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ENSO Quick
Look IRI ENSO Update / Forecast Technical ENSO Update IRI Probabilistic ENSO Forecast ENSO Prediction Plume Monthly archive ENSO Update17 May 2007SummaryAs of mid-May conditions in the tropical Pacific are suggestive of La Nina development. Currently equatorial sea surface temperatures (SSTs) are much below average in the eastern Pacific and along the coast of Peru. Based on the latest observations and forecasts, there is a 55% probability of developing La Nina conditions over the coming season.General DiscussionThe pattern of SST anomalies in the equatorial Pacific resembles that of La Nina conditions. Colder than average waters dominate in the eastern Pacific, with the below-average temperatures now extending to near the dateline. Coolest SST anomalies remain situated near the South American coast. Slightly warmer than average waters are situated in the western Pacific.Consistent with the pattern of surface temperatures, the subsurface ocean structure shows a shallower than average upper ocean mixed layer across the equatorial Pacific, east of 160E. This appears as cold temperature anomalies at the thermocline depth along the equator. The anomalously shallow mixed layer has been in place since February 2007. The upwelling currents acting in the eastern equatorial Pacific, are thus bringing colder than average waters to the surface. These features enhance the chances of developing La Nina conditions in the coming seasons. The current low-level winds are likely to have a mixed impact on the anomalous oceanic conditions. On one hand, a large, strong patch of westerly wind anomalies has been in place over the far western Pacific since early May. These meet up with the enhanced equatorial easterlies near the dateline, and will generate some local deepening of the thermocline. That perturbation could lead to a slight weakening of the subsurface anomalies. On the other hand, the meridional winds have been diverging away from the equator in the central and eastern Pacific, which could enhance the shoaling of the local thermocline as well as lead to cooler surface temperature anomalies. Which process dominates in the coming weeks may decide whether La Nina develops or the system remains ENSO-neutral. Out of a large set of dynamical and statistical forecast models, the majority predict the maintenance of ENSO-neutral conditions through the end of 2007. However, dynamical models that incorporate information about sub-surface ocean conditions give a stonger indication of the development of La Nina. Overall, based on model forecasts and current observations of the ocean surface and subsurface, the probability of a La Niña is estimated at 55% from the coming May-Jun-Jul season through Sep-Oct-Nov 2007. The probability of El Niño conditions re-emerging during the forecast period remains at or below 5% throughout the forecast period. The probability of maintaining ENSO-neutral conditions is below 50% probability, until late-2007. Note 1 - Only models that produce a new ENSO forecast every month are included in the above statement.
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