The control of short-term ice mélange weakening episodes on calving activity at major Greenland outlet glaciers

The dense mixture of iceberg of various sizes and sea ice observed in many of Greenland's fjords, called ice mélange (sikussak in Greenlandic), has been shown to have a significant impact on the dynamics of several Greenland tidewater glaciers, mainly through the seasonal support it provides to the glacier terminus in winter. However, a clear understanding of shorter-term ice mélange dynamics is still lacking, mainly due to the high complexity and variability of the processes at play at the ice–ocean boundary. In this study, we use a combination of Sentinel-1 radar and Sentinel-2 optical satellite imagery to investigate in detail intra-seasonal ice mélange dynamics and its link to calving activity at three major outlet glaciers: Kangerdlugssuaq Glacier, Helheim Glacier and Sermeq Kujalleq in Kangia (Jakobshavn Isbræ). In those fjords, we identified recurrent ice mélange weakening (IMW) episodes consisting of the up-fjord propagation of a discontinuity between jam-packed and weaker ice mélange towards the glacier terminus. At a late stage, i.e., when the IMW front approaches the glacier terminus, these episodes were often correlated with the occurrence of large-scale calving events. The IMW process is particularly visible at the front of Kangerdlugssuaq Glacier and presents a cyclic behavior, such that we further analyzed IMW dynamics during the June–November period from 2018 to 2021 at this location. Throughout this period, we detected 30 IMW episodes with a recurrence time of 24 d, propagating over a median distance of 5.9 km and for 17 d, resulting in a median propagation speed of 400 m d−1. We found that 87 % of the IMW episodes occurred prior to a calving event visible in spaceborne observations and that ∼75 % of all detected calving events were preceded by an IMW episode. These results therefore present the IMW process as a clear control on the calving activity of Kangerdlugssuaq Glacier. Finally, using a simple numerical model for ice mélange motion, we showed that a slightly biased random motion of ice floes without fluctuating external forcing can reproduce IMW events and their cyclic influence and explain observed propagation speeds. These results further support our observations in characterizing the IMW process as self-sustained through the existence of an IMW–calving feedback. This study therefore highlights the importance of short-term ice mélange dynamics in the longer-term evolution of Greenland outlet glaciers

Self-sufficient seismic boxes for monitoring glacier seismology

Glacier seismology is a valuable tool for investigating ice flow dynamics, but sufficient data acquisition in remote and exposed glaciated terrain remains challenging. For data acquisition on a highly crevassed and remote outlet glacier in Greenland we developed self-sufficient and easily deployable seismic stations, "SG-boxes". The SG-boxes contain their own power supply via solar panel, a three-component omni-directional geophone and a GNSS receiver. The SG-boxes can be deployed and retrieved from a hovering helicopter, allowing for deployment in difficult terrain. To assess their performance we conducted a field test comparing the SG-boxes to established on-ice geophone installations at Gornergletscher in Switzerland. Moreover, data from a first SG-box deployment in Greenland were analyzed. The SG-boxes exhibit consistently higher noise levels relative to colocated conventional geophones and a correlation between noise levels, wind and air temperature is found. Despite their noise susceptibility, the SG-boxes detected a total of 13,114 Gornergletscher icequakes over 10 days, which is 30% of the total number of icequakes detected by conventional geophone stations. Hence, even in sub-optimal weather conditions and without additional noise reduction measures, the SG-boxes can provide unique and valuable data from challenging glaciated terrain where no conventional seismic installations are possible

Self-sufficient seismic boxes for monitoring glacier seismology

Glacier seismology is a valuable tool for investigating ice flow dynamics, but sufficient data acquisition in remote and exposed glaciated terrain remains challenging. For data acquisition on a highly crevassed and remote outlet glacier in Greenland we developed self-sufficient and easily deployable seismic stations, "SG-boxes". The SG-boxes contain their own power supply via solar panel, a three-component omni-directional geophone and a GNSS receiver. The SG-boxes can be deployed and retrieved from a hovering helicopter, allowing for deployment in difficult terrain. To assess their performance we conducted a field test comparing the SG-boxes to established on-ice geophone installations at Gornergletscher in Switzerland. Moreover, data from a first SG-box deployment in Greenland were analyzed. The SG-boxes exhibit consistently higher noise levels relative to colocated conventional geophones and a correlation between noise levels, wind and air temperature is found. Despite their noise susceptibility, the SG-boxes detected a total of 13,114 Gornergletscher icequakes over 10 days, which is 30% of the total number of icequakes detected by conventional geophone stations. Hence, even in sub-optimal weather conditions and without additional noise reduction measures, the SG-boxes can provide unique and valuable data from challenging glaciated terrain where no conventional seismic installations are possible

