The planet's subtropical jet streams are creeping toward the poles, boding ill for life in temperate climes. If the trend continues, some of the world's driest regions, such as the Sahara desert in Africa and the arid regions of the southwestern United States, will expand, and droughts could strike formerly lush regions in Europe and elsewhere.
Jet streams are Earth's superwinds. Relatively narrow bands that circle the globe at altitudes of up to 10 kilometers, they can move at more than 320 kilometers an hour and exert tremendous influence over weather conditions at ground level. In particular, patterns of ground temperature and rainfall depend strongly on the jet streams.
Reporting in today's issue of Science, researchers at the universities of Washington and Utah examined satellite data from the past 27 years and discovered that the jets have been wandering. During this time, the subtropical jets have moved as much as 1° of latitude, or 112 kilometers, away from the equator and toward the poles. The researchers also found what they think might be causing the migration: The troposphere--the layer of atmosphere reaching from the surface to an altitude of about 12 kilometers--has warmed faster than the rest of the atmosphere over the subtropics in bands centering about 30° north and south of the equator. Simultaneously, the stratosphere--which overlays the troposphere, extending to 50 kilometers--has been cooling.
How exactly these changes have affected jet stream movements is unclear, says co-author John Wallace, an atmospheric sciences professor at the University of Washington, Seattle. But it's a mystery that bears figuring out, because the long-term impact on regional rainfall patterns could be serious. "The jet streams mark the edge of the tropics, so if they are moving poleward, that means the tropics are getting wider," Wallace says. "If they move another 2° to 3° poleward in this century, very dry areas ... could nudge farther toward the poles, perhaps by a few hundred miles."
"I was not surprised by these results, since models do generally predict poleward movement of the subtropical boundary ... as the climate warms," says Isaac Held, a senior research scientist with the National Oceanic and Atmospheric Administration's Geophysical Fluid Dynamics Laboratory in Princeton, New Jersey. The study is somewhat limited, he cautions, by the narrow timeline of satellite observations, which can make trends hard to distinguish.
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