Observations in the boundary layer under the sea ice in McMurdo Sound

Mitchell, W. M.; Bye, John A. T.

doi:10.1029/ar043p0167

Cited by 14 publications

(22 citation statements)

References 8 publications

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“…During March and April, near surface flow in the east switches northward, and out of the cavity [ Leonard et al ., ; Mahoney et al ., ], this is thought to result from large‐scale changes in horizontal density gradients along the Ross Ice Shelf front that also drive changes to the deep HSSW flows [ Assmann et al ., ]. In contrast, the western flank of the sound is dominated by flow out from beneath the ice shelf [ Assmann et al ., ], typically ice‐laden and supercooled near the surface [ Dunbar and Leventer , ; Barry and Dayton , ; Mitchell and Bye , ; Lewis and Perkin , ].…”

Section: Introductionmentioning

confidence: 86%

Evolution of a supercooled Ice Shelf Water plume with an actively growing subice platelet matrix

et al. 2014

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We use new observations in Western McMurdo Sound, combined with longitudinal hydrographic transects of the sound, to identify a northward-flowing Ice Shelf Water (ISW) plume exiting the cavity of the McMurdo-Ross Ice Shelf. We estimate the plume's net northward transport at 0.4 6 0.1 Sv, carving out a corridor approximately 35 km wide aligned with the Victoria Land Coast. Basal topography of the McMurdo Ice Shelf is such that the plume is delivered to the surface without mixing with overlying warmer water, and is therefore able to remain below the surface freezing temperature at the point of observation beneath first-year ice. Thus, the upper ocean was supercooled, by up to 50 mK at the surface, due to pressure relief from recent rapid ascent of the steep basal slope. The 70 m thick supercooled layer supports the growth and maintenance of a thick, semirigid, and porous matrix of platelet ice, which is trapped by buoyancy at the ice-ocean interface. Continued growth of individual platelets in supercooled water creates significant brine rejection at the top of the water column which resulted in convection over the upper 200 m thick, homogeneous layer. By examining the diffusive nature of the intermediate water between layers of ISW and High Salinity Shelf Water, we conclude that the ISW plume must have originated beneath the Ross Ice Shelf and demonstrate that it is likely to expand eastward across McMurdo Sound with the progression of winter.

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Section: Introductionmentioning

confidence: 86%

Evolution of a supercooled Ice Shelf Water plume with an actively growing subice platelet matrix

et al. 2014

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“…[3] Earlier studies [Crocker and Wadhams, 1989] note that platelet ice makes its first appearance at the ice-water interface between July and September in McMurdo Sound and they relate its genesis to the heat content of the nearsurface ocean. However, this heat budget is influenced by currents that are notoriously variable and dominated by tidal effects [Deacon, 1975;Gilmour et al, 1962;Tressler and Ommundsen, 1962;Littlepage, 1965;Heath, 1971Heath, , 1977Jacobs et al, 1981;Lewis and Perkin, 1985;Mitchell and Bye, 1985;Gow et al, 1998;Barry and Dayton, 1988;Hunt et al, 2003]. A contribution to the water mass of the Sound originates from beneath the Ross Ice Shelf, reflecting subice-shelf processes [Jacobs et al, 1979].…”

Section: Introductionmentioning

confidence: 99%

Observations of platelet ice growth and oceanographic conditions during the winter of 2003 in McMurdo Sound, Antarctica

et al. 2006

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[1] Platelet ice is the name given to ice crystals that nucleate in the ocean and grow either at depth or loosely attached to the ice-water interface. Related to the proximity of ice shelves, it is known to form in supercooled seawater. This study details the conditions for its growth during the austral winter of 2003 in McMurdo Sound, Antarctica. A key finding is that the presence of platelet ice in the sea ice cover is conclusively linked to the time history of the appearance of ice crystals in the water column, monitored using the strength of the backscattered signal from an acoustic Doppler current profiler as a proxy. Generally, these crystals appeared from mid-May as water near the interface became supercooled. This near-surface supercooling appears to be related to a simultaneous, abrupt change in the structure of the upper 250 m of the water column, from one that behaved dynamically and contained both the warmest and coolest measured temperatures to one in which the water was essentially isothermal near its surface freezing temperature. Near-surface ocean temperature was also affected by tidal mixing and by an increase in the thickness and density, over the course of winter, of the surface mixed layer created by salt rejection during ice growth. These processes allowed cold water, trapped by buoyancy in a band at the base of the mixed layer in early winter, to gain access to the ice-water interface by midwinter. This band of cold water probably had its origin beneath the McMurdo Ice Shelf.

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“…The length scales L K , L e , and L Oz can be resolved using either profiles through the water column (e.g. Robertson et al, 1995;Fer and Widell, 2007;Rainville and Winsor, 2008) or, as has been more typical in sub-sea ice turbulence studies, through the use of timeseries of velocity fluctuations at fixed depths (Mitchell and Bye, 1985;McPhee, 1992McPhee, , 2004. A number of studies have sought to place turbulence observations in the Fr t −Re t domain (Imberger and Ivey, 1991;Stacey et al, 1999;Keeler et al, 2005) but none so far in a polar context.…”

Section: Scales Of Turbulencementioning

confidence: 99%

“…At smaller scales, Jacobs et al (1981) examined glacier tongue-affected water 15 km to the north of the present site and resolved diffusive-convective layering over horizontal scales of 5 km or more. Some of the first under-ice turbulence observations were recorded in McMurdo Sound but from very inshore locations (Mitchell and Bye, 1985). Very recently Muench et al (2009) quantified turbulence on the shelf slope 800 km to the north.…”

Section: Introductionmentioning

confidence: 99%

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

et al. 2009

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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.

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Observations in the boundary layer under the sea ice in McMurdo Sound

Cited by 14 publications

References 8 publications

Evolution of a supercooled Ice Shelf Water plume with an actively growing subice platelet matrix

Evolution of a supercooled Ice Shelf Water plume with an actively growing subice platelet matrix

Observations of platelet ice growth and oceanographic conditions during the winter of 2003 in McMurdo Sound, Antarctica

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

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