Simulation of stratospheric water vapor trends: impact on stratospheric ozone chemistry

[摘要] A transient model simulation of the 40-year time period 1960 to 1999with the coupled climate-chemistry model (CCM) ECHAM4.L39(DLR)/CHEM shows astratospheric water vapor increase over the last two decades of0.7 ppmv and, additionally, a short-term increase after majorvolcanic eruptions. Furthermore, a long-term decrease in globaltotal ozone as well as a short-term ozone decline in the tropicsafter volcanic eruptions are modeled. In order to understand theresulting effects of the water vapor changes on lower stratosphericozone chemistry, different perturbation simulations were performedwith the CCM ECHAM4.L39(DLR)/CHEM feeding the water vapor perturbations only to thechemistry part. Two different long-term perturbations of lowerstratospheric water vapor, +1 ppmv and +5 ppmv, and ashort-term perturbation of +2 ppmv with an e-folding time oftwo months were applied. An additional stratospheric water vaporamount of 1 ppmv results in a 5–10% OH increase inthe tropical lower stratosphere between 100 and 30 hPa. As adirect consequence of the OH increase the ozone destruction by theHO_x cycle becomes 6.4% more effective.Coupling processesbetween the HO_x-family and the NO_x/ClO_x-family also affect theozone destruction by other catalytic reaction cycles. The NO_x cyclebecomes 1.6% less effective, whereas the effectiveness ofthe ClO_x cycle is again slightly enhanced. A long-term water vaporincrease does not only affect gas-phase chemistry, but alsoheterogeneous ozone chemistry in polar regions.The model resultsindicate an enhanced heterogeneous ozone depletion during antarcticspring due to a longer PSC existence period. In contrast, PSC formation inthe northern hemisphere polar vortex and therefore heterogeneousozone depletion during arctic spring are not affected by the watervapor increase, because of the less PSC activity.Finally, thisstudy shows that 10% of the global total ozone decline inthe transient model run can be explained by the modeled water vaporincrease, but the simulated tropical ozone decrease after volcaniceruptions is caused dynamically rather than chemically.

[学科分类] 大气科学