A transient model simulation of the 40-year time
period 1960 to 1999 with the coupled climate-chemistry
model (CCM) ECHAM4.L39(DLR)/CHEM shows a stratospheric
water vapor increase over the last two decades of
0.7 ppmv and, additionally, a short-term increase after major
volcanic eruptions. Furthermore, a long-term decrease in
global total ozone as well as a short-term ozone decline in the
tropics after volcanic eruptions are modeled. In order to understand
the resulting effects of the water vapor changes on
lower stratospheric ozone chemistry, different perturbation
simulations were performed with the CCM ECHAM4.L39-
(DLR)/CHEM feeding the water vapor perturbations only to
the chemistry part. Two different long-term perturbations of
lower stratospheric water vapor, +1 ppmv and +5 ppmv, and a
short-term perturbation of +2 ppmv with an e-folding time of
two months were applied. An additional stratospheric water
vapor amount of 1 ppmv results in a 5–10% OH increase in
the tropical lower stratosphere between 100 and 30 hPa. As
a direct consequence of the OH increase the ozone destruction
by the HOx cycle becomes 6.4% more effective. Coupling
processes between the HOx-family and the NOx/ClOxfamily
also affect the ozone destruction by other catalytic
reaction cycles. The NOx cycle becomes 1.6% less effective,
whereas the effectiveness of the ClOx cycle is again
slightly enhanced. A long-term water vapor increase does
not only affect gas-phase chemistry, but also heterogeneous
ozone chemistry in polar regions. The model results indicate
an enhanced heterogeneous ozone depletion during antarctic
spring due to a longer PSC existence period. In contrast,
PSC formation in the northern hemisphere polar vortex and
therefore heterogeneous ozone depletion during arctic spring
are not affected by the water vapor increase, because of the
less PSC activity. Finally, this study shows that 10% of the
global total ozone decline in the transient model run can
be explained by the modeled water vapor increase, but the
simulated tropical ozone decrease after volcanic eruptions is
caused dynamically rather than chemically