Southern Oscillation and El Niño
The Southern Oscillation is an atmospheric event that is strongly related to the prediction and observation of the El Nino Southern Oscillation phenomenon — generally known simply as El Nino.
Changes to surface air pressure are caused by movements of air mass in the atmosphere.
The Southern Oscillation is a “see–saw” effect where surface air pressure (and therefore air mass) reduces at one location while a corresponding increase occurs at another site. This represents a mass of air that is oscillating back and forth across the International Date Line in the tropics and subtropics.
The clearest sign of the SO is the inverse relationship between surface air pressures at two sites: Darwin Australia, and the South Pacific island of Tahiti.
Over periods of a month or longer, higher pressure than normal at one site is almost always concurrent with lower pressure at the other, and vice versa. The pattern reverses every few years.
The Southern Oscillation Index
The Southern Oscillation Index (SOI) measures how abnormal the pressure difference between Tahiti and Darwin is. Negative values of this index correspond to El Niño conditions, while positive SOI values coincide with La Niña episodes.
First documentation of the Southern Oscillation
While El Nino effects all aspects of the climate system it was the atmospheric part — the Southern Oscillation — that first attracted the attention of scientists.
Sir Gilbert Walker documented and named the SO in the 1930s.
The Southern Oscillation is one of several persistent patterns of high and low pressures around the globe.