Frigid South Pole atmosphere reveals flaw in global circulation models

Atmospheric measurements made at Earth’s geographic poles provide a convenient way of validating and calibrating global circulation models. Such measurements also might provide some of the first conclusive evidence of global change in the middle and upper atmospheres. But new data shows that the current models are wrong: Temperatures over the South Pole are much colder in winter than scientists had anticipated.

As reported in the Aug. 28 issue of Geophysical Research Letters, scientists have found that temperatures during mid-winter in the stratopause and mesopause regions at the South Pole are about 40-50 degrees Fahrenheit colder than model predictions.

The work was performed by Chester Gardner, a professor of electrical and computer engineering at the University of Illinois at Urbana-Champaign; Weilin Pan, a doctoral student at Illinois; and Ray Roble, a senior scientist at the High Altitude Observatory of the National Center for Atmospheric Research in Boulder, Colo.

“Our results suggest that wintertime warming due to sinking air masses is not as strong as the models have assumed,” Gardner said. “But, in all fairness, since no one had made these measurements before, modelers have been forced to estimate the values. And, in this case, their estimates were wrong.”

Gardner’s group was the first to make upper atmosphere temperature measurements over the South Pole. From December 1999 until October 2001, the scientists operated a laser radar (lidar) system at the Amundsen-Scott South Pole Station. By combining the lidar data with balloon measurements of the troposphere and lower stratosphere, the scientists recorded temperatures from the surface to an altitude of about 70 miles.

“After the autumnal equinox in March, radiative processes begin cooling the polar atmosphere,” Gardner said. “During the long polar night, the atmosphere above Antarctica receives little sunlight and is sealed off by a vortex of winds that spins counterclockwise. This stable polar vortex prevents the transport of warmer air from lower latitudes into the pole, and leads to extreme cooling of the lower stratosphere.”

In May, June and July, the stratopause region near 30 miles altitude was considerably colder than model predictions, Gardner said. “The greatest difference occurred in July, when the measured stratopause temperature was about 0 degrees Fahrenheit, compared to about 40 degrees Fahrenheit predicted by the models.”

With no sunlight to warm the polar atmosphere, the only source of heat in the wintertime is the adiabatic compression of down welling air masses. This heating effect partially offsets the effects of radiative cooling of greenhouse gases — particularly carbon dioxide — in the middle and upper atmospheres.

“Current global circulation models apparently overpredict the amount of down-welling, because they show warmer temperatures than we observed,” Gardner said.

To test this hypothesis, the researchers reduced the amount of down-welling over the polar cap using the NCAR Thermosphere-Ionosphere-Mesosphere-Electrodynamics General Circulation Model. Developed by Roble and his colleagues, it is the latest in a series of sophisticated three-dimensional, time-dependent models that simulate the circulation, temperature, and compositional structure of the upper atmosphere and ionosphere.

“With the reduced down-welling, the predicted mesopause temperature near 60 miles altitude decreased from about minus 120 degrees Fahrenheit to about minus 140 degrees Fahrenheit, in better agreement with our measurements for mid-winter conditions,” Gardner said. “In the stratopause region, the predicted temperature decreased from about 35 degrees Fahrenheit to about 12 degrees Fahrenheit, also in better agreement with our measurements.”

The recent measurements establish a baseline for polar temperatures, which can then be compared against future changes as greenhouse gases continue to accumulate, Gardner said. “The measurements also show that we have a flaw in some of our global atmospheric circulation models. Now we can go back and improve those models to better predict the temperatures in the middle and upper atmospheres throughout both hemispheres.”

The National Science Foundation funded the work.