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ENSO Update

15 April 2009

Summary

The equatorial Pacific has returned to ENSO-neutral this month, after exhibiting weak La Niña conditions since December 2008. During the Apr-Jun season there is an approximately 75% probability of maintaining ENSO-neutral conditions, and that is predicted to be the most likely situation through the end of 2009 and into early 2010. The likelihood of returning to La Niña conditions remains below 25% probability.

General Discussion

La Niña conditions that were evident since early December have been weakening since early 2009. ENSO-neutral conditions, as indexed by the NINO3.4 SST anomaly index, have been observed since the beginning of April 2009. The atmospheric anomalies still show some characteristics of La Nina, such as in the equatorial Southern Oscillation index and the strengthened low-level equatorial winds. However, based on the ocean's current state, a return to cooler than average sea temperatures, would require persistence of those atmospheric anomalies.

Out of a large set of dynamical and statistical forecast models, two-thirds indicate maintenance of ENSO-neutral conditions during the Apr-May-Jun season in progress. Overall, based on model forecasts and current observations of the ocean surface and subsurface, the probability of ENSO-neutral conditions is estimated at 75%, of El Niño conditions about 2%, and the probability of returning to La Niña conditions is about 23% for the Apr-May-Jun season in progress.

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