Regimes of rotating convection in an experimental model of the Earth's tangent cylinder

Abstract: Earth's fast rotation imposes the Taylor-Proudman Constraint that opposes fluid motion across an imaginary cylindrical surface called the Tangent Cylinder (TC) obtained by extruding the equatorial perimeter of the solid inner core along the rotation direction, and up to the core-mantle boundary (CMB). To date however, the influence of this boundary is unknown and this impedes our understanding of the flow in the polar regions of the core. We reproduce the TC geometry experimentally, where the CMB is modelled as a cold, cylindrical vessel, with a hot cylinder inside it acting as the inner solid core. The vessel is filled with water so as to optically map the velocity field in regimes of criticality and rotational constraint consistent with those of the Earth. We find that the main new mechanism arises out of the baroclinicity near the cold lateral boundary of the vessel, which drives inertia at the outer boundary of the TC, as convection in the equatorial regions of the Earth's core does. The baroclinicity just outside the TC suppresses the classical wall modes found in solid cylinder and the inertia there causes an early breakup of the TPC at the TC boundary. The flow remains dominated by the Coriolis force even up to criticality $\Rt=191$, but because of inertia near the TC boundary, geostrophic turbulence appears at much lower criticality than in other settings. The heat flux escapes increasingly through the TC boundary as the TPC becomes weaker. Hence inertia driven by baroclinicity outside the TC provides a convenient shortcut to geostrophic turbulence, which is otherwise difficult to reach in experiments. These results also highlight a process whereby the convection outside the TC may control turbulence inside it and bypass the axial heat transfer. We finally discuss how Earth's conditions, especially its magnetic field may change how this process acts within the Earth's core.