Why Much of North America Skipped Winter

Cherry blossoms on March 4, a sign of early spring in Brooklyn. CREDIT: Wynne Parry

For parts of North America, this winter was the winter that nearly wasn’t.

January ranked as the fourth-warmest for the 48 U.S. states on record since 1895. December, too, was above average, although not as significantly. The final analysis for February is not yet in, but weather watchers expect last month to rank above average temperature-wise as well.

Of course, this year hasn’t brought early beach weather for everyone; just ask residents of Alaska and Europe, where a frigid cold snap is blamed for hundreds of deaths. And the warmth has been blamed for contributing to the slew of devastating tornadoes that hit the Midwest and southern U.S. on Friday (March 2).

While scientists have said that global warming willcause an uptick in extreme weather, they are hesitant to link any one event or even an unusual season to climate change. Even so, they say, global warming may play a role in the weird winter weather.

The jet stream

The key to understanding the unusually warm winter lies in the jet stream. It is made up of high-altitude, westerly winds. Its polar branch, the one important for determining winter weather, travels over the mid-latitudes of the Northern Hemisphere in winter, according to Jeff Masters, director of meteorology at the weather service and news site Weather Underground.

The polar jet stream divides cold Arctic air to the north from warmer air to the south. This year, meteorologists say, the jet stream has kept the cold air bottled up farther north than usual.

As a result, warmer-than-usual temperatures this year have graced much of the United States, particularly in New England, the Great Lakes and the Upper Plains, according to Mark Paquette, a meteorologist with AccuWeather.com. Southern Canada, too, has gotten its share of mild winter weather

The polar jet stream is influenced by natural patterns, the most prominent being fluctuations in the Northern Annular Mode, also called the Arctic Oscillation. When the mode is in its so-called positive phase, air pressure over the far north remains low, leading to a stronger jet stream. This keeps the cold Arctic air bottled up to the north. The negative phase, meanwhile, is associated with a weaker, meandering jet that allows cold air to spill south.

Reversals

Until late January, the mode was in its positive phase, resulting in warmer temperatures farther north.

But a reversal of phase allowed the jet stream to meander some, and let Arctic cold air move down into Eastern Europe. The result was a cold snap that is blamed for killing hundreds.

The mode has shifted again since then. In fact, a strong jet stream contributed to the tornados that hit the south and the Midwest last week, as did the arrival of warm, moist air from the Gulf of Mexico, according to Masters, who discusses the tornados on his blog.

La Niña and the future

Another large-scale atmospheric pattern, one related to temperatures in the equatorial Pacific Ocean, is also at play. This winter La Niña, associated with cooler water temperatures in the Pacific, has been in effect. La Niña is typically associated with drier-than-normal conditions for the southern and eastern U.S. — largely consistent with precipitation this winter, according to Deke Arndt, chief of the climate monitoring branch at the National Climatic Data Center (NCDC) in Boulder, Colo.

“The dryness looks like what we would expect with La Niña, the warmth we saw is consistent with the positive Arctic Oscillation,” Arndt said. “The two of them tougher, all else being equal, would tend to produce a warmer and drier winter, especially east of the Rockies.”

Paquette predicts an end to this trend. “Mother Nature or weather patterns have a way of evening themselves out. I think it only matter of time before this mild dry pattern flips and we get into a much different weather pattern.”

from:    http://www.livescience.com/18877-mild-winter-weather.html