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

点击翻译按钮获取中文摘要

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.