impact of iceberg calving on the retreat of thwaites glacier west antarctica over the next century with different calving laws and ocean thermal forcing

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

response of southern ocean ventilation to changes in midlatitude westerly winds abstractchanges in ventilation of the southern hemisphere oceans in response to changes in midlatitude westerly winds are examined by analyzing the ideal age tracer from global eddy permitting ocea

testing the sensitivity of the east antarctic ice sheet to southern ocean dynamics past changes and future implications the stability of antarctic ice sheets and their potential contribution to sea level under projected future warming remains highly uncertain the last interglacial 135 000 116 000 years ago provides a potential analogue with global temperatures 2uc higher and rates of sea level rise 5 6m ka 1 leading to sea levels 6 6 9 4m higher than present the source s of this sea level rise remain fiercely debated here we report a series of independent model simulations exploring the effects of migrating southern hemisphere westerlies shws on southern ocean circulation and antarctic ice sheet dynamics we suggest that southerly shifts in winds may have significantly impacted the sub polar gyres inducing pervasive warming 0 2 0 8uc in the upper 1200m adjacent to sectors of the east antarctic ice sheet eais which due to their geometries and connectivity to the southern ocean are highly sensitive to ocean forcing we conclude that the eais potentially made a substantial hitherto unsuspected contribution to interglacial sea levels and given 21st century projections in the southern annular mode and associated shw migration we highlight how pervasive circum antarctic warming may threaten eais stability copyright 2013 john wiley sons ltd

responses of basal melting of antarctic ice shelves to the climatic forcing of the last glacial maximum and co2 doubling abstractbasal melting of the antarctic ice shelves is an important factor in determining the stability of the antarctic ice sheet this study used the climatic outputs of an atmosphere ocean general circulation model to force a circumpolar ocean model that resolves ice shelf cavity circulation to investigate the response of antarctic ice shelf melting to different climatic conditions i e to a doubling of co2 and to the last glacial maximum conditions sensitivity experiments were also conducted to investigate the roles of both surface atmospheric change and changes of oceanic lateral boundary conditions it was found that the rate of change of basal melt due to climate warming is much greater by an order of magnitude than that due to cooling this is mainly because the intrusion of warm water onto the continental shelves linked to sea ice production and climate change is crucial in determining the basal melt rate of many ice shelves sensitivity experiments showed that changes of atmospheric heat

ice shelf melt response to changing winds and glacier dynamics in the amundsen sea sector antarctica it has been suggested that the coastal southern ocean subsurface may warm over the 21st century in response to strengthening and poleward shifting winds with potential adverse effects on west antarctic glaciers however using a 1 128 ocean regional model that includes ice shelf cavities we find a more complex response to changing winds in the amundsen sea simulated offshore subsurface waters get colder under strengthened and poleward shifted winds representative of the sam projected trend the buoyancy driven circulation induced by ice shelf melt transports this cold offshore anomaly onto the continental shelf leading to cooling and decreased melt below 450 m in the vicinity of ice shelf fronts ekman pumping contributes to raise the isotherms in response to changing winds this effect overwhelms the horizontal transport of colder offshore waters at intermediate depths between 200 and 450 m and therefore increases melt rates in the upper part of the ice shelf cavities which reinforces the buoyancydriven circulation and further contributes to raise the isotherms then prescribing an extreme grounding line retreat projected for 2100 the total melt rates simulated underneath thwaites and pine island are multiplied by 2 5 such increase is explained by a larger ocean ice interface exposed to cdw which is then amplified by a stronger melt induced circulation along the ice draft our main conclusions are that 1 outputs from ocean models that do not represent ice shelf cavities e g cmip5 models should not be directly used to predict the thermal forcing of future ice shelf cavities 2 coupled ocean ice sheet models with a velocity dependent melt formulation are needed for future projections of glaciers experiencing a significant grounding line retreat

co2 induced ocean warming of the antarctic continental shelf in an eddying global climate model national oceanic and atmospheric administration noaa cpo project na13oar4310128 national science foundation opp 1513411 university of arizona faculty startup funding university of arizona department of geosciences national aeronautics and space administration nasa nnx14al40g princeton environmental institute pei grand challenge initiative southern ocean carbon and climate observations and modeling soccom project under the national science foundation plr 1425989

instruments for particle size and settling velocity observations in sediment transport in this paper we describe two sensors for measurement of particle size distribution and settling velocity distribution these measurements are critical to the correct estimation of the true sediment concentration in the field as well as to evaluating models for transport rates of sediments a multi angle measurement of laser scattering is made and inverted to obtain the particle size distribution since small angle scattering is relatively insensitive to particle composition the size distribution measurements are robust and do not require particle refractive index it is shown that with a knowledge of the size distribution true particulate volume concentration can be obtained unaffected in calibration by changes in particle size distribution the data from bottom boundary layer experiments using the instrument show the presence of temporal variability in size distribution associated with the strength of forcing of the boundary layer the importance of these observations lies in the implication that historical data acquired with single parameter optical or other sensors needs to be revisited in the second instrument analyzing the observation of size distributions during settling in a settling column produces settling velocity estimates in this case the history of concentration of each size class is examined to determine the settling velocity without invoking any assumptions of settling regime settling velocity data from a field experiment off the new jersey coast fit the model wf na 0 45 10 23 a 1 2 n

regional impacts of the westerly winds on southern ocean mode and intermediate water subduction subduction processes in the southern ocean transfer oxygen heat and anthropogenic carbon into the ocean interior the future response of upper ocean subduction in the subantarctic mode water samw and antarctic intermediate water aaiw classes is dependent on the evolution of the combined surface buoyancy forcing and overlying westerly wind stress here the recently observed pattern of a poleward intensification of the westerly winds is divided into its shift and increase components samw and aaiw formation occurs in regional hot spots in deep mixed layer zones primarily in the southeast indian and pacific it is found that the mixed layer depth responds differently to wind stress perturbations across these regional formation zones an increase only in the westerly winds in the indian sector steepens isopycnals and increases the local circulation driving deeper mixed layers and increased subduction conversely in the same region a poleward shift and poleward intensification of the westerly winds reduces heat loss and increases freshwater input thus decreasing the mixed layer depth and consequently the associated samw and aaiw subduction in the pacific sector all wind stress perturbations lead to increases in heat loss and decreases in freshwater input resulting in a net increase in samw and aaiw subduction overall the poleward shift in the westerly wind stress dominates the samwsubduction changes rather than the increase in wind stress the net decrease in samw subduction across all basins would likely decrease anthropogenic carbon sequestration however the net aaiwsubduction changes across the southern ocean are overall minor

separating the influence of projected changes in air temperature and wind on patterns of sea level change and ocean heat content we present ocean model sensitivity experiments aimed at separating the influence of the projected changes in the thermal near surface air temperature and wind near surface winds forcing on the patterns of sea level and ocean heat content in the north atlantic the distribution of sea level change is more due to the thermal forcing whereas it is more due to the wind forcing in the north pacific in the southern ocean the thermal and wind forcing have a comparable influence in the ocean adjacent to antarctica the thermal forcing leads to an inflow of warmer waters on the continental shelves which is somewhat attenuated by the wind forcing the structure of the vertically integrated heat uptake is set by different processes at low and high latitudes at low latitudes it is dominated by the heat transport convergence whereas at high latitudes it represents a small residual of changes in the surface flux and advection of heat the structure of the horizontally integrated heat content tendency is set by the increase of downward heat flux by the mean circulation and comparable decrease of upward heat flux by the subgrid scale processes the upward eddy heat flux decreases and increases by almost the same magnitude in response to respectively the thermal and wind forcing regionally the surface heat loss and deep convection weaken in the labrador sea but intensify in the greenland sea in the region of sea ice retreat the enhanced heat flux anomaly in the subpolar atlantic is mainly caused by the thermal forcing

