• European Marine Science
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Ice shelves around Antarctica can restrict outlet glaciers and control ice-sheet mass loss. They often contain narrow, curvilinear tracts of thin ice termed ice-shelf channels. Their surface depressions display a morphology including deflections from flowlines and junctions. We investigate ice-shelf channels in the Roi Baudouin Ice Shelf using the radar data and ice-flow modeling. In the shallow radar stratigraphy near the ice-shelf front, syncline and anticline stacks occur beneath the northeastern (i.e. upwind) and the southwestern (i.e. downwind) flanks of the surface depression, respectively. These structures are horizontally coherent and occur over the entire ice column, except near an ice-shelf channel junction where patterns change structurally with depth. Truncation of layers near basal incisions occurs both near the grounding line and farther seawards. Using ice-flow modelling, we show that the stratigraphy is $\sim$9 times more sensitive to atmospheric than to oceanic perturbations, and interpret synclines and anticlines in the shallow stratigraphy with preferential snow deposition on the windward and wind erosion at the downwind side. This causes downwind deflection of ice-shelf channels of several meters per year. The depth variable structures show active modification of an ice-shelf channel junction by ocean melting or by differential flow. We conclude that many ice-shelf channels are seeded at the grounding line; however, their morphology farther seawards is shaped on different length scales by ice dynamics, the ocean, and the atmosphere. These processes act on sub-kilometer scales and outside the realm of most ice-, atmosphere- and ocean models yet they may have broader implications in terms of ice-shelf stability.

Computer models are necessary for understanding and predicting marine ice sheet behaviour. However, there is uncertainty over implementation of physical processes at the ice base, both for grounded and floating glacial ice. Here we implement several sliding relations in a marine ice sheet flow-line model accounting for all stress components and demonstrate that model resolution requirements are strongly dependent on both the choice of basal sliding relation and the spatial distribution of ice shelf basal melting.Sliding relations that reduce the magnitude of the step change in basal drag from grounded ice to floating ice (where basal drag is set to zero) show reduced dependence on resolution compared to a commonly used relation, in which basal drag is purely a power law function of basal ice velocity. Sliding relations in which basal drag goes smoothly to zero as the grounding line is approached from inland (due to a physically motivated incorporation of effective pressure at the bed) provide further reduction in resolution dependence.A similar issue is found with the imposition of basal melt under the floating part of the ice shelf: melt parameterisations that reduce the abruptness of change in basal melting from grounded ice (where basal melt is set to zero) to floating ice provide improved convergence with resolution compared to parameterisations in which high melt occurs adjacent to the grounding line.Thus physical processes, such as sub-glacial outflow (which could cause high melt near the grounding line), impact on capability to simulate marine ice sheets. If there exists an abrupt change across the grounding line in either basal drag or basal melting, then high resolution will be required to solve the problem. However, the plausible combination of a physical dependency of basal drag on effective pressure, and the possibility of low ice shelf basal melt rates next to the grounding line, may mean that some marine ice sheet systems can be reliably simulated at a coarser resolution than currently thought necessary.