射計を用いてアルベドを求め, 得られた密接度と比較し比例関係が求められた。さらに, 海氷域接地気層の鉛直温度傾度を求め, 密接度と比較した。During the 30th, 31st, and 32nd Japanese Antarctic Research Expeditions (JARE-30,JARE-31,and JARE-32), sea-ice conditions were recorded by video camera on board the SHIRASE. Then, the sea-ice images were used to estimate compactness and thickness quantitatively. Analyzed legs are those toward Breid Bay and from Breid Bay to Syowa Station during JARE-30 and JARE-31,and those toward the Prince Olav Coast, from the Prince Olav Coast to Breid Bay, and from Breid Bay to Syowa Station during JARE-32. The results show yearly variations of ice compactness and thickness, latitudinal variations of thickness, and differences in thickness histograms between JARE-30 and JARE-32 in Lutzow-Holm Bay. Albedo values were measured simultaneously by a shortwave radiometer. These values are proportional to those of ice compactness. Finally, we examined the relationship between ice compactness and vertical gradient of air temperature above sea ice. - 牛尾収輝, 滝沢隆俊, 大島慶一郎, 河村俊行 南極資料 41 (1) 329 -334 1997年03月 [査読無し][通常論文]
南極リュツォ・ホルム湾周辺における海洋観測データを用いて, 大陸沿岸に形成されるポリニヤ(海氷野内の開水面または薄氷域)の海洋構造の特性を調べた。この沿岸ポリニヤ域では, 人工衛星画像や航空機からの目視観測結果によると, ほぼ通年にわたって開水面が形成されており, 冬季は新成氷が生産され続けていること, 海洋上層の水温は結氷温度に達していることがわかった。また, ポリニヤ域の対流混合層は, 沖合流氷域のそれよりも厚く発達している。さらに, 対流混合層内の溶存酵素量データから, 深層水の取り込み(entrainment)が生じている可能性も示唆された。A coastal polynya often forms around Lutzow-Holm Bay even in severe winter. To clarify the characteristics of oceanic structure in the polynya, the water mass of the convective-mixed layer was analyzed. Haline convection by the high ice production in the winter polynya contributes to the formation of a thicker mixed layer than that in the pack ice region. Furthermore, the active convection leads to entrainment of the oxygen-poor deep water underlying the winter mixed layer. Consequently, in the polynya located over the continental shelf break, the oxygen content of the mixed layer is somewhat lower than that in the other polynya, Breid Bay, where haline convection reaches only to the shallow sea bottom of the continental shelf. - 滝沢隆俊, 大島慶一郎, 牛尾収輝, 河村俊行, 榎本浩之 南極資料 41 (1) 335 -346 1997年03月 [査読無し][通常論文]
コスモノート海(60-68°S, 35-65°Eの水域)において, コスモノート・ポリニヤの特徴を明らかにするため, 1987年から1992年の間, JAREの夏観測で得られた水温データの解析を行った。沿岸域には水温-1.5℃以下の冷い水塊が広範囲に存在した。その沖合い北東から北西にかけて, 水温1℃以上の暖かい周極深層水が150m以深に認められた。1987年から1991年のSSM/I画像の解析によると, 持続性のある典型的コスモノート・ポリニヤが1988年に現れた。その他の年は小規模なポリニヤが散発的に出現するのみでポリニヤ活動は弱い年であった。一方, 約66°S, 50-60°Eの沿岸域には例年頻繁に沿岸ポリニヤが現れた。また, コスモノート・ポリニヤから東に向けて, 点在するポリニヤ列がしばしば認められた。このポリニヤ列は, 大気の南極収れん線と海洋の南極発散域に沿って形成されると考えられる。Water temperature structure in the Cosmonaut Sea between 60-68°S, 35-65°E in 1987-1992 shows that cold water with temperature below -1.5℃ was present in the coastal region. The Circumpolar Deep Water with temperature higher than 1.0℃ was found below about 150m in depth from northeast to northwest of the cold water area. The SSM/I images in 1987-1991 indicate that polynya activity was intensive in 1988,and a typical Cosmonaut Polynya was observed; due to weak activity, small and sporadic Cosmonaut Polynyas formed in 1987,1989,1990 and 1991. A coastal polynya frequently was observed in every year at about 66°S, 50-60°E. A train of polynyas to the east of the Cosmonaut Polynya often appeared. It is considered that the Atmospheric Convergence Line and Antarctic Divergence Region are responsible for polynya activity in the Cosmonaut Sea. - 大島慶一郎, 滝沢隆俊, 牛尾収輝, 河村俊行 南極資料 41 (1) 311 -328 1997年03月 [査読無し][通常論文]
ACRのもとで, リュツォ・ホルム湾周辺において通年にわたる海洋観測を1990年-1992年の間行った。これらの観測によって, 低温・低塩・高溶存酸素で特徴づけられる南極冬季水の厚さが秋に最大(500m程度), 夏に最小(350-400m程度)になることが示された。この厚さの変化に伴って生じる比容の変化は岸での水位の季節変化の約1/3の変化分にしか相当しない。これは順圧の沿岸流が大きな季節変化をすることを示唆している。一方, 秋には冬季水層において著しい低塩化が起こる。これは前の夏に海氷融解によって表層が低塩化した冬季水が蓄積されたことによると考えられる。この低塩(従って低密度)水の蓄積がこの周辺の海域で南極底層水ができないことの理由の一つになっていると思われる。冬季水の厚さの季節変化は主に沿岸域での風の場の季節変化によることを提出した。すなわち, 沿岸域では卓越風である東風が強まる秋季に冬季水のエクマン収束が強化され, 東風が弱まる夏季にエクマン収束が弱まる, と考えた。Under the Japanese Antarctic Climate Research (ACR) program, year-round oceanographic observations were conducted in the vicinity of Lutzow-Holm Bay, during 1990-1992. It was found that the thickness of the Winter Water (WW) layer, characterized by a cold fresh oxygen-rich water, exhibits its maximum in the austral fall (typically 500m) and its minimum in the austral summer (typically 350-400m). The associated density variation of the water column explains only about one third of the seasonal variations in sea level at the coast, which suggests a large seasonal variation in barotropic coastal flow. Prominent freshening occurs in the WW layer during fall. This appears to be caused by the accumulation of WW, whose upper portion is freshened in the preceding summer. We think that this accumulation of the freshened (accordingly less dense) water is a possible reason why bottom water formation can not occur in this area. We propose that the seasonal variation in the WW layer is mainly caused by the seasonal variation in the wind over the coastal ocean. In fall the prevailing easterly wind intensifies, which increases the Ekman convergence of WW in the coastal ocean, while in summer the opposite occurs. - OHSHIMA K. I. Continental Shelf Res. 1997年
- USHIO Shuki, TAKIZAWA Takatoshi, OHSHIMA Kay I., KAWAMURA Toshiyuki Proceedings of the NIPR Symposium on Polar Meteorology and Glaciology 10 168 -168 1996年09月Aerial observations off East Queen Maud Land, Antarctica, have verified that an open polynya exists and that frazil-ice production occurs there. From the airborne-XBT data, the mixed layer temperature of the polynya attains its freezing point. This polynya is therefore regarded as a latent-heat polynya. Thickness of the convective-mixed layer measures 350m or more, which is much deeper than in the offshore pack ice regions. Also, the mixed layer is more homogeneous in salinity profile as compared with those under fast ice regions near Syowa Station. Such a water structure of the polynya suggests active haline convection accompanied by high ice production. Furthermore, the haline convection causes entrainment of the oxygen-poor deep water underlying the winter mixed layer. Thus, the deep-water entrainment decreases the oxygen content of the mixed layer. The average oxygen saturation of the mixed layer in the polynya is 86%, which is almost the same as that of the pack ice region in the Weddell Sea. The oxygen saturation of the deep water is also equal in both regions. However, the mixed-layer thickness of the coastal polynya is much larger than that of the Weddell Sea. This shows that the entrainment rate in the open polynya region is intense. It is estimated that the entrainment rate is three times greater than that in the Weddell Sea.
- OHSHIMA K. I. Pre. Wadati Conference on Global Change and the Polar Climate 158 -161 1995年
- Upward growth of Antarctic sea ice during summer and autumnKawamura, T, Takizawa, T, Ohshima K. I., Ushio, S Pre. Wadati Conf. on Global Change and the Polar Climate 150-153 1995年 [査読無し][通常論文]
- Data of sea-ice cores obtained in Lutzow-Holm Bay from 1990 to 1992 (JARE-31, -32) in the period of Japanese Antarctic Climate ResearchKawamura, T, Takizawa, T, Ohshima K. I., Ushio, S JARE Data Reports 204 (Glaciology 24) 1995年 [査読無し][通常論文]
- カワムラ トシユキ, オオシマ, ウシオ シュウキ, タキザワ タカトシ, Toshiyuki KAWAMURA, Kay I. OHSHIMA, Shuki USHIO, Takatoshi TAKIZAWA Proceedings of the NIPR Symposium on Polar Meteorology and Glaciology 8 194 -194 1994年11月As part of the Antarctic Climate Research (ACR) program, a study of the atmosphere/sea-ice/ocean interaction was conducted off Queen Maud-Enderby Land, Antarctica. Two-year field observations were carried out to clarify the effect of snow cover on growth processes of sea ice in Ongul Strait from April, 1990 to December, 1991. Routine measurements of the length of snow stakes, snow depth and ice thickness were made about every month at three stations across the Strait. Sea ice cores were collected to assess their structure and temperature. Then, salinity and oxygen isotope concentration profiles of the melted ice samples were determined at intervals of about 0.1m. The snow depth varies consistently from 0 to about 1 meter across the Strait, increasing with distance from the Antarctic Continent. The snow depth affects both the growth and structure of sea ice. In winter, the growth rate is higher in bare ice regions than in snow-covered regions. However, over the year, the ice thickness itself is lower in bare ice regions than in snow-covered regions, in contrast to the common notion that snow cover is associated with a decrease in the growth rate of sea ice. Sea ice in the snow-covered regions increased in thickness in spring rather than in winter. The analyses of the sea-ice structure and the oxygen isotope concentration suggest that such growth processes are caused by the formation of snow ice and by ice formation related to the melting of snow cover. Ice thickness decreases drastically in bare ice regions in summer owing to melting at the bottom surface and inside the ice due to oceanic heat flux and absorption of solar radiation, respectively. In contrast, the decrease is not so great in snow-covered regions due to thermal insulation by the snow cover, leading to thicker sea ice. Major ion concentration profiles of the melted sea-ice samples were also determined. These measurements show that the sea ice samples have a tendency to enrich the concentrations of K^+ and Na^+ and to reduce that of SO_4^<2-> relative to sea water.
