Roles of Upwelling on the Dynamics and Freshwater Transport of a River Plume Over the Inner Shelf

Abstract

Apart from the influences of wind and tides, upwelling can also modulate the dispersal and dynamics of river plumes. However, the specific responses to upwelling processes over the inner shelf remain unclear. This study employs a numerical model to investigate the effects of upwelling over the northeastern shelf of the South China Sea and its barotropic influences on the dynamics and freshwater transport of a medium-scale river plume. In a model experiment with extremely weakened upwelling (tiny upwelling case), plume water spreads across a wide coastal region driven by upwelling-favorable winds. With the barotropic effects of upwelling, plume water occupies a much smaller area, and the alongshore geostrophic current is enhanced. In the Control Run, plume water disperses further offshore in a stratified seawater column, as the Ekman layer thins and the offshore Ekman current intensifies in the upwelling region. The upwelling process not only accelerates the downwind advection of offshore extended far-field plume water due to the drop in coastal sea level but also continually upwells high-salinity water from the lower layer to the upper layer, enhancing mixing with plume water and leading to a smaller river plume area. Under upwelling-favorable wind conditions, freshwater is primarily transported offshore in tiny upwelling case, while the proportion of downwind freshwater transport increases in the Control Run or the barotropic case. In the tiny upwelling case, the contribution of advection and vertical shear to freshwater flux gradually decreases and increases, respectively, with the duration of upwelling-favorable winds, while the tendency reverses in the Control Run.