The unique features of the tropical Pacific and its mean annual cycle set the stage for ENSO to occur. ENSO: El Niño–Southern Oscillation is the simultaneous occurrence of an El Niño and Southern Oscillation events. El Niño is the warm ocean component of ENSO and refers to an anomalous warming of the surface tropical Pacific Ocean east of the dateline to the South American coast. The Southern Oscillation corresponds to a global-scale pattern in mean sea-level pressure, and hence surface winds, that is the atmospheric component of ENSO. Historically ‘El Niño’ referred to the appearance of unusually warm water off the coast of Peru, where it was readily observed as an enhancement of the normal warming about Christmas (hence Niño, Spanish for “the boy Christ-child”), but in the last half-century the term came to be regarded as synonymous with the basin-wide phenomenon. The oceanic and atmospheric conditions in the tropical Pacific fluctuate somewhat irregularly between El Niño and the cold phase of ENSO: a basin-wide cooling of the tropical Pacific, named “La Niña” (“the girl” in Spanish). The most intense phase of each event typically lasts half of a year.