rheology of the ronne ice shelf antarctica inferred from satellite radar interferometry data using an inverse control method 1 the antarctic ice sheet is surrounded by large floating ice shelves that spread under their own weight into the ocean ice shelf rigidity depends on ice temperature and fabrics and is influenced by ice flow and the delicate balance between bottom and surface accumulation here we use an inverse control method to infer the rigidity of the ronne ice shelf that best matches observations of ice velocity from satellite radar interferometry ice rigidity or flow law parameter b is shown to vary between 300 and 900 kpa a 1 3 ice is softer along the side margins due to frictional heating and harder along the outflow of large glaciers which advect cold continental ice melting at the bottom surface of the ice shelf increases its rigidity while freezing decreases it accurate numerical modelling of ice shelf flow must account for this spatial variability in mechanical characteristics citation larour e e rignot i joughin and d aubry 2005 rheology of the ronne ice shelf antarctica inferred from satellite radar interferometry data using an inverse control method geophys res lett 32 l05503 doi 10 1029 2004gl021693

a model study of ocean circulation beneath filchner ronne ice shelf antarctica implications for bottom water formation 1 an isopycnic coordinate ocean circulation model has been applied to the southern weddell sea including the cavity beneath filchner ronne ice shelf with the aim of investigating the buoyancy forced circulation on the continental shelf buoyancy forcing is associated with both the annual growth and decay of sea ice and the interaction between ice shelf and ocean in the model a generally anti cyclonic circulation develops beneath the ice shelf so that new shelf waters entering the cavity in the west emerge colder and fresher in the east the outflow contributes to a dense current that spills off the continental shelf and descends the slope oceanographic observations from the region are consistent with this picture and highlight the overflow as a major source of weddell sea bottom water

the response of ice shelf basal melting to variations in ocean temperature abstract a three dimensional ocean general circulation model is used to study the response of idealized ice shelves to a series of ocean warming scenarios the model predicts that the total ice shelf basal melt increases quadratically as the ocean offshore of the ice front warms this occurs because the melt rate is proportional to the product of ocean flow speed and temperature in the mixed layer directly beneath the ice shelf both of which are found to increase linearly with ocean warming the behavior of this complex primitive equation model can be described surprisingly well with recourse to an idealized reduced system of equations and it is shown that this system supports a melt rate response to warming that is generally quadratic in nature this study confirms and unifies several previous examinations of the relation between melt rate and ocean temperature but disagrees with other results particularly the claim that a single melt rate sensitivity to warming is universally valid the hypothesized

amundsen sea ice production and transport 1 drift and variability of sea ice in the amundsen sea are investigated with ice buoys deployed in march 2000 and a coupled ice ocean model the bremerhaven regional ice ocean simulations brios model results are compared with in situ ocean atmosphere and sea ice measurements satellite observations and 8 19 months of buoy drift data we identify a zone of coastal westward drift and a band of faster eastward drift separated by a broad transition region characterized by variable ice motions the model represents drift events at scales approaching its resolution but is limited at smaller scales and by deficiencies in the national centers for environmental prediction forcing two thirds of the modeled ice production in the southern amundsen moves westward near the coast its transport modulated by meridional wind strength damping sea ice formation in the eastern ross sea half of the ice exported from the ross moves eastward into the northern amundsen sea a net sea ice sink that also receives more than one third of the ice generated to its south a low rate of exchange occurs with the bellingshausen sea which must have a more independent ice regime snow ice formation resulting from high precipitation accounts for one quarter of the ice volume in the amundsen sea aiding the formation of thick ice in a region with generally divergent ice drift freshwater extraction by sea ice formation is roughly balanced by precipitation and ice shelf melting but a positive trend in the surface flux is consistent with an amundsen source for reported freshening in the ross sea

warm ocean is eroding west antarctic ice sheet 1 satellite radar measurements show that ice shelves in pine island bay have thinned by up to 5 5 m yr 1 over the past decade the pattern of shelf thinning mirrors that of their grounded tributaries the pine island thwaites and smith glaciers and ocean currents on average 0 5 c warmer than freezing appear to be the source the synchronised imbalance of the inland glaciers is the result of reduced lateral and basal tractions at their termini and the drawdown of grounded ice shows that antarctica is more sensitive to changing climates than was previously considered

