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Nature Gallery (Earth [Climate / Oceans])
El Niño Effect

The Humboldt Current (Peru Current)

Pacific Ocean Temperatures

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These images, taken directly from the U.S. National Oceanic and Atmospheric Administration (NOAA), cover the Pacific Ocean along the Equator (shown as a horizontal line). Australia and New Guinea appear at left, Central and South America at right. The vertical line indicates the International Date Line.

TOP: Warm water (red) is associated with El Niño, cool water (blue) with La Niña.

BOTTOM: Red shows water that is warmer than the seasonal average. Blue indicates cooler water.

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The cool Humboldt Current, named after the German explorer Alexander von Humboldt (1769–1859), flows north along the west coast of South America. It is part of the anti-clockwise flow of the South Pacific gyre, a circular current on the ocean’s surface. As the wind and current push sea water away from the coast, upwelling occurs, which allows cool, deep, nutrient-rich waters to rise to the sunlit zones. This, in turn, creates a food-rich marine environment that includes penguins and sea lions, and productive fishing grounds.

The cool upwelling also produces fog, but little rainfall for the desert coasts of Peru and northern Chile. Occasionally, the pattern is broken, the upwelling stops and is overridden by warm tropical waters from the central Pacific. This reversal of marine temperature conditions is known as El Niño (Spanish, “the child”). It is so named because it usually occurs around Christmas. This causes great loss of marine life and changes conditions from dry and desert-like to stormy and humid. On rare occasions, such as in 1982 and 1983, a prolonged El Niño may extend beyond the Humboldt Current region to as far north as California and as far south as the Antarctic.

The El Niño Effect
El Niño is a severe atmospheric and oceanic disturbance in the Pacific Ocean that occurs every seven to fourteen years. It is called El Niño, meaning “the Child”, because it usually appears near the Christmas season. Warm surface waters flow from the central Pacific towards the eastern Pacific, suppressing the cold, nutrient-rich upwelling of the Humboldt Current off the coast of South America. This disturbance leads to a fall in the number of plankton, wreaking havoc upon the entire ocean food chain and devastating the fishing industry.

The influence of these currents also leads to a complete reversal of the trade winds, bringing torrential rain, flooding, and mudslides to the usually dry coastal areas of Peru and Ecuador. Another result is the collapse of the monsoons in Asia, which can bring severe drought to Indonesia and northern Australia. The phenomenon also causes severe weather disturbances in other parts of the world, such as droughts in areas of Africa and central North America.

Nature's Vicious Cycle (by National Geographic)

It rose out of the tropical Pacific in late 1997, bearing more energy than a million Hiroshima bombs. By the time it had run its course eight months later, the giant El Niño of 1997-98 had deranged weather patterns around the world, killed an estimated 2,100 people, and caused at least 33 billion [U.S.] dollars in property damage.

Isaias Ipanaqué Silva knew none of that. All he and the other peasant farmers in the Peruvian hamlet of Chato Chico could see was that after weeks of incessant rain the adjacent Piura River had not stopped rising. The rainfall itself was no surprise. Every three to seven years, for as long as anyone could remember, the same rainfall had arrived after a pool of hot seawater the size of Canada appeared off the west coast of the Americas. The ocean would heat up right around Christmastime, so fishermen called the phenomenon El Niño, for the Christ Child. Then that titanic storm source would pour vast amounts of precipitation onto Peru’s normally arid northwestern coast.

But few had ever seen this much rain—five or six inches a day in some places.

Finally, on February 15, 1998, the river broke its banks. The sodden ground could hold no more, and water swept into the riverside homes of Chato Chico. The swirling torrent was first knee-deep and soon chest high. “Suddenly we were surrounded from all directions,” Ipanaqué Silva says. “It took all the little animals. Then my house just fell down completely.

