Twenty years of volume transport from satellite gravimetry in the Atlantic and Southern Ocean

With an observation period of almost twenty years and global data coverage, satellite gravimetry has become a crucial tool for monitoring the state of our planet in a changing climate. Gravimetry-derived mass change has seen numerous applications in different geoscientific disciplines and has fundamentally improved our understanding of the Earth system. One such application is the determination of meridional and zonal volume transport variability based on ocean bottom pressure (OBP) variations, which can provide key insights into climate-relevant ocean currents like the Atlantic Meridional Overturning Circulation (AMOC) or the Antarctic Circumpolar Current (ACC). However, the limited spatial resolution, signal leakage from other geophysical subsystems like the hydrosphere, cryosphere or solid Earth make satellite gravimetry-derived transport estimates difficult to interpret. In this study we investigate geostrophic volume transport variability based on observations of the Gravity Recovery and Climate Experiment (GRACE) and GRACE Follow-On (GRACE-FO) for selected cross sections in the Atlantic and Southern Ocean. We focus on interannual transport variations in the deep ocean, where the more moderately sloping topography poses less stringent requirements on the spatial resolution of the OBP fields, and the lower temporal resolution reduces the impact of observation noise by providing longer averaging periods. Basis for the derived transport variations are high-resolution OBP fields determined in an ensemble Kalman filter approach. This allows us to also propagate the inherent observational noise to transport level and together with glacial isostatic adjustment (GIA) und hydrological model statistics quantify the uncertainty and sensitivity of the derived transport time series. We further contrast results for the Atlantic and Southern Ocean and show the different impact of the satellite observation geometry on meridional and zonal transport estimates.