A selection from Stockholm University publication database

• The thermal structure of small and shallow Arctic Fennoscandian lakes

  2025. Mingzhen Zhang (et al.). Arctic, Antarctic and Alpine research 57 (1)

  Article

  A continuous three-year field study, focusing on the thermal regime and the heat budget of twelve shallow Arctic lakes in northwest Finland, was conducted between 2019 and 2022. The results reveal diverse thermal regimes among these lakes, ranging from cold monomictic to discontinuous cold polymictic and dimictic patterns, reflecting the unique lake responses to their environmental settings. The heat budget of these lakes was predominantly influenced by the strong seasonality of the radiation balance, with latent and sensible heat fluxes consistently exhibiting negative values during the ice-free period, peaking in the summer or late fall. Air temperature and solar radiation were the primary drivers affecting lake thermal structures, at both local and regional scales. The influence of wind speed and cloudiness was more significant for lakes in the treeless tundra, but their regional impact remains relatively weak, along with the impact of precipitation. Additionally, we emphasize the critical role of lake location, geography, and morphology, and particularly altitude, lake size, and water column transparency, in determining changes in stratification and mixing dynamics, overshadowing the influence of lake depth. In conclusion, this study provides new insights into the evolving thermal dynamics of lakes in the European Arctic.

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• Drivers of spatio-temporal variations in summer surface water temperatures of Arctic Fennoscandian lakes (2000–21)

  2024. Mingzhen Zhang (et al.). Polar Research 43

  Article

  The Arctic region is covered with numerous small lakes whose ecosystems are vulnerable to current climate warming and resultant changes in water temperature, ice-cover duration and lake levels. Data on thermal features of these lakes are sparse, which hinders our understanding of the possible ecosystem impacts of the warming climate and climate feedbacks at larger spatial scales. We investigated spatial–temporal variations of lake surface water temperatures (LSWT) in 12 Arctic lakes in north-west Finnish Lapland and explored the predominant drivers of LSWTs by continuous year-round observations. The lake surface temperature data were recorded using thermistors at bi-hourly resolution during the years 2000, 2007–08 and 2019–2021. A large regional heterogeneity was observed in the timing of the maximum and minimum LSWTs and the overall patterns of the annual cycle. Our results reveal that July air temperature, maximum lake depth and altitude explained most of the variance in the summer LSWT (> 85%). The remaining variance was related to geographic location (longitude and latitude), lake morphometric features, such as lake area and catchment area, and certain physico-chemical characteristics, such as Secchi depth and dissolved organic carbon content. Our results provide new insights into thermal responses of different types of small Arctic lakes to climate change.

  Read more about Drivers of spatio-temporal variations in summer surface water temperatures of Arctic Fennoscandian lakes (2000–21)

• Modelled frontal ablation and velocities at Kronebreen, Svalbard, are sensitive to the choice of submarine melt rate scenario

  2024. Felicity Alice Holmes, Eef van Dongen, Nina Kirchner. Journal of Glaciology 70

  Article

  Both submarine melt and calving are important for the overall mass balance of marine-terminating glaciers, but uncertainty is rife with regards to the magnitude of the processes. Modelling allows for these processes to be investigated without the need to visit inaccessible ice marginal zones. This study looks at the impact of different submarine melt and sea-ice back pressure scenarios on modelled calving activity and dynamics at Kronebreen, Svalbard, by running separate summer and winter simulations with various submarine melt parameterisations and sea-ice characteristics. It is found that submarine melt is an important driver of seasonal variation in modelled glacier dynamics and calving activity, with the choice of sliding law also exerting a significant influence on results.

  Read more about Modelled frontal ablation and velocities at Kronebreen, Svalbard, are sensitive to the choice of submarine melt rate scenario

• Short-term calving front dynamics and mass loss at Sálajiegna glacier, northern Sweden, assessed by uncrewed surface and aerial vehicles

  2024. Florian Vacek (et al.). Journal of Glaciology 70

  Article

  Uncrewed aerial vehicles (UAVs) are frequently used in glaciological applications, among other things, for photogrammetric assessments of calving dynamics at glacier termini. However, UAVs are often limited by battery endurance and weight constraints on the scientific payload that can be added. At Sálajiegna, the largest freshwater calving glacier in Sweden, we explored the combined use of a versatile maritime robot (uncrewed surface vehicle, USV) and a UAV to characterise Sálajiegna's short-term and seasonal calving front dynamics and mass loss. For this, a photogrammetric payload suite was integrated into the USV. Consecutive USV surveys of Sálajiegna's front, followed by point cloud based calving detection and surface-reconstruction based volume quantification, allowed for a detailed description of calving-induced terminus changes and is hence suggested as a viable alternative to the differencing of digital elevation models. By combining USV and UAV measurements, we identify sectors of high and low calving activity, a calving front retreat of up to 56 m and a thinning rate in the terminus region of 5.4 cm d⁻¹ during the summer of 2022.