Hundreds of families splashed frantically through the muddy flood to save what they could. In most cases, says another villager, Rosa Jovera Charo, “we just grabbed clothes for the children.” Everything else—chickens and goats, pots and pans, religious icons and personal treasures—washed away. Compared with other places in Peru and around the world, the residents of Chato Chico were fairly lucky. Some were evacuated on barges, a few in helicopters, to a barren but dry refugee camp in the desert. Nearly all survived.

That was not the case some 60 miles [100 kilometers] to the south, in a 3-acre [1.2-hectare] pocket of one-room houses called Motse outside the city of Chiclayo. “We thought that the water couldn’t come here,” says Flora Ramirez, “but we lost practically everything.” Ramirez’s neighborhood was overrun in a matter of minutes. “They strung ropes from one house to another to rescue people,” recalls Manuel Guevara Sanchez. “Some spent three days on the roof. Those who knew how to swim brought them food.” When the flood finally receded, they could begin to count the dead: ten out of a village of just 150.

The runoff from the floods poured into the coastal Sechura Desert. Where there had been nothing but arid hardscrabble waste for 15 years, suddenly—amazingly—lay the second largest lake in Peru: 90 miles [145 kilometers] long, 20 miles [30 kilometers] wide, and ten feet [three meters] deep, with occasional parched domes of sand and clay poking up eerily from the surface.

In other areas the water simply pooled. The mosquitoes that thrived in these places caused rampant malaria—some 30,000 cases in the Piura region alone, three times the average for its 1.5 million residents.

Peru was where it all began, but El Niño’s abnormal effects on the main components of climate—sunshine, temperature, atmospheric pressure, wind, humidity, precipitation, cloud formation, and ocean currents—changed weather patterns across the equatorial Pacific and in turn around the globe. Indonesia and surrounding regions suffered months of drought. Forest fires burned furiously in Sumatra, Borneo, and Malaysia, forcing drivers to use their headlights at noon. The haze traveled thousands of miles to the west into the ordinarily sparkling air of the Maldive Islands, limiting visibility to half a mile [0.8 kilometer] at times.

Temperatures reached 108°F [42°C] in Mongolia; Kenya’s rainfall was 40 inches [100 centimeters] above normal; central Europe suffered record flooding that killed 55 in Poland and 60 in the Czech Republic; and Madagascar was battered with monsoons and cyclones. In the U.S. mudslides and flash floods flattened communities from California to Mississippi, storms pounded the Gulf Coast, and tornadoes ripped Florida.

By the time the debris settled and the collective misery was tallied, the devastation had in some respects exceeded even that of the El Niño of 1982-83, which killed 2,000 worldwide and caused about 13 billion dollars in damage.

And that’s not the end of it. It is not uncommon for an El Niño winter to be followed by a La Niña one—where climate patterns and worldwide effects are, for the most part, the opposite of those produced by El Niño. Where there was flooding there is drought, where winter weather was abnormally mild, it turns abnormally harsh. La Niñas have followed El Niños three times in the past 15 years—after the 1982-83 event and after those of 1986-87 and 1995. Signs of another La Niña began to show up by June 1998.

Learn how wind and water drive El Niño and La Niña (by National Geographic).

Peru was where it all began, but El Niño’s abnormal effects on the main components of climate—sunshine, temperature, atmospheric pressure, wind, humidity, precipitation, cloud formation, and ocean currents—changed weather patterns across the equatorial Pacific and in turn around the globe. Indonesia and surrounding regions suffered months of drought. Forest fires burned furiously in Sumatra, Borneo, and Malaysia, forcing drivers to use their headlights at noon. The haze traveled thousands of miles to the west into the ordinarily sparkling air of the Maldive Islands, limiting visibility to half a mile [0.8 kilometer] at times.

Temperatures reached 108°F [42°C] in Mongolia; Kenya’s rainfall was 40 inches [100 centimeters] above normal; central Europe suffered record flooding that killed 55 in Poland and 60 in the Czech Republic; and Madagascar was battered with monsoons and cyclones. In the U.S. mudslides and flash floods flattened communities from California to Mississippi, storms pounded the Gulf Coast, and tornadoes ripped Florida.

