A dense water outflow from the Ross Sea, Antarctica: Mixing and the contribution of tides
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We use hydrographic, current, and microstructure measurements, and tide-forced ocean models, to estimate benthic and interfacial mixing impacting the evolution of a bottom-trapped outflow of dense shelf water from the Drygalski Trough in the northwestern Ross Sea. During summer 2003 an energetic outflow was observed from the outer shelf ( 500m isobath) to the 1600m isobath on the continental slope. Outflow thickness was as great as 200m, and mean speeds were 0.6m s- 1 relative to background currents exceeding 1m s- 1 that were primarily tidal in origin. No outflow was detected on the slope in winter 2004, although a thin layer of dense shelf water was present on the outer shelf. When the outflow was well-developed, the estimated benthic stress was of order one Pascal and the bulk Froude number over the upper slope exceeded one. Diapycnal scalar diffusivity (Kz) values in the transition region at the top of the outflow, estimated from Thorpe-scale analysis of potential density and measurements of microscale temperature gradient from sensors attached to the CTD rosette, were of order 10- 3-10- 2m2 s- 1. For two cases where the upper outflow boundary was particularly sharply defined, entrainment rate we was estimated from Kz and bulk outflow parameters to be 10- 3m s- 1 ( 100m day- 1). A tide-forced, three-dimensional primitive equation ocean model with Mellor-Yamada level 2.5 turbulence closure scheme for diapycnal mixing yields results consistent with a significant tidal role in mixing associated with benthic stress and shear within the stratified ocean interior. 2008 Elsevier B.V. All rights reserved.
