IRI ENSO Forecast
IRI Technical ENSO Update
Published: September 19, 2019
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-September 2019, coolish but neutral ENSO SST conditions were observed in the NINO3.4 region. The August SST anomaly was 0.20 C, in the neutral range, and for Jun-Aug it was 0.40 C, in the upper portion of the neutral range. 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 -0.3 C, in the lower portion of the ENSO-neutral range. From October 2018 to June 2019 borderline or weak El Niño conditions prevailed in the tropical Pacific, and transitioned to neutral levels by July. SST anomalies have become slightly negative in the eastern equatorial Pacific, but remain above-average in the central and most of the western part of the basin. Atmospheric variables have also been neutral for the last couple of months, including low-level and upper-level zonal wind anomalies and the anomalous convection. Subsurface temperature anomalies from the dateline eastward in the equatorial Pacific have been near to just slightly below average during the last month, with strongest negative anomalies well east from the dateline.
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 states that the currently neutral conditions are most likely to continue through the fall, winter and spring. The latest set of model ENSO predictions, from mid-September, now available in the IRI/CPC ENSO prediction plume, is next discussed: As of mid-September, 12% of the dynamical or statistical models predict El Niño conditions for the Sep-Nov season, and 82% predict ENSO-neutral. Going forward, this large probability difference decreases, and from Nov-Jan through May-Jul 2020, about 55-70% predict neutral and 30-45% predict El Niño, leaving none predicting La Niña.
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 10% for all forecast seasons. Probabilities for neutral conditions begin at 75% for Sep-Nov, and drop to 53-59% for Nov-Jan through May-Jul. Probabilities for El Niño begin at 23% for Sep-Nov and rise to near 40% from Nov-Jan through the final season of May-Jul 2020. 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 tilt of the odds toward neutral ENSO conditions for the entire forecast period of Sep-Nov through May-Jul 2020, with El Niño most competitive from Nov-Jan onward when it hovers near 40%. Probabilities for La Niña are no higher than 7% throughout the 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.
IRI/CPC Mid-Month Model-Based ENSO Forecast Probabilities
Season |
La Niña |
Neutral |
El Niño |
SON 2019 |
2% |
75% |
23% |
OND 2019 |
4% |
61% |
35% |
NDJ 2019 |
6% |
56% |
38% |
DJF 2019 |
7% |
53% |
40% |
JFM 2020 |
5% |
55% |
40% |
FMA 2020 |
3% |
56% |
41% |
MAM 2020 |
2% |
59% |
39% |
AMJ 2020 |
3% |
56% |
41% |
MJJ 2020 |
7% |
54% |
39% |