Revisiting the Tropical Atlantic Influence on El Nino-Southern Oscillation

The influence of the tropical Atlantic on El Nino-Southern Oscillation (ENSO) is examined using sensitivity experiments with the SINTEX-F general circulation model with prescribed sea surface temperature (SST) distributions based on observations for the period 1982-2018. In the control experiment (CTRL) observed SSTs are prescribed over the global oceans; in the sensitivity experiment observed SSTs are prescribed in the tropical Atlantic only, while in other regions the climatological annual cycle is prescribed. A composite analysis of the model output suggests that cold SST events in the northern tropical Atlantic during boreal spring are associated with near-surface wind changes over the equatorial and subtropical Pacific that are conducive to the development of El Nino, consistent with previous studies. The amplitude of these changes, however, is at most 20% of those observed during typical El Nino events. Likewise, warm events in the equatorial Atlantic produce only about 10% of the wind changes seen in the western equatorial Pacific during the developing phase of typical La Nina events. Similar results are obtained from a partial regression analysis performed on an ensemble of atmosphere-only simulations from phase 6 of the Atmospheric Model Intercomparison Project (AMIP) although the equatorial Atlantic influence is stronger in AMIP. Further analysis of the AMIP models indicates that model biases do not have a major impact on the Atlantic-to-Pacific influence. Overall, the results suggest that the tropical Atlantic has a rather weak influence on ENSO development and mostly acts to modulate ongoing events rather than initiate them.