wave attenuation over coastal salt marshes under storm surge conditions salt marshes protect coastlines against waves wave flume experiments show that marsh vegetation causes substantial wave dissipation and prevents erosion of the underlying surface even during extreme storm surge conditions

tide model accuracy in the amundsen sea antarctica from radar interferometry observations of ice shelf motion 1 this study assesses the accuracy of tide model predictions in the amundsen sea sector of west antarctica tide model accuracy in this region is poorly constrained yet tide models contribute to simulations of ocean heat transfer and to the removal of tidal signals from satellite observations of ice shelves we use two satellite based interferometric synthetic aperture radar insar methods to measure the tidal motion of the dotson ice shelf at multiple epochs a single difference technique that measures tidal displacement and a double difference technique that measures changes in tidal displacement we use these observations to evaluate predictions from three tide models tpxo7 1 cats2008a opt and fes2004 all three models perform comparably well exhibiting root mean square deviations from the observations of 9 cm single difference technique and 10 cm double difference technique care should be taken in generalizing these error statistics because 1 the dotson ice shelf experiences relatively small semidiurnal tides and 2 our observations are not sensitive to all tidal constituents an error analysis of our insar based methods indicates measurement errors of 7 and 4 cm for the single and double difference techniques respectively a model based correction for the effect of fluctuations in atmospheric pressure yields an 6 improvement in the agreement between tide model predictions and observations this study suggests that tide model accuracy in the amundsen sea is comparable to other antarctic regions where tide models are better constrained these methods can be used to evaluate tide models in other remote antarctic waters

reconstruction of basal properties in ice sheets using iterative inverse methods inverse problems are used to estimate model parameters from observations many inverse problems are ill posed because they lack stability meaning it is not possible to find solutions that are stable with respect to small changes in input data regularization techniques are necessary to stabilize the problem for nonlinear inverse problems iterative inverse methods can be used as a regularization method these methods start with an initial estimate of the model parameters update the parameters to match observation in an iterative process that adjusts large scale spatial features first and use a stopping criterion to prevent the overfitting of data this criterion determines the smoothness of the solution and thus the degree of regularization here iterative inverse methods are implemented for the specific problem of reconstructing basal stickiness of an ice sheet by using the shallow shelf approximation as a forward model and synthetically derived surface velocities as input data the incomplete gauss newton ign method is introduced and compared to the commonly used steepest descent and nonlinear conjugate gradient methods two different stopping criteria the discrepancy principle and a recent improvement threshold are compared the ign method is favored because it is rapidly converging and it incorporates the discrepancy principle which leads to optimally resolved solutions

modeling the instantaneous response of glaciers after the collapse of the larsen b ice shelf following the disintegration of the larsen b ice shelf antarctic peninsula in 2002 regular surveillance of its 20 tributary glaciers has revealed a response which is varied and complex in both space and time the major outlets have accelerated and thinned smaller glaciers have shown little or no change and glaciers flowing into the remnant scar inlet ice shelf have responded with delay in this study we present the first area wide numerical analysis of glacier dynamics before and immediately after the collapse of the ice shelf combining new datasets and a state of the art numerical ice flow model we simulate the loss of buttressing at the grounding line and find a good qualitative agreement between modelled changes in glacier flow and observations through this study we seek to improve confidence in our numerical models and their ability to capture the complex mechanical coupling between floating ice shelves and grounded ice

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

supraglacial lakes on the larsen b ice shelf antarctica and at paakitsoq west greenland a comparative study we acknowledge the support of the u s national science foundation under grant ant 0944248

acceleration and spatial rheology of larsen c ice shelf antarctic peninsula the disintegration of several antarctic peninsula ice shelves has focused attention on the state of the larsen c ice shelf here we use satellite observations to map ice shelf speed from the years 2000 2006 and 2008 and apply inverse modeling to examine the spatial pattern of ice shelf stiffness results show that the northern half of the ice shelf has been accelerating since 2000 speeding up by 15 between 2000 and 2006 alone the distribution of ice stiffness exhibits large spatial variations that we link to tributary glacier flow and fractures our results reveal that ice down flow from promontories is consistently softer with the exception of churchill peninsula where we infer a stabilizing role for marine ice we conclude that although larsen c is not facing imminent collapse it is undergoing significant change in the form of flow acceleration that is spatially related to thinning and fracture copyright 2011 by the american geophysical union

sensitivity analysis of pine island glacier ice flow using issm and dakota assessing output errors of ice flow models is a major challenge that needs to be addressed if we are to increase our confidence level in projections of mass balance in antarctica and greenland major inputs to ice flow models include geometry ice thickness and surface elevation constitutive laws and boundary conditions geothermal flux basal drag coefficient surface temperature these inputs can be either measured in which case they carry errors due to instruments or inferred using inverse methods such as basal drag which is inverted using insar surface velocities in which case they carry additional errors generated by the inversion process itself in both cases these input errors will result in uncertainties that propagate throughout a forward model and that influence output diagnostics in order to estimate the resulting error margins on diagnostics such as mass flux we develop a new framework based on the design analysis kit for optimization and terascale applications dakota which we interface to the ice sheet system model issm we present results on the pine island glacier west antarctica for which we evaluate error margins of mass flux across the whole glacier given currently known error margins on ice thickness basal friction and ice hardness our results suggest errors in these inputs propagate linearly through the ice flow model providing a way to 1 calibrate measurement requirements for field campaigns collecting data such as bedrock or surface topography 2 quantify uncertainties in projections of mass balance and 3 assess the sensitivity of model outputs to input parameters this new error propagation model should help quantify confidence levels that we assign to model projections for the mass balance of antarctica and greenland which will ultimately improve our projections of future sea level rise in a warming climate copyright 2012 by the american geophysical union

catastrophic ice shelf breakup as the source of heinrich event icebergs 1 heinrich layers of the glacial north atlantic record abrupt widespread iceberg rafting of detrital carbonate and other lithic material at the extreme cold culminations of bond climate cycles both internal glaciologic and external climate forcings have been proposed here we suggest an explanation for the iceberg release that encompasses external climate forcing on the basis of a new glaciological process recently witnessed along the antarctic peninsula rapid disintegrations of fringing ice shelves induced by climate controlled meltwater infilling of surface crevasses we postulate that peripheral ice shelves formed along the eastern canadian seaboard during extreme cold conditions would be vulnerable to sudden climate driven disintegration during any climate amelioration ice shelf disintegration then would be the source of heinrich event icebergs

effect of orbital scale climate cycling and meltwater drainage on ice sheet grounding line migration 1 ice shelf response to climate cycling introduces hysteretic behavior in outlet glaciers with the evolution of meltwater drainage networks enhancing this effect the 2002 collapse of the larsen b ice shelf one of the floating extensions formed from glaciers flowing off the antarctic peninsula into the weddell sea followed the development of widespread poorly drained surface melt ponds during the atypical warmth of the 2001 2002 summer melt season yet for at least the past half century the floating termini of outlet glaciers in greenland did not collapse despite being exposed to summer mean temperatures and melt rates that far exceed those recorded in 2001 2002 on the antarctic peninsula we suggest that this paradox can be explained by the advection of well developed englacial drainage networks within greenland s extensive ablation zone the efficiency of this plumbing system limits meltwater ponding within crevasses and the subsequent ice shelf failure observed on the antarctic peninsula whether progressive or abrupt ice shelf removal leads to near synchronous glacier retreat conversely glaciation of fjords is delayed during ice shelf formation ultimately the relatively rapid loss and gradual establishment of ice shelf buttressing amid climate cycling leads to an asymmetric response in outlet glacier dynamics this asymmetry may have implications for the future of existing and recently calved ice shelves eustatic sea level and the hysteretic behavior of ice sheets

