impact of iceberg calving on the retreat of thwaites glacier west antarctica over the next century with different calving laws and ocean thermal forcing

the retreat of a tidewater glacier observations and model calculations on hansbreen spitsbergen based on observations and model calculations the retreat over the last two decades of hansbreen a tidewater glacier in southern spitsbergen svalbard is investigated the observations of the calving front position between 1982 and 1998 show an abrupt retreat in 1990 which is suggested to be related to a depression in the glacier bed the observed seasonal variations of the front position are mainly due to variations of the calving rate the observations of hansbreen further indicate that during periods of slow front position changes melting at the water line may play an important role in triggering the process of calving the evolution of hansbreen between 1982 and 1998 is simulated with a numerical model for the dynamics of tidewater glaciers using a flotation criterion for calving in which for each time step the part of the glacier terminus which is below a critical height above buoyancy is removed we are able to reproduce the observed rapid retreat of hansbreen through the depression in the glacier bed from the observations and model calculations we conclude that the rapid retreat is mainly an effect of basal topography in the terminus region and not a direct response to a change in mass balance

grounding line movement and ice shelf buttressing in marine ice sheets 1 understanding the dynamics of marine ice sheets is integral to studying the evolution of the antarctic ice sheet in both the short and long terms an important component of the dynamics grounding line migration has proved difficult to represent in numerical models and most successful attempts have made use of techniques that are only readily applicable to flow line models however to capture the stress transmission involved in another important component the buttressing of a marine ice sheet by its ice shelf the transverse direction must also be resolved we introduce a model that solves the time dependent shelfy stream equations and makes use of mesh adaption techniques to overcome the difficulties typically associated with the numerics of grounding line migration we compare the model output with a recent benchmark for flow line models and show that our model yields an accurate solution while using far less resources than would be required without mesh adaption we also show that the mesh adapting techniques extend to two horizontal dimensions experiments are carried out to determine how both ice shelf buttressing and ice rises affect the marine instability predicted for an ice sheet on a foredeepened bed we find that buttressing is not always sufficient to stabilize such a sheet but collapse of the grounded portion is still greatly delayed we also find that the effect of an ice rise is similar to that of narrowing the ice shelf

continued retreat of thwaites glacier west antarctica controlled by bed topography and ocean circulation the amundsen sea sector is experiencing the largest mass loss glacier acceleration and grounding line retreat in antarctica enhanced intrusion of circumpolar deep water onto the continental shelf has been proposed as the primary forcing mechanism for the retreat here we investigate the dynamics and evolution of thwaites glacier with a novel fully coupled ice ocean numerical model we obtain a significantly improved agreement with the observed pattern of glacial retreat using the coupled model coupled simulations over the coming decades indicate a continued mass loss at a sustained rate uncoupled simulations using a depth dependent parameterization of sub ice shelf melt significantly overestimate the rate of grounding line retreat compared to the coupled model as the parameterization does not capture the complexity of the ocean circulation associated with the formation of confined cavities during the retreat bed topography controls the pattern of grounding line retreat while oceanic thermal forcing impacts the rate of grounding line retreat the importance of oceanic forcing increases with time as thwaites grounding line retreats farther inland

greenland flow variability from ice sheet wide velocity mapping using radarsat synthetic aperture radar data we have mapped the flow velocity over much of the greenland ice sheet for the winters of 2000 01 and 2005 06 these maps provide a detailed view of the ice sheet flow including that of the hundreds of glaciers draining the interior the focused patterns of flow at the coast suggest a strong influence of bedrock topography differences between our two maps confirm numerous early observations of accelerated outlet glacier flow as well as revealing previously unrecognized changes the overall pattern is one of speed up accompanied by terminus retreat but there are also several instances of surge behavior and a few cases of glacier slowdown comprehensive mappings such as these at regular intervals provide an important new observational capability for understanding ice sheet variability

