Abstract. Accurately representing abyssal water masses and their inter-basin pathways remains a challenge for global ocean models and reanalyses. In this study, we investigate how horizontal and vertical resolution influence the simulation of abyssal waters by first evaluating three ocean reanalyses and one forward ocean model, focusing on Antarctic Bottom Water pathways from the Weddell Sea to the Argentine and Brazil basins and through the Vema Channel. Model outputs are evaluated against WOA18 climatology and in situ observations from moorings and hydrographic sections. Based on these results, we conduct four targeted experiments with the Finite-Volume Sea Ice-Ocean Model (FESOM 2), modifying horizontal and vertical grid resolution while keeping all other model components unchanged. The experiments show that increasing vertical resolution substantially improves the representation of cold and dense abyssal waters and their inter-basin connectivity, whereas horizontal refinement alone does not systematically improve the representation of abyssal properties and can even degrade it when mixing parameterizations are not adequately tuned. Combining vertical and horizontal refinement improves specific local features, including the structure of the abyssal flow and the realistic eastward deflection of the Antarctic Bottom Water core within the Vema Channel, but does not outperform vertical refinement alone at the basin scale.

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.

The Northeast Greenland Ice Stream (NEGIS) drains through two major outlet glaciers: the 79 North Glacier (79NG) and Zachariae Isstrøm (ZI). Since the 2000s, these glaciers have exhibited contrasting behavior: while the ZI ice shelf has retreated dramatically and transitioned to a tidewater glacier, the 79NG ice tongue has remained relatively stable in extent despite significant thinning. The retreat and thinning of both glaciers have accelerated the upstream ice stream, with important implications for global sea level rise.We present a novel coupled model that integrates the Ice-sheet and Sea-level System Model (ISSM) with the Finite volumE Sea Ice-Ocean Model version 2 (FESOM2). The ice sheet model domain encompasses the NEGIS region, while the global ocean model features enhanced mesh resolution on the Northeast Greenland continental shelf and explicitly resolves the ice shelf cavities of both 79NG and ZI. This coupling enables dynamic representation of ice sheet-ocean-sea ice interactions, including grounding line migration and ice geometry evolution.A hindcast simulation spanning 2008-2023, forced by atmospheric reanalysis data, reproduces the observed calving front retreat at ZI with good fidelity, validating our modeling approach. Beyond validation, this experiment reveals that the rapid ZI retreat is driven primarily by internal ice dynamics rather than changes in oceanic forcing. We extend our analysis through climate projection simulations using atmospheric forcing from CMIP6 scenarios. Applying both low and high emission scenarios (SSP126 and SSP585), we are able to assess the possible future evolution of these glaciers until the end of this century.

The floating ice tongue of the 79N Glacier in Northeast Greenland has been thinning over the past two decades, with warning signs of a potential onset of disintegration. While previous studies primarily attribute the thinning of the ice shelf to oceanic heat flux, limited attention has been given to the significant role of ice shelf plume dynamics as a mechanism for distributing the heat beneath the ice shelf. Here, we develop a horizontal two-dimensional plume model to assess the effects of key factors influencing plume dynamics and, consequently, the estimation of a high-resolution basal melt rate. We examine the effect of ice basal topography roughness and the presence of basal channels, that is extreme roughness of the base in the hinge zone, as well as the impact and pathways of subglacial discharge on melt rates. Our model results show good agreement with observation-based melt rate estimates and indicate that basal channels in the hinge zone are the dominant control on the ice shelf's basal melt rates. In combination with subglacial discharge, the melt rate is increased to at the grounding line, intensifying the channelized melt rate pattern created by basal channels and increasing spatial variability. Additionally, our results indicate that incorporating wet-dry algorithms and calculating a variable drag coefficient are crucial for accurately estimating melt rates during low subglacial discharge season, as well as for determining friction and turbulent exchange coefficients.

Abstract. 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.

