A global El Nino composite and the 1997–98 event

One of the challenges of assessing the impacts of El Niño events, in a world that experiences a variety of medium to large scale climate anomalies every season, is to develop ways of identifying which anomaly patterns are characteristic of El Niño events. We suggest that one strategy is first to determine the anomaly patterns that have been characteristic of many events and then to compare the anomalies of any particular El Niño period with these anomalies. We here examine the global SST, surface wind, and SLP anomalies of the 1997-98 El Niño, in terms of the statistically significant anomalies of a 10-event-average El Niño composite. Within the tropics, and particularly the tropical Pacific, many aspects of the 1997-98 event follow the expected patterns quite well; it appears that the composite may offer a very useful framework for thinking about the 'life-cycle' of El Niño events in these regions. Futher, coupled models should be able to reproduce the composite life-cycle if they are to be considered realistic in their ENSO variability. Outside of the tropics there were major anomalies in 1997-98 that had no counterparts in the composite patterns. We briefly summarize those that did and did not exhibit composite-type behavior.