twenty first century warming of a large antarctic ice shelf cavity by a redirected coastal current the redirection of warm water under the filchner ronne ice shelf during the second half of this century could cause the ice shelf base to melt at a rate 20 times higher than at present warm ocean currents are known to erode ice shelves from below but changes in currents can be forced by many different mechanisms leading to uncertain outcomes this study highlights the vulnerability to climate change of a small antarctic coastal region which has potentially severe consequences for the mass balance of a large antarctic ice shelf hellmer et al use climate modelling to show that the projected loss of sea ice in the weddell sea east of the antarctic peninsula leads to an increase in wind stress which in turn accelerates a warm ocean current far underneath the vast filchner ronne ice shelf the authors predict that the increased warmth could increase melt by a factor of 20 with possible consequences for ice stream dynamics in the east antarctic ice sheet the antarctic ice sheet loses mass at its fringes bordering the southern ocean at this boundary warm circumpolar water can override the continental slope front reaching the grounding line1 2 through submarine glacial troughs and causing high rates of melting at the deep ice shelf bases3 4 the interplay between ocean currents and continental bathymetry is therefore likely to influence future rates of ice mass loss here we show that a redirection of the coastal current into the filchner trough and underneath the filchner ronne ice shelf during the second half of the twenty first century would lead to increased movement of warm waters into the deep southern ice shelf cavity water temperatures in the cavity would increase by more than 2 degrees celsius and boost average basal melting from 0 2 metres or 82 billion tonnes per year to almost 4 metres or 1 600 billion tonnes per year our results which are based on the output of a coupled ice ocean model forced by a range of atmospheric outputs from the hadcm35 climate model suggest that the changes would be caused primarily by an increase in ocean surface stress in the southeastern weddell sea due to thinning of the formerly consolidated sea ice cover the projected ice loss at the base of the filchner ronne ice shelf represents 80 per cent of the present antarctic surface mass balance6 thus the quantification of basal mass loss under changing climate conditions is important for projections regarding the dynamics of antarctic ice streams and ice shelves and global sea level rise

ice pumps and their rates an ice pump is a heat engine driven by the change of freezing point with pressure which will melt ice at depth in the ocean and deposit it at a shallower location it is self starting calculations of the maximum magnitude of this effect are made which show good agreement with field data available for sea and lake ice the discussion is applied to the general case of a moving pack ice sheet with a well mixed surface layer and to floating ice shelves the rate of melt from an 11 m deep pressure ridge keel due to ice pumping is estimated as 26 cm year and that from the front of the ross ice shelf at mcmurdo sound antarctica is estimated as 5 m year for the level of water movement noted in the authors field observations far from the ice front pumping between shelf areas of different thickness will still occur with tidal motion providing the necessary water exchange but its magnitude is now limited by the ability to remove the potentially stable layer of melt water out of the system it is important to realize that the pumping does not depend on the availability of sensible heat in the water column and its effects are additional to any melting caused by the advection of warmer water to the ice water interface

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

modeling the formation and deposition of frazil ice beneath filchner ronne ice shelf large areas of the filchner ronne ice shelf are underlain by layers of marine ice which form when supercooled seawater circulating beneath the ice shelf freezes the freezing process initially produces a suspension of disc shaped frazil ice crystals and these are subs quently deposited onto the ice shelf base in areas where the flow of water is slack enough this has been modeled assuming that the freezing takes place within buoyant plumes of ice shelf water ascending the ice shelf base from source regions near the grounding lines of the major inlet glaciers the deposition of the majority of the suspended frazil ice is found to occur in spatially discrete bursts where peak rates of accumulation at the ice shelf base exceed 1 m yr 1 of solid ice there is a good correlation between the location of the zones of crystal deposition and the position of the upstream limits of the marine ice layers the high rates of localized accumulation account for the rapid buildup observed in the layer thickness which then gradually declines as the marine ice is carried downstream with the flow of the ice shelf model results also suggest an origin for the ice platelets observed at depth in the water column near the filchner ice shelf

