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| Titel |
Geothermal heating, diapycnal mixing and the abyssal circulation |
| VerfasserIn |
J. Emile-Geay, G. Madec |
| 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. 2 ; Nr. 5, no. 2 (2009-06-19), S.203-217 |
| Datensatznummer |
250002424
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| Publikation (Nr.) |
 copernicus.org/os-5-203-2009.pdf |
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| Zusammenfassung |
| The dynamical role of geothermal heating in abyssal circulation is reconsidered
using three independent arguments.
First, we show that a uniform geothermal heat flux close to the observed average
(86.4 mW m−2) supplies as much heat to near-bottom water as
a diapycnal mixing rate of ~10−4 m2 s−1 – the canonical
value thought to be responsible for the magnitude of the present-day abyssal
circulation. This parity raises the possibility that geothermal heating could
have a dynamical impact of the same order. Second, we estimate the magnitude
of geothermally-induced circulation with the density-binning method (Walin, 1982),
applied to the observed thermohaline structure of Levitus
(1998). The method
also allows to investigate the effect of realistic spatial variations of the
flux obtained from heatflow measurements and classical theories of lithospheric
cooling. It is found that a uniform heatflow forces a transformation of
~6 Sv at σ4=45.90, which is of the same order as current best estimates of AABW
circulation. This transformation can be thought of as the geothermal circulation in
the absence of mixing and is very similar for a realistic heatflow, albeit shifted
towards slightly lighter density classes. Third, we use a general ocean circulation
model in global configuration to perform three sets of experiments: (1) a thermally
homogenous abyssal ocean with and without uniform geothermal heating; (2) a more
stratified abyssal ocean subject to (i) no geothermal heating, (ii) a constant heat
flux of 86.4 mW m−2, (iii) a realistic, spatially varying heat
flux of identical global average; (3) experiments (i) and (iii) with enhanced
vertical mixing at depth. Geothermal heating and diapycnal mixing are found to
interact non-linearly through the density field, with geothermal heating eroding
the deep stratification supporting a downward diffusive flux, while diapycnal
mixing acts to map near-surface temperature gradients onto the bottom, thereby
altering the density structure that supports a geothermal circulation.
For strong vertical mixing rates, geothermal heating enhances the AABW cell by
about 15% (2.5 Sv) and heats up the last 2000 m by ~0.15°C,
reaching a maximum of by 0.3°C in the deep North Pacific. Prescribing a
realistic spatial distribution of the heat flux acts to enhance this temperature
rise at mid-depth and reduce it at great depth, producing a more modest increase
in overturning than in the uniform case. In all cases, however, poleward heat
transport increases by ~10% in the Southern Ocean. The three approaches
converge to the conclusion that geothermal heating is an important actor of
abyssal dynamics, and should no longer be neglected in oceanographic studies. |
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