The "weather" in the stratosphere has for decades mysteriously matched the 11-year cycle of sunspots--dark splotches on the sun's surface that mark an increase in solar activity. Now a group of climate modelers presents in the current Science the most promising mechanism yet for amplifying the effects of the solar cycle--and they suggest that sunspots' effects may even work their way down to the surface.
More than a decade ago, meteorologists first reported that when sunspots hit their peak, a ring of relatively high pressure encircles a cap of low pressure over the North Pole in the winter stratosphere (Science, 11 May 1990, p. 684). When the sun's output falls, the pressure pattern reverses. Yet the sunspot cycle alters the sun's total output by only 0.1%, too little for any direct effect on Earth's climate. What could be causing the connection?
The mysterious amplifier, say modelers Drew Shindell of NASA's Goddard Institute for Space Studies in New York City and his colleagues, is the stratosphere's much lamented ozone. Ozone warms the stratosphere by absorbing ultraviolet light, and the sun's UV output rises and falls significantly during the sunspot cycle. Because the north polar region is cloaked in darkness during the winter, the UV-induced warming is limited to lower latitudes. That geographical disparity can drive circulation in the stratosphere, raising atmospheric pressure there and so boosting the westerly stratospheric winds that blow around the pole at 30 to 50 degrees north.
By including ozone and its ability to absorb the sun's ultraviolet radiation in their computer model, Shindell and colleagues also found subtle climate change at the surface. At solar maximum, the mid-latitude, high-pressure ring deflects atmospheric waves that propagate up from the troposphere. The deflection of these waves back into the troposphere alters circulation to build a high-pressure ridge at 40°N that intensifies winds at the surface and redirects storms into Canada and northern Eurasia. The net result is to warm the high latitudes by a few tenths of a degree.
The stratospheric effect seems reasonable enough, says modeler Jerry D. Mahlman of the Geophysical Fluid Dynamics Laboratory in Princeton, New Jersey, who regards the mechanism as the first plausible means of linking sunspots and Earth's atmosphere. But "there's some skepticism" of a trickle-down effect stretching all the way to the surface, says theoretician Lorenzo Polvani of Columbia University. Meteorologists have long doubted that the vanishingly thin stratosphere can affect the massive, turbulent lower atmosphere, called the troposphere.