SEARCH

Search Details

Nakamura Tomohiro

Institute of Low Temperature Science　Pan-Okhotsk Research CenterLecturer

Researcher basic information

■ Degree
  • D.Sc., Kyoto University, Mar. 2001
■ URL
researchmap URLホームページURL■ Various IDs
J-Global ID■ Research Keywords and Fields
Research Keyword
  • 数値シミュレーション
  • 潮汐
  • 渦
  • オホーツク海
  • 内部重力波
  • アリューシャン列島
  • 散乱
  • 鉛直混合
  • 混合
  • 海洋物理
  • 通気
  • 北太平洋中層水
  • 熱塩循環
  • 東樺太海流
  • 中層循環
  • 潮汐混合
  • 北太平洋
  • 鉄仮説
  • 接地境界層
  • 風成循環
  • 高密度水
  • 宗谷暖流
  • 千島列島
  • 物質循環
  • 大気海洋相互作用
  • 風成循環と熱塩循環の結合
Research Field
  • Natural Science, Atmospheric and hydrospheric sciences
  • Environmental Science/Agriculture Science, Environmental dynamic analysis
■ Educational Organization

Career

■ Career
Career
  • 2005 - Present
    Hokkaido University, Institute of Low Temperature Science, 講師
  • 2004 - 2005
    JAMSTEC地球環境フロンティア研究センター, ポスドク研究員
  • 2001 - 2003
    日本学術振興会特別研究員(PD), 日本学術振興会特別研究員
Educational Background
  • 1998 - 2001, Kyoto University, Graduate School of Science, Department of Geophysics (DC)
  • 1996 - 1998, Kyoto University, Graduate School of Science, Department of Geophysics (MC)
  • 1992 - 1996, Kyoto University, Faculty of Science

Research activity information

■ Awards
  • 2007, 日本海洋学会, 岡田賞
    中村知裕
■ Papers
■ Other Activities and Achievements
■ Books and other publications
  • 低温環境の科学事典
    中村知裕, オホーツク海の下層雲・霧と大気海洋相互作用, 第10章, 第2節
    朝倉書店, 2016, [Contributor]
  • 低温科学便覧
    中村知裕, 環オホーツク海域における海洋循環, 第8章, 第2節
    丸善出版株式会社, 2015, [Contributor]
  • オホーツクの生態系とその保全
    三寺史夫; 内本圭亮; 中村知裕; 西岡純; 三角和弘; 津旨大輔, オホーツク海および親潮域における物質循環のモデリング
    北海道大学出版会, 2013, [Contributor]
  • 環オホーツク海地域の環境と経済
    田畑 伸一郎; 江淵 直人; 大島 慶一郎; 西岡 純; 三寺 史夫; 中村 知裕; 庄子 仁; 南 尚嗣; 八久保 晶弘; 白岩 孝行; 本村 眞澄; 西内 修一; 封 安全; 田畑 朋子
    北海道大学出版会, 2012, 9784832967700, Japanese
■ Syllabus
  • 気候ﾓﾃﾞﾘﾝｸﾞ特論, 2024年, 修士課程, 環境科学院
■ Affiliated academic society
  • American Meteorological Society
  • American Geophysical Union
  • METEOROLOGICAL SOCIETY OF JAPAN
  • THE OCEANOGRAPHIC SOCIETY OF JAPAN
■ Research Themes
  • マルチスケールモデリングによる陸域－沿岸域－外洋域間の淡水・物質輸送過程の解明
    科学研究費助成事業
    16 Jun. 2022 - 31 Mar. 2027
    松村 義正; 干場 康博; 田中 潔; 中村 知裕; 古市 尚基
    日本学術振興会, 学術変革領域研究(A), 東京大学, 22H05204
  • Formation of water nutritional property including iron and silicate at the termination of global ocean conveyor belt
    Grants-in-Aid for Scientific Research
    05 Jul. 2021 - 31 Mar. 2026
    西岡 純; 中村 知裕; 三寺 史夫; 山下 洋平; 鈴木 光次; 小畑 元; 近藤 能子; 安田 一郎
    Japan Society for the Promotion of Science, Grant-in-Aid for Scientific Research (S), Hokkaido University, 21H05056
  • 自走する渦対の力学と輸送混合：古典的渦対から乱流的渦対への発展と河川水への応用
    科学研究費助成事業
    01 Apr. 2021 - 31 Mar. 2024
    中村 知裕
    本研究は、沿岸から外洋へ河川水・物質を効果的に輸送する「渦対」について、その力学と輸送・混合効果の解明を目的とする。沿岸は陸や人と外洋のインターフェースであり、多くの陸・人為起源物質が沿岸を介して外洋へ広がる。渦対とは、ペアとなった互いに逆回転の渦で、渦に伴う誘導速度で互いに相手を同じ方向に動かし「自走」する。渦対は、この「自走」により水を遠方まで輸送できる。海洋では、狭い海峡を速い潮流が流れると、海峡下流にジェットができ、その両側に渦対がしばしば形成されることが知られている。
    本年度は、（１）鳴門海峡を想定した数値実験と鳴門海峡の衛星画像解析、および（２）厚岸湾での観測を行った。
