Circulation at South China Sea based on the 15-year NCOM East Asian Seas Reanalysis

Monsoon winds, which are SW in summer and NE in winter, drive the seasonally varying, basin-wide upper layer circulation in the South China Sea (SCS), first described by Wyrtki (1961). The monsoon winds produce a basin-wide cyclonic gyre in the winter and a double gyre in the summer that is cyclonic in the north and anticyclonic in the south as shown in the model reanalysis. These gyres tend to intensify on their western sides due to the earth's β effect. Seasonal eddies in the SCS upper layer circulation include: a cyclonic eddy that occurs NW of Luzon Island in winter; and a pair of counter-rotating eddies that occur off central Vietnam in summer, of which the northern is cyclonic and the southern anticyclonic, and which propel a jet eastward from the coast of Vietnam around 13°N. Within the deep part of the SCS, the China Sea Basin, where the depths are 2000-5560 m, there is a generally cyclonic flow driven by the inflow of North Pacific Deep Water (NPDW) into the SCS through Luzon Strait. The NPDW within the deep basin of the SCS upwells and exits the SCS within the upper and intermediate layers. The cyclonic circulation within the China Sea Basin forms an intensified deep western boundary current (DWBC) on its NW side. The model reanalysis indicates that the velocity within the DWBC tends to increase with depth.