measurements and analyses of velocity profiles and frazil ice crystal rise velocities during periods of frazil ice formation in rivers the vertical concentration distribution of frazil ice crystals in a stream during the form ation and gro lth of frazil ice was discussed in a prel imi nary way by gosi nk and osterkamp 1981 thi s paper extends and completes the analysis of buoyant rise velocities of frazil ice crystals and applies the results to an interpretation of measured veloc ity profiles in rivers during frazil ice events additional experimental data are also presented two tillle scales are defined the buoyant time scale tb whi ch represents the time requi red for a frazil crystal to rise huoyantly from the river bottom to the water surface and the diffusive time scale to which represents the time required for a frazil crystal to be transported by turbulence through the derth it is shown that the ratio of the time scales tb td defines the nature of the layering processes in particular if tb to l i11 be layered conversely if tb to l turbulent mixing will proceed faster than buoyant 1 ifti ng and the fl ow ii 11 be well mixed this ratio for frazil particles of di arneter 2 illfll or more corresponds to rul e of thumb velocity criteria developed in non ay and canada to di sti ngui sh 1 ayered frazil i ce l later fl o i frol l we11mixed flow the development of this theory depends in large part upon the deterr lination of tb which depends upon the rise velocity of frazil ice crystals a force ba lance node 1 was developed for the ri se velocity of a frazil crystal field observations during frazil ice formation in goldstream creek and in the chatanika river north of fairbanks are reported including a series of measure ents of the rise velocities of frazil ice crystals typical particle size of frazil ice was about 2 n with a rise velocity of about 10 0 mm s l the agreement of measured rise velo cities with the theoretical model is good considering uncertainties in the drag coefficient and in the deterlllination of frazil crystal sizes under field conditions velocity profiles in the chatanika river and in sol dstream creek duri ng frazil formati on suggest that the time scale ratio may serve as a transition criter ioi1 betvlee l layered frazil ice water flow and we11iilixed flo l this ratio was calculated with the rise velocity of frazil ice crystals arbitrarily chosen to be 0 01 m s i i ntrooucti on observations of ice formation in northern rivers during the freeze up period show that three ice flol regimes are common sheet ice floating frazil ice and a well mixed flow frazil ice crystals and water in the last case if the velocity of the river is sufficiently high open water conditions may persist throughout the winter designers of hydraulic structures such as canals water transportation facilities hydroelectric power structures water intake structures etc may require information on which flow regime to expect under a given set of flow conditions and on the trans ition from an ice cover to open water flow thus there is substantial interest in developing criteria which can be used to predict the conditions that lead to each of these three flovl regimes and the presence or absence of an ice cover it could be argued that the criteria for the transition from floating frazil ice to a well mixed flow should be the same as the criteria for the form ation of an ice cover the reason for this is that if the frazil crystals remain mixed with the flow then they cannot accumulate on the surface to form an ice cover however it is clear that other hydrological aspects of rivers enter into criteria on the pres ence or absence of an ice cover these include the sinuosity channel and slope variability bed rough ness and also the mechani cs of fraz il crystal floc pan and floe interactions osterkamp and gosink in press and changes of these parameters during the growth of an ice cover for many locations rule of thumb flow velocities have been determined which define the transitions from well mixed flow to floating frazil ice and the formation of a coherent ice cover for example in norway wllere rivers are often wide shallow with steep slopes and underlain with rocks or boulders the water surface is usually covered with moving frazil slush if the water velocity is 0 5 and 1 2 m s l carstens 1971 on the saint lawrence waterway in canada layered frazil ice flow may be expected at river velocities 0 8 and 1 0 m s i

frazil ice formation in an ice shelf water plume 1 we present a model for the growth of frazil ice crystals and their accumulation as marine ice at the base of antarctic ice shelves the model describes the flow of buoyant water upward along the ice shelf base and includes the differential growth of a range of crystal sizes frazil ice formation starts when the rising plume becomes supercooled initially the majority of crystals have a radius of similar to0 3 mm and concentrations are below 0 1 g l depending on the ice shelf slope which controls the plume speed frazil crystals increase in size and number typically crystals up to 1 0 mm in radius are kept in suspension and concentrations reach a maximum of 0 4 g l the frazil ice in suspension decreases the plume density and thus increases the plume speed larger crystals precipitate upward onto the ice shelf base first with smaller crystals following as the plume slows down in this way marine ice is formed at rates of up to 4 m yr in some places consistent with areas of observed basal accumulation on filchner ronne ice shelf the plume continues below the ice shelf as long as it is buoyant if the plume reaches the ice front its rapid rise produces high supercooling and the ice crystals attain a radius of several millimeters before reaching the surface similar ice crystals have been trawled at depth north of antarctic ice shelves but otherwise no observations exist to verify these first predictions of ice crystal sizes and volumes

