implications of till deformation on glacier dynamics the dynamics of glacier motion are governed to a large extent by the properties of the basal interface in this paper we address the interaction of a glacier with a layer of till at its bed in an attempt to test whether our physical understanding of till is sufficient to explain general features of the observed flow field and changes in geometry of black rapids glacier alaska u s a we also investigate whether or not a till layer has a clear surface observable signature in the dynamics of the glacier towards this end we use a finite element ice flow model with a coulomb failure criterion within the basal till layer we find that simple till physics can be used to describe decadal seasonal and short term hours to days velocity variations and possibly uplift events mechanisms for each of these variations involve an increase in the extent of till at failure a transfer of shear stress across the bed and a consequent increase in ice deformation effective shape factors are calculated that permit a simple incorporation of this boundary condition into glacier response models our analyses however have not resulted in the identification of a clear and unique signature of a till layer in the surface dynamics of a glacier

organized structures in subsurface bubble clouds langmuir circulation in the open ocean observations of the movement and distribution of subsurface bubbles reveal the structure of coherent motions near the ocean surface the measurements were obtained in the open ocean from a freely drifting self contained acoustical instrument equipped with side scan sonar dual frequency echo sounders and ambient sound recording systems the bubble clouds are organized into long narrow plumes aligned with the wind consistent with windrows caused by langmuir circulations they have widths of about 3 to 5 m and lengths up to 100 m with multiple scales coexisting their mean separation depends upon wind speed below 5 m s 1 mean spacings were about 5 m increasing to 10 m when winds exceeded 10 m s 1 the maximum depth to which the plumes were observed to penetrate depended to some extent on wind speed with the greatest penetrations of 12 m occurring at the highest wind speeds 13 m s 1 however the time averaged depth of observed bubble plume penetration bears only a weak dependence on wind speed whenever winds exceeded 5 m s 1 a mean penetration depth of 6 m was observed associated with these bubble plumes were maximum downward velocities of about 0 06 m s 1 these speeds were detected at 8 m depth in the center of the plumes magnitudes decreased to zero near the surface and horizontally toward the boundaries of the plumes

late surge glacial conditions on bakaninbreen svalbard and implications for surge termination 1 bakaninbreen is a polythermal glacier in southern spitsbergen svalbard that last surged between 1985 and 1995 seismic reflection data were acquired during early quiescence in spring 1998 just upstream of the surge front the results were combined with complementary ground penetrating radar data to investigate the glacial structure and basal conditions we find no difference between the ice thickness values determined from the seismic and radar methods suggesting that any layer of basal ice cannot be greater than 5 m thick interpretation of the amplitude of the seismic reflections indicates the presence of permafrost close to the glacier base a thin layer of thawed deforming sediment separates the glacier from this underlying permafrost in an area just upstream of the surge front the permafrost becomes discontinuous and may even be absent the ice being underlain by 10 15 m of thawed sediments overlying deeper bedrock high pressure water is believed to have been required to maintain the propagation of the surge and this area of thawed sediment is interpreted as a route for that water to escape from the basal system when the surge front passed over this thawed bed the escaping water reduced the pressure in the subglacial hydraulic system initiating the termination of the surge surge termination was therefore primarily controlled by the presurge permafrost distribution beneath the glacier rather than any feature of the surge itself this termination mechanism is probably limited to surges in polythermal glaciers but the techniques used may have wider glaciological applications

jokulhlaups a reassessment of floodwater flow through glaciers 1 in glaciated catchments glacier generated floods jokulhlaups put human activity at risk with large sporadic jokulhlaups accounting for most flood related fatalities and damage to infrastructure in studies of jokulhlaup hydrodynamics the view predominates that floodwater travels within a distinct conduit eroded into the underside of a glacier however some jokulhlaups produce subglacial responses wholly inconsistent with the conventional theory of drainage by focusing on icelandic jokulhlaups this article reassesses how floodwater flows through glaciers it is argued that two physically separable classes of jokulhlaup exist and that not all jokulhlaups are an upward extrapolation of processes inherent in events of lesser magnitude and smaller scale the hydraulic coupling of multiple nonlinear components to the flood circuit of a glacier can induce extreme responses including pressure impulses in subglacial drainage representing such complexity in mathematical form should be the basis for upcoming research as future modeling results may help to determine the glaciological processes behind heinrich events moreover such an approach would lead to more accurate predictive models of jokulhlaup timing and intensity

surface wave effects on high frequency currents over a shelf edge bank abstractseveral acoustic doppler current profilers and vertical strings of temperature conductivity and pressure sensors deployed on and around the east flower garden bank efgb were used to examine surface wave effects on high frequency flows over the bank and to quantify spatial and temporal characteristic of these high frequency flows the efgb about 5 km wide and 10 km long is located about 180 km southeast of galveston texas and consists of steep slopes on southern and eastern sides that rise from water depths over 100 m to within 20 m of the surface three dimensional flows with frequencies ranging from 0 2 to 2 cycles per hour cph were observed in the mixed layer when wind speed and stokes drift at the surface were large these motions were stronger over the bank than outside the perimeter the squared vertical velocity w2 was strongest near the surface and decayed exponentially with depth and the e folding length of w2 was 2 times larger than that of stokes drift the 2 h averaged w2 i

