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• 1. [Article] 'Sticky spots' and subglacial lakes under ice streams of the Siple Coast, Antarctica

  Locations of subglacial lakes discovered under fast-moving West Antarctic ice streams tend to be associated with topographic features of the subglacial bed or with areas that have strong variations in ...

  Citation × Citation Citation Abstract Copy Title: 'Sticky spots' and subglacial lakes under ice streams of the Siple Coast, Antarctica Author: Sergienko, Olga V., Hulbe, Christina L. Year: 2011 Locations of subglacial lakes discovered under fast-moving West Antarctic ice streams tend to be associated with topographic features of the subglacial bed or with areas that have strong variations in basal conditions. Inversion of ice-stream surface velocity indicates that basal conditions under ice streams can be highly variable and that there can be widespread regions where basal traction is high. To seek an explanation for why lakes appear to be sited near areas with high basal traction, we use numerical models to simulate ice-stream dynamics, thermodynamics and subglacial water flow. We demonstrate that the ice flow over high basal traction areas produces favourable conditions for the ponding of meltwater. Energy dissipation associated with ice sliding over a region with high basal traction constitutes a water source supplying a lake, and ice-thickness perturbations induced by ice flow over variable traction create local minima in hydraulic potential. Variations in thermodynamic processes caused by such ice flow could be responsible for limiting the horizontal extent of the subglacial lakes. Title: 'Sticky spots' and subglacial lakes under ice streams of the Siple Coast, Antarctica Author: Sergienko, Olga V., Hulbe, Christina L. Year: 2011 Locations of subglacial lakes discovered under fast-moving West Antarctic ice streams tend to be associated with topographic features of the subglacial bed or with areas that have strong variations in basal conditions. Inversion of ice-stream surface velocity indicates that basal conditions under ice streams can be highly variable and that there can be widespread regions where basal traction is high. To seek an explanation for why lakes appear to be sited near areas with high basal traction, we use numerical models to simulate ice-stream dynamics, thermodynamics and subglacial water flow. We demonstrate that the ice flow over high basal traction areas produces favourable conditions for the ponding of meltwater. Energy dissipation associated with ice sliding over a region with high basal traction constitutes a water source supplying a lake, and ice-thickness perturbations induced by ice flow over variable traction create local minima in hydraulic potential. Variations in thermodynamic processes caused by such ice flow could be responsible for limiting the horizontal extent of the subglacial lakes. Close

• 2. [Article] Tributaries to West Antarctic ice Streams: Characteristics Deduced from Numerical Modelling of Ice Flow

  A network of relatively fast-flowing tributaries in the catchment basins of the West Antarctic ice streams transport ice from the inland reservoir to the heads of the ice streams. Branches of the network ...

  Citation × Citation Citation Abstract Copy Title: Tributaries to West Antarctic ice Streams: Characteristics Deduced from Numerical Modelling of Ice Flow Author: Hulbe, Christina L., Joughin, Ian R., Morse, David L., Bindschadler, R. A. Year: 2000 A network of relatively fast-flowing tributaries in the catchment basins of the West Antarctic ice streams transport ice from the inland reservoir to the heads of the ice streams. Branches of the network follow valleys in basal topography, but not all valleys contain tributaries. We investigate the circumstances favoring tributary flow upstream of Ice Streams D and E, using a combination of observation and numerical modelling. No consistent pattern emerges. The transition from tributary to ice-stream flow occurs smoothly along the main tributary feeding into the onset of Ice Stream D, with ice thickness being relatively more important upstream, and sliding being relatively more important downstream. Elsewhere, the downstream pattern of flow is more complicated, with local increases and decreases in the contribution of sliding to ice speed. Those changes may be due to variations in basal water storage, subglacial geologic properties or a combination of the two. Title: Tributaries to West Antarctic ice Streams: Characteristics Deduced from Numerical Modelling of Ice Flow Author: Hulbe, Christina L., Joughin, Ian R., Morse, David L., Bindschadler, R. A. Year: 2000 A network of relatively fast-flowing tributaries in the catchment basins of the West Antarctic ice streams transport ice from the inland reservoir to the heads of the ice streams. Branches of the network follow valleys in basal topography, but not all valleys contain tributaries. We investigate the circumstances favoring tributary flow upstream of Ice Streams D and E, using a combination of observation and numerical modelling. No consistent pattern emerges. The transition from tributary to ice-stream flow occurs smoothly along the main tributary feeding into the onset of Ice Stream D, with ice thickness being relatively more important upstream, and sliding being relatively more important downstream. Elsewhere, the downstream pattern of flow is more complicated, with local increases and decreases in the contribution of sliding to ice speed. Those changes may be due to variations in basal water storage, subglacial geologic properties or a combination of the two. Close

• 3. [Article] West Antarctic ice stream discharge variability: mechanism, controls, and pattern of grounding line retreat

  West Antarctic ice streams show pronounced flow variability in their downstream reaches, with changes stranding formerly fast-flowing ice and redirecting discharge. A simple model, in which the temperature ...

