The dominant mode of coupled variability over the South Atlantic Ocean is known as "South Atlantic Dipole" (SAD) and is characterized by a dipole in sea surface temperature (SST) anomalies with centers over the tropical and the extratropical South Atlantic. Previous studies have shown that variations in SST related to SAD modulate large-scale patterns of precipitation over the Atlantic Ocean. Here we show that variations in the South Atlantic SST are associated with changes in daily precipitation over eastern South America. This study is based on observational and regional atmospheric modeling. Rain gauge precipitation, satellite derived sea surface temperature and reanalysis data are used to investigate the variability of the subtropical and tropical South Atlantic and impacts on precipitation. SAD phases are assessed by performing Singular Value Decomposition (SVD) analysis of sea level pressure and SST anomalies.

We show that during neutral El Nino-Southern Oscillation (ENSO) events, SAD plays an important role in modulating cyclogenesis and the characteristics of the South Atlantic Convergence Zone (SACZ). Positive SST anomalies over the extratropical South Atlantic (SAD negative phase) are related to increased cyclogenesis near southeast Brazil as well as the migration of extratropical cyclones further north. As a consequence, these systems organize convection and increase precipitation over eastern South America. Numerical experiments forced with prescribed SST anomalies showed that even though the Atlantic SST affects the position of the cyclone associated with the SACZ, the atmospheric response and precipitation patters over land were opposite from the observational results.

On the other hand, experiments forced with prescribed anomalous driving fields showed that the atmospheric component of SAD plays a significant role for the right position and intensity of precipitation associated with the SACZ. SAD negative anomalies provide the low-level and upper-level atmospheric support for the intensification of the cyclone at surface and for the increase in precipitation over the land portion of the SACZ. Therefore, the numerical experiments suggest that, during ENSO neutral conditions, the SACZ precipitation variability associated with SAD is largely dependent on the atmospheric variability rather than the underlying SST.