Counter‐Wind Currents in the China Shelf Seas: The Alongshore Ageostrophic Trigger and Cross‐Shore Geostrophic Balance

Abstract During the non‐summer seasons, persistent northeastward counter‐wind currents (CWCs) exist over the broad China Shelf Seas, flowing against the northeasterly Southeast Asian monsoon. Using a well‐validated, high‐resolution China Seas model, this study provides novel dynamic explanations for the formation and spatiotemporal variability of these CWCs. We found that alongshore CWCs are triggered (accelerated) by the counter‐wind alongshore net (ageostrophic) pressure gradient force (PGF) and are subsequently sustained by cross‐shore PGF via geostrophic adjustment. The alongshore PGF is driven by two processes. First, the northward decreasing air‐sea heat flux generates the alongshore variable steric height, while coastline orientation determines the rate of latitudinal change and alters the steric height gradient. Second, the alongshore wind establishes a dynamic height gradient through arrested topographic wave dynamics; spatially varying alongshore wind magnitude then induces alongshore variability in this gradient. Consequently, the interaction between variable heat fluxes, wind forcing, and coastline orientation determines the spatiotemporally variable northeastward alongshore net PGF, and the associated CWCs over the China Shelf Seas.