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ENSO Quick
Look IRI ENSO Update / Forecast Technical ENSO Update IRI Probabilistic ENSO Forecast ENSO Prediction Plume Monthly archive ENSO Update20 September 2006SummaryThresholds for El Niño conditions, in terms of tropical Pacific SST anomalies, have been crossed recently. As of mid-September sea surface temperatures (SSTs) exceed 0.5C above average throughout much of the equatorial Pacific, meet the IRI's threshold of 0.7C for this time of year in the eastern-central Pacific, and are more than 1.0C above-average near the dateline. Note that the actual criteria for El Niño conditions includes the provision that the anomalies, especially in the eastern-central Pacific, remain at or above these levels for at least three months -- longer to be considered an El Niño event. Based on the latest observations and forecasts, the probability of developing an El Niño even during September-October-November 2006 season is approximately 55%.General DiscussionSince mid-August SSTs in the central and in the eastern equatorial Pacific have been increasing, with temperatures near the dateline reaching 1.0deg C above average. The increasing SSTs are due largely to persistent westerly wind anomalies over the west Pacific during July and August. Since early September, however, the westerly wind anomalies have subsided. The SSTs in the far eastern equatorial Pacific have varied between about 0.5-2.0C above average over the last several weeks, although the spatial structure of the anomalies has been changeable from week to week. The equatorial heat content is also above-average and has been slowly increasing over the last several months, relative to the 1970-2001 climatology period. The departures from normal of the sub-surface oceanic temperature field, associated with the increasing heat content, and resulting from the westerly wind anomalies, are relatively large-scale but not of large magnitude.Still, they have the potential to further increase SSTs as they are brought to the surface in the eastern Pacific by the local upwelling currents. These sub-surface anomalies will likely need further reinforcement from the wind if they are to contribute to development of an El Niño event in the eastern Pacific. The observations and models have been increasing in their concensus of El Niño development based on the set of forecasts over the last six months. They currently suggest that the development of El Niño conditions is more likely than not to occur by the end of the year. Out of a large set of dynamical and statistical forecast models, slightly more than half predict an El Niño will develop by the end of 2006. Overall, based on model forecasts and current observations of the ocean surface and subsurface, the probability of a La Niña is much less than the climatologically expected odds of 25%. The probability of developing El Niño conditions is 55-60%, and that of maintaining neutral conditions through the end of 2006 is approximately 40%. Note 1 - Only models that produce a new ENSO forecast every month are included in the above statement.
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