circulation mixing and production of antarctic bottom water abstract the antarctic source of bottom water to the abyssal layer of the world ocean is examined as well as its large scale flow pattern and ultimate entrainment rate into the deep water above we make use of the available high quality station data in the southern ocean to construct bottom maps of neutral density and mean property maps including chlorofluorocarbon cfc for the abyssal layer underneath a selected neutral density surface the maximum density at the sill depth of drake passage is used to distinguish between the voluminous deep water mass that is a continuous component of the antarctic circumpolar current from the relatively denser bottom water originated along the antarctic continental margins based on water density antarctic bottom water aabw is defined here generically to include all volumes of non circumpolar water of antarctic origin over the shelf regime multiple localized sources of specific aabw types contribute to the abyssal layer of the adjacent antarctic basins characteristics of these dense bottom waters reflect closely those observed on the parent shelf water mass spreading paths of newly formed deep and bottom waters over the slope regime and their subsequent oceanic circulation patterns are analyzed on the basis of global property maps for the aabw layer interior mixing and interbasin exchanges of aabw are deduced from mean characteristic curves following the southern streamline of the antarctic circumpolar current outflow and mixing of aabw from the weddell sea to the argentine basin is depicted using density and cfc distributions of two zonal hydrographic lines recirculation and mixing of deep and bottom waters within the weddell gyre are also detailed using a meridional section along the greenwich meridian the strength of all localized sources of aabw combined is estimated by two independent approaches an estimate of the total production rate of aabw is calculated based on the oceanic cfc budget for the aabw layer offshore of the 2500 m isobath the sum of all downslope inputs of well ventilated bottom water types underneath the top isopycnal must account for the measured cfc content in the bottom layer the resulting total aabw production rate is about 8 sv which is a conservative figure that neglects the loss of cfc bearing waters across the top isopycnal in recent years whereas about 9 5 sv is calculated assuming a well mixed bottom layer making use of their transient nature cfc distributions at the top of the aabw layer indicate that more direct and rapid entrainment of cfc rich bottom waters below occurs over localized areas with relatively strong upwelling rates and enhanced vertical mixing a second more ad hoc but independent oceanic mass budget of the bottom layer is also constructed a typical basin wide rate of deep upwelling of 3 10 7 m s 1 requires 10 sv 1 sv 106 m3 s 1 of newly formed aabw to sink down the slope around antarctica we have also formulated a spatial distribution of deep upwelling on the isopycnal at the top of the aabw it is expressed as a combination of wind topographic and turbulent components which in turn are functions of the isopycnal depth bottom depth and bottom layer thickness this non uniform upwelling field yields about 12 sv of aabw exported across the top isopycnal fortuitously the overall average of upward speed at the top isopycnal 3 7 10 7 m s 1 compares well with previous estimates of deep upwelling in the northern basins a series of likely sites for strong vertical entrainment of aabw are clearly identified in the modeled distribution of deep upwelling consonant with the observed cfc distributions on the top isopycnal altogether regions with relatively high upwelling rates 5 10 7 m s 1 occupy only a quarter of the total areal extent of the top isopycnal but they account for as much as 45 of the total vertical transport

frazil dynamics and precipitation in a water column with depth dependent supercooling when seawater becomes supercooled collections of small ice crystals known as frazil ice form and grow a model of frazil ice dynamics is presented that deals explicitly with the buoyant settling of frazil crystals onto an overlying surface this yields further insight into transport associated with the ice pump mechanism whereby ice is melted at depth and transferred to a shallower location as a result of the pressure variation of seawater s freezing temperature the model is applied to a vertical cross section through an ice shelf water plume beneath filchner ronne ice shelf antarctica and helps to elucidate the depth variation in its properties for the first time as well as predicting the precipitation rate of frazil crystals the model predicts that frazil ice should be preferentially located in a narrow layer near the ice shelf base as a result of the maximum supercooling there and an influx of crystals rising under their own buoyancy the deposition of these crystals onto the ice shelf is governed by the balance between crystal rising and turbulent transfer of frazil away from the shelf which is investigated in some detail

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

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

the 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

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

a model of ice shelf ocean interaction with application to the filcher ronne and ross ice shelves a simple analytical model has been developed to study the formation of ice shelf water isw isw is assumed to flow as a buoyancy driven layer underneath the ice shelf a relation between potential temperature and salinity in the isw layer is calculated from the mass and energy balance this temperature salinity relation is shown to depend only on the temperature and the salinity of the source water mass and to be practically independent of entrainment and melt rates the model results are obtained without making any assumptions about entrainment and melt rates the model is in good agreement with observations under the ronne ice shelf and it indicates that isw in the filchner depression is formed from western shelf water wsw with salinity higher than 34 75 practical salinity units such high salinity water is only observed in the ronne depression in the western part of the continental shelf this implies a circulation of wsw under the filchner ronne ice shelf from the ronne depression into the filchner depression similarly the model shows that the isw observed under j9 at the ross ice shelf has been formed from low salinity shelf water lssw from the eastern parts of the ross sea continental shelf lssw must therefore circulate under the eastern parts of the ross ice shelf