- USHIO Shuki, TAKIZAWA Takatoshi, OHSHIMA Kay I., KAWAMURA Toshiyuki Proceedings of the NIPR Symposium on Polar Meteorology and Glaciology 8 191 -191 1994年11月In a coastal region where strong offshore winds and currents prevail, newly-formed sea ice is removed as quickly as it forms. Consequently, the open water areas are maintained for a long period of time, despite considerable ice production. Such open waters, or coastal polynyas, release latent heat of fusion of ice and moisture from the ocean to the atmosphere. High-salinity brine is also excluded to the ocean by sea-ice formation. The polynyas may have great influence on the atmosphere-ocean interaction. Satellite images have suggested the presence of winter coastal polynyas between fast ice (or ice shelf) and pack ice regions off Lutzow-Holm Bay and Breid Bay, Antarctica. In 1990-1992,oceanographic observations were intensively conducted by the 31st and 32nd Japanese Antarctic Research Expeditions to understand sea-ice and oceanic processes in the polynya off Lutzow-Holm Bay. Under windy conditions, aircraft observations revealed many new-ice streaks formed approximately parallel to the wind direction and generation of frost smoke from the open water. Water temperature profiles in the polynya were obtained by using aircraft-deployed expendable bathythermographs (AXBT) with a depth capability of 460m. Based on the temperature data, it is estimated that the thickness of the convective-mixed layer with its freezing point is 350-450m. The mixed layer has a tendency to deepen in winter. According to the austral summer oceanographic data acquired on board the icebreakers FUJI and SHIRASE, winter convective-mixed layers remain cold and oxygen-rich in the polynya regions. These mixed layers were evidently thicker than those in pack ice regions. Such water structures were also formed in the Breid Bay polynya. The thick convective-mixed layer is considered to reflect the processes of vigorous sea-ice production and the resultant convective mixing in the open polynyas.
- OHSHIMA Kay I, KAWAMURA Toshiyuki, TAKIZAWA Takatoshi, USHIO Shuki Proceedings of the NIPR Symposium on Polar Meteorology and Glaciology 8 193 -193 1994年11月We made current measurements at three sites under fast ice in Lutzow-Holm Bay during 1990-1991. In spite of no direct wind forcing and negligible thermohaline forcing, the current velocity is not small. Especially in Ongul Strait, the velocity typically reaches the order of 0.3 m/s. It is also noted that the direction of the mean current changes drastically from southward to northward in May. The currents at all sites have some correlation with the wind, atmospheric pressure and tide at Syowa Station. We constructed a numerical ocean model to understand the current system under fast ice. We modeled Lutzow-Holm Bay and the continental shelf which extends eastward. The model is barotropic, and the westward wind stress is applied to the ocean. In the case of no fast ice on the east shelf (corresponding to the before-May case), mainly the first mode of shelf wave is excited; while, when the margin of the fast ice is located at the shelf break (corresponding to the after-May case), the second mode of shelf wave is preferentially excited. Correspondingly, the current direction near the coast becomes opposite between these cases, which agrees with the observations in Ongul Strait. These studies suggest that the current variability under fast ice is mostly governed by shelf wave (or coastally trapped waves) generated by the wind stress offshore, and the dominant mode of shelf wave depends strongly on the location of the fast ice margin.
- Kawamura Toshiyuki, Ohshima Kay I., Ushio Shuki, Takizawa Takatoshi Proceedings of the NIPR Symposium on Polar Meteorology and Glaciology 7 116 -116 1993年09月As part of the Antarctic Climate Research (ACR) program, a two-year study of atmosphere/sea-ice/ocean interaction was conducted off Queen Maud-Enderby Lands, Antarctica in 1990 and 1991. Research on the spatial characteristics of snow depth and sea-ice thickness and of the growth processes of sea ice was carried out in Lutzow-Holm Bay. The lengths of snow stakes, snow depths and sea-ice thicknesses were all measured at ten offshore stations on two latitudinal lines in April, August and October of both years. Sea ice cores were also collected to assess structure, temperature and salinity in 1991. Spatial variations of snow depth and sea ice thickness were clearly noticed. The stations near the Antarctic Continent have little snow cover. However, the snow depth increased consistently with distance from the Continent, reaching a nearly constant maximum value of 1 to 1.5m during the winter. The increase of ice thickness paralleled that of snow depth, the maximum thickness at the offshore stations being 2 to 3m. No differences in these variations were recognized between the two observation lines. The snow depth was found to significantly affect the sea-ice growth processes. At the station with little snow cover, the sea ice grew in the austral winter months and decayed in summer to the thickness of the previous year. The sea ice repeats this growth and decay cycle annually and consequently is limited in thickness to approximately 2m. On the contrary, in the heavily snow-covered regions, the sea ice showed little growth even in the winter, because the thick snow cover and sea ice itself reduced the vertical heat flux and accordingly the growth rate due to bottom freezing. In summer, however, an extreme increase of sea ice thickness of 0.5 to 1m was observed. The growth is probably caused by refreezing of low-salinity meltwater from the snow cover. An aquifer about 0.2m deep was found to exist on the snow/ice interface in January 1992. This fact suggests that the sea ice growth can be attributed to the formation of superimposed ice.
- 大島 慶一郎, Kay I. Ohshima, 北海道大学低温科学研究所, The Institute of Low Temperature Science Hokkaido University 海の研究 = Umi no Kenkyu (Oceanography in Japan) 2 (4) 281 -282 1993年08月25日
- Cosmonaut Polynya region in the AntarcticTakizawa, T, Ohshima K. I., Ushio, S, Kawamura, T, Enomoto, H Proc. Intl. Sympos on ISY Polar Ice Extent 82-86 1993年 [査読無し][通常論文]
- Seasonal variations of the flow and oceanic structure under fast ice Lutzow-Holm Bay, AntarcticaOhshima K. I., Kawamura, T, Takizawa, T, Ushio, S Proc. Fourth Intl. Conf. on Southern Hemisphere Meteorol. Oceanogr. 4, 467-468 1993年 [査読無し][通常論文]
- 昭和基地沖に形成される沿岸ポリニアの水温構造南極資料 37 (3) 252 -259 1993年 [査読無し][通常論文]
- ウシオ シュウキ, タキザワ タカトシ, カワムラ トシユキ, オオシマ, Shuki USHIO, Takatoshi TAKIZAWA, Toshiyuki KAWAMURA, Kay I. OHSHIMA Proceedings of the NIPR Symposium on Polar Meteorology and Glaciology 6 161 -161 1992年12月Some open water areas between fast ice and pack ice zones are often observed off the Prince Olav Coast and Riiser-Larsen Peninsula all year round. In such open waters, called coastal polynyas, which are exposed to the cold air, sea ice newly forms with high growth rate. It is considered that coastal polynyas serve as more efficient ice factories than ice-covered areas where sheet ice gradually grows. So heat loss to the atmosphere is very large. In addition, large salt flux occurs there as sea ice excludes brine. In this way large quantities of heat to the air and salt to the ocean are supplied through the processes of sea ice production in polynyas, which have great influence on global climate. In 1990-1991 (JARE-31) we carried out oceanographic observations to clarify the thermal structure in the polynya region off Lutzow-Holm Bay using aircraft-deployed expendable bathythermographs (AXBT, made by the Tsurumi Seiki Co., Japan) from a Cessna plane. This polynya is commonly called 'Otone Suiro' and is located about 50 miles north of Syowa Station. Temperature and depth accuracies of the AXBT are ±0.1℃ and 5m or ±2% of depth, respectively. Sixteen temperature profiles down to about 500m in depth were obtained from March 1990 to January 1991 except for the polar night season. The thickness of the convective mixed-layer with near freezing temperature was 300-500m. These mixed layers were clearly thick and cold as compared with those in fast ice and pack ice zones. From spring to early summer, the temperature of the surface 50m layer rose to about -1.5∿-1.0℃ because of absorption of solar radiation.
- ISHIKAWA Takayuki, USHIO Shuki, OHSHIMA Kay I., YAMANOUCHI Takashi, SEKO Katsumoto, TAKIZAWA Takatoshi Proceedings of the NIPR Symposium on Polar Meteorology and Glaciology 4 135 -135 1991年05月The purpose of this study is to investigate sea ice distribution off Queen Maud Land on the basis of NOAA AVHRR data received from 1987 to 1989 at Syowa Station. These data are very useful for examining the mesoscale phenomena. There are several characteristics such as coastal polynyas and sea ice eddies in the image. A coastal polynya was observed along the shelf break. Sea ice eddies were found at the ice edge which is located 60-200km off the Antarctic coast from December to February. These eddies are characterized by horizontal scale of 30-50km and small propagation of a few cm/s westward.
- OHSHIMA K. I., WAKATSUCHI Masaaki Proceedings of the NIPR Symposium on Polar Meteorology and Glaciology 3 108 -108 1990年01月Off the coast of Hokkaido in the Sea of Okhotsk, shore-based radars have revealed that an eddy-like pattern (ice-ocean eddy) or backward wave breaking pattern is sometimes observed in the ice floe distribution. A typical ice-ocean eddy is shown in an aerial photograph in the Asahi Shinbun, January 12,1987. It shows that the ice-ocean eddy is composed of numerous pancake ice floes of diameter about 10m. First we investigated the characteristics of these eddies using radar images. These patterns seem to be generated when the ice concentration is small and the wind is weak after strong northerly wind blows. They are mostly observed off the coast of Esashi and Ohmu, near the Soya Strait. They often appear as a vortex train. The scale of the eddies is about 20-30km and the wavelength of the vortex train is about 50km. Next we examine the generation mechanism of these eddies, assuming that sea ice floes act as a tracer for the ocean velocity. The Soya Warm Current which flows in this region is modeled. The model suggests that on this current wave motion is induced by barotropic instability. Using this model, we simulate the behavior of sea ice floes which are driven by wind and oceanic flow. We can reproduce quite similar pattern to observed eddy-like pattern or backward wave breaking pattern. This suggests that the wave motion occurs in the ocean and that the sea ice floes visualize such motion as a tracer.
