Numerical modeling of ice sheet motion and hence projections of global sea level rise require information about the evolving subglacial environment, which unfortunately remains largely unknown due to ...its difficulty of access. Here we advance such subglacial observations by reporting multi‐year observations of seismic tremor likely associated with glacier sliding at Helheim Glacier. This association is confirmed by correlation analysis between tremor power and multiple environmental forcings on different timescales. Variations of the observed tremor power indicate that different factors affect glacial sliding on different timescales. Effective pressure may control glacial sliding on long (seasonal/annual) timescales, while tidal forcing modulates the sliding rate and tremor power on short (hourly/daily) timescales. Polarization results suggest that the tremor source comes from an upstream subglacial ridge. This observation provides insights on how different factors should be included in ice sheet modeling and how their timescales of variability play an essential role.
Plain Language Summary
The Greenland Ice Sheet has lost ice increasingly in the past few decades, contributing significantly to global sea level rise. However, large uncertainties remain in computer simulations of such ice mass loss and hence sea level rise. This uncertainly is mainly attributed to the lack of information on what is happening at the bottom of the ice sheet and how that affects ice sheet movement. For example, how much water is there and whether it is in isolated pockets or is well distributed as a thin sheet can have an important effect on ice movement. In this paper, we observe small ground motions that are generated during glacier movement at a marine‐terminating glacier in Greenland. By analyzing the energy variation of the small ground motions over multiple years as well as observing the reasons that cause the variations, we learn that subglacial water pressure may control ice flow speeds on a seasonal/annual scale, and that ocean tides can change ice flow on an hourly/daily scale. This observation provides important constraints for computer simulations of future sea level rise in terms of the impacting factors and their respective timescales.
Key Points
Multi‐year seismic records reveal glacial slip and tremor variability at Helheim Glacier
Tremor correlates with high effective pressure at tidal timescales, opposite to the expectation at longer timescales
The tremor source points to an upstream subglacial ridge
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FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SAZU, SBCE, SBMB, UL, UM, UPUK
Iceberg calving is a major contributor to Greenland's ice mass loss. Pro-glacial mélange (a mixture of sea ice, icebergs, and snow) may be tightly packed in the long, narrow fjords that front many ...marine-terminating glaciers and can reduce calving by buttressing. However, data limitations have hampered a quantitative understanding. We develop a new radar-based approach to estimate time-varying elevations near the mélange-glacier interface, generating a factor of three or more improvement in elevation precision. We apply the technique to Jakobshavn Isbræ, Greenland's major outlet glacier. Over a one-month period in early summer 2016, the glacier experienced essentially no calving, and was buttressed by an unusually thick mélange wedge that increased in thickness towards the glacier front. The extent and thickness of the wedge gradually decreased, with large-scale calving starting once the mélange mass within 7 km of the glacier front had decreased by >40%.
When glaciers calve icebergs, a fraction of the released potential energy is radiated away via gravity waves. The characteristics of such waves, caused by iceberg calving on Helheim Glacier in east ...Greenland, are investigated. Observations were collected from an array of five high-frequency bottom pressure meters placed along Sermilik Fjord. Calving-generated tsunami waves were identified and used to construct a calving event catalog. Calving events are observed to cluster around high and low semidiurnal tides and around high and prior-to-low semimonthly tides. In the postcalving ocean state, discrete spectral peaks associated with calving events are observed, and they are consistent among all the events. A numerical model is used to compute the resonant modes of the fjord and to simulate calving-generated ocean waves. Damped oscillator boundary forcing with 5- to 10-min periods is found to reproduce well the observed properties of calving waves. These observations and modeling are relevant for better understanding of wave dynamics in glacier fjords.
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DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
Totten Glacier is a fast‐moving East Antarctic outlet with the potential for significant future sea‐level contributions. We deployed four autonomous phase‐sensitive radars on its ice shelf to monitor ...ice‐ocean interactions near its grounding zone and made active source seismic observations to constrain gravity‐derived bathymetry models. We observe an asymmetry in basal melting with mean melt rates along the grounding zone differing by up to 20 m/a. Our new bathymetry model reveals that this melt rate asymmetry coincides with an asymmetry in water column thickness and that the low‐melting ice‐shelf portion is shielded from the main cavity circulation. A 2‐year record yields year‐to‐year melt rate variability of 7–9 m/a with no seasonal cycle. Our results highlight the key role of bathymetry near grounding lines for accurate modeling of ice‐shelf melt, and the importance of sustained multi‐year monitoring, especially at ice‐shelf cavities where the dominant melt rate drivers vary primarily inter‐annually.
