Wind‐Induced Coastal Upwelling Off Southeast Japan Supplying Surface Nutrients to the Kuroshio Downstream

Abstract Wind‐induced coastal upwelling plays a significant role in ocean circulations and ecosystems. Although coastal upwelling along eastern boundaries by trade winds is well known, coastal upwelling along western boundaries and its effects in the North‐East Pacific Ocean have not been well investigated in previous studies. This study presents surface nutrient supply owing to coastal upwelling along the southeast coast of the Japanese mainland in summer investigated with a coupled physical‐biogeochemical model. The model is validated against satellite‐based observations (surface temperature and chlorophyll‐a), showing good agreement between the model and observations. The summer monsoon (southerly wind) induces coastal upwelling along the coast, leading to rich nutrient conditions near the surface and phytoplankton blooms in the study area. The model results show two hotspots of coastal upwelling on the east sides of the Kii and Boso Peninsulas. Although vertical current transport and turbulent diffusion contribute equally to the surface nitrate supply along the peninsulas, coastal upwelling is identified as the primary contributor to variations in the nutrient supply. In addition to the direct advective surface nutrient supply, coastal upwelling strengthens the vertical nutrient gradient, thereby enhancing vertical diffusive nutrient transport. The Kuroshio Current passes near the upwelling regions, resulting in highly concentrated nutrient/phytoplankton water uptake to the sea surface downstream of the Kuroshio, which results in the nutrient/phytoplankton stream.