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ENSO Quick
Look IRI ENSO Update / Forecast Technical ENSO Update IRI Probabilistic ENSO Forecast ENSO Prediction Plume Monthly archive ENSO Update16 August 2007SummaryAs of mid-August the gradual cooling trend suggestive of La Niņa 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. Based on the latest observations and forecasts, there is a 60% probability of developing La Niņa conditions over the coming season.General DiscussionFollowing a weakening of the negative SST anomalies in the east-central equatorial Pacific during June, a re-strengthening and westward expansion took place in July and continues to do so currently in mid-August. Slightly warmer than average waters are observed in parts of the western Pacific, westward of the dateline.Negative anomalies in the subsurface ocean structure have continued over the last five months (with a relative weakening in June), and are now positioned in the east-central Pacific. 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. A tendency toward enhanced Trade Winds has been observed over the last several months and continues in mid-August, resulting in sustained negative SST anomalies in the eastern tropical Pacific. 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. Weekly mean SST anomalies in the NINO3.4 region have been at borderline La Niņa levels during the most recent two weeks, following weaker anomalies in previous weeks. Continuation of the generally enhanced Trade Winds will be important for further amplification of La Niņa conditions in the coming months. Out of a large set of dynamical and statistical forecast models, slightly more models indicate weak or moderate La Niņa conditions than ENSO-neutral conditions through the end of 2007. Dynamical models that incorporate information about sub-surface ocean conditions give a relatively stonger indication of strengthening of La Niņa conditions. Overall, based on model forecasts and current observations of the ocean surface and subsurface, the probability of a La Niņa is estimated at 60% for the Aug-Sep-Oct season in progress, and remains at least 50% through the remainder of 2007. The probability of El Niņo conditions re-emerging during the forecast period is at or below 10% until Mar-Apr-May 2008, and the probability of maintaining ENSO-neutral conditions is slightly 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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