|Harry van Loon.|
The two scientists had previously found that a 10- to 12-year oscillation in the stratosphere of the Northern Hemisphere corresponded to four 11-year solar cycles, beginning in 1958. Now, with the help of a vast data reanalysis conducted by NCAR and the National Centers for Environmental Prediction, they have revealed a mirror image of the solar-stratosphere correlation in the Southern Hemisphere, spanning three solar cycles from 1968 to 1996. Van Loon and Labitzke presented their results at the February meeting of the American Association for the Advancement of Science.
Solar activity cycles from one minimum to the next about every 11 years. During the intervening maximum, explosive activity on the sun intensifies, radiative output increases, and more sunspots are visible on the solar surface. As the measure of this cycle, van Loon and Labitzke used the flux in the 10.7-centimeter radio waveband, an objectively observed quantity highly correlated with the 11-year cycle. They compared these radio data with FUB's daily analyses of the stratosphere. The results show a strong correlation between the solar cycle and the 10- to 12-year oscillation of the lower stratosphere's mean temperatures and constant pressure heights above sea level. "The emergence of a correlation in the Southern Hemisphere similar to that in the Northern Hemisphere has increased our confidence that the solar-stratospheric relationship is more than a statistical coincidence," says van Loon.
The sun's output has varied about 0.1% over one solar cycle during the past several decades. Over centuries, however, larger variations may occur. For example, an extended quiet period on the sun may have chilled the earth during the Little Ice Age between the mid-1550s and mid-1800s. During the long, severe winters and short, wet summers of that period, alpine glaciers advanced down river canyons, Dutch canals froze over, and farming became difficult farther north.
Van Loon and Labitzke found that the highest correlations of the stratosphere's pressure heights with the solar cycle are concentrated in two well-defined latitude zones, which move from lower latitudes in winter to higher latitudes in summer, thus tracking the sun's interseasonal journey.
The annual mean temperatures of the lower stratosphere are well correlated with the solar cycle in the summer of either hemisphere, but only weakly correlated in winter. That is, during the summer months in either hemisphere, the average temperatures of the lower stratosphere rise and fall with the waxing and waning of the sun's energy output over its 11-year cycle.
Before van Loon and Labitzke's research on the stratosphere, researchers had tried for years to find an earthly link to the sun's 11-year cycle. Previous attempts turned up humorous correspondences to the number of Republicans in the U.S. House of Representatives and the length of women's skirts. Even serious scientific stabs at the problem eventually proved false. A solid link takes on added significance now as scientists search for a clear sun-earth connection for computer models used to predict climate change.
"The role of the sun in climate change is still an unsolved problem," says van Loon. "Any relationship between changes in solar output and what happens here on earth is important for understanding long-term climate."