Assessment of degree-2 order-1 gravitational changes from GRACE and GRACE Follow-on, Earth rotation, satellite laser ranging, and models

We carry out a comprehensive analysis and assessment of degree-2 gravitational changes ΔC₂₁, and ΔS₂₁, estimated using the Gravity Recovery and Climate Experiment (GRACE) and GRACE Follow-On (GFO), satellite laser ranging (SLR), Earth Orientation Parameters (EOP), and geophysical models over the period April 2002–February 2020. The four independent estimates of ΔC₂₁ and ΔS₂₁ variations agree well over a broad band of frequencies. The GRACE/GFO Release 6 (RL06) solutions show major improvements over the previous RL05 solutions at both seasonal and intra-seasonal time scales, when compared with EOP and SLR estimates. Among the four independent estimates, highest correlation coefficients and smallest RMS residuals are found between GRACE/GFO and EOP estimates of ΔC₂₁ and ΔS₂₁ variations. GRACE/GFO and EOP ΔC₂₁ and ΔS₂₁ estimates exhibit slightly different trends, which are related to the implementation and interpretation of the pole tide correction in GRACE/GFO data processing. This study provides an important early validation of GFO ΔC₂₁ and ΔS₂₁ solutions, especially the new pole tide correction applied in GRACE/GFO RL06 solutions using independent estimates.