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 共同研究推進部
教授

基本情報

所属

  • 低温科学研究所 共同研究推進部

職名

  • 教授

学位

  • 理学博士(北海道大学)
  • 理学修士(北海道大学)

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

  • 海洋物理学   極域海洋学   Polar Oceanography   Physical Oceanography   

研究分野

  • 自然科学一般 / 大気水圏科学
  • 環境・農学 / 環境動態解析

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

  • 特別編集委員   日本海洋学会(評議員)   Journal of Geophysical Research   日本海洋学会   AGU   American Geophysical Union   Meteorological Society of Japan   Oceanographic Society of Japan   日本気象学会   

研究活動情報

論文

  • Masato Ito, Kay I. Ohshima, Yasushi Fukamachi, Daisuke Hirano, Andrew R. Mahoney, Joshua Jones, Toru Takatsuka, Hajo Eicken
    Journal 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 Sascha
    REMOTE SENSING OF ENVIRONMENT 233 2019年11月 [査読有り][通常論文]
  • Kashiwase Haruhiko, Ohshima Kay I, Fukamachi Yasushi, Nihashi Sohey, Tamura Takeshi
    JOURNAL OF ATMOSPHERIC AND OCEANIC TECHNOLOGY 36 8 1623 - 1641 2019年08月 [査読有り][通常論文]
  • Preusser Andreas, Ohshima Kay I, Iwamoto Katsushi, Willmes Sascha, Heinemann Guenther
    JOURNAL OF GEOPHYSICAL RESEARCH-OCEANS 124 8 5503 - 5528 2019年08月 [査読有り][通常論文]
  • Mensah Vigan, Ohshima Kay I, Nakanowatari Takuya, Riser Stephen
    DEEP-SEA RESEARCH PART I-OCEANOGRAPHIC RESEARCH PAPERS 144 115 - 131 2019年02月 [査読有り][通常論文]
  • Nakata Kazuki, Ohshima Kay I, Nihashi Sohey
    IEEE 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 Tamura
    Journal 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 Takenobu
    ANNALS 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. Dieckmann
    Biogeosciences 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).
  • Yasushi Fukamachi, Daisuke Simizu, Kay I. Ohshima, Hajo Eicken, Andrew R. Mahoney, Katsushi Iwamoto, Erika Moriya, Sohey Nihashi
    JOURNAL 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 Tamura
    IEEE 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 Saitoh
    DEEP-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 Wakatsuchi
    POLAR 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 Eicken
    SCIENTIFIC 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 Kashiwase
    JOURNAL 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. Ohshima
    JOURNAL 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 Nishioka
    JOURNAL 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 Iwamoto
    Geoscience 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 George
    CONTINENTAL 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. Hindell
    NATURE 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. Williams
    JOURNAL 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. Ohshima
    POLAR 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.
  • Daisuke Hirano, Yasushi Fukamachi, Eiji Watanabe, Kay I. Ohshima, Katsushi Iwamoto, Andrew R. Mahoney, Hajo Eicken, Daisuke Simizu, Takeshi Tamura
    JOURNAL 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 Tamura
    JOURNAL 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 Nobre
    DEEP-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.
  • Sohey Nihashi, Kay I. Ohshima
    JOURNAL 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 Wakatsuchi
    JOURNAL 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 Fukamachi
    JOURNAL 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 Ushio
    ANNALS 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 Jones
    ANNALS 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 Eicken
    ANNALS 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. Ohshima
    ANNALS 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 Wakatsuchi
    JOURNAL 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 Hasumi
    JOURNAL 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. Ohshima
    JOURNAL 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 Wakatsuchi
    PROGRESS 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 Nihashi
    PROGRESS 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 Katsumata
    PROGRESS 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 Mizuta
    PROGRESS 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. Ohshima
    PROGRESS 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.
  • Katsushi Iwamoto, Kay I. Ohshima, Takeshi Tamura
    JOURNAL 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. Ohshima
    GEOPHYSICAL 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.
  • Aoki S, Kitade Y, Shimada K, Ohshima K. I, Tamura T, Bajish C. C, Moteki M, Rintoul S. R
    JOURNAL 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 Wakatsuchi
    DEEP-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 Yamaguchi
