AMOC stability and diverging response to Arctic sea ice decline in two climate models
We compare the impacts of Arctic sea ice decline on the Atlantic meridional overturning circulation (AMOC) in two configurations of the Community Earth System Model (CESM) with different horizontal resolutions. In a suite of model experiments, we impose radiative imbalance at the ice surface, replicating a loss of sea ice cover comparable to that observed during 1979–2014, and we find dramatic differences in the AMOC response between the two models. In the lower-resolution configuration, the AMOC weakens by about one-third over the first 100 years, approaching a new quasi-equilibrium. By contrast, in the higher-resolution configuration, the AMOC weakens by ~10% during the first 20–30 years followed by a full recovery driven by invigorated deep water formation in the Labrador Sea and adjacent regions. We investigate these differences using a diagnostic AMOC stability indicator, which suggests that the AMOC in the lower-resolution model is less stable and more sensitive to surface perturbations. Differences between the models’ mean states, including the Atlantic Ocean mean surface freshwater fluxes, control the differences in AMOC stability.
Variations in the AMOC are critical for global and regional climate. The decreasing Arctic sea ice can contribute to the AMOC slowdown by inducing warm and fresh surface anomalies that could spread into the subpolar region on multidecadal time scales. This study shows the AMOC response to transient buoyancy forcing induced by Arctic sea ice decline in two CESM models and investigates how the different behaviors can be related to the AMOC stability properties. Our results demonstrate that the AMOC stability indicator is indeed useful for evaluating AMOC sensitivity to perturbations. We emphasize that, despite the differences in the long-term adjustment, both models simulate a multidecadal AMOC weakening caused by Arctic sea ice decline, relevant to climate change.
In this study, we compared the AMOC response to a modeled sea ice decline that is comparable to the observed during the past three decades in two different configurations of CESM1. These two models differ in their horizontal resolution and several other important model properties. The AMOC responses are dramatically different: the AMOC in the low-res model responds to the imposed Arctic sea ice decline with a robust 30% strength reduction, reaching a new quasi-equilibrium state with a weaker AMOC in roughly 100 years; the AMOC in the high-res model undergoes a modest transient weakening over the first 20–30 years, within the range of its decadal variability, followed by a recovery and slight intensification. We argue that the contrasting AMOC behaviors in the two models can be related to the AMOC stability properties, as described by the AMOC stability indicator, which are largely controlled by the differences in the model mean states, including the basin-wide mean surface freshwater fluxes and the location of North Atlantic Deep Water formation sites.