Abstract. We present two decades of monthly Antarctic mass change at a spatial resolution of (100 km)2, representing an order of magnitude improvement over the current state-of-the-art for gravity-derived estimates. This advancement is achieved by leveraging the accuracy of satellite gravimetry over larger spatial scales and the spatial resolution of altimetry at finer spatial scales. We do this through a formal data combination of satellite gravimetry and altimetry observations at the level of the gravimetry normal equations, estimating 1° circular disk mascon elements. The data combination yields independent mascon estimates, yielding effective spatial resolution of 1°. Gravimetry observations dominate the solution at long wavelengths, while altimetry observations are the dominant contributor at the spatial scale of an individual mascon. This yields a mass change solution that is closely aligned with gravimetry-only solutions from GRACE and GRACE-FO over larger spatial scales, yet spatially resolves mass change variability at finer scales. Additionally, we provide a set of gain factors to further downscale the mass within each mascon, providing monthly mass change estimates at a 1.92 km spatial scale. Uncertainties from measurement systems and model corrections (glacial isostatic adjustment, firn air content) are propagated through the data combination and presented. The methodology presented is extensible to other regions around the globe.

Abstract. Bottom topography strongly constrains ocean circulation in the Arctic, and both theory and numerical modeling suggest that nonlinear flow–topography interactions influence slope-following currents. Yet, how such interactions modify the circulation response to time-variable surface forcing remains poorly understood. Using idealized shallow-water simulations of flow over a corrugated slope in a re-entrant channel, we investigate how nonlinear features arise and evolve under oscillatory forcing. We observe both a persistent prograde flow bias (aligned with topographic Rossby wave propagation) relative to linear estimates, and an asymmetry in the circulation response, with retrograde flow (opposing wave propagation) exhibiting a saturation of flow strength once the flow reaches sufficiently strong velocities. To identify the mechanisms responsible for these behaviors, we evaluate integrated momentum budgets. Which terms appear as dynamically relevant, in addition to linear surface and bottom stresses, depends on the choice of integration path: when integrated along constant-depth contours, the nonlinear dynamics appear as a cross-slope relative vorticity flux, whereas integration along straight transects instead highlights momentum flux convergence and topographic form stress. These perspectives can be unified under quasi-geostrophic scaling as describing a flux of potential vorticity (PV). This PV flux is strongest during retrograde flow and predominantly down-slope, explaining the prograde bias. When retrograde velocities approach the arrest speed of topographic Rossby waves with wavelengths comparable to the corrugation scale, the flux increases sharply, halting further acceleration and producing the observed asymmetry. These results show how flow–topography interactions shape time-variable slope circulation, biasing the flow toward prograde states and limiting retrograde flow strength. Such effects are likely underrepresented in coarse-resolution numerical simulations, and highlight the need for improved representations of unresolved topographic interactions.

The integration of data from multiple satellite altimetry missions, each offering unique observational characteristics, has enabled us to discern both short-term variability and long-term climate trends affecting Greenland's peripheral glaciers and the Greenland Ice Sheet (GrIS). Our methodology, informed by lessons learned from analogous efforts in Antarctica and by improved incorporation of external velocity and elevation data to reduce bias in elevation-change estimates, ensures the consistency and reliability of the derived dataset. An additional enhancement of this product is the inclusion of a digital elevation model, which facilitates the analysis of changes in absolute elevation. The dataset covers the years 1992-2023 and is publicly available as part of NASA's Making Earth System Data Records for Use in Research Environments (MEaSUREs) Inter-Mission Time Series of Land Ice Velocity and Elevation (ITS_LIVE) project (10.5067/ICFVI7DKHZJV, Nilsson et al., 2026). Our analysis reveals significant patterns of mass loss across the GrIS. We find that the ice sheet and peripheral glaciers experienced average mass losses of -160 +/- 17 and -23 +/- 5 Gt a-1, respectively, over the 1992-2023 period, with notable temporal variations. Specifically, the early years of the record exhibit a positive mass balance, likely driven by anomalously positive surface mass balance. However, this trend reverses in later years, with a pronounced increase in mass-loss rates that highlights the accelerating impact of climate change on ice-sheet dynamics and surface mass balance. Moreover, our analysis underscores the importance of considering peripheral glaciers alongside the continental ice sheet when assessing overall mass trends. By incorporating data from peripheral glaciers, we provide a more comprehensive understanding of Greenland's total contributions to global sea-level rise. Our findings reveal not only the magnitude of mass loss but also its evolution through time, emphasizing the need for continued monitoring and research to better understand the impacts of climate change on Earth's cryosphere.

