El Niño is a warming of the tropical Pacific Ocean that occurs roughly every 3 to 7 years. It develops in association with swings in atmospheric pressure known as the Southern Oscillation. During El Niño, the tradewinds weaken along the Equator as atmospheric pressure rises in the western Pacific and falls in the eastern Pacific. This condition allows warm water, normally confined to the far western Pacific, to migrate eastward. Upwelling, a process which brings nutrient-rich cold water to the surface along the coast of South America and along the Equator, is shut down, and sea surface temperatures warm in the central and eastern Pacific. Deep cumulus clouds and heavy rains, normally occurring in the western Pacific over the warmest water, migrate eastward in response to these surface temperature changes. These changes leave the western Pacific dry but bring torrential rains to the islands of the central Pacific and the west coast of South America.