IRI ENSO Forecast
IRI Technical ENSO Update
Published: April 16, 2015
Recent and Current Conditions
The SST anomaly in the NINO3.4 region has been mostly at a weak El Niño level from mid-October through mid-April 2015. For March the average NINO3.4 SST anomaly was 0.58 C, indicative of weak Niño conditions, and for Jan-Mar it was 0.56 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 0.7 C, in the category of weak El Niño. Accompanying this SST has been an atmospheric pattern with supporting indications of an El Niño condition, including westerly low-level wind anomalies and positive anomalies of convection in the vicinity of the dateline. The Southern Oscillation Index (SOI) and the equatorial SOI have also been indicative of weak El Niño, with values of roughly -1.0.
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 an approximately 70% likelihood for El Niño conditions continuing through summer 2015. The latest set of model ENSO predictions, from mid-April, now available in the IRI/CPC ENSO prediction plume, is discussed below. Currently, Nino3.4 SST anomalies are in the weak El Niño cagtegory. Subsurface temperature anomalies across the eastern equatorial Pacific have increased to well above average levels during the past two months as the downwelling phase of a Kelvin wave has been moving eastward at depth in response to westerly low-level wind anomalies during the last 2 to 3 months. The positive heat content anomaly may portend increases in SST over the coming few months, and has already been doing so to some degree. In the atmosphere, the basin-wide sea level pressure anomaly pattern (e.g. the SOI) has been at weak El Niño levels, and anomalous convection (as measured by OLR) has been above average near the dateline. Together, the oceanic and atmospheric features reflect a weak El Niño condition during late March and through mid-April; the ENSO status for the SST alone has been that of weak El Niño for an even longer time period–i.e., over the last five months.
As of mid-April, none of the dynamical or statistical models models predicts La Niña SST conditions for the initial Apr-Jun 2015 season, 75% predicts El Niño conditions, and 25% 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 Jul-Sep 2015 season, among models that do use subsurface temperature information, 95% predicts El Niño SST conditions, 5% predicts ENSO-neutral conditions and none predicts La Niña conditions. For all model types, the probabilities for El Niño are 79% for May-Jul, 83% for Jun-Aug, and 80-89% from Jul-Sep through the end of 2015. No models predict La Niña conditions for any forecast period during 2014. The season having highest probability for El Niño SST conditions is Oct-Dec, when the probability is 89%.
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 less than 2% from Apr-Jun through Aug-Oct 2015, rising to near 5% by Nov-Jan and Dec-Feb 2015-16. Model probabilities for neutral ENSO conditions are approximately 20% through Nov-Jan 2015-16, rising to 24% for Dec-Feb. Probabilities for El Niño are 80-81% for Apr-Jun through Aug-Oct 2015, dropping to 70-75% from Sep-Nov through Dec-Feb 2015-16. The models collectively favor El Niño over other ENSO conditions by the largest margin during from Apr-Jun through Aug-Oct 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 models on the IRI/CPC plume describe, on average, weak El Niño conditions for the Apr-Jun and May-Jul seasons, with strengthening El Niño conditions suggested through northern autumn season, reaching moderate strength. However, model spread is still noteworthy, reflecting the presence of the northern spring ENSO predictability barrier. 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 |
37% |
28% |
35% |
| JFM |
34% |
37% |
29% |
| FMA |
30% |
48% |
22% |
| MAM |
26% |
54% |
20% |
| AMJ |
24% |
54% |
22% |
| MJJ |
25% |
51% |
24% |
| JJA |
25% |
50% |
25% |
| JAS |
27% |
46% |
27% |
| ASO |
29% |
40% |
31% |
| SON |
32% |
34% |
34% |
| OND |
34% |
31% |
35% |
| NDJ |
37% |
27% |
36% |
IRI/CPC Mid-Month Plume-Based ENSO Forecast Probabilities
| Season |
La Niña |
Neutral |
El Niño |
| AMJ 2015 |
~0% |
19% |
81% |
| MJJ 2015 |
~0% |
20% |
80% |
| JJA 2015 |
~0% |
19% |
81% |
| JAS 2015 |
1% |
19% |
80% |
| ASO 2015 |
1% |
19% |
80% |
| SON 2015 |
4% |
22% |
74% |
| OND 2015 |
4% |
21% |
75% |
| NDJ 2015 |
5% |
22% |
73% |
| DJF 2015 |
4% |
24% |
72% |