Abstract Ice–ocean interactions at Greenland Ice Sheet outlet glaciers exert critical control on ice-sheet mass loss and, consequently, global sea-level rise. The GEOEO North of Greenland 2024 icebreaker expedition acquired a breadth of data in the unsurveyed Victoria Fjord, north-western Greenland, to assess the causes of the rapid break-up of the C.H. Ostenfeld ice tongue in 2002, and the environmental changes since. The observations reveal inflows of warm (>0 °C) Atlantic water reaching the grounded ice margin. We propose that Atlantic warm water pulses entering the Arctic Ocean reach north-western Greenland, where they interact with glacier fronts, enhance ice discharge, and amplify the climate-driven retreat of marine-terminating glaciers. The collapse of C.H. Ostenfeld’s ice tongue, unlike the slower retreats of the ice tongues of neighbouring Petermann and Ryder glaciers, reflects unimpeded Atlantic water inflow due to the absence of a shielding bathymetric sill, a function of the region’s bedrock geology.

Ocean forcing of basal melt at the Greenland and Antarctic ice sheets remains a major source of uncertainty in climate ice sheet modelling. Previous efforts to represent these effects focused mainly on the properties of the ocean waters reaching the marine terminating glaciers as well as the near-ice boundary layer flows and processes at the ice-ocean interface. We use high resolution, three dimensional modelling to show the influence that rotational effects have on the fjords circulation and the melt rate distribution and compare the total melt to earlier estimates from two dimensional simulations. Furthermore we investigate the influence that the along and across fjord bathymetry of Greenlandic glacial fjords has on the exchange flow of the warm ocean waters towards the ice sheets and the glacially modified water toward the open ocean. We find that the circulation pattern produced by rotational effects has a profound effect on the distribution of the melt rate at the ice base, producing a concentrated outflow and a melt maximum at the eastern side of a fjord that opens to the open ocean in the north even in narrow fjords (width of the order of the local Rossby Radius). The bathymetry in the fjord has a restricting effect on the inflow of warm Atlantic water and hence on the temperature forcing at the ice base. We compare the inflow strengths for different fjord bathymetries to theoretical estimateion using hydraulic theory (Whitehead, 1998).

Submarine glacier melt rates of the Greenland Ice Sheet remain a major uncertainty in climate model projections of future sea level rise. Development of submarine melt parameterizations has to a high degree relied on ocean circulation modelling of glacial fjords, designed to quantify effects such as ocean thermal forcing and fjord-glacier geometry. Greenlandic fjords are relatively narrow, and it is frequently assumed that across-fjord flow variations are small enough to allow marine melt to be quantified with two-dimensional ocean-circulation models. Here, we present three-dimensional model simulations showing that the interplay between fjord-glacier geometry, side wall friction, and Earth's rotation makes the circulation in ice-shelf cavities three-dimensional even in narrow fjords. Remarkably, we find that Earth's rotation changes the flow pattern in the cavity below the ice shelf, leading to a decrease in the marine melt on a 10 km wide ice shelf by a factor of five compared to a non-rotating simulation. Our study prompts using three-dimensional model configurations of Greenlandic fjords.

