Abstract Thwaites Glacier is one of the fastest‐changing ice‐ocean systems in Antarctica. Basal melting beneath Thwaites' floating ice shelf, especially around pinning points and at the grounding line, sets the rate of ice loss and Thwaites' contribution to global sea‐level rise. The rate of basal melting is controlled by the transport of heat into and through the ice–ocean boundary layer toward the ice base. Here we present the first turbulence observations from the grounding line of Thwaites Eastern Ice Shelf. We demonstrate that contrary to expectations, the turbulence‐driven vertical flux of heat into the ice–ocean boundary layer is insufficient to sustain the basal melt rate. Instead, most of the heat required must be delivered by lateral fluxes driven by the large‐scale advective circulation. Lateral processes likely dominate beneath the most unstable warm‐cavity ice shelves, and thus must be fully incorporated into parameterizations of ice shelf basal melting.

At present, considerable uncertainty surrounds the details of how Earth’s ice sheets interact with the surrounding ocean. This inhibits the reliability of future sea level rise projections from ice sheet models and highlights a need to better constrain ice-ocean interactions with in situ observations. Here, we present detailed ice and ocean data from beneath Thwaites Eastern Ice Shelf, Antarctica, collected with the underwater vehicle Icefin as part of the ITGC MELT project. The observations are a subset of the full data set that focus on the ice-ocean interactions within one 4-m-tall and 200-m-wide terrace formation in the ice base. We present ocean conditions in the terrace from 18 hydrographic profiles that reached within 1 cm of the ice along the feature’s flat roof and 13 cm from its steep sidewall. The ocean observations depict highly stable near-ice ocean stratification within 1 m of the terrace roof that break down near its sidewall, allowing warmer and more saline water to contact the ice there. The ocean observations are combined with ice base elevations and scaled morphological melt patterns in the ice to understand the dominant mechanisms driving ice-ocean interactions within this feature. We then input these data into the three-equation melt parameterization to estimate spatial variability in melt rates within these topographic features. We test various parameterizations for ocean heat flux into the flat and sloped ice surfaces, and compare the results to melt rates sampled along a nearby terrace sidewall and roof with a phase sensitive radar. This work in progress aims to better understand how ocean conditions interact with ice slope on small scales to drive variable melting in warm, highly stratified environments, with hopes of refining existing parameterizations of this process. We expect regions beneath much of the ice shelves occupying West Antarctica to interact similarly with the underlying ocean to what we observe beneath Thwaites. Hence, our observations hold relevance for how ice sheet models parameterize ocean-driven melting in this type of melt-driven regime.

Knowledge gaps about how the ocean melts Antarctica’s ice shelves, borne from a lack of observations, lead to large uncertainties in sea level predictions. Using high-resolution maps of the underside of Dotson Ice Shelf, West Antarctica, we reveal the imprint that ice shelf basal melting leaves on the ice. Convection and intermittent warm water intrusions form widespread terraced features through slow melting in quiescent areas, while shear-driven turbulence rapidly melts smooth, eroded topographies in outflow areas, as well as enigmatic teardrop-shaped indentations that result from boundary-layer flow rotation. Full-thickness ice fractures, with bases modified by basal melting and convective processes, are observed throughout the area. This new wealth of processes, all active under a single ice shelf, must be considered to accurately predict future Antarctic ice shelf melt.

Abstract Understanding ice shelf water (ISW) structure is crucial for studying the basal melting of ice shelves. In this study, we performed large‐eddy simulation experiments to assess ISW structure and basal melt patterns under different current velocity scenarios observed in the Larsen C ice shelf, Antarctica. The LES results revealed that the thickness of ISW is primarily determined by the meridional velocity (perpendicular to the grounding line), while the zonal velocity influences the potential temperature and salinity of ISW. We found that a key parameter determining the basal melt rate was northward meltwater advection which originates from variances in meltwater generation. This advection, in turn, leads to the tilted isopycnals and the potential for thermohaline interleaving in a diffusive convection regime. The different slopes of isopycnals induce distinct heat fluxes, resulting in different basal melt rates far from and near the grounding line (∼0.44 and 1.59 m yr−1, respectively).

