Separating Direct Heat Flux Forcing and Freshwater Feedback on AMOC Change Under Global Warming

The Atlantic meridional overturning circulation (AMOC) is predicted to weaken under global warming. Whether it is caused by heat flux or freshwater flux is under debate. Here we separate these two processes in changing the AMOC under global warming. The simulated AMOC is weakened during the first 600 years and then gradually recovered to its initial state, with heat flux and freshwater feedback dominating at different timescales. Global warming immediately puts freshwater into the Southern Ocean, which triggers the initial AMOC weakening via altering surface temperature. Concurrently, the extensive heat into the ocean surface increases the temperature over the subpolar North Atlantic, reducing the deep convection and thus the AMOC in the subsequent 50–150 years. Meanwhile, the Arctic sea ice melt leads to the AMOC shutdown. Subsequently, the salinity accumulation in the subtropical North Atlantic propagating northward to restart the North Atlantic deep convection is responsible for the AMOC recovery. Plain Language Summary The Atlantic meridional overturning circulation (AMOC) is projected to slow down under anthropogenic warming. However, whether this weakening is caused by heat or freshwater flux is of great debate. Here we use a fully coupled Earth System model to separate the role of direct heat flux forcing and freshwater feedback on AMOC behavior under idealized anthropogenic forcing. The simulated AMOC slows down during the first 600 years and then gradually recovers to its initial strength, with heat flux and freshwater feedback dominating at different timescales: The initial AMOC weakening during the first 50 years is caused by the freshwater feedback, its subsequently weakening during 50–150 years is dominated by direct heat flux forcing, while its continuous weakening and recovery is again attributed by freshwater feedback. This work quantifies the individual role of heat and freshwater flux in AMOC change, highlighting their different contributions at different timescales. Key Points The first study to separate the direct heat effect from freshwater feedback on the thermohaline circulation under global warming The initial Atlantic meridional overturning circulation (AMOC) weakening is driven by freshwater feedback, while its continuous weakening is attributed to heat flux forcing The AMOC collapse and recovery after 200 years is driven by the freshwater feedback