ENSO UPDATE 16 May 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

During the 4-week period from mid-April to mid-May, sea surface temperatures in the eastern tropical Pacific have cooled slightly to become just weakly above average. The atmosphere has continued to lack the large-scale behavior characteristic of developing El Niño events. While generally above average sea surface temperatures still stretch from the coast of South America westward to beyond the dateline, there are now large portions in the eastern half of the basin having only weak departures from the average. Ocean temperatures have cooled considerably from their abnormally warm levels near the immediate coasts of Peru and Ecuador. 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 55% probability of an El Niño by middle to late 2002, lasting 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 decreased to a moderate level, since movement toward El Nino conditions has not continued and the seasonal window of opportunity (February to May) is beginning to close. Compared to one month ago, the range of likely scenarios for tropical Pacific sea surface temperatures has narrowed somewhat. It now appears that the most likely outcome will be either a weak El Niño or a near-neutral condition during the remainder of 2002. If an El Niño develops this northern summer, it would likely be weak, and past events suggest it would continue for at least the remainder of the year and likely through March of 2003.

General Discussion

For the month of April, near to slightly above normal sea surface temperature (SST) was observed, overall, in the eastern and central equatorial Pacific. SSTs averaged close to or greater than one-half degree C above average from about 150 west longitude to west of the date line, and also in the far eastern Pacific from the coast of Peru and Ecuador westward to about 110 west longitude. In between these regions, which includes the Niño 3 region, sea surface temperatures averaged close to or only slightly above normal. While temperatures rose to at least one-half degree above normal across the entire equatorial Pacific during the second week of April, they have cooled somewhat since then. Now, in mid-May, temperature departures from average are weaker, overall, than those of the April average. In mid-May, locally high temperatures that caused unusually heavy precipitation along the west coasts of Ecuador and northern Peru have weakened considerably. Near the dateline, SST anomalies have slightly weakened but are still more than one-half degree above normal. Ocean conditions seen from January through March of 2002 have been observed prior to the onset of many past El Niño events, but most of these cases had developed coupled ocean-atmosphere interactions leading to clearer El Niño development by mid-May than we see this year. The current SST state is not conducive to the initiation of El Niño behavior in the atmosphere, although this is still possible. The main seasonal window of opportunity from February to May is beginning to close. The evolution of the system during the remainder of May and June may lead to greater certainty in the forecasts by the end of June.

The above assessment is reflected in the differences among forecast models as to whether an El Niño episode will develop. While only moderate skill is expected for the model forecasts between February and April, differences among the forecasts made in May are due to the recently observed borderline condition of the ocean-atmosphere system, in which a definite path toward an El Niño or toward a neutral condition still has not been determined. The expected skill this month is greater than it was in April, and it will be still greater next month. With this in mind, depending on the type of forecast model, between 60% and 70% of these models predict El Niño conditions existing by the July-August-September period. All of the remaining models call for a continuation of near-neutral conditions throughout 2002; none indicates 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 are still suggesting an enhanced likelihood, relative to an average year, that the temperatures in the central and eastern tropical Pacific will reach the levels associated with at least a minimal El Niño by the end of northern summer of 2002.

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