How does heat get into the deep ocean?

The role of the ocean's overturning circulation in transporting heat vertically is uncertain. Classical theories describe a thermally direct circulation where cold water downwells and warmer water upwells, cooling the deep ocean. More recently, the upper limb of the overturning circulation has been described as adiabatic, where water downwells and upwells at the same temperature leading to little vertical transport of heat. Here, possible states of the overturning circulation are described ranging from thermally direct to a combination of thermally direct and mechanically direct. In the latter case relatively warm water of the upper limb is pumped downward and mixes in the deep ocean with cold waters of the lower limb. The various states have contrasting driving mechanisms and imply contrasting climate sensitivity. A range of multi-millennial climate simulations and forced eddying ocean hindcasts are diagnosed. Apparently credible simulations can exhibit the full range of states. Strategies are proposed for determining which models and hence which thermodynamic state best represents the real ocean.