the relationship of weddell polynya and open ocean deep convection to the southern hemisphere westerlies abstractthe weddell polynya of the mid 1970s is simulated in an energy balance model ebm sea ice ocean coupled general circulation model gcm with an abrupt 20 increase in the intensity of southern hemisphere sh westerlies this small upshift of applied wind stress is viewed as a stand in for the stronger zonal winds that developed in the mid 1970s following a long interval of relatively weak zonal winds between 1954 and 1972 following the strengthening of the westerlies in this model the cyclonic weddell gyre intensifies raising relatively warm weddell sea deep water to the surface the raised warm water then melts sea ice or prevents it from forming to produce the weddell polynya within the polynya large heat loss to the air causes surface water to become cold and sink to the bottom via open ocean deep convection thus the underlying layers cool down the warm water supply to the surface eventually stops and the polynya cannot be maintained anymore during the 100 yr long model simulation

underlying fracture zone nature of astrid ridge off antarctica s queen maud land new geophysical data are used to refine the extent and setting of astrid ridge a volcanic edifice lying off the princess astrid coast section 5 e to 20 e of queen maud land in antarctica new findings presented here include a well defined fracture zone occurring as a narrow basement ridge which extends northeast from astrid ridge and an adjacent set of mesozoic magnetic anomalies m0 to m9 which are offset by 120 km from an identical previously described set these two sets of anomalies are combined with the conjugate mozambique basin anomalies to define an africa antarctica rotation pole at 10 s 29 w for anomaly m2 time the northern edge of astrid ridge and the sharp southern margin of the mozambique ridge on the african plate are probably related to the same cessation of excess volcanic activity near the end of the cretaceous normal polarity interval both features had their eastern and western limits confined by a contiguous pair of fracture zones a possible major swing in fracture zone trend prior to anomaly m9 is also suggested

explaining the weddell polynya a large ocean eddy shed at maud rise satellite observations have shown the occasional occurrence of a large opening in the sea ice cover of the weddell sea antarctica a phenomenon known as the weddell polynya the transient appearance position size and shape of the polynya is explained here by a mechanism by which modest variations in the large scale oceanic flow past the maud rise seamount cause a horizontal cyclonic eddy to be shed from its northeast flank the shed eddy transmits a divergent ekman stress into the sea ice leading to a crescent shaped opening in the pack atmospheric thermodynamical interaction further enhances the opening by inducing oceanic convection a sea ice ocean computer model simulation vividly demonstrates how this mechanism fully accounts for the characteristics that mark weddell polynya events

winter upper ocean stability and ice ocean feedbacks in the sea ice covered southern ocean abstractin this study under ice ocean data from profiling floats instrumented seals and shipboard casts are used to assess wintertime upper ocean stability and heat availability in the sea ice c

polynya dynamics a review of observations and modeling 1 polynyas are nonlinear shaped openings within the ice cover ranging in size from 10 to 105 km2 polynyas play an important climatic role first winter polynyas tend to warm the atmosphere thus affecting atmospheric mesoscale motions second ocean surface cooling and brine rejection during sea ice growth in polynyas lead to vertical mixing and convection contributing to the transformation of intermediate and deep waters in the global ocean and the maintenance of the oceanic overturning circulation since 1990 there has been an upsurge in polynya observations and theoretical models for polynya formation and their impact on the biogeochemistry of the polar seas this article reviews polynya research carried out in the last 2 decades focusing on presenting a state of the art picture of the physical interactions between polynyas and the atmosphere sea ice ocean system observational and modeling studies the surface heat budget and water mass transformation within these features are addressed

preconditioning and formation of maud rise polynyas in a high resolution earth system model abstractopen ocean polynyas oops in the southern ocean are ice free areas within the winter ice pack that are associated with deep convection potentially contributing to the formation of antarct

