The Response of the Global Overturning Circulation to Recent and Projected Antarctic Meltwater Changes

Abstract The Antarctic ice sheet and ice shelves are losing mass and release freshwater to the ocean, affecting the ocean state. The response of ocean circulation to recent and projected increases in Antarctic meltwater is still uncertain, because of uncertainties in meltwater forcing and deficiencies in global ocean and climate models. Here, we use an equilibrated one‐degree global ocean configuration that realistically represents Antarctic Bottom Water (AABW) formation and abyssal ventilation. We perform three meltwater hosing experiments covering the period 1990–2100: an “Idealized” experiment in which exactly 0.1 Sv (1 Sv = ) of freshwater is released uniformly around the Antarctic coast; a “Realistic” experiment, where reconstructed and projected freshwater anomalies are distributed in time and space based on recent estimates of melting rates and processes; and a “Semi‐realistic” experiment in which the same time‐varying freshwater anomalies are released uniformly around the Antarctic coast. In all experiments, AABW transport at 64S decreases, by 5–12 Sv by 2100. Meanwhile, the Atlantic Meridional Overturning Circulation (AMOC) strengthens by approximately 2 Sv, driven by enhanced Southern Ocean upwelling. This upwelling increase results from weaker compensation between northward near‐surface flow and southward mesoscale‐induced flow in the stratifying upper Southern Ocean. In the idealized experiment, the AMOC ultimately weakens due to the propagation of fresh surface anomalies to the subpolar North Atlantic. The timing of simulated overturning changes depends strongly on meltwater forcing; changes appear latest in the most realistic experiment.