Technical ENSO Update

18 July 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 and autumn 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-June, now available in the IRI/CPC ENSO prediction plume, is discussed below. Currently, Nino3.4 SST anomalies are just slightly negative, but SST is slightly above average in the far western part of the basin and became noticeably below average in the eastern quarter of the basin since May. Subsurface temperature anomalies across the equatorial Pacific have been mixed, but overall have averaged slightly above average since June. In the atmosphere, the basin-wide sea level pressure pattern (e.g. the SOI), has been somewhat above average (indicative of somewhat cool ENSO conditions) but the low-level zonal winds have not been far from average across much of the basin. 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, with a very slight tendency toward the cool part of average, in part due to the negative anomalous SST in the eastern quarter of the basin. However, it should be kept in mind that the subsurface sea temperatures, overall, have not been in the cool part of average for nearly 2 months now.

As of mid-July 22% of the set of dynamical and statistical models models predicts weak La Niña SST conditions for the Jul-Sep 2013 season, none predicts El Niño conditions, and 78% 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 Oct-Dec season, among models that do use subsurface temperature information, 67% predicts ENSO-neutral SSTs, 11% predicts El Niño conditions and 22% predicts La Niña conditions. For all model types, the probability for neutral ENSO conditions ranges from 65% (for Oct-Dec 2013) to 82% (for Feb-Apr 2014) through the end of the forecast period in northern spring 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 26% for Jul-Sep 2013, remaining between 20% and 29% through Oct-Dec and then between 10% and 19% from Nov-Jan 2013-14 to Mar-May 2014. Model probabilities for ENSO-neutral conditions are 72% for Jul-Sep 2013, and ranging between 66% and 73% through early 2014. Meanwhile, probabilities for El Niño are 1% for Jul-Sep 2013, 4% for Aug-Oct, remaining below 10% through Nov-Jan 2013-14 and then rising to 23% for the final forecast period of Mar-May 2014. Clearly, the models collectively favor neutral ENSO conditions through to early 2014; La Niña is very slightly favored over El Niño during the remainder 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 into 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. 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 August by CPC and IRI, which will include some human judgement in combination with the model guidance.

See also: 

• Summary of model predictions