Mississippi River plume dynamics

Northern Gulf of Mexico Hybrid Coordinate Ocean Model (NGoM-HYCOM)
Mississippi River plume dynamics

The distribution and dispersion of Mississippi River freshwater along the Northern Gulf of Mexico shelf is controlled by a variety of factors, which include coastal and bottom topography, wind forcing and ambient shelf/deep basin currents.  Particular characteristics of the Mississippi River delta morphology and dynamic setting make it a unique study case to compare to the well established knowledge of large river plume dynamics. Moreover, the understanding and prediction of environmental issues in the region, such as the recurrent summertime hypoxia in the vicinity of the delta, can be tackled trough understanding the dynamics of the Mississippi River plume.
Process-oriented model simulations are used to study the circulation of the plume, by employing only buoyancy forcing under variable bottom topography and in the presence/absence of winds, in order to examine the factors that influence the plume development and evolution. This study builds on upon idealized process oriented river plume studies with the HYCOM model.

Surface salinity snapshots of different process-oriented experiments of the Mississippi River plume. Gray lines represent selected isobaths. (A)Influence of bottom topography on plume development when river discharge is the only forcing mechanism, after 30 days of plume evolution. When the plume evolves over a 20m deep, flat bottom, it presents enhanced offshore extension, reduced northward intrusion and the development or a meandering pattern near the freshwater source.  In the presence of realistic bottom topography, the plume presents changes that are in agreement with the development of buoyant plumes in the presence of a slope. The across-shelf slopping bottom elongates the plume in the along-shore direction and reduce the offshore expansion due to potential vorticityconstrains.  Freshwater is also retained in the vicinity of the delta.
(B)Influence of simplified wind forcing on a well-developed buoyant plume in the presence of realistic bottom topography. 72h of constant wind forcing is imposed after 30 days of plume development.  The impact of the wind on the plume structure is consistent with the development of surface Ekmandynamics. Easterly winds (upwelling-favorable) promote an offshore surface Ekmantransport that enhances the offshore extension of the plume, beyond the shelfbreak(solid gray line).  Southerly winds also promote offshore transport of plume waters due to the development of an eastward Ekmantransport in the vicinity of the delta. Westerly (downwelling-favorable) winds reduce the offshore expansion and attach the plume to the coast, with an enhancement of the coastal current transport.

Villy Kourafalou — vkourafalou@rsmas.miami.edu
Rafael Schiller — rschiller@rsmas.miami.edu