Abstract
Abstract. The response of the Atlantic Meridional Overturning Circulation (AMOC) to freshwater perturbations critically depends on its mean state. Large swaths of icebergs melting in the North Atlantic during the last deglaciation constituted such perturbations and can, thus, provide important constraints on the stability of the AMOC. However, the mean AMOC state during the Last Glacial Maximum (LGM), preceding the rapid disintegration of the ice sheets during the deglaciation, as well as its response to these perturbations remain debated. Here, we investigate the evolution of the AMOC as it responds to freshwater perturbations under improved LGM boundary conditions in the Bern3D intermediate complexity model. Particularly, we consider the effect of an open versus a closed Bering Strait and the effect of increased tidal dissipation as a result of the altered bathymetry due to the lower glacial sea level stand. The vigorous and deep AMOC under these glacial boundary conditions, consistent with previous simulations with different models, reacts more strongly to North Atlantic freshwater forcings than under preindustrial conditions. This increased sensitivity is mostly related to the closed Bering Strait that cuts off the freshwater escape route through the Arctic into the Pacific, thereby facilitating faster accumulation of freshwater in the North Atlantic and halting deep-water formation. Proxy reconstructions of the LGM AMOC instead indicate a weaker and possibly shallower AMOC than today, which is in conflict with the particularly strong and deep circulation states coherently simulated with ocean circulation models for the LGM. Simulations with reduced North Atlantic deep-water formation, as a consequence of potentially increased continental runoff from ice sheet melt and imposed changes in the hydrological cycle, more closely resemble the overturning circulation inferred from proxies. These circulation states also show bistable behavior, where the AMOC does not recover after North Atlantic freshwater hosing. However, no AMOC states are found here that either comprise an extreme shoaling or vigorous and concurrent shallow overturning as previously proposed based on paleoceanographic data.
Highlights
The Atlantic Meridional Overturning Circulation (AMOC) redistributes heat, nutrients, and carbon between the hemispheres and constitutes an important tipping element in Earth’s climate system (Lenton et al, 2008; Stocker and Wright, 1991)
The zonally integrated Atlantic circulation is characterized by two overturning cells that are driven by the southward transport of North Atlantic Deep Water (NADW) and northward flowing Antarctic Bottom Water (AABW) occupying abyssal depths
In order to achieve topologically more realistic glacial boundary conditions, we investigate the impact of a closed Bering Strait on the AMOC, which is represented in the model by a single grid cell with a depth of about 40 m and a mean simulated meridional throughflow of 0.5 Sv from the Pacific to the Arctic, which is at the lower end of the observed range of 0.4 to 1.2 Sv (Woodgate et al, 2005)
Summary
The Atlantic Meridional Overturning Circulation (AMOC) redistributes heat, nutrients, and carbon between the hemispheres and constitutes an important tipping element in Earth’s climate system (Lenton et al, 2008; Stocker and Wright, 1991). A handful of studies have tried to reconcile proxy data and model simulation by an integrated intercomparison approach mainly based on stable carbon isotope data (e.g., Gebbie, 2014; Hesse et al, 2011; Menviel et al, 2017, 2020) These studies require additional forcings such as freshwater or moisture transport fluxes to achieve a reasonable model–data agreement, and results remain ambiguous regarding parameters such as circulation strength (Menviel et al, 2020). The latter may be responsible for transient changes in the hysteresis structure during the transition from the glacial to the Holocene (Stocker and Marchal, 2000)
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