El Niño is the occasional appearance of warm water off the coast of Peru; its presence results in catastrophic consequences in the fishing industry. A new theory for the occurrence of El Niño is presented. It isshown that El Niño is not due to a weakening of the southeast trades over the waters off Peru, but that during the two years preceding El Niño, excessively strong southeast trades are present in the central Pacific.These strong southeast trades intensify the subtropical gyre of the South Pacific, strengthen the SouthEquatorial Current, and increase the east-west slope of sea level by building up water in the western equatorial Pacific. As soon as. the wind stress in the central Pacific relaxes, the accumulated water flows eastward,probably in the form of an internal equatorial Kelvin wave. This wave leads to the accumulation of warmwater off Ecuador and Peru and to a depression of the usually shallow thermocline. In total, El Niño is theresult of the response of the equatorial Pacific Ocean to atmospheric forcing by the trade winds.