El Niño is the warm phase of the El Niño–Southern Oscillation (ENSO): every few years the surface waters of the central and eastern tropical Pacific become unusually warm. That extra heat shifts rainfall and the atmospheric circulation around the globe — bringing floods to some regions and drought to others — and tends to push global temperatures to new records.
How is El Niño measured?
Scientists watch the sea surface temperature in the Niño 3.4 region (5°N–5°S, 170°W–120°W). NOAA declares El Niño conditions when the anomaly is at least +0.5 °C and the atmosphere responds. The official historical index is the Oceanic Niño Index (ONI), a 3-month running mean of that anomaly. An El Niño episode needs five consecutive overlapping seasons at or above +0.5 °C.
How strong can it get?
- Weak: +0.5 to +0.9 °C
- Moderate: +1.0 to +1.4 °C
- Strong: +1.5 to +1.9 °C
- Very strong: +2.0 °C or more — like 1982-83, 1997-98 and 2015-16.
How long does it last?
Events typically develop between April and June, peak between November and January, and fade the following spring, lasting around 9–12 months; some persist for two years.
Why the name?
Peruvian fishermen called the warm current that appeared around Christmas "El Niño", after the Christ child.