Summer and fall are hurricane season, but for the storms known as polar lows, prime time falls in the dead of winter, when frigid air blows off sea ice to collide with warmer, moister air in the North Atlantic. Polar lows are a lot smaller and weaker than hurricanes, they’re generally shorter-lived, and the only danger they generally pose is to shipping and oil rigs.
However, according to a new study in Nature Geoscience, the dozens of polar lows that roil the Greenland, Iceland and Norwegian seas every year may have an effect on the climate of North America and Europe. And if polar lows move northward with the changing climate, as some studies have predicted, winters in both places could become colder, even as the planet warms.
A polar low northeast of Scandinavia in the Barents Sea. Credit: Erik Kolstad/flickr.
As if that weren’t bad enough, a northward displacement of these Arctic storms could also raise sea level higher along America’s mid-Atlantic coast than the average increase of 3 feet or so projected for the world as a whole by 2100.
It all has to do with the Atlantic meridional overturning circulation (AMOC), a vast conveyer belt of sea water that includes the Gulf Stream. Warm equatorial water travels up along the U.S. East Coast, and then peels off toward the northeast to bathe England and Western Europe with relatively balmy water and air. Without this current’s moderating influence, Madrid, for example, which is as far north as Chicago, would be a lot colder.
When the water reaches the region between Greenland and Norway, it finally cools, becomes denser, and sinks to the bottom of the ocean, where it flows south again, eventually to rise, warm, and begin the journey all over again.
The cooling and sinking would happen in any case, but polar lows help it along by churning the ocean surface, forcing the water to give up its heat faster than it otherwise would; it’s like stirring a cup of tea to make it cool off.
“We think that perhaps 4 or 5 percent of the cooling is due to polar lows,” said lead author Alan Condron, of the University of Massachusetts, in an interview.
As the planet warms, sea ice is likely to diminish in the Arctic — especially in summer, but also, as you go further south, in winter as well. “A couple of previous studies have shown that if the ice migrates northward, these storms could migrate northward as well,” said co-author Ian Renfrew, of the University of East Anglia, in the U.K.
If that happens, their ocean-roiling, water-cooling effect on the AMOC would diminish and the current would presumably slow. That would mean less warm water and warm air for the U.S. East Coast and for Europe — and because water would back up like cars on a congested freeway, the New York-New Jersey region could experience greater sea-level rise by 2100 than the projected worldwide average.
This will be especially true if another effect kicks in. As melting ice caps and glaciers inundate the seas between Greenland and Norway with fresh water, the saltiness of those seas will diminish — and since fresh water is less dense than salt water, it will be even more difficult for surface water to sink for the return southward.
Dilution and the loss of polar storms could therefore combine to increase both the cooling of air and the backup of water — although when it comes to the storms, Condron and Renfrew aren’t making any firm predictions about how much. “That will have to wait for more research,” Renfrew said.
Once the scientists have a better idea of what changes in polar lows might do to ocean temperatures, that information still needs to be incorporated into climate models.
But since the storms are so small and localized, it will take more powerful models than any currently in existence to come up with an overall projection for how the changes would do to climate overall. “It will probably take 10 or even 20 years before we have the raw computing power to do that,” Renfrew said.
No comments:
Post a Comment