Collaborative Research: Assessing the Impact of Tidal Mixing on the Meridional Overturning Circulation of the Oceans during the Last Glacial Maximum

在当代海洋中，大部分潮汐能量在浅层大陆架上消散，而一小部分在深海中引起混合，从而为经向翻转环流（MOC）提供动力。古潮汐研究表明，末次盛冰期（Last Glacial Maximum， LGM）由于海平面下降约120 m，这种情况发生了巨大变化，耗散从浅海架向深海转移。这一发现提出了末次盛冰期经向翻转环流一定更强的假设。然而，最近旨在量化这种耗散位移对LGM MOC的影响的研究结果得出了相互矛盾的结论，从可以忽略的影响到MOC的大幅增加。本项目旨在解决这些差异，并检验上述假设。它还将首次量化现实的、数据受限的、LGM分层对湍流扩散系数、混合和MOC的影响。其他不确定性也将被量化，从而导致对潮汐混合变化及其对LGM MOC的影响的综合估计。因此，这个项目将使人们更好地了解在根本不同的气候条件下控制行星尺度海洋环流变化及其相关的生物地球化学循环的过程。更好地了解LGM期间MOC的驱动机制有可能改善其量化，这对量化冰川海洋的努力具有重要意义。美国碳循环和解决冰期-间冰期大气二氧化碳变化的巨大难题。这个项目的一个有益的副作用可能是，它改进了目前科学界广泛使用的两种气候模式对潮汐混合、扩散和环流的模拟，并影响未来古气候模式比较项目的设计。将支持和培训一名博士后科学家运行和分析潮汐模型。本科生将通过暑期实习接触研究。将组织一次会议，目的是汇集现代物理海洋学家和古海洋学家，促进跨学科的思想交流。一个详细的模拟研究，以调查潮汐混合对当前和末次极盛期（LGM）经向翻转环流（MOC）的影响将使用一个层次的数值模式进行。利用全球潮汐模式的模拟将计算潮汐能量耗散的分布，这些分布将提供给两个全球气候/海洋环流模式，以量化它们对混合和MOC的影响。敏感性实验将探索不同参数化方案对结果的不确定性，如内波阻力、潮汐模式分辨率、LGM分层、浮冰、海平面空间变化以及海底以上混合的垂直衰减。气候模式模拟将采用中等复杂模式和最先进的地球系统模式进行。目前的模拟将通过与观测估计的地表扩散率和示踪剂分布进行比较来评估。不同的循环对生物地球化学循环和碳、氮同位素的影响也将被模拟。最后，将LGM模式的模拟结果与已有代理记录的古重建结果进行比较，以评估模拟的环流。

In the contemporary ocean, most of the tidal energy is dissipated on shallow continental shelves, whereas a smaller portion causes mixing in the deep ocean, which powers the Meridional Overturning Circulation (MOC). Studies of paleo-tides suggest that during the Last Glacial Maximum (LGM), due to the sea level drop of about 120 m, this situation was drastically different and dissipation was shifted from the shallow shelves into the deep ocean. This finding has prompted the hypothesis that the meridional overturning circulation during the Last Glacial Maximum must have been stronger. However, recent research results aimed at quantifying the effects of this dissipation shift on the LGM MOC came to conflicting conclusions, ranging from negligible effects to a large increase in the MOC. This project seeks to resolve these differences and test the aforementioned hypothesis. It will also provide the first quantification of the effects of realistic, data constrained, LGM stratification on turbulent diffusivities, mixing and the MOC. Other uncertainties will also be quantified thus leading to a comprehensive estimate of changes in tidal mixing and its impacts on the LGM MOC. Thus this project will lead to a better understanding of the processes that control planetary-scale ocean circulation changes and their associated biogeochemical cycles in fundamentally different climates. A better understanding of the driving mechanisms for the MOC during the LGM has the potential to improve its quantification with important implications for efforts to quantify the glacial ocean?s carbon cycle and the resolution of the great puzzle of the glacial - interglacial variations in atmospheric carbon dioxide. A beneficial side effect of this project may also be that it improves the present day simulation of tidal mixing, diffusivities and circulation in two climate models that are widely used by the scientific community and influence the design of future Paleoclimate Modeling Intercomparison Projects. A post-doctoral scientist will be supported and trained in running and analyzing a tide model. An undergraduate student will be exposed to research through a summer internship. A conference session will be organized with the goal to bring together modern physical oceanographers and paleoceanographers to foster interdisciplinary exchange of ideas.A detailed modeling study to investigate effects of tidal mixing on the present day and Last Glacial Maximum (LGM) Meridional Overturning Circulation (MOC) will be conducted using a hierarchy of numerical models. Simulations with a global tide model will calculate distributions of tidal energy dissipation, which will be supplied to two global climate/ocean circulation models to quantify their effects on mixing and the MOC. Sensitivity experiments will explore uncertainties due to different proposed parameterizations of internal wave drag, tide model resolution, LGM stratification, floating ice, spatial variations in sea level, and the vertical decay of mixing above the sea floor on the results. Climate model simulations will be conducted with an intermediate complexity model and a state-of-the-science earth system model. Present day simulations will be evaluated by comparison to observational estimates of diapycnal diffusivities and tracer distributions. The effects of different circulations on biogeochemical cycles and isotopes of carbon and nitrogen will also be simulated. Finally, the LGM model results will be compared to paleo-reconstructions from available proxy records in order to evaluate the simulated circulations.