recent sea level contributions of the antarctic and greenland ice sheets after a century of polar exploration the past decade of satellite measurements has painted an altogether new picture of how earth s ice sheets are changing as global temperatures have risen so have rates of snowfall ice melting and glacier flow although the balance between these opposing processes has varied considerably on a regional scale data show that antarctica and greenland are each losing mass overall our best estimate of their combined imbalance is about 125 gigatons per year of ice enough to raise sea level by 0 35 millimeters per year this is only a modest contribution to the present rate of sea level rise of 3 0 millimeters per year however much of the loss from antarctica and greenland is the result of the flow of ice to the ocean from ice streams and glaciers which has accelerated over the past decade in both continents there are suspected triggers for the accelerated ice discharge surface and ocean warming respectively and over the course of the 21st century these processes could rapidly counteract the snowfall gains predicted by present coupled climate models

sensitivity of 21st century sea level to ocean induced thinning of pine island glacier antarctica 1 pine island glacier pig antarctica is rapidly losing mass supporting arguments that it may play a major role in 21st century sea level rise yet this glacier s quantitative contribution to sea level based on theoretical and computational models is poorly known we have developed a basin scale glaciological model to examine the sensitivity of pig to a range of environmental forcings while oceanic melt likely played the leading role in recent thinning and retreat we find that the particular grounding line geometry with an extended ice plain in the 1990s made it susceptible to such forcing our model further indicates that while the rate of grounding line retreat should diminish soon the glacier s mass loss may continue at rates similar to or moderately elevated from the present while substantial our model derived maximum rate of 2 7 cm century is considerably smaller than previous heuristically derived bounds on the sea level contribution

an upper cretaceous sag pond deposit implications for recognition of local seismicity and surface rupture along the kaibab monocline utah in the bull flat area of grand staircase escalante national monument utah united states sedimentological analysis of the tip line area of a late cretaceous normal fault reveals that the fault was active during the deposition and preservation of a sag pond deposit sag ponds deposits are probably under recognized in the rock record and as demonstrated in this study they are helpful in chronicling activity of ancient faults the sag pond sequence is located just below the contact between the upper and capping sandstone members of the wahweap formation and it occurs within close proximity to other features of seismogenic origin the sag pond deposit is a 2 2 m thick succession of carbonaceous interlaminated mudstones and siltstones intruded by sublithic sandstone seismogenic dikes and sills at least two seismic events are required to produce the observed sequence the initial event ruptured the surface probably tilting the underlying strata and produced the down drop for the sag pond after infilling of the sag pond a second seismogenic event mobilized sediment in the underlying upper member and generated the dikes and sills that intrude the sag pond deposit subsequently a minor channel scoured the sag pond deposit with flow parallel to the fault trace infilling of the sag pond ended before deposition of the capping sandstone member

ice ocean processes over the continental shelf of the southern weddell sea antarctica a review 1 interactions between the southern ocean and the weddell sea ice shelves are important both to the antarctic ice sheet and to the production of globally significant water masses here we review the interaction between the filchner ronne ice shelf and the shelf sea in which it floats the continental shelf processes leading to the production of weddell sea deep and bottom waters from the original off shelf source waters are discussed and a new view is offered of the initial production of high salinity shelf water data from ship based measurements at the ice front from glaciological methods and from measurements made within the sub ice shelf cavity itself are used to describe the pattern of flows beneath the ice shelf we also consider the variability observed within the cavity from tidal to interannual time scales and finish with a discussion of future research priorities in the region

numerical modeling of ocean ice interactions under pine island bay s ice shelf 1 a two dimensional numerical model is used to simulate the dynamics of buoyant meltwater rich plumes flowing beneath the ice shelf occupying much of pine island bay west antarctica recent studies have shown that this ice shelf along with all others fringing the amundsen sea is thinning rapidly in the model both the coriolis effect and subshelf topography are important in controlling plume dynamics and the spatial distribution of ice melt melt is concentrated in a narrow zone within 20 km of the grounding line where steep subshelf slopes and access to warm ambient water allow melt rates to exceed 100 m yr 1 the plume generated by entrainment of ambient water into the meltwater in these areas is guided by the topography of the ice shelf underside and exits the ice shelf at three distinct outflow locations melt rates generated along the course of the plume are higher approximately 2 5 than rates elsewhere the model suggests that the observed ice shelf thinning rates could have resulted from a hypothetical instantaneous 0 25 c warming of the ambient water entrained by the plume a context for this value is provided by the 40 year warming trend documented by jacobs et al 2002 for circumpolar deep water in the nearby ross sea

