Technical ENSO Update

21 June 2012

Recent and Current Conditions

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 carried an El Niño watch, and gave a probability of exactly 50% for El Niño conditions developing during late northern summer or early fall. It also stated that neutral conditions are expected to persist at least into part of northern summer. Now, in the middle of June, 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 in ENSO-neutral territory, and are just slightly in the warm direction in terms of SST and the basin-wide sea level pressure pattern. Low-level zonal winds and anomalous convection have been near average and have not yet swung toward an El Niño state. Subsurface temperatures across the equatorial Pacific have now risen to become mildly to moderately above average in the upper part of the ocean from just east of the date line eastward to 90W, and also in the western Pacific. A weakness in this subsurface warmth is found near the dateline. The thermocline depth along the equator is now above average across most of the tropical Pacific, an exception being a stretch of longitude near the dateline where the thermocline is just slightly shallower than average as a weak remnant of the La Niña that dissipated in early April, more than 2 months ago.

As of mid-June, some of the dynamical models and all of the statistical models predict ENSO-neutral conditions for the Jun-Aug season. However, nearly all models then indicate warming from their starting anomaly values (for Jun-Aug) onward, so that for the subsequent seasons most dynamical models predict development of El Niño conditions. However, by contrast, most statistical models predict persistence of neutral conditions through the remainder of 2012. For the Jun-Aug season, 69% of models indicate neutral ENSO conditions, while 31% indicate development of El Niño conditions. By late northern summer/autumn in Aug-Oct, 46% 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 Sep-Nov season, among models that do use subsurface temperature information, 36%  predict ENSO-neutral SSTs, and 64% predict El Niño conditions. For all models, the preference for El Niño conditions maximizes for the Oct-Dec season (at 56%), and declines to levels below 50% for Nov-Jan 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 2% or less from Jun-Aug throughout the remainder of 2012. Model probabilities for ENSO-neutral conditions are 68% for Jun-Aug, dropping to 51% for Jul-Sep, and settling into the 35-40% range from Sep-Nov 2012 through Dec-Feb 2012-13. Probabilities for El Niño are 31% for Jun-Aug, 48% for Jul-Sep, 56% for Aug-Oct, and between 59% and 65% from Aug-Oct to Nov-Jan 2012-13. In words, the models collectively favor El Niño development in late northern summer, lasting through autumn and ending during early northern winter 2012-13. By Jan-Mar 2013, the El Niño probability is down to 48% and declines further thereafter. (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 nontrivial probability (35-40%). 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 during northern winter 2012/13 is that the dynamical models tend to have shorter maximum lead times than the statistical models, so that by the winter of 2012/13 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 to have negligible probability, according to the models, despite that at least one model does indicate cool-neutral conditions through the Nov-Jan season. 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.

See also: 

• Summary of model predictions