  Read more about Short-term calving front dynamics and mass loss at Sálajiegna glacier, northern Sweden, assessed by uncrewed surface and aerial vehicles

• Water temperature, mixing, and ice phenology in the arctic-alpine Lake Darfáljávri (Lake Tarfala), northern Sweden

  2024. Nina Kirchner (et al.). Arctic, Antarctic and Alpine research 56 (1)

  Article

  In the rapidly warming circumpolar Arctic, recent research of lakes has focused on their climatology and ecology but is challenged by sparsity of wintertime data. At the c. 48-m-deep and c. 0.5-km2 large proglacial Darfaljavri (Lake Tarfala), located in an arctic-alpine environment in the Scandinavian Mountains, year-round water temperatures were previously reported for 2016 to 2019. Here, this record is continued for 2019-2020 and 2021-2022, complemented by time-lapse imagery records of the state of the lake surface, as well as degree-day modeling of ice phenology (timing of ice-on and ice-off). Darfaljavri is cryostratified during winter, with interannual variations in the thermocline's thickness and temperature range. The ice season lasts from October to July. Modeled ice-on dates match observed ones reasonably well; however, observed ice-off dates occur much later than modeled ones, likely because of cold impact from Darfaljavri's glacial environment as inferred from a comparison with a close tundra lake. Though new insights into the complex lake mixing and ice phenology are provided, it remains to attribute the characteristics of Darfaljavri's winter stratification to additional potential drivers, such as lake ice thickness, atmospheric heat fluxes, and the water balance of the lake.

  Read more about Water temperature, mixing, and ice phenology in the arctic-alpine Lake Darfáljávri (Lake Tarfala), northern Sweden

• Weichselian–Holocene glacial history of the Sjuøyane archipelago, northern Svalbard

  2024. Anders Schomacker (et al.). Boreas

  Article

  The Sjuøyane archipelago is the northernmost land area of Svalbard; thus, it provides a window to study the terrestrial glacial history and dynamics of the Svalbard–Barents Sea Ice Sheet and complement marine geological studies in the region. To reconstruct the glacial history of Sjuøyane, we describe coastal sedimentary sections in Quaternary sediments and constrain their chronology by radiocarbon and optically stimulated luminescence ages. Erratic boulders and bedrock are sampled for ¹⁰Be cosmogenic exposure dating, aiming to determine the deglaciation age and exposure history. Holocene environments are studied based on lake sediments and emerging vegetation from retreating snow patches. The sedimentary sections largely consist of shallow (glacio-)marine and/or littoral sediments deposited during high relative sea levels. The radiocarbon and luminescence ages suggest they formed during a Middle Weichselian interstadial and after the Late Weichselian glaciation. A wave-washed bedrock erosional notch and rounded boulders at 36±1 m a.h.t. most likely formed during this interstadial. Most of the cosmogenic ¹⁰Be ages are older than the last deglaciation, likely indicating a complex exposure history. One boulder sample suggests that the lowlands were deglaciated 14.7±1.82 ka ago, and two boulder samples with ages of 18.94±3.26 and 22.89±4.05 ka suggest that the highlands were possibly ice-free at this time. The lake sediments from Isvatnet, Phippsøya, consist of glaciolacustrine silt and clay overlain by gyttja. The gyttja has accumulated at least since 7.0 cal. ka BP. Two radiocarbon ages from emerging vegetation suggest Neoglacial cooling since 3.8 cal. ka BP. A patchy glacial drift at the surface of Sjuøyane and well-preserved pre-Late Weichselian sediments suggest that the Late Weichselian glaciation was non-erosive and/or cold-based at this part of the north margin of the Svalbard–Barents Sea Ice Sheet.

