In April–June 2026 the same ocean produced two El Niño readings. One said 0.95. The other said 0.49. On the first, the Pacific was well into El Niño territory. On the second, it had not yet crossed the line. Both numbers were correct, both were published, and a reader who met them side by side would reasonably conclude that one of them was broken.
Two indices, one season
The first number is the Oceanic Niño Index, ONI: the three-month average sea-surface temperature anomaly in the central equatorial Pacific, the Niño-3.4 box. It is the index most news coverage means when it says "El Niño", and it has been for decades.
The second is the Relative Oceanic Niño Index, RONI. It starts from the same Niño-3.4 anomaly and then subtracts the average anomaly across the whole tropical ocean. NOAA's Climate Prediction Center publishes both. The question each one answers is different, and the gap between them is the story.
Why subtract the tropics
El Niño matters because the central Pacific becomes warm relative to the water around it. That contrast is what shifts where the rain falls. But when the entire tropical ocean warms, the Niño-3.4 box warms too, and ONI rises even if the contrast has not changed at all.
RONI removes that shared warming so that what remains is the contrast itself. In a warming climate the two indices drift apart, and the size of the drift says how much of ONI's reading is the whole ocean rather than the pattern that drives rainfall.
Why the threshold makes it sharp
NOAA's working definition of an El Niño episode is a reading of at least 0.5 on ONI across five overlapping seasons. At 0.95, ONI was nearly double that. At 0.49, RONI was a hundredth short of it. Put plainly, one index said "El Niño" in the same season that the other said "not yet".
A difference that large, at the one line everyone is watching, is not rounding. It changes the answer to the question a planner actually asks.
Which one to use
Neither index is the true one. ONI answers "how warm is this patch of ocean compared with its past". RONI answers "how much warmer is it than the tropics around it". For rainfall in Southeast Asia, the second question is usually the one that matters, because the contrast is what moves the weather.
What matters more than the choice is saying which one you used. A number reported as "El Niño index" with no name attached forces the reader to guess, and in 2026 the two guesses gave opposite answers.
What we do
When GlobMaps reports an ENSO figure, we name the index next to it, because a reading without its index is a number the reader has to fill in for themselves. We also do not move a threshold from one index to the other: a line drawn on ONI is not a line on RONI, even when both happen to be written as 0.5.
None of this makes the forecast more certain. It makes the disagreement visible, which is where it belongs, instead of hiding it behind a single number that sounds more confident than the ocean is.