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

replicating the 1970s weddell polynya using a coupled ocean sea ice model with reanalysis surface flux fields the 1970s weddell polynya is simulated in the framework of a coupled ocean sea ice model forced by reanalysis surface flux fields a rapid emergence of strongly negative wind stress curl over the weddell sea intensifies the cyclonic weddell gyre and thus causes the relatively warm and salty weddell deep water wdw to upwell generating an open ocean polynya by melting sea ice or hindering its formation once the polynya occurs in the austral winter the underlying water column is destabilized due to the combined effect of the high salinity wdw a massive cooling at the air sea interface and the ensuing brine rejection from newly forming ice thus inducing open ocean deep convection further analysis shows that the buildup of a large heat reservoir at depth by the mid 1970s was a necessary condition to establish the weddell polynya of the 1970s

a possible link between the weddell polynya and the southern annular mode abstract shortly after the advent of the first imaging passive microwave sensor on board a research satellite an anomalous climate feature was observed within the weddell sea during the years 1974 1976 a 250 103 km2 area within the seasonal sea ice cover was virtually free of winter sea ice this feature the weddell polynya was created as sea ice formation was inhibited by ocean convection that injected relatively warm deep water into the surface layer though smaller less persistent polynyas associated with topographically induced upwelling at maud rise frequently form in the area there has not been a reoccurrence of the weddell polynya since 1976 archived observations of the surface layer salinity within the weddell gyre suggest that the weddell polynya may have been induced by a prolonged period of negative southern annular mode sam during negative sam the weddell sea experiences colder and drier atmospheric conditions making for a saltier surface layer with reduced pycnocline stability t

preconditioning of the weddell sea polynya by the ocean mesoscale and dense water overflows the weddell sea polynya is a large opening in the open ocean sea ice cover associated with intense deep convection in the ocean a necessary condition to form and maintain a polynya is the presence of a strong subsurface heat reservoir this study investigates the processes that control the stratification and hence the buildup of the subsurface heat reservoir in the weddell sea to do so a climate model run for 200 years under preindustrial forcing with two eddying resolutions in the ocean 0 25 cm2 5 and 0 10 cm2 6 is investigated over the course of the simulation cm2 6 develops two polynyas in the weddell sea while cm2 5 exhibits quasi continuous deep convection but no polynyas exemplifying that deep convection is not a sufficient condition for a polynya to occur cm2 5 features a weaker subsurface heat reservoir than cm2 6 owing to weak stratification associated with episodes of gravitational instability and enhanced vertical mixing of heat resulting in an erosion of the reservoir in contrast in cm2 6 the water column is more stably stratified allowing the subsurface heat reservoir to build up the enhanced stratification in cm2 6 arises from its refined horizontal grid spacing and resolution of topography which allows in particular a better representation of the restratifying effect by transient mesoscale eddies and of the overflows of dense waters along the continental slope

globsed updated total sediment thickness in the world s oceans we present globsed a new global 5 arc minute total sediment thickness grid for the world s oceans and marginal seas globsed covers a larger area than previously published global grids and incorporates updates for the ne atlantic arctic southern ocean and mediterranean regions which results in a 29 7 increase in estimated total oceanic sediment volume we use this new global grid and a revised global oceanic lithospheric age grid to assess the relationship between the total sediment thickness and age of the underlying oceanic lithosphere and its latitude an analytical approximation model is used to mathematically describe sedimentation trends in major oceanic basins and to allow paleobathymetric reconstructions at any given geological time this study provides a much needed update of the sediment thickness distribution of the world oceans and delivers a model for sedimentation rates on oceanic crust through time that agrees well with selected drill data used for comparison

