reservoir sedimentation i delta and density current deposits in china where rivers draining loess soils have some of the highest sediment concentrations in the world reservoir sedimentation is a serious problem that has been the subject of intensive investigation since 1957 this paper describes patterns of sediment deposition and erosion drawing on field measurements in china at the sanmenxia guanting shanyiujiang and liujiaxia reservoirs as well as the shanshenggorig barrage and xijin hydropower station the longitudinal and lateral geometry of reservoir deltas is described it is shown that a marked change in grain size can occur at the downstream limit of the delta even when this limit is not apparent from geometry density current data for guanting reservoir are presented problems caused by reservoir sedimentation in china are described including aggredation in the backwater region causing increased flood levels elevated ground water levels and navigation impairment deposition from or into a tributary entry of sediments into hydropower turbines a

fluvial processes and morphological response in the yellow and weihe rivers to closure and operation of sanmenxia dam abstract the fluvial and morphological processes induced by impoundment of the sanmenxia reservoir and relevant human activities on the yellow river and its tributaries are complex the long term annual sediment load of the yellow river was 1 6 billion tons ranking first of all the world s rivers in 1960 sanmenxia dam began filling sediment transport in the river then was greatly disturbed and a new cycle of the fluvial processes was induced first the dam caused not only anticipated sedimentation in the reservoir but also serious sedimentation in the largest tributary of the river the weihe river the response of fluvial process to the dam closure varies in space and time second the downstream reaches of the dam experienced erosion and resiltation changes of river pattern and development of meanders moreover the downstream reaches of the dam have experienced more and more water diversion which has induced readjustment of the longitudinal profile of the river the study reveals that sedimentation in the sanmenxia reservoir enhanced the bed elevation at tongguan where the weihe river flows into the yellow river the rising tongguan s elevation caused retrogressive siltation waves in the weihe river which propagated at a speed of about 10 km yr an equilibrium sedimentation model is proposed which agrees well with the data of sedimentation in the weihe river in the reaches below the dam the river changes from braided to wandering or from wandering braided to wandering meandering the discharge released to the downstream reaches has been regulated by the reservoir and it decreases along the course because the quantity of water diversions is more than the inflow from tributaries the reduction in discharge causes readjustment of the longitudinal bed profile by using the minimum stream power theory we prove that the riverbed profile is developing toward an s shape profile

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

predicting the morphodynamic response of silt laden rivers to water and sediment release from reservoirs lower yellow river china the high sediment load of the yellow river results in rapid infilling of its reservoirs when sediment is not regularly flushed simultaneously the downstream reaches of the yellow river experience extremely high siltation rates which are reduced when sediment is retained in its reservoirs to minimize siltation in the reservoirs and the downstream river bed water and sediment are released from the reservoir in a controlled way through flushing experiments in this paper we analyze the effect of such a flushing event on the downstream river bed through data analysis and numerical modeling sedimentation may be minimized by relating the amount of sediment released from the reservoir to the sediment available for release through operational monitoring and by releasing relatively clear water after turbid water despite this flushing of sediment the reservoir will eventually fill up and more sediment released again into the lower yellow river the change in discharge magnitude and frequency brought about by the reservoir will then probably result in increased siltation rates in the lower yellow river compared to the predam situation

the hafnium and neodymium isotope composition of seawater in the atlantic sector of the southern ocean we present the first combined dissolved hafnium hf and neodymium nd concentrations and isotope compositions of deep water masses from the atlantic sector of the southern ocean eight full depth profiles were analyzed for hf and twelve for nd hafnium concentrations are generally depleted in the upper few hundred meters ranging between 0 2 pmol kg and 0 4 pmol kg and increase to relatively constant values of around 0 6 pmol kg in the deeper water column at the stations north of the polar front pf nd concentrations increase linearly from about 10 pmol kg at depths of 200 m to up to 31 pmol kg close to the bottom indicating particle scavenging and release within the weddell gyre wg however nd concentrations are essentially constant at 25 pmol kg at depths greater than 1000 m the distributions of both elements show a positive correlation with dissolved silicon implying a close linkage to diatom biogeochemistry hafnium essentially shows invariant isotope compositions with values averaging at ehf 4 6 whereas nd isotopes mark distinct differences between water masses such as modified north atlantic deep water nadw end 11 to 10 and antarctic bottom water aabw end 8 6 to 9 6 but also waters locally advected via the agulhas current can be identified by their unradiogenic nd isotope compositions mixing calculations suggest that a small fraction of nd is removed by particle scavenging during mixing of water masses north of the pf nevertheless the nd isotope composition has apparently not been significantly affected by uptake and release of nd from particles as indicated by mixing calculations a mixing envelope of an approximated north pacific and a north atlantic end member shows that nd isotope and concentration patterns in the lower circumpolar deep water lcdw can be fully explained by 30 70 percentage contributions of these respective end members

tracing the thermohaline circulation in the western north atlantic using chlorofluorocarbons abstract measurement of chlorofluorocarbons cfcs f 11 and f 12 were obtained from vertical sections extending across the deep western boundary current dwbc along the western margin of the north atlantic ocean between 32 45 n in 1983 and 1986 these sections revealed an upper and a deep cfc maximun with highest concentrations along the continental slope the upper maximum had a potential temperature ranging from 4 2 4 6 c a core density of 1 5 34 56 and occurred at a depth of about 800m adjacent to the continental slope water with classical labrador sea water s characteristics occurs just above the cfc minimum between the upper and lower maxima and indicates that a significant volume of classical labrador sea water has not entered the region south of the grand banks for the last 3 decades the deep cfc maximum is from dense water overflowing the greenland iceland scotland ridge it occurred at about 3500m depth and had a potential temperature range of 2 02 2 18 c and a core density of 4 45 87 a source function of f 11 and f 12 for the deep maximum was estimated using temperature salinity and tracer data from previous studies the core of the dwbc defined by the deep cfc maximum was estimated to consist of roughly 2 3 gibbs fracture water and 1 3 denmark strait overflow water however 80 or more of the f 11 and f 12 signal was derived from denmark strait overflow water ages for the upper and lower cfc maxima in the dwbc were estimated from the cfc data these ages ranged from 11 12 years at the northernmost section to 18 years at the southernmost section for both features this corresponds to apparent mean current speeds of about 0 8cm s 1 for the upper maximum and 1 4cm s 1 for the deep maximum an age of 8 9 years with a corresponding apparent mean current speed of 2 2 2 5cm s 1 was also estimated for the deep cfc maximum from the observed increase in the f 11 concentration at 41 n between 1983 and 1986 the apparent mean current speeds are significantly less than current speeds measured with long term current meter moorings two reasons for this are 1 mixing between the dwbc core and adjacent water causes ages estimated from cfc data to be overestimated and hence the associated current speed to be underestimated pickart hogg and smethie 1989 and 2 water propagating along the western boundary spends some time in recirculation gyres the northern recirculation gyre south of the eastern north american continental margin is clearly apparent in the lateral distributions of f 11 and f 12 the apparent mean current speed in the northern recirculation gyre was estimated to be 1 3cm s 1 from the cfc data seaward of the dwbc north of the new england seamount chain high cfc low silica parcels of water were observed interspersed with low cfc high silica parcels of water along the bottom the high cfc low silica water is overflow water with a slightly greater component of denmark strait overflow water than contained in the dwbc deep cfc maximum the low cfc high silica water is of southern origin and has a component of antarctic bottom water

