基本情報

所属

• 低温科学研究所　共同研究推進部

職名

• 教授

学位

• 博士（理学）

科研費研究者番号

• 80281583

J-Global ID

• 201101039637625217

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

担当教育組織

•  修士課程　環境科学院
•  博士課程　環境科学院

所属学協会

• 海洋理工学会   日本海洋学会   

研究活動情報

論文

• Oceanographic factors determining the distribution of nutrients and primary production in the subpolar Southern Ocean

  Manami Tozawa, Daiki Nomura, Kaihe Yamazaki, Masaaki Kiuchi, Daisuke Hirano, Shigeru Aoki, Hiroko Sasaki, Hiroto Murase

  Progress in Oceanography 103266 - 103266 2024年04月
• Observing Antarctic Bottom Water in the Southern Ocean

  Alessandro Silvano, Sarah Purkey, Arnold L. Gordon, Pasquale Castagno, Andrew L. Stewart, Stephen R. Rintoul, Annie Foppert, Kathryn L. Gunn, Laura Herraiz-Borreguero, Shigeru Aoki, Yoshihiro Nakayama, Alberto C. Naveira Garabato, Carl Spingys, Camille Hayatte Akhoudas, Jean-Baptiste Sallée, Casimir de Lavergne, E. Povl Abrahamsen, Andrew J. S. Meijers, Michael P. Meredith, Shenjie Zhou, Takeshi Tamura, Kaihe Yamazaki, Kay I. Ohshima, Pierpaolo Falco, Giorgio Budillon, Tore Hattermann, Markus A. Janout, Pedro Llanillo, Melissa M. Bowen, Elin Darelius, Svein Østerhus, Keith W. Nicholls, Craig Stevens, Denise Fernandez, Laura Cimoli, Stanley S. Jacobs, Adele K. Morrison, Andrew McC. Hogg, F. Alexander Haumann, Ali Mashayek, Zhaomin Wang, Rodrigo Kerr, Guy D. Williams, Won Sang Lee

  Frontiers in Marine Science 10 2023年12月08日 

  Dense, cold waters formed on Antarctic continental shelves descend along the Antarctic continental margin, where they mix with other Southern Ocean waters to form Antarctic Bottom Water (AABW). AABW then spreads into the deepest parts of all major ocean basins, isolating heat and carbon from the atmosphere for centuries. Despite AABW’s key role in regulating Earth’s climate on long time scales and in recording Southern Ocean conditions, AABW remains poorly observed. This lack of observational data is mostly due to two factors. First, AABW originates on the Antarctic continental shelf and slope where in situ measurements are limited and ocean observations by satellites are hampered by persistent sea ice cover and long periods of darkness in winter. Second, north of the Antarctic continental slope, AABW is found below approximately 2 km depth, where in situ observations are also scarce and satellites cannot provide direct measurements. Here, we review progress made during the past decades in observing AABW. We describe 1) long-term monitoring obtained by moorings, by ship-based surveys, and beneath ice shelves through bore holes; 2) the recent development of autonomous observing tools in coastal Antarctic and deep ocean systems; and 3) alternative approaches including data assimilation models and satellite-derived proxies. The variety of approaches is beginning to transform our understanding of AABW, including its formation processes, temporal variability, and contribution to the lower limb of the global ocean meridional overturning circulation. In particular, these observations highlight the key role played by winds, sea ice, and the Antarctic Ice Sheet in AABW-related processes. We conclude by discussing future avenues for observing and understanding AABW, impressing the need for a sustained and coordinated observing system.
• Distribution of stable oxygen isotope in seawater and implication on freshwater cycle off the coast from Wilkes to George V Land, East Antarctica

  Shigeru Aoki, Kaihe Yamazaki, Daisuke Hirano, Daiki Nomura, Hiroto Murase

  Progress in Oceanography 217 103101 - 103101 2023年09月
• Modeling seasonal-to-decadal ocean-cryosphere interactions along the Sabrina Coast, East Antarctica

  Kazuya Kusahara, Daisuke Hirano, Masakazu Fujii, Alexander Fraser, Takeshi Tamura, Kohei Mizobata, Guy Williams, Shigeru Aoki

  2023年08月21日 

  Abstract. The Totten Ice Shelf (TIS) and Moscow University Ice Shelf (MUIS), along the Sabrina Coast of Wilkes Land, are the floating seaward terminuses of the second-largest freshwater reservoir in the East Antarctic Ice Sheet. Being a marine ice sheet, it is vulnerable to the surrounding ocean conditions. Recent comprehensive oceanographic observations, including bathymetric measurements off the Sabrina Coast, have shed light on the widespread intrusion of warm modified Circumpolar Deep Water (mCDW) onto the continental shelf and the intense ice-ocean interaction beneath the TIS. However, the spatiotemporal coverage of the observation is very limited. Here, we use an ocean–sea ice–ice shelf model with updated bathymetry to better understand the regional ocean circulations and ocean-cryosphere interactions. The model successfully captured the widespread intrusions of mCDW, local sea-ice production and the ocean heat and volume transports into the TIS cavity, facilitating an examination of the overturning ocean circulation within the cavities and the resultant ice-shelf basal melting. We found notable differences in the temporal variability of ice-shelf basal melting across the two adjacent ice shelves of the TIS and the western part of the MUIS. Ocean heat transport by mCDW controls the low-frequency interannual-to-decadal variability in ice-ocean interactions, but the sea-ice production in the Dalton Polynya strongly modifies the signals, explaining the regional difference between the two ice shelves. The formation of a summertime eastward-flowing undercurrent beneath the westward-flowing Antarctic Slope Current is found to play an important role in the seasonal delivery of ocean heat to the continental shelf.
• Biogeochemical characteristics of brash sea ice and icebergs during summer and autumn in the Indian sector of the Southern Ocean

  Daiki Nomura, Reishi Sahashi, Keigo D. Takahashi, Ryosuke Makabe, Masato Ito, Manami Tozawa, Pat Wongpan, Ryo Matsuda, Masayoshi Sano, Michiyo Yamamoto-Kawai, Natsumi Nojiro, Aiko Tachibana, Norio Kurosawa, Masato Moteki, Takeshi Tamura, Shigeru Aoki, Hiroto Murase

  Progress in Oceanography 214 103023 - 103023 2023年04月 [査読有り]
  
• Diffusion of Circumpolar Deep Water towards Antarctica

  Kaihe Yamazaki, Shigeru Aoki, Kohei Mizobata

  Journal of Geophysical Research: Oceans 2023年02月06日 [査読有り]
  
• The biogeochemistry of zinc and cadmium in the Amundsen Sea, coastal Antarctica

  Hung-An Tian, Mathijs van Manen, Flora Wille, Jinyoung Jung, SangHoon Lee, Tae-Wan Kim, Shigeru Aoki, Charlotte Eich, Corina P.D. Brussaard, Gert-Jan Reichart, Tim M. Conway, Rob Middag

  Marine Chemistry 249 104223 - 104223 2023年02月 [査読有り]
  
• On-Shelf Circulation of Warm Water Toward the Totten Ice Shelf, East Antarctica

  Hirano, D, T. Tamura, K. Kusahara, M. Fujii, K. Yamazaki, Y. Nakayama, K. Ono, T. Itaki, Y. Aoyama, D. Simizu, K. Mizobata, K. I. Ohshima, Y. Nogi, S. R. Rintoul, E. Wijk, J. S. Greenbaum, D. D. Blankenship, K. Saito, S. Aoki

  Nature Communications 14 4995 2023年 [査読有り][通常論文]
   

  Abstract The Totten Glacier in East Antarctica, with an ice volume equivalent to >3.5 m of global sea-level rise, is grounded below sea level and, therefore, vulnerable to ocean forcing. Here, we use bathymetric and oceanographic observations from previously unsampled parts of the Totten continental shelf to reveal on-shelf warm water pathways defined by deep topographic features. Access of warm water to the Totten Ice Shelf (TIS) cavity is facilitated by a deep shelf break, a broad and deep depression on the shelf, a cyclonic circulation that carries warm water to the inner shelf, and deep troughs that provide direct access to the TIS cavity. The temperature of the warmest water reaching the TIS cavity varies by ~0.8 °C on an interannual timescale. Numerical simulations constrained by the updated bathymetry demonstrate that the deep troughs play a critical role in regulating ocean heat transport to the TIS cavity and the subsequent basal melt of the ice shelf.
• The role of the Dotson Ice Shelf and Circumpolar Deep Water as driver and source of dissolved and particulate iron and manganese in the Amundsen Sea polynya, Southern Ocean

  Mathijs van Manen, Shigeru Aoki, Corina P.D. Brussaard, Tim M. Conway, Charlotte Eich, Loes J.A. Gerringa, Jinyoung Jung, Tae-Wan Kim, SangHoon Lee, Youngju Lee, Gert-Jan Reichart, Hung-An Tian, Flora Wille, Rob Middag

  Marine Chemistry 246 104161 - 104161 2022年10月 [査読有り]
  
• Impacts of basal melting of the Totten Ice Shelf and biological productivity on marine biogeochemical components in Sabrina Coast, East Antarctica

  Tetsuya P. Tamura, Daiki Nomura, Daisuke Hirano, Takeshi Tamura, Masaaki Kiuchi, Gen Hashida, Ryosuke Makabe, Kazuya Ono, Shuki Ushio, Kaihe Yamazaki, Yoshihiro Nakayama, Keigo D. Takahashi, Hiroko Sasaki, Hiroto Murase, Shigeru Aoki

  Global Biogeochemical Cycles 2022年09月13日 [査読有り]
   

