研究者データベース

青木 茂(アオキ シゲル)
低温科学研究所 共同研究推進部
准教授

基本情報

所属

  • 低温科学研究所 共同研究推進部

職名

  • 准教授

学位

  • 博士(理学)

科研費研究者番号

  • 80281583

J-Global ID

研究キーワード

  • Physical oceanography   大気海洋相互作用   気候変動   南大洋   

研究分野

  • 環境・農学 / 環境動態解析

研究活動情報

論文

  • Shigeru Aoki, Tomoki Takahashi, Kaihe Yamazaki, Daisuke Hirano, Kazuya Ono, Kazuya Kusahara, Takeshi Tamura, Guy D. Williams
    Communications Earth & Environment 3 1 2022年12月 
    Abstract Melting ice shelves around Antarctica control the massive input of freshwater into the ocean and play an intricate role in global heat redistribution. The Amery Ice Shelf regulates wintertime sea-ice growth and dense shelf water formation. We investigated the role of warm Antarctic Surface Water in ice shelf melting and its impact on dense shelf water. Here we show that the coastal ocean in summer 2016/17 was almost sea-ice free, leading to higher surface water temperatures. The glacial meltwater fraction in surface water was the highest on record, hypothesised to be attributable to anomalous ice shelf melting. The excess heat and freshwater in early 2017 delayed the seasonal evolution of dense shelf water. Focused on ice shelf melting at depth, the importance and impacts of warming surface waters has been overlooked. In a warming climate, increased surface water heating will reduce coastal sea-ice production and potentially Antarctic Bottom Water formation.
  • Kazuki Nakamura, Shigeru Aoki, Tsutomu Yamanokuchi, Takeshi Tamura
    Science of Remote Sensing 100064 - 100064 2022年08月
  • Reishi Sahashi, Daiki Nomura, Takenobu Toyota, Manami Tozawa, Masato Ito, Pat Wongpan, Kazuya Ono, Daisuke Simizu, Kazuhiro Naoki, Yuichi Nosaka, Takeshi Tamura, Shigeru Aoki, Shuki Ushio
    Journal of Geophysical Research: Oceans 127 7 2022年07月
  • Keishi Shimada, Yujiro Kitade, Shigeru Aoki, Kohei Mizobata, Lingqiao Cheng, Kunio T. Takahashi, Ryosuke Makabe, Jota Kanda, Tsuneo Odate
    Communications Earth & Environment 3 120 2022年05月 [査読有り]
  • Nakayama, Y, J. S. Greenbaum, P. Wongpan, K. Yamazaki, T. Noguchi, D. Simizu, H. Kashiwase, D. D. Blankenship, T. Tamura, S. Aoki
    Geophysical Research Letters in press  2022年 [査読有り][通常論文]
  • Kazuki Nakamura, Shigeru Aoki, Tsutomu Yamanokuchi, Takeshi Tamura, Koichiro Doi
    IEEE Journal of Selected Topics in Applied Earth Observations and Remote Sensing 15 3269 - 3281 2022年
  • Manami Tozawa, Daiki Nomura, Shin ichiro Nakaoka, Masaaki Kiuchi, Kaihe Yamazaki, Daisuke Hirano, Shigeru Aoki, Hiroko Sasaki, Hiroto Murase
    Journal of Geophysical Research: Oceans 127 1 2022年01月 
    To quantitatively assess the inorganic carbon cycle in the eastern Indian sector of the Southern Ocean (80–150°E, south of 60°S), we measured ocean surface temperature, salinity, total alkalinity (TA), the partial pressure of carbon dioxide (pCO2), and concentrations of chlorophyll-a (chl a), dissolved inorganic carbon (DIC), and nutrients during the KY18 survey (December 2018–January 2019). The sea–air CO2 flux in this region was −8.3 ± 12.7 mmol m−2 day−1 (−92.1 to +10.6 mmol m−2 day−1). The ocean was therefore a weak CO2 sink. Based on the DIC and TA in the temperature minimum layer, we estimated the change of pCO2 from winter to summer (δpCO2) due to changes in water temperature, salinity, and biological activity (photosynthesis). The spatial distribution of pCO2 in the western part (80–110°E) of the study area was mainly driven by biological activity, which decreased pCO2 from December to early January, and in the eastern part (110–150°E) by temperature, which increased pCO2 from January to February. We also examined the changes in the CO2 concentrations (xCO2) over time by comparing data from 1996 with our data (2018–2019). The oceanic and atmospheric xCO2 increased by 23 and 45 ppm in 23 years, respectively. These changes of ocean xCO2 were mainly driven by an increase in CO2 uptake from the atmosphere as a result of the rise in atmospheric xCO2 and increase in biological activity associated with the change in the water-mass distribution.
  • Daisuke Hirano, Kohei Mizobata, Hiroko Sasaki, Hiroto Murase, Takeshi Tamura, Shigeru Aoki
    Communications Earth & Environment 2 1 2021年12月 [査読有り]
     
    AbstractIce mass loss in the Wilkes Land sector of East Antarctica and the Amundsen and Bellingshausen Sea sectors of West Antarctica has contributed to a rise in sea levels over several decades. The massive continental ice behind the Totten Ice Shelf, equivalent to a few meters of sea-level rise, is grounded well below sea level and therefore, potentially vulnerable to oceanic heat. Here, we present analyses of comprehensive hydrographic observations at the continental slope and shelf break regions off Totten Ice Shelf. We provide robust evidence that the relatively warm Circumpolar Deep Water that originates at intermediate depths in the Antarctic Circumpolar Current is transported efficiently towards the shelf break by multiple cyclonic eddies. We propose that these semi-permanent cyclonic circulations play a critical role in transporting the available ocean heat towards Totten Ice Shelf, and melting it from underneath, thus eventually influencing the global climate.
  • Masahiro Minowa, Shin Sugiyama, Masato Ito, Shiori Yamane, Shigeru Aoki
    Nature Communications 12 1 2021年12月 [査読有り]
     
    Basal melting of ice shelves is considered to be the principal driver of recent ice mass loss in Antarctica. Nevertheless, in-situ oceanic data covering the extensive areas of a subshelf cavity are sparse. Here we show comprehensive structures of temperature, salinity and current measured in January 2018 through four boreholes drilled at a ~3-km-long ice shelf of Langhovde Glacier in East Antarctica. The measurements were performed in 302–12 m-thick ocean cavity beneath 234–412 m-thick ice shelf. The data indicate that Modified Warm Deep Water is transported into the grounding zone beneath a stratified buoyant plume. Water at the ice-ocean interface was warmer than the in-situ freezing point by 0.65–0.95°C, leading to a mean basal melt rate estimate of 1.42 m a−1. Our measurements indicate the existence of a density-driven water circulation in the cavity beneath the ice shelf of Langhovde Glacier, similar to that proposed for warm-ocean cavities of larger Antarctic ice shelves.
  • Nobuo Kokubun, Yukiko Tanabe, Daisuke Hirano, Vigan Mensah, Takeshi Tamura, Shigeru Aoki, Akinori Takahashi
    Limnology and Oceanography 66 10 3740 - 3753 2021年10月 [査読有り]
     
    Ocean circulation plays a key role in structuring marine ecosystems in the Southern Ocean. However, the seasonal dynamics of ocean circulation are poorly understood in the ice-covered continental shelves due to difficulties in conducting observations. We, therefore, investigated spatial and temporal variations in oceanographic conditions and their biological effects on the continental shelf off East Antarctica (35°E–50°E) by deploying conductivity–temperature–depth (CTD) tags on Weddell seals. The seals moved up to 633 km east from the tagging location. We successfully obtained 1254 CTD casts from seven seals. Winter Water (WW) was most prevalent (77.4% of the total data), followed by Supercooled Water (14.2%), Antarctic Surface Water (AASW: 7.4%), Modified Circumpolar Deep Water (mCDW; 0.9%), and Modified Shelf Water (0.1%). During our study period, landfast ice broke up extensively, and the easterly wind was most prevalent during autumn. AASW was observed in the subsurface layer over the shelf in autumn, suggesting that AASW intruded from the surface of off-shelf areas through Ekman transport. Particular water masses (mCDW, AASW, and WW below the AASW) had positive effects on the seals' foraging behavior. These results highlight the importance of easterly wind-driven shoreward intrusion of oceanic surface waters onto the shelf in autumn. This physical process may enhance transport and accumulation of additional prey and increase local prey availability during winter. Such a process may play important roles in the Antarctic coastal marine ecosystems that are influenced by landfast ice.
  • Mi Hae Jeon, Jinyoung Jung, Mi Ok Park, Shigeru Aoki, Tae Wan Kim, Seung Kyu Kim
    Marine Chemistry 235 2021年09月20日 [査読有り]
     
    The Amundsen Sea is the most rapidly melting part of the West Antarctic Ice Sheet, due to increased heat transport by Circumpolar Deep Water (CDW). Tracing CDW and resulting glacial meltwater is important since glacial meltwater may change the water mass properties, leading to the change of the biogeochemical cycles. In this study, in order to investigate the potential for using the humic-like component of fluorescent dissolved organic matter (FDOM) as a tracer for CDW and glacial meltwater in the Amundsen Sea, a hydrographic survey was conducted during the austral summer of 2018 aboard the Korean icebreaker IBR/V Araon. The meteoric water and CDW fractions calculated using the humic-like component (fmw_humic and fcdw_humic) were compared to those using oxygen isotope (δ18O) (fmw) and optimum multiparameter analysis (OMP) (fcdw_OMP), respectively. The fluorescence intensity of the humic-like component varied from 0.007 to 0.021, with higher values in the deeper layer and lower intensities in the surface waters. The range of fcdw_humic (0.5–1.0) was narrower than that of fcdw_OMP (0.1–1.0), indicating that the fcdw_humic values were overestimated due to the remained humic-like C1. To minimize the effect of the remained humic-like C1 on the calculation of CDW fraction, we used newly derived empirical equations (i.e., fcdw_OMP = 105.17 × C1–1.14 for transect 1 and fcdw_OMP = 126.04 × C1–1.41 for transect 2). The CDW fraction calculated using the empirical equations (fcdw_humic_empirical) was in good agreement with the fcdw_OMP. We also found a significant positive relationship between fmw and fmw_humic, indicating that a reasonable method can be applied with a high percentage of explained variance and that fmw can be largely explained by fmw_humic. Our results show that the humic-like component can be a useful tracer for identifying CDW and glacial meltwater in the Amundsen Sea.
  • Yoshihiro Nakayama, Chad A. Greene, Fernando S. Paolo, Vigan Mensah, Hong Zhang, Haruhiko Kashiwase, Daisuke Simizu, Jamin S. Greenbaum, Donald D. Blankenship, Ayako Abe-Ouchi, Shigeru Aoki
    Geophysical Research Letters 48 17 2021年09月16日 [査読有り]
     
    The Totten ice shelf (TIS) in East Antarctica has received increasing attention in recent years due to high basal melt rates, which have been linked to a presence of warm modified Circumpolar Deep Water (mCDW) observed at the ice front. We show that mCDW on-shelf intrusions towards the TIS strengthen when the Antarctic Slope Current (ASC) weakens. This demonstrates that the ASC has a blocking effect and ASC weakening leads to on-shelf intrusions, as proposed by previous observational studies. The interannual variability of the ASC is controlled primarily by atmospheric and oceanic conditions beyond our regional model domain. We further show that heat intrusions onto the continental shelf off the TIS are not influenced by off-shelf warming but are enhanced with coastal freshening, suggesting positive feedback whereby ice melt and freshening upstream could start a chain reaction, leading to increased melt, and further coastal freshening.
  • Kaihe Yamazaki, Shigeru Aoki, Katsuro Katsumata, Daisuke Hirano, Yoshihiro Nakayama
    SCIENCE ADVANCES 7 24 2021年06月 [査読有り]
     
    The southern boundary (SB) of the Antarctic Circumpolar Current, the southernmost extent of the upper overturning circulation, regulates the Antarctic thermal conditions. The SB's behavior remains unconstrained because it does not have a clear surface signature. Revisited hydrographic data from off East Antarctica indicate full-depth warming from 1996 to 2019, concurrent with an extensive poleward shift of the SB subsurface isotherms (>50 km), which is most prominent at 120 degrees E off the Sabrina Coast. The SB shift is attributable to enhanced upper overturning circulation and a depth-independent frontal shift, generally accounting for 30 and 70%, respectively. Thirty years of oceanographic data corroborate the overall and localized poleward shifts that are likely controlled by continental slope topography. Numerical experiments successfully reproduce this locality and demonstrate its sensitivity to mesoscale processes and wind forcing. The poleward SB shift under intensified westerlies potentially induces multidecadal warming of Antarctic shelf water.
  • Masaaki Kiuchi, Daiki Nomura, Daisuke Hirano, Takeshi Tamura, Gen Hashida, Shuki Ushio, Daisuke Simizu, Kazuya Ono, Shigeru Aoki
    Journal of Geophysical Research: Biogeosciences 126 5 2021年05月 [査読有り]
     
