rapid circulation of warm subtropical waters in a major glacial fjord in east greenland the recent rapid increase in mass loss from the greenland ice sheet is primarily attributed to an acceleration of outlet glaciers oceanographic data obtained in summer 2008 show that subtropical waters that reside year round in the shelf ocean off greenland continuously enter a large glacial fjord in east greenland and contribute to melting at the glacier terminus the recent rapid increase in mass loss from the greenland ice sheet1 2 is primarily attributed to an acceleration of outlet glaciers3 4 5 one possible cause of this acceleration is increased melting at the ice ocean interface6 7 driven by the synchronous warming8 9 10 of subtropical waters offshore of greenland however because of the lack of observations from greenland s glacial fjords and our limited understanding of their dynamics this hypothesis is largely untested here we present oceanographic data collected in sermilik fjord east greenland by ship in summer 2008 and from moorings our data reveal the presence of subtropical waters throughout the fjord these waters are continuously replenished through a wind driven exchange with the shelf where they are present all year the temperature and renewal of these waters indicate that they currently cause enhanced submarine melting at the glacier terminus key controls on the melting rate are the volume and properties of the subtropical waters on the shelf and the patterns of along shore winds suggesting that the glaciers acceleration has been triggered by a combination of atmospheric and oceanic changes our measurements provide evidence for a rapid advective pathway for the transmission of oceanic variability to the ice sheet margins

subaqueous melting of store glacier west greenland from three dimensional high resolution numerical modeling and ocean observations we present three dimensional high resolution simulations of ice melting at the calving face of store glacier a tidewater glacier in west greenland using the massachusetts institute of technology general circulation model we compare the simulated ice melt with an estimate derived from oceanographic data the simulations show turbulent upwelling and spreading of the freshwater laden plume along the ice face and the vigorous melting of ice at rates of meters per day the simulated august 2010 melt rate of 2 0 0 3 m d is within uncertainties of the melt rate of 3 0 1 0 m d calculated from oceanographic data melting is greatest at depth above the subglacial channels causing glacier undercutting melt rates increase proportionally to thermal forcing raised to the power of 1 2 1 6 and to subglacial water flux raised to the power of 0 5 0 9 therefore in a warmer climate store glacier melting by ocean may increase from both increased ocean temperature and subglacial discharge key points simulated melt agrees with estimates from a hydrographic survey of the glacier with more runoff glaciers will melt faster at constant ocean temperature 2013 american geophysical union all rights reserved

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

distinct patterns of seasonal greenland glacier velocity predicting greenland ice sheet mass loss due to ice dynamics requires a complete understanding of spatiotemporal velocity fluctuations and related control mechanisms we present a 5 year record of seasonal velocity measurements for 55 marine terminating glaciers distributed around the ice sheet margin along with ice front position and runoff data sets for each glacier among glaciers with substantial speed variations we find three distinct seasonal velocity patterns one pattern indicates relatively high glacier sensitivity to ice front position the other two patterns are more prevalent and appear to be meltwater controlled these patterns reveal differences in which some subglacial systems likely transition seasonally from inefficient distributed hydrologic networks to efficient channelized drainage while others do not the difference may be determined by meltwater availability which in some regions may be influenced by perennial firn aquifers our results highlight the need to understand subglacial meltwater availability on an ice sheet wide scale to predict future dynamic changes

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

ice speed of a calving glacier modulated by small fluctuations in basal water pressure ice flow acceleration has played a crucial role in the rapid retreat of calving glaciers in alaska greenland and antarctica high frequency measurements of ice speed and basal water temperatures from a calving glacier in patagonia show that changes in basal water pressure by a few per cent can significantly affect ice flow speed ice flow acceleration has played a crucial role in the rapid retreat of calving glaciers in alaska1 2 greenland and antarctica3 4 glaciers that calve in water flow much faster than those that terminate on land as a result of enhanced basal ice motion where basal water pressure is high5 however a scarcity of subglacial observations in calving glaciers limits a mechanistic understanding here we present high frequency measurements of ice speed and basal water pressures from glaciar perito moreno a fast flowing calving glacier in patagonia we measured water pressure in boreholes drilled at a site where the glacier is 515 5 m thick and where more than 60 of the ice is below the level of proglacial lakes we found that the mean basal water pressure was about 95 of the pressure imposed by the weight of the overlying ice moreover changes in basal water pressure by a few per cent drove nearly 40 of the variations in ice flow speed the ice speed was strongly correlated to air temperature suggesting that glacier motion was modulated by water pressure changes as meltwater entered the system we conclude that basal water pressure in calving glaciers is important for glacier dynamics and closely connected to climate conditions

distributed subglacial discharge drives significant submarine melt at a greenland tidewater glacier this is the publisher s final pdf the article is copyrighted by american geophysical union and published by john wiley sons inc it can be found at http agupubs onlinelibrary wiley com agu journal 10 1002 28issn 291944 8007

the effect of meltwater plumes on the melting of a vertical glacier face abstractfreshwater produced by the surface melting of ice sheets is commonly discharged into ocean fjords from the bottom of deep fjord terminating glaciers the discharge of the freshwater forms upwelling plumes in front of the glacier calving face this study simulates the meltwater plumes emanated into an unstratified environment using a nonhydrostatic ocean model with an unstructured mesh and subgrid scale mixing calibrated by comparison to established plume theory the presence of an ice face reduces the entrainment of seawater into the meltwater plumes so the plumes remain attached to the ice front in contrast to previous simple models ice melting increases with height above the discharge also in contrast to some simple models and the authors speculate that this overcutting may contribute to the tendency of icebergs to topple inwards toward the ice face upon calving the overall melt rate is found to increase with discharge flux only up to a critical value which depends on the channel size

a simple mechanism for irreversible tidewater glacier retreat 1 tidewater glacier retreat is abrupt and typically irreversible the mechanics of abrupt onset are evaluated by considering the behavior of a sliding law that depends inversely upon effective pressure pice pwater for such a law flux can increase with decreasing thickness if the loss of bed traction exceeds the loss of driving stress characteristic values of increased flux and glacier thinning are shown to propagate upstream if ice flux increases with decreasing thickness below an ice thickness threshold determined by the chosen sliding law upstream propagation leads to instability described in earlier literature in which increased flow leads to thinning loss of bed traction and continued acceleration in simplest terms the instability arises when alterations in glacier geometry act to reduce resistive stresses more than driving stresses the effects of diffusion are also analyzed and it is shown that for the form of sliding law considered here diffusion will not necessarily eliminate upstream propagating instabilities application of this theory shows that a transition from stable to unstable conditions occurred at columbia glacier alaska near the time of onset of its retreat in the early 1980s the theory also successfully predicts stable or rapidly retreating states for 12 other alaska tidewater glaciers and its applicability to greenland outlet glaciers is discussed

positive degree day factors for ablation on the greenland ice sheet studied by energy balance modelling ice ablation is related to air temperature by the positive degree day factor variations of the positive degree day factor in west greenland are studied using an energy balance model to simulate ablation under different conditions degree day factors for simulated and measured ice ablation at nordbogletscher and qaman rssup sermia agree well with values around 8 mm d 1 c 1 degree day factors for snow are less than half those for ice energy balance modelling shows that degree day factors vary with summer mean temperature surface albedo and turbulence but there is only evidence of large positive degree day factors at lower temperatures and with low albedo 0 3 the greatest effect of albedo variations 0 3 0 7 is at lower temperatures while variations in turbulence have greater effect at higher temperatures current models may underestimate runoff from the greenland ice sheet by several tenths because they use a degree day factor for melting ice that is too small for the colder parts of the ice sheet i e the upper ablation area and the northerly margin