By the time the debris settled and the collective misery was tallied, the devastation had in some respects exceeded even that of the El Niño of 1982-83, which killed 2,000 worldwide and caused about 13 billion dollars in damage.

And that’s not the end of it. It is not uncommon for an El Niño winter to be followed by a La Niña one—where climate patterns and worldwide effects are, for the most part, the opposite of those produced by El Niño. Where there was flooding there is drought, where winter weather was abnormally mild, it turns abnormally harsh. La Niñas have followed El Niños three times in the past 15 years—after the 1982-83 event and after those of 1986-87 and 1995. Signs of another La Niña began to show up by June 1998.

Over the years, the appearance of La Niña has been less predictable than that of El Niño, and fewer of its effects have been recorded. But both patterns are now far better understood than ever before. That is because the most recent El Niño will be the first to be remembered for more than just a litany of disasters. The 1997-98 El Niño marked the first time in human history that climate scientists were able to predict abnormal flooding and droughts months in advance, allowing time for threatened populations to prepare. The U.S. National Oceanic and Atmospheric Administration (NOAA) first announced a possible El Niño as early as April 1997; Australia and Japan followed a month later. By summer detailed predictions were available for many regions.

See the progression of El Niño and La Niña through ocean temperatures (by National Geographic).

In northern Peru warnings allowed many farmers and fishermen to make the best of El Niño’s effects. Grass grew on land that is usually barren, and farmers raised cattle. Rice and beans could be planted in areas normally too dry to support them; fishermen were able to plan for shrimp harvests in coastal waters, generally too cold for the shellfish.

“The potential uses of advance information are almost limitless,” says Michael H. Glantz of the National Center for Atmospheric Research (NCAR) in Boulder, Colorado, pointing out how governments and industries around the world can make planning for El Niño and La Niña pay off. For example, Kenyan coffee growers find their product in greater demand when droughts affect coffee harvests in Brazil and Indonesia. Palm oil production in the Philippines typically declines during El Niño, as does the squid catch off the California coast. Countries that anticipate these developments can fill the gaps and prosper.

At the very least, preparation can save lives. Even in poverty-ridden Peru constructing storm drains and stockpiling emergency supplies saved hundreds of lives during 1997 and ’98. Forewarnings brought timely international aid to such places as Papua New Guinea, where highland populations were threatened with starvation after frost and drought combined to destroy subsistence crops. Many affected areas could prepare for floods and fires, population migration, and the spread of disease.

There are written records of El Niño’s effects in Peru at least as far back as 1525, and researchers have found geologic evidence of El Niños in Peruvian coastal communities from at least 13,000 years ago. “We know the Inca knew about them,” says Adm. Giampietri Rojas of Peru’s Institute of the Sea. “They built their cities on the tops of hills, and the population kept stores of food in the mountains. If they built on the coast, it was not near rivers. That’s why so many of their dwellings are standing today.” But it was not until about 25 years ago that the rest of the world began to pay attention to El Niño. And after the surprise devastation of 1982-83, climate experts intensified efforts to understand how the process works globally. Governments invested in equipment to monitor the particular conditions in the Pacific that trigger El Niño. Perhaps the most important effort was the development of the TAO (tropical atmosphere/ocean) array of 70 moored buoys to span the equatorial Pacific. Completed in 1994, the TAO buoys are now the world’s premier early-warning system for change in the tropical ocean. They monitor water temperature from the surface down to 1,600 feet [500 meters], as well as winds, air temperature, and relative humidity.

The data collected by the buoys are transmitted to polar-orbiting satellites and then to NOAA’s Pacific Marine Environmental Laboratory in Seattle. Supplemented with temperature measurements taken by research ships, the data help create a comprehensive portrait of the upper ocean and lower atmosphere.

Meanwhile the TOPEX/Poseidon satellite, a U.S.-French mission begun in 1992, orbits Earth at a height of 830 miles [1,300 kilometers], measuring sea-surface elevation and relaying information about ocean circulation, including the enormous rhythmic sloshings called Kelvin and Rossby waves that travel back and forth across the entire Pacific.

