Northeast Greenland Research Articles

Atlantic Water warming increases melt below Northeast Greenland’s last floating ice tongue

The 79 North Glacier (79NG) features Greenland’s largest floating ice tongue. Even though its extent has not changed significantly in recent years, observations have indicated a major thinning of the ice tongue from below. Both ocean warming and an increase in subglacial discharge from the ice sheet induced by atmospheric warming could increase the basal melt; however, available observations alone cannot tell which of these is the main driver. Here, we employ a global simulation which explicitly resolves the ocean circulation in the cavity with 700 m resolution to disentangle the impact of the ocean and atmosphere. We find that the interannual variability of basal melt below 79NG over the past 50 years is mainly associated with changes in the temperature of the Atlantic Intermediate Water inflow, which can be traced back across the Northeast Greenland continental shelf to the eastern Fram Strait with a lag of 3 years.

Shear margins in upper half of Northeast Greenland Ice Stream were established two millennia ago

Only a few localised ice streams drain most of the ice from the Greenland Ice Sheet. Thus, understanding ice stream behaviour and its temporal variability is crucially important to predict future sea-level change. The interior trunk of the 700 km-long North-East Greenland Ice Stream (NEGIS) is remarkable due to the lack of any clear bedrock channel to explain its presence. Here, we present a 3-dimensional analysis of the folding and advection of its stratigraphic horizons, which shows that the localised flow and shear margins in the upper NEGIS were fully developed only ca 2000 years ago. Our results contradict the assumption that the ice stream has been stable throughout the Holocene in its current form and show that upper NEGIS-type development of ice streaming, with distinct shear margins and no bed topography relationship, can be established on time scales of hundreds of years, which is a major challenge for realistic mass-balance and sea-level rise projections.

Weakened pelagic-benthic coupling on an Arctic outflow shelf (Northeast Greenland) suggested by benthic ecosystem changes

Arctic marine ecosystems are becoming more boreal due to climate change. Predictions of ecosystem change focus mainly on Arctic inflow or interior shelves, with few comprehensive studies on Arctic outflow regions. During September–October 2017, soft-bottom communities were sampled and benthic ecosystem processes were quantified at 12 stations on the Northeast Greenland shelf (outflow shelf) and compared to the last regional ecosystem study, conducted in 1992 and 1993. The benthic habitat was characterized in terms of sediment granulometry, pigment concentrations, and porewater chemistry (dissolved inorganic carbon, nutrients). Total abundance and biomass of macrobenthos and meiobenthos, bacterial abundance, porewater dissolved inorganic carbon and ammonium concentrations were higher on the outer shelf compared to locations adjacent to the Nioghalvfjerdsfjorden glacier at 79°N and the inner shelf stations (e.g., macrofauna: 1,964–2,952 vs. 18–1,381 individuals m−2). These results suggest higher benthic production in the outer parts of the NEG shelf. This difference was also pronounced in macrobenthic and meiobenthic community structure, which was driven mainly by food availability (pigments with 1.3–4.3 vs. 0.3–0.9 µg g−1 sediment, higher total organic carbon content and bacterial abundance). Compared to the early 1990s, warmer bottom water temperatures, increased number of sea-ice-free days and lower sediment pigment concentrations in 2017 were accompanied by decreased polychaete and increased nematode abundance and diversity, and a different community structure of nematode genera. The present study confirms previous reports of strong pelagic-benthic coupling on the NEG shelf, but highlights a possible weakening since the early 1990s, with a potential shift in importance from macrofauna to meiofauna in the benthic community. Increasing inflow of Atlantic water and decreasing volume transport and thickness of sea ice through the Fram Strait, probably affecting the Northeast Water Polynya, may be responsible, suggesting ecosystem-wide consequences of continued changes in sea-ice patterns on Arctic shelves.

