A dense water outflow from the Ross Sea, Antarctica: Mixing and the contribution of tides

Abstract

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 (∼ 500 m isobath) to the ∼ 1600 m isobath on the continental slope. Outflow thickness was as great as ∼ 200 m, and mean speeds were ∼ 0.6 m s − 1 relative to background currents exceeding ∼ 1 m 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 ( K z ) 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 − 2 m 2 s − 1 . For two cases where the upper outflow boundary was particularly sharply defined, entrainment rate w e was estimated from K z and bulk outflow parameters to be ∼ 10 − 3 m s − 1 (∼ 100 m 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.