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

investigation of land ice ocean interaction with a fully coupled ice ocean model 1 model description and behavior 1 antarctic ice shelves interact closely with the ocean cavities beneath them with ice shelf geometry influencing ocean cavity circulation and heat from the ocean driving changes in the ice shelves as well as the grounded ice streams that feed them we present a new coupled model of an ice stream ice shelf ocean system that is used to study this interaction the model is capable of representing a moving grounding line and dynamically responding ocean circulation within the ice shelf cavity idealized experiments designed to investigate the response of the coupled system to instantaneous increases in ocean temperature show ice ocean system responses on multiple timescales melt rates and ice shelf basal slopes near the grounding line adjust in 1 2 years and downstream advection of the resulting ice shelf thinning takes place on decadal timescales retreat of the grounding line and adjustment of grounded ice takes place on a much longer timescale and the system takes several centuries to reach a new steady state during this slow retreat and in the absence of either an upward or downward sloping bed or long term trends in ocean heat content the ice shelf and melt rates maintain a characteristic pattern relative to the grounding line

ice sheet grounding line dynamics steady states stability and hysteresis 1 the ice sheet ice shelf transition zone plays an important role in controlling marine ice sheet dynamics as it determines the rate at which ice flows out of the grounded part of the ice sheet together with accumulation this outflow is the main control on the mass balance of the grounded sheet in this paper we verify the results of a boundary layer theory for ice flux in the transition zone against numerical solutions that are able to resolve the transition zone very close agreement is obtained and grid refinement in the transition zone is identified as a critical component in obtaining reliable numerical results the boundary layer theory confirms that ice flux through the grounding line in a two dimensional sheet shelf system increases sharply with ice thickness at the grounding line this result is then applied to the large scale dynamics of a marine ice sheet our principal results are that 1 marine ice sheets do not exhibit neutral equilibrium but have well defined discrete equilibrium profiles 2 steady grounding lines cannot be stable on reverse bed slopes and 3 marine ice sheets with overdeepened beds can undergo hysteresis under variations in sea level accumulation rate basal slipperiness and ice viscosity this hysteretic behavior can in principle explain the retreat of the west antarctic ice sheet following the last glacial maximum and may play a role in the dynamics of heinrich events

how ice shelf morphology controls basal melting 1 the response of ice shelf basal melting to climate is a function of ocean temperature circulation and mixing in the open ocean and the coupling of this external forcing to the sub ice shelf circulation because slope strongly influences the properties of buoyancy driven flow near the ice shelf base ice shelf morphology plays a critical role in linking external subsurface heat sources to the ice in this paper the slope driven dynamic control of local and area integrated melting rates is examined under a wide range of ocean temperatures and ice shelf shapes with an emphasis on smaller steeper ice shelves a 3 d numerical ocean model is used to simulate the circulation underneath five idealized ice shelves forced with subsurface ocean temperatures ranging from 2 0 c to 1 5 c in the sub ice shelf mixed layer three spatially distinct dynamic regimes are present entrainment of heat occurs predominately under deeper sections of the ice shelf local and area integrated melting rates are most sensitive to changes in slope in this initiation region some entrained heat is advected upslope and used to melt ice in the maintenance region however flow convergence in the outflow region limits heat loss in flatter portions of the ice shelf heat flux to the ice exhibits 1 a spatially nonuniform superlinear dependence on slope and 2 a shape and temperature dependent internally controlled efficiency because the efficiency of heat flux through the mixed layer decreases with increasing ocean temperature numerical simulations diverge from a simple quadratic scaling law

on the coupled response to ice shelf basal melting ice shelf basal melting is tightly coupled to ice shelf morphology ice shelves in turn are coupled to grounded ice via their influence on compressive stress at the grounding line ice shelf buttressing here we examine this interaction using a local parameterization that relates the basal melt rate to the ice shelf thickness gradient this formulation permits a closed form solution for a steady state ice tongue time dependent numerical simulations reveal the spatial and temporal evolution of ice shelf ice stream systems in response to changes in ocean temperature and the influence of morphology dependent melting on grounding line retreat we find that a rapid 1 year re equilibration in upstream regions of ice shelves establishes a spatial pattern of basal melt rates relative to the grounding line that persists over centuries coupling melting to ice shelf shape generally but not always increases grounding line retreat rates relative to a uniform distribution with the same area average melt rate because upstream ice shelf thickness gradients and retreat rates increase nonlinearly with thermal forcing morphology dependent melting is more important to the response of weakly buttressed strongly forced ice streams grounded on beds that slope upwards towards the ocean e g those in the amundsen sea

