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ENSO Update

17 February 2011

Summary

Moderate to strong La Niña conditions have been observed since mid-August 2010, and have slightly weakened to moderate strength as of mid-February 2011. For the February-April season currently in progress, there is an approximate 94% probability for continuing La Niña conditions, and a 6% probability for returning to neutral ENSO conditions. Probabilities for La Niña conditions continue at 74% for the March-May season, and do not drop to below 50% until the April-June season.

General Discussion

In mid-February 2011, the NINO3.4 SST anomaly index continues at values indicative of moderate La Niña levels (1.2 degrees C below average). Zonal wind anomalies in the equatorial Pacific continue to feature stronger than average easterlies, and subsurface oceanic heat content continues to be below average, but to a somewhat lesser extent than during January. These observations indicate a high probability of the continuation of La Niña conditions for at least another one to three months.

Out of a large set of dynamical and statistical prediction models, 83% of them indicate maintenance of La Niña conditions during the upcoming March-May 2011 season. Overall, based on model predictions and current observations of the ocean surface and subsurface, for the February-April, March-May and April-June seasons the probability for La Niña conditions is approximately 94%, 74% and 49%, respectively. By the May-July season the probability for La Niña conditions is estimated at approximately 34%, decreasing to less than 30% for June-August and beyond. Probabilities for El Niño conditions are 10% or less through April-June, rising to the 20%-25% range from the June-August season onward.

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