Simulated stability of the Atlantic Meridional Overturning Circulation during the Last Glacial Maximum

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.

The Effect of Past Saturation Changes on Noble Gas Reconstructions of Mean Ocean Temperature

“…Second, we investigated the response of the noble gases to the colder climate of the LGM. The applied boundary conditions for the LGM are described in detail by Pöppelmeier et al (2021) and include changes in orbital and To assess the impact of even colder temperatures on the noble gases, we performed further experiments with additional top-of-the-atmosphere radiative forcing between −0.5 and −1.5 W/m 2 . The effect of different saturation anomalies on the atmospheric noble gas ratios under LGM boundary conditions was investigated by changing the residual air-sea gas exchange under sea-ice (as for the tuning of the PI state) in sensitivity experiments.…”

“…Next to overturning circulation and sea-ice an additional process is important at the LGM in determining the change in noble gas saturation anomalies relative to PI. The massive continental ice-sheets caused average surface wind speeds at the northern high-latitudes to increase (Muglia & Schmittner, 2015) (see Figure S4 in Supporting Information S1 and Pöppelmeier et al (2021) for details on the implementation), which also increases gas transfer velocities in the model. Decreasing saturation due to larger sea-ice extent (Figures S2 and S3 in Supporting Information S1) and increasing saturation as a result of a more sluggish ocean circulation and increased gas transfer velocities can partly compensate.…”

Hysteresis of the Glacial Atlantic Meridional Overturning Circulation Controlled by Thermal Feedbacks

“…Finally, we should mention that not all models show the same features of the hysteresis as our model (Muglia et al., 2018; Pöppelmeier et al., 2021; Schmittner et al., 2002). Although our findings were confirmed in two simulations, model dependency should be considered.…”

Neodymium budget in the Mediterranean Sea: evaluating the role of atmospheric dusts using a high-resolution dynamical-biogeochemical model

“…This study suggested that the contributions of the Nd sources are ∼ 60 % boundary/benthic source, ∼ 32 % riverine source, and ∼ 9 % dust; however, the coarse resolution of this model limits its ability to sufficiently resolve the processes affecting the Nd oceanic cycle. Later, the same authors investigated the evolution of the AMOC as it responds to freshwater perturbations under improved LGM boundary conditions in the Bern3D intermediate complexity model (Pöppelmeier et al, 2021). Pasquier et al (2022) presented the first inverse model of the global marine biogeochemical cycle of Nd using the Global Neodymium Ocean Model (GNOM) v1.0.…”