Length-of-day, geostrophic motions in the core, and the conductance of the mantle

Numerical simulations of the geodynamo are used to investigate the transfer of angular momentum between the core and the mantle, in the presence of both an electro-magnetic torque and a gravitational torque between the inner core and the mantle. Over a broad range of periods, angular momentum changes are accurately explained when deriving geostrophic motions (organized as cylinders co-axial with the Earth’s rotation vector) from core surface motions. Inverted core surface flows can thus be used to infer the core angular momentum budget. In the dynamo, the largest part of the EM torque variations is associated with the evolution of the solid body rotation, with little contribution from electrical currents originating deep inside the core (the "leakage torque”, formerly associated with the westward drift of the geomagnetic field). A 1D model of geostrophic motions is furthermore used to estimate the efficiency of torsional eigenmodes to couple with the solid Earth. From this we revisit the estimate of the mantle conductance from 1D simulations of geostrophic motions stochastically forced in the volume. We then discuss the ability of torsional waves to trigger the interannual oscillations observed in the length-of-day series, and the relative importance of forcing versus resonances in angular momentum changes at various time-scales.
The 28th IUGG General Assembly (IUGG2023) (Berlin 2023)