Seasonal and interannual global surface mass variations from multisatellite geodetic data

[ 1] Monthly global surface mass distribution changes are estimated in the spherical harmonic domain with a complete spectrum up to degree and order 50. The estimates are derived by inverting GPS displacement series measured at roughly 450 continuously tracking sites and ocean bottom pressure (OBP) estimates of a data assimilated ocean circulation model from 1993 to 2004. The inversion uses a hybrid estimator after singular value decomposition of the normalized measurement equations with reduced reliance on a priori spectral information. The results are then compared and combined with Gravity Recovery and Climate Experiment ( GRACE) gravity data to provide enhanced spectral and geographic coverage. High fidelity degree-1 surface mass variation coefficients are recovered, corresponding to equivalent geocenter motion with better than 0.5 mm annual amplitude precisions in all three components. A clear annual surface mass cycle occurs in both GPS/OBP- and GRACE-derived results. There is very good agreement among the low-degree spherical harmonic coefficients and in global geographic pattern but with significant regional differences in amplitudes. Annual variations of average mass over the global oceans, Antarctica, and Greenland are then derived. Total surface mass over both Antarctica and Greenland peak in their respective summers due to increased atmospheric mass. Large interannual variations have also been found involving several continents in the 11-year GPS/OBP solution. The patterns of variation suggest that the recent satellite laser ranging (SLR) observation of a reversal in trend of the Earth's oblateness may be largely an interannual surface mass cycle with considerable contribution from the northern continents.