Formation and maintenance of the 4-day circulation in the Venus middle atmosphere

The Venus 4-day circulation, with wind speeds 60 times greater than the surface rotation, is located at the top of the cloud layer (65-70 km) in the middle atmosphere of the planet. This dramatic phenomenon is related to the combination of two processes, the meridional circulation induced by solar heating in the cloud layer and its associated poleward momentum transport, and the additional momentum from a 4-day wave originating in the equatorial lower atmosphere. Numerical simulations are conducted for two cases of 4-day wave forcing at the lower boundary of the middle atmosphere. If no wave is forced, no significant zonal-mean wind appears in the equatorial middle atmosphere, while a midlatitudinal jet develops due to centrifugal force. When an appropriate range of the 4-day wave forcing is supplied, a large zonal-mean flow can be realized from the equator to midlatitudes. Planetary Rossby waves having periods of 5-6 days are likely to be generated in baroclinically unstable regions. An ultraviolet cloud pattern appears at the cloud top, forming a dark horizontal Y-shaped pattern. The brightness of the pattern oscillates with a 4-day period at low latitudes and a 5-day period at midlatitudes. The wave pattern, which is formed by the combined effect of the forced 4-day equatorial wave and newly generated planetary Rossby wave, maintains the constant Y-shaped pattern. Adjusted superposition of the two waves plays an important role in the formation of the Y-shaped cloud pattern.