rossby waves mediate impacts of tropical oceans on west antarctic atmospheric circulation in austral winter recent studies link climate changearound antarctica to the sea surface temperature of tropical oceans with teleconnections from the pacific atlantic and indian oceans making different contributions to antarctic climate in this study the impacts of each ocean basin on the wintertime southern hemisphere circulation are identified by comparing simulation results using a comprehensive atmospheric model an idealized dynamical core model and a theoretical rossby wave model the results herein show that tropical atlantic ocean warming indian ocean warming and eastern pacific cooling are all able to deepen the amundsen sea low located adjacent to west antarctica while western pacific warming increases the pressure to the west of the international date line encompassing the ross sea and regions south of the tasman sea in austral winter these tropical ocean basins work together linearly to modulate the atmospheric circulation around west antarctica further analyses indicate that these teleconnections critically depend on stationary rossby wave dynamics and are thus sensitive to the background flow particularly the subtropical midlatitude jet nearthesejets wind shearis amplified which strengthens the generation of rossby waves on the other hand near the edges of the jets the meridional gradient of the absolute vorticity is also enhanced as a consequence of the rossby wave dispersion relationship the jet edge may reflect stationary rossby wave trains serving as a waveguide the simulation results not only identify the relative roles of each of the tropical ocean basins in the tropical antarctica teleconnection but also suggest that a deeper understanding of teleconnections requires a better estimation of the atmospheric jet structures

sustained increase in ice discharge from the amundsen sea embayment west antarctica from 1973 to 2013 we combine measurements of ice velocity from landsat feature tracking and satellite radar interferometry and ice thickness from existing compilations to document 41 years of mass flux from the amundsen sea embayment ase of west antarctica the total ice discharge has increased by 77 since 1973 half of the increase occurred between 2003 and 2009 grounding line ice speeds of pine island glacier stabilized between 2009 and 2013 following a decade of rapid acceleration but that acceleration reached far inland and occurred at a rate faster than predicted by advective processes flow speeds across thwaites glacier increased rapidly after 2006 following a decade of near stability leading to a 33 increase in flux between 2006 and 2013 haynes smith pope and kohler glaciers all accelerated during the entire study period the sustained increase in ice discharge is a possible indicator of the development of a marine ice sheet instability in this part of antarctica key points sustained ase mass flux increase 77 since 1973 thwaites accelerated by 33 during the last 6 years speed changes are pervasive and rapid major implications for ice flow modeling 2014 american geophysical union all rights reserved

submarine melting at the terminus of a temperate tidewater glacier leconte glacier alaska u s a heat fresh and sea water balances indicate that the late summer rate of submarine melting at the terminus of tidewater leconte glacier alaska u s a in 2000 was about 12md 1 w e averaged over the submerged face this is 57 of the estimated total ice loss at the terminus calving plus melting at this time submarine melting may thusprovidea significantcontributiontotheoverallablationofatidewaterglacier oceano graphic measurements conductivity temperature depth made 200 500m from the ter minus identified an isohaline 27ppt and isothermal 7 2 c layer extending from 130m depthto the fjord floor capping this is a 40mthick overflow plume distinguishedby high outflow rates low salinity 22 25ppt and lower temperatures 5 6 c mixing models indicate that fresh water comprised about11 of this plume it originates mostly as sub glacial discharge whose buoyancy drives convection at the terminus deep warm saline watersareincorporatedintotheplumeasitascends causingsubstantialmeltingoficealong the submarine face the calving terminus undergoes seasonal changes that coincide with changes in subglacial discharge and fjord water temperatures and we suggest that these fluctuations interminuspositionare directly relatedto changes in submarine melting

a physically based calving model applied to marine outlet glaciers and implications for the glacier dynamics we present results from numerical ice flow models that include calving criteria based on penetration of surface and basal crevasses which in turn is a function of longitudinal strain rates near the glacier front the position of the calving front is defined as the point where either 1 surface crevasses reach the waterline model cdw or 2 surface and basal crevasses penetrate the full thickness of the glacier model cd for comparison with previous studies results are also presented for a height above buoyancy calving model qualitatively both models cdw and cd produce similar behaviour unlike previous models for calving the new calving criteria are applicable to both grounded termini and floating ice shelves and tongues the numerical ice flow model is applied to an idealized geometry characteristic of marine outlet glaciers results indicate that grounding line dynamics are less sensitive to basal topography than previously suggested stable grounding line positions can be obtained even on a reverse bed slope with or without floating termini the proposed calving criteria also allow calving losses to be linked to surface melt and therefore climate in contrast to previous studies in which calving rate or position of the terminus is linked to local water depth the new calving criterion is able to produce seasonal cycles of retreat and advance as observed for greenland marine outlet glaciers the contrasting dynamical behaviour and stability found for different calving models suggests that a realistic parameterization for the process of calving is crucial for any predictions of marine outlet glacier change