Benthic primary production in Young Sound, Northeast Greenland

The extreme and variable light climate of polar marine environments imposes a substantial limitation on benthic primary production and demands efficient adaptive capacities of the primary producers. This chapter reviews the composition, abundance, primary production and adaptive strategies of benthic primary producers in Young Sound. Benthic primary producers occurred in the 0-50 m depth range and the relative importance of microalgae, crustose coralline macroalgae and foliose macroalgae varied systematically with depth. On a summer day with optimal light conditions benthic primary production showed a maximum of c.
 70 mmol O2 m-2 d-1 in shallow water. Production rates declined gradually to c. 20 mmol O2 m-2 d-1at 10–20 m depth and to 2.5 mmol O2 m-2 d-1 at 30 m depth.
 Foliose macroalgae contributed markedly to primary production in shallow water but became insignificant at water depths >15 m, while benthic diatoms contributed most to primary production at intermediate water depths (5-30 m). At water depths greater than 30 m only coralline algae occurred, but their production was low because of their low abundance, low Pmax and the low ambient irradiance at those depths. All algal groups were well adapted to the ambient irradiance and could, within minutes, acclimate their photosynthetic performance to changing light conditions. The benthic primary production in Young Sound markedly surpassed the pelagic primary production down to water depths of 20 m and the results thereby underline the potential importance of benthic primary production in shallow-water Arctic ecosystems.

Major metallic elements (Al, Ca, and Fe) and size distribution of mineral particles in recent snow in inland Northeast Greenland

Major metallic elements (Al, Ca, and Fe) and size distribution of mineral particles in recent snow in inland Northeast Greenland

Stratigraphy of the Upper Jurassic to lowermost Cretaceous in the Rødryggen-1 and Brorson Halvø-1 boreholes, Wollaston Forland, North-East Greenland

Two shallow cores drilled in northern Wollaston Forland, North-East Greenland, provide a combined section covering the upper Kimmeridgian (Upper Jurassic) – Barremian (Lower Cretaceous) and comprising the Bernbjerg, Lindemans Bugt, Palnatokes Bjerg and Stratumbjerg Formations. A new lithostratigraphic unit, the Storsletten Member, is defined within the Lindemans Bugt Formation. The black mudstone-dominated intervals are dated primarily by dinoflagellate cysts and ammonites, whereas the calcareous mudstones of the Palnatokes Bjerg Formation – sandwiched between the black mudstones – are dated by calcareous nannofossils. The stratigraphy demonstrates an almost complete succession in the Rødryggen-1 core, representing a deeper position in the basin, where the hiatus at the latest Jurassic rift climax predicted in previous models for the eastern Wollaston Forland Basin is absent. In contrast, the Brorson Halvø-1 core represents a position closer to a block crest where unconformities developed. In combination, the cores provide a key biostratigraphic reference section for the Jurassic–Cretaceous boundary interval in the Arctic.

Organic geochemistry of an Upper Jurassic – Lower Cretaceous mudstone succession in a narrow graben setting, Wollaston Forland Basin, North-East Greenland

The Oxfordian–Ryazanian was a period of widespread deposition of marine organic-rich mudstones in basins formed during the early phases of the rifting that heralded the formation of the present-day North Atlantic. Occasionally, uninterrupted deposition prevailed for 20 million years or more. Today, mudstones of this time interval are found on the shelves bordering the North Atlantic and adjacent areas from Siberia to the Netherlands. Here, we report data on two fully cored boreholes from Wollaston Forland (North-East Greenland, approx. 74° N), which represent an uninterrupted succession from the upper Kimmeridgian to the Hauterivian. The boreholes record basin development at two different positions within an evolving halfgraben, located at the margin of the main rift, and thus partially detached from it. Although the overall depositional environment remained an oxygen-restricted deep-shelf setting, rifting-related changes can be followed through the succession. The Kimmeridgian was a period of eustatic highstand and records the incipient rifting with a transgressive trend straddling the transition to the lower Volgian by a gradual change from deposits with high levels of total organic carbon (TOC) and kerogen rich in allochthonous organic matter to deposits with lower TOC and a higher proportion of autochthonous organic matter. This is followed by a slight regressive trend with lower TOC and increased proportions of allochthonous organic matter until rifting culminated in the middle Volgian–Ryazanian, indicated by increasing autochthonous organic matter and higher TOC, which prevailed until basin ventilation occurred towards the end of the Ryazanian. The properties of the reactive kerogen fraction remained rather stable irrespective of TOC, underlining the effect of terrigenous matter input for TOC. These variations are also captured by biological markers and stable carbon isotopes. The deposits are very similar to equivalent successions elsewhere in the proto-North Atlantic region, albeit the proportion of terrigenous kerogen is greater.