    JOURNAL 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 Wakatsuchi
    Nature 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. Nihashi
    INTERNATIONAL 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.
  • T. Tamura, G. D. Williams, A. D. Fraser, K. I. Ohshima
    NATURE 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.
  • Sohey Nihashi, Kay I. Ohshima, Noriaki Kimura
    JOURNAL 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.
  • Chris Petrich, Hajo Eicken, Jing Zhang, Jeremy Krieger, Yasushi Fukamachi, Kay I. Ohshima
    JOURNAL 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 Fukamachi
    JOURNAL 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.
  • K. Shimada, S. Aoki, K. I. Ohshima, S. R. Rintoul
    OCEAN 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 Nakasato
    JOURNAL 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]
  • Annals of Glaciology 57 177 - 184 2011年 [査読無し][通常論文]
  • Yasushi Fukamachi, Kay I. Ohshima, Yuji Mukai, Genta Mizuta, Masaaki Wakatsuchi
    Annals 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. Ohshima, Sohey Nihashi, Hiroyasu Hasumi
    SOLA 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 Nagai
    JOURNAL 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. Ohshima
    JOURNAL 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 Sano
    JOURNAL 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 Wakatsuchi
    DEEP-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. Ohshima
    CONTINENTAL 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.
  • Naoto Ebuchi, Yasushi Fukamachi, Kay I. Ohshima
    2010 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 Saitoh
    JOURNAL 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. Ohshima
    ATMOSPHERE-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.
  • Yasushi Fukamachi, Kunio Shirasawa, Anatoliy M. Polomoshnov, Kay I. Ohshima, Ervin Kalinin, Sohey Nihashi, Humfrey Melling, Genta Mizuta, Masaaki Wakatsuchi
    CONTINENTAL 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. Ohshima
    JOURNAL 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 Wakatsuchi
    JOURNAL 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. Ebuchi
    INTERNATIONAL 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. Ohshima
    JOURNAL 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 Wakatsuchi
    JOURNAL 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. Ishima, Genta Mizuta, Yasushi Fukainachi, Masaaki Wakatsuchi
    CONTINENTAL 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 Nihashi
    GEOPHYSICAL 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 Simizu
    JOURNAL 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.
  • Takeshi Tamura, Kay I. Ohshima, Thorsten Markus, Donald J. Cavalieri, Sohey Nihashi, Naohiko Hirasawa
    JOURNAL 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. Ohshima
    JOURNAL 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 Wakatsuchi
    JOURNAL 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 Wakatsuchi
    GEOPHYSICAL 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 Uto
    IGARSS: 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).
  • Kay I. Ohshima, Sohey Nihashi, Eisuke Hashiya, Tomohiro Watanabe
    JOURNAL 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 Masaaki
    Journal of Geophysical Research 111, C09018/,doi:10.1029/2005JC003327 C9 2006年09月14日 [査読有り][通常論文]
  • 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]
  • J Ono, KI Ohshima, G Mizuta, F Yasushi, M Wakatsuchi
    DEEP-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 Wakatsuchi
    JOURNAL 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. Massom
    ANNALS 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 Wakatsuchi
    2006 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 Wakatsuchi
    Journal 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 Wakatsuchi
    JOURNAL 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 Saitoh
    JOURNAL 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).
  • KI Ohshima, Y Fukamachi, T Mutoh, M Wakatsuchi
    JOURNAL 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 Wakatsuchi
    GEOPHYSICAL 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 KI, Jeffries MO, Kawamura T
    JOURNAL OF GEOPHYSICAL RESEARCH-OCEANS 110 C2 2005年02月05日 [査読有り][通常論文]
  • KI Ohshima, S Nihashi
    JOURNAL 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.
  • SHIMADA Yoichi, KUBOKAWA Atsushi, OHSHIMA Kay I
    Journal of Oceanography 61 5 913 - 920 2005年 [査読無し][通常論文]
  • KI Ohshima, S Martin
    JOURNAL OF GEOPHYSICAL RESEARCH-OCEANS 109 C9 doi:10.1029/2004JC002604  2004年09月 [査読無し][通常論文]
