El Niño 2026 and the ONI and RONI indices

On 12 November NOAA's Climate Prediction Center (CPC) recorded a further strengthening of El Niño, with the Niño-3.4 index at 2.5 °C above average, but lowered the chance of an event stronger than any since 1950 to about 60% for October to December and November to January. A month earlier it put that chance at 83%. Labels such as "super El Niño" are circulating in the media at the same time, and the gap between them and the CPC's assessment comes mainly from the index each one uses, because since February 2026 NOAA officially measures El Niño with the Relative Oceanic Niño Index (RONI) instead of the traditional Oceanic Niño Index (ONI). On the ONI, August to October 2026 is the third warmest three-month season since 1950, while on the RONI it sits below the three big events of 1982/83, 1997/98 and 2015/16.

The ONI measures temperature against a fixed average

The ONI is the three-month running mean of the sea surface temperature anomaly in the Niño-3.4 region of the central and eastern equatorial Pacific, measured against the average of a 30-year period, as described by the US National Integrated Drought Information System (NIDIS). The ocean as a whole is warming, however. When the entire tropical belt is warmer, the same region looks warmer than the past average even if its difference from the surrounding oceans is small, while according to NOAA rainfall in the tropical Pacific is more closely linked to that relative difference.

NOAA switched to the RONI on 1 February 2026

In a notice from the US National Weather Service, the CPC implemented the RONI from 1 February 2026. The RONI is the three-month running mean of the relative Niño-3.4 index, meaning the temperature of the Niño-3.4 region compared with the average of the whole tropical belt. According to the CPC, this reduces the dependence on the reference period, which in the ONI caused problems mainly for real-time estimates. The National Weather Service adds that the new index is more closely linked to rainfall changes in the tropical Pacific and that the records of the old ONI will continue to be updated.

The definition of an event stayed the same and now applies to the new index. El Niño is present when the relative anomaly reaches +0.5 °C or more, and a historical event is recorded when the RONI stays above +0.5 °C for five or more consecutive, overlapping three-month seasons. According to NIDIS, in most cases the RONI lowers the strength of El Niño events and raises the strength of La Niña events compared with the ONI. The period from 2020 to 2023 had three consecutive La Niña events on the ONI, while on the RONI it was a single three-year event. The 2023/24 El Niño remained an El Niño but slightly weaker, and the winters of 2024/25 and 2025/26, neutral on the ONI, are recorded as La Niña on the RONI.

The 2026 El Niño sits below the big events on the RONI and near the top on the ONI

In the CPC files for the ONI and the RONI, the two indices rose in parallel all summer, about half a degree apart. The ONI went from +1.39 °C in May to July to +1.80 in June to August, +2.16 in July to September and +2.46 in August to October. The RONI went from +0.97 to +1.36, +1.69 and finally +1.97 °C. On the ONI, the August to October value is the third highest three-month value since 1950, and only two seasons of 2015/16 are higher.

Event	ONI peak (°C)	RONI peak (°C)
1982/83	+2.14	+2.40
1997/98	+2.37	+2.28
2015/16	+2.59	+2.25
2026 (to October)	+2.46	+1.97

Source: NOAA Climate Prediction Center, ONI and RONI files, three-month values, November 2026 update

In 1982/83 the RONI was in fact higher than the ONI by 0.26 °C, while in 2015/16 it was lower by 0.34 °C and this year by 0.49 °C. This fits the NIDIS description that the RONI removes the long-term warming of the ocean that the ONI does not take into account.

The CPC gives a historic event a 60% chance

The CPC discussion of 12 November defines a historic event as a three-month RONI value of +2.5 °C or more, meaning above every previous El Niño since 1950. The chance of this is now put at about 60% for October to December and November to January, while the chance of a strong to very strong El Niño lasting through January to March 2027 remains above 79%. In the October discussion the corresponding values were 83% for October to December, 70% for November to January and above 83% for a strong to very strong event.

In October the readings were already very high, with Niño-3.4 at +2.1 °C, Niño-3 at +3.0 °C and Niño-1+2, close to the coast of Peru, at +3.9 °C. In November they rose to +2.5, +3.6 and +4.4 °C respectively, and the warm sea anomaly, above 4 °C, spread farther west. According to the CPC, El Niño will strengthen further before it begins to weaken in early 2027. The CPC's strength probability table splits RONI values into half-degree classes, with the lowest, from 0.5 to 1.0 °C, corresponding to a weak event and the highest, 2.0 °C and above, to a very strong one. In the November issue the chance of the highest class is 100% for October to December, 96% for November to January and 79% for December to February, while neutral conditions become the most likely outcome from April to June 2027, at 61%.

The choice of index changes the comparisons with past events

For crops it is rainfall that matters, and the US National Weather Service notes that the RONI is more closely linked to rainfall in the tropical Pacific. So when an article or a market analysis compares this year's event with 1997/98 or 2015/16 to predict effects on droughts, harvests or prices, the index used for the comparison should be stated. On the ONI, 2026 already exceeds 1997/98, while on the RONI it is 0.31 °C lower. FAO's maps of the regions where El Niño usually brings drought or extra rain are based on the history of many events, and the June forecasts for crops were written before the event reached its current strength.

The two indices also differ for earlier years, since according to NIDIS the RONI sees 2023/24 as a slightly weaker El Niño and the winters of 2024/25 and 2025/26 as La Niña. An analysis that uses the ONI to find "similar years" can therefore end up with a different list of events from an analysis that uses the RONI.

Frequently asked questions

Which index is official for El Niño? Since 1 February 2026 NOAA's CPC uses the RONI. The ONI records are still updated, which is why many analyses and news reports still use it.

Is the 2026 El Niño the strongest on record? Not up to October. On the ONI it is third since 1950, behind 2015/16, and on the RONI it sits below 1982/83, 1997/98 and 2015/16. The CPC gives about a 60% chance that it will exceed +2.5 °C on the RONI during the northern hemisphere winter.

What does a historic El Niño event mean? In the CPC's wording, an event with a three-month RONI value of +2.5 °C or more, meaning stronger than any El Niño since 1950.

When will the 2026 El Niño end? The CPC expects it to weaken from early 2027, and its probability table shows a return to neutral conditions in the northern hemisphere spring or early summer of 2027. The effects on rainfall and crops continue after the peak.

Why is the RONI usually lower than the ONI in recent events? Because it compares the Niño-3.4 region with the whole tropical belt, which is also warming, while the ONI compares it with a fixed 30-year average. General warming raises the ONI without raising the RONI.

Was 2023/24 an El Niño? Yes, on both indices. On the RONI it is recorded as slightly weaker than on the ONI.

Important notes

The values in the article are three-month values from the CPC files as updated in November 2026 and they change every month. The CPC's probabilities are forecasts and are revised in each monthly discussion, as the drop from 83% to 60% within one month showed. According to the CPC, the strength of an event does not necessarily match the intensity of its effects, and a stronger El Niño raises confidence in its typical effects on rainfall without guaranteeing them.