In it, the researchers show how these unusual temperatures arose: three different air currents met over the North Sea between Scotland and southern Norway, carrying warm air northwards at high speed as though on a "highway." (see illustration)
One air current originated in the Sahara and brought near-surface warm air with it. To begin with, temperature of this air was about 20 degrees Celsius. While it cooled off on its way to the Arctic, it was still above zero when it arrived. "It's extremely rare for warm, near-surface subtropical air to be transported as far as the Arctic," says Binder.
The second air current originated in the Arctic itself, a fact that astonished the scientists. To begin with, this air was very cold. However, the air mass – which also lay close to the ground – moved towards the south along a curved path and, while above the Atlantic, was warmed significantly by the heatflux from the ocean before joining the subtropical air current.
The third warm air current started as a cold air mass in the upper troposphere, from an altitude above 5 kilometres. These air masses were carried from west to east and descended in a stationary high-pressure area over Scandinavia. Compression thereby warmed the originally cold air, before it entered the "highway to the Arctic."
