Extended Data Fig. 10: Mechanism of South Atlantic salinity pile-up.

a,b, Mechanism illustrated by the salinity budget of the upper South Atlantic in the ocean model experiment HFX (psu year⁻¹). Control climatology (a) and HFX experiment (b) with a weakening AMOC (climatology of the last 20 years). Blue arrows are net E−P flux, red arrows indicate meridional salinity transport and green arrows indicate vertical salinity transport (including a small contribution by mixing). Red shadings are the symbolic salinity gradient across the South Atlantic (note that the northern side is climatologically saltier than the southern side, also see Extended Data Fig. 2c). The accumulated salinity budget of HFX is shown in Extended Data Fig. 9d. The salinity pile-up is caused primarily by the reduced northward salinity transport associated with the reduced AMOC, which is more reduced downstream (northern side) than upstream (southern side) because of a greater mean salinity in the former. This AMOC-induced salinity pile-up is robust for the South Atlantic overall basin mean, while the detailed pattern of salinity changes can be affected by other processes, especially in the coupled model.