The mechanism of the locking of the El Nino event onset phase to boreal spring (from April to June)in an intermediate coupled ocean-atmosphere model is investigated. The results show that the seasonal variation of the zonal wind anomaly over the equatorial Pacific associated with the seasonal variation of the ITCZ is the mechanism of the locking in the model. From January to March of the El Nino year, the western wind anomaly over the western equatorial Pacific can excite the downwelling Kelvin wave that propagates eastward to the eastern and middle Pacific by April to June. From April to December of the year before the El Nino year, the eastern wind anomaly over the equatorial Pacific forces the downwelling Rossby waves that modulate the ENSO cycle. The modulation and the reflection at the western boundary modulate the time of the transition from the cool to the warm phase to September of the year before the El Nino year and cause the strongest downwelling Kelvin wave from the reflected Rossby waves at the western boundary to arrive in the middle and eastern equatorial Pacific by April to June of the El Nino year. The superposition of these two kinds of downwelling Kelvin waves causes the El Nino event to tend to occur from April to June.
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