Technical ENSO Update

19 July 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 55% for El Niño conditions developing most likely during the July-September period. Now, in the middle of July, 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 borderline El Niño level, and the basin-wide sea level pressure pattern has also been in the direction of El Niño, although not as clearly as the SST. Low-level zonal winds and anomalous convection have been near average and have not yet swung toward an El Niño state with consistency. 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 90W, and also in the western Pacific to a lesser extent. A relative weakness in the subsurface warmth is found near the dateline. The thermocline depth along the equator is above average in the western and eastern tropical Pacific, but not in the central one-third of the basin. where the thermocline is just slightly shallower than average.

As of mid-July, most (but not all) of the dynamical models predict El Niño conditions for the Jul-Sep season, continuing through most or all of the remainder of 2012. However, many statistical models predict either just borderline El Niño conditions or warm-neutral ENSO conditions throughout this same period. For the Jul-Sep season, 38% of models indicate neutral ENSO conditions, while 62% indicate development of El Niño conditions. For Aug-Oct, 31% indicate neutral conditions and 68% 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 Oct-Dec season, among models that do use subsurface temperature information, 23% predict ENSO-neutral SSTs, and 77% predict El Niño conditions. For all models, the preference for El Niño conditions in fact maximizes for the Oct-Dec season, and remains at 76% for Nov-Jan. It then declines to levels below 50% for Jan-Mar 2013 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 less than 1% from Jul-Sep throughout the remainder of 2012. Model probabilities for ENSO-neutral conditions are 38% for Jul-Sep, 28% for Aug-Oct, and settling to near 20% for Sep-Nov 2012 through Nov-Jan 2012-13. Probabilities for El Niño are 62% for Jul-Sep, 72% for Aug-Oct, 77% for Sep-Nov, and maximize at 81% for Oct-Dec, not to drop below 50% until Feb-Apr 2013. In words, the models collectively favor El Niño development during Jul-Sep, lasting through autumn and ending during middle to late northern winter 2012-13. By 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 July or August. Howeveer, maintenance of neutral ENSO conditions still has a nonnegligible probability (20-25%). In fact, there is a distinction between the average foreacast of the statistical versus the dynamical models, with many of the statistical models calling for only borderline El Niño conditions developing, or even persistence of neutral conditions, and most of the dynamical models predicting El Niño development. Redevelopment of La Niña appears to have nearly zero probability, according to the models, despite that at least one model does indicate cool-neutral conditions through the Oct-Dec 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.

----- LN N EN -----Climatological Probabilities

DJF 37 28 35

JFM 34 37 29

FMA 30 48 22

MAM 26 54 20

AMJ 24 54 22

MJJ 25 51 24

JJA 25 50 25

JAS 27 46 27

ASO 29 40 31

SON 32 34 34

OND 34 31 35

NDJ 37 27 36

See also: 

• Summary of model predictions 

Note 1 - Only models that produce a new ENSO prediction every month are included in the above statement.
 
 

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