Studies of Dynamics for El Nino-Southern Oscillation (ENSO) Diversity and Complexity

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El Nino refers to an abnormal warming of the ocean surface in the equatorial Pacific, while the Southern Oscillation is a seesaw pattern in sea level pressure in which abnormally low pressure on the eastern side of the Pacific is matched by high pressure on the western side of the basin and vice versa. As El Nino events occur in association with fluctuations of the Southern Oscillation, the combined phenomenon is referred to by the acronym ENSO. While ENSO is an equatorial Pacific phenomenon, its impacts are felt worldwide and include disruptions of the tropical monsoons, heavy rain and mudslides in California, and increases in the frequency of winter storms over the southeastern US. Given its worldwide impacts, ENSO has been a topic of intense scientific research over the past three decades, and skillful ENSO forecasts can now be made up to a few months in advance. However, the full range of ENSO behaviors has not yet been satisfactorily explained, and there is still much room for improvement in ENSO prediction.This project seeks to understand the diversity found in ENSO events, where the term "diversity" denotes the rich variability seen in the amplitude and frequency of events, as well as differences in the pattern of sea surface temperature (SST) anomalies that appear in different events. A key form of diversity is the existence of two distinct types of ENSO events. One is the conventional ENSO event in which sea surface temperature SST anomalies are concentrated in eastern equatorial Pacific. The other, which is referred to as "El Nino Modoki", "Dateline El Nino", "Central Pacific El Nino", and Warm Pool El Nino", has its largest SST imprint in the central equatorial Pacific, rather than the eastern side of the basin, and its impacts over the globe are quite different from those of the conventional El Nino. Research in this project would attempt to determine if the two types can be ascribed to two distinct dynamical modes, driven by different physical mechanisms, or whether the two are produced by a single dynamical mode with a single set of mechanisms which can produce either type depending on stochastic forcing which varies from event to event.Another factor contributing to diversity in ENSO behavior is the nonlinear interaction between the ENSO cycle and the annual cycle. Work performed here examines this interaction and its impact on ENSO diversity. Preliminary work suggests that the interaction creates ENSO-related variability at frequencies which correspond to the sum and difference of the ENSO frequency and the annual cycle. A third factor to be investigated is the extent to which diversity in ENSO behavior is generated by multiplicative noise forcing, in which the forcing of ENSO by stochastic noise (for instance, the effect of winds from westerly wind bursts) is dependent on the state of ENSO SST anomalies. Such dependence is expected to result in events of extreme amplitudes, and may explain why El Nino events are generally stronger than La Nina events (ENSO events of opposite sign).As noted above, the work has broader impacts due to the global effects of ENSO events and their consequences for human activity. ENSO diversity is an important issue for understanding and predicting ENSO events and their effects on weather and climate around the world, and the results of this work could contribute to improved ENSO prediction. In addition, the work will support and train two graduate students, thereby providing for the future workforce in this research area.