seasat wind and wave observations of northeast pacific hurricane iva august 13 1978 seasat wind and wave observations collected in the eye of hurricane iva on august 13 1978 in the northeast pacific are examined the seasat a scatterometer system sass and scanning multichannel microwave radiometer smmr observations indicate a maximum wind speed of 25 30 m s sass derived isotachs indicate a classic banana shaped high wind speed distribution and the synthetic aperture radar sar imagery displays a large scale mean intensity pattern that is in qualitative agreement two dimensional fourier transforms of selected sar scenes show a dominant wave field characterized by wavelengths of 166 211 m with a fanlike distribution of propagation directions a simple geometric model is proposed as an explanation of this dominant wave field in terms of prior storm intensity and location this geometric model the ross parametric hurricane wave model and the seasat wind and wave data are then employed to obtain estimates of 43 km for the radius to maximum winds and 6 4 m for the corresponding significant wave height at that radius

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

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 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

magnetic polarity stratigraphy and biostratigraphy of paleocene and lower eocene continental deposits clark s fork basin wyoming paleomagnetic samples were collected at 273 sites within a 2012 m continental sedimentary sequence in the clark s fork basin near powell wyoming the lower 1158 m is in the polecat bench fm while the upper 854 m is within the willwood fm although significant secondary components of magnetization were present alternating field demagnetization to 300 oe peak field revealed the polarity of the primary component of the natural remanent magnetism a well defined polarity zonation is observed the polecat bench fm is dominantly of reversed polarity but does contain two normal polarity zones the willwood fm in this section is entirely of reversed polarity abundant tiffanian clarkforkian and wasatchian faunas indicate that the age range of the sediments is from late paleocene through early eocene these age constraints along with the characteristic polarity sequence allow the magnetic polarity sequence to be correlated with the magnetic polarity time scale the section correlates with the magnetic polari

winter use of sea ice and ocean water mass habitat by southern elephant seals the length and breadth of the mystery understanding the responses of animals to the environment is crucial for identifying critical foraging habitat elephant seals mirounga leonina from the kerguelen islands 49 20 s 70 20 e have several different foraging strategies why some individuals undertake long trips to the antarctic continent while others utilize the relatively close frontal zones is poorly understood here we investigate how physical properties within the sea ice zone are linked to foraging activities of southern elephant seals ses to do this we first developed a new approach using indices of foraging derived from high temporal resolution dive and accelerometry data to predict foraging behaviour in an extensive low resolution dataset from ctd satellite relay data loggers ctd srdls a sample of 37 post breeding ses females were used to construct a predictive model applied to demersal and pelagic dive strategies relating prey encounter events pee to dive parameters dive duration bottom duration hunting time maximum depth ascent speed descent speed sinuosity and horizontal speed for each strategy we applied these models to a second sample of 35 seals 20 males and 15 females during the post moult foraging trip to the antarctic continental shelf between 2004 and 2013 which did not have fine scale behavioural data the females were widely distributed with important foraging activity south of the southern boundary front while males predominately travelled to the south eastern part of the east antarctica region combining our predictions of pee with environmental features sea ice concentration water masses at the bottom phase of dives bathymetry and slope index we found higher foraging activity for females over shallower seabed depths and at the boundary between the overlying antarctic surface water aasw and the underlying modified circumpolar deep water mcdw increased biological activity associated with the upper boundary of mcdw may provide overwintering areas for ses prey male foraging activity was strongly associated with pelagic dives within the antarctic slope front where upwelling of nutrient rich circumpolar deep water onto surface water may enhance and concentrate resources a positive association between sea ice and foraging activity was found for both sexes where increased biological activity may sustain an under ice ecosystem variability of the east antarctic sea ice season duration is likely a crucial element to allow air breathing predators to benefit from profitable prey patches within the pack ice habitat

