Instability of shelf-break fronts and cross-shelf exchange in the northern Gulf of Mexico.

摘要

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 the variability of the shelfbreak front and quantifies cross-frontal exchange. Frontal dynamics are 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 (Naval Research Laboratory) model. To explain the essential physics that drives phenomena predicted by the realistic simulations, idealized numerical experiments are performed. The response of the shelfbreak front, during winter in the Mississippi Bight, under the influence of different types of forcing is studied using a three-dimensional numerical model ECOM (Estuarine and Coastal Ocean Model). Analysis of a 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. An analysis of model energetics suggests that the flow has both baroclinic and barotropic instabilities. Computed eddy heat fluxes indicate that the shelfbreak front enhances exchange between the shelf and slope. Furthermore, the onshore heat flux is more intense at the frontal position. The results demonstrate that frontal circulation and turbulent heat exchange are very sensitive to the following: (1) bottom topography, (2) degree of flow 1 stratification, (3) local winds, and (4) interaction with oceanic eddies. The physical mechanisms controlling frontal dynamics that are predicted by idealized experiments are identifiable in the realistic simulations, and provide insight into the complex and highly variable circulation within the Mississippi Bight.