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Just to make things even more complicated, it now seems that some of the variations in El Nino cycles come from outside, imposed by other components of the world's intricately interconnected climate system. "We've been treating El Nino as a purely tropical problem, but what if it isn't?" asks Princeton University Oceanographer George Philander. What if some external force--a wind-driven current, say, that sweeps warm water down from the north--were to make it easier for the next El Nino to start?
Such a current, Philander thinks, could explain the unusual spate of El Ninos that marked the first part of this decade. Think of the cycle as one of the strings on the climate's violin, he suggests. "When something changes the tension on the string, the frequency of the vibration also changes."
External forces may also help explain why El Nino has a different impact on the weather from one cycle to the next. Recently, for example, Ed Cook of Lamont-Doherty and Julie Cole of the University of Colorado used tree rings from hundreds of sites to see how El Nino affected North America in the past. Before 1920, they found, El Nino appears to have affected a much larger region of the U.S. than it does today, channeling winter rain and snow all the way up into the Great Lakes and Great Plains. Afterward, however, its sphere of influence retreated to northern Mexico and the American Southwest. Why the shift? It may be, Cole suggests, that El Nino is overlaid on a different climate cycle that is even more important.
One prime suspect is something known as the Pacific decadal oscillation. Since 1977, say researchers from the University of Washington, it has been locked into a mode that has made winters in the Pacific Northwest warm and dry, just as El Nino tends to do. But according to climatologist Nathan Mantua, the Pacific oscillation was in a different phase between 1947 and 1976, and as a result winters in Washington State were cold and rainy.
Yet another player in the El Nino drama is a cycle in the tropical Atlantic that involves a flip-flop between twin pools of water--one warm, one cool--that sit on opposite sides of the equator. Depending on the configuration, farmers in northeastern Brazil could either suffer greatly at the hands of El Nino or feel very little at all.
And then there's the North Atlantic oscillation, which makes El Nino's effect on the eastern U.S. as unpredictable as its influence over Brazil. This climate system, says Gerry Bell of the Climate Prediction Center, changes the position of the jet stream over the ocean. Until recently, the North Atlantic oscillation, which strongly influences Europe's weather as well, was considered to be primarily a manifestation of the atmosphere. But researcher Michael McCartney of the Woods Hole Oceanographic Institution thinks it too is heavily influenced by the sea--in this case by an ocean gyre, a surface current that follows a sweeping circular route. This gyre, he believes, affects the atmosphere by shuttling parcels of warm and cold water between the tropics and northern latitudes.