Jet streams off track, may affect weather patterns

Seattle researchers have discovered that warming of the Earth's atmosphere seems to be shoving jet streams out of their normal tracks — a change that could expand deserts and profoundly affect the world's weather patterns.

Over the past 27 years, the high-speed air currents that steer storms to temperate zones in both hemispheres have shifted about one degree toward the poles, or about 70 miles, scientists estimate in a paper published today in the journal Science.

"This gives direct, observational evidence of massive atmospheric circulation changes," said University of Washington climate scientist Qiang Fu, the paper's lead author.

The researchers stopped short of attributing the shift to global warming.

It's also impossible to say whether the shift is playing a role in recent droughts in the so-called subtropics, including the American Southwest, the Middle East and the Mediterranean, said co-author John M. Wallace, also a UW atmospheric scientist.

But if the jet streams continue to migrate away from the equator, wider swaths of the planet will almost certainly become hotter and drier, Wallace said.

"If the jets move another two or three degrees over the course of the next century, that would be enough to have a significant impact on climate in the subtropics."

More study is needed to sort out possible contributions from natural cycles, random fluctuations and temperature increases caused by greenhouse gases from cars and industry, Wallace added.

But the finding meshes with predictions of global-warming models, said Isaac Held, a senior researcher for the National Oceanic and Atmospheric Administration (NOAA) who was not involved in the study.

The Seattle team analyzed atmospheric temperature data collected by weather satellites between 1979 and 2005. The data had long been cited by global-warming skeptics because it seemed to show the atmosphere wasn't heating up in tandem with Earth's surface.

Fu and other scientists recently put that debate to rest by pointing out flaws in earlier analyses. When those problems were corrected, it became clear the atmosphere is warming overall about as quickly as the ground.

At the Earth's surface, the most rapid and intense warming is taking place in the Arctic. Fu and his colleagues are the first to examine in detail warming patterns in the atmosphere, particularly its 7.5-mile-thick lower layer where most weather occurs.

In the atmosphere, they found the most rapid heating in the subtropics — zones located at about 30 degrees north and south. These regions, which already have warm climates, also include southern China, North Africa, southern Australia and the southern end of South America.

The exact pattern was a surprise. It doesn't jibe with climate models, which predict more uniform warming of the atmosphere over the tropics, Wallace said.

But the pattern of warming is directly linked to the jet-stream changes.

Zipping along at altitudes of about 30,000 feet, the jet streams form at the boundary between warm, tropical air nearer the equator and cooler air closer to the poles. The temperature difference generates winds and storms, said UW meteorologist Cliff Mass.

"Storms are born and evolve right near the jet stream ... and then are steered by the flow aloft," he said.

That's why the jet streams also mark the transition between the world's most arid regions, like the Sahara desert, and temperate zones with adequate rainfall, like much of the United States and Europe.

Washington residents are well aware of the northern jet stream's drenching power.

"When the jet stream is coming at you, it's like a hose," Mass said.

As the subtropics have warmed in recent years, the boundary between cold and warm air has shifted toward the poles, taking the jet streams with it, the Seattle researchers report.

Another way to think of it is as an expansion of the tropics, Wallace said.

If the hose that funnels moisture to temperate regions continues moving, places like Europe could become more like North Africa, and the American Midwest could resemble the desert Southwest.

Possible impacts in the Pacific Northwest are harder to predict because regional topography allows the jet stream to meander widely from Southern California to Alaska, Wallace said.

Held, the NOAA scientist, said temperature records on Earth go back more than 100 years, and tree rings and ocean cores can help researchers estimate climate even further back in time. But satellite measurements didn't start until the late 1970s. That makes it harder to differentiate natural fluctuations in the jet streams from human-induced changes.

The ancient Greeks and Romans built aqueducts in parts of North Africa that today are nothing but desert, Wallace said. So it's possible jet streams were shifting long before people started pumping out massive quantities of greenhouse gases.

"At this point, it's possible what we're seeing is a fluke. It's possible it's a real, long-term trend unrelated to global warming," he said. "It's also certainly possible it is connected to global warming."

Sandi Doughton: 206-464-2491 or sdoughton@seattletimes.com