Decoupled Freshwater Transport and Meridional Overturning in the South Atlantic

Abstract

Abstract 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.

Keywords

• AMOC
• overturning
• gyre
• AAIW
• ocean reanalyses
• bistability