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The Green's function approach is well‐established and widely used for modeling the surface mass loading displacements. Global mean Green's functions (MGFs) are commonly applied without considering local variations of the crustal structure. Derived from the modified layered Earth structure, the local Green's functions (LGFs) are theoretically beneficial to generate more accurate deformation, since they consider interior information of the local crust. This paper analyzed the differences among MGFs from Gutenberg–Bullen A model and two sets of LGFs derived from modified PREM Earth models consolidating with two crust models TEA12 and CRUST1.0, hereafter called PREMTEA and PREMCRU, respectively. Utilizing MGFs and two sets of LGFs, we modeled the corresponding 3D atmospheric loading displacements for 984 ITRF2014 stations and compared them with the ITRF2014 residuals. The results show that LGFs from PREMTEA and PREMCRU perform well in further promoting scatter reduction for ∼72%, ∼56%, and ∼85% of stations for the Up, East and North components, respectively. The improvements for the North components are significant (up to 3.6%). In particular, stations in the east coastal areas of North America and the west edge of Greenland exhibit further promoting scatter reduction for the East components (up to ∼2.5%), while those located in west coastline of North America show better performance for the North components. Nevertheless, there are significant anomalies in northern Europe for PREMCRU, the mutation margin of which should be carefully considered when using a resolution higher than 1°. In the area around station MORP (358.31°W, 55.21°N, sited at coastline of Britain), we suggest using PREMTEA model. Plain Language Summary: In any location worldwide, we employ a consistent set of numbers, known as load Green's functions that characterize the elastic properties of the Earth, to model the deformation caused by mass transfer on Earth's surface, which is referred to as surface mass loading effects. Most research and data products adopt same set of global mean Green's functions for any location on Earth, which neglects the differences in elastic properties of various regions of the Earth, leading to modeling errors to some extent. This study aims to optimize the modeling using local load Green's functions, with the objective of obtaining results that are more theoretically valuable. "Local" means that the Green's functions for each site are derived from an Earth model that takes into account the local crustal structure. The modeled atmospheric loading displacements exhibit enhanced alignment with GNSS position time series. This, in turn, further elucidates that deformations can be more attributed to atmospheric loading. Moreover, the model shows improved performance in the horizontal components when compared to the vertical components. Key Points: Local load Green's Functions enable a more precise modeling of loading displacements than global mean Green's FunctionsThe modeled atmospheric loading displacements based on Local Green's Functions can improve scatter reduction of GNSS position time seriesTwo sets of local load Green's Functions, based on CRUST1.0 model and TEA12 model, are compared. We recommend TEA12 for European region [ABSTRACT FROM AUTHOR]

Numerical modeling of ice sheet motion and hence projections of global sea level rise require information about the evolving subglacial environment, which unfortunately remains largely unknown due to its difficulty of access. Here we advance such subglacial observations by reporting multi‐year observations of seismic tremor likely associated with glacier sliding at Helheim Glacier. This association is confirmed by correlation analysis between tremor power and multiple environmental forcings on different timescales. Variations of the observed tremor power indicate that different factors affect glacial sliding on different timescales. Effective pressure may control glacial sliding on long (seasonal/annual) timescales, while tidal forcing modulates the sliding rate and tremor power on short (hourly/daily) timescales. Polarization results suggest that the tremor source comes from an upstream subglacial ridge. This observation provides insights on how different factors should be included in ice sheet modeling and how their timescales of variability play an essential role. Plain Language Summary: The Greenland Ice Sheet has lost ice increasingly in the past few decades, contributing significantly to global sea level rise. However, large uncertainties remain in computer simulations of such ice mass loss and hence sea level rise. This uncertainly is mainly attributed to the lack of information on what is happening at the bottom of the ice sheet and how that affects ice sheet movement. For example, how much water is there and whether it is in isolated pockets or is well distributed as a thin sheet can have an important effect on ice movement. In this paper, we observe small ground motions that are generated during glacier movement at a marine‐terminating glacier in Greenland. By analyzing the energy variation of the small ground motions over multiple years as well as observing the reasons that cause the variations, we learn that subglacial water pressure may control ice flow speeds on a seasonal/annual scale, and that ocean tides can change ice flow on an hourly/daily scale. This observation provides important constraints for computer simulations of future sea level rise in terms of the impacting factors and their respective timescales. Key Points: Multi‐year seismic records reveal glacial slip and tremor variability at Helheim GlacierTremor correlates with high effective pressure at tidal timescales, opposite to the expectation at longer timescalesThe tremor source points to an upstream subglacial ridge [ABSTRACT FROM AUTHOR]

This paper summarizes the results of the first investigation into the effect of particle‐laden plumes on glacier melting using laboratory experiments. We find that the melt rate, when the ice is exposed to a particle‐laden plume, can be larger than when exposed to an equivalent plume without particles. The increased melt rate is linked to an increase in the plume velocity in response to the presence of suspended particles. Including this increased velocity in a plume model improves melt rate predictions from the "three‐equation model" by approximately 45% for the range of particle concentrations used in this study. Plain Language Summary: Ice loss from the Greenland ice sheet is more rapid in locations where fresh water is released at the base of marine terminating glaciers. The fresh water forms a buoyant plume that rises vertically next to the ice face. Previous observations of these plumes have shown that they can contain significant concentrations of suspended sediment. We show, using laboratory experiments, that the melt rate of a vertical ice face can be increased by the presence of suspended particles in the vertically rising plume. This observation suggests that the effect of such plumes could be larger than current modeling studies predict. Key Points: Laboratory experiments show that melting of an ice face can be increased by the presence of sediment in a subglacial discharge plumeThe increased melting is linked to an increase in the plume velocity in response to the presence of suspended particlesAccounting for the increased plume velocity within the three‐equation model significantly improves predictions of the melt rate [ABSTRACT FROM AUTHOR]

Over the past 20 years, the Gravity Recovery and Climate Experiment (GRACE), and its successor mission, GRACE‐Follow On (GRACE‐FO) have made significant contributions to time‐variable gravity field modeling. A Chinese low‐low satellite‐to‐satellite tracking gravimetry mission (i.e., Chinese future gravimetry mission) has been confirmed to be selected as the polar‐orbiting satellite gravimetry mission for China, because of the capability to collect gravity data globally. However, the analysis of potential contributions to geosciences from GRACE‐FO coupling with the Chinese future gravimetry mission is still limited. This study combines GRACE‐FO and Chinese future gravimetry missions as the Dual GRACE‐like Polar satellite Constellation (DGPC). By carefully choosing the initial orbit parameters of the Chinese future gravimetry mission with the differential evolution algorithm, the DGPC is expected to mitigate the temporal aliasing effects by improving the temporal resolution of time‐variable gravity solutions (i.e., 1‐day and 3‐day solutions). Regarding the spectral‐domain evaluation, zonal, tesseral, and sectorial coefficients estimated by the DGPC show approximately 6.01%–13.42% noise reductions compared with GRACE‐FO. Regarding the spatial‐domain evaluation, the DGPC can suppress noises of about 39.44% and 31.12% in annual amplitude and long‐term trend, respectively. On this basis, this paper analyzes the effectiveness of the DGPC in potential contributions to geosciences (e.g., hydrology, glaciology, and seismology). Specifically, the DGPC can improve accuracy by about 36.96%, 25.85%, and 33.16% with respect to GRACE‐FO for signals in the subhumid basin, signals of ice‐sheet mass balance over Greenland, and coseismic displacement of the fault zone, respectively. In general, the potential capability for high‐frequency signals recovery of the DGPC would facilitate contributions of satellite gravimetry to geosciences. Plain Language Summary: The Gravity Recovery And Climate Experiment (GRACE) mission and its successor GRACE Follow On (GRACE‐FO) have made significant contributions to time‐variable gravity field modeling. A polar gravity satellite mission (i.e., Chinese future gravimetry mission) has been confirmed to be selected as the polar‐orbiting satellite gravimetry mission for China because of its capability to collect gravity data globally. Including the GRACE‐FO mission, it is expected that there will be two polar pairs of gravity satellites (i.e., dual GRACE‐like polar satellite constellation) simultaneously within the next few years. What can we expect from the dual GRACE‐like polar satellite constellation? In this study, we introduce an optimization algorithm (i.e., differential evolution algorithm) to carefully choose the initial orbit parameters of the Chinese future gravimetry mission. The dual GRACE‐like polar satellite constellation has been demonstrated to have the potential to mitigate the temporal aliasing effects of time‐variable gravity solutions. It is found that the dual GRACE‐like polar satellite constellation can improve accuracy by about 36.96%, 25.85%, and 33.16% with respect to GRACE‐FO for signals in the subhumid basin, signals of ice‐sheet mass balance over Greenland, and coseismic displacement of the fault zone, respectively. The temporal improvement of Earth's mass transport estimated by coupling GRACE‐FO with a Chinese polar gravity satellite mission would facilitate contributions of satellite gravimetry to geosciences. Key Points: This paper introduces the differential evolution algorithm into the design of the dual Gravity Recovery And Climate Experiment (GRACE)‐like polar satellite constellationImproving the temporal resolution of time‐variable gravity solutions can potentially contribute to mitigating the temporal aliasing effectsThe dual GRACE‐like polar satellite constellation has high‐frequency signal recovery capabilities and stimulates potential contributions to geosciences [ABSTRACT FROM AUTHOR]

A chain of magnetometers has been placed in Antarctica for comparisons with magnetic field measurements taken in the Northern Hemisphere. The locations were chosen to be on magnetic field lines that connect to magnetometers on the western coast of Greenland, despite the difficulty of reaching and working at such remote locations. We report on some basic comparisons of the similarities and differences in the conjugate measurements. Our results presented here confirm that the conjugate sites do have very similar (symmetric) magnetic perturbations in a handful of cases, as expected. Sign reversals are required for two components in order to obtain this agreement, which is not commonly known. More frequently, a strong Y component of the Interplanetary Magnetic Field (IMF) breaks the symmetry, as well as the unequal conductivities in the opposite hemispheres, as shown in two examples. In one event the IMF Y component reversed signs twice within 2 hours, while the magnetometer chains were approaching local noon. This switch provided an opportunity to observe the effects at the conjugate locations and to measure time lags. It was found that the magnetic fields at the most poleward sites started to respond to the sudden IMF reversals 20 min after the IMF reaches the bow shock, a measure of the time it takes for the electromagnetic signal to travel to the magnetopause, and then along magnetic field lines to the polar ionospheres. An additional 9–14 min is required for the magnetic perturbations to complete their transition. Plain Language Summary: Space science research has long relied on magnetometer measurements in the Northern Hemisphere to detect and observe the flow of currents in the ionosphere and magnetosphere. In the past few years it has become possible to acquire magnetic field measurements in the southern polar region as well, as a result of the placement of a chain of magnetometer stations in a remote part of Antarctica. Each of these magnetometers were placed where the Earth's magnetic field connects to an existing magnetometer in the Northern Hemisphere, on the western coast of Greenland. The locations follow a roughly north‐south meridian in geomagnetic coordinates. These "conjugate" magnetometer chains are useful for observing the similarities and differences between the ionospheric currents flowing in opposite hemispheres as a result of the solar wind's interaction with the Earth's magnetosphere. This paper presents results showing how the inter‐hemispheric measurements are very similar in some cases, but only if the signs of two of the vector components are reversed. In other cases the magnetic fields in the two polar hemispheres are different, mainly due to the summer‐winter differences in conductivity. The conjugate measurement will be useful for future space science research. Key Points: Magnetic field measurements are obtained from magnetic conjugate points in both hemispheresUnder optimal conditions the conjugate magnetic fields are very similar, with signs reversed on two components due to coordinate geometriesMore often the fields differ due to different seasonal conductivities and asymmetrical driving by the magnetic field in the solar wind [ABSTRACT FROM AUTHOR]