Mudstone diagenesis and sandstone provenance in an Upper Jurassic – Lower Cretaceous evolving half-graben system, Wollaston Forland, North-East Greenland

The influence of rifting on the composition of Kimmeridgian to Barremian mudstones from northern Wollaston Forland, North-East Greenland is investigated by petrographic and mineralogical analyses of the Brorson Halvø-1 and Rødryg­gen-1 cores, and provenance of analysis by zircon U-Pb age dating of nearby sandstones. Mudstone composition varies systematically as a function of the timing of rifting progression and position in the half-graben depositional system. Pyrite primarily precipitated in the early rift to rift climax phases. Euhedral pyrite overgrowths on framboids formed only during the rift climax phase (Lindemans Bugt Formation). Dolomite is the dominant carbonate cement, except for the sediments deposited in the early waning rift phase (Palnatokes Bjerg Formation) where calcite is dominant, and in the late waning rift phase (Stratumbjerg Formation) where siderite dominates. The highest-temperature reactions with precipitation of illite, quartz, ankerite and barite signify sediment burial depths of >2 km prior to exhumation. Uplift-induced fracturing occurred mainly in the early rift to rift acceleration succession (Bernbjerg Formation). Mudstones in the proximal part of the half-graben (Rødryggen-1) include more detrital kaolinite than the distal mudstones (Brorson Halvø-1), which contain more mixed-layer illite-smectite and illite. Vermiculite was deposited only in the proximal part of the basin in the rift climax and waning rift successions. Chlorite was deposited proximally and distally during the waning rift phase, though supply began earlier in the distal part. Fine-grained sediment in the distal part of the half-graben was therefore probably supplied by axial transport from Palaeoproterozoic crystalline rocks and Meso- to Neoproterozoic metamorphic rocks located to the north and north-west. This agrees with the zircon provenance signature from outcropping sand-rich facies, where zircon grains with U-Pb ages of 2.0–1.6 Ga are dominant, in addition to common 1.6–0.9 Ga ages, and fewer 2.8–2.6 Ga and 0.47–0.36 Ga ages.

Upper Jurassic – Lower Cretaceous of eastern Wollaston Forland, North-East Greenland: a distal marine record of an evolving rift

Two drill cores covering the Upper Jurassic – Lower Cretaceous succession in Wollaston Forland, NE Greenland, offer an exceptional insight into mud-accumulation in an evolving distal fault block. Previous studies have revealed the presence of long-lasting black mudstone accumulation extending through the oxygen-restricted early rift and rift climax phases (Bernbjerg and Lindemans Bugt Formations). Here, we revisit the depositional evolution recorded in these cores to present a detailed description of the sedimentary succession extending into the late syn-rift settings (Palnatokes Bjerg and Stratumbjerg Formations). The results indicate that the Kimmeridgian – lower Volgian early rift-phase was characterized by suspension settling, laminae-scale event deposition in a tectonically-affected, prodeltaic offshore setting. The event-related depositional processes are expressed by starved wave-ripples, scour-and-fill structures, putative mud floccule ripples, and mud-dominated gravity-flow deposits. During the middle Volgian – Ryazanian rift climax phase, the depositional environment evolved into a narrow half-graben that was detached from the proximal depocentre flanking the coarse sediment fueled deltaic coastline. The correlative sedimentary facies in the detached half-graben are bioclastic and pyrite-rich black mudstones documenting suspension settling and gravity flow/mass wasting deposition in sub-storm wave-base slope and basin-floor environments. Black shale sedimentation ended abruptly in the late Ryazanian when the accumulation of condensed, bioturbated deep sea marls was initiated linked to broader oceanographic reorganization concomitant with waning rift activity in the west. Deposition of red bioclastic mudstones with a common gravity-flow component characterized the Hauterivian, potentially representing final draping of the submerged fault block crest. The top of the cored succession is demarcated by the appearance of dark grey bioturbated mudstones of Barremian age, reflecting the onset of regionally continuous deep-sea mud accumulation in thermally subsidizing basins. Although superficially monotonous, the mudstone-dominated succession reveals a highly dynamic depositional system that reflects shifting marine processes during almost a full rift cycle.