balances of heat and momentum at 33 5 n 127 w in the upper ocean during the mixed layer dynamics experiment the balances of momentum and heat were studied for an 18 day period in october november 1983 during the mixed layer dynamics experiment at a site 650 km off central california data were collected from a drifting platform from which surface winds air temperature and ocean currents from a string of vector measuring current meters were measured the position of the unattended drifter was determined by loran c on the platform currents in the surface layers were partially wind driven and partially coherent with low frequency currents at deeper levels the latter currents though coherent were sheared turned significantly with depth and were presumably nearly geostrophically balanced vertically differencing velocity in the mixed layer improved the balance between wind and current by removing a portion of the geostrophic flow the complex correlation coefficient between wind stress and current integrated to 38 m was 0 76 for current relative to 38 m yet only 0 36 for the undifferenced case surface heat flux magnitudes were not clearly reflected in the heat content of the near surface layers although variability was over the first 13 days of the experiment the correlation coefficient between daily averaged heat flux and heat content change was 0 89 but average net surface heating was 60 w m 2 while the average heat content change in the top 38 m was only 10 w m 2 heat balance must have been restored by advection vertical advection was rejected as an explanation because heat content changes in layers defined by isotherm depths showed similar imbalances with regard to surface heating horizontal advection was estimated by two methods 1 by using sparse thermistor chain tows around the current meter drifter to construct a history of horizontal temperature gradients and 2 by using current profiles at moderate depths to estimate pressure gradients essentially through geostrophy hence buoyancy gradients by the hydrostatic relation hence temperature gradients the two methods gave very different estimates though both tended to redress the imbalance between surface heat flux and local heat storage

tidal forcing of basal seismicity of ice stream c west antarctica observed far inland the seismicity rate beneath the downglacier 85 km of ice stream c west antarctica is modulated by the tide the tide beneath the ross ice shelf modifies the force balance of the ice stream basal environment enough to change the rate of basal microearthquake generation by an order of magnitude this tidal forcing travels up the ice stream as an attenuating wave at approximately 1 6m s 1 and is detectable 85km from the grounding line we successfully model this behavior as an elastic ice stream underlain by a viscous substrate of viscosity and thickness hb and calculate that the substrate has an apparent stiffness hb of o 108 pa s m 1 this finding suggests that the conditions of the till layer at the bed of ice stream c are similar to those of ice stream b and that the reason for the recent stagnation of ice stream c is other than loss of till we further find that the ice stream at the grounding line is more strongly affected by ice shelf processes than by the basal shear stress

turbulent large eddy momentum flux divergence during high wind events abstractmomentum transport by energy containing turbulent eddies in the oceanic mixed layer were investigated during high wind events in the northern gulf of alaska off kayak island sixteen high wind events with magnitudes ranging from 7 to 22 m s 1 were examined winds from the southeast prevailed from one to several days with significant wave heights of 5 9 m and turbulent langmuir numbers of about 0 2 0 4 surface buoyancy forcing was much weaker than the wind stress forcing the water column was well mixed to the bottom depth of about 73 m spectral analyses indicate that a major part of the turbulent momentum flux was concentrated on 10 30 min time scales the ratio of horizontal scale to mixed layer depth was from 2 to 8 turbulent shear stresses in the mixed layer were horizontally asymmetric the downwind turbulent stress at 10 20 m below the surface was approximately 40 of the averaged wind stress and was reduced to 5 10 of the wind stress near the bottom turbulent kinetic energy in the cros

seismic reflection evidence for a deep subglacial trough beneath jakobshavns isbrae west greenland seismic reflection methods were used to determine the ice thickness and basal topography of jakobshavns isbrae a large fast moving ice stream outlet glacier in west greenland a method of data analysis was developed which involves the pointwise migration of data from a linear seismic array and a single explosive source the method yields the depth horizontal position and slope of the basal reflector a deep u shaped subglacial trough was found beneath the entire length of the well defined ice stream the trough is incised up to 1500 m into bedrock and its base lies 1200 1500 m below sea level for at least 70km inland center line ice thickness along most of the channel is about 2500 m or about 2 5 times that of the surrounding ice sheet this prominent bedrock trough was not apparent in existing radio echo sounding data reflection coefficients indicate that much of the basal interface is probably underlain by compacted non deforming sediment the large ice thickness coupled with relatively steep surface slopes leads to high basal shear stresses 200 300 kpa along the ice stream the large shear stresses and lack of a deformable bed imply that internal deformation plays a dominant role in the dynamics of jakobshavns isbrae

velocity structure in the mixed layer during mildex abstract during october november 1983 a mixed layer dynamics experiment mildex took place off the coast of california as a part of mildex a number of sensors were deployed from the research platform flip in an effort to monitor the flow structure in the near surface mixed layer profiling current meters vmcms measured velocities down to 150 m on an hourly basis other vmvms were set at fixed depths depths of interest were examined using a package which measured three component velocities and displayed them in flip s laboratory in real time the current meters often observed sequences of downwind directed jets with maximum velocities exceeding 25 cm s 1 downwelling velocities of equal magnitude were also observed the strongest currents were found 10 to 35 m below the surface at mid depth in the mixed layer surface convergences associated with these jets were visualized by scattering tracers computer cards on the sea surface these features suggest langmuir circulation six doppler sonars wer

the influence of basal processes on the dynamic behaviour of cold based glaciers abstract there exists within glaciology a widely held assumption that basal sliding and bed deformation do not operate beneath cold based ice and that their basal velocity is therefore zero irrespective of bed conditions consequently their ability to erode entrain and transport sediment and thereby alter the landscape is assumed to be limited consequently very little research has been focused towards describing and understanding the motion of cold based ice masses and our knowledge of their behaviour remains poor and the assumption of zero basal velocity unchallenged in this review paper it is argued that this assumption is not universally applicable and that in certain circumstances basal processes not only remain active at sub freezing temperatures but can significantly influence glacier motion this is particularly the case in glaciers where sub freezing basal thermal conditions coincide with the presence of fine grained ice rich subglacial sediments due to the lack of work undertaken on contemporary cold based glaciers much of the information used to support this argument is derived from field research in permafrost areas and on quaternary glacial sediments and from the laboratory testing of ice sediment mixtures it is concluded that the assumption of zero basal velocity beneath cold based ice is overly simplistic and that in reality the situation is likely to be much more complex work is therefore urgently required firstly to investigate the circumstances in which basal motion remains active at sub freezing temperatures and to determine their influence on glacier motion and secondly to examine the likely extent of subglacial permafrost in both the contemporary and quaternary glacial environment

