The robustness of observed El Nino and La Nina lifecycles.

Seasonal climate prediction has largely been predicated on an understanding of the El Nino Southern Oscillation (ENSO). The lifecycle of an El Nino or La Nina event provides perhaps the strongest basis for short term climate prediction, but requires an understanding of the patterns of variability associated with such an event. Recent events and more advanced observational campaigns have raised questions about the utility of a canonical El Nino(La Nina) event, and the possibility that each event must be treated as a unique episode. Recent efforts to officially define El Nino and La Nina events have expanded these questions. We present updated and expanded El Nino and La Nina composites that utilize statistics and time series analyses to identify the robust phenomenology of El Nino and La Nina events. Through this work a robust lifecycle of tropical Pacific sea surface anomalies emerges that applies to even extreme events such as the 1997-98 El Nino. Robust anomalies in other oceans and outside the tropics are also identified. Substantial differences between the phenomenology of La Nina and El Nino events are found; these asymmetries are often overlooked through the use of correlation based analytical techniques. Many of the most robust signals are found in the eastern tropical Pacific, which is often overlooked in monitoring efforts focused farther to the west such as the NINO3.4 index. The patterns identified point to the utility of regarding El Nino and La Nina events as having separate distinct robust lifecycles, and the need to utilize these separate lifecycles in defining climate model skill.