new palaeomagnetic results from icelandic neogene lavas summary in this paper some statistical properties of new collections of palaeomagnetic direction and intensity data are presented and compared with results from older collections the new data come mostly from three composite surveys in the basalt lava pile of iceland totalling about 1000 lavas of ages 7 14 myr the palaeomagnetism of all these lavas converges on a reproducible long term pattern in several important features of the neogene field behaviour previous conclusions which are confirmed in this paper include 1 the distribution of intermediate virtual geomagnetic poles vgps is not confined to preferred longitude intervals 2 normal and reverse polarity states of the geomagnetic field are essentially equivalent as regards the intensity of the field mean field directions and secular variation 3 the virtual geomagnetic dipole moment fdls off with decreasing vgp latitude by a factor of 3 5 4 between polar and equatorial latitudes 4 a significant reduction in the between flow angular dispersion of vgps may have occurrcd during the last 14 myr and 5 palaeomagnetic directions from icelandic basalt lavas are stable and reliable

rapid submarine melting of the calving faces of west greenland glaciers widespread glacier acceleration has been observed in greenland in the past few years oceanographic observations taken in summer 2008 show that ocean waters melted a substantial fraction of ice along the calving fronts of three west greenland glaciers indicating that submarine melting has a profound influence on grounding line stability widespread glacier acceleration has been observed in greenland in the past few years1 2 3 4 associated with the thinning of the lower reaches of the glaciers as they terminate in the ocean5 6 7 these glaciers thin both at the surface from warm air temperatures and along their submerged faces in contact with warm ocean waters8 little is known about the rates of submarine melting9 10 11 and how they may affect glacier dynamics here we present measurements of ocean currents temperature and salinity near the calving fronts of the eqip sermia kangilerngata sermia sermeq kujatdleq and sermeq avangnardleq glaciers in central west greenland as well as ice front bathymetry and geographical positions we calculate water mass and heat budgets that reveal summer submarine melt rates ranging from 0 7 0 2 to 3 9 0 8 m d 1 these rates of submarine melting are two orders of magnitude larger than surface melt rates but comparable to rates of iceberg discharge we conclude that ocean waters melt a considerable but highly variable fraction of the calving fronts of glaciers before they disintegrate into icebergs and suggest that submarine melting must have a profound influence on grounding line stability and ice flow dynamics

a numerical investigation of surface crevasse propagation in glaciers using nonlocal continuum damage mechanics 1 we use a nonlocal viscoelastic damage model to investigate the conditions that enable water free surface crevasse propagation in grounded marine terminating glaciers our simulations on idealized rectangular ice slabs in contact with the ocean show that crevasses propagate faster in thicker ice slabs we find that 1 the fraction of ice slab thickness penetrated by surface crevasses decreases with increasing seawater depth near the terminus 2 a no slip fixed basal boundary condition retards crevasse growth and 3 crevasses form closer to the terminus when the seawater depth is larger or when the glacier base is fixed to the bedrock which could lead to calving of smaller icebergs however water free surface crevasses can penetrate nearly the entire ice thickness only in thicker ice slabs terminating in shallow seawater depths this leads us to the conclusion that surface crevasses alone are not responsible for calving events in marine terminating and thin glaciers

modeling of store gletscher s calving dynamics west greenland in response to ocean thermal forcing glacier front dynamics is an important control on greenland s ice mass balance warmer ocean waters trigger ice front retreats of marine terminating glaciers and the corresponding loss in resistive stress leads to glacier acceleration and thinning here we present an approach to quantify the sensitivity and vulnerability of marine terminating glaciers to ocean induced melt we develop a plan view model of store gletscher that includes a level set based moving boundary capability a parameterized ocean induced melt and a calving law with complete and precise land and fjord topographies to model the response of the glacier to increased melt we find that the glacier is stabilized by a sill at its terminus the glacier is dislodged from the sill when ocean induced melt quadruples at which point the glacier retreats irreversibly for 27 km into a reverse bed the model suggests that ice ocean interactions are the triggering mechanism of glacier retreat but the bed controls its magnitude

