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

摘要

The first turbulence profiler observations beneath land fast sea ice whichis directly adjacent to an Antarctic ice shelf are described. Thestratification in the 325 m deep water column consisted of a layer ofsupercooled water in the upper 40 m lying above a quasi-linearly stratifiedwater column with a sharp step in density at mid-depth. Turbulent energydissipation rates were on average 3×10−8 m2 s−3 with peakbin-averaged values reaching 4×10−7 m2 s−3. The localdissipation rate per unit area was estimated to be 10 m Wm−2 on averagewith a peak of 50 m Wm−2. These values are consistent with a moderatebaroclinic response to the tides. The small-scale turbulent energetics lieon the boundary between isotropy and buoyancy-affected. This will likelyinfluence the formation and aggregation of frazil ice crystals within thesupercooled layer. The data suggest that the large crystals observed inMcMurdo Sound will transition from initial growth at scales smaller than theKolmogorov lengthscale to sizes substantially (1–2 orders of magnitude)greater than the Kolmogorov scale. An estimate of the experiment-averagedvertical diffusivity of mass Kρ yields a coefficient of around2×10−4 m2s−1 although this increased by a factor of 2 nearthe surface. Combining this estimate of Kρ with availableobservations of average and maximum currents suggests the layer ofsupercooled water can persist for a distance of ~250 km from the frontof the McMurdo Ice Shelf.