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.
Highlights
The increasing sea ice extent in the Ross Sea is the main contributor to the overall positive trend in the Antarctic sea ice cover as recorded over the satellite observational period (Parkinson and Cavalieri, 2012)
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 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)
We focus on total freeboard measurements using Global Navigation Satellite System (GNSS) data to estimate sea ice thickness and, given our estimate of the sf, demonstrate how these GNSS-based estimates are influenced by the presence of a sub-ice platelet layer
Summary
The increasing sea ice extent in the Ross Sea is the main contributor to the overall positive trend in the Antarctic sea ice cover as recorded over the satellite observational period (Parkinson and Cavalieri, 2012). Spatial anomalies in sea ice thickness may be interpreted as indictors of the presence of a sub-ice platelet layer, which in turn may infer the presence of supercooled ice shelf water (ISW) (Hughes et al, 2014) As it is very common for sea ice to abut ice shelves in the Antarctic (Bindschadler et al, 2011), and the extent and persistence of the sub-ice platelet layer is substantially unknown, we consider here the effects of this layer on estimates of sea ice thickness. We focus on total freeboard (ice-plus-snow) measurements using Global Navigation Satellite System (GNSS) data to estimate sea ice thickness and, given our estimate of the sf, demonstrate how these GNSS-based estimates are influenced by the presence of a sub-ice platelet layer. Fore, conclusions about its influence may be considered at the larger scale
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