The climate of high midlatitude mountains appears to be warming faster than the global average, but evidence for such elevation-dependent warming (EDW) at higher latitudes is presently scarce. Here, ...we use a comprehensive network of remote meteorological stations, proximal radiosonde measurements, downscaled temperature reanalysis, ice cores, and climate indices to investigate the manifestation and possible drivers of EDW in the St. Elias Mountains in subarctic Yukon, Canada. Linear trend analysis of comprehensively validated annual downscaled North American Regional Reanalysis (NARR) gridded surface air temperatures for the years 1979–2016 indicates a warming rate of 0.028°C a−1 between 5500 and 6000 m above mean sea level (MSL), which is ∼1.6 times larger than the global-average warming rate between 1970 and 2015. The warming rate between 5500 and 6000 m MSL was ∼1.5 times greater than the rate at the 2000–2500 m MSL bin (0.019°C a−1), which is similar to the majority of warming rates estimated worldwide over similar elevation gradients. Accelerated warming since 1979, measured by radiosondes, indicates a maximum rate at 400 hPa (~7010 m MSL). EDW in the St. Elias region therefore appears to be driven by recent warming of the free troposphere. MODIS satellite data show no evidence for an enhanced snow albedo feedback above 2500 m MSL, and declining trends in sulfate aerosols deposited in high-elevation ice cores suggest a modest increase in radiative forcing at these elevations. In contrast, increasing trends in water vapor mixing ratio at the 500-hPa level measured by radiosonde suggest that a longwave radiation vapor feedback is contributing to EDW.
Celotno besedilo
Dostopno za:
BFBNIB, DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
Sediment yields from glacierized basins are used to quantify erosion rates on seasonal to decadal timescales as well as conditions at the glacier bed, and eskers hold valuable information about past ...subglacial hydraulic conditions in their spatial organization, geometry, and sedimentary structures. Ultimately, eskers are a record of past glacio‐fluvial sediment transport, but there is currently no physical model for this process. We develop a 1‐D model of morphodynamics in semicircular bedrock‐floored subglacial channels. We adapt a sediment conservation law developed for mixed alluvial‐bedrock conditions to subglacial channels. Channel evolution is a function of melt opening by viscous heat dissipation from flowing water and creep closure of the overlying ice, to which we add the closure or enlargement due to sediment deposition or removal, respectively. We apply the model to an idealized land‐terminating glacier and find that temporary sediment accumulation in the vicinity of the terminus, or the formation of an incipient esker, is inherent to the dynamics of the channelized water flow. The alluviation of the bed combined with the pressurized channel flow produces unexpected patterns of sediment evacuation: We show that the direction of hysteresis between sediment and water discharge is not necessarily linked to a supply‐ or transport‐limited system, as has been hypothesized for proglacial sediment yields. We also find that the deposition of an incipient esker is a function of a compromise between water discharge and sediment supply, but perhaps more importantly, ice‐surface slope and the temporal pattern of water delivery to the bed.
Plain Language Summary
Glaciers and ice sheets are changing rapidly, impacting sea levels, landscapes, and ecosystems. These changes are tightly linked to the meltwater routing through glaciers' plumbing systems. If this plumbing is pressurized by water flowing into crevasses and moulin (which act like water wells), the ice base can move faster downstream, possibly leading to enhanced ice loss, or vice versa. As glaciers retreated at the end of the last glaciation, they left clues of their passage, including sediment casts of their plumbing system: eskers. Eskers are elongated ridges that snake across the landscape and can be hundreds of kilometers long. Although understanding their deposition can help us understand contemporary ice sheet plumbing systems, their origin has been puzzling for several decades. We build a numerical model tracking sediment as they move with the water under ice. Glaciers naturally produce a sediment bottleneck and tend to form such eskers, producing the first process‐based model for their deposition. We identify ice geometry and temporal patterns of water input into the plumbing system as critical factors, when combined with sufficient sediment and water supplies. This model helps to reconcile contemporary glacier processes and sediment records, which is key to better understand glaciers' plumbing system.
