Signatures of supercooling: McMurdo Sound platelet ice

Abstract

ABSTRACT. Near ice shelves around Antarctica the ocean becomes supercooled and has been observed to carry small suspended ice crystals. Our measurements demonstrate that these small crystals are persistently present in the water column beneath the winter fast ice, and when incorporated in sea ice they reduce the mean grain size of the sea-ice cover. By midwinter, larger ice crystals below the ice/water interface are observed to form a porous sub-ice platelet layer with an ice volume fraction of 0.25 ± 0.06. The magnitude and direction of the oceanic heat flux varied between (5 ± 6) W m

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

“…The ISW plume is the second mechanism. The influence of this plume on sea ice processes has been documented in studies of sea ice structure and growth Dempsey et al, 2010;Mahoney et al, 2011;Gough et al, 2012). Satellite altimeter observations have indicated that the locations of the largest increases in multiyear sea ice thickness from 2003-2009 during the NASA ICESat mission (Price et al, 2013) were coincident with the greatest abundance of platelet ice (Dempsey et al, 2010).…”

“…Using this method in McMurdo Sound, Gough et al (2012) report ρ i as 934 kg m −3 . Using an amended method at seven of our measurement sites in 2011 (where ρ i estimate for each site is the mean of ρ i derived from five drill-hole measurements -see Fig.…”

“…This was deployed vertically through the drillhole and allowed to rotate to a horizontal alignment when exiting the bottom of the drill-hole at the ice-ocean interface. From this position, and as described in Gough et al (2012) the anchor is slowly pulled upward until some resistance is met and the first measurement is taken. This resistance is taken to mark the sub-ice platelet layer/ocean interface.…”

“…At the large scale, the interaction of water sourced from ice shelf basal melting, which freshens the surface ocean, has been suggested as a potential contributor to increasing sea ice extent in the Southern Ocean (Bintanja et al, 2013). Of further interest, it is well known that the outflow of supercooled water from the ice shelf cavity creates an additional heat sink to the ocean promoting sea ice growth (Trodahl et al, 2000;Hellmer, 2004;Purdie et al, 2006;Gough et al, 2012), which increases sea ice thickness in close proximity to ice shelves (Hellmer, 2004;Hughes et al, 2014). This additional ice that forms as a direct result of oceanic heat flux driven by the availability of supercooled water can be split into three components: platelet (or frazil) crystals suspended in the water column, an unconsolidated porous layer of sub-ice platelets directly beneath the sea ice and a layer of consolidated platelet ice incorporated into the sea ice (Dempsey et al, 2010).…”

Response of sub-ice platelet layer thickening rate to variations in Ice Shelf Water supercooling in McMurdo Sound, Antarctica

“…To calculate SIPL thickness at the n th time interval, we adopt the assumptions of HU14 that solid ice fraction within 15 the SIPL is 0.25 (Gough et al, 2012) and that the ice crystals double in volume after precipitation:…”