Abstract
From 2002 to 2017, the Gravity Recovery and Climate Experiment (GRACE) mission’s twin satellites measured variations in the mass redistribution of Earth’s superficial fluids, which disturb polar motion (PM). In this study, the PM excitation estimates were computed from two recent releases of GRACE monthly gravity field models, RL05 and RL06, and converted into prograde and retrograde circular terms by applying the complex Fourier transform. This is the first such analysis of circular parts in GRACE-based excitations. The obtained series were validated by comparison with the residuals of observed polar motion excitation (geodetic angular momentum (GAM)–atmospheric angular momentum (AAM)–oceanic angular momentum (OAM) (GAO)) determined from precise geodetic measurements of the pole coordinates. We examined temporal variations of hydrological excitation function series (or hydrological angular momentum, HAM) in four spectral bands: seasonal, non-seasonal, non-seasonal short-term, and non-seasonal long-term. The general conclusions arising from the conducted analyses of prograde and retrograde terms were consistent with the findings from the equatorial components of PM excitation studies drawn in previous research. In particular, we showed that the new GRACE RL06 data increased the consistency between different solutions and improved the agreement between GRACE-based excitation series and reference data. The level of agreement between HAM and GAO was dependent on the oscillation considered and was higher for long-term than short-term variations. For most of the oscillations considered, the highest agreement with GAO was obtained for CSR RL06 and ITSG-Grace2018 solutions. This study revealed that both prograde and retrograde circular terms of PM excitation can be determined by GRACE with similar levels of accuracy. The findings from this study may help in choosing the most appropriate GRACE solution for PM investigations and can be useful in future improvements to GRACE data processing.
Highlights
Polar motion (PM) is disturbed by many processes with diverse temporal variability ranging from several days to many decades [1]
With the new Gravity Recovery and Climate Experiment (GRACE) solutions, only the Jet Propulsion Laboratory (JPL) and GFZ series revealed a reduction in amplitudes, whereas very little amplitude change was detected for other GRACE data
We showed an alternative method of presenting the hydrological polar motion excitation function
Summary
Polar motion (PM) is disturbed by many processes with diverse temporal variability ranging from several days to many decades [1]. For time scales of a few years or less, the major contributors to changes in PM are angular momentum changes induced by mass redistribution of Earth’s surficial fluids (atmosphere, ocean, and land hydrosphere). These contributions are described as PM excitation functions or angular momentum functions, namely atmospheric angular momentum (AAM), oceanic angular momentum (OAM), and hydrological angular momentum (HAM), and can be determined with several geophysical models. The main reason for the discrepancies between estimations of HAM by different models is the differences in meteorological model forcing data, processing algorithms, temporal and spatial resolution, and the number of parameters estimated [16]. Other geophysical effects, such as earthquake-induced co- and post-seismic deformations [17] or Earth’s core–mantle coupling [18], are usually not considered by models in a rigorous way
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