Atlantic overturning inferred from air-sea heat fluxes indicates no decline since the 1960s

The Atlantic Meridional Overturning Circulation (AMOC) is crucial for global ocean carbon and heat uptake, and controls the climate around the North Atlantic. Despite its importance, quantifying the AMOC’s past changes and assessing its vulnerability to climate change remains highly uncertain. Understanding past AMOC changes has relied on proxies, most notably sea surface temperature anomalies over the subpolar North Atlantic. Here, we use 24 Earth System Models from Coupled Model Intercomparison Project Phase 6 (CMIP6) to demonstrate that these sea surface temperature anomalies cannot robustly reconstruct the AMOC on annual, decadal or centennial timescales. Instead, we find that the net air-sea heat flux anomaly between the Arctic and any given latitude between 26.5°N and 50°N in the North Atlantic is tightly linked to the AMOC anomaly at that latitude on decadal and centennial timescales. On these timescales, the air-sea heat flux proxy works through the conservation of energy, in which energy transferred laterally into the region is typically released to the atmosphere through surface heat fluxes. On annual timescales, however, air-sea heat flux anomalies are modulated more so by atmospheric variability and less by AMOC anomalies. Based on the here identified relationship and observation-based estimates of the past air-sea heat flux in the North Atlantic from reanalysis products, we show the decadal averaged AMOC at 26.5°N has not weakened from 1963 to 2017 although substantial variability exists. Furthermore, we find no decline in the AMOC at any other latitude, though the decadal variability appears distinct between subtropical and subpolar latitudes. This result aligns with previous work that has shown the lack of meridional coherence in the AMOC, and the presence of distinct overturning cells.