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.

中文翻译：

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在实际边界条件下，AMOC在最后一次冰期最大值期间的稳定性

摘要。大西洋子午线翻转循环（AMOC）对淡水扰动的响应主要取决于其平均状态。在最后一次冰消融化期间北大西洋融化的大量冰山构成了这种扰动，因此可能对AMOC的稳定性提供重要的限制。然而，在冰消融冰期间冰盖快速崩解之前的最后一次冰期最大值（LGM）期间，AMOC的平均状态及其对这些扰动的反应仍存在争议。在这里，我们研究了在Bern3D中间复杂度模型中，改进的LGM边界条件下AMOC对淡水扰动的响应。特别是，我们考虑开放白令海峡与封闭白令海峡的影响。在这些冰川边界条件下充满活力的AMOC，与以前使用不同模型进行的模拟一致，其对北大西洋淡水强迫的反应要比工业化前的状况更强烈。这种增加的敏感性主要与封闭的白令海峡有关，白令海峡切断了通过北极进入太平洋的淡水逃逸路线，从而促进了北大西洋更快的淡水蓄积，阻止了深水形成。相反，LGM AMOC的代理重建表明AMOC比今天更弱，甚至更浅，这与LGM海洋环流模型一致模拟的特别强而深的环流状态相冲突。由于冰盖融化可能增加了大陆径流，并增加了水文循环的变化，因此模拟了北大西洋深水形成减少的情况，与代理所推翻的流通更相似。这些循环状态还显示出双稳态行为，其中AMOC在北大西洋淡水冲刷后无法恢复。但是，这里没有发现AMOC状态，包括先前根据古海洋学数据提出的极端浅滩或剧烈并发的浅层倾覆。