- 北海道南方陸棚域及び噴火湾における風による循環に関する数値的研究海と空 66 53 -66 1990年 [査読無し][通常論文]
受賞
- 2018年08月 内閣府、文部科学省他 第11回海洋立国推進功労者（内閣総理大臣賞）
「極域海洋での中深層水形成・循環の解明」受賞者: 大島 慶一郎 - 2016年04月 文部科学省 文部科学大臣表彰 科学技術賞(研究部門)
「海氷域における中深層水形成と海洋循環に関する研究」受賞者: 大島 慶一郎 - 2015年03月 日本海洋学会 日本海洋学会賞
「海氷域の変動とその海洋循環に与える影響に関する研究」 - 2008年11月 日本気象学会 堀内賞
「海氷の気候システムに果たす役割に関する研究」受賞者: 大島 慶一郎 - 2008年03月 日本海洋学会 奨励論文賞
- 1996年04月 日本海洋学会 岡田賞
「極域・亜極域の沿岸流の変動の研究」
共同研究・競争的資金等の研究課題
- 日本学術振興会：科学研究費助成事業 基盤研究(S)研究期間 : 2020年08月 -2025年03月代表者 : 大島 慶一郎, 深町 康, 西岡 純, 二橋 創平, 三谷 曜子, 木村 詞明, 田村 岳史, 真壁 竜介高海氷生産域であるフラジルアイス域を検知できるマイクロ波放射計AMSRのアルゴリズムを開発し、南大洋全域で初めてフラジルアイス域のマッピングを行った。フラジルアイス域を検知できたことで、今まで過小評価されていた沿岸ポリニヤでの海氷生産量をより高精度で見積もることができ、従来のアルゴリズムより、20－50％高い海氷生産量が示された。30年あまり継続してあるマイクロ波放射計SSM/I-SSMISデータに対し、薄氷タイプを識別することでより高精度の薄氷厚が推定できるアルゴリズムを開発した。南大洋をベースに開発したこのアルゴリズムは北極海でも適用可能であることも示され、全球に汎用性のあるアルゴリズムであることが示された。この開発により、長期間での全球での沿岸ポリニヤの海氷生産量の見積もりに道が開けた。プロファイリングフロートの溶存酸素量の変動から、初めてオホーツク海の広範囲で正味の生物生産量（純群集生産量）を見積もることに成功し、春の植物プランクトンの大増殖（春季ブルーム）は海氷融解によってもたらされていることを定量的指標をもって明らかにした。さらに、海氷融解時に放出される物質（鉄分）がブルームを誘起していることを示唆し、海氷による物質輸送の重要性を示した。観測としては、2021年3月に小型砕氷船ガリンコ号を傭船し、海氷サンプリングや氷縁での観測を実施し、海氷による物質輸送の観測研究に着手した。この他、CTDタグを購入し、データ通信用のアルゴスシステムに登録するなど、アザラシ、トドによるバイオロギング観測を行う準備を整えた。
- 日本学術振興会：科学研究費助成事業研究期間 : 2022年04月 -2023年03月代表者 : 川村 賢二, 大島 慶一郎, 池原 実, 茂木 正人, 野木 義史, 阿部 彩子, 菅沼 悠介, 田村 岳史
- 日本学術振興会：科学研究費助成事業研究期間 : 2019年04月 -2023年03月代表者 : 深町 康, 大島 慶一郎, 二橋 創平, 伊東 素代2019年夏季には、研究代表者を中心とする北海道大学のグループがアラスカ北部に出向き、チュクチ海北東部の沿岸域に2017年夏季に設置していた超音波氷厚計を含む2系の係留系を回収し、良好なデータの取得に成功した。これで本海域における継続した海氷厚のデータは世界的に見ても稀な10年分となった。 超音波氷厚計の生データからの海氷厚の導出は非常に手間と時間がかかる作業であるが、2015-17年に本海域において取得したデータについて、この作業を実施した。 本海域におけるこれまでの係留観測データを、春季に取得した海氷のコアサンプルデータと合わせて、海氷初期に生成されるフラジルアイスが、巻き上げられた海底堆積物と接触することを示し、海底堆積物が海氷内部に取り込まれるプロセスが発生していることを示した。このことは、海氷の生成域と融解域が異なる場合の海氷による物質循環の役割を示唆している (Ito, Ohshima, Fukamachi et al., 2019, Journal of Geophysical Research: Oceans) 。 また、超音波氷厚計による係留観測データとマイクロ波放射計による衛星観測データの比較・検討を行い、後者による薄氷厚推定の妥当性を示した (Kashiwase, Ohshima, Fukamachi et al., 2019, Journal of Atmospheric and Oceanic Technology)。
- 日本学術振興会：科学研究費助成事業 挑戦的研究(萌芽)研究期間 : 2020年07月 -2022年03月代表者 : 大島 慶一郎, 深町 康, 伊藤 優人超音波流速計ADCPの体積後方散乱強度データから、生物(主に動物プランクトン)活動、堆積物の巻き上がり、海中内部で生成される海氷(フラジルアイス)、等の海中浮遊物を検知・識別する手法を開発した。識別手法としては、複素EOF(CEOF)解析とバンドパスフィルタリングを主として用いた。中心周波数24時間のバンドパスフィルタをかけ、CEOF解析を施すことで、動物プランクトンの日周活動を明瞭に抽出する汎用性の高い手法を開発した。この方法では、潮流の強い海域では堆積物巻き上がりも抽出されるが、位相差の違いから両者は識別可能となる。フラジルアイス及び海底堆積物の巻き上がりに関しては、上層(フラジルアイスの場合)、下層(巻き上がりの場合)に重みをつけたCEOF解析、風速(フラジルアイスの場合)・海底流速(巻き上がりの場合)との結合EOF解析等を組み合わせることで、抽出はある程度可能となることもわかった。以上の識別手法を、オホーツク海、北極チュクチ海、南極ケープダンレーポリニヤ域で取得された係留系ADCPデータに適用した。南極ケープダンレー沖沿岸の係留データからは、日が昇らない冬季に動物プランクトンの日周鉛直活動が月の満ち欠けに呼応する1か月周期の変動を示すことを、南極域では初めて明らかにした。また、南極域及びオホーツク海の係留データからは、冬季海氷期は、海氷のない時期とは逆に、沿岸域より沖合域の方が動物プランクトンの日周鉛直活動が活発となることが示された。一方、過去にサンプリングした海氷の粒度粒径分布の解析を継続して行い、フラジルアイスによる堆積物取り込み過程の検討を行った。オホーツク海では2022年2月、砕氷巡視船そうやにより、新たな海氷サンプリングも行った。
- 日本学術振興会：科学研究費助成事業研究期間 : 2017年06月 -2022年03月代表者 : 川村 賢二, 大島 慶一郎, 池原 実, 茂木 正人, 福田 洋一, 野木 義史, 阿部 彩子, 菅沼 悠介, 田村 岳史, 末吉 哲雄＜全体・研究統括＞各年度の総括と今後の計画を共有し連携を促進するため、全体ワークショップと拡大総括班会合、運営委員会をオンラインで開催した。＜連携支援＞南大洋における生態系班と底層水班の観測連携や、南極氷床における氷床班と固体地球班の観測連携を調整した。また、データとモデルの統合を支援し、生態系研究におけるデータ・モデル連携を強化するためのタスクチームを立ち上げた。ほかに、３つのリザーバおよび古環境分野のそれぞれに対応するワーキンググループを立ち上げ、連携と成果取りまとめに向けた検討を行った。各タスクチームやワーキンググループが取りまとめた進捗状況や連携に関する検討結果は、領域全体や拡大総括班での共有と議論を行った。＜国際・若手支援＞若手人材育成支援策(海外派遣支援、若手会支援など)や国際動向調査支援については、新型コロナウイルス関連の推移を見つつ可能な範囲で実施した。＜アウトリーチ＞領域ホームページを継続的に更新し、ニュースレターを発行した。学部生向けの企画として、対面式のスクールはコロナウイルスの影響により中止し、代替として動画配信の企画を行った。
- 南極底層水を起点とする熱塩循環・物質循環のダイナミクス日本学術振興会：新学術領域研究 計画研究研究期間 : 2017年06月 -2022年03月代表者 : 大島 慶一郎
- ポリニヤを起点とする熱塩／物質循環日本学術振興会：基盤研究(A)研究期間 : 2017年04月 -2020年03月代表者 : 大島 慶一郎
- 日本学術振興会：科学研究費助成事業研究期間 : 2015年04月 -2019年03月代表者 : 深町 康, 大島 慶一郎, 伊東 素代, プニュシコフ アンドレイ, 平野 大輔アラスカ北部沿岸域で実施した係留観測により、海氷厚の時系列データがそれ以前の観測と合わせて8年分蓄積された。そのデータの解析により、海氷厚に大きな経年変動が存在することを示し、その要因がこの海域の風による海氷の漂流パターンの変動にあることを明らかにした。
係留観測によって得られた海洋の時系列データを用いて、この海域に存在する薄氷域の形成・維持は海氷の沖向きの風による輸送だけでなく、同じ沖向きの風によって湧昇した中層の高温水によっても成されており、この海域で冬季に生成される海水は太平洋起源だけでなく、大西洋起源の海水の影響も受けることを示した。 - 海氷生成とリンクする南極底層水・深層循環の再描像日本学術振興会：基盤研究(A)研究期間 : 2013年04月 -2017年03月代表者 : 大島 慶一郎
