The Impact of Arctic Ozone Depletion on Northern Middle Latitudes: Interannual Variability and Dynamical Control

We have investigated the effect of the export of Arctic ozone loss, or ʹdilutionʹ, on mid latitude ozone depletion during the 1990s, and its relation to interannual meteorological variability. A stratospheric chemical-transport model incorporated a simple gas-phase ozone scheme with the addition of a parameterisation of polar depletion which depended only on temperature and duration of sunlight. The model was forced with the U.K. Meteorological Office analyses from 1991 to 1999 covering eight Northern Hemisphere winters. The modelled Arctic ozone column losses were about half the magnitude of those in the Antarctic and showed a considerable variation from year to year. The northern middle latitudes (40°–60° N) were mainly affected through dilution and experienced a variable 5– 20% depletion. Year-round there is a depletion of about 1% in northern middle latitudes due to activation at the pole but there is no evidence that this depletion increases with time during this integration. A series of inert tracer experiments for the winters from 996 to 1999 showed that the dilution occurs primarily at the 560 K and 465 K isentropic levels where up to 30% of the air originating northward of 67° N on 1 March is found at 47° N later in spring. The strength and persistence of the Arctic vortex were crucial in determining the severity and the timing of the ozone dilution every year by influencing, respectively, the magnitude of the high-latitude depletion and the effectiveness of mixing Awave activity indicating the important role of dynamical processes in regulating the plar-driven mid-latitude ozone depletion.