Interaction between Tropical Atlantic variability andEl

The interaction between tropical Atlantic variability and El Ni~ no-Southern Oscillation (ENSO) is investigated using three ensembles of atmospheric general circulation model integrations. The integrations are forced by specifying observed sea surface temperature (SST) variability over a forcing domain. The forcing domain is the global ocean for the rst ensemble, limited to the tropical ocean for the second ensemble, and further limited to the tropical Atlantic region for the third ensemble. The ensemble integrations show that extratropical SST anomalies have little impact on tropical variability, but the eeect of ENSO is pervasive in the tropics. Consistent with previous studies, the most signiicant innuence of ENSO is found during the boreal spring season and is associated with an anomalous Walker circulation. Two important aspects of ENSO's innuence on tropical Atlantic variability are noted. First, the ENSO signal contributes signiicantly to the \dipole" correlation structure between tropical Atlantic SST and rainfall in the Nordeste Brazil region. In the absence of the ENSO signal, the correlations are dominated by SST variability in the southern tropical Atlantic|resulting in less of a \dipole" structure. Second, the remote innuence of ENSO also contributes to positive correlations between SST anomalies and downward surface heat ux in the tropical Atlantic during the boreal spring season. However, even when ENSO forcing is absent, the model integrations provide evidence for a positive surface heat ux feedback in the deep tropics, which is analyzed in a companion study by Chang et al. (1999). Our analysis shows that interannual atmospheric variability in the tropical Paciic-Atlantic system is dominated by the interaction between two distinct sources of tropical heating: (i) an equatorial heat source in the eastern Paciic associated with ENSO, and (ii) an oo-equatorial heat source associated with SST anomalies in the Caribbean. Modelling this Caribbean heat source accurately could be very important for seasonal forecasting in the Central American/Caribbean region.