wind driven internal waves and langmuir circulations in a numerical ocean model of the southern baltic sea 1 a one dimensional numerical ocean model of the southern baltic sea is used to investigate suitable parameterizations of unresolved turbulence and compare with available observations the turbulence model is a k e model that includes extra source terms piw and plc of turbulent kinetic energy tke due to unresolved breaking internal waves and langmuir circulations respectively as tides are negligible in the baltic sea topographic generation of internal wave energy iwe is neglected instead the energy for deepwater mixing in the baltic sea is provided by the wind at each level the source term piw is assumed to be related to a vertically integrated pool of iwe e0 and the buoyancy frequency n at the same level according to piw z e0n z this results in vertical profiles of e the dissipation rate of tke and kh the eddy diffusivity according to e n and kh n 2 below the main pycnocline earlier observations are inconclusive as to the proper value of and here a range of values of is tested in hundreds of 10 year simulations of the southern baltic sea it is concluded that 1 0 0 3 and that a mean energy flux density to the internal wave field of about 0 9 0 3 10 3 w m 2 is needed to explain the observed salinity field in addition a simple wind dependent formulation of the energy flux to the internal wave field is tested which has some success in describing the short and long term variability of the deepwater turbulence the model suggests that 16 of the energy supplied to the surface layer by the wind is used for deepwater mixing finally it is also shown that langmuir circulations are important to include when modeling the oceanic boundary layer a simple parameterization of langmuir circulations is tuned against large eddy simulation data and verified for the baltic sea

glacier motion dominated by processes deep in underlying till black rapids glacier is a 40 km long surge type glacier in the central alaska range in spring 1997 a wireline drill rig was set up at a location where the measured surface velocities are high and seasonal and annual velocity variations are large the drilling revealed a layer of subglacial till up to 7 m thick that is believed to be water saturated at one location a string of instruments containing three dual axis tiltmeters and one piezometer was successfully introduced into the till the tiltmeters monitored the inclination of the borehole at the ice till interface and at 1 and 2 m into the till for 410 days they showed that no significant deformation occurred in the upper 2 m of the till layer and no significant amount of the basal motion was due to sliding of the ice over the till the measured surface velocity at the drill site is about 60 m a 1 of which 20 30 m a can be accounted for by ice deformation almost the entire amount of basal motion 30 40 m a 1 was taken up at a depth of 2 m in the till possibly in discrete shear layers or as sliding of till over the underlying bedrock we propose that the large scale mobilization of such till layers is a key factor in initiating glacier surges

suspended sediment deposition and trapping efficiency in a delaware salt marsh abstract this study examined sediment deposition and retention in a section of salt marsh in the st jones river estuary in delaware sediment traps siphon samplers a lisst particle sizer and four water level sensors were deployed in a 100 m by 200 m grid over the course of a spring tide in june 2007 the objective was to determine the relative influences of suspended sediment concentration settling velocity tidal hydroperiod and vegetation density on sediment deposition and trapping efficiency within a small section of marsh hydroperiod is not a major contributor to sediment deposition in the study plot spatial patterns in depositional processes on tidal timescales are instead primarily controlled by overmarsh ssc settling velocity and distance from the tidal channel trapping efficiency or the ratio of measured deposition and the amount of sediment available for deposition decreased with distance away from the tidal channel indicating that less of the available suspended sediment was deposited in the marsh interior the suspended particles were flocculated but floc sizes decreased with distance away from the tidal channel

ekman drift in homogeneous water measurements made with satellite tracked buoys drogued in different layers between the sea surface and 30 m depth under homogeneous winter conditions in the north sea allow analysis of the ekman currents under a large variety of wind conditions the experiment lasted from november 20 1991 until february 29 1992 the first 4 weeks of this period during which the buoys stayed close together are used to determine the ekman stresses the total current field is a superposition of barotropic currents due to sea level variations and ekman currents the classical ekman theory is not able to describe properly the observed deflection of the currents to the right of the wind direction and their decay with depth this deflection is 10 near the sea surface and increases to approximately 50 in 25 m depth the relation between wind stress and the stress field in the interior of the water is given by a tensor which describes the rotation and the variation of the stress with increasing depth the concept of eddy viscosity is applicable if a viscosity tensor is used to relate stress and vertical shear the viscosity tensor is a function of the vertical coordinate only and is independent from the wind stress it shows maximum values in 15 to 20 m depth and may be due to langmuir circulation cells further studies are needed to determine the physics of this tensor its magnitude in the interior of the mixed layer exceeds 1000 cgs units consequently ekman currents are weak and may not be the dominant currents within the mixed layer

open ocean surface wave measurement using doppler sonar in october november 1983 doppler sonars mounted on the research platform flip were used to scatter 75 khz sound from the underside of the sea surface at low angle as well as from the interior of the mixed layer surface gravity waves were seen in velocity estimates from the surface scattering sonar even though the wave conditions were unusually calm valid measurements of sea surface motion were obtained over the range interval 600 to 1400 m from flip a second sonar which transmitted horizontally and scattered from the interior of the mixed layer also sensed surface waves although with amplitude reduced in proportion to the decay of the motions with depth wave number frequency spectra of the observed motions are consistent with linear theory the existence emergence of this technology enables the synthesis of kilometer long surface wave arrays in the open sea without the cost and logistical support usually associated with large aperture arrays in addition the doppler acoustic approach can provide information on lower frequency surface currents and on the spatial variations in these low frequency currents such as langmuir cells which might affect wave propagation

air compression as a mechanism for the underdamped slug test response in fractured glacier ice 1 artificial perturbations of borehole water levels known as slug tests are a useful means of characterizing the glacier hydrologic system slug tests were performed on bench glacier alaska in 21 boreholes over three field seasons during the transition from a winter to a summer drainage mode fifty four slug tests were conducted with water level monitoring in up to five boreholes adjacent to the slugged borehole seven of the slug tests were performed in conjunction with dye dispersion tests to identify water pathways within the slugged borehole following perturbation nearly 60 of monitored adjacent boreholes showed a hydraulic connection to the slugged borehole via the glacier bed the nature and degree of connectivity was temporally variable suggesting that the drainage network at the bed was highly dynamic on a daily timescale and spatial scale of tens of meters the variability of slug test responses over time and space limit the feasibility of six alternative explanations for the oscillatory water level behavior characteristic of the underdamped response we propose a seventh that is that coherent air packages are a reasonable means of producing the compliance needed to generate the underdamped slug test responses on bench glacier and that these air packages may exist within the glacier at the tips of subglacially propagated fractures