lower eocene alluvial paleosols willwood formation northwest wyoming u s a and their significance for paleoecology paleoclimatology and basin analysis abstract the lower eocene willwood formation of northwest wyoming is a 700 m thick accumulation of alluvial floodplain and channel mudstones and sandstones nearly all of which show paleopedogenic modifications pedogenesis of willwood sandstones is indicated by taproot and vertebrate and invertebrate bioturbation early local cementation by calcium carbonate and thin illuviation cutans on clastic grains pedogenesis in willwood mudstones is indicated by plant bioturbation insect and other invertebrate burrow casts and lebensspuren free iron aluminum and manganese mobilization including hydromorphic gleying sesquioxide and calcareous glaebule formation in lower parts of the solum presence of clay rich and organic carbon rich zones and well differentiated epipedons and albic and spodic horizons probable a horizons are also locally well developed occurrence of variegated paleosol units in thick floodplain mudstone deposits and their association with thin lenticular and unconnected fluvial sandstones in the willwood formation of the central and southeast bighorn basin suggest that these soils formed during times of rapid sediment accumulation the tabular geometry and lateral persistence of soil units as well as the absence of catenization indicate that willwood floodplains were broad and essentially featureless all willwood paleosols were developed on alluvial parent materials and are complex in that b horizons of younger paleosols were commonly superimposed upon and mask properties of suspected a and b horizons of the next older paleosols the soils appear to be wet varieties of the spodosol and entisol groups aquods and ferrods and aquents respectively though thick superposed and less mottled red purple and yellow paleosols resemble some ultisols most willwood paleosols resemble warm temperate to subtropical alluvial soils that form today under alternating wet and dry conditions and or fluctuating water tables the up section decrease in frequency of gley mottles increase in numerical proportion and thickness of red versus orange coloration and increase in abundance of calcrete glaebules indicate better drained soils and probably drier climate in late willwood time this drying is believed to be related to creation of rain shadows and spacing of rainfall but not necessarily decrease in absolute rainfall due to progressive tectonic structural elevation of the mountainous margins of the bighorn basin

abrupt deep sea warming palaeoceanographic changes and benthic extinctions at the end of the palaeocene a remarkable oxygen and carbon isotope excursion occurred in antarctic waters near the end of the palaeocene 57 33 myr ago indicating rapid global warming and oceanographic changes that caused one of the largest deep sea benthic extinctions of the past 90 million years in contrast the oceanic plankton were largely unaffected implying a decoupling of the deep and shallow ecosystems the data suggest that for a few thousand years ocean circulation underwent fundamental changes producing a transient state that although brief had long term effects on environmental and biotic evolution

circulation and meltwater distribution in the bellingshausen sea from shelf break to coast west antarctic ice shelves have thinned dramatically over recent decades oceanographic measurements that explore connections between offshore warming and transport across a continental shelf with variable bathymetry toward ice shelves are needed to constrain future changes in melt rates six years of seal acquired observations provide extensive hydrographic coverage in the bellingshausen sea where ship based measurements are scarce warm but modified circumpolar deep water floods the shelf and establishes a cyclonic circulation within the belgica trough with flow extending toward the coast along the eastern boundaries and returning to the shelf break along western boundaries these boundary currents are the primary water mass pathways that carry heat toward the coast and advect ice shelf meltwater offshore the modified circumpolar deep water and meltwater mixtures shoal and thin as they approach the continental slope before flowing westward at the shelf break suggesting the presence of the antarctic slope current constraining meltwater pathways is a key step in monitoring the stability of the west antarctic ice sheet

upper ocean variability in west antarctic peninsula continental shelf waters as measured using instrumented seals abstract temperature profile data for the west antarctic peninsula wap continental shelf waters collected from freely ranging instrumented seals crabeater lobodon carcinophagus and leopard hydrurga leptonyx were used to demonstrate that these platforms can be used to supplement traditional oceanographic sampling methods to investigate the physical properties of the upper water column the seal derived profiles were combined with temperature profiles obtained from ship based ctd measurements and from a numerical circulation model developed for the wap to describe changes in temperature structure heat content and heat flux in the upper ocean waters of the wap continental shelf the seal derived data documented the fall to winter transition of the surface waters and the shelf wide presence of modified circumpolar deep water cdw below 150 200 m on the wap continental shelf the heat content of the upper 200 m calculated from the seal derived temperature profiles ranged between 1000 and 1500 mj m 2 similar estimates were obtained from simulated temperature distributions the seal derived temperature measurements provided broader space and time resolution than was possible using any other currently available oceanographic sampling method as such the seal derived measurements provided a valuable dataset for evaluation of temperature fields obtained from a numerical circulation model

correlation between isotope records in marine and continental carbon reservoirs near the palaeocene eocene boundary changes in the isotope content of the large marine carbon reservoir can force shifts in that of the smaller carbon pools in the atmosphere and on land the carbon isotope compositions of marine carbonate sediments from the late palaeocene vary considerably exhibiting a sudden decrease close to the palaeocene eocene boundary which coincides with deep sea benthic extinctions1 and with changes in ocean circulation here we report that these fluctuations in the marine carbon isotope record are closely tracked by the terrestrial records provided by palaeosol carbonates and mammalian tooth enamel in using palaeosol carbonates to reconstruct the co2 content of the ancient atmosphere2 isotope shifts of this sort will have to be taken into account the sharp decrease in 13c 12c ratios in the late palaeocene provides a datum for precise correlation of marine and continental records and suggests that abrupt climate warming at this time may have played an important role in the evolution of land mammals