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• Commentary: Glacial history and depositional environments in little Storfjorden and Hambergbukta of Arctic Svalbard since the younger dryas

  2023. Riko Noormets, Nina Kirchner. Frontiers in Earth Science 11

  Article

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• Impact of the Nares Strait sea ice arches on the long-term stability of the Petermann Glacier ice shelf

  2023. Abhay Prakash (et al.). The Cryosphere 17 (12), 5255-5281

  Article

  One of the last remaining floating tongues of the Greenland ice sheet (GrIS), the Petermann Glacier ice shelf (PGIS), is seasonally shielded from warm Atlantic water (AW) by the formation of sea ice arches in the Nares Strait. However, continued decline of the Arctic sea ice extent and thickness suggests that arch formation is likely to become anomalous, necessitating an investigation into the response of PGIS to a year-round mobile and thin sea ice cover. We use a high-resolution unstructured grid 3-D ocean-sea ice-ice shelf setup, featuring an improved sub-ice-shelf bathymetry and a realistic PGIS geometry, to investigate in unprecedented detail the implications of transitions in the Nares Strait sea ice regime, that is, from a thick and landfast sea ice regime to a mobile, and further, a thin and mobile sea ice regime, with regard to PGIS basal melt. In all three sea ice regimes, basal melt near the grounding line (GL) presents a seasonal increase during summer, driven by a higher thermal driving. The stronger melt overturning increases the friction velocity slightly downstream, where enhanced friction-driven turbulent mixing further increases the thermal driving, substantially increasing the local melt. As the sea ice cover becomes mobile and thin, wind and (additionally in winter) convectively upwelled AW from the Nares Strait enter the PGIS cavity. While its effect on basal melting is largely limited to the shallower (<200ĝ€¯m) drafts during winter, in summer it extends to the GL (ca. 600ĝ€¯m) depth. In the absence of an increase in thermal driving, increased melting under the deeper (>200ĝ€¯m) drafts in winter is solely driven by the increased vertical shear of a more energetic boundary layer current. A similar behaviour is noted when transitioning from a mobile to a thin mobile sea ice cover in summer, when increases in thermal driving are negligible and increases in melt are congruent with increases in friction velocity. These results suggest that the projected continuation of the warming of the Arctic Ocean until the end of the 21st century and the accompanying decline in the Arctic sea ice extent and thickness will amplify the basal melt of PGIS, impacting the long-term stability of the Petermann Glacier and its contribution to the future GrIS mass loss and sea level rise.

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• Impact of tides on calving patterns at Kronebreen, Svalbard – insights from three-dimensional ice dynamical modelling

  2023. Felicity A. Holmes (et al.). The Cryosphere 17 (5), 1853-1872

  Article

  Understanding calving processes and their controls is of importance for reducing uncertainty in sea level rise estimates. The impact of tidal fluctuations and frontal melt on calving patterns has been researched through both modelling and observational studies but remains uncertain and may vary from glacier to glacier. In this study, we isolate various different impacts of tidal fluctuations on a glacier terminus to understand their influence on the timing of calving events in a model of Kronebreen, Svalbard, for the duration of 1 month. In addition, we impose a simplified frontal melt parameterisation onto the calving front in order to allow for an undercut to develop over the course of the simulations. We find that calving events show a tidal signal when there is a small or no undercut, but, after a critical point, undercut-driven calving becomes dominant and drowns out the tidal signal. However, the relationship is complex, and large calving events show a tidal signal even with a large modelled undercut. The modelled undercut sizes are then compared to observational profiles, showing that undercuts of up to ca. 25 m are plausible but with a more complex geometry being evident in observations than that captured in the model. These findings highlight the complex interactions occurring at the calving front of Kronebreen and suggest further observational data and modelling work is needed to fully understand the hierarchy of controls on calving.

  Read more about Impact of tides on calving patterns at Kronebreen, Svalbard – insights from three-dimensional ice dynamical modelling

• Modelled dynamic retreat of Kangerlussuaq Glacier, East Greenland, strongly influenced by the consecutive absence of an ice mélange in Kangerlussuaq Fjord

