The oceanic boundary layer beneath ice shelves

Lead Research Organisation: NERC British Antarctic Survey
Department Name: Science Programmes

Abstract

Antarctic ice shelves are the part of the Antarctic ice sheet that goes afloat at the coast of Antarctica. The ice shelves act as a restraint on the flow of ice from the interior into the ocean, and as such act as a control on the Antarctic ice sheet's contribution to global sea level. Satellite data have shown one part of the ice sheet to be reducing in size, indicating increased ice flow into the ocean. The continental shelf in that sector of Antarctica is flooded with relatively warm water, resulting in high melt rates at the base of the ice shelves. Recent modelling work has strongly hinted that the amount of warm water flooding on to the continental shelf is closely related to the wind conditions. We need to be able to predict the response of these ice shelves to the changing ocean conditions in order to predict how the Antarctica's contribution to sea level change will be affected by possible future changes in climate. These predictions will ultimately be made using numerical models of the ocean that include the cavities beneath the floating ice shelves. The key driver for the circulation of water in the cavities is the release of buoyant meltwater at the base of the ice shelves as the ice melts. So the crucial process is the one by which the heat gets from the ocean up to the ice base through the ice-ocean boundary layer, that is, the layer of water, some 10's of metres thick that is affected by the presence of the ice base. The boundary layer beneath an ice shelf is unique: different to sea ice in some important respects. The physics of the boundary layer beneath rapidly melting ice shelves is particularly poorly understood, and also inadequately represented in numerical models. One of the problems is that the melting itself increases the buoyant flow up inclined the ice shelf base, and the increased speed increases the transfer of heat towards the ice. At the same time, the increased buoyancy near the ice base makes it more difficult for the denser, warm water to be lifted through the boundary layer. The subtle interplay between competing effects results in a complicated, but fascinating, geophysical problem. The aim of this project is to drill an access hole through a rapidly melting ice shelf and a slowly melting ice shelf and make measurements in the boundary layers that will enable us to improve the way they are represented in models. The ice will be around 350 m thick, and the instruments specially designed to be able to work through a 25-cm borehole. The measurements will be very detailed, enabling us to detect turbulent eddies right down to millimetres in diameter. Instruments will be left suspended beneath the ice shelf so that they can monitor the speed of flow of the boundary layer, its temperature, and the rate of basal melting for at least one year. A subset of the data will be transmitted to the U.K. using a satellite data link so that we don't need to wait until the full dataset is recovered from the data loggers during the following field season. The data will be used to provide, for the first time, a comprehensive view of the boundary layer beneath a rapidly melting ice shelf, to be contrasted with the slowly melting counterpart, providing a step forward in our understanding of the physics of a unique environment. The data will be used to calculate the vertical heat transport through the boundary layers, and, for the first time, the different ways in which the heat transport is calculated in models will be tested and calibrated using observations. An additional component of the project is to collaborate with a colleague who makes direct numerical simulations (DNS) of oceanic turbulence. Most models need to make crude approximations of the effects of turbulence, but DNS methods can calculate them directly. Combining field observations with this modelling approach effectively allows us to extend the range of conditions over which the present models can be tested.

Publications

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Davis P (2019) Turbulence Observations Beneath Larsen C Ice Shelf, Antarctica in Journal of Geophysical Research: Oceans

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Venables E (2014) Measuring Turbulent Dissipation Rates Beneath an Antarctic Ice Shelf in Marine Technology Society Journal

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Venables E (2015) Estimation of Ice Shelf Melt Rate in the Presence of a Thermohaline Staircase in Journal of Physical Oceanography

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Venables Emily (2014) Measuring Turbulent Dissipation Rates Beneath an Antarctic Ice Shelf in MARINE TECHNOLOGY SOCIETY JOURNAL

