Surface Mass Variations From GPS/Ocean Bottom Pressure Model and GRACE

We study seasonal surface mass variations and the resulting load-induced deformation as well as time-variable gravity. Monthly global GPS data and an altimetry-assimilated relative ocean bottom pressure model are inverted for spherical harmonic surface mass variations. The concurrent low-degree spherical harmonic time series are compared with corresponding GRACE-derived series. Geographic comparison of the independent surface mass variation results is carried out after applying a Gaussian spatial filter. Good agreements are seen in both spherical harmonic and geographical domains. We also obtain a joint solution of surface mass harmonics using GPS and GRACE data combination. The two independent data sets are highly complementary to each other. Combining them can achieve a more complete picture of the surface mass variations by including robust degree-1 surface mass estimates, which reflect the longest-wavelength hemispherical mass exchange, but are not included in the gravity solution. The joint solution also improves long-wavelength estimates and serves to cross-validate the two techniques from their overlapping strengths.