Elevated nutrient supply caused by the Kuroshio approaching to the continental shelf associated with submesoscale flows and near-inertial waves

Abstract:

While surface layers of the Kuroshio Current have been known for being nutrient poor, previous studies reported that the Kuroshio carries a large amount of nutrients in dark subsurface layers. However, these abundant subsurface nutrients along the stream are not available for phytoplankton unless they are injected to the sunlit surface layer through a mechanism that is still unclear. Results published recently, by the authors, showed that the Kuroshio flowing on the continental shelf induces very strong turbulent dissipation rates O(10^-7 W kg^-1) associated with bands of large amplitude near-inertial internal wave shear, leading to a large diffusive nitrate flux to euphotic zone of >O(1 mmol N m^-2 day^-1). Nevertheless, since this previous study was conducted for just one transect survey, questions remain of how long this enhanced turbulent diffusive nitrate flux extends further to the downstream and how the Kuroshio path modulations influence on the nutrient injection (Submitted image). In this study, using a high-resolution nested simulation coupled with a N₂PZD₂ ecosystem model, we successfully reproduced these observed features, including banded shear near the model Kuroshio. When the model Kuroshio flows closer to the coast, it hits the small bump in the upstream region of the Hyuganada Sea producing negative potential vorticity, and associated disturbances seem to generate near-inertial waves which are trapped in the streak of a lowered lowest internal wave frequency, elevating the internal wave energy and mixing. As a result, the elevated vertical mixing diffuses up the nitrate from dark subsurface layers to sunlit shallow layers. The results of this study can improve our understanding of the role of the Kuroshio in providing nutrients and enhancing biological productions including fish abundances, providing a better predicting of the ecosystem responses to the Kuroshio path modulations.