 
Description The thinning of Larsen C Ice Shelf results from enhanced atmospheric-driven melting, not from enhanced ocean-driven melting. Over the last decade or so there has been debate in the community as to whether the observed surface lowering of Larsen C Ice Shelf is a result of increasing melting at its upper surface, or an increase in ocean-driven melting at its base. The distinction between the two mechanisms is important: a thinning due to increasing surface melting, and the firn densification that results, implies no loss of ice from the ice shelf, just an increase in average ice-column density that results in the ice shelf floating lower in the water. A thinning due to increasing basal melting implies a very significant loss of ice to the ocean, a much more rapid loss of ice thickness, and therefore a loss in the ice shelf's capacity to buttress the inland ice sheet. The key finding reported here is that at two sites drilled through Larsen C Ice Shelf, there was no evidence for waters entering the sub-ice shelf cavity at temperatures significantly above the surface freezing point. This despite the fact that ship-based observations have shown the presence of relatively warm waters in the vicinity of the ice shelf. The water masses at the two drill sites have now been monitored for over a year. As the water entering the cavity is at the surface freezing point, it clearly can't have been warming over the last decade or two. The implication is that the melt rate at the ice base can't have been increasing as a result of an increase in temperature of inflowing waters. There remain two possibilities for ocean-driven increases in melting. One is that currents in the cavity have increased, the other is that the flushing with warm water is episodic, and we have not yet observed it. The first possibility seems unlikely, as our data confirm that the current regime is heavily dominated by tides. The second possibility requires longer datasets to confirm or deny, but the present results suggest that the observed surface elevation change is most likely to be atmospherically-derived, and therefore unlikely to be resulting in a dramatically thinner ice shelf.
Exploitation Route The novel datasets we acquired are available for others to make use of, although we plan to press forward with their analysis. The instrument strings beneath the ice shelf are still working, asn we plan to monitor them as long as possible into the future, looking for interannual vairiability in sub-ice shelf conditions.
Sectors Environment

 
Description The principal impact of the findings has been in the academic community: it is now clear that previously used methods to estimate melt rates at the base of ice shelves in some circumstances (ie, those exising beneath the southern George VI Ice Shelf) are entirely inadequate.
First Year Of Impact 2013
Sector Environment
 
Title Sub-ice shelf microstructure profiling 
Description A technique has been developed to obtain dissipation rates from the boundary layer beneath a floating ice shelf. This involves using a narrow microstructure profiler, available off the shelf, and deploying it through a hot-water drilled hole through the ice shelf. The hole is enlarged using a purpose-designed basal reamer, allowing the production of a large diameter "garage" at the ice base. This allows the profiler to arrive at its correct measurement velocity soon after it enters the boundary layer, yielding usable dissipation measurements. 
Type Of Material Improvements to research infrastructure 
Year Produced 2014 
Provided To Others? Yes  
Impact For the first time, dissipation rates have been measured beneath an Antarctic ice shelf, allowing a direct test of boundary layer parameterisations of vertical heat and salt flux. The impact is therefore on the ice-ocean modelling community which seeks to predict with greater accuracy the basal meltrates of ice shelves. 
 
Title Turbulence beneath Larsen C Ice Shelf, Antarctica (2012) 
Description Turbulent velocity fluctuations in the ice shelf-ocean boundary layer beneath Larsen C Ice Shelf were observed using two turbulence instrument clusters (TICs) deployed 2.5 m and 13.5 m beneath the ice shelf base in December 2011. Each TIC sampled the velocity fluctuations at a rate of 5 Hz, and were operated in burst mode with 15 minutes of data being collected every two hours. 4600 bursts were collected over a period of 392 days. 320 bursts failed the quality control checks, and were removed from the dataset. The TICs were deployed as part of the UK Natural Environment Research Council (NERC) Sub Ice Shelf Boundary Layer Experiment. Funding was provided by the NERC grant NE/H009205/1. 
Type Of Material Database/Collection of data 
Year Produced 2019 
Provided To Others? Yes