Abstract
Abstract. The future evolution of tropical ozone in a changing climate is investigated by analysing time slice simulations made with the chemistry–climate model EMAC. Between the present and the end of the 21st century a significant increase in ozone is found globally for the upper stratosphere and the extratropical lower stratosphere, while in the tropical lower stratosphere ozone decreases significantly by up to 30%. Previous studies have shown that this decrease is connected to changes in tropical upwelling. Here the dominant role of transport for the future ozone decrease is confirmed, but it is found that in addition changes in chemical ozone production and destruction do contribute to the ozone changes in the tropical lower stratosphere. Between 50 and 30 hPa the dynamically induced ozone decrease of up to 22% is amplified by 11–19% due to a reduced ozone production. This is counteracted by a decrease in the ozone loss causing an ozone increase by 15–28%. At 70 hPa the large ozone decrease due to transport (−52%) is reduced by an enhanced photochemical ozone production (+28%) but slightly increased (−5%) due to an enhanced ozone loss. It is found that the increase in the ozone production in the lowermost stratosphere is mainly due to a transport induced decrease in the overlying ozone column while at higher altitudes the ozone production decreases as a consequence of a chemically induced increase in the overlying ozone column. The ozone increase that is attributed to changes in ozone loss between 50 and 30 hPa is mainly caused by a slowing of the ClOx and NOx loss cycles. The enhanced ozone destruction below 70 hPa can be attributed to an increased efficiency of the HOx loss cycle. The role of ozone transport in determining the ozone trend in this region is found to depend on the changes in the net production as a reduced net production also reduces the amount of ozone that can be transported within an air parcel.
Highlights
Studying the evolution of stratospheric ozone (O3) in the 21st century is still of great interest to estimate the effect of the Montreal Protocol and its amendments and to understand how increasing anthropogenic greenhouse gas (GHG) emissions will affect ozone
For the annual mean extratropical total ozone column a future increase is predicted by state-of-the-art chemistry–climate models (CCMs), which will result in a recovery to 1980 values by the end of the 21st century (e.g. Austin et al, 2010; SPARC CCMVal, 2010; Oman et al, 2010; WMO, 2011)
The maximum absolute changes in the tropical lower stratosphere of −0.41 ppmv at 30 hPa are slightly larger than in other CCMs, while the ozone increase above 10 hPa lies in the range of the models included in the study of Oman et al (2010)
Summary
Studying the evolution of stratospheric ozone (O3) in the 21st century is still of great interest to estimate the effect of the Montreal Protocol and its amendments and to understand how increasing anthropogenic greenhouse gas (GHG) emissions will affect ozone. For the annual mean extratropical total ozone column a future increase is predicted by state-of-the-art chemistry–climate models (CCMs), which will result in a recovery to 1980 values by the end of the 21st century The models show no return of the ozone column to levels before 1980. This different evolution is due to a significant decrease of the partial ozone column in the lower stratosphere Austin et al, 2010), which is explained by an increase in tropical upwelling related to increasing concentrations of long-lived GHGs and a concomitant increase in the sea surface temperatures
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