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The recent surge of the Bering-Bagley Glacier System (BBGS), Alaska, in 2008-2013 provided a rare opportunity to study surging in a large and complex system. We simulate glacier evolution for a 20 year quiescent phase, where geometrical and hydrological changes lead to conditions favorable for surging, and the first two years of a surge phase where a surge-front propagates through the system activating the surging ice. For each phase, we analyze the simulated elevation-change and ice-velocity pattern, and infer information on the evolving basal drainage system through hydropotential analysis. During the quiescent phase simulation, several reservoir areas form at locations consistent with those observed. Up-glacier of these reservoir areas, water drainage paths become increasingly lateral and hydropotential wells form indicating an expanding storage capacity of subglacial water. These results are attributed to local bedrock topography characterized by large subglacial ridges that act to dam the down-glacier flow of ice and water. Based on the BBGS’s end-of-quiescence state, we propose several surge initiation criteria to predict when the system is set to surge. In the surge simulation, we model surge evolution through Bering Glacier’s trunk by implementing a new friction law that mimics a propagating surge-wave. Modeled surge velocities share spatial patterns and reach similar peaks as those observed in 2008-2010. As the surge progresses through the glacier, drainage efficiency further degrades in the active surging zone from its already inefficient, end-of-quiescence state. Satellite observations from 2013 indicate hydraulic drainage efficiency throughout the glacier was restored after the surge had ended.
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Propagating speedups during quiescence escalate to the 2020–2021 surge of Sít’ Kusá, southeast Alaska
Abstract We use satellite image processing techniques to measure surface elevation and velocity changes on a temperate surging glacier, Sít’ Kusá, throughout its entire 2013–2021 surge cycle. We present detailed records of its dynamic changes during quiescence (2013–2019) and its surge progression (2020–2021). Throughout quiescence, we observe order-of-magnitude speedups that propagate down-glacier seasonally from the glacier's upper northern tributary, above a steep icefall, into the reservoir zone for the surging portion of the glacier. The speedups initiate in fall and gradually accelerate through winter until they peak in late spring, ~1 − 2 months after the onset of melt. Propagation distance of the speedups controls the distribution of mass accumulation in the reservoir zone prior to the surge. Furthermore, the intensity and propagation distance of each year's speedup is correlated with the positive degree day sum from the preceding melt season, suggesting that winter melt storage drives the seasonal speedups. We demonstrate that the speedups are kinematically similar to the 2020–2021 surge, differing mainly in that the surge propagates past the dynamic balance line at the lower limit of the reservoir zone, likely triggered by the exceedance of a tipping point in mass accumulation and basal enthalpy in the reservoir zone.
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- PAR ID:
- 10526536
- Publisher / Repository:
- Journal of Glaciology
- Date Published:
- Journal Name:
- Journal of Glaciology
- ISSN:
- 0022-1430
- Page Range / eLocation ID:
- 1 to 12
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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null (Ed.)Abstract Glacier surges are periodic episodes of mass redistribution characterized by dramatic increases in ice flow velocity and, sometimes, terminus advance. We use optical satellite imagery to document five previously unexamined surge events of Sít’ Kusá (Turner Glacier) in the St. Elias Mountains of Alaska from 1983 to 2013. Surge events had an average recurrence interval of ~5 years, making it the shortest known regular recurrence interval in the world. Surge events appear to initiate in the winter, with speeds reaching up to ~25 m d −1 . The surges propagate down-glacier over ~2 years, resulting in maximum thinning of ~100 m in the reservoir zone and comparable thickening at the terminus. Collectively, the rapid recurrence interval, winter initiation and down-glacier propagation suggest Sít’ Kusá's surges are driven by periodic changes in subglacial hydrology and glacier sliding. Elevation change observations from the northern tributary show a kinematic disconnect above and below an icefall located 23 km from the terminus. We suggest the kinematic disconnect inhibits drawdown from the accumulation zone above the icefall, which leads to a steady flux of ice into the reservoir zone, and contributes to the glacier's exceptionally short recurrence interval.more » « less
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Abstract The Bering‐Bagley Glacier System (BBGS), Alaska, Earth's largest temperate surging glacier, surged in 2008–2013. We use numerical modeling and satellite observations to investigate how surging in a large and complex glacier system differs from surging in smaller glaciers for which our current understanding of the surge phenomenon is based. With numerical simulations of a long quiescent phase and a short surge phase in the BBGS, we show that surging is more spatiotemporally complex in larger glaciers with multiple reservoir areas forming during quiescence which interact in a cascading manner when ice accelerates during the surge phase. For each phase, we analyze the simulated elevation‐change and ice‐velocity pattern, infer information on the evolving basal drainage system through hydropotential analysis, and supplement these findings with observational data such as CryoSat‐2 digital elevation maps. During the quiescent simulation, water drainage paths become increasingly lateral and hydropotential wells form indicating an expanding storage capacity of subglacial water. These results are attributed to local bedrock topography characterized by large subglacial ridges that dam the down‐glacier flow of ice and water. In the surge simulation, we model surge evolution through Bering Glacier's trunk by imposing a basal friction representation that mimics a propagating surge wave. As the surge progresses, drainage efficiency further degrades in the active surging‐zone from its already inefficient, end‐of‐quiescence state. Results from this study improve our knowledge of surging in large and complex systems which generalizes to glacial accelerations observed in outlet glaciers of Greenland, thus reducing uncertainty in modeling sea‐level rise.more » « less
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Abstract Glacier speedups occur on daily to centennial timescales. While basal water and subglacial drainage configuration are thought to drive glacier speedups across these timescales, it remains unclear whether this forcing always occurs through the same mechanisms. Here, we explore whether the enthalpy model of glacier surging can explain speedups over a broader range of timescales if modified to account for seasonality in surface melt and continuous water supply to the glacier bed. We simulate velocity oscillations that range from seasonal to years. Our model results more closely resemble observations of surges than previous model versions because ice flow variability at seasonal and multi‐year timescales is reproduced simultaneously through hydrological forcing. Under favorable conditions, seasonal water delivery to the bed gradually accumulates in a poorly‐connected basal drainage system, priming the glacier to surge. Surges themselves are marked by high water fluxes and enthalpy drainage from the glacier base.more » « less
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The unique measurement capabilities of ICESat-2 allow high spatiotemporal resolution of complex ice-dynamic processes that occur during a surge. Detailed and precise mapping of height changes on surge glaciers has previously escaped observations from space due to limited resolution of space-borne altimeter data and the surface characteristics of glaciers during surge such as heavy crevassing. This makes geophysical interpretation of deformation and assessment of mass transfer difficult. In this paper, we present an approach that facilitates analysis of the evolution of geophysical processes during a surge, including height changes, crevassing, mass transfer and roughness evolution. We utilize all data from 2 years of ICESat-2 observations collected during the mature phase of the Negribreen Glacier System (NGS) surge in 2019 and 2020. The progression of the NGS surge has resulted in large-scale elevation changes and wide-spread crevassing making it an ideal case study to demonstrate ICESat-2 measurement capabilities, which are maximized when coupled with the Density Dimension Algorithm for Ice (DDA-ice). Results show the expansion of the surge in upper Negribreen which demonstrates the unique ability of ICESat-2/DDA-ice to measure a rapidly changing surge glacier and provide the best estimates for cryospheric changes and their contributions to sea-level rise.more » « less
- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1017/jog.2023.99
