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

Abstract

Abstract. This is an investigation to quantify the influence of the sub-ice platelet layer on satellite measurements of total freeboard and their conversion to thickness of Antarctic sea ice. The sub-ice platelet layer forms as a result of the seaward advection of supercooled ice shelf water from beneath ice shelves. This ice shelf water provides an oceanic heat sink promoting the formation of platelet crystals which accumulate at the sea ice-ocean interface. The build-up of this porous layer increases sea ice freeboard, and if not accounted for, leads to overestimates of sea ice thickness from surface elevation measurements. In order to quantify this buoyant effect, the solid fraction of the sub-ice platelet layer must be estimated. An extensive in situ data set measured in 2011 in McMurdo Sound in the southwestern Ross Sea is used to achieve this. We use drill-hole measurements and the hydrostatic equilibrium assumption to estimate a mean value for the solid fraction of this sub-ice platelet layer of 0.16. This is highly dependent upon the uncertainty in sea ice density. We test this value with independent Global Navigation Satellite System (GNSS) surface elevation data to estimate sea ice thickness. We find that sea ice thickness can be overestimated by up to 19 %, with a mean deviation of 12 % as a result of the influence of the sub-ice platelet layer. It is concluded that within 100 km of an ice shelf this influence might need to be considered when undertaking sea ice thickness investigations using remote sensing surface elevation measurements.

Variability in the Distribution of Fast Ice and the Sub‐ice Platelet Layer Near McMurdo Ice Shelf

“…Twelve percent of consolidated sea ice thickness within 10 km of the MIS has been attributed to an effective negative ocean heat flux induced by supercooled ISW (Gough et al, 2012). In the central-western region of the sound, sea ice cores were composed of 60-100% incorporated platelet ice (Dempsey et al, 2010;Hughes et al, 2014;Purdie et al, 2006) and SPL thicknesses exceeding 7 m were observed (Hughes et al, 2014;Price et al, 2014). Historical records suggest that there had been no significant change in the volume or temperature of supercooled ISW or the thickness of the platelet ice layer in eastern McMurdo Sound from 1902 to 2013 (Langhorne et al, 2015).…”

The Inferred Formation of a Subice Platelet Layer Below the Multiyear Landfast Sea Ice in the Wandel Sea (NE Greenland) Induced by Meltwater Drainage

“…The phenomenon of platelet ice crystals forming a porous matrix (hereafter referred to as a platelet layer) under an existing ice cover has been observed in many coastal regions around Antarctica (e.g., Eicken & Lange, 1989;Hoppmann et al, 2015a;Langhorne et al, 2015;Price et al, 2014). In Antarctic waters, the formation of a platelet layer is related to the so-called ice-pumping process (Lewis & Perkin, 1986) in which cool, fresh water from the basal melt of an ice-shelf becomes supercooled as it rises, due to increasing buoyancy, and leads to the formation of platelet crystals (Foldvik & Kvinge, 1974).…”

Referee comments

“…See for C4 example Hughes et al (2014), Dempsey et al (2010), Robinson et al (2014) and Cheng et al (2019). I would also suggest the authors consider producing spatially interpolated plots of the measured variables, similar to what is presented in Price et al (2014) so that they could better present spatial trends in their data. This would help to strengthen the discussion where they consider mechanisms driving changes in the measured variables, e.g.…”

“…Printer-friendly version Discussion paper Line 71. I suggest the authors include the work of Price et al (2014) and Brett et al (2020) as they also report on the accumulation of sub-ice platelet layers under the sea ice.…”