Keith W. Nicholls

Surface melting and runoff account for about half of the current mass loss of the Greenland Ice Sheet. Regional climate models (RCMs) project runoff to increase further over the 21st century, but the magnitude of this trend varies strongly between different models. This variability arises because RCMs rely on simplified representations of the complex firn hydrological system in Greenland’s percolation zone. However, key parameters for parametrizing meltwater retention and runoff processes remain poorly constrained due to a lack of time-resolved, in situ observations of firn liquid water content.We address this gap by demonstrating that the Autonomous phase-sensitive Radio-Echo Sounder (ApRES) can continuously trace the amount of liquid water in the firn. At three automatic weather station sites on the ice sheet (KAN_U, DYE-2 and Camp Century), we acquired hourly ApRES time series between spring 2023 and 2025, covering two melt seasons. By analyzing these observations in combination with a firn model, we quantify rates of lateral meltwater flow. Comparison with runoff simulations from three RCMs shows that all models overestimate local runoff at KAN_U, and that some even predict runoff at DYE-2 (2124 m a.s.l.), where our observations indicate that all meltwater is refrozen. Expanding these observations will support the development of improved representations of Greenland’s firn hydrological system in RCMs and ultimately enhance the accuracy of GrIS mass balance projections.

Continuous moored time series of temperature, salinity, pressure and current speed and direction are of great importance for understanding the continental shelf and under-ice-shelf dynamics and thermodynamics that govern water mass transformations and ice melting in and around Antarctic marginal seas. In these regions, icebergs and sea ice make ship-based mooring deployment and recovery challenging. Nevertheless, over decades, expeditions around the fringe of Antarctica sporadically deployed and recovered hundreds of moored instruments, including those facilitated through ice shelves boreholes. These datasets tend to be archived in a wide range of data centres, with, to our knowledge, no clear format standardisation. As a result, systematic analysis of historical mooring time series in the marginal seas is often challenging. Here we present the first version of a standardised pan-Antarctic moored hydrography and current time series compilation, with broad international contributions from data centres, research institutes and individual data owners. The mooring records in this compilation span over five decades, from the 1970s to the 2020s, providing an opportunity for a systematic study of the pan-Antarctic water mass transport and shelf connectivity. As a demonstration of the utility of this compilation, we present spectral analysis of the compiled current velocity time series, which unsurprisingly shows the dominating presence of tidal variability within most records. This component of the variability is fitted using multi-linear regression to tidal frequencies, and the tidal fit is removed from the original time series to leave de-tided variability. Given the limited record durations to months to years, de-tided variability is dominated by synoptic (3–10 d period), intraseasonal (10–80 d) and seasonal (∼6 months–1 year) signals. The spatial distribution of the kinetic energy integrated within frequency bands is presented and discussed within respective regional contexts, and future avenues of research are proposed. This data compilation is assembled under the endorsement of Ocean-Cryosphere Exchanges in ANtarctica: Impacts on Climate and the Earth System (OCEAN ICE) project (https://ocean-ice.eu/, last access: 23 October 2025) funded by the European Commission and UK Research and Innovation. It is available and regularly updated in NetCDF format with the SEANOE database at https://doi.org/10.17882/99922 (Zhou et al., 2024a).

This study examines the interplay between water column structure, tidal currents, and basal melting at a site beneath Ronne Ice Shelf, using a 3‐year data set of oceanographic measurements, and a collocated year‐long time series of radar‐derived melt rate estimates. Currents at the site are characterized by mixed semidiurnal tides with strong spring‐neap variability, superimposed on a nontidal flow. The product of current speed and thermal driving, both measured approximately 19 m from the ice base, explains 88% of the melt rate variability. Although current speed is the dominant driver of this variability, thermal driving also contributes non‐negligibly on spring‐neap and longer timescales. The semidiurnal tidal ellipses feature marked vertical variations, transitioning from nearly rectilinear in the mid‐water column to more circular and clockwise (CW)‐rotating near the ice. This depth‐dependence of the semidiurnal tide is attributed to the differential influence of boundary friction on the CW and anticlockwise (ACW) rotary components near the critical latitude (where the tidal frequency equals the Coriolis frequency). A theoretical model, which assumes depth‐independent eddy viscosity, successfully reproduces the observed 3‐year mean vertical structure of the tidal ellipses. Considering the total tidal current rather than individual constituents, ice base friction damps both the time‐mean flow speed and the tidal fluctuations, with attenuation varying over the spring‐neap cycle, peaking during spring tides. The observed latitude‐ and time‐dependent effects of ice base friction on the barotropic tide are not captured in parameterizations that estimate tide‐induced friction velocity by scaling the time‐averaged barotropic tidal speed with a constant drag coefficient.

