Oscillations in the Earth’s Figure axis from 50-year SLR data and polar motion

Summary The Earth’s figure axis is the axis of maximum inertia for the deformed (oblate) Earth, as described by the degree-two, order-one geopotential coefficients C21 and S21. An extended mean-pole model is presented for evaluating solid-Earth and ocean-pole tides. 50-year Satellite Laser Ranging (SLR) data and 24-year GRACE/GRACE-FO data were analyzed to determine variations in Earth’s figure axis, as reflected in changes in the C21 and S21 coefficients. This study reveals that a significant atmosphere-ocean motion induced a variation in C21 that is captured by SLR data but does not appear in the GRACE solution. The current glacial isostatic adjustment (GIA) ICE-6G model requires improvement to account for the observed linear rates of C21 and S21. A significant 30-year and 60-year signal with an amplitude of ∼ 3×10−11 in the Earth’s figure axis is observed using SLR and a ∼10 mas in the PM (polar-motion), which could be predominantly driven by a 0.05-degree tilt of the inner-core figure axis relative to the figure axis of the entire core and is linked to partial electromagnetic core-mantle coupling.