A Coupled Atmosphere-Ocean Mode Driving Multicentennial AMOC Variability in a Climate Model of Intermediate Complexity

Internal climate variability on centennial timescales has been linked to variations in the thermohaline circulation, more specifically to the Atlantic Meridional Overturning Circulation (AMOC), in numerous modeling studies. Recently, multicentennial AMOC variability has been identified in several CMIP6 models, with a discernible influence on global surface temperatures. However, only a subset of global climate models exhibits a significant mode of AMOC variability at these timescales, and proposed physical mechanisms differ considerably. Therefore, a better process understanding of low-frequency AMOC variability is needed.

Here, we analyze multicentennial oscillations of AMOC strength under constant boundary conditions in PlaSim-LSG, a climate model of intermediate complexity. We propose a coupled atmosphere-ocean mode in which these AMOC variations are driven by salinity anomalies from the Arctic Ocean, which are in turn attributed to changes in high-latitude precipitation. The low computational cost of PlaSim-LSG readily allows for millennial-length sensitivity experiments, which, together with the analysis of a three-box dynamical system model, further corroborate our mechanism. Our results reveal that freshwater flux variations in the Arctic are a physically plausible driver of centennial-scale AMOC variability, which raises the question whether this variability might occur over a wider range of climate states than previously reported in the literature.