As the number of in situ measurements of ocean currents in the central Arctic Ocean remains very limited, much of our understanding of the Arctic Ocean circulation is based on idealized wind-driven models. In this paper, we make use of the latest available hydrography and Mean Dynamic Topography to study the Arctic Ocean time-mean circulation. Key concepts such as to what degree the flow is steered by bathymetry, equivalent barotropic, and consistent in direction along isobaths are evaluated. Comparing along- and cross-isobath velocities, we find that while the former generally has a larger magnitude, they are locally comparable in many regions. In these regions, the estimated cross-isobath velocities imply vertical velocities that can be orders of magnitude larger than typical surface Ekman pumping velocities. Even so, we find that the surface and bottom flow is generally well-aligned along closed ambient potential vorticity contours spanning local basins as well as the entire Arctic Ocean. In some regions of the central Arctic Ocean, where the hydrographic coverage is relatively sparse, bathymetric alignment is stronger in the surface. Despite this, our results suggest that the circulation is essentially equivalent barotropic and even barotropic in some regions. At some locations where water enters or exits the deep Arctic Basin, along-isobath flow reversals are observed at the surface as well as the bottom. Finally, the direction and magnitude of the bottom flow in regions of anti-cyclonic surface circulation are found to be sensitive to the choice of data set.

The northern sector of the Greenland Ice Sheet contains some of the ice sheet's last remaining glaciers with floating ice tongues. One of these glaciers is Ryder Glacier, which has been relatively stable in recent decades, in contrast to the neighbouring Petermann and C.H. Ostenfeld glaciers. Understanding Ryder Glacier's future behaviour is important as ice-tongue loss could lead to acceleration and increased ice discharge. Meanwhile, it is unclear whether Greenland-wide modelling attempts are able to accurately resolve the influence of fjord and bedrock topography and small-scale variations in ice dynamics for a glacier like Ryder. To fill these gaps, here we conduct targeted high-resolution modelling of Ryder Glacier until the year 2300. We find that mass loss is dominated by discharge under a low-emissions scenario all the way to 2300, leading to a sea level contribution of between 0.8 and 2 mm depending on the amount of ocean warming. Discharge also plays a key role under a high-emissions scenario up until 2100, after which a strongly negative surface mass balance becomes the dominant driver of mass loss. This negative surface mass balance leads to a much higher sea level rise contribution by 2300 of between 44 and 52 mm, with little sensitivity to the range of ocean warming scenarios used in this study.

The Greenland Ice Sheet is currently the largest single contributor to global sea level rise, with recent decades having been characterised by an acceleration of mass loss. The Northern sector of the Greenland Ice Sheet has been relatively understudied, but is also the sector containing several of the last remaining ice tongues in Greenland. If these floating ice tongues are lost, the associated reduction in buttressing has the potential to lead to large increases in velocities and mass loss. One such glacier is Ryder glacier which, in contrast to the nearby Petermann glacier, has been reasonably stable in recent decades. As such, this glacier was targeted during the Ryder 2019 expedition with Swedish Icebreaker Oden, leading to a wealth of data on its present-day setting and Holocene history. In conjunction with this observational data, the numerical Ice Sheet and Sea Level System Model (ISSM) is used to investigate both the controls on glacier behaviour since 1900 and the likely trajectory of Ryder glacier towards 2100 under different emissions scenarios. The key focus is on understanding under which circumstances Ryder glacier may lose its ice tongue and what the impacts of this are likely to be in terms of glacier dynamics and sea level rise contribution.

