Zonal winds reach in excess of 100 m/s in the middle atmosphere of Venus; the cloud-level atmosphere takes little more than 4 days to complete one rotation, while the solid planet below has a 243-day rotation period. This phenomenon, known as superrotation, is the central problem of the Venus atmosphere. The question we address is how the superrotation maintains itself against friction in the middle atmosphere, and in particular, what are the eddy processes that maintain equatorial superrotation. Also of interest in the cloud-top atmosphere is a feature known as the polar dipole, a pair of hot spots that orbit the pole with a period of about 3 days. Previous linear analyses have suggested that the dipole could be due to barotropic instability.
In the first part of the thesis, we examine the interaction of barotropically unstable polar Rossby waves with the mean cloud-top zonal circulation. Linear, quasi-linear, and nonlinear regimes are investigated in order to isolate the factors that control the properties of these waves. We find that the observed properties of the dipole at high latitudes on Venus may not be explained by barotropic instability theory. In the second part of the thesis we employ a zonally-truncated three-dimensional model of the Venus middle atmosphere. We find that the interaction of the diurnal and semidiurnal thermal tides with the zonal mean flow can maintain the observed cloud top superrotation. The model system tends toward a zonal wind pattern similar to that observed, despite model initializations that were either faster or slower than the final winds. Moreover, our model suggests that the origin of the observed midlatitude `5-day wave' is baroclinic instability within the lower cloud. Finally, the diurnal tidal winds have large amplitude at the cloud tops. This may distort the interpretation of cloud-tracked winds, which use dayside observations only.