Thanks to the TAO buoys, the TOPEX/ Poseidon satellite, and a variety of other tools, climate scientists now have information of unprecedented range and accuracy, which has enabled them to confirm and expand their theories about what occurs both during normal weather patterns and during sea changes that herald the periodic—and inevitable—arrivals of El Niño and La Niña.

Ground and satellite data show how abnormal El Niño winds send warm air toward California (See the image - by National Geographic).

Weather is so variable that it’s hard to call any situation normal. But in most years climate in the equatorial Pacific is governed by one generally dependable pattern. Sunlight heats the uppermost layer of seawater in the western ocean around Australia and Indonesia, causing huge volumes of hot, moist air to rise thousands of feet and creating a low-pressure system at the ocean’s surface. As the air mass rises and cools, it sheds its water content as rain, contributing to monsoons in the area.

Now much drier and far aloft, the air heads east, guided by winds in the upper atmosphere, cooling even more and increasing in density as it travels. By the time it reaches the west coast of the Americas, it is cold and heavy enough that it starts to sink, creating a high-pressure system near the water’s surface. The air currents then flow as trade winds back toward Australia and Indonesia. This giant circulatory loop, moving from west to east in the upper air and from east to west at low altitudes, is called the Walker circulation, for Sir Gilbert Walker, the British scientist who studied the process in the 1920s.

As the trade winds blow westward over the Pacific, they push the warm top layer of the ocean with them, causing the hottest water to pile up around Indonesia, where, because of both wind action and thermal expansion, the sea level is usually about 18 inches [46 centimeters] higher than it is off the west coast of Mexico. All along the eastern Pacific, and especially off Ecuador and Peru, colder subsurface water wells up to replace the sheared-off top layer, bringing up a bevy of nutrients from the deep ocean. That chemical bounty sustains an enormous food web and makes the coastal waters off Peru one of the world’s most prolific fisheries.

El Niño changes all that. For reasons that scientists still do not comprehend, every few years the trade winds subside or even disappear. The usual air-pressure pattern reverses itself in a phenomenon called the southern oscillation, making barometer readings higher in Australia than they are in the central Pacific. The resulting pattern—known as ENSO, for El Niño/Southern Oscillation—involves only one-fifth of the circumference of the planet. But it transforms weather around the globe.

Without the trade winds the top layer of the eastern Pacific does not move west. It stays in place, getting hotter and hotter, swelling as it warms. Eventually it hits the threshold for what meteorologists call deep convection—the point at which the steamy surface air blasts into the upper atmosphere. (In some places during 1997-98, sea levels off South America were 10 inches [25 centimeters] above normal and surface temperatures reached almost 86°F [30°C].) When that happens, water in the upper atmosphere condenses and falls as torrential rain on the west coast of the Americas.

This, in turn, reduces the salinity of the coastal seas, where deepwater upwelling has already declined or stopped. Marine life that customarily thrives off Ecuador and Peru, including economically essential anchovy populations, heads south in search of cooler, richer waters—to the great benefit of fishermen in Chile. Off North America exotic warmwater species suddenly appear farther north. In 1997, apparently for the first time, a fisherman caught a marlin in the ordinarily chilly seas off Washington State. Californians started pulling in bonito and albacore tuna, species normally found only far offshore. Other tuna were netted in the Gulf of Alaska.

Because El Niño moves the rains that would normally soak the western Pacific toward the Americas, such places as Australia, Indonesia, and India may experience severe drought. According to historical records, 600,000 people died in just one region of India from the epic droughts of the 1789-1793 El Niño.

In Africa the altered wind, heat, and moisture patterns of El Niño portend drought—generally in the east and extreme south. In particular, cooling of the southwestern Indian Ocean customarily strengthens a high-pressure area that keeps rainfall from reaching the south.

