Abstract. We present the first multi-model study of the Antarctic sea ice response to enhanced meltwater due to dynamic mass loss from the Antarctic ice sheet. This meltwater flux (and its future increase under global warming) is not included in the most recent state-of-the-art climate model simulations used in CMIP6, representing a missing source of freshwater to the Southern Ocean. Previous climate model simulations have shown a wide range of responses in Antarctic sea ice and climate when this missing meltwater is introduced. Here, we analyze a new suite of 11 models comprising 43 ensemble members to assess the response to 0.1 Sv of Antarctic meltwater input at the ocean surface, evenly distributed around the Antarctic coastline under pre-industrial control forcing. Antarctic sea ice area increases in all models. However, there is a wide range in the response, with annual mean increases ranging from 0.71 to 4.14 million km2. There is also substantial variation in both the spatial distribution and the time scale of the sea ice response. The intermodel spread in sea ice response is influenced by the model mean-state sea ice area and volume, the prevalence of open-ocean deep convection, and the mean-state stratification of the ocean. These findings highlight the importance of model mean-state biases in determining the response to a missing Antarctic meltwater boundary condition.

Ocean-driven basal melting of Antarctic ice shelves plays an important role in the mass loss of the Antarctic Ice Sheet. Ice shelf cavity-resolving ocean models are a valuable tool for understanding ice shelf-ocean interactions and for simulating projections of ice shelf and ocean states under future climate. Designed to assess the current state of ice shelf-ocean modelling, the second Ice Shelf-Ocean Model Intercomparison Project, ISOMIP+, consists of 12 ocean model configurations submitted with a common, idealised experimental setup. Here, we focus on the experiments Ocean0-2 , which are ocean models with idealised, static ice shelf geometries, but where the ocean reaches a balance with prescribed far-field ocean conditions. Different thermal transfer coefficient values (ranging from 0.011 to 0.2) are used for each model in the melting parameterisation to achieve a common, tuned melt rate since the models cover a range of types of vertical coordinates, ice-ocean boundary layer treatments, and numerical schemes. These model differences lead to spread in the resultant ocean properties, circulation, boundary-layer structure and spatial distribution of melting. We also highlight similarities between models, such as a shared linear relationship across most models between melt rate and overturning and barotropic streamfunctions during the spin-up and spin-down, demonstrating a robust relationship between melt and circulation across models and forcing conditions. The ISOMIP+ results provide a systematic comparison of ice shelf cavity-capable ocean models. However, we also demonstrate the need for realistic ice shelf-ocean model intercomparison projects (some already underway) to assess model biases and inter-model variation against sparse observations. Further research is needed to understand the differences between models and further improve our modelled representations of the ice-ocean boundary layer and ice shelf cavity circulation.

In past decades, substantial advances on understanding ocean–ice-shelf interactions have been made, and a number of parameterisations that provide sub-shelf melting for ice sheet modelling studies have been developed. Through ISMIP6, it was found that the choice of parameter values in melt parameterisations can influence the order of magnitude of melt rate changes in projections. Moreover, it has been shown that constraining those parameters with present-day observations is not sufficient to constrain melt rate changes under future warming. For ISMIP7, we hence propose a “come-as-you-are” approach for the choice of the sub-shelf melt parameterisation, but suggest a protocol for calibrating parameters using ocean model simulations and observations that show large changes in cavity temperatures as additional constraints. This is embedded into an updated and revised protocol for processing CMIP model data for the ISMIP7 Antarctic ice-ocean forcing protocol. Come to the presentation if you are interested to learn (more) about the protocol, discuss your testing experiences, or provide feedback.

Ocean microstructure measurements collected during three austral summers (2023-2025) along the Antarctic continental slope off Dronning Maud Land show enhanced subsurface mixing. Mean turbulent dissipation between 100 and 800 m depth is an order of magnitude higher than in the open ocean, with an extreme event reaching W at mid-depth. Elevated dissipation coincides with peaks in vertical velocity shear during periods of strong tidal acceleration associated with spring-tide flow reversals. Enhanced continental slope mixing drives a mean upward heat flux of 3 W into the base of cold surface waters, in agreement with independent estimates from an internal tide model. Combined with reanalysis data, the model suggests tidal mixing along the Antarctic continental slope could produce a circumpolar mean vertical heat flux of 9 W . This upward heat transport may warm the upper ocean and limit sea ice formation around Antarctica.

