The physical mechanism underlying ENSO's phase locking to the seasonal cycle is examined in three parameter regimes: the fast SST limit, the fast wave limit, and the mixed SST - wave dynamics regime. The seasonal cycle is imposed on simple ODE models for each physical regime as either a seasonal ocean-atmosphere coupling strength obtained from the CZ-model using assimilation of observed ENSO data, or as a climatological seasonal upwelling. In all three parameter regimes, the seasonal variations in the ocean-atmosphere coupling strength force the events to peak toward the end of the calendar year, whereas the effect of upwelling is shown to be less important. The phase locking mechanism in the mixed mode and fast SST regimes relies on the seasonal excitation of the Kelvin and the Rossby waves by wind stress anomalies in the central Pacific basin. The peak time of the events is set by the dynamics to allow a balance between the warming and cooling trends due to downwelling Kelvin and upwelling Rossby waves. This balance is obtained because the warming trend due to the large amplitude Kelvin waves, amplified by a weak winter-time coupled ocean-atmosphere instability, balances the cooling trend due to weak Rossby waves, amplified by a strong summer-time coupled instability. The difference between the locking mechanisms in the mixed mode regime and in the fast SST regime is used to understand the effect of the SST adjustment time on the timing of the phase locking. Within the simple models used here, treating ENSO as a noise-driven damped oscillation rather than a self sustained oscillation, results in a different phase locking mechanism that cannot account for ENSO's phase locking to the end of the year. Finally, in the less realistic fast wave regime, a different physical mechanism for ENSO's phase locking is revealed through the SST adjustment time and the interaction between the east Pacific region and the central Pacific region.
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12th Conference on Atmospheric and Oceanic Fluid Dynamics