Turbulence Observations Beneath Larsen C Ice Shelf, Antarctica

• Published in: Journal of geophysical research. Oceans 2019-08, Vol.124 (8), p.5529-5550
• Main Authors: Davis, Peter E.D., Nicholls, Keith W.
• Format: Article
• Language: English
• Subjects: Acceleration; Antarctic ice sheet; Antarctic ice shelves; Boundary layers; Climate; Climate models; Computational fluid dynamics; Drag coefficient; Drag coefficients; Freshwater; Geophysics; Glaciation; Global climate; Global climate models; Global sea level; Ice; Ice sheets; Ice shelves; Inland water environment; Instruments; Kinetic energy; Land ice; Law of the wall; Low flow; Melting; Oceans; Parameterization; Roughness; Roughness length; Scaling; Sea level; Sea level rise; Stratification; Turbulence; Turbulence measurement; Turbulent flow; Turbulent kinetic energy; Turbulent mixing; Turbulent transfer; Uncertainty
• ISSN: 2169-9275; 2169-9291
• DOI: 10.1029/2019JC015164

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. Plain Language Summary Antarctic ice shelves are the floating extensions of the Antarctic ice sheet. They help control the future contribution of Antarctica to global sea level rise by restricting the flow of grounded ice into the ocean. Many ice shelves are increasingly being melted from beneath by relatively warm ocean waters, leading to an acceleration in the flow of ice from the ice sheet interior into the ocean. The very small‐scale turbulent processes that control the rate of basal melting, however, are poorly represented in global climate models, resulting in large uncertainties in projections of future sea level rise. Here we present a yearlong record of the turbulent processes that drive basal melting beneath an Antarctic ice shelf. We show that at our measurement site the turbulence generated by the mean flow is dissipated entirely by small‐scale molec