Abstract. Satellite radar altimetry has provided continuous observations of Antarctic Ice Sheet (AIS) surface elevation change since 1992. However, uncertainties in radar-derived elevation estimates remain substantial, primarily due to the influence of local surface topography and time-variable signal penetration into snow and firn. The launch of the ICESat-2 laser altimetry mission in late 2018 established a new benchmark for high-accuracy surface elevation measurements, enabling inter-comparison with radar altimetry results and improved assessment of associated uncertainties. In this study, we use the ICESat-2 measurements to evaluate radar altimetry-derived elevation change estimates from CryoSat-2 over the 6 905 000 km2 large and relatively flat interior of the AIS, where topography-related errors are small. We apply a suite of radar-specific correction methods to the CryoSat-2 measurements, including multiple retracking algorithms and empirical corrections for the time-variable surface and volume scattering of the radar signal. We analyse a 5.5-year overlap period between ICESat-2 and CryoSat-2 (April 2019-October 2024) to assess how the different correction methods influence the CryoSat-2 surface elevation change estimates and their uncertainties. ICESat-2 observations indicate a thickening of 97 ± 4 km3 yr-1, coinciding with several events of excess snowfall during 2019-2024. All CryoSat-2 solutions yield systematically lower thickening trends, with the smallest bias (0.6 ± 1.0 cm yr−1 or 42 km3 yr-1) obtained using the AWI-ICENet1 convolutional neural network retracker. The remaining trend differences correlate with the ICESat-2 trend signal itself. We discuss possible causes of these systematic differences, one of which is the hypothesis that temporal variations in radar signal penetration associated to temporal variations in snow properties continue to induce systematic errors in inferred surface elevation changes. If the mean trend difference here were representative of the entire grounded AIS (12 352 700 km2), it would correspond to an underestimation of AIS volume and mass trends by approximately 74 km3 yr−1 and 28 Gt yr−1, respectively. These results underscore the challenges of using radar altimetry to resolve subtle, long-term trends related to surface mass balance changes, while also demonstrating the potential of combined laser-radar altimetry analysis to reduce uncertainties in AIS volume and mass balance estimates.

The Greenland Ice Sheet's negative mass balance is driven by a sensitivity to a warming atmosphere and ocean. The fidelity of ice-sheet models in accounting for ice-ocean interaction is inherently uncertain and often constrained against recent fluctuations in the ice-sheet margin from the previous decades. The geological record can be used to contextualise ice-sheet mass loss and understand the drivers of changes at the marine margin across climatic shifts and previous extended warm periods, aiding our understanding of future ice-sheet behaviour. Here, we use the Ice-sheet and Sea-level System Model (ISSM) to explore the Holocene evolution of Ryder Glacier draining into Sherard Osborn Fjord, North Greenland. Our modelling results are constrained with terrestrial reconstructions of the paleo-ice-sheet margin and an extensive marine sediment record from Sherard Osborn Fjord that details ice dynamics over the past 12.5 ka years. By employing a consistent mesh resolution of <1 km at the ice-ocean boundary, we assess the importance of atmospheric and oceanic changes to Ryder Glacier's Holocene behaviour. Our simulations show that the initial retreat of the ice margin after the Younger Dryas cold period was driven by a warming climate and the resulting fluctuations in surface mass balance. Changing atmospheric conditions remain the first-order control in the timing of ice retreat during the Holocene. We find ice-ocean interactions become increasingly fundamental to Ryder's retreat in the mid-Holocene, with higher-than-contemporary melt rates required to force grounding line retreat and capture the collapse of the ice tongue during the Holocene Thermal Maximum. Regrowth of the tongue during the neoglacial cooling of the late Holocene is necessary to advance the terrestrial and marine margins of the glacier. Our results stress the importance of accurately resolving the ice-ocean interface in modelling efforts over centennial and millennial timescales, in particular the role of floating ice tongues and submarine melt, and provide vital analogies for the future evolution of Ryder in a warming climate.

Ocean-driven submarine basal melt of the Greenland Ice Sheet remains a major source of uncertainty in climate projections of future sea level rise based on ice sheet modeling. State-of-the-art parameterizations focus on the properties of the ocean water reaching the marine-terminating glaciers, processes at the ice-ocean interface and subglacial discharge to estimate submarine basal melt. This study uses 2-dimensional, nonrotating, high-resolution ocean simulations complemented by theory and observations to quantify two effects of fjord bathymetry (sills) on the thermal forcing with consequences for the basal melt. These effects are the recirculation (reflux) of glacially modified water due to hydraulic control at the sill (which we study in detail) and the cooling of the inflow due to the restriction of the deepest and warmest Atlantic water inflow. For a fixed sill depth, an increase in subglacial discharge can make the exchange flow hydraulically controlled. Our results suggest that basal melt parameterizations should account for fjord bathymetry, and outline a road map to guide future parameterization developments relevant to narrow fjords.

