the tidal spectrum underneath antarctic ice shelves a year long tidal record has been obtained from beneath the george vi ice shelf antarctica an unusual feature of the record is a significant response in tidal species 3 to 7 these harmonics are practically absent from records further north on the west coast of the antarctic peninsula but are present in all tidal height records from george vi sound a strong ter diurnal signal also exists in the tidal currents under the ice shelf nonlinearity also occurs in the tidal motion of the ronne and ekstrom ice shelves but has not been reported from the ross ice shelf the tidal dynamics of several antarctic ice shelves have therefore been modified by a region of strong nonlinear response to tidal forcing if nonlinear tides in the semidiurnal band are present on the ross ice shelf they could account for difficulties in modeling the area s weak semidiurnal tides an anelastic component in the deformation of the ice at the grounding line is tentatively proposed as the mechanism responsible the positioning of recording pressure sensors in pairs on the seafloor and at the ice shelf base will allow this hypothesis to be tested and also provide a value for the power dissipated by tidally induced flexure at the grounding line

spatial arrangement of soil mantle in glacis de buenavista mexico as a product and record of landscape evolution abstract factors controlling spatial soil organization have recently attracted the attention of soil scientists because they can contribute to regional management of soil resources we present here a case study in the glacis de buenavista mexico an extended volcanic piedmont in order to develop an integral understanding of its soil cover as a product and at the same time as a record of the main stages of landscape evolution three phenomena were considered luvisol and vertisol type soils and pedosediments all of them analyzed in terms of their development degree clay content macro and micromorphology selective extractions of fe fe d fe o silicon si o and aluminum al o weathering index and clay mineralogy luvisols represented in the ahuatenco mexicapa and buenavista sections are located in the northern and central part of the area they are red clayey polygenetic soils that show strong weathering with high kaolinitic clay content and high weathering index values clay illuviation and reductomorphic processes combined with vertic features in some parts of the profiles vertisols dominate the central and southern portion of the glacis they exhibit cracking angular blocks with wedge like shapes slickensides and stress cutans the pedogenesis of the vertisol type is clearly associated with the presence of smectitic minerals in the clay fraction as a product of neoformation factors controlling this pedodiversity are relief which affects bioclimatic differentiation within the studied landsurface and defines the lateral redistribution of moisture and dissolved substances and time luvisols represent a longer pedogenetic phase that started in the late pleistocene according to the age of buried paleosols while vertisols mainly originated in the late holocene according to the age of buried organic horizons pedosediments are located in the central area of the glacis here past and present geomorphic processes interact to produce the greatest soil diversity the recent human induced erosion partly destroyed polycyclic luvisols and exposed ancient subsurface pedosedimentary strata at the same time it produced patches of unconsolidated pedosediments consisting mostly of redeposited luvisol materials

basal conditions at the grounding zone of whillans ice stream west antarctica from ice penetrating radar we present a comprehensive ice penetrating radar survey of a subglacial embayment and adjacent peninsula along the grounding zone of whillans ice stream west antarctica through basal waveform and reflectivity analysis we identify four distinct basal interfaces 1 an ice water saturated till interface inland of grounding 2 a complex interface in the grounding zone with variations in reflectivity and waveforms caused by reflections from fluting sediment deposits and crevasses 3 an interface of anomalously low reflectivity downstream of grounding in unambiguously floating areas of the embayment due to basal roughness and entrained debris and 4 a high reflectivity ice seawater interface that occurs immediately seaward of grounding at the subglacial peninsula and several kilometers seaward of grounding in the embayment occurring after basal debris and grounding zone flutes have melted off the ice bottom sediment deposition via basal debris melt out occurs in both locations the higher basal melt rate at the peninsula contributes to greater grounding line stability by enabling faster construction of a stabilizing sediment wedge in the embayment the low slopes of the ice bottom and bed prevent development of a strong thermohaline circulation leading to a lower basal melt rate and less rapid sediment deposition thus grounding lines in subglacial embayments are more likely to lack stabilizing sediment deposits and are more prone to external forcing whether from the ocean the subglacial water system or large scale ice dynamics our conclusions indicate that subglacial peninsulas and embayments should be treated differently in ice sheet ocean models if these models are to accurately simulate grounding line response to external forcing

influence of tides on melting and freezing beneath filchner ronne ice shelf antarctica an isopycnic coordinate ocean circulation model is applied to the ocean cavity beneath filchner ronne ice shelf investigating the role of tides on sub ice shelf circulation and ice shelf basal mass balance including tidal forcing causes a significant intensification in the sub ice shelf circulation with an increase in melting 3 fold and refreezing 6 fold the net melt rate and seawater flux through the cavity approximately doubles with tidal forcing the spatial pattern and magnitude of basal melting and freezing generally match observations the 0 22 m a 1 net melt rate is close to satellite derived estimates and at the lower end of oceanographic values the ice shelf water outflow mixes with shelf waters forming a cold 1 9 degrees c dense overflow 0 83 sv that spills down the continental slope these results demonstrate that tidal forcing is fundamental to both ice shelf ocean interactions and deep water formation in the southern weddell sea citation makinson k p r holland a jenkins k w nicholls and d m holland 2011 influence of tides on melting and freezing beneath filchner ronne ice shelf antarctica geophys res lett 38 l06601 doi 10 1029 2010gl046462

non linear responses of rutford ice stream antarctica to semi diurnal and diurnal tidal forcing modulation of the flow of rutford ice stream antarctica has been reported previously at semi diurnal diurnal fortnightly and semi annual periods a model that includes non linear response to tidal forcing has been shown to fit closely observations at fortnightly frequencies here we examine that model further and test its fit at a larger set of observed frequencies including the large semi annual displacement we show analytically that when forced by major tidal terms the model using a basal shear stress exponent m 3 predicts several discrete response periods from 4 hours to 0 5 years we examine a 1 5 year gps record from rutford ice stream and find that the model when forced by a numerical tide model is able to reproduce the reported semi annual signal we confirm that about 5 of the mean flow is due solely to the m 3 non linear response to tidally varying basal shear stress our best fitting set of model parameters is similar to those originally reported using a much shorter data record although with noticeably improved fit suggesting these parameters are robust we find that m 3 fits the data well but that m 2 does not furthermore we find that a small variation in flow over the 18 6 year lunar node tide cycle is expected fits to semi diurnal and diurnal terms remain relatively poor possibly due to viscoelastic effects that are not included in the model and reduced gps data quality at some discrete periods for comparison we predict the response of bindschadler ice stream and lambert glacier and show given identical model parameters a similar response pattern but with 1 2 orders of magnitude smaller variability these may still be measurable and hence useful in testing the applicability of this model to other locations

larsen ice shelf has progressively thinned the retreat and collapse of antarctic peninsula ice shelves in tandem with a regional atmospheric warming has fueled speculation as to how these events may be related satellite radar altimeter measurements show that between 1992 and 2001 the larsen ice shelf lowered by up to 0 27 0 11 meters per year the lowering is explained by increased summer meltwater and the loss of basal ice through melting enhanced ocean driven melting may provide a simple link between regional climate warming and the successive disintegration of sections of the larsen ice shelf