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

potential antarctic ice sheet retreat driven by hydrofracturing and ice cliff failure abstract geological data indicate that global mean sea level has fluctuated on 103 to 106 yr time scales during the last 25 million years at times reaching 20 m or more above modern if correct this implies substantial variations in the size of the east antarctic ice sheet eais however most climate and ice sheet models have not been able to simulate significant eais retreat from continental size given that atmospheric co2 levels were relatively low throughout this period here we use a continental ice sheet model to show that mechanisms based on recent observations and analysis have the potential to resolve this model data conflict in response to atmospheric and ocean temperatures typical of past warm periods floating ice shelves may be drastically reduced or removed completely by increased oceanic melting and by hydrofracturing due to surface melt draining into crevasses ice at deep grounding lines may be weakened by hydrofracturing and reduced buttressing and may fail structurally if stresses exceed the ice yield strength producing rapid retreat incorporating these mechanisms in our ice sheet model accelerates the expected collapse of the west antarctic ice sheet to decadal time scales and also causes retreat into major east antarctic subglacial basins producing 17 m global sea level rise within a few thousand years the mechanisms are highly parameterized and should be tested by further process studies but if accurate they offer one explanation for past sea level high stands and suggest that antarctica may be more vulnerable to warm climates than in most previous studies

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

the cretaceous superchron geodynamo observations near the tangent cylinder abstract if relationships exist between the frequency of geomagnetic reversals and the morphology secular variation and intensity of earth s magnetic field they should be best expressed during superchrons intervals tens of millions of years long lacking reversals here we report paleomagnetic and paleointensity data from lavas of the cretaceous normal polarity superchron that formed at high latitudes near the tangent cylinder that surrounds the solid inner core the time averaged field recorded by these lavas is remarkably strong and stable when combined with global results available from lower latitudes these data define a time averaged field that is overwhelmingly dominated by the axial dipole octupole components are insignificant these observations suggest that the basic features of the geomagnetic field are intrinsically related superchrons may reflect times when the nature of core mantle boundary heat flux allows the geodynamo to operate at peak efficiency

some statistical properties of palaeomagnetic directions in icelandic lava flows summary this paper extends the efforts of kristjansson mcdougall to obtain a semi quantitative description of various long term 0 1 10 myr properties of the geomagnetic field in iceland due to the long time intervals between eruptions of the lavas recording the field local average and stochastic properties are emphasized rather than detailed aspects such as pole paths these properties can be represented by smooth functions of virtual pole latitude it is possible to describe both the frequency distribution of virtual poles in latitude and the estimated frequency of geomagnetic reversals by a simple process of random walk of the pole this is also in agreement with the observed long term symmetry of the reversal process and demonstrates that memory effects are unnecessary to account for the behaviour of the field the average transition time for the field isofthe order of 5 6000yr right handedness is a pervasive property of the field in iceland apparently increasing with age it is demonstrated that the size of palaeomagnetic collections in lava sequences has to be much larger than 20 units in order to yield consistent values of statistical parameters for mean palaeofield directions

detailed paleomagnetic study of two volcanic polarity transitions recorded in eastern iceland abstract two sequences of 45 and 49 individual lava flows respectively have been sampled in eastern iceland the two sections range in age from 12 09 to 10 21 ma as reported by watkins and walker watkins n walker g p l 1977 magnetostratigraphy of eastern iceland am j sci 277 513 584 and are labelled as profiles c and d stepwise alternating field af and thermal demagnetizations accompanied by investigations of rock magnetic properties indicate that the magnetization is primarily carried by titanomagnetite demagnetization experiments have identified eight and six transitional lavas respectively for these profiles the transitional virtual geomagnetic poles vgps of the r n profile c reversal are located mainly between patagonia and antarctica with a tendency of the virtual poles to move towards west coast of south america subsequently traveling to the northern hemisphere through several discrete steps located in the middle part of south america then to west africa and on to central asia before the poles settle into normal polarity the second and younger vgp path corresponding to profile d is a path characterized by a reverse to normal to reverse r n r motion of the virtual poles this path is characterized by poles located in west antarctica patagonia and the western and eastern part of south america the passage from the southern to the northern hemisphere is also through a discrete sets of steps along the southwestern pacific followed by a rapid motion to the northern siberian region continuing to the western equatorial part of south america before moving on to the central region of asia followed by a motion to the western pacific prior to the final move to the eastern part of antarctica the data from eastern iceland based on highly reliable transitional results as indicated by the demagnetizations and rock magnetic experiments derived from relatively spaced sites and different ages seem to indicate that the eastern icelandic high latitude 66 n profiles studied have several persistent transitional paleofield features that are uniquely observed at those sites particularly when compared to other volcanic records located at lower latitudes

