El Niño and the Southern Oscillation   El Niño   La Niña    DEFINITION    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. In recent decades, it has been recognized that El Niño is linked with other shifts in global weather patterns in particular to the Southern Oscillation (SO). The SO denotes the pressure difference in the tropical Pacific between Darwin and Tahiti. GENERAL CHARACTERISTICS OF El NIÑO (from Climate Prediction Center page)   Warm (El Niño)Episodes in the Tropical Pacific   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 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 cold 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 La Niña, 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).     GENERAL CHARACTERISTICS OF LA NIÑA     Cold (La Niña) Episodes in the Tropical Pacific   At times ocean surface temperatures in the equatorial Pacific are colder than normal. These cold episodes, sometimes referred to as La Niña episodes, are characterized by lower than normal pressure over Indonesia and northern Australia and higher than normal pressure over the eastern tropical Pacific. This pressure pattern is associated with enhanced near-surface equatorial easterly winds over the central and eastern equatorial Pacific. During cold (La Niña) episodes the normal patterns of tropical precipitation and atmospheric circulation become disrupted. The abnormally cold waters in the equatorial central give rise to suppressed cloudiness and rainfall in that region, especially during the Northern Hemisphere winter and spring seasons. At the same time, rainfall is enhanced 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 stronger than normal. By studying past cold 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. During cold episodes, the colder than normal ocean temperatures in the equatorial central Pacific act to inhibit the formation of rain-producing clouds over that region. Wetter than normal conditions develop farther west over northern Australia, Indonesia and Malaysia, during the northern winter, and over the Philippines during the northern summer. Wetter than normal conditions are also observed over southeastern Africa and northern Brazil, during the northern winter season. During the northern summer season, the Indian monsoon rainfall tends to be greater than normal, especially in northwest India. Drier than normal conditions during cold 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 their respective winter seasons. Mid-latitude low pressure systems tend to be weaker than normal in the region of the Gulf of Alaska, during a cold episode winter. This favors the build-up of colder than normal air over Alaska and western Canada, which often penetrates into the northern Great Plains and the western United States. The southeastern United States, on the other hand, becomes warmer and drier than normal. Since anomaly patterns during cold 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 La Niña, 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 cold episodes in the tropical Pacific can be found in Ropelewski and Halpert (1989, J. Climate, 2, 268-284), and Halpert and Ropelewski (1992, J. Climate, 5, 577-593).         NAME="TABLE">TABLE of Economic     IMPACTS of ENSO 1997      Location  Anomaly  Mayor Social impact Costs Ecuador & Northern Peru  Flooding 600 dead  $650 million Southern Peru & Western Bolivia  Drought   n/a  $240 million  Southern Brazil, Northern Argentina & Eastern Paraguay Flooding 170 dead, 600,000 evacuated  $3 billion Bolivia  Flooding 50 dead, 26,00 homeless  $300 million