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At times ocean surface temperatures in the equatorial
Pacific are colder than normal. These cold episodes, sometimes referred
to as La Niņa 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 Niņa) 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
Hemispherel
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 Niņa, go to
ENSO Advisory (issued when appropriate) or the latest monthly
Climate Diagnostics Bulletin.
NOAA/
National Weather Service
National Centers for Environmental Prediction
Climate Prediction Center
5200 Auth Road
Camp Springs, Maryland 20746
Page Author:
Climate Prediction Center Internet Team
Page last modified: December 19, 2005 |