breakup and conditions for stability of the northern larsen ice shelf antarctica the breakup of ice shelves has been widely regarded as an indicator of climate change1 with observations around the antarctic peninsula having shown a pattern of gradual retreat associated with regional atmospheric warming and increased summer melt and fracturing processes2 3 4 5 6 7 8 9 the rapid collapse of the northernmost section of the larsen ice shelf larsen a over a few days in january 1995 indicated that after retreat beyond a critical limit ice shelves may disintegrate rapidly here we use a finite element numerical model that treats ice as a continuum without fracture10 to examine the breakup history2 between 1986 and 1997 of the two northern sections of larsen ice shelf larsen a and larsen b from which we establish stability criteria for ice shelves analysis of various ice shelf configurations reveals characteristic patterns in the strain rates near the ice front which we use to describe the stability of the ice shelf on larsen a only the initial and final ice front configurations show a stable pattern larsen b at present exhibits a stable pattern but if the ice front were to retreat by a further few kilometres it too is likely to enter an irreversible retreat phase

catastrophic ice shelf break up by an ice shelf fragment capsize mechanism two disintegration events leading to the loss of larsen a and b ice shelves antarctic peninsula in 1995 and 2002 respectively proceeded with extreme rapidity order of several days and reduced an extensive seemingly integrated ice shelf to a jumble of small fragments these events strongly correlate with warming regional climate and accumulation of surface meltwater supporting a hypothesis that meltwater induced propagation of pre existing surface crevasses may have initiated ice shelf fragmentation we address here an additional subsequent mechanism that may sustain and accelerate the ice shelf break up once it begins the proposed mechanism involves the coherent capsize of narrow less than thickness ice shelf fragments by rolling 90 in a direction toward or away from the ice front fragment capsize liberates gravitational potential energy forces open ice shelf rifts and contributes to further fragmentation of the surrounding ice shelf

the evolving instability of the remnant larsen b ice shelf and its tributary glaciers 2015 elsevier b v following the 2002 disintegration of the northern and central parts of the larsen b ice shelf the tributary glaciers of the southern surviving part initially appeared relatively unchanged and hence assumed to be buttressed sufficiently by the remnant ice shelf here we modify this perception with observations from icebridge altimetry and insar inferred ice flow speeds our analyses show that the surfaces of leppard and flask glaciers directly upstream from their grounding lines lowered by 15 to 20 m in the period 2002 2011 the thinning appears to be dynamic as the flow of both glaciers and the remnant ice shelf accelerated in the same period flask glacier started accelerating even before the 2002 disintegration increasing its flow speed by 55 between 1997 and 2012 starbuck glacier meanwhile did not change much we hypothesize that the different evolutions of the three glaciers are related to their dissimilar bed topographies and degrees of grounding we apply numerical modeling and data assimilation that show these changes to be accompanied by a reduction in the buttressing afforded by the remnant ice shelf a weakening of the shear zones between its flow units and an increase in its fracture the fast flowing northwestern part of the remnant ice shelf exhibits increasing fragmentation while the stagnant southeastern part seems to be prone to the formation of large rifts some of which we show have delimited successive calving events a large rift only 12 km downstream from the grounding line is currently traversing the stagnant part of the ice shelf defining the likely front of the next large calving event we propose that the flow acceleration ice front retreat and enhanced fracture of the remnant larsen b ice shelf presage its approaching demise

experimental study on vegetation flexibility as control parameter for wave damping and velocity structure abstract vegetation can contribute to coastal defence by damping incoming waves however prior studies have shown that attenuation varies greatly among plant species plant flexibility is a mechanical property that is commonly omitted but varies considerably between shrubs and grasses on salt marshes therefore we present an experimental study in a laboratory wave flume with artificial vegetation that differs in flexibility only we measured wave attenuation and water particle velocities around rigid and flexible salt marsh vegetation waves were measured using a series of gauges and particle image velocimetry piv was used to measure spatio temporal variations of water particle velocities in the x z plane around the vegetation our results show that flexible vegetation attenuates waves up to 70 less than rigid vegetation due to swaying of flexible plants furthermore we find that rigid vegetation modifies the velocity structure whereas flexible vegetation does not specifically a mean current in the direction of wave propagation develops around the canopy and the horizontal particle velocities are amplified directly above the canopy these results indicate that plant flexibility is a key parameter in the wave vegetation interaction that controls wave damping and velocity structure

geometrical and mechanical properties of four species of northern european brown macroalgae abstract aquatic vegetation interacts with the flow by posing an obstruction and in return experiences drag and is reconfigured by acting forces it is suggested that plant buoyancy and stiffness affect these bio physical interactions and hence should be considered when including vegetation in physical and numerical hydrodynamic models however data on these parameters is either lacking for many species or existing information is insufficient to model flexible aquatic vegetation correctly previous studies have focused on plant stems but did not take plant foliage into account the present study extends the existing knowledge base by providing data for four northern european brown macroalgae and also provides a comparison between the mechanical properties of stem and blade tissue for these species specimens of alaria esculenta laminaria digitata fucus serratus and fucus vesiculosus were collected from a small tidal inlet in norway other than f serratus the macroalgae were positively buoyant and all stems showed bending moduli in agreement with previous studies with respect to their order of magnitude only l digitata exhibited a significant difference between bending moduli for stems and blades and also between the tip and the base of the blades however there were differences in flexural rigidities product of bending modulus and second moment of cross sectional area of both stems and blades and the tip and the base of blades of all but f serratus this highlights the important influence of cross sectional area and cross sectional shape on stiffness

wave dissipation by flexible vegetation 1 dissipation of waves propagating through natural salt marsh vegetation was about half the dissipation expected for rigid vegetation this low dissipation was predicted by a theoretical model that accounts for bending of vegetation motions a transect of 3 pulse coherent acoustic doppler profilers recorded water velocity and pressure at 8 hz within the dense 650 stems m2 canopy of semi flexible single stem vegetation schoenoplectus americanus most wave energy 56 81 was dissipated within 19 m of the marsh edge two dissipation models the first assuming rigid vegetation and the second simulating wave forced vegetation motion using the theory for bending of linearly elastic beams were tested after choosing optimal drag coefficients both models yielded a good fit to the observed dissipation skill score 0 96 0 99 however fitted drag coefficients for the rigid model 0 58 0 78 were below the range 0 98 2 2 expected for the observed reynolds numbers 13 450 and canopy densities accounting for interactions between stem wakes whereas drag coefficients for the flexible model 0 97 1 6 were nearer the expected range indicating that prediction of wave dissipation was improved by simulating vegetation motion