thermohaline circulation and interaction between ice shelf cavities and the adjacent open ocean the circulation system in an ice shelf cavity is driven by buoyancy fluxes due to melting and freezing of ice and horizontal pressure gradients at the interface between the cavity and the open ocean hence the inflow and outflow pattern and the hydrography in the open ocean influence the general hydrographic condition in the cavity which at least provides the potential for melting and freezing processes applying a three dimensional ocean general circulation model to an idealized ice shelf cavity geometry coupled with an open ocean at a topographic ice shelf barrier we found an important parameter controlling the interaction between these two systems idealized studies for different ice shelf and sea bottom topographies and forcing mechanisms for the open ocean show that the ice shelf edge represents a natural barrier for barotropic interaction because of the sudden decrease in water column thickness since the water column thickness and the coriolis force determine the characteristics for geostrophic flow separated circulation systems arise for the open ocean and the ice shelf cavity only in areas where constant water column thickness and from the oceanographic point of view constant f h contours can be observed across the barrier an increased barotropic current can surmount the ice edge and ventilate the water mass beneath the ice shelf this is only the case at lateral sloping sidewalls or at deep depressions which can be found for example in the southern weddell sea in all other cases the circulation in the ice shelf cavity is closed and almost unaffected by the hydrography outside the barrier

greenland ice sheet high elevation balance and peripheral thinning aircraft laser altimeter surveys over northern greenland in 1994 and 1999 have been coupled with previously reported data from southern greenland to analyze the recent mass balance of the greenland ice sheet above 2000 meters elevation the ice sheet is in balance on average but has some regions of local thickening or thinning thinning predominates at lower elevations with rates exceeding 1 meter per year close to the coast interpolation of our results between flight lines indicates a net loss of about 51 cubic kilometers of ice per year from the entire ice sheet sufficient to raise sea level by 0 13 millimeter per year approximately 7 of the observed rise

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

observations beneath pine island glacier in west antarctica and implications for its retreat thinning ice in west antarctica is currently contributing about 10 of the observed rise in global sea level observations obtained from an autonomous underwater vehicle operating beneath pine island glacier west antarctica reveal that the glacier was recently grounded on a transverse ridge in the sea floor but now warm sea water flows through the widening gap above the ridge thinning ice in west antarctica resulting from acceleration in the flow of outlet glaciers is at present contributing about 10 of the observed rise in global sea level1 pine island glacier in particular has shown nearly continuous acceleration2 3 and thinning4 5 throughout the short observational record the floating ice shelf that forms where the glacier reaches the coast has been thinning rapidly6 driven by changes in ocean heat transport beneath it as a result the line that separates grounded and floating ice has retreated inland7 these events have been postulated as the cause for the inland thinning and acceleration8 9 here we report evidence gathered by an autonomous underwater vehicle operating beneath the ice shelf that pine island glacier was recently grounded on a transverse ridge in the sea floor warm sea water now flows through a widening gap above the submarine ridge rapidly melting the thick ice of the newly formed upstream half of the ice shelf the present evolution of pine island glacier is thus part of a longer term trend that has moved the downstream limit of grounded ice inland by 30 km into water that is 300 m deeper than over the ridge crest the pace and ultimate extent of such potentially unstable retreat10 are central to the debate over the possibility of widespread ice sheet collapse triggered by climate change11 12

large fluctuations in speed on greenland s jakobshavn isbrae glacier it is important to understand recent changes in the velocity of greenland glaciers because the mass balance of the greenland ice sheet is partly determined by the flow rates of these outlets jakobshavn isbrae is greenland s largest outlet glacier draining about 6 5 per cent of the ice sheet area and it has been surveyed repeatedly since 1991 ref 2 here we use remote sensing data to measure the velocity of jakobshavn isbrae between 1992 and 2003 we detect large variability of the velocity over time including a slowing down from 6 700 m yr 1 in 1985 to 5 700 m yr 1 in 1992 and a subsequent speeding up to 9 400 m yr 1 by 2000 and 12 600 m yr 1 in 2003 these changes are consistent with earlier evidence for thickening of the glacier in the early 1990s and rapid thinning thereafter our observations indicate that fast flowing glaciers can significantly alter ice discharge at sub decadal timescales with at least a potential to respond rapidly to a changing climate