a model of marine ice formation within antarctic ice shelf rifts large rifts that open in antarctic ice shelves are known to be filled by snow accumulations ice shelf fragments and sea ice this work demonstrates how these rifts may also be filled from below through their interaction with ocean water by marine ice the model presented here quantifies both the rate of marine ice accumulation at the top of the rift which results from melt driven convection at its sides and the impact of this process on waters occupying the confined environment of the rift the results show that such a system could fill the rift with tens of meters of marine ice in the process the temperature and salinity of the ambient water in the rift evolve through two distinct phases the first is characterized by rapid change that takes the properties of the ambient water to near equilibrium values followed by a second phase of much slower change the melting rate at the lower part of the walls of the rift emerges as the principal factor influencing many aspects of model behavior testing the model against field observations from an antarctic rift showed it to be robust and successful in reproducing the main observed features including the presence of a thick layer of supercooled ocean water inside the rift

dynamic in stability of thwaites glacier west antarctica 1 thwaites glacier west antarctica has the potential to directly contribute 1 m to sea level and currently is losing mass and thinning rapidly here we report on regional results for the sea level response to ice sheet evolution searise experiments and investigate the impact of i spatial resolution within existing data sets ii grounding zone processes and iii till rheology on the dynamics of this outlet glacier in addition to the searise data sets we use detailed aerogeophysical and satellite data from thwaites glacier as input to a coupled ice stream ice shelf ocean plume model that includes oceanic influences across a several kilometers wide grounding zone suggested by new high resolution data our results indicate that the ice tongue provides limited stability and that while future atmospheric warming will likely add mass to the surface of the glacier strong ice stream stabilization on bedrock highs narrower than the length of the grounding zone may be ephemeral if circulating waters substantially reduce basal resistance and enhance melting beneath grounded ice within this zone however we find that stability is significantly enhanced by effectively plastic till beds accurate projections of future sea level change relies on correct understanding of the till rheology as well as local basal processes near the grounding line

ice shelf grounding zone structure from icesat laser altimetry 1 we present a technique for investigating the grounding zone gz of antarctic ice shelves using laser altimetry from the ice cloud and land elevation satellite icesat most surface height variability in the gz is easily resolved by the icesat laser s 65 m footprint and 172 m along track spacing comparisons of repeated tracks sampled at different phases of the ocean tide identify the landward and seaward limits of tide forced ice flexure providing gz location and width information for each track using icesat data in the institute ice stream region of southern ronne ice shelf we demonstrate that the location of the gz based on feature identification in satellite imagery or digital elevation models may be in error by several km our results show that icesat will contribute significantly to improving knowledge of gz structure and to studies requiring accurate gz locations e g ice mass balance calculations and ice sheet ocean modeling

tidally controlled stick slip discharge of a west antarctic ice a major west antarctic ice stream discharges by sudden and brief periods of very rapid motion paced by oceanic tidal oscillations of about 1 meter acceleration to speeds greater than 1 meter per hour and deceleration back to a stationary state occur in minutes or less slip propagates at approximately 88 meters per second suggestive of a shear wave traveling within the subglacial till a model of an episodically slipping friction locked fault reproduces the observed quasi periodic event timing demonstrating an ice stream s ability to change speed rapidly and its extreme sensitivity to subglacial conditions and variations in sea level

a novel method for predicting fracture in floating ice basal crevasses may play an important precursory role in determining both the location and propagation of rifts and iceberg dimensions for example icebergs calved recently from thwaites glacier antarctica have the same width as surface undulations strengthening the connection between basal crevasses rifting and calving we explore a novel method for estimating the heights of basal crevasses formed at the grounding lines of ice shelves and ice streams we employ a thin elastic beam teb formulation and tensional yielding criterion to capture the physics of flexed ice at grounding lines observations of basal crevasse heights compare well with model predictions in the siple coast region of the ross ice shelf we find that the teb method is most accurate in areas of low strain rate we also test the method in other areas of antarctica to produce order of magnitude maps of grounding line basal crevasses and find general agreement with reported observations assuming basal crevasses develop in spatio temporal sequence and are advected downstream this method is computationally cheap and could be relatively easy to implement into damage oriented large scale ice models which aim at physically simulating calving and fracture processes

modeling tidal current profiles and vertical mixing beneath filchner ronne ice shelf antarctica abstract one of the warmest water masses beneath filchner ronne ice shelf fris is dense high salinity shelf water hssw that flows into the sub ice shelf cavity from the ice front and occupies the lower portion of the water column a one dimensional turbulence closure ocean model has been applied to this high latitude sub ice shelf environment to demonstrate that tidal currents mix hssw vertically through the water column and cause melting at the bottom of the ice shelf significantly fris lies near the critical latitude for the semidiurnal tide where the coriolis frequency equals the tidal frequency resulting in a strongly depth dependent tidal current and thick boundary layers using the model the effect of the critical latitude stratification and the polarization of the tidal current ellipse on boundary layer structure and subsequent vertical mixing are examined the model shows that stratification significantly affects how the shape of the tidal current ellipse varies with depth and that both

ice stream d flow speed is strongly modulated by the tide beneath the ross ice shelf the flow velocity of ice stream d west antarctica has been measured to vary by a factor of three over the course of a day these fluctuations are measured at the grounding line as well as upstream of the grounding line in the ice plain of ice stream d the diurnal velocity fluctations appear to be driven by the tide beneath the ross ice shelf these results suggest that there is significant and heretofore poorly understood influence of the ocean tide and of the ice shelf on the dynamics of ice stream flow

enabling clean access into subglacial lake whillans development and use of the wissard hot water drill system clean hot water drill systems chwdss are used with clean access protocols for the exploration of subglacial lakes and other subglacial aquatic environments e g ice shelf cavities in antarctica a chwds developed for the whillans ice stream subglacial access research drilling wissard project by the science management office at the university of nebraska lincoln unl smo usa was specifically designed for use in west antarctica where the us antarctic program s south pole traverse could assist with logistical support the initial goal was to provide clean access holes through ice up to 1000 m thick following environmental stewardship guidelines however the existing design allows this chwds to be used for ice thicknesses up to 2000 m following modifications to accommodate longer hose lengths in january 2013 the wissard chwds successfully provided for the first time a clean access borehole through 800 m of ice into subglacial lake whillans beneath the west antarctic ice sheet for the deployment of scientific instruments and sampling tools the development and initial use of the wissard chwds required the project team to address a number of constraints while providing contingencies to meet the defined project scope schedule and budget

ice shelf elevation changes due to atmospheric pressure variations the inverse barometer effect ibe is the isostatic response of ocean surface height to changes in atmospheric pressure p air at a rate of about 1 cm hpa 1 the ibe is a significant contributor to variability of ice shelf surface elevation ice as we demonstrate with simultaneous global positioning system measurements of ice and local measurements of p air from the amery brunt and ross ice shelves antarctica we find that an ibe correction is justified for frequencies covering the weather band 0 03 0 5 cpd cycles per day the ibe correction reduces the standard deviation of the weather band signal of ice from 9 cm to 3 cm with this correction the largest remaining high frequency error signal in ice is the inaccuracy of the present generation of antarctic tide models estimated to be of order 10 cm for most of antarctica

tide induced lateral movement of brunt ice shelf antarctica 1 accurate position data show that brunt ice shelf has an irregular three dimensional motion driven by the ocean tide a 30 day gps record from halley station shows not only the expected vertical tidal displacement but also a significant variation in horizontal velocity the maximum forward velocity occurs about four hours before the maximum tidal height a phase relationship that suggests ocean currents may be responsible however values of ice water friction coefficients needed to make this explanation plausible appear to be too high to be physically reasonable alternative explanations invoke interactions between the ice shelf and a grounded area at the ice front our findings suggest that significant errors might result if these motions are ignored when interpreting double difference satellite sar interferograms where the common assumption is that the horizontal velocity of an ice shelf has no tidal dependence