The Last Glacial Maximum extent of the North American Ice Sheets is well constrained empirically but has proven to be challenging to simulate with coupled Climate‐Ice Sheet models. Coupled Climate‐Ice Sheet models are often too computationally expensive to sufficiently explore uncertainty in input parameters, and it is unlikely that values calibrated to reproduce modern ice sheets will reproduce the known extent of the ice at the Last Glacial Maximum. To address this, we run an ensemble with a coupled Climate‐Ice Sheet model (FAMOUS‐ice), simulating the final stages of growth of the last North American Ice Sheets' maximum extent. Using this large ensemble approach, we explore the influence of numerous uncertain ice sheet albedo, ice sheet dynamics, atmospheric, and oceanic parameters on the ice sheet extent. We find that ice sheet albedo parameters determine the majority of uncertainty when simulating the Last Glacial Maximum North American Ice Sheets. Importantly, different albedo parameters are needed to produce a good match to the Last Glacial Maximum North American Ice Sheets than have previously been used to model the contemporary Greenland Ice Sheet due to differences in cloud cover over ablation zones. Thus, calibrating coupled climate‐ice sheet models on one ice sheet may produce strong biases when the model is applied to a new domain. Plain Language Summary: At the peak of the last ice age, an ice sheet covered much of North America. The extent of this ice sheet is well‐understood after decades of intensive data collection, but producing a computer simulation of the ice sheet which matches our observations has been a challenge. This is partly because of uncertainty about the "correct" model set‐up to create the best simulation, and partly because the computer models used in the simulations require large computing resources. In this paper, we present a series of simulations of the North American Ice Sheet at the peak of the last ice age using a fast‐running computer model in which the atmosphere and ice sheets interact. We run hundreds of simulations to tackle the uncertainty about the optimum values for unknown input parameters. We find that the model's representation of how reflective the ice sheet surface is has the most impact on the size and shape of the simulated ice sheet. Importantly, the parameter values that produce the best simulations of modern‐day Greenland produce poor simulations of the North American Ice Sheets during the last ice age, calling into question whether the parameters chosen for modern Greenland are appropriate from simulating ice sheets under different conditions. Key Points: Simulating the Last Glacial Maximum North American Ice Sheets successfully requires de‐tuning the coupled Climate‐Ice Sheet model FAMOUS‐iceAfter running a large ensemble, we identify multiple acceptable simulations of the LGM North American Ice SheetThe LGM North American cloud cover revealed overtuned albedo parameters to compensate for modern Greenland cloud biases [ABSTRACT FROM AUTHOR]

Ice surface speed increases dramatically from upstream to downstream in many ice streams and glaciers. This speed-up is thought to be associated with a transition from internal distributed deformation to highly localized deformation or sliding at the ice-bedrock interface. The physical processes governing this transition remain unclear. Here, we argue that highly localized deformation does not necessarily initiate at the ice-bedrock interface, but could also take the form of an internal shear band inside the ice flow that connects topographic highs. The power-law exponent n in the ice rheology amplifies the feedback between shear heating and shear localization, leading to the spontaneous formation of an internal shear band that can create flow separation within the ice. We model the thermomechanical ice flow over a sinusoidal basal topography by building on the high-resolution Stokes solver FastICE v1.0. We compile a regime diagram summarizing cases in which a sinusoidal topography with a given amplitude and wavelength leads to shear band formation for a given rheology. We compare our model results to borehole measurements from Greenland and find evidence to support the existence of an internal shear band. Our study highlights the importance of re-evaluating the degree to which internal deformation contributes to total deformation in the ice column and to the flow-to-sliding transition. Plain Language Summary On its way toward the ocean, ice speeds up dramatically from less than 1 m/year upstream to more than a kilometer per year downstream. In this paper, we investigate the physical processes controlling this speed-up. Specifically, we focus on the role of the bedrock topography and rheology in facilitating the transition from this slow to rapid motion. We use a two-dimensional numerical model to simulate the flow field within a slab of ice flowing down a ramp over a simplified topography. We find that including the bedrock topography can lead to a zone of highly localized deformation within the ice above topographic highs. We also find that a non-linear rheology amplifies this localization. We compare our model results to borehole measurements from Greenland and find evidence that supports the existence of this highly localized deformation zone. This study indicates that the localized deformation induced by bedrock topography and amplified by non-linear rheology could be one physical mechanism that governs the speed-up of the ice flow. [ABSTRACT FROM AUTHOR]

GRACE‐D accelerometer data show significant bias jumps since one month after the launch of the GRACE Follow‐On (GRACE‐FO) satellites in May 2018, making them inapplicable for correcting GRACE‐FO products for non‐gravitational accelerations. The GRACE‐FO Science Data System (SDS) compensated this issue by transplanting the GRACE‐C accelerometer data toward that of GRACE‐D. Recently, GRACE‐FO SDS implemented an updated transplant method, used in the latest release of GRACE‐FO data. Here, we examine the impact of updated accelerometer transplant data (ACH) on GRACE‐FO measurements at all levels: (a) Level‐1B inter‐satellite laser ranging residuals measured along satellite orbit, (b) Level‐2 time‐variable gravity solutions from all SDS centers (JPL, CSR, and GFZ), and (c) Level‐3 mascon solutions. We show that inter‐satellite laser ranging residuals are modified at low frequencies below 1 mHz, affecting the along‐orbit analysis of large‐scale time‐variable gravity signals. When mapped into monthly Level‐2 spherical harmonic coefficients of geopotential, the low‐frequency change in inter‐satellite ranging residuals leads to substantial improvement of coefficients associated with resonant orders (i.e., 15, 30, 45, etc.) and C30. We also present an improved SLR‐derived C30 which significantly improves the agreement with updated GRACE‐FO C30 at seasonal and interannual timescales. Moreover, we demonstrate the noise reduction in mass change estimates from new GRACE‐FO Level‐2 data over oceans, Greenland, and Antarctica for all SDS solutions. GRACE‐FO mascon solutions show a moderate change in the updated release. Our comprehensive analyses demonstrate high‐quality estimates of non‐gravitational accelerations by the updated transplant method, resulting in more accurate GRACE‐FO time‐variable gravity and mass change observations. Plain Language Summary: GRACE Follow‐On (GRACE‐FO) consists of two satellites that orbit around the Earth at ∼500 km altitude, following each other at a distance of ∼200 km. The inter‐satellite distance between the two satellites changes because of the gravitational as well as non‐gravitational forces (e.g., air drag), and it is measured by high‐precision sensors. To estimate time‐variable gravity caused by mass change in the Earth system from inter‐satellite ranging data, the effect of non‐gravitational forces is reduced using on‐board accelerometer measurements. The accelerometer on‐board the GRACE‐D satellite started to return erroneous measurements about 1 month after the launch. The GRACE‐FO Science Data System (SDS) compensated this issue by transplanting the GRACE‐C accelerometer data toward that of GRACE‐D. An updated transplant accelerometer data set for GRACE‐D was recently released by the GRACE‐FO SDS and, consequently, new time‐variable gravity and mass change solutions were released. In this paper, we demonstrate the improved accuracy of the new GRACE‐FO time‐variable gravity and mass change solutions caused by high‐quality estimates of non‐gravitational accelerations by the updated transplant method. Key Points: GRACE Follow‐On (GRACE‐FO) laser ranging residuals are modified at low frequencies by ACH, affecting the along‐orbit analysis of large‐scale mass changesGeopotential spherical harmonic coefficients associated with resonant orders (i.e., ∼15, 30, 45, etc.) show the largest improvement by ACHMonthly gravity fields show significantly lower noise for months when the angle between GRACE‐FO orbital plane and Sun‐Earth direction is ∼0 [ABSTRACT FROM AUTHOR]

The appearance of new Arctic islands and straits as the result of landscape transformation due to tidewater glacier recession under climate warming continues. Analysis of available maps and satellite images has served herein as the main research method. Six new islands, each covering an area of 0.4–248 km2, appeared in the years 2018–2021, and are described in this paper. Three other new islands, ranging in area from 0.4 to 3.2 km2, appeared along the Greenland coast in 1997, 2001, and 2013, and are described here, and they have not yet been described elsewhere. Seven of the nine aforementioned new islands appeared in Greenland and the European Arctic, where all earlier 35 new islands and straits had appeared due to ice melting. However, two of these appeared in the Severnaya Zemlya area in 2018, becoming the first new islands in the Asiatic (Siberian) Arctic. Additional new islands are now at an advanced stage of formation, and several will appear in the 2020s in the case of a stabilization of the current climate or further warming. [ABSTRACT FROM AUTHOR]

Background: Research suggests that head‐mounted displays (HMD) can spark situational interest when they are used to provide science learning experiences that are not possible in traditional classroom settings. However, few studies have investigated the lasting effects of using HMDs in an authentic instructional intervention. Objectives: We investigated the effects of a one‐time experience of a virtual field trip to Greenland in a sample of 105 middle school students. Methods: Students used either a standard 2D video (video condition; N = 50) or an HMD (HMD condition; N = 55) as part of a six‐lesson educational activity on the topic of climate change. Informed by social cognitive career theory (SCCT), we investigated the effects of the different conditions (video vs. HMD) on the outcomes of self‐efficacy, outcome expectations, interest, and science intentions across three time points. Results and Conclusions: The results showed that using the HMD‐based virtual field trip, compared to the video, had a positive immediate effect on self‐efficacy and interest, and total later effects on self‐efficacy, outcome expectations, and interest an average of two and a half weeks after the virtual field trip. The results suggest that HMD‐based virtual field trips can influence self‐efficacy, outcome expectations, and interest more than a video‐based virtual field trip when measured approximately two and a half weeks after the intervention. Lay Description: What is currently known about the subject matter: Social Cognitive Career Theory provides a framework for understanding students' learning.Few studies have examined the effect of head‐mounted displays on science interest in the framework of Social Cognitive Career Theory. What this paper adds: The results suggest that virtual field trips based in head‐mounted display can foster self‐efficacy, outcome expectations, and interest more than video‐based virtual field trips. Implications of the findings for practitioners: Fostering learning through head‐mounted displays may increase students' interest in science. [ABSTRACT FROM AUTHOR]

In East Greenland, sedimentation during the Middle Devonian‐earliest Carboniferous interval was characterized by the Old Red Sandstone Molasse Basin deposits. The continental succession recorded folding and intrabasinal unconformities associated with the activation of extensional detachments and core complex formation which post‐dated high‐P granulite facies metamorphism at c. 403 Ma. The extensional collapse responsible for the initiation of the Devonian basin and associated bimodal magmatism must be understood in the context of continued continental convergence during the Early Carboniferous, required by ultrahigh‐pressure metamorphism in North‐East Greenland at c. 355 Ma. Along a small nunatak within the Wordie Gletscher in Payer Land, NE Greenland (lat. 74°N), Precambrian gneisses recording Devonian granulite‐facies metamorphism are in tectonic contact with the molasse deposits along an ESE‐ward dipping thrust fault: the Scotstounhill Thrust. This paper will discuss the role of Devonian contractional structures and contemporaneous extensional faulting and present a new tectonic model that accounts for the exhumation of deep crustal rocks by these coupled mechanisms. [ABSTRACT FROM AUTHOR]

At the Pingorsuit Glacier in North‐West Greenland, an organic‐rich deposit that had recently emerged from the retreating ice cap was discovered at an elevation of 480 m above sea level. This paper reports on macrofossil analyses of a coarse detritus gyttja and peaty soil, which occurred beneath a thin cover of till and glacifluvial deposits. The sediments contained remains of vascular plants, mosses, beetles, caddisflies, midges, bryozoans, sponges and other invertebrates. The flora includes black spruce, tree birch, boreal shrubs and wetland and aquatic taxa, which shows that mires, lakes and ponds were present in the area. We describe a new extinct waterwort species Elatine odgaardii. The fossils were deposited in a boreal environment with a mean July air temperature that was at least 9 °C higher than at present. The fossil assemblages show strong similarities with others from Greenland that have been assigned an Early Pleistocene age, and we suggest a similar age for the sediments found at the margin of the Pingorsuit Glacier. At the Pingorsuit Glacier in North‐West Greenland, an organic‐rich deposit was discovered at an elevation of 480 m above sea level. The sediments contained remains of vascular plants, mosses, beetles, caddisflies, midges, bryozoans, sponges and other invertebrates. The fossils were deposited in a boreal environment with a mean July air temperature that was at least 9 °C higher than at present. [ABSTRACT FROM AUTHOR]

The Bering‐Bagley Glacier System (BBGS), Alaska, Earth's largest temperate surging glacier, surged in 2008–2013. We use numerical modeling and satellite observations to investigate how surging in a large and complex glacier system differs from surging in smaller glaciers for which our current understanding of the surge phenomenon is based. With numerical simulations of a long quiescent phase and a short surge phase in the BBGS, we show that surging is more spatiotemporally complex in larger glaciers with multiple reservoir areas forming during quiescence which interact in a cascading manner when ice accelerates during the surge phase. For each phase, we analyze the simulated elevation‐change and ice‐velocity pattern, infer information on the evolving basal drainage system through hydropotential analysis, and supplement these findings with observational data such as CryoSat‐2 digital elevation maps. During the quiescent simulation, water drainage paths become increasingly lateral and hydropotential wells form indicating an expanding storage capacity of subglacial water. These results are attributed to local bedrock topography characterized by large subglacial ridges that dam the down‐glacier flow of ice and water. In the surge simulation, we model surge evolution through Bering Glacier's trunk by imposing a basal friction representation that mimics a propagating surge wave. As the surge progresses, drainage efficiency further degrades in the active surging‐zone from its already inefficient, end‐of‐quiescence state. Results from this study improve our knowledge of surging in large and complex systems which generalizes to glacial accelerations observed in outlet glaciers of Greenland, thus reducing uncertainty in modeling sea‐level rise. Plain Language Summary: The Bering‐Bagley Glacier System (BBGS), Alaska, Earth's largest temperate surging glacier, recently surged in 2008–2013. A surge glacier cycles between a long period of normal flow and a short period of accelerated flow where large‐scale deformations, such as crevasses, occur. This paper focuses on investigating a surge in a large and complex system rather than a small glacier where most studies on surges have been conducted. We use a numerical model to simulate glacier evolution for both the quiescent phase and the initial surge phase of the BBGS. For each phase, we analyze the simulated elevation‐change and ice‐velocity, and infer information on the evolving hydrologic drainage system. During the quiescent phase, ice‐mass builds up at locations consistent with those observed and water drainage paths become longer with expanding capacity to store subglacial water. These results are attributed to local bedrock topography characterized by large subglacial ridges that act to dam the down‐glacier flow of ice and water. In the surge simulation, we model surge evolution through Bering Glacier by implementing a new friction representation that mimics a propagating wave. As the surge progresses through the glacier, drainage efficiency further degrades in the areas of fast‐moving ice. Key Points: Using a full‐Stokes approach informed by satellite observations, a quiescent and surge phase of the Bering‐Bagley Glacier System (BBGS) are simulatedLocal bed topography controls the formation of several reservoir areas, which lengthen down‐glacier drainage paths during quiescenceA friction representation for the surge phase is implemented based on observed properties of kinematic surge waves in the BBGS [ABSTRACT FROM AUTHOR]