  2023. Jamie Barnett, Felicity A. Holmes, Nina Kirchner. Journal of Glaciology 69 (275), 433-444

  Article

  Mass loss at the Greenland Ice Sheet is influenced by atmospheric processes controlling its surface mass balance, and by submarine melt and calving where glaciers terminate in fjords. There, an ice mélange - a composite matrix of calved ice bergs and sea ice - may provide a buttressing force on a glacier terminus and control terminus dynamics. Kangerlussuaq Glacier is a major outlet of the Greenland Ice Sheet, for which recent major retreat events in 2004/2005 and 2016-2018 coincided with the absence of an ice mélange in Kangerlussuaq Fjord. To better understand the response of Kangerlussuaq Glacier to climatic and oceanic drivers, a 2D flowline model is employed. Results indicate that an ice mélange buttressing force exerts a major control on calving frequency and rapid retreat. When an ice mélange forms in Kangerlussuaq Fjord, it provides stabilising forces and conditions favourable for winter terminus re-advance. When it fails to form during consecutive years, model results indicate that Kangerlussuaq Glacier is primed to retreat into the large overdeepenings in Kangerlussuaq Fjord, and to terminus positions more than 30 km farther inland, implying that excessive mass loss from Kangerlussuaq Glacier by the year 2065 cannot be excluded.

  Read more about Modelled dynamic retreat of Kangerlussuaq Glacier, East Greenland, strongly influenced by the consecutive absence of an ice mélange in Kangerlussuaq Fjord

• A nested high-resolution unstructured grid 3-D ocean-sea ice-ice shelf setup for numerical investigations of the Petermann ice shelf and fjord

  2022. Abhay Prakash (et al.). MethodsX 9

  Article

  Three-dimensional numerical simulation of circulation in fjords hosting marine-terminating ice shelves is challenging because of the complexity of processes involved in such environments. This often requires a comprehensive model setup. The following elements are needed: bathymetry (usually unknown beneath the glacier tongue), ice shelf draft (impacting water column thickness), oceanographic state (including tidal elevation, salinity, temperature and velocity of the water masses), sea ice and atmospheric forcing. Moreover, a high spatial resolution is needed, at least locally, which may be augmented with a coarser and computationally cheaper (nested) model that provides sufficiently realistic conditions at the boundaries. Here, we describe procedures to systematically create such a setup that uses the Finite Volume Community Ocean Model (FVCOM) for the Petermann Fjord, Northwest Greenland. The first simulations are validated against temperature and salinity observations from the Petermann Fjord in September 2019. We provide

  •Complete bathymetry, ice-draft and water column thickness datasets of the Petermann Fjord, with an improved representation of the topography underneath the glacier tongue.

  •Boundary conditions for ocean, atmosphere and sea ice derived from a suite of high-resolution regional models that can be used to initialize and run the regional ocean model with realistic geophysical settings.

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• Climate change in the Baltic Sea region: a summary

  2022. H. E. Markus Meier (et al.). Earth System Dynamics 13 (1), 457-593

  Article

  Based on the Baltic Earth Assessment Reports of this thematic issue in Earth System Dynamics and recent peer-reviewed literature, current knowledge of the effects of global warming on past and future changes in climate of the Baltic Sea region is summarised and assessed. The study is an update of the Second Assessment of Climate Change (BACC II) published in 2015 and focuses on the atmosphere, land, cryosphere, ocean, sediments, and the terrestrial and marine biosphere. Based on the summaries of the recent knowledge gained in palaeo-, historical, and future regional climate research, we find that the main conclusions from earlier assessments still remain valid. However, new long-term, homogenous observational records, for example, for Scandinavian glacier inventories, sea-level-driven saltwater inflows, so-called Major Baltic Inflows, and phytoplankton species distribution, and new scenario simulations with improved models, for example, for glaciers, lake ice, and marine food web, have become available. In many cases, uncertainties can now be better estimated than before because more models were included in the ensembles, especially for the Baltic Sea. With the help of coupled models, feedbacks between several components of the Earth system have been studied, and multiple driver studies were performed, e.g. projections of the food web that include fisheries, eutrophication, and climate change. New datasets and projections have led to a revised understanding of changes in some variables such as salinity. Furthermore, it has become evident that natural variability, in particular for the ocean on multidecadal timescales, is greater than previously estimated, challenging our ability to detect observed and projected changes in climate. In this context, the first palaeoclimate simulations regionalised for the Baltic Sea region are instructive. Hence, estimated uncertainties for the projections of many variables increased. In addition to the well-known influence of the North Atlantic Oscillation, it was found that also other low-frequency modes of internal variability, such as the Atlantic Multidecadal Variability, have profound effects on the climate of the Baltic Sea region. Challenges were also identified, such as the systematic discrepancy between future cloudiness trends in global and regional models and the difficulty of confidently attributing large observed changes in marine ecosystems to climate change. Finally, we compare our results with other coastal sea assessments, such as the North Sea Region Climate Change Assessment (NOSCCA), and find that the effects of climate change on the Baltic Sea differ from those on the North Sea, since Baltic Sea oceanography and ecosystems are very different from other coastal seas such as the North Sea. While the North Sea dynamics are dominated by tides, the Baltic Sea is characterised by brackish water, a perennial vertical stratification in the southern subbasins, and a seasonal sea ice cover in the northern subbasins.

