El Niño and La Niña

El Niño and La Niña

Discovery and study of the El Niño Southern Oscillation

Regional Effects of El Niño and La Niña

Predicting El Niño and La Niña

Resources

El Niño and La Niña are disruptions of the oceanic and atmospheric systems of the equatorial Pacific Ocean that have far reaching effects on Earth’s weather patterns. El Niño and La Niña do not change with the regularity of the seasons; instead, they repeat about every two to seven years. They are the extremes in an aperiodic, or irregular, cycle called the El Niño Southern Oscillation (ENSO), during which warm surface waters from the western Pacific Ocean spread toward the South American coastline.

During non-ENSO periods, southeasterly (east to west) trade winds push equatorial surface waters into western half of the Pacific, driving the northwest-flowing Southern Equatorial Current, and creating a mound of warm water around Indonesia. Sea surface temperatures near Indonesia are typically about 46°F (8°C) higher than those near Ecuador, and the sea surface is about 2 ft (0.7 m) higher. The pool of warm water in the western Pacific warms the air above it, creating an upward current of moist air that rises to form rain clouds. The coastal areas and islands of the western Pacific typically enjoy abundant rainfall and support lush, biologically diverse rainforests, including those of Borneo and New Guinea. Meanwhile, along the coast of South America, cold, nutrient-rich waters from the deep ocean rise to the sea surface, since the warmer surface waters have been blown westward. The result is called an upwelling, which nourishes abundant phytoplankton and zooplankton, the microscopic plants and animals that form the base of the planktonic food chain. As a result, the South American upwelling is a very ecologically productive region. The cold water of an upwelling cools the air above it, creating high-pressure zones of sinking dry air. Regions near upwellings, like coastal Peru and Ecuador, tend to be arid (desertlike).

An ENSO event begins with a slackening of the trade winds in the equatorial Pacific, and a corresponding collapse of the sea surface slope between Indonesia and South America. The pile of warm water in the western Pacific sloshes toward the coast of South America, and shuts down the South American upwelling. A dramatic warming of the waters off of South America and a corresponding decline of marine productivity indicates the El Niño phase of the southern oscillation. La Niña, the opposite phase of an ENSO cycle, occurs when the southeast trade winds are particularly strong. La Niña events are accompanied by colder-than-normal temperatures off South America, and an intensification of the South American upwelling. La Niña events often, but not always, follow El Niño events.

El Niño events occurred during 1982–1983, 1986– 1987, 1991–1992, 1993, 1994, 1997–1998, 2002–2003 and 2006. (It is unusual to have two El Niños in row, as happed in 1993 and 1994. La Niña episodes occurred recently in 1995–1996 and 1998–1999.

Discovery and study of the El Niño Southern Oscillation

The name El Niño comes from nineteenth century Peruvian and Ecuadorian fishermen. They noticed that some years, within a few months of the Christmas holiday, the seawater off the South American coast became warmer, the nearshore ocean currents assumed new patterns, and the fishing became poorer. Every few years, the changes were strong enough to wipe out a fishing season, and to bring significant, long-lasting changes in the weather. For example, normally dry areas on shore could receive abundant rain, turning deserts into lush grasslands for as long as these strong El Niños lasted. Because the phenomenon happened close to Christmas each year, the fishermen dubbed it El Niño, Spanish for “the boy child,” after the Christ child. Only in the 1960s did scientists begin to realize that the strong El Niño events were more than a local South American phenomenon, and were rather one half of a multi-year atmospheric-oceanic cycle that affects the entire tropical Pacific Ocean. The other half of the ENSO cycle has been named La Niña, the girl child, or, less commonly, El Viejo, the old man.

