Assessment of Degree-2 Zonal Gravitational Changes from GRACE, Earth Rotation, Climate Models, and Satellite Laser Ranging

Four independent time series of degree-2 zonal gravitational variations Delta C-20 are compared for the period April 2002 to February 2008. We examine estimates from the Gravity Recovery and Climate Experiment (GRACE), Earth rotation variations, climate models (AOW), and satellite laser ranging (SLR). At the annual period, all Delta C-20 estimates agree remarkably well, and good correlation is found among these time series at nonseasonal time scales as well. SLR and AOW Delta C-20 time series show the best agreement in a broad band of frequencies with the maximum cross-correlation coefficient of 0.86 at nonseasonal time scales. GRACE monthly Delta C-20 estimates are subject to significant aliasing effects due to errors in high-frequency tide models, especially the S-2 and K-2 tides. Correctly removing winds and ocean currents and other motion related excitations from length-of-day (LOD) observations plays a key role in estimating Delta C-20 from LOD, especially at interannual or longer time scales.