Key Points
A framework is presented for sediment transport in subglacial channels over semialluvial beds
Hysteresis in sediment and water flux is inherent to R‐channels regardless of bed alluviation
The formation of eskers is caused by a sediment bottleneck at the glacier terminus
Modeling a response over a nonconvex design region is a common problem in diverse areas such as engineering and geophysics. The tools available to model and design for such responses are limited and ...have received little attention. We propose a new method for selecting design points over nonconvex regions that is based on the application of multidimensional scaling to the geodesic distance. Optimal designs for prediction are described, with special emphasis on Gaussian process models, followed by a simulation study and an application in glaciology. Supplementary materials for this article are available online.
Five decades of radioglaciology — CORRIGENDUM Schroeder, Dustin M.; Bingham, Robert G.; Blankenship, Donald D. ...
Annals of glaciology,
09/2021, Letnik:
62, Številka:
85-86
Journal Article
Significant intra-annual variability in flow rates of tidewater-terminating Arctic glaciers has been observed in recent years. These changes may result from oceanic and/or atmospheric forcing through ...(1) perturbations at the terminus, such as enhanced submarine melt and changes in sea-ice buttressing, or (2) increased surface melt, in response to atmospheric warming, reaching the bed and promoting glacier slip. We examine the influence of these processes on Belcher Glacier, a large fast-flowing tidewater outlet of the Devon Island ice cap in the Canadian Arctic. A hydrologically-coupled higher-order ice flow model is used to estimate changes in glacier flow speed as a result of changes in sea-ice buttressing and hydrologically-driven melt-season dynamics. Daily run-off from five sub-catchments over the 2008 and 2009 melt seasons provides meltwater forcing for the model simulations. Model results are compared with remotely-sensed and in situ ice-surface velocity measurements. Sea-ice effects are found to have a minor influence on glacier flow speed relative to that of meltwater drainage, which is clearly implicated in short-term velocity variations during the melt season. We find that threshold drainage is essential in determining the timing of these short-lived accelerations.
Outburst floods from ice‐marginal lakes are poised to become more prevalent in a warming climate. As glaciers thin and tributaries detach, water can be impounded in these unstable lakes at valley ...confluences. To characterize the role of the little‐studied englacial hydrological system during an outburst flood cycle, we deployed a variety of geophysical and hydrometeorological instruments in and around an ice‐marginal lake dammed by the Kaskawulsh Glacier, Yukon, Canada, to capture its 2017 filling and drainage. The subaerial lake reached a maximum volume of 9.9
±0.5×106 m
3 on 17 August before draining over the course of
∼19 days. During lake filling, abrupt changes in ice shelf uplift rates are associated with the formation of faults and fractures. These hydromechanical interactions are closely linked to a redistribution of englacial water as evidenced by fluctuations in shallow borehole water pressures, and changes in radar internal reflection power. Water balance calculations reveal that the subaerial, subglacial, and englacial reservoirs respectively store 20 (17–23)%, 40 (25–50)%, and 40 (30–60)% of water in the catchment at peak lake level. The calculated englacial storage volume implies saturated porosities up to 4–10% locally or 2–4% catchment‐wide. The englacial system is therefore both volumetrically important and influenced by the hydromechanical interactions between the lake and glacier. Its spatial extent and storage capacity may play a role in lake drainage initiation, modulation of the flood hydrograph, and lake refilling.
Key Points
Englacial water is stored in evolving fracture networks near the surface and at depth adjacent to an ice‐marginal lake
Exchange of water between subaerial, subglacial, and englacial reservoirs is driven by abrupt glacier uplift and fracturing
Thirty to sixty percent of water in the lake catchment is stored englacially prior to the initiation of drainage
Glacier surging has been studied extensively and is understood as a dynamic instability at the glacier bed. Yet an explanation for the heterogeneous distribution of surge‐type glaciers at the scale ...of a mountain range remains elusive. Here we investigate bedrock discontinuity properties in the basins of 16 surge‐type and nonsurge‐type glaciers in the St. Elias Mountains of Yukon, Canada. Using scaled photographs of bedrock outcrops at the margins of each glacier, we digitize traces of the bedrock discontinuities and with automated purpose‐built software, quantify discontinuity properties that are a function of length, orientation, and spacing of bedrock fractures. We obtain an unexpected result: outcrops in the basins of surge‐type glaciers are less fractured than those in the basins of nonsurge‐type glaciers. We hypothesize that the degree of bedrock fracture may control the extent and location of a clast‐rich till transition zone at the glacier bed. This zone would provide flow resistance conducive to the development of an ice reservoir and thus to surging behavior. To reconcile our observations with the global distribution of surge‐type glaciers, we speculate that surge‐type glaciers may occur in geological settings characterized by an intermediate range of bedrock fracture.
