Loop Current - Mississippi River plume interactions

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

The proximity of the Mississippi delta to the shelf break and to neighboring deep regions (DeSoto Canyon) dictate strong topographic control over the dynamics of the Mississippi River plume. However, it is the same proximity that allows the plume, in certain conditions, to interact with the rich offshore circulation of the Gulf of Mexico. The presence of a boundary current (Loop Current) and a rich eddy field in deep areas very close to the delta provide a dramatic offshore removal mechanism of low-salinity / high-chlorophyll waters towards the open Gulf of Mexico. These waters can be transported for long distances along the edge of the Loop Current. They can reach the Straits of Florida and interact with the Florida Keys coastal and reef areas.
Numerical simulations with the NGOM-HYCOM domain forced by realistic atmospheric forcing and by realistic lateral boundary conditions (extracted from the GOM-HYCOM domain) are used to study the offshore removal and interaction of the buoyant plume with the offshore circulation of the Gulf of Mexico, namely the Loop Current and its associated eddies.

(Upper and Middle): Snapshots of Sea Surface Salinity and surface velocity vectors in August 2004, showing an event of offshore removal of plume waters by the Loop Current (A) and after the shedding of a Loop Current Eddy (B). Associated Sea Surface Height fields from the GOM-HYCOM domain are presented on the right panel.
(Lower): Snapshot of temperature and velocity vectors at 100m depth, illustrating the intrusion of the Loop Current Eddy in the NGOM-HYCOM domain. The zonal velocity field and the displacement of the isopycnal layers caused by the eddy are shown on the right panel (vertical section along 87.5 ºW, represented by the red line).
The realistic simulation results demonstrate events of strong interaction between the Mississippi River plume and offshore circulation processes. For instance, during an event of Loop Current in close proximity to the Mississippi River Delta (August 2004), strong offshore removal of plume waters was enacted for approximately 15 days. This process was facilitated by weak winds during that period and by the steep shelf break in the vicinity of the delta (DeSoto Canyon). The dipole circulation generated by the edge of the Loop Current and a cyclonic eddy (Panel A) generated strong offshore currents that effectively entrained plume waters. Later in the month, these plume waters were transported directly offshore along the edge of the Loop Current Eddy (Panel B). At the same time, this Loop Current Eddy transported warm waters to the NGOM region and promoted strong temperature gradients at depth (Panel C).

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