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Technical ENSO Update

17 March 2011


>	Current conditions
>	Expected conditions

Current Conditions

As of mid-March 2011, SSTs indicate weak to moderate strength La Niña conditions in the central and eastern equatorial Pacific. For February the SST anomaly in the NINO3.4 region was -1.30 C, indicative of moderate La Niña conditions, and for the December-February season the anomaly was -1.52 C. 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 NINO3.4 anomaly necessary to qualify as La Niña or El Niño conditions for the Mar-Apr-May and the Apr-May-Jun seasons are approximately (-0.40C, 0.40) and (-0.45, 0.45), respectively.

Expected Conditions

The most recent weekly SST anomaly in the NINO3.4 region is -1.0 C, indicating weak to moderate La Niña conditions in the tropical Pacific; this is slightly weaker than the -1.30 C level observed in February. What is the outlook for the ENSO status going forward? March is a time of the year during which the observed ENSO state is often weakening toward a neutral state, particularly if an ENSO episode has been occurring. Currently, a moderate to strong La Niña is in the process of weakening, and has declined to a weak to moderate strength. One might ask how much longer this event will be considered a weak (or stronger) La Niña condition as opposed to an ENSO-neutral condition. In the current case, negative subsurface sea temperature anomalies have weakened considerably and a volumetric average now indicates near-average temperatures. The negative subsurface temperature anomlies that remain have moved farther to the eastern part of the tropical Pacific as positive anomalies prevail at depth in the western and central tropical pacific, slowly edging eastward. Although SSTs and particularly subsurface sea temperatures have weakened, low-level wind anomalies still indicate enhanced trades in the western and west-central tropical Pacific, and the traditional and equatorial SOI indices remain well above their average. These observations indicate that the atmospheric component of the event is still quite robust. Therefore, while the oceanic indicators continue to weaken, the climate effects associated with La Niña may be expected to linger for a month or so longer than when the SST anomalies show a return to ENSO-neutral levels. Following dissipation of the La Niña we ask what might come later, such as in June and July. Whether the eastward expansion of positive subsurface sea temperature anomalies will eventually surface and induce El Niño conditions later this year is very uncertain, because this is the time of year when future ENSO evolution is difficult to predict.

Presently, the models and observations taken together indicate probabilities of approximately 75% for maintaining La Niña conditions, near 24% for returning to ENSO-neutral conditions, and 1% for developing El Niño conditions during the Mar-May 2011 season in progress. Probabilities for La Niña decrease to 49% for Apr-Jun, and to 30% for May-Jul. In late northern spring the probabilities for La Niña decline to 26% for Jun-Aug and 25% for the following several seasons. Beginning with May-Jul, neutral ENSO conditions are seen as being the most likely, with at least 50% probability.

The above assessment was made in part on the basis of an examination of the current predictions 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 boreal late-spring to early-summer season and as high as 0.75 degrees C in late boreal 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.

The majority of the dynamical and statistical models show continuation of La Niña conditions for the Mar-May season, as they forecast a weakening of the negative anomalies. Many models call for the event to dissipate to neutral sometime between the Mar-May season and the May-Jul season. For the Mar-May season, 82% of the models still indicate at least weak La Niña conditions, and 18% show neutral conditions. By May-Jul, only 32% still show La Niña conditions, 64% neutral, and 4% El Niño conditions. At lead times of 4 or more months into the future, statistical and dynamical models that incorporate information about the ocean's observed subsurface thermal structure generally exhibit higher predictive skill than those that do not. Among models that do use subsurface temperature information, 9 of 15 (60%) predict ENSO-neutral SSTs for the Jul-Sep seasons, 2 of 15 (13%) predict La Niña conditions, and 4 of 15 (27%) predict El Niño conditions. (Note 1). (Note that La Niña conditions for Jul-Sep require a NINO3.4 SST anomaly of -0.50 or stronger, and El Niño conditions require 0.45 or stronger.) Caution is advised in interpreting the distribution of model predictions 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 predictions 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. This method shows probabilities for La Niña at 95% for Mar-May, declining to 51% for Apr-Jun and 30% for May-Jul. The forecasters believe, however, that the northern spring model-based probabilities for La Niña may be higher than they should be, due to a common model bias of persisting ENSO episodes for too long a duration at the end of their typical seasonal cycle. Model probabilities for La Niña and decrease to 25% or slightly less for Jun-Aug and beyond. The same cautions mentioned above for the distribution of model predictions 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, and not the range across the models, nor the ensemble range within individual models.

The IRI's probabilistic ENSO prediction 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 75% probability for La Niña conditions in the Mar-May season in progress, decreasing to 49% for Apr-Jun. Probabilities for neutral ENSO conditions rise from 24% in Mar-May to 45% in Apr-Jun, and to 54% by May-Jul. The probabilities for a return to El Niño conditions remain at low levels from the present through middle northern spring 2011, rising to 16% for May-Jul and to 25% for Jul-Sep and for the following several seasons.