    （１）鳴門海峡は、潮流が速く、渦潮・渦対が形成される。衛星画像より、播磨灘側（海峡の北側）では、海峡部のジェットの向きがおよそ北北西なのに対し、渦対は形成ののち西へ大きく左折する。一方、紀伊水道側（南側）の渦対は、概ね直進するが、播磨灘側に比べ小さい。このような振舞の違いの理由を調べるため、播磨灘側に注目して数値実験を行った。その結果、沿岸地形、海底地形（水深変化）、密度成層が、渦対の進行方向、直径、進行速度に影響することが明らかになった。こうした効果は流量保存と渦管の伸縮に起因していた。
    （２）厚岸湾では、引き潮時に河川水・湖水が厚岸湾に流れ出し、物質を輸送するとともに、厚岸湖の水を入れ替える。現場観測と衛星画像から、引き潮に伴う流出時に、ジェットと渦対そして鳴門の「渦潮」と似た小さい渦の列が形成されることが明らかになった。渦潮は潮流が最大5m/sと極めて速い鳴門海峡が有名だが、流速0.7m/s程度の厚岸湾でも見つかった。実は、渦潮は珍しい現象ではないことが判明した。また、厚岸湾の渦対は河川水を多く含む海水で形成されており、河川水流出も渦対を形成しうることを示唆する。
    日本学術振興会, 基盤研究(C), 北海道大学, 21K03655
  • 表層と中層をつなぐ北太平洋オーバーターン：大陸からの淡水供給を介した陸海結合系
    科学研究費助成事業
    01 Apr. 2021 - 31 Mar. 2024
    三寺 史夫; 白岩 孝行; 植田 宏昭; 中村 知裕
    日本学術振興会, 基盤研究(B), 北海道大学, 21H01154
  • Elucidation of hotspot formation mechanism of marine top predators in the waters around Shiretoko
    Grants-in-Aid for Scientific Research
    01 Apr. 2020 - 31 Mar. 2023
    三谷 曜子; 山村 織生; 綿貫 豊; 山口 篤; 中村 知裕
    2020年6月に予定していた調査がCOVIDで中止になったため，2021年5月に観光船をチャーターし，調査を実施した．また，2019年度に実施したおしょろ丸調査のデータ解析を進めた．目視調査で最も多く記録された海鳥はハシボソミズナギドリであった（6月29日知床岬沖で65,000羽）．鯨類では，イシイルカ，シャチ，マッコウクジラ，ミンククジラが知床半島の東西で見られたが，ナガスクジラは西側のみで見られた．音響調査により，プランクトンの反応を示す120 kHzの周波数では，能取岬沖と知床半島周辺の東西で大きな反応が見られた一方，魚類の反応を示す38 kHzの周波数では知床半島東側に大きな反応が見られた．CTD-LADCPとXBT観測からオホーツク海沿岸は宗谷暖流水が分布し，根室海峡北部にも海面から水深150dbar付近まで，既に入り込んでいることが明らかになった．また，植物プランクトンの優占分類群では，宗谷暖流が卓越する沿岸域および中冷水では珪藻類，沖合域ではナノ鞭毛藻類と，海域により異なっていた．このような海域差は，水塊間の栄養塩および鉄濃度の違いに起因すると考えられた．動物プランクトンバイオマスには，中型動物プランクトンのカイアシ類Metridia okhotensisが優占し，この種の排泄する糞粒が鉛直的な物質輸送に大きく貢献することが示された．11地点で実施したMOHTネットによる採集では，斜里沖陸棚斜面上部の2地点（B2およびB3）でソコイワシ科に属する魚類マイクロネクトンのまとまった採集がみられた．しかし，両地点は必ずしも鯨類や海鳥類との高密度域とは一致しておらず，これら高次捕食者の分布との関連が薄い可能性が示された．
    Japan Society for the Promotion of Science, Grant-in-Aid for Scientific Research (B), 20H03054
  • Freshwater discharge from the Kamchatka Peninsula controls Pan-Okhotsk land-ocean linkage system
    Grants-in-Aid for Scientific Research
    01 Apr. 2017 - 31 Mar. 2020
    Mitsudera Humio
    Oyashio is one of most highly productive ocean in the world. This is owing to the Pan-Okhotsk land-ocean linkage system that provides the Oyashio with iron, an essential micronutrient, originating in wetlands in the Amur River basin via intermediate-layer circulation in the Sea of Okhotsk. This project aimed to elucidate roles of precipitation, snow and glacier over the Kamchatka Peninsula, which transfers information of atmospheric variations to the ocean through riverine discharge. We hypothesized that the riverine discharage from the peninsula may control the Pan-Okhotsk land-ocean linkage system. We have found that (1) the total amount of discharge from the Kamchatka Peninsula is approximately 80% of that of the Amur River, and (2) freshwater from the Kamchatka Peninsula gives more impacts on salinity of the dense shelf water that drives the intermediate-layer circulation than that from the Amur River does.
    Japan Society for the Promotion of Science, Grant-in-Aid for Scientific Research (A), Hokkaido University, 17H01156
  • Observations of ocean fine structures with scales between submeso-scale and turbulence using the three-dimensional current and turbidity observation system
