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.

RONI, the Relative Oceanic Niño Index, is NOAA's answer. It takes the standard ONI calculation and subtracts the average sea-surface-temperature anomaly of the whole tropical belt. What remains is the Pacific's excess warmth relative to its competition — the quantity that actually shifts rainfall and drives teleconnections.

In practice RONI reads systematically lower than ONI in the modern era, and the gap grows as the baseline warms. An event can post a record raw ONI while its RONI ranks below older giants — part of the nuance in comparing 2015-16 with 1997-98, and now 2026-27 with both. When this site says an event is "strong," the classification follows the official ONI; RONI is the second opinion careful readers check. Both ultimately summarize the same water tracked weekly on the dashboard.