Impact of the negative Arctic Oscillation on land surface temperatures throughout the Northern Hemisphere. Acquired December 1 - 31, 2009 from Moderate Resolution Imaging Spectroradiometer (MODIS) on NASA’s Terra satellite

The Arctic Oscillation is unusually strong right now.  The consequences, a warm arctic and cold N Europe and E North America, are illustrated in the image Winter Temperatures and the Arctic Oscillation from NASA’s Earth Observatory’s Image of the Day:

If you live nearly anywhere in North America, Europe, or Asia, it’s no news that December 2009 and early January 2010 were cold. This image illustrates how cold December was compared to the average of temperatures recorded in December between 2000 and 2008. Blue points to colder than average land surface temperatures, while red indicates warmer temperatures. Much of the Northern Hemisphere experienced cold land surface temperatures, but the Arctic was exceptionally warm. This weather pattern is a tale-tell sign of the Arctic Oscillation.

The Arctic Oscillation is a climate pattern that influences winter weather in the Northern Hemisphere. It is defined by the pressure difference between air at mid-latitudes (around 45 degrees North, about the latitude of Montreal, Canada or Bordeaux, France) and air over the Arctic. A low-pressure air mass dominates the Arctic, while high pressure systems sit over the mid-latitudes. The strength of the high- and low-pressure systems oscillates. When the systems are weaker than normal, the pressure difference between the Arctic and mid-latitudes decreases, allowing chilly Arctic air to slide south while warmer air creeps north. A weaker-than-normal Arctic Oscillation is said to be negative. When high and low pressure systems are strong, the Arctic Oscillation is positive.