  Citation × Citation Citation Abstract Copy Title: West Antarctic ice stream discharge variability: mechanism, controls, and pattern of grounding line retreat Author: Hulbe, Christina L., Fahnestock, Mark A. Year: 2004 West Antarctic ice streams show pronounced flow variability in their downstream reaches, with changes stranding formerly fast-flowing ice and redirecting discharge. A simple model, in which the temperature gradient in basal ice provides control of fast sliding in the downstream reach, can explain this behavior. Downstream thinning steepens the temperature gradient near the bed, increasing upward heat flow and the tendency toward basal freezing. The basal temperature gradient is steepest and the tendency toward basal freezing the strongest in ice that has experienced the most rapid downstream thinning, that is, the fastest-flowing ice. The most ?successful? rapid outflows are regions where basal water from elsewhere is likely to be consumed. Freezing here leads to episodic slow-downs and redirections of flow, the history of which appears in satellite imagery as ice rises, distorted streaklines, and margin jumps created when discharge migrates to areas with more favorable basal conditions. One compelling consequence of this process is that it makes catastrophic collapse less likely; if discharge currents are forced to slow when they become too fast (thin), then there may be an upper bound on the retreat rate and discharge flux of the West Antarctic ice sheet (WAIS) ice-stream system under the present climate. Title: West Antarctic ice stream discharge variability: mechanism, controls, and pattern of grounding line retreat Author: Hulbe, Christina L., Fahnestock, Mark A. Year: 2004 West Antarctic ice streams show pronounced flow variability in their downstream reaches, with changes stranding formerly fast-flowing ice and redirecting discharge. A simple model, in which the temperature gradient in basal ice provides control of fast sliding in the downstream reach, can explain this behavior. Downstream thinning steepens the temperature gradient near the bed, increasing upward heat flow and the tendency toward basal freezing. The basal temperature gradient is steepest and the tendency toward basal freezing the strongest in ice that has experienced the most rapid downstream thinning, that is, the fastest-flowing ice. The most ?successful? rapid outflows are regions where basal water from elsewhere is likely to be consumed. Freezing here leads to episodic slow-downs and redirections of flow, the history of which appears in satellite imagery as ice rises, distorted streaklines, and margin jumps created when discharge migrates to areas with more favorable basal conditions. One compelling consequence of this process is that it makes catastrophic collapse less likely; if discharge currents are forced to slow when they become too fast (thin), then there may be an upper bound on the retreat rate and discharge flux of the West Antarctic ice sheet (WAIS) ice-stream system under the present climate. Close

• 4. [Article] Post-Stagnation Behavior in the Upstream Regions of Ice Stream C, West Antarctica

  The region where two active tributaries feed into the now stagnant Ice Stream C (ISC), West Antarctica, is thickening. In this region, we observe a correlation between faster ice flow (the tributaries) ...

