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article imageUnderstanding the Arctic's 'ice-ocean governor'

By Karen Graham     Oct 17, 2018 in Science
Long-term melting of sea ice in the Arctic may lead to the release of huge volumes of cold, fresh water into the North Atlantic, impacting on global climate.
As we learn more and more about how global warming affects interactions between different parts of our environment and the overall impacts this has on our climate, we can begin to understand what is actually happening to our planet.
We have talked about the Beaufort gyre before. It is an enormous 600-mile-wide pool of swirling cold, fresh water, a wind-driven circular ocean current located just north of Alaska and Canada. in the Arctic Ocean's polar region. The gyre contains both ice and water.
During the winter, the current is covered in a thick covering of ice. In the summer, the ice melts, allowing the gyre to gather up pieces of sea ice, along with river run-off, drawing it down to create a vast reservoir of frigid fresh water, equal in volume to all the water in the Great Lakes of North America.
Central Arctic Ocean Currents: Transpolar Drift and Polar Gyre
Central Arctic Ocean Currents: Transpolar Drift and Polar Gyre
Brn-Bld
The "Ice-ocean Governor"
Scientists at MIT have now identified a key mechanism, which they call the “ice-ocean governor,” that controls how fast the Beaufort Gyre spins and how much fresh water it stores. The research findings were published on Wednesday, October 17, in Geophysical Research Letters.
The MIT researchers are saying that the Arctic ice cover essentially sets the speed the gyre current turns, and this can have great implications for the climate.
Why the speed of the Beaufort gyre is important
For the past two or three decades, as global temperatures have progressively warmed, the Arctic's summer ice cover has shrunk in size. This means that with less ice available to act as a governor in controlling the gyre's spin, the current has sped up in recent years, drawing in more pieces of ice and expanding in both volume and depth.
Azimuthally averaged salinity distribution (in color). Overimposed is a schematic view of the gyre f...
Azimuthally averaged salinity distribution (in color). Overimposed is a schematic view of the gyre features: wind-driven Ekman pumping at the surface, opposing eddy-driven circulation (arrows), and the resulting halocline layer that is deeper in the interior.
Woods Hole Oceanographic Institution
The research team suggests that if global temperatures continue to rise, the ice-ocean governor will eventually disappear, and with no governor to control or limit its speed, the Beaufort gyre will transition into a "new regime," eventually spilling over, sort of like an overflowing bathtub.
And when the gyre spills over, huge volumes of cold fresh water will be released into the North Atlantic - and this will undoubtedly affect our global climate and ocean circulation.
"This changing ice cover in the Arctic is changing the system which is driving the Beaufort Gyre, and changing its stability and intensity," says Gianluca Meneghello, a research scientist in MIT's Department of Earth, Atmospheric and Planetary Sciences. "If all this fresh water is released, it will affect the circulation of the Atlantic."
Co-author John Marshall, the Cecil and Ida Green Professor of Oceanography, puts it very succinctly: "If this ice-ocean governor goes away, then we will end up with basically a new Arctic ocean," Marshall says.
The loss of Arctic sea ice is not only a consequence of global warming  but also an accelerant when ...
The loss of Arctic sea ice is not only a consequence of global warming, but also an accelerant when millions of square kilometres of snow reflecting the Sun's radiation back into space are replaced with dark blue ocean that absorbs it instead
MARIO TAMA, GETTY IMAGES NORTH AMERICA/AFP/File
The actual study
The researchers have been studying the Beaufort gyre for a number of years, using measurements taken by satellites between 2003 and 2014 to track the Arctic ice cover and the speed of the Arctic wind. They were able to use these measurements of ice and wind speed to estimate how much the gyre was spinning down beneath the ice.
But they ran into a problem. The numbers they came up with were much smaller than they expected. “We thought there was a coding error,” Marshall recalls. “But it turns out there was something else kicking back.” In other words, there must be some other mechanism that was limiting or slowing down the gyre’s spin.
The scientists recalculated the gyre's speed using additional satellite data that included estimates of ocean current activity in and around the gyre. The numbers they came up with were far more reasonable.
Image of Arctic sea ice taken by  NASA Goddard researcher Linette Boisvert of the holes and openings...
Image of Arctic sea ice taken by NASA Goddard researcher Linette Boisvert of the holes and openings in the sea ice cover that expose the warm ocean below where more heat and moisture are put into the atmosphere, helping to warm the Arctic.
NASA - Operation IceBridge
According to the paper, the researchers found that "in the spring, as the Arctic ice melts away, the gyre is exposed to the wind, which acts to whip up the ocean current, causing it to spin faster and draw down more fresh water from the Arctic’s river runoff and melting ice. In the winter, as the Arctic ice sheet expands, the ice acts as a lid, shielding the gyre from the fast-moving winds. As a result, the gyre spins against the underside of the ice and eventually slows down."
“The ice moves much slower than wind, and when the gyre reaches the velocity of the ice, at this point, there is no friction — they’re rotating together, and there’s nothing applying a stress [to speed up the gyre],” Meneghello says. “This is the mechanism that governs the gyre’s speed.”
Unlike other oceans, where deeper layers tend to have colder temperatures, the Arctic has been known to have a warmer subsurface, as described in Digital Journal in August this year. An increasingly unstable Beaufort Gyre could also disrupt what is called the Arctic’s halocline.
This is the layer of ocean water underlying the gyre’s cold freshwater, that insulates it from much deeper, warmer, and saltier water. If the halocline layer is disrupted by a more unstable Beaufort gyre, this could cause the warmer water to rise, further hastening the melting of the Arctic ice cover.
More about Beaufort Gyre, iceocean governor, Climate change, Arctic currents, Ice loss
 
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