long term effects of water diversions on the longitudinal flow and bed profiles abstractwater diversions along alluvial rivers cause water and sediment loss and thereby affect morphological development assuming spatially continuous diversions along a constant width channel previous studies suggest a longitudinally convex bed at the equilibrium state however the validity of a convex bed profile for practically discrete diversions in natural channels of longitudinally varying width remains to be justified moreover such equilibrium analysis does not reveal the morphological time scale mts associated with water diversions to solve these issues a general theoretical framework is proposed for predicting the equilibrium state of the fluvial system which is applicable to both continuous and discrete water diversions in a longitudinally width varying channel numerical experiments complement the mts studies for water diversions the effects of diversion intensity diversion placement discrete and continuous and diversion scheme pure water and water sediment mixture are also s

data based estimation of anthropogenic carbon and acidification in the weddell sea on a decadal timescale 1 the amount of anthropogenic co2 cant that entered the weddell sea between 1992 and 2008 cant1992 2008 was assessed using the extended multiple linear regression emlr method in the warm deep water wdw and the weddell sea bottom water wsbw cant1992 2008 values were insignificant whereas values as high as 8 mol kg 1 were observed over the shelf cant1992 2008 concentrations in the surface layer varied with latitude between 2 and 11 mol kg 1 weak intrusion of anthropogenic co2 into weddell sea deep water wsdw was demonstrated cant1992 2008 yields 1 5 2 mol kg 1 that more cant1992 2008 was found in the wsdw than in the wsbw is surprising but can be explained by intense ventilation of the wsdw originating from east of the weddell gyre the invasion of cant1992 2008 provokes a shift in the equilibria of the carbonate system resulting in acidification and reduction of co32 concentration the mean decrease of ph in the upper 200 m layer was 0 016 the largest decrease of calcite and aragonite saturation states was observed at the surface this implies that surface waters might become undersaturated with respect to aragonite in the future while the underlying wdw is still saturated results of this analysis suggest that complete undersaturation of surface waters in the weddell sea will be reached after the 21st century

significant effect of sediment cohesion on delta morphology delta morphology is thought to be controlled by factors such as river discharge tides and waves numerical modelling shows that sediment cohesion also strongly influences the development of a delta s characteristics the morphologies of the world s deltas are thought to be determined by river discharge tidal range and wave action 1 more recently sea level rise 2 3 and human engineering4 have been shown to shape delta evolution the effects of factors such as sediment type and the overall amount of sediment carried by rivers are considered secondary4 5 6 in particular the role of sediment cohesion which is controlled by sediment size and type of vegetation is unclear here we use a numerical flow and transport model7 8 9 10 to show that sediment cohesiveness also strongly influences the morphology of deltas we find that holding all other factors constant highly cohesive sediments form bird s foot deltas with rugose shorelines and highly complex floodplains whereas less cohesive sediments result in fan like deltas with smooth shorelines and flat floodplains in our simulations sediment cohesiveness also controls the number of channels that form within the deltas and the average angle of bifurcation of those channels as vegetation generally acts as a cohesive agent we suggest that deltas that formed before the expansion of land plants in the devonian period should show fan like characteristics a finding consistent with the limited data from the sedimentological record11

global ocean meridional overturning abstract a decade mean global ocean circulation is estimated using inverse techniques incorporating air sea fluxes of heat and freshwater recent hydrographic sections and direct current measurements this information is used to determine mass heat freshwater and other chemical transports and to constrain boundary currents and dense overflows the 18 boxes defined by these sections are divided into 45 isopycnal neutral density layers diapycnal transfers within the boxes are allowed representing advective fluxes and mixing processes air sea fluxes at the surface produce transfers between outcropping layers the model obtains a global overturning circulation consistent with the various observations revealing two global scale meridional circulation cells an upper cell with sinking in the arctic and subarctic regions and upwelling in the southern ocean and a lower cell with sinking around the antarctic continent and abyssal upwelling mainly below the crests of the major bathymetric ridges

horizontal circulation and volume transports in the weddell gyre derived from argo float data argo floats provide both hydrographic and trajectory data affording the opportunity to investigate surface to mid depth ocean dynamics here argo float data are used to determine the absolute geostrophic velocity field of the upper 50 2000 m of the weddell gyre from which the overall circulation pattern is investigated the weddell gyre plays a pivotal role in the modification of climate by advecting heat towards the antarctic ice shelves and by modifying the water masses that feed into the lowest limb of the global ocean overturning circulation warm deep water the source water mass that delivers heat to the weddell gyre is conveniently located within the upper 2000 m domain covered by the floats we investigate its volume transport as it circulates the gyre full depth volume transports are estimated by applying a quadratic function to extrapolate the relative dynamic height field component to the full ocean depth using ctd profiles from ship based surveys to determine an extrapolation error major new insights are provided by this study there is an established double gyre structure to the circulation with a strong eastern cell and a weaker western cell regional variation of the baroclinic component of the flow field is revealed indicating a northeast to southwest reduction in the baroclinic flow along with a strong meridional gradient of baroclinic flow along the northern limb of the gyre especially east of 25 w the zonal mean gyre strength away from the shelf edge is 32 5 sv 1 sv 1 106 m3 s 1 of which 13 3 sv are associated with the advection of warm deep water there is a considerable amount of recirculation within the gyre interior where water does not traverse the full zonal extent of the gyre the recirculation is stronger in the eastern cell of the observed double gyre structure the interior circulation cells partly explain the large variations in previous gyre strength estimates we provide an extensive review of previous estimates in context of the new results obtained

a field investigation of the sediment transport characteristics of a high sediment load intermittent river in taiwan most rivers in taiwan are intermittent rivers with relatively steep slopes and carry rapid sediment laden flows during typhoon or monsoon seasons a series of field experiments was conducted to collect suspended load data at the tzu chiang bridge hydrological station of the lower cho shui river which is a major river with the highest sediment yield in taiwan the river reach was aggrading with a high aspect ratio during the 1980s because of sand mining and extreme floods it was incised and has had a relatively narrow main channel in recent years the experimental results indicated that typical sediment transport equations can correctly predict the bed material load for low or medium sediment transport rates e g less than about 1000 tons day m however these equations far underestimate the bed material load for high sediment transport rates the effects of cross sectional geometry change i e river incision and earthquakes on the sediment load were investigated in this study an empirical sediment transport equation with consideration of the aspect ratio was also derived using the field data collected before and after river incision copyright 2012 john wiley sons ltd

