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| Titel |
Mixed layer sub-mesoscale parameterization – Part 1: Derivation and assessment |
| VerfasserIn |
V. M. Canuto, M. S. Dubovikov |
| 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 ; 6, no. 3 ; Nr. 6, no. 3 (2010-07-16), S.679-693 |
| Datensatznummer |
250003579
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| Publikation (Nr.) |
 copernicus.org/os-6-679-2010.pdf |
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| Zusammenfassung |
Several studies have shown that sub-mesoscales (SM ~1 km horizontal
scale) play an important role in mixed layer dynamics. In particular, high
resolution simulations have shown that in the case of strong down-front
wind, the re-stratification induced by the SM is of the same order of the
de-stratification induced by small scale turbulence, as well as of that
induced by the Ekman velocity. These studies have further concluded that it
has become necessary to include SM in ocean global circulation models
(OGCMs), especially those used in climate studies.
The goal of our work is to derive and assess an analytic
parameterization of the vertical tracer flux under baroclinic instabilities and
wind of arbitrary directions and strength. To achieve this goal, we have divided the
problem into two parts: first, in this work we derive and assess a
parameterization of the SM vertical flux of an arbitrary tracer for ocean
codes that resolve mesoscales, M, but not sub-mesoscales, SM. In Part 2,
presented elsewhere, we have used the results of this work to derive a
parameterization of SM fluxes for ocean codes that do not resolve either M
or SM.
To carry out the first part of our work, we solve the SM dynamic equations
including the non-linear terms for which we employ a closure developed and
assessed in previous work. We present a detailed analysis for down-front and
up-front winds with the following results:
(a) down-front wind (blowing in the direction of the surface geostrophic velocity) is the most
favorable condition for generating vigorous SM eddies; the de-stratifying
effect of the mean flow and re-stratifying effect of SM almost cancel each
other out,
(b) in the up-front wind case (blowing in the direction opposite to the surface geostrophic
velocity), strong winds prevents the SM generation while weak winds hinder
the process but the eddies amplify the re-stratifying effect of the
mean velocity,
(c) wind orthogonal to the geostrophic velocity. In this case,
which was not considered in numerical simulations, we show
that when the wind direction coincides with that of the horizontal buoyancy
gradient, SM eddies are generated and their re-stratifying effect partly
cancels the de-stratifying effect of the mean velocity. The case when wind
direction is opposite to that of the horizontal buoyancy gradient, is
analogous to the case of up-front winds.
In conclusion, the new multifaceted implications on the mixed layer
stratification caused by the interplay of both strength and directions of
the wind in relation to the buoyancy gradient disclosed by high resolution
simulations have been reproduced by the present model.
The present results can be used in OGCMs that resolve M but not SM. |
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