The downwelling circulation over the continental shelf in the Northern South China Sea (NSCS) is simulated with a three-dimensional, free-surface, hydrostatic, primitive equation ocean model, Regional Ocean Model System (ROMS). The NSCS model has a spatial resolution of approximately 3 km horizontal spacing and 30 vertical levels. An idealized spatially uniform downwelling favorable wind forcing and the buoyancy intrusion from the East China Sea are utilized for the purpose of process-oriented study during the downwelling. The study aims to identify the spatial and temporal responses of the three-dimensional downwelling flow field to the wind and external forcing in the NSCS. The temporal response in the NSCS to downwelling can be divided into two stages. The first stage is the period...[ Read more ]

The downwelling circulation over the continental shelf in the Northern South China Sea (NSCS) is simulated with a three-dimensional, free-surface, hydrostatic, primitive equation ocean model, Regional Ocean Model System (ROMS). The NSCS model has a spatial resolution of approximately 3 km horizontal spacing and 30 vertical levels. An idealized spatially uniform downwelling favorable wind forcing and the buoyancy intrusion from the East China Sea are utilized for the purpose of process-oriented study during the downwelling. The study aims to identify the spatial and temporal responses of the three-dimensional downwelling flow field to the wind and external forcing in the NSCS. The temporal response in the NSCS to downwelling can be divided into two stages. The first stage is the period before the denser waters (cold and fresher) intrusion and is mainly wind-forced shelf circulation. The second stage is the period after the denser waters intrusion and both wind forcing and the intruded waters regulate the NSCS circulation. For the spatial response, the study reports on the horizontal characteristics and vertical structures of the density and flow field in these two stages respectively, and focus on the coastal waters off Guangdong where the shelf width varies alongshore.

For the horizontal characteristics in the first stage (“onset” stage), east of Shanwei over the widened shelf, the circulation pattern is mainly regulated by the shelf topography, while it is mainly governed by Ekman dynamics in the boundary layers and geostrophic balance in the interior west of Shanwei. In the second stage (the steady stage), the denser waters intrude into domain, and they are largely constrained within the widened shelf east of Shanwei but cross the shelf into the deep sea at the head of the widened shelf where isobaths converge. In surface layer, when the intruding denser waters reach the western side of Shanwei, a strong variability occurs at the frontal zone between these denser waters and the ambient waters. In the depth integrated velocity field west of Shanwei, the geostrophic balance existed in the first stage is destroyed by the strong diffusion of the denser intruding waters.

Vertical structure and alongshore variability of the downwelling circulation in NSCS are illustrated by the velocity and density cross-shelf sections along the lines in Peal River Estuary (PRE), Shanwei and Taiwan Shoals. In response to the northeasterly wind forcing in the first stage, the most notable feature of the vertical structure of downwelling circulation is the formation of downwelling front. In the density field, the front forms below the surface at the offshore edge of the near-shore region where the density is well mixed. An alongshore coastal jet is formed near the front and extends vertically over the full depth. The displacement of the downwelling front for these three sections is different. For PRE line, the front keeps moving offshore at a speed of about 4 km day^-1. For the Shanwei line, the speed of offshore shifting of the front decreases with time. For the Taiwan Shoals line, the front is stationary at the location of 22.3⁰ N, over the continental slope. After the denser waters intrude into the NSCS at the second stage, they replace the vertically uniform density in the near-shore region and the downwelling front previously existed in the onset stage. In response to the intruding waters, a strong alongshore flow appears in the near-shore region and extends downward over the bottom of continental shelf. The cross-section averaged alongshore velocities in both Shanwei and PRE decrease with time, and their cross-shore velocities also have the same variation. The cross-shore velocity in Taiwan Shoals varies periodically in the offshore region of the Taiwan Shoals, namely in the deep sea.