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Abstract Parameterization of submarine melting represents a large source of uncertainty in modeling ice sheet response to climate change. Here we present in situ observations of melt at near‐vertical ice faces using a novel instrument platform mounted rigidly to icebergs. We investigate boundary layer dynamics controlling melt across 31 measurement periods that span a range of momentum and thermal forcing (1–12 cm/s flows and 3–10 K). While melt generally scales with velocity and temperature, we find substantially enhanced melt linked with unsteady forcing. Several implementations of the three‐equation melt parameterization show melt can be predicted within a factor of 2 if the model is evaluated with peak near‐boundary velocities and flows are quasi‐steady. However, if flows are unsteady or the model is evaluated with low‐resolution velocities, melt is underpredicted by 2– We conclude that understanding the detailed character of near‐boundary flows is critical for submarine melt predictions. 

Basal channels, which are troughs carved into the undersides of ice shelves by buoyant plumes of water, are modulators of ice-shelf basal melt and structural stability. In this study, we track the evolution of 12 large basal channels beneath ice shelves of the Amundsen and Bellingshausen seas region in West Antarctica using the Landsat record since its start in the 1970s through 2020. We observe examples of channel growth, interactions with ice-shelf features, and systematic changes in sinuosity that give insight into the life cycles of basal channels. We use the last two decades of the record, combined with contemporary ice-flow velocity datasets, to separate channel-path evolution into components related to advection by ice flow and those controlled by other forcings, such as ocean melt or surface accumulation. Our results show that ice-flow-independent lateral channel migration is overwhelmingly to the left when viewed down-flow, suggesting that it is dominated by Coriolis-influenced ocean melt. By applying a model of channel-path evolution dominantly controlled by ice flow and ocean melt, we show that the majority of channels surveyed exhibit non-steady behavior that serves as a novel proxy for increased ocean forcing in West Antarctica starting at least in the early 1970s. 

Abstract. In late March 2011, landfast sea ice (hereafter, “fast ice”) formed in the northern Larsen B embayment and persisted continuously as multi-year fast ice until January 2022. In the 11 years of fast-ice presence, the northern Larsen B glaciers slowed significantly, thickened in their lower reaches, and developed extensive mélange areas, leading to the formation of ice tongues that extended up to 16 km from the 2011 ice fronts. In situ measurements of ice speed on adjacent ice shelf areas spanning 2011 to 2017 show that the fast ice provided significant resistive stress to ice flow. Fast-ice breakout began in late January 2022 and was closely followed by retreat and breakup of both the fast-ice mélange and the glacier ice tongues. We investigate the probable triggers for the loss of fast ice and document the initial upstream glacier responses. The fast-ice breakup is linked to the arrival of a strong ocean swell event (>1.5 m amplitude; wave period waves >5 s) originating from the northeast. Wave propagation to the ice front was facilitated by a 12-year low in sea ice concentration in the northwestern Weddell Sea, creating a near-ice-free corridor to the open ocean. Remote sensing data in the months following the fast-ice breakout reveals an initial ice flow speed increase (>2-fold), elevation loss (9 to 11 m), and rapid calving of floating and grounded ice for the three main embayment glaciers Crane (11 km), Hektoria (25 km), and Green (18 km). 

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 Feedbacks between ice melt, glacier flow and ocean circulation can rapidly accelerate ice loss at tidewater glaciers and alter projections of sea-level rise. At the core of these projections is a model for ice melt that neglects the fact that glacier ice contains pressurized bubbles of air due to its formation from compressed snow. Current model estimates can underpredict glacier melt at termini outside the region influenced by the subglacial discharge plume by a factor of 10–100 compared with observations. Here we use laboratory-scale experiments and theoretical arguments to show that the bursting of pressurized bubbles from glacier ice could be a source of this discrepancy. These bubbles eject air into the seawater, delivering additional buoyancy and impulses of turbulent kinetic energy to the boundary layer, accelerating ice melt. We show that real glacier ice melts 2.25 times faster than clear bubble-free ice when driven by natural convection in a laboratory setting. We extend these results to the geophysical scale to show how bubble dynamics contribute to ice melt from tidewater glaciers. Consequently, these results could increase the accuracy of modelled predictions of ice loss to better constrain sea-level rise projections globally. 

Abstract Rift propagation, rather than basal melt, drives the destabilization and disintegration of the Thwaites Eastern Ice Shelf. Since 2016, rifts have episodically advanced throughout the central ice-shelf area, with rapid propagation events occurring during austral spring. The ice shelf's speed has increased by ~70% during this period, transitioning from a rate of 1.65 m d⁻¹in 2019 to 2.85 m d⁻¹by early 2023 in the central area. The increase in longitudinal strain rates near the grounding zone has led to full-thickness rifts and melange-filled gaps since 2020. A recent sea-ice break out has accelerated retreat at the western calving front, effectively separating the ice shelf from what remained of its northwestern pinning point. Meanwhile, a distributed set of phase-sensitive radar measurements indicates that the basal melting rate is generally small, likely due to a widespread robust ocean stratification beneath the ice–ocean interface that suppresses basal melt despite the presence of substantial oceanic heat at depth. These observations in combination with damage modeling show that, while ocean forcing is responsible for triggering the current West Antarctic ice retreat, the Thwaites Eastern Ice Shelf is experiencing dynamic feedbacks over decadal timescales that are driving ice-shelf disintegration, now independent of basal melt. 