    Grants-in-Aid for Scientific Research
    01 Apr. 2016 - 31 Mar. 2019
    Nakamura Tomohiro; OOKI Atsushi; SHIMODA Chikara
    Three-dimensional fine structures with scales smaller than O (1 km) are closely related to mixing, which is a fundamental process in the oceans. However, such structures are not well understood due to the lack of observation methods. In this study, we have made successful observations of three-dimensional structures of transitional processes from a breaking of large-amplitude internal waves to turbulence, using the three-dimensional current and turbidity observation system we developed.
    Japan Society for the Promotion of Science, Grant-in-Aid for Scientific Research (C), Hokkaido University, 16K05549
  • 内部波と渦の相互作用による海洋上層の鉛直混合過程
    科学研究費助成事業
    01 Apr. 2016 - 31 Mar. 2018
    中村 知裕
    海洋の生態系とそれにかかわる物質循環には、表層に近い海面混合層直下の鉛直混合が重要となる。本研究では、この鉛直混合の素過程の一つである「渦と内部波の相互作用」に着目する。渦による流れは通常上層ほど強く、この相互作用は表層近くの鉛直混合を促進する。しかし、海洋には渦も内部波も遍在しているにもかかわらず、それらの相互作用はごく一部のパラメタ領域を除き全く分かっていない。そこで本研究は、「渦と内部波の相互作用」を広いパラメタ領域で調べ、相互作用により混合が促進される場所の分布を見積もることを目的とする。
    平成29年度は、(3) 我々が見出した「相互作用と混合の指標」の分布と季節変動を見積もった。その結果、相互作用により混合を引き起こしうる渦が、年中、広く分布しており、渦と内部波の相互作用が鉛直混合へ有意に寄与することが示唆された。加えて(4) 入射する内部波の振幅が大きい場合について数値実験を行い、入射波が渦中で砕波し、波の運動量が解放されて平均流が形成され、渦が変形され、混合が促進されるという新しい内部波と渦の相互作用を見つけた。
    昨年度の成果と合わせ本研究により「渦と内部波の相互作用」の基礎が解明された。本研究の成果は、以下の発展に繋がる基盤、ないしはそれに必要な素過程の一つに関する基礎的知見となる：1. 海洋生態系に重要な上層の鉛直混合の解明、2. これまでの「静止した海洋」を仮定した内部波研究から、流れのある海洋中の内部波の解明への発展、3. 生成域から遠方へ伝播する内部波の行方とそれによる混合の解明、4.渦の季節・経年・長周期変動による相互作用の変動、ひいては、海洋循環の変動による混合の変動という興味深い可能性への発展。
    以上の成果は学会で発表し査読付国際誌への投稿準備中である。
    （研究協力者：伊藤薫（北海道大学大学院環境科学院博士課程）：研究全般）
    日本学術振興会, 新学術領域研究(研究領域提案型), 北海道大学, 16H01587
  • Overturning circulation that connects the surface and intermediate layers in the North Pacific: A new perspective on its three-dimensional structure and variability
    Grants-in-Aid for Scientific Research
    01 Apr. 2014 - 31 Mar. 2017
    Mitsudera Humio; Fujii Yousuke; Nakanowatari Takuya; Nishikawa Hatsumi
    This study aimed to elucidate the overturning circulation originating in the Sea of Okhotsk that ventilates the intermediate layer of the North Pacific Ocean, where strength of the overturn is determined by salinity. However, our knowledge regarding salt transport pathways from low latitude to high latitude was still lacking. Here we present a salt pathway paying attention to the quasi-stationary jets in the transition region between the subtropical and subarctic gyres. The quasi-stationary jets flow northeastward regulated by bottom topography whose height is only 500 m although the ocean is deep. We have elucidated this process using theoretical models, numerical experiments and drifting buoy measurements. Numerical and chemical tracer analyses show that the saline Kuroshio waters are transported to the subarctic gyre through the quasi-stationary jets. The overturn is likely strengthened when the westerly wind is strengthened because of increase of the northward salt flux.