  Abstract To clarify the impacts of basal melting of the Antarctic ice sheet and biological productivity on biogeochemical processes in Antarctic coastal waters, concentrations of dissolved inorganic carbon (DIC), total alkalinity (TA), inorganic nutrients, chlorophyll a, and stable oxygen isotopic ratios (δ¹⁸O) were measured from the offshore slope to the ice front of the Totten Ice Shelf (TIS) during the spring/summer of 2018, 2019, and 2020. Modified Circumpolar Deep Water (mCDW) intruded onto the continental shelf off the TIS and flowed along bathymetric troughs into the TIS cavity, where it formed a buoyant mixture with glacial meltwater from the ice shelf base. Physical oceanographic processes mostly determined the distributions of DIC, TA, and nutrient concentrations. However, photosynthesis and dilution by meltwater from sea ice and the ice shelf base decreased DIC, TA, and nutrient concentrations in surface water near the ice front. These causes also reduced the CO₂ partial pressure in surface water by more than 100 μatm with respect to mCDW in austral summer of 2018 and 2020, and the surface water became a strong CO₂ sink for the atmosphere. Phytoplankton photosynthesis changed DIC and TA in a molar ratio of 106:16. Thus, CO₂ partial pressure decreased mostly as a result of photosynthesis while dilution by glacial and sea ice meltwater had a small effect. The nutrient consumption ratio suggested that photosynthesis was stimulated by iron in the water column, supplied to the surface layer via buoyancy‐driven upwelling and basal ice shelf meltwater in addition to sea ice meltwater.
• Interactive movements of outlet glacier tongue and landfast sea ice in Lützow-Holm Bay, East Antarctica, detected by ALOS-2/PALSAR-2 imagery

  Kazuki Nakamura, Shigeru Aoki, Tsutomu Yamanokuchi, Takeshi Tamura

  Science of Remote Sensing 100064 - 100064 2022年08月 [査読有り]
  
• Effects of Snow and Remineralization Processes on Nutrient Distributions in Multi‐Year Antarctic Landfast Sea Ice

  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 [査読有り]
  
• Warm surface waters increase Antarctic ice shelf melt and delay dense water formation

  Shigeru Aoki, Tomoki Takahashi, Kaihe Yamazaki, Daisuke Hirano, Kazuya Ono, Kazuya Kusahara, Takeshi Tamura, Guy D. Williams

  Communications Earth & Environment 3 1 2022年06月 [査読有り]
   

  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.
• Shoaling of abyssal ventilation in the Eastern Indian Sector of the Southern Ocean

  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]
  
• Seasonal Variation and Governing Dynamics of the Mixed Layer in the Indian Sector of the Southern Ocean

  Matheus Ferreira Azevedo, Shigeru Aoki, Yujiro Kitade

  Journal of Geophysical Research: Oceans 127 4 2022年04月04日 [査読有り]
  
• Validation for Ice Flow Velocity Variations of Shirase Glacier Derived From PALSAR-2 Offset Tracking

  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04月 [査読有り]
  
• Helicopter-based ocean observations capture broad ocean heat intrusions towards the Totten Ice Shelf

  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 50 17 in press  2022年 [査読有り][通常論文]
   

  Abstract The recent discovery of warm ocean water near the Totten Ice Shelf (TIS) has increased attention to the Sabrina Coast in East Antarctica. We report the result of 6‐day helicopter‐based observations conducted during the 61st Japanese Antarctic Research Expedition (JARE61), revealing warm ocean water (0.5–1°C) occupying a large previously unsampled area of the Sabrina Coast (116.5°E−120°E) below 550–600 m. Along the TIS front, we observe modified Circumpolar Deep Water (mCDW) well above freezing (∼−0.7°C), consistent with previous work. We identify glacial meltwater outflow from the TIS cavity west of 116°E. No signs of mCDW intrusions toward the Moscow University Ice Shelf cavity are observed; however, those observations were limited to only two shallow (∼330 m) profiles. We also highlight the advantages of helicopter‐based observations for accessibility, speed, maneuverability, and cost‐efficiency. The combination of ship‐ and helicopter‐based observations using the JARE61 approach will increase the potential of future polar oceanographic observations.
• Seasonal Variations and Drivers of Surface Ocean pCO2 in the Seasonal Ice Zone of the Eastern Indian Sector, Southern Ocean

  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 [査読有り]
   

  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.
• Thermohaline structure and circulation beneath the Langhovde Glacier ice shelf in East Antarctica

  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.
• Shoreward intrusion of oceanic surface waters alters physical and biological ocean structures on the Antarctic continental shelf during winter: Observations from instrumented seals

  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.
• Tracing Circumpolar Deep Water and glacial meltwater using humic-like fluorescent dissolved organic matter in the Amundsen Sea, Antarctica

  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.
• Antarctic Slope Current Modulates Ocean Heat Intrusions Towards Totten Glacier

  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.
• Poleward eddy-induced warm water transport across a shelf break off Totten Ice Shelf, East Antarctica

  Daisuke Hirano, Kohei Mizobata, Hiroko Sasaki, Hiroto Murase, Takeshi Tamura, Shigeru Aoki

  Communications Earth & Environment 2 1 2021年08月06日 [査読有り]
   

  Abstract Ice 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.
• Multidecadal poleward shift of the southern boundary of the Antarctic Circumpolar Current off East Antarctica

  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.
• The Effect of Basal Melting of the Shirase Glacier Tongue on the CO2 System in Lützow-Holm Bay, East Antarctica

  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.
• Reversal of freshening trend of Antarctic Bottom Water in the Australian-Antarctic Basin during 2010s

  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.
• Using under-ice hyperspectral transmittance to determine land-fast sea-ice algal biomass in Saroma-ko Lagoon, Hokkaido, Japan

  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 83 454 - 463 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 chlorophylla(Chl.a), respectively. Spatial analyses unveiled the patch size of sea-ice Chl.ato 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.
• Strong ice-ocean interaction beneath Shirase Glacier Tongue in East Antarctica

  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08月24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.
• Freshening of Antarctic Bottom Water Off Cape Darnley, East Antarctica

  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.
• Structure of the Subpolar Gyre in the Australian-Antarctic Basin Derived From Argo Floats

  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.
• The Cyclonic Eddy Train in the Indian Ocean Sector of the Southern Ocean as Revealed by Satellite Radar Altimeters and In Situ Measurements

  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.
• Continuous winter oceanic profiling in the Cape Darnley Polynya, East Antarctica

  Shigeru Aoki, Kazuya Ono, Daisuke Hirano, Takeshi Tamura

  Journal of Oceanography 76 5 365 - 372 2020年05月25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.
• Vertical distribution of water mass properties under the influence of subglacial discharge in Bowdoin Fjord, northwestern Greenland

  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.
• Saroma-ko Lagoon observations for sea ice physico-chemistry and ecosystems 2019 (SLOPE2019)

  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.
• Fluctuations of the Ice Flow Velocity of Shirase Glacier and its Surrounding Landfast Ice Displacement in East Antarctica Derived from alos-2/Palsar-2 Image Correlation

  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.
• Recent research results and future project in the Antarctic Ocean by Umitaka-Maru research group for physical oceanography

  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.
• Freshening by glacial meltwater enhances melting of ice shelves and reduces formation of Antarctic Bottom Water

  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.
• Influence of Sea Ice Crack Formation on the Spatial Distribution of Nutrients and Microalgae in Flooded Antarctic Multiyear Ice

  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.
• Changes in water properties and flow regime on the continental shelf off the Adélie/George V Land coast, East Antarctica, after glacier tongue calving

  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.
• Breakup of land-fast sea ice in Lützow-Holm Bay, East Antarctica, and its teleconnection to tropical Pacific sea surface temperatures

  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.
• Modeling ocean-cryosphere interactions off adélie and George V Land, East Antarctica

  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.
• Creation of a gridded dataset for the southern ocean with a topographic constraint scheme

  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.
• Oceanic fronts and jets around Japan: A review

  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.
• Atlantic-Pacific asymmetry of subsurface temperature change and frontal response of the antarctic circumpolar current for the recent three decades

  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.
• Spatial and temporal variations in high turbidity surface water off the Thule region, northwestern Greenland

  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.
• Atlantic–Pacific asymmetry of subsurface temperature change and frontal response of the Antarctic Circumpolar Current for the recent three decades

  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.
• Multidecadal warming of Antarctic waters

  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.
• Active water exchange and life near the grounding line of an Antarctic outlet glacier

  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.
• Antarctic bottom water production from the Vincennes Bay Polynya, East Antarctica

  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.
• Observations: Ocean

  Rhein, M, S.R. Rintoul, S. Aoki, E. Campos, D. Chambers, R. A. Feely, S. Gulev, G. C. Johnson, S. A. Josey, A. Kostianoy, C. Mauritzen, D. Roemmich, L. D. Talley, F. Wang

  Climate Change 2013: The Physical Science Basis. Contribution of Working Group I to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change 255 - 316 2014年01月 [査読有り]
  
• Use of L-band SAR data for monitoring of ice sheet marginal zone on East Antarctica

  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).
• Distribution of oxygen isotope ratio of precipitation in the Atlantic-Indian sectors of the Southern Ocean

  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.
• Widespread freshening in the Seasonal Ice Zone near 140°E off the Adélie Land Coast, Antarctica, from 1994 to 2012

  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.
• Antarctic Bottom Water production by intense sea-ice formation in the Cape Darnley polynya

  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.
• Quasi-decadal circumpolar variability of antarctic sea ice

  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.
• Monitoring of the changes of glacier and ice sheet on polar region by L-band SAR data

  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.
• Influence of Ross Sea Bottom Water changes on the warming and freshening of the Antarctic Bottom Water in the Australian-Antarctic Basin

  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.
• Incorporation of nitrogen compounds into sea ice from atmospheric deposition

  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.
• Monitoring of east Antarctic marginal zone using ALOS/PALSAR data

  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.
• The cyclonic circulation in the Australian-Antarctic basin simulated by an eddy-resolving general circulation model