numerical simulation of longitudinal and lateral channel deformations in the braided reach of the lower yellow river bank erosion frequently occurs in the lower yellow river lyr playing an important role in the evolution of this braided river a two dimensional 2d composite model is developed herein that consists of a depth averaged 2d flow and sediment transport submodel and a bank erosion submodel the model incorporates a new technique for updating bank geometry during either degradational or aggradational bed evolution allowing the two submodels to be closely combined using the model the fluvial processes in the braided reach of the lyr between huayuankou and laitongzhai are simulated and the calculated results generally agree with the field measurements including the water surface elevation variation of water surface width and variations of cross sectional profiles the calculated average water surface elevation in the study reach was 0 09 m greater than the observed initial value and the calculated mean bed elevation for six cross sections was 0 11 m lower than the observed value after 24 days these errors are attributed to the large variability of flow and sediment transport processes sensitivity tests of three groups of parameters are conducted and these groups of parameters are related to flow and sediment transport bank erosion and model application respectively analysis results of parameter sensitivity tests indicate that bank erodibility coefficient and critical shear stress for bank material are sensitive to the simulated bank erosion process the lateral erosion distance at huayuankou will increase by 19 as the value of bank erodibility coefficient changes from 0 1 to 0 3 and it will decrease by 57 as the value of critical shear stress for bank increases from 0 6 to 1 2 n m limited changes of other parameters have relatively small effects on the simulated results for this reach and the maximum change extent of calculated results is less than 5 because the process of sediment transport and bank erosion in the braided reach of the lyr is very complicated further study is needed to verify the model

anthropogenic co2 estimates in the southern ocean storage partitioning in the different water masses the research leading to these results was supported through eu fp7 project carbochange changes in carbon uptake and emissions by oceans in a changing climate which received funding form the european commission s seventh framework under grant agreement no 264879 and by the spanish ministry of sciences and innovation and was co founded by the fondo europeo de desarrollo regional 2007 2012 feder through the catarina project ctm2010 17141 mar

distribution and circulation of labrador sea water abstract labrador sea water is the final product of the cyclonic circulation of subpolar mode water in the open northern north atlantic mccartney and talley 1982 the temperature and salinity of the convectively formed subpolar mode water decrease from 14 7 c 36 08 to 3 4 c 34 88 on account of the cumulative effects of excess precipitation and cooling the coldest mode water is labrador sea water which spreads at mid depths and is found throughout the north atlantic ocean north of 40 n and along its western boundary to 18 n a vertical minimum in potential vorticity is used as the primary tracer for labrador sea water labrador sea water is advected in three main directions out of the labrador sea 1 northeastward into the irminger sea 2 southeastward across the atlantic beneath the north atlantic current and 3 southward past newfoundland with the labrador current and thence westward into the slope water region crossing under the gulf stream off cape hatteras the labrador sea water core is

dynamic characteristics of the yellow river mouth the yellow river delta is dynamic because the river carries a heavy sediment load and serious siltation occurs in the area massive sediment transport brings fertile soil to the river mouth leading to its expansion by 20 25 km2 per year the dramatic reduction in runoff and sediment in the delta due to increasing water demand has extended the period of flow cut off the river changes its delta channel about once every ten years the channel shifts affect the upstream flood stages since the river shifted to the present delta channel in 1976 the shape of the delta has changed remarkably sediment laden flow has been building channels by depositing sediment in low velocity areas and has extended the river mouth into the sea for more than 40 km the velocity of sea currents is enhanced by the growth of the river mouth spit at a rate of 1 m s 1 per 10 km extension which results in high capacity of sea currents carrying sediment away from the mouth the new river mouth area was silted up by 14 m and the old river mouth area was eroded by 5 m siltation takes place in the low flow season and scouring occurs in the flood season scouring occurs during the rising limb of a flood and siltation occurs during the receding limb of a flood the roughness of the river changes largely following the increase in discharge the minimum manning roughness occurs at non siltation and non erosion discharge tidal influence in the river is restricted to a section extending over only a few kilometres unlike in many other coastal areas the river water with its high sediment concentration develops into density current and flows underneath the seawater during flood this mechanism contributes to sediment transportation in the delta copyright 2000 john wiley sons ltd

bottom water export from the western ross sea 2007 through 2010 bottom water export from the ross sea february 2007 to january 2011 exhibits seasonal and interannual variability temperature minima coupled to salinity maxima in late austral summer into the fall indicate input from high salinity shelf water hssw secondary temperature minima lacking the high salinity trait characteristic of low salinity shelf water lssw appear in the spring warmer bottom water similar to modified circumpolar deep water mcdw is observed in winter and in early summer the lssw and mcdw may be drawn from the drygalski basin as the hssw pool retreats poleward from the shelf break in response to increased winter polar easterlies allowing these less dense overlying waters to spill into the deep ocean within the benthic layer bottom salinity decreased from 2007 to 2011 by 0 007 year 1 significantly higher than regional decadal trends which we propose is a result of hssw retreat induced by strengthening polar easterlies

exchange of water between the ross gyre and acc assessed by lagrangian particle tracking to reach upwelling and downwelling zones deep within the southern ocean seasonal sea ice cover water masses must move across the antarctic circumpolar current and through current systems including the ross gyre weddell gyre and antarctic slope current in this study we focus our attention on the lagrangian exchange between the ross gyre and surrounding current systems we conducted numerical experiments using five day 3 d velocity fields from the southern ocean state estimate with a particle tracking package to identify pathways by which waters move from near the antarctic coastal margins or antarctic circumpolar current into the interior of the ross gyre and to identify the time scales of variability associated with these pathways waters from near the antarctic margins enter the ross gyre along the western and northern boundaries of gyre until the gyre separates from the pacific antarctic ridge near fracture zones at this juncture antarctic circumpolar current derived inflow dominates the across gyre transport up to the antarctic margin transport and exchange associated with different time average components of flow are calculated to determine the relative contributions of high and low frequency and time mean components

river sedimentation and channel adjustment of the lower yellow river as influenced by low discharges and seasonal channel dry ups fluvial sedimentation can be classified as one of the three types in terms of different morphological units i e vertical overbank deposition vertical deposition on the riverbed and lateral deposition on the riverbank with the example of the lower yellow river the three types of sedimentation have been studied in relation to channel adjustment since 1986 the lower yellow river has entered a new period that is characterized by small discharges with seasonal desiccations dry ups and strong sedimentation has occurred based on the measurement of 36 cross sections along the lower yellow river the rates of the three types of deposition have been calculated and are reported in this study multiple regression has been used to express the relationship between the three rates of deposition and the channel shape adjustment index influenced by the difference in channel sediment and by downstream variations in water discharge and sediment load the indices of channel deposition rate show a clear trend of downstream variation controlled by this the manner of channel adjustment also shows clear trends downstream under the hydrological regime dominated by small discharges the tendencies of adjustment of riverbed slope and bankfull slope are inverse i e the channel slope tends to decrease but the slope at bankfull stage to increase at present the former tendency is dominant thus the sediment carrying capacity of the lower yellow river tends to decrease which intensifies channel sedimentation

geomorphic tectonic control of sediment discharge to the ocean the importance of small mountainous rivers analysis of data from 280 rivers discharging to the ocean indicates that sediment loads yields are a log linear function of basin area and maximum elevation of the river basin other factors controlling sediment discharge e g climate runoff appear to have secondary importance a notable exception is the influence of human activity climate and geology on the rivers draining southern asia and oceania sediment fluxes from small mountainous rivers many of which discharge directly onto active margins e g western south and north america and most high standing oceanic islands have been greatly underestimated in previous global sediment budgets perhaps by as much as a factor of three in contrast sediment fluxes to the ocean from large rivers nearly all of which discharge onto passive margins or marginal seas have been overestimated as some of the sediment load is subaerially sequestered in subsiding deltas before the proliferation of dam construction in the latter half of this century rivers

