ENSO UPDATE 17 June 2002

Summary

During the 4-week period from mid-May to mid-June, sea surface temperatures in the central and eastern tropical Pacific warmed from weakly above average to levels that, if continued for several months, are high enough to represent El Niño ocean conditions. The atmosphere also has developed features suggestive of El Niño, such as reduced trade winds and a below normal Southern Oscillation Index. However, a pattern of anomalous convection characteristic of El Niño episodes is still lacking. There continues to be an enhanced likelihood of an El Niño in 2002 relative to an average year. The IRI's assessment is that there is an approximately 75% probability of an El Niño lasting from the middle of 2002 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 20%, since movement toward El Niño conditions has progressed. Compared to one month ago, the range of likely scenarios for tropical Pacific sea surface temperatures has continued to narrow. The most likely outcome is a weak El Niño, and the second most likely is a slightly warm but near-neutral condition during the remainder of 2002.  The chances of a moderate El Nino are not high, and chances of a strong one are tiny. If a weak El Niño continues through this northern summer, past events suggest it would continue for at least the remainder of the year and likely through March of 2003.

General Discussion

Between the third week of May and mid-June, sea surface temperatures (SSTs) in the east-central equatorial Pacific rose from less than one-half degree to more than 1 degree C above normal. This warming, which included all of the NINO3.4 region and much of the NINO3 region, was a result of a substantial, nearly basin-wide weakening of the easterly trades in late May and early June, largely in association with the Madden-Julian Oscillation (MJO). By mid-June, equatorial SSTs were at least one degree above normal from 160 east to 115 west longitude. Above normal SST also continued in the western tropical Pacific, as well as in much of the Indian Ocean. The atmosphere has now begun showing signs of El Niño-like behavior as well, as the trades have weakened and the Southern Oscillation Index has decreased to somewhat below normal values. However, a significant absence of anomalous convection across much of the central and eastern equatorial Pacific signals a lack of complete coupling of the ocean and atmosphere expected in a multi-season El Niño episode. This may be a result of the fact that the SST in the western Pacific and also those more than 10 degrees off the equator at some east-central Pacific longitudes are also significantly above normal—i.e. a gradient with respect to the equatorial SST anomalies is not sufficient to be conducive to low-level wind convergence, which would immediately result in anomalous convection. By mid-June, the weaker than normal trades recovered toward normal in part of the basin, but remained weak in a small part of the NINO3.4 region

The above assessment is reflected in continuing differences among forecast models as to whether the current El Niño SST state will continue for at least the several months needed to attain the status of an El Niño episode. Depending on the type of forecast model, between 50% and 60% of these models predict El Niño conditions existing by the August-September-October period. All of the remaining models call for a continuation of near-neutral conditions throughout 2002, and most of these indicate slightly above normal temperatures; none indicates the development of La Niña conditions. (Only models that produce a new ENSO forecast every month are included in the above statements.) Later in 2002, the proportion of the models indicating at least minimal El Niño conditions increases further. Thus, overall, the models continue to suggest an enhanced likelihood, relative to an average year, of at least minimal El Niño conditions throughout the second half of 2002.
 

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