Common mechanisms of centennial-scale AMOC variability in CMIP6 models

It has been hypothesized that climate variability on centennial timescales – in the North Atlantic region and beyond – is linked to unforced variability of the Atlantic Meridional Overturning Circulation (AMOC). Because of the presence of external forcings, uncertainties in proxy reconstructions of the AMOC and the short observational record, coupled climate models represent a key tool in assessing low-frequency AMOC variability. However, sufficiently long pre-industrial control (piControl) simulations with state-of-the-art climate models have only become widely available during the past decade. While significant centennial-scale AMOC variability has been identified in several single-model studies, proposed physical mechanisms differ considerably.

Here, we assess mechanisms of AMOC variability on centennial timescales in the CMIP6 multi-model piControl ensemble. We find that a relatively large number of models – 11 out of the 15 analyzed – exhibit a statistically significant mode of centennial-scale MOC variability in the Atlantic. We review previously proposed mechanisms for centennial-scale AMOC variability and test whether their key elements are present in the CMIP6 ensemble.

We find that salinity exchanges between the Arctic and North Atlantic basins, which have previously been proposed as drivers of multi-centennial AMOC variability in two CMIP6 models (IPSL-CM6A-LR and EC-Earth3), can also be identified in other CMIP6 models using the same ocean component (NEMO). However, we find only a weak or no signature of this mechanism in models that do not include NEMO. Even among NEMO models, the amplitude and timescale of centennial-scale AMOC variability is model-dependent, and we assess the relative role of deep-water formation sites in shaping these differences. Because AMOC fluctuations are linked to surface temperature anomalies and related impacts over land, our results motivate the need for more paleoclimate evidence at sub-centennial resolution, which would help constrain the CMIP6 inter-model spread in centennial-scale AMOC variability.