Societal adaptation to rising sea levels requires robust projections of the Antarctic Ice Sheet’s retreat, particularly due to ocean-driven basal melting of its fringing ice shelves. Recent advances in ocean models that simulate ice-shelf melting offer an opportunity to reduce uncertainties in ice–ocean interactions. Here, we compare several community-contributed, circum-Antarctic ocean simulations to highlight inter-model differences, evaluate agreement with satellite-derived melt rates, and examine underlying physical processes. All but one simulation use a melting formulation depending on both thermal driving (T ^⋆) and friction velocity (u^⋆), which together represent the thermal and ocean current forcings at the ice–ocean interface. Simulated melt rates range from 650 to 1277 Gt year⁻¹ (m = 0.45 − 0.91 m year⁻¹), driven by variations in model resolution, parameterisations, and sub-ice shelf circulation. Freeze-to-melt ratios span 0.30 to 30.12 %, indicating large differences in how refreezing is represented. The multi-model mean (MMM) produces an averaged melt rate of 0.60 m year⁻¹ from a net mass loss of 842.99 Gt year⁻¹ (876.03 Gt year⁻¹ melting and 33.05 Gt year⁻¹ refreezing), yielding a freeze-to-melt ratio of 3.92 %. We define a thermo-kinematic melt sensitivity, ζ = m/(T ^⋆ u^⋆) = 4.82 × 10⁻⁵ °C⁻¹ for the MMM, with individual models spanning 2.85 × 10⁻⁵ to 19.4 × 10⁻⁵ °C⁻¹. Higher melt rates typically occur near grounding zones where both T ^⋆ and u^⋆ exert roughly equal influence. Because friction velocity is critical for turbulent heat exchange, ice-shelf melting must be characterised by both ocean energetics and thermal forcing. Further work to standardise model setups and evaluation of results against in situ observations and satellite data will be essential for increasing model accuracy, reducing uncertainties, to improve our understanding of ice-shelf–ocean interactions and refine sea-level rise predictions.

Societal adaptation to rising sea levels requires robust projections of the Antarctic Ice Sheet’s retreat, particularly due to ocean-driven basal melting of its fringing ice shelves. Recent advances in ocean models that simulate ice-shelf melting offer an opportunity to reduce uncertainties in ice–ocean interactions. Here, we compare several community-contributed, circum-Antarctic ocean simulations to highlight inter-model differences, evaluate agreement with satellite-derived melt rates, and examine underlying physical processes. All but one simulation use a melting formulation depending on both thermal driving (T ^⋆) and friction velocity (u^⋆), which together represent the thermal and ocean current forcings at the ice–ocean interface. Simulated melt rates range from 650 to 1277 Gt year⁻¹ (m = 0.45 − 0.91 m year⁻¹), driven by variations in model resolution, parameterisations, and sub-ice shelf circulation. Freeze-to-melt ratios span 0.30 to 30.12 %, indicating large differences in how refreezing is represented. The multi-model mean (MMM) produces an averaged melt rate of 0.60 m year⁻¹ from a net mass loss of 842.99 Gt year⁻¹ (876.03 Gt year⁻¹ melting and 33.05 Gt year⁻¹ refreezing), yielding a freeze-to-melt ratio of 3.92 %. We define a thermo-kinematic melt sensitivity, ζ = m/(T ^⋆ u^⋆) = 4.82 × 10⁻⁵ °C⁻¹ for the MMM, with individual models spanning 2.85 × 10⁻⁵ to 19.4 × 10⁻⁵ °C⁻¹. Higher melt rates typically occur near grounding zones where both T ^⋆ and u^⋆ exert roughly equal influence. Because friction velocity is critical for turbulent heat exchange, ice-shelf melting must be characterised by both ocean energetics and thermal forcing. Further work to standardise model setups and evaluation of results against in situ observations and satellite data will be essential for increasing model accuracy, reducing uncertainties, to improve our understanding of ice-shelf–ocean interactions and refine sea-level rise predictions.

The southern Weddell Sea and the Filchner Ice Shelf cavity are locations of dense bottom water production and are thus connected to the global climate system. However, it has been suggested that increased heat transport from the deep ocean onto the continental shelf and towards the ice cavities would disrupt the dense water production and increase ice shelf melt rates. Processes that affect the southward heat transport are, therefore, important to understand. Sudden strong westward ocean surface stress events – "storms" – are suggested to drive enhanced southward transport of modified Warm Deep Water across the continental shelf in the Filchner Trough region in the southeastern Weddell Sea. We use a mooring network with up to four-year-long mooring records from the region to investigate how the ocean circulation responds to storm events. We find that about 70 % of the events that last longer than four days, have a cumulative westward stress increase larger than 0.4N m⁻² day⁻¹, and a maximum stress above 0.25N m⁻² leads to a significant increase in the speed of the Antarctic Slope Current (ASC) just upstream of Filchner Trough. Roughly one-third of the identified storm events cause an increased southward current speed on the shelf. At the southernmost mooring, 76° S, storm responses are observed mainly during the latter part of the record (mid-2019 to early 2021). This interannual variability in storm response indicates a potential dependency on background hydrography and circulation that remains to be fully explained. This study highlights the potential importance of storms for southward heat transport towards the Antarctic ice shelves. Warm water that is present on the continental shelf during a storm will likely be pushed southward by the enhanced circulation, increasing the southward heat transport and the likelihood that it reaches the ice shelf front before the heat is lost to the atmosphere during winter.

