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 Niņo", 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 Niņo" began to be used in reference to
these major warm episodes.
El Niņo is closely related to a global atmospheric
oscillation known as the Southern Oscillation (SO). During El Niņo
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 Niņo/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
spring seasons. At the same time, rainfall is reduced over
Indonesia, Malaysia and northern Australia. Thus, the normal Walker
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
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).
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 Niņa, go to
ENSO Advisory (issued when appropriate) or the latest monthly
Climate Diagnostics Bulletin.
National Weather Service
National Centers for Environmental Prediction
Climate Prediction Center
5200 Auth Road
Camp Springs, Maryland 20746
Climate Prediction Center Internet Team
Page last modified: December 19, 2005