Usually the Earth’s warmest water can be found in the western Pacific, with the east-to-west trade winds pushing the warm water toward Indonesia. Every so often, however, the trade winds weaken and the warm water ends up sloshing eastward against the coast of South America. These oceanic changes cause disruptions in jet-stream winds and effect global weather conditions. This is the occasional weather pattern is known as El Niño. The opposite effect (a sustained cooling instead of warming) is called La Niña.
Surfnetkids » Resources » Science » El Niño