230th normalization an essential tool for interpreting sedimentary fluxes during the late quaternary there is increasing evidence indicating that syndepositional redistribution of sediment on the seafloor by bottom currents is common and can significantly affect sediment mass accumulation rates notwithstanding its common incidence this process generally referred to as sediment focusing is often difficult to recognize if redistribution is near synchronous to deposition the stratigraphy of the sediment is not disturbed and sediment focusing can easily be overlooked ignoring it however can lead to serious misinterpretations of sedimentary fluxes particularly when past changes in export flux from the overlying water are inferred in many instances this problem can be resolved at least for sediments deposited during the late quaternary by normalizing to the flux of 230th scavenged from seawater which is nearly constant and equivalent to the known rate of production of 230th from the decay of dissolved 234u we review the principle advantages and limitations of this method notwithstanding its limitations it is clear that 230th normalization does provide a means of achieving more accurate interpretations of sedimentary fluxes and eliminates the risk of serious misinterpretations of sediment mass accumulation rates

ocean basin evolution and global scale plate reorganization events since pangea breakup we present a revised global plate motion model with continuously closing plate boundaries ranging from the triassic at 230 ma to the present day assess differences among alternative absolute plate motion models and review global tectonic events relatively high mean absolute plate motion rates of approximately 9 10 cm yr 1 between 140 and 120 ma may be related to transient plate motion accelerations driven by the successive emplacement of a sequence of large igneous provinces during that time an event at 100 ma is most clearly expressed in the indian ocean and may reflect the initiation of andean style subduction along southern continental eurasia whereas an acceleration at 80 ma of mean rates from 6 to 8 cm yr 1 reflects the initial northward acceleration of india and simultaneous speedups of plates in the pacific an event at 50 ma expressed in relative and some absolute plate motion changes around the globe and in a reduction of global mean plate speeds from about 6 to 4 5 cm yr 1 indicates that an increase in collisional forces such as the india eurasia collision and ridge subduction events in the pacific such as the izanagi pacific ridge play a significant role in modulating plate velocities

a convective model for the weddell polynya abstract mechanisms are considered which may induct the large over 105 km2 area of open water or polynya which frequently occurs within the weddell sea winter sea ice we propose that when surface cooling and ice formation decrease the temperature and increase the salinity of the surface water the latter by salt rejection during ice formation in a preconditioned area static instability with intense vertical mixing can occur the upwelled warm salty deep water can then supply enough heat to melt the ice or prohibit its formation even in the middle of winter a simple two level model is derived to test this theory and is found to agree well with observations the process is found to be irregular due to different times of ice onset from one year to the next and to a lesser extent from variations in surface heating and cooling further it is shown that unless the freshwater input exactly balances the increased salinity from the overturn each year the system will either gain or lose salt yearly an

topographic preconditioning of open ocean deep convection abstract evidence for oceanic convection over maud rise in the weddell sea suggests that bottom topography may select the location and scale of deep convecting oceanic chimneys forced by seasonal large scale atmospheric cooling in this paper the role of bottom topography in open mean deep convection is studied using an idealized three dimensional primitive equation model a barotropic mean flow impinges on a gaussian shaped seamount in a stratified domain generating a taylor cap a region of topographically trapped fluid uniform surface cooling is applied throughout the domain when the taylor cap is tall enough to interact with the surface mixed layer the local isolation from the advection of heat by the mean flow forms a conduit into the deep water convection within this region is significantly enhanced relative to ambient levels away from the seamount and to similar numerical simulations performed without bottom topography given uniform background stratification domed isopycnals are not importa

halo of low ice concentration observed over the maud rise seamount 1 a distinctive halo of low sea ice concentration has been observed above the maud rise seamount in the eastern weddell sea the 300 km circular halo is seen most clearly in the monthly mean ice concentration for the months july through november the mean was computed from satellite based passive microwave measurements over a 23 year period the halo is most distinct in october even then however the mean ice concentration in the halo is just 10 less than in the center where it is very near 100 the halo may reflect the existence of a taylor cap circulation over the seamount or other topographically induced mechanisms

