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Technical ENSO Update

21 April 2011


>	Current conditions
>	Expected conditions

Current Conditions

As of mid-April 2011, SSTs indicate weak La Niña conditions in the central and eastern equatorial Pacific. For March the SST anomaly in the NINO3.4 region was -1.00 C, indicative of weak to moderate La Niña conditions, and for the January-March season the anomaly was -1.30 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 Apr-May-Jun and the May-Jun-Jul seasons are approximately (-0.45C, 0.45) and (-0.50, 0.45), respectively.

Expected Conditions

The most recent weekly SST anomaly in the NINO3.4 region is -0.6 C, indicating weak La Niña conditions in the tropical Pacific; this is slightly weaker than the -1.00 C level observed in March. What is the outlook for the ENSO status going forward? April and May represent a time of the year during which the ENSO state is often in transition, such as when ENSO episodes are ending or new episodes are just beginning to grow. Currently, what had been a moderate to strong La Niña several months ago is in the process of dissipating, having now declined to a weak level. Dissipation to neutral ENSO conditions is likely within the next month (by mid-May or earlier). What had been negative subsurface sea temperature anomalies within a large volume of water in the central and eastern tropical Pacific several months ago has not only weakened, but has reversed to become somewhat above average temperatures. A fairly shallow layer of weakly negative subsurface temperature anomlies remains in the central Pacific, encompassing the surface, underlain by above-average sea temperatures. Although subsurface temperatures are generally no longer below average and SSTs are only weakly below average, 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 La Niña event is still quite robust. Therefore, while the SST indicators continue to weaken, the climate effects associated with La Niña may be expected to linger for a month or more longer than when the SST anomalies 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 somewhat positive subsurface sea temperature anomalies will surface and induce El Niño conditions in the coming few months is uncertain; this is the time of year when future ENSO evolution is most difficult to predict.

Presently, the models and observations taken together indicate probabilities of approximately 48% for maintaining La Niña conditions, near 49% for returning to ENSO-neutral conditions, and 3% for developing El Niño conditions during the Apr-Jun 2011 season in progress. Probabilities for La Niña decrease to 27% for May-Jul, and to 23% for Jun-Aug. Meanwhile, probabilities for El Niño conditions increase to 15% for May-Jul, and to 23% for Jun-Aug. From Aug-Oct onward, probabilities for La Niña, neutral and El Niño conditions are 22%, 52% and 26%, respectively, making neutral conditions the most likely scenario during most of the second half of 2011.

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 weak La Niña conditions or cool/neutral conditions for the Apr-Jun season, with a warming trend thereafter. For the Apr-Jun season, 52% of the models indicate at least weak La Niña conditions, and 48% show neutral conditions. By May-Jul, 14% still show La Niña conditions, and 86% show neutral 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 Aug-Oct 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 Aug-Oct require a NINO3.4 SST anomaly of -0.55 or stronger, and El Niño conditions require 0.50 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 48% for Apr-Jun, declining to 26% for May-Jul and 18% for Jun-Aug. Model probabilities for El Niño increase to 17% for Jun-Aug and to 26% for Jul-Aug, remaining between 25 and 30% thereafter. 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 48% probability for La Niña conditions in the Apr-Jun season in progress, decreasing to 27% for May-Jul. Probabilities for El Niño conditions rise from 3% in Apr-Jun to 15% in May-Jul, 23% by Jun-Aug and 25% by Jul-Sep. Probabilities for ENSO-neutral conditions are higher than those of the other two categories for all seasons, and settle to between 50 and 55% during the second half of 2011.