The El Niño-Southern Oscillation (or ENSO) refers to the effects of a band of sea surface temperatures which are anomalously warm or cold in a quasi-annual cycle that develops off the Pacific coast of South America and impacts climate and weather across the tropics and subtropics.
During an El Niño event, the trade winds weaken, leading to a rise in sea surface temperature in the eastern equatorial Pacific and a reduction in upwelling off the coast of South America. This causes rainfall patterns to change (convection centres move further east) and also influences the location of tropical cyclogenesis.
During a La Niña event, the easterly trade winds strengthen and this causes a stronger gradient between the cool waters of the eastern Pacific and the warmer waters of the western Pacific. As a result, the major convection centres shift further west.