Scientists find feedback loop in Arctic ice loss
January 7th, 2010 by Jim JustThe rapid loss of summer ice cover over the Arctic Ocean is creating internal waves in the Arctic waters that could dramatically accelerate the sea ice loss.
That’s what Luc Rainville of the University of Washington’s Applied Physics Laboratory in Seattle and his colleague Rebecca Woodgate report in a study just published in the latest issue of Geophysical Research Letters.
Underwater waves in the Pacific and Atlantic, stirred up by surface winds, keep the oceans from becoming stratified in layers. Near Hawaii, for example, such underwater waves have been measured at depths of up to 200 meters, preventing the ocean from becoming a quiet pool with warm waters on top and colder waters below. The ice-topped Arctic Ocean, on the other hand, is just such a stratified, calm place because sea ice muffles all waves “like a big damper,” as Rainville explains. But that is becoming less the case as summer sea ice melt is opening up ever wider expanses of water.
Unlike any other ocean basin, the Arctic has a lot of very fresh, very cold water on top from melted ice – the cold halocline layer. But below about 100 meters the waters become very salty and slightly warmer. If internal waves become powerful enough to mix these waters, then the warmer surface could accelerate the melting of sea ice.
Here’s the abstract:
The Arctic is generally considered a low energy ocean. Using mooring data from the northern Chukchi Sea, we confirm that this is mainly because of sea ice impeding input of wind energy into the ocean. When sea ice is present, even strong storms do not induce significant oceanic response. However, during ice free seasons, local storms drive strong inertial currents (>20 cm/s) that propagate throughout the water column and significantly deepen the surface mixed layer. The large vertical shear associated with summer inertial motions suggests a dominant role for localized and seasonal vertical mixing in Arctic Ocean dynamics. Our results imply that recent extensive summer sea ice retreat will lead to significantly increased internal wave generation especially over the shelves and also possibly over deep waters. This internal wave activity will likely dramatically increase upper layer mixing in large areas of the previously quiescent Arctic, with important ramifications for ecosystems and ocean dynamics.
The National Snow and Ice Data Center reports temperatures over the Arctic this winter have so far been unusually warm. Paradoxically, this could affect the winds that push the ice out of the Arctic to warmer waters, helping to retain some of the second- and third-year ice through the winter and potentially rebuild some of the older, multiyear ice that has been lost over the past few years. However, NSIDC cautions it is not known whether these weather conditions will persist through the winter or what the net effect will be.
As the chart below shows, Arctic sea ice extent has been declining steadily over the past 30 years.
Arctic sea ice is now tracking at or near record lows.
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