Bolus generation by a river plume in coastal Gulf of Mexico

Abstract:

On December 13, 2013, satellite imagery revealed a packet of internal waves propagating along the coast and was assumed to be connected to the Choctowachee Bay plume in the northern Gulf of Mexico. In situ profiles of current velocity, density and TKE dissipation were obtained to investigate the plume during the Surfzone Coastal Ocean Pathways Experiment. On December 17^th, elevated river discharge, spring tidal velocities and weak winds (< 2.5 m/s) created a freshwater plume that propagated as a buoyant gravity front approximately 6.7 km west of Destin Pass inlet. Results revealed a subcritical gravity front that was dominated in the along-shore direction by Coriolis, pressure gradient, and friction forces. An unsteady trapped core (referred herein as a bolus) was observed ahead (westward) of the gravity front head. The bolus was identified by a bulge of westward near-surface flow that transported less-dense plume water in a turbulent trapped core. As the bolus propagated away from the gravity front head, momentum was reduced through turbulent mixing of the less dense fluid in the core with the ambient water at the trailing edge of the bulge. Eventually, the bolus arrested near the along-shore extent of residual stratification from a previous tidal plume. Bolus formation likely occurred when the gravity front decelerated below the first mode internal wave speed. This research posits that during wave fission, an internal wave radiated away from the gravity front head, transporting less dense plume water with it. The disturbance initially propagated as a surging front until it was arrested. We postulate that the bolus may eventually evolve into internal waves, similar to those observed in the SAR image, but only under the condition that ambient waters are stratified. These results expose an important coastal transport mechanism, particularly in context of an oil spill inside a bay.