North Atlantic Meridional Overturning Circulation: Signatures in Ocean Bottom Pressure and the Earth’s Time-Variable Gravity Field

The North Atlantic Meridional Overturning Circulation (AMOC) comprises a net northward transport of relatively warm water at depths < 1000 m that is compensated by a colder net southward return flow in the abyss. Fluctuations in these transport variables induce weak but measurable changes in ocean bottom pressure that are, in principle, observable from space with dedicated satellite gravity missions. Based on 5 decades of data from the high-resolution ocean model VIKING20X of the North Atlantic (Biastoch et al., 2021), we will outline the dynamical relations that allow us to infer AMOC variations using ocean bottom pressure changes from either the basin interior with depths up to 5000 m (Landerer et al., 2015) or from the narrow continental slope at the western boundary of the North Atlantic at depths between 1000 and 3000 m (Bingham and Hughes, 2008). By means of a recent series of monthly GRACE/GRACE-FO gravity fields (Kvas et al., 2019), we will show, however, that reliable identification of such signals in satellite observations is difficult with data from a single-pair mission operating in a polar orbit. Error budgeting for GRACE-FO based on full-scale end-to-end satellite simulations (Flechtner et al., 2016) suggests that besides accelerometer noise also imperfect tidal and non-tidal background models limit the accuracy of the gravity fields particularly via characteristic along-track error correlations. Whereas continuous improvements in non-tidal (Dobslaw et al., 2017) and tidal (Schindelegger et al., 2018) background modelling will eventually remove some noise from the GRACE/GRACE-FO data, it appears likely that fundamental breakthroughs in space-based AMOC monitoring will only be possible with data from a future mission with enhanced instrument sensitivities and spatially isotropic error characteristics.