Technical ENSO Update

19 September 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, and into winter of 2013-14, 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 exactly at their average. 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 have become slightly above average since June. 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 are leaning toward enhanced easterlies in the eastern part of the tropical Pacific, but with some residual enhanced westerlies in the western part. Anomalous convection (as measured by OLR) has generally been negative in the central and eastern 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 has weakened over the last month, and now most of the ENSO-related fields indicate more "middle-of-the road" neutral conditions.

As of mid-September, only 8% of the set of dynamical and statistical models models predicts weak La Niña SST conditions for the Sep-Nov 2013 season, none predicts El Niño conditions, so that 92% 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 Dec-Feb season, among models that do use subsurface temperature information, 63% predicts ENSO-neutral SSTs, 26% predicts El Niño conditions and 11% predicts La Niña conditions. For all model types, the probability for neutral ENSO conditions ranges from 62% (for Dec-Feb 2013-14 and Jan-Mar 2014) to 92% (for Sep-Nov 2013) through the end of the forecast period in northern early summer 2014. (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 8% for Sep-Nov 2013, remaining near 10% through the end of the forecast period in early northern summer 2014. Model probabilities for ENSO-neutral conditions are 91% for Sep-Nov 2013, and ranging between 69% and 81% thereafter, through Mar-May 2014. By early summer 2014 probabilities for neutral drop to 51% due to the slightly rising probability for El Niño. Probabilities for El Niño are 1% for Sep-Nov 2013, 2% for Oct-Dec, and remain no higher than 10% through Jan-Mar 2014 and then rise to 36% for the final forecast period of May-Jul 2014. Clearly, the models collectively favor neutral ENSO conditions through to early 2014; La Niña is slightly favored over El Niño through early northern winter 2013-14, but then the tables turn and El Niño is favored from Jan-Mar 2014 through early northern summer 2014. 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 northern winter of 2013-14. Some uncertainty exists, but it is not great because we are past the period typically most likely for new ENSO event evolution. The model forecast spread expresses that uncertainty, ranging between weak La Niña and weak El Niño conditions, even though the majority of the forecasts is in the neutral range. Statistical models tend to call for development of weak La Niña conditions during late 2013 more than dynamical models. From late northern winter 2013-14 onward, more dynamical than statistical models begin forecasting weak El Niño conditions. 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