Ocean variability beneath the Filchner‐Ronne Ice Shelf inferred from basal melt rate time series

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

The nature of ice intermittently accreted at the base of Ronne Ice Shelf, Antarctica, assessed using phase‐sensitive radar

In-situ phase-sensitive radar measurements from the Ronne Ice Shelf (RIS) reveal evidence of intermittent basal accretion periods at several sites that are melting in the long-term mean. Periods when ice is accreted at the ice-shelf base coincide with a decrease in the amplitude of the basal return of up to 4 dB. To quantify basal accretion we constrain simultaneously the dielectric constant, electrical conductivity, and thickness of the accreted ice. We do this by exploring the sensitivity of the received basal echo strength and phase to different transmit frequencies using the radar data in combination with a simple model. Along the western RIS we detect episodic basal accretion events leading to ice accumulation at a rate equivalent to 1-3 mm of meteoric ice per day. The inferred accumulation rates and electromagnetic properties of the accreted ice imply that these events are caused primarily by the deposition of frazil ice crystals. Our findings offer the possibility of monitoring and studying the evolution of boundaries between ice-shelf basal melting and accretion regimes using remote observations, collected from the ice-shelf surface

Depth-dependent artifacts resulting from ApRES signal clipping

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

An Intensive Observation of Calving at Helheim Glacier, East Greenland

Calving of glacial ice into the ocean from the Greenland Ice Sheet is an important component of global sea level rise. The calving process itself is relatively poorly observed, understood, and modeled; as such, it represents a bottleneck in improving future global sea level estimates in climate models. We organized a pilot project to observe the calving process at Helheim Glacier in East Greenland in an effort to better understand it. During an intensive one-week survey, we deployed a suite of instrumentation including a terrestrial radar interferometer, GPS receivers, seismometers, tsunameters, and an automated weather station. This effort captured a calving process and measured various glaciological, oceanographic, and atmospheric parameters before, during, and after the event. One outcome of our observations is evidence that the calving process actually consists of a number of discrete events, spread out over time, in this instance over at least two days. This time span has implications for models of the process. Realistic projections of future global sea level will depend on accurate parametrization of calving, which will require more sustained observations

Suppressed basal melting in the eastern Thwaites Glacier grounding zone

Thwaites Glacier is one of the fastest-changing ice–ocean systems in Antarctica1,2,3. Much of the ice sheet within the catchment of Thwaites Glacier is grounded below sea level on bedrock that deepens inland4, making it susceptible to rapid and irreversible ice loss that could raise the global sea level by more than half a metre2,3,5. 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 afloat3,6, 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 base7,8, 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

The Soul in Temporality - the Soul in Infinity (May 20-21), May 21, session B: 1. Ewa Kocój "The Soul’s Journey to the Afterlife: About Funeral Rituals in the Romanian Tradition", 2. Irena Vaňková "The Luminous Something That Lives On after a Person’s Death: The Soul in Today’s Czech Conceptualizations", 3. Lucie Šťastná "A White Dove Flew Out of the Dead Body and Soared into the Blue Sky: The Image of the Soul as a Dove in the Czech Linguistic and Cultural Environment".

Wystąpienia prezentowane na międzynarodowej konferencji naukowej "Dusza w doczesności - dusza w nieskończoności" / The Soul in Temporality - the Soul in Infinity (20-21 maja 2021). Organizatorzy: Instytut Slawistyki PAN, Fundacja Slawistyczna. Dofinansowano z programu „Doskonała nauka” Ministra Edukacji i Nauki. 21 maja, sekcja B: 1. Ewa Kocój "Wędrówka duszy w zaświaty, czyli o obrzędowości pogrzebowej w tradycji rumuńskiej" / "The Soul’s Journey to the Afterlife: About Funeral Rituals in the Romanian Tradition", 2. Irena Vaňková "Světélkující cosi, co po smrti člověka žije dale. Duše v dnešních českých konceptualizacích" / "The Luminous Something That Lives On after a Person’s Death: The Soul in Today’s Czech Conceptualizations", 3. Lucie Šťastná "Z mrtveho tela vyletela bila holubicka a do modre oblohy se vznesla. Obraz duše jako holubice v českém jazykově-kulturním prostředí" / "A White Dove Flew Out of the Dead Body and Soared into the Blue Sky: The Image of the Soul as a Dove in the Czech Linguistic and Cultural Environment"

Line of Sight (LOS) velocity obtained from terrestrial radar interferometer at Helheim Glacier

1h-interval LOS ice flow speed processed using TRI dataset collected at Helheim Glacier in August, 2016

Processed ApRES displacement of internal reflectors at Helheim Glacier in August, 2016.

This is the processed ApRES product using dataset collected in August, 2016. A readme file is included to extract displacement time series at a given point

Rapid Iceberg calving following Removal of Tightly Packed Pro-Glacial Mélange

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 \u3e40%