calving processes and the dynamics of calving glaciers abstract calving of icebergs is an important component of mass loss from the polar ice sheets and glaciers in many parts of the world calving rates can increase dramatically in response to increases in velocity and or retreat of the glacier margin with important implications for sea level change despite their importance calving and related dynamic processes are poorly represented in the current generation of ice sheet models this is largely because understanding the calving problem involves several other long standing problems in glaciology combined with the difficulties and dangers of field data collection in this paper we systematically review different aspects of the calving problem and outline a new framework for representing calving processes in ice sheet models we define a hierarchy of calving processes to distinguish those that exert a fundamental control on the position of the ice margin from more localised processes responsible for individual calving events the first order control on calving is the strain rate arising from spatial variations in velocity particularly sliding speed which determines the location and depth of surface crevasses superimposed on this first order process are second order processes that can further erode the ice margin these include fracture propagation in response to local stress imbalances in the immediate vicinity of the glacier front undercutting of the glacier terminus by melting at or below the waterline and bending at the junction between grounded and buoyant parts of an ice tongue calving of projecting submerged ice feet can be regarded as a third order process because it is paced by first or second order calving above the waterline first order calving can be represented in glacier models using a calving criterion based on crevasse depth which is a function of longitudinal strain rate modelling changes in terminus position and calving rates thus reduces to the problem of determining the ice geometry and velocity distribution realistic solutions to the problem of modelling ice flow therefore depend critically on an appropriate choice of sliding law models that assume that basal velocities are controlled by basal drag can replicate much of the observed behaviour of calving glaciers with grounded termini but an important limitation is that they cannot be used to model floating glacier termini or ice shelves alternative sliding laws that parameterise drag from the glacier margins provide more flexible and robust ways of representing calving in ice sheet models such models can explain a remarkable range of observed phenomena within a simple unifying framework including downglacier increases in velocity and strain rates where basal and or lateral drag diminishes flow acceleration in response to thinning through time the tendency for glaciers to stabilise at pinning points in relatively shallow water or fjord narrowings the constraints on ice shelf stability and the contrasts in calving rates between tidewater and freshwater calving glaciers many unresolved issues remain however including the role played by the removal of backstress in the acceleration of retreating calving glaciers and the controls on melting at and below the waterline

the role of lateral drag in the dynamics of ice stream b antarctica the partitioning of resistive force between the bed and sides of ice stream b antarctica is obtained for three large areas that have been measured using repeat aerial photogrammetry problems associated with data errors and local variations in ice strength and velocity are reduced by considering the areally averaged budget of forces for each photo block results indicate that the bed under ice stream b must be very weak and unable to provide muche resistance mechanical control on this ice stream emanates almost entirely from the lateral margins

northern larsen ice shelf antarctica further retreat after collapse changes of larsen ice shelf antarctica and the surrounding glaciers after its collapse in 1995 were investigated using satellite radar imagery with emphasis onchanges in the glaciers which previously nourished the ice shelf north of seal nunataks the large glaciers retreated several kilometres inland of the previousgrounding line the velocity field of drygalski glacier the largest glacier in this area was mapped by means of interferograms derived from pairs of ers syntheticaperture radar images from 1995 and 1999 the main part of the glacier showed significant acceleration of flow over these 4 years with an increase of veocity upto three fold at the terminus similar accelerations were observed by means of interferometry on several other grounded glaciers suggesting that the removal ofice shelves could lead to an effect on eustatic sea level for larsen b the northernmost surviving part of larsen ice shelf the retreat of the ice front to october2000 is documented

large scale ice flow over a viscous basal sediment theory and application to ice stream b antarctica recent seismic studies of ice stream b antarctica and field analysis of mid latitude glacial deposits suggest that deformable basal sediments e g water saturated till are important in determining ice sheet flow if the ratio of till viscosity to effective ice viscosity is small vertical shear associated with horizontal flow is confined to the deforming bed alone ice flow over a deformable bed is thus akin to that of floating ice shelves because ice shelves flow over inviscid seawater for some antarctic ice streams and possibly for portions of the late wisconsin ice sheets in north america and eurasia basal drag associated with deforming basal sediment does not induce significant vertical gradients of horizontal velocity instead basal drag affects the flow as if it were a horizontal body force balanced by longitudinal and transverse deviatoric stress gradients comparison of the observed flow of ice stream b to finite element simulations incorporating a viscous basal till suggest that a simple till theology is sufficient to explain the current velocity profile these simulations also highlight the importance of horizontal deviatoric stress in regions where driving stress and basal stress do not locally balance while bed deformation is critical to ice stream existence sensitivity tests suggest that ice shelf back pressure is still a crucial control affecting ice stream response to atmospheric and oceanic climate

numerical modelling and data assimilation of the larsen b ice shelf antarctic peninsula in this study the flow and rheology of pre collapse larsen b ice shelf are investigated by using a combination of flow modelling and data assimilation observed shelf velocities from satellite interferometry are used to constrain an ice shelf model by using a data assimilation technique based on the control method in particular the ice rheology field and the velocities at the inland shelf boundary are simultaneously optimized to get a modelled flow and stress field that is consistent with the observed flow the application to the larsen b ice shelf shows that a strong weakening of the ice in the shear zones mostly along the margins is necessary to fit the observed shelf flow this pattern of bands with weak ice is a very robust feature of the inversion whereas the ice rheology within the main shelf body is found to be not well constrained this suggests that these weak zones play a major role in the control of the flow of the larsen b ice shelf and may be the key to understanding the observed pre collapse thinning and acceleration of larsen b regarding the sensitivity of the stress field to rheology the consistency of the model with the observed flow seems crucial for any further analysis such as the application of fracture mechanics or perturbation model experiments

climatically induced retreat and collapse of northern larsen ice shelf antarctic peninsula the areal changes of the northern larsen ite shelf lis antarctic peninsula between march 1986 and march 1997 have been analyzed based on synthetic aperture radar images of the european remote sensing satellites ers 1 and ers 2 and on landsat images this analysis is complemented by data on ice motion and surface mass balance which have been obtained during several field campaigns since the early 1980s after a period of retreat coinciding with atmospheric warming and with decreasing net accumulation at the surface due to melt losses the two northernmost sections of lis disintegrated almost completely within a few days in january 1995 recent observations of the ice shelf section north of jason peninsula which is presently the northernmost section of lis show increased summer melt and intensification of the rifting processes probably causing accelerated retreat of this section in the near future the retreat and the disintegration event of lis indicate high sensitivity of ice shelves to prolonged perturbations of the mass balance

representation of sharp rifts and faults mechanics in modeling ice shelf flow dynamics application to brunt stancomb wills ice shelf antarctica ice shelves play a major role in buttressing ice sheet flow into the ocean hence the importance of accurate numerical modeling of their stress regime commonly used ice flow models assume a continuous medium and are therefore complicated by the presence of rupture features crevasses rifts and faults that significantly affect the overall flow patterns here we apply contact mechanics and penalty methods to develop a new ice shelf flow model that captures the impact of rifts and faults on the rheology and stress distribution of ice shelves the model achieves a best fit solution to satellite observations of ice shelf velocities to infer the following 1 a spatial distribution of contact and friction points along detected faults and rifts 2 a more realistic spatial pattern of ice shelf rheology and 3 a better representation of the stress balance in the immediate vicinity of faults and rifts thus applying the model to the brunt stancomb wills ice shelf antarctica we quantify the state of friction inside faults and the opening rates of rifts and obtain an ice shelf rheology that remains relatively constant everywhere else on the ice shelf we further demonstrate that better stress representation has widespread application in examining aspects affecting ice shelf structure and dynamics including the extent of ice melange in rifts and the change in fracture configurations all are major applications for better insight into the important question of ice shelf stability