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

implications of initial conditions and ice ocean coupling for grounding line evolution abstract ice sheet grounding lines are sensitive to initial conditions and to small perturbations in boundary conditions based on new model results coupling ocean and ice flow to study ice ocean dynamics near ice stream grounding lines we couple an ocean plume model that simulates ice shelf basal melting with a two dimensional isothermal model of ice stream and ice shelf flow the notable results of the coupled model experiments are to reveal grounding line migration sensitivities to i specific aspects of modeling derived and history dependent initial conditions ii to the overall melt magnitude and iii to a positive feedback between focused melting and local slopes of basal ice that is eventually stabilized by buttressing for lengthening ice shelves these interactions can lead to multiple steady states for ice flow over a bed that shallows in the along flow direction and have an important bearing on the effects of bedrock bumps when in the vicinity of bedrock highs grounding lines tend to rapidly advance or retreat towards the basal asperity a significant delay or cessation of de glaciation occurs once the grounding line reaches the leeward side of the bedrock crest however while bedrock bumps can offer stability in the grounding zone minor changes in ocean conditions can easily offset their effect through basal melting feedbacks

glaciological and oceanographic evidence of high melt rates beneath pine island glacier west antarctica satellite imagery indicates that the floating terminus of pine island glacier has changed little in extent over the past two decades data on the velocity and thickness of the glacier reveal that calving of 28 4 gt a 1 accounts for only half of the ice input near the grounding line the apparently steady configuration implies that the remainder of the input is lost by basal melting at a mean rate of 12 3 m a 1 ocean circulation in pine island bay transports 1 c waters beneath the glacier and temperatures recorded in melt laden outflows show that heat loss from the ocean is consistent with the requirements of the calculated melt rate the combination of iceberg calving and basal melting lies at the lower end of estimates for the total accumulation over the catchment basin drawing into question previous estimates of a significantly positive mass budget for this part of the ice sheet

processes involved in the propagation of rifts near hemmen ice rise ronne ice shelf antarctica interferometric radar images collected by ers 1 ers 2 and radarsat 1 are used to observe the rupture tip of rifts that propagate along hemmen ice rise on the ronne ice shelf antarctica interferograms generated in 1992 and 1997 allow for the observation of ice deformation accumulated over 9 and 24 days respectively these interferograms are combined in order to separate the continuous process of creep deformation from the more cyclic motion caused by variations in ocean tide an examination of local gradients in creep deformation reveals the pattern of ice deformation around and near the rupture tips and rifts with great precision up to 10 cm a 1 we compare the observations with a deformation model for ice and obtain the following results 1 the tidal oscillation of the ronne ice shelf only yields small deformations along the rifts and near the rupture tips 2 along the ice front the rifts and at the rupture tips vertical bending is observed which is well explained by a model of viscous deformation of ice furthermore the model indicates that the deformation pattern observed at the rupture tips is a sensitive indicator of the propagation state of the rifts i e active vs inactive 3 the viscous adjustment of ice is the dominant mode of deformation masking the deformation pattern predicted by linear elastic fracture mechanics lefm 4 yet at a spatial scale equivalent to the length of a rift the propagation rate is well predicted by lefm

stability of the west antarctic ice sheet in a warming world equivalent to 0 28 to 0 56 mm yr 1 sea level rise with the rate growing over at least the past two decades 1 4 these observations intensify concerns about the ice sheet s stability that were first raised more than four decades ago 5