Thwaites Glacier is one of the fastest-changing ice–ocean systems in Antarctica 1 – 3 . Much of the ice sheet within the catchment of Thwaites Glacier is grounded below sea level on bedrock that deepens inland 4 , making it susceptible to rapid and irreversible ice loss that could raise the global sea level by more than half a metre 2 , 3 , 5 . The rate and extent of ice loss, and whether it proceeds irreversibly, are set by the ocean conditions and basal melting within the grounding-zone region where Thwaites Glacier first goes afloat 3 , 6 , both of which are largely unknown. Here we show—using observations from a hot-water-drilled access hole—that the grounding zone of Thwaites Eastern Ice Shelf (TEIS) is characterized by a warm and highly stable water column with temperatures substantially higher than the in situ freezing point. Despite these warm conditions, low current speeds and strong density stratification in the ice–ocean boundary layer actively restrict the vertical mixing of heat towards the ice base 7 , 8 , resulting in strongly suppressed basal melting. Our results demonstrate that the canonical model of ice-shelf basal melting used to generate sea-level projections cannot reproduce observed melt rates beneath this critically important glacier, and that rapid and possibly unstable grounding-line retreat may be associated with relatively modest basal melt rates.

Thwaites Glacier represents 15% of the ice discharge from the West Antarctic Ice Sheet and influences a wider catchment 1 – 3 . Because it is grounded below sea level 4 , 5 , Thwaites Glacier is thought to be susceptible to runaway retreat triggered at the grounding line (GL) at which the glacier reaches the ocean 6 , 7 . Recent ice-flow acceleration 2 , 8 and retreat of the ice front 8 – 10 and GL 11 , 12 indicate that ice loss will continue. The relative impacts of mechanisms underlying recent retreat are however uncertain. Here we show sustained GL retreat from at least 2011 to 2020 and resolve mechanisms of ice-shelf melt at the submetre scale. Our conclusions are based on observations of the Thwaites Eastern Ice Shelf (TEIS) from an underwater vehicle, extending from the GL to 3 km oceanward and from the ice–ocean interface to the sea floor. These observations show a rough ice base above a sea floor sloping upward towards the GL and an ocean cavity in which the warmest water exceeds 2 °C above freezing. Data closest to the ice base show that enhanced melting occurs along sloped surfaces that initiate near the GL and evolve into steep-sided terraces. This pronounced melting along steep ice faces, including in crevasses, produces stratification that suppresses melt along flat interfaces. These data imply that slope-dependent melting sculpts the ice base and acts as an important response to ocean warming.

Filchner-Ronne Ice Shelf (FRIS) is the world's largest ice shelf by volume. It helps regulate Antarctica's contribution to global sea level rise, and water mass transformations within the sub-ice-shelf cavity produce globally important dense water masses. Rates of ice shelf basal melting are relatively low, however, as the production of cold (-1.9 degrees C) and dense High Salinity Shelf Water over the Weddell Sea continental shelf isolates the ice shelf from large-scale inflow of warm water. Nevertheless, a narrow inflow of relatively warm (-1.4 degrees C) Modified Warm Deep Water (MWDW) that hugs the western flank of Berkner Bank is observed to reach Ronne Ice Front, although the processes governing its circulation and fate remain uncertain. Here we present the first observations taken within the ice shelf cavity along this warm water inflow using the Autosub Long Range autonomous underwater vehicle. We observe a core of MWDW with a south-westward velocity of 4 cm s(-1) that reaches at least 18 km into the sub-ice cavity. The hydrographic properties are spatially heterogeneous, giving rise to temporal variability that is driven by tidal advection. The highest rates of turbulent dissipation are associated with the warmest MWDW, with the vertical eddy diffusivity reaching 10(-4) m(2) s(-1) where the water column is fully turbulent. Mixing efficiency is close to the canonical value of 0.2. Modeling studies suggest MWDW may become the dominant water mass beneath FRIS in our changing climate, providing strong motivation to understand more fully the dynamics of this MWDW inflow.