role of tides on the formation of the antarctic slope front at the weddell scotia confluence the structure of the antarctic slope front asf and the associated antarctic slope current asc on the scotia sea side of the weddell scotia confluence wsc is described using data from a hydrographic survey and three 1 year long moorings across the continental slope the asc in this region flows westward along isobaths with an annual mean speed of 0 2 m s 1 with time variability dominated by the k1 and o1 tidal diurnal constituents a narrowband oscillation with 2 week period attributable to the spring neap tidal cycle and seasonal variability realistic and idealized high resolution numerical simulations are used to determine the contribution of tides to the structure of the asf and the speed of the asc two simulations forced by realistic atmospheric forcing and boundary conditions integrated with and without tidal forcing show that tidal forcing is essential to reproduce the measured asf asc cross slope structure the time variability at our moorings and the reduced stratification within the wsc two idealized simulations run with tide only forcing one with a homogeneous ocean and the other with initial vertical stratification that is laterally homogeneous show that tides can generate the asc and asf through volume flux convergence along the slope initiated by effects including the lagrangian component of tidal rectification and mixing at the seabed and in the stratified ocean interior climate models that exclude the effects of tides will not correctly represent the asf and asc or their influence on the injection of intermediate and dense waters from the wsc to the deep ocean

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

double thresholds in scour fill processes and some implications in channel adjustment abstract through an analysis of data collected from the yellow river and its tributaries on the loess plateau of china the phenomenon of double thresholds in scour fill processes of wide range water sediment two phase flows has been shown thresholds located in non hyperconcentrated flows may be called the lower threshold and that in hyperconcentrated flows the upper threshold this double threshold phenomenon leads to complicated sediment transport behavior of heavily sediment laden rivers with an increase in suspended sediment concentration the channel sediment delivery ratio increases initially and becomes higher than 1 followed by a decrease and finally becomes lower than 1 again controlled by the double thresholds in the scour fill processes channel adjustment of the lower yellow river is non linear and complex when the suspended concentrations were lower than the lower threshold or higher than the upper threshold scour or bed downcutting was the dominant channel forming process channel shape tends to be narrower and deeper and the channel thalweg became more sinuous when the suspended concentrations lay between the lower and upper thresholds deposition of sediment was the dominant channel forming process channel shape tended to be shallower and wider and channel thalweg became less sinuous

the diabatic deacon cell abstract southern ocean air sea fluxes from the coads dataset are examined for compatibility between buoyancy gain and northward ekman transport an analysis in density classes points to an upwelling of upper circumpolar deep water and subsequent buoyancy gain from air sea exchange as water flows northward in the ekman layer the upper circumpolar deep water layer is too shallow to admit southward geostrophic flow along topography and an eddy mass flux mechanism for southward transport in this layer to replenish the upwelling is advanced based on observed large scale potential vorticity gradients estimates of the strength and vertical structure of the meridional flow are given using repeated hydrographic sections and a new climatology

weddell sea phytoplankton blooms modulated by sea ice variability and polynya formation author s vonberg lauren prend channing j campbell ethan c mazloff matthew r talley lynne d gille sarah t

grain size and sediment concentration effects on channel patterns of silt laden rivers abstract the yellow river in china is characterized by channel erosion and a meandering channel pattern at both low sediment concentrations and at hyperconcentrations whereas deposition and braiding tends to dominate at intermediate sediment concentrations a 3d numerical model delft3d is used to analyze the effect of sediment induced density effects on the formation of channel patterns using a highly simplified river geometry results show that the effect of vertical sediment density stratification and hindered settling significantly influence river channel patterns at some critical concentration the saturation concentration vertical mixing is totally damped by the density gradients at the lutocline a strong vertical gradient in sediment concentration this increases the secondary circulation in the river channel significantly which in turn promotes meander development when the flow is not close to saturation the main channel forming process seems to be channel avulsion through blocking of old channels and subsequent channel cut off when the concentrations are so high that sediment is primarily held in suspension by hindered settling sedimentation is so much reduced that channels are no longer blocked and therefore the river course becomes confined to a single meandering channel

breaching in fine sands and the generation of sustained turbidity currents in submarine canyons natural moderately loosely packed sands can only erode from the surface of the bed after an increase in pore volume because of this shear dilatancy negative pore pressures are generated in the bed in cases of low permeability these negative pressures are released relatively slowly which retards the maximum rate of erosion this effect is incorporated in a new analytically derived pick up function that can explain the observation of gradual retrogressive failure of very steep subaqueous slopes sometimes more than 5 m high in fine non cohesive sands this process termed breaching in the field of sediment dredging may produce large failures in sand bars or river banks the analytical function that describes the breaching process in fine sand is incorporated in a one dimensional steady state numerical model of turbidity currents describing the spatial development of flow this model is applied to simulate a large flushing event in scripps submarine canyon pacific coast of california breach retrogradation and the successive evolution in time of the resulting turbidity current in the canyon are predicted in a sequence of discrete steps predicted velocities are compared with values measured during a flushing event implications for the interpretation of deep water massive sands are discussed

stratification effects by cohesive and noncohesive sediment this study focuses on stratification effects induced by the interaction between suspensions of fine grained cohesive and noncohesive sediment and the turbulent flow in estuarine and coastal environments a concise literature review reveals that this interaction may cause an appreciable modification of the vertical profiles of velocity vertical eddy viscosity diffusivity and reynolds stresses this interaction is further studied with a one dimensional vertical 1dv numerical model which includes the standard k turbulence model with buoyancy destruction terms application of this model to the experimental results by coleman 1981 shows that the measured changes in the velocity profile can be explained entirely by sediment induced buoyancy effects it is argued that capacity for cohesive sediment is cc e a fluid mud layer is formed upon the deposition of the cohesive sediment flocs contrary to the case with noncohesive sediment where the depositing grains immediately form a rigid bed thus a two layer fluid system develops causing significant damping of the vertical mixing processes decreasing the carrying capacity further this positive feed back results in a catastrophic collapse of the turbulence field and the concentration profile a scaling law for this saturation behavior is derived from classical stratified flow theory this law is sustained with a series of numerical experiments with the 1dv model it also predicts the suspended sediment concentrations observed in the yellow river to the right order of magnitude it is concluded that sediment induced buoyancy yields appreciable stratification effects at already moderate suspended sediment concentrations in the case of cohesive sediment these stratification effects can result in a catastrophic collapse of the turbulent flow field and the vertical profile of suspended sediment concentration

modelling hyperconcentrated flow in the yellow river a large amount of the total sediment load in the chinese yellow river is transported during hyperconcentrated floods these floods are characterized by very high suspended sediment concentrations and rapid morphological changes with alternating sedimentation and erosion in the main channel and persistent sedimentation on the floodplain however the physical mechanisms driving these hyperconcentrated floods are still poorly understood numerical modelling experiments of these floods reveal that sedimentation is largely caused by large vertical concentration gradients both in the channel during the rising stage of the flood as well as on the floodplains during a later stage of the flood these vertical concentration gradients are large because the turbulent mixing rates are reduced by the increased sediment induced density gradients resulting in a positive feedback mechanism that produces high deposition rates erosion prevails when the sediment is largely held in suspension due to hindered settling and is strengthened by the reduced wetted cross section caused by massive sedimentation on the floodplain observed patterns of erosion and sedimentation during these floods can be qualitatively reproduced with a numerical model in which sediment induced density effects and hindered settling are included copyright 2009 john wiley sons ltd