force perturbation analysis of recent thinning and acceleration of jakobshavn isbrae greenland observations between 1997 and 2001 of a 30 velocity increase and up to 60 m thinning of downstream parts of jakobshavn isbrae greenland immediately following calving of about 4 km of its 15 km floating ice tongue suggest that acceleration may have been initiated by the calving assuming that the force perturbation associated with such weakening is swiftly transmitted far up glacier i develop equations to estimate the perturbation initially the observed changes are consistent with the comparatively small perturbation associated with the calving thereafter it was probably sustained by thinning of the remaining ice tongue at rates of about 80 m a 1 otherwise the force perturbation would soon have been balanced by reduction in the hydrostatic driving force for longitudinal creep as the glacier thinned with velocities dropping to their former values the calculated force perturbation increases to a maximum about 10 km inland of the grounding line consistent with decreasing weight forces as the glacier thins over bedrock that slopes uphill seawards further inland it progressively decreases probably because marginal drag increased as the glacier accelerated both here and on the floating tongue marginal ice appears to have been softened by the influence of locally intense shear on ice temperature and or fabric more recent observations show continued acceleration and thinning and most of the remaining ice tongue calved away in april 2003 so thinning is likely to continue

simulation of subice shelf melt rates in a general circulation model velocity dependent transfer and the role of friction two parameterizations of turbulent boundary layer processes at the interface between an ice shelf and the ocean beneath are investigated in terms of their impact on simulated melt rates and feedbacks the parameterizations differ in the transfer coefficients for heat and freshwater fluxes in their simplest form they are assumed constant and hence are independent of the velocity of ocean currents at the ice shelf base an augmented melt rate parameterization accounts for frictional turbulence via transfer coefficients that do depend on boundary layer current velocities via a drag law in simulations with both parameterizations for idealized as well as realistic cavity geometries under pine island ice shelf west antarctica significant differences in melt rate patterns between the velocity independent and velocity dependent formulations are found while patterns are strongly correlated to those of thermal forcing for velocity independent transfer coefficients melting in the case of velocity dependent coefficients is collocated with regions of high boundary layer currents in particular where rapid plume outflow occurs both positive and negative feedbacks between melt rates boundary layer temperature velocities and buoyancy fluxes are identified melt rates are found to increase with increasing drag coefficient inline image in agreement with plume model simulations but optimal values of cd inferred from plume models are not easily transferable uncertainties therefore remain both regarding simulated melt rate spatial distributions and magnitudes

a commented translation of the paper by c w correns and w steinborn on crystallization pressure crystallization of salts is recognized to be a major factor of the degradation of porous materials such as stone and concrete on the theoretical side there is now general acceptance about the thermodynamic origin of this phenomenon however on the experimental side there are only scarce quantitative data in this respect the reference work par excellence is that of correns discuss faraday soc 5 267 271 1949 who shows a good correlation between calculated and measured crystallization pressures however concerns about both the thermodynamic derivation and the experimental conditions raise the question about why correns could have obtained such a good correlation this issue is discussed extensively in this paper which is organized as a commented translation of a former paper by correns co authored by steinborn and that is much richer in experimental details

a turbulent bottom boundary layer code for use in numerical ocean models abstract a model for a turbulent bottom boundary slab layer is described the model is designed to fit under a standard depth coordinate ocean general circulation model with a view to improving its response both for local and climate problems the depth of the layer varies temporally and spatially both analytical and numerical versions of the model conserve energy the model is tested using a source of dense water on a slope and performs satisfactorily with the plume spreading far more than the equivalent case without a bottom layer

autonomous underwater vehicles auvs and investigations of the ice ocean interface in antarctic and arctic waters limitations of access have long restricted exploration and investigation of the cavities beneath ice shelves to a small number of drillholes studies of sea ice underwater morphology are limited largely to scientific utilization of submarines remotely operated vehicles tethered to a mother ship by umbilical cable have been deployed to investigate tidewater glacier and ice shelf margins but their range is often restricted the development of free flying autonomous underwater vehicles auvs with ranges of tens to hundreds of kilometres enables extensive missions to take place beneath sea ice and floating ice shelves autosub2 is a 3600 kg 6 7 m long auv with a 1600 m operating depth and range of 400 km based on the earlier autosub1 which had a 500 m depth limit a single direct drive d c motor and five bladed propeller produce speeds of 1 2 m s 1 rear mounted rudder and stern plane control yaw pitch and depth the vehicle has three sections the front and rear sections are free flooding built around aluminium extrusion space frames covered with glass fibre reinforced plastic panels the central section has a set of carbon fibre reinforced plastic pressure vessels four tubes contain batteries powering the vehicle the other three house vehicle control systems and sensors the rear section houses subsystems for navigation control actuation and propulsion and scientific sensors e g digital camera upward looking 300 khz acoustic doppler current profiler 200 khz multibeam receiver the front section contains forward looking collision sensor emergency abort the homing systems argos satellite data and location transmitters and flashing lights for relocation as well as science sensors e g twin conductivity temperature depth instruments multibeam transmitter sub bottom profiler aqualab water sampler payload restrictions mean that a subset of scientific instruments is actually in place on any given dive the scientific instruments carried on autosub are described and examples of observational data collected from each sensor in arctic or antarctic waters are given e g of roughness at the underside of floating ice shelves and sea ice

