Thermal coupling mode in mantle-outer core convection predicted from an ultra-high-resolution numerical simulation of two-layer convection with a large viscosity contrast

Abstract

Previous global mantle seismic tomography analyses have revealed the large-scale horizontal structure of the mantle. On the other hand, the large-scale horizontal structure of the outer core could not be well determined using seismic wave analysis due to its liquid nature. Therefore, at present, numerical simulations are the only method for understanding the large-scale horizontal structure of the outer core. Previous numerical studies on two-layer Rayleigh-Bénard convection with an infinite Prandtl number have shown that the coupling mode between the two layers changes from mechanical coupling to a transitional mode to thermal coupling as the viscosity contrast between the two layers increases. This study presents an ultra-high-resolution numerical simulation of two-layer convection with a viscosity contrast of 10⁴. The effective Rayleigh number of convection in the inner low-viscosity layer is approximately 2 × 10¹⁰. A spatiotemporal analysis of convection confirmed a new thermal coupling mode in the two-layer convection, primarily driven by downwelling plumes. When applied to the coupling between the mantle and outer core of Earth's interior, whose geophysical and geochemical structures are considered nearly hemispherical relative to the axis of rotation, this coupling mode effectively cools Earth’s core and releases heat from the interior to the surface.