El Nino and La Nina

El Nino and La Nina are opposite phases of ENSO (El Nino Southern Oscillation), a coupled ocean-atmosphere oscillation in the Tropical Pacific Ocean that exerts an influence on weather throughout much of the world. Being an oscillation, it moves between these two phases every few years, triggered initially by changes in the atmospheric circulation with ocean temperature changes following. It is measured by the Southern Oscillation Index, which compares surface pressures at Tahiti and Darwin, Australia.