Diurnal and seasonal cycles of aeolian activity are well‐constrained for low latitude dust source regions and provide valuable insights into relationships between dust emissions and environmental drivers. Such cycles have received little systematic attention in high latitude dust source areas (≥50°N and ≥40°S), and understanding them will aid the modelling of atmospheric dust over different timescales. This paper examines the timing and drivers of atmospheric dust concentration close to source at ice‐free locations c. 6 km and c. 37 km from the Greenland Ice Sheet margin. Dust concentration, and associated environmental drivers including wind speed and velocity, air temperature and humidity, was measured from April–October 2018 and April–August 2019. Measured dust concentrations were a similar order of magnitude to source‐proximal values measured globally and varied from 0 to ≥1000 μg m−3. Diurnal cycles in the environmental drivers of dust emission were similar to those observed in low latitudes, but unlike in the low latitudes, there was no clear diurnal pattern of dust concentration. Only 3 out of 17 recorded dust events showed a significant positive relationship between wind speed and dust concentration, and factors such as wind direction, humidity and sediment availability are the over‐riding controls on dust activity at the event and seasonal scales. Whereas 7 dust events are attributed to down‐valley katabatic winds, 10 events were associated with a westerly sea breeze blowing up‐valley towards the ice sheet and high atmospheric moisture content. If deposited on the ice, this dust will alter ice albedo both directly and indirectly, (e.g. through the promotion of algal blooms) affecting cryospheric melt rates. Our results demonstrate an intricate relationship between first‐order controls and dust concentration that raises challenges for modelling the uplift and subsequent dust transport from high latitude sources, especially the appropriateness of assumptions based on emissions behaviour at low latitudes. [ABSTRACT FROM AUTHOR]

The centennial response of land‐terminating glaciers in Greenland to climate change is largely unknown. Yet, such information is important to understand ongoing changes and for projecting the future evolution of Arctic subpolar glaciers, meltwater runoff, and sediment fluxes. This paper analyses the topography, geomorphology, and sedimentology of prominent moraine ridges and the proglacial areas of ice cap outlet glaciers on the Qaanaaq peninsula (Piulip Nunaa). We determine geometric changes of glaciers since the neoglacial maximum; the Little Ice Age (LIA), and we compare glacier behaviour during the LIA with that of the present day. There has been very little change in the rate of volume loss of each outlet glacier since the LIA compared with the rate between 2000 and 2019. However, the percentage of each glacier that is likely composed of cold‐based ice has increased since the LIA, typically by 20%. The LIA moraines comprise subrounded, striated, and faceted clasts that evidence subglacial transport, and outwash plains, flutes, kames, and eskers that evidence subglacial motion and meltwater within temperate ice. Contrastingly, contemporary ice margins and their convex ice surfaces comprise pronounced primary foliation, ephemeral supraglacial drainage, sediment drapes from thrust plane fractures, and an absence of open crevasses and moulins. These calculations and observations together lead us to interpret that these outlet glaciers have transitioned towards an increasingly cold‐based thermal regime despite a warming regional climate. Thermal regime transitions control glacier dynamics and therefore should be incorporated into glacier evolution models, especially where polythermal glaciers prevail and where climate is changing rapidly. [ABSTRACT FROM AUTHOR]

Ice cores provide detailed records of past climate change. Water stable isotopes are the most commonly used ice core climate proxy, but their quantitative interpretation remains challenging. Here, I argue that the gas age‐ice age difference (Δage) is a powerful proxy for past surface temperature. An analytical framework is derived that directly links past temperature to firn properties that can be reliably reconstructed (Δage, lock‐in depth). The framework is calibrated using both present‐day spatial patterns and last glacial maximum temperatures reconstructed via borehole thermometry. The usefulness of the method is demonstrated using three case studies from Greenland and Antarctic ice cores. The calibration suggests that several firn densification models, with the possible exception of the Herron‐Langway model, have insufficient sensitivity to accumulation rates. This low sensitivity, in combination with large amplitude temperature forcing, can explain historical difficulties of densification models in simulating ice age firn thickness in East Antarctica. Plain Language Summary: It is important for scientists to understand past natural climate change. Ice cores drilled in the polar regions contain ancient ice up to 800,000 years old and can be used to reconstruct past climate. Ice from the polar regions contains air bubbles that are trapped at the bottom of the thick (50–120 m) perennial snow pack called the firn. The air in these bubbles is younger than the ice that surrounds it, and this age difference is called Δage. This paper develops a new method to estimate past temperatures of the ice sheet surface using our knowledge of changes in ice core Δage. The method is very simple and fast and agrees very well with independent temperature reconstructions where available. Key Points: The ice core gas age‐ice age difference (Δage) is a powerful proxy for past surface temperatureA simple analytical framework allows for reliable past temperature estimation using empirical estimates of ΔageLow firn model sensitivity to accumulation rate contributes to published model‐data mismatch of ice age firn thickness in East Antarctica [ABSTRACT FROM AUTHOR]

Using the depth (z) and density (ϱ) frameworks, we analyze local contributions to AMOC variability in a 900‐year simulation with the AWI climate model. Both frameworks reveal a consistent interdecadal variability; however, the correlation between their maxima deteriorates on year‐to‐year scales. We demonstrate the utility of analyzing the spatial patterns of sinking and diapycnal transformations through depth levels and isopycnals. The success of this analysis relies on the spatial binning of these maps which is especially crucial for the maps of vertical velocities which appear to be too noisy in the main regions of upwelling and downwelling because of stepwise bottom topography. Furthermore, we show that the AMOC responds to fast (annual or faster) fluctuations in atmospheric forcing associated with the NAO. This response is more obvious in the ϱ than in the z framework. In contrast, the link between AMOC and deep water production south of Greenland is found for slower fluctuations and is consistent between the frameworks. Plain Language Summary: The variability and trends in Atlantic meridional overturning circulation (AMOC) are some of the most important characteristics of an ocean model simulation that are relevant for climate. Usually, the AMOC is computed as a meridional transport (from bottom to top) of zonally integrated flow along constant depth levels (z‐AMOC). However, water in the ocean generally flows along the density surfaces. In low latitudes, they are nearly flat and the general picture given by z‐AMOC is highly relevant. In high latitudes, where the most important processes of dense water formation and sinking take place, density surfaces are sloping, and the z‐AMOC gives a distorted image of the real circulation. This is why recent studies underlined the importance of computing the AMOC as a function of density in addition to AMOC as the function of vertical coordinate. In this paper, we analyze the fundamental differences with respect to AMOC variability in both frameworks in a 900‐year run with the AWI climate model. We demonstrate that the latitudinal position of overturning maxima, amplitude, and variability show substantial differences between the frameworks. We suggest that the ϱ‐AMOC and watermass transformation framework should be used routinely in standard analyses, including forthcoming intercomparison projects. Key Points: Patterns of sinking and diapycnal transformation across depth levels and isopycnals, respectively, can be used to study AMOC variabilityThe AMOC subpolar maximum is largely driven by internal transformationsThe density framework illustrates the interplay between surface buoyancy flux and interior‐mixing [ABSTRACT FROM AUTHOR]

Predation is an important ecological process that can significantly impact the maintenance of ecosystem services. In arctic environments, the relative ecological importance of predation is thought to be increasing due to climate change, partly because of increased productivity with rising temperatures. Therefore, understanding predator–prey interactions in arctic ecosystems is vital for the sustainable management of these northern regions. Network theory provides a framework for quantifying the structures of ecological interactions. In this study, we use dietary observations on mammalian and avian predators in a high arctic region, including isolated peninsulas on Ellesmere Island and north Greenland, to construct bipartite trophic networks. We quantify the complexity, specialization, and nested as well as modular structures of these networks and also determine if these properties varied among the peninsulas. Mammal prey remains were the dominant diet item for all predators, but there was spatial variation in diet composition among peninsulas. The predator–prey networks were less complex, had more specialized interactions, and were more nested and more modular than random expectations. However, the networks displayed only moderate levels of modularity. Predator species had less specialized interactions with prey than prey had with predators. All network properties differed among the peninsulas, which highlights that ecosystems often show complex responses to environmental characteristics. We suggest that gaining knowledge about spatial variation in the characteristics of predator–prey interactions can enhance our ability to manage ecosystems exposed to environmental perturbations, particularly in high arctic environments subject to rapid environmental change. [ABSTRACT FROM AUTHOR]

Microbial communities from extreme environments are largely understudied, but are essential as producers of metabolites, including enzymes, for industrial processes. As cultivation of most microorganisms remains a challenge, culture‐independent approaches for enzyme discovery in the form of metagenomics to analyse the genetic potential of a community are rapidly becoming the way forward. This study focused on analysing a metagenome from the cold and alkaline ikaite columns in Greenland, identifying 282 open reading frames (ORFs) that encoded putative carbohydrate‐modifying enzymes with potential applications in, for example detergents and other processes where activity at low temperature and high pH is desired. Seventeen selected ORFs, representing eight enzyme families were synthesized and expressed in two host organisms, Escherichia coli and Aliivibrio wodanis. Aliivibrio wodanis demonstrated expression of a more diverse range of enzyme classes compared to E. coli, emphasizing the importance of alternative expression systems for enzymes from extremophilic microorganisms. To demonstrate the validity of the screening strategy, we chose a recombinantly expressed cellulolytic enzyme from the metagenome for further characterization. The enzyme, Cel240, exhibited close to 40% of its relative activity at low temperatures (4°C) and demonstrated endoglucanase characteristics, with a preference for cellulose substrates. Despite low sequence similarity with known enzymes, computational analysis and structural modelling confirmed its cellulase‐family affiliation. Cel240 displayed activity at low temperatures and good stability at 25°C, activity at alkaline pH and increased activity in the presence of CaCl2, making it a promising candidate for detergent and washing industry applications. [ABSTRACT FROM AUTHOR]

The Ice, Cloud, and Land Elevation Satellite‐2 (ICESat‐2) mission has collected surface elevation measurements for over 5 years. ICESat‐2 carries an instrument that emits laser light at 532 nm, and ice and snow absorb weakly at this wavelength. Previous modeling studies found that melting snow could induce significant bias to altimetry signals, but there is no formal assessment on ICESat‐2 acquisitions during the melting season. We performed two case studies over the Greenland Ice Sheet to quantify bias in ICESat‐2 signals over snow: one to validate Airborne Topographic Mapper (ATM) data against Next Generation Airborne Visible/Infrared Imaging Spectrometer (AVIRIS‐NG) grain sizes, and a second to estimate ICESat‐2 bias relative to ATM. We used snow optical grain sizes derived from ATM and AVIRIS‐NG to attribute altimetry bias to snowpack properties. For the first case study, the mean and standard deviation of optical grain sizes were 340 ± 65 µm (AVIRIS‐NG) and 670 ± 420 µm (ATM). A mean altimetry bias of 4.81 ± 1.76 cm was found for ATM, with larger biases linked to increases in grain size. In the second case study, we found a mean grain size of 910 ± 381 µm and biases of 6.42 ± 1.77 cm (ICESat‐2) and 9.82 ± 0.97 cm (ATM). The grain sizes and densities needed to recreate biases with a model are uncommon in nature, so we propose that additional surface attributes must be considered to characterize ICESat‐2 bias over snow. The altimetry biases are within the accuracy requirements of the ICESat‐2 mission, but we cannot rule out more significant errors over coarse‐grained snow. Plain Language Summary: The Ice, Cloud, and Land Elevation Satellite‐2 (ICESat‐2) mission has been used to measure changes in land ice, vegetation cover, and sea ice, and there is growing interest to use ICESat‐2 for snow science. ICESat‐2 uses a green laser to estimate the elevation of the Earth's surface, and recent work has shown that snow can introduce errors in green laser measurements such as ICESat‐2. In this study, we used ICESat‐2 and airborne data to (a) identify errors in the ICESat‐2 data and (b) link the errors to snow properties over the Greenland Ice Sheet. We found that ICESat‐2 errors are linked to two snow properties: the size of individual snow grains and the density of the snow. Two retrieval methods were used to estimate snow grain size, though the methods show significant differences when large snow grains are observed. Snow grain size and density are insufficient to fully explain ICESat‐2 biases, so we propose that other factors also play a role. The ICESat‐2 errors are within the accuracy requirements of ICESat‐2, but more significant errors may be possible during the Northern Hemisphere melting season. Key Points: Very large snow grains and densities are needed to explain centimeter‐level differences between ICESat‐2 and airborne lidarBiases from snow grain size and other factors may be influenced by different height measurement methods between ICESat‐2 and airborne lidarSnow grain size retrievals strongly differ between airborne lidar and AVIRIS‐NG over regions with coarse‐grained snow [ABSTRACT FROM AUTHOR]

