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Subglacial freshwater discharge from beneath Antarctic glaciers likely has a strong impact on ice shelf basal melting. However, the difficulty in directly observing subglacial flow highlights the importance of modeling these processes. We use an ocean model of the Totten Ice Shelf cavity into which we inject subglacial discharge derived from a hydrology model applied to Aurora Subglacial Basin. Our results show (a) discharge increases melting in the vicinity of the outflow region, which correlates with features observed in surface elevation maps and satellite‐derived melt maps, with implications for ice shelf stability; (b) the change in melting is driven by the formation of a buoyant plume rather than the addition of heat; and (c) the buoyant plume originating from subglacial discharge‐driven melting is far‐reaching. Basal melting induced by subglacial hydrology is thus important for ice shelf stability, but is absent from almost all ice‐ocean models. Plain Language Summary Grounding line subglacial discharge is the outflow of freshwater at the region where Antarctic ice shelves begin to float. This process is difficult to observe as the outflow region is typically below ice up to several kilometers thick. Using a computer model, we show how the outflow of this fresh water melts the underside of an Antarctic ice shelf. We demonstrate that the outflow leads to a strong increase in melting, which is driven not by the heat in the outflow, but rather by the low salinity water's buoyancy. This buoyancy leads to the formation of a plume that rapidly ascends the underside of the ice shelf and can carry even warmer water to the ice, leading to increased melting. Given that melting of ice shelves can destabilize the grounded ice upstream of the floating region, we show that the relationship between subglacial discharge and ice shelf melt is important for understanding the stability of Antarctic ice. Key Points Subglacial discharge has a strong local effect on melting beneath the Totten Ice Shelf Addition of buoyancy explains the change in melting Discharge has far‐reaching impacts due to the extent of the buoyant plume

Totten Glacier is a fast‐moving East Antarctic outlet with the potential for significant future sea‐level contributions. We deployed four autonomous phase‐sensitive radars on its ice shelf to monitor ice‐ocean interactions near its grounding zone and made active source seismic observations to constrain gravity‐derived bathymetry models. We observe an asymmetry in basal melting with mean melt rates along the grounding zone differing by up to 20 m/a. Our new bathymetry model reveals that this melt rate asymmetry coincides with an asymmetry in water column thickness and that the low‐melting ice‐shelf portion is shielded from the main cavity circulation. A 2‐year record yields year‐to‐year melt rate variability of 7–9 m/a with no seasonal cycle. Our results highlight the key role of bathymetry near grounding lines for accurate modeling of ice‐shelf melt, and the importance of sustained multi‐year monitoring, especially at ice‐shelf cavities where the dominant melt rate drivers vary primarily inter‐annually. Plain Language Summary The point were the Antarctic Ice Sheet goes afloat on the ocean represents a critical region, where minor variations in melt rates can impact glacier flow and influence the rate of sea‐level rise. East Antarctica's Totten Glacier holds the potential to raise global sea level by several meters. Therefore, to understand the conditions it is exposed to, we measured melt rates for 2 years in several key locations near the point where the ice first touches the ocean. Our new measurements of the shape of the Totten Ice Shelf cavity help explain an observed spatial pattern of basal melting and together with local melt rate data resolve a disagreement between existing melt rate estimates from remote‐sensing methods. Key Points Totten Glacier melt rates vary spatially between 0 and over 20 m/a; differences are explained by water column thickness variations from updated bathymetry Temporal melt rate variability is primarily inter‐annual; melt rates differ by 7–9 m/a over two observed years and there is no clear seasonal cycle Contrary to previous findings, we find no topographic barriers to the intrusion of warm water to the Totten Glacier grounding zone

The Antarctic Ice Sheet is losing mass as a result of increased ocean-driven melting of its fringing ice shelves. Efforts to represent the effects of basal melting in sea level projections are undermined by poor understanding of the turbulent ice shelf–ocean boundary layer (ISOBL), a meters-thick layer of ocean that regulates heat and salt transfer between the ocean and ice. To address this shortcoming, we perform large-eddy simulations of the ISOBL formed by a steady, geostrophic flow beneath horizontal ice. We investigate melting and ISOBL structure and properties over a range of free-stream velocities and ocean temperatures. We find that the melting response to changes in thermal and current forcing is highly nonlinear due to the effects of meltwater on ISOBL turbulence. Three distinct ISOBL regimes emerge depending on the relative strength of current shear and buoyancy forcing: “well-mixed,” “stratified,” or “diffusive-convective.” We present expressions for mixing-layer depth for each regime and show that the transitions between regimes can be predicted with simple nondimensional parameters. We use these results to develop a novel regime diagram for the ISOBL which provides insight into the varied melting responses expected around Antarctica and highlights the need to include stratified and diffusive-convective dynamics in future basal melting parameterizations. We emphasize that melting in the diffusive-convective regime is time dependent and is therefore inherently difficult to parameterize.

