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

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

flow processes near smooth and rough concave outer banks in curved open channels flow processes near the concave outer bank in curved river reaches are investigated in a laboratory flume with focus on the influence of the bank roughness an outer bank cell of reversed secondary flow occurs for all the investigated roughness configurations of the outer bank the cell widens the outer bank boundary layer which reduces the flow forcing on the bank but also advects high momentum fluid toward the lower part of the bank which enhances the flow forcing on the bank increasing the roughness of the outer bank causes a considerable widening and strengthening of the outer bank cell which amplifies both effects on the flow forcing the widening of the outer bank boundary layer induced by the outer bank cell reduces the effective width of the channel i e the width where most of the discharge is conveyed which increases significantly the sediment transport capacity in the central part of the channel the near bank processes are qualitatively similar but show considerable quantitative variations around the bend the outer bank cell reaches its maximum strength in the zone that is most vulnerable to bed scour and bank erosion which corroborates its morphological relevance the outer cell is mainly generated by reversed near surface gradients in the profile of the streamwise velocity the anisotropy of the cross stream turbulence is the major mechanism opposed to the outer bank cell increasing roughness of the outer bank amplifies all dominant mechanisms but does not modify their relative importance the relevance of the experimental results for natural curved river configurations is discussed

flow separation at convex banks in open channels laboratory experiments in an open channel bend provide insight into the physics of convex bank flow separation occurring in a variety of channel configurations including confluences and bifurcations the edge of the zone of flow separation is characterized by a shear layer enhanced velocity gradients tke turbulent shear stresses and reversal of the streamwise vorticity and vertical velocity the latter result from turbulence induced secondary flow near the convex bank when bankline curvature abruptly increases flow tends to move away from the convex bank along a straight path as represented by the inertial forces including the centrifugal force in the transverse momentum equation written in curvilinear coordinates mass accumulation at the opposite bank leads to a transverse tilting of the water surface and a pressure gradient towards the convex bank that causes the flow to change direction the pressure gradient force lags spatially behind the inertial forces which promotes flow separation how separation typically occurs downstream of the location of maximum change in the bankline curvature because an abrupt increase in bankline curvature also leads to water surface gradients that cause local flow redistribution towards the convex bank that opposes flow separation the zone of convex bank flow separation is shaped by the secondary flow induced by streamline curvature and turbulence the latter is conditioned by the production rate of tke which crucially depends on the accurate description of the reynolds stresses hydrodynamic geometric and sedimentologic control parameters of convex bank flow separation are identified and discussed

structural characteristics of congelation and platelet ice and their role in the development of antarctic land fast sea ice ice cores were obtained in january 1990 from the land fast ice in mcmurdo sound for a study of variations in texture fabric sub structure composition and development two primary ice types were observed congelation and platelet with a minor amount of frazil ice congelation ice growth precedes platelet ice accretion congelation ice fabrics show frequent moderate to strong alignments a phenomenon believed to be due to water current control of selective ice crystal growth platelet ice originates at the base of the congelation ice initially as a porous latticework of tabular ice crystals which subsequently consolidate by congelation of the interstitial water interstitial congelation ice fabrics generally have little or no alignment indicating the reduced effect of currents within the platelet latticework prior to solidification platelet crystal textures range from small wavy edged forms to large blade like forms platelet crystal fabrics indicate that in addition to being randomly oriented the platelet latticeworks commonly include many crystals with their flat 0001 faces oriented both parallel and normal to the base of the overlying ice plate width data suggest that the interstitial congelation ice growth rates remain similar to those of the overlying congelation ice this effective increase in growth rates probably happens because the latticework of accumulating platelets ahead of the freezing interface ensures that the water within the platelet layer is at the freezing point and less heat has to be removed from platelet rich water than from platelet free water for a given thickness of congelation ice to grow the negative oceanic heat flux associated with platelet ice formation in mcmurdo sound explains why mcmurdo sound fast ice is thicker than ross sea pack ice and also why it reaches a greater thickness than arctic fast ice grown in a similar polar marine climate plate widths in the mcmurdo sound congelation ice suggest however that it grows no faster than arctic congelation ice

a glimpse beneath antarctic sea ice platelet layer volume from multifrequency electromagnetic induction sounding in antarctica ice crystals emerge from ice shelf cavities and accumulate in unconsolidated layers beneath nearby sea ice such sub ice platelet layers form a unique habitat and serve as an indicator for the state of an ice shelf however the lack of a suitable methodology impedes an efficient quantification of this phenomenon on scales beyond point measurements in this study we inverted multifrequency electromagnetic em induction soundings obtained on fast ice with an underlying platelet layer along profiles of 100 km length in the eastern weddell sea em derived platelet layer thickness and conductivity are consistent with other field observations our results suggest that platelet layer volume is higher than previously thought in this region and that platelet layer ice volume fraction is proportional to its thickness we conclude that multifrequency em is a suitable tool to determine platelet layer volume with the potential to obtain crucial knowledge of associated processes in otherwise inaccessible ice shelf cavities