- 日本学術振興会：科学研究費助成事業研究期間 : 2013年04月 -2016年03月代表者 : 大島 慶一郎, FRASER ALEXANDER, FRASER Alexander本課題は、東南極で行っていたModerate-Resolution Imaging Spectro radiometer (MODIS)の可視・赤外線データから定着氷を検出する手法を、全南極に適用することが主なテーマである。昨年度までに開発したMODISデータから半自動化して南極の定着氷の検出する手法を全南極海に適用して、2000年から2014年の14年間の全南極の定着氷データセットを完成させた。完成されたデータセットは、全南極域では初めての高い（2-3km）分解能を持つ定着氷データセットとなり、海洋・海氷・棚氷結合モデルの境界条件データセットとしてもすでに利用されている。南極ケープダンレーポリニヤでは、作成された定着氷を用いて、海氷生産量の季節変動と経年変動が定着氷と強くリンクしていることを明らかにした。また、作成された定着氷データと海氷生産量データ、日豪による現場海洋観測データを合わせた解析から、アデリーランド沖での底層水生成システムとメルツ氷河崩壊によるインパクトを定量的なレベルで明らかにした。氷河大崩壊により海氷生産量は40％減少したこと、氷河崩壊によって定着氷分布が大きく変わったことも海氷生産量の変化に大きく効いていること、などを明らかにした。これらの研究を含めた、代表的なポリニヤでの定着氷と海氷生産量の経年変動に関する研究はJournal of Geophysical Researchに受理された。作成された定着氷時系列データから、南極昭和基地のあるリュッツォ・ホルム湾での定着氷の季節・経年変動が今までよりはるかに高精度で明確になった。これらは、「しらせ」の航行戦略や南極観測隊のオペレーションにも大きく役立つこととなる。
- 日本学術振興会：科学研究費助成事業研究期間 : 2010年04月 -2015年03月代表者 : 若土 正曉, 三寺 史夫, 西岡 純, 中村 知裕, 江淵 直人, 渡邉 豊, 鈴木 光次, 黒田 寛, 大島 慶一郎, 津旨 大輔, 的場 澄人, 江淵 直人, 小埜 恒夫オホーツク海から親潮域の豊かな水産資源を支える“奇跡ともいえる仕組み”を明らかにするため、オホーツク海と北太平洋をつなぐ熱塩／物質循環システムの実態解明を進めた。熱塩循環の強さを決めるオホーツク海北部の高密度水（DSW）の塩濃度は、亜寒帯循環を跨ぐ塩濃度偏差の長距離伝播、海氷生産量変動、降水量変動との「せめぎ合い」により決まる。アムール川河口・オホーツク北部陸棚上に堆積する光合成に必須の栄養物質・鉄は、このDSWに取り込まれ、北太平洋の広範囲に供給される。DSWと潮汐混合を介し強く結合した熱塩循環と物質循環が生物生産を北太平洋規模で規定するという、絶妙なシステムが本研究により明らかとなった。
- 日本学術振興会：科学研究費助成事業研究期間 : 2010年04月 -2015年03月代表者 : 三寺 史夫, 中村 知裕, 浮田 甚郎, 木村 詞明, 小木 雅世, 金子 正美, 田口 文明, 小守 信正, 大島 慶一郎, 川島 正行, 高谷 康太郎冬季の季節海氷域の形成、および夏季のオホーツク海高気圧とそれに伴う下層雲の発生という、オホーツク海・北極圏における特徴的な季節性に関わる大気海洋（海氷）相互作用について探求した。夏季のオホーツク海では下層雲－海面水温フィードバックにより、海面水温が低く維持されることが明らかとなった。また、冬季のオホーツク海に特徴的なのは、その海氷変動が北極海・東シベリア海における先行する夏の海氷変動と連動している、ということである。気圧偏差の年平均で定義した北極振動（Annual AO）という新しい視点から、北極域・オホーツク海域における、海域・季節を越えてリンクするフィードバックシステムの解明を進めた。
- 日本学術振興会：科学研究費助成事業研究期間 : 2011年 -2013年代表者 : 深町 康, 大島 慶一郎一般に、海氷厚の係留観測には特殊な測器である超音波氷厚計が用いられる。海洋観測に広く用いられている超音波ドップラー流速計を用いた海氷厚の計測手法の開発を行うために、アラスカ北部の沖合で、これらの両方の測器を含む係留系による観測を実施した。3年間に延べ4箇所で両方の測器で良好なデータを取得することに成功し、一部のデータについて超音波氷厚計の生データから海氷厚を算出した。 本研究の観測で取得された海氷・海洋のデータを用いて、沿岸定着氷の崩壊、ポリニヤにおける過冷却水とフラジルアイスの生成、電磁誘導センサーによる海氷厚計測とレーダーによる海氷漂流速度の計測との比較研究についての成果をまとめた。
- 海氷生産量のグローバルマッピングとモニタリング構築日本学術振興会：基盤研究(S)研究期間 : 2008年06月 -2012年03月代表者 : 大島 慶一郎
- 日本学術振興会：科学研究費助成事業研究期間 : 2007年 -2009年代表者 : 三寺 史夫, 大島 慶一郎, 中村 知裕, 小野 数也, 小埜 恒夫オホーツク海から北太平洋にかけての中層温暖化の実態を明らかにするとともに、熱塩(中層)循環の力学過程とその変動メカニズムの解明、および、その基礎となる高密度陸棚水(DSW)生成過程の解明を目的とした。北西陸棚域で生成されるDSWはここ50年間で約0.1PSU減少し軽くなっており、これが26.8-27.0σ_θでは温暖化シグナルとして現れている。数値実験の結果、DSWの塩分は、気温の上昇(海氷生成量の減少)、降水量の増加、風応力の変動の影響を受けて変動することが示された。一方、低緯度の亜熱帯循環中層では逆に低温化が顕著である。これは、70年代半ば以降、亜寒帯循環が強化され、親潮を通した亜寒帯から亜熱帯への低温水流入量が増加したためである。
- オホーツク海における、海水・海氷・油の流動予測システムの開発日本学術振興会：基盤研究(B)研究期間 : 2005年04月 -2008年03月代表者 : 大島 慶一郎
- 日本学術振興会：科学研究費助成事業研究期間 : 2006年 -2008年代表者 : 江淵 直人, 中塚 武, 西岡 純, 三寺 史夫, 大島 慶一郎, 中村 知裕オホーツク海および北西北太平洋親潮域の高い海洋生物生産性を支えている物質循環のメカニズムを,海洋中層(400~800m)の循環と鉄の輸送過程に注目して,現場観測と数値モデルによる研究を行った.その結果,オホーツク海北西大陸棚起源の鉄分が,海氷の生成とともに作られるオホーツク海中層水によって移送され,千島海峡で広い深度層に分配された後,西部北太平洋に送り出されている様子が定量的に明らかとなった.
- 日本学術振興会：科学研究費助成事業研究期間 : 2005年 -2007年代表者 : 若土 正暁, 大島 慶一郎, 青木 茂, 深町 康, 牛尾 収輝, 木村 詞明本研究の最重要課題は、南極海インド洋セクターで生成する南極底層水の北上流の季節変動を初めて捉えることに成功した現地観測データを解析することにある。我々は、ウエッデル海やロス海とともに南極底層水の主要な生成域の一つであり、これまで南北輸送流量のデータの全く無いアデリーランド沖をターゲットとして、冬季沿岸ポリニア域で生成する南極底層水が陸棚斜面を下降し、やがて西岸境界流としてケルゲレン海台東岸沖の海底近くを北上する海流量の季節変動を直接捉えるための流速計を中心とする係留観測を実施した。これは豪州CSIROとの共同研究として実施したものであり、2003年2月から2005年1月までの2年間にわたる、合計8観測点での流向、流速、水温、塩分、溶存酸素量のデータ取得に初めて成功した。極めて貴重な、そして膨大なデータの解析を現在精力的に進めている。これまでの解析から、特徴的なのは、どこの観測点でも流向が一様であり、ケルゲレン海台東岸沖斜面域では北西流が、遥か沖合の深海域では全く逆向きの南東流が卓越していることである。つまり、この海域では、ケルゲレン海台東岸沖の南極底層流の西岸境界流としての北上流と、その沖合での再循環による南下流の二つの卓越する循環系から構成されていることが分かった。 本研究ではまた、南極底層水生成域である沿岸ポリニア域で形成する海氷の氷厚分布を人工衛星データから検出して、熱収支計算を行うことによって、南大洋における海氷生産量をマッピングすることに初めて成功した。これにより、熱塩フラックスの条件を与えることも可能になった。一方、近年の地球温暖化によると思われる南極底層水の低塩化の原因として、上記マッピング手法により求められた、南大洋最大の海氷生産域であるロス海沿岸ポリニアでの海氷生産量が1990年代から2000年代にかけて約30%減少していることが大きく関わっている可能性を示唆した。 尚、本研究で得られた観測データはぼう大なものであり、より高いレベルでの解析には充分な時間を必要とする。そのため、本研究期間内には必ずしも十分な成果は得られていないが、1,2年以内にはインパクトある研究成果を世界に公表したい。
- 日本学術振興会：科学研究費助成事業研究期間 : 2005年 -2007年代表者 : 安田 一郎, 日比谷 紀之, 大島 慶一郎, 川崎 康寛, 渡辺 豊H19年度においては、昨年度に引き続きロシア船を傭船し、約1週間(2007年8月下旬から9月上旬)にわたり千島列島海域の乱流計を用いた乱流直接観測を行うことができた。本年度観測では、昨年度機器の故障により十分な深さまで観測ができなかったブッソル海況西水道において1日連続観測を行うことができた。また、深い混合が予想されたブッソル海況東水道の1点、昨年度オホーツク海側で実施したウルップ海況太平洋側の3点、及び、ムシル海況4点で1日連続観測を行ったほか、重要な観測点で乱流・CTD観測を行うことができた。これらの観測により、千島列島海域には、一般的な中深層での乱流強度の約1万倍にも及ぶ1000cm2/secを越える鉛直拡散が起きていることが実証された。一方、1日の中でも潮汐流に応じて乱流強度は大きく変化すること、また、場所ごとに大きく変化することが明らかとなった。今後、さらに観測を行うとともに、長期観測データやモデルなどを用いてこの海域全体の寄与を明らかにしてゆく必要があることもわかった。また、これら乱流の直接観測データとCTDで取得された密度の鉛直方向の逆転から得られた間接的に乱流強度を比較した結果、乱流強度1桁の誤差範囲で間接的に乱流強度を見積もることができる手法を開発することができた。さらに、木の年輪から得られた長期気候指標北太平洋10年振動指数PDOに有意な18.6年潮汐振動周期を見出し、潮汐振動と気候との位相関係を明らかにした。これにより、日周潮汐の強い時期に、赤道域ではラニーニャ傾向、日本東方海域では高温、アリューシャン低気圧は弱い傾向になることがあきらかとなった。また、千島列島付近の日周潮汐が強い時期に表層の層厚が薄くなり、それが日本南岸まで伝搬することに関連して、本州南岸沿岸海域の栄養塩が上昇する傾向があることが明らかとなった。このように、千島列島付近および亜寒帯海域の潮汐混合は、広く北太平洋に大きな影響を与えていることが示唆された。
