ONI is a standard metric used to define the phase of the El Niño Southern Oscillation (ENSO).
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 is 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 (1856-1885, ..., 1986-2015, 1991-2020).
- Calculate a monthly anomaly with years centered in the climatology (1871-1875 uses 1856-1885 climo, ..., 2001-2005 uses 1986-2015 climo, 2006-2010 uses 1991-2020 climo, 2011-2025 also uses 1991-2020 climo because 1996-2025 climo does not exist yet).
- Note that values prior to 1950 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 1856-1885 base period were still used, we would literally be in a perpetual El Niño!
Data file with monthly & seasonal values since 1854
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
