Mooring observations and numerical modeling of thermal structures in the South China Sea

摘要

Three sets of Autonomous Temperature Line Acquisition Systems were deployed in the South China Sea. Gaps aside, the data covered nearly 3 years at the northern station and about 2 years farther south. Fluctuations ranged from episodic to interannual. Internal tides, more diurnal than semidiumal, were active basinwide. Twelve typhoons passed during measurement periods. The most severe one, typhoon Babs in 1998, caused a temperature drop of over 7°C at 50 m. Despite strong monsoons, only near-surface temperature showed clear seasonal variations. Intraseasonal variations induced by mesoscale eddy stood out much better at subsurface depths. Propagating eddies aside, some eddies were seasonal and nearly stationary. From daily archives of an eddy-resolving, data-assimilating ocean model (East Asian Seas Nowcast/Forecast System), we identified two paradigms leading to the generation of a persistent spring-summer warm eddy in the central-western basin. In normal years, a complete cyclonic gyre was driven by a strong winter northeast monsoon. Water piled up along the periphery of the South China Sea. Afterward, a warm eddy could be generated from west of Luzon Island and propagated westward while intensifying. Under a weak northeast monsoon, such as in El Nino years, piled-up water tended to stay in the southern basin. When the wind relaxed in spring, warm water returned northward to form a warm eddy in the central-western basin. Transition from SW to NE monsoon also often led to a warm eddy generation in southern latitudes, when the summer eastward jet departing from central Vietnam broke up.