Processing Choices Affect Ocean Mass Estimates From GRACE

Accurate estimates of ocean mass change are necessary to infer steric sea level change from sea level changes measured with satellite altimeters. Published studies using the Gravity Recovery and Climate Experiment (GRACE) satellite mission indicated a large range in trends (approximate to 1-2mm/year) with reported standard errors of 0.1-0.3mm/year. Here we show that a large part of this discrepancy (up to 0.6mm/year) can be explained by which model is used to account for the effect of glacial isostatic adjustment (GIA). The second largest contribution (0.3-0.4mm/year) is related to the way how different studies have restored atmospheric and oceanic signals which have been removed during the GRACE gravity estimation process. Here two processing strategies, which previously resulted in differing ocean mass trends, are considered. The direct method uses the standard GRACE Stokes coefficients, while the inverse method applies a joint inversion of data from GRACE and altimetry. After accounting for differences in processing corrections, global mean ocean mass estimates from the direct, the mascon, and inverse approach agree with each other on global scales within less than 0.1mm/year. Using the A et al. (2013; ) GIA model, we provide a reconciled monthly time series of global mean ocean mass, which suggests that ocean mass has increased by 1.43mm/year over 2002.6-2014.5, with an amplified rate of 1.75mm/year over 2002.6-2016.5 which covers almost the complete GRACE time span. However, we note that estimates as low as 1.05mm/year cannot be ruled out when other published GIA corrections with lower mass-equivalent signals over Antarctica are used. Plain Language Summary Since 1993, it is possible to estimate highly accurate global sea level rise (currently about 3.2mm/year) from satellite altimetry. However, partitioning the measured total sea level change into steric (volumetric) and mass contributions is challenging. Here we study the computation of the integrated mass contribution, that is, the sum of melting and mass imbalance of the ice sheets in Greenland and Antarctica, of the land glaciers, and variations in terrestrial water storages. We investigate two different approaches which previously provided diverging estimates of ocean mass change of up to 1mm/year: (1) the direct approach which estimates ocean mass change directly from satellite gravity data collected by the Gravity Recovery and Climate Experiment (GRACE) mission and (2) the inverse approach which combines the GRACE gravity data with satellite altimetry data in a joint estimation. We identify the reason for the differences between the approaches by showing that certain individual processing corrections were applied for each method differently. After applying these corrections consistently, ocean mass rates from both approaches agreed well within less than 0.1mm/year and we were able to provide a consistent ocean mass rate of 1.43mm/year for the period 2002-2014. These results have direct implications for estimating the global warming of the oceans.