Climate-state dependence of multi-centennial variability in the Atlantic Meridional Overturning Circulation

Multi-centennial variability of the Atlantic Meridional Overturning Circulation (AMOC) is a fundamental feature of the Earth system, influencing climate variability across past and future warmer climate states. Yet it remains unclear how such low-frequency, internally generated variability responds to changes in the background climate state under greenhouse-gas forcing. Here we investigate the sensitivity of multi-centennial AMOC variability using multi-millennial equilibrium simulations with the EC-Earth3 climate model under three CO2 concentrations (280, 400 and 560 ppm). All simulations exhibit pronounced low-frequency variability in global mean surface air temperature, with the strongest amplitudes in the subpolar North Atlantic and Arctic, closely linked to AMOC fluctuations. Spectral analyses reveal a dominant self-sustained AMOC mode with a period of approximately 200–300 years, primarily maintained by subtropical-subpolar salinity-advection feedback, which persists across all climate states examined. In addition, a secondary mode with a period of 100–150 years is identified, associated with Arctic sea-ice-freshwater interactions. This Arctic-related mode weakens substantially under elevated CO2 forcing as Arctic sea ice retreats. These results demonstrate that multi-centennial AMOC variability arises from both climate-state-independent and climate-state-dependent mechanisms. While the mean AMOC weakens under greenhouse-gas forcing, internal low-frequency variability remains an active component of the climate system, with important implications for interpreting long-term climate variability in both past and future warm climates.