Technical ENSO Update

17 October 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 high likelihood of neutral ENSO conditions enduring through the northern autumn into winter 2013-14, and through spring 2014, with probabilities of El Niño or La Niña each less than 30% through that time. The latest set of model ENSO predictions, from mid-September, now available in the IRI/CPC ENSO prediction plume, is discussed below. Currently, Nino3.4 SST anomalies are slightly negative but in the ENSO-neutral range. The SST continues to be slightly above average in the far western part of the basin, but has been below average in the eastern quarter of the basin since May. Subsurface temperature anomalies across the central and eastern equatorial Pacific were slightly above average from mid-May through September, but are now very close to average in October. In the atmosphere, the basin-wide sea level pressure pattern (e.g. the SOI), and the low-level zonal winds have been approximately average across much of the basin. The upper level zonal winds indicate enhanced westerlies in part of the eastern tropical Pacific. Anomalous convection (as measured by OLR) has generally been negative in the central tropical Pacific, and positive in the far western part of the basin. Together, these features reflect ENSO-neutral conditions. A tendency toward the cool part of the neutral range continues, after a relative weakening of even slightly cool conditions during September.

As of mid-October, only 11% of the set of dynamical and statistical models models predicts weak La Niña SST conditions for the Oct-Dec 2013 season, none predicts El Niño conditions, so that 89% indicates neutral ENSO. 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 Jan-Mar 2014 season, among models that do use subsurface temperature information, 79% predicts ENSO-neutral SSTs, 7% predicts El Niño conditions and 14% predicts La Niña conditions. For all model types, the probability for neutral ENSO conditions is near 90% for Oct-Nov-Dec 2013 to Dec-Jan Feb 2013-14, between 80% and 90% from Jan-Feb-Mar through Apr-May-Jun 2014, and between 70% and 80% for May-Jun-Jul and Jun-Jul-Aug 2014 at the end of the forecast period. (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 5% for Oct-Dec 2013, remaining between 5% and 11% through the end of the forecast period in northern summer 2014. Model probabilities for ENSO-neutral conditions are between 90% and 95% from Oct-Dec 2013 to Jan-Mar 2014, dropping steadily during northern spring 2014, to become near or just below 50% by northern summer. The probabilities for ENSO-neutral drop due to a rising (but never dominating) probability for El Niño. Probabilities for El Niño are 5% or less from Oct-Dec 2013 to Jan-Mar 2014, thereafter steadily increasing to exceed 30% by Apr-Jun 2014 and to 42% by the final forecast period of Jun-Aug. It is clear that the models collectively favor neutral ENSO conditions through northern spring 2014; then by Apr-Jun El Niño probabilities become more competitive with ENSO-neutral probabilities, until they are approximately equally likely by Jun-Aug 2014 (both just shy of 50% due to the lingering 9% likelihood of La Niña). 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 models on the IRI/CPC plume describe, on average, maintenance of neutral ENSO conditions during the coming months, continuing into the northern spring of 2014. A continuing mean disagreement between statistical and dynamical models has diminished somewhat for this latest set of model forecasts. A caution regarding this latest set of model-based ENSO plume predictions, is that 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