    To clarify the effect of basal melting of ice tongues/ice shelf on the CO2 system in the Antarctic continental margin, seawater samples were collected for analysis of dissolved inorganic carbon (DIC), total alkalinity (TA), nutrients, chlorophyll a (chl.a), and oxygen isotopic ratios at the ice front of the Shirase Glacier Tongue (SGT) in Lützow-Holm Bay (LHB), East Antarctica during the summers of 2017 and 2018. At depths greater than ∼20 dbar in LHB, CO2 system parameters were strongly influenced by the dilution effect of SGT basal melting and the inflow of modified Circumpolar Deep Water (mCDW). The distributions of DIC and TA agreed well with an offshore origin of mCDW that flowed beneath the SGT and mixing of basal meltwater and mCDW beneath the SGT. The fraction of SGT meltwater was highest at the station near the ice front. Dilution by SGT basal meltwater reduced the partial pressure of CO2 in the mCDW from 431 to 387 μatm. The water then became a sink rather than source of atmospheric CO2. In the sea surface, DIC and TA were strongly influenced by biological processes. Salinity-normalized DIC decreased with the increase of salinity-normalized TA in accord with 106:16 C:N molar stoichiometry; the chl.a concentration at the sea surface was as high as 31 μg L−1.
  • S. Aoki, K. Yamazaki, D. Hirano, K. Katsumata, K. Shimada, Y. Kitade, H. Sasaki, H. Murase
    Scientific Reports 10 1 2020年12月01日 [査読有り][通常論文]
     
    © 2020, The Author(s). The Antarctic continental margin supplies the densest bottom water to the global abyss. From the late twentieth century, an acceleration in the long-term freshening of Antarctic Bottom Waters (AABW) has been detected in the Australian-Antarctic Basin. Our latest hydrographic observations reveal that, in the late 2010s, the freshening trend has reversed broadly over the continental slope. Near-bottom salinities in 2018–2019 were higher than during 2011–2015. Along 170° E, the salinity increase between 2011 and 2018 was greater than that observed in the west. The layer thickness of the densest AABW increased during the 2010s, suggesting that the Ross Sea Bottom Water intensification was a major source of the salinity increase. Freshwater content on the continental slope decreased at a rate of 58 ± 37 Gt/a in the near-bottom layer. The decadal change is very likely due to changes in Ross Sea shelf water attributable to a decrease in meltwater from West Antarctic ice shelves for the corresponding period.
  • Daisuke Hirano, Takeshi Tamura, Kazuya Kusahara, Kay I. Ohshima, Keith W. Nicholls, Shuki Ushio, Daisuke Simizu, Kazuya Ono, Masakazu Fujii, Yoshifumi Nogi, Shigeru Aoki
    Nature Communications 11 1 2020年12月01日 [査読有り][通常論文]
     
    © 2020, The Author(s). Mass loss from the Antarctic ice sheet, Earth’s largest freshwater reservoir, results directly in global sea-level rise and Southern Ocean freshening. Observational and modeling studies have demonstrated that ice shelf basal melting, resulting from the inflow of warm water onto the Antarctic continental shelf, plays a key role in the ice sheet’s mass balance. In recent decades, warm ocean-cryosphere interaction in the Amundsen and Bellingshausen seas has received a great deal of attention. However, except for Totten Ice Shelf, East Antarctic ice shelves typically have cold ice cavities with low basal melt rates. Here we present direct observational evidence of high basal melt rates (7–16 m yr−1) beneath an East Antarctic ice shelf, Shirase Glacier Tongue, driven by southward-flowing warm water guided by a deep continuous trough extending to the continental slope. The strength of the alongshore wind controls the thickness of the inflowing warm water layer and the rate of basal melting.
  • Shigeru Aoki, Kazuya Ono, Daisuke Hirano, Takeshi Tamura
    Journal of Oceanography 76 5 365 - 372 2020年10月01日 [査読有り][通常論文]
     
    © 2020, The Oceanographic Society of Japan and Springer Nature Singapore Pte Ltd. Antarctic coastal polynyas provide extremely dense water to the global abyss, but the dense water formation process has been poorly monitored in winter. This study developed a new tethered profiler to realize time-series observations of the water column in the Cape Darnley Polynya, East Antarctica. The system successfully obtained temperature and salinity profiles at 10-day intervals from March to November 2017. From March to April, significant cooling and vertical mixing started, while stratification collapsed. Salinity increased rapidly from April to late-May and then gradually increased until October. Salinity development was largely consistent with the cumulative salt increase due to sea-ice production at the initial stage, but not at latter stages, indicating the influence of cross-shelf exchange. These results highlighted the potential of the measurement platform to fill the remaining gap in the global ocean monitoring network.
  • Pat Wongpan, Daiki Nomura, Takenobu Toyota, Tomonori Tanikawa, Klaus M. Meiners, Tomomi Ishino, Tetsuya P. Tamura, Manami Tozawa, Yuichi Nosaka, Toru Hirawake, Atsushi Ooki, Shigeru Aoki
    Annals of Glaciology 61 83 1 - 10 2020年09月24日 [査読有り][通常論文]
     
    Abstract Sea ice, which forms in polar and nonpolar areas, transmits light to ice-associated (sympagic) algal communities. To noninvasively study the distribution of sea-ice algae, empirical relations to estimate its biomass from under-ice hyperspectral irradiance have been developed in the Arctic and Antarctica but lack for nonpolar regions. This study examines relationships between normalised difference indices (NDI) calculated from hyperspectral transmittance and sympagic algal biomass in the nonpolar Saroma-ko Lagoon. We analysed physico-biogeochemical properties of snow and land-fast sea ice supporting 27 paired bio-optical measurements along three transects covering an area of over 250 m × 250 m in February 2019. Snow depth (0.08 ± 0.01 m) and ice-bottom brine volume fraction (0.21 ± 0.02) showed low (0.06) and high (0.58) correlations with sea-ice core bottom section chlorophyll a (Chl. a), respectively. Spatial analyses unveiled the patch size of sea-ice Chl. a to be ~65 m, which is in the same range reported from previous studies. A selected NDI (669, 596 nm) explained 63% of algal biomass variability. This reflects the bio-optical properties and environmental conditions of the lagoon that favour the wavelength pair in the orange/red part of the spectrum and suggests the necessity of a specific bio-optical relationship for Saroma-ko Lagoon.
  • S. Aoki, K. Katsumata, M. Hamaguchi, A. Noda, Y. Kitade, K. Shimada, D. Hirano, D. Simizu, Y. Aoyama, K. Doi, Y. Nogi
    Journal of Geophysical Research: Oceans 125 8 2020年08月01日 [査読有り][通常論文]
     
    © 2020. American Geophysical Union. All Rights Reserved. Recently, a source of Antarctic Bottom Water (AABW) was identified off Cape Darnley at the eastern end of the Weddell-Enderby Basin. However, the behavior and long-term variability of Cape Darnley Bottom Water (CDBW) are not clearly understood. Hydrographic observations from 1974 to 2016 were compared, and a decade-long bottom temperature record was analyzed to clarify multidecadal changes in the CDBW in this region and its downstream influences. In the Cooperation Sea, CDBW spread northwestward with its deepest part reaching to approximately 4,900 dbar. CDBW freshening of 0.001–0.003 decade−1 was revealed. In the Cosmonaut Sea, long-term AABW warming of approximately 0.01–0.03°C decade−1 was prominent in the deep basin, while freshening was detected on the upper continental slope. Spatial patterns suggest that an interbasin deep transport of excess freshwater is carried by CDBW and fed into the Weddell Gyre, which might act as an abyssal freshwater buffer.
  • Kaihe Yamazaki, Shigeru Aoki, Keishi Shimada, Taiyo Kobayashi, Yujiro Kitade
    Journal of Geophysical Research: Oceans 125 8 2020年08月01日 [査読有り][通常論文]
     
    ©2020. The Authors. The climatological structure of the subpolar cyclonic circulation off East Antarctica is delineated with Argo float data from the past decade. Up to 40% of the float profiles in the seasonal ice zone have been without satellite positioning. We refined their position data as following the bathymetry to get appropriate positions in the continental margin. The error of the terrain-following interpolation was estimated by using positioned data to be 23 ± 27 (78 ± 70) km for 90 (390) day period. Profiles with the under-ice period shorter than 360 days are adopted. The float trajectories reveal the extent of the subpolar gyre adjoined to the westward Antarctic Slope Current to its south and the southernmost eastward jet of the Antarctic Circumpolar Current along 4,000 m isobath to its north. The subpolar circulation in the Australian-Antarctic Basin comprises of a series of quasi-barotropic subgyre circulations, which are bounded by bathymetric spurs in the continental slope. The temperature field reveals shoreward excursions of Circumpolar Deep Water associated with the subgyres, effectively supplying heat to the continental shelves. An along-slope temperature variation up to 1°C in 27.7–27.8 kg/m3 σθ indicates an active cross-slope exchange within the layer. Provided the velocity field and the water mass structure, the subsurface water mass exchange is likely accomplished by a combination of topographically controlled mean flow and the eddy transports. Our findings suggest that the bathymetry primarily determines the structure of the subpolar gyre.
  • K. Mizobata, K. Shimada, S. Aoki, Y. Kitade
    Journal of Geophysical Research: Oceans 125 6 2020年06月01日 [査読有り][通常論文]
     
    ©2020. American Geophysical Union. All Rights Reserved. We investigated dynamic ocean topography (DOT) to reveal the oceanic subpolar circulation and possible pathway of modified Circumpolar Deep Water (CDW) in the Indian Ocean sector of the Southern Ocean. Satellite radar altimeter data sets were employed to develop monthly DOT in both open water and ice-covered areas throughout the course of a year. A 0.2° × 0.2° gridded DOT was reconstructed for investigation of the oceanic structure at fine scale. The reconstructed DOT exhibited a “cyclonic eddy train” consisting of the Vincennes eddy, Poinsett eddy, and Sabrina eddies. The relationship between the depth of each eddy and the wind curl showed negative correlations for the Vincennes eddy and Sabrina eddies during wintertime only. Therefore, winds are not the main drivers of the eddies. A hydrographic section in 2017 clearly revealed the vertical structure of the Vincennes eddy. The absolute velocity estimated by the DOT and in situ geostrophic velocity showed equatorward Antarctic Bottom Water (AABW) transport at 6–8 cm s−1 on the western side and poleward transport of CDW at 3–4 cm s−1 on the eastern side of the Vincennes eddy. The net transports of the AABW and CDW layers in the Vincennes eddy were 0.6 ± 0.4 Sv (equatorward) and 0.5 ± 0.36 Sv (poleward), respectively. The order of magnitude of the AABW current speed was confirmed by in situ mooring data. The vertical profiles of absolute velocity suggest the possibility of estimating CDW transport using a satellite altimeter.
  • Yoshihiko Ohashi, Shigeru Aoki, Yoshimasa Matsumura, Shin Sugiyama, Naoya Kanna, Daiki Sakakibara
    Ocean Science 16 3 545 - 564 2020年05月04日 [査読有り][通常論文]
     
    © Author(s) 2020. Subglacial discharge has significant impacts on water circulation, material transport, and biological productivity in proglacial fjords of Greenland. To help clarify the fjord water properties and the effect of subglacial discharge, we investigated the properties of vertical water mass profiles of Bowdoin Fjord in northwestern Greenland based on summer hydrographic observations, including turbidity, in 2014 and 2016. We estimated the fraction of subglacial discharge from the observational data and interpreted the observed differences in subglacial plume behavior between two summer seasons with the numerical model results. At a depth of 15-40 m, where the most turbid water was observed, the maximum subglacial discharge fractions near the ice front were estimated to be &tild; 6 % in 2014 and &tild; 4 % in 2016. The higher discharge fraction in 2014 was likely due to stronger stratification, as suggested by the numerical experiments performed with different initial stratifications. Turbidity near the surface was higher in 2016 than in 2014, suggesting a stronger influence of turbid subglacial discharge. The higher turbidity in 2016 could primarily be attributed to a greater amount of subglacial discharge, as inferred from the numerical experiments forced by different amounts of discharge. This study suggests that both fjord stratification and the amount of discharge are important factors in controlling the vertical distribution of freshwater outflow.
  • Daiki Nomura, Pat Wongpan, Takenobu Toyota, Tomonori Tanikawa, Yusuke Kawaguchi, Takashi Ono, Tomomi Ishino, Manami Tozawa, Tetsuya P. Tamura, Itsuka S. Yabe, Eun Yae Son, Frederic Vivier, Antonio Lourenco, Marion Lebrun, Yuichi Nosaka, Toru Hirawake, Atsushi Ooki, Shigeru Aoki, Brent Else, Francois Fripiat, Jun Inoue, Martin Vancoppenolle
    Bulletin of Glaciological Research 38 1 - 12 2020年 [査読有り][通常論文]
     
