3560 The triggers of the disintegration of Voyeykov Ice Shelf (2007), Wilkes Land, East Antarctica, and its subsequent evolution Jennifer Arthur, Chris Stokes, Stewart Jamieson, Bertie Miles, Rachel Carr, Amber Leeson Corresponding author: Jennifer Arthur Corresponding author e-mail: [email protected] Understanding the controls on the dynamics and disintegration of ice shelves and glacier tongues is crucial to predicting future inland ice loss from the Antarctic Ice Sheet and its contribution to sea-level rise. Voyeykov Ice Shelf (VIS) in Wilkes Land, East Antarctica, lost ~2445 km2 of ice during a disintegration event in late March/early April 2007. Here, we investigate changes in its structure, thickness and velocity during the seven years preceding its disintegration and its evolution until April 2020 using Landsat, Envisat ASAR and MODIS imagery and annual velocity mosaics. We also investigate potential external controls by comparing the timing and scale of changes in a range of datasets including air temperature and wind-field anomalies from climate reanalysis, surface melt predicted by a regional climate model and sea ice concentrations. We show that disintegration was preceded by widening and propagation of major rifts extending to the calving front, but that following disintegration, an immediate speed-up did not occur, indicating the disintegrated portion of VIS exerted minimal buttressing. Our analysis suggests ice shelf disintegration was likely triggered by mélange removal linked to an atmospheric warm anomaly and a less extensive latent heat polynya, which triggered multiyear landfast sea ice (MYLI) breakout and the intrusion of warmer surface waters. Katabatic wind-driven ponding weakened the MYLI adjacent to VIS and primed it for breakout. The example of VIS highlights the need for ice shelf and ice sheet models to account for mélange-ice shelf interactions in the context of predicted future warming. An important implication is that surface melt pond- driven hydrofracturing is not always a necessary precursor to ice shelf collapse. 3661 A 3-D Model of Antarctic Ice Shelf Surface Hydrology Sammie Buzzard, Alex Robel Corresponding author: Sammie Buzzard Corresponding author e-mail: [email protected] The formation of surface meltwater has been linked with the disintegration of many ice shelves in the Antarctic Peninsula over the last several decades. Despite the importance of surface meltwater production and transport to ice shelf stability, knowledge of these processes is still lacking. Understanding the surface hydrology of ice shelves is an essential first step to reliably project future sea level rise from ice sheet melt. In order to better understand the processes driving meltwater distribution on ice shelves, we present results from case studies using a new 3-D model of surface hydrology for Antarctic ice shelves. It is the first comprehensive model of surface hydrology to be developed for Antarctic ice shelves, enabling us to incorporate key processes such as the lateral transport of surface meltwater. Recent observations suggest that surface hydrology processes on ice shelves are more complex than previously thought, and that processes such as lateral routing of meltwater across ice shelves, ice shelf flexure and surface debris all play a role in the location and influence of meltwater. Our model allows us to account for these and is calibrated and validated through both remote sensing and field observations. Here we present results from in depth studies from selected ice shelves with significant surface melt features. This community-driven, open-access model has been developed with input from observations, and allows us to provide new insights into surface meltwater distribution on Antarctica’s ice shelves. This enables us to answer key questions about their past and future evolution under changing atmospheric conditions and vulnerability to meltwater driven hydrofracture and collapse. 3762 Analysing palaeocirque glacier equilibrium line altitudes as indicators of palaeoclimate across the southern Scandinavian Mountains Rachel Oien, Matteo Spagnolo, Brice Rea, Iestyn D. Barr, Robert G. Bingham Corresponding author: Rachel Oien Corresponding author e-mail: [email protected] The equilibrium line altitudes (ELAs) of palaeocirque glaciers are used to obtain quantitative palaeoclimatic information from alpine environments. The dimensions of these glaciers, and their ELAs, are reconstructed based on the topography of, now ice-free, cirques. Typically, this approach is used to derive palaeoclimatic data for a particular time period, typically glossing over the fact that cirques are time- transgressive landforms, shaped over multiple glacial cycles. In this study, we test the validity of using palaeocirque ELAs as indicators of palaeoclimate by comparing them to modern cirques and their relationship to climate (Oien et al., 2020). To achieve this, we reconstruct ELAs from 800 palaeocirques across the southern Scandinavian Mountains. The cirques are mapped in GIS, and their ELAs calculated using the cirque floor altitude and also via a more sophisticated approach, using the GlaRe and ELA GIS tools. The population of cirques is analysed to investigate whether sub-divisions can be made on the basis of floor altitude, aspect, and links to palaeoclimatic patterns. This study will then be compared to 255 extant cirques. 