The dynamics of the Mississippi River plume: Impact of topography, wind and offshore forcing on the fate of plume waters

Abstract

[1] High-resolution numerical simulations of the northern Gulf of Mexico region using the Hybrid Coordinate Ocean Model (HYCOM) were employed to investigate the dynamical processes controlling the fate of the Mississippi River plume, in particular the conditions that favor cross-marginal transport. The study focuses on the effects of topography, wind-driven and eddy-driven circulation on the offshore removal of plume waters. A realistically forced simulation (nested in a data-assimilative regional Gulf of Mexico HYCOM model) reveals that the offshore removal is a frequent plume pathway. Eastward wind-driven currents promote large freshwater transport toward the shelf break and the DeSoto Canyon, where eddies with diameters ranging from 50 to 130 km interact with the buoyant plume and effectively entrain the riverine waters. Our estimates show that the offshore removal by eddies can be as large as the wind-driven shelf transport. The proximity of eddies to the shelf break is a sufficient condition for offshore removal, and shelf-to-offshore interaction is facilitated by the steep bottom topography near the delta. Strong eddy-plume interactions were observed when the Loop Current System impinged against the shelf break, causing the formation of coherent, narrow low-salinity bands that extended toward the gulf interior. The offshore pathways depend on the position of the eddies near the shelf edge, their life span and the formation of eddy pairs that generate coherent cross-shelf flows. This study elucidates the dynamics that initiate a unique cross-marginal removal mechanism of riverine low-salinity, nutrient-rich waters, allowing their export along connectivity pathways, induced by a large-scale current system. Citation: Schiller, R. V., V. H. Kourafalou, P. Hogan, and N. D. Walker (2011), The dynamics of the Mississippi River plume: Impact of topography, wind and offshore forcing on the fate of plume waters,

Relative role of wind forcing and riverine nutrient input on the extent of hypoxia in the northern Gulf of Mexico

“…From left to right: t Uwind only ( Figures 1a-1d); F May + t Uwind (Figures 1e-1h); F spring + t Uwind (Figures 1i-1l); F 11 + t Uwind (Figures 1m-1p); May-June NO 3+2 + t Uwind (Figures 1q-1t); May NO 3+2 + t Uwind (Figures 1u-1x). et al, 2011], a persistent west wind reduces the areal extent by ultimately moving the river plume offshore, away from the continental shelf and into deep water [Lentz, 2004;Morey et al, 2003aMorey et al, , 2003bSchiller et al, 2011]. This movement removes the two factors necessary for coastal ocean hypoxia formation: nutrient supply and density stratification.…”

Phytoplankton community structure in the river‑influenced continental margin of the northern Gulf of Mexico

“…In addition to the influence of river outflow on shelf waters, a number of studies have observed significant offshore transport events involving Mississippi− Atchafalaya river-influenced waters (Walker et al 2005, Schiller et al 2011, Zhang et al 2012. To date, information about phytoplankton community composition in such offshore river-influenced features is limited to a single event (Wawrik & Paul 2004), and the relationship between environmental conditions and phytoplankton community structure has yet to be examined in such features for different seasons and river discharge conditions.…”

Linking Heterotrophic Microbial Activities with Particle Characteristics in Waters of the Mississippi River Delta in the Aftermath of Hurricane Isaac

“…In the Gulf of Mexico, most of the land-sea carbon fluxes are driven by Mississippi River runoff and subsequent biogeochemical organic matter processing that occurs on the Louisiana Shelf near the bird-foot delta, where most of the riverine discharge enters the coastal ocean (e.g., Green et al, 2006). Buoyant freshwater plumes of Mississippi River water generally extend westward from the delta, following the Louisiana coastal current; however, wind-driven changes in the direction of surface water currents near the delta can also lead to an eastward offshore transport of Mississippi River plumes (Schiller et al, 2011).…”