Mechanisms of Interannual Changes of the Summertime Upwelling System in Central South China Sea based on Generalized Q-Vector Omega Equation

The summertime upwelling system off the southern Vietnamese coast is a major oceanographic feature of the South China Sea, highly sensitive to climate variability on both regional and larger scales, and strongly influencing the sustainability of the local fishing-ground productivity. This upwelling system is divided into two coastal regions: the Southern Coastal Upwelling (SCU; south of 12.5°N) and the Northern Coastal Upwelling (NCU; north of 12.5°N), and one offshore region: the Offshore Upwelling (OU; east of 110°E). Based on the high-resolution three-dimensional HYCOM ocean reanalysis product, we investigate the characteristics of the upwelling system and identify the controlling factors in each region on the interannual timescale. The generalized Q-vector ω-equation is adopted to reconstruct vertical velocity, providing a direct means to quantify upwelling intensity and evaluate the primary dynamical processes responsible for driving vertical motion. The summertime vertical velocities under climatological conditions in the central areas of the SCU, NCU, and OU are estimated at 0.16 m d^-1, −0.08 m d^-1, and 0.003 m d^-1, respectively, and can increase to 0.32 m d^-1, 0.07 m d^-1, and 0.08 m d^-1 during strong upwelling events. Analysis of vertical velocity component provides a detailed explanation of the primary roles of the key controlling factors and incorporates the interaction between the two coastal currents along the Vietnamese coast, the combined effect of horizontal shear and density gradient in the overall dynamical mechanism of this upwelling system. Moreover, this framework quantifies the relative contributions of physical processes involved in generating vertical velocity in this region.