El Niño

Defining El Niño

The 1997 to 1998 El Niño

Dangerous science: What causes an El Niño?

Aftermath: The effects of El Niño

The human factor: A possible link between El Niño and global warming

Technology connection: Predicting El Niños

For More Information

In the late 1990s, the weather news was dominated by a phenomenon called El Niño (pronounced el-NEE-nyo). El Niño is a disruption of the ocean-atmosphere system in the tropical Pacific characterized by elevated water temperatures off the coast of Peru. Water temperatures may rise by 5 to 11°F (3 to 6°C). During an El Niño event, the Pacific coast of South America may experience floods, while Australia may suffer drought conditions. The event may also trigger unusual weather disturbances all over the Western Hemisphere. While El Niño is not new—indeed, its occurrence has been recorded for centuries—only in the past few decades have scientists begun to understand the mechanics and scope of this climate-altering event.

The term "el niño" is Spanish for "the child." When it is capitalized as "El Niño," it means "the Christ child." The name was given by sailors in the late 1800s to a weak warm current that appeared off the coast of Ecuador and Peru each year around Christmastime. The term "El Niño" first appeared in print in a Peruvian scientific journal in 1892.

There is evidence that strong El Niños have occurred periodically for thousands of years. In the late 1990s an unusually strong El Niño was blamed for floods, droughts, wildfires, storms, and unseasonable temperatures around the world.

Defining El Niño

El Niño has traditionally been defined as the annual warming of the waters off the coast of Peru. A massive pool of warm water—containing twenty to thirty times as much water as all the Great Lakes combined—arrives from the western Pacific equatorial region, replacing the cold water that typically resides on the South American coast. Most years the warm water only persists for a month or so before the cold water returns.

The water off the coast of Peru is typically about 68°F (20°C). During El Niño years, the water warms—sometimes just slightly and other times by several degrees. During El Niño conditions in December 1997, for example, the water temperature off the coast of Peru was 77°F (25°C).

A telltale sign that El Niño has arrived is the dwindling of fish populations (anchovies, in particular). Large numbers of fish die off or migrate to less-affected areas because the warm water is unable to sustain the tiny animals that fish eat. In scientific terms, the warm water is nutrient poor. The cold water, which is considered nutrient rich, does support tiny marine organisms. As the warm water moves in and these microorganisms die, it becomes nutrient poor.

In recent times the term "El Niño" has come to mean only extraordinarily strong El Niño episodes. Such episodes generally occur every three to seven years, but sometimes happen as often as every two years or as infrequently as every ten years. During these events, coastal waters become significantly warmer than usual—up to 10°F (5.6°C) higher than normal. In addition, the warm waters last longer than a few months (the longest recorded El Niño lasted four years, from 1991–1995) and occupy much of the eastern Pacific Ocean.

Commonly accepted standards for what constitutes a major El Niño event (that from hereon will simply be called an "El Niño") were developed by the Japanese Meteorological Agency (JMA). Those criteria, or conditions that must be met, are as follows: 1. Pacific Ocean temperatures, along the equator from Papua New Guinea in the west to the Galápagos Islands in the east, must be an average of 1°F (0.5°C) above normal; 2. The warm waters must persist for more than six months.

The 1997 to 1998 El Niño

In April 1997 the strongest El Niño in recorded history got underway. It produced heavy rain and flooding on the Pacific coast of South America, in California, along the U.S. Gulf Coast, in Eastern Europe, and in East Africa. Drought and wildfires spread in Australia, Southeast Asia, Mexico, Central America, Texas, Florida, and northeastern Brazil. A series of hurricanes swept through the eastern and western Pacific.

More acres of tropical rain forest burned during the El Niño of 1997–98 than at any other time in recorded history. By the time the episode ended in May 1998, the worldwide death toll due to El Niño—related weather was approximately 23,000, and property damage totaled at least thirty-three billion dollars.

WORDS TO KNOW

air pressure:

the pressure exerted by the weight of air over a given area of Earth's surface. Also called atmospheric pressure or barometric pressure.

ecosystem:

a community of plants and animals, including humans, and their physical surroundings.

El Niño:

means "the Christ child" in Spanish. A period of unusual warming of the Pacific Ocean waters off the coast of Peru and Ecuador. It usually starts around Christmas, which is how it got its name.

ENSO:

stands for El Niño/Southern Oscillation. It describes the simultaneous warming of the waters in the eastern Pacific Ocean and the shifting pattern of air pressure between the eastern and western edges of the Pacific.

food chain:

transfer of food energy from one organism to another. It begins with a plant species, which is eaten by an animal species; it continues with a second animal species, which eats the first, and so on.

jet stream:

the world's fastest upper-air winds. Jet streams travel in a west-to-east direction, at speeds of 80 to 190 miles (130 to 300 kilo-meters) per hour, around 30,000 feet (9,150 meters) above the ground. Jet streams occur where the largest differences in air temperature and air pressure exist. In North America, jet streams are typically found over southern Canada and the northern United States, as well as over the southern United States and Mexico. The northern jet stream is called the polar jet stream, and the southern jet stream is called the subtropical jet stream.

La Niña:

Spanish for "little girl," a period of unusual cooling of the Pacific Ocean waters off the coast of Peru and Ecuador. It often follows an El Niño.

monsoon:

a name for seasonal winds that result in a rainy season occurring in the summer on tropical continents, when the land becomes warmer than the sea beside it.

numerical prediction model:

a computer program that mathematically duplicates conditions in nature. It is often used to predict the weather.

trade winds:

dominant surface winds near the equator, generally blowing from east to west and toward the equator.

upwelling:

the rising up of cold waters from the depths of the ocean, replacing the warm surface water that has moved away horizontally.

Droughts and wildfires

Southeast Asia, plagued by drought (prolonged periods of unusually dry conditions) and wildfires, was the hardest hit area of the 1997–98 El Niño. Indonesia was hit with its worst drought in fifty years. Fires claimed more than twelve million acres of Indonesian and Malaysian rain forest in 1997, and 7.5 million acres in early 1998. (The fires predominantly burned on the islands of Sumatra and Borneo. Sumatra is part of Indonesia and Borneo is divided between the countries of Indonesia and Malaysia.) Relief, in the form of rain, came in May 1998.

The fires were set by farmers who burn the land to clear it for planting. Most years, any lingering flames are put out by the September monsoon. In 1997, however, the monsoon rains were delayed until December. The fires spread rapidly through the parched trees, propelled by hot winds. This method of clearing land for planting, called slash-and-burn agriculture, was outlawed after the 1997 fires. (The law, however, is proving hard to enforce, especially in remote regions).

The smoke from the fires was so thick that the Sun was blocked for days and drivers turned on their headlights at noon. Schools and businesses were shut down, and birds fell from the sky. People were advised to wear face masks outdoors in Indonesia, Malaysia, Singapore, the Philippines, and Thailand. Hundreds of people died of respiratory ailments, and tens of thousands were sickened.

In Sumatra, poor visibility in September 1997 was responsible for the crash of an airplane that killed 234 people. The smoke traveled thousands of miles to the west, affecting air quality and visibility on the Maldives, an island group in the Indian Ocean. Crop losses plagued much of the region. In Indonesia and Malaysia millions of people suffered shortages of food and drinking water. The fires carried a price tag of at least 1.3 billion dollars; the damages due to the haze alone tallied another one billion dollars.

Australia was hit by a severe drought in 1997. Cattle ranchers were forced, due to the lack of water and food, to slaughter their herds. In neighboring Papua New Guinea, drought resulted in widespread hunger. By the end of 1997 several hundred people had died as a result of famine and famine-related diseases, and half a million people were in urgent need of food and water.

In the spring of 1998, drought in the United States struck the Southwest and the Southeast. Texas's rainy winter of 1997–98 gave way to its third driest spring on record, in 1998. Accompanying Texas's drought was one of its worst heat waves of the century, claiming thirty lives.

In March, April, and May 1998, drought and wildfires came to Mexico, Central America, and northeast Brazil. Mexico lost more than 1.25 million wooded acres to fire. Smoke from those fires reduced visibility throughout much of Texas and spread haze and soot as far away as Wisconsin, Florida, and Oklahoma.

The source of Mexico's fires, like those of Southeast Asia, was a combination of slash-and-burn agriculture and exceedingly dry conditions. Fires also burned out of control in Guatemala, Honduras, El Salvador, Nicaragua, and Costa Rica, leading each of those countries to be declared disaster areas. The fires in Mexico and Central America were finally extinguished in May and June, when El Niño faded and the rains fell.

Floods

Rain and snow fell heavily in South America in 1997 and 1998. Ten times the normal amount of rain for the period had fallen in central Chile by mid-August 1997. Floods and mud slides damaged crops, buildings, roads, and bridges. In Chile's Atacama Desert, one of the driest places in the world, heavy rains washed out roads. Rain that drenched the central Andes caused serious flooding in Chile's capital city, Santiago. Paraguay, Uruguay, northeastern Argentina, and southern Brazil also endured heavy rain and sustained damages to buildings and crops.

From November 1997 through May 1998, storms, floods, and mud slides in Peru and neighboring Ecuador claimed 450 lives and did more than three billion dollars in damage to crops, roads, and buildings. Along the coast of Peru storms washed away some 300,000 homes, downed power lines, and destroyed roads and bridges. Waterborne diseases such as cholera and hepatitis, and mosquito-borne diseases such as malaria and dengue fever spread rapidly in coastal Peru.

The town of Chato Chico, Peru, was washed away completely. In the Peruvian coastal town of Mampuesto, where floodwaters caused erosion in a cemetery, caskets and skeletons floated down the streets. A similarly macabre event occurred in Peru's third-largest city, Trujillo, where flooding in a cemetery emptied 123 graves and washed the corpses through the town. In the Peruvian Andes severe snowstorms caused the deaths of 2,500 alpacas, a cold-adapted, llama-like species.

In Peru's coastal Sechura Desert, normally so dry that the ground is hard and cracked, floodwaters formed a lake 90 miles (145 kilometers) long, 20 miles (32 kilometers) wide, and 10 feet (3 meters) deep. To ease food shortages, government officials stocked the temporary lake with fish.

California had its seventh-wettest winter of the century. Between December 1997 and March 1998, the state received between two and four times its normal rainfall. The Sierra Nevada mountain range in eastern California had nearly 200 inches (500 centimeters) of snow; in some spots the snowpack in March measured 20 feet (6 meters).

