Flowers are sprouting in January in New Hampshire, the Sierra Mountains in California are nearly snow-free, and lakes in much of Michigan still have not frozen. It's 2012, and the new year is ringing in another ridiculously wacky winter for the U.S...
This winter's remarkable AO/NAO pattern stands in stark contrast to what occurred the previous two winters, when we had the most extreme December jet stream patterns on record in the opposite direction (a strongly negative AO/NAO). The negative AO conditions suppressed westerly winds over the North Atlantic, allowing Arctic air to spill southwards into eastern North America and Western Europe, bringing unusually cold and snowy conditions. The December Arctic Oscillation index has fluctuated wildly over the past six years, with the two most extreme positive and two most extreme negative values on record. Unfortunately, we don't understand why the AO varies so much from winter to winter, nor why the AO has taken on such extreme configurations during four of the past six winters.
