Atmospheric mountain wave generation on Venus and its influence on the solid planet's rotation rate

The Akatsuki spacecraft observed a 10,000-km-long meridional structure at the top of the cloud deck of Venus that appeared stationary with respect to the surface and was interpreted as a gravity wave. Additionally, over four Venus solar days of observations, other such waves were observed to appear in the afternoon over equatorial highland regions. This indicates a direct influence of the solid planet on the whole Venusian atmosphere despite dissimilar rotation rates of 243 and 4 days, respectively. How such gravity waves might be generated on Venus is not understood. Here, we use general circulation model simulations of the Venusian atmosphere to show that the observations are consistent with stationary gravity waves over topographic highs-or mountain waves-that are generated in the afternoon in equatorial regions by the diurnal cycle of near-surface atmospheric stability. We find that these mountain waves substantially contribute to the total atmospheric torque that acts on the planet's surface. We estimate that mountain waves, along with the thermal tide and baroclinic waves, can produce a change in the rotation rate of the solid body of about 2 minutes per solar day. This interplay between the solid planet and atmosphere may explain some of the difference in rotation rates (equivalent to a change in the length of day of about 7 minutes) measured by spacecraft over the past 40 years.