The Role of Time- and Spatially Varying Wind in Coastal Circulation Based on High-Resolution Oceanic and Atmospheric Simulations

Wind forcing plays a central role in oceanic circulation, including circulation in small bays where complex coastal orography modulates local winds both in time and space. However, their role in the circulation of such bays remains unclear because local winds are difficult to observe directly over the sea. Here, we used oceanic and atmospheric simulations with 100 m horizontal grid spacing, together with observations, to investigate wind-driven circulation under time- and spatially varying wind forcing in Otsuchi Bay. The simulation forced by time- and spatially varying winds reproduced the observed SSS distribution more closely than the simulation forced by time-varying but spatially uniform wind, which is a common simplification in coastal oceanic simulations. This improvement was explained by bay-interior upwelling away from the coastline, which was not evident under spatially uniform wind forcing. This upwelling was driven by wind stress divergence rather than by wind stress curl. Spectral analysis showed that the relative importance of two mechanisms varied across timescales. Divergence-driven vertical motion was strongest at periods shorter than the local inertial period, but it became relatively weaker than curl-driven motion at longer periods. This study demonstrates that short-timescale wind stress divergence can drive bay-interior upwelling and modify the SSS distribution in small bays. This finding further suggests that upwelling in small bays is not limited to coastline-confined upwelling or curl-driven Ekman upwelling. Accounting for divergence-driven upwelling is therefore important for understanding and modeling coastal circulation in small bays, with potential implications for the vertical transport of biogeochemical materials.

Journal of Geophysical Research: Oceans, in press