recent antarctic ice mass loss from radar interferometry and regional climate modelling observed estimates of ice losses in antarctica combined with regional modelling of ice accumulation in the interior suggest that east antarctica is close to a balanced mass budget but large losses of ice occur in the narrow outlet channels of west antarctic glaciers and at the northern tip of the antarctic peninsula

are the near antarctic easterly winds weakening in response to enhancement of the southern annular mode abstractprevious studies have highlighted the sensitivity of the southern ocean circulation to the strengthening poleward shifting westerlies associated with the increasingly positive southern an

modeling ocean processes below fimbulisen antarctica 1 model simulations of circulation and melting beneath fimbulisen antarctica obtained using an isopycnic coordinate ocean model are presented model results compare well with available observations of currents and hydrography in the open ocean to the north of fimbulisen and suggest that warm deep water exists above the level of a sub ice shelf bedrock sill the principal pathway for warm waters to enter the sub ice shelf cavity the model shows a southward inflow of warm deep water over this sill and into the cavity producing a mean cavity temperature close to 1 0 c this leads to high levels of basal melting 10 m a at the grounding line of jutulstraumen and an average melting over the ice shelf base close to 1 9 m a the southward inflow is a compensating flow caused by the northward outflow of fresh cold water produced by the basal melting results on inflow and melting are difficult to validate since no in situ measurements yet exist in the cavity if such high melt rates are realistic the mass balance of fimbulisen must be significantly negative and the ice shelves along dronning maud land must contribute about 4 4 msv of melt water to the weddell sea about 15 of the total antarctic meltwater input to the southern ocean

cmip5 temperature biases and 21st century warming around the antarctic coast projections of ice sheet mass balance require regional ocean warming projections derived from atmosphere ocean general circulation models aogcms however the coarse resolution of aogcms 1 may lead to systematic or aogcm specific biases and 2 makes it difficult to identify relevant water masses here we employ a large scale metric of antarctic shelf bottom water asbw to investigate circum antarctic temperature biases and warming projections in 19 different coupled model intercomparison project phase 5 cmip5 aogcms forced with two different representative concentration pathways rcps for high emissions rcp 8 5 the ensemble mean 21st century asbw warming is 0 66 0 74 and 0 58 c for the amundsen ross and weddell seas as rs and ws respectively rcp 2 6 ensemble mean projections are substantially lower 0 21 0 26 and 0 19 c all distributions of regional asbw warming are positively skewed for rcp 8 5 four aogcms project warming of greater than 1 8 c in the rs across the ensemble there is a strong rcp independent correlation between ws and rs warming as warming is more closely linked to warming in the southern ocean we discuss possible physical mechanisms underlying the spatial patterns of warming and highlight implications of these results on strategies for forcing ice sheet mass balance projections

the calibration of optical backscatter sensors for suspended sediment of varying darkness levels a series of calibration experiments were run in order to determine the effect of the colour of suspended sediment on the response of optical backscatter sensors obs munsell notation describes the properties of soil and sediment colour using the three parameters hue chroma and value which represent respectively the spectral position lightness level and amount of pure chromatic colour present the exposure of the obs to munsell soil chart patches revealed that the obs is sensitive to variations in munsell value and not munsell hue or chroma since obs output decreased with darkness level and increased with lightness level and munsell value it is suggested that the level of blackness of particles acts to absorb the near infrared ir signal of the obs modifying its output a strong correlation r2 0 984 was found between the observed and predicted obs outputs of varying concentrations and ratios of black and white suspended sediment the obs output was predicted using a linear mixture model incorporating end member calibrations of black and white sediment similar to green and boon green m o boon j d iii 1993 the measurement of constituent concentrations in nonhomogeneous sediment suspensions using optical backscatter sensors marine geology 110 pp 73 81 a strong correlation r2 0 907 was also observed between luminance factor and ir reflectance for various minerals suggesting a link between munsell notation and obs output formulae were developed to predict the ir reflectance of sediment of known munsell value concentration and size models predicting sediment transport should consider the effect of the level of blackness of particles on the obs response

in situ high frequency long term measurements of suspended sediment concentration in turbid estuarine system seine estuary france optical turbidity sensors response to suspended sediment characteristics abstract the aim of this study is to investigate the complex response of optical turbidity sensors side and back scattering sensors to suspended particle matter spm characteristics and the consequences when investigating spm dynamics from long term high frequency monitoring networks our investigation is based on the analysis of a unique dataset of monthly 12 h cycle measurements of spm characteristics such as turbidity concentration floc size distribution floc density and organic matter content in the macrotidal seine estuary france between february 2015 and june 2016 results reveal that despite calibration to a formazin standard turbidity sensor response to spm concentrations in the range of 7 7000 mg l 1 are strongly variable from the tidal scale to the annual scale and in different compartments of the seine estuary the variability in the calibration relationships is related to changes in the sensor sensitivity according to i the sensor intern technology mainly due to optical geometry and ii the variability in inherent optical properties iop of spm side scattering optical instruments measuring scattering at 90 provide at the annual scale a more stable optical response than backscattering sensor measuring scattering at angles larger than 100 for a wide variety of floc size and density in the estuarine environment while at the tidal scale the backscatter sensors are the most accurate sensor sensitivity is strongly affected by floc characteristics i e their median size d50 dry density and the scattering efficiency qb results highlight that the median particle diameter contribute to modify the scattering efficiency qb as well as the dry density qb increases with increasing floc size and for a given floc size qb increases with floc density in this study the results are next applied to turbidity data from the long term automated monitoring network in order to estimate spm concentrations and estimate the related uncertainties

a laboratory evaluation of optical backscatterance suspended solids sensors exposed to sand mud mixtures abstract optical suspended solids sensors were studied in the laboratory in order to obtain calibration factors and evaluate the general behavior of the instrument when exposed to suspensions of mud and sand the sensors were found to be capable of measuring the time averaged concentration of either suspended sand or suspended mud but the relative sensitivity of the sensor to these two different types of sediment varied by approximately an order of magnitude in mixtures with a constant or known mud concentration the instrument response to the mud may be treated as an offset and gains appropriate to sand may be used to estimate the suspended sand concentrations semi quantitative observations of the effects of flow direction relative to sensor orientation indicate that significant concern is warranted

a fiber optic sensor for monitoring suspended sediment a new instrument that can be used to continuously monitor concentration of suspended particulates in close proximity to the sea bed z 3 5 cm is described the instrument is composed of a narrow band source light a photodiode detector and glass fibers that transmit and receive light individual sensors can be sampled rapidly 8 hz and respond linearly to increasing concentrations of sediment over the range 0 100 gl 1 an array of sensors can be deployed from z 0 5 to 3 5 cm above the seabed providing simultaneous time series of suspended sediment concentration at five discrete elevations results of a field deployment in the swash zone of a dissipative beach indicate that the performance of the instrument is comparable to that of the standard optical backscatter sensor obs the data show that se the standard optical backscatter sensor obs the data show that sediment resuspension in the swash zone is strongly correlated with low frequency cross shore flows with peak concentrations exceeding 200 gl 1