basal melting at the ekstrom ice shelf antarctica estimated from mass flux divergence we characterize the basal mass balance of the ekstr om ice shelf dronning maud land antarctica using interferometrically derived surface velocities and ice thickness measurements from radio echo sounding res the surface velocities are based on data from european remote sensing satellites 1 and 2 ers 1 2 during 1994 97 the ice thickness grid consists of 136 res profiles acquired between 1996 and 2006 mass fluxes are calculated along selected res profiles where possible to reduce uncertainties from ice thickness interpolation elsewhere large scale mass fluxes are calculated using interpolated ice thickness data themass flux into the ekstr om ice shelf from themain grounded drainage basins is estimated to be 3 19 0 4gt a 1 the mass flux near the ice shelf front is 2 67 0 3gt a 1 assuming steady state and based on the equation of continuity we interpret the residual mass flux as a combined effect of snow accumulation and subglacial melting refreezing using net snow accumulation rates from previous studies we link the mass flux divergence in irregular shaped polygons to processes beneath the ice shelf the highest subglacial melt rates of 1 1ma 1 are found near the grounding zone of two main inflow glaciers and around the german station neumayer iii the detection of unlikely refreezing in a small area 15 km west of neumayer iii is attributed to both dataset inaccuracies and a possibly past violation of the steady state assumption in general the method and input data allow mapping of the spatial distribution of basal melting and the results are in good agreement with several previous studies

the response of ice shelf basal melting to variations in ocean temperature abstract a three dimensional ocean general circulation model is used to study the response of idealized ice shelves to a series of ocean warming scenarios the model predicts that the total ice shelf basal melt increases quadratically as the ocean offshore of the ice front warms this occurs because the melt rate is proportional to the product of ocean flow speed and temperature in the mixed layer directly beneath the ice shelf both of which are found to increase linearly with ocean warming the behavior of this complex primitive equation model can be described surprisingly well with recourse to an idealized reduced system of equations and it is shown that this system supports a melt rate response to warming that is generally quadratic in nature this study confirms and unifies several previous examinations of the relation between melt rate and ocean temperature but disagrees with other results particularly the claim that a single melt rate sensitivity to warming is universally valid the hypothesized

platelet ice the southern ocean s hidden ice a review basal melt of ice shelves is not only an important part of antarctica s ice sheet mass budget but it is also the origin of one of the most peculiar types of sea ice found in the polar oceans platelet ice in many regions around coastal antarctica tiny ice crystals form and grow in supercooled plumes of ice shelf water releasing heat into the surrounding ocean they usually rise towards the surface eventually becoming trapped under an ice shelf as marine ice frequently masses of those crystals are advected out of the ice shelf cavity and accumulate below a solid sea ice cover to form a semiconsolidated layer when the overlying sea ice grows into this so called sub ice platelet layer the loose crystals are consolidated adding additional thickness to the sea ice these phenomena are generally referred to as platelet ice although confusion about the terminology is widespread in the literature the presence of platelet ice has a profound impact on sea ice properties and processes in several regions of antarctica with numerous implications for the local polar marine biosphere most notably sub ice platelet layers provide a stable sheltered nutrient and food rich habitat which usually results in a highly productive and uniquely adapted ecosystem it has also been hypothesised that platelet ice may be an indicator of the state of an ice shelf although comprehensive time series are limited to the ross sea this paper clears up the terminology by providing exact definitions of the relevant terms we review platelet ice formation observational methods as well as geographical and seasonal occurrence the physical properties and ecological implications are merged in a way understandable for physicists and biologists alike to lay the foundation for the interdisciplinary research that is necessary to tackle the current knowledge gaps

world wide delivery of river sediment to the oceans new data and new estimates from old data show that rivers with large sediment loads annual discharges greater than about 15 times 10 6 tons contribute about 7 times 10 9 tons of suspended sediment to the ocean yearly extrapolating available data for all drainage basins the total suspended sediment delivered by all rivers to the oceans is about 13 5 times 10 9 tons annually bedload and flood discharges may account for an additional 1 2 times 10 9 tons about 70 of this total is derived from southern asia and the larger islands in the pacific and indian oceans where sediment yields are much greater than for other drainage basins

detailed seismic bathymetry beneath ekstrom ice shelf antarctica implications for glacial history and ice ocean interaction the shape of ice shelf cavities are a major source of uncertainty in understanding ice ocean interactions this limits assessments of the response of the antarctic ice sheets to climate change her

eddy overturning of the antarctic slope front controls glacial melting in the eastern weddell sea 1 the eastern weddell sea is characterized by narrow continental shelves and warm deep water wdw is located in close proximity to the ice shelves in this region the exchange of wdw across the antarctic slope front asf determines the rate of basal ice shelf melting here we present a unique data set consisting of 2351 vertical profiles of temperature and salinity collected by southern elephant seals mirounga leonina and a profile beneath the fimbul ice shelf fis obtained via drilling through 395 m of ice this data set reveals variations in salinity and temperature through winter and using a conceptual model of the coastal salt budget we quantify the main exchange processes our data show that modified wdw with temperatures below 1 5 c is advected onto the shelf and into the ice shelf cavities by an eddy overturning of the asf the onshore ekman flux of surface waters during summer is the main source of freshwater that leads to the formation of low salinity shelf waters in the region the modified wdw that reaches beneath the ice shelves is too cold for basal ice shelf melting to create such low salinity water a high resolution model of an idealized asf continental shelf ice shelf system supports the conclusions from the data analysis the inflow of wdw onto the continental shelf and into the ice shelf cavity occurs within a bottom boundary layer where the eddy advection in the model is particularly strong in close agreement with the observed vertical profile of temperature beneath the fis citation nost o a m biuw v tverberg c lydersen t hattermann q zhou l h smedsrud and k m kovacs 2011 eddy overturning of the antarctic slope front controls glacial melting in the eastern weddell sea j geophys res 116 c11014 doi 10 1029 2011jc006965

