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In page El Niño–Southern Oscillation:

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El Niño conditions are established when the Walker circulation weakens or reverses and the Hadley circulation strengthens,[citation needed] leading to the development of a band of warm ocean water in the central and east-central equatorial Pacific (approximately between the International Date Line and 120°W), including the area off the west coast of South America,[1][2] as upwelling of cold water occurs less or not at all offshore.[3]