Submarine melt can account for substantial mass loss at tidewater glacier termini. However, the processes controlling submarine melt are poorly understood due to limited observations of submarine ...termini. Here at a tidewater glacier in central West Greenland, we identify subglacial discharge outlets and infer submarine melt across the terminus using direct observations of the submarine terminus face. We find extensive melting associated with small discharge outlets. While the majority of discharge is routed to a single, large channel, outlets not fed by large tributaries drive submarine melt rates in excess of 3.0 m d−1 and account for 85% of total estimated melt across the terminus. Nearly the entire terminus is undercut, which may intersect surface crevasses and promote calving. Severe undercutting constricts buoyant outflow plumes and may amplify melt. The observed morphology and melt distribution motivate more realistic treatments of terminus shape and subglacial discharge in submarine melt models.
Key Points
We find heterogeneous melt across the submarine terminus of a Greenland outlet glacier
Discharge through small subglacial outlets can drive large melt rates
Terminus undercutting increases calving and may alter plume dynamics
Surface melting on the Greenland Ice Sheet is common up to ∼1400 m elevation and, in extreme melt years, even higher. Water produced on the ice sheet surface collects in lakes and drains over the ice ...sheet surface via supraglacial streams and through the ice sheet via moulins. Water delivered to the base of the ice sheet can cause uplift and enhanced sliding locally. Here we use ice‐penetrating radar data to observe the effects of significant basal melting coincident with moulins and calculate how much basal melt occurred. We find that more melting has occurred than can be explained by the release of potential energy from the drainage of surface meltwater during one melt season suggesting that these moulins are persistent for multiple years. We find only a few persistent moulins in our study area that drain the equivalent of multiple lakes per year and likely remain active over several years. Our observations indicate that once established, these persistent moulins might be capable of establishing well‐connected meltwater drainage pathways.
Meltwater from the Greenland Ice Sheet often drains subglacially into fjords, driving upwelling plumes at glacier termini. Ocean models and observations of submarine termini suggest that plumes ...enhance melt and undercutting, leading to calving and potential glacier destabilization. Here we systematically evaluate how simulated plume structure and submarine melt during summer months depends on realistic ranges of subglacial discharge, glacier depth, and ocean stratification from 12 Greenland fjords. Our results show that grounding line depth is a strong control on plume‐induced submarine melt: deep glaciers produce warm, salty subsurface plumes that undercut termini, and shallow glaciers produce cold, fresh surface‐trapped plumes that can overcut termini. Due to sustained upwelling velocities, plumes in cold, shallow fjords can induce equivalent depth‐averaged melt rates compared to warm, deep fjords. These results detail a direct ocean‐ice feedback that can affect the Greenland Ice Sheet.
Key Points
We simulate subglacial plumes and submarine melt in 12 Greenland fjords spanning grounding line depths from 100 to 850 m
Deep glaciers produce warm, salty subsurface plumes that undercut ice; shallow glaciers drive cold, fresh surface plumes that can overcut
Plumes in cold, shallow fjords can induce comparable depth‐averaged melt rates to warm, deep fjords due to sustained upwelling velocities
Greenland's bed topography is a primary control on ice flow, grounding line migration, calving dynamics, and subglacial drainage. Moreover, fjord bathymetry regulates the penetration of warm Atlantic ...water (AW) that rapidly melts and undercuts Greenland's marine‐terminating glaciers. Here we present a new compilation of Greenland bed topography that assimilates seafloor bathymetry and ice thickness data through a mass conservation approach. A new 150 m horizontal resolution bed topography/bathymetric map of Greenland is constructed with seamless transitions at the ice/ocean interface, yielding major improvements over previous data sets, particularly in the marine‐terminating sectors of northwest and southeast Greenland. Our map reveals that the total sea level potential of the Greenland ice sheet is 7.42 ± 0.05 m, which is 7 cm greater than previous estimates. Furthermore, it explains recent calving front response of numerous outlet glaciers and reveals new pathways by which AW can access glaciers with marine‐based basins, thereby highlighting sectors of Greenland that are most vulnerable to future oceanic forcing.
Key Points
We present a comprehensive, seamless bed topography across the ice‐ocean margin around Greenland
Two to 4 times more glaciers have calving fronts grounded below 200 m compared to previous mappings
Total ice volume of Greenland is 2.99 ± 0.02 times 106 km3, yielding a potential sea level rise of 7.42 m, 7 cm greater than previous estimates
At tidewater glaciers, plume dynamics affect submarine melting, fjord circulation, and the mixing of meltwater. Models often rely on buoyant plume theory to parameterize plumes and submarine melting; ...however, these parameterizations are largely untested due to a dearth of near‐glacier measurements. Here we present a high‐resolution ocean survey by ship and remotely operated boat near the terminus of Kangerlussuup Sermia in west Greenland. These novel observations reveal the 3‐D structure and transport of a near‐surface plume, originating at a large undercut conduit in the glacier terminus, that is inconsistent with axisymmetric plume theory, the most common representation of plumes in ocean‐glacier models. Instead, the observations suggest a wider upwelling plume—a “truncated” line plume of ∼200 m width—with higher entrainment and plume‐driven melt compared to the typical axisymmetric representation. Our results highlight the importance of a subglacial outlet's geometry in controlling plume dynamics, with implications for parameterizing the exchange flow and submarine melt in glacial fjord models.
