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ENSO Update

19 December 2007

Summary

La Niña conditions continue in the central and eastern equatorial Pacific. Below-average sea surface temperatures (SSTs) exist from the west coast of South America to west of the dateline. Based on the latest observations and forecasts, there is a 97% probability of maintaining La Niña conditions over the coming season.

General Discussion

La Niña conditions, indicated by substantially below-average sea surface temperatures in the central and eastern equatorial Pacific, enhanced Trade Winds, and reduced equatorial heat content, have been in place since early August. Moderately strong La Niña conditions exist presently. The sea surface temperature anomalies are below -2 C throughout much of the eastern-central equatorial Pacific. The low-level wind anomalies are the greatest since those accompanying the strong and prolonged La Niña event that followed the 1997-98 El Niño. Persistence of La Niña conditions is highly likely through at least the next couple months as the Trade Winds have been particularly strong and persistent across the western-central equatorial Pacific over the past several months. This should provide the needed reinforcement to the ocean anomalies to allow the La Niña conditions to continue at least through early 2008.

Out of a large set of dynamical and statistical forecast models, most models indicate moderately strong La Niña conditions through early 2008. Some dynamical models that incorporate information about sub-surface ocean conditions yield stronger La Niña conditions. Overall, based on model forecasts and current observations of the ocean surface and subsurface, the probability of a La Niña is estimated at 97% for the Dec-Jan-Feb season in progress. The probability of El Niño conditions re-emerging is at or below 10%, and the probability of returning to ENSO-neutral conditions is much below 50%, until northern hemisphere spring, when ENSO conditions typically transition into or through ENSO-neutral conditions.

Note 1 - Only models that produce a new ENSO forecast every month are included in the above statement.

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