The oceanic boundary layer beneath ice shelves

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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.