global tidal residual mean circulation does it affect a climate ogcm a global description of the ocean tidal residual mean circulation trmc is provided and its effect on a global ocean climate model is evaluated the trmc is computed from a global barotropic tidal model based on pure hydrodynamical equations and forced by the major tidal constituents it thus includes both stokes drift and non linear tidal dynamics the trmc map exhibits remarkable basin scale structures in the indonesian archipelago australian region and on the malvinas shelf regions of largest transport are characterized by sites of high tidal amplitudes abrupt topography and western boundaries yet transport magnitudes remain at a few tenths of a sverdrup this trmc is introduced as an external forcing in an ocean climate model at equilibrium the thermohaline structure and the poleward heat transport of the ocean are almost insensitive to the trmc it is concluded that the trmc is negligible for climate purposes in an ocean climate model

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

modeling the basal melting and marine ice accretion of the amery ice shelf the basal mass balance of the amery ice shelf ais in east antarctica is investigated using a numerical ocean model the main improvements of this model over previous studies are the inclusion of frazil formation and dynamics tides and the use of the latest estimate of the sub ice shelf cavity geometry the model produces a net basal melt rate of 45 6 gt year 1 0 74 m ice year 1 which is in good agreement with reviewed observations the melting at the base of the ice shelf is primarily due to interaction with high salinity shelf water created from the surface sea ice formation in winter the temperature difference between the coldest waters created in the open ocean and the in situ freezing point of ocean water in contact with the deepest part of the ais drives a melt rate that can exceed 30 m of ice year 1 the inclusion of frazil dynamics is shown to be important for both melting and marine ice accretion refreezing frazil initially forms in the supercooled water layer adjacent to the base of the ice shelf the net accretion of marine ice is 5 3 gt year 1 comprised of 3 7 gt year 1 of frazil accretion and 1 6 gt year 1 of direct basal refreezing

ocean circulation beneath filchner ronne ice shelf from three dimensional model results a high resolution three dimensional ocean circulation model is applied to the cavity beneath filchner ronne ice shelf fris the model predicts predominantly barotropic currents which form a series of cyclonic gyres in the deep basins and anticyclonic circulations around the islands the surface circulation can be such that the water moves in the direction of decreasing or increasing ice thicknesses in the former case leading to freezing while melting at the ice shelf base results in the latter case the pattern of melting and freezing is consistent with known distributions of marine ice and melting areas beneath fris an anticyclonic circulation around the korff and henry ice rises with melting west of korff ice rise and freezing on the eastern side and north of henry ice rise is the main source for an ice pumping mechanism that produces the observed large marine ice body in the central filchner ronne ice shelf the estimates for net melting for realistic conditions at the open ocean boundary are 40 50 km 3 yr 1 indicating that ice shelf ocean interaction is an important contribution to the mass balance of the ice shelf

accuracy assessment of global barotropic ocean tide models the accuracy of state of the art global barotropic tide models is assessed using bottom pressure data coastal tide gauges satellite altimetry various geodetic data on antarctic ice shelves and independent tracked satellite orbit perturbations tide models under review include empirical purely hydrodynamic forward and assimilative dynamical i e constrained by observations ten dominant tidal constituents in the diurnal semidiurnal and quarter diurnal bands are considered since the last major model comparison project in 1997 models have improved markedly especially in shallow water regions and also in the deep ocean the root sum square differences between tide observations and the best models for eight major constituents are approximately 0 9 5 0 and 6 5 cm for pelagic shelf and coastal conditions respectively large intermodel discrepancies occur in high latitudes but testing in those regions is impeded by the paucity of high quality in situ tide records long wavelength components of models tested by analyzing satellite laser ranging measurements suggest that several models are comparably accurate for use in precise orbit determination but analyses of grace intersatellite ranging data show that all models are still imperfect on basin and subbasin scales especially near antarctica for the m2 constituent errors in purely hydrodynamic models are now almost comparable to the 1980 era schwiderski empirical solution indicating marked advancement in dynamical modeling assessing model accuracy using tidal currents remains problematic owing to uncertainties in in situ current meter estimates and the inability to isolate the barotropic mode velocity tests against both acoustic tomography and current meters do confirm that assimilative models perform better than purely hydrodynamic models

