Measured against a fixed 30-year average, the central Pacific has just had its warmest May–July since 1950. Measured against the rest of the tropical ocean — NOAA's official method since February — three past El Niños were warmer at the same stage.
El Niño is a recurring warm phase in the central Pacific that shifts rainfall, winds and storm tracks across much of the world. Since the 1990s, NOAA has judged whether one is under way using the Oceanic Niño Index, which compares sea temperatures in one stretch of the central Pacific against the average for 1991–2020. On 1 February 2026 the National Weather Service replaced it with the Relative Oceanic Niño Index.1
The new index starts from the same temperatures, then subtracts the average warmth of the entire tropical ocean, from 20°N to 20°S, and rescales the result so it swings as widely as the old one did.1 Put plainly: the old measure asks how warm the Pacific patch is against a fixed benchmark. The new one asks how warm it is compared with the rest of the tropics right now. The temperature thresholds for declaring an El Niño are unchanged.2
NOAA has had an El Niño Advisory in force since June. Its 9 July assessment recorded Pacific temperatures 1.2°C above normal, with winds and rainfall across the ocean shifted into the pattern an El Niño requires.3 That makes 2026 the first El Niño judged by the new measure — and the first case where the two disagree about how big it is.
Because the old benchmark is fixed in the past, warming that has spread across the whole tropical ocean gets counted as El Niño signal — even though it has nothing to do with the Pacific cycle. The new index strips that out. So the gap between the two readings is close to a direct measure of how far the tropics as a whole have warmed since the 1991–2020 baseline.
Across the twenty May–July seasons from 1950 to 1969, that gap averaged −0.11°C. Across the twenty from 2007 to 2026 it averaged +0.25°C. This year's is +0.41°C — large, though not the largest; May–July 2024 reached +0.63°C.
This is not just bookkeeping. El Niño moves weather through contrast — it is the Pacific patch being warmer than the ocean around it that shifts rainfall and storms. NOAA's stated reason for the switch is that the relative measure tracks observed rainfall, storm tracks and winds more closely.2 Rescoring the record drops three weak El Niño winters — 1958–59, 2014–15, 2019–20 — and adds 1992–93.2
They also disagree about when this event began: the old index crossed the threshold in April–June, the new one a season later.45
One reading of Exhibit 01 is that the old index's record is partly an artefact of a warming baseline rather than an unusually strong Pacific event — which is what the change was designed to correct for.2 It remains an interpretation: these are two different measures, not a right one and a wrong one. NOAA's outlook — a 97% chance El Niño lasts into early spring 2027, and 81% odds of a very strong event during October–December — is a forecast, and appears in no figure here.3
Recent readings on the new index are provisional: NOAA warns they can be revised for up to two months after publication, so +0.98°C is an estimate.6 The data files used here also run about 0.1°C below the tables on NOAA's web pages, because each draws on a different version of the sea temperature record.2 Both exhibits use one consistent pair of files rather than mixing the two. This El Niño has not peaked, so where it finally ranks is unknown, and nothing here establishes what weather it will bring to any particular place.
oni.ascii.txt), values through May–July 2026. cpc.ncep.noaa.gov. Retrieved 9 Aug 2026.RONI.ascii.txt), values through May–July 2026. cpc.ncep.noaa.gov. Retrieved 9 Aug 2026.