glacier generated earthquakes from prince william sound alaska analysis of seismic records from stations near the prince william sound supports the association of monochromatic low frequency signals with tidewater glaciers in the area thirty three of the larger better recorded signals with corresponding earthquake magnitudes of ml 1 5 to 2 5 were located using routine seismological techniques epicenters for these glacierquakes clustered in the vicinity of the harvard columbia yale and barry glaciers monthly counts from one station for 1975 1979 and part of 1976 range from less than 4 to 248 events a sudden decrease in the number of events per month in mid 1979 raises the possibility of a single source or a common source mechanism for these glacier related signals the monochromatic nondispersive waveforms which characterize these 1 to 2 hz signals may result from harmonic resonance of the glaciers in the epicentral region we speculate that a variety of source mechanisms within or near the glaciers may provide the initial impulse for these events however the characteristic observed waveform is most likely the result of seismic energy which is internally reflected within the ice at fundamental resonant frequencies boundary conditions associated with tidewater glaciers may also allow the ice body to act as a wave guide for shear or surface waves the signal we observe therefore appears to be a source site phenomenon

langmuir circulation driving sediment entrainment into newly formed ice tank experiment results with application to nature lake hattie united states kara sea siberia 1 langmuir circulation lc was generated under freezing conditions in saltwater tank experiments through surface wind stress and cross waves interacting with subsurface return flow fine grained sediments distributed in the tank prior to frazil crystal formation were aligned in parallel streaks in lc bottom convergence zones downwelling at lc surface convergence zones aligned floating frazil in wind parallel rows and individual crystals rotated on helical paths down to the tank bottom and up again to the surface the crystals interacted with suspended particles in the water column and with sediment on the tank bottom preferentially collecting fine grained particles and enhancing their entrainment into new ice evidence includes higher sediment concentrations in ice and ice interstitial water ice pore water as compared to the tank water both tank ice and ice interstitial water contain more silt sized particles than tank water suspension load and tank bottom sediment sand is reduced in the ice and clay is about the same concentration in all samples this points to preferential entrainment of fine particles in newly formed ice supported by lc driven circulation comparable results of lc supported ice particle entrainment were found in lake hattie comparison of ice sediment from tank experiments run with kara sea material to ice particles from the natural kara setting showed both types of ice sediment have very similar grain size distributions and mineralogical compositions results from experiments and nature help to better understand the potentially lc driven entrainment of sediment into ice formed in shallow freezing waters

till beneath ice stream b 2 structure and continuity during the 1984 1985 antarctic field season 8 3 km of high resolution seismic reflection data were collected in order to image a thin till layer beneath ice stream b west antarctica two parallel seismic reflection lines were oriented transverse to ice flow these data show that the till layer varies in thickness but is continuous over almost the entire length of the profiles with an average thickness of 6 5 m the upper surface of the till layer is smooth but the lower boundary is fluted parallel to flow these till filled flutes are as much as 13 m deep and 1000 m across nowhere on the profiles can any feature be discerned to penetrate more than a few meters into the ice from the bed reflection events from lithified sediments of unknown type are observed to be truncated by the till in an angular unconformity

propagating strain anomalies during mini surges of variegated glacier alaska u s a analyse des deplacements acceleres observes a l aide de jauges de deformation et de geophones 1979 1980 1981 evolution des mini surges deformations glissement basal

in situ measurements of till deformation and water pressure a newly developed hammer was used to insert two autonomous probes 0 8 m and 2 1 m into clast rich subglacial till under black rapids glacier alaska usa both probes were instrumented with a dual axis tilt sensor and a pore water pressure transducer the data are compared to a 75 day record of surface velocities till deformation at depth was found to be highly seasonal it is significant during an early season speed up event but during long periods thereafter measured till deformation rates are negligible both tilt records show rotation around the probe axis which indicates a change in tilt direction of about 30 the tilt records are very similar suggesting spatial homogeneity on the scale of the probe separation 4 m horizontal and 3 3 m vertical there is evidence that during much of the year sliding of ice over till or deformation of a thin till layer 20 cm accounts for at least two thirds of total basal motion basal motion accounts for 50 70 of the total surface motion the inferred amount of ice till sliding is larger than that found at the same location in a previous study when surface velocities were about 10 lower we suggest that variations in ice till coupling account for the observed variations in mean annual speed

hydrological discharges and motion of fels and black rapids glaciers alaska u s a implications for the structure of their drainage systems characteristics of the hydrology and motion of black rapids and fels glaciers alaska were observed from 1986 to 1989 hydrological measurements included stage electrical conductivity and suspended sediment concentration in the discharge stream of each glacier and were made at 0 5 1 h intervals continuously through most of the melt seasons variations in the glacier speed were monitored through the full year at a number of locations along the length of each glacier using time lapse photography 1 d time resolution strain meters 0 5 1 h resolution and seismometers set up to count acoustic emissions both glaciers show similar seasonal diurnal and short term event changes in hydrological discharges and ice speed the hydrological behavior is analyzed in terms of a fast sub system composed of surface streams moulins and large tunnels with discharge that responds rapidly and a slow sub system composed of heterogeneous small passageways through the ice and distributed over the bed that maintain approximately uniform discharge over a day the timing and amplitude of water discharge in the diurnal cycle indicate that roughly 10 40 of the water is routed directly into the fast system the remaining 90 60 of the water enters the slow system dilution of the solute discharged from the slow system by the variable discharge in the fast system results in changes in water discharge and solute concentration that are approximately equal in relative amplitude and inversely related a small time lag from discharge maximum minimum to solute minimum maximum suggests that the fast system is confined to roughly the lowermost 30 40 of the full glacier length the residence time of water in the fast system is short compared to 1 d the slow system contains both short and long residence time passages characteristics of the diurnal cycles are somewhat variable through the melt season but no systematic evolutionary patterns were discerned even though large changes in the mean discharges of water and solutes occur which suggests parallel evolution of the variables that control the response of the fast system events were characterized by contemporaneous increases in suspended sediment concentration in the discharge water and distinct changes in straining on the glaciers events caused by increases in melt or precipitation related to weather and events related to release from reservoirs internal to the glaciers could be distinguished based on the changes in electrical conductivity of the discharge water the correlated changes in sediment discharge and motion of the glaciers indicate that the events were associated with temporary modifications of the slow passages distributed over the bed that allowed enhanced sliding and access of basal water flow to erosion products hydrological differences between black rapids and fels glaciers can be explained by differences in the size of the glaciers if there is a difference in bed structure that explains the difference in dynamics surge black rapids glacier versus non surge fels glacier it does not affect the hydrological parameters that were observed

