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What is La Niña?
Current Status and Forecast
Comparing
La Niñas
Global Effects
Canadian Effects
Further Readings
El Niño
Other Educational Sites
La Niña
What is La Niña?
Brief history of the event, why La Niña occurs, the antithesis El Niño, further reading, and links to other La Niña sites.
Brief History
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La Niña, meaning the little girl, names the appearance of cooler than normal waters in the eastern and central Pacific Ocean. Sometimes called El Viejo, anti-El Niño, or simply "a cold event", it is the antithesis of El Niño.
At the turn of this century, a connection between La Niña, El Niño, and other weather patterns had yet to be established. During the 1920s, the head of the Indian Meteorological Service, Sir Gilbert Walker, recognized patterns to the rainfall in South America. His discovery led him to theorize additional associations with the change in the ocean temperatures, and with atmospheric pressure changes measured at stations at different parts of the Pacific (Darwin, Australia, and Tahiti).
Noticing that as pressure rises in the east, there is typically an accompanying decrease in the west, with the reverse also true, he coined the term Southern Oscillation to categorize his find.
Further study led to the realization that Asian monsoon seasons under certain barometric conditions were often linked to drought in Australia, Indonesia, India and parts of Africa, and to mild winters in western Canada.
(see : Global & Local Effects)
Not until the late 1960s did a Norwegian meteorologist, Jacob Bjerknes, a professor at the University of California, establish the connection between the changes in sea surface temperatures and the weak winds from the east and heavy rainfall that accompany low pressure conditions.
Ultimately, Bjerknes' discovery led to the recognition that the warm waters of El Niño (with the often cool waters of La Niña) and the pressure variance of Walker's Southern Oscillation are interrelated, leading to the full naming of the phenomenon as : "El Niño Southern Oscillation (ENSO)".
Why La Niña Occurs
La Niña is thought to occur due to increases in the strength of the normal patterns of trade wind circulation. Under normal conditions, these winds move westward, carrying warm surface water to Indonesia and Australia and allowing cooler water to upwell along the South American coast. For reasons not yet fully understood, periodically these trade winds are strengthened, increasing the amount of cooler water toward the coast of South America and reducing water temperatures.
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The increased amount of cooler water toward the coast of South America, causes increases in the deep cloud buildup towards southeast Asia, resulting in wetter than normal conditions over Indonesia during the northern hemisphere winter.
The changes in the tropical Pacific are accompanied by large modulations of the jet stream within the middle latitudes, shifting the point at which the stream normally crosses North America. The shifted jet stream contributes to large departures from the normal location and strength of storm paths. The overall changes in the atmosphere result in temperature and precipitation anomalies over North America which can persist for several months.
Atmospheric Circulation Departure
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Typical circulation anomaly of air at 500 hectopascals (at an altitude of about 5 km) during past strong La Niñas. During strong La Niña winters, the airflow pattern would include a stronger jetstream moving across the eastern Pacific ocean and into southern Canada. The arrow on the map indicates approximate position of the jetstream. During El Niño winters, the jetstream is split over Canada, keeping most of southern Canada in a dry and warm regime.
The annual cycle of the jetstream averaged over nine years shows intensification and weakening of the jetstream from the cold to the warm season in the Northern Hemisphere.
The previous La Niña, during the winter of 1995-96, was partly to blame for the flooding in Northern California, Oregon, and Washington states, and higher-than-normal snowfall in the northern plains and Atlantic states.
El Niño
About every four years, a pool of warmer-than-normal water develops off South America. The effects of this phenomenon, called El Niño, can be learned at Environment Canada's El Niño Site.
Links
Some of material on this page originates with organizations not subject to the Official Languages Act and is only available in the language in which it was written.
/education/lanina/what_is/index_e.cfm
The Green LaneTM,
Environment Canada's World Wide Web Site.