Abstract
Abstract The mature phases of El Niño events show a strong tendency of locking to the end of the calendar year. The roles of seasonal variations of the basic state and the relative contributions of individual components of the basic state in this phase locking are investigated using the Zebiak–Cane model. It is shown that seasonal variations of the mean state from July to November have a positive contribution and those from December to June have a negative contribution to preexisting warm SST anomalies. Among the basic-state parameters, the sea surface temperature (SST) is a major factor for the locking of the El Niño mature phase through the anomalous advection of the mean SST gradient. This result differs from previous studies that attribute the El Niño phase locking mainly to seasonal changes in the mean wind divergence. The present result indicates the importance of a proper simulation of mean SST and its seasonal evolution for the simulation of El Niño phase locking in coupled models. Further experiments show that the phase locking of the El Niño mature phase cannot be explained by a balance between the warming trend due to downwelling Kelvin waves and the cooling trend due to upwelling Rossby waves.
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