    © 2020 Japanese Society of Snow and Ice. Saroma-ko Lagoon, located on the Okhotsk Sea coast of Hokkaido, is seasonally covered by flat, homogeneous, easily accessible and safe sea ice. As such, it proves a very useful experimental site for the study of sea ice processes, the inter-comparison of methods, the testing of equipment, and the training of researchers new to the Polar regions. In this contribution, we describe a physical, chemical, and ecosystem survey at Saroma-ko Lagoon, conducted over February 23-28, 2019 under the auspices of the SLOPE2019 (Saroma-ko Lagoon Observations for sea ice Physico-chemistry and Ecosystems 2019) program. Sea ice cores were collected to examine temperature, salinity, oxygen isotopic ratio, thin sections, and chemical and biological parameters such as carbonate chemistry, CH4, nutrients, chlorophyll a concentrations, and ice algae community assemblage. Broadband and spectral irradiance measurements were carried out above/under the sea ice, and different sensors were inter-compared at close positions and environments. Equipment such as spectrometers, air-sea ice CO2/CH4 flux chamber, and under-ice turbulent heat flux systems were tested for future Arctic and Antarctic expeditions. Finally, an artificial pool was dug into the sea ice to understand the effect of snow particles on ice growth and to compare the gas exchange process over sea ice with an ice-free water surface. Our SLOPE2019 field campaign activities provided useful information for inter-comparison work and future sea ice research in the polar oceans.
  • Kazuki Nakamura, Shigeru Aoki, Tsutomu Yamanokuchi, Takeshi Tamura, Shuki Ushio, Koichiro Doi
    International Geoscience and Remote Sensing Symposium (IGARSS) 4172 - 4174 2019年07月 [査読有り][通常論文]
     
    © 2019 IEEE. This study presents that temporal variations in ice flow velocities for Shirase Glacier and its surrounding landfast ice in Liitzow-Holm Bay, East Antarctica were estimated using synthetic aperture radar (SAR) images acquired by the Phased Array L-band SAR-2 (PALSAR-2) onboard Advanced Land Observing Satellite-2 (ALOS-2) in 2015-2018. The images were analyzed using image correlation. The flow velocity at the 20 km downstream from the grounding line (GL) of Shirase Glacier is mean of 2.32 km a-1 in 2015-2018, which show that flow velocity is considered to be less related to the landfast ice stability. However, the flow velocity around 50 km downstream from the GL show changes related to landfast ice condition. The flow velocity is more increasable of 0.3 km a-1 under the unstable landfast ice condition than the stable landfast ice condition. Therefore, our results show the velocity of Shirase Glacier are associated with the condition of landfast ice stability or instability.
  • Yujiro Kitade, Keishi Shimada, Shigeru Aoki, Kay I. Ohshima
    Oceanography Challenges to Future Earth: Human and Natural Impacts on our Seas 123 - 135 2019年02月15日 [査読有り][通常論文]
     
    © Springer Nature Switzerland AG 2019. All rights reserved. Training research vessel Umitaka-Maru belonging to the Tokyo University of Marine Science and Technology has implemented the research cruises in the Southern Ocean, counting the 19th times in the 2015 cruise. Particularly in recent years, Umitaka-Maru has greatly contributed to various findings in oceanography in the Antarctic Ocean, such as clarification of the bottom water formation and monitoring in Lutzow Holm Bay, cooperating with the Japanese Antarctic Research Expedition. This article reviews such scientific progresses implemented by Umitaka-Maru, and further describes the recent trend of Antarctic Bottom Water from the newly obtained data. Most important findings by Umitaka-Maru are the discovery of the Antarctic bottom water (AABW) formation off the Cape Darnley with a high sea ice production polynya and the subsequent discovery of AABW formation off the Vincennes Bay with a medium sized polynya. The base oceanographic observation for physical and chemical properties has been carried out continuously along 110°E line in Australian Antarctic Basin, providing the monitoring of water mass transformation. Freshening of AABW in Australia-Antarctica Basin has been caught in a series of the observation in recent years, it is suggested that the AABW originating from the Vincennes Bay Polynya gives significant effects on the AABW in the basin. To grasp an effect of freshening of AABW on the global ocean circulation, intense hydrographic observations and deployment of a huge mooring system are now in consideration.
  • Alessandro Silvano, Stephen Rich Rintoul, Beatriz Peña-Molino, William Richard Hobbs, Esmee Van Wijk, Shigeru Aoki, Takeshi Tamura, Guy Darvall Williams
    Science Advances 4 4 2018年04月 [査読有り][通常論文]
     
    © 2018 The Authors. Strong heat loss and brine release during sea ice formation in coastal polynyas act to cool and salinify waters on the Antarctic continental shelf. Polynya activity thus both limits the ocean heat flux to the Antarctic Ice Sheet and promotes formation of Dense Shelf Water (DSW), the precursor to Antarctic Bottom Water. However, despite the presence of strong polynyas, DSW is not formed on the Sabrina Coast in East Antarctica and in the Amundsen Sea in West Antarctica. Using a simple ocean model driven by observed forcing, we show that freshwater input from basal melt of ice shelves partially offsets the salt flux by sea ice formation in polynyas found in both regions, preventing full-depth convection and formation of DSW. In the absence of deep convection, warm water that reaches the continental shelf in the bottom layer does not lose much heat to the atmosphere and is thus available to drive the rapid basal melt observed at the Totten Ice Shelf on the Sabrina Coast and at the Dotson and Getz ice shelves in the Amundsen Sea. Our results suggest that increased glacial meltwater input in a warming climate will both reduce Antarctic Bottom Water formation and trigger increased mass loss from the Antarctic Ice Sheet, with consequences for the global overturning circulation and sea level rise.
  • Daiki Nomura, Shigeru Aoki, Daisuke Simizu, Takahiro Iida
    Journal of Geophysical Research: Oceans 123 2 939 - 951 2018年02月 [査読有り][通常論文]
     
    © 2018. American Geophysical Union. All Rights Reserved. Cracks are common and natural features of sea ice formed in the polar oceans. In this study, a sea ice crack in flooded, multiyear, land-fast Antarctic sea ice was examined to assess its influence on biological productivity and the transport of nutrients and microalgae into the upper layers of neighboring sea ice. The water inside the crack and the surrounding host ice were characterized by a strong discoloration (brown color), an indicator of a massive algal bloom. Salinity and oxygen isotopic ratio measurements indicated that 64–84% of the crack water consisted of snow meltwater supplied during the melt season. Measurements of nutrient and chlorophyll a concentrations within the slush layer pool (the flooded layer at the snow-ice interface) revealed the intrusion of water from the crack, likely forced by mixing with underlying seawater during the tidal cycle. Our results suggest that sea ice crack formation provides conditions favorable for algal blooms by directly exposing the crack water to sunlight and supplying nutrients from the under-ice water. Subsequently, constituents of the crack water modified by biological activity were transported into the upper layer of the flooded sea ice. They were then preserved in the multiyear ice column formed by upward growth of sea ice caused by snow ice formation in areas of significant snow accumulation.
  • S. Aoki, R. Kobayashi, S. R. Rintoul, T. Tamura, K. Kusahara
    Journal of Geophysical Research: Oceans 122 8 6277 - 6294 2017年08月 [査読有り][通常論文]
     
    © 2017. American Geophysical Union. All Rights Reserved. Oceanic changes before and after the relocation of iceberg B9B and calving of the Mertz Glacier Tongue (MGT) in February 2010 are examined on the continental shelf off the Adélie Land/George V Land coast, East Antarctica. Summer hydrographic observations, including stable oxygen isotope ratio (δ18O), in 2001/2008 and 2011/2015 and results of a numerical model are used. Along the western flank of the MGT, temperature decreased between 2001 and 2015 for most of the water column in the Adélie Depression. δ18O generally decreased, especially at the MGT draft depths on the northern side. West of the MGT, temperature, salinity, and δ18O decreased in the intermediate layer. East of the MGT, in contrast, temperature increased between 2001 and 2011 at intermediate depths, salinity increased in the intermediate and deep layers, and δ18O slightly decreased in the deep layer but did not change much around 300 dbar. The numerical experiment exhibits a change in ocean circulation, revealing an increase in modified Circumpolar Deep Water (mCDW) inflow in the east and a decrease in the west. The contrasting changes in mCDW intrusion are consistent between the observations and numerical model, and are indicative of the effect of removal of the ice barriers. The contrast is overlain by overall decreases in salinity and δ18O, which suggests an increase in the continental meltwater fraction of 5–20% and might reveal a wide-ranging influence from West Antarctica. The oxygen isotope ratio is, hence, effective in monitoring the increase in continental melt over the Antarctic shelf.
  • S. Aoki
    Geophysical Research Letters 44 7 3219 - 3227 2017年04月 [査読有り][通常論文]
     
    ©2017. American Geophysical Union. All Rights Reserved. A large land-fast sea ice breakup occurred in 2016 in Lützow-Holm Bay, East Antarctica. The breakup caused calving from the Shirase Glacier Tongue. Although similar breakups and calving have been observed in the past, the timing and magnitudes are not well-constrained. The ice's breakup latitude during 1997–2016 was analyzed to investigate the variables controlling breakup and examine correlation with local calving for a longer period. The breakup latitude in April had a persistently high correlation with sea surface temperature (SST) in the tropical Pacific, which exceeds correlations with local atmospheric variables. The years of five out of six observed calving events from the mid-20th century can correspond to those of warm SST episodes and calving-front retreat in the 1980s to warmer SST shift. Our proposed teleconnection between tropical SST and Antarctic sea ice could lead to better predictions of breakup and might impact the glacier flux for a wider region.
  • 松村 義正, 大橋 良彦, 青木 茂, 杉山 慎
    低温科学 75 77 - 84 低温科学第75巻編集委員会 2017年 
    グリーンランド氷床の表面融解水は,氷河を貫通するムーランを通過して岩盤に到達し,陸上の堆積物を懸濁物質として取り込みながら氷河末端で海洋に流出する.淡水である融解水は海水より軽いため直ちに湧昇し,氷河末端で高濁度の融解水プルームを形成してフィヨルド内の鉛直循環を駆動する.本研究では粒子追跡法を組み込んだ非静力学海洋モデルによる理想化シミュレーションによってこの高濁度水プルームを再現し,特にフィヨルド内の懸濁物質輸送過程について解析した.The surface meltwater of the Greenland ice sheet is drained through englacial channels called moulins that reachthe bedrock, and hence it contains substantial amounts of terrestrial sediments. The meltwater runoff from marineterminatingglaciers form an upwelling turbid plume at the glacier front and drives overturning circulation in the fjord.We perform idealized numerical experiments on the turbid meltwater plume and associated sediment transport byusing a non-hydrostatic ocean model coupled with a Lagrangian particle tracking system that simulates the dynamicsof suspended sediments.
  • Kazuya Kusahara, Hiroyasu Hasumi, Alexander D. Fraser, Shigeru Aoki, Keishi Shimada, Guy D. Williams, Robert Massom, Takeshi Tamura
    Journal of Climate 30 1 163 - 188 2017年 [査読有り][通常論文]
     
    © 2017 American Meteorological Society. Ocean-cryosphere interactions along the Adélie and George V Land (AGVL) coast are investigated using a coupled ocean-sea ice-ice shelf model. The dominant feature of the Mertz Glacier Tongue (MGT), located at approximately 145°E, was a highly productive winter coastal polynya system, until its calving in February 2010 dramatically changed the regional "icescape." This study examines the annual mean, seasonal, and interannual variabilities of sea ice production; basal melting of the MGT; ice shelves, large icebergs, and fast ice; Dense Shelf Water (DSW) export; and bottom water properties on the continental slope and rise, and assesses the impacts of the calving event. The interannual variability of the winter coastal polynya regime is dominated by the regional offshore winds and air temperature, which are linked to activity of the Amundsen Sea low pressure system. This is the main driver of the interannual variability of DSW exported from the AGVL region. The calving event led to a decrease in sea ice production that resulted in a decrease in the density of DSW export. Subsequently, there is extensive freshening downstream over the continental shelf and slope regions. In addition, it is found that the calving event causes a significant decrease in the mean melt rate of the MGT, resulting from a decrease in ocean heat flux into the cavity due to ocean circulation changes.
  • Keishi Shimada, Shigeru Aoki, Kay I. Ohshima
    Journal of Atmospheric and Oceanic Technology 34 3 511 - 532 2017年 [査読有り][通常論文]
     
    © 2017 American Meteorological Society. 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.
  • Shinichiro Kida, Humio Mitsudera, Shigeru Aoki, Xinyu Guo, Shin Ichi Ito, Fumiaki Kobashi, Nobumasa Komori, Atsushi Kubokawa, Toru Miyama, Ryosuke Morie, Hisashi Nakamura, Tomohiro Nakamura, Hideyuki Nakano, Hajime Nishigaki, Masami Nonaka, Hideharu Sasaki, Yoshi N. Sasaki, Toshio Suga, Shusaku Sugimoto, Bunmei Taguchi, Koutarou Takaya, Tomoki Tozuka, Hiroyuki Tsujino, Norihisa Usui
    Hot Spots in the Climate System: New Developments in the Extratropical Ocean-Atmosphere Interaction Research 1 - 30 2016年01月01日 [査読有り][通常論文]
     
