The role of double-diffusive convection in basal melting of Antarctic ice shelves

Rosevear, Madelaine G.; Gayen, Bishakhdatta

doi:10.1073/pnas.2007541118

Cited by 33 publications

(31 citation statements)

References 52 publications

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“…Observations show that there are many places where a cold, fresh layer exists between floating ice and the warm, salty seawater. In such circumstances, basal melt still occurs through double-diffusive convection (Kimura et al, 2015;Begeman et al, 2018), though it is lower than it would be if a fully mixed and turbulent boundary layer existed at the ice-ocean interface (Rosevear et al, 2021). Where there is intrusion of a warm, saline layer under fresh, cold subglacial discharge beneath grounded ice, with subcritical water flow and limited entrainment (as may exist for seawater intrusion over hard beds), we argue that double-diffusive convection may occur and maintain distinct water layers while driving some ice melt that is dependent on properties of the saline layer, as has been shown in experiments and direct numerical simulations of the sub-ice boundary layers (Martin and Kauffman, 1977;Turner and Veronis, 2004).…”

Section: Melt From Seawater Intrusion Beneath Grounded Ice Sheetsmentioning

confidence: 99%

Layered seawater intrusion and melt under grounded ice

2022

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Abstract. Increasing melt of ice sheets at their floating or vertical interfaces with the ocean is a major driver of marine ice sheet retreat and sea level rise. However, the extent to which warm, salty seawater may drive melting under the grounded portions of ice sheets is still not well understood. Previous work has explored the possibility that dense seawater intrudes beneath relatively light subglacial freshwater discharge, similar to the “salt wedge” observed in many estuarine systems. In this study, we develop a generalized theory of layered seawater intrusion under grounded ice, including where subglacial hydrology occurs as a macroporous water sheet over impermeable beds or as microporous Darcy flow through permeable till. Using predictions from this theory, we show that seawater intrusion over flat or reverse-sloping impermeable beds may feasibly occur up to tens of kilometers upstream of a glacier terminus or grounding line. On the other hand, seawater is unlikely to intrude more than tens of meters through permeable till. Simulations using the Ice-sheet and Sea-level System Model (ISSM) show that even just a few hundred meters of basal melt caused by seawater intrusion upstream of marine ice sheet grounding lines can cause projections of marine ice sheet volume loss to be 10 %–50 % higher. Kilometers of intrusion-induced basal melt can cause projected ice sheet volume loss to more than double. These results suggest that further observational, experimental and numerical investigations are needed to determine the conditions under which seawater intrusion occurs and whether it will indeed drive rapid marine ice sheet retreat and sea level rise in the future.

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Section: Melt From Seawater Intrusion Beneath Grounded Ice Sheetsmentioning

confidence: 99%

Layered seawater intrusion and melt under grounded ice

2022

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“…There is a rich literature on stably stratified boundary layers (typically under constant stabilizing flux boundary conditions), but the dependence of heat, salt, and momentum fluxes on stratification remains a difficult problem, especially for strongly stratified regimes (Zonta and Soldati, 2018). IOBL turbulence has been explored through laboratory experiments, direct numerical simulations, and large-eddy simulations (Middleton et al, 2021;Mondal et al, 2019;Vreugdenhil and Taylor, 2019;McConnochie and Kerr, 2018;Rosevear et al, 2021). However, this body of work has not yet matured to setting a new standard for ice-shelf melt parameterization in ocean models.…”

