The southern branch of the South Equatorial Current (SSEC) is the northern limit of the South Atlantic Subtropical Gyre. When this current reaches Brazil around 14°S it bifurcates into a southward flow as the Brazil Current (BC) and the surface portion of the northward flowing North Brazil Undercurrent (NBUC). The SSEC system is a key component of the western boundary supply, influencing the NBUC/BC variability and, therefore, global climate through the Meridional Overturning Circulation. In this study, using altimetry satellite data and reanalyzes outputs (1993–2018), we revisit the SSEC mean state and show this current arriving at the South Atlantic western boundary as a multi‐banded flow with surface signatures resulting from different subsurface cores. These bands have velocities between 0.02 and 0.07 m s−1 and, as shown by ADCP data from the PIRATA project, their signature in synoptic scenarios is obscured by eddies and waves with velocities between 0.1 and 0.3 m s−1. In addition, the SSEC annual cycle analysis shows that the seasonality of the bands is out of phase with each other, presenting westward transport anomalies between 0.4 and 2.6 Sv. Finally, our results show that the seasonality of this multi‐banded flow both defines where the BC is born, and modulates the seasonality of semi‐permanent mesoscale eddies off Brazil.

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.

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