  Citation × Citation Citation Abstract Copy Title: Post-Stagnation Behavior in the Upstream Regions of Ice Stream C, West Antarctica Author: Price, Stephen F., Bindschadler, R. A., Hulbe, Christina L., Joughin, Ian R. Year: 2001 The region where two active tributaries feed into the now stagnant Ice Stream C (ISC), West Antarctica, is thickening. In this region, we observe a correlation between faster ice flow (the tributaries) and elevated topography. We conclude that stagnation of ISC resulted in compression and thickening along the tributaries, eventually forming a "bulge" on the ice-sheet surface. Modern hydraulic potential gradients would divert basal meltwater from ISC to Ice Stream B (ISB). These gradients are primarily controlled by the bulge topography, and so likely formed subsequent to trunk stagnation. As such, we argue against "water piracy" as being the cause for ISC's stagnation. Kinematic-wave theory suggests that thickness perturbations propagate downstream over time, but that kinematic-wave speed decreases near the stagnant trunk. This and modest diffusion rates combine to trap most of the tributary-fed ice in the bulge region. Using interferometric synthetic aperture radar velocity measurements, we observe that half of the ice within ISC's southern tributary flows into ISB. That flow pattern and other observations of non-steady flow in the region likely result from stagnation-induced thickening along upper ISC combined with a longer period of thinning on upper ISB. If current trends in thickness change continue, more ice from upper ISC will be diverted to ISB. Title: Post-Stagnation Behavior in the Upstream Regions of Ice Stream C, West Antarctica Author: Price, Stephen F., Bindschadler, R. A., Hulbe, Christina L., Joughin, Ian R. Year: 2001 The region where two active tributaries feed into the now stagnant Ice Stream C (ISC), West Antarctica, is thickening. In this region, we observe a correlation between faster ice flow (the tributaries) and elevated topography. We conclude that stagnation of ISC resulted in compression and thickening along the tributaries, eventually forming a "bulge" on the ice-sheet surface. Modern hydraulic potential gradients would divert basal meltwater from ISC to Ice Stream B (ISB). These gradients are primarily controlled by the bulge topography, and so likely formed subsequent to trunk stagnation. As such, we argue against "water piracy" as being the cause for ISC's stagnation. Kinematic-wave theory suggests that thickness perturbations propagate downstream over time, but that kinematic-wave speed decreases near the stagnant trunk. This and modest diffusion rates combine to trap most of the tributary-fed ice in the bulge region. Using interferometric synthetic aperture radar velocity measurements, we observe that half of the ice within ISC's southern tributary flows into ISB. That flow pattern and other observations of non-steady flow in the region likely result from stagnation-induced thickening along upper ISC combined with a longer period of thinning on upper ISB. If current trends in thickness change continue, more ice from upper ISC will be diverted to ISB. Close

• 5. [Article] An Ice Shelf Mechanism for Heinrich Layer Production

  The effect of an ice shelf in the Labrador Sea on ice-rafted sediment delivery to the glacial North Atlantic is investigated using a finite element numerical model of ice shelf flow. Discharge into the ...

  Citation × Citation Citation Abstract Copy Title: An Ice Shelf Mechanism for Heinrich Layer Production Author: Hulbe, Christina L. Year: 1997 The effect of an ice shelf in the Labrador Sea on ice-rafted sediment delivery to the glacial North Atlantic is investigated using a finite element numerical model of ice shelf flow. Discharge into the shelf from Hudson Strait creates a thick central core, extending downstream into the shelf, that is flanked by relatively thin ice. Melting at the base of the deep keel would produce cool, fresh water which would rise to refreeze along the keel's flanks. Debris deposited by melting deep central ice could create the Heinrich layers observed in the Labrador Sea while the debris-rich ice protected by basal freezing on the flanks of the thick ice plume could transport sediment over large distances. Heinrich layers would be produced solely by external climate forcing, without changes in ice sheet flow. The mechanism is plausible if the lifetime of the ice shelf is about 1000 years. Title: An Ice Shelf Mechanism for Heinrich Layer Production Author: Hulbe, Christina L. Year: 1997 The effect of an ice shelf in the Labrador Sea on ice-rafted sediment delivery to the glacial North Atlantic is investigated using a finite element numerical model of ice shelf flow. Discharge into the shelf from Hudson Strait creates a thick central core, extending downstream into the shelf, that is flanked by relatively thin ice. Melting at the base of the deep keel would produce cool, fresh water which would rise to refreeze along the keel's flanks. Debris deposited by melting deep central ice could create the Heinrich layers observed in the Labrador Sea while the debris-rich ice protected by basal freezing on the flanks of the thick ice plume could transport sediment over large distances. Heinrich layers would be produced solely by external climate forcing, without changes in ice sheet flow. The mechanism is plausible if the lifetime of the ice shelf is about 1000 years. Close

• 6. [Article] Assessment of the Hydrologic Response to Climate Change in the Upper Deschutes River Basin, Central Oregon

  Effects of climate change in the Cascade Range will likely include more rain, less snow, and earlier snowmelt in the Cascade Range as compared to present conditions. These changes, in turn, will affect ...

