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Submesoscale circulation in the Northern Gulf of Mexico: Deep phenomena and dispersion over the continental slope

This study examines the mesoscale and submesoscale circulations along the continental slope in the northern Gulf of Mexico at depths greater than 1000 m. The investigation is performed using a regional model run at two horizontal grid resolutions, 5 km and 1.6 km, over a three year period from January 2010 to December 2012. Ageostrophic submesoscale eddies, and vorticity filaments populate the continental slope, and they are stronger and more abundant in the simulation at higher resolution, as to be expected. They are formed from horizontal shear layers at the edges of highly intermittent, bottom-intensified, along-slope boundary currents and in the cores of those currents when they are confined on steep slopes. The horizontal resolution influences the representation of the lateral and vertical transport of neutrally buoyant tracers. This is exemplified by a passive dye deployed near the Macondo Prospect, where the Deepwater Horizon rig exploded in 2010, and at the largest known natural hydrocarbon seep in the northern Gulf, few hundred kilometers to the west of the rig wellhead. The first site is characterized by weak mean currents and, in the high resolution run, by intense submesoscale eddies that form to the west of the Macondo Prospect. GC600, on the other hand, is surrounded by a complicated bathymetry, and its flow is highly variable. The trapping exerted by the submesoscale eddies and vorticity filaments near the rig constrain the spreading in the vicinity of the Macondo Prospect, and as a result, patchiness in the passive tracer distributions differs in the two integrations, despite similar diffusivities expressing the average spreading rate. The tracer released at GC600, on the other hand, is dispersed more effectively in the first six months in the higher resolution case due to enhanced near-bottom turbulent mixing.