eddy formation by dense flows on slopes in a rotating fluid properties of the flow generated by a continuous source of dense fluid on a slope in a rotating system are investigated with a variety of laboratory experiments the dense fluid may initially flow down the slope but it turns under the influence of rotation to flow along the slope and initial geostrophic adjustment gives it an anticyclonic velocity profile some of the dense fluid drains downslope in a viscous ekman layer which may become unstable to growing waves provided that the viscous draining is not too strong cyclonic vortices form periodically in the upper layer and the dense flow breaks up into a series of domes three processes may contribute to the formation of these eddies first initial downslope flow of the dense current may stretch columns of ambient fluid by the taylor column process which we term capture secondly the initial geostrophic adjustment implies lower layer collapse which may stretch the fluid column and thirdly viscous drainage will progressively stretch and spin up a captured water column overall this last process may be the most significant but viscous drainage has contradictory effects in that it progressively removes dense lower layer fluid which terminates the process when the layer thickness approaches that of the ekman layer the eddies produced propagate along the slope owing to the combined effects of buoyancy coriolis balance and beta gyres this removes fluid from the vicinity of the source and causes the cycle to repeat the vorticity of the upper layer cyclones increases linearly with l d where l is the rossby deformation radius the bottom slope and d the total depth reaching approximately 2 f in the experiments presented here the frequency at which the eddy dome structures are produced also increases with while the speed at which the structures propagate along the slope is reduced by viscous effects the flow of dense fluid on slopes is a very important part of the global ocean circulation system and the implications of the laboratory experiments for oceanographic flows are discussed

circulation and water masses beneath the northern ronne ice shelf antarctica 1 we present oceanographic data from beneath the northern ronne ice shelf the data were collected during the austral summer of 2002 2003 from four sites located near the ice front in the ronne depression they consist of conductivity temperature depth ctd profiles and time series from moored instruments that vary in length from 9 to 20 weeks a strong tidally modulated inflow of relatively fresh water was found at the eastern margin of the ronne depression this low density inflow powers high basal melt rates that are responsible for a substantially thinned area of ice shelf a northward flow of ice shelf water along the western margin of the depression the antarctic peninsula coast was inferred from the ctd data from the new ctd and current meter data and from published results from cruises along the ice front we suggest that the flows at the margins of the ronne depression establish east west density gradients that drive an anticyclonic circulation within the depression the barotropic component of the circulation forms a gyre of strength 5 105 m3 s 1 and occupies a bowl in the field of water column thickness in the northern portion of the depression all water masses sampled had temperatures below the surface freezing point and are therefore classified as ice shelf water the relatively complex nature of the oceanographic regime in the ronne depression is overlain by a seasonal variability that is hinted at by the available time series probably explaining the apparent absence of inflowing hssw at the time of the measurements

investigations of the mass balance of the southeastern ronne ice shelf antarctica data from the filchner v campaign were used to investigate the mass balance conditions in the southeastern ronne ice shelf ris antarctica radio echo sounding and seismic measurements over this area show a maximum ice thickness of 2000 m close to the grounding line of foundation ice stream the measurements also revealed that the position of this grounding line is 40 km further south than previously thought new mass flux calculations result in an estimate or 51 km 3 a 1 for the ice stream transport from the ice sheet into the eastern ice shelf the mollereisstrom mes west of foundation ice stream shows a maximum ice thickness of 1100 1200 m in the grounding line area and a mass flux of 23 km 3 a 1 assuming steady state conditions mass balance calculations based on the new data result in a mean melt rate of about 1 m a 1 at the ice shelf base for the entire southeastern part of the ris the melt rate in the grounding line area of foundation ice stream exceeds 9 m a 1 in contrast other ice streams draining into the filchner ronne ice shelf show maximum melt rates from 1 2 m a 1 mes to 4 m a 1 rutford ice stream our calculations indicate that nearly all of the ice deposited in the drainage area of the eastern ris on the ice sheet does not reach the ice shelf front as original meteoric ice but is melted at the ice shelf base

modeling the effects of frazil ice crystals on the dynamics and thermodynamics of ice shelf water plumes the seawater that comes into contact with the base of a floating ice shelf is modified as a result of the phase changes that occur melting is prevalent in the deepest parts of the subice cavity and this drives a buoyant flow of ice shelf water isw along the sloping ice shelf base the ascent of the isw toward the surface of the ocean causes supercooling because the freezing point rises with the falling pressure and this induces a change from melting to freezing assuming that seed crystals exist the isw now fulfills the condition for the rapid growth of disc shaped frazil ice crystals which may subsequently settle upward out of suspension under the action of gravity a simple numerical model of these processes has been developed based on the theory of inclined plumes the isw is treated as a turbulent particle laden gravity current ascending a reactive boundary and containing a suspended crystal load which evolves in response to the supercooling of the water and the inverted sedimentation of the crystals the frazil ice has two important effects on the behavior of the isw plume because the generation of crystals through out the plume provides such a large area over which phase changes can occur the conversion of supercooling into ice happens much more readily than is possible through turbulent transfer of heat and mass at the ice shelf base a suspended crystal load reduces the bulk density of the isw so that a growing suspension causes the plume to accelerate while deposition of crystals onto the ice shelf effects a deceleration there are positive feedbacks in that an acceleration of the plume induces more rapid crystal growth while a deceleration allows suspended crystals to settle out more easily