a decade of west antarctic subglacial lake interactions from combined icesat and cryosat 2 altimetry we use cryosat 2 interferometric satellite radar altimetry over the mercer and whillans ice streams west antarctica to derive surface elevation changes due to subglacial lake activity at monthly resolution for the period 2010 to 2013 we validate cryosat 2 elevation measurements trends and spatial patterns of change using satellite image differencing and in situ vertical movement from global positioning system gps data two subglacial lake discharge events occur in the same subglacial hydrological catchment within a 9 month period using gps measurements that are spanning the gap between the ice cloud and land elevation satellite and cryosat 2 missions we cross calibrate the two missions to establish the efficacy of cryosat 2 altimetry to measure dynamic changes on the ice sheets

on the interpretation of ice shelf flexure measurements tidal flexure in ice shelf grounding zones has been used extensively in the past to determine grounding line position and ice properties although the rheology of ice is viscoelastic at tidal loading frequencies most modelling studies have assumed some form of linear elastic beam approximation to match observed flexure profiles here we use density radar and dinsar measurements in combination with full stokes viscoelastic modelling to investigate a range of additional controls on the flexure of the southern mcmurdo ice shelf we find that inclusion of observed basal crevasses and density dependent ice stiffness can greatly alter the flexure profile and yet fitting a simple elastic beam model to that profile will still produce an excellent fit estimates of the effective young s modulus derived by fitting flexure profiles are shown to vary by over 200 depending on whether these factors are included even when the local thickness is well constrained conversely estimates of the grounding line position are relatively insensitive to these considerations for the case of a steep bed slope in our study region by fitting tidal amplitudes only and ignoring phase information elastic beam theory can provide a good fit to observations in a wide variety of situations this should however not be taken as an indication that the underlying rheological assumptions are correct

variable deceleration of whillans ice stream west antarctica the whillans ice stream ice plain wip has been slowing since at least 1963 prior constraints on this slowdown were consistent with a constant long term deceleration rate new observations of ice velocity from 11 continuous and 3 seasonal gps sites indicate the deceleration rate varies through time including on interannual time scales between 2009 and 2012 wip decelerated at a rate 6 1 to 10 9 2 m yr2 that was double the multidecadal average 3 0 to 5 6 2 m yr2 to identify the causes of slowdown we used new and prior velocity estimates to constrain longitudinal and transverse force budget models as well as a higher order inverse model all model results support the conclusion that the observed deceleration of wip is caused by an increase in basal resistance to motion at a rate of 10 to 40 pa yr subglacial processes that may be responsible for strengthening the ice stream bed include basal freeze on changes in subglacial hydrology or increases in the area of resistant basal substrate through differential erosion the observed variability in wip deceleration rate suggests that dynamics in subglacial hydrology plausibly driven by basal freeze on and or activity of subglacial lakes plays a key role in modulating basal resistance to ice motion in the region

on the short term grounding zone dynamics of pine island glacier west antarctica observed with cosmo skymed interferometric data using radar satellite data from the italian cosmo skymed csk constellation and the german tandem x formation we present comprehensive measurements of the bi weekly grounding line dynamics of pine island glacier west antarctica from august to december 2015 the one day repeat cycle of csk reveals tidally induced grounding line migration on the scale of kilometers and extensive seawater intrusion within the grounding zone which significantly exceeds that predicted for a stiff bed but are consistent with that calculated for a deformable bed the deformable bed also explains the continuous draining filling of subglacial lakes proximal to the grounding line after correction for oceanic tides we estimate a retreat rate for 2011 2015 of 0 3 km yr at the glacier center and 0 5 km yr on the sides which is three times slower than for 1994 2011 1 2 km yr at the center we attribute the decrease in retreat rate to colder ocean conditions in 2012 2013 relative to 2000 2011

modeling tidal currents beneath filchner ronne ice shelf and on the adjacent continental shelf their effect on mixing and transport a depth averaged tidal model has been applied to the southern weddell sea the model domain covers the southern continental shelf including the ocean cavity beneath filchner ronne ice shelf reasonable agreement with the available current meter data has been achieved our results confirm that in areas with shallow water and large topographic gradients tidal oscillations with peak velocities up to 1 m s 1 play a significant role in the vertical mixing and transport of water masses the estimated energy dissipation beneath filchner ronne ice shelf due to surface friction is 25 gw approximately 1 of the world s total tidal dissipation tidally induced lagrangian residual currents converging at the ice front an area of strong mixing draw together water masses from the continental shelf and sub ice shelf cavity the model indicates that lagrangian residual currents have fluxes of up to 250 000 m3 s 1 and speeds of over 5 cm s 1 along the ice front with over 350 000 m3 s 1 being exchanged between the sub ice shelf cavity and adjacent continental shelf these currents are particularly efficient in ventilating the sub ice shelf cavity within 150 km of ronne ice front such strong tidal mixing will significantly modify the properties of water masses that flow through this region particularly to the west of berkner island the model predictions indicate that tidal processes strongly influence the oceanographic conditions in the vicinity of ronne ice front shipborne observations along the ice front support many of the model predictions concerning the effect of tides on the hydrography

tidally induced variations in vertical and horizontal motion on rutford ice stream west antarctica inferred from remotely sensed observations to better understand the influence of stress changes over floating ice shelves on grounded ice streams we develop a bayesian method for inferring time dependent 3 d surface velocity fields from synthetic aperture radar sar and optical remote sensing data our specific goal is to observe ocean tide induced variability in vertical ice shelf position and horizontal ice stream flow thus we consider the special case where observed surface displacement at a given location can be defined by a 3 d secular velocity vector a family of 3 d sinusoidal functions and a correction to the digital elevation model used to process the sar data using nearly 9 months of sar data collected from multiple satellite viewing geometries with the cosmo skymed 4 satellite constellation we infer the spatiotemporal response of rutford ice stream west antarctica to ocean tidal forcing consistent with expected tidal uplift inferred vertical motion over the ice shelf is dominated by semidiurnal and diurnal tidal constituents horizontal ice flow variability on the other hand occurs primarily at the fortnightly spring neap tidal period m sf we propose that periodic grounding of the ice shelf is the primary mechanism for translating vertical tidal motion into horizontal flow variability causing ice flow to accelerate first and most strongly over the ice shelf flow variations then propagate through the grounded ice stream at a mean rate of 29 km d and decay quasi linearly with distance over 85 km upstream of the grounding zone

high basal melting forming a channel at the grounding line of ross ice shelf antarctica antarctica s ice shelves are thinning at an increasing rate affecting their buttressing ability channels in the ice shelf base unevenly distribute melting and their evolution provides insight into changing subglacial and oceanic conditions here we used phase sensitive radar measurements to estimate basal melt rates in a channel beneath the currently stable ross ice shelf melt rates of 22 2 0 2 m a 1 2500 the overall background rate were observed 1 7 km seaward of mercer whillans ice stream grounding line close to where subglacial water discharge is expected laser altimetry shows a corresponding steadily deepening surface channel two relict channels to the north suggest recent subglacial drainage reorganization beneath whillans ice stream approximately coincident with the shutdown of kamb ice stream this rapid channel formation implies that shifts in subglacial hydrology may impact ice shelf stability

