JMA: El Niño Continuation Through Autumn Now 'Virtually Certain' as NINO.3 Hits +1.9°C

The Japan Meteorological Agency has removed any residual doubt about the trajectory of the 2026 El Niño, assigning a probability of 100% — "virtually certain," in the agency's phrasing — that the event continues through the boreal autumn.

The assessment came in JMA's monthly El Niño Outlook issued on 10 July. The agency judges that "El Niño conditions are considered to have persisted in the equatorial Pacific since Spring 2026," and its flagship index has climbed steeply since then.

A different yardstick, the same story

JMA is distinctive among major forecast centres in defining El Niño by the NINO.3 region of the eastern equatorial Pacific, rather than the Niño 3.4 region used by NOAA, the WMO and others. Because NINO.3 sits further east — closer to the South American coast, where this event has warmed hardest — its numbers are not interchangeable with Niño 3.4 values, a distinction worth keeping in mind when comparing agency reports.

On JMA's yardstick, the event's acceleration is stark. The monthly NINO.3 anomaly rose from +1.2°C in May to +1.9°C in June. The agency's formal event criterion — the five-month running mean of the NINO.3 index — stood at +0.8°C for April 2026, comfortably past the threshold and, given the monthly values now feeding into it, set to climb substantially as the year progresses.

The atmosphere is cooperating with the ocean. JMA reports a Southern Oscillation Index of −2.5 for June — a strongly negative value indicating the sea-level pressure seesaw between Tahiti and Darwin has swung into its El Niño configuration, with weakened trade winds across the basin. That ocean–atmosphere coupling is the engine of the event, as explained in the science of El Niño.

Warm water in the pipeline

JMA's outlook also addresses the mechanism sustaining the event. The agency points to warm subsurface water in the central and eastern equatorial Pacific, which continues to feed surface warming through ocean–atmosphere interaction. In practical terms, the heat needed to maintain — and likely intensify — the surface anomaly is already banked below the surface, which is a key reason the forecast confidence is so high.

The 100% continuation probability is derived from JMA's seasonal ensemble prediction system, JMA/MRI-CPS4, which runs a suite of coupled ocean–atmosphere simulations. For a probabilistic forecast system to concentrate its entire ensemble on one outcome is unusual; it happens only when an event is large, well-established and dynamically locked in.

What it means for Japan

El Niño years carry well-known tendencies for Japan's climate — most classically a warmer autumn and a milder winter, as the winter monsoon flow from the continent tends to weaken. JMA issues its Japan-specific seasonal outlooks separately, but the El Niño Outlook is the foundation on which those forecasts rest. With continuation through autumn now assessed as certain, the question for Japanese planners shifts to the event's peak amplitude and its duration into 2027.

JMA's assessment aligns with the international consensus. NOAA's July discussion put a 97% probability on the event persisting through early spring 2027, as covered in NOAA's July diagnostic discussion, and the WMO expects a peak around September–November. On every agency's index — however defined — the direction of travel through mid-2026 has been the same: up.

Next update

JMA issues its El Niño Outlook monthly, around the 10th of each month. The August edition will show whether the NINO.3 index has extended its May-to-June surge, and will carry the first extended-range guidance reaching into the Northern Hemisphere winter — the season when, historically, eastern-Pacific warm events reach their peak.