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

mass loss of totten and moscow university glaciers east antarctica using regionally optimized grace mascons author s mohajerani y velicogna i rignot e abstract 2018 american geophysical union all rights reserved totten and moscow university glaciers in the marine based sector of east antarctica contain enough ice to raise sea level by 5 m obtaining precise measurements of their mass balance is challenging owing to large area of the basins and the small mass balance signal compared to west antarctic glaciers here we employ a locally optimized processing of gravity recovery and climate experiment grace harmonics to evaluate their mass balance at the sub basin scale and compare the results with mass budget method mbm estimates using regional atmospheric climate model version 2 3 racmo2 3 or modele atmospherique regional version 3 6 4 mar3 6 4 the sub basin mass loss estimate for april 2002 to november 2015 is 14 8 4 3 gt yr which is weakly affected by glacial isostatic adjustment uncertainties 1 4 gt yr this result agrees with mbm racmo2 3 15 8 2 0 gt yr whereas mbm mar3 6 4 underestimates the loss 6 6 1 6 gt yr for the entire drainage the mass loss for april 2002 to august 2016 is 18 5 6 6 gt yr or 15 4 of its ice flux these results provide unequivocal evidence for mass loss in this east antarctic sector

different magnitudes of projected subsurface ocean warming around greenland and antarctica accurate projections of global sea level rise require information of future ocean warming in the vicinity of the large ice sheets an analysis of 19 climate model projections suggests that subsurface ocean warming near both polar ice sheets will be substantial with the potential to lead to significant increases in ice mass loss

potential for southern hemisphere climate surprises climate model results suggest that future climate change in antarctica will be accompanied by continued strengthening and poleward contraction of the southern ocean westerly wind belt paleoclimate records suggest past changes in the westerly winds can be abrupt and that healing of the antarctic ozone hole could lead to poleward contraction of the westerlies and increased meridional atmospheric transport of warm air regionally into antarctica an abrupt shift to more meridional circulation could lead to notable changes in moisture availability for extra antarctic regions increased antarctic ice sheet disintegration and more rapid sea level rise copyright 2015 the authors journal of quaternary science published by john wiley sons ltd

new instrumentation for the investigation of sediment suspension processes in the shallow marine environment abstract an instrumentation system has been developed that continuously measures flow velocity water depth and suspended sediment concentration profiles in the nearshore zone the system consists of four components 1 an array of electronic sensors including an electromagnetic current meter a surface piercing resistance wave gauge and a five element array of optical back scatterance sensors 2 a signal processing electronics unit 3 a guyed mast support structure and 4 a data acquisition system the small size of the sensors and their response characteristics make the array suitable for monitoring sediment transport to within a few centimeters of the bed under moderately large breaking waves 2 m results from an experiment on an exposed ocean beach on the coast of washington u s a indicate that sand suspension events of long duration 15 40 sec relative to the period of incident waves contribute a significant quantity of sand to the total littoral drift work is now in progress to modify the system for self contained operation on the continental shelf field investigations with the modified system will focus on the acquisition of a data set to test the validity of the suspended sediment distribution equation

rapid freshening of antarctic bottom water formed in the indian and pacific oceans 1 repeat hydrographic sections occupied in 1995 and 2005 reveal a rapid decline in the salinity and density of antarctic bottom water throughout the australian antarctic basin the basin wide shift of the deep potential temperature salinity s relationship reflects freshening of both the indian and pacific sources of antarctic bottom water the s curves diverge for waters cooler than 0 1 c corresponding to a layer up to 1000 m thick over the antarctic continental slope and rise changes over the last decade are in the same direction but more rapid than those observed between the late 1960s and the 1990s when combined with recent observations of similar freshening of north atlantic deep water these results demonstrate that dense water formed in both hemispheres is freshening in response to changes in the high latitude freshwater balance and rapidly transmitting the signature of changes in surface climate into the deep ocean

ice loss from the east antarctic ice sheet during late pleistocene interglacials understanding ice sheet behaviour in the geological past is essential for evaluating the role of the cryosphere in the climate system and for projecting rates and magnitudes of sea level rise in future warming scenarios1 4 although both geological data5 7 and ice sheet models3 8 indicate that marine based sectors of the east antarctic ice sheet were unstable during pliocene warm intervals the ice sheet dynamics during late pleistocene interglacial intervals are highly uncertain3 9 10 here we provide evidence from marine sedimentological and geochemical records for ice margin retreat or thinning in the vicinity of the wilkes subglacial basin of east antarctica during warm late pleistocene interglacial intervals the most extreme changes in sediment provenance recording changes in the locus of glacial erosion occurred during marine isotope stages 5 9 and 11 when antarctic air temperatures11 were at least two degrees celsius warmer than pre industrial temperatures for 2 500 years or more hence our study indicates a close link between extended antarctic warmth and ice loss from the wilkes subglacial basin providing ice proximal data to support a contribution to sea level from a reduced east antarctic ice sheet during warm interglacial intervals while the behaviour of other regions of the east antarctic ice sheet remains to be assessed it appears that modest future warming may be sufficient to cause ice loss from the wilkes subglacial basin studies of an antarctic marine sediment core suggest that the east antarctic ice sheet retreated in the vicinity of the wilkes subglacial basin during extended warm periods of the late pleistocene when temperatures were similar to those predicted to occur within this century

a model study of circumpolar deep water on the west antarctic peninsula and ross sea continental shelves abstract transport of relatively warm nutrient rich circumpolar deep water cdw onto continental shelves around antarctica has important effects on physical and biological processes however the characteristics of the cdw along the shelf break as well as what happens to it once it has been advected onto the continental shelf differ spatially in the present study high resolution 4 5 km regional models of the ross sea and the west antarctic peninsula coastal ocean are used to compare differences in cdw transport the models compared very well with observations from both regions examining the fluxes not only of heat but also of a simulated dye representing cdw shows that in both cases cdw crosses the shelf break in specific locations primarily determined by the bathymetry but eventually floods much of the shelf the frequency of intrusions in marguerite trough was ca 2 3 per month similar to recent mooring observations a significant correlation between the along shelf break wind stress and the cross shelf break dye flux through marguerite trough was observed suggesting that intrusions are at least partially related to short duration wind events the primary difference between the cdw intrusions on the ross and west antarctic peninsula shelves is that there is more vigorous mixing of the cdw with the surface waters in the ross sea especially in the west where high salinity shelf water is created the models show that the cdw moving across the antarctic peninsula continental shelf towards the base of the ice shelves not only is warmer initially and travels a shorter distance than that advected towards the base of the ross ice shelf but it is also subjected to less vertical mixing with surface waters which conserves the heat available to be advected under the ice shelves this difference in vertical mixing also likely leads to differences in the supply of nutrients from the cdw into the upper water column and thus modulates the impacts on surface biogeochemical processes

relations between annular modes and the mean state southern hemisphere winter in a zonally symmetric climatology with a single eddy driven jet such as prevails in the southern hemisphere summer the midlatitude variability is dominated by fluctuations of the jet around its mean position as described by the southern hemisphere annular mode sam to study whether this result holds for a zonally asymmetric climatology the observed variability of the southern hemisphere winter is analyzed the mean state in this case is characterized by relatively weak stationary waves yet there exist significant zonal variations in the mean strength and meridional structure of the subtropical jet stream as in summer the winter sam signature is annular in shape and the corresponding wind anomalies are dipolar but it is associated with two different behaviors of the eddy driven jet in different longitudinal ranges over the indian ocean the sam is associated primarily with a latitudinal shift of the jet around its mean position over the pacific sector it is instead characterized by a seesaw in the wind speed between two distinct latitudes corresponding to the positions of the midlatitude and subtropical jets composites of eddy forcing and baroclinicity over both sectors appear consistent with the two different behaviors as in the zonal mean case high frequency eddies both force and maintain the low frequency wind anomalies associated with the sam the positive feedback by eddies is however not local changes in the eddy forcing are influenced most strongly by zonal wind anomalies located upstream