An early December storm caused street flooding in southern California, dumping 6.81 inches (17.30 centimeters) of rain in a 24-hour period. In February 1998 a storm packing 90 mile-per-hour (145 kilometer-per-hour) wind gusts invaded southern and central California. It downed trees and power lines and produced flooding and mud slides. Waves up to 35 feet (10.7 meters) high pounded the coast. Just south of San Francisco, mud slides on the cliffs of Pacifica washed homes into the bay.

Also due to heavy rains in California, some two thousand cattle were stuck in knee-deep mud and died. The flooding also led to crop losses of strawberries, tomatoes, and lettuce. Agricultural losses were estimated at 200 million dollars.

Seventeen people lost their lives in storms and flooding during the California winter. Total El Niño-related property damages (including agricultural losses) throughout the state ran approximately one billion dollars.

The damage toll would have been far greater had Californians not been preparing for the onslaught of El Niño. Since early 1997, residents had been building dikes to keep back ocean water, clearing flood-control channels, and repairing roofs. In Redondo Beach, for example, high waves washed away the sand barriers built to protect the beach but left the beach intact.

East Africa, which usually suffers from drought during El Niños, was inundated with rain. The rain, which fell from October 1997 through January 1998, drowned crops and led to famine throughout the region. Relief agencies set up soup kitchens in Sudan, Somalia, and Kenya, all hard hit by flooding. In Somalia floods claimed the lives of more than 2,300 people, left 250,000 people homeless, and submerged entire villages. Some 100 people were killed in Kenya due to flooding in January 1998.

People and farm animals in Kenya and Somalia also suffered from mosquito-borne diseases that spread as a result of El Niño's rains. Many farmers lost up to 90 percent of their camels, cows, sheep, and goats to an illness, characterized by severe bleeding, called Rift Valley fever. There was also an outbreak of Rift Valley fever among humans—some eighty-nine thousand people were infected. Although the illness is not usually fatal to humans, it exacted a death toll of two hundred people that season.

A strengthened jet stream (the world's fastest upper-air winds) brought abundant rainfall to much of Europe in May and June of 1997. Southwestern Poland, the Czech Republic, eastern Germany, northeastern Hungary, Romania, and northern Slovakia experienced some of their worst floods of the century. Floods claimed the lives of fifty-five people in Poland and sixty people in the Czech Republic. There was more than two billion dollars in damage in the region. Some 300,000 cattle were lost in the Czech Republic and around one million chickens died when Polish farms were flooded.

Hurricanes and typhoons

As predicted for an El Niño year, Atlantic hurricane activity in 1997 was greatly reduced (the hurricane season for the Northern Hemisphere runs from June through December). The only Atlantic hurricane to make landfall in the United States was Hurricane Danny. Danny struck the Gulf Coast in July, bringing high winds and flooding to Louisiana and Alabama. Danny caused four deaths and 100 million dollars in damage.

In contrast, in 1997 there were seven major hurricanes in the Pacific Ocean (those that occur in the western North Pacific are called typhoons). A major hurricane is defined as one that is at least a Category 3 on the Saffir-Simpson scale of hurricane intensity. A category 3 storm does extensive damage and has winds between 111 and 130 miles per hour (179 and 209 kilometers per hour). In an average year the Pacific spawns five major hurricanes.

Hurricane Linda, which lashed Mexico's west coast in mid-September 1997, packed 185 mile-per-hour (298 kilometer-per-hour) winds. By way of comparison, Hurricane Andrew, which did extensive damage to Florida in 1992, had winds of 130 miles (209 kilometers) per hour. Linda—one of the strongest hurricanes ever recorded in the eastern Pacific—was so severe that meteorologists proposed adding a new category, Category 6, to the Saffir-Simpson Scale (the scale presently goes up to 5).

In late September 1997, Hurricane Nora made landfall on Mexico's Baja California peninsula. It then headed north, bringing heavy rains and flooding to Los Angeles and Arizona. Three people were killed in traffic accidents in Los Angeles and San Diego during the storm, and 1,000 people had to evacuate their homes in Arizona.

Weather report: El Niño is hardest on the poor

Around the world, poor people bear the brunt of El Niño. One reason for this reality is that it is more difficult for poor people to escape affected areas. Poor people are therefore more likely to suffer, and die, from drought-related famine or flooding-related illness. Poor people often live in flimsy or substandard housing that is vulnerable to floods; slums sustain the greatest damage due to mud slides and coastal flooding. For people who have little or no financial safety net, a loss of one's home or farm means financial ruin. Another factor is that governments often tend first to the needs of their most powerful constituencies—wealthy and middle-class people—while the needs of the poor are overlooked.

During the 1998 El Niño, when northeast Brazil suffered from drought and famine, poor people in that nation fought back against a neglectful government. Hungry crowds, angered at the failure of the government to distribute relief supplies, looted supermarkets and food trucks. Many of the nation's Catholic priests supported the rebellion, claiming that looting is justified when survival is at stake.

The following month Mexico's west coast was devastated by Hurricane Pauline. Heavy rains, floods, and mud slides killed as many as 230 people in Acapulco and surrounding areas, and destroyed thousands of homes. Tourists and residents faced shortages of food and drinking water for several days before rescue crews could reach them.

Typhoon Winnie whirled across the western Pacific in August 1997, striking the Philippines, Taiwan, and China. Winnie claimed nearly three hundred lives. In early November Typhoon Linda struck the southern tip of Vietnam. Nearly 100,000 homes were lost to the storm's winds and rains. Some three thousand people, most of them fishermen, died in the storm and flooding.

In December 1997, a strong hurricane formed over the El Niño-warmed waters south of Hawaii. Hurricanes rarely form in this region. Typhoon Paka, as it was named, soaked Guam in the southern Mariana Islands on December 16. Winds were measured at 108 miles per hour (174 kilometers per hour) before they destroyed instruments at a weather station on Guam.

Ice storms, tornadoes, and other unusual events

In January 1998 southeastern Canada and New England experienced the region's worst ice storm in recent history. (Meteorologists are divided as to whether or not El Niño is to blame for this event.) Freezing rain fell for five days, bringing down trees, power lines, and high-voltage towers in Eastern Ontario, New Brunswick, Nova Scotia, Maine, Vermont, New Hampshire, and upstate New York. At least twenty-five people lost their lives to the storm. Power was lost for more than four million people—including more than half the population of Maine—most of them for over a week. There was more than one billion dollars in damage to homes, businesses, and power systems throughout the region. More than 18 million acres of forestland were damaged in Maine, New Hampshire, Vermont, and upstate New York. The damage cost was estimated at between 650 million and 1.4 billion dollars.

Weather report: United States has typical El Niño winter in 1997–98

In general, the winter of 1997–98 in the United States was typical of an El Niño winter. The polar jet stream, which runs over central Canada, ran farther north than usual, through central Canada, keeping cold, polar air out of the northern United States and southern Canada. From Bismarck, North Dakota, to Buffalo, New York, temperatures were between 5 and 10°F (3 and 6°C) higher than normal. One way to gauge the severity of a winter is by looking at heating bills; northerners spent approximately 10 percent less on fuel in the winter of 1997–98 than they do on average.

Also as predicted, a strengthened subtropical jet stream brought increased precipitation and below-normal temperatures to the southern United States. Several southern cities, such as Tampa, Florida; New Orleans, Louisiana; and Charleston, South Carolina, had record rainfall during the winter of 1997–98. The state of Florida recorded its wettest winter ever. The Deep South experienced a rare snowstorm in December 1997. Four-to-eight inches of snow blanketed central Mississippi and central Alabama. Heavy snow also fell on the Southwest and the southern Plains states that month.

During the unusually warm spring, which was blamed on El Niño, the ice melted and caused flooding through much of central Canada. Around 5,000 people were forced to evacuate their homes as channels overflowed their banks, turning streets into rivers, in Quebec and Ontario provinces.

The southeastern United States experienced a spate of deadly tornadoes in the spring of 1998. On the night of February 22, forty-one people were killed, many as they slept, as tornadoes tore through central Florida. The tornadoes destroyed more than 800 homes and damaged more than 3,500 more, at a cost of more than 500 million dollars. A tornado that struck northeast Georgia on March 20 took twelve lives. On April 8 and 9, a string of tornadoes in Georgia and Alabama left thirty-four people dead and some five thousand acres of forests destroyed.

Dangerous science: What causes an El Niño?

While scientists have made great strides in recent years toward understanding and predicting El Niño, the origins of El Niño remain a mystery. At present, there are three primary theories as to what triggers an El Niño event: undersea volcanic eruptions, sunspots (magnetic storms on the Sun's surface), and the previous El Niño.

The first theory rests on the assumption that eruptions and lava leaks from volcanoes that dot the floor of the eastern Pacific Ocean provide enough heat to put an El Niño in motion. This theory is supported by the large number of earthquakes that have occurred on the ocean floor, west of South America, during recent El Niños. There is a strong link between the occurrence of undersea earthquakes and volcanic eruptions.

The second theory suggests that the ocean warming at the start of an El Niño is connected to the cycle of sunspots. Sunspots are areas of magnetic disturbance on the surface of the Sun, sometimes referred to as storms. When these storms reach maturity they eject plasma—an extremely hot substance made of charged particles—into space. A link has been established between increased sunspot activity and warmer temperatures on Earth. Scientists are attempting to determine whether the amount of warming during increased sunspot activity is sufficient to trigger an El Niño.

The third theory is that El Niños occur in cycles, with each successive El Niño being set in motion by the one before it. The theory goes like this: as an El Niño weakens, it generates long ocean waves, called Rossby waves, that travel westward across the Pacific. The Rossby waves carry with them the warm surface waters. As the warm layer thins in the eastern Pacific, cold water upwells to take its place.

The mass of warm water, driven by westward-blowing trade winds, then piles up in the western Pacific. When the pile of water in the west becomes so steep (up to five feet above mean sea level) that the trade winds can stack it no higher, the water is drawn down by gravity and flows back to the east. The shifting position of the warm waters creates a change in the air pressure gradient, and the trade winds weaken or reverse direction. The next El Niño is underway.

The influence of oceans on global weather

As scientists have recently discovered, El Niño is second only to the changing seasons as the strongest factor influencing world weather patterns. The reason for El Niño's strength has to do with the role of oceans in regulating weather, and with the enormous amount of energy contained in El Niño's warm waters.

Oceans cover more than 70 percent of the Earth's surface and are responsible for about one-third of total heat distribution around the planet. It is estimated that the top ten feet (three meters) of the ocean water contains as much heat as the total atmosphere.

The heat that is stored in oceans rises into the air above. The warm, moist air ascends and forms clouds. The water vapor within the clouds condenses and falls to the ground as rain. Therefore, it stands to reason that the world's wettest zones are the regions in which ocean temperatures are highest.