the global s1 tide the small s1 ocean tide is caused primarily by diurnal atmospheric pressure loading its excitation is therefore unlike any other diurnal tide in particular pressure loading is maximum near the equator where the diurnal gravitational potential is zero the global character of the s 1 tide is here determined by numerical modeling and by analysis of ocean topography experiment topex poseidon satellite altimeter data the two approaches yield reasonably consistent results amplitudes exceeding 1 cm in several regions are further confirmed by comparison with coastal tide gauges notwithstanding their excitation differences s 1 and other diurnal tides are found to share several common features such as relatively large amplitudes in the arabian sea the labrador sea the sea of okhotsk and the gulf of alaska the most noticeable difference is the lack of an s 1 antarctic kelvin wave these similarities and differences can be explained in terms of the coherences between neardiurnal oceanic normal modes and the underlying tidal forcings whereas gravitational diurnal tidal forces excite primarily a 28 h antarctic pacific mode the s 1 air tide excites several other near diurnal modes none of which has large amplitudes near antarctica

tidal pressurization of the ocean cavity near an antarctic ice shelf grounding line mass loss from the antarctic ice sheet is sensitive to conditions in ice shelf grounding zones the transition between grounded and floating ice to observe tidal dynamics in the grounding zone we moored an ocean pressure sensor to ross ice shelf recording data for 54 days in this region the ice shelf is brought out of hydrostatic equilibrium by the flexural rigidity of ice yet we found that tidal pressure variations at a constant geopotential surface were similar within and outside of the grounding zone this implies that the grounding zone ocean cavity was overpressurized at high tide and underpressurized at low tide by up to 10 kpa with respect to glaciostatic pressure at the ice shelf base phase lags between ocean pressure and vertical ice shelf motion were tens of minutes for diurnal and semidiurnal tides an effect that has not been incorporated into ocean models of tidal currents below ice shelves these tidal pressure variations may affect the production and export of meltwater in the subglacial environment and may increase basal crevasse heights in the grounding zone by several meters according to linear elastic fracture mechanics we find anomalously high tidal energy loss at the k1 constituent in the grounding zone and hypothesize that this could be explained by seawater injection into the subglacial environment at high tide or internal tide generation through interactions with topography these observations lay the foundation for improved representation of the grounding zone and its tidal dynamics in ocean circulation models of sub ice shelf cavities

impact of tide topography interactions on basal melting of larsen c ice shelf antarctica 1 basal melting of ice shelves around antarctica contributes to formation of antarctic bottom water and can affect global sea level by altering the offshore flow of grounded ice streams and glaciers tides influence ice shelf basal melt rate wb by contributing to ocean mixing and mean circulation as well as thermohaline exchanges with the ice shelf we use a three dimensional ocean model thermodynamically coupled to a nonevolving ice shelf to investigate the relationship between topography tides andwb for larsen c ice shelf lcis in the northwestern weddell sea antarctica using our best estimates of ice shelf thickness and seabed topography we find that the largest modeled lcis melt rates occur in the northeast where our model predicts strong diurnal tidal currents 0 4 m s 1 this distribution is significantly different from models with no tidal forcing which predict largest melt rates along the deep grounding lines we compare several model runs to explore melt rate sensitivity to geometry initial ocean potential temperature 0 thermodynamic parameterizations of heat and freshwater ice ocean exchange and tidal forcing the resulting range of lcis averagedwb is 0 11 0 44 m a 1 the spatial distribution of wb is very sensitive to model geometry and thermodynamic parameterization while the overall magnitude of wb is influenced by 0 these sensitivities in wbpredictions reinforce a need for high resolution maps of ice draft and sub ice shelf seabed topography together with ocean temperature measurements at the ice shelf front to improve representation of ice shelves in coupled climate system models

fortnightly variations in the flow velocity of rutford ice stream west antarctica ice sheets like that of antarctica lose much of their ice via fast flowing ice streams hilmar gudmundsson has monitored the flow in one such stream the rutford ice stream in west antarctica over a period of two months the findings are a surprise surface flow velocities here fluctuate by as much as 20 over a period of two weeks perhaps under the influence of the spring neap fortnightly tidal cycle although the mechanism that couples these tides to ice flow has yet to be elucidated these observations caution against the use of velocity measurements over limited periods to infer long term changes in flow rate monitoring of the flow of an ice sheet in antarctica finds that the surface velocity oscillates by as much as 20 per cent every two weeks the spring to neap tidal cycle seems to be the cause although the mechanism through which these tides couple to ice flow has yet to be elucidated these observations caution against the use of velocity measurements over limited periods to infer long term changes in flow rate most of the ice lost from the antarctic ice sheet passes through a few fast flowing and highly dynamic ice streams1 quantifying temporal variations in flow in these ice streams and understanding their causes is a prerequisite for estimating the potential contribution of the antarctic ice sheet to global sea level change2 3 here i show that surface velocities on a major west antarctic ice stream rutford ice stream vary periodically by about 20 per cent every two weeks as a result of tidal forcing tidally induced motion on ice streams has previously been thought to be limited to diurnal or even shorter term variations4 5 6 7 8 9 the existence of strong fortnightly variations in flow demonstrates the potential pitfalls of using repeated velocity measurements over intervals of days to infer long term change

motion of an antarctic glacier by repeated tidally modulated earthquakes subglacial seismicity reveals information about glacier behaviour analysis of repeated seismic events beneath an antarctic outlet glacier is consistent with sliding of debris laden ice over a bedrock asperity with an event frequency that is modulated by the ocean tides

modelling the response of ice shelf basal melting to different ocean cavity environmental regimes we present simulation results from a version of the regional ocean modeling system modified for ice shelf ocean interaction including the parameterisation of basal melting by molecular diffusion alone simulations investigate the differences in melting for an idealised ice shelf experiencing a range of cold to hot ocean cavity conditions both the pattern of melt and the location of maximum melt shift due to changes in the buoyancy driven circulation in a different way to previous studies tidal forcing increases both the circulation strength and melting with the strongest impact on the cold cavity case our results highlight the importance of including a complete melt parameterisation and tidal forcing in response to the 2 4 c ocean warming initially applied to a cold cavity ice shelf we find that melting will increase by about an order of magnitude 24 with tides and 41 without tides

semi automatic mapping of linear trending bedforms using self organizing maps algorithm abstract increased application of high resolution spatial data such as high resolution satellite or unmanned aerial vehicle uav images from earth as well as high resolution imaging science experiment hirise images from mars makes it necessary to increase automation techniques capable of extracting detailed geomorphologic elements from such large data sets model validation by repeated images in environmental management studies such as climate related changes as well as increasing access to high resolution satellite images underline the demand for detailed automatic image processing techniques in remote sensing this study presents a methodology based on an unsupervised artificial neural network ann algorithm known as self organizing maps som to achieve the semi automatic extraction of linear features with small footprints on satellite images som is based on competitive learning and is efficient for handling huge data sets we applied the som algorithm to high resolution satellite images of earth and mars quickbird worldview and hirise in order to facilitate and speed up image analysis along with the improvement of the accuracy of results about 98 overall accuracy and 0 001 quantization error in the recognition of small linear trending bedforms demonstrate a promising framework

a new tide model for the antarctic ice shelves and seas we describe a new tide model for the seas surrounding antarctica including the ocean cavities under the floating ice shelves the model uses data assimilation to improve its fit to available data typical peak to peak tide ranges on ice shelves are 1 2 m but can exceed 3 m for the filchner ronne and larsen ice shelves in the weddell sea spring tidal ranges are about twice these values model performance is judged relative to the 5 10 cm accuracy that is needed to fully utilize ice shelf height data that will be collected with the geoscience laser altimeter system scheduled to be launched on the ice cloud and land elevation satellite in late 2002 the model does not yet achieve this level of accuracy except very near the few high quality tidal records that have been assimilated into the model some improvement in predictive skill is expected from increased sophistication of model physics but we also require better definition of ice shelf grounding lines and more accurate water column thickness data in shelf seas and under the ice shelves long duration tide measurements bottom pressure gauge or gps in critical data sparse areas particularly near and on the filchner ronne and ross ice shelves and pine island bay are required to improve the performance of the data assimilation model

