A South China Sea warm core eddy with sea surface temperatures (SST) higher than 29.5 C, recently reported by Chu and Chang (1995), appears in the central South China Sea (west of Luzon Island) in boreal spring, and strengthens until the onset of the summer monsoon (mid-May), then weakens and disappears at the end of May. Although its size and intensity varies, the warm core eddy releases large moisture and heat fluxes into the atmosphere and in turn affects the monsoon circulation. The transient features and interannual variabilities of the warm core eddy have not yet been studied. In this study we use 189,059 temperature profiles from the U.S. Navy's Master Oceanographic Observation Data Set (MOODS) to investigate the eddy features. We use Optimal Interpolation to create a ten-day interval synoptic data set for 1964-84 on a 0.5 x 1 deg. grid (finer resolution in zonal direction) from the MOODS SST data. Then, we formulate an Emprical Orthogonal Functions (EOF) analysis. The first EOF mode accounts for 35.5% of the variance and is characterized by a field which is similar to the mean field. The second EOF mode accounts for 21.4% of the variance and is characterized by a meso-scale eddy (1l6 - 1l9 E, 16 - 19 N) to the west of Luzon Island. The size of the eddy is around 100,000 km2. The time series of EOF2 behave differently between El Niflo and non-El Niflo years: a warm-core eddy often appears during the non- El Niflo years, and a cold-core eddy usually exists during the El Niflo years. A new scenario of the air sea feedback is proposed for the warm core eddy formation in non-El Nino years and the cold core eddy formation in El Nino years.
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