Vassiliki H. Kourafalou

The fate of river discharge on the continental shelf: 1. Modeling the river plume and the inner shelf coastal current

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 models turbulence closure scheme and wind forcing. In the absence of wind forcing, the modeled river 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.

The fate of river discharge on the continental shelf: 2. Transport of coastal low‐salinity waters under realistic wind and tidal forcing

A three-dimensional numerical simulation of shelf circulation is presented. We employ realistic forcing for the Southeast U.S. Continental Shelf during the spring season. We show that the strongest offshore transport of river-induced, coastal, low-salinity waters and associated materials occurs near the surface. The preferred mean pathway is in the northeastward direction, and it takes about 2 months to cross the entire shelf. Owing to the mean direction of surface transport and the topography of the South Atlantic Bight shelf, the preferred location for springtime removal is off Charleston, South Carolina, and presumably in the vicinity of the Charleston Bump. The transport and fate of the river-induced, coastal, low-salinity waters during the spring season are determined by (1) the stratification of nearshore waters, which is due to the high river runoff and causes the decoupling between “near-surface” and “near-bottom” layers; (2) the prevailing northeastward winds, which cause significant offshore transport within the shallow near-surface Ekman layer; and (3) the tidally induced bottom stirring (M2 tides). Comparison of model and data time series of currents shows very good agreement. Standard deviations of the model and data-computed empirical orthogonal functions are almost identical, while the respective variance-conserving spectra agree both in amplitude and phase.

Evolution of the Macondo well blowout: Simulating the effects of the circulation and synthetic dispersants on the subsea oil transport

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.

Process studies on the Po River plume, North Adriatic Sea

The coastal processes in the Adriatic Sea and in particular the development and evolution of the Po River plume are studied with a three-dimensional, primitive equation model. Experiments are performed to examine the buoyancy-driven flow and the interaction with topography, wind stress, and ambient stratification. The pathways of particles released from the Po are computed to simulate the transport of land-drained materials. In the absence of wind forcing, the Po River plume consists of an offshore bulge that occupies most of the north part of the basin and a southward coastal current that is confined within the narrow coastal region along the Italian peninsula. When all Adriatic rivers are included, basinwide, buoyancy-driven, cyclonic coastal flow is established that diminishes the seaward removal of Po waters and promotes their southward advection through the coastal current. Wind stress modifies the above. Winds that are downwelling-favorable for the Po discharge site (like Bora) restrict offshore plume development, enhance the southward coastal current, and confine the plume within the narrow western Adriatic shelf. Conversely, upwelling-favorable winds (like Scirocco) eliminate the baroclinic coastal current and advect low-salinity Po waters toward the shallow parts of the Northern Adriatic. When preexisting stratification of basin waters is described, the density field is influenced by both temperature and salinity. During winter, when the coastal waters tend to be colder and fresher than offshore waters, the cyclonic circulation is sustained owing to input from rivers and particularly the Po. The intense spatial and temporal variability of wind stress and river runoff and the resulting variability in the circulation of the coastal Adriatic Sea are elaborated through a realistic simulation during January 1994.

Surface evolution of the deepwater horizon oil spill patch: Combined effects of circulation and wind-induced drift

Following the Deepwater Horizon blowout, major concerns were raised about the probability that the Loop Current would entrain oil at the surface of the Gulf of Mexico toward South Florida. However, such a scenario did not materialize. Results from a modeling approach suggest that the prevailing winds, through the drift they induced at the ocean surface, played a major role in pushing the oil toward the coasts along the northern Gulf, and, in synergy with the Loop Current evolution, prevented the oil from reaching the Florida Straits. This implies that both oceanic currents and surface wind-induced drift must be taken into account for the successful forecasting of the trajectories and landfall of oil particles, even in energetic environments such as the Gulf of Mexico. Consequently, the time range of these predictions is limited to the weather forecasting range, in addition to the range set up by ocean forecasting capabilities.

Circulation On the Continental Shelf of the Southeastern United States. Part I: Subtidal Response to Wind and Gulf Stream Forcing During Winter

Abstract Subtidal current and sea level response to wind and Gulf Stream forcing are investigated for the South Atlantic Bight shelf during winter conditions. Low-frequency flow variability in the outer shelf results primarily from wavelike meanders and eddies in the Gulf Stream front that occur in a 2-day to 2-week period band. Current meter derived vertically integrated momentum balances indicated that these large amplitude flow events are in approximate geostrophic balance with baroclinic pressure gradients induced by northward propagating Gulf Stream disturbances. Low-frequency flow at midshelf is primarily a local Ekman response to wind forcing. Cross-shelf momentum balance for the total water column is between the along-shelf geostrophic current and the cross-shelf barotropic pressure gradient resulting from wind induced sea level changes at the coast. This balance holds for both mean and fluctuating parts of the flow, with the along-shelf barotropic current lagging sea level by 6 to 12 hours and al...

