RONI (Relative Oceanic Niño Index)

Global warming created a measurement problem for El Niño watchers: if the whole tropical ocean is warmer than it used to be, a given Niño 3.4 anomaly no longer means what it once did. The atmosphere responds to contrasts — how much warmer the central Pacific is than the tropics around it — not to absolute readings against a past climatology. An index that ignores the warming baseline risks calling events "stronger" than their real atmospheric punch, especially decades apart.

RONI, the Relative Oceanic Niño Index, is the fix. It takes the standard ONI calculation — the three-month running mean of Niño 3.4 anomalies — and subtracts the average sea-surface-temperature anomaly of the whole tropical belt over the same window. What remains is the Pacific's excess warmth relative to its competition, net of the ocean's shared warming trend. NOAA CPC now maintains RONI as the relative ENSO index, publishing it seasonally (three-month values, not weekly) alongside the conventional ONI. Its thresholds carry over unchanged: at or above +0.5°C is El Niño territory, at or below −0.5°C is La Niña.

The practical effect is a re-ranking of the historical record. Under the conventional ONI, 2015-16 holds the modern peak; under RONI, 1982-83 — a shorter event riding a cooler baseline, so less of its reading was "just" background warming — reclaims the top spot instead. It is the clearest evidence that RONI and ONI answer genuinely different questions, not the same question twice.

The current reading is +0.5 for AMJ 2026 — a fresh crossing of the RONI threshold, and typically a touch lower than the same season's conventional Niño 3.4 anomaly on the dashboard. Both indices track the same ocean; RONI just asks how it compares to the rest of a warmer world.

NOAA isn't alone in asking that question. Australia's Bureau of Meteorology publishes its own BOM relative Niño 3.4 index every week — same warming-baseline logic, a different agency, and its own +0.8°C threshold, not RONI's ±0.5°C.