a laboratory examination of floc characteristics with regard to turbulent shearing abstract turbulent shear generated within the water column is recognised as having a controlling influence over both the flocculation of fine grained cohesive sediments within estuarine waters and their respective aggregate break up this study examines the inter relationships between floc characteristics over increasing turbidity 80 200 mg l 1 and turbulent shear 0 0 6 n m 2 environments by the use of a laboratory flume within which a suspension can be sheared at a controlled rate and with an unintrusive macro lens miniature video camera mounted in a viewing port on the flume channel wall the camera enables the direct simultaneous measurement of both floc size and settling velocity from which accurate estimates of floc effective density and porosity can be made measurements were made 120 s after the induced turbulence has ceased the instrument has an upper viewing turbidity limit of 210 mg l 1 and a lower resolution of 20 m the sediment was collected from the inter tidal mudflats at weir quay on the tamar estuary in devon southwest england the results indicated that increasing turbidity at low shear levels encouraged floc growth but the effect of the increasing turbulent shear 0 35 n m 2 together with increasing concentration in suspension causes disruption rather than enhancing the flocculation process at shears up to 0 35 n m 2 the largest size and settling velocity flocs were produced at high concentrations whereas above 0 35 n m 2 disruption caused smaller flocs at higher concentrations the use of algorithms which were based either on a single floc characteristic i e size or settling velocity or a mean fractal dimension were seen not to accurately approximate the experimental data a multiple regression analysis of the experimental data produced the following formula based on mean values of the 20 largest flocs sampled under each of the imposed environmental conditions referred to as max20size mean values settling velocity 0 301 0 00337 rms of the gradient in turbulent velocity fluctuations 0 000606 spm 0 00335 floc size which proved to be the most accurate representation with an r 2 value of 0 95 a similar formula was determined for the average value of the four fastest settling flocs within each sample group max4w s this highlights the importance of modelling algorithms that are developed from data that take into account effective density variations i e simultaneous size and settling velocity measurement

ice sheet grounding line dynamics steady states stability and hysteresis 1 the ice sheet ice shelf transition zone plays an important role in controlling marine ice sheet dynamics as it determines the rate at which ice flows out of the grounded part of the ice sheet together with accumulation this outflow is the main control on the mass balance of the grounded sheet in this paper we verify the results of a boundary layer theory for ice flux in the transition zone against numerical solutions that are able to resolve the transition zone very close agreement is obtained and grid refinement in the transition zone is identified as a critical component in obtaining reliable numerical results the boundary layer theory confirms that ice flux through the grounding line in a two dimensional sheet shelf system increases sharply with ice thickness at the grounding line this result is then applied to the large scale dynamics of a marine ice sheet our principal results are that 1 marine ice sheets do not exhibit neutral equilibrium but have well defined discrete equilibrium profiles 2 steady grounding lines cannot be stable on reverse bed slopes and 3 marine ice sheets with overdeepened beds can undergo hysteresis under variations in sea level accumulation rate basal slipperiness and ice viscosity this hysteretic behavior can in principle explain the retreat of the west antarctic ice sheet following the last glacial maximum and may play a role in the dynamics of heinrich events

eddy generation and jet formation via dense water outflows across the antarctic continental slope along various stretches of the antarctic margins dense antarctic bottom water aabw escapes its formation sites and descends the continental slope this export necessarily raises the isopycnals associated with lighter density classes over the continental slope resulting in density surfaces that connect the near freezing waters of the continental shelf to the much warmer circumpolar deep water cdw at middepth offshore in this article an eddy resolving process model is used to explore the possibility that aabw export enhances shoreward heat transport by creating a pathway for cdw to access the continental shelf without doing any work against buoyancy forces in the absence of a net alongshore pressure gradient the shoreward cdw transport is effected entirely by mesoscale and submesoscale eddy transfer eddies are generated partly by instabilities at the pycnocline sourcing their energy from the alongshore wind stress but primarily by instabilities at the cdw aabw interface sourcing their energy from buoyancy loss on the continental shelf this combination of processes induces a vertical convergence of eddy kinetic energy and alongshore momentum into the middepth cdw layer sustaining a local maximum in the eddy kinetic energy over the slope and balancing the coriolis force associated with the shoreward cdw transport the resulting slope turbulence self organizes into a series of alternating along slope jets with strongly asymmetrical contributions to the slope energy and momentum budgets cross shore variations in the potential vorticity gradient cause the jets to drift continuously offshore suggesting that fronts observed in regions of aabw down slope flow may in fact be transient features

an eddy permitting southern ocean state estimate abstract an eddy permitting general circulation model of the southern ocean is fit by constrained least squares to a large observational dataset during 2005 06 data used include argo float profiles ctd synoptic sections southern elephant seals as oceanographic samplers seaos instrument mounted seal profiles xbts altimetric observations envisat geosat jason 1 and ocean topography experiment topex poseidon and infrared and microwave radiometer observed sea surface temperature an adjoint model is used to determine descent directions in minimizing a misfit function each of whose elements has been weighted by an estimate of the observational plus model error the model is brought into near agreement with the data by adjusting its control vector here consisting of initial and meteorological boundary conditions although total consistency has not yet been achieved the existing solution is in good agreement with the great majority of the 2005 and 2006 southern ocean observations and better repr

sea level changes forced by southern ocean winds 1 on regional scales changes in sea level are significantly affected by local dynamical changes westerly winds over the southern ocean have been strengthening and shifting southward in recent decades and this change is projected to continue in the future this study applies wind forcing anomalies to an eddy permitting ocean model to study the dynamical response to a southern hemisphere westerly wind increase and or southward shift it is shown that the applied wind anomalies result in a change in sea surface slope across the antarctic circumpolar current such that a fall in sea level occurs around the antarctic continental margin the antarctic circumpolar current transport and regional sea level are particularly sensitive to latitudinal shifts in the wind with a much more muted response found when only wind strengthening is applied in addition to the local sea level changes southern ocean winds also have a global effect through changing ocean heat content and the global overturning circulation

a laboratory investigation of particle size effects on an optical backscatterance sensor abstract the effect of particle size on an optical backscatterance sensor s obs response to suspended sediment concentration was investigated in the laboratory sieved ground glass was used in place of natural sediments to eliminate mineralogical effects highly reliable linear regressions of obs output versus suspended sediment concentration were obtained for suspensions of constant particle size regression analyses were conducted to determine an empirical relationship between obs gain and particle size an inverse power law regression best fit the data a regression based upon mie s theory also fit within the error bands of the data suggesting the theory may be applicable to obs performance results indicate that the obs is most sensitive to particle size changes for particles 100 m or less

a model for the structure of the antarctic slope front the antarctic slope front is an oceanic front that appears to be a semi permanent feature situated above or near long sections of the shelf break of the antarctic continental shelf these sections include the shelf break of the ross and weddell seas it is distinguished by a v shaped structure unique to coastal antarctica in which isopycnals slope downward from near the surface on both the shallow continental shelf and the deep continental slope sides toward the bottom near the upper part of the slope in the regions where it is observed the overflow of dense water from the shelf down the slope is observed or presumed to occur such overflows entrain fluid from the overlying stratified environment into the downflow and this entrainment acts as a distributed sink of the overlying fluid a model of this frontal structure is constructed here consisting of the downslope flow modelled as a plume and the overlying fluid governed by quasi geostrophic dynamics with the effect of the plume on the latter being represented by a suitable distribution of sinks the alongslope currents follow from geostrophy and potential vorticity conservation two distinct flow regimes are found in the first regime a realistic depiction of the two sided v shaped structure is obtained where most of the fluid flowing off the shelf enters the downflow at the top of the slope constituting the shallow side of the v and fluid from offshore enters the deep side of the v alongslope flow in this region is mostly eastward in the second regime where most of the fluid flowing off the shelf passes over the downflow only the shallow side of the v is present and alongslope flow is westward near the shelf break and eastward further offshore

controls on circulation cross shelf exchange and dense water formation in an antarctic polynya circulation on the antarctic continental shelf influences cross shelf exchange antarctic bottom water formation and ocean heat flux to floating ice shelves the physical processes driving the shelf circulation and its seasonal and interannual variability remain poorly understood we use a unique time series of repeat hydrographic observations from the adelie land continental shelf and a box inverse model to explore the relationship between surface forcing shelf circulation cross shelf exchange and dense water formation a wind driven northwestward coastal current set up by onshore ekman transport dominates the summer circulation during winter strong buoyancy loss creates dense shelf water this dense water flows off the shelf with a compensating on shelf flow that is an order of magnitude larger in winter than in summer the results demonstrate the importance of winter buoyancy loss in driving the shelf circulation and cross shelf exchange as well as dense water mass formation

