967 / 2024-09-19 22:27:15

Dynamic adjustment of coastal upwelling to surface waves over a steep shelf

surface wave,coastal upwelling,pressure gradient force,Stokes vortex force,Stokes Coriolis force

Session 24 - Estuaries and coastal environments stress - Observations and modelling

The impacts of surface waves on coastal upwelling have rarely been examined in a real ocean. We used a coupled sub-mesoscale wave-circulation model to investigate the variability and dynamics of wave effects on the upwelling over a steep shelf east of Hainan. During a typical upwelling season, the onshore Stokes transport cumulatively raises (reduces) sea level nearshore (offshore) and geostrophically weakens the upwelling jet. This is because the offshore anti-Stokes current induced by the Stokes Coriolis force is weaker than the onshore Stokes drift due to wave-modulated turbulent stress. Meanwhile, the onshore Stokes drift interacts with positive (negative) vorticity along the onshore (offshore) flank of the upwelling jet and yields a Stokes vortex force (SVF) that enhances (weakens) the surface offshore Eulerian flow. Consequently, this SVF further raises the sea level nearshore, weakening the onshore pressure gradient force (PGF) and the upwelling jet. The subsequent baroclinic response caused by the wave-modulated density gradient competitively counteracts this barotropic effect. In addition to this dominant geostrophic adjustment, waves influence the onshore PGF by correcting the surface pressure and by inducing set-up in shallow waters due to varying radiation stress. Overall, wave effects reduce the along-isobath transport by ~8.0%, leading to reductions in vertical shear stress and subsequent bottom onshore transport by ~23.0% nearshore. This study highlights the importance of wave impacts on upwelling over the shelf and reveals the intricate dynamics of a coupled wave-circulation system.