basal shear stress of the ross ice streams from control method inversions 1 we used control method inversions to determine the basal shear stress beneath the ross ice streams where new high resolution velocity data sets have recently become available the inversion algorithm was adapted from an earlier viscous bed algorithm to allow solution for the basal shear stress corresponding to a weak plastic bed we performed several experiments using synthetic data to determine the quality of the inversions these experiments indicate that with high quality surface elevation data e g errors 5 m the inversions are relatively robust with respect to errors in ice flow velocity and bed topography the inversions are consistent with seismic and borehole observations and indicate that the ross ice streams lie atop a bed that is nearly everywhere weak in contrast the tributaries feeding these ice streams overlie alternating patches of strong and weak bed

large scale 3 d experiments of wave and current interaction with real vegetation part 2 experimental analysis abstract this paper assesses the influence of different flow and vegetation parameters on the wave attenuation provided by two contrasting salt marsh species puccinellia maritima and spartina anglica different water depths and wave parameters height and period are considered for both regular and irregular waves with and without an underlying uniform current coming from different directions the study of the submergence ratio h h v influence shows that wave damping coefficient rapidly decreases as the plant submergence ratio increases the high nonlinearities found in the wave current interaction lead to different wave damping patterns in comparison to wave only conditions a smaller wave damping is found for waves and current acting in the same direction and an increase in the wave damping rate is obtained for waves and current flowing in the opposite direction these wave and current tests allow for the studying of the energy dissipation produced by the vegetation increasing our knowledge about flow and plant interaction in estuarine conditions the biomechanical properties of the two real salt marshes used in the experiments are also evaluated and related to wave damping revealing a higher attenuation for stiffer vegetation both the vegetation density and the biomass strongly influence wave damping higher density and biomass values lead to higher attenuation rates for both species

marine ice in larsen ice shelf 1 it is argued that larsen ice shelf contains marine ice formed by oceanic freezing and other mechanisms missing basal returns in airborne radar soundings and observations of a smooth and healed surface coincide downstream of regions where an ocean model predicts freezing visible imagery suggests that marine ice currently stabilizes larsen c ice shelf and implicates failure of marine flow bands in the 2002 larsen b ice shelf collapse ocean modeling indicates that any regime change towards the incursion of warmer modified weddell deep water into the larsen c cavity could curtail basal freezing and its stabilizing influence

greenland ice sheet volume sensitivity to basal surface and initial conditions derived from an adjoint model we extend the application of control methods to a comprehensive three dimensional thermomechanical ice sheet model sicopolis simulation code for polythermal ice sheets lagrange multipliers i e sensitivities are computed with an exact efficient adjoint model that has been generated from sicopolis by rigorous application of automatic differentiation the case study uses the adjoint model to determine the sensitivity of the total greenland ice volume to various control variables over a 100 year period the control space has of the order 1 2 10 6 elements consisting of spatial fields of basal flow parameters surface and basal forcings and initial conditions reliability of the adjoint model was tested through finite difference perturbation calculations for various control variables and perturbation regions ascertaining quantitative inferences of the adjoint model as well as confirming qualitative aspects of ice sheet sensitivities e g expected regional variations we detect regions where model sensitivities are seemingly unexpected or counter intuitive albeit real in the sense of actual model behavior an example is inferred regions where sensitivities of ice sheet volume to basal sliding coefficient are positive i e where a local increase in basal sliding parameter increases the ice sheet volume similarly positive generally negative ice temperature sensitivities in certain parts of the ice sheet are found the detection of which seems highly unlikely if only conventional perturbation experiments had been used the object of this paper is largely a proof of concept available adjoint code generation tools now open up a variety of novel model applications notably with regard to sensitivity and uncertainty analyses and ice sheet state estimation or data assimilation

ice flow sensitivity to geothermal heat flux of pine island glacier antarctica model projections of ice flow in a changing climate are dependent on model inputs such as surface elevation bedrock position or surface temperatures among others of all these inputs geothermal heat flux is the one for which uncertainty is greatest in the area of pine island glacier antarctica available data sets differ by up to a factor of 2 5 here we evaluate the impact of such uncertainty on ice flow using sampling analyses based on the latin hypercube method first we quantify the impact of geothermal heat flux errors on ice hardness a thermal parameter that critically controls the magnitude of ice flow second we quantify the impact of the same errors on mass balance specifically on the mass flux advecting through thirteen fluxgates distributed across pine island glacier we contrast our results with similar uncertainties generated by errors in the specification of ice thickness model outputs indicate that geothermal heat flux errors yield uncertainties on ice hardness on the order of 5 7 with maximum uncertainty reaching 15 resulting uncertainties in mass balance remain however below 1 we discuss the uncertainty distribution and its relationship to the amount of heat available at the base of the ice sheet from friction viscous and geothermal heating we also show that comparatively errors in ice thickness contribute more to model uncertainty than errors in geothermal heat flux especially for fast flowing ice streams 2012 american geophysical union all rights reserved

interferometric analysis of the deformation pattern of the northern larsen ice shelf antarctic peninsula compared to field measurements and numerical modeling the motion field of the northern larsen ice shelf antarctic peninsula was analyzed using radar interferometry in combination with field measurements and finite element model calculations the ice shelf between jason peninsula and seal nunataks has been in steady retreat since january 1995 model calculations suggest that the ice shelf is in a stage of irreversible retreat since the last calving events in summer 1998 99 the interferometric analysis is based on synthetic aperture radar data of the tandem mission of european remote sensing satellites ers 1and 2 in austral spring 1995 the phase contributions due to tidal motion were estimated from the vertical displacement at those parts of the grounding zone where the horizontal motion is close to zero in order to separate the vertical and horizontal motion components over the ice shelf satellite derived velocities compare well with the long term field measurements along a transverse and a longitudinal profile the real interferograms and synthetic interferograms calculated from model velocities show reasonable agreement over the main parts of the ice shelf but differ in the boundary zones where the details are not resolved by the model

an ice shelf model test based on the ross ice shelf a standard numerical experiment featuring the ross ice shelf antarctica is presented as a test package for the development and intercomparison of ice shelfmodels the emphasis of this package is solution of stress equilibrium equations for an ice shelf velocity consistent with present observations as ademonstration we compare five independently developed ice shelf models based on finite difference and finite element methods our results suggest thatthere is little difference between finite element and finite difference methods in capturing the basic large scale flow features of the ice shelf we additionallyshow that the fit between model and observed velocity depends strongly on the ice shelf temperature field for which there is presently little observationalcontrol the main differences between model results are due to the equations being solved the boundary conditions at the ice front and the discretisationmethod finite element vs finite difference

glacier acceleration and thinning after ice shelf collapse in the larsen b embayment antarctica 1 ice velocities derived from five landsat 7 images acquired between january 2000 and february 2003 show a two to six fold increase in centerline speed of four glaciers flowing into the now collapsed section of the larsen b ice shelf satellite laser altimetry from icesat indicates the surface of hektoria glacier lowered by up to 38 6 m in a six month period beginning one year after the break up in march 2002 smaller elevation losses are observed for crane and jorum glaciers over a later 5 month period two glaciers south of the collapse area flask and leppard show little change in speed or elevation seasonal variations in speed preceding the large post collapse velocity increases suggest that both summer melt percolation and changes in the stress field due to shelf removal play a major role in glacier dynamics