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

effects of silt clay content on the susceptibility of river banks to subaerial erosion abstract the aim of this paper is to demonstrate how subaerial processes vary with the silt clay content of river bank soil and to consider this variation in the context of erosion observed in the field in particular spatial and temporal variations in erosion interactions with other bank erosion processes and the implications for bank morphology are explored a number of soil blocks of known silt clay content were subjected to a series of freeze thaw and drying wetting cycles changes in the dimensions of the soil blocks were recorded and the mass of soil eroded from each block together with maximum size of eroding aggregates were measured limitations to this methodology are acknowledged including a limited ability to replicate the forces to which a river bank may be subject restrictions imposed by the available equipment in producing temperature and humidity cycles which reflect those occurring in the field and issues of scale the results indicate that river banks with high silt clay contents are the most susceptible to erosion by subaerial processes high susceptibility may influence spatial variations in bank erosion processes and rates including downstream changes in the effectiveness and significance of subaerial erosion a vertical zoning of processes on a river bank is recognised whereby the upper part of the bank is subject to subaerial erosion and the lower part to fluvial erosion given that a high silt clay content tends to indicate increased susceptibility to subaerial erosion but increased resistance to fluvial erosion this vertical zoning may have implications for bank morphology

characterization of complex fluvial systems using remote sensing of spatial and temporal water level variations in the amazon congo and brahmaputra rivers the surface water and ocean topography swot satellite mission will provide global space based estimates of water elevation its temporal change and its spatial slope in fluvial environments as well as across lakes reservoirs wetlands and floodplains this paper illustrates the utility of existing remote sensing measurements of water temporal changes and spatial slope to characterize two complex fluvial environments first repeat pass interferometric sar measurements from the japanese earth resources satellite are used to compare and contrast floodplain processes in the amazon and congo river basins measurements of temporal water level changes over the two areas reveal clearly different hydraulic processes at work the amazon is highly interconnected by floodplain channels resulting in complex flow patterns in contrast the congo does not show similar floodplain channels and the flow patterns are not well defined and have diffuse boundaries during inundation the amazon floodplain often shows sharp hydraulic changes across floodplain channels the congo however does not show similar sharp changes during either infilling or evacuation second shuttle radar topography mission measurements of water elevation are used to derive water slope over the braided brahmaputra river system in combination with in situ bathymetry measurements water elevation and slope allow one to calculate discharge estimates within 2 3 accuracy these two studies illustrate the utility of satellite based measurements of water elevation for characterizing complex fluvial environments and highlight the potential of swot measurements for fluvial hydrology copyright 2010 john wiley sons ltd

development and properties of sea ice in the coastal regime of the southeastern weddell sea from october to december 1986 a program consisting of sea ice core analysis in combination with sea ice observations was carried out from the icebreaker r v polarstern as part of the winter weddell sea project the ship operated in the central and southeastern weddell sea with interests focusing on the ice shelf front between 70 s and 77 s where a system of coastal polynyas extended for several tens to hundreds of kilometers opened and maintained despite temperatures well below freezing by offshore wind forcing three wind controlled ice formation regimes may be distinguished 1 for wind speeds 10 m s the ice cover is heavily ridged and rafted 50 and formation of granular ice predominates of the total length of cores sampled in the coastal regime 32 consisted of columnar and 35 of granular ice in addition we observed two unique ice classes along the coast platelet ice formed through consolidation of underwater ice platelets underneath the solid ice cover contributed 11 to our core samples another 22 consisted of intermediate columnar granular ice which originates under the dynamic hydrographic conditions of the near ice shelf region

the seasonal appearance of ice shelf water in coastal antarctica and its effect on sea ice growth 1 in this paper we report measurements from the first year round mooring underneath sea ice in mcmurdo sound antarctica which we combine with full depth ocean profiles to identify the incremental appearance of potentially supercooled ice shelf water isw we investigate the effects of isw on sea ice using observations of sea ice growth and crystal structure together with under ice photography we show that the appearance of isw at the surface leads to a disruption in the columnar texture of the sea ice but that persistent growth enhancement occurs only once the entire water column has cooled to the surface freezing point in doing so we demonstrate the possibility of inferring the presence of isw beneath sea ice through crystallographic analysis of cores these findings will be useful for both modeling and observing the extent of isw enhanced ice growth in addition we found that the local growth of first year landfast sea ice only accounted for half of the observed increase in salinity over the water column which indicates that polynyas are responsible for approximately half of the salt flux into mcmurdo sound

towards an estimation of sub sea ice platelet layer volume with multi frequency electromagnetic induction sounding ice platelet clusters modify the heat and mass balance of sea ice near antarctic ice shelves and provide a unique habitat for ice associated organisms the amount and distribution of these ice crystals below the solid sea ice provide insight into melt rates and circulation regimes in the ice shelf cavities which are difficult to observe directly however little is known about the circum antarctic volume of the sub sea ice platelet layer because observations have mostly been limited to point measurements in this study we present a new application of multi frequency electromagnetic em induction sounding to quantify platelet layer properties combining in situ data with the theoretical response yields a bulk platelet layer conductivity of 1154 271msm 1 and ice volume fractions of 0 29 0 43 calibration routines and uncertainties are discussed in detail to facilitate future studies our results suggest that multi frequency em induction sounding is a promising method to efficiently map platelet layer volume on a larger scale than has previously been feasible