the biogeography of major diatom taxa in southern ocean sediments 2 open ocean related species diatoms from 228 southern ocean core top sediment samples were examined to determine the geographic distributions of 32 major diatom species taxa preserved in the sediments of three zonally distinct regions sea ice open ocean and the tropical subtropical in the first of three papers 14 species taxa occurring in the region where sea ice covers the ocean surface on an annual basis are geographically documented comparisons are drawn between the diatom abundances on the sea floor sea ice parameters annual duration and concentration in february and september and february sea surface temperature such parameters are commonly used in reconstructing past oceanographic conditions in the sea ice and open ocean zones analysis of the geographic patterns and sea surface parameter correlations reveals species specific distributions regulated primarily by sea ice coverage and sea surface temperature which support the use of diatom remains for the estimation of these past seasurface environmental parameters comparison with reliable accounts of the 14 species from the sediments or plankton also provides the first glimpses into species specific ecology and habitat linkages d 2005 elsevier b v all rights reserved

antarctic offshore polynyas linked to southern hemisphere climate anomalies offshore antarctic polynyas large openings in the winter sea ice cover are thought to be maintained by a rapid ventilation of deep ocean heat through convective mixing these rare phenomena may alter abyssal properties and circulation yet their formation mechanisms are not well understood here we demonstrate that concurrent upper ocean preconditioning and meteorological perturbations are responsible for the appearance of polynyas in the weddell sea region of the southern ocean autonomous profiling float observations collected in 2016 and 2017 during the largest polynyas to form near the maud rise seamount since 1976 reveal that the polynyas were initiated and modulated by the passage of severe storms and that intense heat loss drove deep overturning within them wind driven upwelling of record strength weakened haline stratification in the upper ocean thus favouring destabilization in 2016 and 2017 we show that previous weddell polynyas probably developed under similarly anomalous conditions which are associated with a mode of southern hemisphere climate variability that is predicted to strengthen as a result of anthropogenic climate change measurements collected during recent polynya events in the southern ocean reveal that these sea ice openings formed as a result of weakened stratification and severe storms and were sustained by deep overturning

multi centennial variability controlled by southern ocean convection in the kiel climate model a quasi oscillatory multi centennial mode of open ocean deep convection in the atlantic sector of the southern ocean in the kiel climate model is described the quasi periodic occurrence of the deep convection causes variations in regional and global surface air temperature southern hemisphere sea ice coverage southern ocean and north atlantic sea surface height the antarctic circumpolar current and the atlantic meridional overturning circulation amoc the deep convection is stimulated by a strong built up of heat at mid depth when the heat reservoir is virtually depleted a coincidental strong freshening event at the sea surface shuts down the convection the heat originates from relatively warm deep water formed in the north atlantic the several decades lasting recharge process of the heat reservoir depends on the amoc and the weddell gyre and sets a minimum delay for the deep convection to recur while the strength of the amoc increases the weddell gyre weakens during the non convective regime convection onset and shutdown also depend on the stochastic occurrence of favorable sea surface conditions which contributes to the multi centennial period of the phenomenon the shutdown triggers a century long deviation in amoc strength caused by significant reductions in bottom water formation and surface salinity in the southern ocean s atlantic sector additional numerical experimentation reveals that sea ice has an important effect on the frequency of occurrence and intensity of the deep convection further we find intriguing similarities to the weddell polynya observed during the 1970s

global sediment thickness data set updated for the australian antarctic southern ocean we present a new 5 min sediment thickness grid for the australian antarctic region 60 e 155 e 30 s 70 s new seismic reflection and refraction data have been used to add detail to the conjugate australian and antarctic margins and intervening ocean floor where regional sediment thickness patterns were poorly known previously on the margins sediment thickness estimates were computed from velocity depth functions from sonobuoy refraction velocity solutions ground truthed against seismic reflection data for the southeast indian ridge abyssal plains sediment thickness contours from geli et al 2007 were used the new regional minimum sediment thickness grid was combined with the global national geophysical data center ngdc sediment grid to create an updated global grid even using the minimum estimates sediment accumulations on the extended australian and antarctic continental margins are 2 km thicker across large regions and up to 9 km thicker in the ceduna basin compared to the global ngdc compilation of sediment thickness data

