Variation of the equatorial moments of inertia associated with a 6-year westward rotary motion in the Earth

The westward-propagating rotary wave-2 displacement field in the mantle with the period of 6 years (Ding and Chao, 2018) predicts an ensuing 6-year variation in the Earth's two equatorial principle moments of inertia. In this paper we search for such signals in the sectoral Stokes coefficients C₂₂ and S₂₂ of the Earth's gravitational field. We analyze the monthly ΔC₂₂ and ΔS₂₂ data series derived from the satellite laser-ranging measurements for 1992-2018 (and the shorter series from the GRACE satellite, 2002-2017). We report unequivocal evidences found in ΔC₂₂ and ΔS₂₂ that conform to the predicted scenario in both amplitude and relative phasing. Conversely, this further corroborates the 6-year westward wave-2 motion in the Earth.