Zonal winds in the gas planets driven by convection above a stably stratified layer

The analysis of the recent gravity measurements of Jupiter and Saturn reveal that the zonal winds observed on their surfaces reach several thousand kilometres deep into their atmospheres. However, it remains unclear which mechanism prevents them from penetrating deeper. Recent models suggest that a stably stratified region would yield the desired effect. In this numerical study we systematically explore the dynamics of flow in a rotating spherical shell where the lower third is stably stratified while convection in the outer region drives multiple zonal winds, similar to those observed on Jupiter or Saturn. When using a rigid lower boundary condition, only an equatorial jet pair is formed without an underlying stable layer. When including a stable layer, fierce multiple jets also develop at mid to high latitudes, once the stable stratification is strong enough to effectively decouple the jet dynamics from the lower boundary. We find that the decay of the jet amplitude near the stable layer boundary is controlled by Ω/N, where Ω is the rotation rate and N the Brunt-Väisälä frequency that quantifies the degree of stable stratification. Furthermore, the penetration distance of the jets is proportional to the jet width. In the convective region, the winds are invariant along the axis of rotation, whereas their extension in the stable layer tends to become radially aligned.