ENSO UPDATE 17 July 2002

Summary

The IRI's assessment is that there is an approximately 90% probability of El Niño conditions during the remainder of 2002 and into early 2003—in other words, it is highly likely that current conditions represent the early phase of an El Niño that will continue into early 2003. This assessment is based on the collective forecasts of many computer models of various types, as well as on the experience of the several oceanographers and atmospheric scientists familiar with the El Niño phenomenon. Compared with the statement from one month ago, this probability has increased by 15%, since substantial warm SST anomalies have continued since late May and are considered likely to continue for the next 6-9 months. If these SST conditions do continue as expected, the most likely strength of the El Niño will be approximately one-third to one-half of the strength of the 1997-98 El Niño, and climate impacts will therefore be comparably weaker than those associated with the 1997-98 El Niño. During the period from mid-June to mid-July, sea surface temperatures (SSTs) in the central and eastern tropical Pacific remained near one degree C above average, which is high enough to represent an El Niño episode if continued over a sufficient number of months. (Note 1)  The atmosphere has also maintained features suggestive of El Niño, such as westerly wind anomalies in the western and central Pacific and a somewhat below normal Southern Oscillation Index. However, a persistent pattern of eastward-shifted convection characteristic of El Niño episodes has not yet commenced. This missing feature has often been lacking in the early stages of past El Niño episodes.

Note 1 - The SST averaged over the April through June period has already satisfied the NOAA/National Weather Service’s requirement for this to be called an El Niño, even if it does not continue into the remainder of this calendar year. In this assessment, we are assuming a definition requiring a longer-lasting episode (e.g., 6 or more months).
 

General Discussion

Between mid-June and mid-July, sea surface temperatures (SSTs) in the east-central equatorial Pacific remained close to 1 degree C above normal, decreasing only very slightly from a peak in early June. This warmth has included the NINO4, NINO3.4, and western portion of the NINO3 region, and resulted from spells of westerly wind anomalies in the western and central tropical Pacific. The atmosphere has been showing signs of El Niño-like behavior also, such as a lower than average Southern Oscillation Index. However, anomalous convection across much of the central and eastern equatorial Pacific has not yet been observed, as would be expected in a more fully developed El Niño episode. This may be a result of the persistence of above normal SST in the western half of the Pacific, which lessens the west-to-east SST gradient, which would promote convection.

We are likely in the early stage of a developing El Niño episode. This assessment is reflected in the SST forecasts of a large set of dynamical and statistical forecast models. Moderately high skill is expected for the model forecasts at this time of the year. Depending on the type of forecast model, between 65% and 80% of the models predict El Niño conditions persisting into the September-October-November period and continuing through Northern Hemisphere winter. All of the remaining models call for near-neutral, but slightly above average SST throughout 2002. (Note 2)  Thus, overall, the models suggest a much enhanced likelihood, relative to an average year, of at least minimal El Niño conditions throughout the second half of 2002 and into early 2003.

Note 2 - Only models that produce a new ENSO forecast every month are included in the above statement.

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