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Climate of the Past An interactive open-access journal of the European Geosciences Union
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https://doi.org/10.5194/cp-2020-135
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/cp-2020-135
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

  21 Oct 2020

21 Oct 2020

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This preprint is currently under review for the journal CP.

Simulated stability of the AMOC during the Last Glacial Maximum under realistic boundary conditions

Frerk Pöppelmeier, Jeemijn Scheen, Aurich Jeltsch-Thömmes, and Thomas F. Stocker Frerk Pöppelmeier et al.
  • Climate and Environmental Physics, Physics Institute and Oeschger Center for Climate Change Research, University of Bern, 3012 Bern, Switzerland

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 thus can provide important constraints on the stability of the AMOC. Yet, 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 responding 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. 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 pre-industrial 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, thus facilitating faster accumulation of freshwater in the North Atlantic halting deep water formation. Proxy reconstructions of the LGM AMOC instead indicate a weaker and possibly shallower AMOC than today, 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.

Frerk Pöppelmeier et al.

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Frerk Pöppelmeier et al.

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Short summary
The stability of the Atlantic Meridional Overturning Circulation (AMOC) critically depends on its mean-state. We simulate the response of the AMOC to North Atlantic freshwater perturbations under different glacial boundary conditions. We find that a closed Bering Strait greatly increases the AMOCs sensitivity to freshwater hosing. Further, the shift from mono- to bistability strongly depends on the chosen boundary conditions, with weaker circulation states exhibiting more abrupt transitions.
The stability of the Atlantic Meridional Overturning Circulation (AMOC) critically depends on...
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