impact of two plumes interaction on submarine melting of tidewater glaciers a laboratory study idealized laboratory experiments investigate the glacier ocean boundary dynamics near a vertical glacier in a two layer stratified fluid discharge of meltwater runoff at the base of the glacier subglacial discharge enhances submarine melting in the laboratory the effect of multiple sources of subglacial discharge is simulated by introducing freshwater at freezing temperature from two point sources at the base of an ice block representing the glacier the buoyant plumes of cold meltwater and subglacial discharge water entrain warm ambient water rise vertically and interact within a layer of depth h2 if the distance between the sources x0 is smaller than h2 0 35 where is the entrainment constant the plume water detaches from the glacier face at the interface between the two layers and or at the free surface as confirmed by previous numerical studies and field observations a plume model is used to explain the observed nonmonotonic dependence of submarine melting on the sources separation the distance between the two sources influences the entrainment of warm water in the plumes and consequently the amount of submarine melting and the final location of the meltwater within the water column two interacting plumes located very close together are observed to melt approximately half as much as two independent plumes the inclusion or parameterization of the dynamics regulating multiple plumes interaction is therefore necessary for a correct estimate of submarine melting hence the distribution and number of sources of subglacial discharge may play an important role in glacial melt rates and fjord stratification and circulation

surface lithofacies biofacies and diatom diversity patterns as models for delineation of climatic change in the southeast atlantic ocean abstract distribution of diatom species in surface sediments of the southeast atlantic ocean is regulated by present day oceanographic and hydrodynamic processes five assemblages vectors defined by factor vector analysis reflect different environments and conditions assemblage a is a high diversity flora associated with the nutrient rich relatively cold waters south of the polar front in the diatom ooze belt occurrence of this flora on maud rise and vicinity may reflect periodic occurrence of a polynya in that area assemblage b is a reworked assemblage that dominates the southern portion of the study area productivity there is low reflecting sea ice cover during most of the year the area north of the polar front is dominated by assemblage c whose characteristic species reflect the relatively warm subantarctic surface water winnowing and frustule breakage have altered assemblage d found in three isolated samples by removing relatively delicate forms leaving lag deposits of more robust species assemblage e is a low diversity stress flora reflecting unstable unpredictable environments along the polar front antarctic slope and the northern boundary of winter sea ice these sites are characterized by the sinking of cold water downcore analysis of cores lying adjacent to the polar front and the diatom ooze pelagic clay boundary show evidence for past climatic variation the low trophic level occupied by diatoms and the subsequent sensitivity of these organisms to abiotic environmental parameters such as light quality make relative diatom abundance a useful tool for monitoring fluctuations of winter sea ice and the temperature changes responsible for these fluctuations the position of the polar front has migrated at least three times within the last 0 015 m y b p within the last 0 3 m y b p warm maxima have occurred at approximately 0 0 0 015 0 125 and 0 3 m y b p

21st century evolution of greenland outlet glacier velocities earlier observations on several of greenland s outlet glaciers starting near the turn of the 21st century indicated rapid annual scale and large 100 increases in glacier velocity combining data from several satellites we produce a decade long 2000 to 2010 record documenting the ongoing velocity evolution of nearly all 200 of greenland s major outlet glaciers revealing complex spatial and temporal patterns changes on fast flow marine terminating glaciers contrast with steady velocities on ice shelf terminating glaciers and slow speeds on land terminating glaciers regionally glaciers in the northwest accelerated steadily with more variability in the southeast and relatively steady flow elsewhere intraregional variability shows a complex response to regional and local forcing observed acceleration indicates that sea level rise from greenland may fall well below proposed upper bounds

convection driven melting near the grounding lines of ice shelves and tidewater glaciers subglacial meltwater draining along the bed of fast flowing marine terminating glaciers emerges at the groundingline wheretheiceeithergoesafloattoformaniceshelforterminates inacalvingface theinputof freshwater to the ocean provides a source of buoyancy and drives convective motion alongside the ice ocean interface this process is modeled using the theory of buoyant plumes that has previously been applied to the study of the larger scale circulation beneath ice shelves the plume grows through entrainment of ocean waters and the heat brought into the plume as a result drives melting at the ice ocean interface the equations are nondimensionalized by using scales appropriate for the region where the subglacial drainage rather than the subsequent addition of meltwater supplies the majority of the buoyancy forcing it is found that the melt rate within this region can be approximated reasonably well by a function that is linear in ocean temperature has a cube root dependence on the flux of subglacial meltwater and has a complex dependency on the slope of the ice ocean interface the model is used to investigate variability in melting induced by changes in both ocean temperature and subglacial discharge for a number of realistic examples of ice shelves and tidewater glaciers the results show how warming ocean waters and increasing subglacial drainage both generate increases in melting near the grounding line

deeply incised submarine glacial valleys beneath the greenland ice sheet the bed topography beneath the greenland ice sheet controls the flow of ice and its discharge into the ocean a combination of sparse radar soundings of ice thickness and high resolution ice motion data suggest that many submarine ice covered valleys extend significantly deeper below sea level and farther inland than thought

modeling turbulent subglacial meltwater plumes implications for fjord scale buoyancy driven circulation abstractfjord scale circulation forced by rising turbulent plumes of subglacial meltwater has been identified as one possible mechanism of oceanic heat transfer to marine terminating outlet glaciers this study uses buoyant plume theory and a nonhydrostatic three dimensional ocean ice model of a typical outlet glacier fjord in west greenland to investigate the sensitivity of meltwater plume dynamics and fjord scale circulation to subglacial discharge rates ambient stratification turbulent diffusivity and subglacial conduit geometry the terminal level of a rising plume depends on the cumulative turbulent entrainment and ambient stratification plumes with large vertical velocities penetrate to the free surface near the ice face however midcolumn stratification maxima create a barrier that can trap plumes at depth as they flow downstream subglacial discharge is varied from 1 750 m3 s 1 large discharges result in plumes with positive temperature and salinity anomalies in the upper water column for

modelling albedo and specific balance of the greenland ice sheet calculations for the sondre stromfjord transect glacio meteorological data obtained during the greenland ice margin experiment gimex investigations in west greenland the sondre stromfjord transect have been used to test and calibrate four energy balance mass balance models for the ice snow surface the region is characterised by the development of a wide zone of low surface albedo in the course of the melting season this zone was simulated in one of the energy balance models by including the effect of surficial meltwater on albedo observed mass balance and albedo data were used to constrain the models although all the models are capable of predicting the transect balance reasonably well only the model with the meltwater albedo coupling is able to reproduce the observed albedo pattern and mass balance profile along the transect by including the feedback between surficial meltwater and albedo in the model the sensitivity of the specific balance to changes in air temperature is found to be greatest just below the equilibrium line in contrast to what is generally found for valley glaciers a 1 k warming of the air temperature would increase the mean ablation along the transect by 0 5 m w e year 1

