Latitudinal regionalization of rotating spherical shell convection

Convection occurs ubiquitously on and in rotating geophysical and astrophysical bodies. Prior spherical shell studies have shown that the convection dynamics in polar regions can differ significantly from the lower latitude, equatorial dynamics. Yet most spherical shell convective scaling laws use globally-averaged quantities that erase latitudinal differences in the physics. Here we quantify those latitudinal differences by analysing spherical shell simulations in terms of their regionalized convective heat-transfer properties. This is done by measuring local Nusselt numbers in two specific, latitudinally separate, portions of the shell, the polar and the equatorial regions, Nu(p) and Nu(e), respectively. In rotating spherical shells, convection first sets in outside the tangent cylinder such that equatorial heat transfer dominates at small and moderate supercriticalities. We show that the buoyancy forcing, parameterized by the Rayleigh number Ra, must exceed the critical equatorial forcing by a factor of approximate to 20 to trigger polar convection within the tangent cylinder. Once triggered, Nu(p) increases with Ra much faster than does Nu(e). The equatorial and polar heat fluxes then tend to become comparable at sufficiently high Ra. Comparisons between the polar convection data and Cartesian numerical simulations reveal quantitative agreement between the two geometries in terms of heat transfer and averaged bulk temperature gradient. This agreement indicates that rotating spherical shell convection dynamics is accessible both through spherical simulations and via reduced investigatory pathways, be they theoretical, numerical or experimental.

Automated summary

Convection dynamics in polar regions differ from those in lower latitudes. In spherical shell simulations, polar convection is triggered when the buoyancy forcing exceeds the critical equatorial forcing by a factor of 20. The heat transfer in polar regions increases much faster than in equatorial regions with increasing Rayleigh number. At high Rayleigh numbers, the heat fluxes in polar and equatorial regions become comparable.