halo of ice deformation observed over the maud rise seamount 1 a distinctive halo of sea ice deformation was observed above the maud rise seamount in the eastern weddell sea in the winter of 2005 the deformation halo is coincident with a halo of low mean ice concentration that is often observed in the region monthly mean ice vorticity estimates for the months july through november reveal the deformation zone most clearly in an arc about 100 km northwest of the seamount where there is a strong gradient in the bathymetry at depths of 3000 5000 m the deformation was computed from satellite based ice motion vectors derived from envisat synthetic aperture radar backscatter images the deformation halo is evidence of a taylor cap circulation over the seamount which has been described and analyzed with modeling studies and concurrent oceanographic observations obtained during an extensive field campaign

vigorous deep sea currents cause global anomaly in sediment accumulation in the southern ocean the vigorous current systems in the southern ocean play a key role in regulating the earth s oceans and climate with the record of long term environmental change mostly contained in deep sea sediments however the well established occurrence of widespread regional disconformities in the abyssal plains of the southern ocean attests to extensive erosion of deep sea sediments during the quaternary we show that a wide belt of rapid sedimentation rates 5 5 cm k y along the southeast indian ridge seir is a global anomaly and occurs in a region of low surface productivity bounded by two major disconformity fields associated with the kerguelen plateau to the east and the macquarie ridge to the west our high resolution numerical ocean circulation model shows that the disconformity fields occur in regions of intense bottom current activity where current speeds reach 0 2 m s and are favorable for generating intense nepheloid layers these layers are transported toward and along the seir to regions where bottom current velocities drop to 0 03 m s and fine particles settle out of suspension consistent with focusing factors significantly greater than 1 we suggest that the anomalous accumulation of sediment along an 8000 km long segment of the seir represents a giant succession of contourite drifts that is a major extension of the much smaller contourite east of kerguelen plateau and has occurred since 3 5 ma based on the age of the oldest crust underlying the deposit these inferred contourite drifts provide exceptionally valuable drilling targets for high resolution climatic investigations of the southern ocean

some effects of a seamount on oceanic flows abstract to demonstrate some effects of a seamount an oceanic flows we have considered a uniform two layer flow passing a right circular cylinder of arbitrary height in a rotating fluid in the case of vanishing stratification we first generalize previous results of low obstacles to an obstacle of finite height and then show how the frictional regime provides a transition from partial to total blocking as the obstacle top approaches the surface in the case of general stratification we have discerned various dynamical regimes according to blockage of the flows characterized by distinctive interface signatures for example as the obstacle top rises through the water column the axisymmetric doming of the interface first gives way to a reduced fore and aft symmetry when the lower layer is partially blocked then becomes a net depression when the lower layer is totally blocked and finally returns to its unperturbed level as both layers become totally blocked we have derived the critical stratificati