The central Pacific Ocean (near the equator), the world's longest continuous open body of water, is a tremendous storehouse of solar energy. Most of the time, the warmest water is concentrated in a deep layer in the western central Pacific, near eastern Australia and Indonesia. Accordingly, that region has a rainy climate. The waters in the eastern Pacific, off the coast of Peru and Ecuador, are much cooler. With less heat to rise into the air, comparatively little rain falls in that region.

During El Niño, the pool of warm water moves eastward across the Central Pacific. The South American coast receives the heavy rains that typically fall in the western Pacific.

The thermocline

Meteorologists (scientists who study weather and climate) explain El Niño as a shift in the thermocline. The thermocline is an imaginary dividing line between warm surface water and the cooler water below in the ocean. Under normal conditions, the thermocline is around 500 feet (150 meters) below the surface in the western Pacific and around 165 feet (50 meters) below the surface in the eastern Pacific. In other words, the warm water extends to a depth of 500 feet in the west and 165 feet in the east. Directly off the coast of Peru, cold water churns up from below and cools the surface water—bringing the thermocline nearly to the surface.

Under El Niño conditions, when the warm surface water heads eastward, the thermocline nearly levels out (it remains slightly lower in the western Pacific than in the eastern Pacific). The layer of warm water off the Peruvian coast is so deep that even water churned up from below is warm.

The trade winds connection

In the early 1960s Norwegian-born American meteorologist Jacob Bjerknes (1897–1975) discovered a link between the warming of waters in the eastern Pacific Ocean and a shift in the direction of the major surface winds at the equator, called the trade winds. Trade winds are a class of global winds—winds that bring warm air to cold areas and cold air to warm areas around the planet. Global winds rise and fall and move along the Earth's surface through a series of loops, or cells, on their route from the equator to the poles and back.

Trade winds originate in the Northern Hemisphere at approximately 30° north (this runs through the southern tip of Florida) and in the Southern Hemisphere at approximately 30° south (this runs through the Amazonian region of South America). At those latitudes air sinks to the surface, warming as it descends, and blows toward the equator. In the Northern Hemisphere trade winds blow to the southwest, and in the Southern Hemisphere trade winds blow to the northwest.

Every so often the trade winds weaken or reverse direction (begin blowing toward the east) in the tropical Pacific Ocean. That change is called the Southern Oscillation (oscillation means shift, swing, or variation). The Southern Oscillation is brought about by a shifting pattern of air pressure between the eastern and western edges of the Pacific Ocean. Air pressure, also known as barometric pressure or atmospheric pressure, is the weight of the air over a given area. Wind flows from areas of high air pressure to areas of low air pressure, in an attempt to equalize conditions.

As it turns out, El Niño and the Southern Oscillation occur at the same time. Years marked by El Niño and the Southern Oscillation are called El Niño/Southern Oscillation, or ENSO, years (they are sometimes called warm-phase ENSO years).

Air pressure and water temperature in normal years and ENSO years

During normal years (also called cold-phase ENSO years) air pressure is higher over the eastern Pacific, near South America, and lower over the western Pacific, near Australia. (High pressure is typically associated with clear skies while low pressure is typically associated with cloudy skies and rain.) This pressure gradient (change in air pressure across a horizontal distance) drives the trade winds from east to west, and toward the equator. The winds carry warmth and moisture toward Australia and Indonesia.

At the same time, the winds push along the surface layer of warm water, actually increasing the sea level in the western Pacific. In contrast, the sea level along the coast of tropical South America lowers, and the top layer of warm water thins. Cold water from the depths of the ocean along the South American shores rises to the surface and replaces the warm water. The water in the western Pacific is typically 14°F (8°C) warmer than the water in the eastern Pacific.

Just prior to an ENSO year, the easterly (coming from the east) trade winds weaken and sometimes reverse direction, and the warm waters in the western Pacific begin to move eastward. The warm water flows eastward in a long chain of waves called Kelvin waves. The air pressure in the eastern Pacific decreases, while the air pressure in the western Pacific rises.

Experiment: Measuring atmospheric pressure

Changes in air pressure over the Pacific Ocean help bring about the weakened and reversed trade winds that are a hallmark of the ENSO. Scientists use an instrument called a barometer to measure air pressure. To understand how barometers work, try this experiment.

Get a balloon and a clean glass jar with a wide top (such as a mayonnaise jar or a peanut butter jar). Use scissors to cut off the neck of the balloon. Brush some rubber cement or glue around the rim of the jar, and the stretch the balloon over the jar so that you have a nearly flat piece of rubber stretched over and glued to the mouth of the jar. Tape a straw to the top of the balloon so that one end of the straw is in the middle of the mouth of the jar and the straw sits parallel to the floor.

When air pressure goes down, the air in the jar will expand, causing the balloon to swell and the straw to point down. When the air pressure goes up, outside air will expand into the jar. The balloon will get pushed down, and the straw will point up.

The warm water, stretching for a distance of thousands of miles, piles up on the coasts of Peru and Ecuador. The layer of warm water becomes so deep along the South American coast that upwelling, or rising cold water, only brings up warm water (the cold water that normally rises to the surface on the South American coast is too far submerged). As the warm water evaporates into the air and forms clouds, the coastal South American nations experience an excess of rain, which causes flooding and erosion (removal of soil) while Australia, Indonesia, and the Philippines have unusually dry weather, and sometimes brush fires.

During a strong ENSO, the warm water doesn't stop once it reaches the shores of South America. Instead, it flows northward along the west coast of North America, sometimes as far north as northern Canada and Alaska. The moisture and heat rise from the ocean to the atmosphere, fueling storms that sweep eastward across North America.

Aftermath: The effects of El Niño

With the exception of the changing seasons, El Niño is the single greatest influence upon world weather patterns. El Niño's importance can be understood in terms of the role of oceans in controlling weather and the enormous amount of energy contained in El Niño's warm waters.

Oceans cover more than 70 percent of Earth's surface and are responsible for about one-third of total heat distribution around the planet. It is estimated that the top 10 feet (3 meters) of the ocean water contains as much heat as the entire atmosphere.

The heat that is stored in oceans warms the air just above the ocean's surface. This warm, moist air rises and is blown over land to form clouds. The water vapor within the clouds condenses into droplets and falls to the ground as rain. Because of this process, the world's wettest zones are the regions in which ocean temperatures are highest.

Who's who: Jacob Bjerknes and ENSO

Jacob Bjerknes (1897–1975), a Norwegian-born meteorologist teaching at the University of California, Los Angeles, put together the pieces of Sir Gilbert Walker's puzzle in 1969. Bjerknes had earlier gained fame as a meteorologist in Norway by describing the life cycle of storms in the middle latitudes (the regions of the world that lie between the latitudes of 30 degrees north and south, such as the United States and Europe).

Bjerknes became interested in the tropical Pacific during the strong El Niño of 1957–58. That event coincided with the International Geophysical Year (IGY), a year in which scientists the world over cooperated in a study of the earth (with an emphasis on oceans) and space.

As an IGY participant Bjerknes discovered that trade winds, or dominant surface winds near the equator, in the tropical Pacific Ocean weakened at the same time that waters warmed in the eastern Pacific. Soon thereafter he established a link between the arrival of warm waters and the following: heavy rains in South America; drought in the western Pacific; and the air pressure seesaw across the tropical Pacific (the Southern Oscillation). By 1969 Bjerknes had demonstrated conclusively that all these ocean-atmosphere interactions were intricately connected in a single, large-scale phenomenon that he called the El Niño/Southern Oscillation (ENSO).

The central Pacific Ocean near the equator, the longest open body of water on Earth, is a tremendous storehouse of heat from the Sun. Most of the time, the warmest water is found in a deep layer in the western central Pacific, near Australia and Indonesia. Accordingly, that region has a rainy climate. The waters in the eastern Pacific, off the coast of Peru and Ecuador, are cooler. They put significantly less heat into the air; as a result, comparatively little rain falls in that region.

Who's who: Sir Gilbert Walker and the Southern Oscillation

In the 1920s British mathematician and physicist Sir Gilbert Walker (1868–1958) was the first to point out the connection between unusual weather events around the globe during certain years—now called El Niño years.

Walker began his research in 1903, when he was named head of the Indian Meteorological Service. Walker was charged with predicting when India's annual monsoon rains would fail (drought was a huge, reoccurring problem in India as it led to periodic famine and the starvation of large numbers of people). At that time, weather authorities believed that local factors, such as increased logging in the region's forests, were responsible for monsoon failures.

Walker sifted through local and global weather records, searching for clues that might explain a pattern of monsoon failures. After two decades of research, Walker made an important discovery. He found that in years when Asian monsoons failed, Australia and parts of Africa also experienced droughts.

Walker looked at air pressure readings for Australia (in the western Pacific) and Tahiti (in the eastern Pacific) for a period of several years. He discovered that in years when the monsoon failed, there was a shift in the pressure gradient (change in air pressure across a horizontal distance) across the ocean. Specifically he found that during dry years, pressure in the west was higher than usual and pressure in the east was lower than usual. Walker called this seesaw of the pressure gradient the Southern Oscillation.

Taking things one step further, Walker found a link between increased ocean temperatures and greater precipitation in the eastern Pacific. In reports written in the 1920s and 1930s, Walker provided evidence linking abnormal weather patterns around the world with the Southern Oscillation. He hypothesized that global weather patterns were set in motion by a combination of the Southern Oscillation and a warming of water in the eastern Pacific, but was unable to verify it. The missing ingredient in his equation was data on wind speed and direction across the Pacific. Walker's theory was proven correct in the 1960s, when Jacob Bjerknes collected the necessary information and put the pieces together.

While Walker made important strides toward understanding a global weather phenomenon, he failed in his task to predict monsoon failures in India. Even with today's sophisticated forecasting equipment, scientists are still grappling with that challenge.

During El Niño, the pool of warm water moves eastward across the Central Pacific. The result is that the South American coast receives the heavy rains that usually fall in the western Pacific.

Effects of El Niño on the United States and Canada

In the United States and Canada, El Niño is one factor among many that determines the weather. The effects of any given El Niño depend on the strength of the event, particularly the way in which it affects the positions of the jet streams.

El Niño's influence on the weather is always greatest in the winter. Winter is when El Niño reaches its most mature stage in the Northern Hemisphere. Winter is also when contrasts in temperatures between the north and south of North America are greatest, and when the jet streams are strongest. The following is a list of general weather trends observed during strong and weak El Niño episodes.

