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| Titel |
Exploring the isopycnal mixing and helium–heat paradoxes in a suite of Earth system models |
| VerfasserIn |
A. Gnanadesikan, M.-A. Pradal, R. Abernathey |
| 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 ; 11, no. 4 ; Nr. 11, no. 4 (2015-07-27), S.591-605 |
| Datensatznummer |
250117266
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| Publikation (Nr.) |
 copernicus.org/os-11-591-2015.pdf |
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| Zusammenfassung |
| This paper uses a suite of Earth system models which simulate the
distribution of He isotopes and radiocarbon to examine two paradoxes
in Earth science, each of which results from an inconsistency between
theoretically motivated global energy balances and direct observations.
The helium–heat paradox refers to the fact that
helium emissions to the deep ocean are far lower than would be
expected given the rate of geothermal heating, since both are
thought to be the result of radioactive decay in Earth's
interior. The isopycnal mixing paradox comes from the fact that many
theoretical parameterizations of the isopycnal mixing coefficient
ARedi that link it to baroclinic instability project it
to be small (of order a few hundred m2 s−1) in the
ocean interior away from boundary currents. However, direct
observations using tracers and floats (largely in the upper ocean)
suggest that values of this coefficient are an order of magnitude
higher. Helium isotopes equilibrate rapidly with the atmosphere and thus exhibit large gradients
along isopycnals while radiocarbon equilibrates slowly and thus exhibits
smaller gradients along isopycnals. Thus it might be thought that resolving
the isopycnal mixing paradox in favor of the higher observational estimates
of ARedi might also solve the helium paradox, by increasing the
transport of mantle helium to the surface more than it would radiocarbon. In
this paper we show that this is not the case. In
a suite of models with different spatially constant and spatially
varying values of ARedi the distribution of radiocarbon
and helium isotopes is sensitive to the value of
ARedi. However, away from strong helium sources in the
southeastern Pacific, the relationship between the two is not
sensitive, indicating that large-scale advection is the limiting
process for removing helium and radiocarbon from the deep ocean. The
helium isotopes, in turn, suggest a higher value of
ARedi below the thermocline than is seen in theoretical
parameterizations based on baroclinic growth rates. We argue that
a key part of resolving the isopycnal mixing paradox is to abandon
the idea that ARedi has a direct relationship to local
baroclinic instability and to the so-called "thickness" mixing
coefficient AGM. |
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