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ENSO Quick
Look IRI ENSO Update / Forecast Technical ENSO Update IRI Probabilistic ENSO Forecast ENSO Prediction Plume Monthly archive ENSO Update18 October 2006SummaryAs of mid-October sea surface temperatures (SSTs) are near 1.0C above average throughout much of the equatorial Pacific, and exceed 2.0C in the eastern equatorial Pacific. The IRI threshold for El Niño conditions was exceeded during the course of September. This threshold is defined in terms of tropical Pacific SST anomalies in the NINO3.4 box (170W-120W; 5S-5N), and the magnitude of the threshold is 0.75C for this time of year. The observed SST anomaly averaged over the entire month of September, however, is slightly less than this threshold. Note that the full criteria for El Niño conditions includes the provision that the eastern-central Pacific anomalies remain at or above these levels for at least three months, and longer to be considered an El Niño event. Based on the latest observations and forecasts, the probability of maintaining El Niño conditions for October-November-December 2006 season is approximately 80%.General DiscussionSince mid-August SSTs in the central and in the eastern equatorial Pacific have been increasing, with temperatures near the dateline and in the east Pacific now greater than 1.5C above average. The initial increase of SSTs was due largely to persistent westerly wind anomalies over the west Pacific during July and August. Westerly wind anomalies picked up again in late September and since have been strong and persistent, currently extending east of the dateline. 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. Furthermore, the current westerly wind anomalies should further deepen the thermocline, and that together with the eastward advection of temperatures by the windstress are likely to maintain, if not strengthen, the current SST anomalies.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 El Niño conditions are likely to persist at least through the end of the year. Out of a large set of dynamical and statistical forecast models, more than three-quarters predict El Niño through 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 maintaining El Niño conditions is approximately 80%, and that of returning to neutral conditions through the end of 2006 is just under 20%. Note 1 - Only models that produce a new ENSO forecast every month are included in the above statement.
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