Glacial Ice Sheet Extent Effects on Modeled Tidal Mixing and the Global Overturning Circulation

At present, tides supply approximately half (1 TW) of the energy necessary to sustain the global deep meridional overturning circulation (MOC) through diapycnal mixing. During the Last Glacial Maximum (LGM; 19,000-26,500 years BP), tidal dissipation in the open ocean may have strongly increased due to the 120- to 130-m global mean sea level drop and changes in ocean basin shape. However, few investigations into LGM climate and ocean circulation consider LGM tidal mixing changes. Here, using an intermediate complexity climate model, we present a detailed investigation on how changes in tidal dissipation would affect the global MOC. Present-day and LGM tidal constituents M-2, S-2, K-1, and O-1 are simulated using a tide model and accounting for LGM bathymetric changes. The tide model results suggest that the LGM energy supply to the internal wave field was 1.8-3 times larger than at present and highly sensitive to Antarctic and Laurentide ice sheet extent. Including realistic LGM tide forcing in the LGM climate simulations leads to large increases in Atlantic diapycnal diffusivities and strengthens (by 14-64% at 32 degrees S) and deepens the Atlantic MOC. Increased input of tidal energy leads to a greater drawdown of North Atlantic Deep Water and mixing with Antarctic Bottom Water altering Atlantic temperature and salinity distributions. Our results imply that changes in tidal dissipation need be accounted for in paleoclimate simulation setup as they can lead to large differences in ocean mixing, the global MOC, and presumably also ocean carbon and other biogeochemical cycles.