Projections of Greenland climate change from CMIP5 and CMIP6

As the Arctic continues to warm faster than the global average, the Greenland ice sheet faces a warmer future without restrictions on greenhouse gas emissions. In this study, 24 CMIP5 and 21 CMIP6 models were used to analyze projections of precipitation, 2 m or near-surface air temperature (T2m), and 500 hPa geopotential height (Z500) over Greenland under medium and high emission scenarios. The multi-model ensemble means indicate that CMIP6 is a warmer output with more precipitation in Greenland under medium and high emission scenarios relative to CMIP5. At the end of the 21st century (2071–2100), annual integrated precipitation in Greenland under RCP8.5 (SSP58.5) is projected to increase by 277.9 (281.3) Gt relative to the 1986–2005 climatological mean, with a significant positive trend of 38.5 (42.9) Gt/dec (p < 0.05) post-2020, approximately twice that under RCP4.5 (SSP24.5). Under high emission scenarios, T2m will steadily increase by 5.2 (5.7) °C at the end of the 21st century and the northern amplifying effect of warming is more intense. In this context, northeast Greenland will increase the highest precipitation, with a trend of >30 mm/dec during 2020–2100. The Z500 rises higher from southeast to northwest Greenland due to global warming under medium and high emission scenarios. Model and scenario uncertainties are major sources of uncertainty at the end of the 21st century. Compared to CMIP5, the model uncertainty of CMIP6 decreases more but remained higher at the end of this century. The projections of northern Greenland are more reliable than those of other regions, with a lower total fractional uncertainty and a greater signal-to-noise ratio. Furthermore, we did not find an additional T2m increase due to internal variability. Under medium and high emission scenarios, Greenland near surface air warming is major owed to global warming. Considering the importance of current circulation anomalies and after removing the artificial increase in Z500, the decrease in ΔZ500 indicates that future warming has been underestimated under medium and high emission scenarios.

The role of a low-level jet for stirring the stable atmospheric surface layer in the Arctic

Abstract. In this study, we analyze the transition of a stable atmospheric boundary layer (ABL) with a low-level jet (LLJ) to a traditional stable ABL with a classic Ekman helix in the late-winter central Arctic. Vertical profiles in the ABL were measured with a hot-wire anemometer on a tethered balloon during a 15 h period in March 2018 in northeast Greenland. The tethered balloon allows high-resolution turbulence observations from the ground to the top of the ABL. The core of the LLJ was observed at about 150 m altitude, and its height and strength were associated with the temperature inversion. Increased turbulence was observed in the vicinity of the LLJ, but most of the turbulence does not reach down to the surface, thus decoupling the LLJ from the surface. Only when the LLJ collapses and the ABL again exhibits a more classical Ekman spiral is a coupling to the surface re-established. The LLJ might enhance both advective and turbulent vertical transport of passive tracers such as aerosol particles or moisture in the often stably stratified Arctic ABL.

Response of Arctic benthic foraminiferal traits to past environmental changes

The Arctic is subjected to all-encompassing disruptions in marine ecosystems caused by anthropogenic warming. To provide reliable estimates of how future changes will affect the ecosystems, knowledge of Arctic marine ecosystem responses to past environmental variability beyond the instrumental era is essential. Here, we present a novel approach on how to evaluate the state of benthic marine biotic conditions during the deglacial and Holocene period on the Northeast Greenland shelf. Benthic foraminiferal species were assigned traits (e.g., oxygen tolerance, food preferences) aiming to identify past faunal changes as a response to external forcing mechanisms. This approach was applied on sediment cores from offshore Northeast Greenland. We performed numerical rate-of-change detection to determine significant changes in the benthic foraminiferal traits. That way, the significant abrupt trait changes can be assessed across sites, providing a better understanding of the impact of climate drivers on the traits. Our results demonstrate that during the last ~ 14,000 years, bottom water oxygen is the main factor affecting the variability in the benthic foraminiferal faunas in this area. Our results show that significant changes in the traits correspond to drastic climate perturbations. Specifically, the deglacial-Holocene transition and mid-Holocene warm period exhibited significant change, with several trait turnovers.