- 日本学術振興会：科学研究費助成事業研究期間 : 2005年 -2006年代表者 : 三寺 史夫, 大島 慶一郎, 中村 知裕, 小野 数也, 西垣 肇, 内本 圭亮オホーツク海南部の千島海盆は北太平洋中層水の源であり、物質循環においてもキーエリアである。この海域では時計回り循環や渦が卓越しており、それに対応するように中層水にあたる密度では分厚い混合層(低渦位層)が分布している。また、宗谷暖流、親潮、東樺太海流が流れこみ、非常に複雑な海域である。そのような千島海盆の時計回り循環が生ずるメカニズムを、単純化されたモデルと、現実的なシミュレーションを用いて研究した。 単純化されたモデルでは、千島列島を海嶺と考え、その直上での潮汐混合が引き起こす千島海盆内の循環を調べた。まず、局所的混合が小さければ線形解が成立することを確かめた。混合を大きくすると、渦位のインプットは中規模渦による混合領域から消散とバランスし、密度境界面の深さが決まること、海底地形がある場合には、地形に沿った下層の流れ場が上層の波動の特性曲線を変えるため、境界面深度に対して大きな影響を与えること、が明らかとなった。特に、特性曲線が閉じる場合にはロスビー波が海嶺上で捕捉され、注入された渦位をなかなか解消されず、境界画が大きく深まる。地形に沿った下層の流れ場が上層の波動の特性曲線を変えるため、境界面深度に対して大きな影響を与えること、が明らかとなった。特に、特性曲線が閉じる場合にはロスビー波が海嶺上で捕捉され、注入された渦位をなかなか解消されず、境界画が大きく深まる。地形に沿った時計回り循環は潮汐残差流で生じるので、本研究で述べた境界面深化のメカニズムは現実にも起こっている可能性が高い。 現実的な高解像度モデルでは、風成循環の影響を調べた。東樺太海流の一部がサハリン島南東端で東方へ離岸し続流を形成しており、続流の南方に千島海盆全体にわたるような時計回り循環が生じていた。また、宗谷暖流が知床半島から流出するときには暖水渦が形成していた。宗谷暖流は大きく季節変動するため、それに伴って暖水渦も秋に生じた。これらは観測結果と良い対応を示す。 このように、千島海盆はオホーツク海内の限られた海域ではあるが、内部潮汐による熱塩循環と風成循環が相俟って時計回り循環が作られている複雑な海域であることが明らかとなった。
- 日本学術振興会：科学研究費助成事業研究期間 : 2002年 -2004年代表者 : 石田 邦光, 大島 慶一郎南極昭和基地沖を観測対象に、昭和基地で受信された数多くのMOS-1/1b・MESSR画像データから、アイスバンドの分布およびスケール等の諸特性を抽出し、ECMWFにおける風のデータを使って、それらの特徴と風の関係を統計的な視点から調べた。その結果、アイスバンドの諸特性と風との間には高い相関関係が見られた。先ず、次の重要な2点を挙げることができる。・アイスバンドの形成およびバンドスケールの決定には、アイスバンドが形成されるまでの数日間の風の履歴が強く影響している。・アイスバンドの長軸方向は、数日間の風の履歴が強く影響しており、風向の右側70〜90度に分布する。そして、これらの結果は、アイスバンドの形成においてWave Radiation Theoryをある程度支持するものであるといえる。さらに、次の貴重な観測結果を得た。・アイスバンド域におけるバンドスケールの冬から夏にかけての減少は、冬から夏に向かって生じる氷盤の融解に伴う氷盤スケールの減少のほかに、同様の季節変化を示す風速の減少が関係していることが示唆された。・off-ice windの条件下で、バンドスケールは海氷域内部から氷縁に向かって減少していた。特にこの傾向は、バンド幅において顕著であった。 また、南極海のような季節海氷域では、海氷融解は大気より開水面(海洋混合層)に与えられる熱によってのみ行われるとする、簡略化した海氷・海洋結合モデルを提出した。このモデルは非線型の力学系モデルに帰着される。この系では、変数である海氷密接度(C)と混合層水温(T)の関係が初期値には関係なくある線に収束することが示され(CT-relationship)、東経25゜-45゜セクタにおける夏期観測のCT-plotをよく説明する。CT-plotは、海氷の融解が開水面を通して入ってくる熱によってのみ起こる簡単な海氷-海洋上層結合モデルによって説明できる。 この関係を使って、海氷海洋間のバルク熱交換係数の値(1.2×10-4ms-1)を求める、という新しい手法も提案した。モデルでは、開水面から入った熱のうち、どの程度融解に使われるかは海氷密接度の関数として決まる傾向を示しており、典型的な南極海の融解期ではほぼ0.7-0.9になる。さらに拡散や移流も考慮したモデルで南極の海氷後退をシミュレートし、開水面アルベドフィードバック効果によって、融解期の海氷の年々変動が生じていることを示唆した。
- 日本学術振興会：科学研究費助成事業研究期間 : 2002年 -2004年代表者 : 若土 正曉, 大島 慶一郎, 青木 茂, 長島 秀樹, 牛尾 収輝, 渡邊 修一南大洋(Southern Ocean)は、その地理的特徴(氷大陸である南極大陸をぐるりと取り囲み、低緯度側で太平洋・大西洋・インド洋の世界三大洋と直接交流をもつ)から、地球規模の気候に重要な役割を果たしている。特に、世界最大の季節海氷域(冬季海氷である南極海(Antarctic)は、北大西洋深層水とともに世界の海洋深層大循環の起源である「南極底層水」を形成する海としても広く知られている。このように、南大洋・南極海は地球科学的に非常に重要な海であるにも関わらず、これまで観測が極端に少なく、地球上で最大のデータ空白域の一つになっている。南極底層水は、南極大陸沿岸域の主にウェッデル海、ロス海、アデリーランド沖で生成し、世界三大洋の深層を循環していくと言われている。地球規模の気候に果たす海洋大循環の役割の重要性を考えると、この南極底層水の循環、特にその低緯度への輸送流量を評価することは極めて重要である。近年、欧米の研究者達によって、ウェッデル海やロス海における底層水形成に関する本格的な観測の実施計画が進められている。 本研究は、もう一つの南極底層水生成域であり、インド洋や太平洋の深層水の起源水域の可能性の高い、アデリーランド沖を中心とした南極海インド洋セクターにおいて、底層水の循環とそれの低緯度への輸送流量を直接測定により評価しようとするものである。冬季沿岸ポリニア域で生成した高密度水が、大陸棚斜面を沈降し、南極底層水として低緯度のインド洋さらには太平洋の深層を循環していく際、その流路上で流速が最大(西岸境界流)となるケルゲレン海台東岸沖に、約30台の流速計(水温塩分計付)からなる係留系(8測点)を2年間設置することによって、そこを通過する底層水の低緯度への輸送流量とその季節変動を捉える。 これら南極大陸を取り囲む三つの起源域で生成する「南極底層水」の低緯度への輸送流量を、国際協力による同時期観測によって直接評価することが出来れば、それら観測値を海洋大循環モデルに組み込むことによって、より信頼性の高い「世界の海洋大循環」像が我々にもたらされることになるはずである。 本研究は、豪州CSIROとの共同で約2ケ年にわたる流速計・水温塩分計からなる係留系観測(8測点)とそれらの設置および回収の際の通常海洋観測を実施し、観測のほとんどすべてが成功裡に終了した。特に、2ケ年にわたる係留系観測では30数基の観測機器のうち、1基を除くすべてにおいて完全な形でのデータを取得することができた。これは、勿論世界初のものであり、極めて貴重なデータ取得に成功したといえる。従って、本研究の主な目的は完全に達成することができた。これら貴重なデータ解析を中心とする研究展開を目的とする科学研究費予算申請「南極海インド洋セクターにおける海洋循環と南北熱塩輸送の量的把握」(平成17年-平成19年;合計15,100千円)が、幸いにも採択されたので研究を進めていきたいと考えている。
- 海氷・海洋結合系におけるアイスアルベドフィードバック日本学術振興会：基盤研究(C)研究期間 : 2000年04月 -2003年03月代表者 : 大島 慶一郎
- 日本学術振興会：科学研究費助成事業研究期間 : 1997年 -1999年代表者 : 河村 俊行, 大島 慶一郎, 白澤 邦男, 石川 信敬, 榎本 浩之海氷は極域の大気-海洋間の相互作用に多大な影響を及ぼす。これまで海氷上の積雪は断熱材としての作用をし、海氷の成長を抑制すると考えられてきた。しかし、南極の多雪域では、積雪は海氷の成長を促進しており、その特異な成長に積雪が大いに関与していることが示唆された。 この様式での成長過程とその普遍性を解明するため、以下の観測を行った。 (A)海氷の構造・物理的性質・成長過程の研究 海氷の成長過程を解明するために、採取した海氷と積雪を解析し、その構造・塩分・酸素同位体比等の諸性質を調べた。特に、オホーツク海・サロマ湖では長年にわたり試料採取・観測を行った。バルト海では時間的に密な試料採取を行い、低塩分水の影響を調査した。また、North Water Polynya出現海域・ロス海の海氷も調査した。更に、実験水槽での再現実験も行った。その結果、海氷成長に寄与する積雪の割合はオホーツク海・バルト海でそれぞれ約8,2〜26%と見積もられた。ロス海の海氷で、積雪の融解水が再凍結してできたsuperimposed iceによる成長が認められた。 (B)海氷の放射特性の研究 サロマ湖とバルト海フィンランド湾を実験フィールドにして、海氷が成長・消滅する12月から4月にかけて、海氷の透過・反射・吸収特性の氷厚・海氷構造依存性について現場観測を実施した。また、実験室内で制御した気象条件下で海氷を成長させ、人工太陽光を用いて海氷の放射特性を求めた。本研究では海氷の放射特性が氷厚だけではなく、むしろ海氷の内部構造に大きく依存することを解明した。 (C)衛星による海氷および海氷上積雪の諸量の推定方法の検討 航空機搭載マイクロ波放射計AMR(Airborne Microwave Radiometer)の観測結果をもとに、海氷の氷厚を推定するアルゴリズムの開発している。海氷の厚さと積雪の深さを反映するパラメータが、密接度100%の海氷タイプの判別に有効であることが分かった。また、このパラメータと氷厚・積雪深の関係を推定した。
