Abstract:

The South China Sea, where the dynamics of the upper ocean are strongly driven by wind fields, is a typical marginal sea influenced by monsoon systems. The “wind pump” theory provides a core framework for understanding wind-driven vertical movement and its corresponding biogeochemical responses. However, most existing studies focus primarily on continental shelves or open seas, leaving the key transitional continental slope zone noticeably short of systematic investigation. In particular, the three-dimensional dynamic responses and topographic modulation effects under persistent strong wind events remain poorly constrained. This study focuses on a persistent strong wind event that occurred in the southern South China Sea from March 21 to 26, 2021. Combining in-situ CTD observations, multi-source remote sensing data and statistical analysis, we systematically compare differences in upper ocean response across the continental shelf, continental slope, and open sea, before, during, and after the strong wind event. The case of site-specific analysis in the upper 0-60 m layer demonstrates that, during the wind, the three topographic zones showed different responses to the strong wind event. On the continental shelf, surface cooling occurred during the wind event, while a pulsed chlorophyll-a increase appeared on the fourth day after wind onset. The open sea showed a relatively weak biological response, with chlorophyll-a exhibiting only a brief, weak peak on the sixth day after wind onset, despite experiencing the greatest mixed-layer deepening. In contrast, the continental slope exhibited the strongest response: intermediate-layer temperature rose by more than 0.24 ℃ and salinity by approximately 0.11 PSU, with vertical velocity indicating upwelling. Its chlorophyll-a response was also distinctive—characterized by delayed in onset, multi-peaked, and moderate in amplitude. These findings suggest that steep continental slopes may amplify wind-driven upwelling through the coupling of Ekman suction and the topographic beta effect and generate multi-scale vertical motion by superposing near-inertial internal waves. Building on these results, we will extend the research scope from vertical processes to three-dimensional space and combines multi-year remote sensing data to analyze the statistical characteristics of historical strong wind and typhoon events in the continental slope region. Investigating across three dimensions—vertical mechanism, spatial pattern and long-term statistics—we aim to deepen the understanding of wind-driven upper ocean response under topographic modulation, and provide new reference cases for improving the “wind pump” theory for the South China Sea.