  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.
• Surface oceanography of BROKE-West, along the Antarctic margin of the south-west Indian Ocean (30 - 80^{{ring operator}} E)

  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.
• Strong export of Antarctic Bottom Water east of the Kerguelen plateau

  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.
• Antarctic bottom water from the adélie and George v Land Coast, East Antarctica (140-149°E)

  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.
• State of the antarctic and southern ocean climate system

  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.
• The Southern Annular Mode and opposite-phased Basin Mode of the Southern Ocean circulation

  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.
• Deep western boundary current and southern frontal systems of the Antarctic Circumpolar Current southeast of the Kerguelen Plateau

  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.
• Development of grounding line database using ers-1/2 data by insar technique

  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.
• Formation regions of Subantarctic Mode Water detected by OFES and Argo profiling floats

  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.
• A series of cyclonic eddies in the Antarctic Divergence off Adélie Coast

  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.
• Observations of small-scale disturbances of the Subantarctic Front south of Australia

  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.
• Frontal positions and mixed layer evolution in the seasonal ice zone along 140°E in 2001/02

  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.
• Freshening of the Adélie land bottom water near 140 °E

  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.
• Interdecadal water mass changes in the Southern Ocean between 30°E and 160°E

  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.
• Characteristics of sea ice thickness and snow depth distributions of the land-fast ice during summer in Lützow-Holmbukta, Antarctica, revealed from ship-based observations

  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.
• Combined utilization of GPS and bottom pressure gauge in landfast ice region around Antarctica

  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.
• Seasonal and spatial variations of iceberg drift off Dronning Maud Land, Antartica, detected by satellite scatterometers

  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.
• Eddies revealed by SeaWiFS ocean color images in the Antarctic Divergence zone near 140°E

  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.
• Sea level variation in seismic normal mode band observed with on-ice GPS and on-hand SG at Syowa Station, Antarctica

  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.
• Multidecadal warming of subsurface temperature in the Indian sector of the Southern Ocean

  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.
• Tidal observations on floating ice using a single GPS receiver

  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.
• Coherent sea level response to the Antarctic Oscillation

  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.
• Sea surface height determination by GPS in sea ice region of Lutzow-Holm Bay, Antarctica

  Doi Koichiro, Seo Noritsune, Aoki Shigeru, Shibuya Kazuo

  Polar geoscience 15 104 - 111 国立極地研究所 2002年10月 

  Global Positioning System (GPS) measurements were conducted at five sites on sea ice in Liitzow-Holm Bay, Antarctica We deteimined sea surface height (SSH) at each point aftei correcting for ocean tides, atmospheric pressuie and vertical distance from sea surface to ice suiface The overall erroi of the obtained SSH is estimated as smaller than 10cm We also obtained sea surface dynamic height (SSDT) by subtiacting synthetic geoidal height from the observed SSH The obtained SSDT values show good agreement with those calculated from the fine resolution Antaictic model (FRAM) After taking tempoial variations ot SSH into consideration the values took higher values at the southern end of the bay than at the northern end of the bay This spatial feature corresponds to the southward increase of SSDT across the continental slope
• Evaluation of seasonal sea level variation at Syowa Station, Antarctica, using GPS observations

  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.
• Ocean Tide Observed with Differential GPS Technique in Lützow-Holm Bay, Antarctica

  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.
• Observations of water temperature and salinity in Ongul Strait, Antarctica, in 1998 and investigations of their intraseasonal, seasonal, and interannual variations

  Aoki Shigeru, Hashida Gen

  Polar meteorology and glaciology 14 68 - 77 国立極地研究所 2000年11月 

  Water temperature and salinity variations were observed below fast ice in Ongul Strait, Antarctica, in 1998. Vertical profiles of temperature and salinity were obtained with a conductivity-temperature-depth profiler, and temperature and salinity were continuously observed with moored sensors. Intraseasonal variations were observed in both fields. Especially, temperature variation of about 0.05℃ was apparent on the time scale of about one month near the thermocline depth. Seasonal variations of temperature and salinity were similar to those observed in 1982-1983 and 1990-1991. However, the temporally averaged temperature and salinity for 1998 showed differences from those for previous years; temperature was higher in 1998 than in 1982-1983 and 1990-1991 at middle to deep layers, and salinity was intermediate between 1982 (more saline) and 1990-1991 (fresher) throughout the water column.
• GPS observation of the sea level variation in Lutzow-Holm Bay, Antarctica

  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.
• Installation and operation of PRARE tracking antenna at Syowa Station, Antarctica

  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.
• Precise determination of geoid height and free-air gravity anomaly at Syowa Station, Antarctica

  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.
• Trends and interannual variability of surface layer temperature in the Indian sector of the Southern Ocean observed by Japanese Antarctic Research Expeditions

  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.
• SAR data processing for interferometry using a personal computer

  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.
• Eddy activities of the surface layer in the western North Pacific detected by satellite altimeter and radiometer

  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.
• Baroclinic disturbances propagating westward in the Kuroshio extension region as seen by a satellite altimeter and radiometers

  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

書籍

• 海の温暖化 : 変わりゆく海と人間活動の影響

  日本海洋学会 (担当:分担執筆範囲:第2.3.1節)
  朝倉書店 2017年07月 (ISBN: 9784254161304) viii, 154p, 巻頭図版 [8] p
• 低温環境の科学事典

  青木茂 (担当:分担執筆範囲:第5章、第9章)
  朝倉書店 2016年07月 (ISBN: 9784254963922) 1 online resource (xii, 411p, 図版 [8] p)
• 低温科学便覧

  北海道大学低温科学研究所 (担当:分担執筆)
  丸善出版 2015年10月 (ISBN: 9784621089644) xvii, 383p
• 地球温暖化 : そのメカニズムと不確実性

  日本気象学会地球環境問題委員会, 日本気象学会 (担当:分担執筆範囲:6.6章、6.7章)
  朝倉書店 2014年12月 (ISBN: 9784254161267) iv, 162p, 図版 [8] p
• 南極海ダイナミクスをめぐる地球の不思議

  青木 茂 
  シーアンドアール研究所 2013年06月 (ISBN: 9784863547162) 255p
• 南極海－氷の海の生態系－

  青木茂 (担当:分担執筆範囲:第4章 ロス海で環境異変は起きているのか？)
  東海大学出版会 2013年01月 (ISBN: 9784486018896)

講演・口頭発表等

• Oceanographic and geophysical observations off Sabrina Coast, East Antarctica, in 2019/2020  [招待講演]

  S.Aoki, T.Tamura, Y.Nakayama, K.Ono, P.Wongpan, K.Yamazaki, T.Itaki, Y.Tokuda, S.Sasaki, D.Hirano, Y.Aoyama

  JpGU-AGU 2020 2020年07月
• Roles of warm and cold sea water pumps along the coast of East Antarctica  [招待講演]

  Shigeru Aoki

  The 1st GRAntarctic International Symposium 2018年12月
• 南大洋における大気・海洋結合系の長期変動に関する観測的研究  [招待講演]

  青木茂

  日本気象学会2017年度堀内賞受賞記念講演 2017年10月
• Research of Oceain-ice BOundary InTeraction adn Change around Antarctica (ROBOTICA): A strategy to explore ice-ocean interactions in East Antarctica  [招待講演]

  Shigeru Aoki, Takeshi Tamura

  AOGS 2017 2017年08月
• 南極大陸棚域における淡水供給 -その空間分布と近年の変化-  [招待講演]

  青木茂

  Japan Geoscience Union Meeting 2014 2014年05月
• 近年の極域海洋変動と淡水循環の役割  [招待講演]

  青木茂

  日本気象学会2011年春季大会シンポジウム「変動する地球気候の鍵‐南極・北極‐」 2011年05月
• Recent freshening of the Antarctic Bottom Water in the Australian-Antarctic basin  [招待講演]

  Shigeru Aoki

  EGU General Assembly 2007 2007年04月
• Interdecadal water-mass changes in the;Indian sector of the;Southern Ocean;heir impacts on;sea level rise  [招待講演]

  Aoki, S, N.L. Bindoff, J.A. Church

  IAMAS 2005 2005年08月
• 南極海ダイナミクス -気候変動の鍵を握る南極底層水-  [招待講演]

  青木茂

  第19回電熱大会 2004年11月
• Large-scale variations of sea level in the Southern Ocean  [招待講演]

  Shigeru Aoki

  Asia Oceania Geoscience Society, 1st meeting 2004年07月
• Observations of annular variations of the Southern Ocean  [招待講演]

  Shigeru Aoki

  Southern Ocean Week 2003 2003年09月
• Oceanic response to the Antarctic Oscillation  [招待講演]

  Shigeru Aoki

  IUGG 2003 2003年07月

その他活動・業績

• 南極海と地球の気候

  青木茂 極地 58 (1) 3 -6 2022年03月 [査読無し]
  
• 大気海洋雪氷系における大洋・半球スケールの鉛直結合変動

  中村尚, 行本誠史, 小寺邦彦, 水田亮, 青木茂, 小川史明, 西井和晃, 宮坂貴文, 吉田聡, 宮本歩, 岡島悟, 田口文明, 野中正見, 升永竜介, 村崎万代 気象研究ノート「気候系のHotspot：中緯度大気海洋相互作用研究の新展開」 244 (244) 245 -277 2021年10月
• 南極の海に探る地球の未来

  青木茂 東陽テクニカルマガジン 30 2 -5 2020年09月
• 第61次南極地域観測隊の計画概要

  青木茂 極地 56 79 -88 2020年03月
• 係留系観測で得られたビンセネス湾沖南極底層水の時系列変化

  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
• トッテン棚氷近傍海域における暖水分布

  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
• 南大洋子午面循環の直接測流と定量評価を目指した係留観測

  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
• 極域海洋モニタリングブイの開発‐オホーツク海における試験観測

  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]
  
