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| Titel |
The predictability of large-scale wind-driven flows |
| VerfasserIn |
A. Mahadevan, J. Lu, S. P. Meacham, P. Malanotte-Rizzoli |
| Medientyp |
Artikel
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| Sprache |
Englisch
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| ISSN |
1023-5809
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| Digitales Dokument |
URL |
| Erschienen |
In: Nonlinear Processes in Geophysics ; 8, no. 6 ; Nr. 8, no. 6, S.449-465 |
| Datensatznummer |
250005889
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| Publikation (Nr.) |
 copernicus.org/npg-8-449-2001.pdf |
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| Zusammenfassung |
| The singular values
associated with optimally growing perturbations to stationary and
time-dependent solutions for the general circulation in an ocean basin
provide a measure of the rate at which solutions with nearby initial
conditions begin to diverge, and hence, a measure of the predictability of
the flow. In this paper, the singular vectors and singular values of
stationary and evolving examples of wind-driven, double-gyre circulations
in different flow regimes are explored. By changing the Reynolds number in
simple quasi-geostrophic models of the wind-driven circulation, steady,
weakly aperiodic and chaotic states may be examined. The singular vectors
of the steady state reveal some of the physical mechanisms responsible for
optimally growing perturbations. In time-dependent cases, the dominant
singular values show significant variability in time, indicating strong
variations in the predictability of the flow. When the underlying flow is
weakly aperiodic, the dominant singular values co-vary with integral
measures of the large-scale flow, such as the basin-integrated upper ocean
kinetic energy and the transport in the western boundary current
extension. Furthermore, in a reduced gravity quasi-geostrophic model of a
weakly aperiodic, double-gyre flow, the behaviour of the dominant singular
values may be used to predict a change in the large-scale flow, a feature
not shared by an analogous two-layer model. When the circulation is in a
strongly aperiodic state, the dominant singular values no longer vary
coherently with integral measures of the flow. Instead, they fluctuate in
a very aperiodic fashion on mesoscale time scales. The dominant singular
vectors then depend strongly on the arrangement of mesoscale features in
the flow and the evolved forms of the associated singular vectors have
relatively short spatial scales. These results have several implications.
In weakly aperiodic, periodic, and stationary regimes, the mesoscale
energy content is usually relatively low and the predictability of the
wind-driven circulation is determined by the large-scale structure of the
flow. In the more realistic, strongly chaotic regime, in which energetic
mesoscale eddies are produced by the meandering of the separated western
boundary current extension, the predictability of the flow locally tends
to be a stronger function of the local mesoscale eddy structure than of
the larger scale structure of the circulation. This has a broader
implication for the effectiveness of different approaches to forecasting
the ocean with models which sequentially assimilate new observations. |
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