Abstract
Abstract. Chemistry-climate models (CCMs) project an earlier return of northern mid-latitude total column ozone to 1980 values compared to the southern mid-latitudes. The chemical and dynamical drivers of this hemispheric difference are investigated in this study. The hemispheric asymmetry in return dates is a robust result across different CCMs and is qualitatively independent of the method used to estimate return dates. However, the differences in dates of return to 1980 levels between the southern and northern mid-latitudes can vary between 0 and 30 yr across the range of CCM projections analyzed. Positive linear trends in ozone lead to an earlier return of ozone than expected from the return of Cly to 1980 levels. This forward shift is stronger in the Northern than in the Southern Hemisphere because (i) trends have a larger effect on return dates if the sensitivity of ozone to Cly is lower and (ii) the trends in the Northern Hemisphere are stronger than in the Southern Hemisphere. 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; therefore transport changes are of minor importance. This conclusion is supported by the fact that the spread in the simulated hemispheric difference in return dates across an ensemble of twelve models is only weakly related to the spread in the simulated hemispheric asymmetry of trends in the strength of the Brewer–Dobson circulation. 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.
Highlights
IntroductionAessa result of the the earlier return of ozone to 1980 levels in northern mid- success of the Montreal Protocol and its amendments and latitudes; transport changes are of minor impor- adjustments, tropospheric concentrations of ozone depleting tance
This conclusion is supported by the fact that the spread substances (ODSs) are decreasing (Montzka et al, 1999) and in the simulated hemispheric difference in return dates across an ensemble of twelve models is only weakly related to the aInremeixdp-elacttietuddteos,csotnratitnoOusepchteoeriadcenCcrleySasicse ipoervonejrecctetheed century. by Chemistry-climate models (CCMs) to spread in the simulated hemispheric asymmetry of trends in return to 1980 levels by the middle of the 21st century
Overall the results suggest that the hemispheric differences in ozone trends, and return dates in the lowermost stratosphere are caused by a combination of (i) enhanced ozone production by increasing NOx concentrations, which is stronger in the Northern Hemisphere (NH), and (ii) hemispheric different transport changes due to the hemispheric asymmetric Brewer–Dobson Circulation (BDC) trends
Summary
Aessa result of the the earlier return of ozone to 1980 levels in northern mid- success of the Montreal Protocol and its amendments and latitudes; transport changes are of minor impor- adjustments, tropospheric concentrations of ozone depleting tance. This conclusion is supported by the fact that the spread substances (ODSs) are decreasing (Montzka et al, 1999) and in the simulated hemispheric difference in return dates across an ensemble of twelve models is only weakly related to the aInremeixdp-elacttietuddteos,csotnratitnoOusepchteoeriadcenCcrleySasicse ipoervonejrecctetheed century. The causes tal column ozone (TOZ), on the other hand, is projected by for chemically-induced asymmetric ozone trends relevant for CCMs to return to 1980 values earlier: by the early 2020s the total column ozone return date differences are found to be over northern mid-latitudes and by the mid-2030s over south-
Sign-in/Register to view highlights & summary Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