quantifying energy and mass fluxes controlling godthabsfjord freshwater input in a 5 km simulation 1991 2012 abstractfreshwater runoff to fjords with marine terminating glaciers along the greenland ice sheet margin has an impact on fjord circulation and potentially ice sheet mass balance through increasing heat transport to the glacier front here the authors use the high resolution 5 5 km hirham5 regional climate model allowing high detail in topography and surface types to estimate freshwater input to godthabsfjord in southwest greenland model output is compared to hydrometeorological observations and while simulated daily variability in temperature and downwelling radiation shows high correlation with observations typically 0 9 there are biases that impact the results in particular overestimated albedo leads to underestimation of melt and runoff at low elevations in the model simulation 1991 2012 the ice sheet experiences increasing energy input from the surface turbulent heat flux up to elevations of 2000 m and shortwave radiation at all elevations southerly wind anomalies and declining

seasonal speedup along the western flank of the greenland ice sheet it has been widely hypothesized that a warmer climate in greenland would increase the volume of lubricating surface meltwater reaching the ice bedrock interface accelerating ice flow and increasing mass loss we have assembled a data set that provides a synoptic scale view spanning ice sheet to outlet glacier flow with which to evaluate this hypothesis on the ice sheet these data reveal summer speedups 50 to 100 consistent with but somewhat larger than earlier observations the relative speedup of outlet glaciers however is far smaller 15 furthermore the dominant seasonal influence on jakobshavn isbrae s flow is the calving front s annual advance and retreat with other effects producing outlet glacier speedups an order of magnitude larger seasonal melt s influence on ice flow is likely confined to those regions dominated by ice sheet flow

recent changes on greenland outlet glaciers aircraft laser altimeter surveys during the 1990s showed near coastal parts of the greenland ice sheet to be thinning despite slow thickening at higher elevations the ice sheet lost mass to the ocean many outlet glaciers thinned more rapidly than could be explained by increased melting during the recent warmer summers indicating dynamic imbalance between glacier velocity and upstream snow accumulation results from more recent surveys presented here show that thinning rates have increased in most coastal regions for almost half of the surveys these increases might have resulted from increases in summer melting but rapid thinning on others is indicative of dynamic changes that increased with time in particular thinning rates on the three fastest glaciers increased to tens of m a 1 after 2000 and other observations show an approximate doubling in their velocities the deep beds of these glaciers appear to have a strong influence on rates of grounding line retreat and thickness change with periods of glacier acceleration and rapid thinning initiated by flotation and break up of lightly grounded glacier snouts or break up of floating ice tongues near simultaneous thinning of these widely separated glaciers suggests that warming of deeper ocean waters might be a common cause nearby glaciers without deep beds are thinning far more slowly suggesting that basal lubrication as a result of increased surface melting has only a marginal impact on greenland outlet glacier acceleration

automated remote sensing of sediment plumes for identification of runoff from the greenland ice sheet the viability of employing sediment plumes emanating from outlets along the western margin of the greenland ice sheet as indicators of runoff is assessed an automated sediment plume quantification system based on daily 250 m moderate resolution imaging spectroradiometer modis band 1 reflectance imagery is developed coherent plumes are identified using spectral thresholds and polygon tracing validation employs imagery quality control procedures and manual verification of plume areas outlets at land terminating margins with wide and straight fjord geometries deliver the most accurate and consistent results plume area observations are also possible at marine terminating margins with relatively static fronts and low proximal sea ice concentrations variability in plume area is examined with reference to special satellite microwave imager ssm i derived daily melt extent at the hydrologic catchment scale at annual timescales plume areas tend to co vary with surface melt extent indicating that more mass is lost by runoff during years of extensive melting some synchronicity in plume areas from different catchments is apparent at seasonal and daily timescales plumes from individual outlets primarily relate to catchment specific melting

an improved mass budget for the greenland ice sheet extensive ice thickness surveys by nasa s operation icebridge enable over a decade of ice discharge measurements at high precision for the majority of greenland s marine terminating outlet glaciers prompting a reassessment of the temporal and spatial distribution of glacier change annualmeasurements for 178 outlet glaciers reveal that despite widespread acceleration only 15 glaciers accounted for 77 of the 739 29 gt of ice lost due to acceleration since 2000 and four accounted for 50 among the top sources of loss are several glaciers that have received little scientific attention the relative contribution of ice discharge to total loss decreased from 58 before 2005 to 32 between 2009 and 2012 as such 84 of the increase in mass loss after 2009 was due to increased surface runoff these observations support recentmodel projections that surface mass balance rather than ice dynamics will dominate the ice sheet s contribution to 21st century sea level rise

north atlantic warming and the retreat of greenland s outlet glaciers mass loss from the greenland ice sheet quadrupled over the past two decades contributing a quarter of the observed global sea level rise increased submarine melting is thought to have triggered the retreat of greenland s outlet glaciers which is partly responsible for the ice loss however the chain of events and physical processes remain elusive recent evidence suggests that an anomalous inflow of subtropical waters driven by atmospheric changes multidecadal natural ocean variability and a long term increase in the north atlantic s upper ocean heat content since the 1950s all contributed to a warming of the subpolar north atlantic this led in conjunction with increased runoff to enhanced submarine glacier melting future climate projections raise the potential for continued increases in warming and ice mass loss with implications for sea level and climate

changes in the velocity structure of the greenland ice sheet using satellite radar interferometry observations of greenland we detected widespread glacier acceleration below 66 north between 1996 and 2000 which rapidly expanded to 70 north in 2005 accelerated ice discharge in the west and particularly in the east doubled the ice sheet mass deficit in the last decade from 90 to 220 cubic kilometers per year as more glaciers accelerate farther north the contribution of greenland to sea level rise will continue to increase

sliding with cavity formation we present a model for the determination of a sliding law in the presence of sub glacial cavitation this law determines the basal stress at a clean ice bedrock interf ace in terms of the velocity and effective pressure the method is based on an exact solut ion of the nye kamb linearly viscous sliding problem with cavities and uses ideas of lliboutry 979 to construct via re norma lization methods an approximate law for gene ral bedrock form we show that for a bedrock whose spectrum has a power law behaviour one obtains a sliding law which gives the basal shear stress proportional to a power of the velocity and to a power of the effective pressure the effect of subgl acial cavitation on the drainage system is exam ined using recent ideas of kamb for sufficiently high velocities drainage through a r6thlisberger tunnel system is unstable and drai nage takes place through the linked system of cavities this leads to a reduction of the effective pressure and by taking account of this one can rewrite the sliding law in terms of st ress and velocity only this sliding law can be multi v alued and it is suggested that this unde rlies the dynamic phenomenon of surges

role of the supraglacial snowpack in mediating meltwater delivery to the glacier system as inferred from dye tracer investigations the supraglacial snowpack plays an important role in mediating the delivery of meltwater produced at the snowpack surface to the rest of the glacier system with potential implications for both proglacial hydrograph form and the timing and magnitude of glacier movement induced by changes in subglacial water pressure however there remain few field observations of the hydrological behaviour of supraglacial snowpacks and its seasonal evolution during the melt season data collected during the 2004 melt season at haut glacier d arolla valais switzerland uses both qualitative and quantitative dye tracing techniques to enhance our understanding of supraglacial snowpack hydrology observations of the movement of dye stained water show the complexity of flow patterns and the influence of ice layers and preferential flow zones in controlling percolation through the snowpack and fluorometric techniques yield average flow rates for percolation through the snowpack of between 0 13 and 0 49 m h the changing form of dye return curves and increasing percolation rates reflect an increase over the course of the melt season in the efficiency with whichthesnowpacktransmitsmeltwater snowpackpermeabilitywasfoundtobesignificantlylower 1 67x10 m on average than that assumed in previous modelling studies of glacial hydrology showing the need for improved information about snowpack hydrology if its importance in controlling runoff is to be fully appreciated