wintertime water mass modification near an antarctic ice front abstractunder ice measurements by seals carrying a miniaturized conductivity temperature depth ctd instrument fill an important gap in existing observations here the authors present data from an instrumented weddell seal that spent eight consecutive months february september foraging in close proximity to the filchner ice shelf thus providing detailed information about the evolution of mixed layer hydrography during the austral autumn and winter the resultant time series of hydrography shows strong seasonal water mass modification dominated by an upper ocean 0 300 m salinity increase of 0 31 corresponding to 3 1 m sea ice growth and the development of a 500 m thick winter mixed layer observations furthermore highlight a gradual salinity increase in a slow 3 5 cm s 1 southward flow on the continental shelf toward the site and suggest that the inferred ice production is better considered as a regional average rather than being purely local no clear seasonality is observed in the properties

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

hunting behavior of a marine mammal beneath the antarctic fast ice the hunting behavior of a marine mammal was studied beneath the antarctic fast ice with an animal borne video system and data recorder weddell seals stalked large antarctic cod and the smaller subice fish pagothenia borchgrevinki often with the under ice surface for backlighting which implies that vision is important for hunting they approached to within centimeters of cod without startling the fish seals flushed p borchgrevinki by blowing air into subice crevices or pursued them into the platelet ice these observations highlight the broad range of insights that are possible with simultaneous recordings of video audio three dimensional dive paths and locomotor effort

early eocene biotic and climatic change in interior western north america imprecise correlation of the marine and terrestrial fossil records has been a major obstacle to understanding migration and extinction of continental biotas and early cenozoic climate change new sup 40 ar sup 39 ar data from the willwood formation in the bighorn basin of wyoming establish an age of 52 8 plus minus 0 3 ma for earliest lostcabinian late wasatchian faunas and coeval early eocene floras strata just beneath earliest wasatchian faunas can be correlated with the np9 np10 boundary in marine sedimentary units which has an interpolated age of approximately 55 7 ma this new information allows the authors to estimate the durations of the wasatchian approximately 5 m y and the lostcabinian approximately 2 m y and shows that the continental biotas are coeval with the acme of cenozoic warmth inferred from delta sup 18 o measurements of foraminifera from 58 to 50 ma paleoclimate in the continental interior at about 45 degree n was warm and equable but patterns of temperature change inferred from continental floras do not track precisely the marine delta sup 18 o record

a model for atmospheric co 2 over phanerozoic time a new model has been constructed for calculating the level of atmospheric co sub 2 over phanerozoic time which is much simpler mathematically than the blag model but more complex geologically and biologically results suggest that there has been a notable pattern of varying atmospheric co sub 2 level over the past 570 my with high levels during the mesozoic and early paleozoic and low levels during the permo carboniferous and late cenozoic sensitivity analysis shows that within reasonable limits of the principal governing parameters this qualitative trend of varying co sub 2 is relatively insensitive to the actual values chosen for these parameters causes of the co sub 2 variation are multiple and no single geological or biological process can be called upon to explain all co sub 2 variation for all time the calculated trend of co sub 2 over time agrees well with independent estimations of paleoclimates thus the greenhouse theory of paleoclimate on a long geological time scale is supported by the results of the present study

carbon dioxide in the paleozoic atmosphere evidence from carbon isotope compositions of pedogenic carbonate stable carbon isotope compositions of pedogenic carbonate occurring in three clay rich vertic paleosols within paleozoic red bed successions in central pennsylvania provide a record of past pedogenic environments and can be used to estimate co sub 2 pressure p sub co sub 2 of the paleozoic atmosphere the delta sup 13 c values of carbonate nodules from paleosols in the deltaic lower bloomsburg formation upper silurian reflect the contribution of carbon from marine groundwater or fossils coupled with low biological activity the delta sup 13 c values of carbonate rhizocretions from stratigraphically high paleosols in the bloomsburg formation and in the alluvial catskill upper devonian and mauch chunk upper mississippian formations suggest an extensive c sub 3 flora and significant contribution of atmospheric co sub 2 paleozoic atmospheric co sub 2 levels inferred from delta sup 13 c of pedogenic carbonate are significantly higher than present levels

the international bathymetric chart of the southern ocean ibcso version 1 0 a new bathymetric compilation covering circum antarctic waters the international bathymetric chart of the southern ocean ibcso version 1 0 is a new digital bathymetric model dbm portraying the seafloor of the circum antarctic waters south of 60 degrees s

