Known as the ocean conveyor belt, the thermohaline circulation
(THC) is the process in which cold and salty water sinks at the poles and warmer
and fresher water upwells at low and middle latitudes. In its Third Assessment
Report, the Intergovernmental Panel on Climate Change flagged the potential
sudden collapse of the THC in the Atlantic, which warms Europe to its current
habitable climate, as a significant source of concern (see Geotimes,
April 2005). Climate modelers have been trying to better understand the
circulation, and some researchers have looked toward the salinity record of
the North Atlantic Ocean. They have found that a recent increase in saltiness
could help stabilize the THC.
Salt levels in the northern part of the North Atlantic Ocean started to decrease soon after measurements began more than 50 years ago. Starting in the 1970s, the Arctic was becoming fresher and fresher, says Hjálmar Hátún, a climate scientist at the University of Washington in Seattle. Sources of freshwater came from rain and runoff from Russian, Alaskan and Canadian rivers. If this kept on going, he says, this could potentially reduce the thermohaline circulation.
But the freshening did not continue. In the past 10 years, the trend has reversed, Hátún says, with the northeastern Atlantic reaching the highest salinity ever recorded in the region. Publishing in the Sept.16 Science, Hátún and colleagues suggest that the rise in the North Atlantics salinity is linked to weakened input from a freshwater source.
The research team considered the contribution from two distinct circulation patterns that transport water in the North Atlantic: the subpolar gyre, which has fairly cold and fresh water, and the subtropical gyre, which brings warmer and saltier waters northward with the Gulf Stream system. The two combine in the central northern North Atlantic, Hátún says, to determine the overall composition in the region. The researchers concluded that a weakened subpolar gyre resulted in a reduced contribution from the fresher subpolar gyre, an influence that they say is important to include when considering the THC: The more salt, the faster and more stable the circulation of the THC.
Ruth Curry, a researcher at the Woods Hole Oceanographic Institution in Massachusetts, says that she is not surprised by the subpolar gyres role in controlling the amounts of salty water flowing northward. But she says that the increasing salinity is limited to a thin layer of the upper ocean in the Norwegian Sea and north of Iceland. The present salinity increase may offset the freshening that occurred from 1970 to 1995, but it has a long way to go before it achieves that level completely, Curry says. We have no crystal balls, and cannot know in advance exactly what will happen in the next few decades.
Although it helps control temperature, the THC also is influenced by melting sea ice, which can input more freshwater into the system. The rate of freshening will depend on how fast the melting occurs in the 21st century, Curry says.
"Climate Policy in an Adapting World," Geotimes, April 2005
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