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Technical ENSO Update

14 November 2007


>	Current conditions
>	Expected conditions

Current Conditions

As of mid-November 2007 SSTs in the eastern and central equatorial Pacific remain much below average, indicating La Niña conditions. The cold anomalies extend from the coast of South America to west of the dateline, with peak anomalies of -2.5C between 120W and 110W. Strong, persistent and large-scale easterly wind anomalies, similar to those observed during September, emerged again in mid-October and have even strengthened slightly in mid-November. These wind anomalies extend across most of the equatorial Pacific, from about 130W to 140E, and have been contributing to the maintenance of cold SST anomalies and associated sub-surface temperature anomalies. The coupling exhibited between the thermocline, SSTs and easterly winds led to substantial coupled growth of the La Niña conditions during late September and early October. The persistence of those anomalies has maintained the La Niña conditions since then.
For October 2007, the SSTs in the NINO3.4 region were below average (-1.38 degrees C anomaly), and for the Aug-Sep-Oct season were -0.92 degrees C from 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 are approximately 0.75C and 0.65, respectively.

Expected Conditions

The most recent weekly SST anomaly in the NINO3.4 region, at -1.5 C, suggests moderately strong La Niña conditions, consistent with the -1.4C NINO3.4 anomaly averaged over the past month. However, cold anomalies must persist in the central-eastern equatorial Pacific through the end of the year if these conditions are to be classified as an event. The issue at this point is whether persistence of the La Niña conditions are likely, or equivalently when and how rapidly the tropical Pacific is likely to transition out of La Niña conditions. Factors of relevance to La Niña maintenance are the enhanced easterly winds, which reinforce the shallow thermocline anomalies, and the strongly negative SST anomalies observed east of the dateline. These features suggest that La Niña conditions could remain for at least a couple more months. Furthermore, in terms of La Niña's demise, at this time there are only weak manifestations of deep thermocline anomalies in the western and off-equatorial Pacific that could bring about a rapid transition out of the current La Niña conditions.

Some aspects of the tropical Pacific air-sea system suggest that even if La Niña persists, it is not likely to grow further. First, October is climatologically the month of strongest upwelling, so contributions to cold SST anomalies from mean upwelling across the anomalously shallow thermocline are not likely to increase. Second, on average La Niña events tend to peak in late northern hemisphere fall, which would suggest that the current conditions have probably reached their peak magnitude. The current state of the air-sea system indicates at least moderate La Niña conditions, and portends maintenance in the coming months. A high probability exists that the system will retain La Niña conditions over the coming seasons. Thus, a 97% probability for La Niña conditions is suggested.

Development of El Niño conditions is very unlikely, with probabilities not exceeding 10% through Apr-May-Jun of 2008. There is an approximately 97% likelihood for La Niña conditions during the Nov-Dec-Jan season, and only a 2% probability for ENSO-neutral conditions. As the peak of the La Niña conditions is likely occuring now or very soon, the probabilities for La Niña decline going forward, although La Niña conditions still are indicated as most likely through early 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 0.45 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 fairly good agreement among ENSO model forecasts through the 10-month forecast period. Over the coming several months, the majority of the predictions indicate moderate La Niña conditions; most of the rest predict weak La Niña conditions. No models predict El Niño conditions to develop until mid-2008. For the DJF 2007 season, 21 of 22 models (95%) 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, 10 of 14 (71%) indicate La Niña conditions will have continued into the Mar-Apr-May season, and 4 of 14 (29%) 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 98% likely for the Dec-Jan-Feb season. The probabilities for La Niña then decline going into 2008. This method shows probabilities for ENSO-neutral beginning well below the climatologically expected 50%, and then rising up to that level by Apr-May-Jun 2008. 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 97% probability for La Niña conditions to persist in the coming season.