Niagara Gazette

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June 25, 2013

94 in Alaska? Weather extremes tied to jet stream

(Continued)

Niagara Gazette — The jet stream, or more precisely the polar jet stream, is the one that affects the Northern Hemisphere. It dips down from Alaska, across the United States or Canada, then across the Atlantic and over Europe and "has everything to do with the weather we experience," Francis said.

It all starts with the difference between cold temperatures in the Arctic and warmer temperatures in the mid-latitudes, she explained. The bigger the temperature difference, the stronger the jet stream, the faster it moves and the straighter it flows. But as the northern polar regions warm two to three times faster than the rest of the world, augmented by unprecedented melting of Arctic sea ice and loss in snow cover, the temperature difference shrinks. Then the jet stream slows and undulates more.

The jet stream is about 14 percent slower in the fall now than in the 1990s, according to a recent study by Francis. And when it slows, it moves north-south instead of east-west, bringing more unusual weather, creating blocking patterns and cutoff lows that are associated with weird weather, the Rutgers scientist said.

Mike Halpert, the deputy director of NOAA's Climate Prediction Center, said that recently the jet stream seems to create weather patterns that get stuck, making dry spells into droughts and hot days into heat waves.

Take the past two winters. They were as different as can be, but both had unusual jet stream activity. Normally, the jet stream plunges southwest from western Washington state, sloping across to Alabama. Then it curves slightly out to sea around the Outer Banks, a swoop that's generally straight without dramatic bends.

During the mostly snowless winter of 2011-12 and the record warm March 2012, the jet stream instead formed a giant upside-down U, curving dramatically in the opposite direction. That trapped warm air over much of the Eastern U.S. A year later the jet stream was again unusual, this time with a sharp U-turn north. This trapped colder and snowier weather in places like Chicago and caused nor'easters in New England, Francis said.

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