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Technical ENSO Update

20 December 2006


>	Current conditions
>	Expected conditions

Current Conditions

The current conditions, as of mid-December 2006, indicate that El Niño conditions are still in place. SSTs exceed 1.0C above average throughout the equatorial Pacific, and are more than 1.5C near the dateline and in the eastern equatorial Pacific. Deep, or downward, perturbations exist across the equatorial thermocline, particularly in the east where the anomalies exceed 35m (or equivalently, sub-surface temperature anomalies of 4.0C). Perturbations to the surface and sub-surface temperatures were initially generated by anomalous westerly winds over the western Pacific during July and August. Those temperature anomalies were since reinforced, particularly in late September and October, by westerly wind anomalies with a relatively large magnitude and fetch. Also consistent with the westerly wind anomalies, the western Pacific thermocline has shoaled both on and off the equator. These shallow thermocline perturbations are beginning to adjust onto the equator and eastward, where they can potentially erode the current El Niño conditions. However, while the deep thermocline anomalies remain in the eastern equatorial Pacific, their contribution to the surface temperatures through vertical advection should maintain the warm SST anomalies already in place.

For November 2006, the SSTs in the NINO3.4 region were 1.21 degrees C above average, and for the September-October-November season were 0.93 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 Nov-Dec-Jan and Dec-Jan-Feb seasons is approximately 0.75C and 0.65C away from average, respectively.

Expected Conditions

El Niño conditions exist in the surface and sub-surface tropical Pacific. These include the warm SST anomalies in the central and eastern equatorial Pacific, persistent westerly wind anomalies in the western Pacific and central Pacific, above-normal equatorial heat content, and large-scale deep perturbations to the equatorial thermocline. Interestingly, although the westerly wind anomalies have been persistent, they have not been as strong or extended as far eastward as in other El Niños of the recent past (1991, 1997, 2002). As a result, convection has generally remained west of the dateline. That the convection is not shifting much to the east during this El Niño may impact certain teleconnections, such as those over North and South America.

The current anomalies in the sub-surface ocean structure are beginning to adjust, particularly as the westerly wind anomalies slacken. This will allow the deep perturbations in the eastern Pacific to move towards the South American coast, at which point they will travel poleward as coastal Kelvin waves. Those deep coastal waves can be expected to change coastal temperatures as they pass. The shallow perturbations of the western Pacific are converging toward the equator and beginning to move eastward. Those sub-surface anomalies will begin to erode the current El Niño in early 2007. The rate at which neutral conditions are restored will depend on several factors, such as constructive or destructive interference of atmospheric weather influencing future perturbations to the equatorial upper ocean structure, and the efficiency with which off-equatorial shallow perturbations survive as they convergeonto the equator.

Persistence of El Niño conditions remains the most likely outcome through early 2007. For the DJF and JFM seasons, there is an estimated 90-95% likelihood for El Niño, approximately 1% probability for La Niña conditions to return, leaving less than 10% probability for ENSO-neutral conditions.

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 little variation among ENSO model forecasts through the first quarter of 2007. All predictions indicate persustence of weak to moderate El Niño conditions. No model is forecasting La Niña conditions to occur through the forecast period. For the DJF 2006/07 season, all 20 models (100%) predict El Niño conditions. 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, 9 of 13 models (69%) suggest that El Niño conditions will still be in place during the Apr-May-Jun season; none 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 El Niño conditions until close to mid-2007: DJF 2006/07 (100%) to MJJ 2007 (56%). 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 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 also indicate a probability of El Niño that is higher than the climatological value (25%)-- approximately 95% from Dec-Jan-Feb 2006/07 to FMA 2007. For La Niña the probabilities stay below 15% 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 a very high confidence for the continuation of El Niño conditions through early 2007, lower than expected probabilities for ENSO-neutral, and negligible probability of La Niña conditions.