For decades, NOAA's Climate Prediction Center classified El Niño and La Niña using the Oceanic Niño Index (ONI) — a simple three-month running average of sea surface temperature anomalies in the Niño 3.4 region of the equatorial Pacific. If that anomaly sat at or above +0.5°C for five consecutive overlapping seasons, you had an El Niño. Simple, consistent, and the standard for almost 30 years.
The problem: the global ocean has been warming for decades because of climate change, independent of any ENSO cycle. ONI doesn't separate "this is El Niño" warming from "this is just the whole ocean running hotter than it used to." As baseline temperatures crept up, ONI started reading recent El Niño events as proportionally stronger than they actually were relative to the rest of the tropical Pacific — and La Niña events as weaker.
What RONI Actually Does
The Relative Oceanic Niño Index (RONI) fixes this by subtracting the tropical-mean sea surface temperature anomaly (averaged across 20°N–20°S) from the Niño 3.4 anomaly before classifying anything. In other words: instead of asking "is this region warmer than its 1991-2020 average," RONI asks "is this region warmer than the rest of the tropics are right now." That's a meaningfully different — and more diagnostic — question.
The practical effect, confirmed by NOAA's own analysis: RONI tends to dampen the apparent strength of El Niño events and amplify the apparent strength of La Niña events compared to the old ONI. A textbook example — using ONI, 2020–2023 was read as a choppy "triple-dip" La Niña with several neutral-looking gaps. Using RONI, that same period reads as one continuous, unbroken La Niña.
Why It Matters for You
If you're tracking this site's RONI panel alongside other sources still quoting legacy ONI numbers, don't be surprised if they diverge slightly — especially during a developing event like the current one. Both metrics are tracking real phenomena; RONI is simply the more climate-change-aware version, and it's now NOAA's official standard.