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

ocean tides in the weddell sea new observations on the filchner ronne and larsen c ice shelves and model validation 1 ocean tides under the large weddell sea ice shelves are among the least well observed on earth here we present new spatially extensive observations of the vertical tidal motion of the filchner ronne and larsen c ice shelves using global positioning system gps data spanning a few weeks to years we pay particular attention to the major tidal constituents m2 s2 o1 k1 as well as important grace aliasing periods k2 and s1 we compare the estimated constituents with recent global and regional tide models and find that no single model is the most accurate across all constituents or ice shelves the root sum square errors are 7 8 cm cats2008a and tpxo7 2 and 11 12 cm got4 7 and fes2004 with the energetic m2 rmse 4 8 cm and s2 4 5 cm generally dominating these statistics the fes2004 k1 is particularly inaccurate near the larsen c ice shelf with errors approaching 20 cm meaning that grace release 4 estimates of mass change in the northern antarctic peninsula will be biased we find tidal energy at 3 4 5 6 and weakly at 7 cycles per day at all of our sites the largest amplitudes within these bands are at m4 mo3 and sp3 and approach 30 mm although significant spatial variations exist we show that they generally do not appear to originate in areas of reduced water column in ice shelf grounding zones comparing model estimates with our m4 ms4 and mn4 values shows that models do not accurately represent these terms

basal mass budget of ross and filchner ronne ice shelves antarctica derived from lagrangian analysis of icesat altimetry traditional methods of deriving temporal variability of antarctic ice shelf elevation from satellite altimetry use a fixed eulerian reference frame where the measured changes include advection of ice thickness gradients between measurement epochs we present a new method which removes advection effects by using an independent velocity field to compare elevations in a moving lagrangian reference frame applying the technique to icesat laser altimetry for the period 2003 2009 over the two largest antarctic ice shelves ross and filchner ronne we show that the lagrangian approach reduces the variability of derived elevation changes by about 50 compared to the eulerian approach and reveals clearer spatial patterns of elevation change the method simplifies the process of estimating basal mass budget from the residual of all other processes that contribute to ice shelf elevation changes we use field data and icesat measurements over ice rises and the grounded ice sheet to account for surface accumulation and changes in firn air content and remove the effect of ice flow divergence using surface velocity and ice thickness data the results show highest basal melt rates 5 m a 1 near the deep grounding lines of major ice streams but smaller melt rates 5 m a 1 near the ice shelf fronts are equally important to total meltwater production since they occur over larger areas integrating over the entire ice shelf areas we obtain basal mass budgets of 50 64 gt a 1 for ross and 124 66 gt a 1 for filchner ronne with changes in firn air content as the largest error source

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

internal tides and baroclinicity in the southern weddell sea 1 model description near the continental shelf break in the southern weddell sea warm deep water and western shelf water meet mixing mechanisms such as internal tides have the potential to mix these water masses and form antarctic bottom water a modified version of the princeton ocean model was utilized to investigate the internal tidal fields generated by the interaction of the m 2 barotropic tide with topography for transects across the continental shelf and slope in the southern weddell sea internal tides were generated over the upper continental slope as predicted by linear internal wave theory although the essentially two dimensional domain resulted in differences between the model elevations and the observations exceeding the observational uncertainties the cross slope velocities agreed well with differences less than the uncertainties for 78 of the existing observations