Antarctic Bottom Water (AABW), formed from Dense Shelf Water (DSW) in Antarctic coastal polynyas, partly drives the global overturning circulation. Cape Darnley Polynya in East Antarctica, the most recently‐identified AABW formation site, remains poorly understood. We present simulations of the Cape Darnley region quantifying the main processes responsible for forming DSW and identify two key, opposing influences. Wintertime DSW export from Cape Darnley—mean of 0.28×106$0.28\\times 1{0}^{6}$ m3 s−1 (0.28 Sv)—is suppressed by basal melting of the Amery Ice Shelf, but enhanced by cold, saline preconditioning from Mackenzie Polynya. A doubling of Amery Ice Shelf melting reduces export to 0.26 Sv (7.0% decrease), while a shutdown of Mackenzie Polynya reduces DSW export to 0.18 Sv (a 36% decrease). Our findings reveal the sensitivity of DSW formation at Cape Darnley to oceanic and glaciological conditions, with implications for future AABW production, the global overturning circulation, and climate.

The presence and nature of marine ice in ice shelves is important, yet difficult to determine. We present transient electromagnetic results spanning 10 km across a suture zone of Larsen C Ice Shelf (LCIS), Antarctica, supported by central borehole measurements. Our results indicate the presence of two ice‐shelf layers. The uppermost layer, ∼300 m thick, has resistivity 103−106 Ωm. We interpret this as meteoric ice, overlying a lower shelf layer 25–56 m thick with resistivity 3–20 Ωm, consistent with permeable basal marine ice. This reconstruction closely matches modeled marine‐ice thicknesses in the area. The porosity of this layer is 0.18–0.40, higher than measured farther down‐flow, suggesting the layer consolidates once formed. Within the upper layer at 78.5 m depth, we identify a 2.5 m thick layer, likely linked to brine infiltration. These heterogeneities in ice shelf properties should be accounted for in future LCIS stability assessments.

Basal melting of Antarctic ice shelves is expected to increase during the twenty-first century as the ocean warms, which will have consequences for ice sheet stability and global sea level rise. Here we present future projections of Antarctic ice shelf melting using the Finite Element Sea Ice/Ice-Shelf Ocean Model (FESOM) forced with atmospheric output from models from phase 5 of the Coupled Model Intercomparison Project (CMIP5). CMIP5 models are chosen based on their agreement with historical atmospheric reanalyses over the Southern Ocean; the best-performing models are ACCESS 1.0 and the CMIP5 multimodel mean. Their output is bias-corrected for the representative concentration pathway (RCP) 4.5 and 8.5 scenarios. During the twenty-first-century simulations, total ice shelf basal mass loss increases by between 41% and 129%. Every sector of Antarctica shows increased basal melting in every scenario, with the largest increases occurring in the Amundsen Sea. The main mechanism driving this melting is an increase in warm Circumpolar Deep Water on the Antarctic continental shelf. A reduction in wintertime sea ice formation simulated during the twenty-first century stratifies the water column, allowing a warm bottom layer to develop and intrude into ice shelf cavities. This effect may be overestimated in the Amundsen Sea because of a cold bias in the present-day simulation. Other consequences of weakened sea ice formation include freshening of High Salinity Shelf Water and warming of Antarctic Bottom Water. Furthermore, freshening around the Antarctic coast in our simulations causes the Antarctic Circumpolar Current to weaken and the Antarctic Coastal Current to strengthen.