Warmer and more persistent intrusions of Circumpolar Deep Water (CDW) onto the West Antarctic Peninsula are a key driver of the recent increase in ice shelf mass loss. The relatively warm and salty CDW is thought to be mixed up to the base of the ice shelves via shear‐driven turbulence where it has a high capacity to melt the ice. We analyze data from a year‐long mooring beneath George VI Ice Shelf at a location where double‐diffusive layering was observed. The turbulent dissipation rates do not vary with mean flow speed, suggesting shear‐driven mixing is not the driver of basal melt at this site. Instead, we predict the observed dissipation using a new method that links along‐isopycnal stirring of temperature anomalies with double‐diffusive convection. Our work suggests that along‐isopycnal temperature variance may be a stronger indicator of melt than flow speed within strongly stratified ice shelf‐ocean boundary layers.

The melt rate of Antarctic ice shelves is of key importance for rising sea levels and future climate scenarios. Recent observations beneath Larsen C Ice Shelf revealed an ocean boundary layer that was highly turbulent and raised questions on the effect of these rich flow dynamics on the ocean heat transfer and the ice shelf melt rate. Directly motivated by the field observations, we have conducted large-eddy simulations (LES) to further examine the ocean boundary layer beneath Larsen C Ice Shelf. The LES was initialized with uniform temperature and salinity (T-S) and included a realistic tidal cycle and a small basal slope. A new parameterization based on previous work was applied at the top boundary to model near-wall turbulence and basal melting. The resulting vertical T-S profiles, melt rate, and friction velocity matched well with the Larsen C Ice Shelf observations. The instantaneous melt rate varied strongly with the tidal cycle, with faster flow increasing the turbulence and mixing of heat toward the ice base. An Ekman layer formed beneath the ice base and, due to the strong vertical shear of the current, Ekman rolls appeared in the mixed layer and stratified region (depth approximate to 20-60 m). In an additional high-resolution simulation (conducted with a smaller domain) the Ekman rolls were associated with increased turbulent kinetic energy, but a relatively small vertical heat flux. Our results will help with interpreting field observations and parameterizing the ocean-driven basal melting of ice shelves.

Floating ice shelves are the Achilles' heel of the Antarctic Ice Sheet. They limit Antarctica's contribution to global sea level rise, yet they can be rapidly melted from beneath by a warming ocean. At Filchner-Ronne Ice Shelf, a decline in sea ice formation may increase basal melt rates and accelerate marine ice sheet mass loss within this century. However, the understanding of this tipping-point behavior largely relies on numerical models. Our new multi-annual observations from five hot-water drilled boreholes through Filchner-Ronne Ice Shelf show that since 2015 there has been an intensification of the density-driven ice shelf cavity-wide circulation in response to reinforced wind-driven sea ice formation in the Ronne polynya. Enhanced southerly winds over Ronne Ice Shelf coincide with westward displacements of the Amundsen Sea Low position, connecting the cavity circulation with changes in large-scale atmospheric circulation patterns as a new aspect of the atmosphere-ocean-ice shelf system.

Abstract Subglacial sediments have the potential to reveal information about the controls on glacier flow, changes in ice-sheet history and characterise life in those environments. Retrieving sediments from beneath the ice, through hot water drilled access holes at remote field locations, present many challenges. Motivated by the need to minimise weight, corer diameter and simplify assembly and operation, British Antarctic Survey, in collaboration with UWITEC, developed a simple mechanical percussion corer. At depths over 1000 m however, manual operation of the percussion hammer is compromised by the lack of clear operator feedback at the surface. To address this, we present a new auto-release-recovery percussion hammer mechanism that makes coring operations depth independent and improves hammer efficiency. Using a single rope tether for both the corer and hammer operation, this modified percussion corer is relatively simple to operate, easy to maintain, and has successfully operated at a depth of >2130 m.

