Past and future of the Atlantic Meridional Overturning Circulation under radiative and meltwater forcings

Past trends in the strength of the Atlantic Meridional Overturning Circulation (AMOC) are still debated, and its fate under global warming is uncertain. Observational studies suggest that there has been persistent weakening since the mid-twentieth century, whereas CMIP models systematically simulate a stable circulation. Here, using Earth System and eddy-permitting coupled ocean–sea-ice models, we show that a freshening of the subarctic Atlantic Ocean and weakening of the overturning circulation increase the temperature and salinity of the South Atlantic on a decadal timescale through the propagation of Kelvin and Rossby waves. Applying this finding to observations provides novel evidence on the recent AMOC slow-down. We also show that accounting for upper-end meltwater input in CMIP6 historical simulations significantly improves the data–model agreement on past changes in the Atlantic Meridional Overturning Circulation, indicating the AMOC has weakened by 13% (9 to15%) under a 1ºC global warming, reached in 2017. Including estimates of subarctic meltwater input for the coming century suggests that this circulation could weaken by 33% under a 2ºC global warming, which can be attained over the coming decade, being twice the AMOC weakening projected by radiative forcing only (16%).