Rising sea level poses a significant challenge and threat to our societies, given that coastal regions are densely populated. The Greenland ice sheet has been a major contributor to global sea level rise in the last decades, particularly its marine-terminating glaciers and their extensions into the ocean. The 79 North Glacier (79NG) features Greenland's largest floating ice tongue, stretching over 80 km in length in a 20 km wide fjord. The 79NG and its neighboring glacier, the Zachariæ Isstrøm, drain the Northeast Greenland Ice Stream which covers 12% of the Greenland Ice Sheet area. Its complete melt would lead to a 1.1-m global sea level rise. Though the extent of the 79NG has not changed significantly in recent years, observations have indicated a major thinning of its ice tongue from below. Both ocean warming and an increase in subglacial discharge from the ice sheet induced by atmospheric warming could increase the basal melt; however, available observations alone cannot tell which of these is the main driver.In this study, we present a setup of the Finite-volumE Sea ice-Ocean Model (FESOM2.1) which explicitly resolves the ocean circulation in the cavity of the 79NG with 700 m resolution. With this novel methodology, we seamlessly connect the global and regional ocean circulation to the circulation in the cavity. Our simulation with realistic bathymetry and ice shelf geometry covers the period 1970-2021, allowing us to disentangle the drivers of the upward trend and interannual variability of basal melt. We find that ocean warming in the subsurface Atlantic Intermediate Water layer that enters the cavity below the 79NG has played a dominant role in the basal melt rate over the past 50 years. The temperature variability can be traced back across the continental shelf of Northeast Greenland to the eastern Fram Strait with a lag of 3 years, implying a predictability of the basal melt of the 79NG. In contrast, subglacial discharge has a relatively small contribution to the interannual variation of the basal melt.

The strong Antarctic Slope Front in the southern Weddell Sea limits the present-day transport of modified Warm Deep Water (mWDW) onto the continental shelf and is associated with a characteristic V-shape in the density structure across the continental slope. The mechanisms controlling today's V-shape are well studied, but its future development is not yet well constrained. In this study, we run ocean model simulations for a 21st century Shared Socioeconomic Pathways (SSP) 3-7.0 emission scenario. The forcing is retrieved from atmospheric model output from simulations with a global climate model and from a higher-resolved regional atmospheric model respectively. We find that the resolution of the atmospheric model component influences the simulated future transport of mWDW onto the continental shelf into the Filchner Trough in the southern Weddell Sea through differences in the evolution of the depth and symmetry of the V-shape over the 21st century. In both simulations, reduced sea-ice formation and weakened Ekman downwelling reduce the depth of the V-shape and increase the sensitivity of its position above the slope to seasonal variations in sea-ice production and in the wind field. Using forcing data from an atmosphere model with higher resolution leads to an acceleration of the density redistribution on the continental shelf compared to the simulations forced with coarse-resolution data. This indicates that the SSP3-7.0 climate scenario may have a greater potential for a regime shift from a cold to a warm Filchner Trough through a cross-slope current before the end of the 21st century than suggested by other ocean simulations for the same scenario but with lower atmospheric resolution. As cross-slope currents disturb the continuity of the V-shape, we define a spatial grade of connectivity to quantify the lateral integrity of the V-shape along the continental slope. We find that the integrity of the V-shape reduces with a delay of 3 months after a strong cross-slope current of mWDW enters Filchner Trough. Atmospheric downscaling increases the potential for a regime shift, dominated by warmer summer air temperatures. The Antarctic Slope Front is temporarily disturbed by cross-slope currents but the primary reason for the regime shift is the cross-slope density gradient.

The rate at which the Antarctic ice sheet loses mass is to a large degree controlled by ice–ocean interactions underneath small ice shelves, with the most sensitive regions concentrated in even smaller areas near grounding lines and local pinning points. Sufficient horizontal resolution is key to resolving critical ice–ocean processes in these regions but difficult to afford in large-scale models used to predict the coupled response of the entire Antarctic ice sheet and the global ocean to climate change. In this study we describe the implementation of a framework that couples the ice sheet flow model Úa with the Finite Element Sea Ice Ocean Model (FESOM-1.4) in a configuration using depth-dependent vertical coordinates. The novelty of this approach is the use of horizontally unstructured grids in both model components, allowing us to resolve critical processes directly, while keeping computational demands within the range of feasibility. We use the Marine Ice Sheet Ocean Model Intercomparison Project (MISOMIP) framework to verify that ice retreat and readvance are reliably simulated, and inaccuracies in mass, heat and salt conservation are small compared to the forcing signal. Further, we demonstrate the capabilities of our approach for a global ocean–Antarctic ice sheet domain. In a 39-year hindcast simulation (1979–2018) we resolve retreat behaviour of Pine Island Glacier, a known challenge for coarser-resolution models. We conclude that Úa–FESOM is well suited to improve predictions of the Antarctic ice sheet evolution over centennial timescales.