Abstract Sea ice is a crucial component of polar climate systems and is undergoing substantial changes in both hemispheres due to evolving climatic conditions. Arctic sea ice is transitioning from perennial to seasonal cover, and the Southern Ocean sea ice is exhibiting recent minima and enhanced seasonality. As global warming continues, the role of sea ice in polar climate systems is expected to transform further. However, many theoretical frameworks and parameterizations in current sea‐ice models are based on observations from an earlier era dominated by thicker multi‐year ice. Here, we synthesize the physical processes governing the dynamics and thermodynamics of drift ice—the mobile pack ice—and its coupling with the atmospheric and oceanic boundary layers. Our goal is to provide a coherent theoretical framework of the sea‐ice evolution equations and to summarize parameterizations of sub‐grid processes used across models of varying complexity. These include representations of momentum and scalar fluxes, ice‐thickness distribution and redistribution, snow and melt‐pond processes, wave–ice interactions, and physical–biogeochemical feedbacks. We also examine how sea ice impacts ocean stratification and mixing, as well as the atmospheric boundary layer and clouds. Finally, we highlight recent observational findings and outline priorities for improving the representation of drift‐ice processes, particularly in light of the changing climate and ice state.

The increasing release of Antarctic meltwater represents one of the most profound, yet uncertain, consequences of global climate change. The absence of interactive ice sheets in state-of-the-art climate models prevents the direct calculation of ice-ocean feedbacks, leaving significant uncertainty in the global and regional consequences of meltwater discharge. This study leverages results from the Southern Ocean Freshwater Input from Antarctica (SOFIA) initiative to assess the ocean response to a 0.1 Sv meltwater perturbation and infer the feedback on ice shelf basal melting across 10 CMIP6 models. We analyze meltwater-induced temperature anomalies across distinct continental shelf regimes and compare them with SSP5-8.5 warming-induced anomalies. We then translate these anomalies into basal melt rates using a parameterization calibrated with a new observational climatology, which reveals strongly regional melt sensitivities that cannot be captured with an Antarctic-wide coefficient. Although the meltwater feedback is generally thought to amplify basal melting, our results demonstrate large regional differences, with implied enhanced ice shelf mass loss in some sectors but suppressed basal melting in others. The model ensemble indicates a warming feedback on the continental shelf in most East Antarctic regions, whereas in West Antarctica, most models simulate either cooling or reduced warming, suggesting a negative feedback. This regional contrast implies that East Antarctica may play an increasingly dominant role in future ice shelf mass loss. Simulations support existing hypotheses linking these asymmetric temperature responses to strong regional connectivity and shelf-break dynamics, including a strengthened Antarctic Slope Front, an accelerated Antarctic Slope Current, and reduced dense shelf water formation.

The stability of Antarctic ice shelves, which regulate the flow of grounded ice into the ocean, depends critically on ocean-driven basal melting. Basal channels, widespread features beneath many ice shelves, modulate ice-shelf basal melt rates and influence ice-shelf stability, yet their oceanic drivers remain poorly understood. Using high-resolution simulations of a cold-water ice shelf cavity, we show that interactions between circulation and channelized topography generate localized overturning that traps intruding warm Circumpolar Deep Water (CDW) beneath the ice, amplifying melt rates by an order of magnitude within channels. This ocean-driven process significantly enhances the sensitivity of the ice shelf basal mass loss to ocean warming, and the resulting differential melting promotes channel growth, with the potential to undermine the structural stability of the deeper part of the ice shelf. Our results reveal a key mechanism for basal channel evolution and indicate that even modest CDW intrusions could have important implications for the stability of cold Antarctic ice shelves.

Theme 1 addresses the Southern Ocean’s (south of 30°S) critical role in regulating Earth’s climate through circulation patterns that mediate global exchanges of heat, carbon, freshwater, and nutrients. The region has absorbed over 70% of anthropogenic heat and has contributed to about 40% of the global ocean uptake of human-emitted carbon dioxide (CO₂) while also controlling ice shelf stability and sea level rise. However, fundamental gaps in year-round observations, particularly during austral autumn, winter, and spring, severely limit our understanding of these processes and their responses to rapid climate change. This white paper establishes key knowledge gaps and high-priority recommendations that are tractable within the InSync timeframe through coordinated program execution with strong engagement from national operators and funding agencies. Critical needs include seasonal observations of the marginal ice zone where carbon, heat and nutrient-rich waters upwell, year-round continental shelf measurements where dense water formation and ice-ocean interactions occur, standardized air-sea flux measurements, and strategic monitoring of regional choke points. Success requires international resource sharing, coordinated deployments, and sustained commitment to both process studies and long-term monitoring.

