Tidal effects on dynamics and freshwater transport of a medium-scale river plume with multiple outlets

Abstract

Tides play a crucial role in regulating the dispersal and dynamics of a river plume. However, the impact of tides on the dynamics and transport of freshwater in a medium-scale river plume, particularly with multiple outlets, is still not well understood. Using the Hanjiang River Plume in the northern South China Sea as an example, we analyze the momentum and volume of the plume based on salinity space. We also investigate the effects of tidal advection and tidal mixing. Tidal advection propels plume water from the bulge downstream, resulting in a plume type with intermediate surface-advected and bottom-advected characteristics. Tidal mixing causes the plume to come into contact with the seafloor, leading to bottom-advected plumes. Tidal advection leads to the accumulation of plume water in high salinity space, while tidal mixing mitigates this effect, as the plume water near the estuary with relatively low salinity is effectively mixed. In the absence of tidal forcing, vertical shear is the main contributor to the total freshwater flux. However, when tidal effects are taken into account, the contribution of vertical shear to the total freshwater flux decreases and becomes comparable to the advection term. The downstream buoyant flow is primarily controlled by geostrophic balance. The barotropic current carries freshwater downstream, overpowering the upstream transport by the baroclinic current, resulting in a net downstream freshwater transport. Tidal advection enhances this downstream freshwater transport, while tidal mixing has the opposite effect.