on the structure and the transport of the eastern weddell gyre abstract the circulation pattern and volume transports in the eastern weddell gyre are estimated on the basis of hydrographic data collected by r v polarstern between 1989 and 1996 in the northeastern edge of the weddell gyre eastward flowing water masses from the antarctic circumpolar current and the weddell sea converge due to the strong effect of topographic constraints on ocean currents in the weakly stratified waters of high latitudes the wedge like structure of the southwest indian ridge can cause the convergence the increased shear leads to instabilities of the current at the eastern end of the ridge which produce an intense mesoscale eddy field between 15 and 30 e in the eddies water from the weddell cold regime and the antarctic circumpolar current waters mix and form the water masses of the weddell warm regime these waters are advected southward and flow towards the westward southern rim current which is driven by the antarctic eastwind band hence there is not a continous flow from the northern to the southern rim but a decay of the mean flow in the northeast and a reformation in the south volume transports across the greenwich meridian estimated on the basis of a combined ctd adcp data set result in an eastward flow of 61 sv in the northern rim current and a westward return flow of 66 sv in the southern part of the gyre the transport is about twice as high as previous estimates between kapp norvegia and the northern tip of the antarctic pensinsula indicating a significant gyre circulation north of 70 s

ocean circulation beneath the ronne ice shelf the intimate thermal contact between the base of antarctic ice shelves and the underlying ocean enables changes in climate to have a rapid impact on the outflow of ice from the interior of antarctica1 2 furthermore water modified by passage under ice shelves particularly in the weddell sea is believed to be an important constituent of antarctic bottom water3 a water mass that can be observed as far north as 50 n in the deep oceans4 antarctic bottom water is both cold and oxygen rich and plays an important part in the cooling and ventilation of the world s oceans because of the difficulty in gaining access the oceano graphic regime beneath ice shelves is very poorly sampled5 by successfully drilling through the ice however we were able to obtain oceanographic data from beneath the largest antarctic ice shelf the ronne filchner ice shelf in the southern weddell sea we find that our data agree well with the predictions of a relatively simple oceanographic plume model of sub ice shelf circulation6 this model can therefore be used with some confidence to investigate the links between climate changes ice shelf melting and bottom water production

modeling tidal currents beneath filchner ronne ice shelf and on the adjacent continental shelf their effect on mixing and transport a depth averaged tidal model has been applied to the southern weddell sea the model domain covers the southern continental shelf including the ocean cavity beneath filchner ronne ice shelf reasonable agreement with the available current meter data has been achieved our results confirm that in areas with shallow water and large topographic gradients tidal oscillations with peak velocities up to 1 m s 1 play a significant role in the vertical mixing and transport of water masses the estimated energy dissipation beneath filchner ronne ice shelf due to surface friction is 25 gw approximately 1 of the world s total tidal dissipation tidally induced lagrangian residual currents converging at the ice front an area of strong mixing draw together water masses from the continental shelf and sub ice shelf cavity the model indicates that lagrangian residual currents have fluxes of up to 250 000 m3 s 1 and speeds of over 5 cm s 1 along the ice front with over 350 000 m3 s 1 being exchanged between the sub ice shelf cavity and adjacent continental shelf these currents are particularly efficient in ventilating the sub ice shelf cavity within 150 km of ronne ice front such strong tidal mixing will significantly modify the properties of water masses that flow through this region particularly to the west of berkner island the model predictions indicate that tidal processes strongly influence the oceanographic conditions in the vicinity of ronne ice front shipborne observations along the ice front support many of the model predictions concerning the effect of tides on the hydrography

impact of antarctic ice shelf basal melting on sea ice and deep ocean properties 1 an approximation of antarctica s rocky and icy coastline normally forms the southern boundary in global climate models such a configuration neglects extensive ice shelf areas where ocean ice interaction initiates a net freshwater flux to the circumpolar continental shelf equal to 75 of the annual mean net precipitation in coastal seas the results of a numerical model for the southern ocean using two contrasting configurations with and without caverns beneath major antarctic ice shelves are compared they show that the freshwater flux due to deep basal melting significantly stabilizes the shelf water column in front of an ice shelf as well as downstream due to advection by the coastal current if the freshwater from the caverns is absent sea ice is thinner shelf waters are warmer and saltier and the southern ocean deep basins are flushed by denser waters

