The role of midlatitude mixing barriers in creating the annual variation of total ozone in high northern latitudes

Data from the HIgh Resolution Dynamics Limb Sounder (HIRDLS), the Microwave Limb Sounder (MLS), and the Whole Atmosphere Community Climate Model (WACCM) are used to investigate the annual variation of total column ozone in high northern latitudes. Downward transport of ozone-rich air by the residual mean circulation during autumn and winter bends ozone isopleths down and increases the high-latitude ozone amounts, leading to an ozone maximum at the end of the winter. During the summer months eddy mixing acts to restore pre-fall distributions of ozone. In this study the large-scale mixing in the lower stratosphere is analyzed using Nakamura's (1996) equivalent length formulation with observed and simulated ozone. The analysis of ozone mixing is performed in the tracer equivalent latitude-potential temperature coordinate system. Steep latitudinal gradients of ozone isopleths below about 500 K occur during the winter, where there are minima in the equivalent length, indicating barriers to mixing at 30 degrees N-40 degrees N. This transport barrier allows large ozone maxima to develop poleward of it. The barrier disappears over the summer, permitting latitudinal mixing of the high ozone air. Above 500 K mixing is more effective during the winter, so a large winter maximum does not occur. In both midlatitude and high latitude the lower stratospheric layer from 330 to 500 K doubles its ozone content from autumn to spring, compared with much smaller changes in the layer from 500 to 650 K. Our results confirm that the presence of the winter transport barrier in the lower stratosphere controls the seasonal variation of total ozone.