IRI ENSO Forecast
IRI Technical ENSO Update and Model-Based Probabilistic ENSO Forecast
Published: August 20, 2015
Recent and Current Conditions
The SST anomaly in the NINO3.4 region attained a weak El Niño level beginning late February 2015, strengthened to moderate strength around mid-May, and strengthened further to a strong level beginning around mid-July. For July the average NINO3.4 SST anomaly was 1.60 C, indicative of strong El Niño conditions, and for May-July it was 1.32 C. The IRI’s definition of El Niño, like NOAA/Climate Prediction Center’s, requires that the SST anomaly in the Nino3.4 region (5S-5N; 170W-120W) exceed 0.5 C. Similarly, for La Niña, the anomaly must be -0.5 C or less. The climatological probabilities for La Niña, neutral, and El Niño conditions vary seasonally, and are shown in a table at the bottom of this page for each 3-month season. The most recent weekly SST anomaly in the Nino3.4 region was 2.0 C, in the category of strong El Niño. Accompanying this SST has been an El Niño atmospheric pattern, including westerly low-level wind anomalies and positive anomalies of convection near and east of the dateline. The Southern Oscillation Index (SOI) and the equatorial SOI have also been quite negative, indicative of El Niño conditions.
Expected Conditions
What is the outlook for the ENSO status going forward? The most recent official diagnosis and outlook was issued one week ago in the NOAA/Climate Prediction Center ENSO Diagnostic Discussion, produced jointly by CPC and IRI; it called for a greater than 90% likelihood for El Niño conditions continuing through winter 2015-16, and 85% to last into spring 2016. The latest set of model ENSO predictions, from mid-Aug, now available in the IRI/CPC ENSO prediction plume, is discussed below. Currently, weekly Nino3.4 SST anomalies are in the stromg El Niño cagtegory. Subsurface temperature anomalies across the eastern equatorial Pacific have been well above average levels during the past 5 months as the downwelling phases of several Kelvin waves (one in the earlier months this year, and several more following that) have moved eastward at depth in response to westerly low-level wind anomalies over the course of the last 5 months. The strong positive heat content anomaly has promoted steady increases in SST over the last 3 months, and is likely to lead to still further SST increases in the coming months, depending on the strength and nature of the atmospheric-oceanic feedbacks. In the atmosphere, the basin-wide sea level pressure anomaly pattern (e.g. the SOI) has been decidedly at El Niño levels. Anomalous convection (as measured by OLR) has been above average both near and to the east of the dateline. Together, the oceanic and atmospheric features reflect strong El Niño conditions for late July through mid-August.
As of mid-August, none of the dynamical or statistical models models predicts La Niña or neutral SST conditions for the initial Aug-Oct 2015 season; 100% predicts El Niño conditions. 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 Nov-Jan 2015-16 season, among models that do use subsurface temperature information, 100% predicts El Niño SST conditions, while none predicts ENSO-neutral conditions and none predicts La Niña conditions. For all model types, the probabilities for El Niño are near 100% (i.e., higher than 99.5%) for Aug-Oct through Feb-Apr 2016, dropping to 89% for Mar-May and 78% for Apr-Jun. No models predict La Niña conditions for any forecast period during 2015 and early 2016. Chances for neutral ENSO conditions are near 0% through Feb-Apr 2016, 11% for Mar-May and 22% for Apr-Jun 2016.
Note – Only models that produce a new ENSO prediction every month are included in the above statement.
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 near-zero from Aug-Oct through Mar-May 2016, and 1% for Apr-Jun 2016. Model probabilities for neutral ENSO conditions are 5% or less through Feb-Apr 2016, 9% for Mar-May and 30% for Apr-Jun 2016. Probabilities for El Niño are near 100% for Aug-Oct 2015 to Nov-Jan 2015-16, 99% for Dec-Feb and Jan-Mar 2016, 98% down to 90% for Feb-Apr and Mar-May 2016 respectively, and 69% for Apr-Jun 2016. 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, extremely high certainty for El Niño conditions for the Aug-Oct 2015 through Jan-Mar 2016 seasons, and still very high certainty through Feb-Apr 2016. In terms of magnitude, the models suggest strengthening El Niño conditions through northern autumn season, most likely peaking at the level of strong El Niño. Model forecast spread still exists, and peak event strength could range from about 1.5 to 2.5 C with a slight possibility of being outside of that interval on either side. 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 March by CPC and IRI, which will include some human judgement in combination with the model guidance.
Climatological Probabilities
| Season |
La Niña |
Neutral |
El Niño |
| DJF |
36% |
30% |
34% |
| JFM |
34% |
38% |
28% |
| FMA |
28% |
49% |
23% |
| MAM |
23% |
56% |
21% |
| AMJ |
21% |
58% |
21% |
| MJJ |
21% |
56% |
23% |
| JJA |
23% |
54% |
23% |
| JAS |
25% |
51% |
24% |
| ASO |
26% |
47% |
27% |
| SON |
29% |
39% |
32% |
| OND |
32% |
33% |
35% |
| NDJ |
35% |
29% |
36% |
IRI/CPC Mid-Month Plume-Based ENSO Forecast Probabilities
| Season |
La Niña |
Neutral |
El Niño |
| ASO 2015 |
~0% |
~0% |
100% |
| SON 2015 |
~0% |
~0% |
100% |
| OND 2015 |
~0% |
~0% |
100% |
| NDJ 2015 |
~0% |
~0% |
100% |
| DJF 2015 |
~0% |
1% |
99% |
| JFM 2016 |
~0% |
1% |
99% |
| FMA 2016 |
~0% |
2% |
98% |
| MAM 2016 |
~0% |
9% |
91% |
| AMJ 2016 |
1% |
30% |
69% |
