IRI ENSO Forecast
IRI Technical ENSO Update
Published: July 19, 2024
Note: The SST anomalies cited below refer to the OISSTv2 SST data set, and not ERSSTv5. OISSTv2 is often used for real-time analysis and model initialization, while ERSSTv5 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. These two products may differ, particularly during ENSO events. The difference between the two datasets may be as much as 0.5 °C. Additionally in some years, the ERSSTv5 may tend to be cooler than OISSTv2 in the context of warming trends, because ERSSTv5 is expressed relative to a base period that is updated every 5 years, while the base period of OISSTv2 is updated every 10 years. In February 2021, both datasets were updated to reflect the 1991-2020 climatology period.
Recent and Current Conditions
The SST anomaly for NINO3.4 during the Apr-Jun 2024 season was +0.45 °C, and for the month of June 2024 it was +0.24 °C. The most recent weekly (week centered on 10 July 2024) anomaly in the NINO3.4 region was 0.3 °C, indicating that the tropical Pacific is in the ENSO-neutral state. The IRI’s definition of El Niño, like NOAA/Climate Prediction Center’s, requires that the monthly 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 colder.
Oceanic and atmospheric conditions across the tropical Pacific are indicative of ENSO-neutral conditions. As of July 17, 2024, the 30-day Southern Oscillation Index stands within the ENSO-neutral range, with a value of -1.12, while the equatorial Southern Oscillation Index registers a value of 0.6 for the month of June 2024. The trade winds (at 850 hPa) are slightly above normal across the central-eastern equatorial tropical Pacific, while the upper-level winds (at 200 hPa) are also close to average over the equatorial Pacific. Negative subsurface temperature anomalies have persisted in the eastern equatorial Pacific Ocean and extended to the surface. Below-average temperature remain at depth in the central Pacific Ocean. Weak (less than 50 meters depth, ~ +0.5 °C) positive subsurface temperature anomalies remain prevalent in the western Pacific Ocean. Together, these observed conditions in the coupled ocean-atmosphere system indicate an ongoing ENSO-neutral conditions in the equatorial Pacific.
Expected Conditions:
Note – Only models that produce a new ENSO prediction every month are considered in this statement.
What is the outlook for the ENSO status going forward?
The El Niño/Southern Oscillation (ENSO) Diagnostic Discussion released on 11 July 2024 by the Climate Prediction Center/NCEP/NWS issued a continuation of ENSO-neutral conditions in the tropical Pacific for the next several month, along with a La Niña watch with 70% likelihood for Aug-Oct, 2024.
The latest set of ENSO prediction models from mid-July 2024 is now available in the IRI ENSO prediction plume. These are used to assess the probabilities of the three ENSO categories by using the average value of the NINO3.4 SST anomaly predictions from all models in the plume, equally weighted. A standard Gaussian error is imposed over that averaged forecast, with 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.
According to the ENSO forecast issued by the IRI, ENSO-neutral conditions are forecasted to continue during Jul-Sep (72% chances), and Aug-Oct (55% chances), while chances for La Niña are 25%, and 41% respectively. La Niña onset is most likely in Sep-Nov 2024, though the probability for that is just 49%, while the continuation of the ENSO-neutral state is estimated at 47%. La Niña then remains the most-likely category for the next three seasons with probabilities of 52% in Oct-Dec, 51% in Nov-Jan, and 48% in Dec-Feb, 2025. In Jan-Mar, 2025, ENSO-neutral conditions become once again dominant with 50% chance, while La Niña is estimated at 43%. ENSO-neutral conditions then prevail during Feb-Apr (60% chances) and Mar-May (69% chances). The probability of El Niño remains very low throughout the forecast period, (less than 10%). A plot of the probabilities summarizes the forecast evolution.
The climatological probabilities for La Niña, ENSO-neutral, and El Niño conditions vary seasonally, and are shown by the lines on the plot, and are given in a table at the bottom of this page for each 3-month season.
Caution is advised in interpreting the forecast distribution from the Gaussian standard error as the actual probabilities, due to differing biases and performance of the different models. 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. At longer leads, the skill of the models degrades, and uncertainty in skill must be convolved with the uncertainties from initial conditions and differing model physics, which leads 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.
In summary, the forecast indicates continued ENSO-neutral conditions until Aug-Oct, 2024. The forecast shows La Niña onset in Sep-Nov with only 49% chances, while the continuation of ENSO-neutral state is estimated with 47% chance. La Niña then remains the most likely category until Dec-Feb 2025 with weak probabilities (52-48%). In Jan-Mar, 2025 ENSO-neutral becomes dominant with 50% chances, while the probability for La Niña continuation is only 43%. ENSO-neutral prevail during Feb-Apr, and Mar-May, 2025 with probabilities of 60% and 69% respectively.
A caution regarding the model-based ENSO plume predictions (released mid-month) is that factors such as known specific model biases and recent changes in the tropical Pacific that the models may have missed, are not considered. This approach is purely objective. Those issues are taken into account in CPC’s official outlooks, which are issued early in the month, and which will include some human judgment in combination with the model guidance.
Season |
La Niña |
Neutral |
El Niño |
JAS |
25 |
72 |
3 |
ASO |
41 |
55 |
4 |
SON |
49 |
47 |
4 |
OND |
52 |
42 |
6 |
NDJ |
51 |
42 |
7 |
DJF |
48 |
44 |
8 |
JFM |
43 |
50 |
7 |
FMA |
33 |
60 |
7 |
MAM |
24 |
69 |
7 |