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

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 Cl_y 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 Cl_y 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 NO_x concentrations in the Northern Hemisphere lowermost stratosphere and troposphere, (ii) stronger decreases in the destruction rates of ozone by the NO_x cycle in the Northern Hemisphere lower stratosphere linked to effects of dynamics and temperature on NO_x concentrations, and (iii) an increasing efficiency of heterogeneous ozone destruction by Cl_y in the Southern Hemisphere mid-latitudes as a~result of decreasing lower stratospheric temperatures

Daily ozone cycle in the stratosphere: global, regional and seasonal behaviour modelled with the Whole Atmosphere Community Climate Model

“…In fact, present observations indicate a tendency for a slow recovery of the stratospheric ozone layer (Jonsson et al, 2009;Garny et al, 2013;Chehade et al, 2014;Kyrölä et al, 2014;Gebhardt et al, 2014). Projections of chemistryclimate models indicate a return of stratospheric ozone concentrations to their 1980s level in the period between 2040 7646 A. Schanz et al: Daily ozone cycle in the stratosphere and 2050 (Eyring et al, 2007).…”

“…However, since the expected ozone trends are of the order of 1 % per decade or less (Garny et al, 2013;Chehade et al, 2014), a thorough correction of the diurnal sampling effects in time series of stratospheric ozone is necessary at any location. Such a correction could be guided by a chemistry-climate model.…”

“…Bhartia et al, 2013). The recovery of the stratospheric ozone layer is expected to be approximately 1 % per decade (Garny et al, 2013;Chehade et al, 2014) so that even small diurnal ozone variations can seriously bias the ozone trend estimate. To date, most observations based on satellites with different equator-crossing times are analysed without any consideration of diurnal ozone variations (Bhartia et al, 2013).…”

Life-cycle perspectives for urban water systems planning

“…Each of the non-halocarbon species (N2O, CH4, CO2) influence the ozone layer via different pathways, and each has differing spatial implications for ozone status. Also important is the effect that anthropogenic climate change will have on atmospheric circulation patterns, increasing the rate of bulk air and ozone transfer from the tropics (where ozone generation rates are highest) towards the mid-latitudes and poles (see Portmann and Solomon 2007, Bekki et al 2011, Revell et al 2012b, Garny et al 2013). …”

“…Firstly, tropical ozone levels are expected to decrease below those of pre-industrial times, despite overall global ozone levels recovering strongly. Secondly, 5.1 See Portmann et al (2012) for a concise summary of some important interdependencies that lead to non-linear ozone responses in atmospheric modelling ozone layer recovery will be much more rapid in the mid-latitudes than in polar regions; and occur sooner in the northern hemisphere than in the south , Oman et al 2010, Garny et al 2013). …”