on the fate of the antarctic slope front and the origin of the weddell front 1 data from the deep ocean ventilation through antarctic intermediate layers dovetail and antarctic large scale box analysis and the role of the scotia sea albatross projects are combined in the region of the antarctic peninsula and the south scotia ridge to determine the paths and transports associated with the antarctic coastal current the antarctic slope front and the weddell front the antarctic coastal current flows over the antarctic continental shelf and is not tied to a particular isobath it transports 1 sv westward to the bransfield strait its subsequent course is uncertain but we speculate that it may continue southwest along the western side of the antarctic peninsula close to the coast the antarctic slope front present almost all the way around antarctica tied to the continental slope has not previously been mapped after it reaches the tip of the antarctic peninsula we show that the frontal jet splits into two branches in the northwest powell basin where the isobaths diverge the shoreward portion of the water associated with the front lying above isobaths shallower than 1500 m is able to cross the south scotia ridge and head northwestward and retains many of the properties of the antarctic slope front it is associated with a transport of 7 sv the portion of the front constrained to lie above isobaths deeper than 1500 m becomes the weddell front this takes a path around the northern powell basin and south of the south orkney islands of the 13 sv that circulates around the southern flank of the south orkney plateau 8 sv of weddell sea deep water and warm deep water leaves the front to continue west the remaining shallow and intermediate water retroflects and returns south associated with the weddell front through the orkney passage approximately 5 7 sv is transported eastward associated with the weddell front tied closely to the southern flanks of the bruce and discovery banks above the 2000 2500 m isobaths and forming the southern boundary of the weddell scotia confluence

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

on the total input of antarctic waters to the deep ocean a preliminary estimate from chlorofluorocarbon measurements 1 deep ocean inventories of dissolved chlorofluorocarbon 11 cfc 11 along representative sections off antarctica provide the first estimate of the overall strength of all dense water sources in the southern ocean their formation rates are reported for three density layers that span the main water masses involved in the lower limb of the thermohaline circulation thc the bottom layer is supplied via sinking of antarctic bottom water aabw produced at a few continental shelves the middle layer receives the offshore injection of ventilated modified circumpolar deep water mcdw produced along much of the lengthy antarctic slope front the top layer is ventilated by northward export of antarctic surface water into the upper circumpolar deep water of the antarctic circumpolar current average southern ocean inputs to the upper two layers of the deep ocean for the 1970 1990 period are derived on the basis of the cfc 11 distributions along meridional sections the mean cfc 11 saturations of all water mass ingredients and the inferred mixing leading to production of dense source waters about 5 4 1 7 sv 1 sv 10 6 m 3 s 1 of near freezing shelf water ventilate the bottom layer and 4 7 1 7 sv and 3 6 1 3 sv of cold antarctic surface water ventilate the middle and top layers therefore the total contribution of ventilated southern ocean waters to the lower limb of the global thc is about 14 sv this is close to the about 17 sv estimated for north atlantic near surface sources from cfc 11 inventories their entrainment of 7 4 2 4 sv of cfc poor subsurface lower circumpolar deep water during the formation and sinking of aabw and mcdw raises the total southern ocean input to the deep ocean to about 21 sv

effect of critical latitude and seasonal stratification on tidal current profiles along ronne ice front antarctica 1 the ice front region of ronne ice shelf lies near the critical latitude of the semidiurnal m2 tide the principal tidal constituent in the southern weddell sea here the coriolis frequency almost equals the m2 tidal frequency resulting in a strong dependence of the m2 tidal currents on depth and stratification and a boundary layer that can occupy the entire water column using data from four long term moorings along ronne ice front we confirm the presence of strongly depth dependent semidiurnal tidal currents and their sensitivity to changes in stratification the time series show dramatic seasonal changes in tidal current profiles and significant interannual variability during periods of stratification the amplitude of the semidiurnal tides in the mid water column shows a twofold increase and despite being several kilometers offshore from the ice front the tidal currents clearly show a second boundary layer originating from the adjacent ice shelf base together these two boundary layers occupy most of the water column up to 600 m deep until intense sea ice formation and the production of high salinity shelf water erodes the vertical stratification during winter when homogeneous conditions prevail a single bottom boundary layer occupies the entire water column at some locations this strong seasonality and sensitivity of the m2 tidal current to stratification highlights the difficulties of interpreting current data from short term moorings while demonstrating that it is the best indicator for characterizing changes in stratification after direct observations of density variations

