Formation of spring warm water southwest of the Philippine Islands: Winter monsoon wake effects

From satellite observations and the reanalysis data, the late spring formation of warm water with temperature higher than 30 °C to the southwest of the Philippine Islands (8–18°N, 115–120°E) is investigated. Our analysis suggests that the blockage of the winter monsoon by the Philippine Islands results in this “Luzon warm water” (LWW) to the southwest of the Luzon Island and the “Vietnam cold tongue” (VCT) to the southeast of the Vietnam coast during winter and early spring in the South China Sea (SCS). The VCT is formed by the southward cold advection by the western boundary current and surface heat loss in the SCS. During the winter monsoon, the LWW first forms due to weak winds southwest of the Philippine Islands and the countering effect of warm Ekman advection against cold geostrophic advection. In spring its temperature exceeds 30 °C (LWW30), helped by strong solar radiation and the winter monsoon wake effect lee of the Philippine Islands. With the winter monsoon weakening, LWW30 extends southwestward in late spring but disappears quickly after the summer monsoon onset. Reduced latent heat flux in the winter monsoon wake is the dominant factor for the spring fast warming southwest of the Philippine Islands.Both VCT and LWW persist from winter to early spring as the Philippine Islands block the winter monsoon. Their interannual variations are correlated with the variation of the LWW30 since the blockage of the winter monsoon by the Philippine Islands modifies surface latent heat flux and ocean advection from winter to early spring. These results strongly suggest that the LWW30 is a result of land–sea–winter monsoon interaction.