What is the Southern Oscillation Index (SOI)?
The fluctuations in ocean temperatures during El Niño and La Niña are accompanied by even larger-scale fluctuations in air pressure between the western and eastern tropical Pacific known as the Southern Oscillation. During El Niño higher-than-average air pressure covers Indonesia and the western tropical Pacific and below-average air pressure covers the eastern tropical Pacific. These pressure departures are reversed during La Niña, which features below-average air pressure over Indonesia and the western tropical Pacific and above-average air pressure over the eastern tropical Pacific. The Southern Oscillation Index (SOI) is designed to measure the strength and phase of the Southern Oscillation. The SOI is calculated using departures from normal in the surface air pressure difference between Tahiti, French Polynesia and Darwin, Australia. These stations are used because of their long data records. During El Niño episodes the SOI has a large negative value due to lower-than-average air
