Profiles of diapycnal turbulent diffusivity K are estimated from hydrographic andvelocity shear observations at 200 CTD/LADCP stations in the Scotia Sea, providingsome of the first direct determinations of diapycnal mixing in the deep Southern Ocean.The calculation technique is based on a comparison of the power spectraldensity of the measured velocity shear with that predicted by the Garrett-Munk (GM76)model, and yields estimates of K that are accurate to within a factor of 2-3.
The resulting distribution of diapycnal mixing reveals a clear link to topographicroughness. High (K > 10-4 m2 s-1) diffusivitiesare diagnosed at depths in excess of 500-1000 m over a large sector of the basin, peakingat values close to 10-3 - 10-2 m2 s-1near the sill of Drake Passage, the South Scotia Ridge and the rough sea floorin the eastern Scotia Sea. In these areas, an obvious signature of upward energypropagation emanating from the bottom topography is detected in profiles of velocityshear. The smoother bathymetry of the northern reaches of the basin is associatedwith lower diffusivities of order 5 x 10-5 - 10-4m2 s-1 in the same depth range. There is a broad reductionof mixing rates in the upper ocean, where often K is only weakly heightened relative tothe GM76 prediction of 10-5 m2 s-1.
The diagnosed K distribution shows a good qualitative correlation with estimates ofstrain variance obtained from a spectral analysis of the finestructure in CTD densityprofiles. Careful examination of the relationship between these two variablessuggests a prominent role of semidiurnal internal tides in driving the cascade ofenergy to scales of turbulent dissipation. The interaction of the deep-reachingAntarctic Circumpolar Current jets with the underlying bathymetry, which generatesinternal lee waves, is also indicated as a significant energy source for diapycnalmixing in the region. |
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