hydrography and circulation in the filchner depression weddell sea antarctica cold and dense ice shelf water isw emerging from the filchner ronne ice shelf cavity in the southwestern weddell sea flows northward through the filchner depression to eventually descend the continental slope and contribute to the formation of bottom water new ship born observations of hydrography and currents from filchner depression in january 2013 suggest that the northward flow of isw takes place in a middepth jet along the eastern flank of the depression thus questioning the traditional view with outflow along the western flank this interpretation of the data is supported by results from a regional numerical model which shows that isw flowing northward along the eastern coast of berkner island turns eastward and crosses the depression to its eastern side upon reaching the filchner ice front the ice front represents a sudden change in the thickness of the water column and thus a potential vorticity barrier transport estimates of northward isw flux based on observations ranges from 0 2 to 1 0 sv

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

internal tides and baroclinicity in the southern weddell sea 2 effects of the critical latitude and stratification tidal dynamics affect antarctic bottom water and ice shelf water production in the southern weddell sea and the general circulation and mixing in this region tidal dynamics were found to be extremely sensitive both to the proximity of the continental shelf slope break to the critical latitude crit and to the existence of a shelf break front if the continental slope was not in the vicinity of crit or a shelf break front was absent generation of internal tides was negligible generation of internal tides was greatest when both a shelf break front was present and the continental slope was near and equatorward of crit when a shelf break front was present and the continental slope was near but poleward of crit a strong benthic response occurred this response included development of benthic inertial oscillations at the continental shelf slope break increased benthic shear a thicker benthic boundary layer and increased benthic dissipation a shelf break front with even a slight horizontal density gradient was found to significantly enhance internal tide generation and or the benthic effects

the transport of the weddell gyre across the prime meridian abstract current meter observations from instruments moored from 1996 to 2001 are used to determine the structure and the volume transport of the weddell gyre on the greenwich meridian the record length mean volume transport is 45 sv to the east within the northern and 56 sv to the west within the southern limb of the weddell gyre thus 11 sv must leave the gyre circulation west of the greenwich meridian and enter it east of it the deficit occurs with 8 sv mainly in the circumpolar deep water cdw warm deep water wdw layer surface waters and weddell sea deep water wsdw contribute 2 and 3 sv respectively in contrast to this 3 sv more of weddell sea bottom water wsbw flow in the northern limb to the east than in the southern limb to the west which suggests a wsbw formation rate of about 3 sv in the western weddell sea in the southern boundary of the antarctic circumpolar current acc a volume transport of 35 16 sv was found using geostrophic calculations adjusted with two measurements of the absolute velocity the large heat content of the northern limb points to an extensive heat exchange between the weddell sea and the world ocean west of the prime meridian hence the heat transport as well as the volume transport suggest that the water masses of the weddell gyre are not exclusively replaced in its eastern part but that there is a significant exchange of water masses to the west of the prime meridian

oceanographic conditions south of berkner island beneath filchner ronne ice shelf antarctica we have made oceanographic measurements at two sites beneath the southern filchner ronne ice shelf hot water drilled access holes were made during january 1999 allowing conductivity temperature depth ctd profiling and the deployment of instrument moorings the ctd profiles show that the entire water column is below the surface freezing point we estimate the summer flux of water between the two sites to be 2 106 m3 s 1 the summer potential temperature salinity properties of the water column suggest that this flow is part of a recirculation in the deepest part of the subice shelf cavity and the filchner depression the recirculation is driven by a combination of the melting of deep basal ice and the freezing that results from the depressurization of the cold buoyant water as it ascends the ice shelf base the source of the water was high salinity shelf water hssw produced in the ronne depression this is the water that provides the external heat necessary for the strong melting at the deep grounding lines in the vicinity of foundation ice stream instruments moored at the drill sites show that during the winter hssw formed on the berkner shelf flows beneath the ice shelf and largely displaces the recirculating water from the two sites this provides an externally driven through flow that is warmer nearer the surface freezing point and slower than the internal recirculation and which is low enough in density to escape the filchner depression