    © The Oceanographic Society of Japan and Springer Japan 2016. This article reviews progress in our understanding of oceanic fronts around Japan and their roles in air–sea interaction. Fronts associated with the Kuroshio and its extension, fronts within the area of the Kuroshio-Oyashio confluence, and the subtropical fronts are described with particular emphasis on their structure, variability, and role in air–sea interaction. The discussion also extends to the fronts in the coastal and marginal seas, the Seto Inland Sea and Japan Sea. Studies on oceanic fronts have progressed significantly during the past decade, but many of these studies focus on processes at individual fronts and do not provide a comprehensive view. Hence, one of the goals of this article is to review the oceanic fronts around Japan by describing the processes based on common metrics. These metrics focus primarily on surface properties to obtain insights into air–sea interactions that occur along oceanic fronts. The basic characteristics derived for each front (i.e., metrics) are then presented as a table. We envision that many of the coupled ocean-atmosphere global circulation models in the coming decade will represent oceanic fronts reasonably well, and it is hoped that this review along with the table of metrics will provide a useful benchmark for evaluating these models.
  • Shigeru Aoki, Genta Mizuta, Hideharu Sasaki, Yoshikazu Sasai, Stephen R. Rintoul, Nathaniel L. Bindoff
    Hot Spots in the Climate System: New Developments in the Extratropical Ocean-Atmosphere Interaction Research 157 - 170 2016年01月01日 [査読有り][通常論文]
     
    © The Oceanographic Society of Japan and Springer Japan 2016. For the 32-year period from 1979 to 2010, trends of surface and subsurface temperature and meridional motion of the current system in the Antarctic Circumpolar Current (ACC) region are studied with in situ observations and an eddy-resolving general circulation model. The observed and simulated surface temperature shows a similar pattern between the Atlantic and Pacific: warming to the north of the Subantarctic/Subtropical Fronts in the Atlantic and of the Subtropical Front in the Pacific and cooling to the south of those fronts. The subsurface temperature trend, again from both observation and model, reveals an asymmetric pattern between the Atlantic and Pacific: subsurface warming is dominant over the whole ACC region in the Atlantic, while both warming and cooling are significant in the Pacific, the former located to the north of the Subantarctic Front and the latter to the south. The model reveals that the ACC has generally shifted poleward in the Atlantic, while it has shifted equatorward around Subantarctic Front and Polar Front in the Pacific. The ACC shift is consistent with the overall subsurface temperature trend. The basin-scale difference of the ACC response can be related to the different regime of the trend in meridional gradient of the zonal wind stress to the north and south of 50–55°S and suggests a coupling of the ACC and overlying westerly on the multi-decadal time scale.
  • Yoshihiko Ohashi, Takahiro Iida, Shin Sugiyama, Shigeru Aoki
    Polar Science 10 3 270 - 277 2016年 [査読有り][通常論文]
     
    © 2016 Elsevier B.V. and NIPR Glacial meltwater discharge from the Greenland ice sheet and ice caps forms high turbidity water in the proglacial ocean off the Greenland coast. Although the timing and magnitude of high turbidity water export affect the coastal marine environment, for example, through impacts on biological productivity, little is known about the characteristics of this high turbidity water. In this paper, we therefore report on the spatial and temporal variations in high turbidity water off the Thule region in northwestern Greenland, based on remote sensing reflectance data at a wavelength of 555 nm (Rrs555). The high turbidity area, identified on the basis of high reflectivity (Rrs555 ≥ 0.0070 sr−1), was generally distributed near the coast, where many outlet glaciers terminate in the ocean and on land. The extent of the high turbidity area exhibited substantial seasonal and interannual variability, and its annual maximum extent was significantly correlated with summer air temperature. Assuming a linear relationship between the high turbidity area and summer temperature, annual maximum extent increases under the influence of increasing glacial meltwater discharge, as can be inferred from present and predicted future warming trends.
  • Shigeru Aoki, Genta Mizuta, Hideharu Sasaki, Yoshikazu Sasai, Stephen R. Rintoul, Nathaniel L. Bindoff
    Journal of Oceanography 71 5 623 - 636 2015年10月29日 [査読有り][通常論文]
     
    © 2015, The Oceanographic Society of Japan and Springer Japan. For the 32-year period from 1979 to 2010, trends of surface and subsurface temperature and meridional motion of the current system in the Antarctic Circumpolar Current (ACC) region are studied with in situ observations and an eddy-resolving general circulation model. The observed and simulated surface temperature shows a similar pattern between the Atlantic and Pacific: warming to the north of the Subantarctic/Subtropical Fronts in the Atlantic and of the Subtropical Front in the Pacific and cooling to the south of those fronts. The subsurface temperature trend, again from both observation and model, reveals an asymmetric pattern between the Atlantic and Pacific: subsurface warming is dominant over the whole ACC region in the Atlantic, while both warming and cooling are significant in the Pacific, the former located to the north of the Subantarctic Front and the latter to the south. The model reveals that the ACC has generally shifted poleward in the Atlantic, while it has shifted equatorward around Subantarctic Front and Polar Front in the Pacific. The ACC shift is consistent with the overall subsurface temperature trend. The basin-scale difference of the ACC response can be related to the different regime of the trend in meridional gradient of the zonal wind stress to the north and south of 50–55°S and suggests a coupling of the ACC and overlying westerly on the multi-decadal time scale.
  • Sunke Schmidtko, Karen J. Heywood, Andrew F. Thompson, Shigeru Aoki
    Science 346 6214 1227 - 1231 2014年12月05日 [査読有り][通常論文]
     
    ©2014 by the American Association for the Advancement of Science; all rights reserved. Decadal trends in the properties of seawater adjacent to Antarctica are poorly known, and the mechanisms responsible for such changes are uncertain. Antarctic ice sheet mass loss is largely driven by ice shelf basal melt, which is influenced by ocean-ice interactions and has been correlated with Antarctic Continental Shelf Bottom Water (ASBW) temperature. We document the spatial distribution of long-term large-scale trends in temperature, salinity, and core depth over the Antarctic continental shelf and slope. Warming at the seabed in the Bellingshausen and Amundsen seas is linked to increased heat content and to a shoaling of the mid-depth temperature maximum over the continental slope, allowing warmer, saltier water greater access to the shelf in recent years. Regions of ASBW warming are those exhibiting increased ice shelf melt.
  • Shin Sugiyama, Takanobu Sawagaki, Takehiro Fukuda, Shigeru Aoki
    EARTH AND PLANETARY SCIENCE LETTERS 399 52 - 60 2014年08月 [査読有り][通常論文]
     
    The grounding line (GL) of the Antarctic ice sheet forms the boundary between grounded and floating ice along the coast. Near this line, warm oceanic water contacts the ice shelf, producing the ice sheet's highest basal-melt rate. Despite the importance of this region, water properties and circulations near the GL are largely unexplored because in-situ observations are difficult. Here we present direct evidence of warm ocean-water transport to the innermost part of the subshelf cavity (several hundred meters seaward from the GL) of Langhovde Glacier, an outlet glacier in East Antarctica. Our measurements come from boreholes drilled through the glacier's similar to 400-m-thick grounding zone. Beneath the grounding zone, we find a 10-24-m-deep water layer of uniform temperature and salinity (-1.45 degrees C; 34.25 PSU), values that roughly equal those measured in the ocean in front of the glacier. Moreover, living organisms are found in the thin subglacial water layer. These findings indicate active transport of water and nutrients from the adjacent ocean, meaning that the subshelf environment interacts directly and rapidly with the ocean. (C) 2014 The Authors. Published by Elsevier B.V.
  • 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28日 [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°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 (× 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. © 2014. American Geophysical Union. All Rights Reserved.
  • Tsutomu Yamanokuchi, Koichiro Doi, Kazuki Nakamura, Shigeru Aoki, Kazuo Shibuya
    Proceedings of the European Conference on Synthetic Aperture Radar, EUSAR Proceedings of the European Conference on Synthetic Aperture Radar, EUSAR 102 - 105 2014年 [査読有り][通常論文]
     
    © VDE VERLAG GMBH · Berlin · Offenbach, Germany. This study shows the use of L-band SAR data for the monitoring of ice sheet marginal zone on East Antarctica, especially focusing on the extraction of grounding line. ALOS / PALSAR is a SAR sensor which was the latest L-band spacebone sensor. Its 46days revisit cycle seemed to prevent the fine extraction of grounding line, however, this study showed the potential ability for the extraction of grounding line on East Antarctica under the condition of slow flowing velocity of ice sheet. This result encourages the extraction of grounding line by ALOS-2 / PALSAR-2 data thanks to its shorter revisit cycle (14days).
  • Kayo Nakamura, Shigeru Aoki, Kei Yoshimura, Naoyuki Kurita
    Scientific Online Letters on the Atmosphere 10 1 154 - 157 2014年 [査読有り][通常論文]
     
    © 2014, the Meteorological Society of Japan. Herein, the spatial distribution of stable oxygen isotope ratios (δ18O) of precipitation in the Atlantic to Indian sectors of the Southern Ocean is examined using the results of in situ observations and numerical modeling. In situ observations of 59 precipitation events reveal poleward decrease of δ18O, with a larger meridional gradient south of 60°S. Moreover, the estimates from the observations and model (IsoGSM) agree reasonably well, with a mean absolute difference of 4.3‰. Thus, the IsoGSM results generally support the observed poleward increase in the meridional gradient. These results will prove valuable in investigating the atmospheric water cycle and in studying oceanic processes of water mass formation and transport.
  • S. Aoki, Y. Kitade, K. Shimada, K. I. Ohshima, T. Tamura, C. C. Bajish, M. Moteki, S. R. Rintoul
    Journal of Geophysical Research: Oceans 118 11 6046 - 6063 2013年11月 [査読有り][通常論文]
     
    Long-term water mass changes during 1994-2012 are examined from nine repeat hydrographic sections in the Seasonal Ice Zone along 140°E, off Antarctica. Significant freshening trends are detected within most of the water masses from the bottom to surface. Bottom Water freshened by 0.008-0.009 decade -1 below isopycnal surfaces and its layer thickness decreased by 120-160 dbar decade-1 throughout the study period. In addition to general thinning, the layer thickness was anomalously thin in 2012, suggesting a possible link with the sudden calving of the Mertz Glacier Tongue and subsequent reduction in sea-ice production. Winter Water freshened by 0.03 decade-1 throughout the study period, with significant interannual variability. In the offshore region, a long-term increase in precipitation can explain a substantial portion of the freshening trend. The Lower Circumpolar Deep Water on the continental slope underwent freshening at the same rate as the Bottom Water during the last two decades. Modified Shelf Water also shows robust freshening at a rate of 0.03 decade-1. Combined with the freshening of near-surface and Bottom Water masses in this region, these data indicate freshening of the entire water column over the continental slope. This widespread freshening is broadly consistent with the enhancement of the global hydrological cycle, together with a possible acceleration of land ice melting. Key Points Antarctic Bottom Water freshened off Adelie Coast, Antarctica, during 1994-2012 Precipitation increase can cause a significant part of Winter Water freshening The freshening is consistent with the enhancement of global hydrological cycle ©2013. American Geophysical Union. All Rights Reserved.
  • 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.
  • C. C. Bajish, S. Aoki, B. Taguchi, N. Komori, S. J. Kim
    Scientific Online Letters on the Atmosphere 9 1 32 - 35 2013年 [査読有り][通常論文]
     
    Decadal variability of Antarctic sea ice and a role of ocean dynamics are examined with sea ice concentration (SIC) and sea surface temperature (SST) derived from satellite microwave observation and obtained by a high resolution coupled oceanatmosphere-ice general circulation model (CGCM). Sea ice observations revealed a circumpolar variability of sea ice edge (SIE) on quasi-decadal time scale. SST also showed variation on similar time scale with warm (cool) anomaly roughly corresponding to retreat (extension) of SIE at negative (positive) southern annular mode (SAM). CGCM run without anthropogenic forcing and volcanic eruptions revealed that the leading mode of SIC is quasicircumpolar pattern with a dominant time scale of 12-17 years and the leading mode of SST also has a similar pattern with the SIC showing a high degree of inverse correlation. The modeled SAM significantly correlates with these leading modes of both SIC and SST, representing the same structure with the observations. This indicates that the oceanic natural variability is the key to understand the quasi-decadal variability in sea ice. © 2013, the Meteorological Society of Japan.
  • Tsutomu Yamanokuchi, Koichiro Doi, Kazuki Nakamura, Shigeru Aoki, Kazuo Shibuya
    Conference Proceedings of 2013 Asia-Pacific Conference on Synthetic Aperture Radar, APSAR 2013 198 - 201 2013年 [査読有り][通常論文]
     
    The global warming may affect the cryospheric environment and many reports showed the retreat of glaciers, thinning of ice shelves and ice sheet on Antarctica. Retreat of mountain glaciers also might be explained in the same context. This paper aims to show how useful the ALOS / PALSAR data for cryospheric applications through the case study of two typical areas, one is rock glacier monitoring on Argentine Andes and the other is ice sheet on Dronning Maud Land, Antarctica © 2013 IEICE.
  • 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 γn Combining double low line 28.30 kg mĝ̂'3. Changes in the average salinity and potential temperature below this isopycnal correspond to a basin-wide warming of 1300 A± 200 GW and freshening of 24 A± 3 Gt yearĝ̂'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 (171-158A°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 A± 23% reduction in RSBW transport and the enhancement of the vertical diffusion of heat resulting from a 30 A± 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% is assumed between the year 1995 and 2005) can explain 51 A± 6% of the warming and 84 A± 10% of the freshening observed in AA-AABW. © Author(s) 2012.
  • Daiki Nomura, Andrew McMinn, Hiroshi Hattori, Shigeru Aoki, Mitsuo Fukuchi
    Marine Chemistry 127 1-4 90 - 99 2011年12月20日 [査読有り][通常論文]
     
