Subsurface Salinity Balance in the South China Sea*

The South China Sea (SCS) is often treated as a semienclosed water body, with the Luzon Strait as its only connection to the Pacific Ocean. A branch of the Kuroshio flows northwestward across the Luzon Strait to enter the SCS, carrying North Pacific Tropical Water (NPTW) into the basin. Using the subsurface salinity maximum as a tracer for NPTW, the authors show how important three secondary straits—the Taiwan Strait to the north and the Karimata and Mindoro Straits to the south—are to the NPTW intrusion at the Luzon Strait. The authors demonstrate that the SCS cannot reach an equilibrium state that is consistent with the observed subsurface salinity distribution unless all of the following components are in place: the Kuroshio, transports through the three secondary straits, downward mixing of freshwater, horizontal mixing induced by mesoscale eddies, and forcing by the local monsoonal winds.