How much skill was there in forecasting the very strong 1997-98 El Nino?

The very strong 1997-98 El Nino was the first major event in which numerous forecasting groups participated in its real-time prediction. A previously developed simple statistical tool-the El Nino-Southern Oscillation Climatol ogy and Persistence (ENSO-CLIPER) model-is utilized as a baseline for deter mination of skill in forecasting this event. Twelve statistical and dynamic al models were available in real time for evaluation. Some of the models we re able to outperform ENSO-CLIPER in predicting either the onset or the dec ay of the 1997-98 El Nino, but none were successful at both for a medium-ra nge two season (6-8 months) lead time. There were no models, including ENSO -CLIPER, able to anticipate even one-half of the actual amplitude of the El Nino's peak at medium-range (6-11 months) lead. In addition, none of the m odels showed skill (i.e., lower root-mean-square error than ENSO-CLIPER) at the zero season (0-2 months) through the two season (6-8 months) lead time s. No dynamical model and only two of the statistical models [the canonical correlation analysis (CCA) and the constructed analog (ANALOG)] outperform ed ENSO-CLIPER by more than 5% of the root-mean-square error at the three s eason (9-11 months) and four season (12-14 months) lead time. El Nino impac ts were correctly anticipated by national meteorological centers one half-y ear in advance, because of the tendency for El Nino events to persist into and peak during the boreal winter. Despite this, the zero to two season (0- 8 month) forecasts of the El Nino event itself were no better than ENSO-CLI PER and were, in that sense, not skillful-a conclusion that remains unclear to the general meteorological and oceanographic communities.