the last two geomagnetic polarity reversals recorded in high deposition rate sediment drifts high resolution records of geomagnetic polarity transitions are rare owing to the difficulty of preserving in the geological record details of a process with a duration of only a few thousand years lavas can record the instantaneous field on cooling but lava records are themselves discontinuous and therefore polarity transition fields are seldom captured sedimentary records are more continuous but typical deep sea sedimentation rates of a few cm kyr 1are too low to record transition fields adequately here we report transition records from north atlantic drift deposits with high sedimentation rates 12 cm kyr 1 at the ocean drilling program sites 983 and 984 virtual geomagnetic polar vgp paths are similar within and between sites for the last two geomagnetic reversals attesting to a memory effect spanning back to back reversals the complex vgp paths feature two vgp clusters one located in northeastern asia and the other in the south atlantic ocean off south america and provide a link between the longitudinally constrained transitional vgps often recorded in sediments1 2 which represent highly smoothed renditions of transition fields and the scattered transitional vgps3 and vgp clusters4 observed in lava records

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

inversion of basal friction in antarctica using exact and incomplete adjoints of a higher order model basal friction beneath ice sheets remains poorly characterized and yet is a fundamental control on ice mechanics here we use a complete map of surface velocity of the antarctic ice sheet to infer the basal friction over the entire continent by combining these observations with a three dimensional thermomechanical higher order ice sheet numerical model from the ice sheet system model open source software we demonstrate that inverse methods can be readily applied at the continental scale with appropriate selections of cost function and of scheme of regularization at a spatial resolution as high as 3 km along the coastline we compare the convergence of two descent algorithms with the exact and incomplete adjoints to show that the incomplete adjoint is an excellent approximation the results reveal that the driving stress is almost entirely balanced by the basal shear stress over 80 of the ice sheet the basal friction coefficient which relates basal friction to basal velocity is however significantly heterogeneous it is low on fast moving ice and high near topographic divides areas with low values extend far out into the interior along glacier and ice stream tributaries almost to the flanks of topographic divides suggesting that basal sliding is widespread beneath the antarctic ice sheet 2013 american geophysical union all rights reserved

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

widespread rifting and retreat of ice shelf margins in the eastern amundsen sea embayment between 1972 and 2011 the major outlet glaciers that drain the eastern sector of the amundsen sea embayment smith haynes thwaites and pine island are among the largest fastest flowing and fastest thinning glaciers in west antarctica their recent ice flow acceleration is linked to ocean induced ice shelf thinning but may also arise from additional losses of ice shelf buttressing that are not well understood here we present a comprehensive history of coastal change in the eastern amundsen sea embayment between 1972 and 2011 derived mostly from landsat imagery the termini of all four major outlet glaciers have retreated but retreat is most rapid along the ice shelf margins where progressive rifting has occurred this pattern of retreat coincides with the recent acceleration of grounded ice and contributed to loss of ice shelf buttressing the observed pattern of margin led gradual ice shelf disintegration appears to be common in accelerating ocean terminating outlet glaciers we hypothesize that this pattern is part of a positive feedback between glacier acceleration and rift growth that could drive further buttressing loss in the eastern amundsen sea embayment

