Tidal mixing and the meridional overturning circulation from the Last Glacial Maximum

Using a global tidal model it is shown that the supply of tidal energy to the deep ocean was larger during the Last Glacial Maximum (LGM; 18,000-22,000 years BP). The results were used to modify the rate of vertical mixing in a coupled ocean-atmosphere model set up for the present and LGM oceans. The increased levels of mechanical energy during the LGM were countered by a fresher upper North Atlantic, which led to a reduced circulation and deep water formation in spite of a trebling of the implicit mixing energy. This identifies the significance of accurately representing vertical mixing in climate models to estimate the recovery time-scales and timings of rapid catastrophic paleoceanographic events. From the estimated levels of implicit energy in the vertical mixing scheme an amendment to diffusivity based mixing schemes is suggested.