On the termination of El Nino

A feature of the end-phase of recent El Nino periods is thermocline shallowing to normal depths, in the central and eastern equatorial Pacific, before sea surface temperature (SST) returns to normal. Also characteristic of El Nino periods is a late-in-the-year shift of the El Nino equatorial westerly wind anomalies, from symmetric about the equator to centered south of the equator. We show that the directly forced ocean response to this shift produces thermocline shallowing like that observed. The wind shift may result from the normal seasonal evolution of Pacific waters warmer than 28°C. These shift from being relatively symmetric about the equator in October-November, to concentrated south of the equator in December-January. Thus, the seasonal cycle may be a central part of the mechanism of El Nino termination.