surface strain anomaly induced by the storage and drainage of englacial water in koryto glacier kamchatka russia to investigate short term flow pattern variations of a temperate glacier longitudinal surface strain was measured with a wire strainmeter in the ablation area of koryto glacier kamchatka russia strain rate anomalies were observed in late summer 2000 that were triggered by a water overflow from a moulin near the measurement site followed by the drainage of accumulated water the strain event started with compressive strain rates of 10 3 d 1 lasting for 6 hours which then became tensile similar strain rate variations were observed again on the next day during the event basal sliding speed measured at the margin in the lower reach of the glacier fluctuated by about 50 of the daily mean smaller and larger sliding speeds corresponded to the compressive and tensile surface strains respectively these measurements suggest that the storage and sudden drainage of water caused spatially non uniform water pressure fluctuations along the glacier changing the sliding regime over short time periods

effects of bedrock lithology and subglacial till on the motion of ruth glacier alaska deduced from five pulses from 1973 2012 a pulse is a type of unstable glacier flow intermediate between normal flow and surging using landsat mss tm and etm imagery and feature tracking software a time series of mostly annual velocity maps from 1973 to 2012 was produced that reveals five pulses of ruth glacier alaska peaks in ice velocity were found in the 1981 1989 1997 2003 and 2010 approximately every 7 years during these peak years the ice velocity increased 300 from approximately 40 m yr to 160 m yr and occurred in an area of the glacier underlain by sedimentary bedrock based on the spatio temporal behavior of ruth glacier during the pulse cycles we suggest the pulses are due to enhanced basal motion via deformation of a subglacial till the cyclical nature of the pulses is theorized to be due to a thin till with low permeability that causes incomplete drainage of the till between the pulses followed by eventual recharge and dilation of the till these findings suggest care is needed when attempting to correlate changes in regional climate with decadal scale changes in velocity because in some instances basal conditions may have a greater influence on ice dynamics than climate

quiescent phase evolution of a surge type glacier black rapids glacier alaska u s a black rapids glacier a surge type glacier in the alaska range most recently surged in 1936 37 and is currently in its quiescent phase mass balance ice velocity and thickness change have been measured at three to ten sites from 1972 to 1994 the annual speed has undergone cyclical fluctuations of as much as 45 about the mean speed ice thickness and surface slope did not change enough to cause the speed fluctuations through changes in ice deformation which indicates that they are being driven by changes in basal motion the behavior of black rapids glacier during this quiescent phase is significantly different from that variegated glacier another well studied surge type glacier in alaska the present medial moraine configuration of black rapids glacier indicates that a surge could occur at any time however ice velocity data indicate that the next surge may not be imminent we believe that there is little chance that the next surge will cross and dam the delta river

structure and variability of langmuir circulation during the surface waves processes program a cooperative multiplatform field experiment was conducted in the eastern north pacific during february and march of 1990 as part of the surface waves processes program swapp one of the experimental objectives was to investigate langmuir circulation so that its role in the evolution of the oceanic surface boundary layer could be better understood the concurrent use of different observational techniques ranging from simple surface drifters to complex doppler sonar systems resulted in new information about langmuir circulation structure and variability estimates of langmuir cell spacing indicated that a broad range of scales from about 2 to 200 m was excited during periods of strong surface forcing and that the energy containing scales evolved with time estimates of cell spacing based on doppler velocities from a surface scanning sonar directed crosswind showed this scale evolution but estimates based on backscattered intensity did not this was attributed to the fact that the intensity based estimates were only indirectly related to circulation strength the near surface convergent velocities from the sonar were used to form an objective quantitative measure of the temporal variations in langmuir circulation strength as expected the circulation strength increased dramatically during strong wind events however circulation strength and wind stress did not decrease simultaneously and langmuir circulation was detectable for up to a day after abrupt reductions in wind stress energy from the surface wave field which decayed more slowly than the wind was apparently responsible for maintaining the circulation the variation of circulation strength was found to be better related to u us than to u where u is the friction velocity is the wind stress and us is the surface wave stokes drift this scaling is consistent with wave current interaction theories of langmuir cell generation

seismic detection of transient changes beneath black rapids glacier alaska u s a ii basal morphology and processes using changes observed in daily seismic reflections we have investigated the basal morphology of black rapids glacier alaska u s a the englacial drainage of ice marginal lakes caused significant changes in the daily reflections as well as dramatic increases in basal motion changes in reflection arrival times and amplitudes indicate that there is a basal till layer at least 5 m thick at some locations beneath this surge type glacier rapid changes in the observed reflection coefficients during the drainage events indicate that changes in till properties must occur throughout the entire 5 m thick layer they must last for several days following the lake drainages and they must be completely reversible over as little as 36 min our seismic analysis shows that changes in effective pressure of the till are unlikely to cause the required changes in the reflection coefficients but that a decrease in till saturation is likely we therefore interpret the cause ofthe seismic anomalies as being a temporary decrease in saturation as water is input to the subglacial hydraulic system and propose that such a change may occur quickly and reversibly by a redistribution ofoverburden pressure higher water pressures within the hydraulic system cause that region to support more of the glacier s weight leaving the remaining areas to support less any till within these areas of decreased normal stress would experience a consequent decrease in pore water pressure causing gas to exolve thus decreasing saturation this decrease in saturation would cause a change in the strength of the basal layer and may affect basal dynamics

