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.