Zonal Flow and Vortices in Anelastic Deep Convection Models of Jupiter and Saturn With Shallow Stable Stratification

Abstract:

Planetary jet streams and vortices have been studied for over 350 years, yet their origin and dynamics are still vigorously debated. On both Jupiter and Saturn zonal flow consists of equatorial superrotation and alternating East-West jets at higher latitude. On Jupiter, numerous vortices, the vast majority anticyclones, occur with various sizes and lifetimes, interacting strongly with the zonal flow. Saturn's vortices and jets are also clearly coupled, and its North and South polar vortices are cyclonic. Models of giant planet atmospheres have generally been of two classes. Shallow flow models produce jets and vortices from 2D turbulence in a very thin spherical layer, but require special conditions to reproduce observed equatorial superrotation. In contrast, deep convection models generically reproduce equatorial superrotation, but typically lack coherent vortices, which do not survive the formation of jets. Here, we combine elements of both approaches using a 3D spherical shell compressible fluid numerical model, driven by convection at depth, but grading to a stably stratified shallow layer. In typical model simulations convective plumes rising from the deep interior impinge on the stably stratified layer, diverge near the outer spherical surface, and efficiently create the dominant anticyclones, which are shielded by downwelling cyclonic rings and filaments. These results may explain the dominance of anticyclones and the flow structure of small and medium sized anticyclonic ovals on Jupiter. The largest of our model vortices form in westward anticyclonic shear nearest the equatorial jet, similar to Saturn’s "storm alley" and Jupiter’s Great Red Spot. We also explore conditions under which cyclones, including polar cyclones like those on Saturn, may form.