May 2026 ENSO Discussion
May 2026 ENSO Discussion
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 observed sea-surface temperature (SST) anomaly in the NINO3.4 region was +0.1 °C during the February–April 2026 season, increasing to +0.47 °C in April 2026. The latest weekly NINO3.4 index, centered on May 13, 2026, reached +0.9 °C. Together, these values suggest that Pacific Ocean conditions are currently near the El Niño threshold and are strengthening rapidly toward a developing El Niño event. The CCSR/IRI's definition of El Niño, 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.
Both oceanic and atmospheric indicators are now beginning to align, pointing to a rapid transition from ENSO neutral conditions toward a developing El Niño state. On the atmospheric side, the Southern Oscillation Index (SOI) fell to -11.2 in April 2026, while the latest 30-day SOI value through 16 May remained strongly negative at -11.1, reflecting a clear weakening of the Walker circulation consistent with El Niño development. In addition, the Equatorial SOI declined to -0.3 during April 2026, further indicating that the atmosphere is starting to respond to the warming Pacific Ocean, a sign that ocean-atmosphere coupling associated with El Niño is becoming established. In parallel, there are early but still uncertain indications of reduced trade wind strength and subtle changes in cloudiness across the equatorial Pacific. A sustained weakening of the trade winds would likely accelerate the development and intensification of El Niño conditions, whereas a strengthening of the trades would act in the opposite direction and could slow or disrupt further evolution at this early stage. Further, the subsurface temperature structure in the central–eastern equatorial Pacific shows a pronounced and noteworthy warming signal. Between approximately 150°W and 80°W, temperatures at depths of 50–150 meters have increased substantially, with anomalies locally reaching up to 6°C. This represents a significant reservoir of subsurface heat that could act as a critical energy source for the further development and intensification of El Niño conditions, provided that atmospheric coupling remains favorable. In addition, the ocean heat content anomaly (averaged over 0–300 meters depth) between the Date Line and 80°W is also markedly elevated. For context, current values are more than twice those observed during the same period in mid-May 2023 (during the El Niño event of 2023).
Expected Conditions
Note - Only models that produce a new ENSO prediction every month are considered in this statement.
The mid-May 2026 ENSO prediction plume from CCSR/IRI has now been updated with the latest model forecasts. 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.
The May 2026 CCSR/IRI ENSO plume forecast paints a strikingly confident El Niño outlook, with probabilities peaking at 98% for May–July (MJJ) 2026 and remaining exceptionally high at 97–98% throughout the forecast period (MJJ 2026 to JFM 2027). ENSO-neutral conditions are reduced to a mere 2–3%, while the probability of La Niña development is effectively zero. Although the May 2026 ENSO forecasts indicate exceptionally high confidence in El Niño development, with probabilities remaining remarkably elevated throughout the forecast period, caution is still advised when interpreting these longer-range outlooks. The forecasts are being issued near the end of the boreal spring predictability barrier, a time of year historically associated with lower ENSO forecast skill, meaning some uncertainty remains despite the very strong probabilistic signal. 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 also 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.