modeling channelized and distributed subglacial drainage in two dimensions 1 we present a two dimensional glacier drainage system model glads that couples distributed and channelized subglacial water flow distributed flow occurs through linked cavities that are represented as a continuous water sheet of variable thickness channelized flow occurs through rothlisberger channels that can form on any of the edges of a prescribed unstructured network of potential channels water storage is accounted for in an englacial aquifer and in moulins which also act as point sources of water to the subglacial system solutions are presented for a synthetic topography designed to mimic an ice sheet margin for low discharge all the flow is accommodated in the sheet whereas for sufficiently high discharge the model exhibits a channelization instability which leads to the formation of a self organized channel system the random orientation of the network edges allows the channel system geometry to be relatively unbiased in contrast to previous structured grid based models under steady conditions the model supports the classical view of the subglacial drainage system with low pressure regions forming around the channels under diurnally varying input water flows in and out of the channels and a rather complex spatiotemporal pattern of water pressures is predicted we explore the effects of parameter variations on the channel system topology and mean effective pressure the model is then applied to a mountain glacier and forced with meltwater calculated by a temperature index model the results are broadly consistent with our current understanding of the glacier drainage system and demonstrate the applicability of the model to real settings

ice shelf tidal flexure and subglacial pressure variations abstract we develop a model of an ice shelf ice stream system as a viscoelastic beam partially supported by an elastic foundation when bedrock near the grounding line acts as a fulcrum leverage from the ice shelf dropping at low tide can cause significant 1 cm uplift in the first few kilometers of grounded ice this uplift and the corresponding depression at high tide lead to basal pressure variations of sufficient magnitude to influence subglacial hydrology tidal flexure may thus affect basal lubrication sediment flow and till strength all of which are significant factors in ice stream dynamics and grounding line stability under certain circumstances our results suggest the possibility of seawater being drawn into the subglacial water system the presence of seawater beneath grounded ice would significantly change the radar reflectivity of the grounding zone and complicate the interpretation of grounded versus floating ice based on ice penetrating radar observations

ice flow of the antarctic ice sheet we present a reference comprehensive high resolution digital mosaic of ice motion in antarctica assembled from multiple satellite interferometric synthetic aperture radar data acquired during the international polar year 2007 to 2009 the data reveal widespread patterned enhanced flow with tributary glaciers reaching hundreds to thousands of kilometers inland over the entire continent this view of ice sheet motion emphasizes the importance of basal slip dominated tributary flow over deformation dominated ice sheet flow redefines our understanding of ice sheet dynamics and has far reaching implications for the reconstruction and prediction of ice sheet evolution

sediment deposition at the modern grounding zone of whillans ice stream west antarctica 1 much of the threshold behavior of marine ice sheets is thought to result from processes occurring at the grounding zone where the ice sheet transitions into the ice shelf at short timescales decades to centuries grounding zone behavior is likely to be influenced by ongoing sediment deposition which can stabilize the grounding zone position we present a ground based geophysical study across the modern grounding zone of whillans ice stream within an embayment setting where subglacial meltwater drains we image sedimentary deposits however we do not observe the classic asymmetric grounding zone wedge deposits which are found in the paleorecord and are thought to stabilize the grounding zone position at an adjacent peninsula we image the grounding zone and an ice shelf pinning point and again observe no wedge deposits

tides and the flow of rutford ice stream west antarctica 1 surface speeds of rutford ice stream west antarctica are known to vary by around 10 20 depending on location with a fortnightly periodicity corresponding to a springneap tidal cycle the reasons for these periodic variations in flow are unclear here the possible role of tidal stress transmission upstream of the grounding line in affecting rates of basal motion is investigated it is found that nonlinear rheological effects within the till when coupled with transmission of tidal stresses within the ice that are linearly related to tidal amplitude can give rise to the type of periodic oscillations in flow observed this nonlinear interaction between tidal forcing and till deformation increases the mean ice flux across the grounding line by a few percent above what might be expected in the absence of tidal forcing periodic velocity fluctuations of this type have not been observed on other ice streams however modeling suggests that this may be due to lack of data and that such flow variations are likely to be common features of active ice streams draining into the ronne ice shelf as well as of other ice streams subjected to similar tidal forcing

connected subglacial lake activity on lower mercer and whillans ice streams west antarctica 2003 2008 we examine patterns of localized surface elevation change in lower mercer and whillans ice streams west antarctica which we interpret as subglacial water movement through a system of lakes and channels we detect and measure the lake activity using repeat track laser altimetry from icesat and image differencing from modis image pairs a hydrostatic potential map for the region shows that the lakes are distributed across three distinct hydrologic regimes our analysis shows that within these regimes some of the subglacial lakes appear to be linked with drainage events in one reservoir causing filling and follow on drainage in adjacent lakes we also observe changes near ice raft a in lower whillans ice stream and interpret them as evidence of subglacial water and other changes at the bed the study provides quantitative information about the properties of this complex subglacial hydrologic system and a relatively unstudied component of ice sheet mass balance subglacial drainage across the grounding line

elastic dynamics and tidal migration of grounding lines modify subglacial lubrication and melting 1 we combine the dynamics of ice bed and ocean in a new elastic model for the tidal timescale migration of grounding lines on deformable foundations previous interpretations of tidal flexure using models of elastic ice shelves with fixed grounding lines were found to be inconsistent suggesting an elasticity of ice that varies spatially and temporally and that is significantly smaller than measured experimentally we argue here that with our model a consistent purely elastic interpretation can be made combining this new approach with remote sensing measurements we show that the grounding line migrates several kilometers during a tidal cycle that we can infer the effective elastic properties of the bed and that the elastic pressure of the ice leads to a hydrological barrier near the grounding line that controls subglacial hydrology our findings imply that subglacial lubrication and melting induced by the ocean thermal forcing can increase substantially during high tide

tidal observations on floating ice using a single gps receiver 1 we measure ocean tides as experienced by sea ice or ice shelves using a single gps receiver such a method allows for tidal measurements to be made with the same precision regardless of its distance from a gps base station since such a base station is not required single epoch observations are shown to be accurate to about 50 mm when compared with concurrent bottom pressure gauge bpg observations the differences between the gps and bpg observations are largely free from power at tidal frequencies furthermore harmonic tidal analyses show that these gps observations allow the correct determination of diurnal and semi diurnal tidal constituents meaning that observations of this type can be assimilated into numerical tide models

episodic ice velocity fluctuations triggered by a subglacial flood in west antarctica height change anomalies in satellite altimeter data have been interpreted as the surface expressions of basal water moving into and out of subglacial lakes these signals have been mapped throughout antarctica on timescales of months to years but only broad connections have been made between active lakes and ice dynamics we present the first high frequency observations of ice velocity evolution due to a cascading subglacial lake drainage event collected over 5 years 2010 2015 using global positioning system data on whillans and mercer ice streams west antarctica we observed three episodic ice velocity changes over 2 years where flow speed increased by up to 4 as well as an 11 month disruption of the tidally modulated stick slip cycle that dominates regional ice motion our observations reveal that basal conditions of an antarctic ice stream can rapidly evolve and drive a dynamic ice response on subannual timescales which can bias observations used to infer long term ice sheet changes