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

a numerical study of three dimensional dense bottom plumes on a southern ocean continental slope three dimensional features of dense bottom plumes flowing over continental slopes with or without along slope topographic variations are investigated by simulating the evolution of a density front at a southern ocean continental shelf break using a three dimensional primitive equation numerical model with a second order turbulence closure scheme embedded the focus of our investigation is the role of topography in determining mixing and the offshore transport of dense shelf water during the transient adjustment process of a density front over a continental slope we compare and discuss the numerical simulations for two cases a uniform shelf and slope case and a case with a canyon that leads from the coast to the deep ocean crossing the shelf and the slope the numerical simulations indicate that baroclinic instability planetary rotation bottom friction and topography play major roles in determining the surface and bottom plume formation the growth and penetration depth of the bottom plumes and the characteristics of water mass on the slope mesoscale eddies play a fundamental role in transporting mass heat and salt from shelf to deep ocean the effects of the depth and width of the canyon are examined with two more experiments the presence of a wide and deep canyon in the continental shelf and slope enhances considerably the drainage of coastal shelf water into the deep ocean

basal channels on ice shelves 1 recent surveys of floating ice shelves associated with pine island glacier antarctica and petermann glacier greenland indicate that there are channels incised upward into their bottoms that may serve as the conduits of meltwater outflow from the sub ice shelf cavity the formation of the channels their evolution over time and their impact on ice shelf flow are investigated using a fully coupled ice shelf sub ice shelf ocean model the model simulations suggest that channels may form spontaneously in response to meltwater plume flow initiated at the grounding line if there are relatively high melt rates and if there is transverse to ice flow variability in ice shelf thickness typical channels formed in the simulations have a width of about 1 3 km and a vertical relief of about 100 200 m melt rates and sea water transport in the channels are significantly higher than on the smooth flat ice bottom between the channels the melt channels develop through melting deformation and advection with ice shelf flow simulations suggest that both steady state and cyclic state solutions are possible depending on conditions along the lateral ice shelf boundaries this peculiar dynamics of the system has strong implications on the interpretation of observations the richness of channel morphology and evolution seen in this study suggests that further observations and theoretical analysis are imperative for understanding ice shelf behavior in warm oceanic conditions

temperature and salinity beneath ronne ice shelf antarctica salinity and temperature measurements have been made beneath ronne ice shelf antarctica access to the seawater was gained by hot water drilling through 562 m of ice at a site 300 km from the ice front the ice column is composed of 516 m of meteoric origin ice underlain by 31 m of consolidated saline ice with a further 15 m of unconsolidated slush at the base the 360 m deep water column beneath the ice shelf consists broadly of a 210 m layer of ice shelf water isw at a potential temperature of 2 30 c and salinity 34 53 overlying a 100 m thick layer of modified western shelf water 2 03 c 34 64 the layers are separated by a weak 50 m thick pycnocline isotopic analyses of water samples support this interpretation of the water types comparison of the data with published oceanographic observations from the shore lead indicate that the source of the water is either the western berkner shelf or north of the ice front in the ronne depression the measurements from the drill site broadly match predictions from a simple plume model of the sub ice shelf water circulation in that model the basal slope of the ice shelf plays a central role in determining the properties of the isw plume with the exception of the upper few tens of meters the temperature and salinity of the entire water column vary by about 0 04 c and 0 03 on tidal timescales and there is evidence for a longer term drift presumably connected with shifts in the larger scale circulation there is also much variability from internal wave activity the salinity and deuterium budgets indicate that the meltwater fraction in the isw observed at the drill site was between 5 3 and 6 5 in reasonable agreement with values previously found for isw in the open ocean

the interactions of sea level change terrigenous sediment influx and carbonate productivity as controls on upper cambrian grand cycles of western newfoundland canada the upper cambrian stratal record in western newfoundland is complex and reflects the interplay and feedback between 1 changes in third order sea level 2 episodic incursion of terrigenous fines onto the paleoshelf and 3 productivity levels of the ancient carbonate factory mixed carbonate and terrigenous clastic rocks of western newfoundland are configured in large scale third order stratal patterns called grand cycles such cycles are tens to hundreds of meters in stratigraphic thickness and appear to span millions of years grand cycles are divided here into ribbon half cycles composed principally of ribbon rock and oolitic half cycles dominated by oolite stratigraphic distribution of shale in these rocks however is not everywhere coincident with large and small scale patterns in carbonate lithofacies and it suggests that the incursion of terrigenous clay and silt onto the ancient platform must be evaluated separately in part from the dynamics of carbonate accumulation areal and stratigraphic distribution of rock cycles record shifts in facies belts on the ancient platform presumably in response to third order changes in accommodation potential meter scale cycles in carbonate lithofacies are of two types peritidal cycles that fine upward and shelf cycles that coarsen upward thick successions of shelf cycles composed of ribbon rock are interpreted as highstand deposits and are separated by a sequence boundary marked by peritidal cycles from lowstand deposits comprising thick successions of ootite shelf cycles the former mid grand cycle transitions of previous workers are thereby reinterpreted in this study as sequence boundaries the long term position of sea level and thus the degree of platform inundation influenced the health of the platform carbonate factory during times of low sea level the platform was bathed in water more saline than open ocean sea water and it had a robust non skeletal carbonate factory that was resilient to poisoning by episodic influx of terrigenous fines onto the shelf in contrast the carbonate factory during highstands was less resilient to terrigenous poisoning and was susceptible to localized drowning by the reduction in the rate of carbonate sediment production the thick shale units are therefore interpreted to represent the coincidence of and interaction between 1 incursion of terrigenous fines onto the ancient platform and 2 a weakened highstand carbonate factory unable to recover from this terrigenous poisoning

