Study: Arctic melting might affect storms, hurricanes
By Kate Spinner
Herald Tribune
Sunday, August 5, 2012 at 3:52 p.m.
Rising flows from Arctic rivers, along with melting sea ice, could eventually shift global weather patterns — including those fueling hurricanes, emerging research shows.
Icebergs in the North Atlantic. Rivers and melting sea ice are the biggest contributors of fresh water from the Arctic.
The river flows themselves cannot shut down the Atlantic's hurricane conveyor belt.
But there is mounting evidence that fresh water coming out of the Arctic could eventually increase enough to weaken an important global ocean pattern called the thermohaline circulation.
The pattern, driven by temperature and salinity differences between salt and fresh water, affects wind flow and other weather patterns across the globe.
If the thermohaline does weaken, it could put a big damper on Atlantic hurricanes.
Whether the increase in river flow is enough now to trigger those changes is a matter of debate.
Xiangdong Zhang, an Arctic climate scientist and professor at the University of Alaska, led a recent study that documented a 15 percent increase in Arctic river flows since the late 1940s.
The rising flows are being driven largely by climate change: altered weather patterns that are sending more rainfall northward.
While Zhang's report is sharply focused on the Arctic, the results raise broader questions about how climate changes there will affect global weather patterns, including the frequency of tropical storms and hurricanes.
The thermohaline circulation is driven by wind in the tropics and sinking water in the colder regions of the world. The wind-driven tropical water stays near the surface and cools as it travels north, becoming more dense along the way.
Once it hits cold enough water it sinks into a deepwater current that snakes around the globe.
In the North Atlantic, the current runs up the east coast of the United States and jogs eastward until it sinks near Iceland. From there, the deep, cold water current runs south.
When excessive amounts of fresh water flow from the Arctic, the water becomes more buoyant in the far North Atlantic and current's water does not sink as much.
That effect could slow the thermohaline circulation, creating weather pattern changes that cool the Atlantic and suppress hurricanes during the summer and fall.
"That's the reason why fresh water is important, because fresh water can change the density in the ocean," Zhang said.
Rivers and melting sea ice are the biggest contributors of fresh water from the Arctic. Exactly how much fresh water is needed to trigger the change is what now must be studied, Zhang and other scientists said.
Increasing river discharges naturally increase the amount of fresh water that flows into the Arctic, but the link to salinity changes in the North Atlantic is not well studied, said Rong Zhang, an oceanographer with the Geophysical Fluid Dynamics Laboratory based in Princeton, N.J.
"A direct link to the North Atlantic is not that clear, but the fresh water export from the Arctic is certainly very important to the thermohaline circulation," said Zhang, who is not related to Xiangdong.
Several studies have indicated that a slower circulation would likely cool the Atlantic, leading to thunderstorms that are more shallow and to fewer hurricanes.
Once flows reach levels significant enough to slow the thermohaline circulation, it would take years to see the results.
Hurricane activity has shown no signs of decreasing so far.
The Atlantic has been generally warmer since the mid-1990s and salinity in the far North Atlantic has actually increased during the same time frame, said Phil Klotzbach an atmospheric research scientist at Colorado State University who specializes in seasonal hurricane prediction.
"If it's having an impact, we certainly haven't seen anything yet," Klotzbach said.
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