Primary Study Of The South China Sea Circulation Inter-annual Variation

The NOAA OISST.V2 monthly SST and the FSU monthly pseudo-stresses in South China Sea have been analyzed, and it's found that the South China Sea SSTA and the ENSO index are highly related. Maximum correlation of 0.36 occurs when the SSTA lags the ENSO index by 5 months. The South China Sea is highly affected by the 1997/98 El Nino event. The SSTA in the winter of 1997/98 El Nino event and the following summer have two peaks, while the wind speed reaches maximum minus anomalies.In order to study the variation of the South China Sea circulation in 1997/98 El Nino event, the ECOM hydrodynamic numerical model has been used. The South China Sea circulation in 1995-2000 has been calculated by the NOAA OISST.V2 monthly SST and the FSU monthly pseudo-stresses forcing. It's obvious that the South China Sea circulation have annual period variation: In winter, the whole South China Sea is controlled by a cyclonic circulation, and in summer, the general circulation is anti-cyclonic, with a cyclonic gyre in the north and a stronger anti-cyclonic gyre in the south. The anomaly sea level and velocity of January and August in 1998 have been analyzed, and they have strong anomalies: (1) In January, the whole South China Sea has positive sea level anomaly and anti-cyclonic velocity anomaly, so the cyclonic circulation is weaker than normal. (2) In August, the South China Sea has positive sea level anomaly, especially stronger in the east of Vietnam, and cyclonic velocity anomaly in south, anti-cyclonic velocity in north, especially strong in the east of Vietnam. So the cyclonic gyre in the north and the anti-cyclonic gyre in the south are both weaker than normal, and a anti-cyclonic gyre appears in the east of Vietnam. The Empirical Orthogonal Function (EOF) decomposition method has been used for the inter-annual variation analysis. According the EOF decomposition time coefficients series, there is obvious variation during the 1997-1998 El Nino event: (1) Form November to March next in the winter of 1997/98, the cyclonic gyre in the north is weaker. (2) From July to October in summer of 1998, both the cyclonic gyre in the north and the anti- cyclonic gyre in the south are weaker. The analysis of wind stress curl is the main reason for this inter-annual variation.