Antarctica’s contribution to global sea-level rise is deeply uncertain, with subglacial water suspected to play a critical role, yet its impact remains unclear. We demonstrate that water at the base of ice sheets influences sliding behaviour and that its exclusion from models can underestimate sea-level rise projections and delay the predicted onset of tipping points. Here we use an Antarctic Ice Sheet model (Elmer/Ice) to explore how different assumptions about water pressure at the ice base affect sea-level rise projections from 2015 to 2300. Our results indicate that incorporating subglacial water can amplify ice discharge across the Antarctic Ice Sheet by up to threefold above the standard approach, potentially contributing an additional 2.2 metres to sea-level rise by 2300. Notably, a smoothly decreasing basal drag near the grounding line more than doubles grounding line flux by 2300 relative to scenarios where effective pressure is simplified into a spatially constant coefficient. Basin-specific responses vary significantly, with some scenarios advancing tipping points by up to 40 years. These findings underscore the critical need to integrate evolving subglacial hydrology into ice sheet models. Hidden water beneath Antarctica’s ice can accelerate ice loss, potentially raising sea levels by over 2 meters by 2300. These findings highlight the urgent need to incorporate evolving subglacial hydrology into ice sheet models for more accurate sea-level rise projections.

The delivery of ocean heat to Antarctic ice shelves is due to intrusions of waters warmer than the local freezing point temperature. Changes in the supply of ocean heat will determine how rapidly ice shelves melt at their base, which affects Antarctic Ice Sheet mass loss and future global mean sea-level rise. However, processes driving ice-shelf basal melting are still poorly understood. Here we investigate the drivers of heat convergence along the Antarctic margins by performing an ocean heat budget analysis using a high-fidelity 4 km circum-Antarctic ocean–ice-shelf model. The simulation produces high basal melting in West Antarctica associated with sustained ocean heat convergence driven by advection of relatively warm deep water intrusions, with minimal seasonality in both heat supply and basal melting. For East Antarctica, ice shelves have substantial basal melt seasonality, driven by strong air–sea winter cooling over the continental shelf depressing shallow melting, while in summer, increased heat inflow towards the ice shelves is driven by surface-warmed waters that subduct under shallow regions of ice, increasing melt. The high seasonality of basal melting in East Antarctic ice shelves is responsive to interactions between the atmospheric forcing, the local icescape, and the activity of coastal polynyas. Our results suggest that seasonal changes in future climate change scenarios are critical in determining the duration and intensity of air–sea fluxes with substantial impacts on ice-shelf basal melting and ice-sheet and sea-level budgets.

Previous studies of Totten Ice Shelf have employed surface velocity measurements to estimate its mass balance and understand its sensitivities to interannual changes in climate forcing. However, displacement measurements acquired over timescales of days to weeks may not accurately characterize long-term flow rates wherein ice velocity fluctuates with the seasons. Quantifying annual mass budgets or analyzing interannual changes in ice velocity requires knowing when and where observations of glacier velocity could be aliased by subannual variability. Here, we analyze 16 years of velocity data for Totten Ice Shelf, which we generate at subannual resolution by applying feature-tracking algorithms to several hundred satellite image pairs. We identify a seasonal cycle characterized by a spring to autumn speedup of more than 100 m yr−1 close to the ice front. The amplitude of the seasonal cycle diminishes with distance from the open ocean, suggesting the presence of a resistive back stress at the ice front that is strongest in winter. Springtime acceleration precedes summer surface melt and is not attributable to thinning from basal melt. We attribute the onset of ice shelf acceleration each spring to the loss of buttressing from the breakup of seasonal landfast sea ice.

Over the period 2003–2008, the Totten Ice Shelf (TIS) was shown to be rapidly thinning, likely due to basal melting. However, a recent study using a longer time series found high interannual variability present in TIS surface elevation without any apparent trend. Here we show that low-frequency intrinsic ocean variability potentially accounts for a large fraction of the variability in the basal melting of TIS. Specifically, numerical ocean model simulations show that up to 44% of the modelled variability in basal melting in the 1–5 year timescale (and up to 21% in the 5–10 year timescale) is intrinsic, with a similar response to the full climate forcing. We identify the important role of intrinsic ocean variability in setting the observed interannual variation in TIS surface thickness and velocity. Our results further demonstrate the need to account for intrinsic ocean processes in the detection and attribution of change. Low frequency intrinsic ocean variability has an unknown impact on Antarctic ice shelves, yet can arise even in the absence of varying climate forcing. Here, the authors show that this variability significantly affects modelled basal melting under the Totten Ice Shelf, with implications for the attribution of change.