Impact of assimilating bottom pressure anomalies from GRACE on ocean circulation estimates

The impact of assimilating global ocean bottom pressure (OBP) information from the Gravity Recovery and Climate Experiment (GRACE) gravity anomalies on the circulation estimate was investigated. For this an estimate of the ocean circulation is being inferred by extending the 50-year-long German part of the Estimating the Circulation and Climate of the Ocean (GECCO) ocean synthesis into recent years. The assimilation system is an improved version of the previous GECCO optimization, which now includes a sea ice model, has enhanced resolution on a truly global domain including the Arctic Ocean. By analyzing differences to a synthesis that additionally assimilated OBP, the GRACE data was found to provide complementary information to standard ocean data sets including satellite altimetry when assimilated. Although in principle standard ocean data sets include the OBP information, the reason why this cannot be extracted is the much larger prior errors for hydrographic and altimeter data in comparison to OBP data owing to the fact that only the former two need to include the unresolved eddy signal. The largest impact of gravity data is found to be on the barotropic circulation, particularly in the subtropical gyres and the polar latitudes. Remaining differences between the simulated and observed OBP information are associated with meridional stripes in the GRACE gravity maps and with the leakage of terrestrial hydrological information into the ocean. Additional differences close to the continental boundaries are related to the self-attraction and loading, processes that are not included in the models.