Ice motion across incised fjord landscapes

Abstract. The thermodynamic behaviour of ice-sheet motion over rough landscapes is poorly understood, with most ice-sheet models prescribing a bed smoother than reality, which will not fully capture topographic features. Subglacial fjords striking obliquely to the palaeo flow direction are an extreme case, but are ubiquitous beneath the western margin of the palaeo Scandinavian Ice Sheet, and likely 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, surface velocity above the fjord is reduced substantially while a thick layer of temperate ice occupies the fjord. Moffatt eddies, or spiralling flows, occur in the fjord hollow with reverse-direction slip at the fjord base. When compared to smoothed topography, perpendicular flow over real topography requires ∼41–89 % greater area-averaged driving stress, dependent on the overlying ice thickness, while a switch from fjord-parallel to fjord-perpendicular flow for real topography requires a ∼28–45 % area-averaged driving-stress increase. These results 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 significantly underappreciated. Last, we note that deep fjords provide a clear physically-based example for why bounded basal traction relationships (i.e. regularised-Coulomb) may not hold at the macro scale in rough settings.