The loss of ozone over the Arctic varies widely from year to year, US scientists say.
By Alex Kirby
BBC News Online environment correspondent
Using data collected by the US space agency (Nasa), they say the amount, timing and pattern of ozone loss all fluctuate.
They say their findings will help to show the effectiveness of agreements on limiting ozone depletion.
And they should also give scientists a better understanding of what is happening in the Arctic.
The research team, which analysed data from the Microwave Limb Sounder (MLS) on Nasa's Upper Atmosphere Research Satellite (UARS), report their findings in the Journal of Geophysical Research, published by the American Geophysical Union.
Ozone, a form of oxygen, protects all life on Earth against harmful ultraviolet radiation. Close to the poles it is thinning, largely because of reactions with chlorofluorocarbons (CFCs) and other industrial gases.
The ozone "hole" - more accurately a thinning - is worse over the Antarctic, where colder conditions mean the loss is more pronounced than in the Arctic.
Scientists believe the international agreement on ozone protection, the Montreal Protocol, should see it restored by about the middle of the century.
Trapped near the Pole
The researchers, led by Dr Gloria Manney of Nasa's Jet Propulsion Laboratory, re-analysed MLS observations covering seven Arctic winters between 1991 and 2000.
They developed a model to account for natural variations in ozone from factors like wind variability.
They found the ozone had been depleted each year except 1998 (when temperatures were too high) within the Arctic vortex, a band of strong winds circling the North Pole.
The MLS ready for attachment to the satellite
The chemicals which destroy the ozone are kept inside the vortex, where temperatures are low.
Nasa says: "Ozone loss was most rapid near the vortex edge, with the biggest losses in 1993 and 1996. The greatest losses occurred in the months of February and March."
It says meteorology drives the changes in the vortex's size, location and duration.
"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."
The MLS experiments measure natural microwave thermal emissions from the limb (or edge) of the Earth's atmosphere and remotely sense vertical profiles of selected atmospheric gases, temperature and pressure.
A Nasa ozone flight heads north from Sweden
Nasa says the information is uniquely able to show the three-dimensional evolution of ozone loss over time.
A new MLS, due for launching in 2004, will provide simultaneous observations of ozone and one or more long-lived trace gases.
Dr Neil Harris, of the UK-based European Ozone Research Coordinating Unit (Eorcu), told BBC News Online: "This research confirms a lot of the work we've been doing in the Arctic.
"It's predominantly the meteorology that's responsible for the ozone loss, and the main factor is probably the stability and temperature of the vortex.
"The question is whether it's natural variability or climate change that's behind it.
"The Antarctic ozone hole in the 1980s followed an increase in atmospheric chlorine levels - it was driven by the chemistry.
"The Arctic ozone loss during the nineties wasn't linked to chlorine - it's been driven by the meteorology."
Images courtesy of Nasa