laboratory apparatus for calibrating optical suspended solids sensors abstract an apparatus for testing and calibrating miniature to medium sized optical suspended solids sensors is described stable suspensions of sedimentary particles ranging in size from 10 6 to 4 10 3 m can be produced in water with concentrations exceeding 100 g 1 1 flow velocities in the test volume can be varied continuously from 0 1 to 1 5 m s 1 and samples of suspensions can be withdrawn directly with a peristaltic pump for gravimetric and particle size analyses the apparatus has been used to evaluate the effects of particle size flow speed and air bubbles on the performance of optical backscatterance sensors and transmissometers

measurements of high concentration suspended sediments using the optical backscatterance sensor abstract an optical backscatterance sensor obs was used to measure extremely high concentration suspended sediments on the amazon continental shelf as part of the amasseds project a multidisciplinary amazon shelf sediment study using a pumping system for obtaining water samples to calibrate the sensor in situ the obs output was observed to increase approximately linearly for concentrations 36 g l the obs response was nearly continuous from 0 to 320 g l making it possible to obtain continuous suspended sediment concentration profiles through fluid muds

shelf ocean exchange and hydrography west of the antarctic peninsula a review the west antarctic peninsula wap is a highly productive marine ecosystem where extended periods of change have been observed in the form of glacier retreat reduction of sea ice cover and shifts

the measurement of constituent concentrations in nonhomogeneous sediment suspensions using optical backscatter sensors abstract a method is proposed for estimating the concentrations of the constituents of nonhomogeneous sediment suspensions using optical backscatter sensors the method is predicated on the assumption that constituent interaction e g grain shielding and multiple scattering does not significantly affect sensor response this fundamentally linear approach was verified by laboratory tests of a backscatter sensor in a range of silt sand mixtures implementation of the method will require further development of the optical backscatter sensor or possibly the pairing of an acoustic backscatter sensor with an optical sensor the method has the potential for resolving small scale spatial and temporal changes in the suspended sediment population and therefore could be employed in studies of winnowing and bed armouring for example

accelerated ice discharge from the antarctic peninsula following the collapse of larsen b ice shelf acceleration exceeds 27 km 3 per year and ice is thinning at rates of tens of meters per year we attribute this abrupt evolution of the glaciers to the removal of the buttressing ice shelf the magnitude of the glacier changes illustrates the importance of ice shelves on ice sheet mass balance and contribution to sea level change index terms 1827 hydrology glaciology 1863 1863 hydrology snow and ice 1827 3349 meteorology and atmospheric dynamics polar meteorology 6924 radio science interferometry 9310 information related to geographic region antarctica citation rignot e g casassa p gogineni w krabill a rivera and r thomas 2004 accelerated ice discharge from the antarctic peninsula following the collapse of larsen b ice shelf geophys res lett 31 l18401 doi 10 1029 2004gl020697

interpretation of recent southern hemisphere climate change climate variability in the high latitude southern hemisphere sh is dominated by the sh annular mode a large scale pattern of variability characterized by fluctuations in the strength of the circumpolar vortex we present evidence that recent trends in the sh tropospheric circulation can be interpreted as a bias toward the high index polarity of this pattern with stronger westerly flow encircling the polar cap it is argued that the largest and most significant tropospheric trends can be traced to recent trends in the lower stratospheric polar vortex which are due largely to photochemical ozone losses during the summer fall season the trend toward stronger circumpolar flow has contributed substantially to the observed warming over the antarctic peninsula and patagonia and to the cooling over eastern antarctica and the antarctic plateau

circum antarctic shoreward heat transport derived from an eddy and tide resolving simulation almost all heat reaching the bases of antarctica s ice shelves originates from warm circumpolar deep water in the open southern ocean this study quantifies the roles of mean and transient flows in transporting heat across almost the entire antarctic continental slope and shelf using an ocean sea ice model run at eddy and tide resolving 1 48 horizontal resolution heat transfer by transient flows is approximately attributed to eddies and tides via a decomposition into time scales shorter than and longer than 1 day respectively it is shown that eddies transfer heat across the continental slope ocean depths greater than 1 500 m but tides produce a stronger shoreward heat flux across the shelf break ocean depths between 500 m and 1 000 m however the tidal heat fluxes are approximately compensated by mean flows leaving the eddy heat flux to balance the net shoreward heat transport the eddy driven cross slope overturning circulation is too weak to account for the eddy heat flux this suggests that isopycnal eddy stirring is the principal mechanism of shoreward heat transport around antarctica though likely modulated by tides and surface forcing

dynamic high speed rupture from the onset of the 2004 parkfield california earthquake 1 we investigated the 2004 parkfield earthquake using a multiscale slip inversion the multiscale approach allows us to both focus on the details of the early stages of the rupture process and to model the entire earthquake at a larger scale our model indicates that the parkfield earthquake began with bilateral high speed rupture with fast slip rate up to 4 m s and fast propagation velocity of about 3 0 km s rupture propagation to the southeast was arrested 3 s after onset while propagation to the northwest continued generated a large slip patch at 5 s and ceased within 10 s a similar rapid initiation was observed for the 2004 mid niigata prefecture japan earthquake in a previous study high speed initial rupture suggests that large earthquakes may initiate and grow in the same way as small earthquakes and that the final size is difficult to predict from the early stages of an earthquake

relation between annular modes and the mean state southern hemisphere summer abstract the annular modes emerge as the leading mode of extratropical month to month climate variability in both hemispheres here the influence of the background state on the structure and dynamics of the southern hemisphere annular mode sam during the austral summer when the climatology is characterized by a single well defined eddy driven jet is studied subsets of the climatology are constructed for early and late summer and for contrasting polarities of the enso cycle the analysis is based both on observations and on perpetual state gcm experiments the main differences between the subsets involve variations of the latitude of the mean jet it is found that in all the cases the sam is characterized by latitudinal shifts of the jet about its mean position reinforced by a positive momentum flux feedback from baroclinic waves this result is consistent with previous studies of the dynamics of the zonally averaged circulation but is found here to hold over all longitudes and for different positi

suspended sediment transport in the seine estuary france effect of man made modifications on estuary shelf sedimentology abstract man made modifications of the seine estuary have produced large geometric changes with important hydrographic and sedimentological consequences affecting estuary shelf interrelationships the estuary once had shallow meandering ebb and flood tidal channels and broad tidal flats today after 130 years of landfill marsh reclamation jettying and dredging there is only a narrow deep jettied channel extending to the sea most of the tidal flats and marshes have been filled these modifications have concentrated river discharge and ebb flow into a narrow jettied channel which in turn has produced a seaward migration of the salt intrusion and the turbidity maximum as the estuary volume was reduced river flow dilution was also reduced and its mean residence time in the estuary was shortened at the same time sedimentation and landfill have reduced the area available for sedimentation of fluvial sediments this change has amplified seaward transport of fluvial suspended sediment and increased the size of shelf mud zones off the estuary mouth man has thus changed the geologic role of the estuary from a sink for fluvial and marine sediment to a source of fluvial sediment for the shelf

rapid bottom melting widespread near antarctic ice sheet grounding lines as continental ice from antarctica reaches the grounding line and begins to float its underside melts into the ocean results obtained with satellite radar interferometry reveal that bottom melt rates experienced by large outlet glaciers near their grounding lines are far higher than generally assumed the melting rate is positively correlated with thermal forcing increasing by 1 meter per year for each 0 1 c rise in ocean temperature where deep water has direct access to grounding lines glaciers and ice shelves are vulnerable to ongoing increases in ocean temperature

