El Niño modulation of the South China Sea circulation

Abstract The South China Sea circulation during the 1982–1983 ENSO (El Nino and Southern Oscillation) event was examined through a combined effort of data analysis and numerical modelling. Monthly wind stress and sea surface temperature fields derived from COADS (Comprehensive Ocean-Atmosphere Data Set) were used to drive a three-dimensional primitive-equation ocean model with a free surface. The mean and anomalous circulation fields during 1981–1984 were derived from the model and climatological data. The 1982–1983 El Nino event in the South China Sea began with increased evaporative cooling in late 1982, followed by a weak northeast monsoon in the winter of 1982–1983 and a weak southwest monsoon in the following summer. The surface waters showed persistently higher-than-normal heat content from late 1982 to late 1983. The increased heat content could not be explained by a decrease in latent heat flux. Instead, the surface circulation, which was weaker than normal, played a major role in the increase in sea surface temperature. During maximum warming in October-November 1982, the strengths of both upwelling in the central basin and downwelling around the perimeter of the South China Sea were reduced, resulting in weaker vertical advection of heat and warming of surface waters. Cessation of the year-long warming began with the late arrival of the northeast monsoon in October 1983. The record-strength northeast monsoon in November produced anomalous upwelling in the central basin and anomalous downwelling around the perimeter of the marginal sea below the mixed layer. The abnormal vertical motions accelerated vertical advection of heat and returned the circulation and heat flux to normal conditions by December. A similar sequence of events appeared during 1986–1987 with minor variations, suggesting that the scenario may be typical for most ENSO occurrences.

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