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Technical ENSO Update

20 February 2008


>	Current conditions
>	Expected conditions

Current Conditions

As of mid-February 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, the cold anomalies become somewhat more focussed in the central Pacific, although cold anomalies still exist along the coast of South America. Peak SST anomalies of approximately -2.5C are found approximately between 160W-140W. Strong wind anomalies diverge away from the region of cooler-than average temperatures, leading to easterly wind anomalies in the Western Pacific and westerly wind anomalies in the Eastern Pacific. The divergent wind anomalies act to pull up the thermocline, thus contributing to the maintenance of cold SST anomalies and associated sub-surface temperature anomalies. The coupling exhibited between the anomalous thermocline, SSTs and winds led to substantial coupled growth of the La Niña conditions during late September and early October 2007. The persistence of those anomalies has maintained the La Niña conditions since then.
For January 2008, the SSTs in the NINO3.4 region were much below average (-1.81 degrees C anomaly), and for the Nov-Dec-Jan season were -1.59 degrees C from average, indicating that the La Niña event continued to strengthen in January. 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 Feb-Mar-Apr and Mar-Apr-May seasons are approximately 0.4C.

Expected Conditions

The most recent weekly SST anomaly in the NINO3.4 region, at -2.1 C, suggests strong La Niña conditions, consistent with the -1.8C NINO3.4 anomaly averaged over the past month. 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. Factors of relevance to La Niña maintenance are the diverging low-level 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 are most likely to remain for at least a couple more months. Furthermore, in terms of La Niña's demise, the deep thermocline anomalies that develop in the western and off-equatorial Pacific could bring about a rapid transition out of the current La Niña conditions. So far, those deep perturbations have been held in the west by the persistent easterly winds, and thus have not had the chance to adjust within the equatorial Pacific and intitiate a transition away from La Niña conditions. A high probability exists that the system will retain La Niña conditions at least through the Apr-May-Jun season.

Development of El Niño conditions is very unlikely, with probabilities not reaching 10% until May-Jun-Jul of 2008. There is an approximately 95% likelihood for La Niña conditions during the Feb-Mar-Apr season, and only a 4% probability for ENSO-neutral conditions. The probabilities for La Niña decline going forward, although La Niña conditions still are indicated as most likely through Apr-May-Jun 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, with single outlier models on both the warm and cold ends of the spectrum. Over the coming several months, the majority of the predictions indicate weak-to-moderate La Niña conditions. Only one model predicts El Niño conditions to develop but not until mid-2008. For the MAM 2008 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, 8 of 14 (57%) indicate La Niña conditions will have continued into the Jun-Jul-Aug season, and 5 of 14 (36%) 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 >99.5% likely for the Mar-Apr-May season. The probabilities for La Niña then decline with time through 2008. This method shows probabilities for ENSO-neutral beginning virtually at 0, and reaching the climatological likelihood of 50% by late-2008, while El Niño likelihood approaches 20% (just below 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 95% probability for La Niña conditions to persist in the coming season.