• 南大洋における観測からみた海洋長期変動

  青木茂 低温科学 76 25 -31 2018年03月
• 南大洋における大気・海洋結合系の長期変動に関する観測的研究 ―2017年度堀内賞受賞記念講演―

  青木茂 天気 65 (6) 27 -33 2018年
• 南極・白瀬氷河の20年間の流動速度変動 (衛星観測特集)

  中村 和樹, 山之口 勤, 青木 茂, 土井 浩一郎, 澁谷 和雄 雪氷 79 (1) 3 -15 2017年01月
• 氷河融解水を起源とする高濁度水プルームの数値モデリング (グリーンランド氷床とその気候システムとの相互作用)

  松村 義正, 大橋 良彦, 青木 茂, 杉山 慎 低温科学 75 77 -84 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.
• ビンセネス湾沖における巨大係留系による長期観測

  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
• グリーンランド北西部におけるカービング氷河の末端変動に大気・海洋が与える影響

  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
• 豪州南極海盆における南極底層水の急速かつ持続的な低塩分化

  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
• グリーンランド北西部におけるカービング氷河の末端変動に大気・海洋が与える影響

  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]
  
• 2. 近年の極域海洋変動と淡水循環の役割(2011年度春季大会シンポジウム「変動する地球気候の鍵-南極・北極-」の報告)

  青木 茂 天気 60 (11) 888 -892 2013年
• 南大洋における降雪の酸素安定同位体比の緯度分布と季節変化

  中村 佳代, 青木 茂, 豊田 威信, 青山 雄一 南極資料 54 (2) 190 -202 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.
• 観測から見た南極底層水の変質と氷床融解の影響把握

  青木茂 月刊海洋 号外54 142 -150 2010年06月
• 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]
  
• 南極陸棚斜面横断東経115度ラインにおける酸素安定同位体比分布

  青木茂 九州大学応用力学研究所所報 135 89 -93 2008年09月
• 干渉SAR処理を用いた南極接地線(grounding line)の精密決定

  山之口勤, 土井浩一郎, 澁谷和雄, 青木茂 九州大学応用力学研究所所報 135 95 -99 2008年09月
• 南極を学び、地球を知る－北海道大学 国際南極大学カリキュラムの取り組み―

  青木茂 大阪国際サイエンスクラブ会報 217 4 -6 2008年07月
• しらせ海上気象観測データから見た110°E 線の亜南極フロント上における大気の海洋に対する応答

  飯島裕司, 青木茂, 谷本陽一 月刊海洋 49 66 -73 2008年01月
• 船上観測データに基づく南極リュツォ・ホルム湾定着氷の夏季における氷厚及び積雪深分布特性について(所外発表論文等概要)

  宇都 正太郎, 下田 春人, 泉山 耕, 牛尾 収輝, 青木 茂, 橋田 元, 若林 裕之, 西尾 文彦 海上技術安全研究所報告 5 (2) 261 -261 2005年
• JARE-43 Tangaroa marine science cruise report (Physical oceanography)

  Shigeru Aoki, Toshihiko Sato Antarctic Record 48 (3) 204 -218 2004年11月 [査読無し][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.
• 南極の海と淡水循環

  青木茂 細氷 50 2 -9 2004年01月
• 南極海ダイナミクス 気候変動の鍵を握る南極底層水

  青木茂 ILLUME 31 (16) 4 -21 2004年01月
• Oceanographic Data in Lutzow-Holm Bay from July 1998 to December 1998 (JARE-39)

  Shigeru AOKI, Gen HASHIDA JARE data reports. Oceanography = JARE data reports. Oceanography 22 1 -18 2001年03月
• 南極域の海面変動と地殻変動-観測の現状と課題-

  青木茂 月刊地球 35 149 -156 2001年
• 南極海の環境変動

  青木 茂 日本エネルギー学会誌 = Journal of the Japan Institute of Energy 79 (11) 1050 -1053 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.
• 衛星観測データに基づく西部北太平洋の中規模擾乱の研究

  青木茂, 今脇資郎 九州大学応用力学研究所所報 76 103 -122 1994年09月
• 海面高度計データと現場海洋観測によるジオイド・モデルの改良

  市川香, 青木茂, 今脇資郎 月刊地球 186 616 -621 1994年01月
• 衛星海面高度計と放射温度計で検出された黒潮続流の擾乱

  青木茂, 今脇資郎, 市川香 九州大学応用力学研究所所報 75 47 -67 1993年05月
• Westward propagation of sea surface anomalies in the western North Pacific as seen by the altimeter and radiometer

  Aoki, S, S. Imawaki, K. Ichikawa Proceedings of the PORSEC-'92 in Okinawa 273 -278 1992年08月

受賞

• 2017年08月 日本気象学会 堀内賞
  

共同研究・競争的資金等の研究課題

• グローバル南極学：大変化する氷床と地球環境の連鎖をつなぐ

  日本学術振興会：科学研究費助成事業

  研究期間 : 2024年04月 -2029年03月 

  代表者 : 青木 茂
• グローバル南極学：大変化する氷床と地球環境の連鎖をつなぐ

  日本学術振興会：科学研究費助成事業

  研究期間 : 2024年04月 -2029年03月 

  代表者 : 青木 茂
• 人新世における南極条約体制のレジリエンス研究

  日本学術振興会：科学研究費助成事業

  研究期間 : 2021年07月 -2026年03月 

  代表者 : 柴田 明穂, 原田 尚美, 青木 茂, 木村 ひとみ, 稲垣 治, 來田 真依子

   

  本研究は、地球システムと人類社会システムが一体化する人新世の下での法学研究の先駆として、国際法制度のレジリエンスのあり方を、法学的知見と地球科学を中心とした自然科学的知見を連携させた社会自然科学的手法を駆使して分析し、新たな国際法学術体系を開拓する。そのモデルケースとして、人新世的痕跡が著しい南極地域を管理する南極条約体制(ATS)を題材として、地球科学からの知見を踏まえて100年先の南極の姿を構想しながら、ATSがそのレジリエンスを発揮し強化しうる国際法学的知見を提示する。
  採択通知後の約7ヶ月の本年度の研究は、引き続きコロナ禍の影響により海外研究者との研究交流はほぼストップし、オンラインによる研究メンバーとの意見交換と研究基盤形成（電子書籍等の購入）が中心となった。それら研究基盤を活用して、Oxford Handbook of International Environmental Lawに、国際法における信義誠実原則の機能を検討する論文を掲載することができた。コロナ禍においても、2021年11月に、南極研究科学委員会(SCAR)人文社会科学常設委員会の年次研究大会を神戸大学主催にてハイブリッド形式で開催し、本研究に関連する研究報告及び対面での意見交換が出来たことは有益であった。その成果としてZia Madani氏との共著論文を発表し、フェローとして研究支援してくれたオスロ大学ポスドクのYelena Yermakova氏とAntarctic Science誌における特別号企画を進められたことは重要である。
• 氷床融解と深層循環の揺らぎをつなぐ－東南極亜寒帯循環から沿岸への輸送過程ー

  日本学術振興会：科学研究費助成事業

  研究期間 : 2021年04月 -2026年03月 

  代表者 : 青木 茂, 草原 和弥, 平野 大輔, 松村 義正

   

  南極沿岸海洋は地球の海水位変動と海洋深層循環の鍵を握る。近年加速する南極氷床の流出・融解には、沿岸大陸棚上のホットスポットにおける熱輸送が重要な役割を果たしている。また、ホットスポットでの氷床融解は十年規模で変動し、沿岸コールドスポットから深海底へ沈み込む底層水に指摘されていた低塩化傾向はここ数年で逆転したことも明らかになりつつある。本研究課題では、南極氷床への熱供給メカニズムとその時間的な変動を解明するため、理解の遅れたウェッデル循環東端のコスモノート海を中心に、沿岸ホットスポットへの熱供給プロセスを現場船舶観測により明らかにする。海洋観測データの解析から、ここ30年ほどでトッテン沖では外洋の熱源が大陸棚に近づきつつあることが分かった。こうした熱の輸送経路として、沖合の時計回り循環と南極斜面流の重要性が明らかになった。またコスモノート海では、沖合から大陸棚上への熱供給パスの存在も分かった。我々が強みをもつ氷床融解水のトレーサーである酸素同位体比の国際共同観測網を主導し、氷床から海洋への融解水流出の実態とその十年規模変動を周極的に把握することを目指す。コールドスポットであるケープダンレー沖の解析では、夏季の表層高温と棚氷融解のリンクが明らかになった。数値実験の併用により、底層水形成のタイミングを送らせる働きをすることが見出され、将来の気候では表層水起源の南極氷床融解が海洋深層循環変化につながる可能性が示唆された。
• カービング氷河の末端プロセスと変動メカニズム －湖と海で何が違うのか－

  日本学術振興会：科学研究費助成事業

  研究期間 : 2020年04月 -2025年03月 

  代表者 : 杉山 慎, 菅 浩伸, 古屋 正人, 青木 茂

   