drainage morphometry and its influence on hydrology in an semi arid region using srtm data and gis an attempt has been made to study drainage morphometry and its influence on hydrology of peddavanka watershed south india drainage networks for the sub basins were derived from topographical map 1 50 000 and shuttle radar topographic mission srtm digital elevation model dem data used for preparing elevation slope and aspects maps geographical information system gis was used in evaluation of linear areal and relief aspects of morphometric parameters the study reveals that srtm dem and gis based approach in evaluation of drainage morphometric parameters and their influence on hydrological characteristics at watershed level is more appropriate than the conventional methods the mean bifurcation ratio r b of the entire basin is 3 88 which indicate that the drainage pattern is not much influenced by geological structures viii sub basin have high elongation ratio r e basin relief b h ruggedness number rn and time of concentration t c it indicates that the erosion and peak discharges are high in these basins therefore the construction of the check dams and earth dams will help in reducing peak discharge on the main channel these studies are very useful for implementing rainwater harvesting and watershed management

quantitative geomorphology applied to fluvial dynamic in aviles and moneta basins tierra del fuego southern argentina the morphometric characteristics of the moneta river basin are compared to those of the aviles river basin both are located in the north of the province of tierra del fuego in argentina and they also have their catchment area in the same range the sierras del bosque runoff on the ne oriented slopes forms the aviles river basin while runoff on the se slopes forms the moneta river basin the analysis was made in two different scales the first as a general scale of the basin and the second as the catchment sector upper middle and lower the quantitative analysis was applied in order to know the behavior of these two fluvial systems for the analysis a cartographic basis scale to 1 20 000 was used which was made on a mosaic of images from google earth the altimetric information was obtained throughout a digital elevation model of 45 m spatial resolution srtm45 shuttle radar topographic mission the indices used were rr rb ish and dd to characterize the geological and geomorphological context and rl re if ct and ca to infer the behavior of the runoff in the catchment it was established that the moneta river basin reaches 6 in order of hierarchy its main channel is 65 52 km long the re determines an elongated basin the ct obtained in the basin is 0 96 whereas the sub basin level was identified to a maximum value of the o3 sm 10 1 46 the dd is low 1 94 km km2 which is directly proportional to the intensity of rainfall and the slopes of the area considered the comparison made at the same scale between basins determines specific fluvial characteristics of each sector upper middle and lower

observation and modeling of platelet ice fabric in mcmurdo sound antarctica 1 during the annual growth of landfast ice in mcmurdo sound antarctica an episodic flux of platelet ice crystals from the ocean contributes to the build up of a porous subice platelet layer which is steadily incorporated into the sea ice cover as it thickens over winter in november 2007 we examined the spatial variability of these processes by collecting sea ice cores with simultaneous oceanographic observations along an east west transect in the sound previously identified draped and bladed platelet ice types were observed in addition we identify resumed columnar growth which appears to be a result of geometric selection from the subice platelet layer after the arrival of new platelet crystals from the ocean has ceased a numerical model of mechanical platelet ice processes is developed that predicts crystal texture and c axis distributions producing virtual incorporated platelet ice with known growth history this model demonstrates how a disordered subice platelet layer arises from an initially flat interface and suggests that such a layer is more likely to form later in the growth season the model also suggests how the grain boundary density in incorporated platelet ice responds to changes in the flux of loose platelet crystals from the ocean application of this result to our 2007 platelet ice observations indicates that sea ice in western mcmurdo sound is subject to larger and more persistent platelet fluxes than the ice in the east this is consistent with the pattern of in situ supercooling just beneath the ocean surface

sea ice freeboard in mcmurdo sound antarctica derived by surface validated icesat laser altimeter data 1 previous investigations have linked changes in the multiyear sea ice area of mcmurdo sound antarctica from 1213 km2 in 2003 to 4923 km2 in 2005 to the passage of large tabular icebergs preventing the annual sea ice breakout this maximum coverage then gradually diminished by 2009 covering 1453 km2 this investigation employs the use of the ice cloud and land elevation satellite icesat laser altimeter to derive freeboard of sea ice in mcmurdo sound from 2003 to 2009 and hence infer thickness changes over this time period two techniques for freeboard retrieval are compared method 1 m 1 follows those previously presented in the literature using the lowest elevations to construct an estimate of sea surface height however the lack of leads in the study area motivated the development of method 2 m 2 which utilizes tide models each year is divided into two investigation periods from september to december and february to june and these investigations were further segmented by sea ice type first year fy and multiyear my both applied methods reveal a statistically significant linear increase in multiyear sea ice freeboard for m 1 the mean freeboard increased over the study period from 0 53 to 1 00 m and for m 2 from 0 46 to 0 95 m evidence is presented that the multiyear sea ice freeboard increase is strongly linked to the development and incorporation of a subice platelet layer no statistically significant trends were observed for first year sea ice icesat derived freeboards over first year and multiyear sea ice areas compare within one standard deviation of airborne measured freeboard in november 2009

development of sea ice in the weddell sea we report on the development and physical properties of sea ice in the central and eastern weddell sea the investigations were part of the winter weddell sea project 1986 which extended over the months of july through december major elements of the glaciological part of this study included continuous shipborne observations of sea ice conditions and occasional helicopter reconnaissance flights extensive measurements of snow and ice thicknesses at daily ice stations and detailed analyses of sampled ice cores from each ice station textural investigations of the sampled ice revealed the dominance of frazil ice in the central weddell sea and the occurrence of an additional ice class called platelet ice together with the commonly known frazil and congelation ice in the coastal region of the eastern weddell sea these results in combination with the visual ice observations reveal two major mechanisms for sea ice generation in the antarctic which were not sufficiently well accounted for in previous investigations in the central weddell sea a cycle of pancake ice formation and its growth into consolidated floes seems to be the dominant process of the advancing sea ice edge in the coastal waters the growing sea ice cover consists to a considerable degree of ice platelets which are formed in the underlying water column in front of the ice shelf edges thus congelation ice growth which is mainly controlled by atmospheric thermodynamic forcing seems to be of less importance in the central and south eastern weddell sea than for example in the arctic basin

