IRI ENSO Forecast
IRI Technical ENSO Update
Published: April 21, 2025
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 Dec-Feb 2025 season was -0.56 °C, and for February 2025, it was -0.35 °C. The most recent weekly average (week centered on March 12, 2025) of the NINO3.4 index was 0.3 °C. The latest seasonal, monthly, and weekly values suggest that the tropical Pacific has been experiencing ENSO-neutral conditions. The IRI’s definition of El Niño, similar to NOAA/Climate Prediction Center’s, requires that the monthly SST anomaly in the NINO3.4 region (5°S-5°N; 170°W-120°W) exceed +0.5 °C. Similarly, for La Niña, the anomaly must be -0.5 °C or colder.
As of mid-April 2025, some atmospheric indicators continue to show La Niña conditions, while oceanic variables are closer to ENSO-neutral. For instance, the traditional and equatorial Southern Oscillation Index (SOI) for March 2025 was +13.2 and +1.5 respectively, falling within the La Niña ranges. The trade winds (at 850 hPa) were near normal in the equatorial central-western Pacific but weaker than normal in the eastern Pacific during March 2025. The upper-level winds (at 200 hPa) are close to average over the equatorial Pacific. Atmospheric convection over the central Pacific is below normal, consistent with typical La Niña conditions. Near-to-below-average subsurface temperatures continue in the central Pacific Ocean, while above-average temperatures remain in the western Pacific (at depth) and in a shallow layer near the surface in the eastern (100W-80W) Pacific Ocean. A moderate short-term surface warming in the eastern Pacific has been weakening due to above-normal trade wind anomalies. As discussed last month, the warming was quite shallow in the eastern Pacific, with no apparent subsurface signatures to support sustained surface warming in the Pacific Ocean. As expected, the short-term warming has been attenuated by strengthening trade winds. It is also important to note that this short-term warming has impacted ongoing La Niña conditions at the surface, and currently, there is no evident supply of cold waters underneath. Therefore, together, these observed conditions in the coupled ocean-atmosphere system indicate 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 10 April 2025 by the Climate Prediction Center/NCEP/NWS issued Final La Niña Advisory, and forecasted the ENSO-neutral conditions during the Northern Hemisphere summer.
The latest set of ENSO prediction models from mid-April 2025 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 April 2025, there is a high probability (96%) of ENSO-neutral conditions continuing during Apr–Jun 2025, while the chances for La Niña and El Niño are only 1% and 3%, respectively. For the periods May–Jul to Aug–Oct, ENSO-neutral conditions remain favored at 81%, 66%, 60%, and 54%, respectively. The probability of La Niña gradually increases from 5% to 20% during the same period, while the probability for El Niño ranges from 14% to 26%. For Sep–Nov 2025 to Dec–Feb 2025/26, ENSO-neutral conditions still have the highest probability, with values between 48% and 46%. During this same period, the probabilities for El Niño and La Niña range from 24% to 31%. This indicates that, after the Northern Hemisphere summer, the odds may tilt toward either El Niño or La Niña, depending on the conditions that develop in the upcoming months. 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 In summary, the forecast indicates a high probability of ENSO-neutral conditions for Apr–Jun 2025, with these conditions expected to persist until Aug–Oct 2025. Besides, it is worth noting that the Relative Oceanic Niño Index (RONI: https://www.cpc.ncep.noaa.gov/data/indices/RONI.ascii.txt), which measures sea surface temperature anomalies in the Niño 3.4 region (5°N–5°S, 120°W–170°W) relative to the rest of the equatorial band, has remained below the -0.5 La Niña threshold for the past seven overlapping seasons (-0.50 for Jun-Aug, -0.63 for Jul-Sep, -0.75 for Aug-Oct, -0.81 for Sep-Nov 2024, -0.92 for Oct-Dec, -1.07 for Nov-Jan, -1.12 for Dec-Feb, and -0.90 for Jan-Mar, 2025). Similarly, the Multivariate ENSO-Index (MEI: https://psl.noaa.gov/enso/mei/), which accounts for both oceanic and atmospheric conditions, registered -0.7°C during Feb-Mar 2025. Therefore, while the traditional ENSO index (Niño 3.4) is showing values that are within ENSO-Neutral range in the month of March 2025, other indices that incorporate global warming signals and atmospheric patterns have been indicative of La Niña-like conditions since mid 2024 and still show La Niña-like conditions.
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 |
AMJ |
1 |
96 |
3 |
MJJ |
5 |
81 |
14 |
JJA |
10 |
66 |
24 |
JAS |
15 |
60 |
25 |
ASO |
20 |
54 |
26 |
SON |
24 |
48 |
28 |
OND |
29 |
44 |
27 |
NDJ |
31 |
43 |
26 |
DJF |
30 |
46 |
24 |