Response of a comprehensive climate model to a broad range of external forcings: relevance for deep ocean ventilation and the development of late Cenozoic ice ages
Response of a comprehensive climate model to a broad range of external forcings: relevance for deep ocean ventilation and the development of late Cenozoic ice ages
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DOI：
10.1007/s00382-018-4157-8
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发表时间：
2019-01-01
期刊：
影响因子：
4.6
通讯作者：
de Lavergne, Casimir
中科院分区：
文献类型：
--
作者：
Galbraith, Eric;de Lavergne, Casimir
在过去的几百万年里，地球从上新世相对温暖和稳定的气候下降到更新世越来越戏剧性的冰河时代周期。长期以来，轨道强迫和大气CO2对陆地冰盖的影响被认为是冰期的主要驱动因素，但对它们对深海环流的直接影响却关注较少。在这里，我们提供了一个广泛的观点CO2的影响，轨道强迫和冰盖的大小，根据一个全面的地球系统模型，通过整合模型的平衡下的40种不同的组合的三个外部强迫。我们发现，南极（AABW）与北大西洋（NADW）沃茨的体积贡献的深海之间的模拟变化很大，并可以预测在AABW和NADW深水形成网站的表面密度之间的差异。AABW-NADW密度差和AABW体积的最小值都出现在间冰期CO_2附近（270- 400 ppm）。在低CO2的情况下，南大洋大量的海冰形成并向北输出，导致南极沃茨的盐度和密度非常高，主导着全球深海。此外，当地球变冷时，低冰度（即地球旋转轴倾斜度降低）通过进一步扩大海冰来增加南极的水量。在高CO2，AABW优势是有利的，由于相对温暖的副极地北大西洋沃茨，与更多的依赖于岁差。与此同时，一个大的Laurentide冰盖引导大气环流，以加强大西洋经向翻转环流，但冷却南大洋远程，增强南极海冰输出，并导致非常咸和扩大AABW。总之，这些结果表明，低CO2，低湿度和相对较小的冰盖的“甜蜜点”将使AMOC中断，促进Dansgaard-Oeschger型突变。在最具代表性的冰川状态下模拟的深海温度和盐度与末次盛冰期（LGM）的重建非常吻合，这使人们相信该模型能够估计水团几何形状的大规模变化。该模型还模拟了一个循环驱动的预制放射性碳水库年龄的增加，这可以解释大多数重建的LGM前工业化海洋放射性碳变化。然而，模拟的冰川海洋的放射性碳含量仍然高于重建的末次冰期，该模型并没有重现重建末次冰期深海氧气消耗。这些与通风有关的分歧可能反映了通风和生态系统过程中尚未解决的物理问题，但也提出了LGM海洋环流不平衡的可能性。最后，模拟结果显示，在低CO2下，地面空气温度和AABW体积对轨道强迫的敏感性增加。我们认为，这种增强的轨道敏感性有助于发展的冰河时代的周期，通过放大气候和碳循环的轨道强迫的反应，随后逐渐下降的趋势CO2。
Over the past few million years, the Earth descended from the relatively warm and stable climate of the Pliocene into the increasingly dramatic ice age cycles of the Pleistocene. The influences of orbital forcing and atmospheric CO2 on land-based ice sheets have long been considered as the key drivers of the ice ages, but less attention has been paid to their direct influences on the circulation of the deep ocean. Here we provide a broad view on the influences of CO2, orbital forcing and ice sheet size according to a comprehensive Earth system model, by integrating the model to equilibrium under 40 different combinations of the three external forcings. We find that the volume contribution of Antarctic (AABW) vs. North Atlantic (NADW) waters to the deep ocean varies widely among the simulations, and can be predicted from the difference between the surface densities at AABW and NADW deep water formation sites. Minima of both the AABW-NADW density difference and the AABW volume occur near interglacial CO2 (270-400ppm). At low CO2, abundant formation and northward export of sea ice in the Southern Ocean contributes to very salty and dense Antarctic waters that dominate the global deep ocean. Furthermore, when the Earth is cold, low obliquity (i.e. a reduced tilt of Earth's rotational axis) enhances the Antarctic water volume by expanding sea ice further. At high CO2, AABW dominance is favoured due to relatively warm subpolar North Atlantic waters, with more dependence on precession. Meanwhile, a large Laurentide ice sheet steers atmospheric circulation as to strengthen the Atlantic Meridional Overturning Circulation, but cools the Southern Ocean remotely, enhancing Antarctic sea ice export and leading to very salty and expanded AABW. Together, these results suggest that a sweet spot' of low CO2, low obliquity and relatively small ice sheets would have poised the AMOC for interruption, promoting Dansgaard-Oeschger-type abrupt change. The deep ocean temperature and salinity simulated under the most representative glacial' state agree very well with reconstructions from the Last Glacial Maximum (LGM), which lends confidence in the ability of the model to estimate large-scale changes in water-mass geometry. The model also simulates a circulation-driven increase of preformed radiocarbon reservoir age, which could explain most of the reconstructed LGM-preindustrial ocean radiocarbon change. However, the radiocarbon content of the simulated glacial ocean is still higher than reconstructed for the LGM, and the model does not reproduce reconstructed LGM deep ocean oxygen depletions. These ventilation-related disagreements probably reflect unresolved physical aspects of ventilation and ecosystem processes, but also raise the possibility that the LGM ocean circulation was not in equilibrium. Finally, the simulations display an increased sensitivity of both surface air temperature and AABW volume to orbital forcing under low CO2. We suggest that this enhanced orbital sensitivity contributed to the development of the ice age cycles by amplifying the responses of climate and the carbon cycle to orbital forcing, following a gradual downward trend of CO2.