ENSO Forecast

September 2025 Quick Look

Published: September 19, 2025

A monthly summary of the status of El Niño, La Niña, and the Southern Oscillation, or ENSO, based on the NINO3.4 index (120-170W, 5S-5N)

As of mid-September 2025, the equatorial Pacific remains in an ENSO-neutral state, with sea surface temperatures in the Niño 3.4 region close to average but exhibiting a gradual cooling trend. The IRI ENSO plume forecast indicates a moderate probability (56%) of La Niña conditions developing during September–November 2025. These La Niña conditions are expected to persist through the boreal winter of 2025/2026 (December–February). However, beginning in January–March, ENSO-neutral conditions are forecasted to return, with probabilities ranging from 55% to 74%, while the likelihood of La Niña gradually decreases. The chances of El Niño development remain very low—below 10%—through March–May 2026.

IRI ENSO Forecast

IRI Technical ENSO Update and Model-Based Probabilistic ENSO Forecast

Published: September 19, 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

Expected Conditions

Note – Only models that produce a new ENSO prediction every month are considered in this statement.

The El Niño/Southern Oscillation (ENSO) Diagnostic Discussion released on 11 September 2025 by the Climate Prediction Center (CPC)/NCEP/NWS, maintained a “La Niña Watch,” forecasting that a transition from ENSO-neutral to La Niña conditions is likely in the next couple of months, with a 71% chance of La Niña during October–December 2025. Thereafter, La Niña remains favored, although the probability decreases to 54% for the December 2025–February 2026 period.

The latest set of ENSO prediction models from mid-September 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.

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.

It is worth noting that the Relative Oceanic Niño Index, which measures sea surface temperature anomalies in the eastern equatorial Pacific relative to the rest of the equatorial band, has exceeded the -0.5 La Niña threshold for the past several overlapping seasons (with values of -0.50 for Jun–Aug 2024, -0.63 for Jul–Sep, -0.75 for Aug–Oct, -0.81 for Sep–Nov, -0.92 for Oct–Dec, -1.07 for Nov–Jan, -1.12 for Dec–Feb, -0.89 for Jan–Mar 2025, -0.67 for Feb–Apr, -0.53 for Mar–May, and -0.49 for Apr–Jun 2025); however, it has returned to ENSO-neutral values (-0.40 for May–Jul, and -0.46 for Jun-Aug, 2025). The Pacific Decadal Oscillation Index in August 2025 recorded a value of -3.23, following its record-breaking value of -4.12 in July 2025.

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.

ENSO Forecast

IRI ENSO Predictions Plume

Published: September 19, 2025

Note on interpreting model forecasts

The following graph and table show forecasts made by dynamical and statistical models for SST in the Nino 3.4 region for nine overlapping 3-month periods. Note that the expected skills of the models, based on historical performance, are not equal to one another. The skills also generally decrease as the lead time increases. Thirdly, forecasts made at some times of the year generally have higher skill than forecasts made at other times of the year--namely, they are better when made between June and December than when they are made between February and May. Differences among the forecasts of the models reflect both differences in model design, and actual uncertainty in the forecast of the possible future SST scenario.

Interactive Chart

You can highlight a specific model by hovering over it either on the chart or the legend. Selecting An item on the legend will toggle the visibility of the model on the page. You can also select DYN MODELS or STAT MODELS to toggle them all at once. Clicking on the "burger" menu above the legend will give you options to download the image or expand to full screen. If you have any feedback on this new feature, please let us know at webmaster@iri.columbia.edu.

IOD Forecast

Published: September 19, 2025

Note: The Dipole Mode Index is calculated based on the ERSSTv5 data. To account for evolving background conditions and long-term warming, SST anomalies were calculated relative to a sliding monthly climatology. For each month in the time series, the climatology was computed as the mean SST for that calendar month over the prior 30 years. Then the Dipole Mode Index (DMI) is defined as the difference in sea surface temperature anomalies between the western equatorial Indian Ocean (50°E–70°E, 10°S–10°N) and the southeastern equatorial Indian Ocean (90°E–110°E, 10°S–0°), and it is used to quantify the strength and phase of the Indian Ocean Dipole (IOD).

Current Conditions

The Dipole Mode Index for the June–August 2025 season was -0.18 °C, and the monthly value for August was -0.317 °C. While these recent seasonal and monthly values suggest IOD-neutral conditions, they also point to the potential for a gradual transition toward a negative IOD phase. According to the IRI/CCSR’s criteria for defining positive and negative Indian Ocean Dipole (IOD) events, a positive IOD is classified when the DMI exceeds +0.4 °C, and a negative IOD when it falls below −0.4 °C. The IOD is considered inactive (neutral) when the DMI lies between −0.4 °C and +0.4 °C.

Model-Based IOD Outlook: Deterministic Forecasts from the NMME

Forecasts from the latest set of models in the North American Multi-model ensemble (NMME) project are used to construct deterministic IOD forecasts from each individual model using its ensemble mean Dipole Mode Index (DMI) to form an IOD forecast Plume.

The IOD plume plot illustrates the latest set of predictions from early September (CESM1, CFSv2, CanESM5, GEM-NEMO, GFDL-SPEAR), along with their equally-weighted multi-model mean. The observed DMI (shown in black) indicates a clear downward trend from May to August 2025, entering negative IOD-neutral conditions by late August. From September onward, all models forecast a transition into a negative IOD phase, with most predicting the DMI to fall below the −0.4 °C threshold during October—confirming the likely onset of a negative IOD event. Each model exhibits unique behavior in terms of intensity and recovery. CESM1 predicts the strongest negative dip in October and shows a rapid rebound, reaching neutral and then weak positive conditions by March 2026. CFSv2 and CanESM5 also predict a significant negative phase in early boreal fall but suggest a more gradual recovery through the winter, approaching neutral conditions by January or February. GEM-NEMO projects a somewhat milder negative event with a relatively steady return to neutral DMI by early 2026. In contrast, GFDL-SPEAR maintains a more persistent negative bias throughout the forecast period, with slower recovery and limited transition back to neutral conditions. Overall, while all models agree on the initiation of a negative IOD event, there is considerable spread in the strength and duration of the event, particularly beyond November. This divergence highlights the uncertainty in seasonal forecasting of IOD evolution, especially during transitions out of peak phases.

Probabilistic IOD Forecasts from the NMME

Based on September 2025 initialization data, the model-based probabilistic forecast of the Indian Ocean Dipole (IOD) was generated by CCSR@NASA-GISS/IRI to assess potential phase developments. The probabilities are computed using a counting method, where each ensemble member from the contributing models is treated individually to determine the likelihood of a negative, neutral, or positive IOD phase for the following months. The climatological probabilities for each IOD phase based on historical data are shown as dotted lines for reference.

The forecast shows a strong model consensus toward a negative IOD event during the boreal fall and early winter. For September and October, the probability of a negative IOD is near or at 100%. This high confidence persists through November, where the probability remains above 75%, while neutral and positive phases are comparatively unlikely. By December, the likelihood of a negative IOD begins to decrease (to around 50%), with a corresponding rise in the probability of neutral conditions. From January through March 2026, the forecasts shift toward increasing neutral probabilities, indicating a gradual decay of the negative IOD signal and a return to climatologically typical conditions. Positive IOD probabilities remain very low throughout the entire forecast period.

In summary, the forecast suggests a highly likely negative IOD event unfolding through the remainder of 2025, peaking in strength during October–November and weakening by early 2026. The strong agreement among model members, especially during the early months, lends confidence to the prediction of a significant negative IOD phase.