Topographic Influences on Wind-Driven, Stratified Flow in a β-Plane Channel: An Idealized Model for the Antarctic Circumpolar Current

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  • 1 IFREMER, Brest, France
  • 2 National Center for Atmospheric Research, Boulder, Colorado
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Abstract

Topographic influences are examined in an eddy-resolving model of oceanic channel flow forced by steady zonal winds. With small explicit lateral friction, transient eddies generated by the baroclinic instability of the mean flow transfer momentum downward to the bottom layer. In the flat-bottom case, bottom friction is the only efficient sink of eastward momentum. When bottom topography is present, the topographic form stress can replace the bottom friction sink in the momentum budget, and a large decrease of the zonal transport results. Large wale topography (of the scale of the forcing) provides the largest form stress. Topographic effects decay with height as suggested by the Prandit scaling, and therefore only topographic scales larger than the Rossby radius can affect the whole water column. In that case, the interfaces are deformed by standing eddies on topographic length scales, and standing eddies replace transient eddies in transferring momentum downward. The bottom-layer mean streamfunction tends to be correlated with the topography as in inviscid solutions. Because of this, only a small part of the flow (the larger scales) contributes to the domain-averaged momentum sink. On smaller scales, the topographic form stress is anticorrelated with the Reynolds stress and has no net effect on the transport. The energy level of the transients is less affected by the topography than is the mean energy. With topography, the space scale of the transients decreases and their time scale increases, and the ratio of potential and kinetic energies is higher.

Abstract

Topographic influences are examined in an eddy-resolving model of oceanic channel flow forced by steady zonal winds. With small explicit lateral friction, transient eddies generated by the baroclinic instability of the mean flow transfer momentum downward to the bottom layer. In the flat-bottom case, bottom friction is the only efficient sink of eastward momentum. When bottom topography is present, the topographic form stress can replace the bottom friction sink in the momentum budget, and a large decrease of the zonal transport results. Large wale topography (of the scale of the forcing) provides the largest form stress. Topographic effects decay with height as suggested by the Prandit scaling, and therefore only topographic scales larger than the Rossby radius can affect the whole water column. In that case, the interfaces are deformed by standing eddies on topographic length scales, and standing eddies replace transient eddies in transferring momentum downward. The bottom-layer mean streamfunction tends to be correlated with the topography as in inviscid solutions. Because of this, only a small part of the flow (the larger scales) contributes to the domain-averaged momentum sink. On smaller scales, the topographic form stress is anticorrelated with the Reynolds stress and has no net effect on the transport. The energy level of the transients is less affected by the topography than is the mean energy. With topography, the space scale of the transients decreases and their time scale increases, and the ratio of potential and kinetic energies is higher.

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