Ozone depletion over Earth's Arctic region varies widely from year to year in its amount, timing and pattern of loss. That's the conclusion of a research team using data from the Microwave Limb Sounder on NASA's Upper Atmosphere Research Satellite.

The findings, published in the current issue of the Journal of Geophysical Research, provide the first consistent, three- dimensional picture of ozone loss during multiple Arctic winters. The findings confirm previous Arctic ozone loss estimate variations.

A new study of Arctic ozone loss in the 1990s found the largest decreases in ozone in late winter 1993 and 1996. The study used data from the JPL-managed Microwave Limb Sounder aboard NASA's Upper Atmosphere Research Satellite.

"This work provides a consistent picture of how Arctic ozone loss varies between winters," said lead researcher Dr. Gloria Manney, a senior research scientist with NASA's Jet Propulsion Laboratory, Pasadena, Calif. "Scientists will have a better understanding of current Arctic ozone conditions and be better able to predict variations in the future."

Manney said NASA's unique vantage point in space provides data needed by policy makers. "They need accurate data to show whether current regulations on ozone-depleting substances are having the desired effect," she said. "In this way, NASA is providing a vital piece of the puzzle needed to understand this global phenomenon."

Ozone is a form of oxygen that shields life on Earth from harmful ultraviolet radiation. Earth's stratospheric ozone layer is thinning around the world outside of the tropics. This thinning is a result of chlorofluorocarbons produced by industrial processes, which form reactive compounds like chlorine monoxide in the stratosphere during winter. To date, ozone loss has been most pronounced over Antarctica, where colder conditions encourage greater ozone loss and result in ozone "hole."

Higher temperatures and other differences in atmospheric conditions in the Arctic have thus far prevented similarly large depletions. Nevertheless, as Manney and her colleagues validated in 1994, widespread Arctic ozone loss also occurs, and scientists are eager to understand it better, since formation of Arctic ozone "hole" could negatively affect populations in Earth's far northern latitudes.

Many uncertainties remain regarding ozone depletion. Scientists want to know what is causing ozone decreases in Earth's mid latitudes. They also wish to assess effects of climate change on future ozone loss, especially in the northern hemisphere high latitudes.

In the new study, Manney's team reanalyzed MLS observations during seven Arctic winters (1991 - 2000) to estimate chemical ozone loss. To yield accurate estimates, the team developed a model to account for naturally occurring ozone variations resulting from atmospheric transport processes such as wind variability. Their results show large year-to- year variability in the amount, timing and patterns of Arctic ozone loss. Ozone depletion was observed in the Arctic vortex each year except 1998, when temperatures were too high for chemical ozone destruction. This vortex is a band of strong winds encircling the North Pole in winter like a giant whirlpool. Inside the vortex, temperatures are low and ozone- destroying chemical are confined. Ozone loss was most rapid near the vortex edge, with the biggest losses in 1993 and 1996. The greatest loses occurred in the months of February and March.

The variability in the size, location and duration of the Arctic vortex is driven by meteorological conditions. High mountains and land-sea boundaries in the northern hemisphere interact with wind variations to generate vast atmospheric undulations that displace air as they travel around Earth. These waves form in the troposphere (the lowest atmospheric layer), where they produce our winter storms, and propagate upward, depositing their energy in the stratosphere. The energy from these waves warms the stratosphere, suppressing formation of polar stratospheric clouds necessary for ozone destruction. Arctic ozone loss tends to be greatest in years when these wave motions are unusually weak.

NASA's MLS experiments measure naturally occurring microwave thermal emissions from the limb of Earth's atmosphere to remotely sense vertical profiles of selected atmospheric gases, temperature and pressure. These data are unique in their ability to show the three-dimensional evolution of ozone loss over time. The Microwave Limb Sounder on the Upper Atmosphere Research Satellite was the first such experiment in space. A next-generation MLS, developed and built at JPL for the Aura mission of NASA's Earth Observing System, is scheduled for launch in 2004. That instrument will provide simultaneous observations of ozone and one or more long-lived trace gases, substantially advancing future studies of ozone loss. The California Institute of Technology in Pasadena manages JPL for NASA.