We didn't expect it. It involves greenhouse gases. The ozone loss is tied to clouds. Until now, the Arctic winter stratosphere has been too warm for clouds to last long. This helps the ozone, because without clouds, ozone-destroying compounds such as chlorine and bromine form compounds that have a limited effect.
New evidence shows ozone loss in the Northern Hemisphere.
But if clouds are present, reactions on cloud particles convert these compounds back into ozone-eating forms. This cloud-assisted process opens the Antarctic hole. The Arctic stratosphere, however, has not been quite cold enough to sustain these clouds. Now, this speculation appears to have become fact. The team found unexpected ozone loss this spring when stratospheric clouds lasted longer than ever before.