Near the end of each calendar year ocean surface temperatures warm along the coasts of Ecuador and northern Peru. Local residents referred to this seasonal warming as "El Niño", meaning The Child, due to its appearance around the Christmas season. Every two to seven years a much stronger warming appears, which is often accompanied by beneficial rainfall in the arid coastal regions of these two countries. Over time the term "El Niño" began to be used in reference to these major warm episodes.

El Niño is closely related to a global atmospheric oscillation known as the Southern Oscillation (SO). During El Niño episodes lower than normal pressure is observed over the eastern tropical Pacific and higher than normal pressure is found over Indonesia and northern Australia. This pattern of pressure is associated with weaker than normal near-surface equatorial easterly (east-to-west) winds. These features characterize the warm phase of the SO, which is often referred to as an El Niño/Southern Oscillation (ENSO) episode.

During warm (ENSO) episodes the normal patterns of tropical precipitation and atmospheric circulation become disrupted. The abnormally warm waters in the equatorial central and eastern Pacific give rise to enhanced cloudiness and rainfall in that region, especially during the boreal winter and spring seasons. At the same time, rainfall is reduced over Indonesia, Malaysia and northern Australia. Thus, the normal Walker Circulation during winter and spring, which features rising air, cloudiness and rainfall over the region of Indonesia and the western Pacific, and sinking air over the equatorial eastern Pacific, becomes weaker than normal, and for strong warm episodes it may actually reverse.

The increased heating of the tropical atmosphere over the central and eastern Pacific during warm episodes, affects atmospheric circulation features, such as the jet streams in the subtropics and in the temperate latitudes of the winter hemisphere. The jet streams over the eastern Pacific Ocean are stronger than normal during warm episodes (see seasonal atmospheric circulation features). Also, during warm episodes extratropical storms and frontal systems follow paths that are significantly different from normal, resulting in persistent temperature and precipitation anomalies in many regions.

By studying past warm episodes scientists have discovered precipitation and temperature anomaly patterns that are highly consistent from one episode to another. Significant departures from normal are shown in the accompanying figures for the Northern Hemisphere winter and summer seasons. Within the tropics, the eastward shift of thunderstorm activity from Indonesia into the central Pacific during warm episodes results in abnormally dry conditions over northern Australia, Indonesia and the Philippines in both seasons. Drier than normal conditions are also observed over southeastern Africa and northern Brazil, during the northern winter season. During the northern summer season, Indian monsoon rainfall tends to be less than normal, especially in northwest India where crops are adversely affected. Wetter than normal conditions during warm episodes are observed along the west coast of tropical South America, and at subtropical latitudes of North America (Gulf Coast) and South America (southern Brazil to central Argentina).

During a warm episode winter, mid-latitude low pressure systems tend to be more vigorous than normal in the region of the eastern North Pacific. These systems pump abnormally warm air into western Canada, Alaska and the extreme northern portion of the contiguous United States. Storms also tend to be more vigorous in the Gulf of Mexico and along the southeast coast of the United States resulting in wetter than normal conditions in that region.

Since anomaly patterns during warm episodes tend to persist for several months, accurate long-range forecasts (1 to 3 seasons) are possible for the regions shown in the accompanying figures. For the latest information on the status of El Niño, go to ENSO Advisory (issued when appropriate) or the latest monthly Climate Diagnostics Bulletin.

More technical information on the global patterns of abnormal precipitation and temperature related to warm episodes in the tropical Pacific can be found in Ropelewski and Halpert (1987, Mon. Wea. Rev., 115, 1606-1626), and Halpert and Ropelewski (1992, J. Climate, 5, 577-593). A general description of a warm (ENSO) episode and its composite evolution can be found in Rasmusson and Carpenter (1982, Mon. Wea. Rev., 110, 517-528). Upper-tropospheric circulation features that accompany extreme phases of the Southern Oscillation are discussed in a paper by Arkin (1982, Mon. Wea. Rev., 110, 1393-1404).