By LEE BOWMAN
Scripps Howard News Service
May 26, 2006
Researchers report Friday in the journal Science that the apparent north-south widening of the zone amounts to about 140 miles and that droughts and other unusually dry conditions in recent years for the American Southwest and Mediterranean Europe may be related to the expansion.
The scientists also found that the jet streams of the Northern and Southern hemispheres have moved toward their respective poles by about 1 degree of latitude, or 70 miles. But they said they couldn't yet tell if the changes are being triggered by natural climate swings or by human activity contributing to global warming.
"It's a big deal. The tropics may be expanding and getting larger," said Thomas Reichler, an assistant professor of meteorology at the University of Utah and a co-author of the study. "If this is true, it also would mean that subtropical deserts are expanding into heavily populated mid-latitude regions."
The researchers, led by Qiang Fu, an associate professor of atmospheric sciences at the University of Washington, analyzed the temperatures that were taken at various heights by National Oceanic and Atmospheric Administration satellites between 1979 and 2005.
They found that the air in the troposphere - the first 7.5 miles above the surface where most weather occurs - has warmed by about 1.5 degrees Fahrenheit in a band at 30 degrees north latitude, which crosses the southern United States, southern China and North Africa, and 30 degrees south latitude, which crosses southern Australia, South Africa and southern South America, than at the equator.
Most climate models had predicted that the fastest warming in the 21st century would happen closer to the equator. But Fu said the satellite data indicate the enhanced warming around 30 degrees latitude has helped reshape atmospheric pressures, making the air in those zones bulge in a way that moved the jet streams further toward the poles.
Earth has two polar jet streams, one in each hemisphere, and the two subtropical jet streams closer to the equator. These narrow tubes of high-speed (250 mile an hour) wind generally move west to east at a height of about 30,000 feet. But they meander north and south in patterns that help determine storm tracks and the boundaries between warm tropical air masses and cooler, drier polar air.
"The (subtropical) jet streams mark the edge of the tropics, so if they are moving poleward, that means the tropics are getting wider," explained John Wallace, another co-author and professor of atmospheric sciences at the University of Washington. "If they move another 2 to 3 degrees poleward in this century, very dry areas, such the Sahara Desert, could nudge further toward the poles, perhaps by a few hundred miles."
Wallace said that shifting mid-latitude storm tracks poleward could reduce winter precipitation in regions such as southern Europe, including the Alps, and southern Australia.
Additionally, Fu pointed out that the measurements also show the enhanced warming of the troposphere corresponds closely with enhanced cooling in a higher atmospheric layer called the stratosphere. That layer extends from about 7.5 miles to about 31 miles.
Reichler said the cooling of the stratosphere coupled with the warming of the lower air mass "may actually cause the change of the jet positions."
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