rapid submarine melting driven by subglacial discharge leconte glacier alaska 1 we show that subglacial freshwater discharge is the principal process driving high rates of submarine melting at tidewater glaciers this buoyant discharge draws in warm seawater entraining it in a turbulent upwelling flow along the submarine face that melts glacier ice to capture the effects of subglacial discharge on submarine melting we conducted 4 days of hydrographic transects during late summer 2012 at leconte glacier alaska a major rainstorm allowed us to document the influence of large changes in subglacial discharge we found strong submarine melt fluxes that increased from 9 1 1 0 to 16 8 1 3 m d 1 ice face equivalent frontal ablation as a result of the rainstorm with projected continued global warming and increased glacial runoff our results highlight the direct impact that increases in subglacial discharge will have on tidewater outlet systems these effects must be considered when modeling glacier response to future warming and increased runoff

spatially distributed runoff at the grounding line of a large greenlandic tidewater glacier inferred from plume modelling understanding the drivers of recent change at greenlandic tidewater glaciers is of great importance if we are to predict how these glaciers will respond to climatic warming a poorly constrained component of tidewater glacier processes is the near terminus subglacial hydrology here we present a novel method for constraining near terminus subglacial hydrology with application to marine terminating kangiata nunata sermia in south west greenland by simulating proglacial plume dynamics using buoyant plume theory and a general circulation model we assess the critical subglacial discharge if delivered through a single compact channel required to generate a plume that reaches the fjord surface we then compare catchment runoff to a time series of plume visibility acquired from a time lapse camera we identify extended periods throughout the 2009 melt season where catchment runoff significantly exceeds the discharge required for a plume to reach the fjord surface yet we observe no plume we attribute these observations to spatial spreading of runoff across the grounding line persistent distributed drainage near the terminus would lead to more spatially homogeneous submarine melting and may promote more rapid basal sliding during warmer summers potentially providing a mechanism independent of ocean forcing for increases in atmospheric temperature to drive tidewater glacier acceleration

seasonal speedup of a greenland marine terminating outlet glacier forced by surface melt induced changes in subglacial hydrology we present subdaily ice flow measurements at four gps sites between 36 and 72 km from the margin of a marine terminating greenland outlet glacier spanning the 2009 melt season our data show that 35 km from the margin seasonal and shorter time scale ice flow variations are controlled by surface melt induced changes in subglacial hydrology following the onset of melting at each site ice motion increased above background for up to 2 months with resultant up glacier migration of both the onset and peak of acceleration later in our survey ice flow at all sites decreased to below background multiple 1 to 15 day speedups increased ice motion by up to 40 above background these events were typically accompanied by uplift and coincided with enhanced surface melt or lake drainage our results indicate that the subglacial drainage system evolved through the season with efficient drainage extending to at least 48 km inland during the melt season while we can explain our observations with reference to evolution of the glacier drainage system the net effect of the summer speed variations on annual motion is small 1 this in part is because the speedups are compensated for by slowdowns beneath background associated with the establishment of an efficient subglacial drainage system in addition the speedups are less pronounced in comparison to land terminating systems our results reveal similarities between the inland ice flow response of greenland marine and land terminating outlet glaciers

subglacial hydrology at rink isbrae west greenland inferred from sediment plume appearance marine terminating outlet glaciers discharge most of the greenland ice sheet s mass through frontal ablation and meltwater runoff while calving can be estimated by in situ and remote sensing observations submarine melting and subglacial meltwater transport are more challenging to quantify here we investigate the subglacial hydrology of rink isbrae a fast flowing west greenland tidewater glacier using time lapse photography modeled runoff estimates and daily satellite imagery from 2007 to 2011 we find that sediment plumes appear episodically at four distinct locations across the terminus and last between 2 h and 17 d this suggests short term variability in discharge and the existence of persistent pathways the seasonal onset of sediment plumes occurs before supraglacial lake drainages shortly after the onset of runoff and only after the wintertime ice melange has begun disintegrating plumes were also visible after the cessation of runoff 23 5 d which is indicative of subglacial storage the lack of either a seasonal velocity change or a correspondence between meltwater availability and plume occurrence suggests that the subglacial system persists in a state of inefficient drainage subglacial hydrology at tidewater glaciers is of critical importance in understanding dynamics at the ice front

rapid erosion beneath the greenland ice sheet the pleistocene ice sheets left a clear signature of erosion but the rate at which ice sheets erode is difficult to determine from either paleolandscapes or observations of contemporary processes here we use two years of sediment flux data derived from meltwaters emerging from an outlet glacier in west greenland to calculate an average rate of subglacial erosion across a catchment extending 50 km inland from the ice margin erosion in this zone occurs at 4 8 2 6 mm a 1 a rate 1 2 orders of magnitude greater than previous estimates of erosion rate beneath the greenland ice sheet our results suggest that where surface meltwaters are able to access the bed the rate of erosion by ice sheets is in keeping with the rapid erosion observed at temperate alpine glaciers during deglacial phases when meltwater was abundant ice sheet margins should therefore have acted as highly efficient agents of erosion

modeling the impact of glacial runoff on fjord circulation and submarine melt rate using a new subgrid scale parameterization for glacial plumes the injection at depth of ice sheet runoff into fjords may be an important control on the frontal melt rate of tidewater glaciers here we develop a new parameterization for ice marginal plumes within the massachusetts institute of technology general circulation model mitgcm allowing three dimensional simulation of large 500 km2 glacial fjords on annual or longer time scales we find that for an idealized fjord without shelf driven circulation subglacial runoff produces a thin strong and warm down fjord current in the upper part of the water column balanced by a thick and slow up fjord current at greater depth although submarine melt rates increase with runoff due to higher melt rates where the plume is in contact with the ice front we find that annual submarine melt rate across the ice front is relatively insensitive to variability in annual runoff better knowledge of the spatial distribution of runoff controls on melt rate in those areas not directly in contact with plumes and feedback mechanisms linking submarine melting and iceberg calving are necessary to more fully understand the sensitivity of glacier mass balance to runoff driven fjord circulation

modeling thermodynamic ice ocean interactions at the base of an ice shelf abstract models of ocean circulation beneath ice shelves are driven primarily by the heat and freshwater fluxes that are associated with phase changes at the ice ocean boundary their behavior is therefore closely linked to the mathematical description of the interaction between ice and ocean that is included in the code an hierarchy of formulations that could be used to describe this interaction is presented the main difference between them is the treatment of turbulent transfer within the oceanic boundary layer the computed response to various levels of thermal driving and turbulent agitation in the mixed layer is discussed as is the effect of various treatments of the conductive heat flux into the ice shelf the performance of the different formulations that have been used in models of sub ice shelf circulation is assessed in comparison with observations of the turbulent heat flux beneath sea ice formulations that include an explicit parameterization of the oceanic boundary layer give results that