scale independent assessment of discharge reduction and riparian disconnectivity following flow regulation by dams by using the established hydraulic relationships among flood frequency flood magnitude and river channel capacity we develop a scale independent assessment of the hydrogeomorphic impacts of 21 dams across the united states that have broad ranges in function and contributing drainage area on the basis of generalized extreme value gev analysis of pre and post dam hydrologic records our analysis indicates that the 2 yr discharge has decreased 60 following impoundment exceeding the magnitude of climatically triggered discharge reductions occurring during the holocene reductions in the frequency of the pre dam 2 yr discharge have been equally profound the pre dam 2 yr flood has occurred on average twice per site whereas statistical analysis indicates that it should have occurred 20 times furthermore floods greater than bankfull have been essentially eliminated by dams completely disconnecting the riparian zone from riverine influence our analyses herein suggest that a critical threshold of disconnectivity exists and corresponds approximately to the pre dam 5 yr flood this similar recurrence probability exists independent of region dam type or catchment size

reservoir sedimentation ii reservoir desiltation and long term storage capacity during the past 30 years china has implemented structural and operational procedures to maximize sediment discharge from the impounded reaches of rivers thereby reducing arresting or reversing reservoir sedimentation at many sites hydraulic methods used in china to preserve reservoir capacity are outlined in this paper including sediment routing during floods sediment flushing during floods emptying and flushing and density current venting a method to compute retrogressive erosion during the drawdown of a reservoir is developed and an approximate method for estimating the amount of sediment that can be vented from a reservoir as a density current is proposed the computed values are compared with field data field data on the floodplain slope formed during flood season and the bottom slope of the main channel formed during nonflood season are analyzed empirical formulas for both slopes are presented for estimating long term storage capacity

overflow dynamics and bottom water formation in the western ross sea influence of tides 1 the tidal impact on overflow dynamics and bottom water production in the western ross sea is studied with the finite element ocean model which allows embedding a mesh with 0 5 km resolution in a coarse resolution 30 km setup without nesting the simulated overflow properties inside and downstream of the western ross sea are described the overflow exhibits pronounced variability at both daily and spring neap tidal time scales in the western ross sea tides increase mixing over both the outer shelf and upper slope there plume jets are shaped by tidal currents at a bathymetric bend west of the drygalski trough mouth descending rapidly and supplying the bottom water a fraction of shelf water remains over the shelf and propagates westward from the ross sea but it does not contribute significantly to bottom water formation because of energetic mixing over the upper slope compared to a simulation without tidal forcing tides with the major k1 and o1 constituents increase the outflow rate over the continental slope off cape adare by about 70 a set of sensitivity experiments show that the rate of bottom water production is not a monotonic function of the tidal currents amplitude tidal forcing with intermediate strength leads to the most efficient bottom water formation

watermass properties of the antarctic slope front a simple model abstract in the antarctic dense shelf water is formed in coastal polynyas and is differentiated from the fresher surface water by the wind induced ice motion that displaces offshore the ice melt from production zones where the shelf water discharges into the deep ocean the antarctic slope front asf is v shaped separating the shelf and surface waters referred to as frontal waters from the circumpolar deep water cdw to elucidate basic constraints on frontal properties a minimal model of homogeneous water masses forced by offshore wind and freshwater input in a perpetual winter is considered with the surface water stirred by and hence aligned with the ice cover there is little leakage of ice or meltwater from the frontal system so ice production and melt merely redistribute heat and salt between frontal waters as such the heat loss to the atmosphere needs to be supplied by entraining cdw which then necessitates the shelf water discharge on account of the mass balance because of the fresh

modeling sea ice production and dense shelf water formation in coastal polynyas around east antarctica 1 using an ice ocean coupled model with fine horizontal resolution around east antarctica sea ice production and dense shelf water dsw formation in coastal polynyas are investigated the model reproduces well the locations of coastal polynyas and the high sea ice production there dsw is formed over the continental shelves under a number of coastal polynyas a threshold density beyond which net production of dsw takes place is largely different among coastal polynyas the densest and most vigorous dsw formation occurs in the cape darnley and mertz ninnis glacier polynyas followed by somewhat less but still significant dsw formation in the prydz barrier and vincennes polynyas assuming mixing of the dsw outflowing across the shelf break with typical modified circumpolar deep water over the continental slope the maximum possible formation rate of antarctic bottom water aabw is estimated to be 7 58 sv around east antarctica between 60 e and 150 e with the cape darnley and mertz ninnis glacier polynyas exhibiting the most active formation rates of 2 13 and 1 97 sv respectively from a series of numerical experiments it is found that the treatment of coastline and grounded icebergs has a large impact on both sea ice production and formation of dsw and aabw

case study delayed sedimentation response to inflow and operations at sanmenxia dam this paper presents a study on the reservoir sedimentation processes in response to changes in incoming flow at the upstream and changes in the pool level at the downstream for sanmenxia reservoir which is located on the middle reach of the yellow river in china and has experienced serious sedimentation problems even since its impoundment in 1960 the hysteresis effect in reservoir sedimentation was used as the basis for analysis and its behavior was fully investigated throughout this study the research found that the rise in the elevation of tongguan which is located in the backwater region at a distance of 113 5 km upstream of the sanmenxia dam had a time delay of about 2 years compared with the sediment deposition in the reservoir area downstream of tongguan moreover the accumulated sediment deposition in the reservoir area was closely related not only to the current year s flow and dam operational conditions but also to the preceding 3 4 years flow and dam operational conditions likewise the

quantifying antarctic bottom water and north atlantic deep water volumes 1 a near global census of antarctic bottom water aabw and north atlantic deep water nadw is essayed through a nonnegative least squares analysis of conservative and quasi conservative seawater properties aabw thickness generally decreases from south to north modulated by ocean bathymetry likewise nadw thickness generally decreases from north to south in the atlantic ocean nadw dominates below the thermocline of the atlantic ocean at least as far south as the subtropical gyre of the south atlantic with a lesser but still significant influence around the entire antarctic circumpolar current in the indian ocean and in the pacific ocean however in the pacific and indian oceans aabw dominates below the thermocline in addition measurable quantities of aabw reach into the abyssal north atlantic on both sides of the mid atlantic ridge the census results suggest that aabw occupies roughly twice the volume of nadw in the three main oceans and that aabw is in contact with roughly twice the area of the deep main ocean floor compared with nadw however these results are somewhat sensitive to choices of water masses their values of seawater properties and the weightings of the seawater properties used in the analysis

