THE BOTTOM COLD WATER ON THE SUNDA SHELF SOUTHWEST OF SOUTH CHINA SEA AND ITS SEASONAL VARIATION

Historical hydrological data were used to study water mass characteristics on the Sunda Shelf southwest of the South China Sea(SCS). A bottom cold-saline water mass around the Natuna Islands on the Sunda Shelf was determined. The cold water split into two branches due to local topography effect: one intruded into the Gulf of Thailand, another into the Karimata Strait. The bottom cold water had obvious seasonal variation. It appeared in spring, peaked in summer, weakened in autumn, and disappeared in winter. We determined that the bottom cold water originates from the upwelling of the subsurface water in the SCS. In summer, surface water on the Sunda Shelf flows into the interior of the SCS due to southwest monsoon, driving the topographic upwelling of subsurface low-temperature-high-salinity water in the SCS. The temperature difference between the bottom water and surface water could reach over 8°C. In winter, surface water flows into the Sunda Shelf from the interior of the SCS by northeast monsoon, resulting in water accumulation on the Sunda Shelf, which limited the upwelling of subsurface water in the SCS.