Chapter 6: Mid-latitude Ozone Depletion

The abundance of ozone at mid-latitudes (35°–60° N and S) is controlled by a combination of chemical and dynamical processes. Observations have shown that mid-latitude column ozone decreased in both hemispheres from about 1980 onwards. The column depletion in the southern hemisphere was larger and increased through to 2000; in the north the depletion peaked around 1993. In the profile, ozone loss was observed in both the lower (∼20km) and upper (∼40km) stratosphere. There is strong evidence that this observed depletion in mid-latitude ozone was largely driven by increases in stratospheric chlorine and bromine. The basic fingerprint of modeled loss matches those observations in terms of profile and latitudinal variations and long-term variations. However, there is also strong evidence that dynamical changes have contributed to changes in northern mid-latitudes, which have been more extensively studied that the south. In the north, about 1/3 of the ozone trend over the last few decades can be ascribed to dynamical changes. Stratospheric chlorine and bromine is now starting to decrease following a peak in the late 1990s. This should cause a recovery of the ozone layer, but other factors (e.g. greenhouse gas-induced climate change) will modulate this. Although ozone has been observed to increase over the past decade (in both the column and profile) it has not yet been possible to ascribe this change unequivocally to recovery from depletion casued by chlorine and bromine species.