    Temporal measurements of temperature, salinity, water-oxygen isotopic ratio and nutrient concentrations at Saroma-ko Lagoon, southern Sea of Okhotsk, were made in February-March 2008 to examine the processes by which nitrogen compounds from the atmosphere were incorporated via snowfall into sea ice. Granular ice made up more than half the ice thickness, and the mass fraction of snow in the snow-ice layer on top of the ice ranged from 0.8% to 46.9%. The high concentrations of NO3-+NO2- and NH4+ observed in the snow and snow-ice throughout the study period were likely due to the proximity of the study site, in northern Japan, to the east coast of the Asian continent. Pollutants containing high NO3-and NH4+ concentrations are transported from East Asia and deposited in snowfall over the sea ice in the southern part of the Sea of Okhotsk. Compared with NO3-+NO2- and NH4+ concentrations, PO43- concentrations in the snow and snow-ice were low. The strong correlation between the NO3-+NO2- and NH4+ concentrations in the snow-ice and the mass fraction of snow indicates that the nitrogen compounds on top of the sea ice were controlled mainly by the snow contribution to the sea ice when snow-ice predominated. Our results indicate that chemical cycles in sea ice can be affected by polluted precipitation (snow) originating from a nonpolar sea. © 2011 Elsevier B.V.
  • Tsutomu Yamanokuchi, Kazuo Shibuya, Koichiro Doi, Shigeru Aoki
    2011 3rd International Asia-Pacific Conference on Synthetic Aperture Radar, APSAR 2011 554 - 557 2011年 [査読有り][通常論文]
     
    This study applies various kinds of analytical method of SAR data and evaluate the potential ability of ALOS / PALSAR data for Antarctic research. Offset tracking is almost able to apply for glacier analysis and showed the consistency with manual interpretation results. The flow velocity estimation results show approximately 2100m/a at the grounding line of Shirase glacier and 300-700m/year at the eastern part of Riiser-Larsen Peninsula. From the time series analysis of ice shelf over three decades showed quite stable state of flow velocity around Lützow-Holm Bay. Grounding line analysis by PALSAR is succeeded by applying 3 pass InSAR method. 2pass InSAR method cannot extract grounding line because fringe due to flow of ice overlaid with tidal fringe and it is impossible to distinguish them. However, 3pass InSAR can remove fringe due to flow of ice and successfully extracted the grounding line from the interferogram. These results encourage us for the cryospheric applications for ALOS-2 satellite. © 2011 KIEES.
  • 中村 佳代, 青木 茂, 豊田 威信, 青山 雄一
    南極資料 54 2 190 - 202 国立極地研究所 2010年07月 [査読無し][通常論文]
     
    降雪の酸素安定同位体比(δ^<18>O)の緯度分布と季節変化の存在はよく知られているが,極域海洋上での降雪のδ^<18>O の報告はほとんどない.本研究では,南大洋高緯度域において,夏季と冬季の4つの現場海洋観測の機会を利用し,船上に降った雪を採取し分析した.また,通年のデータとして,2008年における昭和基地での降雪のサンプルを分析した.これらに基づいて,南大洋上の降雪のδ^<18>O の緯度分布と季節変化について考察する.全般に高緯度ほど低く,また夏季より冬季の方が低いδ^<18>O の分布が得られた.また,緯度変化の傾きは冬季の方が強い.夏季および冬季のδ^<18>O は,60°Sにおいて-5.4‰と-11.3‰であるのに対し,66°Sにおいて-10.5‰と-20.8‰であった.これらの結果は,南極海での塩分分布やその変化のメカニズムを解明する上での手助けになると考えられる.
  • Shigeru Aoki, Yoshikazu Sasai, Hideharu Sasaki, Humio Mitsudera, Guy D. Williams
    Ocean Dynamics 60 3 743 - 757 2010年06月 [査読有り][通常論文]
     
    Flow structure in the Australian-Antarctic basin is investigated using an eddy-resolving general ocean circulation model and validated with iceberg and middepth float trajectories. A cyclonic circulation system between the Antarctic Circumpolar Current and Antarctic Slope Current consists of a large-scale gyre in the west (80-110° E) and a series of eddies in the east (120-150° E). The western gyre has an annual mean westward transport of 22 Sv in the southern limb. Extending west through the Princess Elizabeth Trough, 5 Sv of the gyre recirculates off Prydz Bay and joins the western boundary current off the Kerguelen Plateau. Iceberg trajectories from QuickScat and ERS-1/2 support this recirculation and the overall structure of the Antarctic Slope Current against isobath in the model. Argo float trajectories also reveal a consistent structure of the deep westward slope current. This study indicates the presence of a large cyclonic circulation in this basin, which is comparable to the Weddell and Ross gyres. © Springer-Verlag 2010.
  • G. D. Williams, S. Nicol, S. Aoki, A. J.S. Meijers, N. L. Bindoff, Y. Iijima, S. J. Marsland, A. Klocker
    Deep-Sea Research Part II: Topical Studies in Oceanography 57 9-10 738 - 757 2010年05月 [査読有り][通常論文]
     
    Hydrographic CTD and ADCP data were collected during the BROKE-West research voyage (January-March 2006) in the south-west Indian Ocean sector of the Antarctic margin. These data describe the large-scale circulation, water masses, fronts and summertime stratification in the surface layer over the continental shelf, slope and rise region between 30 and 80{ring operator} E that forms CCAMLR Statistical Area 58.4.2. The surface circulation matched the full-depth circulation and consisted of the eastward flowing southern Antarctic Circumpolar Current front to the north, and the westward flowing Antarctic Slope Current associated with the Antarctic Slope Front along the continental slope to the south. Two sub-polar gyres were detected south of the Southern Boundary of the Antarctic Circumpolar Current: the eastern Weddell Gyre in the Cosmonaut Sea (30 - 50{ring operator} E) and the greater Prydz Bay Gyre in the Cooperation Sea (60 - 80{ring operator} E). In the eastern Weddell Gyre, the seasonal mixed layer depths were shallower, warmer and fresher relative to the regions to the east which were deeper, cooler and more saline. This spatial variability is found to be strongly correlated to the large-scale pattern of sea ice melt/retreat in the months preceding the voyage and the accumulated wind stress thereafter. Areas of upwelling warm deep waters into the surface layer are presented from positive anomalies of potential temperature and nutrient concentrations (nitrate and silicate). These anomalies were strongest in the eastern Weddell Gyre in the vicinity of the Cosmonaut Polynya/Embayment, north of Cape Anne and near the Southern Boundary of the Antarctic Circumpolar Current in the eastern sector of the survey. The summertime stratification (seasonal mixed layer, seasonal pycnocline and Tmin layer) are discussed relative to the distributions of chl a and acoustically determined Antarctic Krill (Euphausia superba) densities. Elevated chl a concentrations were found in the surface layer of the marginal ice zone and it is proposed that these are retained south of the fast, narrow jet of enhanced Antarctic Slope Current on the upper continental slope. There is qualitative evidence of these maxima being subducted and transported north in the seasonal pycnocline in response to Ekman convergence from the easterly winds in this region. The seasonal mixed layer within the sub-polar gyres had relatively low chl a concentrations with sub-surface maxima in the seasonal pycnocline and the top of the Tmin layer. Surface concentrations increased once again north of the Southern Boundary in the north-east of the survey. Krill and chl a concentrations were both co-located and decoupled at different locations across the survey. There was no clear oceanographic boundary influencing the distribution of the krill surveyed, though further work is necessary to properly synthesize this and other biological patterns with the oceanographic processes, given the varying time and length scales and intrinsic sampling limitations. © 2009 Elsevier Ltd. All rights reserved.
  • Y. Fukamachi, S. R. Rintoul, J. A. Church, S. Aoki, S. Sokolov, M. A. Rosenberg, M. Wakatsuchi
    Nature Geoscience 3 5 327 - 331 2010年05月 [査読有り][通常論文]
     
    The primary paths for the transport of Antarctic Bottom Water from the Southern Ocean into the global ocean are the deep western boundary currents east of the Antarctic Peninsula and the Kerguelen plateau. Previous ship-based observations documented distinct water properties and velocities associated with a deep western boundary current in the Kerguelen region, but the mean flow is as yet unconstrained. Here we report measurements from a coherent array of eight current-meter moorings that reveal a narrow and intense equatorward flow extending throughout the water column just east of the Kerguelen plateau. Velocities averaged over two years exceed 20 cm s 1 at depths of about 3,500 m, the strongest mean deep western boundary current flow yet observed at similar depths. We estimate the mean equatorward transport of water colder than 0 C at 12.31.2×10 6 m 3 s 1, partially compensated by poleward flow. We also estimate the net equatorward flow of water colder than 0.2 C at about 8×10 6 m 3 s 1, substantially higher than the 1.9×10 6 m 3 s 1 reported from the boundary current that carries dense water from the Weddell Sea into the Atlantic Ocean north of the Falkland plateau. We conclude that the Kerguelen deep western boundary current is a significant pathway of the global oceans deep overturning circulation. © 2010 Macmillan Publishers Limited.
  • G. D. Williams, S. Aoki, S. S. Jacobs, S. R. Rintoul, T. Tamura, N. L. Bindoff
    Journal of Geophysical Research: Oceans 115 4 2010年04月 [査読有り][通常論文]
     
    We report on observations of dense shelf water overflows and Antarctic Bottom Water (AABW) formation along the continental margin of the Adélie and George V Land coast between 140°E and 149°E. Vertical sections and bottom layer water mass properties sampled during two RVIB Nathaniel B Palmer hydrographic surveys (NBP00-08, December 2000/January 2001 and NBP04-08, October 2004) describe the spreading of cold, dense shelf water on the continental slope and rise from two independent source regions. The primary source region is the Adélie Depression, exporting high-salinity dense shelf water through the Adélie Sill at 143°E. An additional eastern source region of lowersalinity dense shelf water from the Mertz Depression is identified for the first time from bottom layer properties northwest of the Mertz Sill and Mertz Bank (146°E-148°E) that extend as far as the Buffon Channel (144.75°E) in summer. Regional analysis of satellite-derived ice production estimates over the entire region from 1992 to 2005 suggests that up to 40% of the total ice production for the region occurs over the Mertz Depression and therefore this area is likely to make a significant contribution to the total dense shelf water export. Concurrent time series from bottom-mounted Microcats and ADCP instruments from the Mertz Polynya Experiment (April 1998 to May 1999) near the Adélie Sill and on the upper continental slope (1150 m) and lower continental rise (3250 m) to the north describe the seasonal variability in downslope events and their interaction with the ambient water masses. The critical density for shelf water to produce AABW is examined and found to be 27.85 kg m-3 from the Adélie Depression and as low as 27.80 kg m-3 from the Mertz Depression. This study suggests previous dense shelf water export estimates based on the flow through the Adélie Sill alone are conservative and that other regions around East Antarctica with similar ice production to the Mertz Depression could be contributing to the total AABW in the Australian-Antarctic Basin. © 2010 by the American Geophysical Union.
  • Yuji Iijima, Shigeru Aoki, Kunio Kutsuwada
    Journal of Oceanography 65 1 53 - 60 2009年 [査読有り][通常論文]
     
    Over the Southern Ocean the dominant modes of the atmospheric field are known as the Southern Annular Mode (SAM) or Antarctic Oscillation, and the Pacific South American (PSA) pattern. Statistical analysis of sea surface wind (SSW) from satellite observation revealed two leading modes of SAM-like and PSA patterns. In the high latitudes, the SAM-like pattern of the SSW had a large amplitude over the Bellingshausen Basin and Australian-Antarctic Basin, with opposite phase between the two basins. On the intraseasonal time scale, large-scale sea surface height (SSH) also had notable variability, showing a basin-scale anti-phase mode over the two basins. To explain the response of oceanic variations to these atmospheric modes, we analyzed the relationship between the dominant modes of wind stress and large-scale SSH on the intraseasonal time scale. The SAM-like pattern of wind stress was correlated with the SSH variation over the two basins. The SSH basin mode was most simply explained by a simple barotropic response to the SAM-like mode of wind stress, with the curl of opposite phase between the two basins. We conclude that the zonal asymmetry of the wind field of the SAM plays an important role in driving the antiphase SSH basin modes. © Springer Science+Business Media B.V. 2009.
  • Shigeru Aoki, Nobuhiro Fujii, Shuki Ushio, Yasushi Yoshikawa, Shuichi Watanabe, Genta Mizuta, Yasushi Fukamachi, Masaaki Wakatsuchi
    Journal of Geophysical Research: Oceans 113 8 2008年08月08日 [査読有り][通常論文]
     
