The relative importance of ENSO and tropical Atlantic sea surface temperature anomalies for seasonal precipitation over South America: a numerical study

The role of tropical Atlantic sea surface temperature (SST) anomalies durin g ENSO episodes over northeast Brazil (Nordeste) is investigated using the CPTEC/COLA Atmospheric General Circulation Model (AGCM). Four sets of integ rations are performed using SST in Fl Nino and La Nina (ENSO) episodes, cha nging the SST of the Atlantic Ocean. A positive dipole (SST higher than nor mal in the tropical North Atlantic and below normal in the tropical South A tlantic) and a negative dipole (opposite conditions), are set as the bounda ry conditions of SST in the Atlantic Ocean. The four experiments are perfor med using El Nino or La Nina SST in all oceans, except in the tropical Atla ntic where the two phases of the SST dipole are applied. Five initial condi tions were integrated in each case in order to obtain four ensemble results . The positive SST dipole over the tropical Atlantic Ocean and Fl Nino cond itions over the Pacific Ocean resulted in dry conditions over the Nordeste. When the negative dipole and Fl Nino conditions over the Pacific Ocean wer e applied, the results showed precipitation above normal over the north of Nordeste. When La Nina conditions over Pacific Ocean were tested together w ith a negative dipole, positive precipitation anomalies occurred over the w hole Nordeste. Using the positive dipole over the tropical Atlantic, the pr ecipitation over Nordeste was below average. During La Nina episodes, the A tlantic Ocean conditions have a larger effect on the precipitation of Norde ste than the Pacific Ocean. In Fl Nino conditions, only the north region of Nordeste is affected by the Atlantic SST. Other tropical areas of South Am erica show a change only in the intensity of anomalies. Central and southea st regions of South America are affected by the Atlantic conditions only du ring La Nina conditions, whereas during El Nino these regions are influence d only by conditions in the Pacific Ocean.