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ENSO Update

14 January 2009

Summary

The equatorial Pacific is presently exhibiting La Niña conditions. The equatorial Pacific region shows mainly below-average sea surface temperatures (SSTs). There is a 55% probability of La Niña conditions persisting over the coming season, and 45-50% for return to ENSO-neutral conditions.

General Discussion

La Niña conditions returned in early December according to the eastern-central Pacific NINO3.4 SST index. Near average conditions had reigned in the tropical Pacific since early June, following the 2007/08 La Niña event. Atmospheric anomalies have suggested that the tropical Pacific should be cool (or cooling), as the Southern Oscillation index has been persistently positive, and the low-level equatorial winds have been persistently easterly since mid-2008. The oceanic response to these atmospheric conditions has finally appeared.

Out of a large set of dynamical and statistical forecast models, over half indicate weak La Niña during the coming Jan-Feb-Mar season, but nearly all are trending back towards neutral thereafter. Overall, based on model forecasts and current observations of the ocean surface and subsurface, the probability of La Niña conditions is estimated at 55%, of El Niño conditions near 0%, and the probability of maintaining ENSO-neutral conditions is 44% for the Jan-Feb-Mar season in progress.

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