During a strong El Niño, the subtropical jet stream (over Mexico and the southern United States) strengthens over the southern United States, and sometimes merges with the polar jet stream. The strong subtropical jet stream can be seen on satellite photos as a band of clouds and moisture moving across Mexico and the southern United States. The jet stream brings greater-than-normal rainfall—and in some cases flash floods, mud slides, and tornadoes—to southern California, the southwestern United States, northern Mexico, and the Gulf Coast.

The jet stream sometimes dips south once it passes the Gulf Coast. If that happens the southeastern U.S. stays dry, sometimes giving way to wildfires, and has colder-than-usual winters.

Weather report: Shifting air pressure patterns in the North Atlantic

In recent years more attention has been paid to the El Niño/Southern Oscillation's (ENSO) less-famous cousin, the North Atlantic Oscillation (NAO). The NAO is a balancing act between air pressure on the northern and southern reaches of the North Atlantic Ocean. Most years, the prevailing winds (winds blowing in the direction that's observed most often during a given time period) blow from a high-pressure region near the Azores (islands west of Portugal) northward to a low-pressure region near Iceland. From time to time, however, the pressure near Iceland declines. In the early years of the twenty-first century the pressure gradient appears to be weakening, which may indicate warmer summers and colder winters in Europe. However, long-term changes in the NAO indices are hard to predict and the winds weaken—this phenomenon is called an NAO event.

A normal pressure gradient has prevailed since the mid 1970s. The winter weather associated with this pattern is that Europe stays relatively warm while Canada stays relatively cold. When there is an NAO event, however, winter conditions cool down in Europe and warm up in Canada.

Through extensive research into the ENSO phenomena, scientists have determined that ENSO occurs every three to seven years. Understanding of NAO, in contrast, is still in its infancy and the phenomenon's period has yet to be discerned.

If the subtropical and polar jet streams merge, the polar jet stream hovers farther south than usual. The polar jet stream acts as a barrier against cold polar air, keeping it to the north. When the jet stream shifts southward, it allows cold air to move farther south than usual. As a result, southern Canada and much of the northern United States experience cold and sometimes wet weather.

If the jet streams don't merge, the polar jet stream heads north to Alaska before heading eastward across central Canada. In that case southern Canada and the northern United States stay relatively warm and dry.

During a mild El Niño, a weaker subtropical jet stream crosses Mexico before swinging north over the southeastern United States. In that case the West Coast and Gulf states stay relatively dry—and often experience wildfires—while the Southeast gets rain and tornadoes.

The polar jet stream during a mild El Niño heads north into Canada on the western edge of the continent, then dips farther south than usual. With this arrangement the Pacific Northwest experiences dry weather, while the states in the Midwest and Northeast, as well as southern Canada, have cold, wet weather, and sometimes flooding.

The warm waters of El Niño also fuel the development of hurricanes in the equatorial eastern Pacific. Occasionally those hurricanes travel north and drench the coast of southern California, then travel eastward to Texas.

Effects of El Niño on Latin America and the Caribbean

El Niño's effect is experienced most directly on the west coast of South America, particularly in Peru and Ecuador. There the warming of the water disrupts the fishing-based economy. Under normal conditions, cold water, rich in nutrients, rises up from the depths of the ocean to the surface along the shore. The cold water contains phosphates and nitrates that sustain tiny marine plants called phytoplankton (pronounced FIE-toe-plank-ton). The phytoplankton are eaten by tiny marine animals called zooplankton. The zooplankton, in turn, are food for fish. Under El Niño conditions, warm water replaces the cold water. The warm water holds few nutrients; it is inhospitable to phytoplankton and, as a result, to zooplankton and fish. When the coastal water is warm, large numbers of fish die off or migrate in search of food.

While El Niño spells misfortune for fishermen in Peru and Ecuador, it is a blessing to coastal farmers. The warm water fuels intense storms that irrigate thirsty crops. To those farmers, El Niño years are known as años de abundancia—years of abundance. In some years, however, El Niño brings so much rain that flash floods occur, washing away homes and destroying fields.

Farther south, through the western and central portion of the continent—in Chile, Paraguay, and Argentina—El Niño brings excess moisture to normally arid (dry) regions. That precipitation falls in the form of rain in the lowlands and snow in the mountains. Water runs down from the mountains and floods low-lying cities. In Chile's Atacama Desert, one of the driest places in the world (it sometimes goes twenty years without a drop of rain), El Niño can bring enough rain to make wildflowers bloom and wash out roads. El Niño also brings heavier-than-usual rainfall to Uruguay and southern Brazil.

During an El Niño, the northeastern portion of Brazil, as well as Central America, the Caribbean, and southern Mexico usually suffer drought. During some El Niños, crop yields in the region are reduced, and the local populace goes hungry. The dry weather also increases the likelihood of forest fires.

The west coast of Mexico, in contrast to the inland portion of the country, experiences storms and Pacific hurricanes fueled by El Niño's warm waters.

Effects of El Niño on Africa, Asia, Australia, and Europe

El Niño brings dry conditions—and often droughts and wildfires—to Australia, Southeast Asia, India, and Africa. In recent years, the worst El Niño-induced droughts occurred in Australia, India, Papua New Guinea, southeast Asia, and southern Africa. Wildfires raged out of control in Australia, Indonesia, and Malaysia. El Niño is unpredictable in eastern Africa; it sometimes brings drought and other times flooding. Central and eastern Europe sometimes experience excessively rainy weather during El Niño.

El Niño also spurs on the development of hurricanes in the Pacific Ocean (or cyclones, as they are called in the western Pacific region) and typhoons (another regional word for hurricane) in the Indian Ocean. During El Niño years, hurricanes and tropical storms (storm systems that form in the tropics and are weaker than hurricanes) dump heavy rains throughout much of Asia.

Exploring: El Niño and the anchovy industry

In the 1950s Peruvian fishermen expanded their anchovy harvesting operations. They sought to exploit markets in the United States and other industrialized nations for fishmeal—ground-up anchovies that are fed to poultry. In 1971 the Peruvian fishing fleet pulled nearly 14.1 million tons (13 million metric tons) of anchovies out of coastal waters, making Peru the world's top fishing nation.

The following year El Niño struck. As warm waters traveled to coastal Peru in 1972 and stayed throughout 1973, the anchovy population was greatly reduced. While the economic effects were felt most strongly by Peruvians, the lack of fishmeal also affected the U.S. poultry industry and other markets around the world. The number of anchovies still has not returned to pre-1972 levels.

Effects of El Niño on animal life

El Niño not only disrupts weather patterns, but it spells disaster for many types of marine life and land animals. As previously explained, the warm waters of El Niño are inhospitable to plankton, which occupy the bottom rung of the food chain. Fish, which feed on plankton, either move to colder waters or starve. (In a normal year there are six to eight million tons of anchovies in Peru's waters, but in an El Niño year there are only three to four million tons).

The repercussions of the lack of fish are felt all the way up the food chain and persist for several years. Marine birds and marine mammals (such as sea lions) that feed on fish throughout the Pacific Ocean face starvation. Populations of animals that prey upon sea birds also decline. The damage to marine life is observed throughout the Pacific region.

The spread of nutrient-poor, warm water is not the only way that El Niño affects animals. Other effects of El Niño imperil animal life in the following ways: river flooding results in the introduction of sediments and contaminants into coastal waters; large ocean waves erode wildlife habitat; and forest fires in drought-plagued regions drive wildlife out of their homes.

Sea birds

The animals most directly affected by the lack of fish are sea birds, primarily terns and gulls. In El Niño years sea birds in the western Pacific have difficulty finding enough food for themselves and their chicks. One of the biggest die-offs occurred during the 1957–58 El Niño, when some 18 million birds off the coast of Peru perished. Among the hardest-hit species were cormorants, boobies, and pelicans. Twenty-five years later, the 1982–83 El Niño drove away or killed 85 percent of Peru's sea birds. The seventeen million sea birds inhabiting Christmas Island (in the middle of the Pacific) also abandoned their homes at that time.

A key reference to: How El Niño reaches Africa and India

In 1982–83 southern Africa experienced severe drought, and India's monsoon rains, which usually occur during the summer, never came. While the lack of moisture in both places was blamed on El Niño, scientists have only recently determined the way in which El Niño influences regions beyond the Pacific.

It turns out that at the same time that waters warm off the coast of Peru, a similar warming occurs in the Indian Ocean. That warming triggers a reversal of the pressure gradient (the rate at which air pressure changes with horizontal distance) in the Indian Ocean. The surface winds that typically blow to the northwest, toward the coast of Africa and India, change direction. They blow toward the southeast, bringing warmth and moisture to western Australia and leaving southern Africa and India high and dry.

The 1997–98 El Niño also had a noticeable effect on sea birds. Albatrosses abandoned their nests in the Galápagos Islands in search of colder waters and more abundant fish. Peru's populations of Inca tern, guanay, and red-legged cormorant also suffered declines.

The brown pelican population in Baja California and the Gulf of California (on Mexico's west coast) dropped to its lowest level in thirty years during the 1997–98 El Niño. In normal years there are between 10,000 and 20,000 nests in the colony. In March and April 1998, researchers found only 280 nests. Just one month later, not a single nest could be found. Biologists expected the pelicans to make a full recovery once the waters cooled and the anchovies, herrings, and sardines returned.

The decline of least terns in California during the El Niño of 1982–83 provides an interesting case study. The fish that terns typically feed on were few in number and small in size. Female terns laid their eggs later than usual, and the eggs were abnormally small. Many females abandoned their nests for lack of food, and many of the chicks that did hatch did not develop properly. Large numbers of the weakened chicks were preyed upon by small hawks called American kestrels.

The California least tern colony did not recover until 1988. The repercussions of the 1982–83 El Niño lasted so long, in part, because least terns do not breed until they are two or three years old. Thus, in 1984 and 1985 the number of breeding terns was smaller than usual.

Weather report: El Niño weather around the world

Below is a summary of El Niño-inspired weather patterns around the world. This information presents general trends, not hard-and-fast rules. Weather during El Niño years is influenced by the strength of the event and a host of other factors. The weather in a given location during a particular El Niño may even be the opposite of what is listed below.

• Increased precipitation and flooding: Peru, Ecuador, Chile, Paraguay, Argentina, Uruguay, southern Brazil, east-central Africa, central and eastern Europe, western Australia, and eastward from California and Arizona through the southern United States.
• Drought: Northeastern Brazil, Central America, the Caribbean, and southern Mexico, Australia (except the west coast), India, southeast Asia, Papua New Guinea, California (during weak El Niño) and southern Africa.
• Increased hurricane activity: West coast of Mexico, southern California to Texas, Asia (along the coast of Indian Ocean), and Madagascar.
• Warmer than usual winter: Northern United States, western Canada, Alaska, northern Europe, southeast Asia, Japan, North Korea, South Korea, Mongolia, southeast Australia, and southeast Africa.
• Colder than usual winter: Southeastern United States.

