The El Niño Briefing #8: All 22 models say 'very strong' through the winter

Wednesday's short issue carried the number: NOAA's weekly Niño 3.4 reached +3.0 °C for the week centred 16 September, equal to the warmest week in its series. NOAA publishes the next weekly value on Monday, so this issue is about what the forecasters did with that reading.

All 22 models: very strong through the winter. The International Research Institute for Climate and Society (IRI) at Columbia published its September outlook on 21 September. All 22 models in its forecast plume put the event in the "very strong" class — Niño 3.4 at +2.0 °C or more — from September–November 2026 through December–February 2026–27. IRI's strength classes stop at +2.0 °C; the institute adds that a majority of the models, dynamical and statistical alike, could reach or exceed +3.0 °C. Its September summary puts the probability of El Niño at 100% through February 2027, falling to 61% by mid-2027.

Why the models agree: the heat below the surface. IRI reports subsurface temperatures 50–150 m down, between 150°W and 80°W, exceeding normal by more than 8 °C. That reservoir is what feeds the surface. Off Peru and Ecuador, NOAA's weekly file has Niño 1+2 at or above +4.0 °C every week since early August, and at +4.6 °C for the week centred 16 September. The atmosphere is coupled to it: IRI puts the 30-day Southern Oscillation Index to 16 September at −20.1, far past the −7 that the Bureau of Meteorology treats as the El Niño threshold.

The relative index held. Australia's Bureau of Meteorology relative Niño 3.4 — which, like NOAA's RONI, removes the warming of the whole tropical ocean — read +2.53 °C for the week ending 20 September, from +2.51 °C a week earlier, against its El Niño threshold of +0.8 °C. NOAA's seasonal RONI still stands at +1.36 for June–August; it moves once a month, not weekly.

One caution about all of this. A plume is a forecast of the index, not an observation, and +3.0 °C is one week's reading. The official odds are still CPC's from 10 September: 75% that October–December 2026 is stronger than any El Niño since 1950 on the three-month RONI.

Next. NOAA's weekly file updates on Monday 28 September with the week centred 23 September. The next CPC monthly discussion is due on 8 October.

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— The El Niño Briefing