The term El Niño refers to the large-scale ocean-atmosphere climate interaction linked to a periodic warming in sea surface temperatures across the central and east-central Equatorial Pacific, as opposed to La Niña, which is characterized by unusually cold ocean temperatures in the Equatorial Pacific. El Niño is an oscillation of the ocean-atmosphere system in the tropical Pacific having important consequences for weather around the globe. In normal, non-El Niño conditions the trade winds blow to the west along the equator from South America towards Asia in the tropical Pacific Ocean. These winds pile up warm surface water off Asia, so that the sea surface is about 1/2 meter (1 1/2 feet) higher at Indonesia than at Ecuador in South America. The sea surface temperature is about 8ºC (14ºF) warmer off the coast of Asia than in the eastern Pacific, due to an upwelling of cold water from deeper levels in the east Pacific. The cooler water off South America is nutrient-rich, supporting high levels of primary productivity, diverse marine ecosystems, and major fisheries. Clouds and rainfall are found in rising air over the warmest water near Asia, whereas the east Pacific is relatively dry. El Niño was originally recognized by fisherman off the coast of South America as the appearance of unusually warm water in the Pacific Ocean, occurring near the beginning of the year. El Niño means The Little Boy or Christ child in Spanish. This name was used for the tendency of the phenomenon to arrive around Christmas. Typically, this anomaly happens at irregular intervals of two to seven years, and lasts nine months to two years. The average period length is five years. When this warming occurs for seven to nine months, it is classified as El Niño “conditions”; when its duration is longer, it is classified as an El Niño “episode”. The vast tropical Pacific Ocean receives more sunlight than any other region on Earth. Much of this sunlight is stored in the ocean in the form of heat. Typically, the Pacific trade winds blow from east to west, dragging the warm surface waters westward, where they accumulate into a large, deep pool just east of Indonesia, and northeast of Australia. Meanwhile, the deeper, colder waters in the eastern Pacific are allowed to rise to the surface, creating an east-west temperature gradient along the equator known as the thermocline tilt.
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