modelling the delivery of supraglacial meltwater to the ice bed interface application to southwest devon ice cap nunavut canada the transfer of surface generated meltwater to the subglacial drainage system through full ice thickness crevassing may lead to accelerated glacier velocities with implications for ice motion under future climatic scenarios accurate predictions of where surface meltwater accesses the ice bed interface are therefore needed in fully coupled hydrodynamic ice sheet models we present a spatially distributed modelling routine for predicting the location and timing of delivery of surface derived meltwater to the ice bed interface through moulins and supraglacial lake drainage the model is explained as it is applied to the croker bay glacial catchment of devon ice cap canada the formation of moulins drainage of lakes and the transfer of meltwater through the full ice thickness are modelled for the 2004 and 2006 ablation seasons through this case study we assess the model s sensitivity to degree day factors fracture toughness tensile strength and crevasse width and confirm that parameters influencing the rate at which water fills a crevasse are the most significant controls on the ability of a crevasse to reach the bed increased surface melt production therefore has the potential to significantly influence the spatial and temporal transfer of meltwater through surface to bed connections in a warmer climate

flow structure in canopy models dominated by progressive waves summary laboratory experiments were carried out to study the flow structure both inside and above different canopy models which were dominated by progressive waves a set of experimental conditions were considered in a laboratory flume three vegetation models submerged rigid submerged flexible and emergent rigid three plant densities 128 640 and 1280 stems m 2 and three wave frequencies f 0 8 1 and 1 4 hz the progressive waves followed the second order stokes theory in the intermediate depth range the observations revealed that submerged and emergent rigid vegetation models modified the wave induced velocities i e both mean current and wave velocity the submerged rigid vegetation model for plant densities higher than 640 plants m 2 and wave frequency higher than 1 hz acted akin to a false floor confining the mean current to above the plant bed a penetration depth around 2 cm below the top of submerged rigid vegetation was found for the other runs the vertical profile of mean current did not present changes with respect to runs without plants the emergent rigid vegetation model reversed the direction of the induced mean current with the highest velocity corresponding to the highest plant density in contrast the submerged flexible vegetation model had a weak effect on the mean current with the vertical velocity profile similar to that found in experiments without vegetation the wave velocities inside the vegetation for the densest submerged rigid vegetation were found to be reduced by 20 when compared to the wave velocities without vegetation while in emergent rigid vegetation this reduction was of 45

geochemistry of highly acidic mine water following disposal into a natural lake with carbonate bedrock abstract acid mine drainage amd from the zn pb ag bi cu deposit of cerro de pasco central peru and waste water from a cu extraction plant has been discharged since 1981 into lake yanamate a natural lake with carbonate bedrock the lake has developed a highly acidic ph of 1 mean lake water chemistry was characterized by 16 775 mg l acidity as caco3 4330 mg l fe and 29 250 mg l so4 mean trace element concentrations were 86 8 mg l cu 493 mg l zn 2 9 mg l pb and 48 mg l as which did not differ greatly from the discharged amd most elements showed increasing concentrations from the surface to the lake bottom at a maximal depth of 41 m e g from 3581 to 5433 mg l fe and 25 609 to 35 959 mg l so4 the variations in the h and o isotope compositions and the element concentrations within the upper 10 m of the water column suggest mixing with recently discharged amd shallow groundwater and precipitation waters below 15 m a stagnant zone had developed gypsum saturation index si 0 25 and anglesite si 0 1 were in equilibrium with lake water jarosite was oversaturated si 1 7 in the upper part of the water column resulting in downward settling and re dissolution in the lower part of the water column si 0 7 accordingly jarosite was only found in sediments from less than 7 m water depth at the lake bottom a layer of gel like material 90 wt water of ph 1 with a total organic c content of up to 4 40 wet wt originated from the kerosene discharge of the cu extraction plant and had contaminant element concentrations similar to the lake water below the organic layer followed a layer of gypsum with ph 1 5 which overlaid the dissolving carbonate sediments of ph 5 3 7 in these two layers the contaminant elements were enriched compared to lake water in the sequence as

present stability of the larsen c ice shelf antarctic peninsula we modelled the flow of the larsen c and northernmost larsen d ice shelves antarctic peninsula using a model of continuum mechanics of ice flow and applied a fracture criterion to the simulated velocities to investigate the ice shelf s present day stability constraints come from satellite data and geophysical measurements from the 2008 09 austral summer ice shelf thickness was derived from bedmap and icesat data and the density depth relationship was inferred from our in situ seismic reflection data we obtained excellent agreements between modelled and measured ice flow velocities and inferred and observed distributions of rifts and crevasses residual discrepancies between regions of predicted fracture and observed crevasses are concentrated in zones where we assume a significant amount of marine ice and therefore altered mechanical properties in the ice column this emphasizes the importance of these zones and shows that more data are needed to understand their influence on ice shelf stability modelled flow velocities and the corresponding stress distribution indicate that the larsen c ice shelf is stable at the moment however weakening of the elongated marine ice zones could lead to acceleration of the ice shelf due to decoupling from the slower parts in the northern inlets and south of kenyon peninsula leading to a velocity distribution similar to that in the larsen b ice shelf prior to its disintegration

rapid collapse of northern larsen ice shelf antarctica in january 1995 4200 square kilometers of the northern larsen ice shelf antarctic peninsula broke away radar images from the ers 1 satellite complemented by field observations showed that the two northernmost sections of the ice shelf fractured and disintegrated almost completely within a few days this breakup followed a period of steady retreat that coincided with a regional trend of atmospheric warming the observations imply that after an ice shelf retreats beyond a critical limit it may collapse rapidly as a result of perturbated mass balance

evaluation of ice stream model sensitivities for parameter estimation abstract large ensemble perturbed parameter forward ice flow modeling can provide useful insights to uncertainties in inversions for basal drag or other ice flow parameters inversion and data assimilation provide estimates of poorly known parameters that are essential for accurate prognostic modeling because ice flow depends on many such parameters with their associated uncertainties which may interact in nonlinear ways full uncertainty assessment for parameter estimates is challenging with rising computational power it is increasingly practicable to explore co dependencies and sensitivities here we use a well characterized higher order flowline model configured for a well lubricated shelfy ice stream to run large ensembles perturbing the magnitude and spatial pattern of basal drag basal topography and input flux we find for steady state that ice stream velocity and thickness along the entire domain are especially correlated to drag at the downstream end but with greater local correlation during transients the modeled effects of basal topographic perturbations on velocity and ice thickness are primarily local perturbations of input ice fluxes interact with the others in interesting ways these insights point to the value of inversions informed by concentrated observations during forced transients such as lake drainage events accumulation rate fluctuations or ice shelf losses and to the care needed when interpreting local results of some inversions for basal drag parameters

plant stiffness and biomass as drivers for drag forces under extreme wave loading a flume study on mimics abstract moving water exerts drag forces on vegetation the susceptibility of vegetation to bending and breakage determines its flow resistance and chances of survival under hydrodynamic loading to evaluate the role of individual vegetation parameters in this water vegetation interaction we conducted drag force measurements under a wide range of wave loadings in a large wave flume artificial vegetation elements were used to manipulate stiffness frontal area in still water and material volume as a proxy for biomass the aim was to compare i identical volume but different still frontal area ii identical stiffness but different still frontal area and iii identical still frontal area but different volume comparison of mimic arrangements showed that stiffness and the dynamic frontal area i e frontal area resulting from bending which depends on stiffness and hydrodynamic forcing determine drag forces only at low orbital flow velocities did the still frontal area dominate the force velocity relationship and it is hypothesised that no mimic bending took place under these conditions mimic arrangements with identical stiffness but different overall material volume and still frontal area showed that forces do not increase linearly with increasing material volume and it is proposed that short distances between mimics cause their interaction and result in additional drag forces a model based on effective leaf length and characteristic plant width developed for unidirectional flow performed well for the force time series under both regular and irregular waves however its uncertainty increased with increasing interaction of neighbouring mimics

