Data-constrained assessment of ocean circulation changes since the middle Miocene in an Earth system model
Data-constrained assessment of ocean circulation changes since the middle Miocene in an Earth system model
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DOI：
10.5194/cp-17-2223-2021
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发表时间：
2021-10-21
影响因子：
4.3
通讯作者：
Pearson, Paul N.
中科院分区：
文献类型：
--
作者：
Crichton, Katherine A.;Ridgwell, Andy;Pearson, Paul N.
自中新世中期（15 Ma，百万年前）以来，地球气候经历了长期的冷却趋势，其特征是海洋温度降低了7-8摄氏度。这种冷却的原因主要被认为是由于构造板块运动驱动大规模海洋环流模式的变化，因此热量重新分布，以及大气温室气体强迫的下降（以及随之而来的冰盖生长和反馈）。在这项研究中，我们评估的潜力，以限制全球海洋环流和冷却的演变模式在过去的15马同化各种海洋沉积物代用数据在地球系统模型。我们这样做，首先汇编表面和海底海洋温度和海底碳-13（delta C-13）的数据在一系列的七个时间片间隔约250万年的时间间隔。然后，我们配对这与相应的一系列的构造和气候边界条件重建的cGENIE（“松饼”释放）地球系统模型，包括开放与封闭的中美洲航道（CAS）从10马起的替代可能性。在cGENIE模型中，我们探讨了温室气体强迫的不确定性和北太平洋北大西洋盐度通量调整的幅度，需要在模型中创建一个特定强度的大西洋经向翻转环流（AMOC），通过一系列的12个（每个构造重建一个）2D参数合奏。每个合奏成员，然后测试对观测到的全球温度和海底三角洲C-13模式。我们确定，一个相对较高的CO2当量迫使1120 ppm是需要在15马cGENIE再现代理温度估计模型中，注意到这种CO2强迫是依赖于cGENIE模型的气候敏感性，它包含了所有温室气体的影响。我们发现，再现所观察到的长期冷却趋势，需要在模型中逐步下降的温室气体强迫。与此同时，AMOC的强度随着时间的推移而增加，尽管在冷却期间北大西洋表面的盐度减少，这是由于水文循环强度下降和极地温度降低，两者都是由CO2驱动的全球冷却造成的。我们还发现，一个封闭的CAS从10马到现在显示出更好的协议之间的底栖三角洲C-13模式和我们的特定系列的模型配置和数据。我们分析的最终结果是一个明显的案例。1.5千分之一的下降发生在大气中（和约。1份/千海洋表面）三角洲C-13，可用于通知未来三角洲C-13为基础的代理重建。
Since the middle Miocene (15 Ma, million years ago), the Earth's climate has undergone a long-term cooling trend, characterised by a reduction in ocean temperatures of up to 7-8 degrees C. The causes of this cooling are primarily thought to be due to tectonic plate movements driving changes in large-scale ocean circulation patterns, and hence heat redistribution, in conjunction with a drop in atmospheric greenhouse gas forcing (and attendant ice-sheet growth and feedback). In this study, we assess the potential to constrain the evolving patterns of global ocean circulation and cooling over the last 15 Ma by assimilating a variety of marine sediment proxy data in an Earth system model. We do this by first compiling surface and benthic ocean temperature and benthic carbon-13 (delta C-13) data in a series of seven time slices spaced at approximately 2.5 Myr intervals. We then pair this with a corresponding series of tectonic and climate boundary condition reconstructions in the cGENIE ("muffin" release) Earth system model, including alternative possibilities for an open vs. closed Central American Seaway (CAS) from 10 Ma onwards. In the cGENIE model, we explore uncertainty in greenhouse gas forcing and the magnitude of North Pacific to North Atlantic salinity flux adjustment required in the model to create an Atlantic Meridional Overturning Circulation (AMOC) of a specific strength, via a series of 12 (one for each tectonic reconstruction) 2D parameter ensembles. Each ensemble member is then tested against the observed global temperature and benthic delta C-13 patterns. We identify that a relatively high CO2 equivalent forcing of 1120 ppm is required at 15 Ma in cGENIE to reproduce proxy temperature estimates in the model, noting that this CO2 forcing is dependent on the cGENIE model's climate sensitivity and that it incorporates the effects of all greenhouse gases. We find that reproducing the observed long-term cooling trend requires a progressively declining greenhouse gas forcing in the model. In parallel to this, the strength of the AMOC increases with time despite a reduction in the salinity of the surface North Atlantic over the cooling period, attributable to falling intensity of the hydrological cycle and to lowering polar temperatures, both caused by CO2-driven global cooling. We also find that a closed CAS from 10 Ma to present shows better agreement between benthic delta C-13 patterns and our particular series of model configurations and data. A final outcome of our analysis is a pronounced ca. 1.5 parts per thousand decline occurring in atmospheric (and ca. 1 parts per thousand ocean surface) delta C-13 that could be used to inform future delta C-13-based proxy reconstructions.