IRI ENSO Forecast
IRI Technical ENSO Update
Published: August 19, 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-August 2020, SSTs were below average, and in the weak La Niña range in the NINO3.4 region. The July SST anomaly was -0.29 C, in the lower half of the ENSO-neutral range, and for Apr-Jun it was -0.30 C, in that same 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 -0.5 C, at the threshold of weak La Niña. SSTs have been somewhat below average in the eastern equatorial Pacific, slightly below average in the central Pacific, and slightly above average in the west Pacific. Some of the key atmospheric variables such as the low-level and upper-level zonal wind anomalies and patterns of cloudiness and rainfall, have been leaning toward coolish conditions. The traditional Southern Oscillation Index, has recently been positive, and anomalously dry conditions are observed around the date line and in the west-central part of the basin. Subsurface temperature anomalies from the dateline eastward in the equatorial Pacific, after a brief hiatus from their below-average period in late May and early June, returned to below-average levels during August. In summary, overall current conditions lean somewhat toward weak La Niña, but a declaration of La Niña conditions has not yet been made.
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 most of summer, followed by a 60% chance for La Niña for fall, and 55% for winter. The latest set of model ENSO predictions from mid-August, now available in the IRI/CPC ENSO prediction plume, is next discussed: As of mid-August 62% of the dynamical or statistical models predicts La Niña conditions for the Aug-Oct season, while 38% predicts neutral. For Sep-Nov through Nov-Jan, 65-70% of models predicts La Niña while the percentage of models predicting neutral ranges from 27% to 35%. By Jan-Mar, the percentage of models predicting La Niña drops below 50%, and for Mar-May and Apr-Jun neutral becomes the strongly dominant probability.
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 La Niña are 53% for the Aug-Oct season, while chances for ENSO-neutral are 47%. Going forward, probabilities for La Niña rise to 55-60% for Sep-Nov through Nov-Jan, and chances for neutral are near 40%. La Niña probabilities fall below 50% from Dec-Feb onward, and become lower than the chances for neutral from Jan-Mar through the final forecast season of Apr-Jun. Chances for ENSO neutral rise to more than 70% for Mar-May and Apr-Jun. El Niño probabilities are less than 10% through Feb-Apr, then rise to 10-15% by Apr-Jun. 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 relatively slim chance for El Niño conditions throughout the forecast period, and a preference for La Niña conditions relative to neutral conditions for the initial season of Aug-Oct and extending through Nov-Jan, during which time the chances for La Niña exceed 50% and peak at 55-60%, and continue to be the dominant probability through Dec-Feb.
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 |
ASO 2020 |
47% |
53% |
0% |
SON 2020 |
57% |
41% |
2% |
OND 2020 |
58% |
38% |
4% |
NDJ 2020 |
55% |
39% |
6% |
DJF 2021 |
49% |
44% |
7% |
JFM 2021 |
38% |
54% |
8% |
FMA 2021 |
30% |
63% |
7% |
MAM 2021 |
17% |
73% |
10% |
AMJ 2021 |
13% |
71% |
16% |