Variations of the Earth's figure axis from satellite laser ranging and GRACE

Satellite laser ranging (SLR) data were used to determine the variations in the Earth's principal figure axis represented by the degree 2 and order 1 geopotential coefficients: C-21 and S-21. Significant variations at the annual and Chandler wobble frequencies appear in the SLR time series when the rotational deformation or "pole tides" (i.e., the solid Earth and ocean pole tides) were not modeled. The contribution of the ocean pole tide is estimated to be only similar to 8% of the total annual variations in the normalized coefficients: (C) over bar (21)/(S) over bar (21) based on the analysis of SLR data. The amplitude of the nontidal annual variation of (C) over bar (21) is only similar to 30% of (S) over bar (21) from the SLR time series. The estimates of the annual variation in (S) over bar (21) from SLR, the Gravity Recovery and Climate Experiment (GRACE) and polar motion excitation function, are in a good agreement. The nature of the linear trend for the Earth's figure axis determined by these techniques during the last several years is in general agreement but does not agree as well with results predicted from current glacial isostatic adjustment (GIA) models. The "fluid Love number" for the Earth is estimated to be similar to 0.9 based on the position of the mean figure axis from the GRACE gravity model GGM03S and the mean pole defined by the IERS 2003 conventions. The estimate of (C) over bar (21)/(S) over bar (21) from GRACE and SLR provides an improved constraint on the relative rotation of the core. The results presented here indicate a possible tilt of the inner core figure axis of similar to 2 degrees and similar to 3 arc sec displacement for the figure axis of the entire core.