The Southern Oscillation was detected, and named, in 1923 by Sir Gilbert Walker. Walker was the director of observatories in India, and was trying to understand the variations in the summer monsoons (rainy seasons) of India by studying the way atmospheric pressure changed over the Pacific Ocean. Based on meteorologists’ previous pressure observations from many stations in the southern Pacific and Indian oceans, Walker established that, over the years, atmospheric pressure oscillated back and forth across the ocean. In some years, pressure was highest in the Indian Ocean near northern Australia, and lowest over the southeastern Pacific, near the island of Tahiti. In other years, the pattern was reversed. He also recognized that each pressure pattern was correlated to certain weather, and the change from one phase to the other could mean the shift from rainfall to drought, or from good harvests to famine. In the late 1960s, Jacob Bjerknes, a professor at the University of California, first proposed that the Southern Oscillation and the strong El Niño sea warming were two aspects of the same atmosphere-ocean cycle, and explained the ENSO phenomenon in terms of physical mechanisms.

Regional Effects of El Niño and La Niña

Although El Niño and La Niña originates the southern tropical Pacific, its effects are felt throughout the world. 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.

During El Nino years, 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 midsouthern 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 1982–83, 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 El Niños 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.

Predicting El Niño and La Niña

The widespread weather impacts of the two extreme phases of the El Niño Southern Oscillation cycle make understanding and predicting ENSO events a high priority for atmospheric scientists and oceanographers. Researchers have developed computer models of the Southern Oscillation that mimic the behavior of the real atmosphere-ocean system, and predict future events. These computer simulations require the input of very large amounts of data about sea and wind conditions in the equatorial Pacific. A large and growing network of instruments, many of them owned and maintained by the National Atmospheric and Oceanographic Administration (NOAA) provides these data. Ocean buoys, permanently moored in a transect across the

Key Terms

El Niño— The phase of the Southern Oscillation characterized by increased sea water temperatures and rainfall in the eastern Pacific, with weakening trade winds and decreased rain along the western Pacific.

ENSO— Abbreviation for El Niño/Southern Oscillation.

Jet streams— High velocity winds that blow at upper levels in the atmosphere and help to steer major storm systems.

La Niña— The phase of the Southern Oscillation characterized by strong trade winds, colder sea water temperatures and dry weather in the eastern Pacific, with increased rainfall along the western Pacific.

Monsoon— An annual shift in the direction of the prevailing wind that brings on a rainy season and affects large parts of Asia and Africa.

Southern oscillation— A large scale variation in the winds, ocean temperatures and atmospheric pressure of the tropical Pacific Ocean which repeats about every three to four years.

equatorial Pacific, constantly relay information on water temperature, wind, and air pressure to weather prediction stations around the world. The buoys are augmented by research ships, island weather stations, and Earth observing satellites. Even with mounting data and improving computer models, El Niño, La Niñaand the Southern Oscillation remain difficult to predict. However, the ENSO models, and analyses of past ENSO cycles, are now being used in several countries to help prepare for the next El Niño. Countries most affected by the variations in El Niño, such as Peru, Australia and India, presently use El Niño prediction to improve agricultural planning.

See also Air masses and fronts; Oceanography.

Resources

BOOKS

D’Aleo, Joseph S. The Oryx Resource Guide to El Niño and La Niña. Phoenix, AZ: Oryx Press, 2002.

Glantz, Michael H. Currents of Change: Impacts of El Niño and La Niña on Climate and Society. Cambridge, U.K.: University of Cambridge Press, 2001.

Hildebrand, Paula E. El Niño and La Niña Events and North Atlantic Tropical Cyclones. Washington D.C.: Storming Media, 2001.

PERIODICALS

McPhaden, M.J. “TOGA-TAO and the 1991–93 El Niño-Southern Oscillation Event.” Oceanography (v. 6, 1993): 36–44.

OTHER

National Oceanographic and Atmospheric Administration. “NOAA El Niño Page.” February 28, 2002. <http://www.elnino.noaa.gov/> (accessed October 18, 2006).

National Oceanographic and Atmospheric Administration. “El Niño Theme Page.” October 18, 2006. <http://www.pmel.noaa.gov/tao/elnino/nino-home.html> (accessed October 18, 2006).

James Marti

Laurie Duncan