estimating shear stress from moving boat acoustic doppler velocity measurements in a large gravel bed river 1 moving boat acoustic doppler current profiling adcp is increasingly used to measure discharge in large rivers we investigate whether useful information about bed shear stress can be recovered from such data alternative ways to estimate local bed shear stress using the logarithmic law of the wall and spatial averaging are tested using adcp transects across lower fraser river canada repeatability is assessed by comparing estimates from outward and return boat tracks the most precise method uses the vertically averaged mean velocity and a zero velocity height based on bed grain size information the accuracy of the assumed zero velocity height can be judged by consistency between estimates using mean velocity and near bed velocity shear stress estimates from unconstrained log law fits are less repeatable and tend to overpredict and mean shear stress estimates using the depth slope product are unreliable in this river because of nonuniform flow

seasonal evolution of an ice shelf influenced fast ice regime derived from an autonomous thermistor chain ice shelves strongly interact with coastal antarctic sea ice and the associated ecosystem by creating conditions favorable to the formation of a sub ice platelet layer the close investigation of this phenomenon and its seasonal evolution remains a challenge due to logistical constraints and a lack of suitable methodology in this study we characterize the seasonal cycle of antarctic fast ice adjacent to the ekstrom ice shelf in the eastern weddell sea we used a thermistor chain with the additional ability to record the temperature response induced by cyclic heating of resistors embedded in the chain vertical sea ice temperature and heating profiles obtained daily between november 2012 and february 2014 were analyzed to determine sea ice and snow evolution and to calculate the basal energy budget the residual heat flux translated into an ice volume fraction in the platelet layer of 0 18 0 09 which we reproduced by a independent model simulation and agrees with earlier results manual drillings revealed an average annual platelet layer thickness increase of at least 4 m and an annual maximum thickness of 10 m beneath second year sea ice the oceanic contribution dominated the total sea ice production during the study effectively accounting for up to 70 of second year sea ice growth in summer an oceanic heat flux of 21 w m 2 led to a partial thinning of the platelet layer our results further show that the active heating method in contrast to the acoustic sounding approach is well suited to derive the fast ice mass balance in regions influenced by ocean ice shelf interaction as it allows subdiurnal monitoring of the platelet layer thickness

terras caidas fluvial erosion or distinct phenomenon in the amazon in the amazon river basin as in other hydrographic regions of brazil and the world fluvial erosion processes occur because of rivers dynamics however the lateral erosion that occurs on banks of amazonian rivers called terras caidas typically is associated with gravitational movements and is mainly related to large landslides that can damage structures or kill people who live along these riverbanks to better understand the erosive processes that occur in rivers of northern brazil especially those located in the amazon river basin this work aims to describe the terras caidas phenomenon and determine whether it is a distinct phenomenon of fluvial erosion processes the investigation includes analysis of hydrometeorological data of the national water agency ana information collected during field expeditions and mapping of geological and hydrological risk areas in the states of amazonas acre rondonia para and amapa performed by the geological survey of brazil cprm from 2011 to 2016 it is concluded that terras caidas is a distinct phenomenon in the amazon rivers that is different from erosive processes in other brazilian rivers because it is associated not only with the lateral erosion but also with mass movements of moderate and large extents which cause landslips undermining creeps and major landslides

three dimensional structure of flow at a confluence of river channels with discordant beds this paper presents three dimensional data of the mean and turbulent structure of flow collected at a natural confluence of rivers with discordant beds to 1 describe the three dimensional flow field of a natural junction of channels 2 assess the role of changes in bed morphology occurring during transport effective events on the structure of flow at a confluence and 3 examine how the three dimensional structure of flow varies with changes in the ratio of momentum flux between the two confluent streams three dimensional measurements of velocity were reconstructed from the measurements obtained with an array of four two component electromagnetic current meters six detailed velocity profiles were taken at five cross sections in a wide range of flow conditions the mean field of flow is characterised by 1 the acceleration of flow in the downstream portion of the post confluence channel but by lower velocities upstream in the mixing layer area 2 a stagnation zone at the apex of the junction 3 a zone of flow deviation and strong fluid upwelling close to the avalanche face and at the margin of the tributary mouth bar and 4 reduced velocities over the depositional bar at the downstream junction corner the position and extent of these zones vary with changes in the ratio of momentum flux very high intensity of turbulence peaks up to 50 and turbulent kinetic energy were observed in the mixing layer region distortion of the mixing layer characteristic of flow where bed discordance is present between the two tributary channels was evident from mean and turbulent flow data this field study suggests that the effects of bed discordance on flow sediment transport and the resultant bed morphology must be incorporated into conceptual and numeric models of these sites of complex flow

platelet ice under arctic pack ice in winter the formation of platelet ice is well known to occur under antarctic sea ice where subice platelet layers form from supercooled ice shelf water in the arctic however platelet ice formation has not been extensively observed and its formation and morphology currently remain enigmatic here we present the first comprehensive long term in situ observations of a decimeter thick subice platelet layer under free drifting pack ice of the central arctic in winter observations carried out with a remotely operated underwater vehicle rov during the midwinter leg of the mosaic drift expedition provide clear evidence of the growth of platelet ice layers from supercooled water present in the ocean mixed layer this platelet formation takes place under all ice types present during the surveys oceanographic data from autonomous observing platforms lead us to the conclusion that platelet ice formation is a widespread but yet overlooked feature of arctic winter sea ice growth