    The structure of the deep western boundary current and southern frontal systems of the Antarctic Circumpolar Current (ACC) southeast of the Kerguelen Plateau were investigated. Top-to-bottom hydrography was conducted around 58°S, and two underway observations were made around 56°S and 61°S. Middepth floats were also used to describe the subsurface flow and temperature fields. Along the 58°S section, a cold dome was located just off the plateau, and northwestward and southeastward flows were found in its western and eastern sides. The observed flow structure was highly barotropic. The maximum volume transport of the bottom water was 22 Sv equatorward, including a possible cyclonic recirculation of 10 Sv. The transport of the deep recirculation can be explained by the potential vorticity homogenization in the closed isopachs formed by the bottom slope off the plateau and the overlying ACC flowing southeastward through the Fawn Trough. The spatial scale of the cold dome, which showed a signature of the southern ACC front, was around 100 km at 58°S, and the similar cold domes were also found around 56°S and 61°S, along with a signature of the southern boundary of ACC around 61°S. The sharp excursion of the southern ACC front was also confirmed by the middepth float trajectories and the distribution of temperature maximum. This indicates the equatorward excursion up to 54°S, with the warmer ACC water just east of the southern frontal systems. Copyright 2008 by the American Geophysical Union.
  • Tsutomu Yamanokuchi, Koichiro Doi, Kazuo Shibuya, Shigeru Aoki
    International Geoscience and Remote Sensing Symposium (IGARSS) 4 1 2008年 [査読有り][通常論文]
     
    We have a plan to build the ocean-continent boundary database at Antarctica mainly to apply InSAR analysis technique using ERS-1 and ERS-2 tandem mission data. This database shows the snapshot of the state of Antarctic ice shelves and ice sheet in 1995-1996, when the tandem mission operated. The ocean-continent boundary data consist of grounding line and coast line, both which have vector property, and these vector data are stored by ESRI shape format (SHP), which is the world's standard vector format. This database will be expected to be used for ice\ mass flux study at Antarctica and is opened public on internet in this year to contribute the International Polar Year (IPY) activities and anyone can access this database freely through web browser. © 2008 IEEE.
  • Shigeru Aoki, Mieko Hariyama, Humio Mitsudera, Hideharu Sasaki, Yoshikazu Sasai
    Geophysical Research Letters 34 10 2007年05月28日 [査読有り][通常論文]
     
    We investigated the distribution of the deep mixed layer in the Southern Ocean, where Subantarctic Mode Water (SAMW) forms, by using Argo profiling floats and the ocean general circulation model for the Earth simulator (OFES). The mixed-layer depth (MLD) generally increased eastward from 50°-180°E and from 180°-80°W, with distinct local maxima near major bathymetric features. A sudden transition from deep to shallow mixed layer was found along the equatorward limit of the deep mixed layer. This diagnosed MLD front coincides with the line of ug · ∇σ 2 = 0, indicating the distributions of the general deepening and local maxima of MLD were affected by near-surface geostrophic flow. The flow is steered by the major bottom topographies, and the local MLD maxima occurred where the flow direction was deflected southward. This implies a link between the bathymetric features and formation of thick SAMW. Copyright 2007 by the American Geophysical Union.
  • Shigeru Aoki, Daisuke Fukai, Toru Hirawake, Shuki Ushio, Stephen R. Rintoul, Hiroshi Hasumoto, Takashi Ishimaru, Hideharu Sasaki, Takashi Kagimoto, Yoshikazu Sasai, Humio Mitsudera
    Journal of Geophysical Research: Oceans 112 5 2007年05月08日 [査読有り][通常論文]
     
    The spatial and temporal characteristics of cyclonic eddies in the Antarctic Divergence off Adélie Coast are described using repeated in situ observations between 2001 and 2003, satellite observations, and results of a high-resolution ocean general circulation model. Satellite ocean color images and altimetry clearly revealed a series of cyclonic eddies, with diameters of about 100-150 km that were separated by about 150 km in the zonal direction. The eddies are found at almost the same locations and can be traced for at least two months in the spring-summer period. Cold and fresh anomalies were found throughout the water column in the cores of the cyclonic eddies. The water properties in the core of the eddies are similar to water found over the continental slope, 150-200 km to the south. The geopotential anomaly of the cold features was smaller than the sea level anomaly detected with satellite altimetry, indicating that significant barotropic flow is associated with the eddies. The Ocean general circulation model for the Earth Simulator (OFES) reproduces a series of cyclonic eddies similar to those observed, including a significant barotropic component. In the model, the series of eddies appear to originate further west over the continental slope region with no consistent phase propagation. The combination of repeat in situ observations, remote sensing and high-resolution model results confirms the existence of persistent cyclonic eddies near 140°E and suggests the eddies play an important role in the exchange of water across the Antarctic Divergence. Copyright 2007 by the American Geophysical Union.
  • Shigeru Aoki, Kazunori Akitomo
    Deep-Sea Research Part I: Oceanographic Research Papers 54 3 320 - 339 2007年03月 [査読有り][通常論文]
     
    High-resolution XCTD and XBT observations were conducted to study eddy variability in the Subantarctic Front (SAF) and its possible impact on the properties of the adjacent Subantarctic Mode Water (SAMW) in the region south of Australia. The stations were occupied along cross-frontal transects in February 2004 and in February and March 2002. Coarse-resolution data from previous WOCE observations were also examined to reveal the water mass properties, including dissolved oxygen (DO). Small-scale (10-20 km) features were detected in the SAF for all high-resolution sections. Fluctuation of the 7 {ring operator} C isotherm was clearly revealed at a depth of 300-600 m for the σθ = 26.8 - 26.9 kg m- 3 density range. Temperature and salinity inversions of a similar spatial scale were also found on the same density surface. The spatial scale and depth range of the small-scale anomalies could be consistent with those of the baroclinic instability in the top several hundred-meter layer. In the potential temperature-salinity (θ-S) diagram, the small-scale fluctuations constitute cold/fresh intrusions from the warm/saline SAMW curve. The intrusions were found on the θ-S line connecting the deeper SAMW and the shallower Antarctic Surface Water (AASW) of the higher latitudes. From the WOCE observations, the presence of SAMW with high DO was indicated north of the SAF, as previously reported. Within the SAF, intrusions in the θ-S diagram with relatively high DO were found at around the 7 {ring operator} C isotherm for 26.8 - 26.9 kg m- 3, the same densities for the small-scale anomalies and the high-DO SAMW. Analysis of heat and salt deficit suggests that the small-scale disturbances have a comparable or larger role than those of meso-scale anomalies. These imply that the small-scale anomalies contribute to the cross-frontal water exchange and to the formation of high-DO SAMW. © 2007 Elsevier Ltd. All rights reserved.
  • Shigeru Aoki, Stephen R. Rintoul, Hiroshi Hasumoto, Hideki Kinoshita
    Polar Bioscience 20 1 - 20 2006年12月 [査読有り][通常論文]
     
    We describe the circulation and seasonal development of the upper ocean in the Seasonal Ice Zone (SIZ) of the Southern Ocean along 140°E. The 140°E section was repeated four times between November 2001 and March 2002, spanning the period from early spring to autumn. The sea ice edge was located at 62°-63°S in November, and retreated to 65°S in January. The circulation in the region is dominated by several fronts: the southern branch of Polar Front (PF-S) was located between 60° and 61.5°S; the northern branch of Southern ACC front (sACCf-N) was located at 61.5°-63°S, and roughly corresponds with the winter sea ice edge; and the southern branch of sACCf, the southern boundary of the ACC, and the Antarctic Slope Front (ASF) were closely spaced and found between 64°S and 65°S. Vigorous cyclonic (clockwise) eddies were identified in the region between the sACCf-N and sACCf-S throughout the period. Changes in salinity made the dominant contribution to changes in density in the SIZ, while changes in temperature made the largest contribution to density changes in the AZ, north of the sACCf. The depth of the mixed layer generally shoaled to the south, in all seasons. The decrease in mixed layer depth occurred in a series of steps. Seasonal variability in the depth of the mixed layer was strongest in the AZ, where summer warming formed a strong seasonal thermocline above the relatively deep (100 m) Winter Water layer. In the SIZ, the mixed layer became warmer, fresher and lighter in summer but the depth of the mixed layer remained at about 50 m throughout the year. The freshest surface waters were observed in the SIZ in January, immediately following the melt and retreat of the sea ice pack. An increase in mixed layer salinity from January to March likely reflects the effect of mixing with saltier waters below the mixed layer. Mixed layer depths south of the ASF were highly variable, both within and between seasons, varying from a minimum of ∼20 m in January to over 500 m in March. © 2006 National Institute of Polar Research.
  • Shigeru Aoki, Stephen R. Rintoul, Shuki Ushio, Shuichi Watanabe, Nathaniel L. Bindoff
    Geophysical Research Letters 32 23 1 - 4 2005年12月16日 [査読有り][通常論文]
     
    Repeat summer hydrographic observations along 140°E are used to document significant changes in the properties of the Adélie Land Bottom Water (ALBW) between the mid-1990s and 2002-2003. Water on the 28.35 kg·m-3 neutral density surface cooled by 0.2°C and freshened by 0.03 psu between 1994 and 2002. By re-occupying the same stations in the same season, the effects of seasonal variability and spatial variability were minimised allowing the signal of water mass changes to be clearly identified. Comparison of the recent data to high quality historical observations shows that the ALBW also freshened between the late 1960s and the mid-1990s. Although there is insufficient data to construct a continuous time series, the simplest explanation of the observed changes is that there has been a long-term (>30 year) and continuing freshening of the source waters supplying bottom water to the Australian-Antarctic basin. Copyright 2005 by the American Geophysical Union.
  • Shigeru Aoki, Nathaniel L. Bindoff, John A. Church
    Geophysical Research Letters 32 7 1 - 5 2005年04月 [査読有り][通常論文]
     
    Interdecadal water mass changes in the Indian - Western Pacific sectors of the Southern Ocean were investigated using the Japanese Antarctic Research Expeditions and historical hydrographic observations from the 1950s to 1990s. Freshening and cooling occurred on the neutral density surfaces of 27.0 kg·m-3 equatorward of Sub-Antarctic Front. Results for the area south of the Polar Front show warm and saline anomalies and oxygen decreases on the surfaces around 27.9 kg·m-3, which correspond to the Upper Circumpolar Deep Water. These latter anomalies are most simply explained by the mixing of these shallow waters with warmer and fresher surface waters. Steric sea level has also increased with an average change of 1mm·yr-1 from the 1970s to 1990s. The changes are larger north of the Sub-Antarctic Front, implying a strengthening of the Antarctic Circumpolar Current. It appears that the observed changes are consistent with the results from coupled climate model results for a similar period. Copyright 2005 by the American Geophysical Union.
  • Shotaro Uto, Haruhito Shimoda, Koh Izumiyama, Shuki Ushio, Shigeru Aoki, Gen Hashida, Hiroyuki Wakabayashi, Fumihiko Nishio
    Antarctic Record 48 3 165 - 179 2004年11月 [査読有り][通常論文]
     
    Sea ice observations have been conducted onboard the Antarctic Research Vessel "Shirase" since 1987. The authors summarize these data to investigate spatial and inter-annual variability of sea ice thickness and snow depth of the summer land-fast ice in Lützow-Holmbukta. Probability density functions of annual total thickness (ice thickness + snow depth) derived from video observations are categorized into two types, i.e. a thin-ice and a thick-ice type. The total thickness distributions observed by the electro-magnetic inductive method have a clear discontinuity, which reflects the past break-up of the land-fast ice. Thin first-year ice develops offshore of this location, while multi-year ice mostly develops onshore. Ice thickness and snow depth gradually decrease toward the coast within about 20km offshore, probably due to snow drift driven by the strong northeasterly wind. It is concluded that the past breakup of the land-fast ice as well as snow depth have dominant influence on the spatial distribution of ice thickness in Lützow-Holmbukta. The extent of break-up varies significantly year by year. The smaller extent enhances the development of thick multi-year ice, while the larger extent, coupled with the influence of snow drift, enhances reduction or extinction of thick multi-year ice. © 2004 National Institute of Polar Research.
  • Shigeru Aoki, Toshihiko Sato
    Antarctic Record 48 3 204 - 218 2004年11月 [査読有り][通常論文]
     
    To understand the seasonal variation of biological and biogeochemical cycles in the seasonal ice zone in the Southern Ocean, the cruise of JARE-STAGE (Japanese Antarctic Research Expedition-Studies on Antarctic Ocean and Global Environment) was conducted in February 2002 with R/V Tangaroa. Physical oceanography implementations of the cruise are described. The results of the manufacturers' CTD conductivity calibrations were consistent between before and after the cruise, and the difference in salinity estimate was expected to be within 0.0014. Two casts were made to validate the XCTD accuracy and comparisons with the CTD are discussed. Generally, it is concluded that reasonably accurate observations were completed in this cruise. © 2004 National Institute of Polar Research.
  • S. Aoki, K. Shibuya, K. Doi
    Ocean '04 - MTS/IEEE Techno-Ocean '04: Bridges across the Oceans - Conference Proceedings 3 1644 - 1648 2004年 [査読有り][通常論文]
     