The widespread High Arctic Large Igneous Province (HALIP) exhibits prolonged melting over more than 50 Myr, an observation that is difficult to reconcile with the classic view that large igneous provinces (LIPs) originate from melting in plume heads. Hence, the suggested plume‐related origin and classification of HALIP as a LIP have been questioned. Here, we use numerical models that include melting and melt migration to investigate a rising plume interacting with lithosphere of variable thickness, that is, a basin‐to‐craton setting applicable to the Arctic. Models reveal that melt migration introduces significant spatial and temporal variations in melt volumes and pulses of melt production, including protracted melting for at least about 30–40 Myr, because of the dynamic feedback between migrating melt and local lithosphere thinning. For HALIP, plume material deflected from underneath the Greenland craton can re‐activate melting zones below the previously plume‐influenced Sverdrup Basin after a melt‐free period of about 10–15 Myr, even though the plume is already ∼500 km away. Hence, actively melting zones do not necessarily represent the location of the deeper plume stem at a given time, especially for secondary pulses. Additional processes such as (minor) plume flux variations or local lithospheric extension may alter the timing and volume of HALIP pulses, but are to first order not required to reproduce the long‐lived and multi‐pulse magmatism of HALIP. Since melting zones are always plume‐fed, we would expect HALIP magmatism to exhibit plume‐related trace element signatures throughout time, potentially shifting from mostly tholeiitic toward more alkalic compositions. Plain Language Summary: Typically, the arrival of a large mantle upwelling ("mantle plume") is expected to cause catastrophic large‐scale volcanism that lasts a few million years. However, a massive past volcanic event now distributed onshore and offshore across the Arctic (the High Arctic Large Igneous Province—HALIP) defies this definition. This wide‐spread magmatism exhibits dates spanning more than 50 Myr, with several pulses of activity. Based on this prolonged magmatism, it has been questioned whether all of it can be attributed to a mantle plume, despite the geochemistry of basalts indicating a plume source. Here, we show that a plume can cause prolonged and multi‐pulse magmatism if it interacts with an increase in lithosphere thickness. Once the plume moves below the thicker lithosphere, hot plume material is channeled along the base of the lithosphere toward the adjacent thinner part, where it can reactivate previous melting regions. At this time, the active plume can be about 500 km away from the melting region, hence plume‐related melt cannot be used as a proxy for the plume position at the given time. Based on the models, we suggest that the prolonged HALIP magmatism was caused by a plume interacting with the edge of a craton. Key Points: Mantle plumes interacting with changes in lithosphere thickness at craton edges can cause prolonged melting with pulses in the same regionRejuvenated melting happens underneath previously melt‐affected thinned lithosphere several hundred km downstream of the plume stemThe timing and duration of rejuvenated melting in models correspond to and therefore may explain observations of magmatic pulses from High Arctic Large Igneous Province [ABSTRACT FROM AUTHOR]

The recently discovered Fåvne vent field, located at 3,040 m depth on the slow‐spreading Mohns mid‐ocean ridge between Greenland and Norway, is a high‐temperature (≥250°C) vent field that is characterized by Fe oxyhydroxide‐rich and S‐poor chimneys and mounds. The vent field is located on both the hanging wall and footwall of a normal fault with a ∼1.5 km throw that forms the western edge of the ∼20 km wide ridge axial valley. Data collected during exploration of the site using a remotely operated vehicle as well as mineralogical and geochemical analyses of rock samples and sediments are used to characterize the geological setting of the vent field and composition of the hydrothermal deposits. The chimney walls are highly porous and lack defined chalcopyrite lined conduits, typical of high‐temperature chimneys. Overall, abundant Fe oxyhydroxide precipitation at high‐temperature vents at Fåvne reflects an excess of Fe over reduced S in the fluid, leading to precipitation of Fe oxide and oxyhydroxide minerals at high to moderate temperature vents (>100°C), and as microbially mediated and abiotic precipitation of Fe oxyhydroxide minerals at low‐temperature diffuse vents (<100°C). The mounds and chimneys exhibit low base metal and reduced S concentrations relative to globally averaged seafloor deposits and suggest subseafloor mixing of hydrothermal fluid with seawater, causing metal sulfide precipitation. Cobalt enrichment at Fåvne may reflect a subsurface influence of an ultramafic substrate on circulating fluids, although ultramafic rocks are absent on the seafloor and no other elements typical of ultramafic deposits are present. Plain Language Summary: Seafloor hydrothermal deposits are mineral deposits formed from seawater penetrating the oceanic crust and becoming enriched in metals by leaching the surrounding rocks as the temperature rises close to a heat source. Once heated, the fluid rises back to the seafloor where it comes into contact with cold seawater and the metals precipitate, forming chimneys and mounds. The minerals and metal concentrations record temperatures of formation and environments, and can help us understand the processes associated with plate tectonics and the formation of ore deposits. Using underwater vehicles, we collected rocks and sediments from the Fåvne vent field and measured the temperature of the chimneys, mounds, and surrounding seafloor to understand how these deposits form on the seafloor. The Fåvne vent field differs from other vent fields because it is enriched in iron‐rich‐minerals and depleted in sulfur‐rich‐minerals. The dominance of iron‐rich minerals and the abundance of fractures in the seafloor at the vent field suggest that the hydrothermal fluid is cooled by seawater percolating along the fractures, accumulating these metals in the subsurface instead of at the seafloor. This process is important for understanding the current land‐based mineral deposits being mined today and for the exploration of seafloor mineral deposits. Key Points: The Fåvne vent field consists of chimneys and mounds composed of Fe oxyhydroxide minerals with minor sulfide mineralsVenting of hot fluids with temperatures up to 267°C is dominantly diffuse, sustaining significant microbial communitiesLow base metal concentrations of the Fåvne deposits suggest subsurface seawater mixing and mineral precipitation [ABSTRACT FROM AUTHOR]

Freshwater input from Greenland ice sheet melt has been increasing in the past decades from warming temperatures. To identify the impacts from enhanced meltwater input into the subpolar North Atlantic from 1997 to 2021, we use output from two nearly identical simulations in the eddy‐rich model VIKING20X (1/20°) only differing in the freshwater input from Greenland: one with realistic interannually varying runoff increasing in the early 2000s and the other with climatologically (1961–2000) continued runoff. The majority of the additional freshwater remains within the boundary current enhancing the density gradient toward the warm and salty interior waters yielding increased current velocities. The accelerated boundary current shows a tendency to enhanced, upstream shifted eddy shedding into the Labrador Sea interior. Further, the experiments allow to attribute higher stratification and shallower mixed layers southwest of Greenland and deeper mixed layers in the Irminger Sea, particularly in 2015–2018, to the runoff increase in the early 2000s. Plain Language Summary: Global warming has accelerated the melting of the Greenland ice sheet over the past few decades resulting in enhanced freshwater input into the North Atlantic. The additional freshwater can potentially inhibit deep water formation and have future implications on ocean circulation. To determine the influence from Greenland melt, we compare two high‐resolution model experiments all with the same forcing but differing input of Greenland freshwater fluxes from 1997 to 2021. We find that in the experiment with realistically increasing Greenland meltwater, the water becomes fresher and cooler along the continental shelf and boundary of the subpolar gyre. The density difference between the shelf and interior increases with more freshwater, resulting in faster West Greenland Current speeds and enhanced eddy formation. Deeper mixed layers are found in the eastern Irminger Sea, particularly in 2015–2018. From 2009 to 2013, there were shallower mixed layers in the Labrador Sea where less Greenland meltwater was mixed downwards and spread eastward, causing mixed layers to deepen in the Irminger Sea. Key Points: The West Greenland Current (WGC) freshens and cools with the observed recent increase in meltwater runoff from GreenlandThe density gradient across the boundary current intensifies, strengthening the WGC and increasing local eddy formationEnhanced meltwater runoff contributed to an eastward shift in deep convection towards the Irminger Sea (2015–2018) [ABSTRACT FROM AUTHOR]

Monitoring the Greenland mass balance (GMB) is crucial in the context of global sea level rise. Currently, three main methods are used to measure GMB, with the primary source of uncertainty arising from the glacial isostatic adjustment (GIA) contribution. Here, we propose a novel approach based on a simple methodology that uses the entire Greenland GNSS network (GNET) as an instrument to monitor the present‐day mass changes. Our method is validated against GRACE‐derived GMB, and we find a very good agreement. This leads to an independent methodology for estimating present‐day mass changes from GNSS, bridging the gap between GRACE and GRACE‐FO in GMB estimates. Through a combined analysis of GMB from GRACE and GNET, we identify a consistency relation between the gravity and uplift signature of GIA, providing a new robust constraint for GIA models. Plain Language Summary: The rate of Greenland ice mass change is constantly monitored by satellites measuring gravity and surface elevation variations, airborne surveys, and a growing network of GNSS stations on the ground. Each technique has its strengths and weaknesses and spatial/temporal resolution. We developed an approach that uses the whole GNSS network as a "virtual instrument", capable of measuring the total ice loss from Greenland. Comparing this with more standard mass balance estimate techniques, we validate the method, and, by combining these techniques, we find a novel constraint on both the mass variations and glacial isostatic adjustment models. Key Points: New methodology for monitoring daily estimates of the Greenland Mass Balance (GMB) derived from the GNSS networkNew glacial isostatic adjustment constraint from combined GRACE and GNSS observationsFilling the gap in the GMB between GRACE and GRACE‐FO using GNSS data [ABSTRACT FROM AUTHOR]

At marine‐terminating glaciers, both buoyant plumes and local currents energize turbulent exchanges that control ice melt. Because of challenges in making centimeter‐scale measurements at glaciers, these dynamics at near‐vertical ice‐ocean boundaries are poorly constrained. Here we present the first observations from instruments robotically bolted to an underwater ice face, and use these to elucidate the interplay between buoyancy and externally forced currents in meltwater plumes. Our observations captured two limiting cases of the flow. When external currents are weak, meltwater buoyancy energizes the turbulence and dominates the near‐boundary stress. When external currents strengthen, the plume diffuses far from the boundary and the associated turbulence decreases. As a result, even relatively weak buoyant melt plumes are as effective as moderate shear flows in delivering heat to the ice. These are the first in‐situ observations to demonstrate how buoyant melt plumes energize near‐boundary turbulence, and why their dynamics are critical in predicting ice melt. Plain Language Summary: Melting glaciers are projected to produce 10s of centimeters of sea level rise over the next few decades. Despite this threat, the fundamental fluid dynamics which drive melt at tidewater glaciers remain poorly characterized. This is primarily attributed to challenges associated with measuring the temperature and velocity of ocean water at the submerged cliffs of actively calving glaciers. To this end, we have developed a robotically deployed instrument that can be bolted to a glacier's face. This instrument is capable of measuring temperature and kinetic energy of ocean waters within a few centimeters of the ice, representing the first measurements of their kind. Our observations demonstrate the ways in which meltwater at ice boundaries can accelerate melt. In particular, the meltwater tends to be less salty (and hence lighter) than the nearby ocean waters (which are salty, warm and heavy), so the meltwater rises along the ice face, creating an energetic, near boundary flow. With our new measurements, we show that these flows are as important as large‐scale currents in providing energy to the ice to fuel melt. We anticipate these data will help our community create more accurate models of ice melt needed to predict the advance or retreat of marine ice cliffs of Greenland, Alaska and Antarctica. Key Points: Robotic observations at a submerged near‐vertical iceberg face capture turbulent dynamics of buoyant melt plumes and background currentsBuoyant plumes extend 20–50 cm from the boundary and generate broad‐spectrum temperature and velocity fluctuations that drive horizontal turbulent transports of heatWhen ambient horizontal flows are weak, buoyant plumes are the dominant source of boundary layer turbulence that drives heat flux to the ice [ABSTRACT FROM AUTHOR]

