Dynamics of tidal effects on coastal upwelling circulation over variable shelves in the northern South China Sea

Background:

Upwelling is a key driver of physical, biogeochemical, and ecological variability in coastal regions, bringing cold, nutrient-rich deep water into the euphotic zone and thriving marine ecosystems. The wind-induced Ekman frictional effect cannot completely explain the observed spatial and temporal variations of the upwelling. Coastal upwelling is largely affected by topographic features, river plumes, and tides. Tides, intrinsically embedded in the coastal circulation, affect the upwelling through tidal mixing. Nonlinear tidal dynamics cause tidal rectification, which generates tidal residual currents that influence the subtidal mean flow. Moreover, energetic tidal motions can lead to significant nonlinear interactions between the wind-driven upwelling circulation and tidal currents. Tidal effects on the wind-driven upwelling circulation are not isolated but can jointly modulate the inter-linked cross-isobath and along-isobath flows through these mechanisms. The impact of tidal forcing on the complex three-dimensional (3D) wind-driven upwelling dynamics over highly variable shelves is important but remains unclear.

Research highlight:
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 tidal modulation on the upwelling circulation is mainly attributed to the residual currents caused by tidal rectification and further amplified by the interaction between tidal and wind-driven currents. Physically, 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 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 bottom boundary layer 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.   

Key points:
•    Tides weaken (enhance) along/cross-isobath transports in the inner (outer) shelf due to residual currents and tidal-subtidal interaction.
•    Tidal modulation on upwelling is dominated by the geostrophic adjustment through competitive barotropic and baroclinic processes.
•    Tidally enhanced baroclinic along-isobath pressure gradients and bottom stress strengthen the bottom onshore transport. 

For more detailed analysis, please refer to Lin, S., & Gan, J. (2024). Dynamics of tidal effects on coastal upwelling circulation over variable shelves in the northern South China Sea. Journal of Geophysical Research: Oceans, 129, e2024JC021193. https://doi.org/10.1029/ 2024JC021193

Figure. Schematic view of tidal effects on the wind-driven upwelling processes and forcing mechanisms in the shelf of the north South China Sea. Profiles of the along-isobath velocity with and without tides are shown by the solid and dashed purple lines, respectively. Solid (hollow) arrows indicate the directions of transports (forces), with red representing an increase and green representing a decrease. Thick green and black lines show the boundary layers in cases with and without tides. Abbreviations are used for PGF, pressure gradient force; PGC, baroclinic pressure gradient force; ADV, advection; HADV, horizontal advection; VVIS, turbulent stress; SBL, surface boundary layer; BBL, bottom boundary layer.