Meanwhile, back in North America, the jet streams that travel 5 to 8 miles [8 to 13 kilometers] above Earth’s surface shift dramatically. The polar jet stream tends to stay farther north over Canada than usual; as a result, less cold air moves into the upper United States. In fact, northern-tier states saved an estimated five billion dollars in heating costs during the 1997-98 El Niño.

At the same time upper-level tropical winds reverse themselves, blowing the tops off cyclones forming in the mid-Atlantic and usually reducing the number of hurricanes that strike land in the U.S. by half—from an average of two a year to one or none, according to studies at Colorado State University and Florida State University. One study indicates that El Niño also generally reduces tornadoes in the southern Plains states.

An unexpected crop of sardines off the coast of Chile? Tuna in the Gulf of Alaska? Lower heating bills in the U.S.? Fewer hurricanes in the Atlantic?

Enter La Niña. During a La Niña event, an abnormal cooling in the eastern Pacific produces conditions more or less the opposite of those created by El Niño—nature’s way, perhaps, of rectifying the heat imbalance that El Niño represents. As with El Niño, the effects of La Niña are most pronounced from December to March.

In La Niña years the easterly winds from the Americas are stronger than usual. That drives more than the normal amount of warm sea-surface water westward, in turn causing larger than normal volumes of deep, chilly water to rise to the surface and producing a “cold tongue” that extends 3,000 miles [4,800 kilometers] along the Equator from Ecuador to Samoa.

With so much warm water flowing toward Asia, the Pacific’s mighty heat engine remains firmly anchored in the west, causing heavier monsoon rains in India, higher than average precipitation in Australia, and wetter than normal conditions as far west as southern Africa. The huge air masses and cloud banks associated with the hot zone also change the path of the jet streams, which move high-altitude air from west to east across the ocean.

Hurricane Linda, spawned during an El Niño, was one of the strongest eastern Pacific storms on record (See entire image).

The polar jet stream, which in an El Niño year stays high in Canada, moves farther south, driving frigid air down into the U.S. Winters are colder, especially in the northwestern and upper midwestern states. The subtropical jet stream that blows across Mexico and the Gulf during El Niño events weakens during La Niña; consequently, far less rain falls in the Gulf and southeastern states. Drought is common in the desert Southwest. Hurricanes in the tropical Atlantic encounter no westerly wind resistance and therefore are twice as likely to strike the U.S. The 1998 La Niña hurricane season was the deadliest in the past two centuries.

As experts use increasingly reliable data to comprehend the forces and patterns of these periodic weather cycles, they are making better predictions of at least the broad contours of the cycle. There are two major ways of forecasting large-scale weather events such as El Niño, and climate scientists use both.

One method is statistical. Analysts pore over past weather records to determine what kind of conditions have the highest probability of occurring simultaneously. For example, lower barometric pressure and higher sea-surface temperatures in, say, Tahiti usually mean more rain for Ecuador and less for northern Brazil. This technique yields results even if the analyst has no idea how the two coexisting conditions are related, and traditionally forecasters have preferred its reassuring mathematical solidity.

But statistical procedures provide very little information about what cause-and-effect relationships may be producing various climate conditions. Moreover, statistical analysis can only determine the likelihood that past conditions will recur—and no two El Niños or La Niñas are the same.

With the advent of supercomputers, scientists have taken advantage of an alternative method of prediction called climate modeling. In this method, software incorporates the fundamental laws of oceanic and atmospheric physics into a simulated world where weather changes over time. Researchers then feed tens of thousands of specific pieces of information about the real world into the model and see how accurately the computer-generated results resemble what actually happens.

In theory, models can reveal the unique or idiosyncratic conditions that will result from a given climate pattern and then fast-forward to see how events related to that pattern will unfold. In practice, most of the results have proved too broad or uncertain to predict weather on even a large regional scale, much less in a local range of 100 to 200 miles [150 to 300 kilometers].

So, historically, statistical predictions have been somewhat more accurate than computer-generated models—until now. The 1997-98 El Niño “was one in which the full climate models were more successful than statistical predictions for the first time,” according to NCAR climate analyst Kevin Trenberth. “The tropical Pacific,” says Trenberth, “appears to be predictable for a year or so in advance.”

