This work examines ways in which the seasonal cycle influences the evolution of El Niņo-Southern Oscillation (ENSO) in the Tropical Pacific. The following hypotheses and associated physical mechanisms are investigated: (H1) The seasonal warming of the cold tongue early in the calendar year (January-April) favors the initial growth of warm ENSO events, (H2) During a warm ENSO event, the warm surface waters migrating in the western basin from the Southern to the Northern Hemisphere during the northern spring (April-May) trigger enhanced convection along the equator, which contributes to reinforce the event, and (H3) The warm surface waters returning in the western basin from the Northern to the Southern Hemisphere towards the end of the calendar year (November-January) favor the demise of ongoing warm ENSO events.

Hypothesis-validation experiments are performed with a coupled atmosphere-ocean general circulation model (CGCM). In this context, the simulated seasonal cycle and interannual variability can be separated and artificially modified to highlight the aspect targeted for examination. The anomaly-coupling technique is applied, thus allowing for comparisons of simulations in which seasonal conditions in the CGCM's atmospheric component are either fixed or time-varying. The Tropical Pacific version of the UCLA CGCM is used. The results obtained in the experiments are supportive of hypotheses (H1) and (H3). No supportive evidence is found for hypothesis (H2).