Plain Language Summary
The point were the Antarctic Ice Sheet goes afloat on the ocean represents a critical region, where minor variations in melt rates can impact glacier flow and influence the rate of sea‐level rise. East Antarctica's Totten Glacier holds the potential to raise global sea level by several meters. Therefore, to understand the conditions it is exposed to, we measured melt rates for 2 years in several key locations near the point where the ice first touches the ocean. Our new measurements of the shape of the Totten Ice Shelf cavity help explain an observed spatial pattern of basal melting and together with local melt rate data resolve a disagreement between existing melt rate estimates from remote‐sensing methods.
Key Points
Totten Glacier melt rates vary spatially between 0 and over 20 m/a; differences are explained by water column thickness variations from updated bathymetry
Temporal melt rate variability is primarily inter‐annual; melt rates differ by 7–9 m/a over two observed years and there is no clear seasonal cycle
Contrary to previous findings, we find no topographic barriers to the intrusion of warm water to the Totten Glacier grounding zone
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FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SAZU, SBCE, SBMB, UL, UM, UPUK
A phase‐sensitive radar (ApRES) was deployed on Totten Ice Shelf to provide the first in situ basal melt estimate at this dynamic East Antarctic ice shelf. Observations of internal ice dynamics at ...tidal time scales showed that early arrivals from off‐nadir reflectors obscure the true depth of the ice shelf base. Using the observed tidal deformation, the true base was found to lie at 1,910–1,950‐m depth, at 350–400 m greater range than the first reflection from an ice‐ocean interface. The robustness of the basal melt rate estimate was increased by using multiple basal reflections over the radar footprint, yielding a melt rate of 22 ± 2.1 m a−1. The ApRES estimate is over 40% lower than the three existing satellite estimates covering Totten Ice Shelf. This difference in basal melt is dynamically significant and highlights the need for independent melt rate estimates using complementary instrumentation and techniques that rely on different sets of assumptions.
Plain Language Summary
Observations of the rate of melting at the base of ice shelves are needed to model accurately ice sheet evolution. Local measurements are scarce, yet necessary for validation of satellite products and ocean models. We deployed a phase‐sensitive radar in the proximity of grounding zone of Totten Ice Shelf in East Antarctica, to measure basal melt in this dynamic region where uncertainties on melt rate estimates are high. We developed a method that accounts for basal geometry complexities and derived a melt rate estimate of ∼22 m per year, which is lower than previous estimates, but it confirms that the basal melt rate Totten Ice Shelf experiences is unusually high for East Antarctica.
Key Points
First in situ radar‐derived basal melt estimate on Totten Ice Shelf yields 22 ± 2.1 m a−1, at least 40% lower than existing satellite estimates
Radar‐derived observation of tidal ice dynamics constrains estimate of ice thickness for a complex base
The use of multiple basal reflections in melt derivation increases robustness of the estimate when early off‐nadir returns are present
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FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SAZU, SBCE, SBMB, UL, UM, UPUK
Ocean-driven melt of Antarctic ice shelves is an important control on mass loss from the ice sheet, but is complex to study due to significant variability in melt rates both spatially and temporally. ...Here we assess the strengths and weakness of satellite and field-based observations as tools for testing models of ice-shelf melt. We discuss how the complementary use of field, satellite and model data can be a powerful but underutilised tool for studying melt processes. Finally, we identify some community initiatives working to collate and publish coordinated melt rate datasets, which can be used in future for validating satellite-derived maps of melt and evaluating processes in numerical simulations.
