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Technical ENSO Update

17 February 2011


>	Current conditions
>	Expected conditions

Current Conditions

As of mid-February 2011, SSTs indicate moderate strength La Niña conditions in the central and eastern equatorial Pacific. For January the SST anomaly in the NINO3.4 region was -1.65 C, indicative of moderate to strong La Niña conditions, and for the November-January season the anomaly was -1.62 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 Feb-Mar-Apr and the Mar-Apr-May seasons are approximately (-0.45C, 0.40) and (-0.40, 0.40), respectively.

Expected Conditions

The most recent weekly SST anomaly in the NINO3.4 region is -1.2 C, indicating moderate La Niña conditions in the tropical Pacific; this is slightly weaker than the -1.65 C level observed in January. What is the outlook for the ENSO status going forward? February is a time of the year during which the observed ENSO state is often beginning to move toward weaker anomaly values, particularly if an ENSO episode has been occurring. One might ask whether the current La Niña condition should therefore be expected to weaken, and if so, at what rate. In the current case, negative subsurface sea temperature anomalies have continued to occupy the central and eastern equatorial Pacific, but have been pushed farther east and have weakened somewhat in the last one month as the event has also been starting to weaken at the sea surface. Sea temperature anomalies below the surface often portend SST anomalies to be expected in the following few months, as they are subject to the climatological upwelling in the eastern part of the basin. Current low-level wind anomalies still indicate enhanced trades in the western and west-central tropical Pacific, but somewhat more weakly than up to one month ago. The traditional and equatorial SOI indices remain well above their average, indicating that the atmospheric component of the event is still quite robust. The SOI and the enhanced trade winds imply above-average upwelling activity in the eastern tropical Pacific, at least for the very near term, and suggest that the still negative subsurface sea temperature anomalies will likely continue to find their way to the surface during the coming month or two. The continued surfacing of below-average subsurface waters implies a likelihood for short-term continuation of the currently moderate La Niña conditions, although the SST anomaly may continue to weaken as the sub-surface negative heat content weakens against a backdrop of the seasonal cycle of ENSO that suggests a decline in strength at this time of year. Above-average subsurface waters are strengthening in the western tropical Pacific, and have been edging eastward during the last few months. As they continue to expand eastward they may begin to displace the still large volume of below-average water to their east over the course of the coming four months. Whether this positive heat content will eventually surface and induce El Niño conditions later this year is very uncertain, as will be discussed below.

Presently, the models and observations taken together indicate probabilities of approximately 94% for maintaining La Niña conditions, near 6% for returning to ENSO-neutral conditions, and nearly 0% for developing El Niño conditions during the Feb-Apr 2011 season in progress. Probabilities for La Niña decrease to 74% for Mar-May, and to 49% for Apr-Jun. In late northern spring the probabilities for La Niña decline to 34% for May-Jul and to 26% for Jun-Aug and for the following several seasons. Beginning with May-Jul, neutral ENSO conditions are seen as being the most likely, with 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 models show nearly unanimous agreement regarding the continuation of La Niña conditions for the Feb-Apr season, but vary somewhat in their predicted rate of weakening negative anomalies between Feb-Apr and the forthcoming seasons. Many of the statistical and dynamical models call for the event to decline to neutral sometime between the Mar-May season and the May-Jul season. For the Mar-May season, 83% of the models still indicate at least weak La Niña conditions, and 17% show neutral conditions. By May-Jul, only 35% still show La Niña conditions, 48% neutral, and 17% weak 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, 7 of 16 (44%) predict ENSO-neutral SSTs for the Jun-Aug seasons, 5 of 16 (31%) predict La Niña conditions, and 4 of 16 (25%) predict El Niño conditions. (Note 1). (Note that La Niña conditions for Jun-Aug 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 99% for Feb-Apr, declining to 89% for Mar-May and 53% for Apr-Jun. 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 are 34% for May-Jul, and decrease to less than 30% 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 94% probability for La Niña conditions in the Feb-Apr season in progress, and somewhat less (74%) for Feb-Apr. Probabilities for neutral ENSO conditions rise from 6% in Feb-Apr to 23% in Mar-May, and to 42% by Apr-Jun. The probabilities for a return to El Niño conditions remain at very low levels from the present through middle northern spring 2011, not exceeding 10% until May-Jul and rising to 24% for Jun-Aug and for the following several seasons.