    Japan Society for the Promotion of Science, Grant-in-Aid for Scientific Research (A), Hokkaido University, 26247076
  • Development of the observation system of three-dimensional small-scale flow structures: Fill the observation gap between sub-mesoscale and turbulence
    Grants-in-Aid for Scientific Research
    01 Apr. 2013 - 31 Mar. 2016
    NAKAMURA Tomohiro; SHIMODA Chikara; KURODA Hiroshi; FUJITA Kazuyuki
    Mixing is a fundamental factor in oceanography, as it controls the transport of heat and dissolved materials in the ocean together with ocean circulation. However, the actual state and nature of small-scale three-dimensional structures (horizontal scales < O (1 km)), which drives mixing, have been obscured by the lack of means of observation. This study is the first achievement of observing such small-scale structures. For this end, we devised and developed an observation system of small-scale three-dimensional flow structures. With this system, we observed small-scale structures of transitional processes toward turbulence from the breaking of large-amplitude internal waves or sub-mesoscale eddies in the Kuril Straits and Oyashio region.
    Japan Society for the Promotion of Science, Grant-in-Aid for Young Scientists (A), Hokkaido University, 25707036
  • Elucidation of thermohaline/biogeochemical circulation systems connecting the Sea of Okhotsk with subarctic North Pacific Ocean
    Grants-in-Aid for Scientific Research
    01 Apr. 2010 - 31 Mar. 2015
    WAKATSUCHI Masaaki; MITSUDERA Humio; NISHIOKA Jun; NAKAMURA Tomohiro; EBUCHI Naoto; WATANABE Yutaka; SUZUKI Koji; KURODA Hiroshi; OHSHIMA Keiichiro; TSUMUNE Daisuke; MATOBA Sumito
    We investigated the thermohaline/biogeochemical circulation systems, connecting the Sea of Okhotsk with the North Pacific, to elucidate a ‘miraculous mechanism’ that supports abundant fisheries resources in the Okhotsk and Oyashio regions. Salinity of the dense shelf water (DSW) over the northern shelf of the Okhotsk Sea controls the strength of the thermohaline circulation of the North Pacific Ocean. DSW salinity is determined by relative contribution among the salinity anomaly that propagates long distance from the subarctic gyre, freshwater anomaly by precipitation, and saline water anomaly rejected during sea ice formation. DSW entrains nutrients such as iron, originating in the Amur River and deposited over the continental shelf, and spreads these materials to the North Pacific Ocean. This study has revealed an exquisite thermohaline-biogeochemical system coupled through DSW and tidal mixing, which controls primary production in the large area of the subarctic North Pacific.
