Technical ENSO Update

17 January 2013

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 called for a very high likelihood of neutral ENSO conditions enduring through the first quarter of 2013, with probabilities of El Niño or La Niña each less than 30% through northern summer 2013. The latest set of model ENSO predictions, from mid-January, is now available in the IRI/CPC ENSO prediction plume, discussed below. Currently, SSTs are in the cold half of the ENSO-neutral range (anomaly of 0 to -0.5 C) and during the most recent week actually broke into the weak La Niña category in the Nino3.4 region in the east-central central Pacific. The Nino4 region, farther to the west, is very close to average, and SST is above average in the far western part of the basin. Subsurface temperatures across the equatorial Pacific average close to the climatological average, but are above average in the western one-third and below average in the central and eastern Pacific. In the atmosphere, the basin-wide sea level pressure pattern (e.g. the SOI), and the low-level zonal winds have been near-average. Anomalous convection (as measured by OLR) has generally been below average in the central tropical Pacific, near to below average in the eastern part, and above average in the far western part of the basin. Together, although these last features exhibit gradients similar to those found during La Niña, all of the the features collectively reflect ENSO-neutral conditions, leaning to the cold side.

As of mid-January, 8% of the set of dynamical and statistical models models predicts La Niña SST conditions for the Jan-Mar 2013 season, none predict El Niño conditions, and 92% indicate neutral ENSO conditions. For Feb-Apr and Mar-May, 4% indicate La Niña conditions and 96% indicate neutral ENSO 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 Apr-Jun season, among models that do use subsurface temperature information, 90% predict ENSO-neutral SSTs, 5% predict El Niño conditions and 5% predict La Niña contitions. For all model types, the probability for neutral ENSO conditions exceeds 90% from Jan-Mar through 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 near 0% for Jan-Mar 2013, 1% for Feb-Apr, and 4% for Mar-May 2013, rising to near 20% from May-Jul through northern autumn. Model probabilities for ENSO-neutral conditions are 97% for Jan-Mar 2013, 89% for Feb-Apr, and 82% for Mar-May 2013, decreasing to below 60% beginning in May-Jul 2013. Probabilities for La Niña are 3% for Jan-Mar 2013, 10% for Feb-Apr, rising to 19% for Apr-Jun and 26% by Jun-Aug. In words, the models collectively favor neutral ENSO conditions quite strongly straight through to the second quarter 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 24 or more models on the IRI/CPC plume describe, on average, maintenance of neutral ENSO conditions during the coming months, continuing through the first half of 2013. Uncertainty increases greatly from around the Mar-May 2013 season onward, when the probablilities for neutral ENSO somewhat decrease as probabilities for non-neutral ENSO increase to approximately one-in-four for La Niña and one-in-five for El Niño. 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 January by CPC and IRI, which will include some human judgement in combination with the model guidance.

See also: 

• Summary of model predictions