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Optimal observations for variational data assimilation: |
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Determining the overturning in the North Atlantic |
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|
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Armin Koehl |
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An important aspect of ocean state estimation is the design of an |
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observing system that allows to efficiently study climate aspects |
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in the ocean. A solution of a design problem is presented |
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here in terms of optimal observations that emerge as nondimensionalized |
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singular vectors of the data resolution matrix. It is demonstrated |
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that such optimal observations when applied to determining the strength |
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of the overturning stream-function in the North Atlantic at 30 N in 900 m |
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perform significantly better than section data in a 1 degree model. |
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Preferred locations for hydrographic observations are primarily located |
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along the western boundary north of 30N and along the eastern boundary |
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south of 30 N and in the Iberian Sea. |
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By revealing the main processes that accompany anomalies of the overturning, |
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optimal observations are ideally suited for studying causes and effects of |
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anomalies. The response on short time-scales is mainly wind driven and |
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accounts for about two thirds of the anomaly, it includes Eckman transport |
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and coastal up- and down-welling. A small part of the response is buoyancy |
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driven and a slow response to primarily winter time anomalies in the Labrador |
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Sea. Temperature and salinity anomalies in the initial conditions manifests |
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induce alterations of the western boundary currents. The overall picture for |
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year 1997 is an east west density contrast that manifests primarily along |
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the boundaries in the upper 1000 m. |
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