3863 Calving multiplier effect controlled by glacier terminus geometry Donald Slater, Doug Benn, Tom Cowton, Joe Todd Corresponding author: Donald Slater Corresponding author e-mail: [email protected] Quantifying the impact of submarine melting on calving is absolutely central to understanding the response of marine-terminating glaciers to ocean forcing. Modeling and observational studies have shown the potential for submarine melting to amplify calving (the calving multiplier effect), but there is little consensus as to under what conditions this occurs. Furthermore, the process has not yet been parameterized so that it can be included in the large-scale models used for sea level projection. Here, with help from full-Stokes Elmer/Ice simulations, we propose an analytical basis for understanding the presence or absence of the calving multiplier effect. We show that as a calving front becomes undercut it becomes more susceptible to both serac failure (calving only of ice that is undercut) and rotational failure (full thickness calving of ice behind the grounding line). By deriving analytical thresholds for these two forms of calving based on the depth-mean shear and terminus pressure moment, respectively, we suggest that the dominant calving style is determined by the terminus geometry and by the shape of melt undercutting in particular. Uniform undercutting extending from the grounding line to the waterline promotes serac failure and no multiplier effect, while glaciers experiencing linear undercutting that is deepest at the grounding line are more likely to experience a multiplier effect. Our study offers a quantitative framework for understanding where and when the calving multiplier effect occurs, and, therefore, a route to parameterizing the effect in ice sheet-scale models. 3964 Seasonally stable temperature gradients through supraglacial debris in the Everest region of Nepal, Central Himalaya Ann Rowan, Lindsey Nicholson, Duncan Quincey, Morgan Gibson, Tris Irvine-Fynn, Scott Watson, Patrick Wagnon, David Rounce, Sarah Thompson Corresponding author: Ann Rowan Corresponding author e-mail: [email protected] Rock debris covers about 30% of glacier ablation areas in the Central Himalaya and modifies the impact of atmospheric conditions on mass balance. The thermal properties of supraglacial debris are diurnally variable but remain poorly constrained for monsoon-influenced glaciers over the timescale of the ablation season. We measured vertical debris profile temperatures at 12 sites on four glaciers in the Everest region with debris thickness ranging from 0.08–2.8 m. Typically, the length of the ice ablation season beneath supraglacial debris was 160 days (15 May to 22 October) – a month longer than the monsoon season. Debris temperature gradients were approximately linear (r2 > 0.83), measured as –40°C m– 1 where debris was up to 0.1 m thick, –20°C m–1 for debris 0.1–0.5 m thick, and – 4°C m–1 for debris greater than 0.5 m thick. Our results demonstrate that the influence of supraglacial debris on the temperature of the underlying ice surface, and therefore melt, is stable at a seasonal timescale and can be estimated from near- surface temperature. These results have the potential to greatly improve the representation of ablation in calculations of debris-covered glacier mass balance and projections of their response to climate change. 3572 94 GHz radar backscatter: measurements of glacier terrain William Harcourt, David Macfarlane, Duncan Robertson, Brice Rea, Matteo Spagnolo Corresponding author: William Harcourt Corresponding author e-mail: [email protected] Ground-based remote sensing measurements are capable of yielding new insights into complex glacier processes due to their ability to generate measurements at high spatial and temporal resolution. Here, we will report on the development of a novel ground-based millimetre-wave radar system that is capable of mapping radar backscatter and generating Digital Elevation Models (DEMs) of glacier surfaces at high angular resolution and in most weather conditions. The sensor, called AVTIS2, is a 94 GHz (~3 mm wavelength)