Abstract
Here, we present the first electromagnetic induction time-series measurements of ice shelf-influenced fast ice and sub-ice platelet layer thickness over winter and in late spring in McMurdo Sound. Significant increases in sub-ice platelet layer thickness (~0.5–1 m) co-occurred with strong southerly wind events and satellite-observed polynya activity suggesting wind-driven surface circulation of supercooled Ice Shelf Water outflow from the McMurdo-Ross ice shelf cavity. Temporal variability observed in sub-ice platelet layer thickness on diurnal timescales correlated with tidally-induced current patterns previously observed in McMurdo Sound. The thickness of the sub-ice platelet layer increased on spring and neap ebb tides corresponding with northward currents circulating out from the ice shelf cavity. The late spring spatial distributions of first-year and second-year fast ice and sub-ice platelet layer thickness in McMurdo Sound were assessed with drill hole and electromagnetic induction surveys and were comparable to a previous four-year dataset. We resolved second-year fast ice thicknesses of 4 m with a substantial sub-ice platelet layer beneath of up to 11 m using electromagnetic induction techniques suggesting that the longer temporal persistence of the two-year-old fast ice allowed a substantially thicker sub-ice platelet layer to form. The variability observed in the sub-ice platelet layer indicates that a combination of the tides, wind-driven polynya activity and the presence of multi-year ice influences the circulation of Ice Shelf Water in the upper surface ocean and consequently sub-ice platelet layer formation over a range of timescales.
Highlights
In the western Ross Sea, sea ice production in coastal polynyas forms High Salinity Shelf Water (HSSW) (Ohshima et al, 2016) which circulates into the conjoined McMurdo-Ross ice shelf cavity and drives basal melting at depth in the grounding zone 35 (Jacobs et al, 1992)
In November 2018, the fast ice in McMurdo Sound consisted of a 7-15 km wide band of SY ice that ran parallel to the McMurdo Ice Shelf (MIS) in the 195 south and more typical FY ice in the north (Fig. 1b)
The late spring spatial distributions of ice shelf-influenced fast ice and the sub-ice platelet layer have been well described in 520 McMurdo Sound and compared on interannual timescales in previous work by the authors
Summary
In the western Ross Sea, sea ice production in coastal polynyas forms High Salinity Shelf Water (HSSW) (Ohshima et al, 2016) which circulates into the conjoined McMurdo-Ross ice shelf cavity and drives basal melting at depth in the grounding zone 35 (Jacobs et al, 1992). ISW is relatively fresh and buoyant, and can rise along the ice shelf base from deep source regions in the cavity (Macayeal, 1984). As ISW rises adiabatically and the pressure decreases, it can become supercooled (Foldvik and Kvinge, 1974; Jenkins and Bombosch, 1995) promoting frazil ice formation (Holland and Feltham, 2005; Robinson et al, 2014). Frazil ice can grow into larger platelet ice crystals with continued bathing in supercooled ISW (Smith et al, 2012)
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