Coupled Model Predictions of ENSO during the 1980s and the 1990s at the National Centers for Environmental Prediction

In this paper, the authors discuss observed climatic variability from 1982 to early 1995 and emphasize the contrasts between the period of strong interannual variability during the 1980s and the period of more persistent features beginning in 1990. Three versions of the NCEP coupled forecast model, which were developed to predict interannual sea surface temperature variability in the equatorial Pacific, are described and their performance compared for those two periods. Climatic variability during 1982–1992 in the tropical Pacific was dominated by strong low-frequency interannual variations characterized by three warm and two cold El Niño episodes. However, beginning in 1990, the climate state has been characterized by a pattern of persistent positive SST anomalies in the tropical Pacific, especially in the central Pacific near the date line, and weaker than normal trade winds. Superimposed on this were several occurrences of short-lived, generally small-amplitude warmings in the eastern equatorial Pacific. Some of the short-lived warmings amplified into mature warm episodes, such as in spring 1993 and in late 1994. The NCEP coupled models showed useful skill in predicting low-frequency SST variability associated with warm episodes in the tropical Pacific during the 1982–1992 period. However, the short-lived warmings in spring 1993 and fall/winter 1994/95 were not well predicted by the NCEP coupled models. Neither were they predicted by most of the other dynamic or statistical forecast models. If these short-lived warmings truly represent a different behavior of the coupled ocean–atmosphere system on intraseasonal timescales, the skill levels that were developed for predicting the strong low-frequency SST variability of the 1980s are probably not relevant. The lead times for skillful forecasts of short-lived episodes such as those observed in recent years will no doubt be only a few months.