Planetary Circulations: 2. The Jovian Quasi-Geostrophic Regime

The characteristics of the two-level quasi-geostrophic model are evaluated for a wide range of parameter values in the Jovian domain. The results support the hypothesis that baroclinic instability energizes the circulation of Jupiter and Saturn and that the blocking effect of planetary wave propagation on quasi-geostrophic turbulent cascades determines the width and zonality of the bands—the degree of zonality being higher in the absence of surface drag. The model circulations consist of multiple westerly jets, separated by strong easterly flows—the result of momentum partitioning by the Kuo vortex separation process. There are no large-scale vertical motions. A cyclic variation occurs (with a time scale of several years) during which phases with intense, large-scale baroclinic activity alternate with longer, more quiescent phases involving weak, small-scale baroclinic instability and neutral baroclinic waves. These neutral waves, generated by quasi-two-dimensional cascades and propagating at speeds of O(1 m s−1), provide the major mode of adjustment in the quasi-steady phase and form the gyres endemic to multiple jet circulations. Similar large-scale motions occur for all the parameter values considered: for weak and strong static-stabilities, for eddy sizes ranging from 2000–9500 km and for pole-to-equator temperature differences varying from 5–90 K. The weak thermal gradients maintain strong dynamical activity by their association, in geostrophic motion, with the large value of the specific-heat constant for hydrogen. For Jupiter, a correspondence between the theoretical perturbation pressure and the observed planetary-scale features suggests that condensation processes related to the geostrophically balanced pressure variations produce the main cloud bands and Great Red Spot, while local temperature changes due to baroclinic instability and frontogenesis create the eddy cloud systems embedded within the main bands. An analogy between the Great Red Spot and the warm high-pressure region of a neutral baroclinic wave leads us to suggest that the scale selectivity and energy source of ultralong, baroclinically unstable waves could explain the size and persistence of the Jovian feature.