
A pronounced shift in Arctic air pressure systems has triggered unusually cold temperatures across North America, Europe, and northern Asia, while allowing a flood of warmer air into Greenland and northeastern Canada, according to NASA.
In recent weeks, the so-called Arctic Oscillation (AO) index — which tracks the relative pressure differential between the Arctic and mid-latitudes — dropped to the fifth-lowest reading ever recorded, NASA scientists said. When the AO reaches this “negative” phase, scientists say, the pressure gradient between the Arctic and mid-latiutudes weakens, allowing Arctic air to stream south. The NASA graphic above shows unusual land surface temperatures across the Northern Hemisphere, with Europe, Russia, and the U.S. experiencing temperatures as high as 5 to 15 degrees C below normal, while temperatures in Greenland were as high as 15 degrees C above normal. The United Kingdom recorded its fourth-coldest March since 1962, Germany experienced its coldest March since 1883, and Moscow had its coldest March since the 1950s.
Jeff Masters at WeatherUnderground discussed one of the early papers on this phenomenon back in 2010, in a report from the American Geophysical Union meeting of that year.
The talk was given by Dr. Jim Overland of NOAA’s Pacific Marine Environmental Laboratory, one of the world’s experts on Arctic weather and climate (I spent many long months flying in the Arctic with him during the three Arctic field programs I participated in during the late 1980s.) Dr. Overland discussed the remarkable winter of 2009 – 2010, which brought record snowstorms to Europe and the U.S. East Coast, along with the coldest temperatures in 25 years, but also brought the warmest winter on record to Canada and much of the Arctic.
He demonstrated that the Arctic is normally dominated by low pressure in winter, and a “Polar Vortex” of counter-clockwise circulating winds develops surrounding the North Pole. However, during the winter of 2009-2010, high pressure replaced low pressure over the Arctic, and the Polar Vortex weakened and even reversed at times, with a clockwise flow of air replacing the usual counter-clockwise flow of air around the pole. This unusual flow pattern allowed cold air to spill southwards and be replaced by warm air moving poleward. This pattern is kind of like leaving the refrigerator door ajar–the refrigerator warms up, but all of the cold air spills out into the house.
Continue reading “The Paradox of Cold Continents, Warm Arctic”




