The roots of the recent cold weather in Britain and eastern North America lie in unusual goings on high in the atmosphere above the North Pole, as this animation from NASA’s Earth Observatory demonstrates (full video here: 6MB .mov file). The left hand image shows vorticity (rotation, roughly) and the right the temperature at 20km. As the animation moves through January into February, we see the polar vortex (the red bit in the middle) split into two, and stratosphere temperatures over the Arctic jump by as much as 50C. The Earth Observatory explains:
The big change in the Arctic came when the polar vortex ripped apart. A developing weather system in the lower atmosphere traveled upward into the stratosphere. The disturbance nudged into the center of the Arctic air mass, elongating it and eventually splitting it like a cell in mitosis. By February 2, two air masses existed, each with a jet of wind circling it counterclockwise […]. Warm air filled the gap between the two colder air masses, and temperatures high over the North Pole climbed […]. Now the colder air had shifted farther south over Canada and Siberia. Over North America, this piece of the stratospheric polar vortex had a deep reach into the lower atmosphere (troposphere), which created strong winds from the north that carried cold Arctic air far south into the United States.
In Europe, the split in the air mass actually changed the direction of winds in the lower atmosphere. The second piece of the polar vortex was centered east of Western Europe […], and it too was surrounded by a jet of strong wind moving counterclockwise. Like the segment of the polar vortex over North America, this piece of the polar vortex also had a deep reach into the lower atmosphere. It caused cold continental air to blow in from the east, replacing the warmer air that typically blows in from the west. As the frigid air moved over the North Sea, it picked up moisture, which fell over the United Kingdom and parts of France as heavy snow.
There’s a full explanation of the polar circulation at the Earth Observatory page. Well worth a read. Any meteorologists care to comment on just how unusual a feature this is? Are the large blocking highs that bring cold easterlies to Western Europe often associated with polar vortex splits? This is weather, not climate, but the Arctic is experiencing rapid climate change, and this will be expressed as changing weather patterns. A new paper in Climate Dynamics examines this and found “large increases in the potential for extreme weather events […] along the entire southern rim of the Arctic Ocean, including the Barents, Bering and Beaufort Seas.”