The interannual variability of the Brazil Current and links to the large-scale circulation in the South Atlantic

Abstract:

The ocean circulation in the western South Atlantic is characterized by the presence of the subtropical gyre, the largest dynamical feature in the region. The Brazil Current, located in the western limb of this gyre, is a weak western boundary current, resulting from the bifurcation of the southern branch of the South Equatorial Current at around 15°S. The spatial and temporal variability of this current is critical to assess the transport of subtropical waters into subpolar regions and of its link to the Atlantic Meridional Overturning Circulation. Here we use data from hydrographic (XBT transects, surface drifters, and Argo floats) and satellite altimetry observations to investigate the interannual variability of the upper circulation in the western South Atlantic, in particular the subtropical gyre and the Brazil Current, since 1992. The main data sets used here correspond to data from three XBT transects: AX18 (along 34°S), AX97 (along 22°S, between Rio de Janeiro and Trinidade Island), and AX08 (across the subtropical gyre), as well as wind stress data and from satellite altimetry observations. Results obtained are compared against numerical model outputs. Wavelet analysis and complex empirical orthogonal functions are used on the altimetry data to provide the characteristics of propagating features for each frequency band. The main bands are represent eddy-scale, Rossby wave and gyre characteristics. Interannual variability of mesoscale activity in the western part of the basin is analyzed using data, model reanalysis and reduced gravity modeling, and associated to the basin-scale patterns. Results show that the Brazil Current has a mean volume transport of ~4 Sv at 22°S and increase to ~ 12 Sv at 34°S, which is consistent with the additional southward flow at intermediate level between the two sections. Basin wide wind stress anomalies produce anomalous Sverdrup circulation, with a delayed response (up to a few years) in the western boundary due to the propagation time of baroclinic Rossby waves. A five year oscillation is apparent in the eddy energy of the Brazil Current region, and also in the mixed layer depth across the basin, suggesting a coherent variability between the two regions.