Sea mammals

Populations of sea lions, fur seals, and other sea mammals also decline during El Niño years. Those animals, which live in colonies on the South American coast, on the California coast, and on the Galápagos Islands (west of Ecuador), subsist mainly on anchovies. The scarcity of anchovies (and secondary food sources such as halibut, lantern fish, rockfish, and squid) during El Niño years has the most serious impact on young animals. Seal and sea lion pups go hungry because their mothers spend much more time than usual (five to six days, instead of the usual one to two days) seeking fish rather than nursing their young. Pups either starve or grow weak. Many of those pups that survive their first season later prove incapable of finding their own food, and die.

Large numbers of adult sea mammals also starve during El Niño years. Many females are unable to sustain themselves, particularly while nursing. Males are adversely affected by their breeding behaviors. They stay on land defending their territory during breeding season, typically going without food for several weeks. Once the males return to sea, they are unable to find sufficient food to regain their strength and survive.

During the 1982–83 El Niño, 90 percent of the fur seal pups in Peru died. In the same season, more than half of the elephant seal pups in California were lost due to storms that flooded pupping beaches (beaches where seals are born).

Seals in colonies on the California cost were especially hard hit during the 1997–98 El Niño. More than six thousand pups from a colony on San Miguel Island died by the end of 1997. The mortality rate of the pups reached 70 percent; in normal years just 25 percent of the young animals die.

Sea mammals were in such distress in 1997 and 1998 that animal rescue groups set up stations on California beaches. Members of these groups fed and cared for mammals they found stranded on beaches and sandbars. When the animals were well, rescuers released them back into the ocean.

At El Niño's peak in early 1998, water temperatures off Peru's Paracas Peninsula, normally 56 to 58°F (13 to 14°C), rose to 81 to 83°F (27 to 28°C). The results of this extreme warming could be witnessed in the thousands of sea lion and seal carcasses littering South America beaches from Chile to Ecuador.

Even the sea mammals as far away as Antarctica do not escape the grip of El Niño. The icy continent's weddell seal population declines significantly during El Niño years.

Galápagos Islands iguanas and penguins

Another casualty of the warm El Niño waters is green algae. Green algae, which thrives in cold water, is the main food source of the marine iguana, a 39-inch-long reptile that lives on the Galápagos Islands. When the water warms, the green algae become stunted and covered with brown algae. Brown algae are not digestible by marine iguanas. During the 1982–83 El Niño, as the green algae went into decline, much of the Galápagos marine iguana population was wiped out. In 1998, when the waters warmed by 10 °F (5.6°C), marine iguanas suffered again.

The 1982–83 El Niño

The El Niño of 1982 and 1983 was the second strongest El Niño in recorded history (second only to the 1998–99 event). The Southern Oscillation (shift in air pressure across the Pacific) was so great that the trade winds not only weakened, they reversed. Storms took the lives of more than 2,000 people and caused between thirteen and fifteen billion dollars in damage worldwide. El Niño brought about devastating droughts, floods, and storms in every continent except Antarctica.

Hawaii, Mexico, northeastern Brazil, southern Africa, the Philippines, India, Australia, and Indonesia all experienced droughts—some had brush fires and dust storms. Australia was hit with its worst drought ever. A dust storm swept more than 100,000 tons of soil from farmland into coastal cities and the ocean, and 60 percent of Australian farms experienced crop loss. Bush fires killed 72 people and more than 300,000 livestock.

Meanwhile, extensive flooding plagued the southwestern United States, the Gulf states, Cuba, Ecuador, Peru, and Bolivia. Five hurricanes pounded the islands of French Polynesia, and one hurricane blew through Hawaii.

The damage toll due to El Niño in Peru, Chile, Ecuador, Bolivia, and Colombia reached six billion dollars. Peru had its greatest rainfall in recorded history; some areas received sixty times more rain than normal. There were mud slides and flooding in the north and a drought in the south. The coastal town of Chulliyachi, in northwest Peru, was wiped off the map. Twenty-foot (six-meter) waves washed away the town church (it remains submerged along the coast today) and three residential blocks, and turned roads into rivers. The town's 1,500 residents had to be airlifted to safety.

In Ecuador, El Niño rains caused landslides and washed away roads, bridges, houses, and farm animals. The rains drowned crops and destroyed the main railroad line leading to the capital city, Quito. The flooding in coastal Ecuador and northern Peru killed 600 people, mainly those living in slums.

Damage due to storms in the United States cost more than two billion dollars. A string of violent storms traveled across the west coast of the United States, drenching California and creating mud slides and floods and washing away beaches. More than thirty houses were washed off hillsides, into the ocean. There were also storms and flooding in the Rocky Mountains and the Gulf states. The East Coast had its warmest winter in twenty-five years, at a savings of 500 million dollars in heating bills.

The penguins that live on the Galápagos Islands also suffer during El Niño. Galápagos penguins make their homes on several of the islands, including the northernmost islands, which are north of the equator. That makes these flightless birds, which measure twenty to twenty-four inches high and weigh four to five pounds, the sole penguin species naturally occurring in the Northern Hemisphere.

The staple of the penguins' diet is small fish, primarily mullet. Those fish are driven away from the islands by warm El Niño waters. Since the nearest land is 600 miles away, penguins are unable to migrate in search of food. During the 1998 El Niño researchers observed skin-and-bones adult penguins and no juveniles—suggesting either that the birds did not breed or, if they did breed, all of the chicks died.

In the aftermath of the El Niños of 1982–83 and 1997–98, the Galápagos penguin population has been reduced to half its 1970 size. At the end of 1998, they numbered less than 8,500.

Secondary effects on land animals

El Niño's effects are felt by various species of land animals, namely those that feed upon marine animals and those whose habitats are affected by the changes in weather conditions. An example of a land animal that declines in number during El Niño years is the red fox. Red foxes that live on Round Island, Alaska, subsist mainly on the sea birds that nest there. When the warm waters appear and the fish disappear, the population of sea birds—especially common murres and black-legged kittiwakes—declines. El Niño's effect on the foxes is evident in their reduced numbers of offspring. In normal years the foxes have up to seven litters, with four or more pups per litter. In El Niño years foxes typically have just one litter of three or four pups.

Some animal species are harmed by drought-induced wildfires during El Niño years. For the animals that inhabit the Indonesian islands of Borneo and Sumatra, the results of the 1997–98 El Niño were disastrous. At least six million acres of rain forest burned, driving out or killing thousands of orangutans and other animals. Across the Pacific in drought-struck Mexico, fires also burned. Those fires destroyed the winter habitat of monarch butterflies. The monarchs either died or made their home elsewhere.

Effects of El Niño on human health

Unusual weather produced by El Niño affects the health of human beings in many ways. For instance, hunger is a problem in areas where crops have failed due to drought. Respiratory ailments are common in regions ravaged by forest fires. Many diseases are spread by organisms that reproduce rapidly during El Niño years.

Cholera, dysentery, and typhoid are diseases that commonly spread during floods, when sewage treatment systems become overloaded and drinking water supplies become contaminated. An overabundance of standing water also enhances the breeding of mosquitoes, which carry malaria, dengue fever, yellow fever, and encephalitis (pronounced en-SEF-a-LIE-tus).

During the 1982–83 El Niño, flooding in Ecuador, Bolivia, Colombia, Peru, India, and Sri Lanka resulted in significant outbreaks of malaria. In early 1998, when El Niño-driven rains produced flooding, Peru experienced a malaria epidemic. In the Piura region of northwest Peru, where 1.5 million people reside, there were some 30,000 cases of malaria.

Also in 1983, the unusually mild and moist spring and summer in California gave rise to record numbers of fleas carrying the bubonic plague, an infectious disease that wiped out one-fourth of Europe's population during the Middle Ages (476–1453). Fleas spread the disease to mammals, which can pass on the illness to humans. In 1983, thirty-six people contracted the plague (all in western states), and six of them died. That outbreak was the most severe in the United States since the 1920s.

Another way that El Niño affects human health was discovered in 1993, following the outbreak of a deadly disease in the southwestern United States. The disease, caused by a type of virus called hantavirus, killed several people in the Four Corners region (the place where Arizona, Utah, Colorado, and New Mexico come together).

The hantavirus is carried by desert-dwelling rodents called deer mice. In normal years, the deer mouse population is small. Food for the rodents is in limited supply; and predators, such as owls and snakes, keep deer mouse numbers down. During the 1992 El Niño, however, the desert in the Four Corners region received a lion's share of rain. Plant life exploded, as did the deer mouse population.

Along with the greater numbers of deer mice came greater numbers of deer mouse droppings. People who either touched the droppings or breathed dust contaminated with the droppings risked exposure to the hantavirus. When the rains stopped and the desert returned to its arid state, the hantavirus outbreak subsided.

During the El Niño of 1997–98, the desert again received exceptional rainfall. With conditions ripe for a deer mouse population increase, the local population was instructed to steer clear of the rodents and their droppings.

The human factor: A possible link between El Niño and global warming

The strongest El Niños on record occurred in the 1980s and 1990s. That reality has prompted scientists to consider whether human activity—namely global warming—has an effect on El Niño.

Did you know: El Niño bleaches coral reefs

Coral reefs, which are undersea ecosystems sometimes referred to as the rain forests of the oceans, are among the most species-rich places on Earth. Coral reefs are colonies of coral polyps (pronounced PALL-ups)—small, tube-shaped animals with hard exterior skeletons coated with colorful algae. The algae, which give the coral reefs the appearance of underwater gardens, are essential to the survival of the polyps.

Coral reefs are found in the warm, shallow waters of tropical oceans and can survive only within a small temperature range. An increase in temperature of just a few degrees can kill the algae. When the algae die, the coral bleaches (turns a whitish color). Bleaching typically leads to the death of a polyp colony.

Many coral reefs were bleached during the 1982–83 El Niño, when eastern Pacific Ocean temperatures increased by 4 to 5 °F (2 to 3 °C). Some coral species were wiped out entirely. The most extensive damage to coral reefs occurred off the coasts of the Galápagos Islands, Ecuador, Colombia, Panama, and Costa Rica. There the losses to three-hundred-year-old coral reefs ranged from 50 to 97 percent. Scientists estimate that it will take centuries for the corals in those areas to recover.