modelling of hyperconcentrated flood and channel evolution in a braided reach using a dynamically coupled one dimensional approach abstract hyperconcentrated sediment laden floods often occur in a braided reach of the lower yellow river usually leading to significant channel evolution a one dimensional 1d morphodynamic model using a dynamically coupled solution approach is developed to simulate hyperconcentrated flood and channel evolution in the braided reach with an extremely irregular cross sectional geometry in the model the improved equations for hydrodynamics account for the effects of sediment concentration and bed evolution which are coupled with the equations of non equilibrium sediment transport and bed evolution the model was validated using measurements from the 1977 and 2004 hyperconcentrated floods furthermore the effects were investigated of different cross sectional spacings and allocation modes of channel deformation area on the model results it was found that a suitable cross sectional distance of less than 3 km should be adopted when simulating hyperconcentrated floods and the results using the uniform allocation mode can agree better with measurements than other two allocation modes

carbon dynamics of the weddell gyre southern ocean the accumulation of carbon within the weddell gyre and its exchanges across the gyre boundaries are investigated with three recent full depth oceanographic sections enclosing this climatically important region the combination of carbon measurements with ocean circulation transport estimates from a box inverse analysis reveal that deep water transports associated with warm deep water wdw and weddell sea deep water dominate the gyre s carbon budget while a dual cell vertical overturning circulation leads to both upwelling and the delivery of large quantities of carbon to the deep ocean historical sea surface pco2 observations interpolated using a neural network technique confirm the net summertime sink of 0 044 to 0 058 0 010 pg c yr 1 derived from the inversion however a wintertime outgassing signal similar in size results in a statistically insignificant annual air to sea co2 flux of 0 002 0 007 pg c yr 1 mean 1998 2011 to 0 012 0 024 pg c yr 1 mean 2008 2010 to be diagnosed for the weddell gyre a surface layer carbon balance independently derived from in situ biogeochemical measurements reveals that freshwater inputs and biological drawdown decrease surface ocean inorganic carbon levels more than they are increased by wdw entrainment resulting in an estimated annual carbon sink of 0 033 0 021 pg c yr 1 although relatively less efficient for carbon uptake than the global oceans the summertime weddell gyre suppresses the winter outgassing signal while its biological pump and deep water formation act as key conduits for transporting natural and anthropogenic carbon to the deep ocean where they can reside for long timescales

impacts of open ocean deep convection in the weddell sea on coastal and bottom water temperature a high resolution global ocean sea ice model is employed to investigate the impacts of open ocean deep convection on coastal and bottom water temperature in the weddell sea the imposed strong and persistent cyclonic wind forcing and the large loss of bottom water weaken the stratification and eventually trigger the occurrence of open ocean deep convection in the southern limb of the weddell gyre in this model the production rate of the bottom water induced by the deep convection is estimated to be about 5 sv 1 sv 106 m3 s for a polynya with a similar size to that of the observed weddell polynya in the mid 1970s the cooling induced by deep convection at mid depth is transported towards the shelf regions by standing meanders or eddies to affect the basal melting of ice shelves and is transported westward by an intensified slope current interior coastal temperature in regions with a broader continental shelf is less affected by the deep convection as the intensified slope current acts to suppress heat exchanges across the shelf break also the deep convection causes warming in the weddell bottom water around the convection site when the simulated polynya size is similar to that of the observed weddell polynya in the mid 1970s this finding sheds light on the observed non monotonic decadal change cooling between 1984 1992 and warming between 1998 2008 in the weddell bottom water temperature when the simulated polynya further develops into a large size across the weddell sea the sustained broad deep convection causes large cooling in the bottom water in the western weddell sea and warming in the eastern weddell sea with the bottom water temperature also being strongly modulated by a greatly intensified weddell gyre

antarctic bottom water production and export by tides in the ross sea 1 current measurements in the western ross sea show that antarctic bottom water is produced at the outer edge of the continental shelf by tidal stirring of antarctic surface water circumpolar deep water and shelf water current amplitudes during spring tides are greater than 1 m s sufficient to carry circumpolar deep water onto the shelf from offshore and stir at least the lower portion of the water column to a nearly homogeneous mixture the antarctic bottom water produced every day during spring tides does not appear to have a seasonal signal its export rate is estimated to be 1 95 1 85 10 6 m 3 s

case study river training and its effects on fluvial processes in the lower yellow river china more than 50 years river training practices in the lower yellow river provide valuable experience in river management for flood control in rivers having rapid flow changes silting beds and active channel migrations and are of importance in understanding the fluvial processes in regulated rivers with high sediment loads planned channel alignments for river training in the lower yellow river usually consist of a series of consecutive moderate bends representing the natural tendency of flows flow guide works namely spur dikes were constructed on the concave banks of the planned bends to protect the channel against scouring and migration by deflecting the current away from bends and further guiding the main flow from one bend to the next one as a result well planned flow guide works can play a crucial role in limiting channel shifting and migration and in establishing a relatively stable channel enough flow guide works on both sides together reaching about 80 of the channel length may change the transitional and braided channel patterns to a confined meandering pattern

unified view of sediment transport by currents and waves i initiation of motion bed roughness and bed load transport attention is given to the properties of sediment beds over the full range of conditions silts to gravel in particular the effect of fine silt on the bed composition and on initiation of motion critical conditions is discussed high quality bed load transport data sets are identified and analyzed showing that the bed load transport in the sand range is related to velocity to power 2 5 the bed load transport is not much affected by particle size the prediction of bed roughness is addressed and the prediction of bed load transport in steady river flow is extended to coastal flow applying an intrawave approach simplified bed load transport formulas are presented which can be used to obtain a quick estimate of bed load transport in river and coastal flows it is shown that the sediment transport of fine silts to coarse sand can be described in a unified model framework using fairly simple expressions the proposed model is fully predictive in the sense that only the basic hydrodynamic parameters dep

radiocarbon based circulation age of the world oceans 1 the distributions of deep ocean 14c data are often used to illustrate the rate of deep ocean circulation the associated conventional 14c ages show a difference of 1000 years between the north atlantic and the southern ocean and a difference of another 1000 years between the southern ocean and the north pacific these differences may be interpreted directly and mistakenly as the timescale of circulation the characterization of the deep ocean circulation being millennial is common using objectively gridded natural 14c data of key et al 2004 i recast 14c in terms of circulation by accounting for 1 long surface ocean reservoir 14c ages and 2 two sources of deep water formed in the north atlantic and around antarctica the new distribution of circulation 14c ages is more consistent with the deep ocean being characterized by a centennial timescale than a millennial timescale also the role of the southern sourced deep water is now made more obvious by accounting for the two important controls on oceanic 14c that are not well known outside the field of chemical oceanography the new map will be useful as an illustration of global deep ocean circulation to the wider scientific community and as a pedagogical tool to new students in earth sciences and oceanography