dependence of century scale projections of the greenland ice sheet on its thermal regime observations show that the greenland ice sheet has been losing mass at an increasing rate over the past few decades which makes it a major contributor to sea level rise here we use a threedimensional higher order ice flow model adaptive mesh refinement and inverse methods to accurately reproduce the present day ice flow of the greenland ice sheet we investigate the effect of the ice thermal regime on 1 basal sliding inversion and 2 projections over the next 100 years we show that steady state temperatures based on present day conditions allow a reasonable representation of the thermal regime and that both basal conditions and century scale projections are weakly sensitive to small changes in the initial temperature field compared with changes in atmospheric conditions or basal sliding we conclude that although more englacial temperature measurements should be acquired to validate the models and a better estimation of geothermal heat flux is needed it is reasonable to use steady state temperature profiles for short term projections as external forcings remain the main drivers of the changes occurring in greenland 2014 publishing technology

a three dimensional calving model numerical experiments on johnsons glacier livingston island antarctica calving from tidewater glaciers and ice shelves accounts for around half the mass loss from both polar ice sheets yet the process is not well represented in prognostic models of ice dynamics benn and others proposed a calving criterion appropriate for both grounded and floating glacier tongues or ice shelves based on the penetration depth of transverse crevasses near the calving front computed using nye s formula the criterion is readily incorporated into glacier and ice sheet models but has not been fully validated with observations we apply a three dimensional extension of benn and others criterion incorporated into a full stokes model of glacier dynamics to estimate the current position of the calving front of johnsons glacier antarctica we find that two improvements to the original model are necessary to accurately reproduce the observed calving front 1 computation of the tensile deviatoric stress opening the crevasse using the full stress solution and 2 consideration of such a tensile stress as a function of depth our modelling results also suggest that johnsons glacier has a polythermal structure rather than the temperate structure suggested by earlier studies

major tectonic rotation along an oceanic transform zone northern iceland evidence from field and paleomagnetic investigations abstract we provide structural and paleomagnetic evidence for major distributed rotational shear adjacent to and including the active husavik flatey transform fault in the southern part of the tjornes fracture zone of north iceland in the flateyjarskagi peninsula immediately south of the fault dike and bedding trends gradually change over about 11 km in approaching the transform fault main trace in map view this pattern is seen as a progressive clockwise curvature of structural elements that equals or in some cases exceeds 90 we present paleomagnetic analyses that verify large clockwise rotations of the same order as the curvature of structural elements observed we compare our field and paleomagnetic observations with results from careful analog modeling that demonstrate how regional curvature of structures results from distributed strain that dominates the early stages of transform shear deformation prior to the appearance of a transform fault system comprised of discrete shears our paleomagnetic results were obtained from one relatively undisturbed reference site and three sites of deformed rocks in flateyjarskagi and two in tjornes peninsula these results indicate large vertical axis rotations and the progressive deflection of dike trends throughout northern flateyjarskagi also serves as a separate quantitative estimate of vertical axis rotation the clockwise vertical axis rotation estimates of 71 97 from dikes are less than those deduced from paleomagnetism but the order of magnitude is correct and we attribute the differences as mainly reflecting imperfect bed attitude adjustments used in analyses the problem is typical for deformed rocks from high latitude sites with steeply inclined paleomagnetic vectors we consider our field measurements of dikes as more accurate indicators of true tectonic rotation this regional deformation of miocene rocks occurred early in the evolution of the tfz transform probably beginning ca 7 ma when an older rift zone present in western north iceland jumped to its present nvz location in northeast iceland the early deformations involved shear strain distributed over a region about 20 km broad and produced the curved structures this phase was followed by the sequential development of more narrowly distributed discrete shears generating the initial husavik flatey transform fault

a mass conservation approach for mapping glacier ice thickness the traditional method for interpolating ice thickness data from airborne radar sounding surveys onto regular grids is to employ geostatistical techniques such as kriging while this approach provides continuous maps of ice thickness it generates products that are not consistent with ice flow dynamics and are impractical for high resolution ice flow simulations here we present a novel approach that combines sparse ice thickness data collected by airborne radar sounding profilers with high resolution swath mapping of ice velocity derived from satellite synthetic aperture interferometry to obtain a high resolution map of ice thickness that conserves mass and minimizes the departure from observations we apply this approach to the case of nioghalvfjerdsfjorden 79north glacier a major outlet glacier in northeast greenland that has been relatively stable in recent decades the results show that our mass conserving method removes the anomalies in mass flux divergence yields interpolated data that are within about 5 of the original data and produces thickness maps that are directly usable in high spatial resolution high order ice flow models we discuss the application of this method to the broad and detailed radar surveys of ice sheet and glacier thickness copyright 2011 by the american geophysical union

