Dynamics and the ozone distribution in the winter stratosphere : modelling the inter-hemispheric differences

Results from a two year simulation of a General Circulation Model are used to illustrate the main differences found in the lower stratosphere dynamics and the ozone distribution between the Southern and the Northern Hemispheres in winter.The model extends from ground to mesospheric levels with a spectral horizontal resolution up to isotropic wavenumber 42. It incorporates a fully interactive scheme for the ozone mixing ratio which accounts for photochemical sources and sinks, advection by the model winds and coupling with radiative calculations.The model reproduces the large scale inter hemispheric differences quite well, with a very stable and cold vortex in the Southern Hemisphere and a warmer vortex often distorted in the Northern Hemisphere. It is concluded that due to interactions between dynamics, polar stratospheric cloud formation and chemistry, there is a possibility that some stratospheric ozone depletion could be effective in late winter near the night terminator in the Northern Hemisphere, whereas significant ozone depletion only occurs in early spring in the Southern Hemisphere.The importance of synoptic scale dynamics on the ozone transport between the high latitudes and the equator is also stressed. The model develops tongues of ozone-rich air from the high latitudes which are irreversibly mixed at mid-latitudes with tongues of ozone-poor air from the low latitudes. Similar tongues or filaments are clearly visible in the TOMS satellite data. They result from the activity of medium scale-waves in the Southern Hemisphere, whereas in the Northern Hemisphere the larger scale planetary waves play a major role in their development, and their size and extension are larger. It is concluded that transport of the ozone depletion to the mid-latitudes could be more effective in the Northern than in the Southern Hemisphere.