tidal controls on the flow of ice streams the flow of many antarctic ice streams is known to be significantly influenced by tides in the past modeling studies have implemented the tidal forces acting on a coupled ice stream ice shelf system in a number of different ways but the consequences that this has on the modeled response of ice streams to tides have until now not been considered here we investigate for the first time differences in model response that are only due to differences in the way tidal forcings are implemented we find that attempts to simplify the problem by neglecting flexural stresses are generally not valid and forcing models with only changes in ocean back pressure will not capture either the correct amplitudes or length scale

a simple model of the ice shelf ocean boundary layer and current abstractocean forced basal melting has been implicated in the widespread thinning of antarctic ice shelves but an understanding of what determines melt rates is hampered by limited knowledge of the buoyancy and frictionally controlled flows along the ice shelf base that regulate heat transfer from ocean to ice in an attempt to address this deficiency a simple model of a buoyant boundary flow considering only the spatial dimension perpendicular to the boundary is presented results indicate that two possible flow regimes exist a weakly stratified geostrophic cross slope current with upslope flow within a buoyant ekman layer or a strongly stratified upslope current with a weak cross slope flow the latter regime which is analogous to the steady solution for a katabatic wind is most appropriate when the ice ocean interface is steep for the gentle slopes typical of antarctic ice shelves the buoyant ekman regime which has similarities with the case of an unstratified density current on a slope p

hinge line migration of petermann gletscher north greenland detected using satellite radar interferometry the synthetic aperture radar interferometry technique is used to detect the migration of the limit of tidal flexing or hinge line of the floating ice tongue of petermann gletscher a major outlet glacier of north greenland the hinge line is detected automatically from differential interferograms using a model fitting technique based on an elastic beam theory the statistical noise of the model fit is less than 3 mm and the hinge line is mapped with a precision of 30 m following automatic registration of multidate image data to a precision of 5 m hinge line migration is subsequently detected with a precision of 40 m in the horizontal plane across the glacier width the results show that the hinge line of petermann gletscher migrates back and forth with tide by 70 m in excellent agreement with the migration calculated from ocean tides predicted by a tidal model combined with the glacier surface and basal slope measured by an ice sounding radar superimposed on the short term hinge line migration due to tide we detect a hinge line retreat of 270 m in 3 87 years which varies across the glacier width by 120 m the retreat suggests glacier thinning at a rate of 78 35 cm ice a 1 coincidentally an analysis of ice volume fluxes indicates that the hinge line ice flux of petermann gletscher exceeds its balance flux by 0 88 1 km3 ice a 1 which in turn implies glacier thinning at 83 95 cm ice a 1 in the glacier lower reaches both methods therefore suggest that petermann gletscher is currently losing mass to the ocean

wissard at subglacial lake whillans west antarctica scientific operations and initial observations a clean hot water drill was used to gain access to subglacial lake whillans slw in late january 2013 as part of the whillans ice stream subglacial access research drilling wissard project over 3 days we deployed an array of scientific tools through the slw borehole a downhole camera a conductivity temperature depth ctd probe a niskin water sampler an in situ filtration unit three different sediment corers a geothermal probe and a geophysical sensor string our observations confirm the existence of a subglacial water reservoir whose presence was previously inferred from satellite altimetry and surface geophysics subglacial water is about two orders of magnitude less saline than sea water 0 37 0 41 psu vs 35 psu and two orders of magnitude more saline than pure drill meltwater 0 002 psu it reaches a minimum temperature of 0 558c consistent with depression of the freezing point by 7 019 mpa of water pressure subglacial water was turbid and remained turbid following filtration through 0 45 mm filters the recovered sediment cores which sampled down to 0 8 m below the lake bottom contained a macroscopically structureless diamicton with shear strength between 2 and 6 kpa our main operational recommendation for future subglacial access through water filled boreholes is to supply enough heat to the top of the borehole to keep it from freezing

thirteen years of subglacial lake activity in antarctica from multi mission satellite altimetry the ability to detect the surface expression of moving water beneath the antarctic ice sheet by satellite has revealed a dynamic basal environment with implications for regional ice dynamics grounding line stability and fluxes of freshwater and nutrients to the southern ocean knowledge of subglacial activity on timescales important for near term prediction of ice sheet fluctuations decadal to century is limited by the short observational record of nasa s ice cloud and land elevation satellite icesat laser altimetry mission used to generate the last continent wide survey 2003 08 here we use synthetic aperture radar interferometric mode data from esa s cryosat 2 radar altimetry mission 2010 present which samples 45 of the icesat derived subglacial lakes to extend their time series to the end of 2016 the extended time series show that there have been surface height changes at 20 of the 45 lakes since 2008 indicating that some of these features are persistent and potentially cyclic while other features show negligible changes suggesting these may be transient or nonhydrological features continued monitoring of active lakes for both height and velocity changes as well as developing methods for identifying additional lakes is critical to quantifying the full distribution of active subglacial lakes in antarctica

tidal pacing skipped slips and the slowdown of whillans ice stream antarctica we summarize new observations of the deceleration and stick slip motion of whillans ice stream wis antarctica we refine the location of the large sticky spots that resist motion between slip events the locations of which are controlled by the patterns of subglacial water flow our examination of the long term velocity time series for the ice stream reveals that the decadal scale deceleration is not occurring at a steady rate but varies at the sub decadal timescale this unsteady deceleration modulates the temporal evolution of a broad 50 km across surface elevation bulge forming at the junction between the relatively narrow upstream portion of the ice stream and broad ice plain that constitutes the downstream end of wis comparison of observations from april 2003 and november 2010 reveals significant changes in the tidally modulated stick slip cycle that regulates motion on the ice plain we observe that the timing of slip events has become less regular in response to decreased flow speed in the upstream portions of the ice stream the decreased regularity of slip events has reduced the release of stored elastic strain during slip events increasing the rate of deceleration

resonance vibrations of the ross ice shelf and observations of persistent atmospheric waves recently reported lidar observations have revealed a persistent wave activity in the antarctic middle and upper atmosphere that has no counterpart in observations at mid and low latitude locations the unusual wave activity suggests a geographically specific source of atmospheric waves with periods of 3 10 hours here we investigate theoretically the hypothesis that the unusual atmospheric wave activity in antarctica is generated by the fundamental and low order modes of vibrations of the ross ice shelf ris simple models are developed to describe basic physical properties of resonant vibrations of large ice shelves and their coupling to the atmosphere dispersion relation of the long surface waves which propagate in the floating ice sheet and are responsible for its low order resonances is found to be similar to the dispersion relation of infragravity waves in the ice free ocean the phase speed of the surface waves and the resonant frequencies determine the periods and wave vectors of atmospheric waves that are generated by the ris resonant oscillations the altitude dependent vertical wavelengths and the periods of the acoustic gravity waves in the atmosphere are shown to be sensitive to the physical parameters of the ris which can be difficult to measure by other means predicted properties of the atmospheric waves prove to be in a remarkable agreement with the key features of the observed persistent wave activity

flow of the ross ice shelf antarctica is modulated by the ocean tide the ice streams feeding the ross ice shelf antarctica have large tidally modulated sinusoidal and stick slip flow but the interaction with the ice shelf is poorly understood we show that the flow of the ross ice shelf front up to 650 km from the ice streams exhibits smooth sinusoidal motions corresponding to tidal modulation these observations suggest a possible linking of the ice shelf with the ice streams to form a unified system that responds to small perturbations in stresses associated with ocean tides if this is the case the presence of the sinusoidal motion but the absence of stick slip motion suggests there is damping of very high frequency signals the dissimilar signatures of the motions observed in the ice streams and at the front of the ice shelf present challenges to model development aimed at understanding the dynamics of coupled ice stream ice shelf flow and the movement of ice across grounding lines