acceleration and overturning of the antarctic slope current by winds eddies and tides all exchanges between the open ocean and the antarctic continental shelf must cross the antarctic slope current asc previous studies indicate that these exchanges are strongly influenced by mesoscale and tidal variability yet the mechanisms responsible for setting the asc s transport and structure have received relatively little attention in this study the roles of winds eddies and tides in accelerating the asc are investigated using a global ocean sea ice simulation with very high resolution 1 48 grid spacing it is found that the circulation along the continental slope is accelerated both by surface stresses ultimately sourced from the easterly winds and by mesoscale eddy vorticity fluxes at the continental shelf break the asc exhibits a narrow 30 50 km swift 0 2 m s 1 jet consistent with in situ observations in this jet the surface stress is substantially reduced and may even vanish or be directed eastward because the ocean surface speed matches or exceeds that of the sea ice the shelfbreak jet is shown to be accelerated by tidal momentum advection consistent with the phenomenon of tidal rectification consequently the shoreward ekman transport vanishes and thus the mean overturning circulation that steepens the antarctic slope front asf is primarily due to tidal acceleration these findings imply that the circulation and mean overturning of the asc are not only determined by near antarctic winds but also depend crucially on sea ice cover regionally dependent mesoscale eddy activity over the continental slope and the amplitude of tidal flows across the continental shelf break

state of the antarctic and southern ocean climate system this paper reviews developments in our understanding of the state of the antarctic and southern ocean climate and its relation to the global climate system over the last few millennia climate over this and earlier periods has not been stable as evidenced by the occurrence of abrupt changes in atmospheric circulation and temperature recorded in antarctic ice core proxies for past climate two of the most prominent abrupt climate change events are characterized by intensification of the circumpolar westerlies also known as the southern annular mode between 6000 and 5000 years ago and since 1200 1000 years ago following the last of these is a period of major trans antarctic reorganization of atmospheric circulation and temperature between ad1700 and 1850 the two earlier antarctic abrupt climate change events appear linked to but predate by several centuries even more abrupt climate change in the north atlantic and the end of the more recent event is coincident with reorganization of atmospheric circulation in the north pacific improved understanding of such events and of the associations between abrupt climate change events recorded in both hemispheres is critical to predicting the impact and timing of future abrupt climate change events potentially forced by anthropogenic changes in greenhouse gases and aerosols special attention is given to the climate of the past 200 years which was recorded by a network of recently available shallow firn cores and to that of the past 50 years which was monitored by the continuous instrumental record significant regional climate changes have taken place in the antarctic during the past 50 years atmospheric temperatures have increased markedly over the antarctic peninsula linked to nearby ocean warming and intensification of the circumpolar westerlies glaciers are retreating on the peninsula in patagonia on the sub antarctic islands and in west antarctica adjacent to the peninsula the penetration of marine air masses has become more pronounced over parts of west antarctica above the surface the antarctic troposphere has warmed during winter while the stratosphere has cooled year round the upper kilometer of the circumpolar southern ocean has warmed antarctic bottom water across a wide sector off east antarctica has freshened and the densest bottom water in the weddell sea has warmed in contrast to these regional climate changes over most of antarctica near surface temperature and snowfall have not increased significantly during at least the past 50 years and proxy data suggest that the atmospheric circulation over the interior has remained in a similar state for at least the past 200 years furthermore the total sea ice cover around antarctica has exhibited no significant overall change since reliable satellite monitoring began in the late 1970s despite large but compensating regional changes the inhomogeneity of antarctic climate in space and time implies that recent antarctic climate changes are due on the one hand to a combination of strong multi decadal variability and anthropogenic effects and as demonstrated by the paleoclimate record on the other hand to multi decadal to millennial scale and longer natural variability forced through changes in orbital insolation greenhouse gases solar variability ice dynamics and aerosols model projections suggest that over the 21st century the antarctic interior will warm by 3 4 1oc and sea ice extent will decrease by 30 ice sheet models are not yet adequate enough to answer pressing questions about the effect of projected warming on mass balance and sea level considering the potentially major impacts of a warming climate on antarctica vigorous efforts are needed to better understand all aspects of the highly coupled antarctic climate system as well as its influence on the earth s climate and oceans

inability of stratospheric sulfate aerosol injections to preserve the west antarctic ice sheet injection of sulfate aerosols into the stratosphere has the potential to reduce the climate impacts of global warming including sea level rise slr however changes in atmospheric and oceanic circulation that can significantly influence the rate of basal melting of antarctic marine ice shelves and the associated slr have not previously been considered here we use a fully coupled global climate model to investigate whether rapidly increasing stratospheric sulfate aerosol concentrations after a period of global warming could preserve antarctic ice sheets by cooling subsurface ocean temperatures we contrast this climate engineering method with an alternative strategy in which all greenhouse gases ghg are returned to preindustrial levels we find that the rapid addition of a stratospheric aerosol layer does not effectively counteract surface and upper level atmospheric circulation changes caused by increasing ghgs resulting in continued upwelling of warm water in proximity of ice shelves especially in the vicinity of the already unstable pine island glacier in west antarctica by contrast removal of ghgs restores the circulation yielding relatively cooler subsurface ocean temperatures to better preserve west antarctica

connecting antarctic cross slope exchange with southern ocean overturning previous idealized investigations of southern ocean overturning have omitted its connection with the antarctic continental shelves leaving the influence of shelf processes on antarctic bottom water aabw export unconsidered in particular the contribution of mesoscale eddies to setting the stratification and overturning circulation in the antarctic circumpolar current acc is well established yet their role in cross shelf exchange of water masses remains unclear this study proposes a residual mean theory that elucidates the connection between antarctic cross shelf exchange and overturning in the acc and the contribution of mesoscale eddies to the export of aabw the authors motivate and verify this theory using an eddy resolving process model of a sector of the southern ocean the strength and pattern of the simulated overturning circulation strongly resemble those of the real ocean and are closely captured by the residualmean theory over the continental slope baroclinic instability is suppressed and so transport by mesoscale eddies is reduced this suppression of the eddy fluxes also gives rise to the steep v shaped isopycnals that characterize the antarctic slope front in aabw forming regions of the continental shelf furthermore to produce water on the continental shelf that is dense enough to sink to the deep ocean the deep overturning cell must be at least comparable in strength to wind driven mean overturning on the continental slope this results in a strong sensitivity of the deep overturning strength to changes in the polar easterly winds

observations of the antarctic slope undercurrent in the southeastern weddell sea 1 the antarctic slope front presents a dynamical barrier between the cold antarctic shelf waters in contact with ice shelves and the warmer subsurface waters offshore two hydrographic sections with full depth current measurements were undertaken in january and february 2009 across the slope and shelf in the southeastern weddell sea southwestward surface intensified currents of 30 cm s 1 and northeastward undercurrents of 6 9 cm s 1 were in thermal wind balance with the sloping isopycnals across the front which migrated offshore by 30 km in the time interval between the two sections a mid depth undercurrent on february 23 was associated with a 130 m uplift of the main pycnocline bringing warm deep water closer to the shelf break this vertical displacement comparable to that caused by seasonal variations in wind speed implies that undercurrents may affect the exchanges between coastal and deep waters near the antarctic continental margins

sensitivity of circumpolar deep water transport and ice shelf basal melt along the west antarctic peninsula to changes in the winds circumpolardeepwater cdw can be found near the continental shelfbreak around most of antarctica advection of this relatively warm water up to 28c across the continental shelf to the base of floating ice shelves is thought to be a critical source of heat for basal melting in some locations a high resolution 4 km regional ocean sea ice ice shelf model of the west antarctic peninsula wap coastal ocean was used to examine the effects of changes in the winds on across shelf cdw transport and ice shelf basal melt increases and decreases in the strength of the wind fields were simulated by scaling the present day winds by a constant factor additional simulations considered effects of increased antarctic circumpolar current acc transport increased wind strength and acc transport increased the amount of cdw transported onto the wap continental shelf but did not necessarily increase cdw flux underneath the nearby ice shelves the basal melt underneath some of the deeper ice shelves actually decreased with increased wind strength increased mixing over the wap shelf due to stronger winds removed more heat from the deeper shelf waters than the additional heat gained from increased cdw volume transport the simulation results suggest that the effect on the wap ice shelves of the projected strengthening of the polar westerlies is not a simple matter of increased winds causing increased or decreased basal melt a simple budget calculation indicated that iron associated with increased vertical mixing of cdw could significantly affect biological productivity of this region

