The state of the tropical Pacific — where the ocean is warmer or cooler than normal, what that means, and how it has been trending. Data from the NOAA Climate Prediction Center.
Forecasters track sea-surface temperature in four boxes along the equatorial Pacific. Niño 3.4 (the outlined box) is the one the official index is built from.
Oceanic Niño Index (ONI): the 3-month running-mean temperature difference from normal in the Niño 3.4 box. Above +0.5°C (orange zone) trends toward El Niño; below −0.5°C (blue zone) toward La Niña. Five overlapping seasons past the line make it an official episode.
Weekly Niño 3.4 temperature anomaly, last ~2 years. Noisier than the ONI, which smooths it.
The recent sequence of ENSO phases, newest on the right.
| Region | Box | Sea-surface temp | vs. normal | What it tells you |
|---|
Normally, trade winds along the equator push warm surface water toward Asia, letting cool water well up off South America. During El Niño those winds slacken or reverse, warm water sloshes back east, and the central and eastern Pacific run warmer than normal. That shifts where tropical thunderstorms form, which nudges the jet stream and weather patterns worldwide — typically a wetter southern US, drier Indonesia and Australia, a quieter Atlantic hurricane season, and a bump in global average temperature.
La Niña is the opposite phase: stronger-than-usual trade winds, more cold upwelling, and an eastern Pacific that is cooler than normal. Effects tend to mirror El Niño — drier across the southern US, wetter in the Pacific Northwest and Southeast Asia, and a more active Atlantic hurricane season. The two phases plus the in-between "neutral" state make up the El Niño–Southern Oscillation (ENSO), which swings back and forth every 2–7 years.
The Oceanic Niño Index is NOAA's official yardstick: the average sea-surface temperature in the Niño 3.4 box, expressed as a difference from the 1991–2020 average, then smoothed into a 3-month running mean. It crosses into El Niño territory at +0.5°C and La Niña at −0.5°C. An official episode requires the index to stay past the threshold for at least five consecutive overlapping 3-month seasons. Peak values label the strength: weak (0.5–0.9), moderate (1.0–1.4), strong (1.5–1.9), very strong (2.0+).
Each Niño region samples a different part of the equatorial Pacific. Niño 1+2 hugs the South American coast and reacts first and most sharply. Niño 3 covers the eastern Pacific. Niño 3.4 straddles the central Pacific and best captures the atmosphere's response, which is why the ONI uses it. Niño 4 sits in the warm western/central Pacific and matters for "Modoki" or central-Pacific events.