disconformities and paleoceanography in the southeast indian ocean during the past 5 4 million years biostratigraphy and magnetostratigraphy are used to identify deep sea disconformities in the southeast indian ocean the hiatuses are used to define periods of increased bottom water activity and to indicate deep sea circulation patterns during the pliocene and pleistocene seven zones of hiatus are restricted to areas confined by topographic restrictions or by water depth control on bottom water flow an analysis of hiatus frequency reveals that a water depth of about 4 200 m separates disconformities formed by antarctic bottom water 4 200 m and circumpolar deep water 4 200 m hiatus zones in deep basins occur in the tasman emerald balleny and south indian basins and east of the macquarie ridge shallow water hiatus zones 4 200 m occur at the junction of the southeast indian rise the macquarie ridge and the tasman basin and along the antarctic margin changes in the extent of these regional hiatuses are examined by analysis of hiatus occurrence in seven time slices during the past 5 4 m y although the timing of hiatuses produced by deep and shallow water masses are not synchronous the causes of increased bottom water activity may be correlated with paleoclimatic events southern ocean cooling and the proposed re establishment of the west antarctic ice sheet and its fringing ice shelves in the late gilbert chron t 4 2 to 3 6 m y b p were accompanied by increased antarctic bottom water and circumpolar deep water activity as revealed by the frequency of both shallow and deep disconformities the formation of the northern hemisphere barents ice sheet at 3 5 m y b p however led to a decrease in antarctic bottom water production by inhibiting the formation of saline north atlantic deep water which combines with cold antarctic shelf water to form antarctic bottom water cool southern ocean climatic conditions that culminated in the late matuyama chron t 1 66 to 0 72 m y b p resulted in a widespread disconformity produced by increased circumpolar deep water activity and a more restricted disconformity produced by antarctic bottom water the areal extent of both deep and shallow disconformities was reduced after the late matuyama chron t 0 72 m y b p in response to decreased bottom current activity

circulation retention and mixing of waters within the weddell scotia confluence southern ocean the role of stratified taylor columns the waters of the weddell scotia confluence wsc lie above the rugged topography of the south scotia ridge in the southern ocean meridional exchanges across the wsc transfer water and tracers between the antarctic circumpolar current acc to the north and the subpolar weddell gyre to the south here we examine the role of topographic interactions in mediating these exchanges and in modifying the waters transferred a case study is presented using data from a free drifting intermediate depth float which circulated anticyclonically over discovery bank on the south scotia ridge for close to 4 years dimensional analysis indicates that the local conditions are conducive to the formation of taylor columns contemporaneous ship derived transient tracer data enable estimation of the rate of isopycnal mixing associated with this column with values of o 1000 m2 s obtained although necessarily coarse this is of the same order as the rate of isopycnal mixing induced by transient mesoscale eddies within the acc a picture emerges of the taylor column acting as a slow steady blender retaining the waters in the vicinity of the wsc for lengthy periods during which they can be subject to significant modification a full regional float data set bathymetric data and a southern ocean state estimate are used to identify other potential sites for taylor column formation we find that they are likely to be sufficiently widespread to exert a significant influence on water mass modification and meridional fluxes across the southern edge of the acc in this sector of the southern ocean

coherent baroclinic eddies on a sloping bottom a coherent and stable baroclinic eddy in a rotating fluid was produced on a sloping bottom by releasing a dome of salt water into the ambient fresh water a strong cyclonic vortex is produced above the heavy dome the entire eddy system moves north westward with the up slope direction designated north as a taylor column the eddy system displays long lifetimes but it is shown that a theory of isolated systems cannot account for the experimental observations instead it is demonstrated that the vortex flow above the lens is along the lines of constant depth producing a net pressure force on the lens which approximately balances the buoyancy force when ekman friction is also included it accounts for the northward motion of the dome

quantifying the opal belt in the atlantic and southeast pacific sector of the southern ocean by means of 230th normalization 1 a set of 114 samples from the sediment surface of the atlantic eastern pacific and western indian sectors of the southern ocean has been analyzed for 230th and biogenic silica maps of opal content th normalized mass flux and th normalized biogenic opal flux into the sediment have been derived significant differences in sedimentation patterns between the regions can be detected the mean bulk vertical fluxes integrated into the sediment in the open southern ocean are found in a narrow range from 2 9 g m 2 yr 1 eastern weddell gyre to 15 8 g m 2 yr 1 indian sector setting upper and lower limits to the vertically received fraction of open ocean sediments the silica flux to sediments of the atlantic sector of the southern ocean is found to be 4 2 1 4 1011 mol yr 1 just one half of the last estimate this adjustment represents 6 of the output term in the global marine silica budget

