GRACE era variability in the Earth's oblateness: a comparison of estimates from six different sources

Abstract

We study fluctuations in the degree-2 zonal spherical harmonic coefficient of the Earth's gravity potential, C 20 , over the period 2003-2015. This coefficient is related to the Earth's oblateness and studying its temporal variations, C 20 , can be used to monitor large-scale mass movements between high and low latitude regions. We examine C 20 inferred from six different sources, including satellite laser ranging (SLR), GRACE and global geophysical fluids models. We further include estimates that we derive from measured variations in the length-of-day (LOD), from the inversion of global crustal displacements as measured by GPS, as well as from the combination of GRACE and the output of an ocean model as described by Sun et al. We apply a sequence of trend and seasonal moving average filters to the different time-series in order to decompose them into an interannual, a seasonal and an intraseasonal component. We then perform a comparison analysis for each component, and we further estimate the noise level contained in the different series using an extended version of the three-cornered-hat method. For the seasonal component, we generally obtain a very good agreement between the different sources, and except for the LOD-derived series, we find that over 90 per cent of the variance in the seasonal components can be explained by the sum of an annual and semiannual oscillation of constant amplitudes and phases, indicating that the seasonal pattern is stable over the considered time period. High consistency between the different estimates is also observed for the intraseasonal component, except for the solution from GRACE, which is known to be affected by a strong tide-like alias with a period of about 161 d. Estimated interannual components from the different sources are generally in agreement with each other, although estimates from GRACE and LOD present some discrepancies. Slight deviations are further observed for the estimate from the geophysical models, likely to be related to the omission of polar ice and groundwater changes in the model combination we use. On the other hand, these processes do not seem to play an important role at seasonal and shorter timescales, as the sum of modelled atmospheric, oceanic and hydrological effects effectively explains the observed C 20 variations at those scales. We generally obtain very good results for the solution from SLR, and we confirm that this well-established technique accurately tracks changes in C 20 . Good agreement is further observed for the estimate from the GPS inversion, showing that this indirect method is successful in capturing fluctuations in C 20 on scales ranging from intra-to interannual. Obtaining accurate estimates from LOD, however, remains a challenging task and more reliable models of atmospheric wind fields are needed in order to obtain high-quality C 20 , in particular at the seasonal scale. The combination of GRACE data and the output of an ocean model appears to be a promising approach, particularly since corresponding C 20 is not affected by t...

Improved geophysical excitation of length-of-day constrained by Earth orientation parameters and satellite gravimetry products

“…Chen et al 2007Chen et al , 2013bRiva et al 2009;Chambers & Willis 2010;Rio et al 2011;Schrama et al 2014;Ries 2015, personal communication). The reliability of SLR-derived C 20 has been demonstrated by Chen & Wilson (2008 and by Meyrath et al (2016). The SLR C 20 time-series solution used here is from M.K.…”

ITRF2014, Earth Figure Changes, and Geocenter Velocity: Implications for GIA and Recent Ice Melting

“…They also showed that these variations are consistent with the last decade observed long-term tendency of the geoid oblateness, the so-called J 2 parameter. J 2 time variations have been observed and studied for more than 30 years (e.g., Yoder et al, 1983;Cheng et al, 2011;Ivins et al, 1993;Meyrath et al, 2017;Mitrovica & Peltier, 1993;Nakada et al, 2016;Trupin, 1993). It is well accepted today that the J 2 long-term variations are most probably induced by glacial isostatic adjustment (GIA) and recent ice melting (RIM) (e.g., Ivins et al, 1993;Mitrovica & Peltier, 1993;Trupin, 1993;Cox and Chao, 2002;Dickey et al, 2002;Nerem & Wahr, 2011;Cheng et al, 2013;Nakada et al, 2015).…”

Using GRACE to Explain Variations in the Earth's Oblateness

“…However, a consideration of only elastic Earth's properties implies that viscoelastic solid Earth signals, such as glacial isostatic adjustment (GIA), are ignored. have demonstrated that a sufficiently accurate J 2 time series can be obtained from GRACE data supported by an oceanic C 20 coefficient time series, derived from ocean bottom pressure (OBP) variations by an ocean circulation model (Meyrath et al, 2017). The recently ended Gravity Recovery And Climate Experiment (GRACE) satellite mission monitored changes in the Earth's gravity field for almost 15 years .…”

“…For more than four decades, J 2 has been monitored by satellite laser ranging (SLR; Cheng et al, 2013). In the last 20 years, new data types, platforms, and methodologies have become available, and this enables comparisons between different solutions (Meyrath et al, 2017). Under the assumption that the Earth deforms elastically (Blewitt, 2003), seasonal J 2 variations can be derived from Global Positioning System-detected solid Earth deformations (Gross et al, 2004;Lavallée et al, 2010) or Earth's rotation variations (Chen & Wilson, 2003).…”

“…Unfortunately, GRACE estimates of J 2 variations are generally less accurate than those based on SLR observations (e.g., Bonin et al, 2018;Chen & Wilson, 2008;Talpe et al, 2017), partly due to thermal-dependent systematic errors in the accelerometers onboard the GRACE satellites (Cheng & Ries, 2017). have demonstrated that a sufficiently accurate J 2 time series can be obtained from GRACE data supported by an oceanic C 20 coefficient time series, derived from ocean bottom pressure (OBP) variations by an ocean circulation model (Meyrath et al, 2017). This approach, which is an extension of the method by Swenson et al (2008), will be from here on referred to as the GRACE-OBP approach.…”