The zonal momentum balance in an eddy-resolving general-circulation model of the Southern Ocean

David P. Stevens, Vladimir O. Ivchenko

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The momentum balance in the zonally unbounded region of the Southern Ocean is examined using an eddy-resolving ocean general-circulation model (namely FRAM). Momentum, which is input at the surface and accelerates the Antarctic Circumpolar Current, is transferred down the water column and removed by topographic form stress. Bottom friction and lateral eddy viscosity are found to be negligible. the poleward flux of eastward momentum has a small effect in redistributing momentum. In spite of this, below the wind-driven surface layer and above the level of topography, the poleward momentum-flux divergence provides the main balance along with the ageostrophic flux of planetary vorticity (although the magnitude of these terms is an order of magnitude smaller than the wind stress). Below the Ekman layer, standing eddies produce a drag on the flow whilst transient eddies accelerate the flow. However, the impact of transient eddies is smaller. the downward transfer of momentum is achieved by interfacial form stress. This can be understood in terms of a poleward density (heat) flux. the main contribution comes from standing eddies, with a smaller contribution from transient eddies. Both contributions assist the transfer. the flux of density (heat) from the neighbouring oceans (to the north and south of the Drake Passage latitudes) influences the depth penetration of zonal momentum, particularly in the upper 1000 m. the Johnson-Bryden theory is generalized to give an additional term which is proportional to the stream function for the residual circulation associated with Eliassen-Palm cross-sections.
Original languageEnglish
Pages (from-to)929-951
Number of pages23
JournalQuarterly Journal of the Royal Meteorological Society
Issue number540
Publication statusPublished - Apr 1997

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