Process studies on the evolution of the Mississippi River plume: Impact of topography, wind and discharge conditions

Abstract

The Mississippi River (MR) outflow is the strongest buoyant input for the Gulf of Mexico (GoM). The high resolution numerical model Northern-GoM (NGoM) HYCOM is used to perform process oriented experiments that examine the relative role of the effects of topography, wind-driven circulation and discharge conditions on the plume waters fate. Simulations showed that the slope over the western and eastern shelf areas enhance the overall alongshore propagation of the plume and reduce the offshore expansion (stronger downstream and upstream currents). The direction of the buoyancy-driven currents is defined as downstream and upstream for the Louisiana-Texas (LATEX) and Mississippi-Alabama-Florida (MAFLA) shelf areas, west and northeast of the MR Delta, respectively. Flooding conditions and/or downwelling-favorable winds strengthen the downstream flow over the LATEX shelf; they also suppress the vertical mixing of the upper low salinity waters with the deeper ocean layers. However, flooding conditions do not enhance the offshore extension. Downwelling-favorable winds support the downstream current and deepen the plume. Upwelling-favorable winds reverse or eliminate the downstream current and enhance the northward transport of plume waters toward the MAFLA shelf, but also offshore. This cross-marginal transport effect is significant, as it facilitates interaction of low-salinity MR plume waters (which are rich in nutrients and sediments) with deep oceanic currents in the GoM basin interior. The separate and combined investigation of the factors that determine MR plume dynamics enlightens the relations and differences among the MR circulation patterns over the NGoM region. Calculations of freshwater transport are carried out to further discuss the dynamics and serve as the basis for quantifying the transport and fate of river-borne waters and associated biogeochemical materials.