The lower limb of the Atlantic meridional overturning circulation (AMOC) is the equatorward flow of dense waters formed through the cooling and freshening of the poleward‐flowing upper limb. In the subpolar North Atlantic (SPNA), upper limb variability is primarily set by the North Atlantic Current, whereas lower limb variability is less well understood. Using observations from a SPNA mooring array, we show that variability of the AMOC's lower limb is connected to poleward flow in the interior Irminger Sea. We identify this poleward flow as the northward branch of the Irminger Gyre (IG), accounting for 55% of the AMOC's lower limb variability. Over 2014–2018, wind stress curl fluctuations over the Labrador and Irminger Seas drive this IG and AMOC variability. On longer (>annual) timescales, however, an increasing trend in the thickness of intermediate water, from 2014 to 2020, within the Irminger Sea coincides with a decreasing trend in IG strength. Plain Language Summary: In the subpolar North Atlantic, warm salty waters get transported northwards by the upper branch of the meridional overturning circulation. As they travel northwards, they transform: cooling, densifying, and sinking. The cooler deeper waters then get transported back southwards toward the equator in the lower branch of the overturning circulation. The transformation and transport of these waters plays a critical role in our climate system. However, the lower branch of the overturning circulation and the mechanisms controlling how it changes are still not well understood. Observations from a fixed array of moorings between Greenland and Scotland are used here to identify the interior (away from land boundaries) Irminger Sea as a region important for the overturning's lower branch. Specifically, we find that a closed system of currents in the western Irminger Sea, known as the Irminger Gyre, plays an important role in the overturning's variability. Gyre strength is then linked to the recirculation of newly transformed waters that get exported as part of the overturning's lower branch. Finally, we investigate the impact of the atmosphere on Irminger Sea circulation and find that fluctuations of the winds are important drivers of change in this gyre and the overturning. Key Points: The interior Irminger Sea, where the poleward limb of the Irminger Gyre (IG) dominates, is a hotspot for the overturning's lower limb variabilityA trend in IG transport is linked to deep intermediate water masses found in the Irminger SeaWind stress curl over the Labrador and Irminger Seas drives IG and Atlantic meridional overturning circulation variability [ABSTRACT FROM AUTHOR]

To advance our comprehension of the complex geological history and mantle dynamics in the North Atlantic region, we employ all available broadband seismic data recorded in Greenland to reveal an abnormal mantle transition zone (MTZ) structure. Central and eastern Greenland exhibits depressed 410 and 660 km discontinuities (d410 and d660, respectively) bordering the MTZ, indicative of a substantial thermal anomaly associated with an underlying plume, surpassing the 1,800°C threshold for post‐garnet phase transitions at the d660. Variations in MTZ thickness across Greenland stem from differing temperature anomalies at the d410 and d660, possibly linked to a tilted plume within the MTZ. These findings corroborate geodynamic models, elucidating the interaction between post‐garnet phase transitions and upwelling plumes. The results shed light on the origin of the enigmatic Icelandic hotspot track and its influence on the thermal and lithospheric structures beneath Greenland. Plain Language Summary: The physical and chemical processes responsible for producing the magnificent volcanic eruptions in Iceland and the North Atlantic region are debated issues in the geoscientific community despite numerous field and laboratory studies over the past several decades. One of the hypotheses for the formation of the volcanoes is that they originate from a column of hot rocks rising from the deep mantle, that is, a mantle plume. The geometry, depth extent, and temperature anomaly of the plume are not well defined but can be constrained using the topography of two phase‐transition boundaries approximately at the globally averaged depths of 410 and 660 km, respectively. By analyzing P‐to‐S converted phases from the two discontinuities recorded by seismic stations, our results support the existence of a mantle plume that is broader than most other plumes on Earth beneath east Greenland. In addition, the results suggest that the nature of the phase transition across the 660 km discontinuity is different between the hottest core of the plume stem and the colder peripheral areas. The existence of the particular type of phase transition in the plume center can explain the large dimension of the plume, the excessive volcanism and other observations in the area. Key Points: The central and east Greenland shows abnormal mantle transition zone (MTZ) thickness, indicating a deep‐seated mantle plumeSeismic data unveil the impact of post‐garnet phase transition at the 660‐km discontinuity on mantle dynamicsVariations in MTZ thickness suggest the existence of a tilted plume and offer insights into the plume behavior [ABSTRACT FROM AUTHOR]

High‐resolution records from past interglacial climates help constrain future responses to global warming, yet are rare. Here, we produce seasonally resolved climate records from subarctic‐Canada using micron‐scale measurements of oxygen isotopes (δ18O) in speleothems with apparent annual growth bands from three interglacial periods—Marine Isotope Stages (MIS) 11, 9, and 5e. We find 3‰ lower δ18O values during MIS 11 than MIS 5e, despite MIS 11 likely being warmer. We explore controls on high‐latitude speleothem δ18O and suggest low MIS 11 δ18O values reflect greater contribution of cold‐season precipitation to dripwater from longer annual ground thaw durations. Other potential influences include changes in precipitation source and/or increased fraction of cold‐season precipitation from diminished sea ice in MIS 11. Our study highlights the potential for high‐latitude speleothems to yield detailed isotopic records of Northern Hemisphere interglacial climates beyond the reach of Greenland ice cores and offers a framework for interpreting them. Plain Language Summary: Few climate records pre‐dating the last ice age exist from high‐latitude North America, which inhibits our understanding of how regions with permafrost responded to past warming and how they might change in the future. Here, we help fill this data gap by using six speleothems (cave mineral deposits) from a cave in the Northwest Territories, Canada to produce climate records that span thousands of years during former warm periods of Earth's history. We find that speleothems that grew during an exceptionally warm super‐interglacial period 400,000 years ago have 3‰ lower oxygen isotope (δ18O) values compared to those that grew during a likely cooler interglacial 125,000 years ago. We explore potential explanations for the difference in δ18O across interglacials, and suggest that lower δ18O values during warmer periods reflect greater infiltration of cool‐season precipitation with longer annual ground thaw durations. This study highlights the importance of high‐latitude speleothems to provide detailed climate records beyond the range available from Greenland ice cores. Key Points: Long high‐latitude terrestrial climate records are rare in the Northern HemisphereHigh‐latitude speleothems can provide ultra‐high‐resolution climate records beyond the reach of Greenland ice coresMean oxygen isotopes of Arctic and subarctic speleothems likely are controlled by annual ground thaw durations [ABSTRACT FROM AUTHOR]

Polar bears (Ursus maritimus) depend on sea ice to hunt their ice‐associated prey. However, climate‐induced sea ice loss is leading to changes in space‐use strategies of polar bears, with bears in some subpopulations spending more time on land or selecting alternative habitats. One such documented alternative habitat is glacier ice, which provides year‐round access to prey, although the feeding habits of polar bears using glacier ice relative to those following the retreating ice and/or seasonally moving onshore are not known. Here, we use adipose tissue from polar bears (n = 104) from the Baffin Bay subpopulation live‐captured in Northwest Greenland during the springs of 2009–2013 to investigate dietary patterns between space‐use strategies inferred from satellite telemetry data, while considering demographic and interannual variation. Using quantitative fatty acid signature analysis (QFASA) to generate diet estimates, ringed seals (Pusa hispida) and bearded seals (Erignathus barbatus) were estimated as the primary and secondary prey of Baffin Bay polar bears for all sex/age classes and sampling years, apart from a single anomalous year (2009) with a relatively high proportion of beluga (Delphinapterus leucas) and narwhal (Monodon monoceros). While demographic and short‐term temporal variation was minimal, fatty acid signatures and QFASA‐generated diet estimates clearly differed between polar bears using coastal and offshore space‐use strategies. "Offshore" adult females (n = 31), which make long‐distance movements across the Baffin Bay pack ice, had high proportions of C22‐chain length monosaturated fatty acids and diet estimates that included beluga, narwhal, harp (Pagophilus groenlandicus), and hooded seal (Cystophora cristata). "Coastal" adult females (n = 6), which remain resident at glacier fronts in Northwest Greenland year‐round including during the sea ice‐free season, consumed proportionally more ringed seals (+13%) and similar proportions of bearded seal, but essentially no beluga and narwhal or harp and hooded seal. Thus, space‐use strategy is a driver of intrapopulation diet variability. As space‐use strategies change with ongoing loss of sea ice habitat, our results suggest important ramifications for polar bear feeding patterns. [ABSTRACT FROM AUTHOR]

This study aims to detect residual thermal anomalies along the path of a hotspot beneath the continental lithosphere using gradiometry data. We calculate perturbations in gravity vectors and gradients by employing a simple thermal anomaly model. The results highlight that the horizontal components of Bouguer gravity gradients, sensitive to source structure and elongation, exhibit substantial magnitudes, exceeding current detection levels. To improve signals from sources of different sizes and orientations, we apply a filtering approach involving wavelet transforms and rotated local frames. We also assess the impact of lateral crustal variations on the gravity field by introducing random density anomalies and a seismic crustal model, testing the method's ability to distinguish between these anomalies and thermal sources. Using Gravity field and steady‐state Ocean Circulation Explorer satellite data, we generated scale‐orientation diagrams aiming to identify signals aligned with the direction of plate movement and with a spatial scale of a few hundred kilometers (scale of the plumes). Maps of filtered Bouguer gravity gradients aligned with hotspot trajectories are generated for all continental hot plumes, with the position and the age of the intraplate volcanism to support the interpretation. While we do not show a clear signal in regions that are too tectonically complex or with lateral variations in the crust or at the lithospheric boundary correlated with the hotspot trace, we found observed signals with expected sign and amplitude over Hoggar, Tibesti, Darfour and Cameroon tracks in Africa and over Iceland and Jan Mayen in Greenland. However, determining corresponding mass source depths remains a complex task. Plain Language Summary: This study aims to detect residual heat patterns from hotspots beneath the continental lithosphere by analyzing gravity changes using gradiometry data. The results reveal significant gravity variations, particularly horizontally, suggesting the presence of heat sources beyond current detection capabilities. To improve signal detection for varied heat source sizes, we employ a filtering approach with wavelet transforms and rotated frames. The impact of crustal gravity variations is also considered, testing the ability to distinguish between different anomalies. Utilizing Gravity field and steady‐state Ocean Circulation Explorer satellite data, we create diagrams identifying signals linked to plate movement and heat source size, and we generate maps displaying filtered gravity gradients changes along hotspot paths on continents. While complex tectonic regions show no clear signal, expected signals are observed over specific tracks in Africa and Greenland. Determining the depth of corresponding mass sources remains a challenging aspect of the study. Key Points: We reconstruct hotspots trajectories on the continental lithosphere and we superimpose the position of the age of intraplate volcanismWe build maps of Bouguer gravity gradients filtered at the spatial scale of a few hundred kilometers and oriented along the trajectoriesWe show that gradiometry data have the potential to track hotspots in the continental lithosphere for tens of millions years [ABSTRACT FROM AUTHOR]

Relic coastal landforms (fossil corals, cemented intertidal deposits, or erosive features carved onto rock coasts) serve as sea‐level index points (SLIPs), that are widely used to reconstruct past sea‐level changes. Traditional SLIP‐based sea‐level reconstructions face challenges in capturing continuous sea‐level variability and dating erosional SLIPs, such as tidal notches. Here, we propose a novel approach to such challenges. We use a numerical model of cliff erosion embedded within a Monte Carlo simulation to investigate the most likely sea‐level scenarios responsible for shaping one of the best‐preserved tidal notches of Last Interglacial age in Sardinia, Italy. Results align with Glacial Isostatic Adjustment model predictions, indicating that synchronized or out‐of‐sync ice‐volume shifts in Antarctic and Greenland ice sheets can reproduce the notch morphology, with sea level confidently peaking at 6 m and only under a higher than present erosion regime. This new approach yields insight into sea‐level trends during the Last Interglacial. Plain Language Summary: Scientists typically investigate the position of sea level in geological time using the elevation, age, and characteristics of fossil marine organisms living in shallow water (e.g., coral reefs), beach deposits, or erosional features that were formed near the sea level. However, these indicators offer only fragmented, if not only point‐like information in time and not a continuous sea‐level record. To overcome this issue, we use a numerical model that reconstructs the shape of tidal notches (i.e., indentations created close to sea level in carbonate cliffs). We compare model‐generated notch shapes with the real shape of the tidal notch, and we produce a set of continuous sea‐level histories that are more likely to have produced one of the best‐preserved fossil tidal notches in the Orosei Gulf, Sardinia, Italy, carved during the Last Interglacial highstand, 125.000 years ago. Our findings suggest that whether the ice sheets in Antarctica and Greenland melted at the same time or separately, both scenarios could reproduce the actual shape of the tidal notch we observe at present. Our findings indicate that the erosion rate during that period was higher than present and the sea level is very likely to have reached up to 6 m. Key Points: Cliff erosion modeling and Monte Carlo analysis indicate tidal notch geometry can offer a continuous record of past sea level variabilityThe geometry of Orosei's tidal notch, Italy can be replicated through simultaneous or asynchronous Antarctic–Greenland ice melting scenariosThe morphology of the Last Interglacial notch is more efficiently replicated using higher‐than‐present erosion rates and a 6 m sea‐level peak [ABSTRACT FROM AUTHOR]

