Turbulent Mixing and Nutrient Supply associated with the Kuroshio Induced Submesoscale Anticyclonic Vorticity over Seamounts and Continental slopes south of Kyushu Japan

Although the Kuroshio is known as nutrient poor in the surface layers, several previous studies have reported that a large amount of nutrients are transported laterally in its subsurface layers as a nutrient stream. On the other hand, the Kuroshio flows over many topographic features along its path. In the Tokara Strait, south of Kyushu, our previous measurements revealed the 100-1000-fold enhancement in turbulent kinetic energy dissipation rates spreading over 100 km along the Kuroshio flowing over seamounts. The strong turbulence is found to coincide with the negative potential vorticity associated with the anticyclonic vorticity streaks generated at the steep seamounts. Although the intense turbulence probably forms a hotspot of nutrient injections, how much nutrients can be supplied remains unclear as there were no simultaneous nutrient measurements. 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 a large amount of nitrate is supplied to the shallower layers at O(1 mmolm-2day-1) over 180 km at the base of euphotic zone on average. Furthermore, subsurface chlorophyll-a maximum is diffused vertically near the seamount and enhanced in the subsurface layers of downstream. The results suggest that the biological responses of lower trophic levels in the upstream Kuroshio occur under the surface in the stratified seasons, that cannot be seen by satellites. On the other hand, the Kuroshio or warm core eddies frequently intrude into the region north of the Yaku Island, forming a branch current through the Osumi Strait, which merges back into the Kuroshio after passing the Tanegashima Island. The southward shoaling bottom slopes along the northern coast of Yaku and Tanegashima Islands can, therefore, form favorable flow conditions to generate anticyclonic vorticity. Using the reanalysis data, it is shown that the Osumi branch current occurs fall to winter. In the presentation, the results from a high-resolution numerical simulation coupled with an ecosystem model are presented to elucidate the nutrient supply mechanisms associated with the anticyclonic vorticity in the Tokara Strait and in the regions north of Yaku and Tanegashima Islands.