Links Between Stream Water Nitrogen and Terrestrial Vegetation in Northeast Greenland

AbstractThe Arctic is warming and significant changes to the landscape, including increased vegetative cover (“greening”), are expected in the near future. These landscape changes may alter nitrogen (N) availability in terrestrial, stream, and coastal ecosystems, where production is often N limited, but the exact changes in nutrient cycling are uncertain. Here, we analyzed the relationship between vegetation greenness (i.e., NDVI) and dissolved inorganic (DIN) and organic (DON) concentrations in streams draining 14 headwater catchments (mean 3.6 km2, range 0.4–11 km2) across three samplings in the Zackenberg area, Northeast Greenland. We found large variation in DIN and DON concentrations across the sampled streams. We further show that this variation is correlated to water temperature and catchment NDVI, such that increased vegetation greenness and temperature correlated with lower DIN, and increased greenness also correlated with higher DON concentrations in streams. The results suggest that increased terrestrial vegetation due to rising air temperature could substantially alter dissolved N concentrations and form in streams, with potentially cascading impacts on coastal areas.

A climatology of Arctic fog along the coast of East Greenland

AbstractThis study presents a comprehensive climatology of coastal fog from four synoptic weather stations operated by the Danish Meteorological Institute along the entire East Greenland coast between 1958 and 2016. Elements investigated include fog frequency, daily timing, temperature, wind, visibility and radiosonde profiles during fog. The spatiotemporal patterns in fog from the low‐ to high‐Arctic locations were related to varying regional seasonal temperatures, surface and upper‐air wind and sea ice conditions, and to correlations with the North Atlantic Oscillation (NAO) and the Greenland Blocking Index (GBI). Results indicate that ˜70–80% of East Greenland fog occurs in summer (MJJA), and yearly fog onset is near‐coincident with the start of sea ice break‐up. This warm‐season fog has the typical characteristics of advection fog, as shown in the radiosonde profiles and the association with a gentle sea breeze. More than 95% of warm‐season fog is warmer than −10°C, and peaks close to 0°C and, therefore, consists of liquid or supercooled water droplets. In the cold season, mixed‐phase fog prevails in the high‐Arctic locations, accounting for ˜70% of observations. Ice fog (T &lt; −30°C) occurs in only 2% of observations and is limited to Northeast Greenland during the cold season. The cold‐season composite radiosonde fog profiles in the high‐Arctic locations are characterized by deep (˜1000 m) and strong (˜6°C) surface‐based temperature inversions. Visibility during most fog conditions is lowest during the warm season (&lt;500 m) and highest during the cold season (&lt;800 m). In Northeast Greenland, visibility during warm‐season fog has decreased by ˜50 m·dec−1 between 1981 and 2016. In Southeast Greenland, fog visibility is high during low GBI and a positive phase of NAO, but no other correlations with climate indices were found.

Gelatinous zooplankton off the Northeast Greenland coast

Gelatinous zooplankton are useful indicators of climate-driven shifts in the ocean; hence our study goal was to determine their diversity and distribution in the poorly investigated area of the Northeast Greenland shelf and adjacent waters. Zooplankton samples were collected vertically using a MultiNet in August and September 2017, at 9 stations along two transects: northern and southern. Eleven taxa were identified, of which Plotocnide borealis had the largest share within the assemblage in both transects. Gelatinous zooplankton biodiversity was the highest in the north. The local bathymetry and hydrology both shaped the distribution of the gelatinous zooplankton, leading to the emergence of three ecological groups: 1) taxa typically associated with, or found exclusively, in the intermediate, colder shelf waters (e.g. Plotocnide borealis); 2) organisms associated with higher temperatures and greater depths, mainly of the off-shelf waters (e.g. Aglantha digitale); 3) organisms whose distribution depended mainly on salinity and oxygen saturation (e.g. Aeginopsis laurentii). Additionally, A. digitale was associated with the presence of the warmer waters of Atlantic origin, the presence of which on both sides of the Fram Strait allowed to compare our findings with better studied gelatinous zooplankton from the West Spitsbergen Current, leading to the conclusion that the existent evidence of progressing Atlantification suggests that such impacts might also be expected off the Northeast Greenland coast.

