A model study of the connection between polar and midlatitude ozone loss in the Northern Hemisphere lower stratosphere

[1] A numerical model has been used to quantify halogen-induced ozone loss at the winter pole and at middle latitudes. This loss is compared with that due to nonhalogen ozone-destroying cycles. The three-dimensional off-line chemical transport model, SLIMCAT, was run at 3.75degrees latitude by 3.75degrees longitude, with United Kingdom Meteorological Office analyzed winds and temperatures. The contribution of polar processes to ozone loss at middle latitudes was investigated with novel tracers mapped to equivalent latitudes. Novel tracers were also included in SLIMCAT to follow ozone loss by reactions with ClOx,BrOx, NOx, and HOx, and to follow ozone production by oxygen photolysis. The interannual variability of the different processes was studied for five winters in the 1990s, which covered a variety of meteorological conditions in and around the polar vortex. Analysis of the ozone loss tracers shows large interannual variability in the relative strengths of particular chemical ozone loss mechanisms depending on the meteorology of the given year. In all cases, the ClO-BrO cycle dominates at polar latitudes. The role of mixing between the pole and middle latitudes also varies with the meteorological conditions. In winters with a cold, stable vortex, like 1996/1997, there is little impact of polar processes on midlatitude loss. In contrast, the cold, disturbed vortex of 1999/2000 contributed significantly to the ozone loss in middle latitudes. In all winters, ozone loss from cycles involving halogens was an important contributor (40-70%) to the modeled midlatitude ozone loss.