southern ocean winter mixed layer austral winter 1986 observations from the polarstern along the greenwich meridian from the ice edge to the antarctic margin show the mixed layer beneath the winter sea ice cover to be significantly depressed in oxygen saturation incorporation of weddell deep water wdw into the winter mixed layer responsible for this undersaturation also introduces heat and salinity into the surface layer which strongly influences the mixed layer sea air exchanges and sea ice formation processes the total wdw transfer into the mixed layer averages 45 myr 1 implying a residence time for the surface water of 2 5 years the associated winter heat flux is 41 w m 2 which limits ice thickness to about 0 55 m agreeing quite well with observations the air temperatures during the cruise are just sufficient to remove the wdw heat input in the presence of observed ice thickness and concentration this suggests that the sea ice cover and wdw heat input into the mixed layer are in approximate balance by midwinter the annual heat flux from wdw to the surface layer and hence into the atmosphere is estimated as 16 w m 2 extrapolation of the greenwich meridian wdw entrainment value to the full circumpolar 60 70 s belt yields total up welling of 24 106 m3 s 1 similar extrapolation of the heat flux value gives a circumpolar total of 2 8 1014 w as a consequence of circulation topography interaction the maud rise water column stands out as an anomaly relative to the surrounding region with a significantly more saline and dense mixed layer below the mixed layer the water column over the crest of the rise is identical to that over the flanks if the latter water column is upwelled by 400 m this uplifting is believed to be a response of the upstream flow encountering the rise increased upstream flow would be expected to increase maud rise upwelling and the dependent salinity density of the mixed layer slight increases in the mixed layer density could trigger a convective mode and generation of a polynya it is hypothesized that spin up of the weddell gyre s barotropic circulation induced by an increase of the regional wind stress curl would enhance the probability of polynya development over maud rise

the dynamics of mixed layer deepening during open ocean convection abstractopen ocean convection is a common phenomenon that regulates mixed layer depth and ocean ventilation in the high latitude oceans however many climate model simulations overestimate mixed l

bedform velocity matrix the estimation of bottom current velocity from bedform observations a wide variety of bedforms both depositional and erosional in origin has been recognized on the deep seafloor and attributed to the influence of bottom currents these range in scale from those visible in bottom photographs centimeter to decimeter to those recorded with seafloor bathymetric imaging meter to kilometer in many cases it has been possible to provide some quantification of substrate grain size and flow velocity responsible for each bedform type we have synthesized both our own and published data in order to present a bedform velocity matrix which facilitates the estimation of bottom current velocity based on bedform type despite imperfections we believe this to be a valuable model for assessing strength and variability of bottom currents that can have a significant influence on the siting of submarine cables pipelines and other seafloor installations

causal interactions between southern ocean polynyas and high latitude atmosphere ocean variability abstractweddell sea open ocean polynyas have been observed to occasionally release heat from the deep ocean to the atmosphere indicating that their sporadic appearances may be an important feature

lithospheric anisotropy on the kerguelen hotspot track inferred from rayleigh wave polarisation anomalies summary rayleigh waves recorded at the geoscope station paf on the kerguelen isles in the indian ocean show strong polarisation anomalies in the period range 20 50 s as demonstrated by dispersion analysis of 3 component recordings the largest and most consistent anomalies are observed for events located in the southern part of the java trench at 25 s the rayleigh waves present transverse components with an amplitude of up to 55 per cent of the amplitude of the longitudinal components the particle motion in the horizontal plane is largely elliptical by comparison very few and mostly small polarisation anomalies are detected at the nearby geoscope stations ais and crzf on the amsterdam and crozet isles respectively wave path deviations from the epicentre receiver great circle as calculated in tomographic models of the indian ocean cannot explain the polarisation anomalies using a multiple scattering scheme for modelling surface waves in 3 d heterogeneous and anisotropic structures we show that wavefield distortion due to the geometrical structure of the kerguelen plateau in the vicinity of the station cannot explain the anomalies either but that anisotropy can we infer the presence of an anisotropic structure in the lithosphere to the north of the kerguelen isles containing 40 per cent oriented pyrolite with fast axis tilting downwards in a north north east direction the anisotropy may be caused by deformation of the lithosphere related to the kerguelen hotspot