Spatial Variability in the Primary Production Rates and Biomasses (Chl a) of Sea Ice Algae in the Canadian Arctic–Greenland Region: A Review

The aims of this review are to elucidate the spatial variation in the primary production rates and biomasses (Chl a) of sea ice algae in the Canadian Arctic–Greenland region, characterized by its comparable physical settings. A database was compiled from 30 studies of the production rates and biomasses (Chl a) of sea ice algae, the snow and ice thicknesses, ice types, nutrients (Si(OH)4, PO4, (NO3 + NO2)), and NH4 concentrations in the ice and below the ice from the region. Production rates were significantly higher (463 mg C m−2 d−1) in Resolute Bay and Northern Baffin Bay (317 mg C m−2 d−1), both in the Canadian Arctic, compared to a rate of 0.2 mg C m−2 d−1 in northeast Greenland. The biomasses reached 340 mg Chl a m−2 in Resolute Bay in comparison to 0.02 mg Chl a m−2 in southwest Greenland. Primary production at other Canadian and Greenland sites was comparable, but sea ice Chl a was higher (15.0 ± 13.4 mg Chl a m−2) at Canadian sites compared to Greenland ones (0.8 ± 0.5 mg Chl a m−2). Resolute and Northern Baffin Bay production rates were significantly higher when compared to other Arctic Ocean sites outside the studied region. The review concludes that the high production rates and biomasses in Resolute and Northern Baffin Bay are related to the inflow and mixing of nutrient-rich waters of Pacific origin. A conceptual model with drivers and inhibitors of the primary production of sea ice algae is proposed, and the database is compiled into a dataset of published data for further studies.

Impacts of glacial and sea-ice meltwater, primary production, and ocean CO2 uptake on ocean acidification state of waters by the 79 North Glacier and northeast Greenland shelf

The waters adjacent to the Nioghalvfjerdsbræ (79 North Glacier, 79NG) are influenced by Greenland Ice Sheet (GrIS) melt, sea-ice meltwater, and waters on the adjacent northeast Greenland shelf (NEGS). We investigated ocean acidification (OA) variables and the role of freshening, primary production, and air-sea CO2 exchange in Dijmphna Sound (DS) and on the NEGS in the summers of 2012 and 2016. The upper 150 m consisted of Polar Water with Arctic origin that was divided into a fresh surface layer (SL&amp;lt;50 m) and a cold halocline layer (CHL, 50 to 150 m). The layer below 150 m was of Atlantic origin. The SL freshwater was larger in 2012 than in 2016, mainly originated from local 79NG (and GrIS) runoff in DS, whereas on the NEGS in both years, it was mainly from sea-ice melt. The lowest aragonite saturation state (ΩAr) of 1.13 was found in the SL in 2012. Biological CO2 drawdown at primary production caused increased ΩAr in SL, which compensated for most of the ΩAr decrease due to the freshwater dilution of carbonate ions reducing total alkalinity, hence preventing corrosive conditions. This was most pronounced near the 79NG front in 2012, where surface stratification was most pronounced coinciding with large glacial meltwater fractions. Freshening decreased ΩAr by 0.4 at the 79NG front was compensated by biological CO2 drawdown by ~0.5. In 2016, a well-mixed water column in DS and NEGS, with dilution by sea-ice meltwater, caused less compensation on ΩAr by biological CO2 drawdown than in 2012. In future with changing climate and changing ocean chemistry, the increased meltwater effects may overcome the alleviating effects of biological CO2 drawdown on OA with unfavorable conditions for calcifying organisms. However, our study also suggests that primary production may be stimulated by stratification from surface meltwater. In addition, Atlantification and subglacial discharge may result in upwelling of inorganic nutrients that could promote primary production.

