Technical ENSO Update

21 November 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 remainder of northern autumn into winter 2013-14, and through spring 2014, with probabilities of El Niño or La Niña each less than 30% until May-Jul 2014 when El Niño probabilities rise above that level but still less than 50%. 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 near the middle of the ENSO-neutral range. The SST continues to be slightly above average in the far western part of the basin, but continues to be below average in the eastern quarter of the basin, but much less strongly so than several months ago. Subsurface temperature anomalies across the central and eastern equatorial Pacific have been slightly above average since mid-May, and are now becoming somewhat more clearly/strongly positive. 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 now also near-average across the tropical Pacific. Anomalous convection (as measured by OLR) has generally been negative in the west-central tropical Pacific, and positive in the far western part of the basin and Indonesia. Together, these features reflect ENSO-neutral conditions.

As of mid-November, only 8% of the set of dynamical and statistical models models predicts weak La Niña SST conditions for the Nov-Jan 2013-14 season, 4% predicts El Niño conditions, and 88% 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 Feb-Apr 2014 season, among models that do use subsurface temperature information, 76% predicts ENSO-neutral SSTs, 24% predicts El Niño conditions and none predicts La Niña conditions. For all model types, the probability for neutral ENSO conditions is near 90% from Nov-Jan 2013-14 to Jan-Mar 2014, between 74% and 83% from Feb-Apr through May-Jul 2014, and 55%-60% for Jun-Aug and Jul-Sep 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 1% for Nov-Jan 2013-14, remaining at 10% or less through the end of the forecast period in northern summer 2014. Model probabilities for ENSO-neutral conditions are more than 90% from Nov-Jan 2013-14 to Jan-Mar 2014, dropping steadily during northern spring 2014, to become less than 50% from May-Jul through the end of the forecast period in Jul-Sep 2014. Probabilities for El Niño are below 10% from Nov-Jan 2013-14 to Jan-Mar 2014, thereafter steadily increasing to exceed 30% by Apr-Jun 2014 and to between 40% and 50% from May-Jul to Jul-Sep 2014 (maximizing at 48% for both Jun-Aug and Jul-Sep, the last two forecast seasons). It is clear that the models collectively favor neutral ENSO conditions into northern spring 2014; then by May-Jul El Niño probabilities become more competitive with ENSO-neutral probabilities, until they are approximately equally likely from May-Jul 2014 onward. 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 northern spring 2014. The possibility of El Niño development is seen starting May-Jul 2014, and the objective model-based probabilities for that still remain just barely shy of 50% for Jun-Aug and Jul-Sep 2014. 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