Jet Stream Shift Is Expanding the Earth's Tropics and Deserts
Washington - Deserts in the American Southwest and around the globe are creeping toward heavily populated areas as the jet streams shift, scientists reported Thursday.
The result: Areas already stressed by drought may get even drier.
Satellite measurements made from 1979 to 2005 show that the atmosphere in the subtropical regions both north and south of the equator is heating up. As the atmosphere warms, it bulges out at the altitudes where the northern and southern jet streams slip past like swift and massive rivers of air. That bulging has pushed both jet streams about 70 miles closer to the Earth's poles.
Since the jet streams mark the edge of the tropics, in essence framing the hot zone that hugs the equator, their outward movement has allowed the tropics to grow wider by about 140 miles. That means the relatively drier subtropics move as well, pushing closer to places like Salt Lake City, where Thomas Reichler, co-author of the new study, teaches meteorology.
"One of the immediate consequences one can think of is those deserts and dry areas are moving poleward," said Reichler, of the University of Utah. Details appear in Thursday's Science Express, the online edition of the journal Science.
The movement has allowed the subtropics to edge toward populated areas, including the American Southwest, southern Australia and the Mediterranean basin. In those places, the lack of precipitation already is a worry.
"The Mediterranean is one region that models consistently show drying in the future. That could be very much related to this pattern that we are seeing in the atmosphere," said Isaac Held, a senior research scientist with the National Oceanic and Atmospheric Administration. He is not connected with the research.
A shift in where subtropical dry zones lie could make climate change locally noticeable for more people, said Karen Rosenlof, a NOAA research meteorologist also unconnected to the study.
"It is a plausible thing that could be happening, and the people who are going to see its effects earliest are the ones who live closer to the tropics, like southern Australia," said Rosenlof. Her own work suggests the tropics have actually compressed since 2000, after growing wider over the previous 20 years.
Reichler suspects global warming is the root cause of the shift, but said he can't be certain. Other possibilities include variability and destruction of the ozone layer. However, he and his colleagues have noted similar behavior in climate models that suggest global warming plays a role.
Moving the jet streams farther from the equator could disrupt storm patterns, as well as intensify individual storms on the poleward side of the jet streams, said lead author Qiang Fu, a University of Washington atmospheric scientist.
In Europe, for example, that shift could mean less snow falling on the Alps in winter. That would be bad news for skiers, as well as for farmers and others who rely on rivers fed by snowmelt.
"This definitely favors or enhances the frequency of droughts," Fu said of such a shift.