observations of platelet ice growth and oceanographic conditions during the winter of 2003 in mcmurdo sound antarctica 1 platelet ice is the name given to ice crystals that nucleate in the ocean and grow either at depth or loosely attached to the ice water interface related to the proximity of ice shelves it is known to form in supercooled seawater this study details the conditions for its growth during the austral winter of 2003 in mcmurdo sound antarctica a key finding is that the presence of platelet ice in the sea ice cover is conclusively linked to the time history of the appearance of ice crystals in the water column monitored using the strength of the backscattered signal from an acoustic doppler current profiler as a proxy generally these crystals appeared from mid may as water near the interface became supercooled this near surface supercooling appears to be related to a simultaneous abrupt change in the structure of the upper 250 m of the water column from one that behaved dynamically and contained both the warmest and coolest measured temperatures to one in which the water was essentially isothermal near its surface freezing temperature near surface ocean temperature was also affected by tidal mixing and by an increase in the thickness and density over the course of winter of the surface mixed layer created by salt rejection during ice growth these processes allowed cold water trapped by buoyancy in a band at the base of the mixed layer in early winter to gain access to the ice water interface by midwinter this band of cold water probably had its origin beneath the mcmurdo ice shelf

the holocene alluvial plain of the middle amazon river brazil abstract the fluvial belt of the amazon river is a complex of quaternary sedimentary units of different ages and formation conditions present position of the channel and the morphology and size of the alluvial plain are related to neo tectonic lineaments in the study area the main channel of the solimoes is nearly straight the river may be considered anastomosed albeit with few islands and anabranches the main holocene units are the impeded floodplain and the channel dominated floodplain the channel dominated floodplain is a complex mosaic of fluvial forms including younger channels active sandbars islands levees scroll dominated plains and abandoned belts the impeded floodplain is a widespread unit characterised by a very flat surface and round or irregularly shaped lakes the impeded drainage flood plain is only partially affected by the present floods and even those parts affected by floods receive little or no sediment from the main channel more important sediment contribution is by alluvial plain channels from the solimoes branches that generate deltas in the impeded floodplain lakes the impeded floodplain is interpreted to be an earlier stage in the evolution of the holocene alluvial belt the formation was marked by widespread vertical accretion and a channel morphogenetically less active than at present the later stage is associated with an active main channel and the paranas and generation of a scroll dominated plain this very large river system had a complex and delayed response to variations in the hydrological conditions induced by changes in the regional climate during the late holocene

variability in the distribution of fast ice and the sub ice platelet layer near mcmurdo ice shelf variability in the volume of supercooled ice shelf water outflow in mcmurdo sound is reflected in the thickness and distribution of fast ice and the sub ice platelet layer beneath ground based electromagnetic induction and drill hole surveys of the distribution and thickness of ice shelf influenced fast ice and the sub ice platelet layer in mcmurdo sound were carried out in late spring of 2011 2013 2016 and 2017 in 2011 and 2017 thicker sub ice platelet layers of up to 7 5 and 6 m were observed respectively fast ice formation throughout the winters of 2011 and 2017 was influenced by a higher occurrence of strong southerly wind events and resultant activity of the ross sea polynya in contrast lower wind conditions in 2016 led to largely undisturbed sea ice growth and anomalously extensive fast ice coverage a thinner sub ice platelet layer of up to 4 m was observed in 2016 in 2011 and 2017 substantial and variable sub ice platelet layers were detected in a region of exchange of water masses between the ross sea and the mcmurdo ross ice shelf cavity which were not observed in 2013 and 2016 we hypothesize that a higher frequency of strong southerly wind events resultant polynya activity and high salinity shelf water production over winter accelerates circulation and increases melting in the proximal shallow mcmurdo ice shelf and the deeper ross ice shelf regions of the conjoined cavity the outflow of supercooled ice shelf water and sub ice platelet layer formation in mcmurdo sound are consequently promoted

three dimensional flow modelling and sediment transport in the river klaralven a three dimensional ow model that uses the rng k e turbulence model and a non equilibrium wall function was applied to the river klaralven in the southwest part of sweden the objectives were to study the nature of the ow in the river bifurcation and to investigate the short term sediment transport patterns in the river the effectiveness of three dimensional ow models depends upon 1 how well the river geometry and it surface roughness are modelled and 2 the choice of the closure model improvements were obtained by modelling the river in two parts the entire river reach and a selected part composite manning coef cients were used to account for roughness properties the method requires a calibration process that ensures the water surface pro les match the eld data the k e model under predicted both the extent of ow separation zones and the number of secondary ow regions having a spiral motion in comparison with the rng k e model the 3 d model could predict with good accuracy both the general and secondary ow elds in the river the results agreed well with the 3 d velocity measurements using an acoustic doppler current pro ler a conceptual model was developed that accounts for the development of secondary ows in a river bifurcation having two bends the main ow feature in the river cross sections was the existence of multiple counter rotating spiral motions the number of spiral motions increased as the river bends were approached the river bends also caused vorticity intensi cation and increased the vertical velocities the application of the 3 d ow model was extended by solving the sediment continuity equation the sediment transport patterns were related to the secondary ow elds in the river the sediment transport patterns at the river bifurcations are characterized by the growth of a sandbank copyright 2004 john wiley sons ltd