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

the role of membrane like stresses in determining the stability and sensitivity of the antarctic ice sheets back pressure and grounding line motion membrane stresses act along thin bodies which are relatively well lubricated on both surfaces they operate in ice sheets because the bottom is either sliding or is much less viscous than the top owing to stress and heat softening of the basal ice ice streams flow over very well lubricated beds and are restrained at their sides the ideal of the perfectly slippery bed is considered in this paper and the propagation of mechanical effects along an ice stream considered by applying spatially varying horizontal body forces propagation distances depend sensitively on the rheological index and can be very large for ice type rheologies a new analytical solution for ice shelf profiles and grounded tractionless stream profiles is presented which show blow up of the profile in a finite distance upstream at locations where the flux is non zero this is a feature of an earlier analytical solution for a floating shelf the length scale of decay of membrane stresses from the grounding line is investigated through scale analysis in ice sheets such effects decay over distances of several tens of kilometres creating a vertical boundary layer between sheet flow and shelf flow where membrane stresses adjust bounded physically reasonable steady surface profiles only exist conditionally in this boundary layer where bounded steady profiles exist adjacent profile equilibria for the whole ice sheet corresponding to different grounded areas occur neutral equilibrium if no solution in the boundary layer can exist the ice sheet profile must change the conditions for existence can be written in terms of whether the basal rate factor sliding or internal deformation is too large to permit a steady solution the critical value depends extremely sensitively on ice velocity and the back stress applied at the grounding line high ice velocity and high stress both favour the existence of solutions and stability changes in these parameters can cause the steady solution existence criterion to be traversed and the ice sheet dynamics to change a finite difference model which represents both neutral equilibrium and the dynamical transition is presented and preliminary investigations into its numerical sensitivity are carried out evidence for the existence of a long wavelength instability is presented through the solution of a numerical eigenproblem which will hamper predictability

a one dimensional model of ice shelf ocean interaction large scale oceanic circulation beneath antarctic ice shelves is driven by the thenmohaline differences which result from mass and energy exchange at the ice ocean interface dense saline waters are drawn underneath the ice shelves and emerge cooled and diluted as plumes of ice shelf water a simple one dimensional model of this process has been developed in which the ice shelf water plume is treated as a turbulent gravity current initiated at the inland margin by a flow of fresh meltwater emerging from beneath the grounded ice subsequent evolution of the plume as it ascends along an ice shelf base of specified geometry can be simulated the model has been applied to a flow line on ronne ice shelf antarctica to explain the observed distribution and rate of basal melting and freezing calculations indicate that the present mean melt rate of 0 6 m yr 1 would increase to 2 6 m yr 1 if the underlying water were to warm by 0 6 c this would not only lead to significant thinning of the ice shelf but could also cause a profound change in ocean circulation on the open continental shelf

adjoint sensitivities of sub ice shelf melt rates to ocean circulation under the pine island ice shelf west antarctica we investigate the sensitivity of sub ice shelf melt rates under pine island ice shelf west antarctica to changes in the oceanic state using an adjoint ocean model that is capable of representing the flow in sub ice shelf cavities the adjoint code is based on algorithmic differentiation ad of the massachusetts institute of technology s ocean general circulation model mitgcm the adjoint model was extended by adding into the ad process the corresponding sub ice shelf cavity code which implements a three equation thermodynamic melt rate parameterization to infer heat and freshwater fluxes at the ice shelf ocean boundary the inferred sensitivities reveal dominant timescales of 30 60 days over which the shelf exit is connected to the deep interior via advective processes they exhibit rich three dimensional time evolving patterns that can be understood in terms of a combination of the buoyancy forcing by inflowing water masses the cavity geometry and the effect of rotation and topography in steering the flow in the presence of prominent features in the bedrock bathymetry dominant sensitivity pathways are found over a sill as well as shadow regions of very low sensitivities to the extent that these transient patterns are robust they carry important information for decision making in observation deployment and monitoring

conditions for a steady ice sheet ice shelf junction abstract this paper investigates the conditions under which a marine ice sheet may adopt a steady profile the ice is treated as a linear viscous fluid caused to flow from a rigid base to and over water treated as a denser but inviscid fluid the solutions in the region around the point of flotation or transition zone are calculated numerically in flow and out flow conditions appropriate to ice sheet and ice shelf flow are applied at the ends of the transition zone and the rigid base is specified the flow and steady free surfaces are determined as part of the solutions the basal stress upstream and the basal deflection downstream of the flotation point are examined to determine which of these steady solutions satisfy contact conditions that would prevent i the steady downstream basal deflection contacting the downstream base and ii the upstream ice commencing to float in the event it was melted at the base in the case that the upstream bed is allowed to slide we find only one mass flux that satisfies the contact conditions when no sliding is allowed at the bed however we find a range of mass fluxes satisfy the contact conditions the effect of backpressure on the solutions is investigated and is found to have no affect on the qualitative behaviour of the junctions to the extent that the numerical linearly viscous treatment may be applied to the case of ice flowing out over the ocean we conclude that when sliding is present weertman s instability hypothesis holds