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