seasonal cycle of hydrography on the eastern shelf of the filchner trough weddell sea antarctica new two year long records from three moorings located at 76 s along the eastern flank and shelf of the filchner trough give insight in the seasonal cycle of hydrography to a region where modified warm deep water mwdw enters the southern weddell sea continental shelf possibly reaching the filchner ronne ice shelf the biggest ice shelf by volume in antarctica a persistent northward flow of ice shelf water isw is found along the slope of the trough at 600 m depth while the data on the shelf indicate a seasonal cycle characterized by four phases a distinct warm inflow period separated in two phases with maximum temperatures of 1 c appears to be related to the seasonal heaving of the antarctic slope front thermocline along the continental shelf break further north and a seasonal extension of the isw layer onto the eastern shelf the density gradients between the isw in the trough and the mwdw on the adjacent shelf drive the southward flow during these phases a flow reversal is found in winter ceasing the southward flow along the eastern flank of the trough weaker density gradients between the trough and the shelf during winter allow a westward flow partly driven by a n s density gradient existing across the eastern shelf during this time from spring through to summer the isw layer in the trough extends onto the eastern shelf where it occupies the bottom layer at our moorings and it is associated with northward flow

wintertime ocean conditions over the southern weddell sea continental shelf antarctica 1 during the austral winter of 2007 a weddell seal tagged with a miniaturized conductivity temperature depth ctd instrument travelled over the central southern weddell sea continental shelf the instrument yielded 750 ctd profiles 250 of them to the sea floor the data show a full depth flow of water onto the shelf via a sill at the shelf break 74 s 44 w the warmth from the core of the flow was able to maintain the surface mixed layer above the freezing point resulting in a band of reduced ice production an estimate of the on shelf flux suggests that this flow accounts for most of the estimated 3 sv of water draining from the southern weddell sea continental shelf

foraging behaviour of weddell seals leptonychotes weddellii in connection to oceanographic conditions in the southern weddell sea the region of the filchner outflow system fos in the southeastern weddell sea is characterized by intensive and complex interactions of different water masses dense ice shelf water isw emerging from beneath the ice shelf cavities on the continental shelf meets modified warm deep water mwdw originating from the antarctic circumpolar current at the sill of the filchner trough these hydrographic features convert the fos into an oceanographic key region which may also show enhanced biological productivity and corresponding aggregations of marine top predators in this context six adult weddell seals leptonychotes weddellii were instrumented with ctd combined satellite relay data loggers in austral summer 2014 by means of these long term data loggers we aimed at investigating the influence of environmental conditions on the seals foraging behaviour throughout seasons focussing on the local oceanographic features weddell seals performed pelagic and demersal dives mainly on the continental shelf where they presumably exploited the abundant bentho pelagic fish fauna diurnal and seasonal variations in light availability affected foraging activities mwdw was associated with increased foraging effort however we observed differences in movements and habitat use between two different groups of weddell seals seals tagged in the pack ice of the fos focussed their foraging activities to the western and partly eastern flank of the filchner trough which coincides with inflow pathways of mwdw in contrast weddell seals tagged on the coastal fast ice exhibited typical central place foraging and utilized resources close to their colony high foraging effort in mwdw and high utilization of areas associated with an inflow of mwdw raise questions on the underlying biological features this emphasizes the importance of further interdisciplinary ecological investigations in the near future as the fos may soon be impacted by predicted climatic changes

the fate of the southern weddell sea continental shelf in a warming climate abstractwarm water of open ocean origin on the continental shelf of the amundsen and bellingshausen seas causes the highest basal melt rates reported for antarctic ice shelves with severe consequences for the ice shelf ice sheet dynamics ice shelves fringing the broad continental shelf in the weddell and ross seas melt at rates orders of magnitude smaller however simulations using coupled ice ocean models forced with the atmospheric output of the hadcm3 sres a1b scenario run co2 concentration in the atmosphere reaches 700 ppmv by the year 2100 and stays at that level for an additional 100 years show that the circulation in the southern weddell sea changes during the twenty first century derivatives of circumpolar deep water are directed southward underneath the filchner ronne ice shelf warming the cavity and dramatically increasing basal melting to find out whether the open ocean will always continue to power the melting the authors extend their simulations applying twentieth century atmospheric

seasonal inflow of warm water onto the southern weddell sea continental shelf antarctica 1 to capture the austral summer to winter transition in water mass properties over the southern weddell sea continental shelf and slope region 19 weddell seals were tagged with miniaturized conductivity temperature depth sensors in february 2011 during the following 8 months the instruments yielded about 9000 temperature salinity profiles from a previously undersampled area this allows for the first time a description of the seasonality of warm water intrusions onto the shelf as well as its southward extent towards the filchner ice shelf a temperature section across the filchner depression and eastern shelf shows a pronounced decrease in warm water inflow from summer to winter further supported by an almost 3 year long time series from a shelf break mooring the seasonal variability is related to the surface wind stress and an associated deepening of the off shelf core of warm deep water

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