If you live almost anywhere in North America, Europe, or Asia, it is probably not news to you that December 2009 and early January 2010 were very cold. The image shows how temperatures during that December differed from the average recorded from 2000 to 2008. Blue indicates areas where temperatures were below the recorded average, while red indicates temperatures above average. Many parts of the Northern Hemisphere were experiencing a cold winter, while polar regions were exceptionally warmer than average.
During winter, the Arctic Oscillation (AO) is an important pattern affecting Northern Hemisphere weather. It is defined by the difference in atmospheric pressure between the middle latitudes—around 45° north, roughly the latitude band between Montreal, Canada, and Bordeaux, France—and the polar regions. [source]
While a low-pressure air mass usually sits over the polar region, higher pressure tends to occur farther south in the middle latitudes. This pressure difference produces winds that help confine intensely cold air to the polar region. At times this pressure pattern weakens, meaning the pressure difference between the polar and middle-latitude regions decreases. This allows cold polar air to move farther south while warmer air from the south moves toward the polar regions. An Arctic Oscillation weaker than average is known as its “negative phase.” When the pressure systems are strong, the Arctic Oscillation is in its “positive” phase.
According to the U.S. weather service, during December 2009 the oscillation over the North Atlantic was strongly negative. The image above shows the effects of this negative phase on surface temperatures across the Northern Hemisphere, observed by the Moderate Resolution Imaging Spectroradiometer (MODIS) aboard NASA's Terra satellite.
As can be seen, polar air made the land surface at middle latitudes considerably colder, while more northerly areas such as Greenland and Alaska were much warmer than usual. [source] [source]
For more information about the Arctic Oscillation, see:
- Polar regions and the Arctic Oscillation
- Effects of an exceptionally negative Arctic Oscillation phase on warmer polar regions