widespread rapid grounding line retreat of pine island thwaites smith and kohler glaciers west antarctica from 1992 to 2011 we measure the grounding line retreat of glaciers draining the amundsen sea sector of west antarctica using earth remote sensing ers 1 2 satellite radar interferometry from 1992 to 2011 pine island glacier retreated 31 km at its center with most retreat in 2005 2009 when the glacier ungrounded from its ice plain thwaites glacier retreated 14 km along its fast flow core and 1 to 9 km along the sides haynes glacier retreated 10 km along its flanks smith kohler glaciers retreated the most 35 km along its ice plain and its ice shelf pinning points are vanishing these rapid retreats proceed along regions of retrograde bed elevation mapped at a high spatial resolution using a mass conservation technique that removes residual ambiguities from prior mappings upstream of the 2011 grounding line positions we find no major bed obstacle that would prevent the glaciers from further retreat and draw down the entire basin 2014 american geophysical union all rights reserved

flow dynamics of tidewater glaciers a numerical modelling approach the dynamics of grounded tidewater glaciers is investigated with a time dependent numerical flow model which solves the full equations for the stress and velocity fields and includes a water pressure dependent sliding law the calving criterion implemented in the model shifts the calving front at each time step to the position where the frontal ice thickness exceeds flotation height by a prescribed value with this model the linear relation between calving rate and water depth proposed on empirical grounds is qualitatively reproduced for the situation of a slowly retreating or advancing terminus but not for situations of rapid changes length changes of tidewater glaciers i e especially rapid changes are dominantly controlled by the bed topography and are to a minor degree a direct reaction to a mass balance change thus accurate information on the near terminus bed topography is required for reliable prediction of the terminus changes due to climate changes the results also confirm the suggested cycles of slow advance and rapid retreat through a basal depression rapid changes in terminus positions preferably occur in places where the bed slopes upwards in the ice flow direction

new boundary conditions for the west antarctic ice sheet subglacial topography of the thwaites and smith glacier catchments airborne radar sounding over the thwaites glacier tg catchment and its surroundings provides the first comprehensive view of subglacial topography in this dynamic part of the west antarctic ice sheet wais and reveals that tg is underlain by a single broad basin fed by a dendritic pattern of valleys while smith glacier lies within an extremely deep narrow trench subglacial topography in the tg catchment slopes inland from a broad low relief coastal sill to the thickest ice of the wais and makes deep connections to both pine island glacier and the ross sea embayment enabling dynamic interactions across the wais during deglaciation simple isostatic rebound modeling shows that most of this landscape would be submarine after deglaciation aside from an island chain near the present day ross amundsen ice divide the lack of topographic confinement along tg s eastern margin implies that it may continue to widen in response to grounding line retreat

extended eof analysis of tropical disturbances toga coare empirical orthogonal function eof analyses are performed of time height series of zonal and meridional winds and of cumulonimbus heating and drying in the tropics the data are from a rawinsonde array in the western pacific located between the equator and 108s during the intensive observation period of the tropical ocean global atmosphere coupled ocean atmosphere response experiment toga coare the eof analyses are performed by applying a window of 20 days to the data and thus calculating the eofs of the time development of the vertical structure the wind time series is found to be well represented by two pairs of eofs each representing an oscillation the first oscillation has a period of approximately 40 days is predominantly in the zonal wind component and has a first internal mode vertical structure with westerly anomalies in the lower troposphere corresponding to easterly perturbations in the upper troposphere this pair describes 48 3 of the variance a second eof pair in the wind is a zonal variation that occurs predominantly in the upper troposphere it has a period of approximately 24 days and describes 13 9 of the variance the heating drying series is described by a dominant oscillation of period 40 days representing 41 of the variance the structure is maximum in the middle troposphere and is associated with the same physical phenomeon as the dominant u y oscillation the second eof pair for heating drying has a period of 13 days so there is a large time separation in periodiocities for heating drying compared to that for winds the second 13 day oscillation in heating drying has the same vertical structure as the dominant 40 day oscillation

