Warm El Nino/Southern Oscillation (ENSO) events are climatic fluctuations in which sea surface temperature (SST) along the central-eastern equatorial Pacific is relatively warm, accompanied locally by lower sea level pressure (SLP), with higher SLP on the western side of the basin. Cold ENSO events have the same pattern but with the SST and SLP anomalies reversed, so that SST is colder and SLP is higher along the central-eastern equatorial Pacific.
Classical theories view ENSO events as the result of an instability of the coupled atmosphere-ocean system, in which the atmospheric circulation anomalies associated with the SLP anomalies reinforce the oceanic anomalies associated with the SST change and vice versa, thereby enabling a weak initial perturbation to grow into a fully-developed ENSO event. Early studies demonstrated that instability theory can account for ENSO-like behavior in highly simplified models of the equatorial Pacific ocean-atmosphere system. More recent research using sophisticated models and observational analysis suggests that various forms of precursors and triggers, some external to the equatorial Pacific, can play a key role in ENSO initiation. But while precursors and triggers can be identified for some ENSO events, other events appear to initiate without them. There are also instances in which conditions appear favorable for an ENSO event but initiation does not occur.
This research seeks to determine the essential elements for ENSO initiation through model experiments in which ENSO events are initiated starting from an atmosphere-ocean state which contains virtually no ENSO activity. The ENSO-free initial state is generated through a spin-up procedure in which an atmosphere-ocean general circulation model (GCM) is modified so that the ocean component model does not feel the effect of atmospheric circulation anomalies (more precisely, surface wind stress anomalies) over the equatorial Pacific. Out of 60 simulations performed in preliminary work for the project, 10 developed strong warm ENSO events and 13 developed strong cold events. Work performed under this award analyzes these experiments and conducts additional experiments to determine why some simulations produced warm events while others developed cold events or near-neutral conditions.
The work has broader societal impacts due to the variety of ways in which ENSO events affect weather and climate over the US and other parts of the world. A better understanding of the key factors in ENSO initiation may lead to better ENSO predictions, and in particular a reduction in the occurrence of "false alarms" in which an ENSO event is predicted but does not materialize. In addition, the project supports a female graduate student, thereby developing the future workforce in this research area, and enhancing the participation of an underrepresented group.
