Jet stream, America's storm maker, moving slowly northward

[April 18, 2008]  WASHINGTON (AP) -- The jet stream -- America's stormy weather maker -- is creeping northward and weakening, new research shows. That potentially means less rain in the already dry South and Southwest and more storms in the North.

And it could also translate into more and stronger hurricanes since the jet stream suppresses their formation. The study's authors said they have to do more research to pinpoint specific consequences.

From 1979 to 2001, the Northern Hemisphere's jet stream moved northward on average at a rate of about 1.25 miles a year, according to the paper published Friday in the journal Geophysical Research Letters. The authors suspect global warming is the cause, but have yet to prove it.

The jet stream is a high-speed, constantly shifting river of air about 30,000 feet above the ground that guides storm systems and cool air around the globe. And when it moves away from a region, high pressure and clear skies predominate.

Two other jet streams in the Southern Hemisphere are also shifting poleward, the study found.

The northern jet stream "is the dominant thing that creates weather systems for the United States," said study co-author Ken Caldeira, a climate scientist at the Carnegie Institution of Washington in Stanford, Calif. "Basically look south of where you are and that's probably a good guess of what your weather may be like in a few decades."

The study looked at the average location of the constantly moving jet stream and found that when looked at over decades, it has shifted northward. The study's authors and other scientists suggest that the widening of the Earth's tropical belt -- a development documented last year -- is pushing the three jet streams toward the poles.

Climate models have long predicted that with global warming, the world's jet streams would move that way, so it makes sense to think that's what happening, Caldeira said. However, proving it is a rigorous process, using complex computer models to factor in all sorts of possibilities. That has not been done yet.

A rate of 1.25 miles a year "doesn't sound like much, but that works out to about 18 feet per day," Caldeira said. "If you think about climate zones shifting northward at this rate, you can imagine squirrels keeping up. But what are oak trees going to do?

"We are seeing a general northward shift of all sorts of phenomena in the Northern Hemisphere occurring at rates that are faster than what ecosystems can keep up with," he said.

Dian Seidel, a research meteorologist for the National Oceanic and Atmospheric Administration who wrote a study about the widening tropical belt last year, said she was surprised that Caldeira found such a small shift. Her study documented that the tropical belt was bulging at a much faster rate. Caldeira said his figures represent the minimum amount of movement.

The jet stream also factors into bumpy air travel. It is a cause of clear air turbulence that airline pilots try to avoid by tracking where the jet stream is.

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Carnegie Institution of Washington's Department of Global Ecology

[Associated Press; By SETH BORENSTEIN]

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