Turbulence Observations Beneath Larsen C Ice Shelf, Antarctica

Abstract

Increased ocean‐driven basal melting beneath Antarctic ice shelves causes grounded ice to flow into the ocean at an accelerated rate, with consequences for global sea level. The turbulent transfer of heat through the ice shelf‐ocean boundary layer is critical in setting the basal melt rate, yet the processes controlling this transfer are poorly understood and inadequately represented in global climate models. This creates large uncertainties in predictions of future sea level rise. Using a hot‐water drilled access hole, two turbulence instrument clusters (TICs) were deployed 2.5 and 13.5 m beneath Larsen C ice shelf in December 2011. Both instruments returned a yearlong record of turbulent velocity fluctuations, providing a unique opportunity to explore the turbulent processes within the ice shelf‐ocean boundary layer. Although the scaling between the turbulent kinetic energy (TKE) dissipation rate and mean flow speed varies with distance from the ice shelf base, at both TICs the TKE dissipation rate is balanced entirely by the rate of shear production. The freshwater released by basal melting plays no role in the TKE balance. When the upper TIC is within the log‐layer, we derive an under‐ice drag coefficient of 0.0022 and a roughness length of 0.44 mm, indicating that the ice base is smooth. Finally, we demonstrate that although the canonical three‐equation melt rate parameterization can accurately predict the melt rate for this example of smooth ice underlain by a cold, tidally forced boundary layer, the law of the wall assumption employed by the parameterization does not hold at low flow speeds.

Ice-shelf ocean boundary layer dynamics from large-eddy simulations

“…In these simulations the destruction of TKE by the stabilizing buoyancy flux is two orders of magnitude smaller than shear production of TKE throughout the IOBL. Our finding that shear production of TKE dominates over the buoyancy term is consistent with Davis and Nicholls (2019) who found that shear production was an order of magnitude greater than buoyancy destruction of TKE in the IOBL below Larsen C Ice Shelf. Caulfield, 2003).…”

Topography generation by melting and freezing in a turbulent shear flow

“…The front of a marine-terminating glacier can melt as fast as several metres per day horizontally (as recently reported for the LeConte Glacier, Sutherland et al 2019), but an ice shelf around Antarctica typically melts at a rate of only a few centimetres per day or less (Dutrieux et al 2014). On the other hand, ocean currents are most often turbulent and exhibit temporal variabilities on fast time scales of just a few seconds (Davis & Nicholls 2019), such that phase changes between ice and water are multi-physics phenomena with large scale separation.…”

“…2014). On the other hand, ocean currents are most often turbulent and exhibit temporal variabilities on fast time scales of just a few seconds (Davis & Nicholls 2019), such that phase changes between ice and water are multi-physics phenomena with large scale separation.…”

Two-timescale response of a large Antarctic ice shelf to climate change

“…To ensure numerical stability on the long timescales used in this study, we do not use the z * coordinate system as in Naughten et al 30 , but instead employ MITgcm’s linear free surface scheme. We use an ice shelf drag coefficient of 0.0025 following new observational evidence 35 . The MITgcm code has also been updated to version 67k.…”