While it is unclear whether global warming is the culprit, the study by the University of Utah and the University of Washington observes that the tropical zone has ballooned by 140 miles north to south since 1979. This bulge also has caused a shift in the locations of subtropical zones, which cover places such as the American Southwest.
Salt Lake City, which was already in the northern reaches of a subtropical dry zone, finds itself deeper into that climate region as a result. If the trend continues, Utah could face a parched future.
"We really don't know what causes the tropical expansion," said Thomas Reichler, a U. meteorology researcher.
The study appearing in today's issue of the journal Science reports findings gleaned from satellite information but offers no explanation for the phenomenon.
"This study really gives us direct observational evidence of atmospheric circulation changes," said Qiang Fu, lead author from the University of Washington, in Seattle.
Fu, who earned his doctorate at the U., added that subtropical jet streams are slowly moving toward the poles. These meandering rivers of high-speed winds typically mark the dividing line between subtropical dry zones and cooler mid-latitude zones, such as those found north of Utah.
In parts of the Southwest, if these climate zones continue to change, it could mean even less precipitation. The Salt Lake Valley could face greater challenges in meeting the water demands of a growing population, Reichler said.
"You shift your precipitation patterns everywhere," he said.
To find details on the expanding tropics, researchers used a satellite-based device known as a microwave sounding unit. As oxygen molecules escape the atmosphere and head toward space, they give off radiation signatures that the MSUs can read. The higher the radiation level, the warmer the oxygen, which corresponds to higher temperatures inside the atmosphere, explained Reichler, an author of the paper.
Information from the study, gathered between 1979 and 2005, helped researchers map how the boundaries of the tropic zone have changed.
Though the Science paper does not place any blame, Reichler suspects global warming may be the cause. Climate prediction models seem to show a widening tropical zone when global warming is incorporated into the computer simulations. Other possible causes include natural climate variation or problems with the planet's ozone layer.
glavine@sltrib.com
