[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,

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Schiller, R.V. and Kourafalou, V.H., 2014. Loop current impact on the transport of Mississippi river waters. Journal of Coastal Research, 30(6), 1287-1306. Coconut Creek (Florida), ISSN 0749-0208.River plume studies have traditionally focused on coastal and shelf circulation. Here, we examine novel interactions of a large river plume with offshore flows, which introduces additional export pathways for the riverine waters. We show the impact of the Loop Current System (LCS) on the offshore transport and fate of Mississippi River (MR) plume waters. The high-resolution Northern Gulf of Mexico (NGoM) model, nested in a data-assimilative, regional, Gulf of Mexico model, was employed for a 5-year, realistically forced simulation. Model results, in tandem with satellite data, demonstrate that the entrainment of MR waters by the LCS varies interannually and extends pathways of shelf cross-marginal transport to the basin interior and beyond. Plume-to-LCS interactions are determined by episodic northward intrusions of the LCS in the NGoM. If the intrusion is very close to the MR Delta, plume waters can be entrained with little contribution from winds. If the intrusion is farther offshore and lasts for months, LCS entrainment may be modulated by wind forcing because plume transport can change from an along-shelf to an exclusively across-shelf regime. Seasonal changes in river discharge did not have a direct influence on the transport induced by LCS intrusions. However, high-frequency (daily) variability of the river discharge induced by the Mississippi and 16 other major rivers was important and played a role in the overall NGoM freshwater transport. During the 5-year period, plume-to-LCS interactions corresponded on average tõ 11% of the yearly net offshore freshwater transport near the MR Delta. Model results were supported by satellite and in situ observations that demonstrate the far-reaching potential of MR waters as they are traced to the SE Gulf of Mexico and the Florida Keys. This study demonstrates how across-basin, connectivity pathways for riverine waters may be affected by local coastal-to-offshore interactions induced by a large-scale current system.

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 buoyancydriven 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 4The riverine waters exit through several passes into NGoM, the largest ones being the Southwest Pass, South Pass and Pass a Loutre [Walker et al., 1994]. The MR outflow is subject to strong vertical mixing due to supercritical conditions at the mouth of the passes [Wright and Coleman, 1971], which act as hydraulic constrictors for the outflow [Armi and Farmer, 1986]. The present study is focused beyond this near-field region at the mouth of the passes, on the continental shelf around the Delta, where the outflow is characterized by a bottom-attached saline front, and over the far-field areas of the Louisiana-Texas (LATEX) shelf, the Mississippi-Alabama-Florida (MAFLA) shelf and southern offshore deeper regions (Figure 1). As the outflow spreads and shoals, it accelerates, increases vertical shear and triggers Kelvin-Helmholtz-like flow instabilities and turbulent mixing [MacDonald and Geyer, 2004]. Mixing and spreading of the MR plume are controlled by a variety of factors.The buoyancy of the plume waters drives three distinct pathways that are common for large-scale rivers (defined as riverine systems where the Coriolis effect is important, Garvine, 1995): an anticyclonic bulge, a "downstream" coastal current in the direction of Kelvin wave propagation (toward the LATEX shelf) and an "upstream" current domain (toward the MAFLA shelf). These patterns have been identified in early studies on river plume dynamics in the presence of buoyancy forcing only [e.g. Chao and Boicourt, 1986;Garvine, 1987;Kourafalou et al., 1996].The influence of local wind stress is a dominant mechanism for driving the circulation over the inner shelf [Cochrane and Kelly, 1986;Li et al., 1997;Morey et al., 2003;Nowlin et al., 2000...

Following the 2010 Deepwater Horizon accident of a massive blow-out in the Gulf of Mexico, scientists from government, industry, and academia collaborated to advance oil spill modeling and share best practices in model algorithms, parameterizations, and application protocols. This synergy was greatly enhanced by research funded under the Gulf of Mexico Research Initiative (GoMRI), a 10-year enterprise that allowed unprecedented collection of observations and data products, novel experiments, and international collaborations that focused on the Gulf of Mexico, but resulted in the generation of scientific findings and tools of broader value. Operational oil spill modeling greatly benefited from research during the GoMRI decade. This paper provides a comprehensive synthesis of the related scientific advances, remaining challenges, and future outlook. Two main modeling components are discussed: Ocean circulation and oil spill models, to provide details on all attributes that contribute to the success and limitations of the integrated oil spill forecasts. These forecasts are discussed in tandem with uncertainty factors and methods to mitigate them. The paper focuses on operational aspects of oil spill modeling and forecasting, including examples of international operational center practices, observational needs, communication protocols, and promising new methodologies.