IRI ENSO Forecast
IRI Technical ENSO Update
Published: September 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 in the NINO3.4 region during the Jun-Aug 2024 season was +0.13 °C, and for the month of August 2024 it was -0.07 °C. The most recent weekly average (week centered on 11 September 2024) of the NINO3.4 index was -0.2 °C. These values indicate that the current tropical Pacific state is ENSO-neutral. 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. However, both traditional and equatorial Southern Oscillation Indices are marginally in La Niña territory. The trade winds (at 850 hPa) are stronger than 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. During July and August 2024 there was a sustained strengthening of the trade winds in the equatorial Pacific. The convection over the central Pacific is near normal, while above normal cloudiness is observed over Indonesia. In response, the colder waters have persisted and strengthened in the central-eastern equatorial Pacific Ocean starting from late August to recent time. Meanwhile, warming in the western Pacific has been substantially reduced. Together, these observed conditions in the coupled ocean-atmosphere system indicate an ongoing ENSO-neutral conditions in the equatorial Pacific, but with evidence of the coupling between the ocean and atmosphere expected during developing La Niña conditions.
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 12 September 2024 by the Climate Prediction Center/NCEP/NWS issued that La Niña is favored to emerge in Sep-Nov, 2024 with 71% chances and is expected to stay through Jan-Mar 2025.
The latest set of ENSO prediction models from mid-September 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 in September 2024, La Niña (50%) and ENSO-neutral (50%) conditions are forecasted with equal chances. La Niña onset is favored in Oct-Dec (60%) as compared to the continuation of ENSO-neutral conditions, which is estimated at 39%. La Niña conditions are then forecasted to continue during Nov-Jan (62%), Dec-Feb (57%), and Jan-Mar (49%), 2025. Subsequently, ENSO-neutral returns as the most-likely category for the rest of the forecast period with probabilities of 59% in Feb-Apr, 68% in Mar-May, 71% in Apr-Jun, and 63% in May-Jul 2025. During the spring and early summer of 2025, La Niña is estimated to be within the range of 22% to 37%. The probability of El Niño remains very low throughout the forecast period (less than 10% except May-Jul, 2025, which is 15%). 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 equal chances for continued ENSO-neutral conditions in Sep-Nov, and a potential transition to La Niña in Oct-Dec, 2024. The forecast then shows weak La Niña conditions that remain dominant till Jan-Mar, 2025, while the continuation of ENSO-neutral state is the second dominant category. In Feb-Apr 2025, ENSO-neutral becomes again the most likely category and remains so for the remainder of the forecast period.
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 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 |
SON |
50 |
50 |
0 |
OND |
60 |
39 |
1 |
NDJ |
62 |
37 |
1 |
DJF |
57 |
41 |
2 |
JFM |
49 |
48 |
3 |
FMA |
37 |
59 |
4 |
MAM |
28 |
68 |
4 |
AMJ |
20 |
71 |
9 |
MJJ |
22 |
63 |
15 |