A: El Niño affects temperature and rainfall patterns around the globe, but affects different regions in different ways. Although it is best known for substantially warmer-than-normal sea surface temperatures off the Pacific Ocean coast of equatorial South America, El Niño originates in a weakening of winds that normally blow across the Pacific from east to west. Normally these winds push the strongest storms and warmest waters into the western Pacific, but when they weaken, the storms and warm water shift thousands of miles to the east, leading to El Niño conditions. The relocation of these storms changes the patterns of atmospheric motion, altering temperature and rainfall patterns in many regions around the globe.
During an El Niño event, storms tend to flow across the United States along a different track than in non-El Niño years. In winter, this brings more storms into California from the tropics, as well as wetter conditions in the southern Midwest and along the Atlantic and Gulf coasts. The jet stream also settles into a more west-to-east flow than normal, preventing cold air in Canada from creeping into the upper plains states and leading to warmer conditions across the Pacific Northwest to the Great Lakes.
Elsewhere in the world, dry conditions prevail over the Western Pacific when El Niño shifts storms to the east, while South and East Asia have warm winters. Eastern Pacific circulations also suppress rainfall over northeast Brazil, and Argentina experiences wetter summers. In addition, hurricanes tend to form more often along the Pacific coast of Central America, but tropical storms are less frequent in the Atlantic and Caribbean.
— Alex Ruane, Climate Research Division, Scripps Institution of Oceanography
Attached file: Voyager: February 2007 - PDF