  前年度に引き続き、2021年度もCOVID-19の影響を受けて、チリおよびアルゼンチンでの氷河観測が実施できない状況となった。そこで当初の計画を一部変更して、南極における野外観測を実施すると共に、人工衛星データを使った解析に力を入れた。その結果、南極のカービング氷河における重要な観測データが得られた他、既存のデータと衛星データの解析で論文出版と学会発表の成果が挙がった。 (1) 野外観測： 2021年12月から2022年1月にかけて、南極ラングホブデ氷河で熱水掘削を含む観測を実施して、カービング氷河の底面観測に成功した。当初計画とは異なる地域、異なる手法による観測であるが、氷河氷床と海洋の相互作用解明につながる重要なデータが得られたと考えている。このほか、パタゴニアでの氷河・氷河湖・海洋調査に向けて、観測機器の調査と準備を行った。特にマルチビームソナーを使った観測に関して、専門業者への観測委託の検討が進んだ。 (2) データ解析： 過去にチリ・グレイ氷河の前縁湖で測定した水温と流速データを解析して、氷河排水が氷河湖の水温季節変動に与える影響を明らかにした。また人工衛星データの解析によって、パタゴニア西部グレーベ湖で2020年に起きた急激な水位低下を発見した。詳しい解析の結果、この現象が近年では世界最大の氷河湖排水イベントであることが判明した。 (3) 論文出版・学会発表：グレイ湖で測定した通年の水温変化をNature Communications誌に出版した他、氷河湖排水イベントをCommunications Earth & Environment誌に投稿中。COVID-19の影響で海外における学会発表は困難となり、国内での学会で成果を発表した。 (4) 研究会合：10月にオスロ大学の研究者とオンラインでワークショップを開催した他、研究チームでの小規模な会合を適宜開催した。
• 未探査領域への挑戦

  日本学術振興会：科学研究費助成事業

  研究期間 : 2017年06月 -2022年03月 

  代表者 : 野木 義史, 吉田 弘, 沖野 郷子, 巻 俊宏, 青山 雄一, 青木 茂, 末吉 哲雄, 田村 岳史

   

  本研究課題は、ロボット技術等の無人観測技術を、未探査領域である南極沿岸の海氷域と沿岸域に適用することで、新たなデータ取得を目指すものである。 2019年度に、全体の会合を3回(2019年4/25, 10/10および2020年3/30(TV会議))開催し、現況や今後の方針等について議論した。また、AUV(Autonomous Underwater Vehicle)の専門的な検討を行うサブワーキンググループの会合も適宜実施し、AUV試験および試験結果に基づく改良点等の検討を行った。以下、2019年度に実施した概要を示す。 前年度にAUVのほぼ全体的な組み上げを終了したAUVの試験を中心に実施し、調整や修正等を進めた。水槽実験や実際の海での試験に加え、特に冬期の北海道紋別港において、氷下海洋域での試験を行い、修正および調整を実施し、今後の改良点を整理した。さらにAUV回収のために、小型の遠隔操縦ロボット（ROV）の基本的な組立を実施、回収機構の設計を行った。南極沿岸浅海域海底地形調査用の小型ROVも基本的な組立を行い、水槽等で試験を行うい、さらに北海道サロマ湖において氷下での試験観測を実施し、概ね良好な結果を得た。また、AUV探査を行う予定である南極周辺の既存海底地形データの収集と整理を継続するとともに、AUV搭載マルチビーム測深機データの解析手法の検討を行った。 南極ケープダンレー沖に設置した時系列データを衛星回線で送信する係留式ブイシステムについては回収を行った。一部システムに不具合等があり、原因解明を進めている。 大陸域での観測では、固定翼・回転翼UAVを用い、昭和基地上空での空撮データを実施し、これまでのデータとともに昭和基地の表面地形図を作成および精度評価を継続した。さらに、固体班と連携し、積雪による地殻変動や重力変化の評価も継続した。また、氷河・棚氷の融解量把握のため白瀬氷河上において、氷河氷厚レーダーApRES(Autonomous phase-sensitive Radio-Echo Sounder) の観測も継続した。
• 酸素同位体観測による南極沿岸海洋への氷床融解水流入の直接評価

  日本学術振興会：科学研究費助成事業

  研究期間 : 2017年04月 -2022年03月 

  代表者 : 青木 茂, 田村 岳史, 松村 義正

   

  南極氷床の融解が海水位上昇や海洋深層循環にもたらす影響を調べるため、氷床融解水の特定に不可欠な特殊トレーサーである酸素安定同位体比の国際的な観測網を構築し、融解水の海洋中での存在量とその変化を直接的に評価した。周極的にみて融解量の多い場所を特定した。アデリーランド沖の海域で氷床融解に起因する淡水量が2010年代中盤まで増加した傾向を捉えた。一方2010年代中盤以降、西南極で棚氷融解量が減少し、その影響がオーストラリア-南極海盆底層に及んだ可能性を示した。同海盆では外洋の構造がここ30年程度高緯度側にシフトした実態も明らかになり、周極沿岸域の長期的な時空間変動の理解が進展した。
• 南極海における定着氷状態とバイオマスの関係性とその経年的変動の実態把握

  日本学術振興会：科学研究費助成事業

  研究期間 : 2018年11月 -2021年03月 

  代表者 : 青木 茂, WONGPAN PAT

   

  沿岸域に発達する定着海氷は極域の熱や物質循環に重要な役割をもつ。特に南極域では、氷河・氷床の流動を抑制する働きや生態系の生息環境の維持など、海洋海氷-氷床相互作用における重要な役割が近年注目を集めており、その物理・生物学的な特性の把握が望まれている。本研究では、これまで申請者がロス海現場観測に基づき開発した放射計による海氷厚および海氷中植物存在量の定式化をベースに、これを他の氷海域に適用して比較検討することで、より汎用性のある氷厚や生物生産量の定式化をめざす。まず、この観測に使用する光学系測器を中心とした観測システムを構築する。これらをもちいて現場海氷でもちいることで実際のスペクトル分布をうると同時に、他の直接的観測手法をもちいて実際の海氷の氷厚・積雪と植物存在量を同時に測定し、これらを比較することで、この関係性の定式化を目指す。 本研究では、光学観測により海氷の物理―生物特性を測定する現場観測システムを構築し、このシステムを北海道サロマ湖定着氷域での集中海氷観測に適用した。これにより、海氷下連続スペクトル測定で正規化したスペクトル変化指数とアイスアルジーの生物量との関係を求めた。物理パラメータと生物量との間には一定の関係性が得られたが、その関係性はこれまで極域において先行研究が求めたものとは異なることが分かった。また、同観測システムを南極リュツォホルム湾およびトッテン氷河沖海域の多年氷域において運用し、サロマ湖で行った観測と同様にして物理パラメータと生物量との関係性を調べた。この結果をこれまでの他の定着氷域で得られた結果と統合することで、グローバルに適用可能な定式化を行った。加えて、南極リュツオホルム湾定着氷域でこれまで実施されてきた観測成果に基づき、十年規模変動の実態と植物存在量との変化の関係についての関係性を議論した。
• ポリニヤを起点とする熱塩／物質循環

  日本学術振興会：科学研究費助成事業

  研究期間 : 2017年04月 -2020年03月 

  代表者 : 大島 慶一郎, 深町 康, 平野 大輔, 二橋 創平, 青木 茂, 西岡 純

   

  南極、北極、オホーツクの沿岸ポリニヤでの係留系データの詳細な解析から、大気からの冷却が強い場合、最大水深100mに及ぶ海中でのフラジルアイス生成が起こることが示され、沿岸ポリニヤでは今まで考えられた以上に大きな海氷生産を生むことが示唆された。また、流れが強い場合は、海底堆積物が巻き上がって海氷内に鉄等の栄養物質が取り込まれることを実測から明確にし、その物質循環における重要性を提示した。これら係留データを検証データに用い、衛星マイクロ波によるフラジルアイス検知・薄氷厚アルゴリズムを開発し、全球の沿岸ポリニヤの海氷生産量を高精度に見積もり、中深層水変動との関係解明に資するデータを作成した。
• 高浮力型氷海リアルタイムモニタリングブイシステムの開発

  日本学術振興会：科学研究費助成事業

  研究期間 : 2016年04月 -2019年03月 

  代表者 : 青木 茂, 小野 数也

   

  極域沿岸氷海域における海洋鉛直プロファイルをリアルタイムに送信する定点ブイの実用化を目ざし、高流速対応のシステム設計と運用性向上を検討した。浮力調整式で対応する場合、躯体が非常に大きく、取り扱いが困難となった。実際の環境下での運用状況を調べるため、現有ブイで南極沿岸ポリニヤ域における1年間の観測を行い、約8ヶ月間の運用に成功した。水平流速は強くはなかったが浮上成功率は50%に届かず、水平流速以外の制限要因が存在した。高流速対応の代替策として、深度をウインチで制御するブイの動作試験を行い、同じ沿岸ポリニヤ域への設置に成功した。今後各手法の得失を検討し、効果的な極域海洋モニタリングを実現したい。
• 南極底層水の昇温・低塩化に伴う深層大循環の変貌予測に関する基礎研究

  日本学術振興会：科学研究費助成事業

  研究期間 : 2015年04月 -2019年03月 

  代表者 : 北出 裕二郎, 溝端 浩平, 青木 茂, 田村 岳史, アゼベド マテウス, 渡辺 和帆, 嶋田 啓資

   

  近年、地球温暖化に伴い南極底層水が昇温・低塩化している。本研究では南極底層水が変質する機構を解明し、今後、深層大循環がどのように変化するかを明らかにすることを目指した基礎研究が実施された。4年間にわたり毎年1月の約1ケ月間、南大洋東経110度に沿った海洋観測、および長期係留系の設置・回収を継続して実施した。その結果、南極底層水の形成における貫入深度の季節的変動特性や子午面方向の物資輸送の主要な機構を明らかにした。さらに、南極氷床溶融水が南極底層水の変質に及ぼす影響を評価した。また、子午面循環の鍵を握る中規模渦の監視技術として、海氷域における衛星海面高度データの補正アルゴリズムの開発が行われた。
• 南極海オーバーターニングにおける淡水量変化の量的把握と原因究明

  日本学術振興会：科学研究費助成事業

  研究期間 : 2013年04月 -2017年03月 

  代表者 : 青木 茂, 深町 康

   