  • K Katsumata, KI Ohshima, T Kono, M Itoh, Yasuda, I, YN Volkov, M Wakatsuchi
    JOURNAL 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 Wakatsuchi
    JOURNAL 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.
  • T Toyota, T Kawamura, KI Ohshima, H Shimoda, M Wakatsuchi
    JOURNAL 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 Wakatsuchi
    CONTINENTAL 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 Wakatsuchi
    JOURNAL 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 Wakatsuchi
    JOURNAL 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.
  • J. Meteor. Soc. Japan 81 4 653 - 677 2003年 [査読無し][通常論文]
  • Two layer model of wind-driven circulation in the Antarctic Ocean
    Polar Meteorol. Glaciol. 17 36 - 47 2003年 [査読無し][通常論文]
  • Distribution and formation of Okhotsk Sea Intermediate Water: An analysis of isopycnal climatology data
    Journal of Geophysical Research 108: 3258 doi: 10.1029/2002JC001590  2003年 [査読無し][通常論文]
  • Variability of sea-ice draft off Hokkaido in the Sea of Okhotsk revealed by a moored ice-profiling sonar in winter of 1999
    Geophysical Research Letters 30 doi: 10.1029/2002GL016197  2003年 [査読無し][通常論文]
  • D Simizu, KI Ohshima
    JOURNAL 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.
  • In-situ ice andmeteorological observations in the southern Sea of Okhotsk in 2001 winter:ice structure, snow on ice, surface temperature, and optical environments
    Polar Meteorol. Glaciol. 16 116 - 132 2002年 [査読無し][通常論文]
  • Near-surface circulation and tidal currents of the Okhotsk Sea observed with the satellite-tracked drifters
    Journal of Geophysical Research 107: 3195 doi:10.1029/2001JC001005  2002年 [査読無し][通常論文]
  • Sohey Nihashi, Kay I. Ohshima
    Geophysical 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 Ohshima
    JOURNAL 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 Wakatsuchi
    Journal 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
    JOURNAL 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 Miyakana
    JOURNAL 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.
  • OHSHIMA K I, KAWAMURA T, TAKIZAWA T, USHIO S, MIYAKAWA T
    Antarctica. Journal of Geophysical Research 105 C7 17121 - 17132 2000年 [査読無し][通常論文]
  • Jun Inoue, Masayuki Kawashima, Kay I. Ohshima, Yasushi Fujiyoshi, Ken-Ichi Maruyama
    Journal 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. Ohshima
    Journal 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.
  • Kunimitsu Ishida, Kay I. Ohshima, Takashi Yamanouchi, Hiroshi Kanzawa
    Journal 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 Muramoto
    JOURNAL 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.
  • Ice production and deep-water entrainment in shelf break polynya off Enderby Land, Antarctica
    Journal of Geophysical Research 104 29 771 - 780 1999年 [査読無し][通常論文]
  • KI Ohshima, K Yoshida, H Shimoda, M Wakatsuchi, T Endoh, M Fukuchi
    JOURNAL 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. Jeffries
    AGU, Washington, D.C. 317 - 324 1998年 [査読無し][通常論文]
  • T Kawamura, KI Ohshima, T Takizawa, S Ushio
    JOURNAL 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 Kawamura
    JOURNAL 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 Ono
    Journal 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 NAGANOBU
    JOURNAL 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 Ushio
    Journal 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 OHSHIMA
    JOURNAL 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, Antarctica
    Annals of Glaciology 19 298 - 306 1994年 [査読無し][通常論文]
  • Seasonal variations in ocean structure and current in Ongul Strait, Antarctica, in 1991
    Proceedings of the NIPR Symposium on Polar Meteorology and Glaciology 7 51 - 59 1993年 [査読無し][通常論文]
  • T KAWAMURA, KI OHSHIMA, S USHIO, T TAKIZAWA
    ANNALS 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 Bay
    Proceedings of the NIPR Symposium on Polar Meteorology and Glaciology 6 106 - 125 1992年 [査読無し][通常論文]
  • A note on water exchange under fast ice in Lutzow-Holm Bay, Antarctica
    Proceedings of the NIPR Symposium on Polar Meteorology and Glaciology 4 74 - 80 1991年 [査読無し][通常論文]
  • KI OHSHIMA, M WAKATSUCHI
    JOURNAL OF PHYSICAL OCEANOGRAPHY 20 4 570 - 584 1990年04月 [査読無し][通常論文]
  • M WAKATSUCHI, KI OHSHIMA
    JOURNAL OF PHYSICAL OCEANOGRAPHY 20 4 585 - 594 1990年04月 [査読無し][通常論文]
  • Keigh I. Ohshima
    Journal 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.
  • Keigh I. Ohshima
    Journal 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 2008
    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 ・・・
    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

