Technical ENSO Update

20 September 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 65 to 70% for El Niño conditions developing (or continuing) through northern winter 2012-13. The latest set of model ENSO predictions, from mid-September, is now available in the IRI/CPC ENSO prediction plume, discussed below. The current east-central tropical Pacific SSTs are at a weak El Niño level, and subsurface temperatures across the equatorial Pacific have been moderately above average in the upper part of the ocean from west of the date line eastward to 100W, although this positive heat content anomaly has slightly weakened in the last few weeks. 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, and shows a relative weakness in the central part of the basin. While the subsurface heat content anomaly is positive in the upper 120 m of ocean, the anomaly is negative deeper than approximately 120 m depth. In the atmosphere, the basin-wide sea level pressure pattern (e.g. the SOI) has been variable and notably slow to develop El Niño-like patterns. Low-level zonal winds and anomalous convection have been only partially suggestive of incipient El Niño features. These atmospheric patterns are not clearly consistent with El Niño.

As of mid-September, most (but not all) of the dynamical models predict El Niño SST conditions for the Sep-Nov season, and most of these show a continuation of El Niño through the remainder of 2012. Although most of the statistical models predict a weaker El Niño strength than the dynamical models, a minority show only warm-neutral condtions through the remainder of 2012 despite weak El Niño levels already in the latest seasonal average observed SSTs. For the Sep-Nov season, 8% of models indicate neutral ENSO conditions, while 92% indicate continuation of El Niño conditions. For Nov-Jan, 16% indicate neutral conditions and 84% 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 Dec-Feb season, among models that do use subsurface temperature information, 20% predict ENSO-neutral SSTs, and 80% predict El Niño conditions. For all models, the preference for El Niño conditions maximizes for the Sep-Nov season, when 92% predict El Niño conditions. This probability declines to 58% by Jan-Mar 2013 and to 20% 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 82% for Sep-Nov, 83% for Oct-Dec, and 81% for Nov-Jan 2012-13, remaining at 50% or higher through Jan-Mar 2013 and dropping to 35% by Mar-May 2013. Model probabilities for ENSO-neutral conditions are 18% for Sep-Nov, 17% for Oct-Dec, 19% for Nov-Jan 2012-13, rising to 41% for Jan-Mar 2013 and exceeding 50% beginning in Feb-Apr 2013. Probabilities for La Niña are near zero through Jan-Mar 2013, rising to 20% by May-Jul 2013. In words, the models collectively favor continuation of El Niño through the Jan-Mar 2013 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, development (or maintenance) of weak El Niño during the present time (September 2012). Such conditions have already developed in the SSTs, but the atmospheric response to the SSTs has been ambiguous so far. Without active participation of the atmosphere, the changes in atmospheric circulation, and the consequent climate teleconnections, cannot occur. Implicit in the model predictions of an approximately 80% chance of a weak El Niño persisting into northern winter 2012-13 is an assumption that the atmosphere will eventually respond to the warmed SSTs so that the event will sustain itself for several months. 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.

See also: 

• Summary of model predictions