Icefin performed the first long range robotic exploration of the grounding zone of Thwaites Glacier from January 9-12 2020. Icefin was part of the MELT project of the International Thwaites Glacier Collaboration deployed to the grounding zone of Thwaites Glacier, West Antarctica over the period December 2019-February 2020. MELT is an interdisciplinary project to explore rapid change across the grounding zone, and in particular basal melting. The subglacial cavity ~2km north of the grounding zone was accessed via hot water drilling on January 7-8, 2020. Icefin, a hybrid autonomous and remotely operated underwater vehicle designed for sub-ice and borehole operations, conducted over 15km of round-trip data collection under the ice along a section of the glacier from the grounding zone extending to a point 4 km oceanward. The vehicle collected data with ten different science sensors including cameras, sonars, conductivity/temperature and dissolved oxygen. Overall, the water column ranged from ~100m downstream that narrowed quickly to an average of 50m that spanned over 2km, to a long segment of ~30m thickness before quickly narrowing over 500m towards the grounding zone. The seafloor structures run roughly parallel to ice flow direction, consisting of furrows, ridges, and grooves in some cases mirrored by the ice structure. The Icefin dives revealed a diverse set of basal ice conditions, with complex geometry, including a range of terraced features, smooth ablated surfaces, crevassing, sediment rich layers of varying kinds, as well as interspersed clear, potentially accreted freshwater ice. The ocean directly beneath the ice varies spatially, from moderately well-mixed near the grounding zone to highly stratified within and below concavities in the ice downstream. Sediments along the sea floor range from fine grained downstream to course angular gravel near the grounding zone distributed between larger boulders. We observed rocky material in the ice that ranged from fine grained layers compressed within the ice to small angular particles volumetrically distributed within ice, to gravel and cobbles, as well as trapped boulders up to meter scale. In addition to the oceanographic, glaciological and sea floor conditions, we also catalogued communities of organisms along the seafloor and ice-ocean interface. We will report the highlights and initial conclusions from Icefin’s in situ data collection, and offer perspectives on change at the grounding zone.

Increased ocean-driven basal melting beneath Antarctic ice shelves causes grounded ice to flow into the ocean at an accelerated rate, with consequences for global sea level. The turbulent transfer of heat through the ice shelf-ocean boundary layer is critical in setting the basal melt rate, yet the processes controlling this transfer are poorly understood and inadequately represented in global climate models. This creates large uncertainties in predictions of future sea level rise. Using a hot-water drilled access hole, two turbulence instrument clusters (TICs) were deployed 2.5 and 13.5 m beneath Larsen C ice shelf in December 2011. Both instruments returned a yearlong record of turbulent velocity fluctuations, providing a unique opportunity to explore the turbulent processes within the ice shelf-ocean boundary layer. Although the scaling between the turbulent kinetic energy (TKE) dissipation rate and mean flow speed varies with distance from the ice shelf base, at both TICs the TKE dissipation rate is balanced entirely by the rate of shear production. The freshwater released by basal melting plays no role in the TKE balance. When the upper TIC is within the log-layer, we derive an under-ice drag coefficient of 0.0022 and a roughness length of 0.44 mm, indicating that the ice base is smooth. Finally, we demonstrate that although the canonical three-equation melt rate parameterization can accurately predict the melt rate for this example of smooth ice underlain by a cold, tidally forced boundary layer, the law of the wall assumption employed by the parameterization does not hold at low flow speeds. Plain Language Summary Antarctic ice shelves are the floating extensions of the Antarctic ice sheet. They help control the future contribution of Antarctica to global sea level rise by restricting the flow of grounded ice into the ocean. Many ice shelves are increasingly being melted from beneath by relatively warm ocean waters, leading to an acceleration in the flow of ice from the ice sheet interior into the ocean. The very small-scale turbulent processes that control the rate of basal melting, however, are poorly represented in global climate models, resulting in large uncertainties in projections of future sea level rise. Here we present a yearlong record of the turbulent processes that drive basal melting beneath an Antarctic ice shelf. We show that at our measurement site the turbulence generated by the mean flow is dissipated entirely by small-scale molecular processes. The freshwater released by basal melting does not significantly influence the turbulent environment. Most importantly, however, we demonstrate directly from our observations that existing representations of basal melting in global climate models can accurately predict the melt rate when turbulent mixing is sufficiently intense to overcome stratification. Much work remains to be done, as the turbulent environment varies widely between ice shelves.

