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, ~20,000 years ago), due to the sea level drop of ~120 m, this situation was drastically different and dissipation was shifted from the shallow into the deep ocean, which has prompted the hypothesis that the LGM MOC must have been stronger. However, previous work quantifying the effects of this dissipation shift on the LGM MOC came to conflicting conclusions, ranging from negligible effects to a large increase. This project will reveal the reasons behind these differences and test the aforementioned hypothesis. It will also provide the first quantification of the effects of realistic, data constrained, LGM stratification on diffusivities, mixing and the MOC. Other uncertainties, as indicated above, will also be quantified thus leading to a comprehensive estimate of changes in tidal mixing on the LGM MOC. Therefore this project will lead to a better understanding of processes that control planetary-scale ocean circulation changes in fundamentally different climates.
In a detailed modeling study we will investigate effects of tidal mixing on the present day and LGM MOC. 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 model. PD simulations will be evaluated by comparison to observational estimates of diapycnal diffusivities and tracer distributions. Effects of different circulations on biogeochemical cyles and isotopes of carbon and nitrogen will also be simulated. Comparisons of LGM model results with paleo-reconstructions will be used to evaluate the simulated paleo-circulations.
the National Science Foundation's Physical Oceanography Program