South China Sea warm pool detected in spring from the Navy's Master Oceanographic Observational Data Set (MOODS)

A South China Sea warm pool with sea surface temperature (SST) higher than 29.5 degrees 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 degrees x 1 degrees grids (finer resolution in zonal direction) from the MOODS SST data. An ensemble mean SST field (T) was established with a rather weak horizontal gradient (28.5 degrees C near the Palawan Island to 26 degrees C near the southeast China coast). Second, we performed a composite analysis to obtain the averaged SST anomaly field (T) over tilde deviating from the ensemble mean for the winter and spring seasons (December-May). During December-March, (T) over tilde is negative almost everywhere throughout the whole South China Sea. In early April, positive (T) over tilde 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 degrees-119 degrees E, 14 degrees-19 degrees N) has (T) over tilde > 1.8 degrees C. The size of the warm pool is around 200,000 km(2). Third, we performed an empirical orthogonal function (EOF) analysis on the residue data ((T) over cap), deviating from T + (T) over tilde, 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 degrees-118 degrees E, 16 degrees-18 degrees 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 degrees C or a cool pool with a maximum strength of -1 degrees C. Combination of (T) over tilde and PC2 x EOF2 leads to an occurrence of a central SCS warm pool from April to May with a warm anomaly varying between 0.8 degrees and 3 degrees C.