On the termination of El Nino

A. feature of the end-phase of recent El Nino periods is thermocline shallo wing to normal depths, in the central and eastern equatorial Pacific, befor e sea surface temperature (SST) returns to normal. Also characteristic of E l Nino periods is a late-in-the-year shift of the El Nino equatorial wester ly wind anomalies, from symmetric about the equator to centered south of th e equator. We show that the directly forced ocean response to this shift pr oduces thermocline shallowing like that observed. The wind shift may result from the normal seasonal evolution of Pacific waters warmer than 28 degree s C. These shift from being relatively symmetric about the equator in Octob er-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 term ination.