Antarctic basal melt is crucial for the future evolution of the Antarctic ice sheet and ocean circulation. However, few Earth system models explicitly simulate ice-shelf cavities. Here, using an Earth system model with interactive Antarctic ice-shelf cavities, we show that regional hydrography and topography determine a cavity tipping point. The Filchner-Ronne ice-shelf cavity will encounter such a tipping point with abrupt warm-water intrusion, rapid basal melt increase and massive freshwater release in response to increasing CO2 levels within this century. Conversely, the Ross Ice Shelf shows a more gradual response. Our results also suggest that previous ice-sheet modelling overestimated future ice-shelf melt, highlighting the need for comprehensive Earth system models with interactive ice-sheet dynamics and cavities for better climate projections.

The Greenland Ice Sheet has steadily lost mass over the past decades, presently representing the second-largest single contributor to global sea-level rise. In line with the rest of the Greenland Ice Sheet, the glaciers draining the northeast Greenland ice stream have been observed to retreat and thin. Here, we present a comprehensive study of processes affecting and being affected by the mass balance of marine-terminating and peripheral glaciers in northeast (NE) Greenland. Our focus is on the 79N Glacier (79NG), which hosts Greenland’s largest floating ice tongue. We provide new insight into the ice surface melt, the ice mass balance, glacier dynamics, the regional solid Earth response, the ocean-driven basal melt and the consequences of meltwater discharge into the ocean. Our study is based on field observations, remote sensing and simulations with numerical models of different complexity, most of them originating from the Greenland Ice Sheet–Ocean Interaction (GROCE) experiment. We find the overall negative climatic mass balance of 79NG to co-vary with summertime volumes of supraglacial lakes and show that the spatial pattern of the overall negative ice mass balance for NE Greenland is mirrored by the pattern of glacial-isostatic adjustment. We find near-coastal mass losses of both marine-terminating and peripheral glaciers in NE Greenland to be of a similar magnitude in the last decade. In contrast to the neighboring Zachariæ Isstrøm, 79NG – despite experiencing massive thinning of the floating tongue – has resisted an acceleration of ice discharge across the grounding line due to buttressing imposed by lateral friction of the 70 km long ice tongue in the narrow glacial fjord. Observations and models employed in this study are consistent in terms of melt rates occurring below the floating ice tongue. Our results suggest that the multidecadal warming of Atlantic Intermediate Water flowing into the cavity below the ice tongue – supplied by the recirculating branch of the West Spitsbergen Current in Fram Strait – is the main driver of the recent major increase in basal melt rates. We find that the meltwater leaving the cavity toward the ocean at subsurface levels quickly dilutes on the wide shelf. The study concludes by summarizing important estimates of changes to the state of the atmosphere, ice, land and ocean domains.

