Basal drag of Fleming Glacier, Antarctica, Part B: implications of evolution from 2008 to 2015

Abstract. The Wordie Ice Shelf-Fleming Glacier system in the southern Antarctic Peninsula has experienced a long-term retreat and disintegration of its ice shelf in the past 50 years. Upstream glacier acceleration and dynamic thinning have been observed over the past two decades, especially after 2008 when only a little constraining ice shelf remained at the Fleming Glacier front. It is important to know whether the substantial speed up and surface draw-down of the glacier since 2008 is a direct response to increasing ocean forcing or driven by the feedback within an unstable marine-based glacier system or both. To explore the mechanism underlying the changes, we use a Stokes (full stress) model to simulate the basal shear stress of the Fleming system in 2008 and 2015. Recent observational studies have suggested the 2008–2015 velocity change was due to the ungrounding of the Fleming Glacier front. Our modelling shows that the fast flowing region of the Fleming Glacier shows a very low basal shear stress in 2008 but with a band of higher basal shear stress along the ice front. It indicates that the ungrounding process might have not started in 2008, which is consistent with the height above buoyancy calculation in 2008. Comparison of our inversions for basal shear stresses for 2008 and 2015 suggests the migration of the grounding line by ~ 9 km upstream from the grounding line position in 1996, a shift which is consistent with the change in floating area deduced from the height above buoyancy in 2015. The southern branch of the Fleming Glacier and the Prospect Glacier apparently have retreated by ~ 1–3 km from 2008 to 2015. The retrograde bed underneath the Fleming Glacier has promoted migration of the grounding line, which we suggest may be triggered by subglacial drainage as a response to the increased basal water supply through greater frictional heating at the ice-bedrock interface further upstream in the fast-flowing region. Improved knowledge of bed topography near the grounding line and further transient simulation is required to predict the future grounding line movement of the Fleming Glacier system precisely and subsequently understand better the ice dynamics and the its future contribution to sea level.