ENSO Current State: Ocean–Atmosphere Overview

The 2026 El Niño event continues to intensify across the central and eastern equatorial Pacific. The traditional Niño 3.4 index shows a pronounced and sustained rise, with the seasonal mean reaching +2.46°C during July–September 2026 and the September monthly value increasing further to +2.84°C. The latest weekly Niño 3.4 index, centered on October 14, 2026, climbed to +3.3°C, providing further evidence that El Niño is already very strong and continuing to strengthen. Weekly sea surface temperature anomalies across the central-eastern equatorial Pacific have remained above +3.0°C for a fifth consecutive week, underscoring the exceptional intensity and persistence of the event.

Oceanic and atmospheric indicators are increasingly aligned, providing compelling evidence that the ongoing El Niño event will continue to intensify. On the atmospheric side, the Southern Oscillation Index (SOI) reached −21.4 in September 2026, while the latest 30-day SOI value, through October 17, remained strongly negative at −18.6. These persistently negative SOI values indicate a pronounced weakening of the Walker circulation, consistent with ongoing El Niño. In addition, the Equatorial SOI reached −2.3 in September 2026, further indicating that the atmosphere is responding to the warming Pacific Ocean. The spatial pattern of convection and winds further reinforces this atmospheric response. Enhanced convection and precipitation extended across the equatorial Pacific from the Date Line to the eastern Pacific, while suppressed convection persisted over the southern Philippines, Indonesia, and Papua New Guinea. Consistent with the exceptionally strong El Niño, low-level (850-hPa) westerly wind anomalies extended from the western to the east-central equatorial Pacific, while upper-level (200-hPa) easterly anomalies prevailed across the same region, with westerly anomalies over the far eastern equatorial Pacific. Together, these signals provide strong evidence that ocean–atmosphere coupling associated with El Niño is firmly established.

The subsurface ocean remains exceptionally warm, reinforcing evidence of the major El Niño's persistence and strength. A vast reservoir of unusually warm water continues to extend across the central and eastern equatorial Pacific, with temperatures at 50–150 m depth between 150°W and 80°W exceeding climatological values by more than 8°C. The exceptional subsurface warming is now becoming evident at the surface in the far eastern Pacific. The Niño 1+2 index has exceeded +5.0°C for three consecutive weeks, signaling the emergence of exceptionally strong coastal El Niño conditions along the South American coast. With substantial subsurface heat still in place, this vast reservoir of ocean heat could fuel further surface warming and intensify El Niño, provided favorable ocean–atmosphere coupling persists. In tandem, subsurface cooling is evident at approximately 100–250 m in parts of the western Pacific, from around 140°E toward the Date Line, although the anomalies there are considerably weaker. The stark contrast between the exceptionally warm central–eastern Pacific and the weaker or negative oceanic anomalies farther west provides strong evidence of a major redistribution of subsurface ocean heat associated with the ongoing El Niño. The magnitude and spatial extent of this warming raise the potential for further increases in surface temperature anomalies through the boreal fall and winter. If this evolution continues, the 2026 El Niño could rank among the strongest events in the historical ENSO record.

ENSO Prediction Plume: Model-by-Model Outlook (Traditional Niño 3.4 Index)

The mid-October 2026 CCSR/IRI ENSO prediction plume has been updated with the latest model forecasts. The outlook shows close agreement among the 27 participating models (16 dynamical and 11 statistical) that El Niño conditions will strengthen further during 2026 and continue into early 2027.

The models deliver a strong and consistent message: an exceptionally strong El Niño is expected to persist through early 2027. The combined average of dynamical and statistical models (thick blue line) forecasts the Niño 3.4 index to reach approximately +3.5°C in both OND and NDJ 2026. Equally striking is the tight spread around the multimodel average from OND 2026 through MAM 2027, indicating strong model agreement on the event's continued persistence. Although individual models differ in the magnitude of the peak, the overall signal points to a very strong El Niño lasting well beyond fall 2026 and into spring 2027.

ENSO Probabilities

To generate the ENSO probability outlook, forecasts from all 27 participating models (16 dynamical and 11 statistical) are combined into an equally weighted multi-model average of Niño 3.4 SST anomalies. A Gaussian error distribution is then applied to the ensemble-mean forecast, with its width determined by the expected forecast skill for the season and lead time. Higher forecast skill results in a narrower distribution and greater confidence, while lower skill produces a broader range of possible outcomes. Probabilities are calculated based on the likelihood of Niño 3.4 SST anomalies falling within the standard ENSO thresholds: El Niño (≥ +0.5°C), ENSO-neutral (−0.5°C to +0.5°C), and La Niña (≤ −0.5°C). This methodology translates the multi-model forecast into probabilities for each ENSO category.

The October 2026 objective ENSO outlook strongly favors El Niño persisting through boreal spring 2027, with a potential continuation into early summer. El Niño probabilities remain at 100% from OND 2026 through MAM 2027, then ease to 92% in AMJ and 60% in MJJ 2027. By JJA 2027, ENSO-neutral becomes the most likely state at 51%, while the El Niño probability drops to 37%, signaling a gradual weakening of the event toward summer.

Notably, no probability is assigned to either La Niña or ENSO-neutral conditions from SON 2026 through MAM 2027, underscoring the exceptionally strong likelihood of continued El Niño conditions through the remainder of 2026 and into early 2027.

ENSO Strength

Forecasts of ENSO strength are estimated through a consensus-based model counting approach using a set of Niño 3.4 sea surface temperature anomaly thresholds, to indicate weak, moderate, strong and very strong El Niño and La Niña categories.

The mid-October 2026 intensity assessment provides a striking indication of the exceptional strength of the ongoing El Niño event. The model ensemble points to an exceptionally strong 2026–27 El Niño, with all 27 models forecasting a "very strong" El Niño (Niño 3.4 ≥ +2.0°C) from OND 2026 through DJF 2026–27. Our current intensity classification is capped at +2.0°C, yet a majority of models (about 10 models) in the plume—including both dynamical and statistical models—forecasts Niño 3.4 values that could reach or exceed +3.5°C, well beyond our highest defined category. This is more than a high-probability El Niño: the ensemble shows an unusually strong and consistent signal for an event of exceptional intensity. Although the forecast shifts toward weaker categories in early 2027, strong-to-moderate El Niño conditions remain through MJJ 2027, underscoring the unusual magnitude and persistence of the 2026 event. Taken together, the mid-October objective ENSO outlook points to an exceptionally strong El Niño that will remain firmly established through the end of 2026 and persist into late spring 2027.

Notes

The SST anomalies cited below are based on the NOAA Optimum Interpolation Sea Surface Temperature (OISSTv2) dataset. The climatology period is 1991–2020.

The primary metric used to monitor the El Niño–Southern Oscillation (ENSO) is the traditional Niño 3.4 index (TONI), defined as the area-averaged sea surface temperature anomaly over the Niño 3.4 region (5°S–5°N, 170°W–120°W).

According to the CCSR/IRI definition, El Niño conditions occur when monthly TONI exceeds +0.5°C, while La Niña conditions occur when monthly TONI falls below −0.5°C.

An ENSO "event" is considered established when the TONI threshold (±0.5°C) persists for at least five consecutive overlapping 3-month seasons (e.g., SON, OND, NDJ, DJF, JFM).