improved 1d inversions for sea ice thickness and conductivity from electromagnetic induction data inclusion of nonlinearities caused by passive bucking the porosity of sea ice is a fundamental physical parameter that governs the mechanical strength of sea ice and the mobility of gases and nutrients for biological processes and biogeochemical cycles in the sea ice layer however little is known about the spatial distribution of the sea ice porosity and its variability between different sea ice types an efficient and nondestructive method to measure this property is currently missing sea ice porosity is linked to the bulk electrical conductivity of sea ice a parameter routinely used to discriminate between sea ice and seawater by electromagnetic em induction sensors here we have evaluated the prospect of porosity retrieval of sea ice by means of bulk conductivity estimates using 1d multifrequency em inversion schemes we have focused on two inversion algorithms a smoothness constrained inversion and a marquardt levenberg inversion which we modified for the nonlinear signal bias caused by a passive bucking coil operated in such a highly conductive environment using synthetic modeling studies 1d inversion algorithms and multiple frequencies we found that we can resolve the sea ice conductivity within 0 01 s m using standard assumptions for the conductivity porosity relation of sea ice we were able to estimate porosity with an uncertainty of 1 2 which enables efficient and nondestructive surveys of the internal state of the sea ice cover

sensitivity of bed shear stress estimated from vertical velocity profiles the problem of sampling resolution bed shear stress in open channel flows is often estimated from the logarithmic vertical velocity profile however most measuring devices used in the field do not allow for flow velocity to be measured very close to the bed the lack of near bed measurements is a critical loss of information which may affect bed shear stress estimates detailed velocity profiles obtained from a field acoustic doppler velocimeter over three different bed roughnesses clearly show that the inclusion of near bed points is critical for the estimation of bed shear stress in a shallow river environment moreover the results indicate that using the full flow depth instead of the bottom 20 per cent of the profile generates an underestimation of the shear stress when flow is uniform 1998 john wiley sons ltd

turbulence structure in a river reach with sand bed measurements and analysis of the three dimensional turbulence structure in a straight lowland river reach are presented accurate measurements of velocity profiles were taken with an acoustic doppier velocimeter adv and a multichannel micropropeller based system it is shown that analytical expressions derived from investigations of laboratory open channel flows agree well with the measured data only in the central part of the river where flow can be considered weakly three dimensional at the same time clear differences of empirical parameters between river and laboratory flows are detected it was established that river turbulence is isotropic for spatial scales smaller than the river depth the data also allowed the detection and analysis of the turbulence anisotropy impact on the formation of secondary currents special attention is paid to the investigation of coherent structures their spatial scales were evaluated applying the conditional average procedure based on uw quadrant analysis on average the scales of ejection and sweep events are as large as 1 5 times the flow depth

antarctic ice sheet loss driven by basal melting of ice shelves accurate prediction of global sea level rise requires that we understand the cause of recent widespread and intensifying glacier acceleration along antarctic ice sheet coastal margins atmospheric and oceanic forcing have the potential to reduce the thickness and extent of floating ice shelves potentially limiting their ability to buttress the flow of grounded tributary glaciers indeed recent ice shelf collapse led to retreat and acceleration of several glaciers on the antarctic peninsula but the extent and magnitude of ice shelf thickness change the underlying causes of such change and its link to glacier flow rate are so poorly understood that its future impact on the ice sheets cannot yet be predicted here we use satellite laser altimetry and modelling of the surface firn layer to reveal the circum antarctic pattern of ice shelf thinning through increased basal melt we deduce that this increased melt is the primary control of antarctic ice sheet loss through a reduction in buttressing of the adjacent ice sheet leading to accelerated glacier flow the highest thinning rates occur where warm water at depth can access thick ice shelves via submarine troughs crossing the continental shelf wind forcing could explain the dominant patterns of both basal melting and the surface melting and collapse of antarctic ice shelves through ocean upwelling in the amundsen and bellingshausen seas and atmospheric warming on the antarctic peninsula this implies that climate forcing through changing winds influences antarctic ice sheet mass balance and hence global sea level on annual to decadal timescales

signatures of supercooling mcmurdo sound platelet ice near ice shelves around antarctica the ocean becomes supercooled and has been observed to carry small suspended ice crystals our measurements demonstrate that these small crystals are persistently present in the water column beneath the winter fast ice and when incorporated in sea ice they reduce the mean grain size of the sea ice cover by midwinter larger ice crystals below the ice water interface are observed to form a porous sub ice platelet layer with an ice volume fraction of 0 25 0 06 the magnitude and direction of the oceanic heat flux varied between 5 6 wm 2 upwards and 15 10 wm 2 downwards in may but by september it settled between 6 2 and 11 2 w m 2 the negative values imply that the ocean acts as a heat sink which is responsible for the growth of 12 of the ice thickness between june and september this oceanic contribution should not be ignored in models of antarctic sea ice thickness close to an ice shelf

the inferred formation of a subice platelet layer below the multiyear landfast sea ice in the wandel sea ne greenland induced by meltwater drainage