Section: B Begeman Et Al: Iobl Dynamicsmentioning

confidence: 99%

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

2022

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Abstract. Small-scale turbulent flow below ice shelves is regionally isolated and difficult to measure and simulate. Yet these small-scale processes, which regulate heat and salt transfer between the ocean and ice shelves, can affect sea-level rise by altering the ability of Antarctic ice shelves to “buttress” ice flux to the ocean. In this study, we improve our understanding of turbulence below ice shelves by means of large-eddy simulations at sub-meter resolution, capturing boundary layer mixing at scales intermediate between laboratory experiments or direct numerical simulations and regional or global ocean circulation models. Our simulations feature the development of an ice-shelf ocean boundary layer through dynamic ice melting in a regime with low thermal driving, low ice-shelf basal slope, and strong shear driven by the geostrophic flow. We present a preliminary assessment of existing ice-shelf basal melt parameterizations adopted in single component or coupled ice-sheet and ocean models on the basis of a small parameter study. While the parameterized linear relationship between ice-shelf melt rate and far-field ocean temperature appears to be robust, we point out a little-considered relationship between ice-shelf basal slope and melting worthy of further study.

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“…The scarp could also be formed by ocean processes: for example, a convective process known as double-diffusive convection can drive differential melting of a vertical ice face (Huppert and Turner, 1978). Evidence of double-diffusive convection has been seen in observations and models of the ocean beneath an ice shelf (Kimura et al, 2015;Middleton et al, 2021;Rosevear et al, 2021), however, it is likely that the currents at AM06 are too strong for this process to dominate circulation near the ice and produce such a significant feature through differential melting.…”

Section: Adcp-derived Basal Morphologymentioning

confidence: 99%

“…In high-resolution models, double-diffusive convection has been shown to drive melting beneath ice shelves under warm, low shear conditions (Middleton et al, 2021), forming a thermohaline staircase beneath the ice (Rosevear et al, 2021). Observations of a thermohaline staircase beneath George VI Ice Shelf (Kimura et al, 2015), which is subject to extremely high thermal driving, suggest that double-diffusive convection may drive melting there.…”

Section: Comparison With Other Direct Melt Rate Measurementsmentioning

confidence: 99%

Evaluation of basal melting parameterisations using in situ ocean and melting observations from the Amery Ice Shelf, East Antarctica

Rosevear

Stevens

2021

Preprint

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Abstract. Ocean driven melting of Antarctic ice shelves is causing grounded ice to be lost from the Antarctic continent at an accelerating rate. However, the ocean processes governing ice shelf melting are not well understood, contributing to uncertainty in projections of Antarctica's contribution to sea level. Here, we analyse oceanographic data and in situ measurements of ice shelf melt collected from an instrumented mooring beneath the centre of the Amery Ice Shelf, East Antarctica. This is the first direct measurement of basal melting from the Amery Ice Shelf, and was made through the novel application of an upwards-facing Acoustic Doppler Current Profiler (ADCP). ADCP data were also used to map a region of the ice base, revealing a steep topographic feature or “scarp” in the ice with vertical and horizontal scales of ~20 m and ~40 m respectively. The annually-averaged ADCP-derived melt rate of 0.51 ± 0.18 m yr−1 is consistent with previous modelling results and glaciological estimates, and there is significant seasonal variation in melting with a maximum in May and a minimum in September. Melting is driven by temperatures ~0.2 °C above the local freezing point and background and tidal currents, which have typical speeds of ~3.0 cm s−1 and 10.0 cm s−1 respectively. We use the coincident measurements of ice shelf melt and oceanographic forcing to evaluate parameterisations of ice-ocean interactions, and find that parameterisations in which there is an explicit dependence of the melt rate on current speed beneath the ice tend to overestimate the local melt rate at AM06 by between 200 % and 400 %, depending on the choice of drag coefficient. A convective parameterisation in which melting is a function of the slope of the ice base is also evaluated and is shown to under-predict melting by 20 % at this site. Using available observations from other ice shelves, we show that a common current speed-dependent parameterisation overestimates melting at all but the coldest, most energetic cavity conditions.

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The role of double-diffusive convection in basal melting of Antarctic ice shelves

Cited by 33 publications

References 52 publications

Layered seawater intrusion and melt under grounded ice

Layered seawater intrusion and melt under grounded ice

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

Evaluation of basal melting parameterisations using in situ ocean and melting observations from the Amery Ice Shelf, East Antarctica

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