    Japan Society for the Promotion of Science, Grant-in-Aid for Scientific Research (S), Hokkaido University, 22221001
  • Atmosphere-ocean-sea ice interaction processes in the Sea of Okhotsk and the Arctic Region
    Grants-in-Aid for Scientific Research
    01 Apr. 2010 - 31 Mar. 2015
    HUMIO Mitsudera; NAKAMURA Tomohiro; UKITA Jinro; KIMURA Noriaki; OGI Masayo; KANEKO Masami; TAGUCHI Bunmei; KOMORI Nobumasa; OHSHIMA Keiichiro; KAWASHIMA Masayuki; TAKAYA Kotaro
    Atmosphere-ocean (-sea ice) interaction processes in the Sea of Okhotsk and the Arctic Sea were investigated, where characteristic seasonality, such as the formation of seasonal sea ice areas in winter as well as the formation of the Okhotsk High and low-level clouds in summer, is exhibited. Sea surface temperature (SST) in the Sea of Okhotsk is kept cool in summer. This is likely attributed to the feedback between the SST and the low-level clouds. As for winter, it was found that the sea ice extent in the Sea of Okhotsk is linked well with the sea ice extent of the preceding summer in the Eastern Siberian Sea within the Arctic Sea. We investigated this linkage from a novel point of view, which is the annual Arctic Oscillation (annual AO), defined by the surface pressure anomaly averaged over one year from January of each year. Feedback processes associated with the annual AO were discussed.
    Japan Society for the Promotion of Science, Grant-in-Aid for Scientific Research on Innovative Areas (Research in a proposed research area), Hokkaido University, 22106010
  • Roles of the Marginal Seas on Atmospheric Disturbances, Clouds, and Circulations
    Grants-in-Aid for Scientific Research
    01 Apr. 2010 - 31 Mar. 2015
    TACHIBANA Yoshihiro; MANDA Atsuyoshi; YAMAMOTO Masaru; KODAMA Yasumasa; MOTEKI Kousaku; YOSHIOKA Mayumi; YOSHIDA Akira; TSUBOKI Kazuhisa; NAKAMURA Tomohiro; ODAMAKI Minoru
    Summertime oceanic influences on the atmosphere are found; 1) Oceanic influence on the Baiu frontal zone, 2) Radiosonde observational evidence of the impact of cold SST on an anticyclone, 3) Weakening of winds above the Kuroshio front, 4) Seasonal migration of the Baiu frontal zone, 5) Marine boundary layer and low-level cloud responses to the Kuroshio Extension sea surface temperature front, 6) Impacts of a warming marginal sea on rainfall organized under the Asian monsoon. Wintertime oceanic influence on the atmosphere are found; 1) Effects of enclosed ocean on cyclogenesis, 2) Two types of cold-air outbreaks and the effect of coastal sea surface temperature, 3) Abnormal winter weather in Japan controlled by small-scale ocean, 4) Rapid merger and recyclogenesis of twin cyclones leading to heavy precipitation around Japan, 5) Cloud-resolving simulation of heavy snowfalls in Japan, 6) Influences of SST anomaly over the East Asian marginal sea on the subpolar and polar regions.
    Japan Society for the Promotion of Science, Grant-in-Aid for Scientific Research on Innovative Areas (Research in a proposed research area), Mie University, 22106003
  • Air-sea interactions with complex oceanographic structure over the marginal seas and their influence on marine ecosystem
    Grants-in-Aid for Scientific Research
    01 Apr. 2010 - 31 Mar. 2015
    ISOBE Atsuhiko; KAKU Shinu; NAKAMURA Hirohiko; HIROSE Naoki; ISHIZAKA Joji; KIDA Shinichiro; KAKO Shinichiro; NAKAMURA Tomohiro; MANDA Atsuyoshi
    It is found that the Kuroshio meander south of the Japan Islands gives fluctuations of extratropical-cyclone paths south of the islands. It is found that winter cooling over the shallow coastal areas in the East China Sea shows a negative feedback process via sea surface pressure (hence, wind) changes caused by low SST. It is found that surface winds over the Seto Inland Sea oscillate in accordance with SST changes due to the tidal mixing at spring and neap tides, and that SST changes also influence on sea and land breeze over the coastal areas. It is therefore concluded that oceanic processes on regional and coastal scales can indeed alter atmospheric processes, and that two-way processes are likely to be accomplished under appropriate conditions. In addition, the accomplishment of coupling between atmosphere, ocean, and biosphere is likely to occur; we found that SST and extratropical cyclone activities are altered by spring phytoplankton boom over the Sea of Japan.
