Measuring Turbulent Dissipation Rates Beneath an Antarctic Ice Shelf

Venables, Emily J.; Nicholls, Keith W.; Wolk, Fabian; Makinson, Keith; Anker, Paul

doi:10.4031/mtsj.48.5.8

Cited by 7 publications

(19 citation statements)

References 13 publications

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“…In line with the increasing energy levels as a function of mean flow speed, the rate of TKE dissipation increases from a minimum of ~10 ‐9 W/kg at both MAVS to a maximum of ~10 ‐6 W/kg at the upper MAVS and ~10 ‐7 W/kg at the lower MAVS (Figures a and b). The rate of TKE dissipation is similar to that observed under sea ice in the Arctic (McPhee, ; Peterson et al, ) and the Antarctic (McPhee, ; McPhee & Martinson, ), but is greater than that observed beneath Pine Island Ice Shelf (~10 ‐9 W/kg; Kimura et al, ) and George VI Ice Shelf (~10 ‐10 W/kg; Venables et al, ). As a function of mean flow speed, ε scales as a power law at the upper MAVS ( r 2 = 0.93) and exponentially at the lower MAVS ( r 2 = 0.93).…”

Section: Resultssupporting

confidence: 72%

Turbulence Observations Beneath Larsen C Ice Shelf, Antarctica

Davis

Nicholls

2019

JGR Oceans

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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.

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Section: Resultssupporting

confidence: 72%

Turbulence Observations Beneath Larsen C Ice Shelf, Antarctica

Davis

Nicholls

2019

JGR Oceans

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“…The primary contribution of tides to the thermodynamic exchange processes described in section 7.2.1 is through increased turbulence (represented by u* ) in the ocean layer adjacent to the ice base. Direct measurements of turbulence under ice shelves are rare (Kimura et al, ; Stanton et al, ; Venables et al, ). In models, u* can be approximated from the velocity magnitude | u s | of the model layer nearest the ice base as u* = C D 1/2 | u s |, where C D is a nondimensional quadratic drag coefficient that is frequently taken to be between 0.002 and 0.003.…”

Section: Effect Of Tides On Ice Shelf Basal Mass Balancementioning

confidence: 99%

Ocean Tide Influences on the Antarctic and Greenland Ice Sheets

Padman

Siegfried

Fricker

2018

Reviews of Geophysics

154

160

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Ocean tides are the main source of high‐frequency variability in the vertical and horizontal motion of ice sheets near their marine margins. Floating ice shelves, which occupy about three quarters of the perimeter of Antarctica and the termini of four outlet glaciers in northern Greenland, rise and fall in synchrony with the ocean tide. Lateral motion of floating and grounded portions of ice sheets near their marine margins can also include a tidal component. These tide‐induced signals provide insight into the processes by which the oceans can affect ice sheet mass balance and dynamics. In this review, we summarize in situ and satellite‐based measurements of the tidal response of ice shelves and grounded ice, and spatial variability of ocean tide heights and currents around the ice sheets. We review sensitivity of tide heights and currents as ocean geometry responds to variations in sea level, ice shelf thickness, and ice sheet mass and extent. We then describe coupled ice‐ocean models and analytical glacier models that quantify the effect of ocean tides on lower‐frequency ice sheet mass loss and motion. We suggest new observations and model developments to improve the representation of tides in coupled models that are used to predict future ice sheet mass loss and the associated contribution to sea level change. The most critical need is for new data to improve maps of bathymetry, ice shelf draft, spatial variability of the drag coefficient at the ice‐ocean interface, and higher‐resolution models with improved representation of tidal energy sinks.

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“…Although the observations from George IV Ice Shelf reported by Kimura et al (2015) showed clear evidence for double-diffusive convection, Venables et al (2014) noted that some turbulent shear profiles taken beneath George VI Ice Shelf showed low dissipation values concurrent with double-diffusive staircases, while others showed no staircases and high dissipation values. One hypothesis is that double-diffusive convection is suppressed when turbulence exceeds a critical threshold.…”

Section: Introductionmentioning

confidence: 82%

“…Certain observations cannot be explained by the damping effect of stratification. Borehole observations made on the George VI Ice Shelf (Venables et al 2014;Kimura et al 2015), found layers (or 'thermohaline staircases') in the temperature and salinity profiles adjacent to the ice. Although layers can form in fluids with a single stratifying component (Phillips 1972), Kimura et al (2015) argued that the staircases observed beneath George VI Ice Shelf are associated with the difference between the molecular diffusivities of temperature and salinity.…”

Section: Introductionmentioning

confidence: 99%

“…To investigate double-diffusion within an adjusting ice shelf-ocean boundary layer we use idealised, high-resolution numerical simulations, inspired by field observations. We force ambient turbulence to reach a target dissipation rate similar to measurements beneath George VI Ice Shelf (Venables et al 2014), then consider the evolution of a dynamically melting boundary under a homogeneous initial condition for temperature and salinity. We vary the far-field temperature and forced dissipation rate across simulations as controlling parameters.…”

Section: Introductionmentioning

confidence: 99%

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Numerical Simulations of Melt-Driven Double-Diffusive Fluxes in a Turbulent Boundary Layer beneath an Ice Shelf

Middleton

Vreugdenhil

Holland

et al. 2021

Journal of Physical Oceanography

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The transport of heat and salt through turbulent ice shelf-ocean boundary layers is a large source of uncertainty within ocean models of ice shelf cavities. This study uses small-scale, high resolution, 3D numerical simulations to model an idealised boundary layer beneath a melting ice shelf to investigate the influence of ambient turbulence on double-diffusive convection (i.e. convection driven by the difference in diffusivities between salinity and temperature). Isotropic turbulence is forced throughout the simulations and the temperature and salinity are initialised with homogeneous values similar to observations. The initial temperature and the strength of forced turbulence are varied as controlling parameters within an oceanographically relevant parameter space. Two contrasting regimes are identified. In one regime double-diffusive convection dominates, and in the other convection is inhibited by the forced turbulence. The convective regime occurs for high temperatures and low turbulence levels, where it is long-lived and affects the flow, melt rate and melt pattern. A criterion for identifying convection in terms of the temperature and salinity profiles, and the turbulent dissipation rate, is proposed. This criterion may be applied to observations and theoretical models to quantify the effect of double-diffusive convection on ice shelf melt rates.

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Measuring Turbulent Dissipation Rates Beneath an Antarctic Ice Shelf

Cited by 7 publications

References 13 publications

Turbulence Observations Beneath Larsen C Ice Shelf, Antarctica

Turbulence Observations Beneath Larsen C Ice Shelf, Antarctica

Ocean Tide Influences on the Antarctic and Greenland Ice Sheets

Numerical Simulations of Melt-Driven Double-Diffusive Fluxes in a Turbulent Boundary Layer beneath an Ice Shelf

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