Submesoscale Eddy-induced Nutrient Upwelling in the Upstream Kuroshio Current Forms Ecological Hotspot

Abstract While the Kuroshio, a western boundary current in the North Pacific, transports a large amount of nutrients in dark subsurface layers, it has been unclear whether these subsurface nutrients are supplied to continental shelves along southern coast of Japan. Recent observations revealed that the upstream Kuroshio flowing through rough topography forms a large-scale turbulence hotspot that may supply nutrients to sunlit surface layers. However, the Kuroshio-Island interaction in this region may also induce eddies and nutrient upwelling which have neither been observed directory nor quantified. Here, through high-resolution in-situ observations, we show submesoscale ~10 km nutrient structures along isopycnals, suggestive of eddy-induced upwelling. High-resolution simulations reproduce these features generated by submesoscale cyclonic eddy-induced nitrate upwelling at O(10 mmol m-2day-1), accounting ~9% of net primary production in the area 400 km downstream. Submesoscale eddies in the upstream Kuroshio partly explain how the Kuroshio sustains high biodiversity and biological production.