ice dammed lake drainage cools and raises surface salinities in a tidewater outlet glacier fjord west greenland the drainage of ice dammed lakes in the form of outburst floods in greenland is detected regularly by remote sensing and these events are expected to occur more frequently in a warmer climate however their impact on ice sheet stability and neighboring water bodies is still unknown in this interdisciplinary study we investigate lake drainages from the greenland ice sheet into a west greenland fjord by analyzing simultaneous time series of satellite observations and direct hydrographic measurements of temperature and salinity in the fjord satellite images show that in general lake drainages have occurred quasiperiodically during the last decade a particular sequence of drainage events was observed by satellite in 2009 and was analyzed together with the first direct hydrographic observations signs of ice dammed lake drainages were observed by a downstream mooring located just below the intertidal zone the release of freshwater occurred at the fjord subsurface at a tidewater outlet glacier the downstream in water sequence of property changes in relation to these drainage events was observed as an almost immediate decrease in surface layer temperature 2 c followed within a week by the arrival of a high saline pulse 5 units with elevated salinity lasting for several days during the passage during lake drainages large amounts of relatively warm and saline intermediate water are brought to the near surface layers by entrainment processes near the glacier front and this influences the hydrography of the fjord but also impacts the ecosystem through upwelling of nutrient rich intermediate water

rapid large area mapping of ice flow using landsat 8 abstract we report on the maturation of optical satellite image based ice velocity mapping over the ice sheets and large glacierized areas enabled by the high radiometric resolution and internal geometric accuracy of landsat 8 s operational land imager oli detailed large area single season mosaics and time series maps of ice flow were created using data spanning june 2013 to june 2015 the 12 bit radiometric quantization and 15 m pixel scale resolution of oli band 8 enable displacement tracking of subtle snow drift patterns on ice sheet surfaces at 1 m precision ice sheet and snowfield snow drift features persist for typically 16 to 64 days and up to 432 days depending primarily on snow accumulation rates this results in spatially continuous mapping of ice flow extending the mapping capability beyond crevassed areas our method uses image chip cross correlation and sub pixel peak fitting in matching landsat path row pairs high pass filtering is applied to the imagery to enhance local surface texture the current high image acquisition rates of landsat 8 725 scenes per day globally reduces the impact of high cloudiness in polar and mountain terrain and allows rapid compilation of large areas or dense temporal coverage of seasonal ice flow variations the results rival the coverage and accuracy of interferometric synthetic aperture radar insar mapping

mechanical and hydrologic basis for the rapid motion of a large tidewater glacier 1 observations the data presented in part 1 of this paper meier et al this issue are here used to assess the role of water input output water storage and basal water pressure in the rapid movement of columbia glacier alaska consistently high basal water pressures mostly in the range from 300 kpa below to 100 kpa above the ice overburden pressure are responsible in an overall way for the high glacier flow velocities 3 5 9 m d 1 which are due mainly to rapid basal sliding caused by the high water pressure diurnal fluctuation in basal water pressure is accompanied by fluctuation in sliding velocity in what appears to be a direct causal relation at the upglacier observation site the water pressure fluctuation tracks the time integrated water input less a steady withdrawal as expected for the diurnally fluctuating storage of water in the glacier far from the terminus at the downglacier site the situation is more complex diurnal peaks in water level which are directly related to intraglacial water storage as well as to basal water pressure are shifted forward in time by 4 hours probably as a result of the effect of diurnal fluctuation in water output from the glacier which affects the local water storage fluctuations near the terminus because of the forward shift in the basal water pressure peaks which at the downglacier site lead the velocity peaks by 6 hours a mechanical connection between water pressure and sliding there would have to involve a 6 hour quarter period delay however the nearly identical nature of the diurnal fluctuations in velocity at the two sites argues for a single consistent control mechanism at both sites the velocity variations in nondiurnal speed up events caused by extra input of water on the longer timescale of several days are only obscurely if at all correlated with variations in basal water pressure but correlate well with water storage in the glacier it appears that small variations in water pressure 100 kpa sufficient to produce the observed velocity variations 15 30 are mostly masked by pressure fluctuations caused by spontaneous local reorganizations of the basal water conduit system on a spatial scale much smaller than the longitudinal coupling length over which basal water pressure is effectively averaged in determining the sliding velocity at the achieved level of observation the clearest though not complication free control variable for the sliding velocity variations is basal water storage by cavitation at the glacier bed

heat sources for glacial melt in a sub arctic fjord godthabsfjord in contact with the greenland ice sheet 1 recent warming of subpolar mode water off greenland has been suggested to accelerate the mass loss from tidal outlet glaciers of the greenland ice sheet we present a comprehensive analysis of water masses dynamics and interannual hydrographic variability in godthabsfjord a sill fjord in contact with tidal outlet glaciers on the west coast of greenland through seasonal observations we recognize an intermediate baroclinic circulation mode driven by tidal currents and an associated important local heat source for the fjord during summer this results in significant warming and freshening of the intermediate layer of the main fjord and the increase in heat content is equivalent to melting of 2 1 km3 of glacial ice this is comparable to 8 km3 glacial ice discharge estimated from the kangiata nun ta sermia calving front per year during winter the external heat source in the west greenland current enters the fjord as intermittent inflows of either cold 2 c dense water in pulses of 1 to 3 months duration four distinct circulation modes are observed in the fjord of which all can contribute to glacial ice melt an important aspect of the ice distribution in the fjord is that only a minor fraction is exported out of the fjord

effect of near terminus subglacial hydrology on tidewater glacier submarine melt rates submarine melting of greenlandic tidewater glacier termini is proposed as a possible mechanism driving their recent thinning and retreat we use a general circulation model mitgcm to simulate water circulation driven by subglacial discharge at the terminus of an idealized tidewater glacier we vary the spatial distribution of subglacial discharge emerging at the grounding line of the glacier and examine the effect on submarine melt volume and distribution we find that subglacial hydrology exerts an important control on submarine melting under certain conditions a distributed system can induce a factor 5 more melt than a channelized system with plumes from a single channel inducing melt over only a localized area subglacial hydrology also controls the spatial distribution of melt which has the potential to control terminus morphology and calving style our results highlight the need to constrain near terminus subglacial hydrology at tidewater glaciers if we are to represent ocean forcing accurately

short term velocity variations on hansbreen a tidewater glacier in spitsbergen spatial and temporal variations of the flow of hansbreen a tidewater glacier in southern spitsbergen svalbard are investigated during summer 1999 surface flow velocities were measured in the ablation zone of hansbreen with a temporal reso lution of 3 4 hours short events with strongly increased surface velocities and a typical duration of 1 2 days were observed these speed up events are related to periods of strongly increased water input to the glacier due to rainfall or enhanced surface melt a close relation is found between the surface velocities and water pressure recorded in a moulin however there are indications from a short time lag between velocity and water pressure peak as well as from observed vertical surface uplifts that basal motion is related to basal water storage rather than directly to basal water pressure the observed short term velocity variations and associated processes on hansbreen are very similar to those observed on land based valley glaciers and suggest that the relevant mechanisms and physical processes that control the flow and its temporal variations are similar in contrast to the flow of land based glaciers sliding velocities on hansbreen are observed to be high all year round and velocities increase towards the calving front

