ENSO Update  15 March 2002

The International Research Institute for Climate Prediction (IRI) issues monthly summaries about the current state of ENSO, and the consensus of models that forecast ENSO developments for the coming 6 or more months.

Summary

The overall condition of the tropical Pacific is neutral but showing movement toward El Niño conditions. Above average sea surface temperatures continue in the central tropical Pacific, and have now also appeared in the far eastern tropical Pacific. While there is an enhanced likelihood of an El Niño due to this overall condition, uncertainty still exists as to whether a full-fledged, basin-wide El Niño will develop in the next few months. This uncertainty is expected to diminish over the next 2 months. Forecast models presently suggest that if an El Niño does develop in the coming months it is likely that it would not be among the very strongest El Niños. If an El Niño should develop, past events suggest it would continue for at least the remainder of the year and likely through March of 2003.

General Discussion

Current observations indicate near-neutral ENSO conditions, although sea surface temperatures (SSTs) are currently slightly above average over much of the tropical Pacific. Observations of ocean conditions over the past few months, however, suggest that a shift towards warmer conditions is occurring. In the central equatorial Pacific, the higher than average SSTs seen over the past few months continued through February. In addition, warmer than average water emerged in early February in the eastern tropical Pacific, causing heavy rainfall in some coastal locations in Ecuador and northern Peru. While locally significant, such developments along coastal South America should be distinguished from those expected in association with a Pacific basin-wide El Niño, which, once developed, can alter seasonal climate patterns around many regions of the globe. Presently, the atmosphere has not yet begun to demonstrate such El Niño-like behavior. Ocean conditions seen so far in 2002 have been observed prior to the onset of many past El Niño events. However, based on historical observations and the performance of computer models at this time of the year, it is not certain that the warmer than average water in the central and far eastern Pacific will be sufficient to lead to a basin-wide El Niño. Thus, while the current ocean conditions exhibit the necessary early conditions for El Niño, there is still some uncertainty as to whether El Niño will develop in the next few months. The evolution of the system during the remainder of March and April is expected to lead to greater certainty in the forecasts by the end of April.

The above interpretation is reflected in the differences among forecast models as to whether an El Niño will occur. These differences are consistent with the fact that the forecast models are known to have only moderate skill in predicting the onset of El Niño events during the current time of year (February through April). With this in mind, depending on the type of forecast model, between 45% and 55% predict El Niño conditions developing by the middle of 2002. All of the remaining models call for a continuation of near-neutral conditions throughout 2002; none indicate the development of La Niña conditions. (Only models that produce a new ENSO forecast every month are included in the above statements.) Overall, the models indicate an enhanced likelihood that the temperatures in the central and eastern tropical Pacific will reach the level of a minimal El Niño by June or July of 2002.

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