Ice motion across incised fjord landscapes

The dynamic behaviour of ice-sheet motion over rough landscapes is poorly understood, with most ice-sheet models prescribing a bed smoother than reality, which does not fully capture topographic features. Subglacial fjords striking obliquely to the palaeo flow direction are an end member of this misrepresentation, but are ubiquitous beneath the western margin of the palaeo Scandinavian Ice Sheet, and provide a useful proxy for areas of the present-day Greenland Ice Sheet. Here, we consider Veafjorden as a characteristic western Norwegian fjord where striations clearly evidence palaeo perpendicular ice flow, and perform 3D thermodynamically-coupled ice-motion simulations across a range of orientations. For perpendicular flow, Moffatt eddies, or spiralling flows, occur within a thick layer of temperate ice in the fjord hollow with reverse-direction slip at the fjord base. Area-averaged driving stress in simulations with high-resolution topography and fjord-perpendicular flow is ∼41 %–89 % greater than in simulations with smoothed control topography for an equivalent surface velocity. In comparison, simulations with fjord-perpendicular flow show ∼28 %–45 % greater area-averaged driving stress than simulations with fjord-parallel flow. The steep slopes of fjords and other similar features also provide a clear physically-based example for why bounded basal traction relationships may not hold at the macro scale in rough settings. Similar topographic features may explain surface velocity variations at many locations towards the margins of the Greenland Ice Sheet, and imply that the role of anisotropic roughness in resisting ice-sheet motion may be under-represented in models.