Dynamic Adjustment of Upwelling Circulation to Conservative Wave Effects over a Steep Shelf

Abstract We investigated conservative wave effects on the dynamic adjustment of upwelling circulation and found that waves reduce the along-isobath transport in the upwelling circulation, leading to reductions in vertical shear stress and subsequent cross-isobath upslope frictional transport over a steep shelf east of Hainan. Specifically, we used a submesoscale coupled wave–circulation model to investigate the variability and dynamics of the wave effects on the upwelling circulation over the shelf. Analyses of data from a wave buoy and remotely sensing surface temperature further demonstrated the critical role of waves in reducing the upwelling. During a typical upwelling season, onshore Stokes transport cumulatively raises sea level nearshore and reduces it offshore, which geostrophically weakens the alongshore upwelling jet and reduces the upslope frictional transport in the bottom boundary layer. We found that concurrent offshore anti-Stokes current induced by the Stokes Coriolis force is not strong enough to offset the onshore Stokes drift due to wave-modulated turbulent stress. The onshore Stokes drift also interacts with positive vorticity at the onshore flank of the alongshore jet and negative vorticity on the offshore flank, which yields a Stokes vortex force (SVF) that converges Eulerian flow on the shoreside of the jet. Consequently, SVF further raises the sea level nearshore, weakening shoreward pressure gradient force (PGF) and the alongshore upwelling jet. The subsequent baroclinic response caused by the wave-modulated density gradient competitively counteracts these barotropic effects. In addition to this dominant geostrophic adjustment, waves influence the PGF through setup by varying radiation stress and correcting the surface pressure in shallow waters.