A further analysis of the tropical Atlantic SST modes and their relations to north-eastern Brazil rainfall during different phases of Atlantic Multidecadal Oscillation

Abstract

This paper examines the climate variability modes in the South American/Atlantic sector accompanying dry and wet years over north-eastern Brazil (NEB) in which the tropical Pacific and Atlantic oceanic-atmospheric conditions usually associated, respectively, with dryness and wetness are absent. The analyses are for several variables and take into account the warm and cold Atlantic Multidecadal Oscillation (AMO) phases (WAMO and CAMO). Four cases are analysed: DRY-WAMO, DRY-CAMO, WET-WAMO and WET-CAMO. The main difference in the sea surface temperature (SST) anomaly patterns in the tropics between the AMO phases responsible for the differences in the precipitation anomaly patterns is the differential longitude positioning of the warming or cooling of the surface waters in the equatorial Atlantic. In consequence, the anomalous Atlantic Hadley and Walker circulations also show differences between the AMO phases, which justify the observed precipitation anomalies over tropical South America for the analysed cases. The strong anomalous Atlantic Hadley cell determines the dipolar structure of the precipitation anomalies between NEB and northern South America for the DRY-WAMO and WET-WAMO cases. The strong anomalous Atlantic Walker cell defines the dryness over NEB for the DRY-CAMO, and both the strong anomalous Atlantic Hadley and Walker cells act together to yield an anomalous dry condition over NEB and wet condition over northern South America. Therefore, the results here provided clear indications that for climate monitoring and forecasting tasks, the AMO phases should be considered. These are new aspects of the tropical Atlantic variability that might be useful for climate monitoring purposes.