Estimating Global Ocean Mass Change from Low-Degree Gravity Field

The Global Mean Ocean Mass (GMOM) change reflects the large-scale mass transport between land and oceans, which has been directly and accurately measured by GRACE and GRACE-FO since 2002. However, for the pre-GRACE era, the estimation of GMOM is mainly based on the mass budget approach by combining individual mass change estimates of land water storage, glaciers, and ice sheets from geophysical models and remote sensing data, which lacks a constraint on the consistency between different components. Prior to GRACE, time-varying gravity fields can also be determined from SLR and DORIS, but their spatial resolution is generally limited to about degrees and orders (d/o) 5, which cannot be used directly to infer regional mass variations, even for regions as large as the global ocean, due to strong signal leakage. In this contribution, we present promising results using the d/o 5 SLR gravity fields for GMOM estimation after appropriately addressing signal leakage. The leakage-corrected GMOM estimates are compared with those from GRACE (d/o 60) between 2005 and 2015, a period when GRACE is considered most reliable, and the results show that they are in remarkable agreement for GMOM change rates. Our proof-of-concept study provides the possibility of using SLR/DORIS-derived low-degree gravity fields to track GMOM changes back to the 1990s, and to cross-validate GRACE/GRACE-FO estimates for the single accelerometer period after 2016.