Using radar data from the Beardmore Glacier grounding zone, we image a narrow subglacial channel ( m wide) that reaches a height of 200 m above the ambient ice‐shelf draft. Using repeat ICESat‐2 observations and Worldview digital elevation models, we show that this channel we observe with radar is part of a system of channels. These channels form near the grounding zone where the axis of the channels runs up‐gradient in smoothed ice base elevation (perpendicular to smoothed ice base elevation contours). Downstream, these features are advected with the flow and expressed as Eulerian surface elevation change in differenced co‐registered digital elevation models. Continuity calculations indicate that melt rates within the channel are at least 20 m . Idealized one‐dimensional plume modeling indicates these melt rates require substantial meltwater discharge and are geographically continuous extensions of subglacial conduits we image upstream of the grounding zone. These basal‐melt rates are higher than the ambient basal‐melt rates in the Ross. Asymmetric melt across the width of the channel suggests there is cross‐channel ocean boundary current that may affect the efficiency of energy exchange across the ice‐shelf ocean boundary layer within the channel. This is consistent with recent model experiments that suggest ice shelf basal channel shape determines channelized ice‐ocean interactions.

Societal adaptation to rising sea levels requires robust projections of the Antarctic Ice Sheet’s retreat, particularly due to ocean-driven basal melting of its fringing ice shelves. Recent advances in ocean models that simulate ice-shelf melting offer an opportunity to reduce uncertainties in ice–ocean interactions. Here, we compare several community-contributed, circum-Antarctic ocean simulations to highlight inter-model differences, evaluate agreement with satellite-derived melt rates, and examine underlying physical processes. All but one simulation use a melting formulation depending on both thermal driving (T ^⋆) and friction velocity (u^⋆), which together represent the thermal and ocean current forcings at the ice–ocean interface. Simulated melt rates range from 650 to 1277 Gt year⁻¹ (m = 0.45 − 0.91 m year⁻¹), driven by variations in model resolution, parameterisations, and sub-ice shelf circulation. Freeze-to-melt ratios span 0.30 to 30.12 %, indicating large differences in how refreezing is represented. The multi-model mean (MMM) produces an averaged melt rate of 0.60 m year⁻¹ from a net mass loss of 842.99 Gt year⁻¹ (876.03 Gt year⁻¹ melting and 33.05 Gt year⁻¹ refreezing), yielding a freeze-to-melt ratio of 3.92 %. We define a thermo-kinematic melt sensitivity, ζ = m/(T ^⋆ u^⋆) = 4.82 × 10⁻⁵ °C⁻¹ for the MMM, with individual models spanning 2.85 × 10⁻⁵ to 19.4 × 10⁻⁵ °C⁻¹. Higher melt rates typically occur near grounding zones where both T ^⋆ and u^⋆ exert roughly equal influence. Because friction velocity is critical for turbulent heat exchange, ice-shelf melting must be characterised by both ocean energetics and thermal forcing. Further work to standardise model setups and evaluation of results against in situ observations and satellite data will be essential for increasing model accuracy, reducing uncertainties, to improve our understanding of ice-shelf–ocean interactions and refine sea-level rise predictions.

Societal adaptation to rising sea levels requires robust projections of the Antarctic Ice Sheet’s retreat, particularly due to ocean-driven basal melting of its fringing ice shelves. Recent advances in ocean models that simulate ice-shelf melting offer an opportunity to reduce uncertainties in ice–ocean interactions. Here, we compare several community-contributed, circum-Antarctic ocean simulations to highlight inter-model differences, evaluate agreement with satellite-derived melt rates, and examine underlying physical processes. All but one simulation use a melting formulation depending on both thermal driving (T ^⋆) and friction velocity (u^⋆), which together represent the thermal and ocean current forcings at the ice–ocean interface. Simulated melt rates range from 650 to 1277 Gt year⁻¹ (m = 0.45 − 0.91 m year⁻¹), driven by variations in model resolution, parameterisations, and sub-ice shelf circulation. Freeze-to-melt ratios span 0.30 to 30.12 %, indicating large differences in how refreezing is represented. The multi-model mean (MMM) produces an averaged melt rate of 0.60 m year⁻¹ from a net mass loss of 842.99 Gt year⁻¹ (876.03 Gt year⁻¹ melting and 33.05 Gt year⁻¹ refreezing), yielding a freeze-to-melt ratio of 3.92 %. We define a thermo-kinematic melt sensitivity, ζ = m/(T ^⋆ u^⋆) = 4.82 × 10⁻⁵ °C⁻¹ for the MMM, with individual models spanning 2.85 × 10⁻⁵ to 19.4 × 10⁻⁵ °C⁻¹. Higher melt rates typically occur near grounding zones where both T ^⋆ and u^⋆ exert roughly equal influence. Because friction velocity is critical for turbulent heat exchange, ice-shelf melting must be characterised by both ocean energetics and thermal forcing. Further work to standardise model setups and evaluation of results against in situ observations and satellite data will be essential for increasing model accuracy, reducing uncertainties, to improve our understanding of ice-shelf–ocean interactions and refine sea-level rise predictions.