The short answer
ONI measures the three-month Niño-3.4 sea-surface temperature anomaly against a climate baseline. RONI starts with the same Niño-3.4 region but compares it with the broader tropical-ocean temperature anomaly, then rescales the result to preserve comparable variability. In February 2026, NOAA's Climate Prediction Center made RONI its primary index for official ENSO monitoring and prediction.
RONI vs ONI at a glance
| Question | ONI | RONI |
|---|---|---|
| Core region | Niño-3.4 | Niño-3.4 |
| Reference | Departure from a centered 30-year climatology | Niño-3.4 departure relative to the broader tropical-ocean departure, then variance-adjusted |
| Official CPC ENSO monitoring in 2026 | Still published for continuity | Primary standard |
| Main advantage | Long-established and familiar | Less sensitive to changes in the tropical mean and climate base period |
| Can the value differ for the same season? | Yes. The indices measure the same Pacific region against different reference frames. | |
Why NOAA changed the index
CPC says the traditional ONI can become harder to interpret when broad tropical sea-surface temperatures are unusually warm or cool relative to the fixed climate baseline. RONI removes the tropical-mean component before classifying ENSO, so the index better isolates how unusual Niño-3.4 is compared with the tropics as a whole.
NOAA also notes that RONI is less sensitive to the choice of 30-year reference period and can align more clearly with the atmospheric patterns that create ENSO impacts. The goal is not to make events look weaker or stronger—it is to make classifications more stable and physically representative.
Why two NOAA El Niño numbers can disagree
You may see one page quoting a conventional Niño-3.4 or ONI value and another NOAA product using RONI. A difference does not automatically mean one page is stale or wrong. Before comparing numbers, check the label: Niño-3.4 anomaly, ONI and RONI are not interchangeable.
This is especially important in 2026 because CPC's official strength probabilities are verified with RONI. When Local Winter Outlook says the event has a very high probability of becoming “very strong,” it is referring to CPC's RONI strength categories, not an older ONI threshold table.
Did NOAA rewrite El Niño history?
CPC has recalculated the historical record with RONI, so some past seasons can move between weak, moderate, strong or neutral labels compared with ONI. The weather that happened did not change. The classification changed because the reference frame changed. NOAA continues to publish historical ONI tables so older studies and records can still be interpreted in their original context.
What RONI does not tell you
- It does not predict your city's winter temperature.
- It does not tell you how much rain or snow will fall.
- A larger RONI value does not guarantee stronger local impacts.
- It does not replace CPC's seasonal temperature and precipitation outlooks.
That last point matters most for planning. CPC explicitly warns that large positive or negative RONI values do not necessarily translate into equally large local effects. Use ENSO strength as a background signal, then use the official seasonal outlook for the regional odds.
Which number should you use?
For the current 2026–27 event, use RONI for official CPC ENSO status and strength. Use ONI when reading older research, comparing with legacy records, or when a source explicitly says it is using ONI. Never mix the two in a single ranking without labeling them.
Primary sources: NOAA CPC RONI adoption announcement ↗, official RONI history ↗, and official ONI history ↗.