Coupled ice sheet and ocean models are vital for projecting the dynamics of the Antarctic Ice Sheet and for predicting future sea level rise. The Filchner-Ronne sector of Antarctica contains a number of deep-bedded ice streams and glaciers potentially vulnerable to the Marine Ice Sheet Instability. Previous work has shown that, in a warming climate, a mode switch in circulation could bring intrusions of warm Circumpolar Deep Water (CDW) that would increase basal melt rates near the deep grounding lines of these vulnerable glaciers. Furthermore, the adjacent Weddell Sea is an important site of global deep water formation that is heavily dependent on the export of Ice Shelf Water. Here, we develop a new ice-ocean coupling framework for linking the global Finite volumE Sea ice Ocean Model (FESOM-2) with the Ice-sheet and Sea-level System Model (ISSM), and we apply this framework to model the Filchner-Ronne sector of Antarctica and the adjacent Weddell Sea. We use adaptive mesh resolution for FESOM-2 ranging from 100 km elements in the far field down to 3 km in the Weddell Sea and the sub-ice cavity. Our ice sheet model resolution varies from 10 km down to ~300 m, with basal friction taken from an inversion fit to present-day surface velocities. We use offline coupling with a timestep of 1 year. We develop an adaptive filtering technique for the transmission of melt rates from the ocean model to the ice model that effectively removes numerical artifacts caused by the z-coordinate representation of the ice base in the ocean model while preserving true structure in the melt rate field. For the adaptation of the ocean model to the updated ice geometry, we develop an iterative horizontal-vertical extrapolation procedure for ocean tracers and a minimal smoothing procedure for ocean velocities to ensure that the ocean model can restart in a manner that is both realistic and numerically stable. Using this coupling architecture, we are able to directly restart the ocean model after the geometry change without requiring either a cold start or a spinup period with reduced timesteps and increased viscosity. We then simulate the evolution of the coupled ice-ocean system, including a moving calving front, over the next century under a range of climate forcing scenarios. We find that the projected mode switch to warm conditions in the Filchner-Ronne cavity happens earlier in our coupled model than in previous projections, with warm CDW first entering the Filchner cavity in ~2035 under SSP585 forcing, followed by ice shelf thinning, grounding line retreat, and grounded ice mass loss in the ensuing decades. By comparison, previous projections in strongly warming scenarios showed the CDW entering the cavity in 2050-2075. These results emphasize the rapid changes in the cryosphere and the Southern Ocean that could arise from continued anthropogenic warming, and the importance of coupled modeling for fully understanding the dynamics of the ice-ocean system.

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 yr−1 (m=0.45-0.91 m yr−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 0.64 m yr−1 from a net mass loss of 843 Gt yr−1 (876 Gt yr−1 melting and 33 Gt yr−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.