  地球規模の海洋オーバーターンをになう底層の南極起源の海水は、近年、淡水化しつつあると指摘される。南極大陸沿岸で生まれる海水が全大洋底層の水のもとの一つとなるので、この沿岸海洋の変質の実態を把握して淡水化の原因を探るために、酸素の同位体をつかった特殊なトレーサーを含む観測研究を推進した。オーバーターンの起点となる海域の一つであるロス海沿岸部は淡水化しているが、これは西南極海洋の暖水化と同期していた。もう一つの起点であるアデリーランド沿岸では、海氷生産の減少に加えて氷床融解成分の増加がみられた。これらより、大規模とローカルな氷況変化の両方が淡水含有量に大きな影響を与える可能性が明らかになった。
• 海氷生成とリンクする南極底層水・深層循環の再描像

  日本学術振興会：科学研究費助成事業

  研究期間 : 2013年04月 -2017年03月 

  代表者 : 大島 慶一郎, 深町 康, 青木 茂, 松村 義正, 北出 裕二郎, 二橋 創平, 牛尾 収輝, 田村 岳史, 高橋 晃周, 二橋 創平, フレーザー アレクサンダー, ウィリアムス ガイ, 佐藤 建

   

  東南極を中心として、沿岸ポリニヤでの高海氷生成による底層水形成の詳細を明らかにした。南大洋第2の海氷生産域であるケープダンレーポリニヤでは、係留系観測等から、海中フラジルアイス生成による高海氷生産過程や、高密度水が混合・変質して底層水に至る過程を明らかにした。第3の海氷生産域であったメルツポリニヤでは、氷河舌の崩壊によって海氷生産量が半減し、底層水生成が大きく減少していることが示された。これら以外に、東南極ではプリッツ湾及びビンセネス湾でも小規模ではあるが高海氷生産による底層水形成が示され、ポリニヤ高海氷生産（正）と棚氷融解（負）の効果の兼ね合いで底層水形成が決まることも示唆された。
• 氷海環境リアルタイムモニタリングブイシステムの開発

  日本学術振興会：科学研究費助成事業

  研究期間 : 2013年04月 -2016年03月 

  代表者 : 青木 茂, 小野 数也, 市川 雅明

   

  本研究では、近年発展した海洋中層ブイの浮力制御技術と衛星通信技術を応用して、氷海域でも運用可能な海洋鉛直観測を行う係留型ブイ観測システムの開発を目的とし、オホーツク海知床沖で通算七ヶ月にわたる実海域試験を行った。センサーブイについては、浮上下降動作、待機深度の制御、衛星通信によるデータ通信、長期連続耐久運転、海氷下の浮上・停止動作といったプログラムした動作のすべてに成功した。また流速の実測により、ブイ浮上の限界となる流速を求めた。これにより実運用に耐えうるシステムの基盤技術を確立し、今後さらに高度な海洋条件下での運用に対応するための基盤を得た。
• 南極海表層水の低塩分化が深層大循環に及ぼす影響評価に関する基礎研究

  日本学術振興会：科学研究費助成事業

  研究期間 : 2011年04月 -2015年03月 

  代表者 : 北出 裕二郎, 田村 岳史, 溝端 浩平, 嶋田 啓資, 青木 茂, 平野 大輔, 芦田 将成, 鈴木 萌, 白井 優, 笠島 克恵

   

  南極海表層水の低塩化に伴う南極底層水の変質と深層循環に及ぼす影響を評価するため、東京海洋大学研究練習海鷹丸により2012年から2015年の毎年1月に南極海110°E及び140°E周辺海域で、水温･塩分場の観測、長期係留観測、乱流観測等を実施した。本観測では、中規模ポリニヤを起源とした南極底層水の生成を世界で初めて明らかにし、オーストラリア南極海盆の底層水上部へと沈み込んでいることを示した。底層水の顕著な低塩化は、140°Eだけでなく110°Eでも認められ、特に、昇温も認められた2014年には海盆底層水の密度が相対的に軽くなり、ビンセネス湾起源底層水が海盆底層水下部まで達していることが分かった。
• モンスーン・アジアにおける大気海洋雪氷系の鉛直結合変動

  日本学術振興会：科学研究費助成事業

  研究期間 : 2010年04月 -2015年03月 

  代表者 : 中村 尚, 青木 茂, 行本 誠史, 高谷 康太郎, 美山 透, 小守 信正, 吉田 聡, 榎本 剛, 三瓶 岳昭, 釜堀 弘隆, 田口 文明, 水田 亮, 村崎 万代, 小坂 優, 西井 和晃

   

  モンスーンに伴い太平洋とアジア大陸との海陸熱コントラストがもたらす「東西の熱的せめぎ合い」の気候系への影響に関する研究を予定通り達成できた．特に，日本北方に停滞性ブロッキング高気圧を形成し，冬季モンスーンの異常な強化をもたらす対流圏循環異常が，惑星規模波動の秋から冬への季節的な増幅の異常な強化の現れで，北極上空の成層圏を寧ろ寒冷化させて成層圏オゾン減少にも寄与し得ることを見出した．一方，梅雨期に夏季モンスーンに伴って熱帯からの気流が流れると，東シナ海の黒潮沿いに積乱雲が組織化されることや，盛夏期にかけての急速な東シナ海北部の温暖化に伴って梅雨末期に集中豪雨が最も起こり易くなる事が見出された．
• ケープダンレー沖は南極底層水の主要な生成域か？

  日本学術振興会：科学研究費助成事業

  研究期間 : 2011年04月 -2014年03月 

  代表者 : 深町 康, 青木 茂, 松村 義正, 清水 大輔

   

  2008-09年に南極海インド洋セクターのケープダンレー沖の斜面域で取得した係留観測による海洋の時系列データを、人工衛星による海氷データ、測器を取り付けたアザラシによる海洋データと合わせて、この海域で沿岸ポリニヤでの高い海氷生産に起因したローカルな南極底層水の生成が起こっており、ウェッデル海、ロス海、アデリーランド沖に続く、第4の主要な南極底層水の生成域であること示した。特に、この海域が他の3つの生成域とは異なり、広い大陸棚、棚氷、大きな窪地の全てが存在しないにも関わらず、高い海氷生産のみによって主要な生成域となっていることは特筆に値する。
• 国際観測ネットワークによる南極底層水淡水化の量的把握と氷河融解の影響検証

  日本学術振興会：科学研究費助成事業

  研究期間 : 2009年 -2012年 

  代表者 : 青木 茂, 深町 康, 嶋田 啓資

   

  酸素安定同位体比測定という新しい手法を標準化した国際共同海洋観測ネットワークの構築と既存のデータセットとの比較解析から、南極海のオーストラリア-南極海盆における南極底層水のここ約20年間にわたる淡水化の実態を把握した。この淡水化の主要な原因は、東側から本海盆に流入する底層水の低塩化と流入する量の低下と考えられる。本観測ネットワークにより、今後の海洋変化の原因を評価するための酸素同位体比分布像を構築することに成功した。
• 海氷生産量のグローバルマッピングとモニタリング構築

  日本学術振興会：科学研究費助成事業

  研究期間 : 2008年 -2012年 

  代表者 : 大島 慶一郎, 江淵 直人, 青木 茂, 深町 康, 豊田 威信, 松村 義正, 北出 裕二郎, 舘山 一孝, 二橋 創平, 小野 数也, 榎本 浩之, 木村 詞明, 田村 岳史

   

  海洋中深層循環及びその変動を決めうる海氷生産量を、衛星データ等から見積もるアルゴリズムを開発し、そのグローバルマッピングを初めて行った。沿岸ポリニヤでの高海氷生産過程を長期係留観測から明らかにし、アルゴリズムの検証も行った。南極第2の高海氷生産域であることが示されたケープダンレー沖が未知の南極底層水生成域であることもつきとめた。南極海とオホーツク海では、海氷生産量の変動が底層水や中層水の変質とリンクしていることを明らかにし、中深層循環弱化の可能性を指摘した。
• 斜面下降流による南極底層水の混合過程の研究

  日本学術振興会：科学研究費助成事業

  研究期間 : 2007年 -2009年 

  代表者 : 青木 茂, WILLIAMS Guv Darvall, WILLIAMS Guy Darvall

   

  南極底層水は世界で最も高密度の水塊であり、深層大循環の主要な構成要素である。南極底層水は、主として沿岸陸棚域での冷却・海氷生成により生じた高密度陸棚水が、沖側の高温高塩分水を取り込みつつ陸棚斜面を流れ下ることで形成される。オーストラリア・南極海盆では、ロス海から西向きに流れてきた底層水に対して、アデリー海谷上を中心とするポリニア起源の高密度陸棚水が陸棚斜面上を下降して加わることによって、低温・低塩の底層水が生じると考えられてきた。このアデリーランド沖における斜面下降流の詳細な特性を把握するため、2001年1月と2004年10月に実施された船舶観測のデータを解析した。その結果、従来言われていたアデリー海谷からの下降流に加えて、より東側に位置するメルツ海谷からの二次的な下降流の存在が新たに明らかになった。この下降流はメルツ海谷上の低塩陸棚水に起因すると考えられる。ただし、底層水の水塊変質に及ぼす影響はアデリー海谷からの下降流に比べて小さい。1998年の係留系の連続データの解析とあわせて、主として冬季の下降流イベントにより、ロス海に起因するやや高温な性質が低温側へ断続的に変質することが分かった。これらにより、アデリー海谷より上流側のメルツ海谷からの下降流も南極底層水の低温・低塩化に関与していることが示された。このメルツ海谷上の高密度水形成の原因を調べるため、海氷生産の役割について検討した。人工衛星観測に基づく海氷生成量との比較から、メルツポリニアでの海氷生産は、係留データから見積もられる海水の密度との間に強い相関をもつことが分かった。人工衛星観測の結果は、マックロバートソンランド沖など東南極沿岸の随所でメルツポリニアに匹敵する海氷生産量を示している。このことは、より広域にわたって海洋底層まで達する下降流が生じている可能性を示唆するものである。
• 酸素同位体比測定による南極底層水低塩化過程と淡水循環変動の実態把握

