The tropical Pacific and Atlantic Oceans share many common climatological features such as easterly trade winds, eastward shoaling thermocline, an eastern cold tongue and a northerly intertropical convergence zone (ITCZ). However, a comparison of climate variability between the two oceans reveals more differences than similarities. The Pacific is dominated by the equatorially symmetric El Nino/Southern Oscillation (ENSO) while the Atlantic ITCZ is controlled by changes in interhemispheric sea surface temperature (SST) gradient. To understand the causes of these differences in variability, a dynamic ocean-atmosphere coupling model is developed that includes both the Bjerknes and wind-evaporation-SST feedbacks. Equatorially symmetric and antisymmetric modes emerge from the model, displaying distinct growth rate dependence on zonal wavenumber. Consistent with observations, an equatorially trapped ENSO mode dominates an ocean with a longitudinal size of the Pacific while in a smaller Atlantic-size one, a monopole mode with a broad meridional scale coexists with an equatorially antisymmetric dipole mode. ¿ 1999 American Geophysical Union