El Niño and La Niña

The atmosphere and the ocean form a coupled system, that is, they respond to each other. Changes in the ocean cause a response in the winds above it, and vice versa. For reasons not yet fully understood, an ENSO event begins with a change in the atmosphere-ocean system of the southern equatorial Pacific. The Southeasterly Trade Winds weaken, and they push less warm water to the western edge of the Pacific, causing farreaching changes. Fewer rain clouds form over Indonesia, the Pacific Islands, Australia, and Southeast Asia. Lush rain forests dry out and become fuel for forest fires. The area of heavy rain shifts to the mid-southern Pacific, where heavy rains inundate usually arid islands. In the eastern Pacific, the surface water becomes warmer. Ocean upwelling is weakened, and the surface water runs low on the nutrients that support the ocean food chain. Many species of fish are driven elsewhere to find food; in severe El Niño years fish populations may be almost completely wiped out. Bird species that depend on fish must look elsewhere, and the human fishing population faces economic hardship. At the same time, the warmer waters offshore encourage the development of clouds and thunderstorms. Normally dry areas in western South America, such as Peru and Ecuador, may experience torrential rains, flooding, and mud slides during the El Niño phase.

The climatic effects of El Niño have long been noted in the tropical Pacific, and are now being studied around the world. The altered pattern of winds, ocean temperatures, and currents during an El Niño is believed to change the high-level winds, called the jet streams, that steer storms over North and South America. El Niños have been linked with milder winters in western Canada and the northern United States, as most severe storms are steered northward toward Alaska. As Californians saw in 1982–1983, and 1998–1999, an El Niño can cause extremely wet winters along the west coast, and bring torrential rains to the lowlands and heavy snows to the mountains. Alteration of the jet streams by El Niño can also contribute to storm development over the Gulf of Mexico, and to heavy rainfall in the southeastern United States. Similar changes occur in countries of South America, such as Chile and Argentina, while droughts may affect Bolivia and parts of Central America.

El Niño also appears to affect monsoons, which are annual shifts in the prevailing winds that bring rainy seasons to India, southeast Asia, and portions of Africa. The rains of the monsoon are critical for agriculture; when the monsoon fails, millions of people risk starvation. It appears that while El Niños do not always determine monsoons, they contribute to weakened monsoons in India and southeastern Africa, and tend to strengthen those in eastern Africa.

In general, the effects of El Niño are reversed during the La Niña extreme of the ENSO cycle. During the 1998–1999 La Niña episode, for example, the central and northeastern United States experienced record snowfall and sub-zero temperatures, rainfall increased in the Pacific Northwest, and a record number of tornadoes plagued the southern states. Not all El Niños and La Niñas have equally strong effects on the global climate because every El Niño and La Niña event is of a different magnitude and duration.