  日本学術振興会：科学研究費助成事業

  研究期間 : 2005年 -2008年 

  代表者 : 青木 茂, 豊田 威信

   

  南極海沿岸の四割に及ぶ領域で南極底層水・沿岸陸棚水の特性変化の実態を把握し、あわせて酸素同位体比解析のための資料の収集とそれによる変動原因の考察を行った。南極底層水は全般的に低塩化しつつある。ロス海では特に顕著で、陸棚水の低塩化と関連している可能性があるが、酸素同位体比解析により氷床融解の影響が示唆された。酸素同位体比は底層水の生成源の推定にも有効であった。インド洋区における起源淡水として降雪と陸氷の分析を実施したところ、平均的には両者は十分区別可能で、本手法の有効性が示された。
• 南極海インド洋セクターにおける海洋循環と南北熱塩輸送の量的把握

  日本学術振興会：科学研究費助成事業

  研究期間 : 2005年 -2007年 

  代表者 : 若土 正暁, 大島 慶一郎, 青木 茂, 深町 康, 牛尾 収輝, 木村 詞明

   

  本研究の最重要課題は、南極海インド洋セクターで生成する南極底層水の北上流の季節変動を初めて捉えることに成功した現地観測データを解析することにある。我々は、ウエッデル海やロス海とともに南極底層水の主要な生成域の一つであり、これまで南北輸送流量のデータの全く無いアデリーランド沖をターゲットとして、冬季沿岸ポリニア域で生成する南極底層水が陸棚斜面を下降し、やがて西岸境界流としてケルゲレン海台東岸沖の海底近くを北上する海流量の季節変動を直接捉えるための流速計を中心とする係留観測を実施した。これは豪州CSIROとの共同研究として実施したものであり、2003年2月から2005年1月までの2年間にわたる、合計8観測点での流向、流速、水温、塩分、溶存酸素量のデータ取得に初めて成功した。極めて貴重な、そして膨大なデータの解析を現在精力的に進めている。これまでの解析から、特徴的なのは、どこの観測点でも流向が一様であり、ケルゲレン海台東岸沖斜面域では北西流が、遥か沖合の深海域では全く逆向きの南東流が卓越していることである。つまり、この海域では、ケルゲレン海台東岸沖の南極底層流の西岸境界流としての北上流と、その沖合での再循環による南下流の二つの卓越する循環系から構成されていることが分かった。 本研究ではまた、南極底層水生成域である沿岸ポリニア域で形成する海氷の氷厚分布を人工衛星データから検出して、熱収支計算を行うことによって、南大洋における海氷生産量をマッピングすることに初めて成功した。これにより、熱塩フラックスの条件を与えることも可能になった。一方、近年の地球温暖化によると思われる南極底層水の低塩化の原因として、上記マッピング手法により求められた、南大洋最大の海氷生産域であるロス海沿岸ポリニアでの海氷生産量が1990年代から2000年代にかけて約30%減少していることが大きく関わっている可能性を示唆した。 尚、本研究で得られた観測データはぼう大なものであり、より高いレベルでの解析には充分な時間を必要とする。そのため、本研究期間内には必ずしも十分な成果は得られていないが、1,2年以内にはインパクトある研究成果を世界に公表したい。
• GRACE衛星により得られる変動重力場の極域からの検証

  日本学術振興会：科学研究費助成事業

  研究期間 : 2004年 -2006年 

  代表者 : 渋谷 和雄, 谷口 真人, 青木 茂, 福田 洋一, 藤本 博巳, 石川 尚人

   

  本研究の目的は、測地学的な地上検証データとGRACE衛星(2003年3月打ち上げ)の重力データを対照させ、南極域での大気・海洋・氷床変動に伴う重力変動を検出し解釈することである。1999-2003年のVLBIデータ解析から昭和基地の地殻隆起速度は4.6±2.2mm/yrと求められ(Fukuzaki et al.,2005)、GPSによるリュツォ・ホルム沿岸露岩域の結果(1.4±0.2mm/yr ; Ohzono et al., 2006)より大きいこと、DORISによる値はAmalvict et al.(2007)による再解析で3.61±0.21mm/yrと求まり、3種の宇宙測地の比較が明らかになった。絶対重力測定によると、昭和基地での重力減少は-0.27μGal/yrであり(Fukuda et al.,2006)、Makinen et al.(2007)が示すとおり、後氷期地殻隆起モデルの規正に使用されよう。小西ら(2007)は過去17年分のJARE海上重力計データの統一的ドリフト補正を行ったので、昭和基地周辺域360,000km^2において行われた日独航空機重力観測データによる重力異常図を基礎に、staticな地域重力場モデルの精度はさらに向上することになり、南極域での水循環によるdynamicな重力変動の詳細解明につながるだろう。一方、(66°51'S,37°49'E,水深4600m)地点に設置した海底圧力計(OBP)には2004年12月26日発生のスマトラ地震津波が明瞭に記録されていて、水位変動による超伝導重力計や広帯域地震計への荷重効果(Nawa et al.,2007)が示された。さらに、OBPデータから潮汐成分を除去した時に残る長期圧力変動(2年分データ)振幅と、GRACEによる月平均重力変動振幅は同位相であることもわかってきた。湧出量計についても、第46次越冬隊の手によりリュツォ・ホルム湾氷海下で800m深度での測定実績が得られ、南極域においても中緯度での測定と同程度(10^<-8> to 10^<-6>m/s)の湧出量が得られることが判明し、これら新しい測器展開の有効性が立証された。
• 海洋の南極振動とその気候変動における役割の解明

  日本学術振興会：科学研究費助成事業

  研究期間 : 2003年 -2004年 

  代表者 : 青木 茂

   

  南半球の大気変動では、大気再解析データや大気-海洋結合モデルの結果から、南極大陸上と低緯度側とが逆位相で変動する南極振動(AAO)モードが卓越していることが明らかになってきた。こうした大気変動は数十日規模から数十年規模にわたって卓越している。南極海洋においても、これに対応した大規模な変動の存在が予想される。 季節内周期帯においては、南極沿岸の水位がコヒーレントに変動することが示された。南極振動の指標であるAAOインデックスと沿岸水位変動は、季節内変動の振幅がともに冬に強い傾向を示す。一方、人工衛星海面高度計により求めた外洋域での水位変動については、AAOインデックスに対する回帰係数は沿岸における値に比べて一桁小さく、応答が南極沿岸域に集中していることを示している。九州大学応用力学研究所と共同で行った再解析データ(ERA40)を用いたモデル実験やGCM(ECCO)によって再現された季節内順圧変動を解析すると、南極振動に対応して、沿岸から水深3000m程度の陸棚斜面上まで同期したコヒーレントな変動が良く再現されていることが分かった。この変動の位相は沿岸で実際に観測された海水位変動のものと一致し、順圧応答の卓越性を示している。 次いで十年規模以上の周期帯について、主として日本南極観測のデータに基づいて、インド洋区における水塊変動および力学的高度変動を調べた。亜南極前線以北では表層等密度面上において低塩・低温化傾向が観測された。これは気温の上昇や降水の増加と調和的であると同時に、西風の強化による高緯度からの低塩分水の輸送とも整合的である。この特徴が周極的であるとすれば、観測されているここ20-30年間におけるAAOの強化が関係している可能性が考えられる。今後は現象の周極性を確認すると同時に、底層における水塊変化や大陸斜面域における水塊混合過程とその径年的変化の影響を調べる必要がある。
• 南極海インド洋セクターにおける深層循環の実態把握-特に南極底層水の低緯度への輸送量の直接評価-

  日本学術振興会：科学研究費助成事業

  研究期間 : 2002年 -2004年 

  代表者 : 若土 正曉, 大島 慶一郎, 青木 茂, 長島 秀樹, 牛尾 収輝, 渡邊 修一

   

  南大洋(Southern Ocean)は、その地理的特徴(氷大陸である南極大陸をぐるりと取り囲み、低緯度側で太平洋・大西洋・インド洋の世界三大洋と直接交流をもつ)から、地球規模の気候に重要な役割を果たしている。特に、世界最大の季節海氷域(冬季海氷である南極海(Antarctic)は、北大西洋深層水とともに世界の海洋深層大循環の起源である「南極底層水」を形成する海としても広く知られている。このように、南大洋・南極海は地球科学的に非常に重要な海であるにも関わらず、これまで観測が極端に少なく、地球上で最大のデータ空白域の一つになっている。南極底層水は、南極大陸沿岸域の主にウェッデル海、ロス海、アデリーランド沖で生成し、世界三大洋の深層を循環していくと言われている。地球規模の気候に果たす海洋大循環の役割の重要性を考えると、この南極底層水の循環、特にその低緯度への輸送流量を評価することは極めて重要である。近年、欧米の研究者達によって、ウェッデル海やロス海における底層水形成に関する本格的な観測の実施計画が進められている。 本研究は、もう一つの南極底層水生成域であり、インド洋や太平洋の深層水の起源水域の可能性の高い、アデリーランド沖を中心とした南極海インド洋セクターにおいて、底層水の循環とそれの低緯度への輸送流量を直接測定により評価しようとするものである。冬季沿岸ポリニア域で生成した高密度水が、大陸棚斜面を沈降し、南極底層水として低緯度のインド洋さらには太平洋の深層を循環していく際、その流路上で流速が最大(西岸境界流)となるケルゲレン海台東岸沖に、約30台の流速計(水温塩分計付)からなる係留系(8測点)を2年間設置することによって、そこを通過する底層水の低緯度への輸送流量とその季節変動を捉える。 これら南極大陸を取り囲む三つの起源域で生成する「南極底層水」の低緯度への輸送流量を、国際協力による同時期観測によって直接評価することが出来れば、それら観測値を海洋大循環モデルに組み込むことによって、より信頼性の高い「世界の海洋大循環」像が我々にもたらされることになるはずである。 本研究は、豪州CSIROとの共同で約2ケ年にわたる流速計・水温塩分計からなる係留系観測(8測点)とそれらの設置および回収の際の通常海洋観測を実施し、観測のほとんどすべてが成功裡に終了した。特に、2ケ年にわたる係留系観測では30数基の観測機器のうち、1基を除くすべてにおいて完全な形でのデータを取得することができた。これは、勿論世界初のものであり、極めて貴重なデータ取得に成功したといえる。従って、本研究の主な目的は完全に達成することができた。これら貴重なデータ解析を中心とする研究展開を目的とする科学研究費予算申請「南極海インド洋セクターにおける海洋循環と南北熱塩輸送の量的把握」(平成17年-平成19年;合計15,100千円)が、幸いにも採択されたので研究を進めていきたいと考えている。
• 南極海における海洋環境の中長期的変動に関する研究

