The Annual Cycle and El Niño Termination

Abstract

Understanding the processes that control the termination of El Niño is essential to representing them properly in dynamical prediction schemes, and to understand how the duration of events may change in changing climate. The processes that set the timing of the end of El Niño events are explored using a combination of data analysis, and forced ocean, atmosphere and coupled general circulation model experiments. It is found that meridional changes to atmospheric anomaly field in both the western Pacific and the eastern Pacific are fundamental to the timing El Niño termination. The near-Dateline changes drive the eastern equatorial Pacific thermocline shoaling which preconditions El Niño terminations; the far eastern equatorial Pacific changes result in the peculiar termination of extreme El Niño events (like 1982-3 and 1997-8). These changes in the meridional structure of atmospheric anomalies result from interactions between the annual cycle of insolation and anomalous El Niño conditions.

The termination of recent El Ninño events has been characterized by (1) a late-year southward shift of near-Dateline westerly wind anomalies, and (2) subsequent cold tongue thermocline shallowing is driven by the wind shift. OGCM experiments establish the shift as the primary cause of cold tongue thermocline shallowing. Reflected equatorial waves and local wind anomaly changes are much less important in setting the timing. Analysis of the GFDL CGCM shows that the late-year southward shift of zonal wind anomalies is also a characteristic feature of the termination of El Niño in the coupled model. Perturbation experiments with the CGCM indicate that interactions between the seasonal cycle of solar insolation and anomalous El Niño conditions are a dominant mechanism for the thermocline shoaling at the end of El Niño events.

The two recent extreme El Niño events (1982-3 and 1997-8) have exhibited a peculiar termination: warm SST anomalies persisted in the eastern Pacific, in spite of significant thermocline shoaling begining in boreal winter at the height of the events; and then there was a sudden cooling of eastern equatorial SST in later boreal-spring. It is found that the unusual extension (and sudden termination) of these extreme El Niño events is associated with the development (and subsequent nothward retreat) of an equatorial ITCZ, which decoupled the cooling subsurface ocean from the warm surface. Atmospheric and coupled GCM experiments indicate that the equatorial ITCZ - and thus unusual termination of extreme El Niño events - results from the interaction of extreme SSTA and the annual cycle of insolation.

Succesful theories and models of El Niño should either represent or parametrize the processes that cause the meridional changes in the atmospheric anomaly field, since these processes are fundamental to the termination of El Niño.