Understanding the Dynamics of El Nino-Southern Oscillation (ENSO) Complexity

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El Nino/Southern Oscillation (ENSO) events occur when sea surface temperature (SST) warms (El Nino) or cools (La Nina) along the equator in the central and eastern Pacific (CP and EP, respectively). These events have worldwide consequences from disruptions in tropical monsoons to changes in the severity of winter weather over the United States and Canada. The interplay of ocean currents, surface winds, and precipitation leading to ENSO events has been intensively studied, and the underlying dynamical mechanisms have been identified. But important questions remain, particularly regarding event-to-event differences in ENSO behavior that compromise prediction efforts. The term "ENSO complexity" has been coined to refer to differences commonly observed between individual events. Past work on the topic has emphasized differences in the SST pattern at the peak of the ENSO event, in particular the location of the maximum SST anomaly in either the CP or EP. Work here complements this analysis by looking at systematic differences in event transition, seeking to explain why some warm events are immediately followed by cold events and vice versa while others, particularly cold events, linger for multiple years.Preliminary research by the Principal Investigator (PI) identifies distinct onset mechanisms for ENSO oscillation between warm and cold events and for persistent events of the same sign. In the charge-discharge (CD) mechanism, warm water accumulates in the upper ocean over the equatorial Pacific prior to the onset of a warm event and is subsequently discharged to higher latitudes. This mechanism leads to oscillations in which warm events are followed directly by cold events. In the seasonal footprinting (SF) mechanism, wintertime weather anomalies in the middle latitudes cause ocean temperature changes which in turn produce changes in the surface winds. The surface wind changes propagate into the tropical Pacific and result in ENSO events, and the PI's work links this mechanism to multi-year cold events. Further work suggests that changes in ENSO behavior since 1980 are associated with enhancement of the SF mechanism, and that climate models overrepresent the CD mechanism and thus produce ENSO events which are too regular.This project will address five related goals: 1) understanding the combined effects on ENSO complexity of the CD and SF onset mechanisms and subsequent ENSO evolution; 2) understanding the cause of the asymmetry between the positive and negative phases of the SF mechanism; 3) understanding the mean state sensitivity and slow variations in the CD and SF mechanisms; 4) using ENSO complexity dynamics to understand future ENSO projections and to develop new metrics for model intercomparisons; and 5) using ENSO complexity dynamics to understand ENSO activity in paleoclimate datasets. The work is performed through analysis of observational datasets and simulations using climate models in a variety of configurations. Work addressing goal 5 uses data from the Last Millennium Reanalysis to examine differences in ENSO complexity between the Medieval Climate Anomaly of the period 950 to 1250 and the Little Ice Age period (1300 to 1850).This research has societal relevance due to the worldwide impacts of ENSO events and the desirability of skillful forecasts of them to allow for appropriate preparations. The work is directly relevant to forecasting given the focus on warm-to-cold transition versus multi-year persistence, a central challenge of ENSO prediction. The finding that the SF mechanism has become more prominent in recent decades also has implications for anticipating future changes in ENSO behavior. The project would support and train a postdoctoral researcher as well as a graduate student and a graduate student, thus providing for the future workforce in this research area.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.