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ENSO Update

19 September 2007

Summary

As of mid-September the cooling trend suggestive of La Niña development continues. Current equatorial sea surface temperatures (SSTs) are below average in most of the eastern tropical Pacific, particularly in the eastern one-third of the basin. If these conditions were to persist, they would be classified as La Niña. Based on the latest observations and forecasts, there is a 65% probability of maintaining La Niña conditions over the coming season.

General Discussion

Negative anomalies in the subsurface ocean structure have been observed in the east-central Pacific throughout most of 2007 (with a relative weakening in June). Stronger than average Trade Winds in the central Pacific have contributed to re-enforcing the negative subsurface anomalies during September. The mixed layer is therefore shallower than normal throughout the east-central equatorial Pacific, and this is evidenced as negative temperature anomalies at the thermocline depth along the equator. In addition, the region of below-normal SSTs has expanded towards the west, and now extends from the coast of South America to the dateline. The westward extent of these below average SSTs has varied month to month, and was relatively small in June, but on a longer time-scale has tended to increase gradually from March to the present - indicative of a developing La Niña. Weekly mean SST anomalies in the NINO3.4 region have been at borderline La Niña levels over the last several weeks. If these SST anomalies persist, or strengthen, it could consitute a La Niña event. Continuation of the generally enhanced Trade Winds will be important for further amplification of La Niña conditions in the coming months.

Out of a large set of dynamical and statistical forecast models, more models indicate weak to moderate La Niña conditions than ENSO-neutral conditions through the end of 2007. Dynamical models that incorporate information about sub-surface ocean conditions give a stonger indication of strengthening of 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 65% for the Sep-Oct-Nov season in progress, and through the remainder of 2007. The probability of El Niño conditions re-emerging during the forecast period is at or below 10% until Apr-May-Jun 2008, and the probability of returning to ENSO-neutral conditions is below 50% during 2007.

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

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