|An El Niño occurs when the atmosphere and ocean waters interact to form warmer than normal water in the eastern and central Pacific Ocean. Sometimes an El Niño is followed by a La Niña whose effects on weather patterns are generally opposite to those caused by an El Niño.
(Top) Normal sea surface temperatures. (Middle) Sea surface temperatures during an El Niño. Notice the red areas to the west of South America. (Bottom) Sea surface temperatures during a La Niña. Notice the blue areas to the west of South America.