The Relative Oceanic Niño Index (RONI) is the standard metric used to define the phase of the El Niño Southern Oscillation (ENSO).
RONI replaced ONI in February 2026.
It is a measure of how anomalously warm or cool the central-to-eastern equatorial Pacific Ocean is relative to the rest of the global tropics, smoothed out across three months to reduce noise and spikes.
Products on this page utilize the NOAA Climate Prediction Center methodology for evaluating ENSO (see first two bullets at bottom of page):

• 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.
• RONI ≥ 0.5°C indicate El Niño, values ≤ -0.5°C indicate La Niña, and values in between indicate a Neutral phase.

Data file with monthly & seasonal values since 1850


Additional information and resources:
    - Announcement of transition from ONI to RONI in February 2026
    - "A Relative Sea Surface Temperature Index for Classifying ENSO Events in a Changing Climate" (L'Heureux et al. 2024)
    - "Extended Reconstructed Sea Surface Temperature Version 6 (ERSSTv6): Part I. An Artificial Neural Network Approach" (Huang et al. 2025)
    - "Extended Reconstructed Sea Surface Temperature Version 6 (ERSSTv6): Part II. Upgrades on Quality Control and Large-scale Filter" (Huang et al. 2025)
    - NOAA/NCEI ERSSTv6 overview
    - NOAA/CPC seasonal RONI values since 1950