Monday, December 20, 2010

Arctic Weather - Arctic Oscillation - 2011? Negative (blue) Phase of High Pressure

Sunday, December 19, 2010

Winter storms in Europe and the Arctic Oscillation

From the Arctic Climatology and Meteorology Education Center
Figure there courtesy of J. Wallace, University of Washington

AO index from 1950 to present
National Weather Service
Climate Prediction Center
For several days now flights have been cancelled into and out of many of Europe's major airports.  London's Heathrow and Gatwick have been amongst the hardest hit, Paris Charles de Gaulle is canceling a quarter of the flights today, and Frankfurt is canceling at least 500 of a planned 1300 flights. Airlines are claiming that they haven't seen storms like this in 20-30 years.  What is going on?

Weather in Europe is strongly controlled by a natural phenomenon called the Arctic Oscillation. This is a pattern of pressures over the Arctic.  In a so-called "positive phase" (illustrated on the left of the figure at the upper right) the pressure over the polar region is low and higher pressure in the midlatitudes drives ocean storms toward the north.  In this phase, Alaska, Scotland and Scandinavia have wetter than normal weather, and the US west and the Mediterranean have drier conditions. Frigid winter air doesn't extend as far into North America as usual, which keeps much of the US east of the Rocky Mountains warmer than normal, but Greenland and Newfoundland are colder than usual. Since the 1970's the oscillation tended to be in this phase, which you can see by the dominance of red on the graph to the right.

In the "negative phase" there is relatively high pressure over the polar region and lower pressures in the midlatitudes, but the difference in the pressures are small, that is, the pressure systems are weak. We are now in a negative phase.  The Arctic is warmer than average, whereas parts of the midlatitudes are colder than normal.  The Arctic Oscillation especially affects patterns over Europe, and is the cause of the current blustery weather as well as that in December a year ago. Ironically, the Arctic Oscillation Index went strongly negative just about the time that the 2009  Copenhagen Climate Conference started, causing unexpectedly cold weather there!

For my European friends, stay warm and safe during these holidays


More info:

Map of the Arctic

North Atlantic Circulation

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