large scale oceanographic constraints on the distribution of melting and freezing under ice shelves previous studies suggest that ice shelves experience asymmetric melting and freezing topography may constrain oceanic circulation and thus basal melt freeze patterns through its influence on the potential vorticity pv field however melting and freezing induce a local circulation that may modify locations of heat transport to the ice shelf this paper investigates the influence of buoyancy fluxes on locations of melting and freezing under different bathymetric conditions an idealized set of numerical simulations the decoupled simulations employs spatially and temporally fixed diapycnal fluxes these experiments in combination with scaling considerations indicate that while flow in the interior is governed by large scale topographic gradients recirculation plumes dominate near buoyancy fluxes thermodynamically decoupled models are then compared to those in which ice ocean heat and freshwater fluxes are driven by the interior flow the coupled simulations near the southern boundary strong cyclonic flow forced by melt induced upwelling drives inflow and melting to the east recirculation is less evident in the upper water column as shoaling of meltwater freshened layers dissipates the dynamic influence of buoyancy forcing yet freezing remains intensified in the west in coupled simulations the flow throughout the cavity is relatively insensitive to bathymetry stratification the slope of the ice shelf and strong meridionally distributed buoyancy fluxes weaken its influence

a numerical model of the weddell sea large scale circulation and water mass distribution a circumpolar model for regional studies of the wind driven and thermohaline circulation of the southern ocean including the major sub ice shelf areas is described a first series of numerical experiments focusing on the weddell sea reveals a pronounced and persistent double cell structure of the weddell gyre with a maximum transport of 60 sv in agreement with observations experiments with artificial passive tracers point to the shallow shelf areas off the filchner ronne ice shelf as main locations for bottom water production the trajectories of lagrangian floats are used to determine the pathways and timescales of water mass spreading in the model in addition the effect of sub ice shelf forcing on the water mass characteristics is evaluated it is shown that water modified in the sub ice cavities contributes significantly to the deep and bottom water formation along the continental slope and affects the water mass characteristics throughout the weddell sea by increasing the stability of the near surface stratification and preventing deep convection

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

modeling ice ocean interaction in ice shelf crevasses ocean freezing within ice shelf basal crevasses could potentially act as a stabilizing influence on ice shelves however ice ocean interaction and ocean dynamics within these crevasses are as yet poorly understood to this end an idealized 2 d model of an ice shelf basal crevasse has been developed using fluidity a finite element ocean model using an unstructured mesh a simple model of frazil ice formation and deposition has been incorporated into fluidity to better represent the freezing process model results show two different flow regimes dependent on the amount of freezing in the crevasse one driven by freezing at the top of the crevasse and the other by the ingress of meltwater from outside the crevasse in the first freezing at the top of the crevasse leads to the formation of an unstable overturning circulation due to the rejection of dense salty water in the second a buoyant layer is formed along the sides and roof of the crevasse stratifying the water column frazil ice precipitation is found to be the dominant freezing process at the top of the basal crevasse in the freeze driven case with direct freezing being dominant in the melt driven case in both cases melting occurs lower down on the walls of the crevasse due to the strong overturning circulation the freezing in ice shelf crevasses and rifts is found to be highly dependent upon ocean temperature providing a stabilizing influence on ice shelves underlain by cold waters that is not present elsewhere

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

evolution of a supercooled ice shelf water plume with an actively growing subice platelet matrix we use new observations in western mcmurdo sound combined with longitudinal hydrographic transects of the sound to identify a northward flowing ice shelf water isw plume exiting the cavity of the mcmurdo ross ice shelf we estimate the plume s net northward transport at 0 4 0 1 sv carving out a corridor approximately 35 km wide aligned with the victoria land coast basal topography of the mcmurdo ice shelf is such that the plume is delivered to the surface without mixing with overlying warmer water and is therefore able to remain below the surface freezing temperature at the point of observation beneath first year ice thus the upper ocean was supercooled by up to 50 mk at the surface due to pressure relief from recent rapid ascent of the steep basal slope the 70 m thick supercooled layer supports the growth and maintenance of a thick semirigid and porous matrix of platelet ice which is trapped by buoyancy at the ice ocean interface continued growth of individual platelets in supercooled water creates significant brine rejection at the top of the water column which resulted in convection over the upper 200 m thick homogeneous layer by examining the diffusive nature of the intermediate water between layers of isw and high salinity shelf water we conclude that the isw plume must have originated beneath the ross ice shelf and demonstrate that it is likely to expand eastward across mcmurdo sound with the progression of winter

