Three-dimensional structure of temperature, salinity, and Velocity of the summertime Vietnamese upwelling system in the South China Sea on the interannual timescale

Summertime upwelling system off the southern Vietnamese coast is one of the most essential oceanographic features in the South China Sea. This system is divided into two regions along the coast, the Southern Coastal Upwelling (SCU; south of 12.5°N) and Northern Coastal Upwelling (NCU; north of 12.5°N), and one in the offshore area, the Offshore Upwelling (OU; east of 110°E). Utilizing the HYCOM ocean reanalysis product in the period of 1994–2015, vertical characteristics of this upwelling system on the interannual timescale are investigated. Furthermore, the omega equation is applied to reconstruct vertical velocity to quantify its intensity and clarify the corresponding leading factors in the three regions. The analysis indicates that the kinematic deformation effect is the primary contributor to coastal upwelling formation while the momentum effect plays the leading role in offshore upwelling. The SCU variability is more sensitive to the momentum effect; however, in the NCU, the kinematic deformation effect is offset by the momentum effect and the upwelling is enhanced as the kinematic deformation (momentum) effect increases (decreases). The summertime mean vertical velocities in the central areas of SCU, NCU, and OU are estimated at 0.16 m/d, −0.08 m/d, and 0.003 m/d, respectively. The vertical velocity speeds up to 0.32 m/d, 0.07 m/d, and 0.08 m/d as the strong upwelling event occurs.