The El Nino Southern Oscillation (ENSO) is a shift in the jet stream over the Pacific Ocean caused by warmer-than-usual ocean water. It is responsible for severe weather, droughts and other effects worldwide.
A warming trend in the Pacific Ocean occurs at intervals of two to seven years, and typically lasts from nine months to two years. This is called “El Niño.” A corresponding cooling trend is termed “La Niña.”
El Niño is a change in air pressure between the eastern and western parts of the Pacific Ocean. Low air pressure occurs over warm water, so a warming trend that extends over the entire east-west expanse of the Pacific Ocean will create a large band of low pressure, feeding thunderstorms. The storms form over the usually cool waters off the coast of South America, causing flooding.
Despite its origin in the Pacific, El Niño alters weather patterns across the globe, bringing excessive flooding to some areas and drought to others. Warmer, drier winters are experienced in the northern half of the U.S., while much wetter winters are felt in the southern U.S. and northern Mexico.