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• Supraglacial lake expansion, intensified lake drainage frequency, and first observation of coupled lake drainage, during 1985–2020 at Ryder Glacier, Northern Greenland

  2022. Jacqueline Otto, Felicity A. Holmes, Nina Kirchner. Frontiers in Earth Science 10

  Article

  Along the Greenland Ice Sheet margin, supraglacial lakes store and redistribute ice sheet surface run off, and comprise an important potential hydrological link between the ice surface and the base, with ramifications for subglacial drainage systems and ice flow. As a consequence of increasing global mean surface air temperatures, these lakes have been predicted to expand further inland and to affect larger areas of the ice sheet. However, as contemporary dynamics of such supraglacial lake expansion are not well studied, any assessment of their future implications remains afflicted with uncertainty. Here, recent changes in supraglacial lake distribution and expansion, and in their drainage behavior and frequency, are presented for Ryder Glacier, Northern Greenland, as concluded from a remote sensing based analysis. The 35-year time span covered in the analysis allows for the detection of trends in lake processes and ice velocity, which otherwise were found to exhibit large inter-annual variability. It also reveals the first occurrence of a coupled lake drainage event in 2002. By linking supraglacial lake expansion, drainage modes, and drainage frequency to the efficiency of the subglacial drainage system and ice flow on seasonal and decadal timescales, a contribution is made to better understand the complexity of coupled glacio-hydrological processes, and to help reduce uncertainties in predictions of future mass loss from the Greenland Ice Sheet.

  Read more about Supraglacial lake expansion, intensified lake drainage frequency, and first observation of coupled lake drainage, during 1985–2020 at Ryder Glacier, Northern Greenland

• A first continuous three-year temperature record from the dimictic arctic-alpine Lake Tarfala, northern Sweden

  2021. Nina Kirchner (et al.). Arctic, Antarctic and Alpine research 53 (1), 69-79

  Article

  Arctic lakes are exposed to warming during increasingly longer ice-free periods and, if located in glaciated areas, to increased inflow of meltwater and sediments. However, direct monitoring of how such lakes respond to changing environmental conditions is challenging not only because of their remoteness but also because of the scarcity of present and previously observed lake states. At the glacier-proximal Lake Tarfala in the Kebnekaise Mountains, northern Sweden, temperatures throughout the water column at its deepest part (50 m) were acquired between 2016 and 2019. This three-year record shows that Lake Tarfala is dimictic and is overturning during spring and fall, respectively. Timing, duration, and intensity of mixing processes, as well as of summer and winter stratification, vary between years. Glacial meltwater may play an important role regarding not only mixing processes but also cooling of the lake. Attribution of external environmental factors to (changes in) lake mixing processes and thermal states remains challenging owing to for example, timing of ice-on and ice-off but also reflection and absorption of light, both known to play a decisive role for lake mixing processes, are not (yet) monitored in situ at Lake Tarfala.

  Read more about A first continuous three-year temperature record from the dimictic arctic-alpine Lake Tarfala, northern Sweden

• Calving at Ryder Glacier, Northern Greenland

  2021. Felicity A. Holmes (et al.). Journal of Geophysical Research - Earth Surface 126 (4)

  Article

  Recent evidence has shown increasing mass loss from the Greenland ice sheet, with a general trend of accelerated mass losses extending northwards. However, different glaciers have been shown to respond differently to similar external forcings, constituting a problem for extrapolating and upscaling data. Specifically, whilst some outlet glaciers have accelerated, thinned, and retreated in response to atmospheric and oceanic warming, the behavior of other marine terminating glaciers appears to be less sensitive to climate forcing. Ryder glacier, for which only a few studies have been conducted, is located in North Greenland and terminates with a floating ice tongue in Sherard Osborn Fjord. The persistence or disintegration of floating ice tongues has impacts on glacier dynamics and stability, with ramifications beyond, including sea level rise. This study focuses on understanding the controls on calving and frontal ablation of the Ryder glacier through the use of time-lapse imagery and satellite data. The results suggest that Ryder glacier has behaved independently of climate forcing during recent decades, with fjord geometry exerting a first order control on its calving.

