Penetration of boundary-driven flows into a rotating spherical thermally-stratified fluid

Abstract: Motivated by the dynamics within terrestrial bodies, we consider a rotating, strongly thermally stratified fluid within a spherical shell subject to a prescribed laterally inhomogeneous heat-flux condition at the outer boundary. Using a numerical model, we explore a broad range of three key dimensionless numbers: a thermal stratification parameter (the relative size of boundary temperature gradients to imposed vertical temperature gradients), $10^{-3} \le S \le 10^{4}$, a buoyancy parameter (the strength of applied boundary heat flux anomalies), $ 10^{-3} \le B \le 10^{6}$, and the Ekman number (ratio of viscous to Coriolis forces), $10^{-6} \le E \le 10^{-4}$. We find both steady and time-dependent solutions and delineate the temporal regime boundaries. We focus on steady-state solutions, for which a clear transition is found between a low $S$ regime, in which buoyancy dominates dynamics, and a high $S$ regime, in which stratification dominates. For the latter case, the radial and horizontal velocities scale respectively as $u_r \sim S^{-1}$, $u_h \sim S^{-\frac{3}{4}}\ B^{\frac{1}{4}}$ and are confined to boundary-induced flow within a thin layer of depth $(S\ B)^{-\frac{1}{4}}$ at the outer edge of the domain. For the Earth, if lower-mantle heterogeneous structure is due principally to chemical anomalies, we estimate that the core is in the high-$S$ regime and steady flows arising from strong outer-boundary thermal anomalies cannot penetrate the stable layer. However, if the mantle heterogeneities are due to thermal anomalies and the heat-flux variation is large, the core will be in a low-$S$ regime in which the stable layer is likely penetrated by boundary-driven flows.