Guides

Understand ENSO and its indices

These guides explain the numbers shown on ENSO Tracker: what each index measures, why the relative and conventional versions differ, and how NOAA decides whether El Niño, La Niña or ENSO-neutral conditions are present. Each guide shows the site’s own latest reading where one exists and links into the chart in the matching mode. See the current ENSO status and observations →

Read the observations

• Niño 3.4 index: current reading and what it means

  The region’s boundaries, the difference between an anomaly and the actual sea surface temperature, and why the relative and conventional weekly values are not the same number.

• Relative Oceanic Niño Index (RONI)

  The seasonal index NOAA uses for ENSO monitoring: how the three-letter season labels translate into dates, what the ±0.5 °C lines mean, and why the newest values can change.

Understand the phases and the terminology

• El Niño, La Niña and ENSO-neutral explained

  The three phases of the El Niño–Southern Oscillation, what changes in the ocean and the atmosphere, and which part of the tracker answers which question.

• RONI vs ONI: what changes and why it matters

  Why NOAA moved from the conventional Oceanic Niño Index to a relative index, how the two definitions differ, and which of them this site displays.

How this site works

• Data sources and methodology

  Dataset versions, baselines, publication cadence, stale-status handling, missing values, precision and how revisions are recorded.