Dual Coastal Downwelling Belts Over the Northwestern South China Sea in Winter

Abstract

This study explores a unique dual coastal downwelling belt in the northwestern South China Sea (NWSCS) during the winter northeasterly monsoon season. Employing both in situ observations and numerical simulations, we delineate how the topography of the Leizhou Peninsula and Hainan Island fundamentally shapes this phenomenon. The study reveals a westward alongshore current, propelled by downwelling-favorable winds and a buoyancy-driven river plume, which fosters seawater convergence on the western shelf. This process establishes a northeastward-directed pressure gradient force that triggers a stronger offshore flow than that driven by the northeasterly monsoon winds alone, as described by Ekman dynamics. On the inner shelf, warmer seawater driven by the northeasterly winds converges with colder, fresher coastal water near the 30-m isobath, forming a pronounced downwelling belt. In contrast, on the outer shelf, a landward reduction in the strength of the onshore current, which is counterbalanced by a landward enhancement of the eastward alongshore pressure gradient force with alongshore wind stress and the eastward Coriolis force, leads to water convergence and subsequent downwelling at the ∼60 m isobath. This dual-downwelling-belt structure, underscored by its dynamic mechanisms, not only deviates from traditional coastal downwelling theories but also highlights the dominant role of regional topography in modulating the downwelling dynamics in the NWSCS.