new oceanographic data from beneath ronne ice shelf antarctica oceanographic data have been obtained via an access hole made through ronne ice shelf the site which is the third in a series of similar studies lies 17 km west of korff ice rise where 825 m of ice overlies a 485 m deep water column measurements included conductivity and temperature profiles and an instrument mooring was deployed for long term measurements of currents temperature and conductivity at the sea floor there was a 150 m layer of well mixed water with a potential temperature and salinity of 1 97 c and 34 72 the water cooled and freshened towards the ice shelf base ultimately reaching 2 41 c and 34 51 the hydrographic and water current data imply a flow into the deepest part of the sub ice shelf cavity of about 200 000 m s 1 of the deeper relatively warm water which would be able to power an average basal melt rate of 0 2 m a 1 for the western portion of ronne ice shelf

the role of tides in ocean ice shelf interactions in the southwestern weddell sea near observed weddell sea gyre continental shelf cavity water masses in interactive ocean sea ice tide ice shelf melt simulations tides amplify basal melt refreeze pattern raising net melt by 50 associated with enhanced time varying kinetic energy at the ice base impacts of additional meltwater production on weddell continental shelf sea ice and newly formed bottom water properties are suggested corresponding author u hausmann uhausc at gmail dot com this article has been accepted for publication and undergone full peer review but has not been through the copyediting typesetting pagination and proofreading process which may lead to differences between this version and the version of record please cite this article as

modeling ice shelf cavities in a z coordinate ocean general circulation model 1 processes at the ice shelf ocean interface and in particular in ice shelf cavities around antarctica have an observable effect on the solutions of basin scale to global coupled ice ocean models despite this these processes are not routinely represented in global ocean and climate models it is shown that a new ice shelf cavity model for z coordinate models can reproduce results from an intercomparison project of earlier approaches with vertical or isopycnic coordinates as a proof of concept ice shelves are incorporated in a 100 year global integration of a z coordinate model in this simulation glacial meltwater can be traced as far as north as 15 s the observed effects of processes in the ice shelf cavities agree with previous results from a coordinate model notably the increase in sea ice thickness however melt rates are overestimated probably because the parameterization of basal melting does not suit the low resolution of this configuration

ice shelf basal melting in a global finite element sea ice ice shelf ocean model the finite element sea ice ocean model fesom has been augmented by an ice shelf component with a three equation system for diagnostic computation of boundary layer temperature and salinity ice shelf geometry and global ocean bathymetry have been derived from the rtopo 1 dataset a global domain with a triangular mesh and a hybrid vertical coordinate is used to evaluate sub ice shelf circulation and melt rates for present day climate the model is forced with ncep reanalysis data basal mass fluxes are mostly realistic with maximum melt rates in the deepest parts near the grounding lines and marine ice formation in the northern sectors of ross ice shelf and filchner ronne ice shelf total basal mass loss for the ten largest ice shelves reflects the importance of the amundsen sea ice shelves getz ice shelf is shown to be a major melt water contributor to the southern ocean despite their modest melt rates the cold water ice shelves in the weddell sea are still substantial sinks of continental ice in antarctica discrepancies between the model and observations can partly be attributed to deficiencies in the forcing data or to sometimes unavoidable smoothing of ice shelf and bottom topographies

the effect of atmospheric forcing resolution on delivery of ocean heat to the antarctic floating ice shelves abstractoceanic melting at the base of the floating antarctic ice shelves is now thought to be a more significant cause of mass loss for the antarctic ice sheet than iceberg calving in this study a 10 km horizontal resolution circum antarctic ocean sea ice ice shelf model based on the regional ocean modeling system roms is used to study the delivery of ocean heat to the base of the ice shelves the atmospheric forcing comes from the era interim reanalysis 80 km resolution and from simulations using the polar optimized weather research and forecasting model 30 km resolution where the upper atmosphere was relaxed to the era interim reanalysis the modeled total basal ice shelf melt is low compared to observational estimates but increases by 14 with the higher resolution winds and just 3 with both the higher resolution winds and atmospheric surface temperatures the higher resolution winds lead to more heat being delivered to the ice shelf cavities from the adjacent ocean and an increase in the

