A large-scale atmospheric and hydrospheric fluctuation centred in the equatorial Pacific Ocean. It exhibits a nearly annual pressure anomaly, alternatively high over the Indian Ocean and high over the South Pacific. Its period is slightly variable, averaging 2. 33 years. The variation in pressure is accompanied by variations in wind strengths, ocean currents, sea-surface temperatures, and precipitation in the surrounding areas. The Southern Oscillation is coupled directly with the El Niño phenomenon, giving rise to the El Niño-Southern Oscillation (ENSO) phenomenon. See El Niño events and El Niño Southern Oscillation (ENSO).