Abstract
Similar to seasonal and intraseasonal variations in polar motion (PM), interannual variations are also largely caused by changes in the angular momentum of the Earth’s geophysical fluid layers composed of the atmosphere, the oceans, and in-land hydrologic flows (AOH). Not only are inland freshwater systems crucial for interannual PM fluctuations, but so are atmospheric surface pressures and winds, oceanic currents, and ocean bottom pressures. However, the relationship between observed geodetic PM excitations and hydro-atmospheric models has not yet been determined. This is due to defects in geophysical models and the partial knowledge of atmosphere–ocean coupling and hydrological processes. Therefore, this study provides an analysis of the fluctuations of PM excitations for equatorial geophysical components χ1 and χ2 at interannual time scales. The geophysical excitations were determined from different sources, including atmospheric, ocean models, Gravity Recovery and Climate Experiment (GRACE) and GRACE Follow-On data, as well as from the Land Surface Discharge Model. The Multi Singular Spectrum Analysis method was applied to retain interannual variations in χ1 and χ2 components. None of the considered mass and motion terms studied for the different atmospheric and ocean models were found to have a negligible effect on interannual PM. These variables, derived from different Atmospheric Angular Momentum (AAM) and Oceanic Angular Momentum (OAM) models, differ from each other. Adding hydrologic considerations to the coupling of AAM and OAM excitations was found to provide benefits for achieving more consistent interannual geodetic budgets, but none of the AOH combinations fully explained the total observed PM excitations.
Highlights
The purpose of this study was to evaluate ability of geophysical excitation functions calculated from different models and Gravity Recovery and Climate Experiment (GRACE) data to close the “geodetic budget”: Geodetic Angular Momentum (GAM) = atmosphere + ocean (AO) + Hydrological Angular Momentum (HAM) at interannual time scales
In AO1 combination, the Oceanic Angular Momentum (OAM) ECCO is forced by Angular Momentum (AAM) NCEP/NCAR model
The geodetic and geophysical AO+HAM excitation models yielded better results when compared with GAM, than those with just the AAM and OAM models
Summary
Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations. Variations in the Earth’s rotation, in relation to the crust of our planet, is commonly expressed by the term polar motion (PM). Such rotations are mainly governed by the redistribution of masses and their motion within surficial fluid layers. These masses are composed of the atmosphere, the oceans, and inland freshwater systems including snow, ice, and soil moisture and ground water flows. Angular momentum exchanges between the Earth’s core and the mantle are responsible for rotation variations
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