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Technical ENSO Update

19 August 2010


>	Current conditions
>	Expected conditions

Current Conditions

As of mid-August 2010, SSTs indicate moderate La Niña conditions in the central and eastern equatorial Pacific, following a period of weak La Niña conditions from mid-June to mid-July. For July the SST anomaly in the NINO3.4 region was -0.94 C, indicative of weak but approaching moderate La Niña conditions, and for the May-July season the anomaly was -0.51 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 Aug-Sep-Oct and the Sep-Oct-Nov seasons are approximately (-0.55C, 0.50) and (-0.75, 0.70), respectively.

Expected Conditions

The most recent weekly SST anomaly in the NINO3.4 region is -1.1 C, indicating weak La Niña conditions in the tropical Pacific; this is just slightly lower than the -0.94 C level observed in July. What is the outlook for the ENSO status going forward? August is a month of the year during which the stage is often already set for the ENSO conditions to be expected during the remainder of the year. With the rapid demise of the moderate(+) 2009/10 El Niño in May and now attainment of moderate La Niña conditions, one might ask whether La Niña conditions are essentially guaranteed to persist over the coming several months, and whether they could strengthen much further in the coming 1 to 3 months. A clue to the answers lie 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 enhanced trades, especially in the western and west-central portion of the tropical Pacific, and 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 moderate La Niña conditions, a process that has already been occurring over the last couple of months.

Presently, the models and observations taken together indicate probabilities of about 96% for maintaining La Niña conditions, 4% for returning to ENSO-neutral conditions, and near 0% for developing El Niño conditions during the August-September-October season in progress. Going forward, probabilities continue at more than 90% levels for the three-month periods extending through November-December-January 2010/11. In early 2011 the probabilities for La Niña begin to weaken, declining to about 64% by March-April-May, and below 50% from April-May-June onward.

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 general agreement regarding very high probabilities for La Niña conditions over the remainder of 2010 and into the early months of 2011, but indicate some disagreement about the strength of the event. Both statistical and dynamical models call for moderate La Niña strength (stronger than -1C), with dynamical models averaging slightly stronger predictions than statistical models. A few models of either type predict a strong event, while a few of either type predict a weak event. Collectively the average of the model predictions is for moderate La Niña conditions with peak strength at approximately -1.4 C during the Oct-Nov-Dec and Nov-Dec-Janseasons. For the current Aug-Oct season, 96% of the models are predicting La Niña conditions, while 4% predict ENSO-neutral conditions. This strong tilt of the odds is roughly maintained in the model predictions throughout the remainder of 2010, becoming slightly weaker in early 2011 and much weaker from at the beginning of 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 Dec-Jan-Feb season, 2 of 13 (15%) predict ENSO-neutral SSTs, and 11 of 13 (85%) predict La Niña conditions. (Note 1). (Note that La Niña conditions for Dec-Jan-Feb require a NINO3.4 SST anomaly of -0.65 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 about 98% and 85% for Aug-Sep-Oct and Sep-Oct-Nov, respectively, and remain at approximately 85% to 90% from Oct-Nov-Dec to Feb-Mar-Apr 2011. (Note that the threshold for La Niña increases from approximately -0.55C to -0.75C between Aug-Sep-Oct and Oct-Nov-Dec, 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 96% probability for La Niña conditions in the Aug-Sep-Oct season in progress, and only slightly less (but still greater than 90%) through Nov-Dec-Jan 2010/11. Probabilities for neutral ENSO conditions rise from 4% in Aug-Sep-Oct to 9% in Dec-Jan-Feb, and to 50% by Apr-May-Jun 2011. The probability for a return to El Niño conditions remain at very low levels from the present until early 2011, reaching the climatological probability of 25% by May-Jun-Jul 2011.