The Oceanic Niño Index (ONI) was the standard metric used to define the phase of the El Niño Southern Oscillation (ENSO).
ONI was replaced by RONI in February 2026.
It is a measure of how anomalously warm or cool the central-to-eastern equatorial Pacific Ocean is compared to "normal", smoothed out across multiple months to reduce noise and spikes. This simple index was used to identify the phase of ENSO which has significant teleconnections all around the globe.
Using the NOAA Climate Prediction Center methodology:
- Calculate a monthly average sea surface temperature in the Nino 3.4 region (5°S-5°N, 170°W-120°W).
- Calculate a monthly 30-year climatological value, updated every 5 years (1851-1880, ..., 1991-2020, 1996-2025).
- Calculate a monthly anomaly with years centered in the climatology (1866-1870 uses 1851-1880 climatology, ..., 2011-2015 uses 1996-2025 climatology, and 2016-present also uses 1996-2025 climatology because 2001-2030 climatology does not exist yet).
- Note that values prior to 1940 have greater uncertainty -- use with caution.
- All values are included in a data file at the bottom of the page.
- ONI ≥ 0.5°C indicate El Niño, values ≤ -0.5°C indicate La Niña, and values in between indicate a Neutral phase.
The monthly 30-year climatological values are updated every five years and are used to calculate the anomalies, which in turn are used to calculate the Oceanic Niño Index (ONI). The updating of the base periods is done to remove the observed warming trend in the Niño 3.4 region (see Climate Watch article). Since El Niño and La Niña are defined as departures from "normal", the normal has to be representative of the current climate if they are to mean anything. If the 1851-1880 base period were still used, we would be in a perpetual El Niño!
Data file with monthly & seasonal values since 1850
Additional information and resources:
- NOAA/NCEI ERSSTv5 overview
- NOAA/CPC Table of seasonal ONI values since 1950
- NOAA/CPC monthly Nino3.4 values since 1950
- NOAA/CPC seasonal ONI values since 1950
