The importance of topographically-induced submesoscale processes on cross-shelf transport

The intrinsic dynamics, geomorphological characteristics and the proximity of nutrients sources from land make continental shelves much more productive than the adjacent deep ocean. Thus, the exchange of waters across continental shelf may play an important role in the distribution of nutrients and biota between these environments. In fact, much of this exchange is hindered by the abrupt topographic variation of the shelf-break as well as by quasi-geostrophic boundary currents that flow mostly along isobaths at the shelf edge. However, small-scale bathymetric features induce instabilities of these currents giving rise to mesoscale eddies and submesoscale filaments that may facilitate shelf-open ocean exchange. We use a regional numerical model in two horizontal resolutions, namely 6km (mesoscale-resolving) and 2km (submesoscale-permitting), in order to show that submesoscale processes and enhanced bathymetry resolution are key for shelf-open ocean exchange along the Brazil Current path between Cape Frio and Cape São Tomé (RJ, 22°S–23°S). On average, over three years of simulations, there is a total net offshore transport of 2 Sv between 22°S and 23°S and 1 Sv onshore between 23°S and 24°S. Much of this exchange is driven by episodic bursts due to meso- and submesoscale processes which disrupt the along-isobath, quasi-geostrophic flow of the Brazil Current. Changes in the bathymetry along the shelf-break induces meandering of the Brazil Current which leads to, due potential vorticity conservation, the shedding of cyclonic eddies that promote cross-shelf exhange. Submesoscale filaments associated with these vortices operate on shorter temporal and spatial scales and are associated with large, episodic cross-shelf fluxes thereby enhancing the exchange of properties between the shelf and the open ocean. In addition, long-term average of the vertical velocity at submesoscale configure hotspots of upwelling enhancing the supply of the nurient-rich South Atlantic Central Water along the shelf-break region were the nutrient-poor Tropical Water is tipically carried out by Brazil Current in the upper levels.