Core-mantle topographic torque: a spherical harmonic approach and implications for the excitation of the Earth's rotation by core motions

Abstract

The interaction of the fluid pressure over the topography of the core mantle boundary generates a torque that leads to perturbations in the Earth's rotation. A general formulation of this topographic torque is proposed with the help of a development in spherical harmonics of the pressure and shape. Various implications of this coupling mechanism are discussed according to the symmetry properties of the dynamic pressure field at the core surface and the distribution of the topographical deviations (bumps) of the core mantle boundary inferred from seismology. Specifically, it is shown that different interaction terms contribute to the equatorial torque; although their sum cannot be precisely quantified using present knowledge, it could be sufficient to excite the Earth's wobble. The axial torque contribution seems to be too large to explain the decade fluctuations of the Earth's rotation rate, suggesting the existence of a mechanism which reduces the axial resulting torque without altering the equatorial one.