The southward freshwater flux through Nares Strait is an important component of the Arctic's freshwater budget. On short time scales, flow through the strait is dominated by the tides, and tidal dynamics may be important for the magnitude of the freshwater flux over longer periods. Here we build upon our existing knowledge of the tides in the region by exploring their propagation and vertical structure using data from four bottom-mounted Acoustic Doppler Current Profilers deployed in Nares Strait between 2003 and 2006. We observe that propagating barotropic semidiurnal tidal waves interact to create a standing wave pattern, explaining the abnormally large tidal amplitudes that are observed in this region. In the along-strait direction, semidiurnal tidal currents exhibit strong variations with depth. In contrast, the diurnal tides propagate northward through the strait as progressive waves, and the tidal currents are broadly depth invariant. Proximity of Nares Strait to the semidiurnal critical latitude and the topographical restriction imposed by the steep side wall of Ellesmere Island are primary drivers behind the observed vertical variability. In the upper part of the water column, baroclinic activity increases the tidal current amplitude by up to 25%. In the across-strait direction, a two-layer structure exists in both the diurnal and semidiurnal tidal flow, with a phase lag of approximately a quarter of a tidal cycle across the strait for the semidiurnal tide. Our results suggest that strong vertical motion exists against the side walls of Nares Strait, as the across-strait flow interacts with the steeply sloping bathymetry.

Abstract A climatically induced acceleration in ocean‐driven melting of Antarctic ice shelves would have consequences for both the discharge of continental ice into the ocean and thus global sea level, and for the formation of Antarctic Bottom Water and the oceanic meridional overturning circulation. Using a novel gas‐tight in situ water sampler, noble gas samples have been collected from six locations beneath the Filchner Ice Shelf, the first such samples from beneath an Antarctic ice shelf. Helium and neon are uniquely suited as tracers of glacial meltwater in the ocean. Basal meltwater fractions range from 3.6% near the ice shelf base to 0.5% near the sea floor, with distinct regional differences. We estimate an average basal melt rate for the Filchner‐Ronne Ice Shelf of 177 ± 95 Gt/year, independently confirming previous results. We calculate that up to 2.7% of the meltwater has been refrozen, and we identify a local source of crustal helium.

Ocean‐driven basal melting of Amundsen Sea ice shelves has triggered acceleration, thinning, and grounding line retreat on many West Antarctic outlet glaciers. Here we present the first year‐long (2014) record of basal melt rate at sub‐weekly resolution from a location on the outer Pine Island Ice Shelf. Adjustment of the upper thermocline to local wind forced variability in the vertical Ekman velocity is the dominant control on basal melting at weekly to monthly timescales. Atmosphere‐ice‐ocean surface heat fluxes or changes in advection of modified Circumpolar Deep Water play no discernible role at these timescales. We propose that during other years, a deepening of the thermocline in Pine Island Bay driven by longer timescale processes may have suppressed the impact of local wind forcing on high‐frequency upper thermocline height variability and basal melting. This highlights the complex interplay between the different processes and their timescales that set the basal melt rate beneath Pine Island Ice Shelf.