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

the effects of rotation and ice shelf topography on frazil laden ice shelf water plumes a model of the dynamics and thermodynamics of a plume of meltwater at the base of an ice shelf is presented such ice shelf water plumes may become supercooled and deposit marine ice if they rise because of the pressure decrease in the in situ freezing temperature so the model incorporates both melting and freezing at the ice shelf base and a multiple size class model of frazil ice dynamics and deposition the plume is considered in two horizontal dimensions so the influence of coriolis forces is incorporated for the first time it is found that rotation is extremely influential with simulated plumes flowing in near geostrophy because of the low friction at a smooth ice shelf base as a result an ice shelf water plume will only rise and become supercooled and thus deposit marine ice if it is constrained to flow upslope by topography this result agrees with the observed distribution of marine ice under filchner ronne ice shelf antarctica in addition it is found that the model only produces reasonable marine ice formation rates when an accurate ice shelf draft is used implying that the characteristics of real ice shelf water plumes can only be captured using models with both rotation and a realistic topography

application of a transient reduced gravity plume model to the denmark strait overflow a hydrostatic reduced gravity two dimensional primitive equation numerical model for the simulation of transient bottom arrested gravity plumes was developed by adopting an algorithm from a wadden sea model a model that simulates the wetting and drying of mud flats sand banks etc with a movable lateral boundary the vertically integrated nonlinear rotational model accounts for the dissipation of momentum due to bottom friction and for the entrainment of water mass properties from a spatially structured but stagnant ambient water body it predicts the temporal and spatial evolution of the flow field and the water mass modification within a gravity plume that descends on an arbitrary topography previous model investigations considered a laterally integrated streamtube whereas our model resolves the plume horizontally in the streamtube approach it was assumed that the plume descends in a stationary almost geostrophic balance modified only by bottom drag and entrainment the results of this model demonstrate that this balance may be disturbed whenever the plume encounters topographic disturbances which for the initial intruding phase in particular accounts for its highly transient character the model results are in good agreement with theoretical and laboratory investigations and with observations as is demonstrated by some basic test experiments an application to the overflow of dense water through the denmark strait is presented

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

supercooling and energy exchange near the arctic ocean surface conductivity temperature and depth ctd measurements made north of svalbard centered around 83 n 10 e show regions in which the surface waters are supercooled to a depth of about 8 m at other locations warm atlantic waters advected into the area by the west spitzbergen current are melting the sea ice the resulting large horizontal salinity gradients cause intrusive layering with layers at depth sometimes being below their surface freezing point supercooling is explained in terms of the pressure dependence of freezing temperature and the existence of ice keels well below the surface it is noted that the potential heat sink for surface freezing provided by supercooling can be of the same order as that available from heat loss to the atmosphere at the same time an equivalent amount of ice is being melted off the keels thus constituting an ice pump

correction to a model study of ocean circulation beneath filchner ronne ice shelf antarctica implications for bottom water formation by adrian jenkins and david m holland

numerical simulations of the ross sea tides tidal currents below the floating ross ice shelf are reconstructed by using a numerical tidal model they are predominantly diurnal achieve maximum strength in regions near where the ice shelf runs aground and are significantly enhanced by topographic rossby wave propagation along the ice front a comparison with observations of the vertical motion of the ice shelf surface indicates that the model reproduces the diurnal tidal characteristics within 20 similar agreement for the relatively weak semi diurnal tides was not obtained and this calls attention to possible errors of the open boundary forcing obtained from global ocean tidal simulations and to possible errors in mapping zones of ice shelf grounding air sea contact below the ice shelf is eliminated by the thick ice cover the dominant sub ice shelf circulation may thus be tidally induced a preliminary assessment of sub ice shelf conditions based on the numerical tidal simulations suggests that 1 strong barotropic circulation is driven along the ice front and 2 tidal fronts may form in the sub ice shelf cavity where the water column is thin and where the buoyancy input is weak

temperature variability beneath ronne ice shelf antarctica from thermistor cables thermistor cables have been deployed at two sites beneath ronne ice shelf antarctica one site is to the east of a submarine ridge that delineates the eastern boundary of the ronne depression and the other is 100 km to the north above the eastern slope of the depression long records from the cables up to 22 months indicate a large difference in the temperature variability at the two sites being an order of magnitude greater in the ronne depression site 2 although the records appear otherwise similar there is no significant correlation between them the high variability in the site 2 record has allowed the construction of a simple descriptive model of the local oceanographic regime winter freezing in the open water north of the ice front generates western shelf water wsw a type of high salinity shelf water which travels southwest beneath the ice shelf appearing at site 2 as a slope trapped current at the bottom of the water column baroclinic instability in the flow manifests itself in the site 2 temperature record as oscillations on time scales of 5 to 15 days the disturbances cause a periodic east west advection of water masses across the ronne depression site 2 is on the eastern slope of the depression where the wave induced eastward motion forces ice shelf water to rise resulting in periodic ice platelet formation in the water column as surmised from conductivity temperature depth measurements at the site the depth of the wsw layer decreases by 40 to 60 m during a 100 day period starting some 4 months after the beginning of the summer assuming an absence of significant wsw production during the summer the 4 month delay implies a minimum average speed of wsw flow of about 0 02 m s 1 the wsw flux into the ronne depression is estimated at 3 105 m3s 1

