IRI ENSO Forecast
IRI Technical ENSO Update
Published: June 19, 2018
Note: The SST anomalies cited below refer to the OISSTv2 SST data set, and not ERSSTv4. OISSTv2 is often used for real-time analysis and model initialization, while ERSSTv4 is used for retrospective official ENSO diagnosis because it is more homogeneous over time, allowing for more accurate comparisons among ENSO events that are years apart. During ENSO events, OISSTv2 often shows stronger anomalies than ERSSTv4, and during very strong events the two datasets may differ by as much as 0.5 C. Additionally, the ERSSTv4 may tend to be cooler than OISSTv2, because ERSSTv4 is expressed relative to a base period that is updated every 5 years, while the base period of OISSTv2 is updated every 10 years and so, half of the time, is based on a slightly older period and does not account as much for the slow warming trend in the tropical Pacific SST.
Recent and Current Conditions
In mid-June 2018, the NINO3.4 SST anomaly showed neutral ENSO conditions. For May the SST anomaly was -0.13 C, indicating neutral conditions, and for February-April it was -0.41 C, indicating cool-neutral ENSO. 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 anomaly in the Nino3.4 region was 0.2, showing neutral conditions. Additionally, most of the key atmospheric variables, including the lower and upper level zonal wind anomalies and the Southern Oscillation Index, suggest neutral condition. The only exception is the anomalies of outgoing longwave radiation (convection), which show weak remnants of the La Niña episode that ended two months ago. The subsurface temperature anomalies across the eastern equatorial Pacific have warmed to moderately above-average, suggesting the possibility of a warming of the SST in the coming months. Given the current and recent SST anomalies, the subsurface profile and the conditions of most key atmospheric variables, we will likely remain in ENSO-neutral conditions at least through Northern Hemisphere summer, with a chance for a warming leading to El Niño development after about August.
Expected Conditions
What is the outlook for the ENSO status going forward? The most recent official diagnosis and outlook was issued approximately one week ago in the NOAA/Climate Prediction Center ENSO Diagnostic Discussion, produced jointly by CPC and IRI; it stated that ENSO-neutral is expected during summer, with about an even chance for El Niño during autumn, rising to about 65% for winter. An El Niño watch accompanied that Discussion. The latest set of model ENSO predictions, from mid-May, now available in the IRI/CPC ENSO prediction plume, is discussed below. Those predictions also suggest that the SST is likely to remain in the ENSO-neutral range until around the Aug-Oct season, when El Niño development is at least 50% likely, rising to higher levels later in the year.
As of mid-June, about 75-80% of the dynamical or statistical models predict neutral conditions for the initial Jun-Aug 2018 season, with about 20-25% showing El Niño conditions. Over the course of the rest of 2018, probabilities for neutral drop below 50% by Aug-Oct and to near 20% from Oct-Dec through Feb-Apr 2019, as the probability for El Niño hovers near 80% during those same seasons. La Niña probabilities are near zero throughout the forecast period. 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 Sep-Nov 2018 season, among models that do use subsurface temperature information, 25-30% of models predicts neutral conditions and 70-75% predict El Niño conditions.
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 approximately 5% or lower for the full range of seasons from Jun-Aug 2018 through to Feb-Apr 2019. Probabilities for neutral conditions begin at about 70% for Jun-Aug, fall below 50% beginning in Aug-Oct, and decrease further to near 20-25% for the last three seasons of Dec-Feb to Feb-Apr. Meanwhile, the probabilities for El Niño, which begin at about 25-30% for May-Jul, rise to exceed 50% from Aug-Oct onward into early 2019, reaching the 70-80% range from Dec-Feb through Feb-Apr. 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.
In summary, the probabilities derived from the models on the IRI/CPC plume describe, on average, a preference for ENSO-neutral for Jun-Aug and Jul-Sep 2018, followed by a tilt of the odds toward El Niño conditions starting in Aug-Oct and increasing to 70% or higher by the end of 2018 and in early 2019. Probabilities for La Niña are about 5% or less throughout the entire forecast period. 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 early next month by CPC and IRI, which will include some human judgment 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 Model-Based ENSO Forecast Probabilities
Season |
La Niña |
Neutral |
El Niño |
JJA 2018 |
1% |
71% |
28% |
JAS 2018 |
3% |
52% |
45% |
ASO 2018 |
4% |
42% |
54% |
SON 2018 |
5% |
35% |
60% |
OND 2018 |
6% |
31% |
63% |
NDJ 2018 |
5% |
29% |
66% |
DJF 2019 |
3% |
26% |
71% |
JFM 2011 |
1% |
23% |
76% |
FMA 2019 |
0% |
20% |
80% |