Mechanisms of centennial AMOC variability in a climate model of intermediate complexity

Centennial-scale climate variability in the North Atlantic is characterized by the absence of a clear external forcing. Hence, identifying mechanisms of internal variability at these timescales is crucial to understand low-frequency climate variations. For this task, long control simulations with coupled climate models represent a key tool.

Although significant spectral peaks in centennial variability in the Atlantic Meridional Overturning Circulation (AMOC) were found among some state-of-the-art models, CMIP6 models disagree on the amplitude, periodicity and even existence of centennial AMOC variability. This disagreement motivates the use of models of reduced complexity with idealized setups and perturbed physics ensembles to elucidate the mechanisms of AMOC variability at long timescales.

Here, we investigate multi-millennial piControl simulations of PlaSim-LSG, an earth system model intermediate complexity (EMIC). For a range of vertical oceanic diffusion parameters, PlaSim-LSG exhibits strong oscillations of AMOC strength, as well as of salinity and surface temperatures in the North Atlantic, with a period of about 270 years.

Lag correlation analysis shows that a positive feedback involving the interplay of surface salinity, freshwater flux and sea ice concentration in the Norwegian Sea and the Arctic Ocean is the key driver behind these oscillations. In contrast to previous studies with other models, interhemispheric coupling only plays a minor role. We discuss preliminary results of sensitivity experiments for testing the proposed mechanism, and compare our results with previously proposed mechanisms of AMOC oscillations in CMIP6 models.