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.
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 allowing deep, cold water to rise to the surface (upwelling) along the South American coast. Due to strong easterly winds pushing water in the west Pacific and thermal expansion of the water, the sea level is usually 50 cm higher than in the eastern Pacific. For reasons not yet fully understood, periodically these trade winds are strengthened enhancing upwelling of cold water along the west coast of South America.
Cross section in the tropical Pacific during normal conditions
During normal conditions, winds from the east, also referred to as easterly winds (indicated by white arrows), transport water westward along the equator where solar radiation contributes to the enhancement of warmer sea surface temperatures in the western Pacific, and allows colder water to upwell along the coast of South America. The warm sea surface water in the western Pacific warms the air above it. A circulation cell (indicated by the convective loop) is set up over the tropical Pacific causing cool air to descend over the eastern tropical Pacific and west coast of South America. The resulting high pressure over the region creates drier conditions.
La Niña conditions
Sometimes, the end of an El Niño cycle sees a return to normal conditions; often however, conditions known as a La Niña cycle take over. The arrival of La Niña can be seen as a return to “more than normal” conditions. An increase in the 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 changes in the northern hemisphere atmospheric circulation. The resulting changes in the subtropical jet stream entering North America contributes to large departures in the location and strength of storm paths. The changes in the atmospheric circulation result in anomalous temperature and precipitation conditions over North America that can persist for several months. In Canada, climatic anomalies during the winter months include: above average precipitation in British Columbia, colder-than-normal temperatures in the Prairies and above average precipitation in Ontario and Quebec. The U.S. can experience, drier conditions in both the western Pacific and the southeastern United States.
- The phrase "La Niña" is Spanish for "the girl" and sometimes called "El Viejo" (old man). It is characterized by unusually cold ocean temperatures in the equatorial Pacific. In comparison, El Niño is characterized by unusually warm ocean temperatures in the Equatorial Pacific.
- La Niña is a natural phenomenon in the climate system that has been occurring for centuries. Detailed observations from ships led to systematic instrumental record keeping in the earlier half of the twentieth century.
- La Niñas appear approximately every 3-5 years and typically last 1-2 years.
- Global climate abnormalities of La Niña are less pronounced and in some areas tend to be the opposite of those associated with El Niño. However, the effects of La Niña are not always opposite to that of El Niño.
- After an El Niño, the climate does not always swing to a La Niña phase. There have been only 17 moderate to strong La Niñas compared to 25 moderate to strong El Niños in the twentieth century.
- It is believed that La Niña's cooling of the equatorial Pacific tends to favour hurricane formation in the western Atlantic. In contrast, El Niño conditions tend to suppress the development of tropical storms and hurricanes in the Atlantic, but increase the number of tropical storms over the eastern and central Pacific Ocean.
- La Niña usually brings colder winters to the Canadian west and Alaska, and drier, warmer weather to the American southeast.
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