    Japan Society for the Promotion of Science, Grant-in-Aid for Scientific Research on Innovative Areas (Research in a proposed research area), 22106002
  • Direct observations of tidal mixing and studies on ocean/climate variability synchronized with 18.6-year period nodal tidal cycle
    Grants-in-Aid for Scientific Research
    2008 - 2012
    YASUDA Ichiro; HASUMI Hiroyasu; KOMATSU Kosei; NISHIOKA Jun; WATANABE Yutaka; NAKATSUKA Takeshi; ITOH Sachihiko; TATEBE Hiroaki; KATSUMATA Katsurou; NAKAMURA Tomohiro; HIROE Yutaka; OSAFUNE Satoshi; TANAKA Yuki; IKEYA Tohru; NISHIKAWA Haruka; TOMOSADA Akira
    Intense tide-induced turbulent mixing that reached deep ocean around the Kuril or Aleutian Straits was observationally evidenced. This strong vertical mixing affects marine ecosystem in the subarctic North Pacific through circulations of nutrients such as iron. Furthermore, observations and climate model experiments revealed that oceanic variations originated from the 18.6-year period modulation of the tidal mixing in those regions are amplified by air-sea interactions such as SST east of Japan and the Aleutian Low Pressure system and regulate bi-decadal climate and ocean variations.
    Japan Society for the Promotion of Science, Grant-in-Aid for Scientific Research (S), The University of Tokyo, 20221002
  • Internal wave scattering with frequency change and associated mixing
    Grants-in-Aid for Scientific Research
    2009 - 2011
    NAKAMURA Tomohiro
    Scattering and generation of oceanic internal waves with frequency change, associated mixing, and the effects of mixing on ocean circulation are shown using theories, numerical models, observations, and data analyses.
    Japan Society for the Promotion of Science, Grant-in-Aid for Young Scientists (B), Hokkaido University, 21740338
  • Elucidating mechanisms of intermediate layer warming in the Sea of Okhotsk and North Pacific.
    Grants-in-Aid for Scientific Research
    2007 - 2009
    MITSUDERA Humio; OHSHIMA Keiichiro; NAKAMURA Tomohiro; ONO Kazuya; ONO Tsuneo
    This study is aimed at elucidating characteristics of intermediatelayer warming in the Sea of Okhotsk and western subarctic gyre of the North Pacific, and its mechanisms from a point of view of thermohaline circulation. Dense shelf water (DSW) produced over the northern shelves in the Okhotsk has freshened about 0.1 PSU in the last 50 years, causing warming in the 26.8-27.0σ_θ. isopycnals Numerical experiments indicate that air temperature rise (which causes decrease in sea ice production), increase of fresh water precipitation and wind variations are responsible for the warming.
    Japan Society for the Promotion of Science, Grant-in-Aid for Scientific Research (B), Hokkaido University, 19340131
  • Study of air-sea interactions on multi temporal scales over oceanic fronts
    Grants-in-Aid for Scientific Research
    2007 - 2009
    MINOBE Shoshiro; KOMORI Nobumasa; KUWANOーYOSHIDA Akira; NAKAMURA Tomohiro
    We reported in a cover article of Nature (13 Mar. 2008) that the Gulf Stream causes a series of atmospheric responses, not limited to the atmospheric boundary layer, but penetrate to the free troposphere. Also, we found two different atmospheric modes, i.e., the shallow heating mode prominent in winter and the deep heating mode in summer. The difference of the summer and winter atmospheric responses is associated with the atmospheric stability between the two seasons. A long-term numerical integration of an Atmosphere-Ocean Coupled General Circulation model realistically reproduces major features of the recent satellite surface-wind observations. Furthermore, the first observational evidence of wave breaking, which is expected to cause low sea-surface temperatures, associated with ocean tide is obtained.
    Japan Society for the Promotion of Science, Grant-in-Aid for Scientific Research (B), Hokkaido University, 19340130
  • Modeling of the Intermediate Circulation and Material Transferin the Pan-Okhotsk Region
    Grants-in-Aid for Scientific Research
    2006 - 2008
    EBUCHI Naoto; NAKATSUKA Takeshi; NISHIOKA Jun; MITSUDERA Humio; OHSHIMA Keiichirou; NAKAMURA Tomohiro
    オホーツク海および北西北太平洋親潮域の高い海洋生物生産性を支えている物質循環のメカニズムを,海洋中層(400~800m)の循環と鉄の輸送過程に注目して,現場観測と数値モデルによる研究を行った.その結果,オホーツク海北西大陸棚起源の鉄分が,海氷の生成とともに作られるオホーツク海中層水によって移送され,千島海峡で広い深度層に分配された後,西部北太平洋に送り出されている様子が定量的に明らかとなった.
