Topographic modulation on the layered circulation in the South China Sea

Abstract

The South China Sea (SCS) is the largest semi-enclosed marginal sea in the western Pacific. It exhibits a unique vertically rotating cyclonic, anticyclonic, and cyclonic circulation in its upper, middle, and deep layers. Over slope topography, these layered currents interact and significantly shape the structure and intensity of the basin circulation. In this study, we employ process-oriented numerical simulations to investigate how upper-layer processes, characterized by greater magnitude and variability, influence the layered circulation over the irregular topographic slope. The simulations reveal that stronger upper intrusion from the open ocean directly enhances upper-layer circulation, which subsequently strengthens the middle and the deep slope currents. Vorticity dynamics illustrate that changes in the middle and deep slope current are largely related to the vertical stretching ($\mathit{\zeta }\mathit{\_}\mathrm{DIV}$) induced by bottom geostrophic cross-isobath transport (${\mathrm{CGT}}_{\mathrm{b}}$). As the upper-layer cyclonic slope current intensifies, it modulates the bottom pressure distribution, resulting in stronger negative $\mathit{\zeta }\mathrm{\_}\mathrm{DIV}$ predominantly over the northwestern slope to intensify the middle anticyclone slope current. Similarly, for the deep cyclonic slope current, the ${\mathrm{CGT}}_{\mathrm{b}}$ maintains downwelling in the northern part and upwelling over the southern slope. Over the southern slope, the strengthening of the positive ${\mathrm{CGT}}_{\mathrm{b}}$ is induced by the increment of the advection of relative vorticity and planetary vorticity in the water column, in which the middle layer has an important contribution, but the upper layer has a minimal impact. Conversely, on the northern slope, the strengthening of the negative ${\mathrm{CGT}}_{\mathrm{b}}$ is primarily influenced by the upper layer.