Weakened AMOC upper limb compensated by strengthened South Atlantic subtropical gyre circulation in CESM1-LE simulations

The upper limb of the Atlantic Meridional Overturning Circulation (AMOC) 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 (SASG). Here, the human-induced response of AMOC and SASG near-surface pathways is illustrated according to CESM1 Large Ensemble simulations from 1920 to 2100, where future projections derive from the most aggressive (yet most realistic) scenario in assumed fossil fuel use and greenhouse gas emissions. In terms of flow redistribution, it is shown that the AMOC upper limb weakens not because less waters are being imported from the adjacent ocean basins — but because they are being mostly directed to recirculate in the southwestern portion of a distorted SASG, turning back southward after reaching the South Atlantic western boundary.