“Record temperatures across North America and Europe, record low snowfall, flowers and birds thinking it’s spring instead of the dead of winter… what the hell’s going on? Is this global warming? Not exactly… this year’s near-complete lack of winter, especially compared to last year, seems to be mostly due to La Niña, keeping the jet stream pushed way north.
The cause of this warm first half of winter is the most extreme configuration of the jet stream ever recorded, as measured by the North Atlantic Oscillation (NAO). The Arctic Oscillation (AO), and its close cousin, the North Atlantic Oscillation (which can be thought of as the North Atlantic’s portion of the larger-scale AO), are climate patterns in the Northern Hemisphere defined by fluctuations in the difference of sea-level pressure in the North Atlantic between the Icelandic Low and the Azores High.
The AO and NAO have significant impacts on winter weather in North America and Europe—the AO and NAO affect the path, intensity, and shape of the jet stream, influencing where storms track and how strong these storms become. During December 2011, the NAO index was +2.52, which was the most extreme difference in pressure between Iceland and the Azores ever observed in December (records of the NAO go back to 1865.) The AO during December 2011 had its second most extreme December value on record, behind the equally unusual December of 2006.
These positive AO/NAO conditions caused the Icelandic Low to draw a strong south-westerly flow of air over eastern North America, preventing Arctic air from plunging southward over the U.S. and Europe.
Damn immigrants! That La Niña, taking a job away from a hard working American.