evolution of the southern ocean winter mixed layer and sea ice open ocean deepwater formation and ventilation an analytical model has been developed to describe the nature of the winter ocean sea ice interaction in the southern ocean to determine the principal processes responsible for maintaining stability in this marginally stable region and to predict the response to changes in the external forcing and initial conditions the model accurately describes the sparse winter observations and the general temporal evolution of the ice mixed layer depth and salinity for the period following the elimination of the seasonal pycnocline by rapid fall ice growth a scale analysis and sensitivity study reveal the system parameter dependence this indicates that the magnitude of entrainment is proportional to fst h0 s where fs is the external salt flux h0 the depth of the winter mixed layer and s the salinity gradient through the pycnocline thermodynamic ice growth reflects predominantly the competition between the oceanic diffusive heat gain across the base of the mixed layer and the loss to the atmosphere through the ice and leads a strong thermal gradient across the pycnocline leads to a significant diffusive flux which balances most of the heat lost to the atmosphere this limits the amount of ice growth possible and plays the principal role in maintaining the system stability since the ice growth is required for destabilization through haline rejection during the growth process the system is also sensitive to the large ratio of heat to salt in the pycnocline this introduces a negative feedback where increasing ice growth induces entrainment by the associated salt flux which in turn drives an entrainment heat flux which compensates for heat lost to the atmosphere this moderates the ability to grow ice and drive further entrainment the absolute strength of the salinity gradient on the other hand dictates the effectiveness which a same magnitude external salt flux will have in driving entrainment the effectiveness with which changes in these parameters as well as several other external parameters can influence the southern ocean stability is explicitly evaluated through the sensitivity study and analysis

post miocene disconformities and paleoceanography in the atlantic sector of the southern ocean abstract the magnetostratigraphy and biostratigraphy of 166 islas orcadas piston cores in the atlantic sector of the southern ocean reveal disconformities in the sedimentary record which are attributed to bottom current erosion the hiatus frequency as a function of water depth reveals both shallow erosion by circumpolar deep water cdw and deep erosion by antarctic bottom water aabw the post miocene history of erosion by both bottom currents is delineated by analysis of areal and temporal fluctuations of hiatus frequency erosion by cdw caused disconformities on the maud rise astrid ridge indian antarctic ridge scotia sea and islas orcadas rise the shallow disconformities increased in frequency and size from 4 0 to 1 0 ma with two periods of distinctly widespread disconformities from 5 5 to 4 5 ma and 2 5 to 1 5 ma after 1 0 ma the frequency and extent of the hiatuses decreased erosion by aabw caused disconformities in the weddell abyssal plain argentine basin and through a narrow conduit in the south sandwich basin downstream from the bullard fracture zone the deep disconformities increased in frequency and size from 4 5 to 5 0 ma 3 5 to 4 0 ma and from 1 0 to 2 5 ma after 1 5 ma the frequency and extent of hiatuses decreased fluctuations in hiatus frequency are consistent with results of a similar study in the southeast indian ocean and may be correlated with paleoceanographic events southern ocean cooling and possible re establishment of the west antarctic ice sheet and fringing ice shelves at 4 3 3 5 ma was accompanied by increased aabw and cdw activity the formation expansion of the northern hemisphere ice sheets at 2 5 3 0 ma resulted in a decrease in aabw velocity and was accompanied by increased cdw activity both deep and shallow bottom currents increased in velocity during the late matuyama chron 2 0 1 5 ma as a result of antarctic cooling after 1 0 ma however the frequency and extent of shallow and deep hiatuses was reduced and the modern extent of hiatuses is the lowest within the post miocene