Key Points
3‐D structure and transport of a subglacial discharge plume from novel near‐glacier surveying
Observed plume is inconsistent with axisymmetric plume theory commonly used to model submarine melt and near‐glacier circulation
Observations point toward a wider plume that drives higher entrainment and more submarine melting
Primary Objective: The primary aim of this study was to determine the frequency of severe impaired self-awareness (ISA) in patients with severe traumatic brain injury (TBI) and the correlates of ...selected clinical, neuropsychiatric and cognitive variables. The secondary aim of the study was to assess depression and apathy on the basis of their level of self-awareness.
Methods: Thirty patients with severe TBI and 30 demographically matched healthy control subjects (HCs) were compared on measures of ISA, depression, anxiety, alexithymia, neuropsychiatric symptoms and cognitive flexibility.
Results: Twenty percent of the patients demonstrated severe ISA. Severe post-acute ISA was associated with more severe cognitive inflexibility, despite the absence of differences in TBI severity, as evidenced by a Glasgow Coma Scale (GCS) score lower than 9 in all cases in the acute phase. Patients with severe ISA showed lower levels of depression and anxiety but tended to show more apathy and to have greater difficulty describing their emotional state than patients with severe TBI who showed minimal or no disturbance in self-awareness.
Conclusion: These findings support the general hypothesis that severe ISA following severe TBI is typically not associated with depression and anxiety, but rather with apathy and cognitive inflexibility.
Calving and submarine melt drive frontal ablation and sculpt the ice face of marine‐terminating glaciers. However, there are sparse observations of submarine termini, which limit estimates of ...spatially varying submarine melt. Here we present a detailed survey of a west Greenland glacier to reveal heterogeneity in submarine terminus morphology. We find that the majority of the terminus (~77%) is undercut, driven by calving in the upper water column and submarine melting at depth. The remaining ~23% of the terminus is overcut, driven by calving alone. We use observations of six subglacial discharge outlets, combined with a plume model, to estimate spatially varying discharge fluxes. While small discharge fluxes (<43 m3/s) feed numerous, deeply undercut outlets with subsurface plumes, ~70% of the net subglacial flux emerges at the terminus center, producing a vigorous, surface‐reaching plume. This primary outlet drives large, localized seasonal retreat that exceeds calving rates at secondary outlets.
Plain Language Summary
Using a sensor to map the shape of a glacier terminus below sea level, we are able to quantify how the terminus shape changes across the glacier. This allows us to identify different classes of terminus shape, which vary from undercut to overcut. Undercut regions are formed through melting focused near the base of the terminus, where freshwater emerges from discrete channels. Using a model, we explain the degree of undercutting found in channels by varying the flux of freshwater emerging from each channel. This allows us to understand the relative role of small channels on total terminus melt compared to much larger, more frequently observed channels. We find that small channels produce significant melting compared to a much larger channel at the glacier center but not as much iceberg calving.
Key Points
We present direct observations of submarine terminus morphology at a Greenland tidewater glacier
Terminus morphology is highly heterogeneous and varies from overcut to strongly undercut
We combine our observations with a plume model to estimate spatially varying subglacial discharge fluxes
Marine‐terminating glaciers play a critical role in controlling Greenland's ice sheet mass balance. Their frontal margins interact vigorously with the ocean, but our understanding of this interaction ...is limited, in part, by a lack of bathymetry data. Here we present a multibeam echo sounding survey of 14 glacial fjords in the Uummannaq and Vaigat fjords, west Greenland, which extends from the continental shelf to the glacier fronts. The data reveal valleys with shallow sills, overdeepenings (>1300 m) from glacial erosion, and seafloor depths 100–1000 m deeper than in existing charts. Where fjords are deep enough, we detect the pervasive presence of warm, salty Atlantic Water (AW) (>2.5°C) with high melt potential, but we also find numerous glaciers grounded on shallow (<200 m) sills, standing in cold (<1°C) waters in otherwise deep fjords, i.e., with reduced melt potential. Bathymetric observations extending to the glacier fronts are critical to understand the glacier evolution.
