ENSO’s Phase Locking to the Seasonal Cycle in the Fast-SST, Fast-Wave, and Mixed-Mode Regimes

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Abstract
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 ordinary differential equation models for each physical regime either as a seasonal ocean–atmosphere coupling strength obtained from the model of Zebiak and Cane 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 Rossb... Abstract 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 ordinary differential equation models for each physical regime either as a seasonal ocean–atmosphere coupling strength obtained from the model of Zebiak and Cane 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 Rossb...

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