El Niño rearranges the distribution of rainfall in the equatorial Pacific. During normal years (top figure) upwelling induced by the easterly surface winds along the equator (arrow) keeps the surface waters of the central Pacific cool (blue). Heavy rainfall is confined to the warm (red) waters surrounding Indonesia at the western end of the Pacific. During El Niño (bottom figure) the easterly surface winds weaken and retreat to the eastern Pacific, allowing the central Pacific to warm, and the rain area to migrate eastward.

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