rapid subsurface warming and circulation changes of antarctic coastal waters by poleward shifting winds the southern hemisphere westerly winds have been strengthening and shifting poleward since the 1950s this wind trend is projected to persist under continued anthropogenic forcing but the impact of the changing winds on antarctic coastal heat distribution remains poorly understood here we show that a poleward wind shift at the latitudes of the antarctic peninsula can produce an intense warming of subsurface coastal waters that exceeds 2 c at 200 700 m depth the model simulated warming results from a rapid advective heat flux induced by weakened near shore ekman pumping and is associated with weakened coastal currents this analysis shows that anthropogenically induced wind changes can dramatically increase the temperature of ocean water at ice sheet grounding lines and at the base of floating ice shelves around antarctica with potentially significant ramifications for global sea level rise

on low latitude virtual geomagnetic poles in icelandic basalt lava sequences abstract in two paleomagnetic studies from the neogene lava sequences of iceland published in 1977 of the order of 10 of the reliably determined remanence directions corresponded to virtual pole positions below 40 latitude this was a larger proportion than found in most other volcanic locations to explain it harrison harrison c g a 1980 secular variation and excursions of the earth s magnetic field j geophys res 85 3511 3522 suggested that some 10 of the observed virtual geomagnetic poles vgp from iceland might be distributed at random over the globe these random poles can be envisaged as dating from periods when the geomagnetic field is weak and dominated by variable non dipole terms much additional data on remanence directions and intensities has been collected since then and the present study analyses results from over 3500 lava flows in iceland the relative frequency and strength of virtual dipoles in this collection vary with the pole latitude in a similar way as in the collection studied by harrison normalizing of remanence intensity data by the use of anhysteretic remanence arm may improve the resolution of this type of analysis

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

some aspects of the late tertiary geomagnetic field in iceland summary in 1972 78 the late n d watkins and others carried out ajoint field programme of geological mapping in the mio pliocene flood basalts of iceland including sampling for k ar dating and palaeomagnetic research the major part of the palaeomagnetic sampling is represented by 2462 lavas in five long composite sections through the lava pile this paper deals with various statistical properties of this data set it is concluded that geomagnetic reversals occur more frequently than is assumed in the current ocean floor polarity time scale there is no evidence for significant asymmetries between normal and reverse polarity states of the field neither as regards chron lengths secular variation or virtual dipole moment magnitude intensities of remanence in these lavas are shown to be well approximated in terms of a hyperbolic distribution the latitude distribution of virtual magnetic poles can be fitted with a bingham function having k 4 5 and low latitude poles do not occur preferentially in any particular longitude interval

bathymetry of the amundsen sea embayment sector of west antarctica from operation icebridge gravity and other data we employ airborne gravity data from nasa s operation icebridge collected in 2009 2014 to infer the bathymetry of sub ice shelf cavities in front of pine island thwaites smith and kohler glaciers west antarctica we use a three dimensional inversion constrained by multibeam echo sounding data offshore and bed topography from a mass conservation reconstruction on land the seamless bed elevation data refine details of the pine island sub ice shelf cavity a slightly thinner cavity beneath thwaites and previously unknown deep 1200 m channels beneath the crosson and dotson ice shelves that shallow 500 m and 750 m respectively near the ice shelf fronts these sub ice shelf channels define the natural pathways for warm circumpolar deep water to reach the glacier grounding lines melt the ice shelves from below and constrain the pattern of past and future glacial retreat

contribution of antarctica to past and future sea level rise polar temperatures over the last several million years have at times been slightly warmer than today yet global mean sea level has been 6 9 metres higher as recently as the last interglacial 130 000 to 115 000 years ago and possibly higher during the pliocene epoch about three million years ago in both cases the antarctic ice sheet has been implicated as the primary contributor hinting at its future vulnerability here we use a model coupling ice sheet and climate dynamics including previously underappreciated processes linking atmospheric warming with hydrofracturing of buttressing ice shelves and structural collapse of marine terminating ice cliffs that is calibrated against pliocene and last interglacial sea level estimates and applied to future greenhouse gas emission scenarios antarctica has the potential to contribute more than a metre of sea level rise by 2100 and more than 15 metres by 2500 if emissions continue unabated in this case atmospheric warming will soon become the dominant driver of ice loss but prolonged ocean warming will delay its recovery for thousands of years