Rainfall variability in northeast South America(1) and the Sahel region of Africa(2-4) is profoundly influenced by the sea surface temperature (SST) of the tropical Atlantic Ocean. Of particular importance are relative changes in SST between the hemispheres on decadal timescales, a phenomenon often called the Atlantic SST dipole(1,5). Here we propose that the decadal variation in the tropical SST dipole may be attributed to an unstable thermodynamic ocean-atmosphere interaction between wind-induced heat fluxes and SST, Using coupled ocean-atmosphere models, we show that the coupled dipole mode has a typical oscillation period of about a decade. The notion that the Atlantic dipole-like SST variability may be related to an oscillatory coupled mode might assist attempts to predict decadal climate variability in the tropical Atlantic region.