Planetary-Scale Waves and the Cyclic Nature of Cloud Top Dynamics on Venus

Abstract

Pioneer Venus OCPP ultraviolet images spanning eight years have been analyzed objectively to derive quantitative information on the properties of planetary-scale wave modes at the Venus cloud tops. We infer propagation characteristics for longitudinal wavenumber 1 by Fourier analyzing time series of longitudinal mean normalized image brightness. The dominant equatorial mode during 1979?80 was the 4-day periodicity associated with zonal motion of the Y-feature. The difference between this and the 4.7-day equatorial rotation period derived from the tracking of small cloud features implies that the Y is a propagating wave with a prograde phase speed of about 15 ms?1 relative to the wind. Simultaneous time series of cloud-tracked wind fluctuations also exhibit a 4-day periodicity, lending support to the wave interpretation. The prograde propagation and equatorial confinement of the wave, and the absence of analogous meridional wind fluctuations, identify it as a Kelvin wave. Zonal winds peak near the leading edge of the Y; gravity wave theory then implies that dark UV features at low latitudes are cold and produced by upwelling or convection associated with the wave. In 1982?83 the Kelvin mode was very weak or absent, replaced by a 5-day equatorial periodicity in brightness that is not significantly different from the 5.0-day cloud-tracked wind rotation period recorded during those years. Zonal wind fluctuations for 1982 show no obvious spectral peak, suggesting that brightness variations at this time are due to advection of a remnant albedo pattern rather than active wave propagation. The Kelvin wave amplitude and implied propagation characteristics suggest that it dissipates at the cloud tops and contributes significantly to the maintenance of the cloud top equatorial superrotation. The disappearance of the Kelvin wave between 1980 and 1982 may therefore explain the coincident 5?10 ms?1 decline in the equatorial zonal wind. The 1985?86 images indicate a return of the 4-day brightness periodicity and a restoration of equatorial wind speeds similar to those in 1979?80. Thus, the cloud level dynamics may be cyclic, with an apparent time scale of 5?10 years. A separate midlatitude planetary-scale transient mode with a period near 5 days also occurs when the 4-day equatorial wave is present. The midlatitude mode retrogrades with respect to the zonal wind and may be a slowly rotating analog to an internal Rossby-Haurwitz wave generated by shear instability of the midlatitude jet. If so, it too may accelerate the equatorial wind. Solar-locked diurnal and semidiurnal tidal modes are also present in both the brightness and cloud-tracked wind data during all imaging periods; their amplitudes appear to be similar to that of the equatorial Kelvin wave. The long-term evolution and maintenance of the Venus cloud top superrotation may therefore reflect a complex balance among at least four eddy momentum transport mechanisms.