Influence of El Nino-Southern Oscillation and Equatorial Atlantic on rainfall over northern and northeastern regions of South America

The impacts of anomalous events in the Pacific Ocean associated with the El Nino - Southern Oscillation (ENSO) on rainfall over northern and northeastern regions of South America were evaluated for the period from 1900 to 2007 using composite analyses. The El Nino (La Nina) events in the Pacific, that together with a cold (warm) Atlantic Equatorial Mode (AEM) form an interbasin gradient between the Pacific and Atlantic were analyzed considering separately those for which the gradient forms during the ENSO onset phase from those for which the gradient forms during the ENSO demise phase. The results show that the rainfall pattern over the northern and northeastern region of South America is reinforced under an interbasin gradient during the initial phase of the ENSO event. In this case, a possible explanation is that the AEM with opposite sign of the ENSO event in its onset stage creates favorable conditions for the development of an interhemispheric gradient in the Tropical Atlantic acting in the same direction of the interbasin gradient, and collaborating to reinforce the El Nino (La Nina) effect on the precipitation. On the other hand, for ENSO events for which the interbasin gradient forms in the demise stage, the impact on the precipitation is more significative in the northern and central-western regions of the basin. A possible explanation for these differences is linked to the alterations in the east-west atmospheric circulation associated with the east-west gradient of the SST anomalies. The result of this study might be useful mainly for climate monitoring purposes.