Atmospheric blocking is essentially an atmospheric traffic jam. Key winds get involved in a sort of blustery chicken fight, and a high-pressure system gets stuck in one place for eight to 10 days, causing extreme weather (such as drought or flood conditions).
According to atmospheric scientist Tony Luop, atmospheric blocking occurs between 20 and 40 times each year globally. For example, blocking patterns over Greenland and northeastern Canada regularly send chilling blasts of air down toward the U.S [source: Evans]. Those chilling blasts are forced south because they have to detour around that pesky high-pressure system. Blocking patterns can be a normal aspect of wintertime atmospheric conditions, so when they didn't occur in late 2011, the floodgates of winter remained unpredictably closed.
But for how long?
The National Weather Service graph on this page shows a very visible increase in blocking patterns from mid-January on, which lines up with the winter weather hitting the Northeast as we write.
But what about La Niña? El Niño's other half occurs when strong trade winds push warm surface water westward, exposing lower cool waters in the east. So far in 2012, La Niña has nudged the North American polar jet stream farther north than normal [source: Sohn].
Think of this jet stream, or current of winds, as the snaking belt that holds up North America's warm-weather pants. These pants typically ride high during the summer (around the Canadian border) and low during the winter (dipping down through the southeastern United States). In winter 2011-2012, the jet stream belt has been too high, resulting in warmer-than-normal weather for parts of the United States, while also blasting Alaska and Europe with deep chills.
Meteorologists also blame Arctic oscillation (AO), a complex weather pattern involving atmospheric circulation over the Arctic. So far in 2012, the AO index has remained positive, helping to tighten that jet stream belt around the upper regions of the Northern Hemisphere [source: Mackrael].
Can we place the blame on climate change? Climate scientists will be the first to warn against pointing to any single weather event as proof of climate change. The atmosphere, after all, is a beast of an equation to work with. Climate change is a problem with multiple dimensions, but rising ocean temperatures and extreme weather are undoubtedly part of the equation [source: Gore].
For now, however, the winterless among us celebrate their "Januly" weather while residents of Alaska and Eastern Europe dream of warmer days.