modelling antarctic fast ice growth an existing thermodynamic ice growth model semtner 1976 has been tested for its ability to predict the growth of fast ice in mcmurdo sound antarctica significant discrepancies between observed and predicted ice thicknesses were found to occur primarily due to the presence of sub ice platelets and the formation of a snow ice layer although these ice growth processes are not well enough understood to permit rigorous physical modelling it is shown that fairly simple modifications to the model greatly improve the accuracy of the thickness predictions and serve to highlight the importance of these processes in the antarctic fast ice environment surface flooding and snow ice formation are assumed to occur immediately upon the establishment of a positive hydrostatic water level and a surface temperature in excess of a critical value above which interconnecting channels in the ice matrix permit the flow of water to the surface the presence of the sub ice platelet layer is assumed to increase columnar ice growth at a rate proportional to the volume fraction of ice in the platelet layer a simple technique but one that permits estimates of platelet enhanced growth without detailed knowledge of oceanographic conditions the resulting model predictions are in close agreement with measurements of fast ice growth and decay in mcmurdo sound however data suitable for testing the model over a complete range of conditions and over multi year cycles are not available at the present time

important role for ocean warming and increased ice shelf melt in antarctic sea ice expansion changes in sea ice significantly modulate climate change because of its high reflective and strong insulating nature in contrast to arctic sea ice sea ice surrounding antarctica has expanded1 with record extent2 in 2010 this ice expansion has previously been attributed to dynamical atmospheric changes that induce atmospheric cooling3 here we show that accelerated basal melting of antarctic ice shelves is likely to have contributed significantly to sea ice expansion specifically we present observations indicating that melt water from antarctica s ice shelves accumulates in a cool and fresh surface layer that shields the surface ocean from the warmer deeper waters that are melting the ice shelves simulating these processes in a coupled climate model we find that cool and fresh surface water from ice shelf melt indeed leads to expanding sea ice in austral autumn and winter this powerful negative feedback counteracts southern hemispheric atmospheric warming although changes in atmospheric dynamics most likely govern regional sea ice trends4 our analyses indicate that the overall sea ice trend is dominated by increased ice shelf melt we suggest that cool sea surface temperatures around antarctica could offset projected snowfall increases in antarctica with implications for estimates of future sea level rise

ice platelets below weddell sea landfast sea ice basal melt of ice shelves may lead to an accumulation of disc shaped ice platelets underneath nearby sea ice to form a sub ice platelet layer here we present the seasonal cycle of sea ice attached to the ekstrom ice shelf antarctica and the underlying platelet layer in 2012 ice platelets emerged from the cavity and interacted with the fast ice cover of atka bay as early as june episodic accumulations throughout winter and spring led to an average platelet layer thickness of 4m by december 2012 with local maxima of up to 10 m the additional buoyancy partly prevented surface flooding and snow ice formation despite a thick snow cover subsequent thinning of the platelet layer from december onwards was associated with an inflow of warm surface water the combination of model studies with observed fast ice thickness revealed an average ice volume fraction in the platelet layer of 0 25 0 1 we found that nearly half of the combined solid sea ice and ice platelet volume in this area is generated by heat transfer to the ocean rather than to the atmosphere the total ice platelet volume underlying atka bay fast ice was equivalent to more than one fifth of the annual basal melt volume under the ekstrom ice shelf

observed platelet ice distributions in antarctic sea ice an index for ocean ice shelf heat flux antarctic sea ice that has been affected by supercooled ice shelf water isw has a unique crystallographic structure and is called platelet ice in this paper we synthesize platelet ice observations to construct a continent wide map of the winter presence of isw at the ocean surface the observations demonstrate that in some regions of coastal antarctica supercooled isw drives a negative oceanic heat flux of 30 wm 2 that persists for several months during winter significantly affecting sea ice thickness in other regions particularly where the thinning of ice shelves is believed to be greatest platelet ice is not observed our new data set includes the longest ice ocean record for antarctica which dates back to 1902 near the mcmurdo ice shelf these historical data indicate that over the past 100 years any change in the volume of very cold surface outflow from this ice shelf is less than the uncertainties in the measurements

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

acoustic velocity profiler for laboratory and field studies a monostatic acoustic doppler velocity profiler advp for use in hydraulic research is presented including signal treatment algorithms the 1 mhz advp used in the present study has an accuracy better than 2 mms s 1 with a range of up to 100 cm for velocities up to 290 cms s 1 and a resolution of 256 points in the profile results of advp measurements in uniform flow in two laboratory open channels with smooth and gravel beds compare favorably with established distribution laws for mean velocity variance and reynold s stress indicating that the turbulence scales are sufficiently resolved by the instrument measurements with the same instrument in a river and a lake illustrate that it is capable of producing excellent results in the field as well it was demonstrated that unaffected by water quality the instrument is a good research tool for nonintrusive and accurate instantaneous profile measurements under turbulent flow conditions

salinity profiles of antarctic sea ice field data and model results in order to describe and compare sea ice salinity distributions which are of interest in the study of physical and biological ice characteristics as well as ice ocean interaction we have computed composite profiles and fitted third degree polynomials to salinity data of 129 cores from the weddell sea four characteristic profile shapes have been recognized and described through the polynomial coefficients c s i and type profiles the field data have been compared with ideal salinity profiles generated by a simulation scheme based on thermodynamic growth under climatological conditions representative of the weddell sea composite salinity profiles agree well with simulations irrespective of the growth mechanism i e frazil or congelation suggesting that ice properties which depend on salinity or porosity may evolve independent of ice growth conditions analysis of salinity and 18o data in conjunction with the simulations demonstrates that apart from meteorological conditions controlling growth velocity and temperature distribution within the ice flooding and upward brine expulsion are important in raising top salinities as frequently observed low top salinities are typical of ice that has undergone retexturing and differential desalination salinity decreases towards the bottom of a floe are a result of high oceanic heat fluxes and snow cover effects as a consequence of these processes the bulk salinity of weddell sea ice cannot be described by a simple linear age thickness relationship as characteristic of arctic first year ice