Weakening of the Atlantic Meridional Overturning Circulation Abyssal Limb in the North Atlantic

The abyssal limb of the global Meridional Overturning Circulation redistributes heat and carbon as it carries Antarctic Bottom Water from the Southern Ocean towards the Northern Hemisphere. Using mooring observations and hydrographic data from multiple sources in the North Atlantic, we show that northward-flowing Antarctic Bottom Water is constrained below 4,500 m with a mean volume transport of 2.40 +/- 0.25 Sv at 16 degrees N. We find that during 2000-2020, the Antarctic Bottom Water northward transport weakened by approximately 0.35 +/- 0.13 Sv, corresponding to a 12 +/- 5% decrease. The weakening of the Atlantic Meridional Overturning Circulation abyssal cell is a probable response to reduced Antarctic Bottom Water formation rates over the past several decades and is associated with abyssal warming observed throughout the western Atlantic Ocean. We estimate that the warming of the Antarctic Bottom Water layer in the subtropical North Atlantic is, on average, 1 m degrees C per year in the last two decades due to the downward heaving of abyssal isopycnals, contributing to the increase of abyssal heat content and, hence, sea-level rise in the region (1 m degrees C = 0.001 degrees C). This warming trend is approximately half of the Antarctic Bottom Water warming trend observed in the South Atlantic and parts of the Southern Ocean, indicating a dilution of the signal as the Antarctic Bottom Water crosses the Equator. Mooring observations and hydrographic data suggest the Atlantic Meridional Overturning Circulation abyssal limb has weakened over the past two decades in the North Atlantic, most likely due to reduced Antarctic Bottom Water formation rates.