seasonal variability of the circulation system in a west greenland tidewater outlet glacier fjord godthabsfjord 64 n many tidewater outlet glacier fjords surround the coast of greenland and their dynamics and circulation are of great importance for understanding the heat transport toward glaciers from the ice sheet thus fjord circulation is a critical aspect for assessing the threat of global sea level rise due to melting of the ice sheet however very few observational studies describe the seasonal dynamics of fjord circulation here we present the first continuous current measurements april november from a deep mooring deployed in a west greenland tidewater outlet glacier fjord four distinct circulation phases are identified during the period and they are related to exchange processes with coastal waters tidal mixing and melt processes on the greenland ice sheet during early summer warm intermediate water is transported toward the glacier at an average velocity of about 7 cm s 1 in late summer the average velocity decreases to 3 cm s 1 during a period with significant subglacial freshwater discharges during this period a large variability in current velocities is also observed the associated average heat transport in an intermediate depth range corresponds to 568 gw in early summer and is reduced to 287 gw in late summer these heat fluxes are at the higher end of previously reported fluxes our measurements show that the intermediate heat transport varies over time and during summer provides a major contribution to the heat budget and thereby potentially to glacial melt we suggest that intermediate heat transport may play a similar important role in other fjords around greenland

acceleration of jakobshavn isbrae triggered by warm subsurface ocean waters observations over past decades show a sudden switch of jakobshavn isbrae a large outlet glacier feeding a deep ocean fjord on greenland s west coast from slow thickening to rapid thinning in 1997 this switch is associated with a doubling in glacier velocity hydrographic data show a concurrent sudden increase in subsurface ocean temperatures along the entire west coast of greenland suggesting that the changes in jakobshavn isbrae were triggered by the arrival of relatively warm water originating from the irminger sea observations over the past decades show a rapid acceleration of several outlet glaciers in greenland and antarctica1 one of the largest changes is a sudden switch of jakobshavn isbrae a large outlet glacier feeding a deep ocean fjord on greenland s west coast from slow thickening to rapid thinning2 in 1997 associated with a doubling in glacier velocity3 suggested explanations for the speed up of jakobshavn isbrae include increased lubrication of the ice bedrock interface as more meltwater has drained to the glacier bed during recent warmer summers4 and weakening and break up of the floating ice tongue that buttressed the glacier5 here we present hydrographic data that show a sudden increase in subsurface ocean temperature in 1997 along the entire west coast of greenland suggesting that the changes in jakobshavn isbrae were instead triggered by the arrival of relatively warm water originating from the irminger sea near iceland we trace these oceanic changes back to changes in the atmospheric circulation in the north atlantic region we conclude that the prediction of future rapid dynamic responses of other outlet glaciers to climate change will require an improved understanding of the effect of changes in regional ocean and atmosphere circulation on the delivery of warm subsurface waters to the periphery of the ice sheets

seasonal changes in ice sheet motion due to melt water lubrication abstract a numerical model is used to calculate how the motion of an idealized ice sheet margin is affected by the subglacial drainage of melt water from its surface the model describes the evolution of the drainage system and its coupling with ice flow through a sliding law that depends on the effective pressure the results predict ice acceleration during early summer when the inefficient drainage system is temporarily overwhelmed the growth of a more efficient drainage system leads to a subsequent slowdown of the ice very close to the margin but high water pressure and ice velocity are maintained through much of the summer further inland annual mean ice velocity increases with the total quantity of melt water except close to the margin where it is almost insensitive to the amount of melting short term variability of melt water input leads to rapid changes in ice velocity that result in a slight increase in the mean velocity relative to a smoother input linked cavity and poroelastic models for the distributed drainage system are compared and their relative merits discussed two different sliding laws are considered and the need for a holistic description of hydraulically controlled sliding is highlighted

seasonal variability in the dynamics of marine terminating outlet glaciers in greenland recent studies indicate that the dynamics of fast flowing marine terminating outlet glaciers of the greenland ice sheet may be sensitive to climate and ocean forcing on sub annual timescales observations of seasonal behavior of these glaciers at such high temporal resolution however are currently few here we present observations of front position flow speed near surface air temperature and ocean conditions for six large marine terminating glaciers in the uummannaq region of west greenland to investigate controls on short term glacier dynamics as proposed by other studies we find that seasonal front advance and retreat correlates with the formation and disappearance of an ice melange our data suggest that high sea surface temperature anomalously low sea ice concentration and reduced melange formation in early 2003 have triggered multi year retreat of several glaciers in the study area which is consistent with other regions in greenland of the stable glaciers only rink isbrae exhibits a seasonal speed variation that correlates with variations in front position with the others undergoing mid summer deceleration that indicates the effects of subglacial meltwater discharge and drainage system evolution drainage of supraglacial lakes and water filled crevasses results in substantial decreases in speed 40 60 on fast flowing glaciers our results demonstrate that attempts to model ice sheet evolution must take into account short timescale flow dynamics resulting from drainage events and oceanographic conditions

deformation of till beneath ice stream b west antarctica the behaviour and possible instability of the west antarctic ice sheet depend fundamentally on the dynamics of the large ice streams which drain it model calculations show that most ice stream velocity arises at the bed1 2 and radar sounding has shown the bed to be wet3 but the basal boundary condition is not well understood seismic evidence from the upstream b camp upb on the siple coast of west antarctica4 shows that the ice stream there rests on a layer of unconsolidated sediment averaging 5 or 6 m thick in which the water pressure is only 50 kpa less than the overburden pressure because this thin layer occurs well inland beneath an active ice sheet and rests on a surface showing flutes4 characteristic of glacial erosion5 we presume that it is glacial till we propose here that deformation within the till is the primary mechanism by which the ice stream moves and we discuss implications of this hypothesis

modeling of snow and ice melt at eth camp west greenland a study of surface albedo 1 the objective of this paper is to present the validation over greenland of a thermodynamic snow ice model that was complemented with the snow metamorphism and albedo parameterizations of the centre d etudes de la neige grenoble france in order to make the surface albedo variable and interactive special attention is given to the surface albedo since it is the most important parameter in energy exchanges with the atmosphere for snow and ice melt the development of an integrated snow ice and water albedo model takes into account the different surface types observed on an ice sheet and the snow albedo is calculated from the simulated surface snow grains the validation for a polar site has been done at eth camp west greenland 1155 m above sea level during the 1990 and 1991 summer seasons although both ablation seasons differed greatly in 1990 it showed a negative mass balance while in 1991 it ended with a positive mass balance a single model configuration was able to provide good results for both 1990 and 1991 these simulations show that the snow metamorphism laws included enable an accurate simulation of the surface albedo and henceforth of the surface mass balance for a polar site provided that the snow model is correctly forced at its surface level and that meltwater retention percolation and drainage are well represented in the snow model the model results are also compared with two other modeling approaches and differences between the three snow models are detailed

