Dynamics of polar vortices at cloud top and base on Venus inferred from a general circulation model: Case of a strong diurnal thermal tide

Polar vortices in the presence of a thermal tide are investigated using a Venusian middle atmosphere general circulation model. Around the cloud top, where the warm polar region is maintained by the thermal wind associated with a high latitude jet, the temperature contrast forms the polar vortex pattern. The cold collar and hot oval (monopole) near the pole are enhanced by the polar diurnal tide, and unstable vortices form the hot dipole and tripole. The centroid of the hot oval is displaced from the pole to around 80° by the diurnal tide. The hot dipole appears and breaks up into a tripole when transient vortical and divergent eddies with zonal wavenumbers 2 and higher are predominant within the polar hot oval region. Because the divergence and temperature are a quarter cycle out of phase with the eddy vorticity, the vortical eddies transport heat toward the cold region. Thus, the cloud-top polar vortices are mainly formed by a combination of the diurnal tide and transient baroclinic wave. At the cloud base, isotherms are almost zonally uniform and the eddy temperature structure is not apparent. In contrast, divergence and vorticity have large amplitudes within this region. The vortical eddies have a comma-shaped pattern, which is stably maintained and rotates with a period of about 5 days. The divergence and vorticity might be important in controlling cloud morphology at the cloud base via material transport.