El Niño << ehl NEEN yoh >> is a part of the interaction between Earth’s atmosphere and the tropical waters of the Pacific Ocean. An El Niño occurs about every two to seven years, and it can affect the climate throughout the world. In the United States, for example, the climate becomes wetter than normal in the south and drier than normal in the Pacific Northwest. Since the early 1980’s, El Niños have become more frequent and more severe. A typical El Niño lasts about 18 months and is often followed by an opposite pattern called La Niña. The change back and forth between El Niño and conditions in which there is no El Niño is known as the Southern Oscillation.
In ocean science, the term El Niño originally referred to a current of warm water that flows southward along the coast of Ecuador and Peru every winter. The current was called El Niño because it usually occurs near Christmas. El Niño is Spanish for the boy and is used to refer to the Christ child. About every two to seven years, the warm current is abnormally strong, lasts for an unusually long time, and is accompanied by changes in the winds and precipitation across the entire tropical Pacific region. For this reason, El Niño gradually came to refer to the entire interaction of the ocean and atmosphere during the period of the stronger-than-normal current.
Climate without El Niño.
When there is no El Niño, the warmest waters in the tropics are on the western side of the Pacific, near Indonesia. The air pressure over those waters is low. On the eastern side of the Pacific, near South America, the pressure of the overlying air is high. Winds in the tropics blow from areas of high pressure to areas of low pressure. Over the tropical Pacific, therefore, the winds normally blow from east to west.
The winds of the tropical Pacific blow the surface waters from east to west. In the east, deeper cold water rises to the surface to replace the water that is blown away. The cold water is rich in minerals and other nutrients that feed tiny organisms drifting at and near the surface. The organisms, in turn, support a huge population of fish. As a result, the waters off Ecuador and Peru are one of the world’s largest commercial fishing areas.
In the west, the warm ocean waters heat the air above them. The heated air is less dense than the cooler air surrounding it. The heated air therefore rises, producing clouds that provide rain to the western Pacific.
Climate with El Niño.
During an El Niño, air pressure is higher than normal in the west and abnormally low in the east. The east-to-west winds over the tropical Pacific therefore weaken—or may even reverse. In either case, the waters off Ecuador and Peru become abnormally warm. Nutrient-rich cold water does not rise to the surface there, and so the fish population declines sharply.
Also during an El Niño, clouds and heavy rainfall occur mainly over the warmer water in the eastern Pacific. Consequently, the coast of South America becomes wetter than normal. To the west, by contrast, the climate in Indonesia and other nations of Southeast Asia is unusually dry. Droughts may even occur.
Climate with La Niña.
La Niña is Spanish for the girl. In general, the climate associated with a La Niña is the opposite of that associated with an El Niño. During a La Niña, for example, the water in the western Pacific is even warmer than it is when there is merely no El Niño.
