Technical ENSO Update

16 May 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 northern summer of 2013, with probabilities of El Niño or La Niña each less than 30% through that time. The latest set of model ENSO predictions, from mid-May, is now available in the IRI/CPC ENSO prediction plume, discussed below. Currently, Nino3.4 SST anomalies are near zero, but SST is slightly above average in the far western part of the basin and has become below average in the eastern quarter of the basin. Subsurface temperatures across the equatorial Pacific average close to the climatological average, but are above average in the western quarter of the basin and below average in part of the eastern tropical Pacific. In the atmosphere, the basin-wide sea level pressure pattern (e.g. the SOI), has been near average and the low-level zonal winds have also been not far from average across much of the basin. 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, with a very slight tendency toward the cool part of average, now largely because of the more strongly negative anomalous SST in the eastern quarter of the basin.

As of mid-May, 17% of the set of dynamical and statistical models models predicts weak La Niña SST conditions for the May-Jul 2013 season, none predicts El Niño conditions, and 83% 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 Aug-Oct season, among models that do use subsurface temperature information, 83% predicts ENSO-neutral SSTs, 0% predicts El Niño conditions and 17% predicts La Niña conditions. For all model types, the probability for neutral ENSO conditions ranges from 63% (for Nov-Jan 2013-14) to 83% (for May-Jun-Jul 2013) to the end of the forecast period in early 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 21% for May-Jul 2013, 26% for Jun-Aug, and 27% for Jul-Sep 2013, remaining between 20% and 28% throughout 2013. Model probabilities for ENSO-neutral conditions are 77% for May-Jul 2013, 67% for Jun-Aug, and 60% for Jul-Sep 2013, decreasing to between 58% and 65% through early 2014. Probabilities for El Niño are 2% for May-Jul 2013, 7% for Jun-Aug, rising to only 13% to 14% through early 2014. In words, the models collectively favor neutral ENSO conditions through to early 2014; La Niña is slightly favored over El Niño during all of the period. 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 second half of 2013. Uncertainty exists, because we are in the period of typically most likely new ENSO event evolution from now until about late June. The model forecast spread expresses that uncertainty, ranging between weak La Nina and borderline El Nino conditions, even though the majority of the forecasts is in the neutral range. 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 January by CPC and IRI, which will include some human judgement in combination with the model guidance.

See also: 

• Summary of model predictions