Counterwind Currents in the China Shelf Seas: The Origin of the Pressure Gradient Driver

Abstract There exist unique northeastward counterwind currents (CWCs) over the China shelf seas, subject to the balance between the northeastward alongshore ageostrophic (effective) pressure gradient force (PGF eff , the residual PGF after balancing the Coriolis force) and southwestward surface wind stress forcing. The underlying physics for the formation of the alongshore PGF remains largely ambiguous. We used process-oriented modeling of the China shelf seas to investigate how the alongshore PGF and subsequent CWC form. Driven by a typical alongshore variable density field and wind forcing, our numerical model produced a realistic shelf current structure and CWC. Our results show that the alongshore sea level elevation gradient, induced by an alongshore variable wind, mainly contributed to the PGF, which is consistent with arrested topographic wave theory. However, the wind-induced elevation gradient alone is insufficient to overcome the frictional effects of wind forcing to form the CWC. The alongshore density gradient, due to heterogeneous heating, enhances the PGF because of the steric effect on sea level. The enhanced PGF produced by the density gradient and wind-induced elevation gradient critically forms the CWC. In addition, the seasonally variable density gradient and wind forcing determine the spatiotemporal variability of the CWC. We found that the dominant intrinsic dynamics of the wind and buoyancy forcing are enough to trigger the CWCs, and the shelf’s topographic features further shape the structure of the CWC. The study provides new insights into the formation of CWC, which has been widely observed over the shelves globally.