The cyclical warming of East Pacific Ocean sea water temperatures off the western coast of South America that can result in significant changes in weather patterns in the United States and elsewhere. This occurs when warm equatorial waters move in and displace the colder waters of the Humbolt Current, cutting off the upwelling process.

Illustrations:

Hotter temperatures are near the equator.
El Niño Global Climate Pattern - Temperature Scale in Degrees Centigrade

Illustrations Courtesy of NOAA and the U.S. Navy