basal friction of ice stream e west antarctica we use surface velocity derived from sequential landsat imagery and a control method to estimate the basal friction distribution of a major west antarctic ice stream the area averaged basal stress is approximately 1 4 10 4 pa or about 29 of the area averaged driving stress of 4 9 10 4 pa uncertainty of the derived area averaged basal stress is difficult to assess and depends primarily on spatial variation of the flow law rate factor in the constitutive law spatial variation associated with depth averaged temperature variation gives an uncertainty of approximately 10 3 pa approximately 60 of the ice stream has a basal stress magnitude less than 10 4 pa and approximately 30 has less than 10 3 pa these characteristics suggest the presence of a mechanically weak water charged subglacial till small scale sticky spots where basal friction exceeds the area averaged driving stress are scattered irregularly across the subglacial regime and comprise approximately 15 of the ice stream area sticky spots cluster in regions where landsat imagery suggests structural features in the underlying bedrock

roles of marine ice rheology and fracture in the flow and stability of the brunt stancomb wills ice shelf marine ice sometimes as part of an ice melange significantly affects ice shelf flow and ice fracture the highly heterogeneous structure of the brunt stancomb wills ice shelf bsw system in the east weddell sea offers a rare setting for uncovering the difference in rheology between meteoric and marine ice here we use data assimilation to infer the rheology of the brunt stancomb wills ice shelf by an inverse control method that combines interferometric synthetic aperture radar measurements with numerical modeling we then apply the inferred rheology to support the hypothesis attributing the observed 1970s ice shelf flow acceleration to a change in the stiffness of the ice melange area connecting brunt proper with stancomb wills and to examine the consequences of frontal rift propagation we conclude that while the brunt stancomb wills system is currently not susceptible to extreme fragmentation similar to that of the larsen b ice shelf in 2002 our inverse and forward modeling results emphasize its vulnerability to destabilization by relatively rapid changes in the ice melange properties resulting from the interaction of its marine ice component with ocean water or by the further propagation of a frontal rift copyright 2009 by the american geophysical union

accelerated ice discharge from the antarctic peninsula following the collapse of larsen b ice shelf acceleration exceeds 27 km 3 per year and ice is thinning at rates of tens of meters per year we attribute this abrupt evolution of the glaciers to the removal of the buttressing ice shelf the magnitude of the glacier changes illustrates the importance of ice shelves on ice sheet mass balance and contribution to sea level change index terms 1827 hydrology glaciology 1863 1863 hydrology snow and ice 1827 3349 meteorology and atmospheric dynamics polar meteorology 6924 radio science interferometry 9310 information related to geographic region antarctica citation rignot e g casassa p gogineni w krabill a rivera and r thomas 2004 accelerated ice discharge from the antarctic peninsula following the collapse of larsen b ice shelf geophys res lett 31 l18401 doi 10 1029 2004gl020697

causes of pre collapse changes of the larsen b ice shelf numerical modelling and assimilation of satellite observations satellite observations revealed that beside a rapid thinning the larsen b ice shelf lbis was undergoing a significant acceleration before its collapse in 2002 this paper investigates the ice shelf acceleration between 1995 and 1999 using a combination of data assimilation and numerical modelling based on a flow model adjusted to the 1995 insar velocities perturbation experiments are performed such as ice front retreat thinning increase in tributary flow and rheological weakening furthermore an inversion for ice shelf rheology and tributary flow velocity is performed for both the 1995 and the 1999 insar velocities the perturbation experiments together with the inversion strongly suggest that the acceleration cannot solely be explained by the retreat of the ice shelf front but relies on a further significant rheological weakening of the already weak shear zones within the lbis minor tributary acceleration is found to be an effect rather than a cause of the ice shelf acceleration furthermore the observed acceleration cannot be explained by the observed recent thinning we conclude that for smaller ice shelves such as the lbis such weak shear margins play a crucial role in controlling their dynamics and are the key to understand changes in the future finally we compare the dynamic thinning likely to be associated with the observed acceleration with the observed thinning for the ice shelf as a whole this thinning accounts for 20 of the observed value which implies that factors such as enhanced basal melt were the primary cause of the observed thinning

recent dramatic thinning of largest west antarctic ice stream triggered by oceans a growing body of observational data suggests that pine island glacier pig is changing on decadal or shorter timescales these changes may have far reaching consequences for the future of the west antarctic ice sheet wais and global sea levels because of pig s role as the ice sheet s primary drainage portal we test the hypothesis that these changes are triggered by the adjoining ocean specifically we employ an advanced numerical ice flow model to simulate the effects of perturbations at the grounding line on pig s dynamics the speed at which these changes are propagated upstream implies a tight coupling between ice sheet interior and surrounding ocean

surface structure and stability of the larsen c ice shelf antarctic peninsula a structural glaciological description and analysis of surface morphological features of the larsen c ice shelf antarctic peninsula is derived from satellite images spanning the period 1963 2007 the data are evaluated in two time ranges a comparison of a 1963 satellite image photomosaic with a modern digital mosaic compiled using 2003 04 austral summer data and an image series since 2003 showing recent evolution of the shelf we map the ice shelf edge rift swarms crevasses and crevasse traces and linear longitudinal structures called flow stripes or streak lines the latter are observed to be continuous over distances of up to 200 km from the grounding line to the ice shelf edge with little evidence of changes in pattern over that distance integrated velocity measurements along a flowline indicate that the shelf has been stable for 560 years in the mid shelf area linear longitudinal features may be grouped into 12 units each related to one or a small group of outlet feeder glaciers to the shelf we observe that the boundaries between these flow units often mark rift terminations the boundary zones originate upstream at capes islands or other suture areas between outlet glaciers in agreement with previous work our findings imply that rift terminations within such suture zones indicate that they contain anomalously soft ice we thus suggest that suture zones within the larsen c ice shelf and perhaps within ice shelves more generally may act to stabilize them by reducing regional stress intensities and thus rates of rift lengthening

development of an antarctic digital elevation model by integrating cartographic and remotely sensed data a geographic information system based approach we present a high resolution digital elevation model dem of the antarctic it was created in a geographic information system gis environment by integrating the best available topographic data from a variety of sources extensive gis based error detection and correction operations ensured that our dem is free of gross errors the carefully designed interpolation algorithms for different types of source data and incorporation of surface morphologic information preserved and enhanced the fine surface structures present in the source data the effective control of adverse edge effects and the use of the hermite blending weight function in data merging minimized the discontinuities between different types of data leading to a seamless and topographically consistent dem throughout the antarctic this new dem provides exceptional topographical details and represents a substantial improvement in horizontal resolution and vertical accuracy over the earlier continental scale renditions particularly in mountainous and coastal regions it has a horizontal resolution of 200 m over the rugged mountains 400 m in the coastal regions and approximately 5 km in the interior the vertical accuracy of the dem is estimated at about 100 130 m over the rugged mountainous area better than 2 m for the ice shelves better than 15 m for the interior ice sheet and about 35 m for the steeper ice sheet perimeter the antarctic dem can be obtained from the authors

glacier surge after ice shelf collapse the possibility that the west antarctic ice sheet will collapse as a consequence of ice shelf disintegration has been debated for many years this matter is of concern because such an event would imply a sudden increase in sea level evidence is presented here showing drastic dynamic perturbations on former tributary glaciers that fed sections of the larsen ice shelf on the antarctic peninsula before its collapse in 1995 satellite images and airborne surveys allowed unambiguous identification of active surging phases of boydell sjogren edgeworth bombardier and drygalski glaciers this discovery calls for a reconsideration of former hypotheses about the stabilizing role of ice shelves

