How and why climate variability differs between the tropical Atlantic and Pacific

The tropical Pacific and Atlantic Oceans share many common climatological f eatures such as easterly trade winds, eastward shoaling thermocline, an eas tern cold tongue and a northerly intertropical convergence zone (ITCZ). How ever, a comparison of climate variability between the two oceans reveals mo re differences than similarities. The Pacific is dominated by the equatoria lly symmetric El Nine/Southern Oscillation (ENSO) while the Atlantic ITCZ i s controlled by changes in interhemispheric sea surface temperature (SST) g radient. To understand the causes of these differences in variability, a dy namic ocean-atmosphere coupling model is developed that includes both the B jerknes and wind-evaporation-SST feedbacks. Equatorially symmetric and anti symmetric modes emerge from the model, displaying distinct growth rate depe ndence on zonal wavenumber. Consistent with observations, an equatorially t rapped 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 meridio nal scale coexists with an equatorially antisymmetric dipole mode.