Instability of shelf-break fronts and cross-shelf exchange in the Northern Gulf of Mexico.
Observational and numerical studies suggest that the shelfbreak front in the Mississippi Bight exhibits complex and meandering motions that can grow to large amplitudes and form eddies. These processes are of interest because they play an important role in cross-frontal exchange as well as in driving a mean flow. This study characterizes variability of the shelfbreak front and quantifies the cross-frontal exchange. Frontal dynamics is examined using two numerical models. Characteristics of the front and eddy heat transport are computed based on the results of realistic simulations of the NRL model. To explain the essential physics that drives phenomena predicted by realistic simulations, a series of idealized numerical experiments is performed. Response of the shelfbreak front, observed during wintertime in the Mississippi Bight, under influence of different types of forcing is studied using a three-dimensional numerical model ECOM. Analysis of sixteen series of numerical experiments reveals that in all cases the flow is baroclinically unstable and undergoes three phases of development: (1) adjustment, (2) meander growth, and (3) eddy detachment. The evolution of the front without external forcing is attributed to flow instability, which significantly contributes to cross-frontal exchange. Analysis of energetics suggests a hybrid baroclinic-barotropic instability of the flow. Computed eddy heat flux indicates that the shelfbreak front enhances the exchange between the shelf and slope. Furthermore, onshore heat flux is more intense at the frontal position. The results demonstrate that frontal circulation and turbulent heat exchange are very sensitive to (1) bottom topography, (2) degree of flow stratification, (3) local winds, and (4) interaction with oceanic eddies. Physical mechanisms controlling frontal dynamics, which are predicted by idealized experiments, are identifiable in the realistic simulations and provide an insight into complex and highly variable circulation in the Mississippi Bight.