[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,
We study the development and evolution of buoyant river plumes on the continental shelf. Our calculations are based on three-dimensional numerical simulations, where the river runoff is introduced as a volume of zero salinity water in the continuity equation and mixing is provided by the model's turbulence closure scheme and wind forcing.In the absence of wind forcing the modeled fiver plumes typically consist of an offshore bulge and a coastal current in the direction of Kelvin wave propagation. We propose a plume classification scheme based on a bulk Richardson number, which expresses the relative magnitude of the buoyancy-induced stratification versus the available mixing. When the ratio of the discharge and shear velocities is greater (less) than 1, the plume is categorized as supercritical (subcritical); that is, the width of the bulge is greater (less) than the width of the coastal current. Supercritical plumes are often characterized by a meandering pattern along the coastal current, caused by a baroclinic instability process. For a given discharge, subcritical plumes are produced by large mixing and/Or shallow water depths. In the presence of wind forcing the favorable conditions for offshore removal of coastal low-salinity waters include high river runoff and strong upwelling-favorable wind stress. When the rivers are treated as individual sources of freshwater ("point source" behavior), the wind-driven flow may exhibit substantial spatial variability. Under the above removal conditions, strong offshore transport takes place in "jetlike" flow regions within the river plume, in contrast to the downwind acceleration of adjacent waters. When the rivers are treated as a long "line source" of freshwater, the plume region resembles a coastal low-salinity band and the above removal conditions trigger offshore transport that is most pronounced at the "head" of the source. control the fate of fivefine waters and related materials afterPaper number 95JC03024. 0148-0227/96/95JC-0302455.00 their release in the coastal ocean. Our main objective is to describe the generation and evolution of a fiver plume on the continental shelf and determine the important factors that govern its offshore expansion and, consequently, the removal mechanism of fiver-borne materials.The frontal structure of a fiver plume has been discussed by several investigators. Earlier studies, such as those by Kao et al. [1977], Kao [1981], Ikeda [1984], and Csanady [1984], recognized the importance of nonlinearity, Coriolis, and friction in the development of the buoyant plume. McClimans [1986] identifi•ed the following three major processes that characterize the dynamics of the seaward expansion of the river flow: (1) acceleration, resulting from the balance between inertia and gravity (buoyancy) forces; (2) mixing, governed by turbulence due to bottom and interfacial friction; and (3) geostrophy, where the balance between Coriolis and the developed cross-shore pressure gradient (due to "freshening" of coastal waters) generates an alongshore coas...
We deployed three replicate larval light traps off the upper Florida Keys from June-September 2001 to measure the delivery of settlement-stage fish larvae to the coral reefs. Nightly measures of larval abundance were compared to water temperatures measured across the outer reef, nearby wind records, and the alongshore and cross-shelf components of the currents measured at the seaward edge of the reef. These time series, together with satellitederived sea surface temperature and color fields indicate that a very large multi-taxa larval pulse on 20 July was directly associated with the passage of a Florida Current (FC) sub-mesoscale frontal eddy embedded within the elongated remnant of a mesoscale eddy. A second large pulse of larvae occurred when a similar mesoscale eddy passed the upper Keys in mid-June. Periods of increased tidal bore activity occurred with the passage of these eddies. Semidiurnal internal tides caused near-bottom onshore intrusions of cooler slope waters during periods of onshore meanders of the FC front when the downstream baroclinic flow and stratification increased at the reef margin. The high abundance and similarity in larval ages within taxa on 20 July indicate a Keys shelf origin, although the temporal and spatial scales of entrainment cannot be resolved. The passage of a third mesoscale eddy in September did not result in a larval pulse, possibly because of a mismatch between biological and physical criteria, several of which must be met for larval transport by mesoscale eddies to be successful.
During the Deepwater Horizon incident, crude oil flowed into the Gulf of Mexico from 1522 m underwater. In an effort to prevent the oil from rising to the surface, synthetic dispersants were applied at the wellhead. However, uncertainties in the formation of oil droplets and difficulties in measuring their size in the water column, complicated further assessment of the potential effect of the dispersant on the subsea-to-surface oil partition. We adapted a coupled hydrodynamic and stochastic buoyant particle-tracking model to the transport and fate of hydrocarbon fractions and simulated the far-field transport of the oil from the intrusion depth. The evaluated model represented a baseline for numerical experiments where we varied the distributions of particle sizes and thus oil mass. The experiments allowed to quantify the relative effects of chemical dispersion, vertical currents, and inertial buoyancy motion on oil rise velocities. We present a plausible model scenario, where some oil is trapped at depth through shear emulsification due to the particular conditions of the Macondo blowout. Assuming effective mixing of the synthetic dispersants at the wellhead, the model indicates that the submerged oil mass is shifted deeper, decreasing only marginally the amount of oil surfacing. In this scenario, the oil rises slowly to the surface or stays immersed. This suggests that other mechanisms may have contributed to the rapid surfacing of oil-gas mixture observed initially. The study also reveals local topographic and hydrodynamic processes that influence the oil transport in eddies and multiple layers. This numerical approach provides novel insights on oil transport mechanisms from deep blowouts and on gauging the subsea use of synthetic dispersant in mitigating coastal damage.
[1] The Florida Current (FC) is the branch of the Gulf Stream system within the Straits of Florida, connected to the Loop Current in the Gulf of Mexico. Cyclonic, cold-core eddies travel along this oceanic current system, entering the Straits of Florida in the vicinity of the Dry Tortugas and evolving along the Florida Keys island chain and coral reefs. The development of the high-resolution ($900 m) hydrodynamic model Florida Straits, South Florida, and Florida Keys (FKeyS), nested within a Gulf of Mexico model (both based on the Hybrid Coordinate Ocean Model), has enabled new findings in eddy variability. Together with high-resolution ($1 km) ocean color imagery, multiyear model archives have been employed to study the changes in the position of the FC front and the relationship with eddy evolution. It was found that eddy interactions and transformations are common, with multiple eddy cells within individual eddies or new cells emerging from existing vortices. Features in the Dry Tortugas area previously thought to be semipermanent are shown to be frequently transformed and/or replenished. A mechanism of local cyclogenesis is also proposed. Incoming eddies interact with and influence the downstream propagation of previous eddies. Systems of eddies, rather than individual vortices, can form the elongated features observed between the FC front and the Atlantic Florida Keys Shelf. Topography plays an important role in eddy dissipation or growth. A close synergy between eddy evolution and FC meandering is revealed. The results have implications on the connectivity of remote coastal and reef ecosystems.
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