undercutting of marine terminating glaciers in west greenland marine terminating glaciers control most of greenland s ice discharge into the ocean but little is known about the geometry of their frontal regions here we use side looking multibeam echo sounding observations to reveal that their frontal ice cliffs are grounded deeper below sea level than previously measured and their ice faces are neither vertical nor smooth but often undercut by the ocean and rough deep glacier grounding enables contact with subsurface warm salty atlantic waters aw which melts ice at rates of meters per day we detect cavities undercutting the base of the calving faces at the sites of subglacial water sgw discharge predicted by a hydrological model the observed pattern of undercutting is consistent with numerical simulations of ice melt in which buoyant plumes of sgw transport warm aw to the ice faces glacier undercutting likely enhances iceberg calving impacting ice front stability and in turn the glacier mass balance

thick sediments beneath greenland s ablation zone and their potential role in future ice sheet dynamics the geological nature of glacier beds plays a key role in ice sheet dynamics whereas little is known about greenland s subglacial geology the presence of basal sediments is a necessary condition for fast flowing antarctic ice streams such sediments sustain subglacial till layers which if water saturated and under high pore water pressure provide little resistance to ice flow using receiver function modeling of teleseismic p waves we report a thick at least tens of meters sediment layer beneath a site in greenland s ablation zone 15 km away from the western ice sheet margin although we do not discuss the origin or detailed nature of these subglacial sediments we suggest that they are capable of sustaining a till layer due to the prevalence of an inefficient pressurized subglacial drainage system this till layer would typically be under high pore pressures and fail at the shear stresses transmitted from the overlaying ice ice flow resistance is thus focused on regions where till is consolidated or absent or where form drag over obstacles takes place in contrast to antarctica greenland s surface melt now affects practically all latitudes and efficient hydraulic connections between ice sheet surface and bed are pervasive throughout the ablation zone this implies that rapid melt induced changes in subglacial water pressure are possible the time required for basal till strength to react to these changes depends on the till s properties we estimate that formation and destruction of flow resistance can occur on time scales of less than a few years this could lead to changes in ice flow that are currently difficult to predict

shear wave velocity structure of the shallow sediments in the bohai sea from an ocean bottom seismometer survey abstractto investigate the seismic velocity structure of the shallow sediments in the bohai sea of china we conducted a shear wave velocity inversion of the surface wave dispersion data from a survey of 12 ocean bottom seismometers obss and 377 shots of a 9000 in3 air gun with obs station spacing of approximately 5 km and air gun shot spacing of approximately 190 m high quality scholte wave data were recorded by the obss within 0 4 5 km offset we retrieved the scholte wave phase velocity dispersion for the fundamental mode and first overtone in the frequency band of 0 9 3 0 hz with the phase shift method and inverted for the shear wave velocity structure of the shallow sediments with a damped iterative least squares algorithm pseudo 2d shear wave velocity profiles with a depth of approximately 400 m revealed coherent features of relatively weak lateral velocity variation we also estimated the uncertainty in shear wave velocity structure based on the pseudo 2d profiles from six trial inversions wi

heat sources for glacial ice melt in a west greenland tidewater outlet glacier fjord the role of subglacial freshwater discharge the melting of tidewater outlet glaciers from the greenland ice sheet contributes significantly to global sea level rise accelerated mass loss is related to melt processes in front of calving glaciers yet the role of ocean heat transports is poorly understood here we present the first direct measurements from a subglacial plume in front of a calving tidewater outlet glacier surface salinity in the plume corresponded to a meltwater content of 7 which is indicative of significant entrainment of warm bottom water and according to plume model calculations significant ice melt energy balance of the area near the glacier showed that ice melt was mainly due to ocean heat transport and that direct plume associated melt was only important in periods with high meltwater discharge rates of 100 m3 s 1 ocean mixing outside of the plume area was thus the primary heat source for melting glacier ice

water flow through temperate glaciers understanding water movement through a glacier is fundamental to several critical issues in glaci ology including glacier dynamics glacier induced floods and the prediction of runoff from glacierized drainage basins to this end we have synthesized a conceptual model of water movement through a temper ate glacier from the surface to the outlet stream pro cesses that regulate the rate and distribution of water input at the glacier surface and that regulate water movement from the surface to the bed play important but commonly neglected roles in glacier hydrology where a glacier is covered by a layer of porous perme able firn the accumulation zone the flux of water to the glacier interior varies slowly because the firn tempo rarily stores water and thereby smooths out variations in the supply rate in the firn free ablation zone in con trast the flux of water into the glacier depends directly on the rate of surface melt or rainfall and therefore varies greatly in time water moves from the surface to the bed through an upward branching arborescent net work consisting of both steeply inclined conduits formed by the enlargement of intergranular veins and gently inclined conduits spawned by water flow along the bottoms of near surface fractures crevasses engla cial drainage conduits deliver water to the glacier bed at a limited number of points probably a long distance downglacier of where water enters the glacier englacial conduits supplied from the accumulation zone are quasi steady state features that convey the slowly varying water flux delivered via the firn their size adjusts so that they are usually full of water and flow is pressurized in contrast water flow in englacial conduits supplied from the ablation area is pressurized only near times of peak daily flow or during rainstorms flow is otherwise in an open channel configuration the subglacial drainage system typically consists of several elements that are distinct both morphologically and hydrologically an up glacier branching arborescent network of channels in cised into the basal ice conveys water rapidly much of the water flux to the bed probably enters directly into the arborescent channel network which covers only a small fraction of the glacier bed more extensive spatially is a nonarborescent network which commonly includes cav ities gaps between the glacier sole and bed channels incised into the bed and a layer of permeable sediment the nonarborescent network conveys water slowly and is usually poorly connected to the arborescent system the arborescent channel network largely collapses during winter but reforms in the spring as the first flush of meltwater to the bed destabilizes the cavities within the nonarborescent network the volume of water stored by a glacier varies diurnally and seasonally small temper ate alpine glaciers seem to attain a maximum seasonal water storage of 200 mm of water averaged over the area of the glacier bed with daily fluctuations of as much as 20 30 mm the likely storage capacity of subglacial cavities is insufficient to account for estimated stored water volumes so most water storage may actually occur englacially stored water may also be released abruptly and catastrophically in the form of outburst floods

direct observations of evolving subglacial drainage beneath the greenland ice sheet simultaneous observations of moulins and boreholes in western greenland show that water delivery to the base of the ice sheet via moulins affects short term ice velocity fluctuations but not late season ice velocity decelerations increased meltwater delivery to the base of the greenland ice sheet will increase the ice sheet velocity accelerating its inevitable rush towards the ocean and subsequent sea level rise or will it debate on this topic is at the forefront of cryospheric research but has been hampered by the lack of simultaneous observations of hydraulic head in moulins vertical shafts that deliver water to the base of the ice sheet and boreholes which monitor basal water pressure lauren andrews and colleagues now provide these observations from a small region in western greenland and reveal that water delivery by moulins into channelized basal flow is indeed linked to short term fluctuations in ice velocity late season decelerations in ice velocity however seem to be controlled by changes in unchannelized flow rather than any shifts in the moulin system seasonal acceleration of the greenland ice sheet is influenced by the dynamic response of the subglacial hydrologic system to variability in meltwater delivery to the bed1 2 via crevasses and moulins vertical conduits connecting supraglacial water to the bed of the ice sheet as the melt season progresses the subglacial hydrologic system drains supraglacial meltwater more efficiently1 2 3 4 decreasing basal water pressure4 and moderating the ice velocity response to surface melting1 2 however limited direct observations of subglacial water pressure4 5 6 7 mean that the spatiotemporal evolution of the subglacial hydrologic system remains poorly understood here we show that ice velocity is well correlated with moulin hydraulic head but is out of phase with that of nearby 0 3 2 kilometres away boreholes indicating that moulins connect to an efficient channelized component of the subglacial hydrologic system which exerts the primary control on diurnal and multi day changes in ice velocity our simultaneous measurements of moulin and borehole hydraulic head and ice velocity in the paakitsoq region of western greenland show that decreasing trends in ice velocity during the latter part of the melt season cannot be explained by changes in the ability of moulin connected channels to convey supraglacial melt instead these observations suggest that decreasing late season ice velocity may be caused by changes in connectivity in unchannelized regions of the subglacial hydrologic system understanding this spatiotemporal variability in subglacial pressures is increasingly important because melt season dynamics affect ice velocity beyond the conclusion of the melt season8 9 10

