Polar winds of change
Are changes in wind flow around the poles a symptom of human-induced climate change?
University of Washington researcher John M. Wallace and colleague David Thompson addressed this question in a paper presented at the American Geophysical Union’s December 1999 meeting in San Francisco.
Examining climate change in the polar regions is important for several reasons. First, most of the planet’s surface warming has been concentrated in the high latitudes, particularly in winter. Second, polar temperatures have a major influence on global wind patterns. Why? Because tropical temperatures don’t change nearly so much as those in the high latitudes, and the “gradient” of the change in temperature from the tropics to the poles dictates the position and strength of the jet stream, the tracks of storms, and the resulting rainfall/snowfall patterns.
As the strongest winds meander around the globe, they form a closed loop—often referred to as the polar vortex. In winter, when the polar regions become bitterly cold, the polar vortex expands southward over the lower latitudes, its associated jet stream strengthens, and storms track farther south across the middle and southern portions of Eurasia and North America. Conversely, this vortex contracts in summer, when the polar cold air source becomes depleted and the temperature gradient weakens.
Wallace and colleagues examined changes in the winter circumpolar vortex since 1970 and detected a significant contraction. This contracted vortex is linked to warmer winters over Europe and Asia and a northward shift in storm tracks and the associated precipitation. According to coauthor Thompson, “The recent trend seems unprecedented in the historical model.”
But this major wind shift was not necessarily caused by greenhouse warming. “We can’t be sure that what we’re seeing is not natural,” Wallace said.
Some closely related research suggests Wallace may well be correct. With two colleagues, Paul C. Knappenberger and Adam Burnett, I have prepared a paper for next month’s meeting of the Association of American Geographers in Pittsburgh that augments Wallace’s findings.
Our team examined the size of the circumpolar vortex in the midlayers of the atmosphere (about three miles above the surface) over time. Because these data are available back to 1948, we provide a more complete picture of observed climate changes.
When averaged over winter months (December, January, and February), the vortex has indeed contracted significantly since 1970. But this contraction followed an equally impressive period of vortex expansion that began in the late 1940s. Since atmospheric greenhouse gas concentrations have been increasing throughout this period of record, it’s difficult to argue that the recent contraction is human-induced without first dismissing the earlier vortex expansion.
The actual causes of these changes in vortex size are not known, but they probably arise from a combination of factors. One possible culprit is changes in tropical Pacific Ocean temperatures (the warm El Niño–cold La Niña cycle), which can influence midlatitude winds over the Pacific and North America. Wallace and colleagues theorize that these vortex changes are linked to wind shifts from above—in the stratosphere (the layer of very thin air from about seven to 30 miles above the surface). If stratospheric circulation changes are, indeed, generating changes in wind patterns below, then other factors, such as potential stratospheric ozone depletion, could also play a role.
Until climatologists sort out these various causes and effects, it’s safe to assume that the recent winter circulation changes are well within the normal range of variability. Which means mild, less snowy winters across the Americas and Eurasia and lowering the handicaps of midlatitude golfers everywhere.
Robert E. Davis is an associate professor of environmental science at the University of Virginia.
Davis, R.E. et al., 2000. Northern Hemisphere temperature trends and linkages to the 500 hPa circumpolar vortex. Invited paper, 96th Annual Meeting of the Association of American Geographers, Pittsburgh, Pennsylvania (forthcoming in April).
Thompson, D.W.J., and J.M. Wallace, 1999. Meteorological aspects of the arctic oscillation. Poster, American Geophysical Union, 1999 Fall Meeting, December 13–17, San Francisco, California.