Centennial‐Scale Variability of the Atlantic Meridional Overturning Circulation in CMIP6 Models Shaped by Arctic–North Atlantic Interactions and Sea Ice Biases.

Climate variability on centennial timescales has often been linked to internal variability of the Atlantic Meridional Overturning Circulation (AMOC). However, due to the scarceness of suitable paleoclimate proxies and long climate model simulations, large uncertainties remain on the magnitude and physical mechanisms driving centennial‐scale AMOC variability. For these reasons, we perform a systematic multi‐model comparison of centennial‐scale AMOC variability in pre‐industrial control simulations of state‐of‐the‐art global climate models. Six out of nine models in this study exhibit a statistically significant mode of centennial‐scale AMOC variability. Our results show that freshwater exchanges between the Arctic Ocean and the North Atlantic provide a plausible driving mechanism in a subset of models, and that AMOC variability can be amplified by ocean–sea ice feedbacks in the Labrador Sea. The amplifying mechanism is linked to sea ice cover biases, which could provide an observational constraint for centennial‐scale AMOC variability. Plain Language Summary: Changes in ocean circulation are often proposed as drivers of natural variations of the Earth's climate on timescales of centuries. However, it is unclear how strong these natural variations of the circulation strength, called internal variability, are in the real world, because reconstructions from the past climate are sparse and climate models are expensive to run for these long timescales. Here, we compare how the latest generation of climate models simulate internal variability of the Atlantic Meridional Overturning Circulation (AMOC)—the ocean circulation that is often thought to be responsible for Europe's comparatively mild climate—on timescales of 100–250 years. We find that many models have stronger variability on these timescales than what would be expected simply from random noise. In several models, AMOC variability appears to be driven by the release of fresh water from the Arctic Ocean and amplified by intermittent sea ice cover in the North Atlantic. However, this amplification only occurs if a model simulates a too extensive sea ice cover in winter. This mechanism shows that sea ice cover—which is easily observable—could be used to constrain variability of the AMOC on timescales longer than the observational record. Key Points: We present a robust multi‐model comparison of internal centennial‐scale AMOC variability in state‐of‐the‐art climate modelsA robust mechanism of Arctic–North Atlantic freshwater exchange is identified only in models that use the NEMO ocean componentSea ice cover biases in convective regions of the North Atlantic amplify AMOC variability and could provide an observational constraint [ABSTRACT FROM AUTHOR]