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• Glacial dynamics and deglaciation history of Hambergbukta reconstructed from submarine landforms and sediment cores, SE Spitsbergen, Svalbard

  2021. Riko Noormets, Anne Flink, Nina Kirchner. Boreas 50 (1), 29-50

  Article

  The submarine landforms and shallow sediment record are presented from Hambergbukta, southeastern Spitsbergen using swath-bathymetric, subbottom acoustic, and sediment core data. The mapped landforms include large terminal and end-moraines with associated debrisflow aprons on their distal flanks, drumlinized till surface, glacial lineations, medial and retreat moraines, crevasse squeeze ridge networks, eskers, as well as iceberg-produced terraces and plough-marks. Analysis of the landforms and landform assemblages in combination with the sediment core data and aerial imagery studies reveal a complex and dynamic glacial history of Hambergbukta. We present a detailed history of Hambergbreen glacier indicating two previously unknown surges as well as new details on the nature of the subsequent ice-margin retreat. The results from two gravity cores combined with the shallow acoustic stratigraphy and high-resolution bathymetry suggest that the c. AD 1900 surge was less extensive than previously thought and the retreat was most likely rapid after the c. AD 1900 and 1957 surges of the Hambergbreen. Mixed benthic foraminifera collected from the outer fjord basin date to 2456 cal. a BP, suggesting older sediments were re-worked by the c. AD 1900 surge. This highlights the importance of exercising caution when using foraminifers for dating surge events in fjord basins enclosed by prominent end-moraines.

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• Is A Common Goal A False Hope in Convergence Research?

  2021. J. G. Ernakovich (et al.). Earth's Future 9 (5)

  Article

  The Arctic faces multiple pressures including climate change, shifting demographics, human health risks, social justice imbalances, governance issues, and expanding resource extraction. A convergence of academic disciplines-such as natural and social sciences, engineering and technology, health and medicine-and international perspectives is required to meaningfully contribute to solving the challenges of Arctic peoples and ecosystems. However, successfully carrying out convergent, international research and education remains a challenge. Here, lessons from the planning phase of a convergence research project concerned with the health of Arctic waters developed by the Arctic Science IntegrAtion Quest (ASIAQ) are discussed. We discuss our perspective on the challenges, as well as strategies for success, in convergence research as gained from the ASIAQ project which assembled an international consortium of researchers from disparate disciplines representing six universities from four countries (Sweden, Japan, Russia, and the United States) during 2018-2020.

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• Sensitivity of ice sheet surface velocity and elevation to variations in basal friction and topography in the full Stokes and shallow-shelf approximation frameworks using adjoint equations

  2021. Gong Cheng, Nina Kirchner, Per Lötstedt. The Cryosphere 15 (2), 715-742

  Article

  Predictions of future mass loss from ice sheets are afflicted with uncertainty, caused, among others, by insufficient understanding of spatiotemporally variable processes at the inaccessible base of ice sheets for which few direct observations exist and of which basal friction is a prime example. Here, we present a general numerical framework for studying the relationship between bed and surface properties of ice sheets and glaciers. Specifically, we use an inverse modeling approach and the associated time-dependent adjoint equations, derived in the framework of a full Stokes model and a shallow-shelf/shelfy-stream approximation model, respectively, to determine the sensitivity of grounded ice sheet surface velocities and elevation to time-dependent perturbations in basal friction and basal topography. Analytical and numerical examples are presented showing the importance of including the time-dependent kinematic free surface equation for the elevation and its adjoint, in particular for observations of the elevation. A closed form of the analytical solutions to the adjoint equations is given for a two-dimensional vertical ice in steady state under the shallow-shelf approximation. There is a delay in time between a seasonal perturbation at the ice base and the observation of the change in elevation. A perturbation at the base in the topography has a direct effect in space at the surface above the perturbation, and a perturbation in the friction is propagated directly to the surface in time.