Observational data kit gathered and reprocessed to facilitate the evaluation of ocean and ice-sheet simulations of the Weddell Sea sector as part of MISOMIP2. __________________________________________ This entire dataset should be cited as: the MISOMIP2 MIPkit-W dataset (https://zenodo.org/doi/10.5281/zenodo.8316180) that includes data collected through multiple cruises of the Polarstern Research Vessel and originally provided by the Alfred Wegener Institute, Bremerhaven, Germany, and glaciological data from the MeaSUREs, MeaSUREs ITS_LIVE projects. For more specific use of some of the MIPkit-W data, we encourage people to cite the original data referenced below. __________________________________________ IceSurfVel_MIPkitW_2000-2019 : annual maps of ice surface velocity Surface-parallel velocity vectors (in m/year) between 2000 and 2019 are provided on the common MISOMIP2 grid with regular grid spacing of 1 km. Data were calculated as a weighted average of all available Earth observation data from the MeaSUREs project (Rignot et al. 2014 and Mouginot et al. 2017) and MeaSUREs ITS_LIVE project (Gardner et al. 2022), with weights corresponding to the inverse square error of the original datasets. Propagated errors and a mask indicating the original data sources for each grid point are also included. __________________________________________ IceSurfElChange_MIPkitW_1992-2019 : annual maps of surface elevation change Surface elevation changes (in m) between 1992 and 2019, relative to 16-Dec-2013, are provided on the common MISOMIP2 grid with regular grid spacing of 1 km. Data were calculated as a weighted average of available Earth observation data from MeaSUREs ITS_LIVE (Nilsson et al. 2023) for the grounded ice and MeaSUREs ITS_LIVE data (Paolo et al. 2023) for floating ice. Propagated errors and a mask indicating the original data sources for each grid point are also included. __________________________________________ Oce3d_MIPkitW_* : 3-dimensional temperature and salinity (horizontal slices every 100m) The hydrographic properties provided on horizontal sections at 15 depths come from the CTD measurements obtained from late December to early March during the Alfred Wegener Institute Polarstern cruises ANT-XII/3 (Schroeder, 2010), PS82 (Schroeder, 2014), PS96 (Schroeder, 2016) and PS111 (Janout, 2019), which cover years 1995, 2014, 2016 and 2018, respectively. __________________________________________ OceSec_MIPkitW_* : vertical sections The first vertical section (OceSec1) goes from the tip of the Antartic Peninsula to Kapp Norvegia (12.33°E). It is known as WOCE-SR04 and has been monitored since 1989. The data provided were collected during Polarstern cruises in Sep.-Oct. 1989, Nov.-Dec. 1990, Dec. 1992-Jan. 1993, Mar.-May 1996, Apr.-May 1998 (Fahrbach and Rohardt, 1990, 1991, 1993, 1996, 1998), Jan.-Apr. 2005 (Rohardt 2010), Feb.-Apr. 2008 (Fahrbach and Rohardt, 2008), Dec. 2010-Jan. 2011 (Rohardt et al. 2011), Dec. 2012-Jan. 2013 (Rohardt 2013), as well as Dec. 2016-Jan. 2017 and Dec. 2018-Feb. 2019 (Rohardt and Boebel, 2017, 2020). The second vertical section (OceSec2) is at approximately 76°S and covers the eastern side of Filchner Through. It was surveyed during some of the Polarstern cruises on 5-8 Jan. 2014 (Schroeder, 2014), 20-24 Jan. 2016 (Schroeder, 2016) and 4-23 Feb. 2018 (Janout, 2019). The third and fourth sections were obtained along the front of Ronne (OceSec3) and Filchner (OceSec4) ice shelves, respectively. The Filchner section was measured on 1-3 Feb. 1977 by the Norwegian Antarctic Research Expedition with Research Vessel Polarsirkel (Foldvik et al., 1985), 7-16 Jan. 1981 (Hubold and Drescher, 1982), 25 Jan.- 4 Mar. 1995 (Schroeder, 2010), 15-17 Jan. 2014 (Schroeder, 2014), 15 Jan. 2016 (only one vertical profile; Schroeder, 2016), and 14-23 Feb. 2018 (Janout, 2019). The Ronne section was measured by some of these expeditions on 25 Jan.