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Technical ENSO Update

20 January 2011


>	Current conditions
>	Expected conditions

Current Conditions

As of mid-January 2011, SSTs continue to indicate moderate to strong La Niña conditions in the central and eastern equatorial Pacific. For December the SST anomaly in the NINO3.4 region was -1.53 C, indicative of moderate to strong La Niña conditions, and for the October-December 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 Jan-Feb-Mar and the Feb-Mar-Apr seasons are approximately (-0.55C, 0.50) and (-0.45, 0.40), respectively.

Expected Conditions

The most recent weekly SST anomaly in the NINO3.4 region is -1.7 C, indicating moderate to strong La Niña conditions in the tropical Pacific; this is just slightly cooler than the -1.53 C level observed in December. What is the outlook for the ENSO status going forward? January 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, and have shown little tendency to weaken even in the last one to two months as the event has been in the process of discharging at the 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 indicate much enhanced trades, especially in the western and west-central portion of the tropical Pacific, and very strongly positive traditional and equatorial SOI indices. These atmospheric features imply above-average upwelling activity in the eastern tropical Pacific, at least for the near term, and suggest that the negative subsurface sea temperature anomalies will likely continue to find their way to the surface during the coming two or more months. The continued surfacing of below-average subsurface waters implies a likelihood for short-term (at least one month) continuation of the strength of the currently moderate to strong La Niña conditions, despite that the seasonal cycle of ENSO suggests a typical decline in strength at this time of year. Above-average subsurface waters exist in the western tropical Pacific, and although they have been edging eastward during the last few months they do not appear poised to displace the large volume of below-average water to their east for at least two or more months, and possibly longer.

Presently, the models and observations taken together indicate probabilities of approximately 98% for maintaining La Niña conditions, near 2% for returning to ENSO-neutral conditions, and nearly 0% for developing El Niño conditions during the Jan-Mar 2011 season in progress. Probabilities for La Niña decrease slightly to 88% for Feb-Apr, and to 67% for Mar-May. In late northern spring the probabilities for La Niña weaken at a faster rate, declining to 46% for Apr-Jun and to 27% for Jun-Aug and for the following several seasons.

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 unanimous agreement regarding the continuation of La Niña conditions into part of the second quarter of 2011, but vary somewhat in their predictions of the rate of decreasing strength starting from the Jan-Mar season. Most statistical and dynamical models call for at least moderate La Niña strength (stronger than -1C) through the Feb-Apr season. For the Jan-Mar, Feb-Apr and Mar-May seasons, 100% of the models are predicting La Niña conditions, while none predicts ENSO-neutral conditions. Following Mar-May, some models begin indicating a return to neutral ENSO conditions, but not to the point of being a majority until the May-Jul season. By Jul-Sep, two of the 23 models call for 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, 6 of 13 (46%) predict ENSO-neutral SSTs for the Jun-Aug seasons, 6 of 13 (46%) predict La Niña conditions, and 1 of 13 (8%) 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 near 100% for Jan-Mar and Feb-Apr, declining to 95% for Mar-May and 69% for Apr-Jun. (Note that the threshold for La Niña weakens from approximately -0.55C to -0.45C between Jan-Mar and Apr-Jun, due to the seasonality of the interannual variance). The forecasters believe, however, that the northern spring model-based probabilities for La Niña may be higher than it 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 46% for May-Jul, and decrease to less than 40% for Jul-Sep 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 98% probability for La Niña conditions in the Jan-Mar season in progress, and only slightly less (88%) for Feb-Apr. Probabilities for neutral ENSO conditions rise from 2% in Jan-Mar to 11% in Feb-Apr, and to 44% 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 reaching 10% until Apr-Jun and rising to 23% for Jun-Aug and for the following several seasons.