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ENSO Update

17 November 2011

Summary

Weak La Niña conditions re-emerged in August after a brief period of ENSO-neutral conditions following the ending of the significant 2010-11 La Niña. The current event has slowly strengthened and is currently of weak to moderate strength. For the November-January season currently in progress, there is an approximately 74% probability for continuing La Niña conditions, a 26% probability for returning to neutral conditions, and virtually no chance for the development of El Niño conditions. Weak to moderate La Niña conditions are the most likely scenario for the remainder of 2011, into the first couple of months of 2012.

General Discussion

In mid-November 2011, the NINO3.4 SST anomaly index was at -0.9 C from average, indicative of weak La Niña conditions. Sea temperatures below the ocean surface remain below average, and zonal wind anomalies in the equatorial Pacific feature a somewhat enhanced strength of easterlies, although recently there was a notable interruption due to intraseasonal variability. The Southern Oscillation Index is slightly above average, at a borderline to weak La Niña level. These observations indicate weak to nearly moderate La Niña conditions, as has been the case since early October.

Out of a large set of dynamical and statistical prediction models, 67% of them indicate La Niña conditions during the November-January 2011 season currently in progress, while 33% indicate neutral ENSO conditions. Based on model predictions and current observations of the ocean surface and subsurface, for the November-January season the probability for La Niña conditions is approximately 74%, while the probability for neutral conditions is 26% and the probability for El Niño development is near zero. For the January-February-March season 2012, the probabilities for La Niña, neutral and El Niño conditions are near 62%, 35% and 3%, respectively.

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