Technical ENSO Update

18 October 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 approximately 50 to 55% for El Niño conditions continuing into northern winter 2012-13. The latest set of model ENSO predictions, from mid-October, is now available in the IRI/CPC ENSO prediction plume, discussed below. The current east-central tropical Pacific SSTs are only in the warm portion of the ENSO-neutral range (anomaly of 0 to 0.5C) and subsurface temperatures across the equatorial Pacific remain above average in the upper part of the ocean from west and near the date line, but only modestly so. Even with this positive heat content anomaly, the thermocline depth along the equator is only slightly above average in some portions of the tropical Pacific. In the atmosphere, the basin-wide sea level pressure pattern (e.g. the SOI) has been variable and not consistent with a typical El Niño pattern. Low-level zonal winds and anomalous convection have been only partially suggestive of an incipient El Niño state. Lately the pattern of outgoing longwave radiation (OLR) has exhibited enhanced convection to the west of the dateline, near the middle of the NINO4 region. This location is west of that more typical of El Niño conditions, but could expand eastward to become more in conformity in the coming weeks. The OLR pattern also includes lower than normal convection over parts of Indonesia. Thus, although these atmospheric patterns are not clearly consistent with El Niño, there is still opportunity for them to become more so. The SST anomaly in the NINO4 region is now highest among the four major NINO regions, at +0.4C.

As of mid-October, just less than half of the set of dynamical and statistical models models predicts El Niño SST conditions (mainly weak) for the Oct-Dec season, and some of these show a continuation of El Niño through into the very beginning of 2013. The statistical models generally predict a slightly lower SST than the dynamical models, and many of them show only warm-neutral condtions through the remainder of 2012. For the Oct-Dec season, 56% of models indicate neutral ENSO conditions, while 44% indicate El Niño conditions. For Dec-Feb, 64% indicate neutral conditions and 36% 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 Jan-Mar season, among models that do use subsurface temperature information, 60% predict ENSO-neutral SSTs, and 40% predict El Niño conditions. For all models, the probability for El Niño conditions maximizes for the Oct-Dec season, with 44% predicting El Niño conditions. This probability declines to 32% by Jan-Mar 2013 and to 25% for Mar-May (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 El Niño at 56% for Oct-Dec, 56% for Nov-Jan, and 48% for Dec-Feb 2012-13, dropping to 33% by Feb-Apr 2013. Model probabilities for ENSO-neutral conditions are 44% for Oct-Dec, 44% for Nov-Jan, 52% for Dec-Feb 2012-13, rising to 60% for Jan-Mar 2013 and remaining at 60% or more until Apr-Jun 2013. Probabilities for La Niña are near zero through Jan-Mar 2013, rising to 18% by May-Jul 2013. In words, the models collectively favor, although only very slightly, El Niño conditions through the Nov-Jan 2012-13 season. 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, maintenance of weak El Niño during the present time (October 2012), despite that current observed conditions have become inadequate to be considered even borderline El Niño conditions. As mentioned above, the pattern of anomalous convection is not consistent with El Niño, but could become consistent with some eastward expansion of the region of enhanced convection. The SST anomaly pattern is also no longer that of El Niño, but this could change if the slightly above average SSTs somewhat west of the dateline would expand eastward and strengthen somewhat. The model predictions show an approximately 55% chance of a weak El Niño returning and lasting into the beginning of northern winter 2012-13. Following this latest model-based ENSO preduction plume prediction, 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 October by CPC and IRI, which will include some human judgement in combination with the model guidance. Currently, given the latest observations, this added human factor appears more likely to weaken than to strengthen the probability for weak El Niño conditions to return in the coming few months.

See also: 

• Summary of model predictions