It's easy for Americans to forget during this particularly frigid and stormy winter that the Arctic is experiencing extremely warm weather. Temperatures peaked 18 degrees Celsius higher than usual in the areas of Siberia, Alaska and the Northwest Passage above the Arctic circle last week. This map compares land temperatures for January 9 to 16 with the same data and dates for the period from 2003 to 2010. Red indicates higher-than-usual temperatures for that area; blue, colder.

A climate mechanism called the Arctic Oscillation (AO) is probably the culprit for the turnaround. The AO fluctuates between positive and negative phases. During a positive phase, air pressure over the Arctic is high, whereas it is low over the middle latitudes of North America, a condition that restricts cold winds to the Arctic. Right now, the AO is in a negative phase, which means air pressure gradients are reversed, allowing cold Arctic winds to race south over the U.S. The AO was in a negative phase during the 2009–10 U.S. east coast winter snowstorms as well.

The phases can change quickly, however. The National Oceanic and Atmospheric Administration predicts the AO will enter a positive phase next month.

–Francie Diep