  日本学術振興会：科学研究費助成事業

  研究期間 : 1999年 -2001年 

  代表者 : 福地 光男, 平譯 享, 青木 茂, 小達 恒夫, 谷口 旭, 谷村 篤, 渡邉 修一

   

  地球規模環境変動への関わりが大きいとされる南極海の海洋環境の中長期的変動について、これまで日本南極観測隊による観測が継続されてきた南極海インド洋区に焦点をあて、国内外に蓄積された観測データを基に解析を実施した。昭和基地への往復航路上に限られた定常的な観測結果ではあるが、インド洋区での海水温の昇温傾向が初めて確認され、また、海表層の植物プランクトン現存量に大きな経年変動が認められた。また、動物プランクトン現存量の変動を解析する上で、連続プランクトン採集器を用い、オーストラリア側と共同で現場観測とデータ解析を実施した結果、動物プランクトン量にも経年変動が認められ、また、同採集方法が中長期的モニタリング観測に有用であることが国際的にも認められた。更に、これら海洋物理と海洋生物をリンクする要因である栄養塩濃度の時空間変動を解析した結果、インド洋区において表面珪酸塩濃度が平均的に高い海域と低い海域とが交互に存在することを初めて指摘した。その変動要因は近年話題となっている南極周極波動現象のような大気振動現象に呼応しているといえる。これらの因果関係を明らかにするには関係する要因のプロセス研究が不可欠であり、そのための現場観測に参加した。オーストラリアの研究者を中心に、国際共同観測体制を作り上げ、2001年から2002年にかけての夏期シーズンに日豪から合計4隻の観測船を導入した時系列観測を実現した。中長期変動解析の鍵となるパラメーターをコア観測項目とし、4航海を通して標準化した観測方法を世界に先駆けて初めて導入した。これまでスナップショット的な観測が主体であったが、初めて夏期間を連続的にカバーした結果が得られ、今後の中長期変動解析の発展に大きく貢献する成果であった。また、データ解析、及び、現場観測を通して、国内外の南極海洋研究者が分野横断型の共同研究体制の基盤を築き上げることが出来たことは大きな成果である。
• 人工衛星合成開口レーダ干渉法による遠隔地地形モデルの作成およびその検証

  日本学術振興会：科学研究費助成事業

  研究期間 : 1997年 -2000年 

  代表者 : 青木 茂, 土井 浩一郎, 小澤 拓, 澁谷 和雄, 青木 茂

   

  本研究は、人工衛星SARデータを用いてアクセスの困難な遠隔地の地形モデルを作成し、GPSなどの独立な手法により、得られた地形モデルの精度検証を行なうことを目的とした。 日本南極地域観測隊が活動するリュツォ・ホルム湾周辺の大陸氷床から沿岸露岩域で得られた人工衛星SARデータに3パス法を適用し、同地域の地形モデルを作成した。GPS等による観測結果を基準として、最新のグローバル地形モデルと比較したところ、一桁程度精度のよい地形モデルが得られることがわかった。一方で、3パス法の適用可能なデータが常に得られるわけではないことや、変動の大きい地域では地形データを復元できないといった問題点を明らかにした。 基盤に着底している氷床域と海洋に浮上している棚氷との境界域(グランディングライン)は地形モデルの境界線を与えるが、この境界地形の精密な導出を目的として干渉SAR法を適用した。リュツォ・ホルム湾西方のボードワン棚氷を対象として、鉛直変動の空間的な相違からグランディングラインを検出することに成功し、これをトレースして精密な空間分布を求めることができた。また、得られた鉛直変動は、海洋潮汐モデルと大気圧の影響を考慮した鉛直変動とよく一致した。 棚氷域や定着氷域など急激な変動を伴う対象に対する現場検証間測手法を確立するため、海氷上と大陸地殻上でGPS観測を行ない、その有効性を調べた。独立な観測による比較から、GPS干渉測位により求められた海氷の鉛直変動は、海水位の変動を1〜2cmの精度で捕らえていることが示された。 SAR干渉法を適用できる地域は広く、本研究によって遠隔地域の地形データを取得に威力を発揮しうることが明らかになった。地上検証点としてGPSを効率的に利用することにより、氷床部のみならず棚氷部を含む雪氷域全域における地形モデルとその変動のより精密な把握が可能になるものと期待される。
• トペックス・ポセイドン海面高度計による海洋と固体地球のダイナミクスの研究

  日本学術振興会：科学研究費助成事業

  研究期間 : 1996年 -1997年 

  代表者 : 今脇 資郎, KOBLINSKV C., 大江 昌嗣, 久保田 雅久, 川村 宏, 福田 洋一, KOBLINSKY C., 土井 浩一郎, 藤 浩明, 田村 良明, 田中 寅男, 小泉 金一郎, 瀬川 爾朗, 大久保 修平, BOWIN C., 今西 祐一, 倉賀野 連, 川崎 康寛, 岡田 喜裕, 江淵 直人, 市川 香, 青木 茂, SMITH W.H.F., TAPLEY B.D., FUKUMORI I., NEREM R.S., LE Traon P.ー, RAPP R.H., BINDOFF N.L.

   

  人工衛星に搭載したマイクロ波海面高度計は,海面の高さを数センチの精度でグローバルに観測できる測器であり,そのデータは海洋と固体地球のカ学に関する研究において極めて有効であると考えられている.本研究では,海洋物理学研究者と固体地球物理学研究者が互いに緊密な連携を取りながら協力研究を行うことを目指した. 昨年度l0月には英国で,今年度10月には仏国で開かれた衛星海面高度計トペックス・ポセイドンの科学検討チームの会合に,研究代表者と研究分担者延べ9名を派遺し,海面高度計に関連した本研究グループの研究成果を披露するとともに,国際的な最新の研究情報を得た.昨年度,今年度とも11月に東京においてグループ全体の研究集会を開催し,衛星海面高度計データの処理や解析法に関する情報交換を行った.この集会に合わせて,昨年度は豪州の,今年度は米国と仏国の研究分担者計4名を招請し国際的な共同研究の促進を図った.その成果を集会の報告書としてまとめた.また,両年度とも2月と3月には米国の研究分担者計3名を招請し,各地で打合せ会を開催して情報交換を行った. これらの活動を通じて以下の成果が得られた.(l)海面高度計データを,現場の海洋観測データと組み合わせることによって,日本南岸における黒潮の流量の長期にわたる連続記録が得られた.(2)同様に現場観測データを併用することによって,北太平洋全域にわたる表層循環とその変動の様子が明らかになった.(3)海面高度計データを海洋の数値モデルに同化する技術を向上させた.(4)海面高度計データから全球的な海洋潮汐の精密なモデルが得られた.(5)従来は機密扱いであったGEOSATの測地ミッション期間中のデータの解放にともない,全球的な高分解能の重力異常分布が得られた.(6)重力異常の現場観測データなどを併用することによって,特に日本近海に関して高精度のジオイド分布が得られた.
• 衛星海面高度計データを用いた全球における絶対海面力学高度場と流速場の推定と評価

  日本学術振興会：科学研究費助成事業

  研究期間 : 1996年 -1996年 

  代表者 : 青木 茂

   

  人工衛星海面高度計のデータから海面高度の時間変動成分の時系列を作成し、別に求めた海面力学高度の時間平均場と組み合わせることにより、全球における絶対的な海面力学高度場を求めた。平均海面高度場(海面流速場についても同様)として、気候学的な密度場に基づいて求めた平均場(以下、気候学平均場)と、最新のジオイドモデルを用いた平均場(以下、ポテンシャル平均場)を作成した。気候学平均場については、Teagueら(1991)の1000db準拠のもの、World Ocean Atlas94の1000db・2500db準拠のものを用い、ポテンシャル平均場についてはTOPEX/POSEIDON-JGM3モデルを用いた。時間平均場の精度を評価するため、海面高度の格子点データから求めた平均流速場について、ドリフタ-ブイの軌跡から求められた平均海面流速場との比較を行った。その東西流速については、南北20°C以内の低緯度域と南大洋において両者間の差が大きい。低緯度域では、気候学平均場を用いた方が差が小さい。地衡流を考えた場合、ジオイドの誤差の影響は低緯度ほど相対的に大きく現れる。南大洋では、東西流速についてはポテンシャル平均場の方が差が小さく、ポテンシャル平均場の方がよい精度を与える可能性を示唆している。また、気候学平均場についても、1000db準拠のものより2500db準拠のものの方が差が小さく、全球について一律の無流面を仮定することが適当でないことを示唆している。南大洋における流速場の性質を詳しく調べるため、3年間のアルゴスブイデータと絶対流速場との比較を行った。東向き流速については、系統的にドリフタ-ブイの方が非常に速い場合が多く、フロントに伴う強い流れが再現されていないことが分かる。現実の東西流速を十分に表現するためには、緯度方向に密な平均場の精度の向上が必要である。

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