We consider upwelling and downwelling dynamics in an idealized ocean model configuration of the Western Gotland Basin in the Baltic Sea, featuring a gently sloping bottom in the west and a steep bathymetry in the east. Typical transient wind conditions and seasonally variable stratification are examined. Upwelling and downwelling jets develop at the coastal boundaries and interact through cross-shore boundary-layer flows. Initial evolution of the coastal jets is consistent with linear theory. The front position and the onset of instability is governed by the wind forcing, with a weak dependence on seasonal stratification. The unstable growth rates and wavelengths over the slope depend on the relative orientation of the slope and isopycnals, consistent with theory. The upwelling jets become baroclinically unstable during the wind-forced phase, whereas instability onset for downwelling on the slope is after 2-3 weeks (during the relaxation phase). The downwelling on the steep side is consistently stable. The regime with unstable upwelling on the slope side with concurrent stable downwelling on the steep side is more frequent (southwesterly winds: 30% occurrence) and leads to strong cross-shore transport. Unstable downwelling on the slope with upwelling on the steep side is a rarer event (northwesterly winds: 10% occurrence) and generates strong vertical mixing on the slope, with implications for oxygen and nutrient fluxes on the inner shelf along the Swedish coast. Baroclinic eddies contribute to elevated vertical mixing in the surface layer.

Coupling paleo numerical simulations of the Greenland Ice Sheet with physical geological evidence of past ice sheet extent can greatly improve our understanding of the factors driving ice loss. Geological observations can be used to reconstruct the state of the Greenland Ice Sheet at snap shots in time, thus acting as constraints to test the fidelity of ice sheet models that can tell a continuous story of retreat over the same geologic timescales. Swedish Ice Breaker Oden’s visit to Sherard Osborn Fjord and Ryder Glacier in 2019 collected a plethora of marine-geological data that describes the glacier’s behaviour and retreat during the Holocene. Here we use a 3D thermo-coupled Higher-Order ice flow module incorporated in the Ice-sheet and Sea-level System Model (ISSM) to simulate the dynamics of Ryder Glacier from 12500 ka to present day. By focusing on a specific individual glacier, we can run the model at resolutions

The Arctic Ocean is a key component of Earth's climate system, and an understanding of ocean dynamics in this region is central for predicting how the Arctic is responding to a changing climate. In this study, we examine the ocean circulation in a high-resolution numerical model of the Arctic Ocean and Nordic Seas. Based on what is observed in this simulation, we reexamine an existing idealized linear model estimating the time-variable large-scale circulation in ocean basins, and test it against the highly nonlinear numerical model. The idealized model is an integral relation derived from the linear momentum equations and assumes that the circulation around a closed depth contour is driven by surface stresses and regulated by bottom friction. We show that the idealized model estimates agree very well with the numerical simulations. This indicates that much of the variability of the large-scale circulation can be explained by linear processes. In particular, a correct description of the net surface stress over partially ice-covered areas improves the correlation between linear model and numerical simulations significantly in the Arctic Ocean compared to a previous study. However, undetected in that previous study, we now find that the linear model might be lacking a cyclonic tendency.

The inflow of warm waters into the Nordic Seas, crucial for sustaining the climate-regulating Atlantic overturning circulation, can be reconstructed from hydrography using a north-south dynamic height gradient across the Greenland-Scotland Ridge. Variations in this influx are herein linked to northward-propagating thermohaline anomalies, initially observed at the intergyre boundary and likely driven by changes in ocean heat transport. As these anomalies reach the eastern subpolar North Atlantic, they modulate the cross-ridge dynamic height difference, thereby influencing both the Atlantic inflow and the Nordic Seas overflows on multi-year to decadal scales. Thus, these thermohaline anomalies play a dynamically active role in modulating the watermass exchanges across the ridge and downstream along the Atlantic Water path, rather than being a simple passive train of signals. This explains why these thermohaline signals are a key source of climate predictability and provides fresh insights into the functioning of the Nordic Seas overturning circulation from observations.