Subglacial hydrology can exert an important control on ice flow by affecting drag at the ice-bedrock interface. Here, we report on a series of subglacial drainage events (outbursts) along the Northeast Greenland Ice Stream (NEGIS), initiating as far inland as ~500 km from the margin of Zachariae Isstrøm. The drainage events are associated with local transient uplift, followed by prolonged subsidence, measured by satellite synthetic aperture radar interferometry (DInSAR). In downstream regions, drainage events are associated with a local speed-up in ice flow. The high spatiotemporal resolution of the DInSAR measurements allows for a detailed mapping of the drainage propagation pathway. We show that multiple drainage cascades have occurred along the same identified pathway over the years 2020-2022. Finally, the propagation speed of subglacial water flow is found to vary greatly along NEGIS, suggesting that fundamental differences may exist in the subglacial environment.

Surface runoff over the Greenland Ice Sheet has been shown to have an impact on ice velocities, both at short as well as decadal timescales. While the short timescales are necessary to comprehend the physical processes connecting subglacial water pressure and ice motion, upscaling to longer timescales is paramount to assessing the future behavior of glaciers in a warming climate. In this study, we assess in a land-terminating part of Southwest Greenland over 2013-2021 the relationship between annual ice velocities derived from optical feature-tracking and surface runoff derived from the ERA5-MAR climate model. The recent time period, providing frequent satellite acquisition, allows for a precise selection of image pairs, while also covering summer melt seasons varying in both intensity and duration. We find that the exact link between runoff anomalies and ice velocity anomalies changes depending on the basin considered and that the relationship also changes with altitude. However, all basins do show a similar overall behavior: at low elevations, while a small increase in runoff leads to faster velocities, a large increase in runoff leads to a slowdown of the glacier ice, but years with even larger runoff would tend to make the ice faster again. As altitude increases, runoff anomalies variations seem to have less impact on ice velocities. We compute for each pixel a simple index to quantify this relationship, presenting here a map displaying how runoff anomalies affected the velocities in 2013-2021 and underlining the spatially varying impact of meltwater depending on altitude and location.

Surface melt forces summertime ice-flow accelerations on glaciers and ice sheets. Here, we show that large meltwater-forced accelerations also occur in winter in Greenland. We document supraglacial lakes (SGLs) draining in cascades at unusually high elevation, causing an expansive flow acceleration over a ~5200 km2 region during winter. The 3-component interferometric surface velocity field and decomposition modeling reveals the underlying flood propagation with unprecedented detail as it traveled over 160 km from the drainage site to the margin, providing novel constraints on subglacial water pathways, drainage morphology, and links with basal sliding. The triggering SGLs continuously grew over 40 years and suddenly released decades of stored meltwater into regions of the bed never previously forced, demonstrating surface melt can impact dynamics well beyond its production. We show these events are common and thus their cumulative impact on dynamics should be further evaluated.

At least half of today’s mass loss of the Greenland ice sheet is due to the retreat of tidewater glaciers. For example, over the past decade Helheim Glacier in southeast Greenland has been one of the largest contributors to total ice discharge across the Greenland ice sheet. There is broad agreement that the acceleration and retreat of these marine terminating glaciers has been triggered by the intrusion of warmer currents in the fjords, however, other processes such as changes in basal conditions, ice rheology, surface mass balance or calving dynamics may have also played important roles in controlling the retreat of these glaciers. Without quantifying the individual contributions of these processes, it is difficult to determine which of these processes should be included in ice sheet models to correctly capture the present and future retreat and associated mass loss of the ice sheet. In this study, we simulate the dynamics of Helheim Glacier, from 2007 to 2020, using the Ice-sheet and Sea-level System Model (ISSM) to investigate the model response to changes in external forcing and boundary conditions. By switching off each of these external forcing components and comparing the numerical solution with observations, we identify that the seasonal to inter-annual variability of Helheim Glacier is relatively insensitive to the choice of friction law or the ice rheology, but that the position of the calving front has a direct and large impact on ice velocity.We then apply automatic differentiation to quantify the transient sensitivity of the ice flux near the terminus to changes in ocean-induced melting rates, basal frictions, ice rheology, calving dynamics and surface mass balance. These sensitivities highlight the regions where each parameter may contribute the most to changes in ice flux and which process should be properly captured by numerical models in order to accurately project the future response of Helheim Glacier. This study, as a result, can be used as a guide for model development of similar glaciers.

Ocean warming around Antarctica has the potential to trigger marine ice-sheet instabilities. It has been suggested that abrupt and irreversible cold-to-warm ocean tipping points may exist, with possible domino effect from ocean to ice-sheet tipping points. A 1/4° ocean model configuration of the Amundsen Sea sector is used to investigate the existence of ocean tipping points, their drivers, and their potential impact on ice-shelf basal melting. We apply idealized atmospheric perturbations of either heat, freshwater or momentum fluxes, and we characterize the key physical processes at play in warm-to-cold and cold-to-warm climate transitions. Relatively weak perturbations of any of these fluxes are able to switch the Amundsen Sea to an intermittent or permanent cold state, i.e., with ocean temperatures close to the surface freezing point and very low ice-shelf melt rate. The transitions are reversible, i.e., cancelling the atmospheric perturbation brings the ocean system back to its unperturbed state within a few decades. All the transitions are primarily driven by changes in surface buoyancy fluxes over the continental shelf, as a direct consequence of the freshwater flux perturbation, or through changes in net sea-ice production resulting from either heat flux perturbations or from changes in sea-ice advection for the momentum flux perturbation. These changes affect the vertical ocean stratification and thereby ice-shelf basal melting. For warmer climate conditions than presently, the surface buoyancy forcing becomes less important as there is a decoupling between the surface and subsurface layers, and ice-shelf melt rates appear less sensitive to climate conditions.

Ice motion and ice boundary are critical information for ice sheet models that project ice evolution in a warming climate. We present four historical, continent-wide, maps of Antarctic ice motion and boundary for the time period 1995-2022. The results reveal no change in 2/3rd of the interior regions, block rotation and rift propagation at ice shelf fronts, and widespread glacier speedup that propagates from 10 to 100 km’s inland. Speedup affects the entire drainage of the Amundsen Sea Embayment sector, in West Antarctica; the entire west coast of the Antarctic Peninsula down to GeorgeVI Ice Shelf; the east coast down to Larsen C Ice Shelf; Getz Ice Shelf, Hull and Land glaciers in West Antarctica; Matusevitch, Ninnis, Mertz and Denman glaciers, glaciers in Porpoise and Vincennes Bay, and Robert, Wilma and Rayner glaciers in Enderby Land, in East Antarctica. We attribute the changes to faster melting by warmer ocean waters.