Impacts of Wind and Current on the Interannual Variation of the Summertime Upwelling Off Southern Vietnam in the South China Sea

Abstract

Summertime upwelling off the southern Vietnamese coast is one of the most essential oceanographic features in the South China Sea. Based on analyzing the 38-year (1982–2019) sea surface temperature (SST) images, locations of summertime Vietnamese upwelling centers are found to be classified into three sub-regions: the Southern Coastal Upwelling (SCU; south of 12.5°N), the Northern Coastal Upwelling (NCU; north of 12.5°N), and the Offshore Upwelling (OU; east of 110°E). Variations of upwelling intensities in the three sub-regions are further quantified via an adaptive SST-based upwelling index, and possible processes relevant to wind field (including wind stress and its curl) and currents are, respectively, proposed. The analyses show that the local along-shore wind stress, inducing offshore Ekman transport, can produce the coastal upwelling-favorable condition but is not the only one of main factors in controlling the interannual variability of the coastal upwelling. The enhancement of wind stress curl dipole off the southern Vietnamese coast, which accompanies the scaling up of the double-gyre structure east of Vietnam and results in the reinforcement and southward shift of eastward-flowing jet, and the exaggeration of the cyclonic gyre-associated southward along-shore current are responsible for the intensification of upwelling in the SCU, but suppress the development of upwelling in the NCU. The well-developed double-gyre structure serves as the essential condition for the OU occurrence and the OU is much more sensitive to the change of cyclonic gyre to the north of the jet.