How Seasonal Variations of High Latitude Total Ozone are Controlled by Transport Barriers

Total ozone in northern mid- and high- latitudes has a large maximum at the end of the northern winter, and minimum at the end of the summer. Data from the High Resolution Dynamics Limb Sounder (HIRDLS) experiment on Aura provide profiles of ozone and temperature with high vertical and along track resolution that extends down into the upper troposphere. Displaying ozone isopleths in a potential temperature - equivalent latitude coordinate system shows the downward advection by the Brewer-Dobson circulation during fall and winter, but also the existence of barriers to transport around 35° N below 500 K. These are in the locations predicted by Nakamura's [1996] effective diffusivity formulation, and are not present above 500 K, or during summer. The major component of the total column change is due to the ozone buildup behind the winter barrier below 500 K, which disappears as a result of mixing in summer. The seasonal variation of total ozone is in very good agreement with direct measurements by the Ozone Monitoring Instrument (OMI), also on Aura. Calculations using the Whole Atmosphere Community Climate Model (WACCM) suggest that in a warming world mid-latitude barriers will be stronger, mid latitude gradients will be steeper and high latitude winter mixing ratios and total column amounts will be noticeably larger.