Topographically Generated Submesoscale Shear Instabilities Associated with Brazil Current Meanders

Abstract

The western boundary current system off southeastern Brazil is composed of the poleward flowing Brazil Current (BC) in the upper 300mand the equatorward flowing Intermediate Western Boundary Current (IWBC) underneath it, forming a first-baroclinic mode structure in the mean. Between 22°S and 23°S, the BC-IWBC jet develops recurrent cyclonic meanders that grow quasi-stationarily via baroclinic instability, though their triggering mechanisms are not yet well understood. Our study, thus, aims to propose a mechanism that could initiate the formation of these mesoscale eddies by adding the submesoscale component to the hydrodynamic scenario. To address this, we perform a regional 1/50°(∼2-km) resolution numerical simulation using CROCO (Coastal and Regional Ocean COmmunity model). Our results indicate that incoming anticyclones reach the slope upstream of separation regions and generate barotropic instability that can trigger the meanders’ formation. Subsequently, this process generates submesoscale cyclones that contribute, along with baroclinic instability, to the meanders’ growth resulting in a submesoscale-to-mesoscale inverse cascade. Lastly, as the mesoscale cyclones grow, they interact with the slope generating inertially and symmetrically unstable anticyclonic submesoscale vortices and filaments.