A generalized methodology to deploy different types of vertical coordinate system in arbitrarily defined time‐invariant local areas of quasi‐Eulerian numerical ocean models is presented. After detailing its characteristics, we show how the general localization method can be used to improve the representation of the Nordic Seas overflows in the UK Met Office NEMO‐based eddy‐permitting global ocean configuration. Three z*‐levels with partial steps configurations localizing different types of hybrid geopotential/terrain‐following vertical coordinates in the proximity of the Greenland‐Scotland ridge are implemented and compared against a control configuration. Experiments include a series of idealized and realistic numerical simulations where the skill of the models in computing pressure forces, reducing spurious diapycnal mixing and reproducing observed properties of the Nordic Seas overflows are assessed. Numerical results prove that the localization approach proposed here can be successfully used to embed terrain‐following levels in a global geopotential levels‐based configuration, provided that the localized vertical coordinate chosen is flexible enough to allow a smooth transition between the two. In addition, our experiments show that deploying localized terrain‐following levels via the multi‐envelope method allows the crucial reduction of spurious cross‐isopycnal mixing when modeling bottom intensified buoyancy driven currents, significantly improving the realism of the Nordic Seas overflows simulations in comparison to the other configurations. Important hydrographic biases are found to similarly affect all the realistic experiments and a discussion on how their interaction with the type of localized vertical coordinate affects the realism of the simulated overflows is provided. Plain Language Summary: Numerical ocean models are arguably one of the most advanced tools the scientific community can use to study the dynamics of the world's oceans. However, the ability of an ocean model to realistically simulate ocean currents depends on the numerical techniques it employs, such as the type of vertical coordinate system. Ocean models typically implement a single type of vertical coordinate throughout the entire model domain, which is often unable to accurately represent the vast variety of physical processes driving the oceans. In this study, we propose a new method that allows different types of vertical coordinates in selected regions of the same model domain. Our method targets a particular class of ocean models (known as quasi‐Eulerian), improving the way they represent the important influence the sea floor exerts on ocean currents. After introducing our novel approach, we present the results of a series of numerical experiments where we test its skill for improving the representation of the Nordic Seas overflows, an important type of ocean current located at depth in the proximity of the Greenland‐Scotland ridge. Key Points: A general methodology to embed distinct types of vertical coordinates in local time‐invariant targeted areas of quasi‐Eulerian ocean modelsThree different hybrid geopotential/terrain‐following coordinates are localized in the Nordic overflows region of a z*‐levels global modelUsing local multi‐envelope terrain‐following levels reduces diapycnal mixing improving the realism of the simulated Nordic overflows [ABSTRACT FROM AUTHOR]

In this study, we investigate the effects caused by interplanetary (IP) shock impact angles on the subsequent ground dB/dt variations during substorms. IP shock impact angles have been revealed as a major factor controlling the subsequent geomagnetic activity, meaning that shocks with small inclinations with the Sun‐Earth line are more likely to trigger higher geomagnetic activity resulting from nearly symmetric magnetospheric compressions. Such field variations are linked to the generation of geomagnetically induced currents (GICs), which couple to artificial conductors on the ground leading to deleterious consequences. We use a sub‐set of a shock data base with 237 events observed in the solar wind at L1 upstream of the Earth, and large arrays of ground magnetometers at stations located in North America and Greenland. The spherical elementary current system methodology is applied to the geomagnetic field data, and field‐aligned‐like currents in the ionosphere are derived. Then, such currents are inverted back to the ground and dB/dt variations are computed. Geographic maps are built with these field variations as a function of shock impact angles. The main findings of this investigation are: (a) typical dB/dt variations (5–10 nT/s) are caused by shocks with moderate inclinations; (b) the more frontal the shock impact, the more intense and the more spatially defined the ionospheric current amplitudes; and (c) nearly frontal shocks trigger more intense dB/dt variations with larger equatorward latitudinal expansions. Therefore, the findings of this work provide new insights for GIC forecasting focusing on nearly frontal shock impacts on the magnetosphere. Plain Language Summary: Space weather effects caused by solar perturbations can increase the intensity of electric currents flowing in the Earth's upper atmosphere. These currents in turn induce geoelectric currents on the ground that couple with ground infrastructure, such as power transmission lines, oil/gas pipelines, and railways, which further cause deleterious consequences. Recent studies have shown that the angle with which solar perturbations impact the Earth closely controls intensifications of electric currents in the upper atmosphere and the subsequent ground magnetic field perturbations causing geoelectric currents. When these space weather drivers impact Earth more frontally, the resulting atmospheric current enhancements and the subsequent ground magnetic field perturbations are generally more intense. In this study, we use a large amount of magnetic field data recorded in North America and Greenland. We find that the most intense geomagnetic field disturbances are caused by the impacts of nearly frontal solar perturbations on the Earth and cover large geographic areas reaching southern Canada and northern United States. The results of this work can be used by space weather forecasters when predicting events with significant magnetic field variations leading to intense geoelectric currents, particularly when tracking space weather drivers that are forecasted to impact Earth nearly frontally. Key Points: Typical substorm‐time ground dB/dt variations (5–10 nT/s) are caused by interplanetary shocks with moderate inclinations (140°–160°)The more frontal the shock, the more intense and the broader the ionospheric current amplitudesThe more frontal the shock, the more intense, larger the area coverage, and the lower the equatorward latitudinal extent of dB/dt variations [ABSTRACT FROM AUTHOR]

Recent observations revealed the existence of ice slabs and aquifers on the Greenland ice sheet (GrIS). Both affect the ice sheet's hydrology: ice slabs facilitate runoff and aquifers modulate drainage to the bed. However, their climatic drivers and history remain unclear, as most observations cover only two decades. Here, we present a model simulation of the evolution of GrIS ice slabs and aquifers (1980–2020), evaluated using radar measurements. The results show that accumulation, melt and rain rates are good predictors for the spatial distribution of ice slabs and aquifers. Both features were already present in the late 1980s, and their extent remained relatively constant until the beginning of this century, after which increased melt led to their expansion. We show that almost any transect from the coast to the ice‐sheet interior will cross either an ice slab region, or an aquifer, or both. Plain Language Summary: The Greenland ice sheet is covered by a layer of old, porous snow called firn, which has the capacity to prevent surface melt runoff. The firn can however, form thick ice slabs or store large amounts of water underground. These change the fate of surface melt by respectively enhancing runoff and draining water to the ice sheet's bed. While observations have shown where these phenomena occur, it is still unknown what the exact atmospheric conditions are that lead to their formation. Here, we tackle this problem using computer simulations. Our results show that the amount of snowfall, melt and rain determine whether ice slabs or aquifers can form. The results also suggest that ice slabs and aquifers are more abundant than previously assumed, and that their observed expansion started only in the early 2000s, and they will continue to expand if the climate gets warmer and melt and rain become more abundant. Key Points: Accumulation, melt and rain are good predictors of ice slab and firn aquifer locationsThe expansion of modeled ice slabs and aquifers on the Greenland ice sheet started in the early 2000sWe show that, in between the ablation zone and the accumulation zone, there are always ice slabs and/or aquifers present [ABSTRACT FROM AUTHOR]

Watermass transformation in the Irminger Sea, a key region for the Atlantic Meridional Overturning Circulation, is influenced by atmospheric and oceanic variability. Strong wintertime atmospheric forcing in 2015 resulted in enhanced convection and the densification of the Irminger Sea. Deep convection persisted until 2018, even though winters following 2015 were mild. We show that this behavior can be attributed to an initially slow convergence of buoyancy, followed by more rapid convergence of buoyancy. This two‐stage recovery, in turn, is consistent with restratification driven by baroclinic instability of the Irminger Current (IC), that flows around the basin. The initial, slow restratification resulted from the weak horizontal density gradients created by the widespread 2015 atmospheric heat loss. Faster restratification occurred once the IC recovered. This mechanism explains the delayed recovery of the Irminger Sea following a single extreme winter and has implications for the ventilation and overturning that occurs in the basin. Plain Language Summary: The Irminger Sea, between Greenland and Iceland, is known to be an important driver of variability in the global ocean circulation that regulates global climate. During the 2015 winter, the Irminger Sea experienced widespread cooling and buoyancy loss down to 1,000 m, resulting in deeper wintertime mixing than had been observed in the region for many years. This low buoyancy state and deep wintertime mixing persisted from 2015 to 2018, despite a return to average atmospheric wintertime conditions. Here we study how processes that typically provide summertime buoyancy gain to balance wintertime buoyancy loss contribute to the multi‐year recovery of the region. We characterize the recovery of the interior of the Irminger Sea as having two stages: initially slow recovery, followed by more rapid recovery. Our findings are consistent with earlier studies stating that the main source of buoyancy to the interior is eddies shed from the current flowing around the Irminger Sea. This study shows that the deep mixing that is important for global circulation and climate is influenced by changes in the current around the basin. Key Points: Widespread buoyancy loss across the Irminger interior and Irminger Current (IC) delayed the recovery of the interior from strong cooling in 2015Baroclinic instabilities shed from the IC are the dominant source of buoyancy restratifying the sub‐surface Irminger interiorIt is important to consider changes in the IC when considering drivers of variability in convection, ventilation, and the Atlantic Meridional Overturning Circulation [ABSTRACT FROM AUTHOR]

Greenland's bedrock responds to ongoing ice loss with an elastic vertical land motion (VLM) that is measured by Greenland's Global Navigation Satellite System (GNSS) Network (GNET). The measured VLM also contains other contributions, including the long‐term viscoelastic response of the Earth to the deglaciation of the last glacial period. Greenland's ice sheet (GrIS) produces the most significant contribution to the total VLM. The contribution of peripheral glaciers (PGs) from both Greenland (GrPGs) and Arctic Canada (CanPGs) has not carefully been accounted for in previous GNSS analyses. This is a significant concern, since GNET stations are often closer to PGs than to the ice sheet. We find that, PGs produce significant elastic rebound, especially in North and East Greenland. Across these regions, the PGs produce up to 32% of the elastic rebound. For a few stations in the North, the VLM from PGs is larger than that due to the GrIS. Plain Language Summary: The solid Earth has long been compressed by the weight of overlying ice caps and ice sheets. As climate change causes ice to melt, and these loads diminish, the solid Earth relaxes, producing instantaneous elastic rebound and delayed viscoelastic rebound of the bedrock. Such displacements are recorded by the 58 permanent Global Navigation Satellite System (GNSS) stations that comprise the Greenland GNSS Network (GNET). So far, only the ice mass changes from the ice sheet have been considered in the analyses of deformation recorded by GNET. Here, we evaluate the contribution from the peripheral glaciers, which are often much closer to the stations than the ice sheet. We find that at many stations, the signal produced by the peripheral glaciers is non‐negligible, especially in North and East Greenland. This allows us to better understand the residual rebound signal produced by the end of the last ice age. Key Points: Elastic rebound due to ice loss from peripheral glaciers can exceed that due to ice sheet lossThis effect is most significant at Global Navigation Satellite System Network (GNET) sites in North and East GreenlandPeripheral glacier loss should be acknowledged when isolating glacial isostatic adjustment from GNET [ABSTRACT FROM AUTHOR]

Rapid warming and human exploitation threaten boreal forests. Understanding links among vegetation, climate, and people in this vast biome requires highly resolved long‐term records that integrate regional inputs. We developed an 850‐year pollen‐based record of supraregional vegetation change using a southern Greenland ice core and atmospheric modeling that identified the boreal and mixed‐conifer forests of eastern Canada as the dominant pollen source regions. Conifer pollen increased ∼1400 CE at the onset of the cooler and drier Little Ice Age. A subsequent decline began ∼1650 CE and a statistically significant pollen change after 1760 CE suggests ecological consequences of the Little Ice Age cooling and initial human exploitation that persisted until recent decades. These supraregional changes are broadly consistent with local records and demonstrate intensification of human impacts on northern forests, suggesting a shift from a climate‐modulated to an increasingly human‐controlled system during recent centuries. Plain Language Summary: Understanding the consequences of climate warming and human exploitation for northern forests requires long‐term regional records of this sparsely populated region. We present a first 850‐year vegetation record from a low‐elevation ice core in Southern Greenland. The ice core integrates pollen from Eastern Canadian forests and other boreal forests, providing temporal precision and regional coverage not available from previous paleoecological investigations. We show that climate change drove changes in the abundances of coniferous species during the Little Ice Age. Subsequent warming coincided with the onset of timber exploitation that brought a shift from a climate‐controlled to an increasingly human‐controlled system. Current warming trends likely will further alter vegetation distribution, productivity, and disturbance regimes of northern forests, but they will be moderated by the legacy of human impacts. Key Points: Atmospheric model FLEXPART identifies boreal and mixed‐conifer forests in Canada as pollen source area for Southern Greenland ice corePollen in Greenland ice track conifer pollen increases ∼1400 CE at the onset of the cooler and drier Little Ice AgeConifer pollen declines ∼1650 CE coincident with cold LIA climate and initial commercial forest exploitation [ABSTRACT FROM AUTHOR]

