Another tough summer for Arctic sea ice
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Enter Jennifer Francis, a researcher at Rutger University's Institute of Marine and Coastal Sciences. She and four colleagues asked a simple question: If you look at real-world measurements, what sort of local and distant effects show up during years where summer sea ice scrapes bottom, figuratively speaking.Skip to next paragraph
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In a phone chat, she summarized what she and her team found.
During periods of larger than average summer sea-ice retreat, of course, more dark ocean is exposed to the sun. So the ocean absorbs heat, then releases it back into the atmosphere as fall sets in. This slows the pace at which the near-surface layer of air, the so-called boundary layer, cools.
In the Arctic, this layer typically tends to be very stable and very low to the ground compared with other parts of the globe. The extra warmth the oceans release, however, keeps this layer relatively warm. The warm air expands, increases the thickness of the boundary layer. This, in turn, enables it to hold more heat, Dr. Francis explains. This whole sequence slows the freezing process, retarding the return of winter ice.
This added warmth also allows the air to hold more moisture as seawater evaporates. Clouds increase. The clouds act as a blanket, trapping heat near the surface. And since the clouds appear in the fall, when the hours of daylight up there rapidly dwindle to zero, this heat-trapping effect offsets any cooling one might expect from cloud tops reflecting sunlight back into space.
So far, this is all local to the Arctic region.
But, Francis continues, her team also saw that the jet stream weakens during the fall and winter following a leaner than normal summer's worth of Arctic sea ice. The jet stream is a high-altitude, high-speed river of air that spins off eddies that become storm systems. It also steers the storms. Its strength is governed by the size of the temperature differences between the southern and northern portions of the hemisphere. And the net warming effect of low-sea-ice seasons reduces that north-south temperature difference.
The weakened jet stream and shifted storm tracks bring drier-than-normal conditions to much of North America, Alaska, and northern Europe. Conditions tend to be wetter than normal along eastern Greenland, through much of the western and central Mediterranean, Japan, and a patch of the Pacific Northwest.
These effects become most obvious when they aren't overwhelmed by big atmospheric features such as El Nino or La Nina.
"Some models do get some of this right," Francis says, pointing to yet-to be published work by scientists at the National Center for Atmospheric Research in Boulder, Colo. The key, she says, is the impact of ice extent on the Arctic's boundary layer.
This budding convergence between measured data and models is encouraging, she says, when it comes time to project future effects of climate change in the region lower latitudes.