Last year on Groundhog’s Day, large swaths of the country were covered in two feet of snow or more after a large storm pounded the eastern United States. This year, Punxsutawney Phil emerged from his den on a balmy day after the third-least snowy January on record. A comparison of snowfall (or lack thereof) so far this season to last year's winter white-out shows what a difference a year makes.
Climate forecasters often describe the Arctic Oscillation as the “wild card” of the winter forecast. So far in 2011, the Arctic Oscillation has been in its positive phase, playing the card that favors a milder winter in the eastern United States.
Large-scale shifting of the weight of the atmosphere between mid- and high latitudes creates climate patterns known as the Arctic and North Atlantic Oscillations. These patterns have a big influence on winter weather in the Eastern U.S.
NOAA's Climate Scene Investigators analyzed why the mid-Atlantic region had record-setting snowstorms this winter. The team looked for but found no human "fingerprints" on the severe weather. Instead, they fingered two naturally occurring climate patterns as co-conspirators in the case.
The Pacific-North American teleconnection pattern influences regional weather by affecting the strength and location of the East Asian jet stream, and subsequently, the weather it delivers to North America.
Over the span of days or weeks, the strength of surface air pressure over the North Atlantic seesaws between Iceland and the Azores Islands. The shifting pressure reflects changes in atmospheric circulation that have a big impact on mid-latitude weather in the U.S. and Europe.
The Arctic Oscillation (AO) refers to an atmospheric circulation pattern over the mid-to-high latitudes of the Northern Hemisphere. The most obvious reflection of the phase of this oscillation is the north-to-south location of the storm-steering, mid-latitude jet stream.