Recovering North Atlantic meridional transport variability from bottom pressure anomalies – a revisit

Significant resources have been and are continuing to be devoted to monitor variability in the North Atlantic Meridional Overturning Circulation (AMOC), an essential component of the climate system. Transport mooring arrays are one means of choice, but given their confinement to individual latitudes, complementary large-scale observations are desirable. Here we follow lines of previous research and explore whether ocean bottom pressure (OBP) estimates from present and future satellite gravimetry missions can add useful constraints on interannual AMOC variability between 25°N and 55°N. Central to these considerations are OBP signals along the continental slope of the basin’s western boundary, which—in principle—allow for recovery of geostrophic AMOC variability above 1000 m, between 1000–3000 m, and in some latitudes also below 3000 m. We apply the geostrophic calculation to latest GRACE (Gravity Recovery and Climate Experiment) monthly OBP solutions, in part to document current system capabilities and highlight known shortcomings (e.g., coarse horizontal resolution and data noise). Despite these issues, there are tentative signs of robust and meridionally connected transport variations at latitudes of the New England continental slope (between 35°N and 42°N), including a positive anomaly of ~2 Sverdrup between 2006 and 2012 below 1000 m. We further assess the feasibility of recovering AMOC variability from boundary pressures in high-resolution coupled climate models. These datasets provide useful information on the variance of the involved quantities as well as locations (i.e., depths and latitudes) where accurate knowledge of OBP changes is most important. Insights gained here will be used in upcoming closed-loop orbit simulations that will make special allowance for mass change recovery on the continental slope through dedicated parameterizations in the gravity field estimation.