Long‐term variation in the cloud‐tracked zonal velocities at the cloud top of Venus deduced from Venus Express VMC images

Abstract

We present observational evidence of the variation of the cloud‐tracked zonal velocity by ~20 m s–1 with a timescale of a few hundred days in the southern low latitude region based on an analysis of cloud images taken by the Venus Monitoring Camera on board Venus Express. A spectral analysis suggests that the variation has a periodicity with a period of about 255 days. Although cloud features are not always passive tracers, the periodical variation of the dynamical state is a robust feature. Superposed on this long‐term variation of the zonal velocity, Kelvin wave‐like disturbances tend to be observed in periods of relatively slow background velocity, while Rossby wave‐like disturbances tend to be observed in periods of fast background velocity. Since the momentum deposition by these waves can accelerate and decelerate the mean flow, these waves may contribute to the suggested long‐term oscillation.