Revisiting the Atlantic South Equatorial Current

Abstract

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.