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 s trong interannual variability during the 1980s and the period of more persistent features beginning in 1990. Three versions of the NCEP coup led forecast model, which were developed to predict interannual sea su rface temperature variability in the equatorial Pacific, are described and their performance compared for those two periods. Climatic variab ility during 1982-1992 in the tropical Pacific was dominated by strong low-frequency interannual variations characterized by three warm and two cold El Nino episodes. However, beginning in 1990, the climate sta te has been characterized by a pattern of persistent positive SST anom alies in the tropical Pacific, especially in the central Pacific near the dare line, and weaker than normal trade winds. Superimposed on thi s were several occurrences of short-lived, generally small-amplitude w armings in the eastern equatorial Pacific. Some of the short-lived war mings amplified into mature warm episodes, such as in spring 1993 and in late 1994. The NCEP coupled models showed useful skill in predictin g low-frequency SST variability associated with warm episodes in the t ropical Pacific during the 1982-1992 period. However, the short-lived warmings in spring 1993 and fall/winter 1994/95 were not well predicte d by the NCEP coupled models. Neither were they predicted by most of t he other dynamic or statistical forecast models. Lf these short-lived warmings truly represent a different behavior of the coupled ocean-atm osphere 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 forec asts of short-lived episodes such as those observed in recent years wi ll no doubt be only a few months.