- 日本学術振興会：科学研究費助成事業研究期間 : 1997年 -1999年代表者 : 大島 慶一郎, 深町 康, 浮田 甚郎夏季南極海海氷域の熱収支から、大気からの熱のインプットはアルベドの違いから海氷面より開水面からのほうがずっと大きく、海氷融解は開水面からの熱が海氷を側面と底面から融かしていく過程が重要であることが定量的に確かめられた。海氷の融解が大気から開水面に入る熱によってのみで行われると成り立つ、(海氷融解率)=(開水面の割合)*(大気からの熱的外力)という単純な関係が、現実のデータ(マイクロ波放射計による海氷密接度データ)からも示唆された。 人工衛星による海氷データを用いて調べたところ、ほとんどの年で融解最盛期の12月と結氷期の4月における海氷密接度のアノマリーの空間分布はよく一致していた。12月は海氷分布の年々変動が最も大きい月でもあるので、この時期に大気から海洋上層中に与えられる熱は、年により大きく異なることになる。解析の結果、融解期における海氷の多少が、海洋上層へ与えられる熱の多寡を生み、それが海に記憶されて、海氷が一度消滅した後、結氷期にも反映されることが示唆された。 海氷の融解は開水面から海洋混合層に入った熱によってのみ起こるとする簡略な海氷・海洋結合モデルを提出した。モデルから、海氷密接度と混合層の水温との関係は、あるカーブに収束することが示され、昭和基地沖やロス海での観測をよく説明している。また、このモデルは南極の子午面方向の海氷後退をある程度説明しえ、第0近似的には海氷融解は1次元バランスで決まっていることも示唆される。さらにモデルを2次元に拡張し、風による移流の効果(力学効果)を組み入れると、「海氷を発散させる風が卓越し海氷密接度が減じると、開水面を通しての熱がますます増大し、さらに密接度が減じる」というアイスアルベドフィードバック効果が表現でき、この効果が年々の海氷後退の違いをもたらしていることが示唆された。
- 日本学術振興会：科学研究費助成事業研究期間 : 1997年 -1998年代表者 : 中塚 武, 深町 康, 大島 慶一郎本科研費を用いた研究は、主に次の2つの部分からなる。一つは、硝酸塩の窒素同位体比を用いた海洋表層における生物活動と水塊混合の履歴の解析であり、もう一つは、栄養塩の精密濃度測定に基づく中・深層水の移流混合過程の解析である。前者には、主にべーリング海やアラスカ湾で日本・カナダ間の往復貨物船により周年的に採取された表層水試料を用い、後者には、オホーツク海南部海盆域でロシア船を使って行われたCTD/RMS観測の試料を用いた。当科研費の大部分は、それぞれ窒素同位体比、栄養塩濃度の測定用の真空ラインの製作、栄養塩自動分析計の購入に充てられた。硝酸塩の窒素同位体比を用いた表層水の混合過程の解析では、硝酸塩の濃度-同位体比プロットにより、ベーリング海においては、アラスカ湾中央部よりも夏季の表層水の鉛直混合が活発である可能性が示唆された。栄養塩の精密測定による中・深層水の移流混合過程の解析では、特に硝酸塩とリン酸塩の濃度を用いた新たな化学量であるN^*(=([NO_<3->]-16・[PO_4^<3->]+2.9)・0.87)が、水塊の起源と混合過程の追跡に極めて有効であることを見いだした。このN^*は、通常は海中では変化せず一部の海域や海底泥中のみでおこる脱窒や窒素固定によってそれぞれ減少・増加する「準保存量」である。このN^*の分布をオホーツク海南部全体で詳細に求めたところ、海底・大陸斜面付近に底泥での脱窒によると見られる低い値を観測しただけでなく、中層(水深200-400m付近のいわゆる低渦位層)に沿って、大陸斜面上部からつながる顕著な極小値の貫入が認められた。これは北太平洋中層水の起源となるオホーツク海中層水が、「オホーツク海北西部の大陸棚(=低N^*域)で形成されて水平流出した水塊であること」の証明を与えるものである。
- 日本学術振興会：科学研究費助成事業研究期間 : 1997年 -1997年代表者 : 大島 慶一郎, 水田 元太, 深町 康, 竹内 謙介, 若土 正曉オホーツク海冬季の現場での海氷・海洋データは今までほとんど得られていなかった。1996,1997,1998年2月の3年にわたって海上保安庁の砕氷船「そうや」により、オホーツク海南西部において、海氷の諸量及び海洋構造の観測を行った。海氷の厚さは1996,1998年は平均20〜30cm程度であったが、1997年は60cmあり、年によって大きく異なることがわかった。従って、衛星データから得られる密接度のデータから海氷の量的な評価を行うことは大きな問題がある。また、この氷厚の違いが何によるのかは今後の興味深い課題である。海氷アルベド(日射に対する反射率)は1996,1997年の観測から、オホーツク海では氷状によらずほぼ0.6〜0.65程度であることがわかった。 1997年2月の海氷域での海氷観測データから以下のことが明らかになった。オホーツク海では冬季、結氷温度で一様な海洋混合層が300m以上に及ぶ場合がある。これは、海氷生成による鉛直対流だけでなく、南風による岸でのエクマン収束のようなmechanicalな効果によることも示唆される。このような厚い混合層を持つ水塊は東カラフト海流として海底斜面に沿ってカラフト沿岸沿いを南下し、それに伴って厚い海氷も南へ運ばれる。移流された海氷の一部は風で周辺の暖水域へ運ばれ、初めはそこで融解され表層を低塩化するが潜熱を奪うことによって、やがてそこを結氷温度まで下げ、新生氷生成域に転じさせることになる。 現在、これらの海氷・海洋データから海氷の生成・移動・融解が熱塩収支過程に及ぼす役割を解析中である。一方、1998年2月の「そうや」の観測では、海氷上とその風下でのゾンデ観測を行い、海氷による大気の変質という観点での解析も現在行っている。
- 日本学術振興会：科学研究費助成事業研究期間 : 1995年 -1997年代表者 : 池田 元美, 日比谷 紀之, 大島 慶一郎, 安田 一郎全地球的な気候変動において、変動の時間スケールが長くなればなるほど、海洋の果たす役割りは増す。特にグリーンランド海で形成される深層水は、全海洋の循環、すなわちコンベアベルトの源になっており、気候変動にもっとも大きな影響を与えている。グリーンランド海には北極海から海氷を含んだ低塩分水が流れ込む。低塩分水が表層を覆っている場合は、冷却されてもその密度は下層の高塩分水より大きくなれない。しかし冷却が非常に強くなると海氷が生成され、それに伴って高塩分水がはきだされるので、下層より密度が大きくなることが可能になる。このように、淡水と塩分が深層水形成の重要なパラメータであることがわかる。いっぽう鉛直混合が起こる海洋は水平方向に一様ではなく、10kmから100kmのスケールをもつ中規模現象が存在している。それに伴って鉛直成層の強弱分布ができ、成層の弱い部分で鉛直混合が起きやすくなっている。高密度水は陸棚上でも海氷生成に伴って形成される。陸棚で形成された高密度水は陸棚斜面に沿って沈降する。 本研究においてとられた手法は、海氷海洋結合モデルを構築し、その中で海氷生成に伴う高密度水の形成と分布を再現することであり、次に鉛直混合、中規模現象、陸棚上の高密度水形成、内部重力波などのプロセスを解明するモデルを用いたプロセス研究であった。結合モデルはラブラドル海とオホーツク海に応用された。両方の場合とも海氷の季節変化を再現することができ、それとともに陸棚上に高密度水が形成された。非静水圧モデルを用いて、海氷下での鉛直混合プロセスを調べたが、開氷面の大きさ、分布、運動によって、高密度水の性質が異なることが示された。中規模渦を解像する陸棚モデルを用いて、海流の不安定のために中規模現象が作られ、陸棚と沖での海水混合を支配していること、そしてこの混合によって陸棚上の高密度水が広がっていくことを示した。鉛直高解像度モデルを用いて、内部重力波が海洋の混合に重要である可能性を示した。これらの研究成果は、深層水形成に関わる重要なプロセスを解き明かし、これからの海洋モデルを用いた気候変動予測に資することができた。