Fourteen phase‐sensitive radars (ApRES) were deployed on the Filchner‐Ronne Ice Shelf (FRIS) to measure variability in its basal melt rate. Melt rates from sites along the Ronne Depression vary ...seasonally, consistent with the dynamics of the propagation of seasonal dense water from the western ice front into the cavity. Several sites at the back of the FRIS cavity feature a signal with two seasonal maxima. Sub‐ice shelf oceanographic data available from one of the sites indicate that this signal is caused by two different pathways followed by the same source water. Inter‐annual variability is strongest along a direct flow pathway between western Ronne Ice Front and western Berkner Island. Highest melting occurred in 1999 and 2018, following anomalously low summer sea‐ice concentrations in front of the ice shelf. Inter‐annual melt rate variability at the back of the FRIS cavity is limited. If present, it is expressed as a suppression or delay in the arrival of the seasonal melt rate minimum, which can be understood in terms of inter‐annual stratification changes and variable inflow pathways toward the sites. Long term mean ApRES melt rates agree with estimates from satellite data over eastern FRIS. However, the satellite estimates overstate the area of active basal freezing in the western part of the ice shelf. Furthermore, the temporal melt rate variability from the satellite estimates exaggerates the range of variability at both seasonal and inter‐annual time scales with any correspondence between the in‐situ and remotely derived inter‐annual variability being limited to a single site.
Plain Language Summary
Ice shelves form the floating extension of the Antarctic Ice Sheet and they are melted at their base by the ocean beneath. Changes in the rate of ice‐shelf melting can affect the ice‐shelf thickness and impact the flow of the grounded ice sheet. Melt rate changes also modify the properties of waters exported to the surrounding ocean. We used multiple ground‐based, downward‐looking radars to monitor the rate of basal melting across the Filchner‐Ronne Ice Shelf (FRIS), the most voluminous ice shelf in Antarctica. We used the basal melt rate time series together with previously acquired ocean data from beneath the ice shelf to infer and understand seasonal and inter‐annual oceanic variability beneath FRIS and to describe its spatial differences across this extensive ice shelf. Additionally, we compared the in‐situ melt rate measurements with estimates from satellite data, concluding that the satellite‐derived melt rates significantly overestimate the range of temporal variability experienced at FRIS.
Key Points
Melt rate time series from Filchner‐Ronne Ice Shelf show multiple time scales of variability with a character that varies spatially
Inter‐annual melt rate variations driven by sea‐ice concentration anomalies are limited to a pathway through the central Ronne Ice Shelf
Existing melt rate estimates from satellite data dramatically overestimate the amplitude of inter‐annual melt rate variability
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BFBNIB, FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SAZU, SBCE, SBMB, UL, UM, UPUK
Several autonomous phase-sensitive radio-echo sounders (ApRES) were deployed at Greenland glaciers to investigate ice deformation. Different attenuation settings were tested and it was observed that, ...in the presence of clipping of the deramped ApRES signal, each setting produced a different result. Specifically, higher levels of clipping associated with lower attenuation produced an apparent linear increase of diurnal vertical cumulative displacement with depth, and obscured the visibility of the basal reflector in the return amplitude. An example with a synthetic deramped signal confirmed that these types of artifacts result from the introduction of harmonics from square-wave-like features introduced by clipping. Apparent linear increase of vertical displacement with depth occurs when the vertical position of a near-surface internal reflector changes in time. Artifacts in the return amplitude may obscure returns from internal reflectors and the basal reflector, making it difficult to detect thickness evolution of the ice and to correctly estimate vertical velocities. Variations in surface melt during ApRES deployments can substantially modulate the received signal strength on short timescales, and we therefore recommend using higher attenuator settings for deployments in such locations.
Thwaites Glacier is one of the fastest-changing ice-ocean systems in Antarctica
. Much of the ice sheet within the catchment of Thwaites Glacier is grounded below sea level on bedrock that deepens ...inland
, making it susceptible to rapid and irreversible ice loss that could raise the global sea level by more than half a metre
. The rate and extent of ice loss, and whether it proceeds irreversibly, are set by the ocean conditions and basal melting within the grounding-zone region where Thwaites Glacier first goes afloat
, both of which are largely unknown. Here we show-using observations from a hot-water-drilled access hole-that the grounding zone of Thwaites Eastern Ice Shelf (TEIS) is characterized by a warm and highly stable water column with temperatures substantially higher than the in situ freezing point. Despite these warm conditions, low current speeds and strong density stratification in the ice-ocean boundary layer actively restrict the vertical mixing of heat towards the ice base
, resulting in strongly suppressed basal melting. Our results demonstrate that the canonical model of ice-shelf basal melting used to generate sea-level projections cannot reproduce observed melt rates beneath this critically important glacier, and that rapid and possibly unstable grounding-line retreat may be associated with relatively modest basal melt rates.
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GEOZS, IJS, IMTLJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBMB, UL, UM, UPUK, ZAGLJ
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