[A-OS16] Frontiers of Ocean Mixing Research

[AOS16-12] Subsurface phytoplankton increase caused by submesoscale instability and associated turbulent mixing in the Kuroshio flowing over seamounts

*Takeyoshi Nagai¹, Silvana Gloria Duran Gomez², Hiroaki Saito³, Hiroshi Ogawa³, Toru Kobari⁴, Naoki Yoshie⁵ (1.Department of Ocean Sciences, Tokyo University of Marine Science and Technology, 2.Graduate School of Marine Science and Technology, Tokyo University of Marine Science and Technology, 3.Atmosphere and Ocean Research Institute, The University of Tokyo, 4.Faculty of Fisheries, Kagoshima University, 5.Center for Marine Environmental Studies, Ehime University)

Keywords:Kuroshio, Nutrient sream, Turbulent Mixing, Submesoscale, Near inertial internal waves

The Kuroshio flows over many topographic features along its path, causing the strong turbulent mixing and plausibly large nutrient diffusive fluxes. In the Tokara Strait, our previous measurements revealed the 100-1000-fold enhancement in turbulent kinetic energy dissipation rates spreading over 100 km scale along the Kuroshio flowing over seamounts. The observed very strong turbulence probably forms a hotspot of nutrient injections in the Kuroshio, that could play important roles in biological production along its path. However, as there were no simultaneous nutrient measurements, it remains unclear how the lower trophic levels response to the nutrient injections. In this study, by using a twin tow-yo turbulence and biogeochemical profiling system along the Kuroshio in the Tokara Strait during October 2020 R.V. Hakuho-Maru cruise, it is shown that the subsurface chlorophyll-a maximum is diffused vertically near the seamount and increased in the subsurface layers of downstream. Furthermore, measured turbulence and nitrate data show that a large amount of nitrate is supplied to the shallower layers at a rate, ~3 mmolm^-2day^-1 over 180 km at the base of euphotic zone on average. The results suggest that the biological responses of lower trophic levels in the upstream Kuroshio occur under the surface in the stratified seasons, that may not be seen by the satellites. We will show the results from the numerical model in the presentation, if possible.