wave attenuation by flexible idealized salt marsh vegetation abstract wave attenuation by vegetation is a highly dynamic process and its quantification is important for understanding shore protection potential and modeling coastal hydrodynamics data documenting the interactions of spartina alterniflora represented by polyolefin tubing and single and double peaked irregular waves were collected in a large scale laboratory flume the laboratory provided a controlled environment to evaluate wave attenuation including the parameters of stem density submergence wave height and peak period wave attenuation appeared to be most dependent on stem density and the ratio of stem length to water depth wave attention increased slightly with wave height while no clear trend with respect to wave period was seen treating double peaked spectra as superimposed wave systems revealed a preferential dissipation of the higher frequency wave system relative to the lower frequency wave system under emergent conditions wave energy loss occurred at all frequencies of both spectral types with dissipation increasing with frequency above the spectral peak parameterizing the spectral equilibrium range as a function of frequency showed a steepening of the spectral tail compared to the 4 power law under emergent conditions an empirical relationship defining the bulk drag coefficient for s alterniflora as a function of the stem reynolds number is found to serve as a first estimate for engineering applications

recent atmospheric warming and retreat of ice shelves on the antarctic peninsula in 1978 mercer1 discussed the probable effects of climate warming on the antarctic ice sheet predicting that one sign of a warming trend in this region would be the retreat of ice shelves on the antarctic peninsula analyses of 50 year meteorological records have since revealed atmospheric warming on the antarctic peninsula2 3 and a number of ice shelves have retreated4 8 here we present time series of observations of the areal extent of nine ice shelves on the antarctic peninsula showing that five northerly ones have retreated dramatically in the past fifty years while those further south show no clear trend comparison with airtemperature data shows that the pattern and magnitude of ice shelf retreat is consistent with the existence of an abrupt thermal limit on iceshelf viability the isotherm associated with this limit having been driven south by the atmospheric warming ice shelves therefore appear to be sensitive indicators of climate change

inland thinning of the amundsen sea sector west antarctica 1 together with the pine island glacier pig the thwaites tg and smith sg glaciers are the principal drainage systems of the amundsen sea as sector of western antarctica here we use satellite radar altimetry and interferometry to show that a rapid thinning of ice has occurred within the fastest flowing sections of all as outlet glaciers the pattern of thinning extends to distances greater than 150 km inland between 1991 and 2001 the tg and sg thinned by more than 25 and 45 m at their grounding lines and a total of 154 16 km 3 of ice or 0 43 mm of eustatic sea level rise was lost from the as sector glaciers to the ocean we show that the thickness changes may have caused the pig tg and sg to retreat inland by over 8 4 and 7 km respectively in line with independent estimates of grounding line migration

isotropic and anisotropic flow relations for ice dynamics a flow relation for the minimum strain rates for isotropic ice is presented this flow relation has been compiled from laboratory tests in shear and compression and covers the temperature and stress ranges typical of natural ice masses however the ice in natural ice masses is mostly anistropic three cores drilled at law dome antarctica are used as examples to illustrate some of the different anisotropies that develop in ice masses in relation to the stress configurations and accumulated strains for the shear or compression stress configurations anistropic flow rates can be derived as enhancements of the minimum isotropic flow rates for the particular stress temperature and stress configuration for compression this enhancement asymptotically approaches 3 for shear the enhancement may be as high as 8 for combinations of shear and compression further tests are required

nature based flood protection the efficiency of vegetated foreshores for reducing wave loads on coastal dikes this paper analyses the effect of vegetation on wave damping under severe storm conditions based on a combination of field measurements and numerical modelling the field measurements of wave attenuation by vegetation were performed on two salt marshes with two representative but contrasting coastal wetland vegetation types cordgrass spartina anglica and grassweed scirpus maritimus the former is found in salty environments whereas the latter is found in brackish environments the measurements have added to the range with the highest water depths and wave heights presented in the literature so far a numerical wave model swan has been calibrated and validated using the new field data it appeared that the model was well capable of reproducing the observed decay in wave height over the salt marsh the model has been applied to compute the reduction of the incident wave height on a dike for various realistic foreshore configurations and hydraulic loading conditions additionally the efficiency of vegetated foreshores in reducing wave loads on the dike has been investigated where wave loads were quantified using a computed wave run up height and wave overtopping discharge the outcomes show that vegetated foreshores reduce wave loads on coastal dikes significantly also for the large inundation depths that occur during storms and with the vegetation being in winter state the effect of the foreshore on the wave loads varies with wave height to water depth ratio on the foreshore the presence of vegetation on the foreshore extends the range of water depths for which a foreshore can be applied for effective reduction of wave loads and prevents intense wave breaking on the foreshore to occur this research demonstrates that vegetated foreshores can be considered as a promising supplement to conventional engineering methods for dike reinforcement

hydrodynamics induced by wind waves in a vegetation field the wave induced kinematics and dynamics of a submerged or emerged vegetation field is analyzed using potential flow and an eigenfunction expansion the problem is solved considering regular as well as irregular incident waves the model takes into account the vegetation motion and solves for the complete wave system on the vegetation field and in its vicinity the model is validated against experimental laboratory data obtained by other authors showing a much better agreement than previous theoretical models in this paper the model is used to evaluate wave height evolution damping vegetation and fluid motion and forces and moments on the vegetation furthermore the inclusion of irregular waves provides force and moment distributions on the vegetation field depending on the wave climate statistics

basal conditions for pine island and thwaites glaciers west antarctica determined using satellite and airborne data we use models constrained by remotely sensed data from pine island and thwaites glaciers west antarctica to infer basal properties that are difficult to observe directly the results indicate strong basal melting in areas upstream of the grounding lines of both glaciers where the ice flow is fast and the basal shear stress is large farther inland we find that both glaciers have mixed bed conditions with extensive areas of both bedrock and weak till in particular there are weak areas along much of pine island glacier s main trunk that could prove unstable if it retreats past the band of strong bed just above its current grounding line in agreement with earlier studies our forward ice stream model shows a strong sensitivity to small perturbations in the grounding line position these results also reveal a large sensitivity to the assumed bed sliding or deforming model with non linear sliding laws producing substantially greater dynamic response than earlier simulations that assume a linear viscous till rheology finally comparison indicates that our results using a plastic bed are compatible with the limited observational constraints and theoretical work that suggests an upper bound exists on maximum basal shear stress

measurement of temperature in a margin of ice stream b antarctica implications for margin migration and lateral drag ice temperature was measured in and around the chaotically crevassed south margin of ice stream b antarctica from 1992 to 1994 the temperatures at 30 m depth in the chaotic zone are about 12 k lower than in the adjacent uncrevassed ice due to the ponding of cold winter air at depths greater than 150 m there is clear evidence of internal heating of the ice due to the large shear deformation rate in the marginal zone analysis of the depth of cooling below the crevasses and of the internal heating gives two pieces of information first over the last half century the lateral shear stress averaged 2 0 x 10 5 pa in the top third of the margin and second the margin moved outward at an average rate of 7 3 m a these values do not involve any assumptions about the flow law of ice the uncertainties are roughly 20 the value of lateral shear stress indicates that the most of the drag on the ice stream is along its sides