  Read more about Sensitivity of ice sheet surface velocity and elevation to variations in basal friction and topography in the full Stokes and shallow-shelf approximation frameworks using adjoint equations

• Ryder Glacier in northwest Greenland is shielded from warm Atlantic water by a bathymetric sill

  2020. Martin Jakobsson (et al.). Communications earth & environment 1 (1)

  Article

  The processes controlling advance and retreat of outlet glaciers in fjords draining the Greenland Ice Sheet remain poorly known, undermining assessments of their dynamics and associated sea-level rise in a warming climate. Mass loss of the Greenland Ice Sheet has increased six-fold over the last four decades, with discharge and melt from outlet glaciers comprising key components of this loss. Here we acquired oceanographic data and multibeam bathymetry in the previously uncharted Sherard Osborn Fjord in northwest Greenland where Ryder Glacier drains into the Arctic Ocean. Our data show that warmer subsurface water of Atlantic origin enters the fjord, but Ryder Glacier's floating tongue at its present location is partly protected from the inflow by a bathymetric sill located in the innermost fjord. This reduces under-ice melting of the glacier, providing insight into Ryder Glacier's dynamics and its vulnerability to inflow of Atlantic warmer water. A bathymetric sill in Sherard Osborn Fjord, northwest Greenland shields Ryder Glacier from melting by warm Atlantic water found at the bottom of the fjord, according to high-resolution bathymetric mapping and oceanographic data.

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• Simulated last deglaciation of the Barents Sea Ice Sheet primarily driven by oceanic conditions

  2020. Michele Petrini (et al.). Quaternary Science Reviews 238

  Article

  The Barents Sea Ice Sheet was part of an interconnected complex of ice sheets, collectively referred to as the Eurasian Ice Sheet, which covered north-westernmost Europe, Russia and the Barents Sea during the Last Glacial Maximum (around 21 ky BP). Due to common geological features, the Barents Sea component of this ice complex is seen as a paleo-analogue for the present-day West Antarctic Ice Sheet. Investigating key processes driving the last deglaciation of the Barents Sea Ice Sheet represents an important tool to interpret recent observations in Antarctica over the multi-millennial temporal scale of glaciological changes. We present results from a perturbed physics ensemble of ice sheet model simulations of the last deglaciation of the Barents Sea Ice Sheet, forced with transient atmospheric and oceanic conditions derived from AOGCM simulations. The ensemble of transient simulations is evaluated against the databased DATED-1 reconstruction to construct minimum, maximum and average deglaciation scenarios. Despite a large model/data mismatch at the western and eastern ice sheet margins, the simulated and DATED-1 deglaciation scenarios agree well on the timing of the deglaciation of the central and northern Barents Sea. We find that the simulated deglaciation of the Barents Sea Ice Sheet is primarily driven by the oceanic forcing, with prescribed eustatic sea level rise amplifying the ice sheet sensitivity to sub-shelf melting over relatively short intervals. Our results highlight that the sub-shelf melting has a very strong control on the simulated grounding-line flux, showing that a slow, gradual ocean warming trend is capable of triggering sustained grounded ice discharge over multi-millennial timescales, even without taking into account marine ice sheet or ice cliff instability.

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• Skewness of Temperature Data Implies an Abrupt Change in the Climate System Between 1985 and 1991

  2020. Alasdair Skelton, Nina Kirchner, I. Kockum. Geophysical Research Letters 47 (23)

  Article

  Instrumental records of mean annual temperature extend back to the seventeenth and eighteenth centuries at multiple sites in Europe. For such long time series, we expect and find that histograms of mean annual temperature data become skewed toward higher temperatures with time because of global warming. However, we also find that skewness changed abruptly and started increasing between 1985 and 1991 (95% confidence) at 17 sites. We argue that this finding may imply an abrupt change in the climate system affecting Europe which probably occurred at this time. We investigate possible causes and find Arctic sea ice loss, potentially linked to reduced sulfate aerosol emissions and coupled to temperature by an albedo feedback mechanism, a likely candidate. This is based on good correlations of sea ice extent and sulfate aerosol emissions with skewness of mean annual temperature data.

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• Water current measurements using oceanographic bottom lander LoTUS

  2020. Maria Kjelldorff (et al.). Applied Ocean Research 94

  Article

  LOTUS is a bottom landing, Long Term Underwater Sensing node made for the observation of ocean water temperatures. LoTUS is moored to the seafloor and measures temperature according to a specified time schedule until, at the end of the mission, it surfaces to transmit the collected data to on-shore recipients using an Iridium link. The paper presents an extension of the sensing capability which includes water current velocity (speed and direction) using a robust, reliable and inexpensive Eulerian method. The method is based on the tilting stick principle where a combination of inertial and magnetic measurement data are used. The paper discusses the principal technique, modeling of the system, practical considerations, optimization of the setup for specific flow conditions, and the verification of experimental data.

