Effects of neutral buoyancy outer boundary condition in models of deep convection in giant planets

Abstract

Cloud motions have revealed strong east–west jet streams on Jupiter and Saturn. Two different paradigms have emerged for the origin of these jets. In the shallow dynamics paradigm zonal flow is driven largely by solar radiation and weather dynamics in the troposphere. In the deep convection paradigm forcing is thought to originate primarily via heat flow from the deep interior. Most previous anelastic models that focus on the development of deep zonal flows implement a constant entropy difference from the inner to outer boundary with no internal heat sources. For models with strong density gradients these constant entropy boundary conditions yield strong convective forcing at the outer boundary. However, observations indicate that giant planet tropospheres are stably stratified or perhaps near neutrally buoyant. Based on these observations we present here a deep convection parameter study with a zero entropy gradient (neutral stability) outer boundary condition and a uniform internal heat sink. We find that near convective onset, a neutrally buoyant shallow layer modifies the flow strongly, pushing active convection to occur near the bottom boundary. Furthermore, deeply seated jets develop efficiently, even with relatively weak forcing. Thus, for near critical and weak forcing the zero entropy gradient top boundary condition yields flows that resemble Boussinesq results, even for strong density gradients. For strongly convecting flows, the jet structure is more similar to cases with constant entropy difference. However, with strong convective mixing, the zero entropy gradient outer boundary condition results in relatively extensive regions of dynamical stability (reversed entropy gradient). Under these conditions, surface flows can develop with larger scale non-zonal features and vorticity structures that have planetary characteristics.