Basal melting of ice shelves is fundamental to Antarctic ice sheet mass loss, yet direct observations remain sparse. We present the first year‐round melt record (2017–2021) from a phase‐sensitive radar on Fimbulisen, one of the fastest flowing ice shelves in Dronning Maud Land, East Antarctica. The observed long‐term mean ablation rate at 350 m depth below the central ice shelf was 1.0 ± 0.5 m yr−1, marked by substantial sub‐weekly variability ranging from 0.4 to 3.5 m yr−1. 36‐h filtered basal melt rate fluctuations closely align with ocean velocity. On seasonal time scales, melt rates peak during austral spring to autumn (September–March), driven by both elevated ocean velocities and thermal driving near the base. The combined effect of thermal driving and current speed explains the majority of the melt rate variability (r = 0.84), highlighting the dominant role of shear‐driven turbulence. This relationship enables parameterization of melt rates for the decade‐long ocean record (2010–2021), although deviations appear under low and high forcing conditions. Both observed and parameterized melt rates show similar yearly mean magnitudes compared to satellite‐derived melt rates but with a tenfold lower seasonal amplitude and a 3‐month delay in seasonality. These detailed concurrent ice–ocean observations provide essential validation data for remote sensing and numerical models that aim to quantify and project ice‐shelf response to a change in ocean forcing. In situ measurements and continued monitoring are crucial for accurately assessing and modeling future basal melt rates, and for understanding the complex dynamics driving ice‐shelf stability and sea‐level change.

The ongoing increase of global mean temperature, caused by anthropogenic CO2 emissions, will most likely lead to enhanced melting and calving of Antarctic ice shelves in the coming decades. As a consequence, the freshwater input into the Southern Ocean is expected to increase as well. The resulting change in ocean salinity could have significant consequences for ocean circulation, water column stratification, and water mass formation in the Southern Ocean, which are all expected to affect the capacity of the surface ocean to remove CO2 from the atmosphere, and the sequestration of carbon in the deep ocean. However, the magnitude and spatio-temporal patterns of these changes and their links to freshwater forcing are not yet well understood. To reduce these uncertainties, increase our understanding, and better quantify the feedbacks on the climate system, the international SOFIA initiative (Swart et al., 2023) defines freshwater input protocols for consistent use in various Earth System Models. Here we study the impact of additional freshwater around Antarctica on circulation and carbon fluxes in a steady preindustrial climate state using four Earth System Models. Most of the models show a decrease in the uptake of CO2 by the surface of the Southern Ocean, caused by a strengthened outgassing of natural CO2 between 50°S and 60°S. The stronger outgassing can be attributed to an increase in sub-surface dissolved inorganic carbon concentration south of the Antarctic Circumpolar Current that is associated with a redistribution of water masses in the Southern Ocean. Furthermore the reduction of the production and downward flow of Antarctic Bottom Water is leading to a decrease of its volume, and the expansion of carbon-rich Circumpolar Deep Water, which increases the carbon content at depth and thus weakens the overall CO2 uptake. However, the models disagree in terms of the intensity of the weakened Southern Ocean CO2 uptake. This difference seems to be mainly linked to the model resolution and the representation of the ocean mean state, e.g. the strength of the stratification, which is a determining factor for the redistribution of the additional freshwater to depth. To pursue this work, experiments with additional freshwater forcing in various climate states are conducted to analyse the ocean carbon cycle’s response and quantify potential climate feedbacks.

The effects of freshwater river runoff on dynamics of ice-covered brackish lakes have not been adequately studied to date. Compared to freshwater lakes, the circulation patterns in brackish lakes are complicated by non-linear effects of temperature and salinity on density stratification and mixing, and as a result on the ice melt. Quantifying these effects is essential for understanding circulation of large endorheic lakes in cold regions and their ecological and physical characteristics. We present modeling results on circulation caused by river runoff in a typical ice-covered brackish lake obtained with the Regional Ocean Modeling System (ROMS). The lake water salinity was set to 14 practical salinity units (PSU). In the initial state, the water temperature increased linearly from the freezing point at the surface to the temperature of maximum density, at the bottom, both accounting for the water salinity. Mixing of cold freshwater river inflow with the warmer saline waters produces negative buoyancy and downslope flow of dense currents near the river inlets with a secondary geostrophically-balanced circulation throughout the lake. We use the modeling results to quantify the contribution of this circulation mechanism on deep lake circulation and ventilation of the near-bottom waters.