- 24 Feb. 1995, 14-15 Jan. 2016, and 9-14 Feb. 2018. The files OceSec_MIPkitW_model_lon_lat.csv contain the coordinates (longitude, latitude) at which model data should be interpolated to be compared to the observational sections. __________________________________________ OceMoor_MIPkitW_* : moorings Temperature, salinity and velocity time series are provided at three moorings placed along the 76°S vertical section and referred to as OceMoor1 (AWI252, 30.47°W), OceMoor2 (AWI253, 30.99°W) and OceMoor3 (AWI254, 31.48°W), and cover the period from Jan. 2014 to Feb. 2018 (Schroeder et al., 2017a,b,c, 2019a,b,c). Temperature, salinity and velocity data were obtained at two depths for AWI252 (335 and 421 m depth for a seafloor at 447 m) and AWI253 (349 and 434 m depth for a seafloor at 456 m), while a single depth is provided for AWI254 (553 m for a seafloor at 581 m). __________________________________________ The archive example_routines.zip contains example of Matlab routines that were used to prepare the MIPkit-W ocean data, as well as GenerateData_MIPkit_Ice.m and writeNC_MIPkit_Ice.m that were used to process glaciological data. __________________________________________ References Fahrbach, E. and Rohardt, G. (1990). Physical oceanography during POLARSTERN cruise ANT-VIII/2 (WWGS) on section SR02 and SR04, PANGAEA, https://doi.org/10.1594/PANGAEA.742580 Fahrbach, E. and Rohardt, G. (1991). Physical oceanography during POLARSTERN cruise ANT-IX/2 on section SR04, PANGAEA, https://doi.org/10.1594/PANGAEA.735277 Fahrbach, E. and Rohardt, G. (1993). Physical oceanography during POLARSTERN cruise ANT-X/7 on section SR04, PANGAEA, https://doi.org/10.1594/PANGAEA.742651 Fahrbach, E. and Rohardt, G. (1996). Physical oceanography during POLARSTERN cruise ANT-XIII/4 on section S04A, PANGAEA, https://doi.org/10.1594/PANGAEA.738489 Fahrbach, E. and Rohardt, G. (1998): Physical oceanography during POLARSTERN cruise ANT-XV/4 (DOVETAIL) on section SR04, PANGAEA, https://doi.org/10.1594/PANGAEA.742626 Fahrbach, E. and Rohardt, G. (2008): Physical oceanography during POLARSTERN cruise ANT-XXIV/3, PANGAEA, https://doi.org/10.1594/PANGAEA.733414 Foldvik, A,, Gammelsrød,T. & Tørresen, T. 1985: Hydrographic observations from the Weddell Sea during the Norwegian Antarctic Research Expedition 1976/77. Polar Research, 3:2, 177-193, https://doi.org/10.3402/polar.v3i2.6951 Hubold, G. and Drescher, H. E. (1982). Die Filchner-Schelfeis-Expedition 1980/81 mit MS "Polarsirkel". Liste der Planktonfänge und Lichtstärkemessungen , Reports on Polar Research, Alfred Wegener Institute for Polar and Marine Research, Bremerhaven, 4, https://epic.awi.de/id/eprint/26181/1/BerPolarforsch19824.pdf Rohardt, G. (2010). Physical oceanography during POLARSTERN cruise ANT-XXII/3, https://doi.org/10.1594/PANGAEA.733664 Rohardt, G. (2013). Physical oceanography during POLARSTERN cruise ANT-XXIX/2, https://doi.org/10.1594/PANGAEA.817255 Rohardt, G. and Boebel, O. (2017). Physical oceanography during POLARSTERN cruise PS103 (ANT-XXXII/2), https://doi.org/10.1594/PANGAEA.881076 Rohardt, G. and Boebel, O. (2020). Physical oceanography during POLARSTERN cruise PS117, https://doi.org/10.1594/PANGAEA.910663 Rohardt, G., Fahrbach, E., and Wisotzki, A. (2011): Physical oceanography during POLARSTERN cruise ANT-XXVII/2, https://doi.org/10.1594/PANGAEA.772244 Schröder, M. (2010). Physical oceanography during POLARSTERN cruise ANT-XII/3, https://doi.org/10.1594/PANGAEA.742581 Schröder, M. and Wisotzki, A. (2014). Physical oceanography during POLARSTERN cruise PS82 (ANT-XXIX/9), https://doi.org/10.1594/PANGAEA.833299 Schröder, M., Ryan, S., and Wisotzki, A. (2016). Physical oceanography during POLARSTERN cruise PS96 (ANT-XXXI/2 FROSN), https://doi.org/10.1594/PANGAEA.859040 Schröder, M., Ryan, S., and Wisotzki, A. (2017a). Physical oceanography and current meter data from mooring AWI252-1, https://doi.org/10.1594/PANGAEA.875931 Schröder, M., Ryan, S., and Wisotzki, A. (2017b). Physical oceanography and current meter data from mooring AWI253-1, https://doi.org/10.1594/PANGAEA.875932 Schröder, M., Ryan, S., and Wisotzki, A. (2017c). Physical oceanography and current meter data from mooring AWI254-1, https://doi.org/10.1594/PANGAEA.875933 Schröder, M., Ryan, S., and Wisotzki, A. (2019a). Physical oceanography and current meter data from mooring AWI252-2, https://doi.org/10.1594/PANGAEA.903104 Schröder, M., Ryan, S., and Wisotzki, A. (2019b). Physical oceanography and current meter data from mooring AWI253-2, https://doi.org/10.1594/PANGAEA.903315 Schröder, M., Ryan, S., and Wisotzki, A. (2019c). Physical oceanography and current meter data from mooring AWI254-2, https://doi.org/10.1594/PANGAEA.903317