genesis of the antarctic slope current in west antarctica the stability of the west antarctic ice sheet wais depends on ocean heat transport toward its base and remains a source of uncertainty in sea level rise prediction the antarctic slope current asc a major boundary current of the ocean s global circulation serves as a dynamic gateway for heat transport toward antarctica here we use observations collected from the bellingshausen sea to propose a mechanistic explanation for the initiation of the westward flowing asc waters modified throughout the bellingshausen sea by ocean sea ice and ocean ice shelf interactions are exported to the continental slope in a narrow topographically steered western boundary current this focused outflow produces a localized front at the shelf break that supports the emerging asc this mechanism emphasizes the importance of buoyancy forcing integrated over the continental shelf as opposed to local wind forcing in the generation mechanism and suggests the potential for remote control of melt rates of wais largest ice shelves

sediment resuspension flocculation and settling in a macrotidal estuary national natural science foundation of china 40876043 papd fund china scholarship council 2010832427 mel young scientist visiting fellowship melrs0938

inflow of warm circumpolar deep water in the central amundsen shelf the thinning and acceleration of the west antarctic ice sheet has been attributed to basal melting induced by intrusions of relatively warm salty water across the continental shelf a hydrographic section including lowered acoustic doppler current profiler measurements showing such an inflow in the channel leading to the getz and dotson ice shelves is presented here the flow rate was 0 3 0 4 sv 1 sv 10 6 m 3 s 21 and the subsurface heat loss was estimated to be 1 2 1 6 tw assuming that the inflow persists throughout the year it corresponds to an ice melt of 110 130 km 3 yr 21 which exceeds recent estimates of the net ice glacier ice volume loss in the amundsen sea the results also show a 100 150 m thick intermediate water mass consisting of circumpolar deep water that has been modified cooled and freshened by subsurface melting of ice shelves and or icebergs this water mass has not previously been reported in the region possibly because of the paucity of historical data

using eulerian and lagrangian approaches to investigate wind driven changes in the southern ocean abyssal circulation this study uses a global ocean eddy permitting climate model to explore the export of abyssal water from the southern ocean and its sensitivity to projected twenty first century poleward intensifying southern oceanwind stress the abyssalflow pathways and transportareinvestigated usinga combinationoflagrangian and eulerian techniques in an eulerian format the equator and poleward flows within similar abyssal density classes are increased by the wind stress changes making it difficult to explicitly diagnose changes in the abyssal export in a meridional overturning circulation framework lagrangian particle analyses are used to identify the major export pathways of southern ocean abyssal waters and reveal an increase in the number of particles exported to the subtropics from source regions around antarctica in response to the wind forcing both the lagrangian particleand eulerian analyses identify transients asplaying a key rolein the abyssal export ofwater from the southern ocean wind driven modifications to the potential energy component of the vorticity balance in the abyss are also found to impact the southern ocean barotropic circulation

marine ice sheet collapse potentially under way for the thwaites glacier basin west antarctica resting atop a deep marine basin the west antarctic ice sheet has long been considered prone to instability using a numerical model we investigated the sensitivity of thwaites glacier to ocean melt and whether its unstable retreat is already under way our model reproduces observed losses when forced with ocean melt comparable to estimates simulated losses are moderate 1 mm per year of sea level rise collapse in the different simulations within the range of 200 to 900 years

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

multidecadal warming of antarctic waters decadal trends in the properties of seawater adjacent to antarctica are poorly known and the mechanisms responsible for such changes are uncertain antarctic ice sheet mass loss is largely driven by ice shelf basal melt which is influenced by ocean ice interactions and has been correlated with antarctic continental shelf bottom water asbw temperature we document the spatial distribution of long term large scale trends in temperature salinity and core depth over the antarctic continental shelf and slope warming at the seabed in the bellingshausen and amundsen seas is linked to increased heat content and to a shoaling of the mid depth temperature maximum over the continental slope allowing warmer saltier water greater access to the shelf in recent years regions of asbw warming are those exhibiting increased ice shelf melt

widespread acceleration of tidewater glaciers on the antarctic peninsula over the last half century the antarctic peninsula ap has been among the most rapidly warming regions on earth this has led to increased summer snowmelt loss of ice shelves and retreat of 87 of marine and tidewater glacier fronts tidewater glacier flow is sensitive to changes in basal water supply and to thinning of the terminus and faster flow leads directly to sea level rise the flow rates of most ap tidewater glaciers have never been measured however and hence their dynamic response to the recent changes is unknown we present repeated flow rate measurements from over 300 glaciers on the ap west coast through nine summers from 1992 to 2005 we show that the flow rate increased by similar to 12 on average and that this trend is greater than the seasonal variability in flow rate we attribute this widespread acceleration trend not to meltwater enhanced lubrication or increased snowfall but to a dynamic response to frontal thinning we estimate that as a result the annual sea level contribution from this region has increased by 0 047 0 011 mm between 1993 and 2003 this contribution together with previous studies that assessed increased runoff from the area and acceleration of glaciers resulting from the removal of ice shelves implies a combined ap contribution of 0 16 0 06 mm yr 1 this is comparable to the contribution from alaskan glaciers and combined with estimated mass loss from west antarctica is probably large enough to outweigh mass gains in east antarctica and to make the total antarctic sea level contribution positive

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

rapid subsurface warming and circulation changes of antarctic coastal waters by poleward shifting winds the southern hemisphere westerly winds have been strengthening and shifting poleward since the 1950s this wind trend is projected to persist under continued anthropogenic forcing but the impact of the changing winds on antarctic coastal heat distribution remains poorly understood here we show that a poleward wind shift at the latitudes of the antarctic peninsula can produce an intense warming of subsurface coastal waters that exceeds 2 c at 200 700 m depth the model simulated warming results from a rapid advective heat flux induced by weakened near shore ekman pumping and is associated with weakened coastal currents this analysis shows that anthropogenically induced wind changes can dramatically increase the temperature of ocean water at ice sheet grounding lines and at the base of floating ice shelves around antarctica with potentially significant ramifications for global sea level rise

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

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

multidecadal freshening and lightening in the deep waters of the bransfield strait antarctica the deep waters of the bransfield strait receive considerable amounts of water from the weddell sea continental shelf the restricted connections to the surrounding ocean and relatively easier access makes the bransfield strait an important proxy region for monitoring changes in the dense weddell sea shelf water masses which are an important precursor of antarctic bottom water aabw long term hydrographic data from the period 1960s 2010s showed freshening and lightening of the deep water masses of the bransfield strait which was likely caused by large freshwater inputs originating from the western shelf of the weddell sea the rates of freshening and lightening were 0 0010 0 0005 yr 1 and 0 0016 0 0014 kg m 3 yr 1 for the central basin respectively and 0 0010 0 0006 yr 1 and 0 0029 0 0013 kg m 3 yr 1 for the eastern basin respectively the deep waters showed a high degree of interannual thermohaline variability which appeared to be caused by changes in the proportions of source water mass mixing between the years statistically significant negative correlations between salinity neutral density fields and the southern annular mode sam were observed 0 56 and 0 62 for the central basin respectively and 0 58 and 0 68 for the eastern basin respectively between 1980 and 2014 during sam positive phases communication between the weddell sea and the bransfield strait is reduced which leads to less saline and lighter water masses in the bransfield strait however the opposite trends are observed during sam negative phases

