The Roles of Wind and Baroclinic Processes in Cross-Isobath Water Exchange Within the Bohai Sea

Considering the significance of water exchange between the nearshore and offshore waters to shallow-water biogeochemical processes, the influences of wind and baroclinic processes on water exchanges across the 10m (E10) and 20-m isobaths (E20) in the Bohai Sea during ice-free cycles of 1998-2019 were investigated using a three-dimensional Finite Volume Community Ocean Model (FVCOM). The two-way analysis of variance illustrated that both E10 and E20 showed significant seasonal and spatial variations. The cross-isobath fluxes were the strongest during summer and the weakest during fall, with offshore (onshore) transport in the upper (bottom) layer. The net volume transport was at O (10-2) Mm3/s (millions of cubic meters per second) in Bohai and Liaodong Bays, which was an order of magnitude greater than that in Laizhou Bay. Further numerical experiments revealed that wind regulated the upper-layer E10 via the wind-induced advection, while it altered E20 mainly through the enhanced mixing and the subsequent weakening of the baroclinic pressure gradient force in the subsurface layer. In addition, the thermal gradient driven cross-isobath volume transports in Bohai and Liaodong Bays were most prominent during summer but negligible during fall. Those along the 10-m isobath in Laizhou Bay, however, were induced by the salinity gradient from the buoyant fluxes of the Yellow River all year round. With supplemental contributions of the baroclinic processes related to the cold pool of the Bohai Sea, and the wind-induced advection and mixing in the surface and subsurface layers, respectively, Laizhou Bay's E20 had a four-layer vertical structure during summer.