Dynamics of Tidal Effects on Coastal Upwelling Circulation Over Variable Shelves in the Northern South China Sea

Abstract

The impact of tidal forcing on the complex three-dimensional (3D) wind-driven upwelling dynamics over highly variable shelves is important but remains unclear. Combining field and satellite observations and a high-resolution numerical model, we investigated tidal effects on the 3D transport and dynamics of the wind-driven upwelling circulation in the northern South China Sea. Our results showed that tides weaken (strengthen) the subtidal along-isobath and cross-isobath transports in the inner (outer) shelf, primarily by modulating the corresponding cross-isobath and along-isobath pressure gradients. The tidally induced geostrophic adjustment is predominantly contributed by its barotropic effect due to sea level gradients above the bottom boundary layer (BBL) in both the inner and outer shelves. The baroclinic process, caused by tidally modulated density gradients, counteracts the barotropic effect and dominates the intensified onshore flow within a thickened BBL from the outer to middle shelves, together with the enhanced bottom stress. Based on the vorticity dynamic, this baroclinic process leads to an amplified joint effect of baroclinicity and relief over the steep shelf that strengthens tidally modulated along-isobath pressure gradients. While tidally enhanced bottom stress curl can be essential in the inner shelf. The intensity of tidal modulation varies across different shelves, depending on tidal strength and bottom topography. The subtidal bottom onshore transport can increase by sevenfold when strong tidal currents persist over a steep outer shelf. This comprehensive study unveiled the dynamics of tidal effects on the upwelling and the importance of an interlinked tidal-subtidal system over complex shelf topography.