scalings for submarine melting at tidewater glaciers from buoyant plume theory abstractrapid dynamic changes at the margins of the greenland ice sheet synchronous with ocean warming have raised concern that tidewater glaciers can respond sensitively to ocean forcing understanding of the processes encompassing ocean forcing nevertheless remains embryonic the authors use buoyant plume theory to study the dynamics of proglacial discharge plumes arising from the emergence of subglacial discharge into a fjord at the grounding line of a tidewater glacier deriving scalings for the induced submarine melting focusing on the parameter space relevant for high discharge tidewater glaciers the authors suggest that in an unstratified fjord the often quoted relationship between total submarine melt volume and subglacial discharge raised to the power is appropriate regardless of plume geometry provided discharge lies below a critical value in these cases it is then possible to formulate a simple equation estimating total submarine melt volume as a function of discharge fjord temperature

the 1958 2009 greenland ice sheet surface melt and the mid tropospheric atmospheric circulation in order to assess the impact of the mid tropospheric circulation over the greenland ice sheet gris on surface melt as simulated by the regional climate model mar an automatic circulation type classification ctc based on 500 hpa geopotential height from reanalyses is developed general circulation correlates significantly with the surface melt anomalies for the summers in the period 1958 2009 the record surface melt events observed during the summers of 2007 2009 are linked to the exceptional persistence of atmospheric circulations favouring warm air advection the ctc emphasizes that summer 500 hpa circulation patterns have changed since the beginning of the 2000s this process is partly responsible for the recent warming observed over the gris

seasonal variability of submarine melt rate and circulation in an east greenland fjord 1 the circulation in a glacial fjord driven by a large tidewater glacier is investigated using a nonhydrostatic ocean general circulation model with a melt rate parameterization at the vertical glacier front the model configuration and water properties are based on data collected in sermilik fjord near helheim glacier a major greenland outlet glacier the approximately two layer stratification of the fjord s ambient waters causes the meltwater plume at the glacier front to drive a double cell circulation with two distinct outflows one at the free surface and one at the layers interface in summer the discharge of surface runoff at the base of the glacier subglacial discharge causes the circulation to be much more vigorous and associated with a larger melt rate than in winter the simulated double cell circulation is consistent in both seasons with observations from sermilik fjord seasonal differences are also present in the vertical structure of the melt rate which is maximum at the base of the glacier in summer and at the layers interface in winter simulated submarine melt rates are strongly sensitive to the amount of subglacial discharge to changes in water temperature and to the height of the layers they are also consistent with those inferred from simplified one dimensional models based on the theory of buoyant plumes our results also indicate that to correctly represent the dynamics of the meltwater plume care must be taken in the choice of viscosity and diffusivity values in the model

evolution of drainage system morphology at a land terminating greenlandic outlet glacier 1 the influence of meltwater on the dynamics and geomorphic impact of the greenland ice sheet is strongly controlled by the morphology of the ice sheet s drainage system however this system and its evolution through the melt season remain poorly understood here we present the results of an intensive programme of dye tracing experiments undertaken along the lower 14 km of a land terminating greenlandic outlet glacier over a period of four months during the 2010 melt season these data are interpreted in conjunction with observations of proglacial discharge englacial water storage surface melt rates and ice velocity to produce a detailed picture of the changing hydrology of the glacier following the onset of melt in the spring inputs to the drainage system regularly exceed outputs causing the englacial water level to rise to the ice sheet surface during this time there is a rapid transition from distributed to channelized drainage in those parts of the drainage system closed by ice deformation over winter as the melt season progresses channel efficiency increases and englacial storage and ice velocity decrease high velocity events continue to be observed following the channelization of the drainage system however indicating that hydrological forcing of ice velocity occurs despite the existence of channels during periods when meltwater inputs exceed the capacity of the subglacial drainage system

seasonal evolution of subglacial drainage and acceleration in a greenland outlet glacier an acceleration of ice mass loss has been observed near the margin of the greenland ice sheet partly as a result of faster ice motion observations by gps receivers reveal high seasonal variability in ice motion with summer motion up to 220 higher than winter background levels the greenland ice sheet contains enough water to raise sea levels by 7 m however its present mass balance and future contribution to sea level rise is poorly understood1 accelerated mass loss has been observed near the ice sheet margin partly as a result of faster ice motion2 3 4 surface melt waters can reach the base of the ice sheet and enhance basal ice motion5 6 however the response of ice motion to seasonal variations in meltwater supply is poorly constrained both in space and time here we present ice motion data obtained with global positioning system receivers located along a 35 km transect at the western margin of the greenland ice sheet throughout a summer melt season our measurements reveal substantial increases in ice velocity during summer up to 220 above winter background values these speed up events migrate up the glacier over the course of the summer the relationship between melt and ice motion varies both at each site throughout the melt season and between sites we suggest that these patterns can be explained by the seasonal evolution of the subglacial drainage system similar to hydraulic forcing mechanisms for ice dynamics that have been observed at smaller glaciers

on the seasonal freshwater stratification in the proximity of fast flowing tidewater outlet glaciers in a sub arctic sill fjord 1 the greenland ice sheet releases large amounts of freshwater into the fjords around greenland and many fjords are in direct contact with the ice sheet through tidewater outlet glaciers here we present the first seasonal hydrographic observations from the inner part of a sub arctic fjord relatively close to and within 4 50 km of a fast flowing tidewater outlet glacier this region is characterized by a dense glacial and sea ice cover freshwater from runoff subglacial freshwater sgfw discharge glacial and sea ice melt are observed above 50 90 m depth during summer sgfw and subsurface glacial melt mixed with ambient water are observed as a layered structure in the temperature profiles below the low saline summer surface layer 1 c intermediate water mass is a significant subsurface heat source for ice melt we analyze the temperature and salinity profiles observed in late summer with a thermodynamic mixing model and determine the total freshwater content in the layer below the summer surface layer to be between 5 and 11 the total freshwater contribution in this layer from melted glacial ice was estimated to be 1 2 while the corresponding sgfw was estimated to be 3 10 the winter measurements in the subsurface halocline layer showed a total freshwater content of about 1 and no significant contribution from sgfw

combined measurements of subglacial water pressure and surface velocity of findelengletscher switzerland conclusions about drainage system and sliding mechanism interpretation des mesures de pression dans les forages et des vitesses superficielles la comparaison avec les modeles suggere que l on a neglige le frottement du aux debris dans les modeles et qu il n existe jamais de chenaux cylindriques rectilignes