subglacial seismic reflection strategies when source amplitude and medium attenuation are poorly known seismic reflection techniques are a powerful way to probe physical properties of subglacial strata inversion of seismic data for physical properties may be hampered however by lack of knowledge of the source amplitude as well as lack of knowledge of the compressional and shear attenuation in the ice new methods are described to measure the source signature that require no a priori knowledge of the ice attenuation profile another new method is described to obtain the angular dependence of the subglacial bed reflection coefficient that is relatively insensitive to knowledge of the ice attenuation finally a correction is provided to a long standing error in the literature regarding measurement of the bed normal incidence reflection coefficient

how much do we really know about glacier surging some of the ideas about glacier surging are considered mainly but not entirely in the light of observations of temperate glaciers in alaska u s a made within the last 15 years climate has an influence on surge recurrence interval climate and weather also affect surge initiation termination and magnitude regional studies lead to the speculation that subglacial till plays a key role in surging and it has been found under all surge type glaciers studied so far including black rapids and variegated glaciers alaska in most of the glaciers studied till deformation processes dominate the motion in quiescence the linked cavity model of surge triggering and rapid motion is not consistent with these observations but the limited coverage of the observations does not rule it out under parts of the glaciers studied the till observations in alaska raise old questions about the interaction between till and the hydraulic systems of temperate glaciers the role of stored water which observations show to be active even in winter on black rapids glacier is noted

the galloping glacier trots decadal scale speed oscillations within the quiescent phase a 28 year record of annual surface speeds has revealed oscillations with a period of roughly 12 years during the quiescent phase of black rapids glacier alaska u s a the original galloping glacier these oscillations are hypothesized to be the manifestation of slowly propagating waves of till failure and till healing with at least the second cycle being initiated by the anomalous advance of a tributary glacier observations support the idea that such dynamics may have occurred but are not conclusive in the conceptual model describing the mechanisms of till failure and healing temporal variations in longitudinal stress gradients are proposed to be more important in causing till failure than temporal variations in effective pressure

time dependent basal stress conditions beneath black rapids glacier alaska usa inferred from measurements of ice deformation and surface motion observations of surface motion and ice deformation from 2002 03 were used to infer mean stress fields in a cross section of black rapids glacier alaska usa over seasonal timescales basal shear stresses in a well defined zone north of the center line orographic left were approximately 7 and 16 lower in spring and summer respectively than in winter correspondingly higher stresses were found near the margins these changes in the basal shear stress distribution were sufficiently large to cause mean surface velocities to be 1 2 and 1 5 times larger in spring and summer than in winter these results were inferred with a simple inverse finite element flow model that can successfully reproduce bulk surface velocities and tiltmeter data stress redistribution between the well defined zone and the margins may also occur over much shorter time periods as a result of rapidly changing basal conditions ice bed decoupling or enhanced till deformation thereby causing large variations in surface velocity and strongly influencing the glacier s net motion during summer

observed growth of langmuir circulation surface velocity patterns and upper ocean density profiles are presented from a period following a sudden increase in wind a prior wind speed of 8 m s failed to produce visible signs of langmuir circulation after the wind speed increased to 13 m s langmuir circulation developed within 15 min the initial scale observed was about 16 m streak to streak and may have been restricted by the depth scale of the measurements this spacing is about two thirds of the dominant wavelength 4 s period waves the streak spacing two cell widths increased at roughly 40 m h for the next hour the density measurements indicate a depth of mixing which increased over the same period at a rate of 20 m h thus the vertical to horizontal aspect ratio was about 1 1 prior stratification was weak buoyancy frequency of about 1 5 cph and probably did not affect the circulation initially mixing at the surface of the oceans is important to a wide variety of concerns including global climate and the health of marine life two main causes of this mixing can be identified 1 when surface water is cooled it becomes denser and sinks mixing downward either to the bottom or until limited by the preexisting stratification and 2 the wind can mechanically stir the surface layer to some depth usually limited by stratification the former can result in mixing to considerable depths but the latter occurs more often over the majority of the world oceans frequently the two driving forces occur together what are the details of this mixing for example are bubbles dragged deep into the mixed layer injecting gases at depth are there repeatable features which could lead to improved models of surface mixing and its effects when wind blows over water lines are often visible on the surface running roughly parallel to the wind in an exemplary series of experiments langmuir 1938 found these to be lines of convergence along the surface with downwelling below each line he found also that maxima in the downwind surface currents occur along these lines thus the mixing layer is organized into rolls of alternating sign aligned with the wind and water parcels follow helical paths downwind this form of circulation in surface layers of water has come to be called langmuir circulation historically this term has not implied any particular mechanism of formation and it generally has not been used in reference to other similar structures e g roll vortices in the atmosphere or in wall bounded convection langmuir 1938 p 123 expressed his belief that the helical vortices set up by wind apparently constitute the essential mechanism by which the epilimnion is produced clearly this form of helical vortices is efficient in transporting momentum energy and matter throughout the surface layer of water three mechanisms are currently considered to drive langmuir circulation 1 an interaction between surface waves and winddriven shear gives rise to this form of circulation as a linear instability a review of this theory and of observations before 1983 is given by leibovich 1983 2 even without surface waves turbulent mixing will occur due to the breakdown of the

langmuir supercells a mechanism for sediment resuspension and transport in shallow seas recent measurements at a cabled sea floor node in 15 meters of water off the coast of new jersey suggest that langmuir supercells langmuir circulations that achieve vertical scales equal to the water depth under extended storms are an important mechanism for major sediment resuspension events on the extensive shallow shelves off the eastern u s coast because sediment resuspension is a prelude to transport supercell events are a necessary condition for major sediment transport such events may also contribute to shelf sea exchange and to offshore gradation of benthic community structure in shallow seas

