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，大部分潮汐能量被耗散在浅层大陆架上，而一小部分潮汐能量在深海引起混合，这为子午线翻转环流(MOC)提供了动力。对古潮汐的研究表明，在末次冰盛期(LGM)，由于海平面下降了约120m，这种情况截然不同，耗散从浅海陆架转移到深海。这一发现引发了一种假设，即在最后一次冰盛期期间，经向翻转环流一定更强。然而，最近的研究结果旨在量化这种耗散变化对末次冰期的MOC的影响，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层结对湍流扩散、混合和MOC的影响。其他不确定因素也将被量化，从而导致对潮汐混合的变化及其对末次冰盖模型MOC的影响的全面估计。因此，该项目将有助于更好地了解在根本不同的气候条件下控制全球范围海洋环流变化的过程及其相关的生物地球化学循环。更好地理解末次盛会期间MOC的驱动机制有可能改进其量化，这对量化冰川海洋-S碳循环和解开冰川-间冰期大气二氧化碳变化这一巨大谜题具有重要意义。该项目的一个有益的副作用还可能是，它改进了目前科学界广泛使用的两个气候模式中的潮汐混合、扩散系数和环流的模拟，并影响到未来古气候模拟国际比较项目的设计。一名博士后科学家将在运行和分析潮汐模型方面得到支持和培训。本科生将通过暑期实习接触到研究。将组织一次会议，目的是将现代物理海洋学家和古海洋学家聚集在一起，促进跨学科的思想交流。将使用一系列数值模型进行详细的模拟研究，以调查潮汐混合对现代和末次冰川最大(LGM)子午线翻转环流(MOC)的影响。全球潮汐模式的模拟将计算潮汐能量耗散的分布，这些分布将被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。敏感性实验将探索由于内波阻力、潮汐模型分辨率、LGM层结、浮冰、海平面的空间变化以及海底以上混合的垂直衰减等不同参数对结果的不确定因素。气候模型模拟将使用中等复杂性模型和最先进的地球系统模型进行。目前的模拟将通过与观测估计的昼夜扩散系数和示踪剂分布进行比较来评估。还将模拟不同循环对生物地球化学循环和碳、氮同位素的影响。最后，将LGM模型的结果与现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.