AN 'INTERMEDIATE' GENERAL CIRCULATION MODEL FOR OZONE CHANGE STUDIES

摘要

Ozone is an important gas in the atmosphere since, although it is only present in tiny quantities, it plays a vital role in protecting mankind from the harmful effects of solar radiation and a dominant part in determining the temperature structure of the atmosphere. The significant depletion of ozone, near-global in extent, which has been observed in recent years has raised concern primarily because of its direct effect of allowing more harmful solar ultraviolet radiation to penetrate to the Earth's surface. However, through its effect on the atmospheric temperature structure, it is possible that the effects of ozone depletion could be far more wide-ranging. When the temperature of the lower stratosphere cools below a critical value polar stratospheric clouds (PSCs) can form. It is well known that heterogeneous chemical reactions occurring on the surfaces of these PSCs play an important role in the removal of ozone from this region (e.g. Crutzen and Arnold (1986), Solomon (1990)). The dominant radiative perturbation to this region as a result of this ozone loss is a cooling; hence it is possible that a positive feedback might be at work, whereby a cooled lower stratosphere, encouraging more ozone loss, leads to further cooling because of the reduced absorption of solar radiation by the reduced ozone. In addition to this, changes in the atmospheric circulation accompanying the ozone change might act to offset or amplify the purely radiative effects. Previous studies (e.g. Kiehl and Boville (1988)) have found that an upper stratospheric warming, attributable to an enhanced descending branch of the diabatic circulation, accompanies ozone depletion and Rind et al. (1998) have shown the potential for radiative-chemical-dynamical coupling. The cooled lower stratosphere might be associated with a strengthened circulation, which becomes more impervious to disturbances such as stratospheric sudden warmings, thus encouraging further cooling. In order to address such issues, suitable state-of-the-art climate models need to be developed and used. This paper briefly describes the development of an 'Intermediate' General Circulation Model and shows examples of how its climatology compares with observations. Some results of experiments comparing a late 1970s ozone scenario with one of the late 1990s are very briefly presented; work on this is ongoing and will be published in the scientific literature as soon as possible.