Fractures in glaciers—Crack tips and their stress fields by observation and modeling

AbstractHigh‐resolution optical camera systems are opening new opportunities to study fractures in ice. Here, we present data obtained from the Modular Aerial Camera System camera system operated onboard of Alfred Wegener Institute Helmholtz Centre for Polar and Marine Research (AWI) polar aircraft in northeast Greenland in 2022. In addition, we are using optical and radar satellite imagery. The study area is the 79°N Glacier (Nioghalvfjerdsbræ, 79NG), an outlet glacier of the Northeast Greenland Ice Stream. We found that crack tips are exhibiting additional isolated cracks ahead of the main crack. Subsequent crack propagation is starting from those isolated cracks, leading to an advance of the crack, with bridges between crack faces. The bridges provide information of the episodic crack propagation. Fractures have typically a length scale of kilometers and the distance of crack faces is in the order of meters to tenths of meters. Fracture modes will be inferred from stress fields computed by an inverse modeling approach using the Ice Sheet and Sea Level System Model. To this end, a surface velocity field derived from satellite remote sensing is used for the optimal control method that constrains model parameters, for example, basal friction coefficient or rheology.

Shifts of the Recirculation Pathways in Central Fram Strait Drive Atlantic Intermediate Water Variability on Northeast Greenland Shelf

AbstractIncreased oceanic heat transport plays a key role in the accelerated mass loss of Greenland's marine‐terminating glaciers. The melt rate of major glaciers in Northeast Greenland (NEG) is controlled by ocean variability, in particular warm Atlantic Intermediate Water (AIW), on the continental shelf. A high‐resolution configuration of the ocean sea‐ice model FESOM2.1 is assessed at local and regional scales, and used to investigate the drivers of AIW temperature variability on the NEG continental shelf. The seasonal to decadal variability of AIW is characterized, featuring both pronounced interannual fluctuations and a long‐term warming trend. A major source of AIW is Atlantic Water (AW) originating from the West Spitsbergen Current that recirculates in Fram Strait. AW anomalies are advected westwards and partly control the AIW temperatures on the continental shelf. Increased AIW temperatures are also connected to pronounced northern and central branches of recirculating AW in Fram Strait, and enhanced AW temperatures more regionally. The strengthening of the pathways brings more warmer AIW onto the northern part of the NEG continental shelf. There, it circulates anti‐cyclonically and results in shelf‐wide warming. Regional atmospheric forcing is connected to the changes in the AW circulation. The strengthening of the northern AW branches is likely caused by anticyclonic wind anomalies over the Barents Sea that drive an enhanced northward AW transport in Fram Strait. Thus, controlled by a combination of both upstream and regionally forced circulation conditions, the changes in local AIW temperatures may also affect the oceanic heat transport reaching the Central Arctic Ocean.

Borehole fibre-optic seismology inside the Northeast Greenland Ice Stream

SUMMARYIce streams are major contributors to ice sheet mass loss and sea level rise. Effects of their dynamic behaviour are imprinted into seismic properties, such as wave speeds and anisotropy. Here, we present results from a distributed acoustic sensing (DAS) experiment in a deep ice-core borehole in the onset region of the Northeast Greenland Ice Stream, with focus on phenomenological and methodological aspects. A series of active seismic surface sources produced clear recordings of the P and S wavefield, including internal reflections, along a 1500 m long fibre-optic cable that was placed into the borehole. The combination of nonlinear traveltime tomography with a firn model constrained by multimode surface wave data, allows us to invert for P and S wave speeds with depth-dependent uncertainties on the order of only 10 m s−1, and vertical resolution of 20–70 m. The wave speed model in conjunction with the regularly spaced DAS data enable a straightforward separation of internal upward reflections followed by a reverse-time migration that provides a detailed reflectivity image of the ice. While the differences between P and S wave speeds hint at anisotropy related to crystal orientation fabric, the reflectivity image seems to carry a pronounced climatic imprint caused by rapid variations in grain size. Further improvements in resolution do not seem to be limited by the DAS channel spacing. Instead, the maximum frequency of body waves below ∼200 Hz, low signal-to-noise ratio caused by poor coupling, and systematic errors produced by the ray approximation, appear to be the leading-order issues. Among these, only the latter has a simple existing solution in the form of full-waveform inversion. Improving signal bandwidth and quality, however, will likely require a significantly larger effort in terms of both sensing equipment and logistics.