observations of seasonal variation in the ekman layer abstract the seasonal variation in the ekman layer is examined using observations from the long term upper ocean study lotus including surface meteorology current and temperature the near surface current is found to be coherent with the wind at low frequencies periods greater than one day using a wind relative averaging method the authors find the observed near surface volume transport to be consistent with ekman transport the mean current profile has a spiral shape that is flat compared to the classic ekman spiral in that it rotates less with depth than does the ekman spiral the mean current e folds over 12 m in the summer and 25 m in the winter with a decay in amplitude that implies eddy viscosities of 60 10 4 and 250 10 4 m2 s 1 respectively diurnal cycling is the dominant mode of variability in the summer and it determines the vertical structure of the spiral in the winter diurnal cycling is almost nonexistent due to greatly reduced solar insolation and stronger winds a persisten

langmuir turbulence in shallow water part 1 observations during extended deployment at an ocean observatory off the coast of new jersey a bottom mounted five beam acoustic doppler current profiler measured large scale velocity structures that we interpret as langmuir circulations filling the entire water column these circulations are the large eddy structures of wind wave driven turbulent flows that occur episodically when a shallow water column experiences prolonged strong wind forcing many observational characteristics agree with former descriptions of langmuir circulations in deep water the three dimensional velocity field reveals quasi organized structures consisting of pairs of surface intensified counter rotating vortices aligned approximately downwind maximum downward velocities are stronger than upward velocities and the downwelling region of each cell defined as a pair of vortices is narrower than the upwelling region maximum downward vertical velocity occurs at or above mid depth and scales approximately with wind speed the estimated crosswind scale of cells is roughly 3 6 times their vertical scale set under these conditions by water depth the long axis of the cells appears to lie at an angle 10 20 to the right of the wind a major difference from deep water observations is strong near bottom intensification of the downwind jets found typically centred over downwelling regions accessible observational features such as cell morphology and profiles of mean velocities turbulent velocity variances and shear stress components are compared with the results of associated large eddy simulations reported in part 2 of shallow water flows driven by surface stress and the craik leibovich vortex forcing generally used to represent generation of langmuir cells a particularly sensitive diagnostic for identification of langmuir circulations as the energy containing eddies of the turbulent flow is the depth trajectory of invariants of the turbulent stress tensor plotted in the lumley triangle corresponding to realizable turbulent flows when langmuir structures are present in the observations the lumley map is distinctly different from that of surface stress driven couette flow again in agreement with the large eddy simulations les unlike the les observed velocity fields contain two distinct and significant scales of variability documented by wavelet analysis of observational records of vertical velocity variability with periods of many minutes is that expected from langmuir cells drifting past the instrument at the slowly time varying crosswind velocity shorter period variability of the order of 1 2 min has roughly the observed periodicity of surface wave groups suggesting a connection with the wave groups themselves and or the wave breaking associated with them in high wind conditions

till beneath ice stream b 1 properties derived from seismic travel times seismic experiments conducted on ice stream b part of the marine ice sheet of west antarctica show a meters thick layer immediately beneath the 1000 m thick ice a seismic experiment consisting of wide angle reflection profiling along a line parallel to ice stream flow was conducted to determine the properties of this layer inversion of seismic travel times yields a compressional wave speed of less than 1700 m s 1 and a shear wave speed less than 160 m s 1 for the layer these very low wave speeds imply that the material in the layer is highly porous and is saturated with water at a high pore pressure based on wave speeds in other saturated unconsolidated sediments we believe that a porosity substantially greater than 0 32 probably around 0 4 and an excess of overburden pressure over pore pressure of only 50 kpa 0 5 bar characterize the layer at this location

subglacial drilling at black rapids glacier alaska u s a drilling method and sample descriptions we employed a commercial wireline drill rig to investigate the subglacial conditions of black rapids glacier a well studied surge type glacier in the central alaska range the four main goals were to assess the capabilities of the commercial drilling industry for sampling subglacial material to investigate the basal morphology to determine the subglacial geology and to emplace borehole instruments the drilling was done in an area where seasonal and secular variations in speed are large and where seismic studies suggested the presence of a till layer four holes were drilled at three locations to a maximum depth of 620 m three holes yielded samples of basal ice and till although recovery of the latter was generally poor bedrock was sampled in one or possibly two of the holes in the area sampled the glacier is underlain by a till layer some 4 7 m thick confirming the seismic interpretation it consists ofa sandy matrix at least 20 30 of which comprises larger clasts limited samples ofthe matrix indicate that near the top ofthe till the porosity is 40 and that some of the pore water is frozen geologic studies suggest that the drilling area lies to the north of the denali fault a major tectonic boundary followed by the glacier and that most of the till is locally derived with transport distances of 2 km

inertial oscillations in the upper ocean during the mixed layer dynamics experiment mildex surface currents in the near inertial frequency band were analyzed from 18 day long measurements taken in october november 1983 as part of the mixed layer dynamics experiment the currents below two drifting platforms separated by 55 km were found to have differed in their inertial response to a frontal passage on november 1 by 10 cm s 1 wind records from the two platforms were used to compute the predicted inertial currents with a simple slab model of the wind driven flow the model currents reproduced most of the features of the observed inertial currents including the disparity between the two locations following the november 1 frontal passage the disparity was due to differences in the weak winds preceding the frontal passage which account for inertial currents of 5 cm s 1 in opposing directions at the two platforms at the time of the front these differences or small scale 50 km structure in the wind field were responsible for the disparity rather than the slight difference in the arrival time 2 hours of the front at the two locations