Dark (low albedo) surface ice on the Greenland Ice Sheet enhances melting and subsequent runoff, a major mass loss contributor during the ablation season. The accumulation of both biological (e.g., glacier ice algae) and abiotic (e.g., mineral dust) light‐absorbing particulates are important darkening factors, that are potentially influenced by the duration of snow‐free, bare ice (a phenological factor), and other geo‐topographical factors such as elevation, slope, aspect and the distance from the ice margin. Here, we present the first medium‐resolution (30 m) analysis of the phenological and geo‐topographical controls on the distribution of dark ice in SE and SW Greenland from statistical analysis of data derived from a harmonized satellite albedo product and ArcticDEM. The duration of bare ice primarily controls the distribution of dark surface ice, allowing for algae growth on inland ice surfaces in particular, whereas geo‐topographical factors are only secondary controls. Plain Language Summary: Meltwater that runs off the Greenland Ice Sheet is currently the largest single glaciological contributor to global sea level rise. Recent darkening of the surface ice has contributed significantly to increased melt rates. We used medium resolution (30m) satellite data to link ice surface darkness (albedo) to the topography (elevation, slope, and aspect) of the ice sheet surface, the distance from the ice margin, and the duration of bare or snow‐free surface ice. Our analysis shows that the duration of bare ice or snow‐free conditions was the most significant factor in driving the darkening of ice. We also found that the impact of topographic factors on the ice darkening varied depending on whether abiotic or biological processes were driving the darkening. This information helps us to better explain where the ice sheet is melting fastest and to predict the loss of ice mass, particularly considering the expected increase in the area of bare, snow‐free ice in our warming climate. Key Points: The duration of snow‐free, bare ice is the primary control on the distribution of dark iceLonger durations of bare ice (median = 40 days) are prerequisites for ice to become darkSurface slope and aspect influence darkening by modulating the duration of bare ice and glacier ice algae growth [ABSTRACT FROM AUTHOR]

Small‐to‐moderate volcanic eruptions can lead to significant surface cooling when they occur clustered, as observed in recent decades. In this study, based on new high‐resolution ice‐core data from Greenland, we produce a new volcanic forcing data set that includes several small‐to‐moderate eruptions not included in prior reconstructions and investigate their climate impacts of the early 19th century through ensemble simulations with the Max Planck Institute Earth System Model. We find that clustered small‐to‐moderate eruptions produce significant additional global surface cooling (∼0.07 K) during the period 1812–1820, superposing with the cooling by large eruptions in 1809 (unidentified location) and 1815 (Tambora). This additional cooling helps explain the reconstructed long‐lasting cooling after the large eruptions, but simulated regional impacts cannot be confirmed with reconstructions due to a low signal‐to‐noise ratio. This study highlights the importance of small‐to‐moderate eruptions for climate simulations as their impacts can be comparable with that of solar irradiance changes. Plain Language Summary: Volcanic eruptions can influence global climate through the emission of sulfuric acids shielding Earth from incoming solar radiation. Previous volcanic reconstructions based on ice‐cores from the polar regions, however, only considered very strong volcanic eruptions. In this study, based on new ice‐core measurements from Greenland, we reconstruct for the first time volcanic sulfur emissions from small to medium‐sized eruptions and investigate their impact on climate in the early 19th century through experiments with the Max Planck Institute Earth System Model (MPI‐ESM1.2‐LR). We find that clustering of small to medium‐sized eruptions can cause significant global surface cooling (∼0.07 K), which during the 1812–1820 period amplified the cooling caused by the two known large eruptions of the period (1809 unidentified and 1815 Tambora). This additional surface cooling from small eruptions helps explain the long‐lasting cooling after the two strong eruptions generally found in the reconstruction, but the simulated regional impacts cannot be fully confirmed with reconstructions that are too noisy. This study highlights the importance of including small‐to‐moderate eruptions for climate model simulations as their impacts are comparable with that of solar irradiance forcing. Key Points: A new ice‐core based reconstruction of volcanic sulfate in the atmosphere (1733–1895) includes small‐to‐moderate eruptionsSmall‐to‐moderate eruptions can induce significant surface cooling and help explain the long‐lasting cooling in the early 19th centuryRegional cooling from small‐to‐moderate eruptions may be influenced by the circulation changes from the 1815 Tambora for over 10 years [ABSTRACT FROM AUTHOR]

Attenuation of ocean waves by ice is a crucial process of the interaction between waves and sea ice in marginal ice zone (MIZ), while such interaction can contribute to the retreating of sea ice in the Arctic. Based on the retrieved two‐dimensional ocean wave spectra by spaceborne Synthetic Aperture Radar, we investigated the attenuation of ocean waves in the MIZ in Svalbard and Greenland. The results show that the energy attenuation rate ranges from 0.126 × 10−4/m to 0.618 × 10−4/m. Quantitative analysis suggests that the attenuation rate is significantly related to wave height and peak wave period of coming waves. It is further found that the waves decay faster in the area with ice thickness exceeding 0.5 m. We compared the derived wave attenuation rates in the present study with those in previous studies based on in situ measurements, which reveals that waves are becoming less attenuated by sea ice in the Arctic. Plain Language Summary: The interaction between sea ice and ocean waves is one of the key processes that accelerates the retreat of sea ice in the Arctic. The attenuation of ocean waves by sea ice is crucial to understanding the wave‐ice interaction mechanism and predicting ice changes. Spaceborne Synthetic Aperture Radar (SAR), capable of imaging ocean waves and sea ice in two‐dimension with high spatial resolution, has shown tremendous potential in studies on wave‐ice interaction. In this study, SAR images acquired in ice‐covered areas near Svalbard and east of Greenland were collected, and then ocean wave spectra were retrieved from these SAR images. Ocean wave spectra depict sea states elaborately by showing the wave energy distribution in different frequencies and directions. Subsequently, we derived the wave attenuation rate in sea ice from these wave spectra. By comparing the derived attenuation rates with previous field observations, the study reveals a lower attenuation rate, which suggests the waves were less attenuated by ice in past decades under ongoing retreating and thinning of sea ice in the Arctic. This indicates that waves can penetrate sea ice easier and deeper, which may further induce the retreating of sea ice. Key Points: Wave attenuation rate in sea ice was derived based on non‐linear inversion of two‐dimensional ocean wave spectra by Synthetic Aperture Radar in the Arctic marginal ice zone (MIZ)The attenuation rate generally follows the exponential law, varying with sea state (wave height and period) and sea ice conditionsCombining previous studies and this one, we may infer that the wave attenuation in the Artic MIZ is weakening due to sea ice retreat [ABSTRACT FROM AUTHOR]

The Greenland Ice Sheet is a leading contributor to global sea‐level rise because climate warming has enhanced surface meltwater runoff. Melt rates are particularly sensitive to air temperatures due to feedbacks with albedo. The primary melt‐albedo feedback, fluctuation of seasonal snowlines, however, is determined not only by melt but also by antecedent snowfall which could delay the onset of dark glacier ice exposure. Here we investigate the role of snowfall versus air temperatures on ice sheet melt‐albedo feedbacks using satellite remote sensing and atmospheric reanalysis data. We find several lines of evidence that snowline fluctuations are driven primarily by air temperatures and that snowfall is a secondary control. First, standardized linear regressions indicate that the timing of glacier ice exposure is nearly twice as sensitive to air temperatures than antecedent snowfall. Second, in 74% of the ablation zone by area, winter snowfall rates are not significantly correlated with winter air temperatures. This relationship implies that ice sheet melt due to climate warming is unlikely to be compensated by higher snowfall rates in the ablation zone. Third, we find no significant change in snowfall rates in the ablation zone during our 1981–2021 study period. Our findings demonstrate that snowfall is unlikely to reduce future ice sheet melt and that ice sheet meltwater runoff should be accurately predicted by air temperatures. Although given the importance of melt‐albedo feedbacks, ice sheet models that parameterize albedo or are coupled with regional climate models are likely to provide the most accurate projections of mass loss. Plain Language Summary: The Greenland Ice Sheet is currently losing mass because rates of mass loss (mainly due to surface meltwater runoff and iceberg discharge into the ocean) are higher than rates of mass gain (mainly due to snowfall). As the climate warms, rates of mass loss are expected to increase non‐linearly because rising air temperatures darken the surface, leading to more solar energy absorption, and more melting. The main process responsible for surface darkening is the exposure of dark glacier ice due to Greenland's seasonally evolving snowline. The position of the snowline is determined not only by summer melt but also by the thickness of the snowpack that accumulates during winter. In this study, we investigated whether summer melt or snowpack thickness was more important for determining the timing of glacier ice exposure using satellite remote sensing and climate model outputs. We found that the glacier ice exposure is much more sensitive to air temperatures than snowfall and that snowfall did not change in the ablation zone during the 1981–2021 study period. Our findings imply that snowfall is unlikely to reduce ice sheet mass loss and that meltwater runoff from the ice sheet should be accurately predicted by air temperatures. Key Points: We investigate the extent to which antecedent snowfall versus summer air temperatures controls melt‐albedo feedbacksWe find that melt‐albedo feedbacks are primarily driven by air temperatures and that antecedent snowfall is of secondary importanceWe find little evidence that snowfall will compensate enhanced ice sheet melt due to climate warming [ABSTRACT FROM AUTHOR]

Surface processes alter the water stable isotope signal of the surface snow after deposition. However, it remains an open question to which extent surface post‐depositional processes should be considered when inferring past climate information from ice core records. Here, we present simulations for the Greenland Ice Sheet, combining outputs from two climate models with an isotope‐enabled snowpack model. We show that surface vapor exchange and associated fractionation imprint a climate signal into the firn, resulting in an increase in the annual mean value of δ18O by +2.3‰ and a reduction in d‐excess by −6.3‰. Further, implementing isotopic fractionation during surface vapor exchange improves the representation of the observed seasonal amplitude in δ18O from 65.0% to 100.2%. Our results stress that surface vapor exchange is important in the climate proxy signal formation and needs consideration when interpreting ice core climate records. Plain Language Summary: The climate information contained in falling snow is modified by exchange processes with the atmosphere after the snow has fallen to the surface. It is important to understand how this modification affects the interpretation of past climate information from ice core isotope records. In this study, we combined outputs from two climate models to simulate the climate signal in a snow core on the Greenland Ice Sheet. We evaluate the snow core model using snow observations from the Greenland Ice Sheet. By simulating snow cores with and without the modification at the surface, we find a considerable impact of the surface modification on the climate signal in the snow core. Further, considering the surface modification causes an improved representation of the seasonal changes compared to observations. Our findings highlight the importance of surface processes in forming climate information contained in ice cores and underscore the need to include these processes in the ice core interpretation. Key Points: Water isotopic fractionation during vapor exchange significantly affects the simulated annual and seasonal isotope climate signal in ice coresThe simulated seasonal amplitude of the δ18O signal in the snowpack improves when including surface vapor exchange induced fractionationA phase shift in the simulated seasonal maximum in d‐excess toward early autumn is induced by vapor exchange, consistent with observations [ABSTRACT FROM AUTHOR]

