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Adjoint Sensitivity of an Ocean General Circulation Model to Bottom Topography

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Losch, M. and Heimbach, P. (2007): Adjoint Sensitivity of an Ocean General Circulation Model to Bottom Topography , Journal of physical oceanography, 37(2), pp. 377-393 . doi: 10.1175/JPO3017.1
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Abstract:

Bottom topography, or more generally the geometry of the ocean basins,is an important ingredient in numerical ocean modeling.With the help of an adjoint model, it is shown that scalardiagnostics in a coarse resolution model, such as the transportthrough Drake Passage, the strength of the Atlantic meridionaloverturning circulation, the Deacon cell, and the meridional heattransport across 32S, are sensitive to bottom topography as muchas to surface boundary conditions. For example, adjoint topography sensititivities of the transportthrough Drake Passage are large in choke points areas such as theCrozet-Kerguelen Plateau and south of New Zealand; the Atlanticmeridional overturning circulation is sensitive to topography in thewestern boundary region of the North Atlantic Ocean and along theScotland-Iceland ridge.The adjoint sensitivities are found to agree to direct perturbationmethods with deviations smaller than 30 % for significantperturbations on time scales of 100 years, so that the assumptionof quasi-linearity that is implicit to the adjoint method holds.The horizontal resolution of the numerical model affects the detailsof the sensitivities to bottom topography, while large scalepatterns and the overall impact of changes in topography appear tobe robust.The relative impact of changes in topography and surface boundaryconditions on the model circulation is estimated by multiplying theadjoint sensitivities with assumed uncertainties.The effects are locally on the same order of magnitude ifuncorrelated uncertainties are assumed. If the changes arecorrelated in space, changing the surface boundary conditions has alarger impact on the scalar diagnostics.Bottom topography ought to be included in the control vector informal state estimation.

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