Interannual Variability of the Summer Surface Salinity in the Southern South China Sea Driven by the Summer Monsoon

During summer, the southern South China Sea (SCS) is continuously freshened by the propagation of an anticyclonic gyre driven by monsoon, which transports low-salinity water from the Gulf of Thailand and Karimata Strait. This study examines the propagation, structure, and dynamics of summer SCS surface salinity and its response to the monsoon variability. Analysis using Global Ocean Physics Reanalysis product and salinity budget indicates that advection from the Gulf of Thailand serves as the primary source of low-salinity water, whereas transport from the Karimata Strait plays a secondary role. The summer surface salinity in the SCS experiences significant interannual changes driven by the summer monsoon associated with the El Niño-Southern Oscillation. Strong summer monsoons during the developing phase of El Niño reduce salinity in the interior region of the southern SCS (SSCS) but create high-salinity regions off the Vietnamese coast (∼11–15˚N); the former is caused by the increased influx of low-salinity water into the interior region of the SSCS along an anticyclonic pathway, whereas the latter results from a combined effect of enhanced Ekman suction and reduced horizontal advection. In contrast, weak summer monsoons during the decaying phase of El Niño disrupt the low-salinity anticyclonic pathway, increasing salinity in the interior region of the SSCS and forming a low-salinity tongue along the Vietnamese coast.