Dynamics of wind-driven upwelling off the northeastern coast of Hainan Island

Both observational and reanalysis sea surface temperature (SST) data revealed that the strong upwelling occurs frequently off the northeastern coast of Hainan Island (downstream of the changing point of topography, off the Tongluo Cape), which cannot be attributed to the along-shelf wind alone. To identify the dynamics of this upwelling, a numerical experiment was conducted using an idealized topography that was simplified from the actual topography off the coast of Hainan Island. Analysis of the vertically integrated momentum balance showed that the upwelling-favorable cross-isobath transport downstream of the changing point of topography was primarily intensified by the along-isobath barotropic pressure gradient force (PGT), but was weakened by the along-isobath baroclinic pressure gradient force (PGC). Based on the vorticity equation, the PGT was linked to the advection of relative vorticity, the bottom stress curl and the gradient of momentum flux. The potential vorticity (PV) balance was applied to diagnose the PGC-related processes. The negative PV terms from the joint effect of baroclinicity and relief (JEBAR) and the baroclinic bottom pressure torque made a similar contribution to weakening onshore transport downstream of the changing point of topography as the negative PV term from wind stress. The onshore transport was intensified by the positive PV from bottom stress. In the shallow nearshore region, the cross-isobath forces played an important role for the cross-isobath transport.