Interannual variability of low-degree gravitational change, 1980-2002

Time variations in the Earth's gravity field at periods longer than 1 year, for degree-two spherical harmonics, Delta C-21, Delta S-21, and Delta C-20, are estimated from accurately measured Earth rotational variations. These are compared with predictions of atmospheric, oceanic, and hydrologic models, and with independent satellite laser ranging (SLR) results. There is remarkably good agreement between Earth rotation and model predictions of Delta C-21 and Delta S-21 over a 22-year period. After decadal signals are removed, Earth-rotation-derived interannual Delta C-20 variations are dominated by a strong oscillation of period about 5.6 years, probably due to uncertainties in wind and ocean current estimates. The model-predicted Delta C-20 agrees reasonably well with SLR observations during the 22-year period, with the exception of the recent anomaly since 1997/1998.