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

  • 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
  • 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
  • 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
  • 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
  • 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
  • Ocean circulation and ice-ocean interaction in high latitude
    研究期間 : 1990年

教育活動情報

主要な担当授業

  • 極域海洋学特論
    開講年度 : 2018年
    課程区分 : 修士課程
    開講学部 : 環境科学院
    キーワード : 北極海, 南極海, オホーツク海, 海氷, 深層水, 中層水, 気候変動 Arctic Ocean, Antarctic Ocean, Okhotsk Sea, Sea ice, Deep water, Intermediate water, Climate change
  • 地球圏科学特別講義Ⅲ
    開講年度 : 2018年
    課程区分 : 修士課程
    開講学部 : 環境科学院
    キーワード : テーマ1 大気海洋系変動の力学 キーワード:大気テレコネクション、大気海洋相互作用 テーマ2 黒潮の流路・流量変動 -源流域から日本南岸までー キーワード:季節・経年・十年規模変動、局所応答と遠隔応答 Theme 1 atmospheric teleconnection, atmosphere-ocean interaction Theme 2 Kuroshio path and volume transport variations from Luzon Island to the southern coast of Japan keywords: Seasonal, interannual and interdecadal variations, Local and remote responses
  • 大学院共通授業科目(一般科目):自然科学・応用科学
    開講年度 : 2018年
    課程区分 : 修士課程
    開講学部 : 大学院共通科目
    キーワード : 北極、地球観測、衛星リモートセンシング
  • 大気海洋物理学基礎論
    開講年度 : 2018年
    課程区分 : 修士課程
    開講学部 : 環境科学院
    キーワード : 気象学, 海洋学, 大気大循環, 地衡風(流), 温帯高低気圧, 熱帯気象学, 数値予報, 大気境界層, 大気化学, オゾン層, 気候変動, 地球温暖化, 熱塩循環, 風成循環, ポテンシャル渦度, 西岸境界流, 海洋混合層, 潮汐 Meteorology, Oceanography, General Circulation, Geostrophic Flow,  Midlatitude Weather System, Tropical Meteorology, Numerical Weather Prediction, Atmospheric Boundary Layer, Atmospheric Chemistry, Ozone Layer, Climate Change, Global Warming, Thermohaline Circulation, Wind-driven Circulation, Potential Vorticity, Western Boundary Current, Ocean Mixed Layer, Tide
  • 一般教育演習(フレッシュマンセミナー)
    開講年度 : 2018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

大学運営

委員歴

  • 2017年04月 - 現在   日本気象学会   評議員
  • 2005年04月 - 現在   日本海洋学会   評議員   日本海洋学会
  • 2009年05月 - 2015年03月   環境省   知床世界自然遺産地域学委員会委員
  • 日本海洋学会   英文誌副編集委員   日本海洋学会


Copyright © MEDIA FUSION Co.,Ltd. All rights reserved.