Ocean circulation and dynamics over the China Shelf Seas

Abstract

The China Shelf Seas (CSS) has complex circulations jointly governed by wind, tides, terrestrial buoyancy and momentum influxes, exchange with the adjacent oceans, and the underlying dynamics of flow-topography interactions. Many unresolved issues about circulation dynamics remain in the CSS. By combining observations, novel numerical simulation approaches, and holistic geophysical fluid dynamics analysis, this thesis study tackled the issues in the following three parts under the wind-driven shelf circulation dynamics framework. In part 1, we investigated the responses of the upwelling circulation to oscillational tidal and persistent lateral buoyancy influx based on a newly implemented two-dimensional model that is enabled with these two forcings. We found that the tidally induced intra-tidal asymmetric friction and nonlinear advection, as well as the buoyancy influx largely alter the cross-shore transport and reshape the alongshore circulation through both baroclinic and barotropic geostrophic adjustments. In part 2, we extended the classic bottom boundary layer (BBL) dynamics by including the effects of variable wind-driven upwelling regimes. We revealed that spatially variable upslope buoyancy advection induced by geostrophic cross-isobath transport (GCT) over the meandering shelf topography greatly changes BBL dynamics by the modulation of bottom frictional shear and restratification and forms variable BBL regime over the shelf. In part 3, we expanded our study to the entire CSS and investigated the dynamics of counter-wind currents (CWCs) by combining observational study and both high-resolution realistic and process-orientated modeling. We found that the net alongshore pressure gradient force, the ensuing ageostrophic alongshore acceleration and cross-shore geostrophic adjustment, controlled by variable wind, stratification, and variable shelf topography, are the intrinsic dynamics for CWCs formation. Multi-mechanism analyses provided new understandings to the existing ambiguous CWCs dynamics. This thesis advances the knowledge of the complex shelf circulation dynamics of the CSS that is applicable to other similar coastal systems worldwide.

Date of Award	2024
Original language	English
Awarding Institution	The Hong Kong University of Science and Technology
Supervisor	Jianping GAN (Supervisor)