on the representation of ice shelf grounding zones in sar interferograms we investigate limitations of the one dimensional elastic beam model to detect grounding line and thickness of an ice shelf from a differential interferogram spatial limitations due to grounding line curvature and variable ice thickness are analyzed by comparison with two dimensional plate flexure temporal limitations from the tide dependent shift of the grounding line are analyzed by superpositions of four tidal flexure profiles representing differential interferograms i at scales greater one ice thickness seaward protrusions of the grounding line are well represented by the elastic beam model while landward embayments of the same scale produce significant misplacements 10 per cent of the ice thickness ii for reasonable spatial variations of shelf thickness the elastic beam model gives reliable estimates of grounding line position and unfractured mean ice thickness near the grounding line iii for about 20 per cent of superpositions of four tidal flexure profiles the resulting grounding line misplacements exceed the physical tidal shift of the grounding line by factors 2 for differential tide levels 2 km examples of real interferometric profiles from west antarctic ice shelves corroborate our results

antarctic ice sheet loss driven by basal melting of ice shelves accurate prediction of global sea level rise requires that we understand the cause of recent widespread and intensifying glacier acceleration along antarctic ice sheet coastal margins atmospheric and oceanic forcing have the potential to reduce the thickness and extent of floating ice shelves potentially limiting their ability to buttress the flow of grounded tributary glaciers indeed recent ice shelf collapse led to retreat and acceleration of several glaciers on the antarctic peninsula but the extent and magnitude of ice shelf thickness change the underlying causes of such change and its link to glacier flow rate are so poorly understood that its future impact on the ice sheets cannot yet be predicted here we use satellite laser altimetry and modelling of the surface firn layer to reveal the circum antarctic pattern of ice shelf thinning through increased basal melt we deduce that this increased melt is the primary control of antarctic ice sheet loss through a reduction in buttressing of the adjacent ice sheet leading to accelerated glacier flow the highest thinning rates occur where warm water at depth can access thick ice shelves via submarine troughs crossing the continental shelf wind forcing could explain the dominant patterns of both basal melting and the surface melting and collapse of antarctic ice shelves through ocean upwelling in the amundsen and bellingshausen seas and atmospheric warming on the antarctic peninsula this implies that climate forcing through changing winds influences antarctic ice sheet mass balance and hence global sea level on annual to decadal timescales

on the disintegration of ice shelves the role of fracture dans une plateforme glaciaire contrairement au cas des glaciers les crevasses de surface peuvent atteindre le niveau de mer et les crevasses de fond remonter jusqu a ce niveau les crevasses jouent alors un role important dans la desintegration de la plateforme la fracturation est examinee en fonction des contraintes generales et locales on applique l etude a la stabilite de plateformes de l antarctique ouest

diurnal and semidiurnal tide induced lateral movement of ronne ice shelf antarctica 1 recent gps observations from a spatially extensive network across ronne ice shelf show significant daily ice flow variations at all sites the almost synchronous horizontal displacements occur at diurnal and semidiurnal tidal periods during spring tides displacements velocities and strains near the ice front have superimposed oscillations that are 300 of their mean values and occur over a six hour period resulting in regular ice shelf flow reversals close to ice stream grounding lines however the horizontal diurnal and semidiurnal signals decay and almost vanish from our analysis we conclude that ice shelves respond primarily elastically to tidal tilting thus accounting for the observed diurnal and semidiurnal flow variations and their amplification toward the ice shelf front our findings suggest that detailed modeling of these data could provide improved ice shelf and ice stream models for correctly simulating ice shelf flow and predicting future ice sheet evolution citation makinson k m a king k w nicholls and g hilmar gudmundsson 2012 diurnal and semidiurnal tide induced lateral movement of ronne ice shelf antarctica geophys res lett 39 l10501 doi 10 1029 2012gl051636

tidal movement of nioghalvfjerdsfjorden glacier northeast greenland observations and modelling nioghalvfjerdsfjorden glacier is a 60 km long and 20 km wide floating outlet glacier located at 79 30 n 22 w draining a large area of the northeast greenland ice sheet climate mass balance and dynamics studies were carried out on the glacier in three field seasons in 1996 1997 and 1998 as part of this work tidal movement observations were carried out by simultaneous differential global positioning system gps measurements at several locations distributed on the glacier surface the gps observations were performed continously over several tidal cycles at the same time tiltmeter measurements were carried out in the grounding zones along the glaciers margins and upstream where the glacier leaves the main ice sheet a tide gauge installed in the sea immediately in front of the glacier front recorded the tide in the open sea during the field seasons the observations show that the main part of the glacier tongue responds as a freely floating plate to the phase and amplitude of the local tide in the sea however kilometre wide flexure zones exist along the marginal and upstream grounding lines attempts to model the observed tidal defectionand tilt patterns in the flexure zone by elastic beam theory are unsuccessful in contrast to previous findings by other investigators the strongest disagreement between our measurements and results derived from elastic beam theory is a significant variation of the phase of the tidal records with distance from the grounding line most clearly displayed by the tilt records we suggest that the viscous properties of glacier ice must be taken into account and consequently that a viscoelastic beam model must be used to adequately describe tidal bending of floating glaciers

estuaries beneath ice sheets interactions between subglacial hydrology and the ocean make the existence of estuaries at the grounding zones of ice sheets likely here we present geophysical observations of an estuary at the downstream end of the hydrologic system that links the active subglacial lakes beneath whillans ice stream to the ocean beneath the ross ice shelf antarctica this subglacial estuary consists of a hydropotential low upstream of the grounding zone which is linked to the ocean by a hydropotential trough and a large subglacial channel this subglacial channel which is imaged using active source seismic methods has an apparent width of 1 km and a maximum depth of 7 m the hydropotential trough continues upstream of the grounding zone and results from an along flow depression in surface elevations pressure differences along the trough axis are within a range that can be overcome by tidally induced processes making the interaction of subglacial and ocean water likely

the supply of subglacial meltwater to the grounding line of the siple coast west antarctica recent satellite studies have shown that active subglacial lakes exist under the antarctic ice streams and persist almost to their grounding lines when the lowest lying lakes flood the water crosses the grounding line and enters the sub ice shelf cavity modeling results suggest that this additional freshwater influx may significantly enhance melting at the ice shelf base we examine the spatial and temporal variability in subglacial water supply to the grounding lines of the siple coast ice streams by combining estimates for lake volume change derived from ice cloud and land elevation satellite icesat data with a model for subglacial water transport our results suggest that subglacial outflow tends to concentrate towards six embayments in the siple coast grounding line although mean grounding line outflow is 60m3 s 1 for the entire siple coast maximum local grounding line outflow may temporarily exceed 300 m3 s 1 during the synchronized flooding of multiple lakes in a hydrologic basin variability in subglacial outflow due to subglacial lake drainage may account for a substantial portion of the observed variability in freshwater flux out of the ross ice shelf cavity the temporal variability in grounding line outflow results in a net reduction in long term average melt rate but temporary peak melting rates may exceed the long term average by a factor of three

