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ENSO Update

19 March 2009

Summary

The equatorial Pacific has exhibited weak but clear La Niña conditions since December 2008. La Niña conditions continue, but are weakening toward the borderline of neutral conditions as of mid-March. There is an approximately 50% probability of La Niña conditions persisting during the March-May season, decreasing to 25-30% for the May-July season as near-neutral conditions become most likely.

General Discussion

La Niña conditions returned in early December according to the eastern-central Pacific NINO3.4 SST index. Near average conditions had prevailed in the tropical Pacific since early June, following the 2007/08 La Niña event. Atmospheric anomalies had suggested that the tropical Pacific should have been cooling since late August 2008, when the Southern Oscillation index became persistently positive and low-level equatorial winds became persistently easterly. However, the oceanic response to these atmospheric conditions lagged by three to four months.

Out of a large set of dynamical and statistical forecast models, close to half indicate weak La Niña during the coming Mar-Apr-May season in progress, and most are trending towards neutral conditions by May or June. Overall, based on model forecasts and current observations of the ocean surface and subsurface, the probability of La Niña conditions is estimated at 50%, of El Niño conditions near 0%, and the probability of maintaining ENSO-neutral conditions is about 50% for the Mar-Apr-May season in progress. Beginning in Jun-Jul-Aug, no tilt away from the climatological odds of 25-50-25% is currently indicated.

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