[A-OS15] [EE] Ocean Mixing Matters

[AOS15-07] How the Tokara strait cultivates the Kuroshio

*DAISUKE HASEGAWA¹, Takeshi Matsuno², Eisuke Tsutumi², Tomoharu Senjyu², Hirohiko Nakamura³, Toru Kobari³, Ayako Nishina³, Naoki Yoshie⁴, Xinyu Guo⁴, Miwa Nakagawa⁴, Takeyoshi Nagai⁵, Takahiro Tanaka⁶, Ichiro Yasuda⁶ (1.Tohoku National Fisheries Research Institute, Japan Fisheries Research and Education Agency, 2.The Research Institute for Applied Mechanics, Kyusyu University, 3.Faculty of Fisheries, Kagoshima University, 4.Center for Marine Environmental Studies, Ehime University, 5.Tokyo University of Marine Science and Technology, 6.Atmosphere and Ocean Research Institute, The University of Tokyo)

Keywords:Turbulence, Mixing, Topography, Nutrient, Nitrate, Flux

Previous studies have reported clear signs of vigorous turbulent mixing in the Kuroshio due to the presences of shallow and steep topographies in its path (e.g., Hasegawa et al., 2004 and 2008, Chang et al., 2016). Turbulent mixing is one of the most important processes supplying nutrients to the surface euphotic zone from the deep water; however, a quantitative understanding of the turbulent vertical nutrient flux is still limited. On November 2016, we have conducted intensive survey around the Tokara strait by drifting the T/V Kagoshima-maru with the Kuroshio's stream and passed across the shallow (∼100 m) sill while deploying a submersible ultraviolet nitrate analyzer (Deep SUNA by Satlantic) attached on a turbulence ocean microstructure profiler (TurboMAP-L by JAC). Occurrence of a flow separation and a hydraulic jump on the sill have been identified from a high resolution velocity survey. The rate of dissipation of kinetic energy reaches O(10^-5 W kg^-1), and the turbulent vertical nitrate flux reaches O(1 mmol m^-2 day^-1), which is the highest value ever reported for the open ocean.