The 4-D structure of upwelling and Pearl River plume in the northern South China Sea during summer 2008 revealed by a data assimilation model

We analyze four-dimensional structures of upwelling and Pearl River plume in the northern South China Sea (NSCS) during the summer of 2008 based on data assimilation. An Ensemble Kalman Smoother scheme is employed in the Princeton Ocean Model. It is found that the Pearl River plume axis extended eastward along with the surface current and swerved offshore twice near (116°E, 22.6°N) and (117.5°E, 22.8°N) before reaching the Taiwan Strait. The vertical transect of salinity along the plume axis indicates that the Pearl River freshwater could affect salinity distribution down to a depth of 10–20 m. Anomalously warm water is found in the upper layer, which could be attributed to the intensified stratification and suppressed vertical mixing caused by the freshwater of the plume capping the upwelling west of 116°E. The varying winds from upwelling favorable to downwelling favorable could induce a low-salinity water lens at the center of the model domain. Upwelling in the NSCS initially occurred at 114.5°E, to the east of the Pearl River Estuary, intensified eastward, and reached its maximum near Shantou (116.7°E, 23.2°N). Since current-induced upwelling appeared mainly in Shantou due to the widened shelf, it is found that even if the wind-induced upwelling was shut down in Shanwei by downwelling favorable wind on July 4, the upwelling still existed in Shantou. Moreover, because the direction of large-scale current was in favor of upwelling in the NSCS that cannot be reversed by varying local winds over a short time period, the upwelling shutdown time is longer for both wind-induced and current-induced upwelling in Shantou than for mainly wind-induced upwelling in Shanwei. The steeper slope in Shanwei also shortens the upwelling shutdown time there.