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 Atlantic’s
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 gyre’s 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.
Kathryn Hansen
Links:
"Climate
Policy in an Adapting World," Geotimes, April 2005
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