Polar Vortex
A vortex is a circulation pattern that is naturally very stable. Examples include water swirling down a drain, tornadoes, and dust devils. This sort of stable flow often forms around very cold polar air and is called a polar vortex. The ozone movie below can help you better visualize the Antarctic polar vortex pattern for 2001. The polar vortex is a very cold environment ideally suited for the formation of polar stratospheric clouds and also provides an isolated region which allows chemical reactions to take place without dilution, much like a giant laboratory beaker.
click image for movie of 2001 ozone development.