larsen b ice shelf rheology preceding its disintegration inferred by a control method a new complete velocity field from satellite remote sensing is combined with numerical modeling to infer the rheology of the larsen b ice shelf before its disintegration the resulting spatial distribution of the flow parameter exhibits large variability which reflects very well observed ice shelf features this variability is explained by factors including advection of colder ice from tributary glaciers bottom melting and the presence of zones of strong shear and fracture the inferred distribution is applied to simulate numerically the flow regime ofthe ice shelf and to examine its modification by the presence of open rifts and by the retreat of ice shelf front between 1996 and 2000 results demonstrate that variable rheology is essential to understanding ice shelf evolution especially the close relationship among frontal retreat fracture ice flow acceleration and the destabilization of ice shelves copyright 2007 by the american geophysical union

modelling of rift propagation on ronne ice shelf antarctica and sensitivity to climate change 1 the calving of icebergs from large antarctic ice shelves is controlled mainly by the formation and propagation of rifts originating from the side margins of the ice shelf and local areas of grounding using insar we observe the evolution of rifts along hemmen ice rise on ronne ice shelf antarctica prior to the large calving event of october 1998 we couple these observations with a computer model combining the viscous flow of an ice shelf with a linear elastic fracture mechanics description of the propagation of rifts the model reveals that the ice melange trapped in between the rifts exerts a major control on the propagation of rifts and in turn on ice shelf stability melting of the ice melange from oceanic or atmospheric warming would significantly increase the propagation rate of rifts and threaten the ice shelf stability

fracture field for large scale ice dynamics recent observations and modeling studies emphasize the crucial role of fracture mechanics for the stability of ice shelves and thereby the evolution of ice sheets here we introduce a macroscopic fracture density field into a prognostic continuum ice flow model and compute its evolution incorporating the initiation and growth of fractures as well as their advection with two dimensional ice flow to a first approximation fracture growth is assumed to depend on the spreading rate only while fracture initiation is defined in terms of principal stresses the inferred fracture density fields compare well with observed elongate surface structures since crevasses and other deep reaching fracture structures have been shown to influence the overall ice shelf dynamics we propose the fracture density field introduced here be used as a measure for ice softening and decoupling of the ice flow in fracture weakened zones this may yield more accurate and realistic velocity patterns in prognostic simulations additionally the memory of past fracture events links the calving front to the upstream dynamics thus the fracture density field proposed here may be employed in fracture based calving parameterizations the aim of this study is to introduce the field and investigate which of the observed surface structures can be reproduced by the simplest physically motivated fracture source terms

assessment of ice flow dynamics in the zone close to the calving front of antarctic ice shelves this work was supported by a british antarctic survey studentship funded by the natural environment research council grant no ne k50094x 1

observation of ocean tides below the filchner and ronne ice shelves antarctica using synthetic aperture radar interferometry comparison with tide model predictions tides near and under floating glacial ice such as ice shelves and glacier termini in fjords can influence heat transport into the subice cavity mixing of the under ice water column and the calving and subsequent drift of icebergs free surface displacement patterns associated with ocean variability below glacial ice can be observed by differencing two synthetic aperture radar sar interferograms each of which represents the combination of the displacement patterns associated with the time varying vertical motion and the time independent lateral ice flow we present the pattern of net free surface displacement for the iceberg calving regions of the ronne and filchner ice shelves in the southern weddell sea by comparing sar based displacement fields with ocean tidal models the free surface displacement variability for these regions is found to be dominated by ocean tides the inverse barometer effect i e the ocean s isostatic response to changing atmospheric pressure also contributes to the observed vertical displacement the principal value of using sar interferometry in this manner lies in the very high lateral resolution tens of meters obtained over the large region covered by each sar image small features that are not well resolved by the typical grid spacing of ocean tidal models may contribute to such processes as iceberg calving and cross frontal ventilation of the ocean cavity under the ice shelf

acceleration of pine island and thwaites glaciers west antarctica recent satellite investigations revealed that in the 1990s the grounding line of pine island and thwaites glaciers west antarctica retreated several km the ice surface on the interior of the basins lowered 10 cm a 1 and pine island glacier thinned 1 6 m a 1 these observations however were not sufficient to determine the cause of the changes here we present satellite radar interferometry data that show the thinning and retreat of pine island glacier are caused by an acceleration of ice flow of about 18 2 in 8 years thwaites glacier maintained a nearly constant flow regime at its center but widened along the sides and increased its 30 15 mass deficit by another 4 in 4 years the combined mass loss from both glaciers if correct contributes an estimated 0 08 0 03 mm a 1 global sea level rise in 2000

application of control methods for modelling the flow of pine island glacier west antarctica the distribution of basal traction on a transect along pine island glacier west antarctica is estimated by inverting observed surface velocities using a control method and a simple numerical stream flow model this model calculates the horizontal flow along a transect based on the assumptions that the horizontal flow is independent of ice depth and that the driving stresses are balanced by resistive forces at the glacier bed and margin and by gradients in longitudinal stress basal traction is assumed to be linearly related to the basal velocity for the lateral shear traction a parameterization based on an inversion of glen s flow law is used the applicationof the control method allowsus to calculate the set of model parameters e g the basal friction coefficient that gives the best fit betweenmodelled andobserved surface velocities themodel is used to investigate the stress regime of pine island glacier in particular to estimate the importance of basal lateral and longitudinal stresses relative to each other in the flat region just behind the grounding line basal drag lateral drag and the longitudinal stress gradient are the same order of magnitude in the steep region up glacier from the grounding line the driving stresses are highest and balanced predominantly by basal resistive stresses further upstream in the trunk of the glacier lateral and basal drag predominate

spontaneous ice front retreat caused by disintegration of adjacent ice shelf in antarctica abstract antarctic ice discharge constitutes the largest uncertainty in future sea level projections floating ice shelves fringing most of antarctica exert retentive forces onto the ice flow while abrupt ice shelf retreat has been observed it is generally considered a localized phenomenon here we show that the disintegration of an ice shelf may induce the spontaneous retreat of its neighbor as an example we reproduce the spontaneous but gradual retreat of the larsen b ice front as observed after the disintegration of the adjacent larsen a ice shelf we show that the larsen a collapse yields a change in spreading rate in larsen b via their connecting ice channels and thereby causes a retreat of the ice front to its observed position of the year 2000 prior to its collapse this mechanism might be particularly relevant for the role of east antarctica and the antarctic peninsula in future sea level

the link between climate warming and break up of ice shelves in the antarctic peninsula a review of in situ and remote sensing data covering the ice shelves of the antarctic peninsula provides a series of characteristics closely associated with rapid shelf retreat deeply embayed ice fronts calving of myriad small elongate bergs in punctuated events increasing flow speed and the presence of melt ponds on the ice shelf surface in the vicinity of the break ups as climate has warmed in the antarctic peninsula region melt season duration and the extent of ponding have increased most break up events have occurred during longer melt seasons suggesting that meltwater itself not just warming is responsible regions that show melting without pond formation are relatively unchanged melt ponds thus appear to be a robust harbinger of ice shelf retreat we use these observations to guide a model of ice shelf flow and the effects of meltwater crevasses present in a region of surface ponding will likely fill to the brim with water we hypothesize building on weertman 1973 hughes 1983 and van der veen 1998 that crevasse propagation by meltwater is the main mechanism by which ice shelves weaken and retreat a thermodynamic finite element model is used to evaluate ice flow and the strain field and simple extensions of this model are used to investigate crack propagation by meltwater the model results support the hypothesis