The Southern Oscillation: the air-pressure see-saw behind El Niño

El Niño is half of a pair. The "Southern Oscillation" is the other half: a see-saw in air pressure across the tropical Pacific. Together they make ENSO, the El Niño–Southern Oscillation.

Two weather stations. The Southern Oscillation Index (SOI) compares sea-level air pressure at Tahiti, in the central Pacific, with Darwin, in northern Australia (NOAA Climate.gov). During El Niño, pressure falls below average at Tahiti and rises above average at Darwin, so the SOI becomes negative. During La Niña, the reverse happens and the SOI turns positive.

How to read it. Australia's Bureau of Meteorology says sustained SOI values below −7 often indicate El Niño, while values above +7 are typical of La Niña. Its SOI usually ranges from about −35 to +35. Monthly values are what count: daily numbers jump around with ordinary weather.

Where it stands now. The Bureau reported a 30-day SOI of −20.2 to 11 October 2026, well inside El Niño territory and in line with the warm Pacific Ocean.

An Indian connection. Sir Gilbert Walker, a former head of India's weather service, traced this pressure see-saw in the 1920s and early 1930s while looking for ways to forecast the monsoon (IMD). The Walker circulation is named after him.

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