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Steady as She Goes for Ocean's Conveyor
26 March 2010 5:10 pm
Europe can rest easy. A new analysis of data from satellites and drifting sensors finds no evidence that the Atlantic portion of the "Conveyor Belt"—the great warm current flowing ultimately from the Pacific toward the frigid far North Atlantic—is slowing. Scientists and the public had worried that global warming might be shutting down the conveyor flow and threatening a big chill for Europe. Now, judging by its behavior, the conveyor appears to be far less susceptible to throttling by climate change than once feared.
Headlines warning of Europe’s coming ice age first appeared 5 years ago. In a 2005 Nature paper, oceanographers analyzed temperature and salinity measurements made during five brief ship surveys between 1957 and 2004. These data suggested a 30% decline in the northward flow of the Atlantic conveyor near 26°N around the turn of the century. But continuous measurements by cable-moored instrument arrays soon revealed fluctuations in conveyor flow in the space of a year that would have swamped the once-a-decade surveys. Signs of an ice age evaporated, at least by scientists’ reckoning if not the public’s.
Physical oceanographer Joshua Willis has now further allayed those fears. Working at NASA's Jet Propulsion Laboratory in Pasadena, California, he combined centimeter-accuracy satellite measurements of the height of the sea surface with observations from subsurface, free-floating Argo drifters. He could then calculate the conveyor flow at 41°N. As he writes in a new paper in Geophysical Research Letters, the different approach at a latitude where the flow is much less variable shows “that substantial slowing of the [conveyor] did not occur during the past 7 years.” Judging by the satellite data alone—before the Argo float program got going in about 2002—substantial slowing “is unlikely to have occurred in the past two decades.”
A finding of no slowing is fine by physical oceanographer Carl Wunsch of the Massachusetts Institute of Technology in Cambridge. The satellite-drifter analysis is only the latest evidence against a slowing, he says. And at this rate, he adds, it will likely be decades before the conveyor changes enough to be detected by in situ or satellite-borne instruments.