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| People walk through the snow in Manhattan's East Village Monday in New York City. | ||
By Doyle Rice, USA TODAY
Teeth-chattering, bitterly cold winds have swept across the eastern half of the USA this month, sending December temperatures to near-record cold levels all the way from Minneapolis to Miami.
Blame it mainly on the North Atlantic Oscillation (NAO) and its close cousin, the Arctic Oscillation (AO). These large-scale climate patterns in the atmosphere over the Arctic and north Atlantic Ocean strongly affect winter weather.
The swings in atmospheric pressure between the polar regions and the mid-latitudes (areas that are populated) "cause a redistribution in temperatures across the Northern Hemisphere," says meteorologist Michelle L'Heureux of the Climate Prediction Center in Camp Springs, Md.
L'Heureux says December's NAO has been in what scientists call its "negative" or "cold" phase, causing Arctic air to surge farther south into the central and eastern USA.
The cold air can also invade northern and western Europe, as it has this month, causing travel troubles in the U.K., Germany and France. Meanwhile, Greenland and much of eastern and northern Canada are experiencing a relatively mild month.
The NAO was in a record negative phase last winter, which supplied the cold air that contributed to the colossal snowfalls in the Mid-Atlantic states, L'Heureux says. "The NAO this winter — so far — is not approaching those record values yet."
When the NAO and AO are in their "positive" or "warm" phase, the USA and northern Europe see milder winters.
Vikings recorded these effects nearly a thousand years ago, according to James Hurrell of the National Center for Atmospheric Research. They noticed severe winters tended to strike Greenland in tandem with mild winters in Denmark, and vice versa — a classic NAO effect, he says.
The NAO and AO can overwhelm the effects of the more well-known El Niño/La Niña climate patterns, which are changes in temperature of the tropical Pacific Ocean that affect weather worldwide. However, El Niño and La Niña are what climatologists primarily rely upon to make their seasonal forecasts.
"The NAO/AO are this winter's — and every winter's — wild card," L'Heureux says. "We know that because they are not predictable past two weeks. They can become quite strong and wreak havoc on the seasonal forecasts."
A slight break from the cold blast should be on the way for the eastern USA this week, according to the Climate Prediction Center, as the NAO and AO have shifted more toward their "positive" or "warm" phase in recent days.
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