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    NATIONAL GEOGRAPHIC MAGAZINE

    Parfit, Michael. “Living with Natural Hazards.” (Order issue.)

    Suplee, Curt. “Unlocking the Climate Puzzle.” (Order issue.)

    “El Niño’s Rampage.” Millennium Moments, Order issue.)

    “Do El Niño’s Ripples Extend to Antarctica?” Earth Almanac, (Order issue.)

    “Is the Sea Warming in the Western Pacific?” Geographica, (Order issue.)

    “El Niño Brightens a Desert Landscape.” Geographica, (Order issue.)

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    Additional Resources

    Bigg, Grant R. The Oceans and Climate. Cambridge University Press, 1996.

    Burroughs, William James. Weather Cycles: Real or Imaginary? Cambridge University Press.

    Diaz, Henry F. and Vera Markgraf. El Niño: Historical and Paleoclimatic Aspects of the Southern Oscillation. Cambridge University Press, 1992.

    Gates, David Murray. Climate Change and its Biological Consequences. Sinauer Associates, 1993.

    Geer, Ira W., ed. Glossary of Weather and Climate, With Related Oceanic and Hydrolic Terms. American Meteorological Society, 1996.

    Graedel, T.E. Atmosphere, Climate, and Change. Scientific American Library, 1995.

    Schneider, Stephen H., ed. Encyclopedia of Climate and Weather. Oxford University Press, 1996.

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  • El Niño/La Niña
    Nature’s Vicious Cycle
    1 | 2 | Part 3

    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
    Hurricane Linda, spawned during an El Niño, was one of the strongest eastern Pacific storms on record.
    Click to 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.”

    Return to top | Credits 1 | 2 | Part 3


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