Chemical contribution to future tropical ozone change in the lower stratosphere

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 ClO_x and NO_x loss cycles. The enhanced ozone destruction below 70 hPa can be attributed to an increased efficiency of the HO_x 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.