Observations of turbulence beneath sea ice in southern McMurdo Sound, Antarctica

Abstract

Abstract. The first turbulence profiler observations beneath land fast sea ice which is directly adjacent to an Antarctic ice shelf are described. The stratification in the 325 m deep water column consisted of a layer of supercooled water in the upper 40 m lying above a quasi-linearly stratified water column with a sharp step in density at mid-depth. Turbulent energy dissipation rates were on average 3×10 −8 m 2 s −3 with peak bin-averaged values reaching 4×10 −7 m 2 s −3 . The local dissipation rate per unit area was estimated to be 10 m Wm −2 on average with a peak of 50 m Wm −2 . These values are consistent with a moderate baroclinic response to the tides. The small-scale turbulent energetics lie on the boundary between isotropy and buoyancy-affected. This will likely influence the formation and aggregation of frazil ice crystals within the supercooled layer. The data suggest that the large crystals observed in McMurdo Sound will transition from initial growth at scales smaller than the Kolmogorov lengthscale to sizes substantially (1-2 orders of magnitude) greater than the Kolmogorov scale. An estimate of the experimentaveraged vertical diffusivity of mass K ρ yields a coefficient of around 2×10 −4 m 2 s −1 although this increased by a factor of 2 near the surface. Combining this estimate of K ρ with available observations of average and maximum currents suggests the layer of supercooled water can persist for a distance of ∼250 km from the front of the McMurdo Ice Shelf.

The sub-ice platelet layer and its influence on freeboard to thickness conversion of Antarctic sea ice

“…Sea ice thickness was overestimated on average by 12 % in southern McMurdo Sound as a result of the buoyant influence of the sub-ice platelet layer on the sea ice cover above. The influence of the ice shelf is expected to extend beyond 200 km from the edge of the McMurdo Ice Shelf (Stevens et al, 2009). Platelet ice observations partly confirm this and have been recorded in sea ice cores 80 km north of the ice shelf edge (Dempsey et al, 2010).…”

Flow and mixing near a glacier tongue: a pilot study

“…During the period of postpulse oscillation N 2 was at detectable limits of ∼10 −8 s −2 . These values bracket other comparable observations in the Sound where N 2 =3 × 10 −6 s −2 was observed over the ridge running off Cape Armitage (Stevens et al, 2009).…”

“…This enabled estimation of the turbulent energy dissipation rate ε units of m 2 s −3 equivalent to W kg −1 ) his device has been used in such conditions previously (Stevens et al, 2009). The main physical modification to the profiler involved the use of a circular drag brush rather than the standard pair of square drag elements as provided by the manufacturer.…”

“…As described in Stevens et al (2009), energy dissipation rates were resolved from the dual shear probe profiler using standard techniques (Prandke, 2005). The profiles were segmented into 5 m thick bins that were overlapped by 50 %.…”

“…The energy dissipation rate (ε) rose, albeit for localized moments in time, as much as three orders of magnitude above what might be regarded as normal background levels (∼10 −8 m 2 s −3 , Stevens et al, 2009). In a situation with similar headland scale and flow speeds Edwards et al (2004) quantified how local turbulent buoyancy flux scaled with ε and that it is likely maximum very close to a headland.…”