Key Points
Bathymetry mapping extending to ice fronts essential in Greenland fjords
Fjords are far deeper than expected and host warm, salty waters where deep enough
Many glaciers are retreated in shallow waters in otherwise deep fjords
Mass loss from the Greenland ice sheet (GrIS) has increased over the last two decades in response to changes in global climate, motivating the scientific community to question how the GrIS will ...contribute to sea‐level rise on timescales that are relevant to coastal communities. Observations also indicate that the impact of a melting GrIS extends beyond sea‐level rise, including changes to ocean properties and circulation, nutrient and sediment cycling, and ecosystem function. Unfortunately, despite the rapid growth of interest in GrIS mass loss and its impacts, we still lack the ability to confidently predict the rate of future mass loss and the full impacts of this mass loss on the globe. Uncertainty in GrIS mass loss projections in part stems from the nonlinear response of the ice sheet to climate forcing, with many processes at play that influence how mass is lost. This is particularly true for outlet glaciers in Greenland that terminate in the ocean because their flow is strongly controlled by multiple processes that alter their boundary conditions at the ice‐atmosphere, ice‐ocean, and ice‐bed interfaces. Many of these processes change on a range of overlapping timescales and are challenging to observe, making them difficult to understand and thus missing in prognostic ice sheet/climate models. For example, recent (beginning in the late 1990s) mass loss via outlet glaciers has been attributed primarily to changing ice‐ocean interactions, driven by both oceanic and atmospheric warming, but the exact mechanisms controlling the onset of glacier retreat and the processes that regulate the amount of retreat remain uncertain. Here we review the progress in understanding GrIS outlet glacier sensitivity to climate change, how mass loss has changed over time, and how our understanding has evolved as observational capacity expanded. Although many processes are far better understood than they were even a decade ago, fundamental gaps in our understanding of certain processes remain. These gaps impede our ability to understand past changes in dynamics and to make more accurate mass loss projections under future climate change. As such, there is a pressing need for (1) improved, long‐term observations at the ice‐ocean and ice‐bed boundaries, (2) more observationally constrained numerical ice flow models that are coupled to atmosphere and ocean models, and (3) continued development of a collaborative and interdisciplinary scientific community.
Plain Language Summary
Increasing mass loss from the Greenland ice sheet (GrIS) in response to changes in global climate has motivated the scientific community to understand how much sea level rise will happen in the coming decades. Observations now indicate that the impact of a melting GrIS are more widespread than just sea‐level rise and include changes to ocean properties and circulation, nutrient and sediment cycling, and ecosystem function. Major uncertainties still hamper accurate predictions of these impacts, particularly for outlet glaciers in Greenland that terminate in the ocean because their flow is strongly controlled by multiple processes that alter their boundary conditions at the ice‐atmosphere, ice‐ocean, and ice‐bed interfaces. Many of these processes change on a range of overlapping timescales and are challenging to observe. Here we review the scientific progress in understanding how GrIS outlet glaciers respond to climate and how our understanding has changed over time as observations have increased. We conclude with recommendations for (1) improved, long‐term observations at the ice‐ocean and ice‐bed boundaries, (2) more observationally‐constrained ice flow models that are linked to atmosphere and ocean models, and (3) continued development of a collaborative and interdisciplinary scientific community.
Key Points
Outlet glacier changes are heterogeneous and result in large uncertainties in future sea‐level rise contribution from Greenland
Uncertain understanding of outlet glacier changes is largely due to ice‐ocean and ice sheet basal processes
Future research needs include expanded observations, improved modeling, and greater inclusion of new researchers
Glacier terminus changes are one of the hallmarks of worldwide glacier change, and thus, there is significant focus on the controls and limits to retreat in the literature. Here we use the ...observational record of glacier terminus change from satellite remote sensing data to characterize glacier retreat in central West Greenland with a focus on the last 30 years. We compare terminus observations of retreat to glacier/fjord geometry from available bed and bathymetry data and find that glacier retreat accelerates through wide, overdeepened parts of the bed characterized by retrograde bed slopes. We find that the morphology of the overdeepening can be used as a predictive measure for the length of retreat and that short regions (less than twice the seasonal change in terminus position) of the bed with prograde bed slopes are not sufficient to stop a retreating terminus. Even narrow overdeepenings can control glacier retreat, likely because they focus subglacial runoff, which entrains warm water in the fjords when it emerges at the grounding line and melts the terminus, creating enhanced local retreat. Future retreat of these glaciers is assessed given upstream fjord geometry.
Plain Language Summary
Glaciers that reach the marine margin of the Greenland Ice Sheet are experiencing increases in mass loss over time. These losses are greater than land‐terminating glaciers and are spatially variable. Even glaciers that drain into the same fjord system can experience different amounts, rates, and durations of retreat and thinning. To explain this, we examined retreat over an ∼30‐year record derived from satellite images of Greenland and compared it to the submarine fjord and subglacial topography across a region containing 15 glaciers. We find good correspondence between glacier retreat and the length of an overdeepened reach of the fjord behind the glacier terminus.
Key Points
Glacier termini in central West Greenland began progressive retreat in 1998 +/‐3 years
Heterogeneous retreat rates and extents can be partially explained by local differences in bed topography
Length of overdeepening behind the terminus controls the amount of retreat observed
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