- 日本学術振興会：科学研究費助成事業研究期間 : 1995年 -1997年代表者 : 若土 正曉, 河村 俊行, 深町 康, 大島 慶一郎, 日比谷 紀之ポリニヤ(Polynya)とは、広大な海氷原の内部(外洋ポリニヤ)や沿岸付近(沿岸ポリニヤ)に定常的に依存する大規模な開水面或いは疎氷域のことをいう。そこでは、大気と海洋の間で熱や各種ガスの交換が活発に行われる。厳冬期になると本来なら海氷で広く覆われてしまうはずの極域海洋において、何故特定の海域だけに開水面が形成し、そのまま維持されるのか、またそれらポリニヤが大気や海洋に果たしている役割は何か、という問題に対する理解を深める事が本研究の目的である。 我々は、研究海域として南極昭和基地周辺とオーストラリア南方の南極沿岸域(アデリーランド沖)を取り上げた。前者では、ポリニヤの成因として海洋よりも大気の影響の方が大きいこと、それも大陸からの斜面下降風(カタバ風)野影響を強く受けるところと、もっと広くシノプテイックな大気場が支配的なところと場所によって異なる事がわかった(Ishikawa et al.,1996)。 一方、後者のアデリーランド沖では、1995年1月と1996年1月の2回の海洋観測とその間(1995年1月から1996年3月まで)の大陸棚斜面での流速計係留観測を行った。この海域は、南極周辺で海氷密接度の最も低いところとして、よく知られている。我々の観測結果から、以下の事が明らかになった。 (1)底層付近に低温、低塩、高酸素、低珪素、高フロンの性質をもつ水塊が2年とも存在した。この水塊の性質は、比較的若い水に特有のものである。 (2)一年を通じて流速が最も大きく、流速や水温の変動の最も大きかったのは、海底付近に設置された流速計のデータであった。また、流速と水温の季節変動の最も大きかったのも、その海底付近に設置した流速計によるものであった。 (3)この海底付近の流速は、8月から12月にかけて増大し、それにともなって水温低下も起こった。 (4)この海域は、南極底層水の生成域の一つであり、その源はすぐ東側の沿岸ポリニヤ域であり、その生成は冬から春にかけての時期にのみ起こっている。
- 日本学術振興会：科学研究費助成事業研究期間 : 1996年 -1996年代表者 : 大島 慶一郎, 深町 康, 河村 俊行, 若土 正曉オホーツク海では冬季海氷が覆われる時期での海氷及び海洋の観測は過去にほとんど行なわれていない。1996年2月と1997年2月に海上保安庁の砕氷船「そうや」により、オホーツク海南西部において海氷域での海氷・海洋観測を行なった。特に海氷に関しては今まで計られていない海氷の厚さ、アルベド(日射に対する反射率)の観測を行った。海氷の厚さは1996年では5〜30cmで平均20cm程度、1997年では20〜80cmで両年で大きな違いがあった。今後、この違いは何によるのかを衛星データと組み合わせた解析を行なう予定である。オホーツク海の海氷のアルベドは平均して0.59と推定された。ただし、ダ-クニラスのアルベドは約0.1という値となり、この低い値はニラスの結晶構造によることも示唆された。海洋構造の観測から、冬季のオホーツク海南西域では北方より水が流れていること、またその冬季の鉛直混合対流は300m以上にも及ぶことが観測された。また、氷縁域ではまず海氷が移流され融解して海水が低温低塩化し、それが現場結氷を生じさせる状況をつくっていることが明らかになった。 マイクロ波放射計データ(DMSP SSM/I)から海氷の密接度だけでなく海氷の種類(Ice Type)の情報を用いて、海氷の結氷域に注目することによって海氷の特徴的な変動過程を調べた。氷縁域での広範囲に及ぶ結氷は、大気場の擾乱により海氷域が急速に後退した後に起こり、氷縁結氷はそのような急速に後退した海氷域を短期間でリカバーすることがわかった。しかし、氷縁域での広範囲な結氷により、新しく海氷域が拡大することはない。比較的長い時間スケールでみた海氷域拡大は、主に風による移流が要因であることが示唆された。年による海氷の拡がりの違いも風による移流によって説明しうる場合が多い。但し、そうでない場合もあり、海がどう影響しているかなどが今後の課題である。
- 日本学術振興会：科学研究費助成事業研究期間 : 1994年 -1994年代表者 : 大島 慶一郎南極昭和基地周辺の定着氷における海氷・海洋・気象データによる解析から、南極定着氷における海氷の融解は底面融解と内部融解による部分が大きく、表面融解は小さいことが示された。これは表面融解が卓越する北極とは著しく異なる結果である。底面融解は、海洋上層からの熱の他に、海氷下面に生育するアイスアルジ(珪藻)が日射を強く吸収することによる効果が非常に重要であることが示唆された。一方、内部融解は海氷上の積雪量に依存するとともに、海氷の性質の非一様性が重要な要因であることが示された。現在、一次元の熱伝導方程式を用いて海氷融解モデルを作成している。境界条件を単純化し、日射等の外力項を固有関数で展開することによって解析解を求めることができた。この解析解でも内部融解・底面融解が南極定着氷域で卓越することが示される。今後はより一般的な場合について、数値計算によって調べる予定である。 流氷域での海氷融解機構を調べる目的で、「しらせ」での南極往路において取得された、連続ビデオ観測による海氷密接度・氷板の大きさ・氷厚のデータと表層水温塩分連続観測・XBTデータを解析した。これらの解析結果によると、南極の沿岸より100Km以上離れている外洋域では第0近似的には海氷融解はローカルなバランスで決まるのに対して、沿岸に近い海域では水平移流の効果が大きいことがわかった。これらの結果に基づいて簡単な流氷域における海氷融解モデルを提出した。モデルでは、まず日射により海面に熱が入りそのうちの何割かが海氷融解に使われる、という定式化をしている。海面に入る熱が側面融解と底面融解に使われる割合を海氷密接度と海氷板の大きさの関数として表すと観測結果をある程度説明しえることがわかった。
- 日本学術振興会：科学研究費助成事業研究期間 : 1992年 -1994年代表者 : 河村 俊行, 榎本 浩之, 大島 慶一郎, 若土 正暁近年注目されている気候変動の研究にとって、極域での大気-海洋相互作用の正確な評価が不可欠となっている。特に実測データの著積が強く要望されているが、観測は地域的・季節的に非常に限られたものとなっている。日本南極地域観測隊ではWCRPの一環として南極気候研究計画を実施し、その後半の1990〜92年に昭和基地のあるリュツォ・ホルム湾において集中的な「大気-海氷相互作用」の観測を行った。2年間連続した越冬観測で得られた現場データと衛星観測の解析から、同湾の海洋および海氷について、以下の事が明らかになった。 (1)南極沿岸域では、南極冬季水の厚さが秋季に最大、夏季に最小となる。 (2)この季節変化は風の季節変化に伴うエクマ収束の変化によると考えられる。 (3)定着氷下の流れは、定着氷縁の位置に大きな影響を受ける。 (4)湾内の定着氷の厚さは、積雪深の増大に対応して大陸から離れるに従って増大していた。 (5)積雪の多い地域での海氷は、厚い積雪と海氷のため冬の間でも殆ど成長していないが、夏の時期に極めて大きな氷厚の増大が認められた。これは、積雪の寄与により、海氷が上方に成長したことによる。 (6)マイクロ波放射計のデータが、湾内の定着氷の表面融解の検知に有効であることが分かった。 (7)マイクロ波放射計データの解析から湾沖の海氷分布は、年により夏季に収束する様子が見いだされた。 (8)湾沖の大利根水路は、コスモノートポリニヤからの複数の低密度海氷域の系列の延長上に位置することが衛星画像より見いだされた。
- 日本学術振興会：科学研究費助成事業研究期間 : 1993年 -1993年代表者 : 大島 慶一郎第31・32次日本南極観測隊(1990〜1992)による通年の海洋観測データから、南極沿岸海洋場は、非常に顕著な季節変化をすることが明らかになった。すなわち、南極冬季水の厚さが秋季に最大(約500m)、夏季に最小(300〜400m)になる。熱塩収支解析によると沿岸域は、秋に非常に大きな負の塩分フラックスがあり、これはローカルなバランス(海氷の融解や降雪)では説明できない。また、一年を通じて上層の一様温度層内は、塩分的には成層している。これらのことから、秋に低塩の水が移流されて上層に蓄積されたと解釈できる。 これらの沿岸域での季節変化は大気場の季節サイクルによることを提出した。南極沿岸域は東風が卓越風であるが、これが極域大気循環場の季節変化に伴って夏季に著しく弱まり、秋季に強まる。秋季東風が強まると、沿岸域へのエクマン輸送が増加し、上層の南極冬季水の厚さが増加する。一方、夏季は東風が弱まるので南極冬季水の厚さが減少する。実際に推定される風の応力を計算するとこの機構は定量的にも説明し得る。 南極沿岸の海洋場はこれら傾圧的応答の他に順圧的応答も重要であることが、昭和基地の潮位データ解析から示唆された。また、南極沿岸流が大気場の季節変化あるいはシノプティックな変化に対して、顕著な応答をすることが、潮位やアルゴス漂流ブイの解析から示された。さらにNOAAの赤外線映像から導き出した海氷漂流ベクトルのパターンから、この沿岸流は海底地形に強く制御されることが明らかになった。特に、沿岸より張り出した海底のリッヂは海洋・海氷場に大きな影響を与える。
- 日本学術振興会：科学研究費助成事業研究期間 : 1993年 -1993年代表者 : 若土 正曉, 深町 康, 大島 慶一郎本研究の目的は、海象データの解析、人工衛星からの画像データによる氷状観測、さらには数値実験などによって、日本海底層水の形成機構を明らかにすることである。特に今回初めて得られたヨーロッパの地球資源衛星(ERS-1)からの合成開口レーダ(SAR)画像をはじめ、AVHRRの可視・赤外画像などを解析することによって、日本海底層水形成に本質的な役割を果たしていると考えられている、間宮海峡やシベリヤ沿岸沖の海氷生成過程についての定量的な情報を得る。幸いにも、今回解析したデータの中には、明らかに気象条件の異なる場合(強風と微風)が含まれていたので、両者を比較検討することによって、以下のような風による海氷過程の違いを観測することが出来た。まず、北風の強い時には、シベリヤ沿岸沖に形成した沿岸ポリニヤ域で、急速なフラジルアイスの生成と風による風下への吹き流しが大規模に起こっていることが観測された。一方、風がほとんど無かった時には、漂流ベクトルを求めた画像解析によって、氷が5〜15km/dayの速度で北向きに移動していることが観測された。この微風条件下にもかかわらず北向きの漂流は、連続性から、その補償流として、やや深い層を南向きに流れるプリュームが存在してるはずであることを示唆している。このプリュームは海氷生成にともなって排出されるブライン(高塩分水)によってもたらされていると考えられる。2層モデルによる数値実験を行った結果によると、上層の北流に対して、下層では、地球の自転効果によるシベリヤ沿岸に沿った南下流が存在することを示唆した。その南下流は、沿岸ポリニヤ域での効率のよいフラジルアイス生成にともなうブライン排出効果によって促進されることもわかった。本研究によって、定性的ではあるが、間宮海峡、特にシベリヤ沿岸沖に形成した沿岸ポリニヤ域の急速海氷生産にともなうブライン排出が、日本海底層水形成に大きく寄与していることが示唆された。
