Tropospheric ozone plays an important role in determining the oxidative capacity of the troposphere. It also impacts air quality and is a greenhouse gas. Changes in the abundance of ozone in the upper troposphere and lower stratosphere (UTLS) are of particular concern as it is in this region of the atmosphere that ozone contributes most strongly to the radiative forcing of the climate system. We use the GEOS-Chem model together with observations from the Atmospheric Chemistry Experiment - Fourier Transform Spectrometer (ACE-FTS) satellite instrument to examine the influence of stratosphere-troposphere exchange (STE) on the abundance of ozone in the upper troposphere and pollutants in the lower stratosphere. We characterize the regional and seasonal variations in mixing in the UTLS in GEOS-Chem and assess its impact on UTLS tracer concentrations. In particular, we focus on quantifying the contribution of STE to the lower stratospheric carbon monoxide budget in the Antarctic during austral spring as well as the ozone budget in the subtropics and mid-latitudes of the southern hemisphere upper troposphere in austral winter. The interannual variability of the stratospheric contribution to the ozone budget in the southern hemisphere is explored using a 27-year simulation (1980-2006) with GEOS-Chem driven by MERRA meteorological fields.