How Much Skill Was There in Forecasting the Very Strong 1997–98 El Niño?

The very strong 1997–98 El Niño caused dramatic worldwide changes to weather patterns, such as drought in Indonesia, extreme rains in Peru and Ecuador, and a shutdown of the Atlantic hurricane season (Bell and Halpert 1998). While past El Niño events have also produced similar though not as large effects, this was the first El Niño in which national meteorological centers made accurate forecasts of its impacts several months in advance (Barnston et al. 1999a,b). One issue that remains unresolved is how much skill did the various El Niño–Southern Oscillation (ENSO) forecast methodologies have for the event itself at various lead times. This issue was originally addressed in Barnston et al. (1999a), which analyzed the eight three-monthly forecast times from June 1996 to March 1998 for the operationally available statistical and dynamical models. They found that some of the models performed worse than a persistence control forecast, while most of the models performed quite well relative to persistence for the duration of the 1997–98 El Niño event. Both statistical and dynamical models were found in each categorization of performance. However, three key issues require a revisitation of this topic. The first is that an explicit analysis of the models’ performance is needed for the onset and decay of the event. Second, stratification of the various forecasts would allow for analysis of how skill changes with increased lead time. Finally, the error analysis should be put into the context of evaluating the available schemes with respect to a common “no-skill” threshold that is more challenging than simple persistence alone: the El Niño– How Much Skill Was There in Forecasting the Very Strong 1997–98 El Niño?