Technical ENSO Update

17 May 2012

Recent and Current Conditions

Since the weak/moderate strength La Niña conditions dissipated in early April 2012, ENSO-neutral conditions have prevailed. As of mid-May, SST anomalies in the east-central tropical Pacific are only very slightly cooler than average. Since February, SSTs have been warmer than average in the eastern part of the tropical Pacific basin. For April the SST anomaly in the NINO3.4 region was -0.39 C, indicative of cool-neutral ENSO conditions, and for the February-April season the anomaly was -0.55 C. Starting in December 2011, the IRI's definition of El Niño conditions began following that of NOAA/Climate Prediction Center, in which the SST anomaly in the NINO3.4 region (5S-5N; 170W-120W) exceeds 0.45 C. Similarly, for La Niña, the anomaly must be -0.45 C or less. The climatological probabilities for La Niña, neutral, and El Niño conditions vary seasonally, and are shown in a table at the bottom of this page for each 3-month season. The most recent weekly SST anomaly in the NINO3.4 region is -0.0 C, indicating exactly average conditions in the tropical Pacific; this is slightly warmer than the -0.39 C level observed in April.

Expected Conditions

What is the outlook for the ENSO status going forward? The official diagnosis and outlook was issued earlier this month in the NOAA/Climate Prediction Center ENSO Diagnostic Discussion, produced jointly by CPC and IRI. It stated that the earlier La Niña conditions had transitioned to neutral conditions, and that the neutral conditions are expected to persist at least through northern summer. Now, in the middle of May, a new set of model ENSO predictions is available as shown in the IRI/CPC ENSO prediction plume, discussed below. The current east-central tropical Pacific SSTs are now in ENSO-neutral territory, and in fact are very close to the exact climatological average. Subsurface temperatures across the equatorial Pacific have now risen to become mildly to moderately above average in the upper part of the ocean from the date line eastward to 100W. The thermocline depth along the equator is now above average across most of the tropical Pacific, an exception being a stretch of longitude near and just east of the dateline where the thermocline is just slightly shallower than average as a weak remnant of the dissipated La Niña.

As of mid-May, nearly all of the dynamical and statistical models predict ENSO-neutral conditions for the May-Jul season. All models then indicate warming from their starting anomaly values (for May-Jul) onward, although one or two show this only negligibly. For the May-Jul season, 92% of models indicate neutral ENSO conditions, while 8% (i.e., one dynamical model) indicate quick development of El Niño conditions. By late northern summer/autumn in Aug-Oct, 42% indicate neutral conditions and 58% predict El Niño conditions. At lead times of 3 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. For the Aug-Oct season, among models that do use subsurface temperature information, 35%  predict ENSO-neutral SSTs, and 65% predict El Niño conditions. For all models, the preference for El Niño conditions maximizes for the Sep-Nov season (at 58%), and declines to below 50% for Dec-Feb 2012-13 and later. (Note 1).

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 more quantitatively precise and less vulnerable to sampling errors than the categorical tallying method used above. This alternative method uses the mean of the predictions of all models on the plume, equally weighted, and constructs a standard error function centered on that mean. The standard error is Gaussian in shape, and has its width determined by an estimate of overall expected model skill for the season of the year and the lead time. Higher skill results in a relatively narrower error distribution, while low skill results in an error distribution with width approaching that of the historical observed distribution. This method shows probabilities for La Niña at less than 5% from May-Jul throughout the remainder of 2012. Model probabilities for ENSO-neutral conditions are 84% for May-Jul, dropping to 59% for Jun-Aug, and settling into the 40-45% range from Aug-Oct 2012 through Nov-Jan 2012-13. Probabilities for El Niño are 13% for May-Jul, 37% for Jun-Aug, 48% for Jul-Sep, and between 50% and 55% from Aug-Oct to Nov-Jan 2012-13. In words, the models collectively favor El Niño development in late northern summer, lasting into autumn but ending by early northern winter 2012-13. By Jan-Mar 2013, the El Niño probability is down to 42%. (See the next paragraph for a non-physical explanation of why the event is predicted, by the average of the models, to end early.) A plot of the probabilities generated from this most recent IRI/CPC ENSO prediction plume using the multi-model mean and the Gaussian standard error method summarizes the model consensus out to about 10 months into the future. The same cautions mentioned above for the distributional count of model predictions apply to this Gaussian standard error 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 total range across the models, nor the ensemble range within individual models.

The probabilities derived from the 25 or more models on the IRI/CPC plume describe, on average, development of weak El Niño around late August or September. Howeveer, maintenance of neutral ENSO conditions still has a substantial probability (40-45%). In fact, there is a distinction between the average foreacast of the statistical versus the dynamical models, with most of the statistical models calling for persistence of neutral conditions and most of the dynamical models predicting El Niño development. A non-physical reason for the predicted early ending to the possible El Niño is that the dynamical models tend to have shorter maximum lead times than the statistical models, so that by the end of autumn 2012 only a few dynamical models remain in the average, creating an average prediction determined more heavily by the statistical models. Redevelopment of La Niña appears very unlikely, according to the models, despite that at least one model indicates cool-neutral conditions through the calendar year. Factors such as known specific model biases and recent changes that the models may have missed will be taken into account in the next official outlook to be generated and issued in early June by CPC and IRI, which will include some human judgement in combination with the model guidance.

Using the 0,.45 C thresholds, the climatological probabilities of La Niña, neutral, and El Niño conditions for each 3-month season are as follows:

See also:

• Summary of model predictions