The ozone content in the polar stratosphere has been shown to be destroyed by CFC's permeating to the stratosphere from the troposphere but there can be other influences as well.
Ozone is important because it absorbs UV radiation which creates warming of the stratosphere. The Ozone is formed in the tropical stratosphere and transported to the polar stratosphere by a system known as the Brewer-Dobson -Circulation. The strength of this circulation varies from year to year and can in turn be dictated by other influences.
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