The bleaching of corals recurred during the 1997–98 El Niño. Significant damage was done to the reefs off the Pacific coasts of Panama, Costa Rica, and Mexico. The warm waters also wiped out an 18-mile (29-kilometer) long coral colony along the Great Barrier Reef of Australia. Varying degrees of bleaching also occurred in coral reefs off the coasts of French Polynesia, Kenya, the Galápagos Islands, the Florida Keys, Baja California, Mexico's Yucatan coast, the Cayman Islands, and the Netherlands Antilles. The corals at most of these sites were expected to recover once the water temperature returned to normal.

Average temperatures around the world have risen over the last 100 years. Global warming is the theory that temperatures have begun to rise, and will continue to rise, due to an increase of certain gases, called greenhouse gases, in the atmosphere. Greenhouse gases are gases that trap heat in the atmosphere. The most abundant greenhouse gases are water vapor and carbon dioxide. Others include methane, nitrous oxide, and chlorofluorocarbons. The atmospheric increase of one particular greenhouse gas—carbon dioxide—is believed to be the primary reason for global warming. Carbon dioxide is produced by the burning of fossil fuels. It is emitted by factory smokestacks and cars.

Orangutans struggle to survive

The effects of the 1997–98 El Niño pushed orangutans toward possible extinction. Orangutans that survived the wildfires brought about by the El Niño-induced drought in Indonesia faced starvation due to dwindling food sources. Baby orangutans, too weak to hold onto their mothers, reportedly dropped from trees and died. Prior to the wildfires, orangutans were already considered an endangered species (their population had been reduced by forest clearing and hunting).

Over the last century, the amount of carbon dioxide released into the atmosphere has increased by 30 percent. During that same time period, the planet has become, on average, slightly more than 1 °F (0.5 °C) warmer. The warmest year in U.S. history since the keeping of detailed records began in 1895 was 1998. The years 1997 and 1998 were the world's warmest two years of the last century.

The Intergovernmental Panel on Climate Change (IPCC), a group consisting of 2,500 of the world's leading climatologists, believes that the balance of evidence suggests that humans do have an effect on global climate. The question remains: Is that human influence affecting El Niño?

Arguments for and against the global warming connection

Scientists were prompted to look at a possible connection between global warming and El Niño when the 1997–98 El Niño showed itself as the strongest episode in recorded history, just two years after the end of a five-year El Niño—the longest episode in recorded history. In addition, the 1997–98 El Niño came just fourteen years after the 1982–83 El Niño—previously the strongest episode on record. According to statistical models, such a sequence would occur naturally just once every two thousand years. That fact suggests that human-induced factors, rather than nature, may be responsible for the pair of unusually strong El Niños.

One theory explaining how global warming and increased El Niño activity are connected is that as the planet warms, heat builds up in the Pacific Ocean. El Niño acts as an escape valve for the excess heat, moving it eastward across the ocean and then releasing it into the atmosphere. Furthermore, computer simulations (computer programs that mimic real-world events) show that increased carbon dioxide levels in the atmosphere lead to an uneven heating of the planet. In the Pacific Ocean, the eastern portion warms to a greater degree than the western portion (exactly the conditions found during El Niño).

While many scientists believe that global warming may affect El Niño, few believe that is the entire reason. El Niño experts point out that huge shifts have occurred in the global climate over the past hundreds of thousands of years, without any human influence such as global warming. Natural variability in climate has ranged from ice ages to warm periods. At certain times in our planet's past, El Niño-like conditions have lasted for thousands of years.

Many scientists refuse to make conclusions about the impact of global warming on El Niño based on one century's worth of data. Those scientists argue that at least another century of careful measurement of El Niños is needed to make such a determination.

Why most scientists won't yet blame global warming

While many scientists think that global warming may affect El Niño, few think there is sufficient evidence to be certain. El Niño experts point to the huge shifts that the global climate has experienced over the past hundreds of thousands of years, without the influence of humans. They refuse to make assessments based on spotty data from one century. (Detailed data collection on El Niño around the world only began in 1986.) They suggest that the recent series of unusually strong El Niños may be within normal limits of shifts in global climate.

One reason why it may be too early to blame global warming for increased El Niño activity is that there are many, many influences on global climate, most of them unrelated to human activity. Throughout the Earth's history, the natural variability in climate has ranged from ice ages to warm periods. At certain times in our planet's past, El Niño-like conditions have lasted for thousands of years.

Many members of the scientific community think that at least another century of careful measurement of El Niños is needed to determine whether or not global warming is a factor.

Why are a few degrees of warming such a big deal?

You may be wondering why a warming of the ocean of just a few degrees has such a tremendous impact on weather. After all, slight changes in air temperature happen frequently with little consequence. In the case of El Niño, it is the heat contained in a tremendous volume of water (twenty to thirty times as much water as all the Great Lakes combined) that matters. A temperature increase of even 1 °F throughout that volume of water results in a significant increase in the heat contained therein.

To understand El Niño's power, it is important to realize that heat and temperature are not the same thing. The difference between heat and temperature has to do with kinetic energy, the energy of motion. As molecules heat up, their kinetic energy increases. Heat is the total kinetic energy of a substance, whereas temperature is the average kinetic energy (also defined as the hotness or coldness) of a substance. In other words, heat takes into account the volume of a substance. If you take two volumes of liquid at the same temperature—say a bathtub and a coffee cup—the bathtub contains more heat.

Just how much kinetic energy does El Niño contain? It contains more energy than can be produced in one year by one million power plants, at 1,000 megawatts each; more energy than all the fossil fuel (gasoline, coal, and natural gas) that has burned in the United States since 1900; and as much energy as 500,000 twenty-megaton hydrogen bombs.

Technology connection: Predicting El Niños

Scientists have spent decades unlocking the secrets of El Niño and the Southern Oscillation. Now that a high level of understanding of these phenomena has been achieved, the most pressing task is predicting when they will occur. With advance warning of destructive weather, societies can make preparations to minimize the damage.

Predictions of El Niños remain of a general nature, such as whether conditions will be wetter, drier, colder, or warmer than usual. Crop growing season forecasts issued by international climate prediction agencies state one of the following: near-normal conditions; a weak El Niño with a slightly wetter than normal growing season; a full-blown El Niño with flooding; or cooler than normal waters offshore, with higher than normal chance of drought (in other words, La Niña conditions).

A key reference to: El Niños since 1950

The definition of a strong El Niño event (otherwise called an El Niño), established by the Japanese Meteorological Society and endorsed by the National Center for Atmospheric Research, is that Pacific Ocean temperatures along the equator from Papua New Guinea to the Galápagos Islands must be an average of 1 °F (0.5 °C) above normal for six months or more. According to that definition, there have been sixteen El Niños since 1950. The El Niños of 1991, 1992, 1993, 1994, and 1995 are separated by brief periods in which ocean temperatures were still above normal, yet less than the required 1°F above normal. Many meteorologists consider these periods to be a single, five-year-long event.

• August 1951–February 1952
• March 1953–November 1953
• April 1957–June 1958
• June 1963–February 1964
• May 1965–June 1966
• September 1968–March 1970
• April 1972–March 1973
• August 1976–March 1977
• July 1977–January 1978
• October 1979–April 1980
• April 1982–July 1983
• August 1986–February 1988
• March 1991–July 1992
• February 1993–September 1993
• June 1994–March 1995
• April 1997–May 1998

In recent years scientists have developed the tools to make predictions about the development, intensity, and effects of El Niño. They use a combination of computer models and measurements of air and water conditions in the tropical Pacific Ocean. The measurements are taken by a network of weather buoys (drifting or anchored floating objects containing weather instruments) and satellites, supplemented by readings taken on ships. With today's technology, meteorologists (scientists who study weather and climate) at weathers prediction centers are able to observe changes in the ocean as they occur.

Computer-based prediction

Beginning in the early 1980s, meteorologists have used computer models of climate change (also called climate modeling) in their attempts to predict El Niños. Climate modeling starts with a sophisticated computer program, called a numerical prediction model. The model incorporates mathematical equations that mimic processes in nature. The equations are based on the laws of oceanic and atmospheric physics, describing motion, thermodynamics (the relation of heat and mechanical energy), and the behavior of water.

When a set of data describing current conditions is entered into the computer, the program tells what is likely to happen up to several months in the future. The computer models are constantly fine-tuned based on data from the weather buoys and satellites.

Tropical Ocean-Global Atmosphere (TOGA)

In the early 1980s the World Meteorological Organization, a Geneva, Switzerland-based agency of the United Nations, developed an ocean-monitoring system called the Tropical Ocean-Global Atmosphere (TOGA). The stated purpose of TOGA was to explore the predictability of the tropical ocean-atmosphere system and the impact on the global atmospheric climate on time scales of months to years. The development of TOGA was hastened by the strong 1982–83 El Niño, which took place during the planning stages of TOGA.

TOGA was coordinated by the National Oceanic and Atmospheric Administration (NOAA) of the United States and weather agencies of France, Japan, Korea, and Taiwan, with the participation of thirteen other nations. The program operated from 1985 to 1994. During that time TOGA researchers observed interactions between the air and sea in the equatorial Pacific and assessed how those interactions would affect changes in climate around the world.

Watch this: "Chasing El Niño"

In 1998, the PBS series NOVA broadcast an episode entitled "Chasing El Niño" that chronicles scientists' attempts to understand the causes of El Niño and make predictions on the magnitude and development of El Niño events. The documentary opens with a look at the Pacific Ocean, which is the largest feature of Earth's surface and has a huge influence on global climate. The program explores the history of El Niño prediction efforts in the Pacific, including the creation of the Tropical Atmosphere Ocean (TAO) array, a massive network of ocean buoys.

"Chasing El Niño" also explores the creation of the massive computer models scientists use to help predict El Niño's intensity. For those with a taste for more hands-on work, meteorologists are shown flying a plane directly into an oncoming storm! The scientists measure rainfall, temperature, wind speed, and air pressure in hopes of forecasting where the storm will fall. "Chasing El Niño" offers an informative and exciting look at the science of predicting one of the world's most mysterious weather effects.

TOGA used the following equipment to collect information: weather buoys, satellites, ships, and tidal gauges (instruments that measure the coming and goings of the tides). These instruments, collectively, measured water temperature at the ocean surface and to a depth of 1,650 feet (500 meters), as well as air temperature, relative humidity, ocean currents, sea level, and the speed and direction of surface winds. All data was transmitted daily, via satellite, to weather prediction centers.