the antarctic slope current in a changing climate the antarctic slope current asc is a coherent circulation feature that rings the antarctic continental shelf and regulates the flow of water towards the antarctic coastline the structure and variability of the asc influences key processes near the antarctic coastline that have global implications such as the melting of antarctic ice shelves and water mass formation that determines the strength of the global overturning circulation recent theoretical modeling and observational advances have revealed new dynamical properties of the asc making it timely to review earlier reviews of the asc focused largely on local classifications of water properties of the asc s primary front here we instead provide a classification of the current s frontal structure based on the dynamical mechanisms that govern both the along slope and cross slope circulation these two modes of circulation are strongly coupled similar to the antarctic circumpolar current highly variable motions such as dense overflows tides and eddies are shown to be critical components of cross slope and cross shelf exchange but understanding of how the distribution and intensity of these processes will evolve in a changing climate remains poor due to observational and modeling limitations results linking the asc to larger modes of climate variability such as el nino show that the asc is an integral part of global climate an improved dynamical understanding of the asc is still needed to accurately model and predict future antarctic sea ice extent the stability of the antarctic ice sheets and the southern ocean s contribution to the global carbon cycle

anthropogenic co2 in the southern ocean distribution and inventory at the indian atlantic boundary world ocean circulation experiment line i6 1 the southern ocean where various water masses are formed mode intermediate deep and bottom waters has a high potential to absorb anthropogenic co2 cant however most data based and model estimates indicate low cant inventories south of 50 s in order to investigate this paradox the distribution of cant is estimated between south africa and antarctica world ocean circulation experiment woce line i6 based on a back calculation technique previously used in the north atlantic kortzinger et al 1998 and adapted here for application in the southern ocean at midlatitudes 30 50 s formation and spreading of mode and intermediate waters results in a deep penetration of cant down to 2000 m south of 50 s significant concentrations of cant were estimated in circumpolar deep water 10 mol kg and antarctic bottom water aabw 20 25 mol kg higher concentrations are detected along the continental slope in aabw presumably formed in prydz bay compared to aabw of weddell sea origin the distribution of cant obtained north of 50 s compares well with previous data based and model estimates but large disagreements are found in the south however although transient tracers are not used in the back calculation technique employed here the distribution of cant is remarkably well correlated with cfcs we reevaluated the column inventories of cant for the southern ocean and found higher values at high latitudes 70 90 mol m2 compared to the subtropical subantarctic region 40 80 mol m2 these results support the idea that deep and bottom water formation in the southern ocean is a key process in the natural sequestration of anthropogenic co2

the role of deformation partitioning in the deformation and recrystallization of plagioclase and k feldspar in the woodroffe thrust mylonite zone central australia abstract in the woodroffe thrust mylonite zone central australia recrystallization in plagioclase and k feldspar involved subgrain rotation assisted by grain boundary or kink band boundary bulging without contribution from a change in the chemical composition from host grains to new grains the size of subgrains and new grains changes across the mylonite zone apparently as a function of the strain rate and the h2o content of the rock the partitioning of deformation into zones of progressive shearing and progressive shortening controls the sites of recovery and recrystallization in feldspar during mylonitization the size of feldspar porphyroclasts in well developed mylonites is governed by the scale of deformation partitioning reached in the earlier stages of mylonitization before the formation of a large proportion of fine grained matrix that can accommodate the progressive shearing component of the deformation recrystallization occurs in microcline apparently without involving a translation to a monoclinic structure as microcline twinned new grains are common adjacent to microcline twinned host grains k feldspar triclinicity values calculated from xrd traces increase from the margins to the interior of the mylonite zone in conjunction with deformation intensity k feldspar host grains locally have cores of orthoclase or untwinned microcline surrounded by mantles of twinned microcline suggesting a relationship between the presence of microcline twinning and the degree of k feldspar triclinicity

thermohaline stratification below the southern ocean sea ice the end of winter stratification within the cold cyclonic trough of the weddell gyre near 60 s between 5 e and the greenwich meridian is resolved with the mikhail somov data set the temperature maximum of the weddell deep water wdw is for the most part less than 0 5 c but warmer cells of wdw are found these warm wdw cells have temperature salinity and oxygen properties similar to the wdw characteristic of the weddell gyre inflow which is situated to the southeast of the mikhail somov study region the warm wdw cells are accompanied by domes in the pycnocline of 40 m amplitude over the surrounding pycnocline while deeper isopycnals are depressed anticyclonic shear below the 27 83 isopycnal within the warm wdw cells is compensated by the cyclonic shear associated with the pycnocline dome the pycnocline domes are exposed to about 50 greater entrainment by the turbulently active winter mixed layer relative to the regional entrainment rate this entrainment can significantly erode the warm cells in a single winter season introducing excess heat and salt into the mixed layer while the heat is lost to the atmosphere the excess salt is not necessarily compensated by increased fresh water introduction it is hypothesized that the warm wdw cells within the weddell gyre trough are derived from instability within the frontal zone which extends from maud rise to the northeast separating the weddell warm regime from the cold regime greater than normal injection of warm wdw cells into the weddell gyre trough would increase the surface salinity which would tend to destabilize the pycnocline increasing the probability of deep convection and polynya events

how much deep water is formed in the southern ocean three tracers are used to place constraints on the production rate of ventilated deep water in the southern ocean the distribution of the water mass tracer po4 phosphate star in the deep sea suggests that the amount of ventilated deep water produced in the southern ocean is equal to or greater than the outflow of north atlantic deep water from the atlantic radiocarbon distributions yield an export flux of water from the north atlantic which has averaged about 15 sv over the last several hundred years cfc inventories are used as a direct indicator of the current production rate of ventilated deep water in the southern ocean although coverage is as yet sparse it appears that the cfc inventory is not inconsistent with the deep water production rate required by the distributions of po4 and radiocarbon it has been widely accepted that the major part of the deep water production in the southern ocean takes place in the weddell sea however our estimate of the southern ocean ventilated deep water flux is in conflict with previous estimates of the flux of ventilated deep water from the weddell sea which lie in the range 1 5 sv possible reasons for this difference are discussed

channel pattern change downstream from a reservoir an example of wandering braided rivers reservoir induced channel pattern change is an issue of significance both in theory and engineering practice based on data from three major wandering braided rivers in china namely the yongdinghe the hanjiang and the yellow rivers as well as a laboratory experimental study this paper deals with channel pattern change downstream from reservoirs complex response in channel adjustment has been observed in the field and as a result a three stage descriptive model has been established to describe the course of channel pattern change also the three stages have been simulated successfully by a laboratory physical model although the observed tendency of channel pattern change of the three rivers is quite different which has been a matter of controversy for many years among chinese researchers it can be satisfactorily explained by the present complex response model because of different boundary conditions the duration of the three stages for the three rivers is different so the channel pattern change of the three rivers shows a different tendency at each specific stage the present study suggests that schumm s qualitative model for river channel metamorphosis should be modified when used for predicting reservoir induced channel pattern change on a wandering braided river because the model has not taken into consideration the changing erosional resistance of channel boundary material and therefore the resultant feedback effect it cannot predict the whole course of channel pattern change over a long period

