Hydrological excitations of polar motion from GRACE gravity field solutions

Abstract

We use the latest time-variable Earth gravity field harmonics from the GRACE satellite mission (Release 5 – RL05) to determine seasonal and nonseasonal scales of polar motion excitation functions from global geophysical fluids, and particularly from the portion from land-based hydrology. Hydrological excitation functions of polar motion from the mass of equivalent water thicknesses (EWT) derived gravimetrically from the solutions of three GRACE processing centers, the Center for Space Research (CSR), the Jet Propulsion Laboratory (JPL) and the GeoforschungsZentrum (GFZ), are intercompared. Additionally, we estimate the hydrological signal as well in a different manner, as a residual from geodetically observed polar motion, by subtracting atmospheric (pressure + wind) and oceanic (bottom pressure + currents) contributions. Among the gravimetric excitation functions based on RL05 there are still significant differences between center results which we attribute mainly to residual signals over ocean areas. It appears that the CSR processing may lead to stronger agreement between the hydrological signal and residual values, surely due to details in the processing, like resolutions to different degree and order used in the static background gravity model.