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Technical ENSO Update

15 February 2007


>	Current conditions
>	Expected conditions

Current Conditions

As of mid-February 2007 conditions indicate that the El Niño event has ended. SSTs are currently observed to be approximately 0.5C above average in the parts of the equatorial Pacific, particularly near the dateline. In the ENSO relevant regions of the central/eastern equatorial Pacific SST anomalies have declined considerably since their peak values in December 2006. Cold SST anomalies have developed in the eastern Pacific, reaching approximately -1.0C below average near 125W. The deep, or downward, thermocline perturbations in the eastern Pacific, associated with the El Niño event, have been replaced over the past month with shallow anomalies. The upwelling Kelvin wave associated with the shallow thermocline anomalies originated in the off-equatorial western Pacific, and has been slightly amplified by large-scale easterly wind anomalies since late December. These easterly wind anomalies are also helping to draw these subsurface temperature anomalies toward the surface, resulting in the localized cold SST anomalies. Currently the thermocline is shallower than normal across most of the equatorial Pacific, and the Trade Winds have been anomalously easterly.

For January 2007, the SSTs in the NINO3.4 region were 0.74 degrees C above average, and for the November-December-January season were 1.08 degrees C above average. Currently the IRI's definition of El Niño conditions rests on an index of SST anomalies, averaged over the NINO3.4 region (5S-5N; 170W-120W), exceeding the warmest 25%-ile of the historical distribution, and similarly for La Niña relative to the 25%-ile coldest conditions in the historical distribution. The magnitude of NINO3.4 anomaly necessary to qualify as La Niña or El Niño conditions for the Jan-Feb-Mar and Feb-Mar-Apr seasons is approximately 0.5C and 0.4C away from average, respectively.

Expected Conditions

The tropical Pacific air-sea system is transitioning out of an El Niño event. Relevant aspects of this include the decline of warm SST anomalies and emergence of cold SST anomalies in the eastern equatorial Pacific, easterly wind anomalies across much of the central and eastern tropical Pacific, and the decline of equatorial upper ocean heat content. Consistent with the declining heat content, shallow thermocline anomalies have migrated from the west Pacific toward the east, experiencing some reinforcement from easterly wind anomalies. Those sub-surface anomalies aided in eroding the current El Niño event. In addition, the easterly wind anomalies have increased equatorial upwelling in the east, pulling the subsurface anomalies to the surface leading to localized cold SST anomalies.

While many of the features in the tropical Pacific air-sea system are consistent with initiation of La Niña conditions -- shallow thermocline in the east, emergence of cold SST anomalies, and easterly wind anomalies -- considerable uncertainty remains over whether such conditions will lead to the growth of a La Niña event. In particular, the eastern equatorial Pacific is tending toward the climatologically warm part of the annual cycle. The ITCZ is closest to the equator at this time of the year, leading to weaker winds, and thus a weaker connection between subsurface and surface conditions. Still, the trend is currently toward cooling, and there is a finite possibility that the system may reach La Niña conditions at least temporarily. Probabilities favor ENSO-neutral conditions throughout the forecast period, however.

A continuation of El Niño conditions is very unlikely likely, with only 10% probability in the coming months. Throughout the forecast period there is an estimated 60% likelihood for ENSO-neutral conditions. An approximately 30% probability exists for La Niña conditions to develop over the next couple months, dropping to climatological odds through the Spring barrier.

The above assessment was made in part on the basis of an examination of the current forecasts of ENSO prediction models as well as the observed conditions. For purposes of this discussion, El Niño SST conditions are defined as SSTs in the NINO3.4 region being in the warmest 25% of their climatological distribution for the 3-month period in question over the 1950-present timeframe. The corresponding cutoff in terms of degrees C of SST anomaly varies seasonally, being close to 0.45 degrees C in northern late-spring/early-summer season and as high as 0.75 degrees C in late northern autumn. La Niña conditions are defined as NINO3.4 region SSTs being in the coolest 25% of the climatological distribution. Neutral conditions occupy the remaining 50% of the distribution. These definitions were developed such that the most commonly accepted El Niño and La Niña episodes are reproduced.

There is some variation among ENSO model forecasts through the 10-month forecast period. Most predictions indicate ENSO-neutral conditions. A few models are forecasting La Niña conditions to develop during the forecast period. For the MAM 2007 season, 7 of 20 models (35%) predict El Niño conditions to continue, while 2 of 20 models (10%) predict La Niña conditions to have emerged. At lead times of more than 4 months into the future, statistical and dynamical models that incorporate information about the ocean's observed sub-surface thermal structure generally exhibit higher predictive skill than those that do not. Among models that do use sub-surface temperature information, 2 of 13 models (15%) suggest that El Niño conditions will still be in place during the Jun-Jul-Aug season; 3 of 13 (23%) indicate SSTs exceeding the threshold for La Niña (Note 1). The majority of the models indicate that the SST conditions in the NINO3.4 region will exhibit ENSO-neutral conditions throughout the forecast period, except at the shortest lead. However, most of the models did not capture the rate at which the tropical Pacific returned to average conditions over the last couple months. Caution is advised in interpreting the distribution of model forecasts as the actual probabilities. At longer leads, the skill of the models degrades, and skill uncertainty must be convolved with the uncertainties from initial conditions and differing model physics, leading to more climatological probabilities in the long-lead ENSO Outlook than might be suggested by the suite of models. Furthermore, the expected skill of one model versus another has not been established using uniform validation procedures, which may cause a difference in the true probability distribution from that taken verbatim from the raw model predictions.

An alternative way to assess the probabilities of the three possible ENSO conditions is to use the mean of the forecasts of all models, and to construct a standard error function centered on that mean. The standard error would be Gaussian in shape, and would have its width determined by an estimate of overall expected model skill for the season of the year and the lead time. Higher skill would result in a relatively narrower error distribution, while low skill would result in an error distribution with width approaching that of the historical observed distribution. When this method is applied to the current model forecasts, results indicate a probability of El Niño that is higher than the climatological value (25%) over the next couple months -- approximately 50% for Feb-Mar-Apr 2007 and 32% for AMJ 2007. For La Niña the probabilities stay below the climatologically expected 25% throughout the forecast period. The same cautions mentioned above for the distribution of model forecasts apply to this alternative method of inferring probabilities, due to differing model biases and skills.

The IRI's probabilistic ENSO forecast takes into account the indications of this set of models, the outcome of the standard error approach described above, and additional factors such as the very latest observations that may have developed after the initialization times of some of the models. It indicates that ENSO-neutral conditions are most likely throughout the forecast period.