The control of short-term ice mélange weakening episodes on calving activity at major Greenland outlet glaciers

The dense mixture of iceberg of various sizes and sea ice observed in many of Greenland's fjords, called ice mélange (sikussak in Greenlandic), has been shown to have a significant impact on the dynamics of several Greenland tidewater glaciers, mainly through the seasonal support it provides to the glacier terminus in winter. However, a clear understanding of shorter-term ice mélange dynamics is still lacking, mainly due to the high complexity and variability of the processes at play at the ice–ocean boundary. In this study, we use a combination of Sentinel-1 radar and Sentinel-2 optical satellite imagery to investigate in detail intra-seasonal ice mélange dynamics and its link to calving activity at three major outlet glaciers: Kangerdlugssuaq Glacier, Helheim Glacier and Sermeq Kujalleq in Kangia (Jakobshavn Isbræ). In those fjords, we identified recurrent ice mélange weakening (IMW) episodes consisting of the up-fjord propagation of a discontinuity between jam-packed and weaker ice mélange towards the glacier terminus. At a late stage, i.e., when the IMW front approaches the glacier terminus, these episodes were often correlated with the occurrence of large-scale calving events. The IMW process is particularly visible at the front of Kangerdlugssuaq Glacier and presents a cyclic behavior, such that we further analyzed IMW dynamics during the June–November period from 2018 to 2021 at this location. Throughout this period, we detected 30 IMW episodes with a recurrence time of 24 d, propagating over a median distance of 5.9 km and for 17 d, resulting in a median propagation speed of 400 m d−1. We found that 87 % of the IMW episodes occurred prior to a calving event visible in spaceborne observations and that ∼75 % of all detected calving events were preceded by an IMW episode. These results therefore present the IMW process as a clear control on the calving activity of Kangerdlugssuaq Glacier. Finally, using a simple numerical model for ice mélange motion, we showed that a slightly biased random motion of ice floes without fluctuating external forcing can reproduce IMW events and their cyclic influence and explain observed propagation speeds. These results further support our observations in characterizing the IMW process as self-sustained through the existence of an IMW–calving feedback. This study therefore highlights the importance of short-term ice mélange dynamics in the longer-term evolution of Greenland outlet glaciers.</p

Induction of early Purkinje cell dendritic differentiation by thyroid hormone requires RORα

<p>Abstract</p> <p>Background</p> <p>The active form (T₃) of thyroid hormone (TH) controls critical aspects of cerebellar development, such as migration of postmitotic neurons and terminal dendritic differentiation of Purkinje cells. The effects of T₃on early dendritic differentiation are poorly understood.</p> <p>Results</p> <p>In this study, we have analyzed the influence of T₃on the progression of the early steps of Purkinje cell dendritic differentiation in postnatal day 0 organotypic cerebellar cultures. These steps include, successively, regression of immature neuritic processes, a stellate cell stage, and the extension of several long and mature perisomatic protrusions before the growth of the ultimate dendritic tree. We also studied the involvement of RORα, a nuclear receptor controlling early Purkinje cell dendritic differentiation. We show that T₃treatment leads to an accelerated progression of the early steps of dendritic differentiation in culture, together with an increased expression of RORα (mRNA and protein) in both Purkinje cells and interneurons. Finally, we show that T₃failed to promote early dendritic differentiation in <it>staggerer </it>RORα-deficient Purkinje cells.</p> <p>Conclusions</p> <p>Our results demonstrate that T₃action on the early Purkinje cell dendritic differentiation process is mediated by RORα.</p

The Determination of the Snow Optical Grain Diameter and Snowmelt Area on the Greenland Ice Sheet Using Spaceborne Optical Observations

The optical diameter of the surface snow grains impacts the amount of energy absorbed by the surface and therefore the onset and magnitude of surface melt. Snow grains respond to surface heating through grain metamorphism and growth. During melt, liquid water between the grains markedly increases the optical grain size, as wet snow grain clusters are optically equivalent to large grains. We present daily surface snow grain optical diameters (dopt) retrieved from the Greenland ice sheet at 1 km resolution for 2017–2019 using observations from Ocean and Land Colour Instrument (OLCI) onboard Sentinel-3A. The retrieved dopt are evaluated against 3 years of in situ measurements in Northeast Greenland. We show that higher dopt are indicative of surface melt as calculated from meteorological measurements at four PROMICE automatic weather stations. We deduce a threshold value of 0.64 mm in dopt allowing categorization of the days either as melting or nonmelting. We apply this simple melt detection technique in Northeast Greenland and compare the derived melting areas with the conventional passive microwave MEaSUREs melt flag for June 2019. The two flags show generally consistent evolution of the melt extent although we highlight areas where large grain diameters are strong indicators of melt but are missed by the MEaSUREs melt flag. While spatial resolution of the optical grain diameter-based melt flag is higher than passive microwave, it is hampered by clouds. Our retrieval remains suitable to study melt at a local to regional scales and could be in the future combined with passive microwave melt flags for increased coveragepublishedVersio

