Below is a graph of the Arctic Oscillation Index over the latest 120 days. This zonally symmetric seesaw pattern between sea level pressures in polar and temperate latitudes was first identified by Lorenz and named in 1998 by David W.J. Thompson and John Michael Wallace. Essentially, air pressure is measured at various places across the Arctic and at the middle latitudes of the Northern Hemisphere - about 45° north, roughly the latitude of Milan, Montreal or Vladivostok. The difference between the average readings for the two latitudes gives the state of the Arctic Oscillation index. In this graph zero is equal to the average for 1979 to 2000.
( from http://www.cpc.noaa.gov/products/precip/CWlink/daily_ao_index/ao_index.html )
In the positive phase, frigid winter air does not extend as far into the middle of North America and Europe as it would during the negative phase of the oscillation. This keeps much of the United States east of the Rocky Mountains warmer than normal, but leaves Greenland and Newfoundland colder than usual. Weather patterns in the negative phase are in general "opposite" to those of the positive phase.
The figure above illustrates that a positive phase of the Arctic Oscillation (left) is associated with strengthening of winds circulating counterclockwise around the North Pole north of 55°N. In winter these winds pull more warm air from oceans to continents causing winter warming, and like a top spinning very fast, they hold a tight pattern over the North Pole and keep frigid air from moving south. Cool winds sweep across eastern Canada while North Atlantic storms bring rain and mild temperatures to Northern Europe. Drought conditions prevail over the Mediterranean region.
During the negative phase of the Arctic Oscillation (right), cool continental air plunges into the Midwestern United States and Western Europe while storms bring rainfall to the Mediterranean region - this is what we experience right now! In this for us cool phase higher than normal atmospheric pressure over the Arctic leads to weaker westerlies in the upper atmosphere. With weak westerlies cold Arctic air reaches USA, Northern Europe, and Asia.
NASA Earth Observatory brought the image below a couple of days ago showing the temperature of the land surface for 3-10 December 2010 compared to the average temperature for the same period between 2002 and 2009.
About a year ago they featured a similar image.