Editorial Type:
Article
Article Type:
Research Article

Available Potential Energy and Irreversible Mixing in the Meridional Overturning Circulation

Graham O. Hughes The Australian National University, Canberra, Australia

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Andrew Mc C. Hogg The Australian National University, Canberra, Australia

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Ross W. Griffiths The Australian National University, Canberra, Australia

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Print Publication:
01 Dec 2009
DOI:
https://doi.org/10.1175/2009JPO4162.1
Page(s):
3130–3146
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Abstract

The overturning circulation of the global oceans is examined from an energetics viewpoint. A general framework for stratified turbulence is used for this purpose; first, it highlights 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 the 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.

Corresponding author address: Graham O. Hughes, Research School of Earth Sciences, The Australian National University, Canberra, ACT 0200, Australia. Email: Graham.Hughes@anu.edu.au

Abstract

The overturning circulation of the global oceans is examined from an energetics viewpoint. A general framework for stratified turbulence is used for this purpose; first, it highlights 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 the 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.

Corresponding author address: Graham O. Hughes, Research School of Earth Sciences, The Australian National University, Canberra, ACT 0200, Australia. Email: Graham.Hughes@anu.edu.au

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