eddy mediated transport of warm circumpolar deep water across the antarctic shelf break the antarctic slope front asf modulates ventilation of the abyssal ocean via the export of dense antarctic bottom water aabw and constrains shoreward transport of warm circumpolar deep water cdw toward marine terminating glaciers along certain stretches of the continental shelf particularly where aabw is exported density surfaces connect the shelf waters to the middepth circumpolar deep water offshore offering a pathway for mesoscale eddies to transport cdw directly onto the continental shelf using an eddy resolving process model of the asf the authors show that mesoscale eddies can supply a dynamically significant transport of heat and mass across the continental shelf break the shoreward transport of surface waters is purely wind driven while the shoreward cdw transport is entirely due to mesoscale eddy transfer the cdw flux is sensitive to all aspects of the model s surface forcing and geometry suggesting that shoreward eddy heat transport may be localized to favorable sections of the continental slope

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

the fluctuations of fixed nh4 n content in the luochuan loess and its paleoclimatic significance abstract the fixed nh4 n content varies from 120 to 202 g g in luochuan loess paleosol sequences this content in the interstratified paleosols is significantly higher than that of the loess fixed nh4 n content in the loess l2 is the lowest varying between 144 to 147 g g whereas the highest in the paleosol s5 reaching 200 g g the distribution of fixed nh4 n is similar to that of the

calving fluxes and basal melt rates of antarctic ice shelves iceberg calving has been assumed to be the dominant cause of mass loss for the antarctic ice sheet with previous estimates of the calving flux exceeding 2 000 gigatonnes per year1 2 more recently the importance of melting by the ocean has been demonstrated close to the grounding line and near the calving front3 4 5 so far however no study has reliably quantified the calving flux and the basal mass balance the balance between accretion and ablation at the ice shelf base for the whole of antarctica the distribution of fresh water in the southern ocean and its partitioning between the liquid and solid phases is therefore poorly constrained here we estimate the mass balance components for all ice shelves in antarctica using satellite measurements of calving flux and grounding line flux modelled ice shelf snow accumulation rates6 and a regional scaling that accounts for unsurveyed areas we obtain a total calving flux of 1 321 144 gigatonnes per year and a total basal mass balance of 1 454 174 gigatonnes per year this means that about half of the ice sheet surface mass gain is lost through oceanic erosion before reaching the ice front and the calving flux is about 34 per cent less than previous estimates derived from iceberg tracking1 2 7 in addition the fraction of mass loss due to basal processes varies from about 10 to 90 per cent between ice shelves we find a significant positive correlation between basal mass loss and surface elevation change for ice shelves experiencing surface lowering8 and enhanced discharge9 we suggest that basal mass loss is a valuable metric for predicting future ice shelf vulnerability to oceanic forcing

eddy driven exchange between the open ocean and a sub ice shelf cavity the exchange between the open ocean and sub ice shelf cavities is important to both water mass transformations and ice shelf melting here we use a high resolution 500 m numerical model to investigate to which degree eddies produced by frontal instability at the edge of a polynya are capable of transporting dense high salinity shelf water hssw underneath an ice shelf the applied surface buoyancy flux and ice shelf geometry is based on ronne ice shelf in the southern weddell sea an area of intense wintertime sea ice production where a flow of hssw into the cavity has been observed results show that eddies are able to enter the cavity at the southwestern corner of the polynya where an anticyclonic rim current intersects the ice shelf front the size and time scale of simulated eddies are in agreement with observations close to the ronne ice front the properties and strength of the inflow are sensitive to the prescribed total ice production flushing the ice shelf cavity at a rate of 0 2 0 4 106 m3 s 1 depending on polynya size and magnitude of surface buoyancy flux eddy driven hssw transport into the cavity is reduced by about 50 if the model grid resolution is decreased to 2 5 km and eddies are not properly resolved

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