    To monitor sea level and sea state change, measurements of the vertical displacement of the landfast ice at Nisi-no-ura Cove in Lützow-Holm Bay, Antarctica, were made between April and December 1998 using GPS. Bottom pressure measurements were made simultaneously at the sea floor beneath the GPS station with a bottom pressure gauge (BPG). Seasonal variations were detected in the ice surface elevation and bottom pressure time series. K was shown that the difference between the potential sea level and ice surface elevation revealed a realistic freeboard. To postulate that the GPS and BPG observations are given, sea ice thickness was now predicted from the sea ice surface elevation and bottom pressure, assuming constant ice and water densities. The derived thickness agreed well with the observed one with an RMS error of 0.06 m. Thus the combined GPS-BPG system can provide a useful estimate for the sea ice thickness. Seasonal variation of the "sensitivity ratio" (the conversion coefficient from bottom pressure to sea level) was detected in the sea ice surface elevation and bottom pressure time series. The sensitivity ratio increased until September-October, and then decreased, consistent with thickening of the sea ice during the winter. Thus it ensures the obvious facts that the sensitivity ratio at a certain time should be used with the information of density stratification and the GPS can provide a height reference for correcting the intrinsic drift nature of the BPG. Therefore, the combined use of GPS and BPG will enable us to monitor long-term sea level change more precisely in landfast ice-covered regions. © 2004 IEEE.
  • Shigeru Aoki
    Journal of Oceanography 59 5 629 - 635 2003年10月 [査読有り][通常論文]
     
    Seasonal and spatial variations of iceberg drift were studied using continuous satellite scatterometer images off Dronning Maud Land, East Antarctica. Generally, iceberg drift speed showed a westward increase to the Greenwich Meridian. Seasonal variations of the drift speed were high in autumn-early winter and low in spring, and their magnitudes also increased westward. Seasonal variations of the drift speed were significantly correlated with variations of sea levels at Syowa and Mawson Stations, and hence qualitatively consistent with geostrophic current variations. Thus, the scatterometer data are demonstrated to be useful in monitoring iceberg trajectory and oceanic current variations.
  • Toru Hirawake, Sakae Kudoh, Shigeru Aoki, Stephen R. Rintoul
    Geophysical Research Letters 30 9 2003年05月01日 [査読有り][通常論文]
     
    Eddy-like distributions of chlorophyll a were clearly detected in the Antarctic Divergence zone around 140°E by SeaWiFS ocean color images. The distribution was also supported by in-situ measurements. The eddy-like structure was associated with changes in surface and subsurface water mass properties. Extremely high levels of chlorophyll in coastal areas were transported and redistributed horizontally offshore by these eddies, and a significant effect of the huge biomass on the biological processes in the investigated area was expected. The lateral exchange between coastal and offshore waters driven by these eddies, may be an essential factor in the observed patchy structures in this region.
  • Kazunari Nawa, Naoki Suda, Shigeru Aoki, Kazuo Shibuya, Tadahiro Sato, Yoshio Fukao
    Geophysical Research Letters 30 7 55 - 1 2003年04月 [査読有り][通常論文]
     
    We analyze sea level variation data acquired by a differential GPS and gravity data acquired by a superconducting gravimeter (SG) at Syowa Station, Antarctica, in an eight month period of 1998. At frequencies between 0.2 and 2.5 mHz in the seismic normal mode band we observe similar spectral peaks in both of the data sets. We also observe high coherence and zero phase between the two data sets at the frequencies of these peaks. The results of response analysis and simple mode calculation suggest that the observed peaks in the SG data are due to the effects of ocean water attraction and loading associated with sea level variation, a possible cause of which is the seiche in Lützow-Holm Bay around the station. Applying a transfer function method to both of the data sets, we can reduce the background noise due to the oceanic effects in the SG data.
  • Shigeru Aoki, Miwako Yoritaka, Akihiro Masuyama
    Journal of Geophysical Research: Oceans 108 4 2003年04月 [査読有り][通常論文]
     
    Long-term variations of subsurface layer temperature were examined for 32 years (1966-1998) of continuous observations in the Indian sector of the Southern Ocean. The data analyzed consist of observations from Japanese Antarctic Research Expeditions and also from other countries. Significant warming trends of about 0.004°C x yr-1 were observed at all depths from 200 to 900 m to the north of the southern Antarctic Circumpolar Current (ACC) front [Orsi et al., 1995]. The rates of warming increase with increasing depth from 0.003°C x yr-1 at 200 m to 0.012°C x yr-1 (statistically significant) at 900 m in the region close to the front. These warming trends were also found after applying a technique to correct for differences in the data sampling locations. The warming tendency in the frontal region is consistent with the assumption of a southward shift of the front and may have a relationship with the observed retreat of sea ice edge.
  • Matt King, Shigeru Aoki
    Geophysical Research Letters 30 3 2003年02月01日 [査読有り][通常論文]
     
    We measure ocean tides as experienced by sea ice or ice shelves using a single GPS receiver. Such a method allows for tidal measurements to be made with the same precision regardless of its distance from a GPS base station, since such a base station is not required. Single epoch observations are shown to be accurate to about 50 mm when compared with concurrent Bottom Pressure Gauge (BPG) observations. The differences between the GPS and BPG observations are largely free from power at tidal frequencies. Furthermore, harmonic tidal analyses show that these GPS observations allow the correct determination of diurnal and semi-diurnal tidal constituents, meaning that observations of this type can be assimilated into numerical tide models.
  • Shigeru Aoki
    Geophysical Research Letters 29 20 11 - 1 2002年10月15日 [査読有り][通常論文]
     
    © Copyright 2002 by the American Geophysical Union. Large-scale sea level variations around Antarctica were studied and their relationship with atmospheric variations was investigated. Tide gauge data at five coastal stations were used. Coherent sea level variations were clearly detected for all stations on intraseasonal time scales. The coherent variations had significant negative correlations with an index of the atmospheric annular mode variation (Antarctic Oscillation). Coherence was significant for periods from 10 to 100 days with negligible time lag. The negative correlation is consistent with the mechanism that a high (low) westerly anomaly leads to a stronger (weaker) northward Ekman drift and causes divergence (convergence) around Antarctica.
  • Shigeru Aoki, Kazuo Shibuya, Akihiro Masuyama, Taku Ozawa, Koichiro Doi
    Journal of Oceanography 58 3 519 - 523 2002年 [査読有り][通常論文]
     
    Global Positioning System (GPS) measurements of the vertical displacement of fast ice near Syowa Station, Antarctica, were conducted between April and December 1998 to evaluate measurements of sea level variation derived with a conventional bottom pressure gauge (BPG). The BPG-derived sea level revealed a seasonal variation of about 0.13 m, with a high in April-June and a low in November-December. The GPS-derived sea level, combined with observed sea ice thickness, supported the BPG result, with an RMS error of 0.007 m. Our result also demonstrates that GPS is a powerful technique for monitoring sea level variations.
  • Shigeru Aoki, Taku Ozawa, Kazuo Shibuya, Akihiro Masuyama
    Journal of the Geodetic Society of Japan 47 1 181 - 186 2001年 [査読有り][通常論文]
     
    Displacement of the fast sea ice was observed with differential kinematic GPS technique at Nisi-no-ura Cove in Lützow-Holm Bay, Antarctica. The observations were carried out for eight months from April to December in 1998. The results were evaluated with bottom pressure gauge (BPG) measurements. Tidal displacements were obtained clearly for both measurements. From the difference between both measurements, the GPS vertical accuracy was estimated to be 1-1.5 cm. Tidal constituents derived from the GPS observations were consistent with those from the BPG observations; the “sensitivity ratios”, conversion coefficient from BPG output to sea level, derived for six major tidal components were around 0.98 and the phase lags for the major components agreed within 1°. Horizontal motions on the tidal time scales were detected, but the motion is less than a few centi-meters and much less than that on the longer time scale. Thus, GPS has demonstrated the ability to monitor tides with sufficient accuracy. © 2001, The Geodetic Society of Japan. All rights reserved.
  • 青木 茂
    日本エネルギー学会誌 = Journal of the Japan Institute of Energy 79 11 1050 - 1053 日本エネルギ-学会 2000年11月 [査読無し][通常論文]
     
    The Southern Ocean is the unique ocean that connects world's major oceans. It plays important roles in global watermass formation and conversion, while it exchanges momentum, heat, and mass with overlying atmosphere, interacting with sea ice. Recent progress in observational techniques and analysis methods revealed that the climate system in the Southern Ocean has significant long-term variations from interannual to decadal time scales. The coupled ocean-atmosphere-sea ice system has a possibility of interannual variation through teleconnections over the entire Southern Ocean (Antarctic Circumpolar Wave). Huge ice-free enclosures (polynya) were observed in the ice-covered Weddell Sea in late-1970s, but they did not appear in recent years. Some of the recent observational topics are presented, and connections with the anthropogenic activities are discussed.
  • Shigeru Aoki, Taku Ozawa, Koichiro Doi, Kazuo Shibuya
    Geophysical Research Letters 27 15 2285 - 2288 2000年08月01日 [査読有り][通常論文]
     
    Differential GPS observations were conducted on the fast ice in Lutzow-Holm Bay, Antarctica. The observations were carried out intermittently from April to December in 1998. The vertical displacement, which is a combination of oceanic tidal variation and high frequency variations of 3-6 minutes period, was clearly detected. Eight hours' comparison of the height variation derived from GPS with that from a video monitoring system showed good agreement with a standard deviation of 2.2 cm. The high frequency fluctuations of about 1 cm were obvious in both observations and had significant correlation with each other. The low-pass filtered variations agreed well with a standard deviation of 1.7 cm. Tidal constituents derived from the GPS observations for the entire period showed good agreement with those from the seven years of pressure gauge observations. Thus, GPS has demonstrated the ability to observe 2 cm accuracy sea level variations.
  • Kazuo Shibuya, Masaki Kanao, Toshihiro Higashi, Shigeru Aoki
    Antarctic Record 44 1 14 - 24 2000年03月 [査読有り][通常論文]
     
    A PRARE (Precise Range and Range-rate Equipment) tracking antenna was installed by the 38th Japanese Antarctic Research Expedition (JARE-38) at Syowa Station (69.0°S, 39.6°E), Antarctica in March 1997. The antenna pillar pin of the PRARE tracking antenna was tied to the International GPS Service of Geodynamics (IGS) antenna reference point by relative GPS positioning, and its International Terrestrial Reference Frame 1994 (ITRF94) coordinates were estimated as X(P) = 1766500.399 m, Y(P) = 1460251.211 m, and Z(P) = -5932214.446 m at the epoch of 1997.0. The total number of normal point data for range measurements attained almost 100% (around 2500 min per week) from March to September, 1997; however, the number of received data decreased significantly thereafter. This may be due to a degraded X-band RF-cable under the cold temperature condition in the radome. Although an on-site tropospheric correction could not be made, post-processing with the surface synoptic data showed improvements of the overall root mean square (rms) range errors from about 50 cm to 30 cm. Application of the PRARE obtained precise orbits to radar altimeter and synthetic aperture radar studies will be discussed in a separate report.
  • Kazuo Shibuya, Koichiro Doi, Shigeru Aoki
    Earth, Planets and Space 51 3 159 - 168 1999年 [査読有り][通常論文]
     
    From the International Terrestrial Reference Frame 1994 (ITRF94) coordinates of the Doppler Orbitography Radiopositioning Integrated by Satellites (DORIS) beacon marker at Syowa Station (69.0°S, 39.6°E), Antarctica, we derived the ellipsoidal height of the Scientific Committee on Antarctic Research (SCAR) Global Positioning System (GPS) point as 42.240 m on the World Geodetic System 1984 (WGS84) associated ellipsoid of a tide free Earth at the epoch of 1993.0. Because the SCAR GPS reference mark was 21.165 m above the local mean sea level at the epoch of 1993.0, and because the sea surface topography is estimated as -1.29 m, the ground data of geoid height can be estimated as 22.37 m on the WGS84 ellipsoid. As for error estimate of the above value, 20-30 cm formal error can be assigned including 3 cm error from the DORIS determination, 1 cm error from the local geodetic tie, and the dominant 20-30 cm error from uncertain modeling of sea surface topography, etc. The EGM96 geoid model gives the synthetic geoid height of 22.10 m at Syowa Station; the discrepancy of 27 cm from the observed value is within the 36 cm cumulative (to the degree 360) rms (root-mean-square) error of the model. We retried similar determination at Breid Bay (70.2°S, 23.8°E) and made a tie to the inland outcropped Seal Rock; the obtained value of 21.4 m has an overall error of 1.8 m. These ground data can be used as test data for generating higher-order (n, m ≥ 360) geopotential models. With the establishment of the International Absolute Gravity Basestation Network (IAGBN) standard value at Syowa Station and gravimetric connection to the Seal Rock, ground data of free-air gravity anomalies of 0.05 mgal accuracy at Syowa Station and 1 mgal accuracy at Seal Rock were obtained. These gravity ground data will also serve as test data for adjusting the satellite- and/or shipborne-derived gravity anomaly maps in the region concerned.
  • Shigeru Aoki
    Journal of Oceanography 53 6 623 - 631 1997年12月 [査読有り][通常論文]
     