a chlorofluoromethane and hydrographic section across drake passage deep water ventilation and meridional property transport new hydrographic and nutrient data obtained on a section across drake passage f s meteor january 1990 world ocean circulation experiment hydrographic program section s1 are in close agreement with property sections reported previously the chlorofluoromethanes cfm 11 and cfm 12 were measured in drake passage for the first time cfm concentrations are found to decrease from the surface down into the upper circumpolar deep water for which they confirm water renewal from the south for the lower circumpolar deep water in which cfm concentrations were above detection limit only south of the polar front very little water renewal on the cfm time scale is implied nonvanishing cfm is again found in the weddell sea deep water and the southeast pacific deep water toward the bottom in the south but recent ventilation for the latter water mass is rejected cfm 11 and cfm 12 concentrations vary essentially in constant proportion down to very low concentrations questioning the possibility of using cfm ratios as age markers the observed ratios are shown to be a natural feature of the upwelling regime of the southern ocean property concentrations on isopycnal surfaces display large undulations reaching down into the upper circumpolar deep water their extrema due to varying contribution of young water of southern origin are situated at the boundaries of the current bands of the antarctic circumpolar current the feature is ascribed to property advection by rings and is taken to support previous claims that rings are an important transport mechanism across the antarctic circumpolar current and that they might assist in maintaining its fronts

formation and export of dense shelf water from the adelie depression east antarctica 1 coastal polynyas in the adelie depression are an important source of antarctic bottom water to the australian antarctic basin we present time series april 1998 to may 1999 and august 1999 to february 2000 of data from temperature salinity sensors in both the adelie depression and the known outflow region of the adelie sill to describe the annual cycle of shelf water densities from april through september salinification beneath the polynya produces dense shelf waters during september october shelf water densities in the depression peak at 27 94 kg m 3 and the cooling and freshening signature of ice shelf water is observed north of buchanan bay in november december shelf water densities decrease as intrusions of warm and relatively fresh modified circumpolar deep water enter east of the adelie sill from january march the surface layer is conditioned by the cooling of the atmosphere which overturns the upper water column at the adelie sill observed daily mean currents were approximately 10 cm s 1 with intense instantaneous currents greater than 50 cm s 1 at the sill depth using an idealized outflow region with a rectangular cross sectional area 6 106 m2 we present the first estimates of shelf water export by potential density class assuming shelf water with a minimum density of 27 88 kg m 3 has sufficient negative buoyancy for downslope mixing and a fourfold volume increase 1 3 mixing ratio from entrainment the dense shelf water export of 0 1 0 5 sv results in an annual average production of bottom water in this region of between 0 4 and 2 0 sv the wide range in bottom water estimate results from data limitations and a narrowing of this range requires further mooring observations

western ross sea continental slope gravity currents antarctic bottom water of the world ocean is derived from dense shelf water that is carried downslope by gravity currents at specific sites along the antarctic margins data gathered by the anslope and clima programs reveal the presence of energetic gravity currents that are formed over the western continental slope of the ross sea when high salinity shelf water exits the shelf through drygalski trough joides trough immediately to the east offers an additional escape route for less saline shelf water while the glomar challenger trough still farther east is a major pathway for export of the once supercooled low salinity ice shelf water that forms under the ross ice shelf the drygalski trough gravity currents increase in thickness from 100 to 400 m on proceeding downslope from 600 m the shelf break to 1200 m upper slope sea floor depth while turning sharply to the west in response to the coriolis force during their descent the mean current pathway trends 35 downslope from isobaths benthic layer current and thickness are correlated with the bottom water salinity which exerts the primary control over the benthic layer density a 1 year time series of bottom water current and hydrographic properties obtained on the slope near the 1000 m isobath indicates episodic pulses of shelf water export through drygalski trough these cold 34 75 pulses correlate with strong downslope bottom flow extreme examples occurred during austral summer fall 2003 comprising concentrated high salinity shelf water 1 9 c 34 79 and approaching 1 5 m s 1 at descent angles as large as 60 relative to the isobaths such events were most common during november may consistent with a northward shift in position of the dense shelf water during austral summer the coldest saltiest bottom water was measured from mid april to mid may 2003 the summer fall export of high salinity shelf water observed in 2004 was less than that seen in 2003 this difference if real may reflect the influence of the large iceberg c 19 over drygalski trough until its departure in mid may 2003 when there was a marked decrease in the coldest saltiest gravity current adjacent to drygalski trough northward transport of cold saline recently ventilated antarctic bottom water observed in march 2004 off cape adare was 1 7 sv including 0 4 sv of high salinity shelf water

diapycnal and isopycnal transports in the southern ocean estimated by a box inverse model quantitative descriptions of circumpolar deep water upwelling and evolution into a lighter mode and heavier bottom waters in the southern ocean are still not well constrained here data from two occupations of eight hydrographic sections are combined and used in a box inverse model to estimate isopycnal and diapycnal transportsinthesouthernocean amixedlayerboxallowsdiapycnaltransportsinthesurfacemixedlayertobe estimated separately current velocity at 1000 dbar was constrained by the mean velocity field estimated from subsurfacefloatdriftdata theestimatedisopycnaltransportsarelargelyconsistentwithpastestimatesandwith outputs of three ocean general circulation models the estimated subduction and upwelling at the base of the southernoceanmixedlayershowthatuppercircumpolardeepwaterupwells 16615and17621sv where 1sv 10 6 m 3 s 21 by different inversion methods and evolves into heavier lower circumpolar deep water 5 6 13 and 6 6 18sv and bottom water 86 9a nd 86 13sv or lighter mode and intermediate waters 96 18 and 13 6 24sv meridional transport in the surface mixed layer is due to northward ekman transport and mostly southward eddy transport in seasonal ice covered areas near antarctica a significant 14 6 14sv southward transport was found the southward eddy transport is largest north of the antarctic circumpolar currentanddecreasespolewardbecauseofthepolewarddecreaseintheeddydiffusivity theinteriordiapycnal transports which can be either upward gaining buoyancy or downward gaining density are comparable in magnitude to the horizontal diapycnal transports within the surface mixed layer

meridional circulation across the antarctic circumpolar current serves as a double 231pa and 230th trap abstract upwelling of circumpolar deep water in the weddell gyre and low scavenging rates south of the antarctic circumpolar current acc cause an accumulation of particle reactive nuclides in the weddell gyre a ventilation reversible scavenging model that successfully described the accumulation of 230th in this area was tested with other particle reactive nuclides and failed to adequately describe the depth distributions of 231pa and 210pb we present here a modified model that includes a nutrient like accumulation south of the antarctic polar front in an upper meridional circulation cell as well as transport to a deep circulation cell in the weddell gyre by scavenging and subsequent release at depth the model also explains depletion of 231pa and 230th in weddell sea bottom water wsbw by ventilation of newly formed deep water on a timescale of 10 years but this water mass is too dense to leave the weddell gyre in order to quantify the processes responsible for the 231pa and 230th composition of newly formed antarctic bottom water aabw we present a mass balance of 231pa and 230th in the atlantic sector of the southern ocean based on new data from the geotraces program the acc receives 6 0 1 5 10 6 dpm s 1 of 230th from the weddell sea similar in magnitude to the net input of 4 2 3 0 10 6 dpm s 1 from the north for 231pa the relative contribution from the weddell sea is much smaller only 0 3 0 1 10 6 compared to 2 7 1 4 10 6 dpm s 1 from the north weddell sea deep water wsdw leaving the weddell gyre northward to form aabw is exposed in the acc to resuspended opal rich sediments that act as efficient scavengers with a th pa fractionation factor f 1 hydrothermal inputs may provide additional removal with low f scavenging in the full meridional circulation across the opal rich acc thus acts as a double 231pa and 230th trap that preconditions newly formed aabw