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On the reduced sensitivity of the Atlantic overturning to Greenland ice sheet melting in projections: a multi-model assessment

机译：关于预测中大西洋倾覆对格陵兰冰盖融化的敏感性降低：多模型评估

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Large uncertainties exist concerning the impact of Greenland ice sheet melting on the Atlantic meridional overturning circulation (AMOC) in the future, partly due to different sensitivity of the AMOC to freshwater input in the North Atlantic among climate models. Here we analyse five projections from different coupled ocean-atmosphere models with an additional 0.1 Sv (1 Sv = 10(6) m(3)/s) of freshwater released around Greenland between 2050 and 2089. We find on average a further weakening of the AMOC at 26 degrees N of 1.1 +/- 0.6 Sv representing a 27 +/- 14 % supplementary weakening in 2080-2089, as compared to the weakening relative to 2006-2015 due to the effect of the external forcing only. This weakening is lower than what has been found with the same ensemble of models in an identical experimental set-up but under recent historical climate conditions. This lower sensitivity in a warmer world is explained by two main factors. First, a tendency of decoupling is detected between the surface and the deep ocean caused by an increased thermal stratification in the North Atlantic under the effect of global warming. This induces a shoaling of ocean deep ventilation through convection hence ventilating only intermediate levels. The second important effect concerns the so-called Canary Current freshwater leakage; a process by which additionally released freshwater in the North Atlantic leaks along the Canary Current and escapes the convection zones towards the subtropical area. This leakage is increasing in a warming climate, which is a consequence of decreasing gyres asymmetry due to changes in Ekman pumping. We suggest that these modifications are related with the northward shift of the jet stream in a warmer world. For these two reasons the AMOC is less susceptible to freshwater perturbations (near the deep water formation sides) in the North Atlantic as compared to the recent historical climate conditions. Finally, we propose a bilinear model that accounts for the two former processes to give a conceptual explanation about the decreasing AMOC sensitivity due to freshwater input. Within the limit of this bilinear model, we find that 62 +/- 8 % of the reduction in sensitivity is related with the changes in gyre asymmetry and freshwater leakage and 38 +/- 8 % is due to the reduction in deep ocean ventilation associated with the increased stratification in the North Atlantic.
机译：未来格陵兰冰盖融化对大西洋经络翻转环流（AMOC）的影响存在很大的不确定性，部分原因是气候模型之间AMOC对北大西洋淡水输入的敏感性不同。在这里，我们分析了2050年至2089年格陵兰岛周围释放的另外0.1 Sv（1 Sv = 10（6）m（3）/ s）淡水的来自不同耦合的海洋-大气模型的五个预测。我们发现，平均而言， AMOC在26度N为1.1 +/- 0.6 Sv时代表2080-2089年的27 +/- 14％的补充减弱，而相比2006-2015年则仅由于外部强迫作用而减弱。这种减弱低于在相同的实验设置中但在最近的历史气候条件下使用相同的模型集合所发现的减弱。这主要归因于两个主要因素。首先，在全球变暖的影响下，由于北大西洋的热分层增加，导致在表面和深海之间出现了去耦趋势。这通过对流引起浅海深层通风，因此仅使中间水平通风。第二个重要影响是所谓的“金丝雀流”淡水泄漏。一个过程，通过该过程，北大西洋中额外释放的淡水沿金丝雀流泄漏，并从对流区流向亚热带地区。这种泄漏在气候变暖的情况下正在增加，这是由于埃克曼泵送的变化导致回旋不对称性降低的结果。我们建议这些修改与更温暖的世界中喷流的北移有关。由于这两个原因，与最近的历史气候条件相比，AMOC在北大西洋不易受到淡水干扰（在深水形成侧附近）。最后，我们提出了一个双线性模型，该模型考虑了前两个过程，从而对由于淡水输入而引起的AMOC敏感性下降提供了概念上的解释。在此双线性模型的极限内，我们发现灵敏度下降的62 +/- 8％与回旋不对称性和淡水泄漏的变化有关，而38 +/- 8％的下降与深海通风相关随着北大西洋分层的增加。

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