ABSTRACT
Thwaites Glacier represents 15% of the ice discharge from the West Antarctic Ice Sheet and influences a wider catchment1-3. Because it is grounded below sea level4,5, Thwaites Glacier is thought to be susceptible to runaway retreat triggered at the grounding line (GL) at which the glacier reaches the ocean6,7. Recent ice-flow acceleration2,8 and retreat of the ice front8-10 and GL11,12 indicate that ice loss will continue. The relative impacts of mechanisms underlying recent retreat are however uncertain. Here we show sustained GL retreat from at least 2011 to 2020 and resolve mechanisms of ice-shelf melt at the submetre scale. Our conclusions are based on observations of the Thwaites Eastern Ice Shelf (TEIS) from an underwater vehicle, extending from the GL to 3 km oceanward and from the ice-ocean interface to the sea floor. These observations show a rough ice base above a sea floor sloping upward towards the GL and an ocean cavity in which the warmest water exceeds 2 °C above freezing. Data closest to the ice base show that enhanced melting occurs along sloped surfaces that initiate near the GL and evolve into steep-sided terraces. This pronounced melting along steep ice faces, including in crevasses, produces stratification that suppresses melt along flat interfaces. These data imply that slope-dependent melting sculpts the ice base and acts as an important response to ocean warming.
ABSTRACT
Temporal variations in ice sheet flow directly impact the internal structure within ice sheets through englacial deformation. Large-scale changes in the vertical stratigraphy within ice sheets have been previously conducted on centennial to millennial timescales; however, intra-annual changes in the morphology of internal layers have yet to be explored. Over a period of 2 years, we use autonomous phase-sensitive radio-echo sounding to track the daily displacement of internal layers on Store Glacier, West Greenland, to millimeter accuracy. At a site located â¼30 km from the calving terminus, where the ice is â¼600 m thick and flows at â¼700 m/a, we measure distinct seasonal variations in vertical velocities and vertical strain rates over a 2-year period. Prior to the melt season (March-June), we observe increasingly nonlinear englacial deformation with negative vertical strain rates (i.e., strain thinning) in the upper half of the ice column of approximately -0.03 a-1, whereas the ice below thickens under vertical strain reaching up to +0.16 a-1. Early in the melt season (June-July), vertical thinning gradually ceases as the glacier increasingly thickens. During late summer to midwinter (August-February), vertical thickening occurs linearly throughout the entire ice column, with strain rates averaging 0.016 a-1. We show that these complex variations are unrelated to topographic setting and localized basal slip and hypothesize that this seasonality is driven by far-field perturbations in the glacier's force balance, in this case generated by variations in basal hydrology near the glacier's terminus and propagated tens of kilometers upstream through transient basal lubrication longitudinal coupling.
ABSTRACT
The average rate of melting at the base of the large Filchner-Ronne Ice Shelf in the southern Weddell Sea is currently low, but projected to increase dramatically within the next century. In a model study, melt rates increase as changing ice conditions cause a redirection of a coastal current, bringing warm water of open ocean origin through the Filchner Depression and into the Filchner Ice Shelf cavity. Here we present observations from near Filchner Ice Shelf and from the Filchner Depression, which show that pulses of warm water already arrive as far south as the ice front. This southward heat transport follows the eastern flank of the Filchner Depression and is found to be directly linked to the strength of a wind-driven coastal current. Our observations emphasize the potential sensitivity of Filchner-Ronne Ice Shelf melt rates to changes in wind forcing.
