Why Does El Niño Tend to Peak in Boreal Winter: I. The Role of the Ocean-Atmosphere Coupling Strength

This study investigates the seasonal phase-locking of El Niño peaks using the model ofSun (1997), https://doi.org/10.1029/97GL01960 that well simulates various characteristics of El Niño–Southern Oscillation (ENSO). Our findings reveal that when the dynamical coupling strength between the atmosphere and ocean in the model varies seasonally, as observed, El Niño events tend to peak during boreal winter. The extent of this phase-locking is influenced by the amplitude of the coupling strength seasonal cycle through the nonlinear frequency locking. In observations, El Niño peaks in winter because the coupling strength is highest in October and lowest in April. The time lag between the peak coupling strength and the peak El Niño magnitude is related to the amplitude of the seasonal cycle in the coupling strength. Furthermore, we found the higher coupling strength in the seasonal cycle may correspond to a higher ENSO growth rate by calculating the seasonal cycle of the model equilibriums.