On the end of El Nino events

One of the noteworthy features of the return to eastern equatorial Pacific SST to normal at the end of El Niño events, is the extent to which it is generally linked ("phase locked") to the seasonal cycle. Warm El Niño eastern Pacific SST anomalies usually return towards climatology in the boreal spring following the onset of the El Niño event; however, the exact timing of the return of eastern Pacific SST to normal varies from event to event. Available data and model results indicate that easterly-wind-forced equatorial upwelling has dominated the initial return to cool conditions. To return SST to normal two conditions must occur: equatorial easterly wind-stress must be present with enough strength to drive upwelling, and the equatorial thermocline must be close enough to the surface to make cool water available to the shallow upwelling circulation. We provide a data and model based overview of the termination of moderate and strong El Niño events, emphasizing the roles of the seasonal cycle, the corresponding meridional shifts of westerly wind events and the shallow wind-forced upwelling circulation of the equatorial Pacific ocean. The processes that cause the eastern equatorial thermocline to shallow during Winter-Spring include 'delayed oscillator' type wave reflections and the seasonal shift of westerly wind anomalies south of the equator, following the waters of warmest SST and most active deep convection. In moderate El Niño events, such as 1991-2, easterlies are present over the eastern equatorial Pacific, so the eastern Pacific SST returns towards climatology as the thermocline shoals. In strong El Niño events, such as 1982-3 and 1997-8, the eastern equatorial zonal stress weakens and even becomes westerly, so even though the thermocline has shoaled SST remains warmer than normal for months. In the absence of large-scale equatorial easterlies, the Madden-Julian Oscillation may provide episodic easterly stress to return SST towards climatology.