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Technical ENSO Update

21 October 2010


>	Current conditions
>	Expected conditions

Current Conditions

As of mid-October 2010, SSTs continue to indicate moderate to strong La Niña conditions in the central and eastern equatorial Pacific. For September the SST anomaly in the NINO3.4 region was -1.56 C, indicative of moderate to strong La Niña conditions, and for the July-September season the anomaly was -1.25 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 Oct-Nov-Dec and the Nov-Dec-Jan seasons are approximately (-0.75C, 0.75) and (-0.70, 0.75), respectively.

Expected Conditions

The most recent weekly SST anomaly in the NINO3.4 region is -1.5 C (but was -1.8 C for the previous two weeks), indicating moderate to strong La Niña conditions in the tropical Pacific; this is similar to the -1.56 C level observed in September. What is the outlook for the ENSO status going forward? October is a time of the year during which the observed ENSO state usually indicates the ENSO conditions to be expected during the remainder of the year and often into the first month or two of the following year. One might ask whether the current La Niña condition is therefore guaranteed to persist over the coming several months, and whether it could strengthen any farther in the coming few months. A clue to the answers lies in the strength and extent of negative subsurface sea temperature anomalies at depth in the central and eastern Pacific, whose upper portion has encompassed the surface waters since approximately mid-June. Sea temperature anomalies below the surface often portend SST anomalies to be expected in the following 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 months. The surfacing of below-average subsurface waters implies some likelihood for further increases in the strength of the currently already moderate to strong La Niña conditions, although substantial additional cooling appears unlikely at this point as we approach the typical time of peak strength.

Presently, the models and observations taken together indicate probabilities of approximately 99% for maintaining La Niña conditions, near 1% for returning to ENSO-neutral conditions, and nearly 0% for developing El Niño conditions during the October-November-December season in progress. Going forward, probabilities for La Niña continue at 96% or greater for the three-month periods extending through December-January-February 2010/11. In northern spring 2011 the probabilities for La Niña begin to weaken at a faster rate, declining to about 62% by March-April-May, and to 40% for April-May-June.

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 very high certainty for La Niña conditions over the remainder of 2010 and into the earliest months of 2011, but vary somewhat in their predictions of the peak strength of the event. All statistical and dynamical models call for at least moderate La Niña strength (stronger than -1C). A few statistical models, and at least half of the dynamical models, predict a strong event (-1.8C or stronger). Collectively the average of the model predictions is for moderate to strong La Niña conditions with peak strength at approximately -1.75 C during the Oct-Nov-Dec and Nov-Dec-Jan seasons. For the current October-December season, 100% of the models are predicting La Niña conditions, while none predict ENSO-neutral conditions. This unanimity continues through the Feb-Mar-Apr season of 2011. From Mar-Apr-May onward, some models begin indicating a return to neutral ENSO conditions, but not to the point of being a majority until late northern spring. 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, none indicate El Niño conditions for the Mar-Apr-May season, 2 of 12 (17%) predict ENSO-neutral SSTs, and 10 of 12 (83%) predict La Niña conditions. (Note 1). (Note that La Niña conditions for Mar-Apr-May require a NINO3.4 SST anomaly of only -0.40 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 October-December, November-January and December-February, remaining at 95% or more through February-April 2011. (Note that the threshold for La Niña weakens from approximately -0.75C to -0.45C between October-December and February-April, due to the seasonality of the interannual variance). 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 99% probability for La Niña conditions in the October-December season in progress, and only slightly less (still at least 96%) through December-February 2010/11. Probabilities for neutral ENSO conditions rise from 1% in October-December to 7% in January-March, and to 47% by April-June 2011. The probability for a return to El Niño conditions remain at very low levels from the present through early 2011, not exceeding 10% until April-June and reaching 23% by June-August.