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

Abstract

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 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 deviating from the ensemble mean for the winter and spring seasons (December–May). During December–March, is negative almost everywhere throughout the whole South China Sea. In early April, positive 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 . The size of the warm pool is around 200,000 km2. Third, we performed an empirical orthogonal function (EOF) analysis on the residue data , deviating from , for the winter and spring seasons, in order to obtain transient and interannual variations of the SST fields. EOFl 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 and PC2×EOF2 leads to an occurrence of a central SCS warm pool from April to May with a warm anomaly varying between 0.8° and 3°C.