Drivers of hemispheric differences in return dates of mid-latitude stratospheric ozone

In a new ACP paper, H. Garny and co-authors investigate the main factors driving the hemispherical asymmetry in ozone return dates. They find that the hemispherical return date differences, which range between 0-30 years across the CCM projections analysed, are affected by both the sensitivity of ozone to Cly (ozone trends have a larger effect on return dates when sensitivity is lower) and the difference in ozone trends between hemispheres. An attribution analysis performed with two CCMs shows that chemically-induced changes in ozone are the major driver of the earlier return of ozone to 1980 levels in northern mid-latitudes. The causes for chemically-induced asymmetric ozone trends relevant for the total column ozone return date differences are found to be (i) stronger increases in ozone production due to enhanced NOx concentrations in the Northern Hemisphere lowermost stratosphere and troposphere, (ii) stronger decreases in the destruction rates of ozone by the NOx cycle in the Northern Hemisphere lower stratosphere linked to effects of dynamics and temperature on NOx concentrations, and (iii) an increasing efficiency of heterogeneous ozone destruction by Cly in the Southern Hemisphere mid-latitudes as a result of decreasing lower stratospheric temperatures. The full abstract can be found here.