Technical ENSO Update

15 November 2012

Recent and Current Conditions

Expected Conditions

What is the outlook for the ENSO status going forward? The most recent 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 discontinued a previously issued El Niño watch, and gave a probability of only approximately 40% for El Niño conditions to re-develop in late northern fall 2012 or winter 2012-13. The latest set of model ENSO predictions, from mid-November, 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 near the date line eastward, 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), low level zonal winds and anomalous convection have not been clearly suggestive of an incipient El Niño state during most of October and the first half of November, although at times they leaned slightly in that direction. 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. The OLR pattern includes lower than normal convection over parts of Indonesia. These atmospheric patterns are not clearly consistent with El Niño, and reflect a more or less neutral ENSO state. The SST anomaly in the NINO4 region is now highest among the four major NINO regions, at +0.6C.

As of mid-November, just under one-quarter of the set of dynamical and statistical models models predicts El Niño SST conditions for the Dec-Feb season, and some of these show a brief continuation of El Niño through into very early 2013. For the Nov-Jan 2012-13 season, 22% of models indicate El Niño conditions, while 78% indicate neutral ENSO conditions. For Dec-Feb, these same forecast proportions continue. 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 Feb-Apr season, among models that do use subsurface temperature information, 78% predict ENSO-neutral SSTs, and 22% predict El Niño conditions. For all models, the probability for El Niño conditions maximizes for the Nov-Jan, Dec-Feb and Jan-Mar season, with 22% predicting El Niño conditions. This probability declines to 17% by Feb-Apr 2013 and to 10% 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 24% for Nov-Jan 2012-13, 21% for Dec-Feb, and 21% for Jan-Mar 2013, remaining in the 20-31% range through the middle of 2013. Model probabilities for ENSO-neutral conditions are 76% for Nov-Jan 2012-13, 79% for Dec-Feb, and 79% for Jan-Mar 2013, decreasing to 71% for Mar-May 2013 and 63% for Apr-Jun. Probabilities for La Niña are near zero through Jan-Mar 2013, rising to 9% by Apr-Jun and 20% for Jun-Aug 2013. In words, the models collectively favor neutral ENSO conditions through the remainder of 2012 and through the first half of 2013. 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 neutral (but slightly warmer than average) ENSO conditions during the coming months, and near-neutral conditions (whether warmer than average or not) well into 2013. Following this latest model-based ENSO 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 December by CPC and IRI, which will include some human judgement in combination with the model guidance.

See also: 

• Summary of model predictions