  Citation × Citation Citation Abstract Copy Title: Assessment of the Hydrologic Response to Climate Change in the Upper Deschutes River Basin, Central Oregon Author: Chang, Heejun, Waibel, M. Scott, Gannett, Marshall W., Hulbe, Christina L. Year: 2011 Effects of climate change in the Cascade Range will likely include more rain, less snow, and earlier snowmelt in the Cascade Range as compared to present conditions. These changes, in turn, will affect the timing of runoff, groundwater recharge, and groundwater discharge to spring-fed streams. This hydrologic response needs to be examined and understood due to implications for water management. In this study, a water- and energy-balance model was used to explore 21st century changes in the water budget in the upper Deschutes Basin, and a groundwater model was used to evaluate the response of the groundwater system to those changes. A Deep Percolation Model (DPM) developed for the basin in the 1990s uses spatially distributed climate data to calculate a daily mass balance for the major components of the hydrologic budget. For this work, we drove the DPM using ensemble means of eight downscaled global climate models with the Intergovernmental Panel on Climate Change?s A1B and B1 emission scenarios. Although similar for both scenarios, greater changes in the timing of runoff and recharge as well as higher reductions in snowpack occur using the A1B scenario. Considering both scenarios, diminished snowpack results in reductions in spring runoff ranging from 40% to 63% and recharge from 21% to 37%. These reductions are offset by late fall and winter increases. Also, spatial changes in the mean annual ratio of recharge to runoff occur due to changes in soil infiltration rates. The modeled response of the groundwater system to changes in the time and amount of recharge varies spatially. Short flow-path systems in the upper part of the basin are most sensitive to change in seasonality of recharge. At regional scales, diffusion along groundwater flow paths partially attenuates the effects of changes in recharge timing. Furthermore, slight increases in total annual groundwater discharge to smaller streams in the upper portion of the basin, and slight decreases in discharge to larger stream systems in the north-central portion of the basin are projected. Title: Assessment of the Hydrologic Response to Climate Change in the Upper Deschutes River Basin, Central Oregon Author: Chang, Heejun, Waibel, M. Scott, Gannett, Marshall W., Hulbe, Christina L. Year: 2011 Effects of climate change in the Cascade Range will likely include more rain, less snow, and earlier snowmelt in the Cascade Range as compared to present conditions. These changes, in turn, will affect the timing of runoff, groundwater recharge, and groundwater discharge to spring-fed streams. This hydrologic response needs to be examined and understood due to implications for water management. In this study, a water- and energy-balance model was used to explore 21st century changes in the water budget in the upper Deschutes Basin, and a groundwater model was used to evaluate the response of the groundwater system to those changes. A Deep Percolation Model (DPM) developed for the basin in the 1990s uses spatially distributed climate data to calculate a daily mass balance for the major components of the hydrologic budget. For this work, we drove the DPM using ensemble means of eight downscaled global climate models with the Intergovernmental Panel on Climate Change?s A1B and B1 emission scenarios. Although similar for both scenarios, greater changes in the timing of runoff and recharge as well as higher reductions in snowpack occur using the A1B scenario. Considering both scenarios, diminished snowpack results in reductions in spring runoff ranging from 40% to 63% and recharge from 21% to 37%. These reductions are offset by late fall and winter increases. Also, spatial changes in the mean annual ratio of recharge to runoff occur due to changes in soil infiltration rates. The modeled response of the groundwater system to changes in the time and amount of recharge varies spatially. Short flow-path systems in the upper part of the basin are most sensitive to change in seasonality of recharge. At regional scales, diffusion along groundwater flow paths partially attenuates the effects of changes in recharge timing. Furthermore, slight increases in total annual groundwater discharge to smaller streams in the upper portion of the basin, and slight decreases in discharge to larger stream systems in the north-central portion of the basin are projected. Close

• 7. [Article] The Link Between Climate Warming and Break-Up of Ice Shelves in the Antarctic Peninsula

  A review of in situ and remote-sensing data covering the ice shelves of the Antarctic Peninsula provides a series of characteristics closely associated with rapid shelf retreat: deeply embayed ice fronts; ...

