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ENSO Update

14 November 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. Those conditions developed over the last couple months to the point of moderately strong La Niña conditions that are observed presently. The sea surface temperature anomalies are now below -2.5C in parts of the eastern 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. The anomalous features of the coupled ocean-atmosphere system must still persist for another couple months for a La Niña event to be declared for this year. This seems highly probably as the Trade Winds have been particularly strong and persistent across the central equatorial Pacific for the past month. This should provide the needed reinforcement to the ocean anomalies to allow the La Niña conditions to continue at least through the end of the year.

Out of a large set of dynamical and statistical forecast models, most models indicate moderately strong La Niña conditions through early 2008. Most 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 98% for the Nov-Dec-Jan 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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