For better projections of sea level rise, two things are needed: an improved understanding of the contributing processes and their accurate representation in climate models. A major process is basal melting of ice shelves and glacier tongues by the ocean, which reduces ice sheet stability and increases ice discharge into the ocean. We study marine melting of Greenland's largest floating ice tongue, the 79° North Glacier, using a high‐resolution, 2D‐vertical ocean model. While our fjord model is idealized, the results agree with observations of melt rate and overturning strength. Our setup is the first application of adaptive vertical coordinates to an ice cavity. Their stratification‐zooming allows a vertical resolution finer than 1 m in the entrainment layer of the meltwater plume, which is important for the plume development. We find that the plume development is dominated by entrainment only initially. In the stratified upper part of the cavity, the subglacial plume shows continuous detrainment. It reaches neutral buoyancy near 100 m depth, detaches from the ice, and transports meltwater out of the fjord. Melting almost stops there. In a sensitivity study, we show that the detachment depth depends primarily on stratification. Our results contribute to the understanding of ice–ocean interactions in glacier cavities. Furthermore, we suggest that our modeling approach with stratification‐zooming coordinates will improve the representation of these interactions in global ocean models. Finally, our idealized model topography and forcing are close to a real fjord and completely defined analytically, making the setup an interesting reference case for future model developments. Plain Language Summary: The global increase of sea levels is a consequence of human‐induced climate change. It presents a threat to coastal regions and demands action to protect human life and infrastructure near the coast. Planning protective measures requires projections of sea level rise, computed with climate models. We present an approach to improve the simulation of an important contributor to sea level rise: melting of floating ice shelves by ocean circulation. Our modeling approach uses a vertical model grid that evolves over time. The temporal evolution depends on the density structure of the ocean. Large density differences appear just below an ice shelf, where fresh meltwater mixes with salty seawater. The adaptive grid of our model resolves this mixing process in great detail. This is important for an accurate computation of the melt rate and enables us to study in depth the ice shelf–ocean interactions. We study them at the glacier tongue of the 79° North Glacier, which is Greenland's largest ice shelf. The physical understanding gained from our simulations is also applicable to other floating glacier tongues and ice shelves. We suggest that using the presented model technique in global ocean models can improve projections of melting and sea level rise. Key Points: Melting of the 79° North Glacier ice tongue by turbulent ocean currents is studied with an idealized 2D‐vertical fjord modelThe subglacial plume behaves like an entraining plume close to the grounding line and like a detraining gravity current further downstreamA vertical resolution finer than 1 m is achieved in the subglacial plume by using adaptive vertical coordinates that zoom to stratification [ABSTRACT FROM AUTHOR]

This report contains terrestrial absolute gravity measurements performed near the village of Kulusuk in southeast Greenland. It represents an exceptionally long time series of nearly two decades. The measurements began in July 1996 until July 2015. The station is located a few meters beside a permanent GNSS station. The final gravity values (g‐values), more precisely the accelerations due to gravity, are listed along their associated uncertainties. A description of the station, a short explanation on the absolute gravimeter, and the data processing are presented. [ABSTRACT FROM AUTHOR]

A predictive model for the variation of ionospheric currents is of great scientific and practical importance to our modern industrial society. To study the response of ionospheric currents to external drivers including geomagnetic indices and solar radiation, we developed a feedforward neural network model trained on the Equivalent Ionospheric Current (EIC) data from 1st January 2007 to 31st December 2019. Due to the highly imbalanced nature of the ionospheric currents data, which means that the data of extreme events are much less than those of quiet times, we utilized different loss functions to improve the model performance. Our model demonstrates the potential to predict the active events of ionospheric currents reasonably well (e.g., EICs during substorms) within a timescale of a few minutes. Although the data used for training are measurements over the North American and Greenland sectors, our model is not only able to predict EICs within this region, but is also able to provide a promising out‐of‐sample prediction on a global scale. Plain Language Summary: Studying the ionospheric current system is important for our modern society, since its drastic change can impact the ground‐based facilities such as power grid. We used a neural network model to predict the influence of geomagnetic activities and upstream condition on the variation of ionospheric currents over the North American and Greenland regions during a whole solar cycle period. Because the data set of ionospheric currents is highly imbalanced, we used different loss function to optimize our model. We found that our model can reasonably well predict the ionospheric current events which are related to the magnetospheric activities. We also found that our model can provide predictions beyond the North American and Greenland regions. Key Points: A neural network is trained to predict the response of Equivalent Ionospheric Currents (EIC) to geomagnetic indicesA comparative study between the mean square error and focal loss has been made to address the EIC's imbalance regression problemOur model can spatially and temporally predict the eastward‐westward EIC component [ABSTRACT FROM AUTHOR]

Greenland's glacial fjords modulate the exchange between the ice sheet and ocean. Subglacial‐discharge‐driven plumes adjacent to glaciers may exert an important influence on fjord water properties, submarine glacier melting and the export of glacially‐modified waters to the shelf. Here we use a numerical plume model in conjunction with observations from proximal to 14 glaciers in northwest Greenland to assess the impact of these plumes on near‐glacier water properties. We find that in late summer, waters emanating from glacial plumes often make up >50% of the fjord water composition at intermediate depths. These plume waters are comprised largely of upwelled Atlantic Water, warming the near‐glacier water profile and likely increasing submarine melting. Our findings demonstrate the key role played by plumes in driving water modification in Greenland's fjords, and the potential for simple models to capture these impacts across a range of settings. Plain Language Summary: Fjords form an important point of contact between the Greenland Ice Sheet and surrounding ocean. Warm ocean waters in these fjords melt the submerged sections of marine‐terminating glaciers, whilst freshwater draining from the ice sheet modifies coastal water properties, impacting ocean circulation. In this study, we use observations of fjord temperature and salinity in combination with numerical modeling to evaluate the mechanisms through which glaciers modify fjord circulation and water properties. Across 14 glaciers in northwest Greenland, we find that the input of glacial meltwater drives the upwelling of warm, deep Atlantic‐origin waters, which mix with the cooler Arctic‐origin waters typically found above ∼250 m depth. These glacially modified waters thus cause a substantial warming of fjord waters relative to those off the coast, a feedback that will amplify the oceanic melting of these glaciers. Key Points: We assess the impact of subglacial‐discharge plumes on water properties proximal to 14 tidewater glaciers in northwest GreenlandIn late summer, waters emanating from glacial plumes often make up >50% of the fjord water composition at intermediate depthsThese plume waters are comprised largely of upwelled Atlantic Water, warming the near‐glacier water profile and likely increasing submarine melting [ABSTRACT FROM AUTHOR]

Water mass transformation in the Nordic Seas and the associated overflow of dense waters across the Greenland‐Scotland Ridge (GSR) acts to maintain the lower limb of the Atlantic meridional overturning circulation. Here, we use ocean and atmospheric reanalysis to assess the temporal variability in the Nordic Seas overturning circulation between 1950 and 2020 and its relation to surface buoyancy forcing. We find that variable surface‐forced transformation of Atlantic waters in the eastern Nordic Seas can explain variations in overflow transport across the GSR. The production of dense water masses in the Greenland and Iceland Seas is of minor importance to overflow variability. The Nordic Seas overturning circulation shows pronounced multidecadal variability that is in phase with the Atlantic Multidecadal Variability (AMV) index, but no long‐term trend. As the AMV is currently transitioning into its negative phase, the next decades could see a decreased overflow from the Nordic Seas. Plain Language Summary: In the Nordic Seas, warm, light waters are transformed into cold, dense waters. This dense‐water formation is important to the global ocean circulation. In this study, we quantify the role of surface heat and freshwater fluxes in this dense‐water formation. We find that variable surface fluxes in the eastern Nordic Seas can largely explain variations in dense‐water formation since 1950. The formation of dense waters in the Nordic Seas and the subsequent export of these waters into the North Atlantic show no long‐term trend. We nevertheless predict that the dense‐water formation will decrease during the next decade as a result of multidecadal variability. Key Points: Variable transport of Nordic overflow waters is determined by surface forcing in the Norwegian SeaNordic Seas overturning circulation shows pronounced multidecadal variability since 1950The overturning circulation and overflow are projected to decrease toward 2040 [ABSTRACT FROM AUTHOR]

Sea ice modulates the energy exchange between the atmosphere and the ocean through its kinematics. Marginal ice zone (MIZ) dynamics are complex and are not well resolved in routine observations. Here, we investigate sea ice dynamics in the Greenland Sea MIZ using in situ and remote sensing Lagrangian drift datasets. These datasets provide a unique view into ice dynamics spanning spatial scales. We find evidence of tidal currents strongly affecting sub‐daily ice motion. Velocity anomalies show abrupt transitions aligned with gradients in seafloor topography, indicating changes in ocean currents. Remote‐sensed ice floe trajectories derived from moderate resolution satellite imagery provide a view of small‐scale variability across the Greenland continental shelf. Ice floe trajectories reveal a west‐east increasing velocity gradient imposed by the East Greenland Current, with maximum velocities aligned along the continental shelf edge. These results highlight the importance of small scale ocean variability for ice dynamics in the MIZ. Plain Language Summary: Sea ice in the Arctic Ocean plays an important role in climate due to its influence on ocean circulation and air‐sea energy exchange. Ice motion results from competing and interacting effects of winds, ocean currents, and internal ice stresses. This study uses two novel observational datasets to analyze ice motion in the Greenland Sea and Fram Strait marginal ice zones. We find abrupt changes in the primary causes of ice motion associated with seafloor topography. In shallow seas, strong tidal currents affect ice drift, resulting in repeated opening and closing of the ice. Near the shelf edge, boundary currents increase ice drift speeds, causing ice pack shear. Sea ice models that ignore small‐scale ocean currents will underestimate ice deformation. Key Points: Ice dynamics measured with two new Lagrangian datasets show strong bathymetry dependent differences over short spatial scalesWind variability is insufficient to describe drift variability in regions with abrupt changes in bathymetry and large gradients in currentsDrifting buoy observations show a spatially heterogeneous imprint of tidal currents on ice dynamics [ABSTRACT FROM AUTHOR]

Rapid warming in the Arctic is drastically impacting marine ecosystems, affecting species communities and food‐web structure. Pelagic Themisto amphipods are a major component of the Arctic zooplankton community and represent a key link between secondary producers and marine vertebrates at higher trophic levels. Two co‐existing species dominate in the region: the larger Themisto libellula, considered a true polar species and the smaller Themisto abyssorum, a sub‐Arctic, boreal‐Atlantic species. Recent changes in abundance and distribution ranges have been detected in both species, likely due to the Atlantification of the Arctic. The ecology and genetic structure of these species are understudied, despite their high biomass and importance in the food web. For both species, we assessed genetic diversity, patterns of spatial genetic structure and demographic history using samples from the Greenland shelf, Fram Strait and Svalbard. This was achieved by analysing variation on the mitochondrial cytochrome c oxidase subunit 1 gene (mtCOI). The results revealed contrasting levels of mtCOI diversity: low levels in T. libellula and high levels in T. abyssorum. A lack of spatial genetic structure and a high degree of genetic connectivity were detected in both species in the study region. These patterns of diversity are potentially linked to the impacts of the Last Glacial Maximum. T. libellula populations may have been isolated in glacial refugia, undergoing gene flow restriction and vicariant effects, followed by a population expansion after deglaciation. Whereas T. abyssorum likely maintained a stable, widely distributed metapopulation further south, explaining the high diversity and connectivity. This study provides new data on the phylogeography of two ecologically important species, which can contribute to predicting how zooplankton communities and food‐web structure will manifest in the rapidly changing Arctic. [ABSTRACT FROM AUTHOR]

Firn on the Greenland Ice Sheet (GrIS) buffers meltwater, and has a variable thickness, complicating observations of volume change to mass change. In this study, we use a firn model (IMAU‐FDM v1.2G) forced by a regional climate model (RACMO2.3p2) to investigate how the GrIS firn layer thickness and pore space have evolved since 1958 in response to variability in the large‐scale atmospheric circulation. On interannual timescales, the firn layer thickness and pore space show a spatially heterogeneous response to variability in the North Atlantic Oscillation (NAO). Notably, a stronger NAO following the record warm summer of 2012 led the firn layer in the south and east of the ice sheet to regain thickness and pore space after a period of thinning and reduced pore space. In the southwest, a decrease in melt dominated after 2012, whereas in the east, the main driver was an increase in snow accumulation. At the same time, the firn in the northwestern ice sheet continued to lose pore space. The NAO also varies on intra‐annual timescales, being typically stronger in winter than in summer. This impacts the amplitude of the seasonal cycle in GrIS firn thickness and pore space. In the wet southeastern GrIS, most of the snow accumulates during the winter, when melting and densification are relatively weak, leading to a large seasonal cycle in thickness and pore space. The opposite occurs in other regions, where snowfall peaks in summer or autumn. This dampens the seasonal amplitude of firn thickness and pore space. Plain Language Summary: Most of the Greenland ice sheet's surface is covered by a layer of firn, which is compressed snow that can retain meltwater within its pores, and in this way buffer runoff into the ocean. Loss of pore space thus amplifies future Greenland ice‐sheet mass loss and accelerates sea‐level rise. We use computer models of the firn layer to investigate how firn has evolved from 1958 to 2020. The firn layer has gotten substantially thinner, less porous, and warmer over this period, but the changes are not the same everywhere. Moreover, after 2012, the firn in some regions recovered some of its lost pore space. These changes are linked to changes in the jet stream, a band of strong westerly winds encircling the Northern Hemisphere. A weaker and more meandering jet stream enhances the loss of pore space and vice versa. The jet stream also impacts the seasonal distribution of snowfall, and in this way, it influences the seasonal cycle of the firn depth and pore space. Key Points: The thickness and pore space of Greenland's firn layer respond spatially heterogeneously to changes in the North Atlantic Oscillation (NAO)After 2012, a more positive NAO led to thickening and cooling of the firn layer in some regions, but not everywhereThe amplitude of the seasonal cycle in firn thickness is linked to the seasonality of the polar jet stream [ABSTRACT FROM AUTHOR]