Abstract
Increasing ice loss of the Antarctic Ice Sheet (AIS) due to global climate change affects the orientation of the Earth’s spin axis with respect to an Earth-fixed reference system (polar motion). Here the contribution of the decreasing AIS to the excitation of polar motion is quantified from precise time variable gravity field observations of the Gravity Recovery and Climate Experiment (GRACE) and from measurements of the changing ice sheet elevation from altimeter satellites. While the GRACE gravity field models need to be reduced by noise and leakage effects from neighboring subsystems, the ice volume changes observed by satellite altimetry have to be converted into ice mass changes. In this study we investigate how much individual gravimetry and altimetry solutions differ from each other. We show that due to combination of individual solutions systematic and random errors of the data processing can be reduced and the robustness of the geodetic derived AIS polar motion excitations can be increased. We investigate the interannual variability of the Antarctic polar motion excitation functions by means of piecewise linear trends. We find that the long-term behavior of the three ice sheet subregions: EAIS (East Antarctic Ice Sheet), WAIS (West Antarctic Ice Sheet) and APIS (Antarctic Peninsula Ice Sheet) is quite different. While APIS polar motion excitations show no significant interannual variations during the study period 2003-2015, the trend of the WAIS and EAIS polar motion excitations increased in 2006 and again in 2009 while it started slightly to decline in 2013. AIS mass changes explain about 45% of the observed magnitude of the polar motion vector (excluding glacial isosatic adjustment). They cause the pole position vector to drift along 59^{circ } East longitude with an amplitude of 2.7 mas/yr. Thus the contribution of the AIS has to be considered to close the budget of the geophysical excitation functions of polar motion.
Highlights
In recent years, climate change has led to increasing ice loss of the Antarctic Ice Sheet (AIS) which has an significant impact on polar motion
In this study we use two data sets of equivalent water height (EWH) anomalies derived from ITSG-Grace2018 by using two different post-processing methods: (1) The method of tailored sensitivity kernels developed at TU Dresden (TUD) in the frame of the Antarctic Ice Sheet project of the European Space Agency’s (ESA) Climate Change Initiative (CCI) (Groh and Horwath 2016) and (2) the filter effect reduction approach on global grid point scale developed in the frame of the German Research Foundation (DFG) project CIEROT (Combination of geodetic space observations for estimating cryospheric mass changes and their impact on Earth rotation) at Deutsches Geodätisches Forschungsinstitut der TU München (DGFI-TUM) (Göttl et al 2019)
To assess the accuracy of the gravimetry derived polar motion excitations we use different Gravity Recovery and Climate Experiment (GRACE) gravity field models based on mass concentration parameters (CSR RL06M, JPL RL06M) or on spherical harmonics (ITSG-Grace2018)
Summary
Climate change has led to increasing ice loss of the Antarctic Ice Sheet (AIS) which has an significant impact on polar motion. Individual geophysical mass-related excitations of polar motion are mathematically described by equatorial angular momentum functions χ1e and χ2e (Barnes et al 1983; Gross 2015; Wahr 2005), where the index e denotes what kind of excitation mechanism is described by the equatorial angular momentum functions (e.g. AIS =: mass effect of the entire Antarctica, EAIS =: mass effect of the East Antarctic Ice Sheet, WAIS =: mass effect of the West Antarctic Ice Sheet, APIS =: mass effect of Antarctic Peninsula Ice Sheet, see Fig. 1) These so-called excitation functions are directly related to the fully normalized subsystem specific degree-2 potential coefficients C 2e1 and S2e1 which are derived from equivalent water heights ewh using the global spherical harmonic analysis (GSHA). Data and data processing This section provides an overview of the GRACE gravity field models and multi-mission satellite altimetry solutions which are used within this study to determine AIS mass changes and their impact on polar motion. The degree-1 coefficients of all gravity field solutions are replaced by estimates listed in the GRACE Technical Note 13 (Swenson et al 2008; Sun et al 2016) to take into account that a redistribution of masses is referred to a coordinate system attached to the Earth’s
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