Exploiting the combined GRACE/GRACE-FO solutions to determine gravimetric excitation of polar motion

Interpretation of variations in polar motion (PM) excitation due to global mass redistribution of atmosphere, oceans, hydrosphere and cryosphere is an important task that takes place on the boundary between geodesy and geophysics. The role of Earth’s surficial fluids in PM excitation is usually assessed by analysing time series of atmospheric, oceanic, hydrological and cryospheric angular momentum (AAM, OAM, HAM and CAM, respectively). With the launch of the Gravity Recovery and Climate Experiment (GRACE) and GRACE Follow-On (GRACE-FO) missions, a new era of using global gravity data to determine gravimetric excitation of PM has begun. This can be done due to linear relationship between degree-2 order-1 coefficients of geopotential and equatorial components of PM excitation.

A number of institutes around the world routinely process and deliver GRACE/GRACE-FO solutions; however, the non-negligible differences between PM excitation  estimates derived from data provided by various data centres exist. The choice of most appropriate GRACE/GRACE-FO solution for the purpose of HAM and CAM determination is a very complex task and depends on several factors like considered time period, analysed oscillation and assumed criterion of evaluation. The use of a combination of multiple GRACE/GRACE-FO solutions is a very common approach in various research tasks.

In this work, we create combined GRACE/GRACE-FO solutions to determine the total effect of HAM and CAM (HAM/CAM) on PM excitation. To do so, we use three-cornered hat method to estimate noise level of single GRACE/GRACE-FO solutions. We also consider unweighted and weighted mean of GRACE/GRACE-FO datasets as well as publicly available combined solutions such as that provided by the International Combination Service for Time-variable Gravity Field (COST-G). Our estimates of HAM/CAM based on the combined data are compared to the HAM/CAM determined from single solutions and to the hydrological plus cryospheric signal in observed (geodetic) excitation called geodetic residuals (GAO). As data form Satellite Laser Ranging (SLR) are also recommended to determine HAM/CAM, especially in the period of lack of GRACE or GRACE-FO measurements, we include SLR solutions to our analyses. Our work aims to check whether the combination of multiple solutions from different computing centres noticeable increases the compliance between HAM/CAM and GAO in several spectral bands like seasonal and non-seasonal oscillations.