Decoupled Freshwater Transport and Meridional Overturning in the South Atlantic

Freshwater transports (Fov) by the Atlantic meridional overturning circulation (AMOC) are sensitive to salinity distributions and may determine AMOC stability. However, climate models show large salinity biases, distorting the relation between Fov and the AMOC. Using free‐running models and ocean reanalyses with realistic salinities but quite different AMOCs, we show that the fresh Antarctic Intermediate Water layer eliminates salinity differences across the AMOC branches at ~1,200 m, ∆S1200m, which decouples Fov from the AMOC south of ~10°N. As the Antarctic Intermediate Water disappears north of ~10°N, a large ∆S1200m allows the AMOC to drive substantial southward Fov in the North Atlantic. In the South Atlantic the 0–300 m zonal salinity contrasts control the gyre freshwater transports Fgyre, which also determine the total freshwater transports. This decoupling makes the southern Fov unlikely to play any role in AMOC stability, leaving indirect Fgyre feedbacks or Fov in the north, as more relevant factors. Plain language summary The Atlantic Ocean has an upper circulation branch transporting warm waters toward the Arctic. These waters sink due to changes in both salinity and temperature, leading to a cold and deep southward circulation branch throughout the Atlantic. This “overturning circulation” moves heat and freshwater over large distances, contributing to regulate Earth's climate, but the circulation strength may also be affected by these transports via feedback effects. It has been proposed that South Atlantic freshwater transports are a sensitive indicator of circulation feedback, which could lead to instability in the climate system. However, models of the ocean and atmospheric circulations used to study climate often show large errors in salinity distributions and freshwater transports and therefore may misrepresent climate stability. We show that with realistic salinities, the overturning circulation produces virtually no freshwater transports throughout the South Atlantic and is unlikely to have any role in feedbacks causing climate instability. Horizontal gyre circulations dominate South Atlantic freshwater transports, which could still have some indirect influence on climate stability. In contrast, the overturning circulation does drive a strong freshwater transport in the North Atlantic, and therefore, salinity feedbacks on the climate stability are much more likely to be important in the north. Key Points Atlantic overturning freshwater t