NECKLACE – an over-engineered acronym for an ambitious programme of field observations. The Network for the Collection of Knowledge on meLt of Antarctic iCe shElves aims to install instruments on all the major Antarctic ice shelves to collect measurements of the rate of ocean-driven melting. (Think of a "necklace" of instruments encircling Antarctica...) These time series of melt rates from point locations will complement the satellite-derived maps of spatially-smoothed time-averaged melt rates that are now becoming available, and will be a valuable source of data for assimilation into, or validation of, models of the interaction between the Southern Ocean and the Antarctic Ice Sheet.
The impact of changes in the Southern Ocean on Antarctic ice shelves has never received as much attention as over the last decade. This burgeoning interest is due largely to the realisation that ice shelves do indeed help buttress the Antarctic Ice Sheet, with changes in ice shelves affecting the flow of ice into the ocean from the inland grounded ice sheet, thereby helping control the ice sheet's contribution to global sea level change. And this is an urgent issue. The flow of ice into the Amundsen Sea via Pine Island Glacier is known to be accelerating, with other glacier systems demonstrating similar behaviour.
For the majority of Antarctica the changes in ice shelf configuration causing most concern result from long term increases in the rate of melting at their bases, caused by changes in sub-ice shelf ocean conditions. To understand, and ultimately to predict the way ice shelf basal melting responds to a changing Southern Ocean, we need to have ocean models that include ice shelves as an integral and fully interactive part of their domain.
Although such ocean models exist, their performance in the sub-ice shelf part of the domain is very difficult to optimise and validate. However, an excellent diagnostic turns out to be the rate of ice-shelf basal melting itself, and more particularly, how that basal melt rate changes as a result of seasonal and interannual changes in the oceanographic forcing.
In a collaboration between University College London and the British Antarctic Survey, a new instrument has been developed that will allow the collection of year-round time series of melt rates. The instrument is a ground-based, downward-looking radar that measures, to millimetre-precision, ice-shelf thickness. Of course, changes in ice-shelf thickness also result from ice flow (as the ice spreads, for example, it becomes thinner), and firn compaction in the upper layers. These effects can be accounted for: internal reflections from inhomogeneities within the ice column are detected by the radar and provide a "ruler" that stretches or compresses, allowing the direct measurement of the effects of compaction and ice flow. This is a tried and tested technique (pRES – phase-sensitive Radio-Echo Sounding); what's new is Auto-pRES, an instrument that can be left alone in standalone mode for months or years at a time. Auto-pRES is lightweight, low-power and relatively low in cost (< 10kUSD). The full dataset, typically one measurement every few hours, is logged on the instrument, with a reduced dataset transmitted over a daily Iridium datalink.
The idea of NECKLACE, a SOOS-endorsed activity, is to exploit the circumpolar logistical reach of the combined Antarctic research community to have ApRES instruments established on all the major ice shelves for an extended period during the interval 2015 to 2020. Although the final model has yet to be decided, the present vision is to have monthly-averaged meltrates made freely available via the SOOS data-portal as soon as they have been processed and quality-checked. The full time series, including compaction and ice strain data, would be returned to the participating group responsible for the procurement and deployment of the instrument.
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