Polar motion of a 3D viscoelastic earth model: Consequences for GIA signals in GRACE-FO

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Surface deformations due to changes in the rotation of the Earth are significantly impacted by glacial isostatic adjustment (GIA). The long-term trend of polar motion contributes to global observations like that of the current satellite gravity mission GRACE-FO. The theory and how to apply this contribution to correct GRACE observational data is well understood and goes back to the concise studies of Mitrovica et al. (2005) and Wahr et al. (2015), respectively. According to the International Earth Rotation Service (IERS), a standard correction method is suggested, where the observed long-term trend of the polar motion is considered to originate from GIA. Recent studies show that the modelled GIA contribution to polar motion strongly depends on structural features of the Earth's interior as well as on the glacial history. Other processes like mantle convection or more recent climatic processes are attributed to contribute as well (Adhikari et al. 2018). In this presentation we focus on the impact of the Earth's viscosity structure on the modelled polar motion. In addition to its radial stratification, we discuss the influence of lateral variability. We apply the numerical 3D viscoelastic lithosphere and mantle model VILMA, which solves the gravitationally self-consistent field equations in a spherical geometry, and which considers the rotational feedback and the sea-level equation. The theory of Martinec and Hagedoorn (2014) applied here is not based on the normal mode theory, but solves the field equations in the time domain. We show the consistency of the chosen approach and rate the influence of lateral changes in viscosity against the impact of radial viscosity stratification. The study was motivated by the ESA Third Party Mission 'GRACE-FO' and contributes to the German Climate Modelling Initiative 'PalMod'. Lit:Adhikari, S, Caron L, Steinberger, B, ..., Ivins, ER (2018). What drives 20th century polar motion? Earth Planet. Sci. Lett. doi:10.1016/j.epsl.2018.08.059Martinec, Z, Hagedoorn, JM (2014). The rotational feedback on linear-momentum balance in glacial isostatic adjustment. Geophys. J. Int. doi:10.1093/gji/ggu369Mitrovica, JX, Wahr, J, Matsuyama, I, Paulson, A (2005). The rotational stability of an ice-age earth. Geophys. J. Int. doi:10.1111/j.1365-246X.2005.02609.xWahr, J, Nerem, RS, Bettadpur, SV (2015). The pole tide and its effect on GRACE time-variable gravity measurements: Implications for estimates of surface mass variations. J. Geophys. Res. Solid Earth. doi:10.1002/2015JB01198