Circulation on the western South Atlantic continental shelf: 2. Spring and autumn realistic simulations

[1] Buoyancy-driven currents are here investigated in a complex scenario, where two river plumes (the La Plata River and the Patos Lagoon plumes) occur in the vicinity of two opposing western boundary currents (the Malvinas and the Brazil currents). The study addresses the contrasting scenarios found during the austral spring and fall seasons, due to variations in river discharge, in wind stress, and in the boundary current transport. The winds blow preferably from the northeast during the spring and from the southwest during the fall. The Brazil Current transport is stronger during the spring, when the Brazil-Malvinas Confluence is displaced southward, and weaker during the fall when the confluence moves northward. The study is conducted as a series of numerical simulations which consider the river discharges, the tides, variable wind stress, and thermohaline fields which are realistic in terms of the Brazil-Malvinas Confluence. Our discussion focuses on the riverine water distribution in each season. The austral fall scenario shows coastally trapped plumes, and the spring scenario shows significant offshore removal, but our fall plumes are not too elongated and narrow as other authors found in the presence of constant winds, no tides, and no boundary currents, and our spring offshore removal is not as strong as found by others.

Circulation on the Continental Shelf of the Southeastern United States. Part II: Model Development and Application to Tidal Flow

Abstract An extensive amount of work has been carried out to characterize the flow on the shelf between Cape Canaveral and Cape Hatteras. Data show that the winter flow in this region is driven mainly by tides and wind, while significant Gulf Stream effects are observed only on the outer shelf. A vertically integrated two-dimensional model is adapted to the South Atlantic Bight shelf and applied to predict tidal and wind-driven currents under vertically well-mixed conditions that are characteristic for the winter months (November to April). The model is based on the equations of motion combined with the continuity equation and appropriate boundary conditions. A numerical finite element method in space is coupled with a simple finite difference scheme in time to integrate the equations. The model is predictive (prognostic) in that apart from boundary conditions only the bottom friction and the wind stress coefficient need be prescribed. In the application of the model to tidal flow conditions, results of a...

Circulation on the Continental Shelf of the Southeastern United States. Part III: Modeling the Winter Wind-Driven Flow

Abstract A vertically integrated two-dimensional model has been adapted for the study of the wind-driven flow on the South Atlantic Bight (SAB) shelf during the winter season. Wind data are used as an input to the model and current data are used to verify the model results. Initially, a constant wind field in time and space is applied over the whole domain; the predicted flow pattern shows good agreement with observations. Model results using observed spatially and temporally varying wind fields are also in good agreement with observations. At the shelf-break the comparison is rather poor due to the strong influence of the Gulf Stream in the outer shelf data, which was neglected in the model. Water particle trajectories resulting from wind forcing, tidal forcing, and from the combined effect of an along-shelf surface slope with tidal forcing are compared. The along-shelf slope seems to have a significant effect on the net displacement of water particles as well as on the along-shelf volume transport. Comp...

Circulation on the western South Atlantic continental shelf: 1. Numerical process studies on buoyancy

[1] Buoyancy-driven currents are investigated in the western South Atlantic Ocean, where a major river plume, La Plata, interacts with a minor one, Patos Lagoon. A series of simulations were performed with a three-dimensional, free-surface numerical model, to better understand the processes which control both plumes evolution and their interaction. The simulations are focused on the importance of turbulent mixing to the evolution of the La Plata plume and its downshelf penetration toward the Patos Lagoon. Satellite observations and historical data show that the La Plata plume has a tendency to expand along the river southern margin, before it turns north in the direction of coastal trapped waves propagation. Our results, on the other hand, show that in the absence of the tides and the wind, this plume tends to leave the estuary attached to its northern margin (downstream), unless the model parameters are tuned to produce unrealistically high turbulent mixing. We then address the role of each parameter that controls mixing, and show that by tuning the model parameters we can get plumes similar to those observed in nature. However, the plumes produced in the experiments with excessively high mixing parameters showed unbounded southward (upstream) expansion. By introducing the tides we show that the plumes produced in the presence of tidal stirring, and normally set mixing parameters, are more realistic, suggesting that tidal stirring is more effective than artificially tuned turbulent mixing on causing the plume lateral expansion.