The current view of the global meridional overturning circulation (MOC) is one where the Atlantic is a dominant basin for deep water formation, but with the potential for bipolar seesaws between the Southern Ocean and the North Atlantic during catastrophic freshening events in either hemisphere. Here we investigate the stability of this paradigm through the response of an intermediate complexity coupled climate model, set in an oceanically more sensitive glacial configuration, to variation in the vertical mixing rates. It is found that the convective basin is set by the upper ocean diffusivity, with higher such diffusivities leading to a Pacific-dominated MOC. The upper ocean diffusivity is found to have a larger impact on the MOC than catastrophic flood events. It is known that deep ocean mixing rates were enhanced during glacial periods due to greater deep ocean tidal dissipation in the shallower oceans, with less extensive continental shelves. It is hypothesized that, combined with modified atmospheric states, there has been potential for the MOC to significantly alter between different glacial periods.