La Niña

This site contains comprehensive information on the past and present impact of La Niña on the climate of Canada and its effects on the global climate. Through animation, this site shows the changes in the Canadian climate and climatic extremes that accompany La Niña. Along with a scientific explanation of the La Niña phenomenon, comparisons between various La Niñas and forecast of La Niña are also provided.

• Brief History
• Why La Niña Occurs
• How La Niña affects North America via Changes in Jet Stream
• Related Section

Brief History

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 the 20th 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 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".

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.

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.

Cross section in the tropical Pacific during normal conditions. Winds from the east (indicated by white arrows) push water westward along the equator where solar radiation produces warmer sea surface temperatures. A convection circulation sets up producing abundant rain in the western tropical Pacific and drier conditions in the eastern tropical Pacific.

Cross section in the tropical Pacific during La Niña conditions. Winds from the east (indicated by white arrows) push water westward along the equator allowing deep sea cold waters to upwell along the coast of south America. Warmer-than-normal sea surface temperatures give rise to cloudiness in the western tropical Pacific. A convection circulation sets up producing abundant rain in western tropical Pacific and drier conditions in the eastern tropical Pacific. 

How La Niña affects North America via Changes in Jet Stream

Typical circulation anomaly of air at 500 hectapascals (at an altitude of about 5 km) during past strong La Niñas in decametres. Blue areas show below normal pressure, while higher-than-normal pressure occurs in the red regions. Thick black lines indicate position of jet streams. With below normal atmospheric pressure, most of western Canada experiences below normal winter temperatures.

Related Section

• El Niño