The researchers believe Saharan air reaching the Arctic was a rare but significant factor in an unusual warming event two years ago.
In the winter of 2015/16, something happened that had never before been seen on this scale: at the end of December, temperatures rose above zero degrees Celsius for several days in parts of the Arctic, reports Phys.org. Temperatures of up to eight degrees were registered north of Svalbard.
Temperatures this high have not been recorded in the winter half of the year since the beginning of systematic measurements at the end of the 1970s. As a result of this unusual warmth, the sea ice began to melt.
“We heard about this from the media,” says Heini Wernli, Professor of Atmospheric Dynamics at ETH Zurich. The news aroused his scientific curiosity, and a team led by his then doctoral student Hanin Binder investigated the issue. In November 2017, they published their analysis of this exceptional event in the journal Geophysical Research Letters.
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.”
Poleward warm air transport
This highway of air currents was made possible by a particular constellation of pressure systems over northern Europe. During the period in question, intense low-pressure systems developed over Iceland while an extremely stable high-pressure area formed over Scandinavia.
This created a kind of funnel above the North Sea, between Scotland and southern Norway, which channelled the various air currents and steered them northwards to the Arctic.
This highway lasted approximately a week. The pressure systems then decayed and the Arctic returned to its typical frozen winter state. However, the warm period sufficed to reduce the thickness of the sea ice in parts of the Arctic by 30 centimetres – during a period in which ice usually becomes thicker and more widespread.
“These weather conditions and their effect on the sea ice were really exceptional,” says Binder. The researchers were not able to identify a direct link to global warming. “We only carried out an analysis of a single event; we didn’t research the long-term climate aspects” emphasises Binder.
via Tallbloke’s Talkshop
January 16, 2018 at 06:39AM