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Why did the entire Eastern U.S. and much of Eurasia experience such a chilly several weeks? You may or may not have heard buzz about the Arctic Oscillation, a climatic phenomenon not unlike El Niño/La Niña. The main difference: The Arctic Oscillation (AO) is a north-south shift in pressure and temperature in the atmosphere and ocean, while El Niño/La Niña is an east-west shift in pressure and temperature. Well, at a basic level.
Check out this thematic map, which compares December 2009 surface temperatures with December 2000-2008 averages around the globe.