The increasing release of meltwater from Antarctica represents one of the most profound yet uncertain consequences of global climate change. The lack of interactive ice sheets in state-of-the-art climate models, including those participating in the Coupled Model Intercomparison Project (CMIP6), combined with the inadequate representation of key processes driving ice shelf basal melting, prevents the direct calculation of ice-ocean feedbacks and leaves a high uncertainty on the magnitude and impacts of meltwater discharge. Previous studies that explored meltwater impacts produced partially contradictory findings, largely relied on experiments with single models, had inconsistent experimental designs, and imposed varying freshwater forcing rates. To address these shortcomings, this study employs results from the new "Southern Ocean Freshwater Input from Antarctica” (SOFIA) initiative to assess the effect of meltwater-induced ocean warming on basal melting and potential future Antarctic mass loss. We evaluate the ocean response to meltwater across a suite of 10 CMIP6 models and compare it to future scenarios simulations without additional meltwater (SSP5-8.5), assessing model bias and both meltwater- and global warming-induced anomalies in the Southern Ocean. Applying these anomalies to a regional basal melting parameterization, constrained by a new observational hydrographic climatology, our findings reveal that meltwater feedbacks amplify warming on the continental shelf and enhance ice loss in many sectors around Antarctica. However, in the West Antarctic regions where the greatest ice mass loss was observed in recent years, most models show either cooling or reduced warming on the shelf, hence indicating a negative feedback due to the meltwater input. Consistent with previous studies, we confirm that regional disparities are driven by advection and acceleration of the Antarctic Slope Current. Our results suggest that mass loss from East Antarctica will become increasingly important under future global warming. The meltwater-induced feedback causes an additional 750 Gt/year of ice loss in the multi-model median response to our perturbation experiments. For comparison, observations estimate current anomalous ice shelf loss at approximately 1,000 Gt/year, while SSP5-8.5 simulations, which account for global warming without additional Antarctic meltwater, project an anomalous 3,400 Gt/year of ice loss by the end of the century.

Enhanced Antarctic ice sheet mass loss yields ocean surface freshening, cooling and sea ice expansion, which result in changes in the atmospheric conditions. Using the Southern Ocean Freshwater Input from Antarctica (SOFIA) multi‐model ensemble, we study the atmospheric response to a 100‐year idealized freshwater release of 0.1 Sv. All models simulate a surface‐intensified tropospheric cooling and lower‐stratospheric warming south of 35°S. Tropospheric cooling is attributed to sea ice expansion and the associated albedo enhancement in winter and a colder sea surface in summer. This cooling yields a downward displacement of the tropopause, reduced stratospheric water vapor content and ultimately warming around 200 hPa. An enhanced southward eddy heat flux explains warming at 10–100 hPa during austral winter. Despite a temporally (and spatially) uniform prescribed freshwater flux, a prominent sea ice seasonal cycle and atmosphere dynamics result in a distinct seasonal pattern in the occurrence and magnitude of the temperature responses.

Cold and dense water from the Greenland Sea, which has been found in the Lofoten Basin in the Norwegian Sea, is an important contributor to the Greenland–Scotland Ridge overflow, which feeds the deep and bottom waters in the North Atlantic. These two basins are divided by the Mohn Ridge, but there is no clear current connecting them. The aim of this study is to investigate how the Greenland Sea water enters the Lofoten Basin. We deployed a mooring on the western flank of the Mohn Ridge to measure the potential transport across the ridge during two periods: 2016/17 and 2017/18. The observation results indicate that the water above 1500 m in the Greenland Sea can be intermittently transported to the Lofoten Basin. In addition, we observed periods of flow reversal, which indicate bidirectional exchange between the two basins across the ridge. Our data from three consecutive seasons indicate that such inflows in August–September are a typical feature of the exchange across the Mohn Ridge. Net exports during these two periods into the Lofoten Basin were eltimated to be 5.86 Sv and 3.00 Sv, exhibiting noticeable interannual variations. We propose two possible mechanisms that could be driving the export. One is due to passing cyclones, which lower the sea level height along the Mohn Ridge and drive outflow. The second is due to the sudden weakening of the wind in summer, which results in outflow from the Greenland Sea through temporary geostrophic deviation.

