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Technical ENSO Update

19 March 2008


>	Current conditions
>	Expected conditions

Current Conditions

As of mid-March 2008 SSTs in the central equatorial Pacific remain much below average. Coinicident with and contributing to the colder than average SSTs, the low-level winds are diverging from the cold anomalies, and the equatorial heat content is much below average. Taken together, these anomalous ocean-atmosphere conditions indicate strong and mature La Niña conditions. Over the past month, however, the cold anomalies have lessened in the eastern equatorial Pacific and become more focussed near the dateline. In addition, warm anomalies have developed off the coast of South America over the past month. On the other hand, the Southern Oscillation index (SOI) continues to be large and positive, indicative of an ongoing strong La Niña event. Likely related to the SOI, strong easterly wind anomalies persist in the Western Pacific, west of the dateline, and continue to enforce the local thermocline structure. Therefore not only have the low-level winds enforced the shallow thermocline and reduced heat content in the central and eastern Pacific, they have also deepened the thermocline and increased the heat content in the far western Pacific. The deep thermocline anomalies in the western Pacific are adjusting onto the equator and are likely contributing to the recent reduction in the La Niña conditions in the far eastern Pacific.

For February 2008, the SSTs in the NINO3.4 region were much below average (-1.90 degrees C anomaly), and for the Dec-Jan-Feb season were -1.74 degrees C from average, indicating that the La Niña event continued to strengthen in February. 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 Mar-Apr-May and Apr-May-Jun seasons are approximately 0.4C and 0.45C, respectively.

Expected Conditions

The most recent weekly SST anomaly in the NINO3.4 region, at -1.1 C, down from -2.1C a month ago, suggests the weakening of the La Niña conditions. The issue continues to be how long La Niña conditions are likely to persist, or equivalently when and how rapidly the tropical Pacific is likely to transition out of La Niña conditions. The diverging low-level winds, that have been reinforcing the shallow thermocline anomalies, are now focussed farther west and are thus having less direct impact on the thermocline. The low-level easterly wind anomalies have been persistent, though, and have built up a mass of warm water in the western equatorial Pacific that is beginning to adjust onto the equator. However, once the deep thermocline anomalies begin to adjust they must overcome the negative heat content in order to bring the equatorial Pacific back to neutral conditions and possibly even to initiate El Niño conditions. Given the current magnitude of the SOI, the negative heat content and the spatial extent of negative SST anomalies, La Niña conditions are most likely to remain for at least through the May-Jun-Jul season.

Development of El Niño conditions is very unlikely, with probabilities not reaching 10% until May-Jun-Jul of 2008. There is an approximately 85% likelihood for La Niña conditions during the Mar-Apr-May season, and a 14% probability for ENSO-neutral conditions. The probabilities for La Niña decline going forward, although La Niña conditions still are indicated as most likely through May-Jun-Jul 2008.

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 0.40 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 considerable agreement among ENSO model forecasts through the 10-month forecast period. Over the coming several months, the majority of the predictions indicate weak-to-moderate La Niña conditions, transitioning to cool ENSO-neutral conditions by mid-2008. No model predicts El Niño conditions to develop within the forecast period. For the AMJ 2008 season, 20 of 22 models (91%) predict La Niña conditions to exist. 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, 8 of 14 (57%) indicate La Niña conditions will have continued into the Jul-Aug-Sep season, and 6 of 14 (43%) predict that SSTs will be ENSO-neutral (Note 1). 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 similar probabilities for La Niña. For those probabilities, La Niña is 92% likely for the Apr-May-Jun season. The probabilities for La Niña then decline with time through 2008. This method shows probabilities for ENSO-neutral beginning low, and reaching the climatological likelihood of 50% by late-2008, while El Niño likelihood approaches its climatological probability of 25%. 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. In particular, this approach considers only the mean of the predictions not their range.

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 85% probability for La Niña conditions to persist in the coming season.