Methodology
ENSO indices: definitions and interpretation
Conventional and relative sea surface temperature anomalies
A conventional sea surface temperature (SST) anomaly represents the departure of a Niño region’s SST from its 1991–2020 climatological baseline. Relative anomalies additionally account for contemporaneous temperature anomalies across the broader tropics. The default display uses NOAA’s published relative anomalies derived from OISST v2.1. The conventional display uses the separate OISST v2.1 archive of conventional anomalies.
Both datasets are reproduced as published by NOAA. No fixed correction is applied, and anomalies are not reconstructed from rounded SST values. The relative weekly dataset provides anomalies for four Niño regions without absolute SST values. Absolute SST values from the conventional dataset are retained in tooltips and downloads.
The Relative Oceanic Niño Index
The Relative Oceanic Niño Index (RONI) is a three-month relative Niño 3.4 index derived from ERSST v6. Its calculation includes adjustment for tropical-mean anomalies and rescaling. NOAA publishes a new overlapping three-month season each month, normally by the fifth day. RONI is a distinct seasonal product; weekly Niño 3.4 anomalies do not constitute a weekly RONI series.
Each RONI season is assigned to the year of its central month. Thus, DJF 1950 covers December 1949–February 1950, whereas NDJ 1950 covers November 1950–January 1951. Seasonal comparisons use twelve labelled three-month periods. Weekly comparisons align observations by month and day on a leap-year calendar, preserving 29 February.
ENSO phase assessment and episode classification
The ENSO phase displayed at the top of the page is taken from the dated NOAA CPC ENSO Diagnostic Discussion. Advisory and watch designations are reported separately. An El Niño watch indicates the potential for development rather than an established El Niño phase. The reported assessment is independent of the selected region and the availability of individual measurements.
The ±0.5 °C lines on the RONI graph mark reference thresholds. NOAA’s historical episode classification requires persistence across at least five consecutive overlapping seasons. The operational assessment also considers the coupled ocean–atmosphere system and expected persistence. A single weekly or seasonal index value is therefore insufficient to establish an ENSO episode.
Colour encoding of seasonal index values
The historical ribbon and background shading represent seasonal RONI values, including when the plotted series is a weekly regional index. Each three-month value is assigned to its central calendar month. Red denotes values at or above +0.5 °C, blue denotes values at or below −0.5 °C, and grey denotes values between these thresholds. Seasons with missing or unpublished values remain unshaded. These colours indicate index ranges; official historical episode classification additionally requires persistence across five consecutive overlapping seasons. The current ENSO phase is reported separately in the NOAA assessment at the top of the page.
Temporal coverage and numerical representation
The relative weekly record begins on 9 September 1981, the conventional weekly record on 2 September 1981, and the RONI record with DJF 1950. Incomplete first and latest years are retained. Graphs connect observations with straight line segments and show gaps at missing measurements; missing values are not assigned a value of zero. Weekly tables, seasonal cell details and CSV downloads preserve source precision. Summary values and matrix cells are rounded to one decimal place.
Data provenance, revisions and validation
Complete source archives are retained with retrieval timestamps and SHA-256 checksums. Each update compares all published periods with the preceding archive and records revisions in the source data. The two most recent RONI values are subject to revision. If an update to the numerical data fails validation, the previously validated dataset remains in use.
If a diagnostic discussion cannot be validated, the most recent verified assessment is retained with its original issue and retrieval dates. In the absence of a verified assessment, the display reads “Status unavailable”. An assessment is labelled “Last confirmed status” and accompanied by an overdue notice more than seven days after the expected next issue, or 45 days after its issue date when no next issue date is supplied. The displayed phase therefore represents an assessment valid at a stated issue date.
NOAA RONI definitions and historical table ↗
NOAA’s February 2026 relative-index transition notice ↗