The Filchner-Ronne-Ice Shelf (FRIS) is the earth’s largest ice shelf by volume and its cavity a crucial part of the southern Weddell Sea ocean circulation. In mid-2017, the Filchner Ice Shelf (FIS) cavity experienced a shift towards a stronger circulation and increased outflow of Ice Shelf Water (ISW) into Filchner Trough. The increase was attributed to enhanced sea ice formation and the associated production of High Salinity Shelf Water (HSSW) in the source region north of Ronne Ice Shelf. The corresponding circulation pattern was termed “Ronne-mode”, which contrasts the “Berkner-mode”, characterized by a more locally-enhanced circulation at the northern FIS edge. Here we employ new time series from two drill hole mooring sites underneath FIS, as well as moorings from the Filchner Trough and Filchner Sill, to highlight the spatial and temporal extent of this recent ISW outflow event. Underneath FIS, the “Ronne-mode” overruled the normally-observed seasonality in currents and hydrography, and resulted in northward ISW transport for about two years. The export led to the subsequent filling of Filchner Trough with ISW from 2018 until mid-2020, which then overflowed across the Sill between late 2018 for nearly one year. Our observations provide new insights into the variability of the southern Weddell Sea shelf and FRIS cavity circulation, which is important for the abyssal water mass export and thus for global ocean circulation.

Increased basal melting of the Petermann Ice Shelf is typically attributed to rising ocean temperatures. While subglacial discharge is known to intensify basal melt, the underlying mechanisms and their evolution in a warming climate remain unresolved. Using a 3-D numerical regional ice shelf-ocean model centered on the Petermann Fjord, we identify a regime shift in heat flux efficiency within the ice shelf cavity when discharge exceeds the current peak summer value. In this regime, thermal driving saturates, and discharge-intensified currents increase melt by enhancing shear-driven turbulent mixing across the ice shelf-ocean boundary layer. Increases in melt are most profound at the crests of basal channels, where vigorous meltwater confluence amplifies friction velocity. Challenging conventional attributions of increased ice shelf basal melting to ocean warming alone, our results demonstrate how atmospheric warming exacerbates ocean-driven melt processes and is likely to play a dominant role in amplifying future basal melt.

Mass loss from the Antarctic and Greenland Ice Sheets could lead to a rise in global mean sea level of 0.25 m by 2100 and several metres by 2300 if greenhouse gas emissions remain unmitigated. Uncertainties in these estimates are strongly related to ocean-driven ice melt, which can lead to grounding line retreat, thinning and acceleration of the fast-flowing regions of both Antarctica and Greenland. The processes of ocean-driven ice melt on large spatial and temporal scales are imperfectly known, and measurements are sparse, impacting the accuracy of ice sheet and ocean model projection studies. The Joint Commission on Ice-Ocean Interactions (JCIOI) hosted the first community workshop in October 2022 with the aims to: (1) identify critical knowledge gaps surrounding processes that govern ocean-driven melt of ice sheets across a range of spatio-temporal scales; and (2) identify options to address the knowledge gaps through observing, parameterising, and modelling ice-ocean interactions, and their impacts on ice mass loss and ocean dynamics. Community discussions from the workshop highlighted the need for concurrent and sustained measurements of ice, ocean and atmosphere properties at the ice sheet-ocean interface, and making best use of existing observations to improve models, capture observed changes, better understand physical mechanisms and improve future projections. Building on the workshop outputs, we propose to develop a framework for ice-ocean observations that details the essential measurements that need to be collected, and the temporal and spatial scales on which to measure. This framework will require widespread community engagement on key scientific questions, agreement and coordination, including protocols for data collection, processing, and sharing.

Basal melting of Antarctic ice shelves significantly contributes to ice sheet mass loss, with distinct regional disparities in melt rates driven by ocean properties. In Dronning Maud Land (DML), East Antarctica, cold water predominantly fills the ice shelf cavities, resulting in generally low annual melt rates. In this study, we present a 4-year record of basal melt rates at the Ekström Ice Shelf, measured using an autonomous phase-sensitive radio-echo sounder (ApRES). Observations reveal a low mean annual melt rate of 0.44 m a−1, with a seasonal variability. Enhanced melting occurs in winter and spring, peaking at over 1 m a−1, while rates are decreased in summer and autumn. We hypothesise that the dense water formed during sea-ice formation erodes the water column stratification during late winter and spring, leading to an increase in the buoyancy of the ice shelf water plume. An idealised plume model supports this hypothesis, indicating that the plume velocity is the primary driver of seasonal basal melt rate variability, while changes in ambient water temperature play a secondary role in the range of oceanographic conditions that are observed below the Ekström Ice Shelf. These findings offer new insights into the dynamics of ice–ocean interactions in East Antarctica, emphasising the need for further observations to refine our understanding of ocean variability within ice shelf cavities and improve assessments of ice shelf mass balance.