This El Niño is second only to 1997 for the time of year — and still short of the four biggest

Second only to 1997

The Relative Oceanic Niño Index (RONI) measures how much warmer the central-eastern equatorial Pacific is than normal, after subtracting the average warming across the whole tropical ocean between 20°N and 20°S.² It is now the primary standard for NOAA's official El Niño advisories and classifications.³ Noodle News explained the switch in August, when the May–July reading of the new index ranked 2026 fourth for that season.

Two months on, it has moved up. NOAA's Climate Prediction Center (CPC) puts the July–September 2026 value at +1.69°C. Among the 77 July–September seasons since 1950, only 1997 was warmer, at +1.84°C. The June–August reading, +1.36°C, also ranks second, again behind 1997.¹

The index rose by 0.72°C between May–July (+0.97°C) and July–September. Over the same two-month step, 1997 rose by 0.64°C and 2015 by 0.52°C.¹ At this point in the year, 2026 is ahead of 2015, which went on to become one of the strongest events in the record.

The warmth is concentrated in the east. In August, on the traditional monthly indices, CPC recorded anomalies of +3.4°C in the Niño-1+2 region off South America, +2.5°C in Niño-3 and +1.8°C in Niño-3.4, but only +0.1°C in Niño-4, the westernmost box. It also reported subsurface anomalies above +10°C, low-level westerly wind anomalies along the equator and negative Southern Oscillation indices — the atmospheric half of the pattern that defines an El Niño.⁴

The four that went above +2.0°C

Since 1950, four El Niños have peaked above +2.0°C on RONI: 1982–83 at +2.40°C, 1997–98 at +2.28°C, 2015–16 at +2.25°C and 1991–92 at +2.12°C.¹ Two others came close without crossing the line: 1965 reached +1.99°C and 1972 reached +1.97°C. No other El Niño since 1950 got above +1.9°C.

All four were still strengthening in July–September. After that season, each added between 0.44°C (1997–98) and 1.29°C (1991–92) before peaking in November–January or December–February.¹ Those gains describe what happened in past events. They are not a forecast for this one, and Noodle News has not applied them to 2026.

The ranking at this stage is also a weak guide to the final peak. In July–September 1982, the index stood at +1.14°C, seventh of the eight seasons in Exhibit 01; that event went on to set the record. The 1991–92 event was further back still, at +0.83°C.¹

The reverse also happens. In 1965 and 1972, July–September readings of +1.49°C and +1.40°C put both events fourth and fifth on this list. Each then added only about half a degree — 0.50°C and 0.57°C — and peaked just under +2.0°C, in October–December.¹ A strong reading at this stage has preceded both outcomes.

What forecasters expect, and how sure they are

Everything in this section is a forecast, not a measurement. In its 10 September discussion, CPC gave a greater than 90% chance of a very strong El Niño through the northern autumn and winter. It put the chance of a historic event — a three-month RONI value of +2.5°C or more in October–December — at 75%.⁴ That was up from 69% in its 13 August discussion.⁵ No El Niño since 1950 has reached that level on RONI.

CPC does not define "very strong" in the discussion text. The International Research Institute for Climate and Society (IRI) at Columbia University uses +2.0°C on the traditional, non-relative Niño-3.4 index. In its mid-September outlook, all 22 models in its forecast set kept the event at or above that level from September–November through December–February, and it put the chance of El Niño conditions at 100% through February 2027.⁶

The two bodies are measuring slightly different things. IRI's figures are standard sea-surface temperature anomalies against a 1991–2020 average, without the tropics-wide adjustment that RONI makes, so its numbers are not directly comparable with those above.⁶ CPC's next discussion is due on 8 October.⁴