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El Niño (/ɛlˈniːnjoʊ/, /-ˈnɪn-/, Spanish pronunciation: [el ˈniɲo]) is associated with a band of warm ocean water temperatures that periodically develops off the Pacific coast of South America. It is a phase of 'El Niño Southern Oscillation' (ENSO), which refers to variations in the temperature of the surface of the tropical eastern Pacific Ocean and in air surface pressure in the tropical western Pacific. The warm oceanic phase, El Niño, accompanies high air surface pressure in the western Pacific.[2][3] Mechanisms that cause the oscillation remain under study.

El Niño is defined by prolonged warming in the Pacific Ocean sea surface temperatures when compared with the average value. The accepted definition is a warming of at least 0.5 °C (0.9 °F) averaged over the east-central tropical Pacific Ocean. Typically, this anomaly happens at irregular intervals of two to seven years, and lasts nine months to two years.[5] The average period length is five years. When this warming occurs for only seven to nine months, it is classified as El Niño "conditions"; when it occurs for more than that period, it is classified as El Niño "episodes".[6] The first signs of an El Niño are a weakening of the Walker circulation and strengthening of the Hadley circulation composing of:[7] Rise in surface pressure over the Indian Ocean, Indonesia, and Australia Fall in air pressure over Tahiti and the rest of the central and eastern Pacific Ocean Trade winds in the south Pacific weaken or head east Warm air rises near Peru, causing rain in the northern Peruvian deserts