Satellite data show tropics are expanding
The two jet streams that mark the north and south edges of the world's tropical circulations each moved poleward by about 110 kilometers (70 miles) between 1979 and 2005, according to an analysis published in the 26 May issue of Science. A team led by Qiang Fu (University of Washington/Lanzhou University) examined global temperatures in the troposphere and stratosphere as reported by microwave sounding unit (MSU) satellites.
Since the 1970s, MSU temperatures have generally warmed in the troposphere and cooled in the stratosphere. Fu's group found that the stratospheric cooling has been most pronounced across the subtropics and midlatitudes, from about 15° to 45°N and S. The tropospheric warming also showed an enhancement in these areas, along with a general trend toward warming at higher northern latitudes. According to Fu and colleagues, these effects combined to push the troposphere upward most strongly around 30°N and 30°S, where the subtropical jet streams are located. In response, the jet streams each shifted poleward by about one degree in latitude.
If confirmed, this trend could mark a poleward expansion of the world's dry subtropics, where descending air produces most of Earth's hottest deserts, including the Sahara, Arabian, Thar, and Mojave. The study did not attempt to attribute the circulation shift to anthropogenic climate change. Most simulations of global warming project a poleward shift in the polar jet streams, whose average locations are around 45°N and S. However, according to Fu and colleagues, "a pattern like the recent trends, with the strongest tropospheric warming centered [at around] 30° latitude, is not recovered in the simulations."
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