adaptation of an isopycnic coordinate ocean model for the study of circulation beneath ice shelves abstract much of the antarctic coastline comprises large floating ice shelves beneath which waters from the open ocean circulate the interaction of the seawater with the base of these ice shelves has a bearing both on the rate at which antarctic bottom water is formed and on the mass balance of the ice sheet an isopycnic coordinate ocean general circulation model has been modified so as to allow the incorporation of a floating ice shelf as an upper boundary to the model domain the modified code admits the introduction of an arbitrary surface pressure field and includes new algorithms for the diagnosis of entrainment into and detrainment from the surface mixed layer special care is needed in handling the cases where the mixed layer and isopycnic interior layers interact with surface and basal topography the modified model is described in detail and then applied to an idealized ice shelf ocean geometry simple tests with zero surface buoyancy forcing indicate that the introduction of the static s

recent changes in the dynamic condition of the ross ice shelf antarctica variations in the amplitude of radar echoes from the bottom of the grid western half of the ross ice shelf have been analyzed contrary to the results of a similar analysis performed for the grid eastern sector of the ice shelf bands of low signal strength downstream from both crary ice rise and the siple coast do not correlate with modern flow lines the difference in direction between the radar bands downstream of crary ice rise and the present velocity vectors and the absence of a comparable trend farther east suggest to us that the grounding line around crary ice rise retreated within the last 1000 years this hypothesis is reinforced by the observation of several domes and hollows in ice thickness downstream of crary ice rise which are similar to a hollow now located in the wake of the ice rise and a dome on its eastern flank we interprete this as evidence for a rapid increase in flow around the ice rise which carried downstream the ice topography formed around the ice rise similar but less detailed evidence found downstream of the siple coast suggests that there was a regional retreat of the west antarctic grounding line

a model of tidally dominated ocean processes near ice shelf grounding lines 1 glaciological processes at grounding lines which divide floating ice shelves from grounded ice sheets may strongly influence the dynamics and evolution of inland ice therefore understanding the oceanic forcing on ice shelves in this region is of importance to predictions of cryospheric change and sea level rise as the ocean cavity shallows toward the grounding line tidal mixing becomes proportionately more important until a tidal front forms beyond which the water properties are vertically homogenized the extent of this mixed zone is relevant to several questions because a fully mixed region behaves differently to the stratified ocean offshore in this study a highly simplified one dimensional model is used to examine the size properties and sensitivities of the mixed zone the model suggests that most grounding line mixed zones are small implying that the usual models representing a stratified ocean are generally valid if tidal mixing is also taken into account modeled mixed zones can be significant in near freezing regions with vigorous tides and a shallowly sloping cavity but even these areas are smaller than previously proposed it therefore seems that upwelling of warm water rather than mixing in tidal zones generally maintains ice shelf basal melting near grounding lines where mixed zones are present the model suggests that they insulate the grounding line from offshore ocean waters the model illustrates the origin of ice shelf water plumes and confirms that unlike elsewhere in the cavity melting in the mixed zone increases linearly in response to ocean warming

ice sheet grounding zone stabilization due to till compaction 1 based on modeling motivated by new gps and radio echo sounding surveys a few kilometers long zone of whillans ice stream west antarctica just inland of the grounding line has higher basal shear stress than the ice farther upstream or the freely slipping ice shelf downstream data from this zone show a few meter high upwarp of the surface overlying a large fold extending through all internal radar layers observed flowband modeling shows that the fold can be generated by decreased basal lubrication beneath the upwarp basal topography alone cannot create this fold physical modeling and available data suggest that low amplitude tidal flexure of the ice shelf extends a few kilometers inland downward flexing of this grounded ice from the rising tide would compact subglacial till resulting in higher basal shear stress this result suggests that important processes influencing grounding line stability are not included in modern whole ice sheet models

tidal bending and strand cracks at the kamb ice stream grounding line west antarctica an extensive set of shore parallel fractures are observed at the grounding line of kamb ice stream kis in west antarctica seismicity measured in the grounding zone is as elsewhere around antarctica tidally forced and moreover strand cracks propagate nearly exclusively on the falling tide measured surface deflection and a model of fracture propagation are used to conclude that bending on the falling tide favors propagation while bending on the rising tide suppresses propagation without the perturbation due to tidal bending strand cracks would be rare and appear farther downstream than observed we speculate that the very large number of cracks observed at kis is due to the stagnant to floating transition at that grounding line which allows cyclic bending of the same ice and relatively large stretching rates

field studies of bottom crevasses in the ross ice shelf antarctica analyse des crevasses observees par sondages radar aeriens et terrestres localisation de leurs points de formation

nonlinear interaction between ocean tides and the larsen c ice shelf system 1 we report on three gps records of flow of the larsen c ice shelf spanning 2 months to 2 years variations in speed are evident at periods from a few hours to 182 days including semi diurnal diurnal and 14 76 days at fortnightly periods the ice shelf varies by 10 from its long term speed but at diurnal timescales it is up to 100 a nonlinear relationship between ocean tides and velocity is required to explain the observed variations in flow as an initial examination we model flow as a non linear function of basal shear stress including tidal perturbations in the ice shelf grounding zone and show that the major features of the observations are reproduced notably the long period signal largely absent from the vertical tidal signal alternative explanations are discussed these observations demonstrate that the ice shelf system is highly sensitive to relatively modest changes in forcing at its boundaries

ocean tides in the weddell sea new observations on the filchner ronne and larsen c ice shelves and model validation 1 ocean tides under the large weddell sea ice shelves are among the least well observed on earth here we present new spatially extensive observations of the vertical tidal motion of the filchner ronne and larsen c ice shelves using global positioning system gps data spanning a few weeks to years we pay particular attention to the major tidal constituents m2 s2 o1 k1 as well as important grace aliasing periods k2 and s1 we compare the estimated constituents with recent global and regional tide models and find that no single model is the most accurate across all constituents or ice shelves the root sum square errors are 7 8 cm cats2008a and tpxo7 2 and 11 12 cm got4 7 and fes2004 with the energetic m2 rmse 4 8 cm and s2 4 5 cm generally dominating these statistics the fes2004 k1 is particularly inaccurate near the larsen c ice shelf with errors approaching 20 cm meaning that grace release 4 estimates of mass change in the northern antarctic peninsula will be biased we find tidal energy at 3 4 5 6 and weakly at 7 cycles per day at all of our sites the largest amplitudes within these bands are at m4 mo3 and sp3 and approach 30 mm although significant spatial variations exist we show that they generally do not appear to originate in areas of reduced water column in ice shelf grounding zones comparing model estimates with our m4 ms4 and mn4 values shows that models do not accurately represent these terms

an improved model for tidally modulated grounding line migration understanding grounding line dynamics is necessary for predictions of long term ice sheet stability however despite growing observations of the tidal influence on grounding line migration this short timescale migration is poorly understood with most modeling attempts assuming beam theory to calculate displacements here we present an improved model of tidal grounding line migration that treats migration as an elastic fracture problem forced by the additional ocean water pressure from the tide this new model predicts that the grounding line cannot be assumed to be in hydrostatic equilibrium and furthermore that migration is inherently asymmetric and nonlinear with migration distances that are not proportional to the tidal load specifically for constant surface slope the grounding line migrates upstream approximately ten times further as the tide rises from mean sea level to high tide than it migrates downstream as the tide falls from mean sea level to low tide and migration distances are substantially larger than simple flotation arguments suggest numerical tests also show that the dependence of migration distance on elastic moduli and ice sheet thickness are inconsistent with predictions of beam theory for a range of realistic values finally applying the new model to observations in antarctica results in new estimates of bed slopes though these estimates remain uncertain due to imperfect knowledge of the relevant rheological parameters