Intensified vertical mixing around various sea mounts along the Kuroshio and its contribution to the ecosystem

Takeshi Matsuno1, Xinyu Guo2, Hirohiko Nakamura3, Eisuke Tsutsumi4, Tomoharu Senjyu5, Takahiro Endoh1, Jing Zhang6, Kaoru Ichikawa1, Toru Kobari3, Naoki Yoshie2, Daisuke Hasegawa7, Takeyoshi Nagai8, Ayako Nishina3, Akie Sakai9, Tadateru Noguchi6, Ming-Huei Chang10, Yiing Jang Yang11, Sen Jan10, Cesar Villanoy12, Keunjong Lee13, Daigo Yanagimoto4 and Ichiro Yasuda4, (1)RIAM, Kyushu University, Fukuoka, Japan, (2)CMES, Ehime University, Matsuyama, Japan, (3)Fac. Fisheries, Kagoshima University, Kagoshima, Japan, (4)AORI, University of Tokyo, Kashiwa, Japan, (5)Research Institute for Applied Mechanics, Kyushu University, Fukuoka, Japan, (6)Fac. Science, University of Toyama, Toyama, Japan, (7)Tohoku National Fisheries Research Institute, Japan Fisheries Research and Education Agency, Shiogama, Japan, (8)Tokyo Univ.Mar.Science&Tech., Tokyo, Japan, (9)Interdisciplinary Graduate School of Engineering Sciences, Kyushu University, Fukuoka, Japan, (10)National Taiwan University, Institute of Oceanography, Taipei, Taiwan, (11)National Taiwan University, Taipei, Taiwan, (12)Marine Science Institute, University of the Philippines, Quezon City, Philippines, (13)Korea Institute of Ocean Science and Technology, Busan, South Korea
The Kuroshio is the western boundary current constituting a part of North Pacific sub-tropical gyre, and transports heat and materials from low to high latitudes. The objectives of this study, a part of OMIX project, are to determine quantitatively the intensity of vertical mixing along the Kuroshio, particularly around various sea mounts or ocean ridges, and evaluate contribution of the mixing to material transport and primary production.

Luzon Strait, I-Lan Ridge/East of Taiwan, shelf break of the East China Sea and Tokara Strait were chosen as places to survey, where predominant vertical mixing would occur associated with the bottom topography along the Kuroshio. Twelve cruises in total were carried out from 2015 to 2019. Among them, intensive studies were performed in Tokara Strait, including measurements of microstructure, current measurements with moored arrays, water sampling to analyze nutrients, chemical tracers and biological factors and to incubate phytoplankton. Strong turbulence was repeatedly observed around the sea mount within the Kuroshio, where turbulent energy dissipation rate ε frequently exceeded 10-6 W kg-1. This intensified turbulence would be generated with hydraulic control, on which high resolution numerical model suggested the appearance of strong turbulence associated with hydraulic jump just behind the sea mount. The intensified vertical mixing enhanced the upward transport of nutrients, for example, vertical flux of nitrate was about 1 mM m-2 day-1 around the sea mount, which would support significant primary production in the euphotic zone and influence the ecosystem downstream of the Kuroshio.

Similar enhanced vertical mixing was examined around the ocean ridge in the Luzon Strait and I-Lan Ridge, east of Taiwan, along the Kuroshio. Largest ε exceeded 10-6 W kg-1 around both ridges similarly to Tokara Strait, while the strong turbulence in the Luzon Strait was caused by tidal motions during our observations.