Model simulations of the northern extravortex ozone column: Influence of past changes in chemical composition

Abstract : A high-resolution three-dimensional chemistry transport model is used to evaluate thecontributions of different mechanisms (in situ chemistry, import of chemically activated orozone-depleted polar air) to the extravortex chemical ozone loss during the winter-springperiod and to show how their efficiencies might have changed in the last 2 decades.Two extreme, but somewhat typical, present-day winters are considered: the 1999–2000winter (cold with a stable and persistent vortex) and the 2001–2002 winter (warm with anweak and distorted vortex). An ozone budget analysis is performed for the partialozone column between 350 and 600 K using a combination of geographical and chemicalozone tracers. The results suggest that the interannual variability in the seasonal ozone lossat midlatitudes is mainly driven by the import of ozone-depleted polar air. Thecontribution from the import of chemically activated polar air is not found to besignificant. The magnitudes of the polar contributions (entirely driven by the halogencycles) differ by about a factor 4 between the simulations. The polar contribution is foundto be responsible for 60% of the total extravortex ozone loss for the exceptional 1999–2000 Arctic winter. In contrast, the magnitude of the in situ destruction (dominated byHOxand NOxcycles) varies little from one winter to the other one. The evolution of theatmospheric chemical composition since 1980 may account for an additional midlatitudeozone loss of 3 to 10 Dobson units depending on the winter considered (representing up toabout 40% of the observed midlatitude ozone trends). The polar contribution is foundto increase by a factor 2 to 3 in the last 2 decades, whereas the in situ destruction increasesby about 20% only.