WINTER UPWELLING OFF LUZON IN THE NORTHEASTERN SOUTH CHINA SEA

A major upwelling region centered about 100 km offshore between 16 deg rees and 19 degrees N off northwest Luzon in the South China Sea is re vealed in the distributions of temperature, salinity, and dissolved ox ygen concentration during a cruise in December 1990. Upward movement o f water low in dissolved oxygen can be identified to a depth of 300 m. Climatological distributions of temperature and upper ocean heat cont ent and tracer distribution obtained from a recent numerical experimen t lend support to the existence of this upwelling region. Results from a general circulation model of the South China Sea further show that upwelling off Luzon occurs between October and January, in agreement w ith the climatological data. Current structures obtained from a simple two-layer model and the general circulation model both suggest that t his upwelling is not driven by local winds but contains a large, remot ely forced component arising from the basin circulation. The upwelling is sustained by offshore Ekman drift from above and a converging nort hward undercurrent from below. The would-be downwind coastal jet in th e surface layer is largely offset by currents associated with the wint er circulation gyre. Upwelled water with high nutrient content spreads southwestward in January and could be an important nutrient source fo r the surface water in the interior South China Sea in spring.