Projected Atlantic overturning slow-down is to be compensated by a strengthened South Atlantic subtropical gyre

The upper limb of the Atlantic meridional overturning circulation brings shallow interocean contributions to replenish the North Atlantic export of deepwaters. It is primarily formed in the southern South Atlantic where the converging entrainment of Pacific and Indian Ocean waters meet and incorporate into the South Atlantic subtropical gyre. Here, we use Community Earth System Model 1 Large Ensemble simulation results along 1920-2100 to investigate the response of the Atlantic meridional overturning circulation upper limb and the South Atlantic subtropical gyre to future human-induced climate warming under business-as-usual greenhouse gas emissions. In terms of flow redistribution, we find that the Atlantic meridional overturning circulation upper limb weakens not because less waters are being imported from the Pacific and Indian basins - but because waters are being mostly directed to recirculate in the southwestern portion of a distorted South Atlantic subtropical gyre, turning back southward after reaching the South Atlantic western boundary. Large ensemble simulations project an intensification of the South Atlantic subtropical gyre with reduced water supply to the North Atlantic due to a weakening of the Atlantic meridional overturning circulation and changes in atmospheric forcing. (AU)