extensive storage of basal meltwater in the onset region of a major west antarctic ice stream a major meltwater body exists beneath a tributary of bindschadler ice stream west antarctica in a region where subglacial lakes have not been mapped but near where rapid vertical motion of the ice sheet surface has suggested shifting of a subglacial water body the water is trapped by a local reversal in ice air surface slope arising from ice flow over variable basal topography and from the positive feedback of basal lubrication from the trapped water strong variations in the water content of the sediments upglacier of the water body arise from a similar process these results are revealed by a novel application of the amplitude variation with offset avo seismic technique the existence of such water bodies and of the strong spatial variation in subglacial sediment properties is not captured in current models of subglacial hydrology lubrication of ice stream motion and sediment transport

seismic detection of transient changes beneath black rapids glacier alaska u s a i techniques and observations to gain new insight into the mechanisms of basal motion we have demonstrated the feasibility of an active seismic technique to measure temporal changes in basal conditions on sub hourly time scales one region of the bed of black rapids glacier alaska u s a was monitored for a period of 45 days using seismic reflections the majority ofthese reflections were nearly identical however three significant anomalies were recorded several days apart these corresponded with the englacial drainage of two ice marginal lakes and one supraglacial pothole each up glacier of the study site as well as dramatic increases in basal motion two of these seismic anomalies revealed identical changes over 1 km 2 of the bed despite the fact that their drainage events occurred at different locations further these two seismic anomalies were followed by records identical to the non anomalous state showing that the seismic changes were reversible in one of these events two records taken 36 min apart revealed that the transition between the anomalous and normal states occurred completely within this short interval

ice thickness measurements of taku glacier alaska u s a and their relevance to its recent behavior using radio echo soundings and seismic reflections we measured cross sections of taku glacier near juneau alaska to resolve inconsistencies in previous measurements and to understand better the glacier s dynamics the maximum thickness is about 1477 m and the minimum bed elevation is about 600 m below sea level which establishes taku glacier as the thickest and deepest temperate glacier yet measured our data indicate that during the 19th century the terminus of taku glacier may have begun its rapid advance at a position where the ice bed was greater than 300 m below sea level and more than 25 km from the inland end of its submarine trough this behavior is uncharacteristic of temperate tide water glaciers the glacier which no longer calves has eroded a sediment layer 100 m thick since 1890 at an average rate of about 3 m a 1 since 1948 this high erosion rate retards advance by entrenching the glacier into the terminal moraine calculations based on ice deformation theory indicate significant basal ice motion near the terminus and high basal shear stress 140 220 kpa along much of its length estimated differences between ice flux and balance flux are consistent with observed thickening and positive net mass balance these data indicate that ice volume is increasing and that further advance is likely

evolution of langmuir circulation during a storm wind stress waves stratification velocity profiles and surface fields of radial velocity and acoustic backscatter intensity were measured along a drift track 50 to 150 km off point arguello california on march 8 1995 the wind increased from calm to 12 m s from the se opposing swell from the nw it increased to 15 m s at noon utc on march 9 remained steady over the next 12 hours briefly dropped and veered by 60 then returned a mixed layer deepened quickly to 25 m then held roughly steady through the next 2 days in spite of gusty winds continuing at 15 25 m s a phased array doppler sonar system took measurements covering 250 m by 150 m of the surface with 5 m by 10 m spatial resolution averages over 6 min removed surface waves permitting continuous assessment of strength orientation spacing and degree of organization of features associated with surface motion e g langmuir circulation even when conditions were too rough for visual assessment several results stand out 1 as found previously most wind mixing arises from inertial shear across the thermocline 2 consistent with wind wave forcing of langmuir circulation plueddemann et al 1996 suggest that surface velocity variance scales like u us where u is friction velocity and us is the surface stokes drift however the measurements here scale with us 1 alone once langmuir circulation is established 3 the velocity variance is weaker here than expected given the magnitudes of wind and waves leading to a smaller estimated mixing effect 4 large vacillations in lc strength are seen just before the brief veering of the wind it is suggested that bubble buoyancy could play a dynamic role 5 mean orientation and spacing can differ for intensity versus radial velocity features

basal conditions on rutford ice stream west antarctica from seismic observations a seismic reflection profile perpendicular to the ice flow direction has been acquired on rutford ice stream antarctica an interpretation of both the amplitude and phase of the seismic reflections from the ice bed interface has been made to investigate the properties of the sub ice material the interpretation assumes that acoustic impedance can be used to imply subglacial sediment porosity multiple reflections from the ice bed interface on a number of seismic wide angle lines allowed a calibration of the reflection coefficient at the bed this enabled the acoustic impedance of the bed material to be calculated from the seismic reflection data mean acoustic impedance in the different sections of the bed ranges from 2 70 106 kg m 2 s 1 to 4 31 106 kg m 2 s 1 almost three quarters of the ice stream bed at this site appears to be saturated deforming sediments the rest of the bed is probably also saturated sediments but they are not deforming to any significant degree ice flow could include a combination of subglacial deformation and basal sliding localized regions which support disproportionately high amounts of basal shear stress may also occur from the seismic data it is not possible to apportion relative amounts of restraint to these different processes there does not appear to be any correlation between the different sections identified on the ice stream bed with either satellite images or nearby surface velocity data a feature which is interpreted as a subglacial drumlin is seen on the seismic section

large eddy simulations of upper ocean response to a midlatitude storm and comparison with observations abstract a large eddy simulation les model is used to investigate an upper ocean response to a fall storm in the open ocean of the north pacific ocean the storm is characterized by rapid increases in wind speed and surface heat loss but a relatively steady wave field the les model shows that surface convergence zones or windrows organize into line patterns aligned with the wind direction evolving from nearly parallel lines to irregular structures featuring y junctions as the wind speed increases the downwelling to upwelling velocity ratio ranges between 1 2 and 1 6 indicating a moderate level of asymmetry between the downwelling and upwelling plumes in langmuir circulation during the storm the turbulent langmuir number lat increases from 0 2 to 0 5 while the vertical turbulence intensity w2 decreases from 1 4 to 0 7 u 2 where u is the friction velocity the order of turbulence intensities in three directions switches from crosswind vertical downwind directions to downwind crosswind ver