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Indeed, this El Nino, like the others that preceded it, has generated as many questions as answers. Why, scientists wonder, does it sometimes torpedo the Indian monsoon and sometimes leave it alone? Is it typical, or very unusual, that as many as four El Ninos have struck over the past seven years? How remarkable is it that two record-breaking El Ninos have occurred within 15 years of each other?
To try and solve these and other puzzles, many scientists have moved beyond their computer models and headed into the field to collect real data. Last week Martin Ralph, a climatologist with NOAA's Environmental Technology Laboratory in Boulder, Colo., spent 25 hours in a P-3 "hurricane hunter" aircraft, flying into the teeth of a Pacific storm to measure temperature, wind and humidity. His goal: to figure out precisely how such storms build, move and interact with the coastline. Along with data from more than a dozen other NOAA experiments, Ralph's information will be fed back into the computer models as a reality check. "We're just learning," he says. "But we've been in the right place at the right time."
By trying to unravel the detailed behavior of El Nino, Ralph and dozens of other researchers are furthering a scientific quest that began in the 1920s, when the British meteorologist Sir Gilbert Walker linked swings in atmospheric pressure over the Pacific to a disastrous failure of the Indian monsoon 50 years earlier. In the 1960s, UCLA meteorologist Jacob Bjerknes suggested that El Nino was governed by the same swings in atmospheric pressure.
The way El Nino works, scientists are now convinced, is that high pressure in the eastern Pacific sends trade winds blowing to the West. Because these winds push water before them like an invisible plow, the sea's surface actually measures about a foot and a half higher around Indonesia and Australia than it does off the coast of Peru. When the pressure drops and trade winds slacken, the water sloshes back downhill, to the east.
This eastward flow is central to the physics that drive El Nino, says Scripps' Nicholas Graham. The sloshing sends waves across the ocean like ripples in a pond. These waves, in turn, push down on the so-called thermocline, a layer of cooler water that normally mingles with the warmer water at the surface. As the thermocline sinks to greater depths, the mixing stops, temperatures at the sea's surface rise, and an El Nino begins.
These ripples can be thousands of miles long, but since they travel 100 ft. or more beneath the surface they're hard to detect directly. So scientists use satellites to pick up the subtle undulations in sea level produced as the ripples pass by. That's how NASA oceanographer Anthony Busalacchi could see early last spring that swarms of undersea waves had started to head out across the Pacific toward the coast of Peru; he followed them as they slammed into the continental shelf, then split, heading sharply south toward Chile and north toward Alaska.
The warm water created by the south-moving ripples created a heat wave that sent residents of Santiago flocking to nearby beaches in the middle of winter, while the north-moving waves triggered a sharp rise in ocean temperatures off California and Washington State, delighting sportfishermen by attracting tropical species like marlin to usually frigid waters.