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| Titel |
On the time to tracer equilibrium in the global ocean |
| VerfasserIn |
F. Primeau, E. Deleersnijder |
| 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. 1 ; Nr. 5, no. 1 (2009-02-03), S.13-28 |
| Datensatznummer |
250002262
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| Publikation (Nr.) |
 copernicus.org/os-5-13-2009.pdf |
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| Zusammenfassung |
| An important issue for the interpretation of data from deep-sea
cores is the time for tracers to be transported from the sea surface
to the deep ocean. Global ocean circulation models can help shed
light on the timescales over which a tracer comes to equilibrium in
different regions of the ocean. In this note, we discuss how the
most slowly decaying eigenmode of a model can be used to obtain a
relevant timescale for a tracer that enters through the sea surface
to become well mixed in the ocean interior. We show how this
timescale depends critically on the choice between a Neumann surface
boundary condition in which the flux of tracer is prescribed, a
Robin surface boundary condition in which a combination of the flux
and tracer concentration is prescribed or a Dirichlet surface
boundary condition in which the concentration is prescribed.
Explicit calculations with a 3-box model and a three-dimensional
ocean circulation model show that the Dirichlet boundary condition
when applied to only part of the surface ocean greatly overestimate
the time needed to reach equilibrium. As a result
regional-"injection" calculations which prescribe the surface
concentration instead of the surface flux are not relevant for
interpreting the regional disequilibrium between the Atlantic and
Pacific found in paleo-tracer records from deep-sea cores. For
tracers that enter the ocean through air-sea gas exchange a
prescribed concentration boundary condition can be used to infer
relevant timescales if the air-sea gas exchange rate is sufficiently
fast, but the boundary condition must be applied over the entire
ocean surface and not only to a patch of limited area. For tracers
with a slow air-sea exchange rate such as 14C a Robin-type
boundary condition is more relevant and for tracers such as
δ18O that enter the ocean from melt water, a Neumann
boundary condition is presumably more relevant. Our
three-dimensional model results based on a steady-state modern
circulation suggest that the relative disequilibrium between the
deep Atlantic and Pacific is on the order of "only" 1200 years or
less for a Neumann boundary condition and does not depend on the size
and location of the patch where the tracer is injected. |
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