IRI ENSO Forecast
IRI Technical ENSO Update
Published: April 20, 2020
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-April 2020, SSTs were above average, and near the threshold of weak El Niño in the NINO3.4 region. The February SST anomaly was 0.56 C, in the weak El Niño range, and for Jan-Mar it was 0.50 C, at the borderline of warm-neutral and weak El Niño. 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 0.6 C, in the lower portion of the weak El Niño range. SSTs are slightly above-average in the eastern equatorial Pacific, slightly more strongly above average in the east-central Pacific, and even more strongly above average just west of the Date Line and in the west-central Pacific. Many of the key atmospheric variables such as the low-level and upper-level zonal wind anomalies, and patterns of cloudiness and rainfall, have been exhibiting neutral ENSO conditions, while some (e.g., the Southern Oscillation Index) currently show weak El Niño-like values. Subsurface temperature anomalies from the dateline eastward in the equatorial Pacific, which were above average for most of the last 6 months, have now returned to average as anomalously negative sub-surface waters have expanded eastward. Despite recent SSTs being at borderline El Niño levels, overall conditions suggest an ENSO-neutral state.
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 neutral conditions are most likely to continue through the summer and fall. The latest set of model ENSO predictions from mid-April, now available in the IRI/CPC ENSO prediction plume, is next discussed: As of mid-April, only 12% of the dynamical or statistical models predicts El Niño conditions for the Apr-Jun season, while 88% predicts ENSO-neutral. Going forward, the percentage of models predicting neutral gradually drops to just under 60% by the Aug-Oct season, and to the 40-50% range for the four final seasons of Sep-Nov through Dec-Feb 2020-21. Percentages of models predicting El Niño are below 20% from Apr-Jun through Aug-Oct and then rise to about 25% for Sep-Nov and Oct-Dec, and to 35-40% for Nov-Jan and Dec-Feb. No model predicts La Niña for Apr-Jun, but the percentage rises to a maximum of 30% for Sep-Nov before receding somewhat by the end of 2020.
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. Using this method, chances for El Niño are 19% for the Apr-Jun season, and chances for ENSO-neutral are 81%. Going forward, probabilities for neutral are higher than those for El Niño or La Niña for all seasons through the final season of Dec-Feb 2020-21. Probabilities for neutral are about 76% for May-Jul, and gradually drop to about 40-45% for the Sep-Nov through Dec-Feb seasons. El Niño probabilities hover near 20% from Apr-Jun through Aug-Oct, then rise somewhat, reaching 30% or slightly higher for Nov-Jan and Dec-Feb. Chances for La Niña are below 20% through Jun-Aug, rise to about 25% for Jul-Sep and to a maximum of nearly 35% for Oct-Dec. 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 neutral ENSO conditions relative to El Niño or La Niña for all forecast seasons. El Niño is more likely than La Niña through the May-Jul forecast period, while La Niña becomes more likely than El Niño from Jul-Sep through Oct-Dec. For Dec-Feb, however, El Niño again becomes slightly more likely than La Niña (33% versus 24%). 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 |
AMJ 2020 |
0% |
81% |
19% |
MJJ 2020 |
5% |
76% |
19% |
JJA 2020 |
17% |
66% |
17% |
JAS 2020 |
24% |
58% |
18% |
ASO 2020 |
30% |
50% |
20% |
SON 2020 |
32% |
44% |
24% |
OND 2020 |
34% |
39% |
27% |
NDJ 2020 |
30% |
40% |
30% |
DJF 2020 |
24% |
43% |
33% |