    Japan Society for the Promotion of Science, Grant-in-Aid for Scientific Research (A), Hokkaido University, 18201002
  • オホーツク海北部から流出入する熱・塩・海水輸送の直接観測
    科学研究費助成事業
    2006 - 2007
    中村 知裕
    2006年8月のオホーツク海中央部の観測に於いて、冬季混合層の名残である中冷水より下、26.7〜26.75sqの中心密度を持つ低温低塩分水が見つかった。この低温低塩分水の起源は、北部陸棚域からシェリコフ湾に分布する東部陸棚高密度水(eDSW)または冬季混合層の可能性が高い。起源がeDSWである場合、eDSWの一部はオホーツク海中央部へ直接流入していることが示唆される。すなわち、従来の、eDSWは陸棚上を西に流れ、西側の陸棚高密度水と合流もしくはその起源となるという知見とは異なる流出経路の存在を示唆している。一方、起源が冬季混合層ならば、積算海氷面積が過去最小を記録するなど比較的暖冬であった前の冬季でさえ、冬季混合層密度が26.75sqに達していたことになり、平年の冬季混合層密度はそれを越える可能性を示唆する。いずれの場合も、ここで報告する低温低塩分水の観測は、北太平洋中層水の主な起源の一つであるオホーツク海モード水またはオホーツク海中層水の上部を通気する新たな経路の存在を示唆している。以上の成果はJournal of Oceanographyに投稿中である。
    また、ロシア側の許可の問題で千島列島における鉛直混合観測が行えなかったので、その代替として千島列島と力学的に状況が似ているアリューシャン列島およびベーリング海陸棚斜面において内部波と鉛直混合の観測を実施した。その結果、振幅200mに達する大振幅内部波を捉えることに成功した。また、鉛直拡散の評価から強い鉛直混合が生じていることが示唆された。これらの内部波過程を数値モデルで定性的に再現し解析した結果、観測された大振幅内部波はシル(海底山脈)上を流れる日周潮流により生成された非定常風下波であることが示唆された。以上の成果は現在国際誌に投稿準備中である。
    日本学術振興会, 若手研究(B), 北海道大学, 18740288
  • Mechanism of clockwise circulations in the southern Sea of Okhotsk
    Grants-in-Aid for Scientific Research
    2005 - 2006
    MITSUDERA H.; OSHIMA Keiichiro; NAKAMURA T.; ONO K.; NISHIGAKI H.
    Clockwise circulations are dominant in the Kuril Basin, the southern portion of the Sea of Okhotsk. Correspondingly, potential vorticity (PV) in the intermediate layer is low in this region. In this project, formation processes of the clockwise circulation were investigated.
    First, effects of localized mixing around the Kuril Islands were considered. A two-layer model with a submarine ridge was used, and mixing is parameterized as diapycnal vertical velocity over the ridge topography. When the mixing commences, the interface deepens over the ridge locally. For weak mixing cases, the depth of the interface corresponds well to a linear beta-plume solution. If the mixing becomes stronger, low PV water forms first over the ridge. Then, the low PV water is carried westward by eddies and waves, letting low PV water occupy the entire region west of the ridge. It is noted that the lower layer flow around the ridge gives significant influences on the circulation through modifying characteristics of the upper-layer wave. In particular, if the characteristics have closed contours, then PV input due to vertical velocity is hardly radiated from the topographic region until the interface becomes so deep that baroclinic instability can take place. Bimodal states in the clockwise circulation are found.
    Second, a high resolution Okhotsk Sea Model, with a 1/12 degree horizontal resolution, was used to investigate wind-driven circulation. The clockwise circulation in the Kuril Basin is formed as a recirculation gyre associated with an eastward jet that extends from the East Sakhalin Current. Both the numerical results and the altimeter data from satellites indicate that this circulation has an annual signal corresponding to the annual variations in wind-stress.
    From these experiments, it has been found that the clockwise circulation in the Sea of Okhotsk has rich structures incorporating both wind-driven and thernmohaline components. An analytical model that investigates impacts on the intermediate layer circulation was also constructed. Further, analysis of data from the intensive CTD survey across the Bussol' Strait were carried out to estimate the vertical mixing strength along the Kuril Islands.
    Japan Society for the Promotion of Science, Grant-in-Aid for Scientific Research (C), Hokkaido University, 17540406