Torsional oscillations in the Earth core: analysis of the core angular momentum by multi-channel Singular Spectrum Analysis

Torsional oscillations in the axial angular momentum of the Earth's liquid core are investigated by dividing the core into twenty idealized equi-volume annuli coaxial with the axis of rotation of the Earth. The temporal variations in the axial angular momentum component of each annulus are determined from the core-mantle boundary (CMB) velocity field under the assumption of rigid rotation of each annulus. With the available velocity fields at the CMB as derived from geomagnetic variation observations (as calculated by A. Jackson), the general characteristics of axial core angular momentum (CAM) are explored over 15 decades (1840 - 1990). Five modes (with a mode being defined as a global oscillation with common variability among the 20 annuli) are isolated and are studied separately by using the Multi-channel Singular Spectrum Analysis. By perturbing the data using a set of technique, we have tested the robustness of the obtained mode. The three first modes have been shown quite robust, while the two last have only succeeded to part of the test implying that their existence needs to be verified by further study.