SOI (Southern Oscillation Index)

The Southern Oscillation Index is the oldest ENSO measurement still in daily use — and the atmosphere's vote on what the ocean is doing. It compares surface air pressure at two stations that bracket the tropical Pacific: Tahiti in the east-central basin and Darwin on Australia's north coast. The standardized difference (Tahiti minus Darwin) is the SOI.

The logic maps straight onto the Walker circulation. In normal and La Niña years, pressure runs relatively high at Tahiti (under sinking air) and low at Darwin (under the rising, rainy branch): a positive SOI. During El Niño the pattern inverts — pressure rises over Australia's side as the rain engine departs eastward, and the SOI swings negative. Sustained strongly negative values are the signature of a coupled, committed El Niño; that swing during April–June 2026 was part of what justified NOAA's Advisory.

Two conventions coexist: NOAA publishes a standardized index where values beyond about ±1 are meaningful, while Australia's Bureau of Meteorology scales its version by ten (±7 as the customary threshold). Same seesaw, different units — worth checking before comparing charts. Our dashboard plots the NOAA series monthly, warm-colored when negative, alongside the ocean indexes it should confirm.