Plain Language Summary
“Surging” is a perplexing form of glacier flow in which ice moves slowly for decades and then suddenly speeds up by a factor of 10 to 1,000. Surging gives us a window into some of the most dramatic forms of mass movement and sheds light on the general mechanics of glacier flow. Here we revisit an early hypothesis that glacier surging is related to fractures in the underlying bedrock. To test this hypothesis, we studied the bedrock characteristics around 16 glaciers—nine of which are known to surge—in the St. Elias Mountains of Yukon, Canada. We quantified the extent of bedrock fracture in each of the glacier basins from photographs, using software developed specifically for this project. Contrary to expectation, we find that surging glaciers in our study area inhabit basins with less fractured bedrock than the basins with nonsurging glaciers. To explain our results, we propose a hypothesis for surging that links bedrock fracture to the nature and distribution of rocky debris under the glacier, creating friction between the ice and bed. To reconcile our results with previous studies, we propose that surging glaciers worldwide may be found on bedrock that is not too fractured, but just fractured enough.
Key Points
Discontinuity properties of bedrock are quantified in 16 glacier basins
Surge‐type glaciers are associated with less fractured bedrock than nonsurge‐type glaciers in our study area
We hypothesize that the extent of bedrock fracture controls a zone of high basal drag during quiescence
We use daily surface velocities measured over several weeks in 2007 and 2008 on a slowly surging glacier in Yukon, Canada, to examine the ordinary melt-season dynamics in the context of the ongoing ...surge. Horizontal velocities within and just below the ~1.5 km-long zone of fastest flow, where the surge is occurring, are often correlated during intervals of low melt. This correlation breaks down during melt events, with the lower reaches of the fast-flow zone responding first. Velocity variability in this lower reach is most highly correlated with melt; velocities above and below appear to respond at least as strongly to the velocity variations of this reach as to local melt. GPS height records are suggestive of ice/bed separation occurring in the fast-flow zone but not below it, pointing to a hydrological cause for the short-term flow variability in the surging region. Independent velocity measurements over 6 years show a maximum July flow anomaly coincident with the location most responsive to melt. Results from a simple model of dashpots and frictional elements lend support to the hypothesis that this zone partly drives the dynamics of the ice above and below it. We speculate that the slow surge may enhance glacier sensitivity to melt-season processes, including short-term summer sliding events.
We describe a portable low-frequency impulse radar system intended for ground-based surveys that employs off-the-shelf hardware integrated with custom-designed software. The hardware comprises a ...1–200 MHz transmitter, digitizer, computer and GPS receiver, which together weigh ∼1.5 kg. The entire system, including waterproof enclosures and batteries suited for >8 hours of continuous operation, weighs <10 kg plus the weight of the antenna housing. The system design is flexible, permitting hardware components such as the digitizer or navigation device to be exchanged. The software includes acquisition parameter control, real-time visual ice-depth rendering and data management capabilities using a hierarchical data format. The system described here has been successfully used to sound polythermal ice up to ∼220 m thick in ski-based surveys in the Yukon, Canada, and temperate ice up to ∼550 m thick in machine-based surveys in Iceland.
Efficient collection of snow depth and density data is important in field surveys used to estimate the winter surface mass balance of glaciers. Simultaneously extensive, high resolution, and accurate ...snow-depth measurements can be difficult to obtain, so optimisation of measurement configuration and spacing is valuable in any survey design. Using in-situ data from the ablation areas of three glaciers in the St. Elias Mountains of Yukon, Canada, we consider six possible survey designs for snow-depth sampling and N = 6–200+ sampling locations per glacier. For each design and number of sampling locations, we use a linear regression on topographic parameters to estimate winter balance at unsampled locations and compare these estimates with known values. Average errors decrease sharply with increasing sample size up to N ≈ 10–15, but reliable error reduction for any given sampling scheme requires significantly higher N. Lower errors are often, but not always, associated with sampling schemes that employ quasi-regular spacing. With both real- and synthetic data, the common centreline survey produces the poorest results overall. The optimal design often requires sampling near the glacier margin, even at low N. The unconventional “hourglass” design performed best of all designs tested when evaluated against known values of winter balance.
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