An Ill Wind

After just a few years' absence, "El Nino" is back in Northwest headlines. Its warm waters are blamed for the failure of this summer's Bristol Bay sockeye salmon run and credited with ferrying swordfish and marlin north from Baja for the convenience of sport-fishermen off Ilwaco. A bad year is predicted for ski areas from Whistler to Sun Valley and a worse one for homeowners in landslide-prone Southern California. All this is somehow connected to Australian droughts and the size of the Peruvian anchovy crop, but just how never quite becomes clear.It's not surprising that we're a little vague about the phenomenon meteorologists call "the Southern Oscillation" and the rest of us know as "El Nino." Though this periodic wobble in world weather patterns got rolling about the same time our African ancestors started experimenting with walking upright, 4 million years passed before anyone noticed. And though it's second only to the grand annual cycle of the seasons in its impact on human society, it wasn't until late in the 19th century that our species' incorrigible habit of collecting apparently useless bits of information yielded enough data to spot El Nino's faint, irregular "signal" in the welter of daily rainfall, temperature, and air-pressure records. The phenomenon got its popular name in 1895 at an international scientific conference, when a Peruvian savant described a warm current that mysteriously appeared off his country's shores every few years around Christmas (the time of El Nino, the Christ child). It brought gifts: sea snakes, dead alligators, and torrential rain on the normally dessicated Peruvian coast.But it was a scientist halfway around the world who found the first evidence that Pacific weather patterns were undergoing more than local variations. Sir Gilbert Walker, an Englishman stationed in India, spent more than 30 years trying to understand and predict the periodic drying-up of the summer monsoon rains on which farmers depend to stave off starvation. Walker sought correlations with phenomena as diverse as sunspots, temperatures in Vancouver, and floods on the Nile. He failed to predict droughts in India, but did discover an unmistakable pattern. When average barometric pressure was abnormally high in Darwin, on the northwest coast of Australia, it was abnormally low in the mid-Pacific near Tahiti, 5,000 miles away. And vice versa: Roughly twice a decade for decades on end, when Darwin got floods, Tahiti dried out. Correlations are fine, but not much use to weather forecasters: When an unexpected downpour sends your house down the bluff, you probably won't be consoled to know it should be clear and dry in Papeete. For effective forecasting, you need a lot more numbers than Walker had at his disposal, and a dynamic "model" to wire those numbers together.It took another 30 years to accumulate enough rainfall and ocean- temperature data to show how Walker's Southern Oscillation determined basic weather patterns all the way from the Indian Ocean to Central America and the Caribbean. It took 20 more (and many millions of dollars investment in supercomputers) to find a reliable way to predict El Nino's lurches.The intermittent weather pattern scientists now call ENSO (for "El Nino/ Southern Oscillation") exists only because, from western Guinea to northern Ecuador, from the Aleutian Islands to the Antarctic Shelf, a nearly unbroken expanse of ocean covers almost a third of the earth's surface. The annual drift of the sun, bringing opposite seasons to the Northern and Southern hemispheres, has little effect in the central Pacific. Here the forecast stays the same, month after month, millennium after millennium: "Sunny and warm with intermittent thundershowers." With the equatorial sun sitting nearly straight above it, the sea's surface absorbs a good 90 percent of the light that hits it. This produces the biggest reservoir of solar heat in the world: millions of square miles of ocean ranging between 70 and 85 degrees Fahrenheit (warmer at the western end) all year long. But the reservoir is shallow, because sunlight penetrates only a few hundred feet into the sea. Below that level (the "thermocline"), cool waters circulate according to their own patterns, mixing little with the warm water above. Warm water evaporates, warming the air above. This air rises, carrying moisture, and cooler air flows in to replace it. This happens more in the western Pacific, which has the warmer waters. So cooler air flows in from the east, producing the globe-girdling equatorial "trade winds." These push the waters of the "Pacific Warm Pool" westward (average sea level is a good foot and a half higher off Australia than South America). This allows deep, cold water to well up off Ecuador and Peru. This cold water is in turn blown westward in a cooling "tongue" measurable in satellite probes as far west as the longitude of Hawaii.But twice a year, as mariners of old knew, the trade winds weaken, as the overhead sun moves from the Northern to Southern hemisphere and back again. When the winds weaken, so does the cold upwelling, and the Pacific Warm Pool sloshes sluggishly back toward South America. Other things being equal, the trades pick up again after a month or so, but things are never equal for long. After a few years soaking up the sun's heat, the Pacific Warm Pool is big and hot enough come autumn to overcome the westward pressure of the trade winds. Cold upwelling all but ceases along the South American coast, and El Nino is back for another indeterminate stay. What seemed to be mere tough luck for Peruvian fishermen and seabirds is now understood to be a wake-up call for everyone. When the heart of the Pacific Warm Pool lies far to the west, it keeps the monsoons on schedule in the Indian Ocean and waters the rain forests of Southeast Asia, Indonesia, and New Guinea. When the warm pool's center shifts eastward, so do the rain clouds, drenching the empty mid-Pacific and points east.For North America, the impact of the shift is indirect but just as powerful. Huge, symmetrical high-pressure systems on both sides of the equatorial mid-Pacific literally suck air away from higher latitudes, sending the jet stream (the boundary where polar and tropical air meet) far south of its "normal" west-to-east path. Winter rains get heavier in a swath from Corvallis, Oregon, through California, across the southern tier of the United States, and up the East Coast to Bangor, Maine. Meanwhile the Pacific Northwest dries out: snowpacks shrink, reservoirs fall, forests lose moisture and become more vulnerable to fire. The thing that makes predicting the impact of an El Nino episode so tricky is that the wind and ocean pattern that produces it is only semi-stable: Weak trade winds allow warm water to flood eastward across the Pacific, which in turn helps keep the trade winds weak. Sooner or later, chance, in the form of quadrillions of random meteorological micro-events, lets the trades pick up enough to push the warm flood back westward. When they do, the great Pacific storm systems obediently trundle back above it, India gets its monsoons back, and the system locks down again in Position ANuntil random variation flips the switch once more. Scientists realize now that neither position is "normal"; what's normal is the endless alternation between the two extremes: El Nino and its less spectacular counterpart, that same Position A, which some scientists call "La Nina." All we humans can do is try to swing along with it. Thanks to greatly improved predictions, Peruvian farmers are already doing that, planting rice instead of cotton when a rainy spring is forecast. Maybe it's time Northwest ski operators and Los Angeles homeowners caught up with those farmers. The Pacific Warm Pool isn't going to change its ways. Or is it? Since 1991, the Pacific pendulum has swung more strongly toward El Nino than La Nina. Some suspect that global warming, if it really exists, may be shifting the balance. But no one knows if this is happening, or even if it could. It took us 4 million years to notice the phenomenon at all; it will take a few more to get used to its ways. For additional information, two first-rate Web sites offer more on El Nino's past and present than you'll ever need to know. The National Oceanographic and Atmospheric Administration (NOAA) maintains a central page with lots of well-organized basic data and links to dozens more pages: Columbia University's Lamont-Dougherty Earth Observatory offers graphic goodies galore, showing month-by-month variations from the early '80s to date: Downey is a senior editor at Seattle Weekly.

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