Expression of X-chromosome linked inhibitor of apoptosis protein in mature purkinje cells and in retinal bipolar cells in transgenic mice induces neurodegeneration

Transgenic mice with overexpression of the caspase-inhibitor, X-chromosome-linked inhibitor of apoptosis protein (XIAP) in Purkinje cell (PC) and in retinal bipolar cells (RBCs) were produced to study the regulation of cell death. Unexpectedly, an increased neurodegeneration was observed in the PCs in these L7-XIAP mice after the third postnatal week with the mice exhibiting severe ataxia. The loss of PCs was independent of Bax as shown by crossing the L7-XIAP mice with Bax gene–deleted mice. Electron microscopy revealed intact organelles in PCs but with the stacking of ER cisterns indicative of cell stress. Immunostaining for cell death proteins showed an increased phosphorylation of c-Jun in the PCs, suggesting an involvement in cell degeneration. Apart from PCs, the number of RBCs was decreased in adult retina in line with the expression pattern for the L7 promoter. The data show that overexpression of the anti-apoptotic protein XIAP in vulnerable neurons leads to enhanced cell death. The mechanisms underlying this neurodegeneration can be related to the effects of XIAP on cell stress and altered cell signaling.Supported by Sigrid Juselius Foundation, Academy of Finland, EU Biotech Grant, Liv och Hälsa, Maud Kuistila, Ylppö Foundation, Uppsala University and Minerva Foundation. We thank Dr. Urmas Arumäe for discussions, and Dr. Patrik Ernfors for the Bax KO mice, and Eeva Lehto for technical assistance. L7AUG was a kind gift from Dr. J. Oberdick, Ohio State University, USA.Peer reviewe

Greenland ice sheet rainfall climatology, extremes and atmospheric river rapids

Greenland rainfall has come into focus as a climate change indicator and from a variety of emerging cryospheric impacts. This study first evaluates rainfall in five state-of-the-art numerical prediction systems (NPSs) (CARRA, ERA5, NHM-SMAP, RACMO, MAR) using in situ rainfall data from two regions spanning from land onto the ice sheet. The new EU Copernicus Climate Change Service (C3S) Arctic Regional ReAnalysis (CARRA), with a relatively fine (2.5 km) horizontal grid spacing and extensive within-model-domain observational initialization, has the lowest average bias and highest explained variance relative to the field data. ERA5 inland wet bias versus CARRA is consistent with the field data and other research and is presumably due to more ERA5 topographic smoothing. A CARRA climatology 1991–2021 has rainfall increasing by more than one-third for the ice sheet and its peripheral ice masses. CARRA and in situ data illuminate extreme (above 300 mm per day) local rainfall episodes. A detailed examination CARRA data reveals the interplay of mass conservation that splits flow around southern Greenland and condensational buoyancy generation that maintains along-flow updraft ‘rapids’ 2 km above sea level, which produce rain bands within an atmospheric river interacting with Greenland. CARRA resolves gravity wave oscillations that initiate as a result of buoyancy offshore, which then amplify from terrain-forced uplift. In a detailed case study, CARRA resolves orographic intensification of rainfall by up to a factor of four, which is consistent with the field data

Greenland ice sheet climate disequilibrium and committed sea-level rise

Ice loss from the Greenland ice sheet is one of the largest sources of contemporary sea-level rise (SLR). While process-based models place timescales on Greenland’s deglaciation, their confidence is obscured by model shortcomings including imprecise atmospheric and oceanic couplings. Here, we present a complementary approach resolving ice sheet disequilibrium with climate constrained by satellite-derived bare-ice extent, tidewater sector ice flow discharge and surface mass balance data. We find that Greenland ice imbalance with the recent (2000–2019) climate commits at least 274 ± 68 mm SLR from 59 ± 15 × 103 km2 ice retreat, equivalent to 3.3 ± 0.9% volume loss, regardless of twenty-first-century climate pathways. This is a result of increasing mass turnover from precipitation, ice flow discharge and meltwater run-off. The high-melt year of 2012 applied in perpetuity yields an ice loss commitment of 782 ± 135 mm SLR, serving as an ominous prognosis for Greenland’s trajectory through a twenty-first century of warming