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What the Niño-3.4 box is and why everyone measures it

In short. When you read that El Niño is "+2. 1°C", that number almost always comes from one patch of ocean: the Niño-3.

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Painted map of the Pacific with a dashed box on the equator labelled Niño-3.4, three cards giving its latitude, longitude and 3-month averaging, a scale showing the plus and minus 0.5°C thresholds, and the latest reading of +2.1°C.

When you read that El Niño is "+2.1°C", that number almost always comes from one patch of ocean: the Niño-3.4 region.

Where is it? The box runs along the equator from 5°N to 5°S, and from 170°W to 120°W in the central-eastern Pacific (NOAA CPC; IMD). Older boxes (Niño-1+2, Niño-3 and Niño-4) were set up along shipping routes. Research in 1997 found Niño-3.4, which overlaps Niño-3 and Niño-4, to be the most representative of ENSO (NOAA Climate.gov).

How is it read? Forecasters average the sea temperature difference from normal over three months. If it is +0.5°C or more, and the atmosphere responds, that points to El Niño. If it is −0.5°C or less, La Niña (IMD; NOAA CPC).

What changed in 2026. The oceans are warming everywhere, which can make a fixed "normal" misleading. So NOAA adopted the Relative Oceanic Niño Index (RONI) in February 2026, which compares the box with the average of all tropical oceans. Australia's Bureau of Meteorology switched to a relative index in September 2025 (IMD FAQ).

Latest reading. NOAA reported +2.1°C for Niño-3.4 in September 2026 (8 October 2026 discussion). That is a monthly value, far above the +0.5°C El Niño line.

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Last checked 17 Oct 2026

Last checked: 17 October 2026. Forecasts can change; check the latest IMD, NOAA, WMO and BoM updates.

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