Technical ENSO Update

16 August 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 75% for El Niño conditions developing most likely during the August-September period, and likely lasting through the remainder of the calendar year. Now, in the middle of August, 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 at a weak El Niño level, and the basin-wide sea level pressure pattern has also been in the direction of El Niño, although notably less clearly than the SST and not sufficiently to describe El Niño conditions. Low-level zonal winds and anomalous convection also have swung toward an El Niño state only slightly. Subsurface temperatures across the equatorial Pacific have risen to become moderately above average in the upper part of the ocean from just east of the date line eastward to 100W. The thermocline depth along the equator is above average in the western and eastern tropical Pacific, but with a relative weakness in the central part of the basin.

As of mid-August, most (but not all) of the dynamical models predict El Niño conditions for the Aug-Oct season, continuing through most or all of the remainder of 2012. However, half or more of the statistical models predict either just borderline El Niño conditions or just neutral ENSO conditions throughout this same period. For the Aug-Oct season, 24% of models indicate neutral ENSO conditions, while 76% indicate development of El Niño conditions. For Sep-Nov, 24% again indicate neutral conditions and 76% 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 Nov-Jan season, among models that do use subsurface temperature information, 15% predict ENSO-neutral SSTs, and 85% predict El Niño conditions. For all models, the preference for El Niño conditions maximizes for the Oct-Dec season, when 80% predict El Niño conditions. This probability declines to 50% for Jan-Mar 2013 and to 17% 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 La Niña at less than 1% from Jul-Sep throughout the remainder of 2012. Model probabilities for ENSO-neutral conditions are 23% for Aug-Oct, 21% to 22% for Sep-Nov through Nov-Jan 2012-13, rising to 42% for Jan-Mar 2013 and exceeding 50% beginning in Feb-Apr 2013. Probabilities for El Niño are 77% for Aug-Oct, 78% for Sep-Nov, and maximize at 79% for Oct-Dec,remaining at 50% or higher through Jan-Mar 2013 and dropping to 35% by Mar-May 2013. In words, the models collectively favor El Niño development at the present time, lasting through northern autumn and ending during late northern winter or early spring 2012-13. For Jan-Mar 2013, the El Niño probability is 58% and declines rapidly thereafter. 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 during the present time (August or early September 2012). Howeveer, maintenance of neutral ENSO conditions still has a nonnegligible probability (20-25%). There still remains some difference between the average foreacast of the statistical versus the dynamical models, with many of the statistical models calling for warm-neutral or only borderline El Niño conditions developing, and most of the dynamical models predicting El Niño development and persistence through the remainder of 2012. Redevelopment of La Niña is predicted to have nearly zero probability, according to the models, despite that at least two statistical models actually indicate cool-neutral conditions through the Nov-Jan 2012-13 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 September by CPC and IRI, which will include some human judgement in combination with the model guidance.

See also: 

• Summary of model predictions