Changes in water mass distribution are considered to be a significant contributor to the atmospheric CO₂ concentration drop to around 186 ppm recorded during the Last Glacial Maximum (LGM). Yet simulating a glacial Atlantic Meridional Overturning Circulation (AMOC) in agreement with paleotracer data remains a challenge, with most models from previous Paleoclimate Modelling Intercomparison Project (PMIP) phases showing a tendency to simulate a strong and deep North Atlantic Deep Water (NADW) instead of the shoaling inferred from proxy records of water mass distribution. Conversely, the simulated Antarctic Bottom Water (AABW) is often reduced compared to its pre-industrial volume, and the Atlantic Ocean stratification is underestimated with respect to paleoproxy data. Inadequate representation of surface conditions, driving deep convection around Antarctica, may explain inaccurately simulated bottom water properties in the Southern Ocean. We investigate here the impact of a range of surface conditions in the Southern Ocean in the iLOVECLIM model using nine simulations obtained with different LGM boundary conditions associated with the ice sheet reconstruction (e.g., changes of elevation, bathymetry, and land–sea mask) and/or modeling choices related to sea-ice export, formation of salty brines, and freshwater input. Based on model–data comparison of sea-surface temperatures and sea ice, we find that only simulations with a cold Southern Ocean and a quite extensive sea-ice cover show an improved agreement with proxy records of sea ice, despite systematic model biases in the seasonal and regional patterns. We then show that the only simulation which does not display a much deeper NADW is obtained by parameterizing the sinking of brines along Antarctica, a modeling choice reducing the open-ocean convection in the Southern Ocean. These results highlight the importance of the representation of convection processes, which have a large impact on the water mass properties, while the choice of boundary conditions appears secondary for the model resolution and variables considered in this study.

中文翻译：

---

LGM期间南大洋表面条件对深海环流的影响：模型分析

水质量分布的变化被认为是大气CO ₂的重要贡献者在末次盛冰期 (LGM) 记录的浓度下降到 186 ppm 左右。然而，根据古示踪数据模拟冰川大西洋经向翻转环流 (AMOC) 仍然是一个挑战，以前古气候建模比对项目 (PMIP) 阶段的大多数模型显示出模拟强大而深的北大西洋深水 (NADW) 的趋势从水团分布的代理记录推断的浅滩。相反，模拟的南极底水 (AABW) 与其工业化前的体积相比经常减少，并且大西洋分层相对于古代理数据被低估了。表面条件的不充分表示，推动了南极洲周围的深层对流，可以解释不准确模拟的南大洋底水特性。我们在此研究了 iLOVECLIM 模型中南大洋一系列表面条件的影响，该模型使用了与冰盖重建相关的不同 LGM 边界条件（例如，海拔、水深和海陆掩膜的变化）获得的九个模拟，并且/或与海冰出口、咸水形成和淡水输入相关的建模选择。基于海面温度和海冰的模型数据比较，我们发现，尽管系统模型偏差在南大洋寒冷且海冰覆盖相当广泛，但模拟结果与海冰代理记录的一致性有所提高。季节性和区域模式。然后我们表明，唯一不显示更深 NADW 的模拟是通过参数化沿南极洲的盐水下沉获得的，减少南大洋公海对流的建模选择。这些结果突出了对流过程表示的重要性，对流过程对水团特性有很大影响，而边界条件的选择对于本研究中考虑的模型分辨率和变量来说似乎是次要的。