    Interannual temperature variation of surface layer (0-400 m) is examined from 12 years (1983-1995) of XBT data obtained by Japanese Antarctic Research Expeditions in the Indian sector of the Southern Ocean. To extract interannual variation, the effect of positional differences in data sampling points is corrected by using the World Ocean Atlas 1994 climatological field. Statistical analysis is made to verify the reliability of the correction qualitatively, and the ambiguity of the obtained signal is discussed. Significant warming trends of 0.02-0.04°C/year are observed at depths of 200-400 m in the western region (50°-90°E, 58°-61°S) from both the observed and positionally corrected time series. Besides these trends, variations of 4-5 year cycles are suggested. The fluctuations at most of the levels except 50-150 m show similar patterns to that at the surface. The effects of different dynamics are suggested in the Winter Water layer.
  • Makoto Omura, Katsuaki Koike, Koichiro Doi, Shigeru Aoki
    International Geoscience and Remote Sensing Symposium (IGARSS) 4 1582 - 1584 1997年 [査読有り][通常論文]
     
    We have constructed a processing system for SAR interferometry using a personal computer. Several images were processed from raw signal data acquired by ERS-1 and JERS-1. An initial interferogram was successfully obtained from ERS-1 SAR image pair. Therefore, the constructed system was proved to be useful. Such a system on a personal computer will contribute to the SAR interferometry in a large range of geoscience.
  • Shigeru Aoki, Shiro Imawaki
    Journal of Oceanography 52 4 457 - 474 1996年08月 [査読有り][通常論文]
     
    Geosat radar altimeter data during the first year (from November 1986 to November 1987) of its Exact Repeat Mission are analyzed to estimate the eddy kinetic energy and propagation characteristics of anomalies of sea surface dynamic topography (SSDT) for the western North Pacific. SSDT anomalies are compared with anomalies of sea surface temperature (SST) derived from NOAA satellite radiometer data. The eddy kinetic energy (Ke) is large in the Kuroshio stationary meander region and Kuroshio Extension region. In the downstream region of the Kuroshio Extension, Ke is especially large on the upstream and downstream sides of prominent bathymetric features. In the interior region of the subtropical gyre is found a zonal tongue of large Ke at around 20-23 °N. Westward propagation is dominant in the SSDT and SST anomaly field at mid-latitudes. Longitude-time lag correlation diagrams reveal the coincidence of SSDT and SST anomalies statistically, which fact suggests the baroclinic nature of the anomalies. Zonal phase speeds of SSDT anomalies are approximately equal to the theoretical speeds of baroclinic first-mode long Rossby waves, but the meridional variation of observed phase speeds does not follow the simple theoretical variation of decreasing speeds monotonously with increasing latitudes.
  • S. Aoki, S. Imawaki, K. Ichikawa
    Journal of Geophysical Research 100 C1 839 - 855 1995年 [査読有り][通常論文]
     
    The Geosat radar altimeter data during the first year of its exact repeat mission are analyzed to investigate the behavior of individual anomalies of sea surface dynamic topography (SSDT) and their statistical properties in the Kuroshio Extension region. The SSDT anomalies are compared with mesoscale anomalies of sea surface temperature (SST) derived from satellite radiometer data. Remarkable baroclinic anomalies are detected in the time series of both SSDT and SST, and their westward propagation is clearly traced. -from Authors
  • 青木 茂
    防食技術 33 9 520 - 526 社団法人 腐食防食協会 1984年 
    Quality of organic materials for chemical equipment like as chemically resistant FRP and organic linings is deeply affected by a degree of skills to apply them. Then, we must care of quality control and inspection for them in order to dissolve any troubles. This paper describes the quality assurance system on organic materials at fablication and their inspection methods for new and used equipment.

書籍

その他活動・業績

  • 溝端浩平, 北出裕二郎, 嶋田啓資, 平野大輔, 松村義正, CHENG Lingqiao, 青木茂, 田村岳史 日本地球惑星科学連合大会予稿集(Web) 2020 2020年
  • 平野大輔, 田村岳史, 溝端浩平, 伊藤優人, 山崎開平, 國府陽一郎, 木内政彰, 村瀬弘人, 佐々木裕子, 青木茂 日本海洋学会大会講演要旨集 2019 2019年
  • 北出裕二郎, 溝端浩平, 渡部和帆, CHEN Linquiao, GUO Gaopin, 青木茂, 牛尾収輝, 嶋田啓資, 大島慶一郎 日本地球惑星科学連合大会予稿集(Web) 2019 2019年
  • 青木茂, 市川雅明, 市川雅明, 小野数也, 深町康, 深町康, 深町康, 大島慶一郎, 大島慶一郎, 中川敏彦, 小林研吾, 小竹正人, 小竹正人, 小澤知史 海洋理工学会誌(Web) 25 (1) 29‐34(J‐STAGE) 2019年 [査読無し][通常論文]
  • 北出裕二郎, 嶋田啓資, 溝端浩平, 青木茂, 田村岳史, 千手智晴, 深町康, 大島慶一郎 日本海洋学会大会講演要旨集 2017 2017年
  • 箕輪昌紘, 箕輪昌紘, 杉山慎, 澤柿教伸, 津滝俊, 榊原大貴, 榊原大貴, 大橋良彦, 大橋良彦, 青木茂 雪氷研究大会講演要旨集(Web) 2016 2016年
  • 嶋田啓資, 北出裕二郎, 溝端浩平, 青木茂, 大島慶一郎, 田村岳史, 小達恒夫 日本地球惑星科学連合大会予稿集(Web) 2016 2016年
  • 箕輪 昌紘, 杉山 慎, 澤柿 教伸, 津滝 俊, 榊原 大貴, 大橋 良彦, 青木 茂 雪氷研究大会講演要旨集 2016 (0) 29 -29 2016年 [査読無し][通常論文]
  • Shinichiro Kida, Humio Mitsudera, Shigeru Aoki, Xinyu Guo, Shin ichi Ito, Fumiaki Kobashi, Nobumasa Komori, Atsushi Kubokawa, Toru Miyama, Ryosuke Morie, Hisashi Nakamura, Tomohiro Nakamura, Hideyuki Nakano, Hajime Nishigaki, Masami Nonaka, Hideharu Sasaki, Yoshi N. Sasaki, Toshio Suga, Shusaku Sugimoto, Bunmei Taguchi, Koutarou Takaya, Tomoki Tozuka, Hiroyuki Tsujino, Norihisa Usui Journal of Oceanography 71 (5) 469 -497 2015年10月29日 [査読無し][通常論文]
     
    © 2015, The Oceanographic Society of Japan and Springer Japan. This article reviews progress in our understanding of oceanic fronts around Japan and their roles in air–sea interaction. Fronts associated with the Kuroshio and its extension, fronts within the area of the Kuroshio-Oyashio confluence, and the subtropical fronts are described with particular emphasis on their structure, variability, and role in air–sea interaction. The discussion also extends to the fronts in the coastal and marginal seas, the Seto Inland Sea and Japan Sea. Studies on oceanic fronts have progressed significantly during the past decade, but many of these studies focus on processes at individual fronts and do not provide a comprehensive view. Hence, one of the goals of this article is to review the oceanic fronts around Japan by describing the processes based on common metrics. These metrics focus primarily on surface properties to obtain insights into air–sea interactions that occur along oceanic fronts. The basic characteristics derived for each front (i.e., metrics) are then presented as a table. We envision that many of the coupled ocean-atmosphere global circulation models in the coming decade will represent oceanic fronts reasonably well, and it is hoped that this review along with the table of metrics will provide a useful benchmark for evaluating these models.
  • 未知の南極底層水生成域の発見と今後の観測に向けて―ケープダンレープロジェクト―
    大島慶一郎, 深町康, 青木茂, 清水大輔, 田村岳史, 牛尾収輝, 橋田元, 北出裕二郎, 若土正暁 月刊海洋 総特集「南大洋の深層水形成と海氷過程」 54、12-20 2010年 [査読無し][通常論文]
  • P. A. Mayewski, M. P. Meredith, C. P. Summerhayes, J. Turner, A. Worby, P. J. Barrett, G. Casassa, N. A.N. Bertler, T. Bracegirdle, A. C. Naveira Garabato, D. Bromwich, H. Campbell, G. S. Hamilton, W. B. Lyons, K. A. Maasch, S. Aoki, C. Xiao, Tas Van Ommen Reviews of Geophysics 47 (1) 2009年03月 [査読無し][通常論文]
     
    This paper reviews developments in our understanding of the state of the Antarctic and Southern Ocean climate and its relation to the global climate system over the last few millennia. Climate over this and earlier periods has not been stable, as evidenced by the occurrence of abrupt changes in atmospheric circulation and temperature recorded in Antarctic ice core proxies for past climate. Two of the most prominent abrupt climate change events are characterized by intensification of the circumpolar westerlies (also known as the Southern Annular Mode) between ̃6000 and 5000 years ago and since 1200-1000 years ago. Following the last of these is a period of major trans-Antarctic reorganization of atmospheric circulation and temperature between A.D. 1700 and 1850. The two earlier Antarctic abrupt climate change events appear linked to but predate by several centuries even more abrupt climate change in the North Atlantic, and the end of the more recent event is coincident with reorganization of atmospheric circulation in the North Pacific. Improved understanding of such events and of the associations between abrupt climate change events recorded in both hemispheres is critical to predicting the impact and timing of future abrupt climate change events potentially forced by anthropogenic changes in greenhouse gases and aerosols. Special attention is given to the climate of the past 200 years, which was recorded by a network of recently available shallow firn cores, and to that of the past 50 years, which was monitored by the continuous instrumental record. Significant regional climate changes have taken place in the Antarctic during the past 50 years. Atmospheric temperatures have increased markedly over the Antarctic Peninsula, linked to nearby ocean warming and intensification of the circumpolar westerlies. Glaciers are retreating on the peninsula, in Patagonia, on the subAntarctic islands, and in West Antarctica adjacent to the peninsula. The penetration of marine air masses has become more pronounced over parts of West Antarctica. Above the surface, the Antarctic troposphere has warmed during winter while the stratosphere has cooled yearround. The upper kilometer of the circumpolar Southern Ocean has warmed, Antarctic Bottom Water across a wide sector off East Antarctica has freshened, and the densest bottom water in the Weddell Sea has warmed. In contrast to these regional climate changes, over most of Antarctica, near-surface temperature and snowfall have not increased significantly during at least the past 50 years, and proxy data suggest that the atmospheric circulation over the interior has remained in a similar state for at least the past 200 years. Furthermore, the total sea ice cover around Antarctica has exhibited no significant overall change since reliable satellite monitoring began in the late 1970s, despite large but compensating regional changes. The inhomogeneity of Antarctic climate in space and time implies that recent Antarctic climate changes are due on the one hand to a combination of strong multidecadal variability and anthropogenic effects and, as demonstrated by the paleoclimate record, on the other hand to multidecadal to millennial scale and longer natural variability forced through changes in orbital insolation, greenhouse gases, solar variability, ice dynamics, and aerosols. Model projections suggest that over the 21st century the Antarctic interior will warm by 3.4° ± 1°C, and sea ice extent will decrease by ̃30%. Ice sheet models are not yet adequate enough to answer pressing questions about the effect of projected warming on mass balance and sea level. Considering the potentially major impacts of a warming climate on Antarctica, vigorous efforts are needed-to better understand all aspects of the highly coupled Antarctic climate system as well as its influence on the Earth's climate and oceans. Copyright 2009 by the American Geophysical Union.
  • アオキ シゲル, ハシダ ゲン, Shigeru AOKI, Gen HASHIDA JARE data reports. Oceanography = JARE data reports. Oceanography 22 1 -18 2001年03月

受賞

  • 2017年08月 日本気象学会 堀内賞

教育活動情報

主要な担当授業

  • 大学院共通授業科目(教育プログラム):南極学
    開講年度 : 2021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 : Polar regions, the Antarctic, the Arctic, sea ice, ice sheet, atmosphere, ocean, climate system, ecosystem, Antarctic expeditions, Antarctic treaty
  • 極域海洋学特論
    開講年度 : 2021年
    課程区分 : 修士課程
    開講学部 : 環境科学院
    キーワード : 南極海, 北極海, オホーツク海, 海氷, 陸氷, 底層水, 深層水, 中層水, 気候変動 Antarctic Ocean, Arctic Ocean, Okhotsk Sea, Sea ice, Land ice, Bottom water, Deep water, Intermediate water, Climate change
  • 大気海洋解析法特論
    開講年度 : 2021年
    課程区分 : 修士課程
    開講学部 : 環境科学院
    キーワード : データ解析,統計,大気,海洋,気候 Data analysis, Statistics, Atmosphere, Ocean, Climate
  • 一般教育演習(フレッシュマンセミナー)
    開講年度 : 2021年
    課程区分 : 学士課程
    開講学部 : 全学教育
    キーワード : 南極海、北極海、オホーツク海、南極・北極、海洋大循環、気候変動、海氷、温暖化、衛星観測、IPCC


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