In fact, “to a certain extent we underplayed what the models were telling us,” says Ants Leetmaa, director of NOAA’s Climate Prediction Center. He believes that if scientists had relied on the models more and the statistical evidence less, their 1997-98 predictions would have been even more accurate.

As encouraging as the model results have been, there is still room for improvement. For example, most of the best models created in advance of the 1997-98 El Niño predicted much smaller monsoons in India than actually occurred and far less rain than actually fell in southeastern Africa and Australia. Kenya and Somalia had heavy and prolonged rains that provoked an epidemic of waterborne Rift Valley fever and dengue fever, among other maladies. “The big question is why,” says Leetmaa. “That’s the challenge for the future.”

It would be far easier to tune the climate models if scientists were able to look through centuries of records. But “we just don’t have hundreds of years of data,” Leetmaa explains. And even if they were available, “data sets aren’t going to give us the full answer. But analyzing the data in combination with computer-simulated experiments is where we’re going to make progress.”

Greater distribution of monitoring equipment would also increase the accuracy of climate-pattern prediction. No observation networks have been established yet for the equatorial Atlantic and Indian Oceans. Since part of the variability among El Niños and their regional impacts can be attributed to activity in these ocean basins, the need for improved data reporting in these areas seems clear. Especially, experts note, if El Niños are becoming more ferocious.

There is a consensus among climate scientists that El Niños have become more frequent and progressively warmer over the past century. Beyond that there is little agreement, particularly about whether human activity might be exacerbating their effects.

In the past 98 years there have been 23 El Niños and 15 La Niñas. Of the century’s ten most powerful El Niños, four—the four strongest—have occurred since 1980. But no one knows whether this indicates a trend or is simply a meaningless random clustering.

And no one can know at this point. Even a hundred years of precise rainfall and temperature observations in the Pacific might not be sufficient to confirm a major tendency one way or the other. Moreover, many experts now suspect that El Niños—and indeed many oceanic weather patterns—may alternate in form and severity on a timescale of decades or even centuries. “By and large,” says NOAA’s Leetmaa, “the El Niño patterns look a lot like the overall changes in U.S. rainfall and temperature patterns from decade to decade.” But no matter what’s happening, “the bottom line is the past 20 years are different from the previous 30.”

It is difficult to imagine how the global warming observed over the past hundred years, which amounts to about one-tenth of a degree Fahrenheit [one-twentieth of a degree Celsius] a decade, could have much effect on the stupefying volume of water in the equatorial Pacific. But it is plausible, some scientists believe.

”El Niño moves heat,” says Tom Karl, one of NOAA’s veteran climate experts, “both in terms of water temperature and in atmospheric convection. This heat is transported out of the oceans and the tropics during the peak of El Niño as global temperatures increase. As the heat is released, the whole El Niño cycle begins again, with less cloudiness in the tropics and with the oceans absorbing more heat. With global warming there is more heat available. So the cycle may be shortened because the recharge time is shorter or because the release of heat is less efficient.”

Whatever the future may bring, the world need never again be taken completely off guard by El Niño or La Niña. Due to the unprecedented foresight that climate science has made possible, the ocean’s thermal moods may not seem so unpredictable and diabolical, but rather an ordinary part of life on the planet. “We have to realize that it’s something natural that’s going to happen again and again,” says Capt. Hector Soldi Soldi, a hydrographics expert with the Peruvian Navy. “And we have to be ready for that.”

Even Isaias Ipanaqué Silva—now living in one of the spare refugee camps in northern Peru, where homes are no more than four woven-straw walls with a plastic tarp for a roof—knows it. He and his neighbors walk three miles each way, every day, to farm the riverside fields that lay right next to their homes before El Niño swept their hamlet away. “We can’t go back,” he says, sad but resigned. “It will happen again. If God wants to save us next time too, we say thanks. But right now, this is where we will stay.”