Of the weather buoys, some were drifting and some were moored (anchored to the ocean floor). The drifting buoys, called Global Lagrangian Drifters, emitted signals that indicated their positions, and thus the direction of surface water motion. They also recorded air pressure and temperature of surface ocean water at various locations. The moored buoys measured surface winds and temperatures at various depths of the ocean.

The information collected by TOGA filled in many gaps of knowledge about El Niño's life cycle. The program also established the first means of monitoring the Pacific Ocean and the atmosphere in real time (at the present).

Tropical Atmosphere Ocean Array (TAO)

Although the TOGA program ended in 1994, it initiated a permanent, international network of ocean-atmosphere monitoring. That system—which includes moored and drifting buoys, satellites, and research ships—is called the El Niño-Southern Oscillation Monitoring System. The central element of the monitoring system is the Tropical Atmosphere Ocean Array (TAO). Completed in December 1994 as TOGA was coming to an end, the TAO takes continuous ocean measurements. The purpose of the TAO is to detect El Niños in their earliest stages and improve forecasting.

One of TOGA's greatest achievements was the development of the TAO array. The TAO project is jointly coordinated by the NOAA and weather agencies in Japan, Taiwan, and France. Its headquarters are at the NOAA's Pacific Marine Environmental Laboratory (PMEL) in Seattle, Washington.

The TAO array consists of sixty-five moored buoys and five current meters (instruments that measure the strength and direction of currents), spanning the equatorial Pacific—covering one-third of the globe. The buoys and meters are stationed at intervals between longitudes of 135° east (near Indonesia) and 95° west (just west of Peru), and latitudes 10° north and 10° south (forming a wide band with the equator in the center). (Degrees of longitude are imaginary lines encircling Earth, perpendicular to the equator, that tell one's position east or west on the globe; degrees of latitude run parallel to the equator and tell one's position north or south on the globe).

TAO's buoys detect air temperature, surface wind speed and direction, relative humidity, sea surface temperature, and ocean temperature to a depth of 1,650 feet (500 meters). The buoys and current meters transmit information, via NOAA satellites, continuously to TAO project headquarters. The data is fed into high-speed computers and is analyzed, after which it is made available to weather prediction centers and climate researchers around the world.

TAO's computers combine the thousands of continuous readings from the buoys into a single picture. That picture appears on researchers' monitors as a checkerboard with different colored squares. The color of each square indicates the instruments' readings of ocean and atmosphere at a given location. New readings are entered, and the picture is updated several times a day. It is possible to view a series of pictures taken previously, in rapid succession—like a movie of ocean conditions. This technology allows researchers to literally watch El Niños unfold.

ATLAS buoys

The TAO array's moored instruments are called ATLAS buoys. At the top of each buoy is a set of sensors measuring wind, humidity, and temperature; a data transmitter; and a satellite antenna. Beneath the floating portion of an ATLAS buoy hangs a 1,722-foot-long (525-meter-long) sensor cable. Temperature sensors are placed at various depths along the length of the cable. The cable ends with a 4,200-pound (1,900-kilogram) anchor.

The original ATLAS buoys had a life expectancy of one year, after which they required servicing or replacing. A new generation of ATLAS buoys was introduced in 1996, boasting longer lifetimes and greater measurement capabilities.

What's next for TAO

The next phase in the prediction of El Niño will likely involve expanding the TAO array to the Indian Ocean, the tropical Atlantic Ocean, and throughout the northern and southern Pacific. Evidence indicates that climate variability in those regions is linked to El Niño. An NOAA program called the Pilot Research Moored Array (PIRATA) proposes to place buoys across the equatorial Atlantic Ocean early in the twenty-first century.

TOPEX/Poseidon satellite

The TOPEX/Poseidon (TOPEX stands for Ocean Topography Experiment) satellite is another key player on the El Niño prediction team. This satellite—a joint project of the U.S. National Aeronautics and Space Agency (NASA) and the French space agency, Centre Nationale d'Etudes Spatiales (National Center of Space Studies)—was launched in 1992.

The TOPEX/Poseidon, orbiting the Earth at a height of 830 miles (1,336 kilometers), uses radar altimeters (instruments that measure altitude by bouncing radar beams off the ocean surface) to measure sea level heights. The measurements are accurate to within a few centimeters. Sea-surface heights are directly related to the heat content of the ocean. A rising sea level in the eastern Pacific is an important clue that an El Niño is underway.

Exploring: Take a virtual cruise on a research ship

A vital element of the TAO array is the National Oceanic and Atmospheric Association's (NOAA) research ship, the Ka'imimoana (means "ocean seeker"). The Ka'imimoana performs maintenance on the TAO's weather buoys. Scientists aboard the ship also take measurements of ocean currents, surface water temperature, and ocean temperature to depths of 4,957 feet (1500 meters).

The Ka'imimoana was constructed in 1989 and purchased by the NOAA in 1993. After being converted to an oceanic research vessel, the Ka'imimoana went into operation in April 1996.

Visit the Ka'imimoana home page at http://www.moc.noaa.gov/ka/index.html. There you will see the ship's officers and crew as well as photographs and data from the ship.

If governments are aware that unusual conditions will lead to crop loss or shortages of drinking water, they may stockpile food and water for their residents. Health precautions may be taken in areas where flooding is expected to produce outbreaks of waterborne or mosquito-borne diseases.

Predictions are also useful along the west coasts of South and North America, where high waves and flooding cause damage. If an El Niño is predicted, residents may build barriers to prevent beach erosion and work to reinforce bridges and other structures. At the same time, they may halt new construction projects.

The United States and France put into orbit another oceanographic (pertaining to the study of oceans) satellite in May 2000. The satellite replaced the TOPEX/Poseidon, which has lasted far longer than expected.

The benefits of El Niño prediction

Once El Niño has been predicted, the challenge to researchers is how to make that prediction meaningful. The question for many experts is how El Niño predictions can be adapted to the needs of specific industries and people.

The task of making El Niño predictions useful has been assigned to the International Research Institute for Climate Prediction. That institute was established in 1996 at Columbia University's Lamont-Doherty Earth Observatory.

With advance warning of the destructive weather El Niño has in store, societies can make preparations to minimize the damage. El Niño predictions are most valuable to people involved in agriculture and fishing, especially in tropical nations. (The tropics generally suffer the greatest consequences of El Niño's droughts and flooding.) Other areas in which El Niño predictions are useful are public health, transportation, forestry, water resources, and energy production.

Among the countries that have used El Niño predictions to manage agriculture are Peru, Australia, Brazil, Ethiopia, and India. Farmers in these countries consider the expected precipitation levels and temperature when deciding which crops, and how much of each crop, to plant.

A key reference to: El Niño warning signs

When any of the following trends are recorded by weather buoys or satellites, researchers take note that an El Niño may be brewing:

• The temperature of the surface water increases at progressively eastward locations.
• The water at great depths of the western Pacific cools (in other words, the pool of warm water in the western Pacific grows shallower).
• The air pressure in the western Pacific rises, or the air pressure in the eastern Pacific falls.
• The sea level in the western Pacific falls, or the sea level in the central or eastern Pacific rises.
• The speed of the easterly (from east to west) winds in the eastern Pacific decreases.
• The current, which typically runs from east to northwest, shifts direction.
• The relative humidity (amount of moisture in the air) falls over the western Pacific or rises over the central or eastern Pacific.

An example of the difference an El Niño prediction can make is the improvement in northeastern Brazilian agricultural yields in 1991 over those of 1987. Farmers in the state of Ceara in northeastern Brazil, in a typical year, produce 716,000 tons of rice, beans, and corn. In 1987 the region suffered from an El Niño-related drought, and crop production fell drastically to 110,000 tons. In 1991, in contrast, farmers heeded warnings of an impending El Niño-related drought. Government officials provided seeds that were drought-resistant and had shorter growing seasons, to willing farmers. As a result, 584,000 tons of crops were harvested.

Farmers and fishermen in northern Peru made the most of predictions issued in advance of the 1997–98 El Niño. Anticipating heavy rains and the grass that would grow on normally dry land, farmers raised cattle. Farmers also planted rice—a crop that thrives in wet conditions (during dry years, in contrast, farmers may plant cotton—a crop that requires little rain). Fishermen planned for a harvest of shrimp, since those marine animals inhabit the warm waters that El Niño brings.

[See AlsoHuman Influences on Weather and Climate; Weather: An Introduction ]

For More Information

BOOKS

Arnold, Caroline. El Niño: Stormy Weather for People and Wildlife. New York: Clarion Books, 1998. Reprint, New York: NY, Clarion Books, 2005.

Fagan, Brian. Floods, Famines and Emperors: El Niño and the Fate of Civilizations. New York: Basic Books, 2000.

Glantz, Michael H. Currents of Change: El Niño's Impact on Climate and Society. 2nd ed. New York: Cambridge University Press, 2001.

Glynn, P. W., ed. Global Ecological Consequences of the 1982–83 El Niño-Southern Oscillation. Amsterdam: Elsevier Science Publishers, 1990.

WEB SITES

"El Niño." National Oceanic and Atmospheric Administration. 〈http://www.elnino.noaa.gov/〉 (accessed August 25, 2006).

"El Niño Resources." USA Today. 〈http://www.usatoday.com/weather/resources/basics/wnino0.htm〉 (accessed August 25, 2006).

"Tracking El Niño." Nova Online. 〈http://www.pbs.org/wgbh/nova/elnino/〉 (accessed August 25, 2006).

El Niño

El Niño (pronounced el-NEEN-yo) is the name given to a change in the flow of water currents in the Pacific Ocean near the equator. El Niño—Spanish for "the child" because it often occurs around Christmas—repeats every three to five years. Although El Niño takes place in a small portion of the Pacific, it can affect the weather in large parts of Asia, Africa, Indonesia, and North and South America. Scientists have only recently become aware of the far-reaching effects of this phenomenon.

What is El Niño?

The rotation of Earth and the exchange of heat between the atmosphere and the oceans create wind and ocean currents. At the equator, trade winds blow westward over the Pacific, pushing surface water away from South America toward Australia and Indonesia. These strong trade winds, laden with moisture, bring life-giving monsoons to eastern Asia. As warm surface water moves west, cold, nutrient-rich water from deep in the ocean rises to replace it. Along the coast of Peru, this pattern creates a rich fishing ground.

Every three to five years, however, the trade winds slacken, or even reverse direction, allowing winds from the west to push warm surface water eastward toward South America. This change is called the Southern Oscillation (oscillation means swinging or swaying), and it is brought about by a shifting pattern of air pressure between the eastern and western ends of the Pacific Ocean. The warm water, lacking nutrients, kills marine life and upsets the ocean food chain. The warm, moist air that slams into the South American coast brings heavy rains and storms. At the same time, countries at the western end of the Pacific—Australia, Indonesia, and the Philippines—have unusually dry weather that sometimes causes drought and wildfires.