  Citation × Citation Citation Abstract Copy Title: The Link Between Climate Warming and Break-Up of Ice Shelves in the Antarctic Peninsula Author: Scambos, Ted A., Hulbe, Christina L., Fahnestock, Mark A., Bohlander, Jennifer Year: 2000 A review of in situ and remote-sensing data covering the ice shelves of the Antarctic Peninsula provides a series of characteristics closely associated with rapid shelf retreat: deeply embayed ice fronts; calving of myriad small elongate bergs in punctuated events; increasing flow speed; and the presence of melt ponds on the ice-shelf surface in the vicinity of the break-ups. As climate has warmed in the Antarctic Peninsula region, melt-season duration and the extent of ponding have increased. Most break-up events have occurred during longer melt seasons, suggesting that meltwater itself, not just warming, is responsible. Regions that show melting without pond formation are relatively unchanged. Melt ponds thus appear to be a robust harbinger of ice-shelf retreat. We use these observations to guide a model of ice-shelf flow and the effects of meltwater. Crevasses present in a region of surface ponding will likely fill to the brim with water. We hypothesize (building on Weertman (1973), Hughes (1983) and Van der Veen (1998)) that crevasse propagation by meltwater is the main mechanism by which ice shelves weaken and retreat. A thermodynamic finite-element model is used to evaluate ice flow and the strain field, and simple extensions of this model are used to investigate crack propagation by meltwater. The model results support the hypothesis. Title: The Link Between Climate Warming and Break-Up of Ice Shelves in the Antarctic Peninsula Author: Scambos, Ted A., Hulbe, Christina L., Fahnestock, Mark A., Bohlander, Jennifer Year: 2000 A review of in situ and remote-sensing data covering the ice shelves of the Antarctic Peninsula provides a series of characteristics closely associated with rapid shelf retreat: deeply embayed ice fronts; calving of myriad small elongate bergs in punctuated events; increasing flow speed; and the presence of melt ponds on the ice-shelf surface in the vicinity of the break-ups. As climate has warmed in the Antarctic Peninsula region, melt-season duration and the extent of ponding have increased. Most break-up events have occurred during longer melt seasons, suggesting that meltwater itself, not just warming, is responsible. Regions that show melting without pond formation are relatively unchanged. Melt ponds thus appear to be a robust harbinger of ice-shelf retreat. We use these observations to guide a model of ice-shelf flow and the effects of meltwater. Crevasses present in a region of surface ponding will likely fill to the brim with water. We hypothesize (building on Weertman (1973), Hughes (1983) and Van der Veen (1998)) that crevasse propagation by meltwater is the main mechanism by which ice shelves weaken and retreat. A thermodynamic finite-element model is used to evaluate ice flow and the strain field, and simple extensions of this model are used to investigate crack propagation by meltwater. The model results support the hypothesis. Close

• 8. [Article] Subglacial thermal balance permits ongoing grounding line retreat along the Siple Coast of West Antarctica

  Changes in the discharge of West Antarctic ice streams are of potential concern with respect to global sea level. The six relatively thin, fast-flowing Ross ice streams are of interest as low-slope end-members ...

  Citation × Citation Citation Abstract Copy Title: Subglacial thermal balance permits ongoing grounding line retreat along the Siple Coast of West Antarctica Author: Parizek, Byron R., Alley, Richard B., Hulbe, Christina L. Year: 2003 Changes in the discharge of West Antarctic ice streams are of potential concern with respect to global sea level. The six relatively thin, fast-flowing Ross ice streams are of interest as low-slope end-members among Antarctic ice streams. Extensive research has demonstrated that these "rivers of ice" have a history of relatively high-frequency (óO(100) years), asynchronous discharge variations with evolving lateral boundaries. Amidst this variability, a ~1300 km grounding-line retreat has occurred since the Last GlacialMaximum. Numerical studies of Ice Stream D (Parizek and others, 2002) indicate that a proposed thermal-regulation mechanism(Clarke and Marshall, 1998; Hulbe and MacAyeal,1999; Tulaczyk and others, 2000a,b), which could buffer the West Antarctic ice sheet against complete collapse, may be over-ridden by latent-heat transport within meltwater from beneath inland ice. Extending these studies to Ice Stream A,Whillans Ice Stream and Ice Stream C suggests that further grounding-line retreat contributing to sea-level rise is possible thermodynamically. However, the efficiency of basal water distribution may be a constraint on the system. Because local thermal deficits promote basal freeze-on (especially on topographic highs), observed short-term variability is likely to persist. Title: Subglacial thermal balance permits ongoing grounding line retreat along the Siple Coast of West Antarctica Author: Parizek, Byron R., Alley, Richard B., Hulbe, Christina L. Year: 2003 Changes in the discharge of West Antarctic ice streams are of potential concern with respect to global sea level. The six relatively thin, fast-flowing Ross ice streams are of interest as low-slope end-members among Antarctic ice streams. Extensive research has demonstrated that these "rivers of ice" have a history of relatively high-frequency (óO(100) years), asynchronous discharge variations with evolving lateral boundaries. Amidst this variability, a ~1300 km grounding-line retreat has occurred since the Last GlacialMaximum. Numerical studies of Ice Stream D (Parizek and others, 2002) indicate that a proposed thermal-regulation mechanism(Clarke and Marshall, 1998; Hulbe and MacAyeal,1999; Tulaczyk and others, 2000a,b), which could buffer the West Antarctic ice sheet against complete collapse, may be over-ridden by latent-heat transport within meltwater from beneath inland ice. Extending these studies to Ice Stream A,Whillans Ice Stream and Ice Stream C suggests that further grounding-line retreat contributing to sea-level rise is possible thermodynamically. However, the efficiency of basal water distribution may be a constraint on the system. Because local thermal deficits promote basal freeze-on (especially on topographic highs), observed short-term variability is likely to persist. Close