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| Titel |
Understanding mixing efficiency in the oceans: do the nonlinearities of the equation of state for seawater matter? |
| VerfasserIn |
R. Tailleux |
| Medientyp |
Artikel
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| Sprache |
Englisch
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| ISSN |
1812-0784
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| Digitales Dokument |
URL |
| Erschienen |
In: Ocean Science ; 5, no. 3 ; Nr. 5, no. 3 (2009-07-20), S.271-283 |
| Datensatznummer |
250002577
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| Publikation (Nr.) |
 copernicus.org/os-5-271-2009.pdf |
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| Zusammenfassung |
| There exist two central measures of turbulent mixing in
turbulent stratified fluids that are both caused by molecular diffusion:
1) the dissipation rate D(APE) of available potential energy APE;
2) the turbulent rate of change Wr, turbulent of background gravitational
potential energy GPEr. So far, these two quantities have often been
regarded as the same energy conversion, namely the irreversible conversion
of APE into GPEr, owing to the well known exact equality D(APE)=Wr, turbulent
for a Boussinesq fluid with a linear equation of state.
Recently, however, Tailleux (2009) pointed out that the above equality
no longer holds for a thermally-stratified compressible, with the ratio
ξ=Wr, turbulent/D(APE) being generally lower than unity
and sometimes even negative for water or seawater, and argued that D(APE)
and Wr, turbulent actually represent two distinct types of energy
conversion, respectively the dissipation of APE into one particular subcomponent
of internal energy called the "dead" internal energy IE0, and the conversion
between GPEr and a different subcomponent of internal energy called
"exergy" IEexergy. In this paper, the behaviour of the
ratio ξ is examined
for different stratifications having all the same buoyancy frequency
N vertical profile, but different vertical profiles of the parameter
Υ=α P/(ρCp),
where α is the thermal expansion coefficient, P the
hydrostatic pressure, ρ the density, and Cp the specific heat capacity
at constant pressure, the equation
of state being that for seawater for different particular constant values of salinity.
It is found that ξ and Wr, turbulent depend critically on the sign
and magnitude of dΥ/dz, in contrast with D(APE), which appears largely
unaffected by the latter. These results have important consequences for how
the mixing efficiency should be defined and measured in practice, which are
discussed. |
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