Another type of unusual weather that often follows an El Niño is called La Niña, which is Spanish for "the girl." El Niño and La Niña are opposite phases in the Southern Oscillation, or the back and forth cycle in the Pacific Ocean. Whereas El Niño is a warming trend, raising the water temperature as much as 10°F (5.6°C) above normal, La Niña is a cooling of the waters in the tropical Pacific, dropping the temperature of the water as much as 15°F (8°C) below normal.

Global effects of El Niño

Meteorologists believe the altered pattern of winds and ocean temperatures during an El Niño changes 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 more severe storms are steered northward to Alaska. The jet streams altered by El Niño can also contribute to storm development over the Gulf of Mexico, which brings heavy rains to the southeastern United States. Similar rains may soak countries of South America, such as Peru and Ecuador, 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 on rainy seasons. The rains of the monsoons are critical for agriculture in India, Southeast Asia, and portions of Africa. When the monsoons fail, millions of people are at risk of starvation. It appears that wind patterns associated with El Niños carry away moist air that would produce monsoon rains.

La Niña can bring cold winters to the Pacific Northwest, northern Plains states, Great Lakes states, and Canada, and warmer-than-usual winters to the southeastern states. In addition, it can bring drier-than-usual conditions to California, the Southwest, the Gulf of Mexico, and Florida, as well as drought for the South America coast and flooding for the western Pacific region.

Not all El Niños and La Niñas have equally strong effects on global climate; every El Niño and La Niña event is different, both in strength and length.

Words to Know

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

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.

Worst El Niños of the century

According the National Oceanic and Atmospheric Administration (NOAA), 23 El Niños and 15 La Niñas took place in the twentieth century. Out of those, the four strongest occurred after 1980. Scientists are unsure if this is an indication that human activity is adversely affecting the weather or if it is simply a meaningless random clustering.

The El Niño event of 1982–83 was one of the most destructive of the twentieth century. It caused catastrophic weather patterns around the world. Devastating droughts hit Africa and Australia while torrential rains plagued Peru and Ecuador. In the United States, record snow fell in parts of the Rocky Mountains; drenching rains flooded Florida and the Gulf of Mexico's coast; and intense storms brought about floods and

mud slides in southern California. French Polynesia in the South Pacific was struck by its first typhoon in 75 years. It is estimated this particular El Niño killed 2,000 people and caused $13 billion worth of property damage.

Less than 15 years later, another destructive El Niño pattern developed. This one, however, was much more devastating than the 1982-83 event. In fact, it was the worst in recorded history. Beginning in late 1997, heavy rain and flooding overwhelmed the Pacific coast of South America, California, and areas along the Gulf Coast. Eastern Europe and East Africa were affected, as well. Australia, Central America, Mexico, northeastern Brazil, Southeast Asia, and the southern United States were all hit hard by drought and wildfires. In the United States, mudslides and flash floods covered communities from California to Mississippi. A series of hurricanes swept through the eastern and western Pacific. Southeast Asia suffered through its worst drought in fifty years. As a result, the jungle fires used to clear lands for farming raged out of control, producing smoke that created the worst pollution crisis in world history. At least 1,000 people died from breathing problems. By the time this El Niño period ended some eight months later in 1998, the unusual weather patterns it had created had killed approximately 2,100 people and caused at least $33 billion in property damage.

[See also Atmospheric pressure; Ocean; Weather; Wind ]

El Niño

El Niño is the most powerful weather event on the earth, disrupting weather patterns across half the earth's surface. Its three- to seven- year cycle brings lingering rain to some areas and severe drought to others. El Niño develops when currents in the Pacific Ocean shift, bringing warm water eastward from Australia toward Peru and Ecuador. Heat rising off warmer water shifts patterns of atmospheric pressure, interrupting the high-altitude wind currents of the jet stream and causing climate changes.

El Niño, "Christ child" or "the child" in Spanish, tends to appear in December. The phenomenon was first noted by Peruvian fishermen in the 1700s, who saw a warming of normally cold Peruvian coastal waters and a simultaneous disappearance of anchovy schools that provided their livelihood.

A recent El Niño began to develop in 1989, but significant warming of the Pacific did not begin until late in 1991, reaching its peak in early 1992 and lingering until 1995–the longest runing El Niño on record. Typically, El Niño results in unusual weather and short-term climate changes that cause losses in crops and commercial fishing . El Niño contributed to North America's mild 1992 winter, torrential flooding in southern California, and severe droughts in southeastern Africa. Wild animals in central and southern Africa died by the thousands, and 20 million people were plagued by famine . The dried prairie of Alberta, Canada, failed to produce wheat, and Latin America received record flooding. Droughts were felt in the Philippines, Sri Lanka, and Australia, and Turkey experienced heavy snowfall. The South Pacific saw unusual numbers of cyclones during the winter of 1992. El Niño's influence also seems to have suppressed some of the cooling effects of Mount Pinatubo's 1991 explosion.

Scientists mapping the sea floor of the South Pacific near Easter Island found one of the greatest concentration of active volcanoes on Earth. The discovery has intensified debate over whether undersea volcanic activity could change water temperatures enough to affect weather patterns in the Pacific. Some scientists speculate that periods of extreme volcanic activity underwater could trigger El Niño.

El Niño ends when the warm water is diverted toward to the North and South Poles, emptying the moving reservoir of stored energy. Before El Niño can develop again, the western Pacific must "refill" with warm water, which takes at least two years.

See also Atmosphere; Desertification

[Linda Rehkopf ]

RESOURCES

BOOKS

Diaz, H. R., ed. El Niño: Historical and Paleoclimatic Aspects of the Southern Oscillation. New York: Cambridge University Press, 1993.

Glynn, P. W., ed. Global Ecological Consequences of the El-Niño Southern Oscillation, 1982-1983. New York: Elsevier Science, 1990.

PERIODICALS

Mathews, N. "The Return of El Niño." UNESCO Courier (July-August 1992): 44-46.

Monastersky, R. "Once Bashful El Niño Now Refuses to Go." Science News 143 (23 January 1993): 53.

El Niño

El Niño, or El Niño-Southern Oscillation (ENSO), a warm current that temporarily raises the surface-water temperature off the Peruvian and Ecuadorian coasts around Christmas, hence the name ("the child"). Occasionally, especially warm waters disrupt the food chain significantly. Notably strong El Niño episodes have occurred in 1541, 1578, 1614, 1624, 1652, 1701, 1720, 1728, 1763, 1770, 1791, 1804, 1814, 1828, 1845, 1864, 1871, 1877–1878, 1884, 1891, 1899, 1911, 1918, 1925–1926, 1941, 1957–1958, 1972–1973, 1982–1983, 1991–1992, 1993, 1994, 1997–1998, 2002–2003, 2004–2005, and 2006–2007. Observers have noted "teleconnections," such as drought in central Chile and northeast Brazil and heavy rain in Peru and Ecuador. During strong El Niño years, eastern Africa gets more rain and Canadian winters are warmer. Moreover, global climate change further exacerbates the effects of El Niño. The research of Sir Gilbert Walker (1868–1958), Jakob Bjerknes (1897–1975), and the 1985–1995 Tropical Ocean and Global Atmosphere (TOGA) study have contributed to a comprehensive explanation for El Niño events and their global implications. In the twenty-first century, many countries employ numerical prediction models to better adapt to and prevent the climate variability effects El Niño produces. International bodies such as the Center on Research El Niño (CIIFEN) headquartered in Guayaquil, Ecuador, likewise generate scientific research to assist on a regional scale.

See alsoFishing Industry .

BIBLIOGRAPHY

William H. Quinn et al., "Historical Trends and Statistics of the Southern Oscillation, El Niño, and Indonesian Droughts," in Fishery Bulletin 76, no. 3 (July 1978): 663-678.

Kevin Hamilton and Rolando R. García, "El Niño/Southern Oscillation Events and Their Associated Midlatitude Teleconnections, 1531–1841," in Bulletin of the American Meteorological Society 67 no. 11 (November 1986): 1354-1361.

M. H. Glantz et al., eds., Teleconnections Linking Worldwide Climate Anomalies: Scientific Basis and Societal Impact (1991).

Additional Bibliography

Caviedes, César. El Niño in History: Storming through the Ages. Gainesville: University Press of Florida, 2001.

Gasparri, Enrico; Carlo Tassara; and Margarita Velasco. El fenómeno de El Niño en el Ecuador, 1997–1999: Del desastre a la prevención. Quito, Ecuador: Ediciones Abya-Yala, 1999.

Lawas, Edward A. El Niño and the Peruvian Anchovy Fishery. Sausalito, CA: University Science Books, 1997.

Sandweiss, Daniel H.; Jeffrey Quilter; and Joanne Pillsbury. El Niño, Catastrophism, and Culture Change in Ancient America: A Symposium at Dumbarton Oaks, 12th-13th October 2002. Washington, DC: Dumbarton Oaks Research Library and Collection, 2008.

Sesé, José María, and Ruth Magali Rosas. El fenómeno "El Niño" en la costa norte del Perú a través de la histoira; Perú-Ecuador: Un espacio compartido. Piura, Peru: Universidad de Piura, 2001.

                                        Robert H. Claxton

El Niño (Sp. (Christ) ‘child’) Warm surface current that flows in the equatorial Pacific Ocean towards the South American coast. It occurs around Christmas time. An easing or reversing of the trade winds over the s Pacific Ocean, causes warm surface waters that have ‘piled-up’ in the w Pacific to flow back and warm the coastal waters of South America by 2 to 3°C. It has a dramatic effect on climate patterns in Australia and Southeast Asia, and may affect rainfall as far away as Africa. In normal years, trade winds blow e to w along the Equator, dragging warm surface waters into a pool off n Australia and monsoon rains to Indonesia. In the w Pacific, the Humboldt Current pushes the surface waters away from the coast of Peru, bringing cold water to the surface. This upwelled, nutrient-rich water stimulates phytoplankton production and swells the population of anchovies, a mainstay of the Peruvian fishing industry. In an El Niño year, the upwelling ceases and the biological productivity of the area collapses. In 1982–83, the anchovy catch fell by 600%. In addition, mean sea-level along the coast of Latin America may increase by as much as 50cm (20in), causing widespread flooding. Some scientists believe that the frequency (presently every 2–10 years) and effects of El Niño may be increasing.

http://www.elnino.noaa.gov