How do scientists know an El Niño has started? The Niño 3.4 index explained
"El Niño has arrived" is announced after a thermometer reading in one box of the Pacific, and the rule behind that reading can be changed when the whole ocean warms.
Guess first
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Scientists know an El Niño has started by watching the temperature of one particular patch of the Pacific. It is called the Niño 3.4 region: a box about 3,000 km southeast of Hawaii, stretching from 120° to 170° west longitude and five degrees of latitude either side of the equator. Each month forecasters work out how much warmer or cooler its sea surface is than normal, averaged over three months. In the United States, an El Niño is declared when this area is forecast to run 0.5 °C above average or more for the next several seasons. A drop of the same size means La Niña, and anything in between counts as .
Two things make the rule less simple than it sounds. First, different countries use different rules. Japan's weather agency uses another region, Niño 3, and wants more than 0.5 °C for six consecutive months. Peru's government uses a region right at its coast and a threshold of 0.4 °C for three months. Second, the baseline can change: in February 2026, because tropical seas had warmed so much, NOAA switched to the Relative Oceanic Niño Index, which subtracts the average warming of the whole tropical belt, so that a generally warmer ocean is not mistaken for El Niño.
United States (NOAA)
- Region: Niño 3.4
- Threshold: 0.5 °C above normal
- Since February 2026: minus the average of the whole tropics (RONI)
Japan and Peru
- Japan: Niño 3 region, over 0.5 °C for six months in a row
- Peru: Niño 1+2 off its coast, 0.4 °C for three months
- Australia also weighs the trade winds and pressure
There is a second, older measure: the pressure see-saw between Tahiti and Darwin, Australia, called the . During an El Niño, pressure is lower over Tahiti and higher in Darwin.
Try it
The Pacific's swing, and the 0.5 °C line
y = 0.5 · sin(1.5x + 0)
Challenge · Make the swing big enough to pass the +0.5 °C El Niño line and the -0.5 °C La Niña line, then set the speed to 1.25 so a full cycle takes about five years.
Quiz me
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Recap
Watch a box of Pacific sea about 3,000 km southeast of Hawaii: 0.5 °C above normal for several months means El Niño, below means La Niña.
💡 A trick to remember it · Think of the Niño 3.4 box as the Pacific's fever thermometer: half a degree over normal and the doctors call it El Niño.
Surprising fact · Different countries use different regions and thresholds, and the US method changed in February 2026.
Sources (4)
No source, no claim. Every fact in this lesson (18 claims) cites at least one of these.
- [1]El Niño–Southern Oscillation · Wikipedia
- [2]Relative Oceanic Niño Index (RONI): Historical El Niño / La Niña episodes (1950–Present) · NOAA Climate Prediction Center
- [3]ENSO Diagnostic Discussion, 8 October 2026 · NOAA Climate Prediction Center
- [4]El Niño set to become very strong, raising risks of extreme weather into 2027 · World Meteorological Organization