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National Geographic : 2005 Aug
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acivt .ho tha fo 19 5 194 whe se-sufc te praue wer low there.were .0 . S..........0^fl~~i3S^5B- - 0-. S .0^^^^^ bewe- h fa another .tani an .h trpis Frqec of mao hurcae rie and falls ona multid cadal ti e frame graph atleft) th t scientist are till try ng to un erstand g. A - so -. - .6 . - .^^^^^^^^^^^^^^^^B S- 5-... - - - a.. . . - . - SS S'ISro~o^^^'BBI^^^^ - years from now, or perhaps thirty (the timetable is difficult to predict), the cycle should reverse, tending to suppress major hurricanes. Why the variation? "Frankly, no one can say with 100 percent certainty, but it appears to be a natural effect," says Thomas Delworth, a cli mate modeler at NOAA's Geophysical Fluid Dynamics Laboratory in Princeton, New Jersey. Delworth is part of a major scientific effort to develop accurate computer climate models, and much of his work focuses on thermohaline circulation-that is, the way ocean currents, and consequently such cycles as the AMO, are driven by heat and salinity. Thermohaline circulation runs the Atlantic conveyor belt, part of a global ocean system in which a continuous flow of upper-level water is drawn from the tropical Atlantic north toward the Pole. There the water cools, sinks, and cycles back to the southern oceans in deepwater currents. As the conveyor belt speeds up, tropical sur face water is drawn north more quickly, and temperatures in the North Atlantic are as much as 2°F warmer. That's good for hurricanes. "A hurricane is essentially an engine that runs on heat," says Chris Landsea, a meteorologist at NOAA's Hurricane Research Division in Miami. "The warmer the sea-surface temperature [it must be at least 80°F for a hurricane to start] and the more warm, moist air that's available, the stronger a hurricane can become." How and where does the conveyor belt speed up, increasing the overturning circulation of warm and cold water? It's at the point where cold surface water sinks that the acceleration of the Atlantic conveyor belt probably happens, Del worth says. Cold, dry air coming off Canada extracts heat from the water. When these winds blow stronger and colder than average over a number of years, increasingly chilled water sinks faster because it is more dense, intensifying the flow rate. Years of weaker and warmer winds have the opposite effect, slowing the conveyor belt. Climate records indicate a correlation between a pattern of increased cold winds and the 1995 upswing in hurricane formation. "From the late sixties to the mid-nineties, westerly winds strengthened" Delworth says. "The overturning IN HOT WATER 79
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