  Sulfur-water chemistry plays an important role in the middle atmosphere of
Venus. Ground based observations have found that simultaneously observed SO2
and H2O at ~64 km vary with time and are temporally anti-correlated. To
understand these observations, we explore the sulfur-water chemical system
using a one-dimensional chemistry-diffusion model. We find that SO2 and H2O
mixing ratios above the clouds are highly dependent on mixing ratios of the two
species at the middle cloud top (58 km). The behavior of sulfur-water chemical
system can be classified into three regimes but there is no abrupt transition
among these regimes. In particular, there is no bifurcation behavior as
previously claimed. We also find that the SO2 self-shielding effect causes H2O
above the clouds to respond to the middle cloud top in a non-monotonic fashion.
Through comparison with observations, we find that mixing ratio variations at
the middle cloud top can explain the observed variability of SO2 and H2O. The
sulfur-water chemistry in the middle atmosphere is responsible for the H2O-SO2
anti-correlation at 64 km. Eddy transport change alone cannot explain the
variations of both species. These results imply that variations of species
abundance in the middle atmosphere are significantly influenced by the lower
atmospheric processes. Continued ground-based measurements of the co-evolution
of SO2 and H2O above the clouds and new spacecraft missions will be crucial for
uncover the complicated processes underlying the interaction among the lower
atmosphere, the clouds and the middle atmosphere of Venus.