The 79 North Glacier (79NG) features Greenland’s largest floating ice tongue. Even though its extent has not changed significantly in recent years, observations have indicated a major thinning of the ice tongue from below. Both ocean warming and an increase in subglacial discharge from the ice sheet induced by atmospheric warming could increase the basal melt; however, available observations alone cannot tell which of these is the main driver. Here, we employ a global simulation which explicitly resolves the ocean circulation in the cavity with 700 m resolution to disentangle the impact of the ocean and atmosphere. We find that the interannual variability of basal melt below 79NG over the past 50 years is mainly associated with changes in the temperature of the Atlantic Intermediate Water inflow, which can be traced back across the Northeast Greenland continental shelf to the eastern Fram Strait with a lag of 3 years.

The transport of water masses with ocean circulation is a key component of the global climate system. In this context, the Filchner Trough in the southern Weddell Sea is critical, as it is a hotspot for the cross-shelf-break exchange of Dense Shelf Water and Warm Deep Water. We present results from Lagrangian particle tracking experiments in a global-ocean–sea-ice model (FESOM-1.4) which includes ice-shelf cavities and has eddy-permitting resolution on the southern Weddell Sea continental shelf. With backward and forward experiments, we assess changes between a present-day and a future (SSP5-8.5) time slice in the origin of waters reaching the Filchner Ice Shelf front and the fate of waters leaving it. We show that particles reaching the ice-shelf front from the open ocean originate from 173 % greater depths by 2100 (median; 776 m as compared to 284 m for the present day), while waters leaving the cavity towards the open ocean end up at 35 % shallower depths (550 m as compared to 850 m for the present day). Pathways of water leaving the continental shelf increasingly occur in the upper ocean, while the on-shelf flow of waters that might reach the ice-shelf cavity, i.e., at deeper layers, becomes more important by 2100. Simultaneously, median transit times between the Filchner Ice Shelf front and the continental shelf break decrease (increase) by 6 (9.5) months in the backward (forward) experiments. In conclusion, our study demonstrates the sensitivity of regional circulation patterns in the southern Weddell Sea to ongoing climate change, with direct implications for ice-shelf basal melt rates and local ecosystems.

Abstract. The Greenland Ice Sheet has steadily lost mass over the past decades, presently representing the second-largest single contributor to global sea-level rise. Even the glaciers draining the Northeast Greenland ice stream have been observed to retreat and thin. Here, we present a comprehensive study of processes affecting and being affected by the mass balance of marine terminating and peripheral glaciers in Northeast Greenland. Our focus is on the 79N Glacier (79NG), which hosts Greenland’s largest floating ice tongue. We provide new insight into the ice surface melt, ice mass balance, glacier dynamics, regional solid earth response, ocean-driven basal melt and the consequences of meltwater discharge into the ocean. Our study is based on observations, remote sensing and simulations with numerical models of different complexity, most of them originating from the Greenland Ice Sheet–Ocean Interaction Experiment (GROCE). We find the overall negative climatic mass balance of the 79NG to co-vary with summertime volumes of supraglacial lakes, and show the spatial pattern of overall negative ice mass balance for NE Greenland to be mirrored by the pattern of glacial isostatic adjustment. We find near coastal mass losses of both marine terminating and peripheral glaciers in NE Greenland to be of similar magnitude in the last decade. In contrast to the neighboring Zachariae Isstrøm, the 79NG – despite experiencing massive thinning of the floating tongue – has resisted an acceleration of ice discharge across the grounding line due to buttressing imposed by lateral friction of the 70 km-long ice tongue in the narrow glacial fjord. Observations and models employed in this study are consistent in terms of melt rates occurring below the floating ice tongue. Our results suggest the multidecadal warming of Atlantic Intermediate Water flowing into the cavity below the ice tongue – supplied by the recirculating branch of the West Spitsbergen Current in Fram Strait – to be the main driver of the recent major increase in basal melt rates. We find the melt water leaving the cavity toward the ocean at subsurface levels to quickly dilute on the wide shelf. The study concludes by summarizing important estimates of changes to the state of the atmosphere, ice, land and ocean domains.

The 2nd Marine Ice Sheet and Ocean Model Intercomparison Project (MISOMIP2) is a natural progression of previous and ongoing model intercomparison exercises that have focused on the simulation of ice-sheet--ocean processes in Antarctica. The previous exercises motivate the move towards more realistic configurations and more diverse model parameters and resolutions. The first objective of MISOMIP2 is to investigate the robustness of ocean and ocean--ice-sheet models in a range of Antarctic environments, through comparisons to interannual observational data. We will assess the status of ocean--ice-sheet modelling as a community and identify common characteristics of models that are best able to capture observed features. As models are highly tuned based on present-day data, we will also compare their sensitivity to abrupt atmospheric perturbations leading to either very warm or slightly warmer ocean conditions than present-day. The approach of MISOMIP2 is to welcome contributions of models as they are, but we request standardised variables and common grids for the outputs. There will be two target regions, the Amundsen Sea and the Weddell Sea, chosen because they describe two extremely different ocean environments and have been relatively well observed compared to other parts of Antarctica. An observational "MIPkit" is provided to evaluate ocean and ice sheet models in these two regions.