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• Calving controlled by melt-under-cutting

  2019. Penelope How (et al.). Annals of Glaciology 60 (78), 20-31

  Article

  We present a highly detailed study of calving dynamics at Tunabreen, a tidewater glacier in Svalbard. A time-lapse camera was trained on the terminus and programmed to capture images every 3 seconds over a 28-hour period in August 2015, producing a highly detailed record of 34 117 images from which 358 individual calving events were distinguished. Calving activity is characterised by frequent events (12.8 events h(-1)) that are small relative to the spectrum of calving events observed, demonstrating the prevalence of small-scale calving mechanisms. Five calving styles were observed, with a high proportion of calving events (82%) originating at, or above, the waterline. The tidal cycle plays a key role in the timing of calving events, with 68% occurring on the falling limb of the tide. Calving activity is concentrated where meltwater plumes surface at the glacier front, and a similar to 5 m undercut at the base of the glacier suggests that meltwater plumes encourage melt-under-cutting. We conclude that frontal ablation at Tunabreen may be paced by submarine melt rates, as suggested from similar observations at glaciers in Svalbard and Alaska. Using submarine melt rate to calculate frontal ablation would greatly simplify estimations of tidewater glacier losses in prognostic models.

  Read more about Calving controlled by melt-under-cutting

• High-resolution bathymetric mapping reveals subaqueous glacial landforms in the Arctic alpine lake Tarfala, Sweden

  2019. Nina Kirchner (et al.). Journal of Quaternary Science 34 (6), 452-462

  Article

  In Arctic alpine regions, glacio-lacustrine environments respond sensitively to variations in climate conditions, impacting, for example,glacier extent and rendering former ice-contact lakes into ice distal lakes and vice versa. Lakefloors may hold morphological records of past glacier extent, but remoteness and long periods of ice cover on such lakes make acquisition of high-resolution bathymetric datasets challenging. Lake Tarfala and Kebnepakte Glacier, located in the Kebnekaise mountains, northern Sweden, comprise a small, dynamic glacio-lacustrine system holding a climate archive that is not well studied. Using an autonomous surface vessel, a high-resolution bathymetric dataset for Lake Tarfala was acquired in 2016, from which previously undiscovered end moraines and a potential grounding line feature were identified. For Kebnepakte Glacier, structure-from-motion photogrammetry was used to reconstruct its shape from photographs taken in 1910 and 1945. Combining these methods connects the glacial landform record identified at the lakefloor with the centennial-scale dynamic behaviour of Kebnepakte Glacier. During its maximum 20(th) century extent, attained c. 1910, Kebnepakte Glacier reached far into Lake Tarfala, but had retreated onto land by 1945, at an average of 7.9 m year(-1).

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• Relating ocean temperatures to frontal ablation rates at Svalbard tidewater glaciers

  2019. Felicity A. Holmes (et al.). Scientific Reports 9

  Article

  Fjord-terminating glaciers in Svalbard lose mass through submarine melt and calving (collectively: frontal ablation), and surface melt. With the recently observed Atlantification of water masses in the Barents Sea, warmer waters enter these fjords and may reach glacier fronts, where their role in accelerating frontal ablation remains insufficiently understood. Here, the impact of ocean temperatures on frontal ablation at two glaciers is assessed using time series of water temperature at depth, analysed alongside meteorological and glaciological variables. Ocean temperatures at depth are harvested at distances of 1 km from the calving fronts of the glaciers Kronebreen and Tunabreen, western Svalbard, from 2016 to 2017. We find ocean temperature at depth to control c. 50% of frontal ablation, making it the most important factor. However, its absolute importance is considerably less than found by a 2013-2014 study, where temperatures were sampled much further away from the glaciers. In light of evidence that accelerating levels of global mass loss from marine terminating glaciers are being driven by frontal ablation, our findings illustrate the importance of sampling calving front proximal water masses.

  Read more about Relating ocean temperatures to frontal ablation rates at Svalbard tidewater glaciers

Show all publications by Nina Kirchner at Stockholm University