ENSO Conditions and Coral Bleaching

I) CPC Official Probabilistic Forecast
(based on a consensus of NOAA's CPC and Columbia University's IRI forecasters, using observational information and model output, along with human judgment):

Septemebr 11, 2025 (updated early-month)

Alert Status: La Niña Watch
(definitions)

A transition from ENSO-neutral to La Niña is likely in the next couple of months, with a 71% chance of La Niña during October - December 2025. Thereafter, La Niña is favored but chances decrease to 54% in December 2025 - February 2026.

DIAGNOSTIC DISCUSSION:

ENSO-neutral continued in August 2025, with near-to-below average sea surface temperatures (SSTs) observed across the central and eastern equatorial Pacific Ocean. The latest weekly Niño SST index values ranged from -0.4°C to -0.2°C. Negative subsurface temperature anomalies (averaged from 180°-100°W) strengthened, with below-average temperatures prevailing from the surface to 200m depth in the central and eastern Pacific.

Low-level wind anomalies were easterly across most of the equatorial Pacific, while upper-level wind anomalies were westerly over the western and eastern equatorial Pacific. Convection was enhanced over Indonesia and was suppressed near the Date Line. Collectively, the coupled ocean-atmosphere system reflected the continuation of ENSO-neutral.

Last month, the IRI multi-model predicteds slightly favored ENSO-neutral through the Northern Hemisphere winter 2025-26. However, all available models from the North American Multi-Model Ensemble favor La Niña to emerge and persist through the winter. Based on this guidance and recently observed trends across the surface and subsurface equatorial Pacific, the team also favors La Niña to develop.

... Read more at the NOAA's Diagnostic Discussion, CPC ENSO Update, and CPC's Official Probabilistic ENSO Forecast sites, all updated at the same time in the early-month.

** Note: IRI's Monthly Ouick Look updates twice per month and presents the latest update between the (I) early-month CPC official Probabilistic Forecast (shown in this section) and (II) mid-month IRI Model-Based Probabilistic Forecast (shown in the following two sections).

(The update of the CPC ENSO diagnostics discussion and forecast is scheduled for the second Thursday of each month.)

II) IRI Model-Based Probabilistic Forecast
(a purely objective ENSO probability forecast that uses as input the model predictions from IRI's ENSO Predictions Plume):

September 19, 2025 (updated mid-month)

A transition from the current ENSO-neutral state to La Niña conditions during September–November 2025, which is expected to persist through the boreal winter of 2025/2026 (December–February).

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.

... Read more at the IRI Technical ENSO Update, IRI Model-Based Probabilistic ENSO Forecast, IRI ENSO Predictions Plume, and IRI Forecast Probability Distribution Based on the IRI ENSO Prediction Plume sites, all updated at the same time in the mid-month.

** Note: IRI's Monthly Ouick Look updates twice per month and presents the latest update between the (I) early-month CPC official Probabilistic Forecast (shown in this section) and (II) mid-month IRI Model-Based Probabilistic Forecast (shown in the following two sections).

(The update of the IRI ENSO forecast is scheduled for the 19th of each month. If the 19th falls on a weekend or holiday, it is released on the closest business day.)

III) IRI ENSO Predictions Plume
The following graph shows forecasts made by a set of dynamical and statistical models for SST in the Niño-3.4 region, for nine, overlapping, three-month periods.

September 19, 2025 (updated mid-month)

The IRI ENSO prediction plume indicates a moderate likelihood of La Niña conditions during September–November 2025, with a 56% chance. The multimodel mean of statistical and dynamical models suggests La Niña conditions are likely to persist through December–February (50%), peaking at 60% during October–December. Thereafter, beginning in January–March, ENSO-neutral conditions are expected to become dominant once again. These forecasts indicate a transition from the current ENSO-neutral state to La Niña conditions during September–November, which are forecasted to persist briefly through the boreal winter at relatively low probabilities, followed by a return to ENSO-neutral conditions that are expected to remain through the end of the forecast period. During this time, the chances of El Niño development remain minimal.

... Read more here, where any specific model can be interactively examined, along with other detailed information.

(The update of the IRI ENSO forecast is scheduled for the 19th of each month. If the 19th falls on a weekend or holiday, it is released on the closest business day.)

Historically Speaking & More:

El Niño and La Niña events tend to develop during the period Apr-Jun and they

Tend to reach their maximum strength during October - February
Typically persist for 9-12 months, though occasionally persisting for up to 2 years
Typically recur every 2 to 7 years

The spring predictability barrier: 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. This is a well-known challenge for forecaster and it can impact statistical models more than dynamical models.

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