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

evidence for basal marine ice in the filchner ronne ice shelf the filchner ronne ice shelf which drains most of the marine based portions of the west antarctic ice sheet is the largest ice shelf on earth by volume the origin and properties of the ice that constitutes this shelf are poorly understood because a strong reflecting interface within the ice and the diffuse nature of the ice ocean interface make seismic and radio echo sounding data difficult to interpret1 2 ice in the upper part of the shelf is of meteoric origin but it has been proposed2 5 that a basal layer of saline ice accumulates from below here we present the results of an analysis of the physical and chemical characteristics of an ice core drilled almost to the bottom of the ronne ice shelf we observe a change in ice properties at about 150 m depth which we ascribe to a change from meteoric ice to basal marine ice the basal ice is very different from sea ice formed at the ocean surface and we propose a formation mechanism in which ice platelets in the water column accrete to the bottom of the ice shelf

outflow of dense water from the storfjord in svalbard a numerical model study recent investigations on deep water renewal point to the important role of dense water formed on the continental shelves surrounding the arctic ocean in this investigation a hydrostatic reduced gravity primitive equation model for the simulation of the spatial and temporal evolution of gravity plumes on a continental slope is applied and compared to the observed outflow of a plume of dense water originally formed in storfjorden svalbard into the greenland sea toward fram strait the vertically integrated nonlinear rotational model accounts for the entrainment of water mass properties from a spatially structured but stagnant ambient water body this study reveals that part of the dense water in accordance with earlier estimates spreads northward along the eastern side of fram strait another branch of the plume guided by the topography flows into a deep trench east of the knipovitch ridge to the west of svalbard during its descent to depths of more than 2000 m the plume entrains three different water masses east spitsbergen water atlantic water and norwegian sea deep water and hence changes its water mass characteristics the volume of deep water produced by the mechanism discussed here depends not only on the amount of initially formed brine enriched shelf water but also on the water mass characteristics of the latter

interannual variability and ventilation timescales in the ocean cavity beneath filchner ronne ice shelf antarctica 1 multiyear time series of ocean current and temperatures from beneath filchner ronne ice shelf antarctica demonstrate both seasonal and interannual variability the seasonal signal is visible at all measurement sites although it was swamped for a 2 year period 1999 2001 when extraordinarily light sea ice cover in the southern weddell sea during the 1997 1998 austral summer caused an anomalously large pulse of high salinity shelf water to flush beneath the ice shelf the pulse was observed twice at an instrumented site near the berkner island coast once on its way to the filchner depression and once after the signal had propagated around the depression and returned to the site as an anomalously large pulse of ice shelf water the timings of the signal allow an estimate of 24 30 months for the flushing timescales of the sub ice shelf ocean cavity indicating that the cavity is highly responsive to external forcing a timescale for the full ventilation of the cavity of 4 5 years is obtained from the length of time the sub ice shelf conditions take to return to their original state a timescale significantly shorter than previous estimates

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

melting and freezing beneath filchner ronne ice shelf antarctica 1 we use remote sensing data sets to evaluate the spatial distribution of melt beneath the filchner ronne ice shelf fris the net melt rate of 83 4 24 8 gtons yr is 2 5 5 times lower than previous glaciological estimates but is similar to existing oceanographic estimates the spatial distribution however differs significantly from standard conceptual and numerical models in which most melt occurs along the grounding lines our results suggest most grounding line melt is refrozen while the dominant ice shelf water isw source is melting near the ice shelf front probably associated with tidal action this suggests that changes in ice shelf extent can impact isw production rates in the weddell sea

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

a model for tidewater glacier undercutting by submarine melting dynamic change at the marine terminating margins of the greenland ice sheet may be initiated by the ocean particularly where subglacial runoff drives vigorous ice marginal plumes and rapid submarine melting here we model submarine melt driven undercutting of tidewater glacier termini simulating a process which is key to understanding ice ocean coupling where runoff emerges from broad subglacial channels we find that undercutting has only a weak impact on local submarine melt rate but increases total ablation by submarine melting due to the larger submerged ice surface area thus the impact of melting is determined not only by the melt rate magnitude but also by the slope of the ice ocean interface we suggest that the most severe undercutting occurs at the maximum height in the fjord reached by the plume likely promoting calving of ice above it remains unclear however whether undercutting proceeds sufficiently rapidly to influence calving at greenland s fastest flowing glaciers

ice shelf water overflow and bottom water formation in the southern weddell sea 1 cold shelf waters flowing out of the filchner depression in the southern weddell sea make a significant contribution to the production of weddell sea bottom water wsbw a precursor to antarctic bottom water aabw we use all available current meter records from the region to calculate the flux of cold water 1 9 c over the sill at the northern end of the filchner depression 1 6 0 5 sv and to determine its fate the estimated fluxes and mixing rates imply a rate of wsbw formation referenced to 0 8 c of 4 3 1 4 sv we identify three pathways for the cold shelf waters to enter the deep weddell sea circulation one path involves flow constrained to follow the shelf break the other two paths are down the continental slope resulting from the cold dense water being steered northward by prominent ridges that cross the continental slope near 36 w and 37 w mooring data indicate that the deep plumes can retain their core characteristics to depths greater than 2000 m probably aided by thermobaricity the plume water at this depth can flow at a speed approaching 1 m s 1 implying that the flow is occasionally supercritical we postulate that such supercriticality acts to limit mixing between the plume and its environment the transition from supercritical to slower more uniform flow is associated with very efficient mixing probably as a result of hydraulic jumps