Ambient Water Circulation Near a River Discharge: Investigating the Formation of a Topographic β-Plume.

Field observations and model outputs of buoyancy-driven coastal currents have shown the presence of a weak along-shelf current flowing against the direction of coastal trapped wave propagation. It usually sits seaward and beneath the coastal current. In simulations, this reverse flow is robust to changes in slope, domain size, boundary conditions, and stratification. However, not much attention has been given to understanding the source of this circulation near a discharge of freshwater. We propose that this current is due to the formation of a topographic β-Plume. The presence of a river discharge implies a localized mass loss of ambient water due to entrainment into the river plume. The vertical velocity related to the entrainment produces a localized vortex stretching. In areas where the background potential vorticity gradient is controlled by the bathymetry slope, a circulation could be initiated by the vorticity change in the ambient flow. Water is transported up the slope towards where the localized entrainment is located. Using ROMS output, we investigate if a topographic β-Plume can explain the observed reverse flow and study how (1) the bathymetry slope, (2) the vertical mixing rate, and (3) the size of the river plume affects the generation of a β-Plume transport. The results of this study help us better understand how river plumes affect the circulation of ambient water and provide insight into how changes in river discharge may affect local coastal circulation