A South China Sea warm pool with sea surface temperature (SST) higher than 29.5 ¿C, recently reported by Chu and Chang <1995a, b> and Chu et al. <1997>, appears in the central South China Sea (west of the Luzon Island) in boreal spring, strengthens until the onset of the summer monsoon (mid-May), and then weakens and disappears at the end of May. The transient features and interannual variabilities of the warm pool have not yet been studied. Here we use a subset of the U.S. Navy's Master Oceanographic Observation Data Set (MOODS) to investigate the surface thermal features. First, we employed an optimal interpolation scheme to build up a 10-day interval synoptic data set for December 1963 to November 1984 on a 0.5¿¿1¿ grids (finer resolution in zonal direction) from the MOODS SST data. An ensemble mean SST field (T&blmac;) was established with a rather weak horizontal gradient (28.5 ¿C near the Palawan Island to 26 ¿C near the southeast China coast). Second, we performed a composite analysis to obtain the averaged SST anomaly field T˜ deviating from the ensemble mean for the winter and spring seasons (December--May). During December--March, T˜ is negative almost everywhere throughout the whole South China Sea. In early April, positive T˜ with closed isoline (warm pool) was evident west of Luzon Island. In May, the central SCS warm anomaly becomes stronger. On May 11--20, the central SCS warm pool (114¿--119 ¿E, 14¿--19 ¿N) has T˜>1.8 ¿C. The size of the warm pool is around 200,000 km2. Third, we performed an empirical orthogonal function (EOF) analysis on the residue data (T⁁), deviating from T&blmac;+T˜, for the winter and spring seasons, in order to obtain transient and interannual variations of the SST fields. EOF1 accounts for 35.5% of the variance and resembles the ensemble mean pattern of nearly parallel contours with a maximum value in the southeast and a minimum value in the northwest. EOF2 accounts for 21.4% of the variance and is characterized by a warm/cool pool (116¿--118 ¿E, 16¿--18 ¿N) west of the Luzon Island. The corresponding principal component (PC2) has strong interannual variability with a maximum value of 10 on February 11--20, 1965 and a minimum of -12 on March 21--31, 1964. This indicates the appearance of either a warm pool with a maximum strength of 1.2 ¿C or a cool pool with a maximum strength of -1 ¿C. Combination of T˜ and PC2¿EOF2 leadsto an occurrence of a central SCS warm pool from April to May with a warm anomaly varying between 0.8¿ and 3 ¿C.