| dc.description.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. Besides, 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. By means of the HYCOM product in the period of 1994-2015, the vertical structures of temperature, salinity, and horizontal velocity in the SCU, NCU, and OU regions are further explored and the vertical velocity field is reconstructed by the omega equation. Under the climatological condition, the isotherms, isohalines, and thermocline layers in the studied region experience an uplifting towards the coast, and a shoaling towards the north in the offshore area. These characteristics are attributed to the presence of 1) the western boundary current (the upstream of eastward-flowing jet), which sets isoline lifted toward the coast and is the western flank of the anti-cyclonic gyre in the southern basin of SCS, 2) the cross-shelf transport, which is driven by Ekman transport, 3) the southward coastal current in 12-16ºN, which belongs to the western flank of cyclonic gyre to the north of eastward-flowing jet and sets the isolines lifted toward the open ocean, and 4) the eastward-flowing jet. Under the strong upwelling condition, which is identified via the SSTUI, pronounced uplifting of isolines and intensified cooling and salting effects are observed in each subregion. During the strong SCU condition, the wind-driven cross-shelf transport is enhanced, accompanying the intensifications of the southward current along the coast and the eastward-flowing jet. In the strong NCU condition, cross-shelf transport is enhanced as well, while the southward coastal current disappears with a northward extension of the western boundary current. In the strong OU condition, the strengthening of a cyclonic eddy is observed to the north of the jet. By using the omega equation, well-defined upwellings in the SCU, NCU, and OU regions are characterized by strong upward velocities. On average, the maximum upward velocities in SCU, NCU, and OU are estimated at 2.7 m/d, 1.7 m/d, and 0.6 m/d, respectively. Enhanced vertical velocities during stronger upwelling conditions are estimated at 3.1 m/d, 2.7 m/d, and 1.4 m/d, respectively. | en_US |