Available potential energy and irreversible mixing in the meridional overturning circulation

The overturning circulation of the global oceans is examined from an energetics viewpoint. The general framework of Winters et al. (1995) for stratified turbulence is used, first to highlight the importance of available potential energy in facilitating the transfer of kinetic energy to the background potential energy (defined as the adiabatically-rearranged state with no motion). Next, it is shown that it is the rate of transfer between different energy reservoirs that is important for maintenance of the ocean overturning, rather than the total amount of potential or kinetic energy. A series of numerical experiments is used to assess which energy transfers are significant in the overturning circulation. In the steady state, the rate of irreversible diapycnal mixing is necessarily balanced by the production of available potential energy sourced from surface buoyancy fluxes. Thus, the external inputs of available potential energy from surface buoyancy forcing and of kinetic energy from other sources (such as surface winds and tides, and leading to turbulent mixing) are both necessary to maintain the overturning circulation.