antarctic ice sheet sensitivity to obliquity forcing enhanced through ocean connections deep sea geological records indicate that antarctic ice sheet growth and decay is strongly influenced by the earth s astronomical variations known as milankovitch cycles and that the frequency of the glacial interglacial cycles changes through time here we examine the emergence of a strong obliquity axial tilt control on antarctic ice sheet evolution during the miocene by correlating the antarctic margin geological records from 34 to 5 million years ago with a measure of obliquity sensitivity that compares the variance in deep sea sediment core oxygen isotope data at obliquity timescales with variance of the calculated obliquity forcing our analysis reveals distinct phases of ice sheet evolution and suggests the sensitivity to obliquity forcing increases when ice sheet margins extend into marine environments we propose that this occurs because obliquity driven changes in the meridional temperature gradient affect the position and strength of the circum antarctic easterly flow and enhance or reduce ocean heat transport across the antarctic continental margin the influence of obliquity driven changes in ocean dynamics is amplified when marine ice sheets are extensive and sea ice is limited our reconstruction of the antarctic ice sheet history suggests that if sea ice cover decreases in the coming decades ocean driven melting at the ice sheet margin will be amplified the sensitivity of the antarctic ice sheet to obliquity increases when ice sheet margins are exposed to the ocean suggests an analysis of sediment core oxygen isotope records

the effect of flocs on optical backscattering measurements of suspended material concentration abstract optical backscattering measurements were made using coagulated kaolin and estuary sediments at various concentrations and subjected to two flow conditions these measurements showed that the size of the flocs can give the same effect as changing the concentration by up to 2 5 times in using optical measurements as indicators of suspended sediment concentration care must be used to evaluate the in situ floc size in estuarine and coastal studies

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

an integrated optic and acoustic ioa approach for measuring suspended sediment concentration in highly turbid environments abstracts accurate measurement of suspended sediment concentration ssc in highly turbid environments has been a problem due to optical or acoustic signal saturation and attenuation the saturation returns a limited measurement range and the attenuation raises an ambiguity problem that a low optical or acoustic output could mean a low or a high ssc in this study an integrated optic and acoustic ioa approach is proposed to i overcome the ambiguity problem ii increase the measurement range to high ssc values and iii obtain high resolution ssc profiles the ioa approach is a combination of argus suspension meter asm optical backscatter sensor obs and acoustic doppler velocimeter adv in this approach the asm derived ssc is preferred because of its lowest relative error followed by obs and adv the asm can produce high resolution 1 cm interval ssc profiles when it is not saturated usually ssc

variability in cohesive sediment settling fluxes observations under different estuarine tidal conditions the mass settling flux which is defined as the product of the concentration and the settling velocity is of prime importance with respect to both stratified and well mixed estuarine conditions the determination of these fluxes for applied modelling purposes in high energy tidal estuarine environments is very problematic this is because the muddy sediments which dominate in estuaries flocculate producing a variety of sizes and settling velocities and this flocculation process is not understood well enough to be fully described theoretically by drawing on examples of floc spectra acquired in situ using the inssev system this study explains how mass settling fluxes in the near bed region can vary by three or four orders of magnitude in meso and macro tidal estuaries throughout a single tidal cycle a floc population representative of dilute suspension conditions on a neap tide indicated only 35 of the floc mass was macroflocs 160 m however the macrofloc settling velocity 2 4 mm s 1 three times faster than the microflocs which meant the former fraction contributed 57 of the 205 mg m 2s 1 settling flux both highly concentrated 4 6 g l 1 and very turbulent spring tide conditions 1 6 n m 2 produced a bi modal distribution in terms of the floc size and dry mass with the former 54 of the mass was contained within the 240 480 m size fraction with a further 25 of the dry floc mass in the flocs over 480 m in diameter these large flocs had settling velocities between 4 8 mm s 1 which meant 99 5 of the settling flux 33 5 g m 2s 1 was accredited to the macroflocs the high turbulence environment saw the dry floc mass distribution shift 60 40 in favour of the microflocs the microfloc settling velocity was 1 45 mm s 1 0 35 mm s 1 faster than the larger macrofloc fraction in terms of the total mass settling flux 0 9 g m 2 s 1 this translates into the microflocs contributing 70 during high turbulence at slack water the flux only reached 12 mg m 2 s 1 and macrofloc growth was mainly attributed to differential settling continuous floc observations made over a complete tidal cycle revealed that the asymmetrical distribution of the tidal energy generated throughout the spring conditions in the tamar estuary demonstrated a distinct control on the flocculation process the less turbulent ebb produced 86 of the total tidal cycle mass settling flux of which only 8 of the settling flux was outside the turbidity maximum an attempt to simulate these large settling fluxes by using a constant settling rate of 0 5 mm s 1 under estimated the tidal cycle settling flux by 78 with less than 15 of the total flux being estimated during the advection of the turbidity maximum on the ebb in contrast using a faster constant settling velocity parameter of 5 mm s 1 representative of the macrofloc fraction resulted in a mass flux over estimate of 116 for the tidal cycle duration

an instrument system for monitoring and sampling suspended sediment in the benthic boundary layer abstract an instrument system has been constructed that can monitor and sample suspended sediment distributions in the benthic boundary layer it consists of miniature nephelometers and suspended sediment samplers placed within one meter of the seabed the system is capable of continuously monitoring suspended sediment profiles at eight levels between 14 and 100 cm above the seabed and collecting suspended sediment samples at four levels 20 50 70 and 100 cm at three times during a deployment period the suspended sediment system is designed to fit onto the instrumented tripod geoprobe which contains four electromagnetic current meters pressure sensor bottom stereo camera two temperature sensors transmissometer and a savonius rotor current meter sensor operation data recording and sediment sampling events are synchronized thus detailed measurements of the near bottom flow conditions are made concurrently with suspended sediment measurements the combined system has been used in sediment transporting environments within san francisco bay california and puget sound washington

grounding line retreat of totten glacier east antarctica 1996 to 2013 2015 american geophysical union all rights reserved totten glacier east antarctica a glacier that holds a 3 9 m sea level change equivalent has thinned and lost mass for decades we map its grounding line positions in 1996 and 2013 using differential radar interferometry insar data and develop precise high resolution topographies of its ice surface and ice draft using nasa operation icebridge data insar data and a mass conservation method we detect a 1 to 3 km retreat of the grounding line in 17 years the retreat is asymmetrical along a two lobe pattern where ice is only grounded a few 10 m above sea level or ice plain which may unground further with only modest amounts of ice thinning the pattern of retreat indicates ice thinning of 12 m in 17 years or 0 7 0 1 m yr at the grounding line on average sustained thinning will cause further grounding line retreat but may not be conducive to a marine instability key points grounding line of totten glacier is retreating not as fast as west antarctica retreat pattern explained by the newly inferred bed geometry if ice thinning maintains bed geometry conducive to further retreat

seasonality of warm water intrusions onto the continental shelf near the totten glacier warm modified circumpolar deep water mcdw from the southern ocean drives rapid basal melt of the totten ice shelf on the sabrina coast east antarctica affecting the mass balance of the grounded totten glacier recent observations show that mcdw intrudes onto the continental shelf through a depression at the shelf break here we investigate such intrusions by combining 1 new oceanographic and bathymetric observations collected for two consecutive years by profiling floats in the depression south of the shelf break 2 oceanographic measurements collected by conductivity temperature depth instrumented seals on continental slope and 3 an ocean model the depression provides a pathway for persistent inflow of warm 0 1 c mcdw to the inner shelf in austral autumn and early winter mcdw intrusions were up to 0 5 c warmer and were are 75 m thicker than in spring and summer the seasonality of the flow on the continental slope explains the seasonality of the intrusions the mcdw layer on the continental slope is warmer and thicker to the east of the depression than to the west in autumn and early winter a strong top to bottom westward current antarctic slope current transports the warmer and thicker mcdw layer along the slope and is diverted poleward at the eastern entrance of the depression a bottom intensified eastward current antarctic slope undercurrent develops in other months allowing cooler and thinner intrusions to enter the depression from the west our study illustrates how circulation on the antarctic slope regulates the ocean heat delivery to the continental shelf and ultimately to the ice shelves

response of the southern ocean overturning circulation to extreme southern annular mode conditions the positive trend of the southern annular mode sam will impact the southern ocean s role in earth s climate however the details of the southern ocean s response remain uncertain we introduce a