The ENSO teleconnection to the tropical Atlantic Ocean: Contributions of the remote and local SSTs to rainfall variability in the tropical Americas

Recent developments in Tropical Atlantic Variability (TAV) identify the El Nino-Southern Oscillation (ENSO) as one of the leading factors in the interannual climate variability of the basin. An ENSO event results in Tropic-wide anomalies in the atmospheric circulation that have a direct effect on precipitation variability, as well as an indirect effect, that is, one mediated by sea surface temperature (SST) anomalies generated in the remote ocean basins. In order to separate the relative contributions of the atmospheric and oceanic components of the ENSO teleconnection to the tropical Atlantic Ocean, results from two ensembles of atmospheric general circulation model (AGCM) experiments, differing in oceanic boundary conditions, are compared. AGCM integrations performed with the Community Climate Model version 3 (CCM3), forced by global, observed SST during 1950-94 reproduce the observed ENSO-related rainfall anomalies over the tropical Americas and adjacent Atlantic. A parallel ensemble of integrations, forced with observed SST in the tropical Atlantic only, and climatology elsewhere, is used to separate the effect of the direct atmospheric teleconnection from the atmosphere's response to the ENSO-forced SST anomalies in the Atlantic basin. It is found that ENSO-related atmospheric and oceanic anomalies force rainfall anomalies of the same sign in northeast Brazil, of opposite sign in the Caribbean basin. The direct atmospheric influence of a warm ENSO event reduces model rainfall as a whole over the tropical Atlantic basin. This observation is consistent with the hypothesis that an ENSO-related Tropic-wide warming of the free troposphere forces the vertical stabilization of the tropical atmosphere. ENSO-related atmospheric anomalies are also known to force a delayed (relative to the mature phase of ENSO) warming of tropical North Atlantic SST through the weakening of the northeasterly trade winds and consequent reduction of surface fluxes. It is found that this delayed oceanic component forces a northward displacement of the Atlantic intertropical convergence zone, resulting in increased precipitation over the Caribbean and reduced precipitation over northeast Brazil during the boreal spring following the mature phase of ENSO.