Improved geocentre motion estimates through the weighted combination of GRACE/GRACE-FO solutions and OBP models

SUMMARY Geocentre motion, defined as the displacement of Earth’s centre of mass relative to its centre of figure, is crucial for maintaining the International Terrestrial Reference Frame origin and quantifying large-scale mass redistribution. However, whether observing geocentre motion by tracking satellite orbits or inferring it using geophysical models, accurately acquiring such subtle motions imposes stringent requirements on the consistency and precision of both tracking data and geophysical models. This study improves geocentre motion estimates derived from the combination of GRACE/GRACE-FO time-variable gravity (TVG) and Ocean Bottom Pressure (OBP) models (the GRACE-OBP method) in two ways. First, we apply a forward modelling technique to mitigate land–ocean leakage in GRACE/GRACE-FO TVG fields, which demonstrably outperforms empirical coastline buffer-zone corrections in controlled simulation experiments. Secondly, we introduce the Bayesian Three-Cornered Hat (BTCH) method to optimally combine geocentre series derived from multiple GRACE solutions and two independent OBP models (Estimating the Circulation and Climate of the Ocean, Phase II and Max Planck Institute Ocean Model), producing an improved geocentre product without requiring a ground-truth reference. Uncertainty analysis shows that the noise level is governed primarily by the GRACE solution, and that BTCH provides a clearer advantage over equal-weighted averaging when the number of input series is limited, reducing the noise level by about 30 per cent. After restoring atmospheric and oceanic contributions, our improved geocentre series shows good agreement with the CSR Satellite Laser Ranging-derived geocentre product. Although uncertainty levels vary among individual solutions, the estimated annual and secular trend signals are broadly consistent and show limited sensitivity to the choice of GRACE TVG solution and OBP model. Using the improved geocentre series, we revisit the annual geocentre oscillation and its drivers; the results indicate that cryospheric mass variability and land–ocean mass exchange (i.e. sea-level fingerprints) provide non-negligible contributions to the annual geocentre cycle and improve consistency with observations. Finally, the improved geocentre series yields the lowest uncertainty in degree-1 mass variations, with a global RMS of 0.55 mm. Incorporating these degree-1 terms into mass budget assessments yields secular trends of 38.8 Gt yr−1 for the Antarctic Ice Sheet and 0.57 mm yr−1 for global mean ocean mass, highlighting the need for accurate geocentre corrections to support reliable long-term climate monitoring.