Dynamics of the Ryukyu Current System and its role in the material circulation in the North Pacific

2018 Fiscal Year Final Research Report

• Project/Area Number: 15H03725
• Research Category: Grant-in-Aid for Scientific Research (B)
• Allocation Type: Single-year Grants
• Section: 一般
• Research Field: Meteorology/Physical oceanography/Hydrology
• Research Institution: Kagoshima University
• Principal Investigator: Nakamura Hirohiko 鹿児島大学, 農水産獣医学域水産学系, 教授 (50284914)
• Co-Investigator(Kenkyū-buntansha): 仁科 文子 鹿児島大学, 農水産獣医学域水産学系, 助教 (80311885) ; 加古 真一郎 鹿児島大学, 理工学域工学系, 助教 (60709624) ; ナ ハンナ 北海道大学, 理学(系)研究科(院), その他 (40726171)
• Research Collaborator: Zhu Xiao-Hua ; Park Jae-Hun ; Min Hong Sik ; Na Hanna
• Project Period (FY): 2015-04-01 – 2019-03-31
• Keywords: 琉球海流 / 黒潮 / 中規模渦 / 季節変動 / 北太平洋中層水 / 栄養塩

Outline of Final Research Achievements

The main results are as follows: 1) The Ryukyu Current was observed in its upstream area through the Japan, China and Korea joint collaboration research. As a result, a quasi-permanent undercurrent with a ~20 cm/s velocity core was identified at ~500 m depth. The intensity of this velocity core was not affected by impingements of westward-propagating mesoscale eddies, suggesting that the undercurrent, i.e., the Ryukyu Current, is not an organization of such eddy impingements. This result was supported by the analysis of a mesoscale eddy trucking dataset. 2) Based on data analysis and numerical experiments, the reason why seasonal velocity variations of the Kuroshio and Ryukyu Current are out of phase was clarified. Namely, the local process due to the East Asian Monsoon is responsible for the seasonal velocity variation of the upper-layer Kuroshio, while the remote process due to the curl of interior wind stresses is responsible for that of the lower-layer Kuroshio and Ryukyu Current.

Free Research Field

海洋物理学

Academic Significance and Societal Importance of the Research Achievements

琉球海流の形成理論については，筆者らが提案した陸棚波理論（Nakamura et al., 2007）は存在するものの，中規模渦が琉球海流の形成に寄与しているか否かという基本的な問いは未解決であった。本研究の結果は，源流域の琉球海流の形成に，同緯度帯を西進してくる中規模渦は寄与していないことを示しており，形成理論を確立する上で大きな前進をもたらしたと言える。さらに，黒潮と琉球海流の流速の季節変動研究から，黒潮の表層流速の季節変動に対する東アジアモンスーンの重要性が指摘され，黒潮研究の新たなパラダイムを開拓した。これらの2点は，海洋物理学における大きな学術的意義と言える。