Barrier layer in the South China Sea during summer 2000

Using temperature–salinity profiles obtained from a cruise in summer 2000, the structure and formation of the barrier layer (BL) in the South China Sea (SCS) are investigated. Fresh water flux, ocean circulation, and wind stirring are important for BL formation, depending on regions. In the eastern SCS, Philippine mountains induce heavy rainfall, resulting in a fresh water cap at the surface and the formation of a thick wide spread BL. In the northwestern basin on the lee of the Annam Cordillera range, by contrast, a rain shadow reduces fresh water flux, which along with wind-induced upwelling, prevents the BL forming. Southeast of Vietnam, a thick BL forms as the Mekong River plume is advected by the northeastward western boundary current and its offshore extension. In the southeastern basin, the surface water is mixed deeply under the strong southwesterly monsoon, unfavorable for the BL formation despite heavy rainfall. In the Luzon Strait, the east/southeastward surface Ekman drift carries fresh SCS surface water, riding on the intruding Kuroshio meander that carries well-mixed, warm and saline water. The vertical overlapping of these two water masses gives rise to a thick BL.