Abstract Frontal ablation, the combination of submarine melting and iceberg calving, changes the geometry of a glacier's terminus, influencing glacier dynamics, the fate of upwelling plumes and the distribution of submarine meltwater input into the ocean. Directly observing frontal ablation and terminus morphology below the waterline is difficult, however, limiting our understanding of these coupled ice–ocean processes. To investigate the evolution of a tidewater glacier's submarine terminus, we combine 3-D multibeam point clouds of the subsurface ice face at LeConte Glacier, Alaska, with concurrent observations of environmental conditions during three field campaigns between 2016 and 2018. We observe terminus morphology that was predominately overcut (52% in August 2016, 63% in May 2017 and 74% in September 2018), accompanied by high multibeam sonar-derived melt rates (4.84 m d −1 in 2016, 1.13 m d −1 in 2017 and 1.85 m d −1 in 2018). We find that periods of high subglacial discharge lead to localized undercut discharge outlets, but adjacent to these outlets the terminus maintains significantly overcut geometry, with an ice ramp that protrudes 75 m into the fjord in 2017 and 125 m in 2018. Our data challenge the assumption that tidewater glacier termini are largely undercut during periods of high submarine melting. 

Abstract. Atmospheric rivers (ARs) transport large amounts of moisture from the mid- to high-latitudes and they are a primary driver of the most extremesnowfall events, along with surface melting, in Antarctica. In this study, we characterize the climatology and surface impacts of ARs on WestAntarctica, focusing on the Amundsen Sea Embayment and Marie Byrd Land. First, we develop a climatology of ARs in this region, using anAntarctic-specific AR detection tool combined with theModern-Era Retrospective analysis for Research and Applications, version 2 (MERRA-2) and the European Centre for Medium-Range Weather Forecasts (ECMWF) Reanalysis v5 (ERA5) atmospheric reanalyses. We find that while ARs are infrequent (occurring 3 % of the time), they cause intense precipitation in short periods of time and account for 11 % of the annual surface accumulation. They are driven by the coupling of a blocking high over the Antarctic Peninsula with a low-pressure system known as the Amundsen Sea Low. Next, we use observations from automatic weather stations on Thwaites Eastern Ice Shelf with the firn model SNOWPACK and interferometric reflectometry (IR) to examine a case study of three ARs that made landfall in rapid succession from 2 to 8 February 2020, known as an AR family event. While accumulation dominates the surface impacts of the event on Thwaites Eastern Ice Shelf (> 100 kg m−2 or millimeters water equivalent), we find small amounts of surface melt as well (< 5 kg m−2). The results presented here enable us to quantify the past impacts of ARs on West Antarctica's surface mass balance (SMB) and characterize their interannual variability and trends, enabling a better assessment of future AR-driven changes in the SMB. 

Abstract. The Thwaites Eastern Ice Shelf buttresses a significant portion of Thwaites Glacier through contact with a pinning point 40 km offshore of the present grounding line. Predicting future rates of Thwaites Glacier’s contribution to sea-level rise depends on the evolution of this pinning point and the resultant change in the ice-shelf stress field since the breakup of the Thwaites Western Glacier Tongue in 2009. Here we use Landsat-8 feature tracking of ice velocity in combination with ice-sheet model perturbation experiments to show how past changes in flow velocity have been governed in large part by changes in lateral shear and pinning point interactions with the Thwaites Western Glacier Tongue. We then use recent satellite altimetry data from ICESat-2 to show that Thwaites Glacier’s grounding line has continued to retreat rapidly; in particular, the grounded area of the pinning point is greatly reduced from earlier mappings in 2014, and grounded ice elevations are continuing to decrease. This loss has created two pinned areas with ice flow now funneled between them. If current rates of surface lowering persist, the Thwaites Eastern Ice Shelf will unpin from the seafloor in less than a decade, despite our finding from airborne radar data that the seafloor underneath the pinning point is about 200 m shallower than previously reported. Advection of relatively thin and mechanically damaged ice onto the remaining portions of the pinning point and feedback mechanisms involving basal melting may further accelerate the unpinning. As a result, ice discharge will likely increase up to 10 % along a 45 km stretch of the grounding line that is currently buttressed by the Thwaites Eastern Ice Shelf. 

Abstract To increase inclusivity, diversity, equity and accessibility in Antarctic science, we must build more positive and inclusive Antarctic field work environments. The International Thwaites Glacier Collaboration (ITGC) has engaged in efforts to contribute to that goal through a variety of activities since 2018, including creating an open-access ‘Field and Ship Best Practices’ guide, engaging in pre-field season team dynamics meetings, and surveying post-field season reflections and experiences. We report specific actions taken by ITGC and their outcomes. We found that strong and supported early career researchers brought new and important perspectives regarding strategies for transforming culture. We discovered that engaged and involved senior leadership was also critical for expanding participation and securing funding to support efforts. Pre-field discussions involving all field team members were particularly helpful for setting expectations, improving sense of belonging, describing field work best practices, and co-creating a positive work culture.