- 日本学術振興会：科学研究費助成事業研究期間 : 1993年 -1993年代表者 : 若土 正曉, 長島 秀樹, 深町 康, 大島 慶一郎本研究では、時間的にも空間的にも最もグローバルな水循環系の一部を占める極域海洋における氷山融解過程を明らかにするために現場観測と水槽実験を試みた。 (1)現場観測;研究分担者大島は第32次日本南極地域観測隊(1990〜1992)に参加し、ACR(南極気候研究)の一貫として、昭和基地周辺の海洋・海氷観測を実施した。そのうち、リュツォ・ホルム湾の定着氷に捕捉された氷山近傍で実施したCTD観測において、水温・塩分の鉛直プロファイルがステップ(階段)構造を示していることを見い出した(Ohshima et al.,1994)。これらT,Sのステップ構造は氷山が喫水している範囲内の深さにおいてのみ見られた。典型的なステップの厚さは20〜30m、その水温差、塩分差はそれぞれ0.05℃、0.05〜0.10%_0であった。このステップ構造は、海洋に対する氷山の何らかの影響を示しており、塩分成層した海洋中での氷山の側壁融解過程に伴って誘起された可能性がある。氷山に限らず、棚氷や海氷などの融解は、夏季の極域海洋における支配的な海洋現象として、その物理過程を調べることは重要である。 (2)水槽実験:海洋中における氷山融解過程の研究は、今までにも極端に単純化した形の水槽実験が試みられている。それらは、密度成層か密度一様か、側壁融解か底面融解かに大きく分類される。そのうち、本研究では、今まで全くなされていない、密度(塩分)成層下における底面融解の物理過程を調べた。アクリル製の水槽(0.2×0.2×0.5m)にNacl溶液を満たし、氷板の融解にともなって生じる対流現象は、シュリーレン法を用いて可視化した。実験結果は以下のとおりである。海水から氷への熱輸送は、氷下面の水中に数層の層形成をともないながらなされる。但し、これら層形成をもたらすための最も重要な条件は、最初に塩分勾配をつくる際の表面塩分に関してその最大密度が結氷温度以下になる塩分値(海水の場合、24.7%_0)以上でなければならない。氷下面の水中に形成する数層(例えば、境界層、二重拡散をともなう対流層など)の物理的特性と熱輸送過程におけるそれらの役割について、初期条件の塩分勾配、水温を変化させて詳細に調べた。二重拡散をともなう対流混合層の厚さは密度成層が小さい程増大し、氷板の融解率は、水温が高い程、密度成層が小さい程増大した。現在、これら実験結果を整理中であり、今後は、理論的考察も加えて、密度成層下における氷板の融解とそれに伴って生じる上方熱輸送過程についてモデル化を計りたい。
- 日本学術振興会：科学研究費助成事業研究期間 : 1991年 -1992年代表者 : 若土 正曉, 藤吉 康志, 大島 慶一郎, 竹内 謙介, 滝沢 隆俊南極大陸のまわりの海は、冬になると海氷で広く覆われてしまう。その中にあって、本研究対象域だけが氷量の極端に少ない状況を毎年繰り返していることが最近の衛星情報によって明らかになった。その原因を探り、広大な海域において展開されているであろう大規模な大気・海洋物理過程の実態を解明することが本研究を始めたそもそもの動機である。 研究対象域が位置する南緯65°付近は、もともと低気圧のたまり場になっており、海洋表面では発散・湧昇が起こり易く、古くからオキアミなどの水産資源の宝庫として知られてはいた。しかし、南極大陸のまわりを取り囲むこの発散・湧昇域は、冬季に海氷で覆われてしまうことなどで観測が困難なこともあって、未だに海洋構造ですらわかっていなかった。本研究では水産庁「開洋丸」や日本南極地域観測隊の砕氷艦「しらせ」など日本の船舶によって得られた海洋観測データを解析し、南極発散線の海洋構造について以下のように明らかにした。 (1)南極発散線は、長径500km規模の南北につぶれた形の低気圧性渦が東西に数個並んだ渦列から構成されている。(2)渦は周極深層水の外洋からの湧昇と沿岸域で海氷生成によってもたらされる低温の高密度水の沈降との重ね合わせによって形成・維持されている。(3)渦列は、ほぼ同じ場所に毎年存在している。(4)特定の渦形成域を定めているのは、沿岸の大陸棚から北向きに延びて存在するridgeにある。(5)渦や南極周極流の蛇行を介して沿岸域と外洋との南北海水交換がなされている。(6)渦列や蛇行を構成しているのは約2,000〜3,000km規模の大きな低気圧性循環である。渦や蛇行を含む全体の大きな循環を通して、熱や塩の東西や南国の循環、さらには湧昇、沈降を含む深層循環がなされている。海氷発散やポリニヤ形成も南極発散線域における海洋循環場での大気-海氷-海洋相互作用の一部物理過程と位置づけられる。
- Ocean circulation and ice-ocean interaction in high latitude研究期間 : 1990年
- 日本学術振興会：科学研究費助成事業研究期間 : 1988年 -1988年代表者 : 小野 延雄, 大島 慶一郎世界の海洋の低層を占める海水は、グリーンランド海と南大洋ウェッデル海盆付近とで生成され、海水からの排出ブラインによる塩分供給が重要な役割を果すと推測されている。本研究の目的は、寒冷海域における深層海水の形成過程を、理論的考察、低温水槽実験、オホーツク海沿岸での冬季現場実験観測、数値実験等により明らかにすることにある。 高密度海水形成機構の研究:低温低塩海水が等密度の高温高塩海水と混合すると、混合水は海水密度の温度・塩分関係が原因でより高密度になる。この密度増加効果はキヤベリングと呼ばれ、低温の海水が関与するほど大きく、また海氷生成を伴うとブラインが排出されるので高密度水の生産効率が一段と高まる。キヤベリングは南極低層水の形成要因とされているが、その効果は水深が増すと減少する。本研究では、海氷から排出されるブラインが下の結氷温度の海水と混合すると、ブラインの結氷温度の塩分依存関係が原因で混合水が過冷却となり、水中氷を生むという新しい高密度海水の形成過程を考えて、その効果の評価を試みた。 低温室内実験:低温室で低温高塩分ブラインを作成して結氷温度の海水に加えて混合し、氷の結晶が生成するのを観察した。種々の条件下で生じる氷の量を、熱量計での潜熱測定によって計測することを現在試みている。また低温送風実験水槽を用いたフラジルアイス生産量の測定は、昨年度に引き続き行った。 オホーツク海沿岸結氷での冬期現場実験観測:前年度、サロマ湖の海氷層を通して人為的に低温高塩分ブラインを流下させ、海氷下面に中空氷柱を生長させた。海氷下に流れがあると、中空氷柱が成長できずに水中氷となって浮上するのが観察された。その際に撮影したビデオを今年度に分析した。今冬は記録的暖冬となり、サロマ湖の氷も薄かったので予定の実験を一部変更して、3月に紋別港内で現場実験観測を実施した。
- 日本学術振興会：科学研究費助成事業研究期間 : 1987年 -1987年代表者 : 小野 延雄, 大島 慶一郎, 滝沢 隆俊, 若土 正暁本研究は氷生成を伴うような寒冷海域において, 全大洋の底層を占める高密度海水がどのように形成されていくかを明らかにすることを目的としている. この目的のために, 低温実験室における水槽実験, 野外における実験観測, モデルによる考察などを実施した. 1.低温実験室における水槽実験:風を吹かせることのできる低温実験水槽(0.4×2.0×0.6m)を用いて, 気温(-10〜30°C)や風速(2〜6m/s)を変えて種々の海氷生成環境を作り出し, 厳寒条件下や強風条件下の氷生成およびブラインの排出過程を調べた. 生じた氷結晶(フラジルアイス)は風下に吹き流されて水面が長期間維持されて, 氷およびブラインの高生産率を持続する. 吹送撹拌によって氷生成を伴いながらも約2度の過冷却が生じているのを測定した. 静かに結氷する場合と比較すると, 数倍の氷生産が起ることを知った. 2.サロマ湖海氷での現場実験観測:1988年2月にサロマ湖(塩湖)の天然結氷を利用した現場実験観測を行った. 湖面結氷とその氷を切り取って作ったプールの薄氷とで氷温と塩分の推移を測定し, 海氷からの塩排出を見積った. また, 湖面結氷に穴を開けてビデオカメラを沈め, 天然海氷の下面から流下するブラインの姿を観察した. 着色したブラインや低温高塩分のブラインを人為的に加え, ブライン流下の様子を可視化して捕えた. ブラインを受けとめる装置を氷の下に入れて注射器でブラインを採取した. ビデオ映像の解析はこれから行うが, 天然海氷の下面から間欠的に流下するブラインや, 海氷下面に垂れ下がるスタラクタイト(中空氷柱)の撮影に成功した. 3.高密度海水の形成モデルの研究:実験結果を考慮しながら流下ブラインと下の海水との混合過程に関する考察を行った. 重い海水を効率よく生産する条件などについて今後は考究したい.
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- 一般教育演習(ﾌﾚｯｼｭﾏﾝｾﾐﾅｰ)開講年度 : 2021年課程区分 : 学士課程開講学部 : 全学教育キーワード : 南極海、北極海、オホーツク海、南極・北極、海洋大循環、気候変動、海氷、温暖化、衛星観測、IPCC