reassessment of net surface mass balance in antarctica abstract recent in situ measurements of surface mass balance and improved calculation techniques are used to produce an updated assessment of net surface mass balance over antarctica a new elevation model of antarctica derived from ers 1 satellite altimetry supplemented with conventional data was used to delineate the ice flow drainage basins across antarctica the areas of these basins were calculated using the recent digital descriptions of coastlines and grounding lines the delineation of drainage basins was achieved using an automatic procedure which gave similar results to earlier hand drawn catchment basins more than 1800 published and unpublished in situ measurements of net surface mass balance from antarctica were collated and then interpolated a net surface mass balance map was derived from passive microwave satellite data being employed as a forcing field to control the interpolation of the sparse in situ observations basinwide integrals of net surface mass balance were calculated using t

impact of the lateral blast on the spatial pattern and grain size characteristics of the 18 may 1980 mount st helens fallout deposit the 18 may 1980 eruption of mount st helens started with a lateral blast that fed a pyroclastic surge which then uplifted to form a co blast plume thirty minutes later plinian activity started at the vent and fluctuated in intensity for 9 h the resulting fallout deposit documented to 600 km from vent presents some striking features 1 displacement of the overall deposit to the north of the vent 2 a secondary thickness and mass maximum at 300 km from vent 3 a total grain size distribution dominated by fine ash 62 wt of the deposit 63 m and 4 individual grain size distributions that vary dramatically in the crosswind direction from strongly bimodal in the south to skewed unimodal in the north results from a new deconvolution of the individual grain size distributions show that they are a combination of a coarse subpopulation that decreases in size with distance from vent and a constant fine subpopulation with a mean of 15 m relative proportions of each subpopulation vary asymmetrically in the crosswind directions with the fine subpopulation preponderant toward the north and the coarse one dominating the south of the deposit both reach their absolute maxima in mass on the deposit axis componentry analyses of selected samples show that blast derived material is greatly enriched toward the north of the deposit these results indicate that the co blast plume dispersed fine grained material over great distances and dominated the fine subpopulation comparison with reanalysis data of atmospheric wind fields and satellite images of the spreading ash cloud suggests contrasting ash transport and depositional processes for the early co blast plume and the later vent derived plinian plumes the co blast plume is displaced to the north it had a high overshoot height and eastward dispersion via strong winds low in the stratosphere 10 15 km the plinian plumes were lower and dispersed most of the material to the southeast as the direction of high velocity winds shifted just before the late climactic plinian eruptive phase fine ash fine subpopulation was deposited continuously throughout the deposit with an increase of sedimentation rate 300 km from the vent where there is a secondary maximum in the deposit mass and thickness fine ash probably settled by a combination of enhanced sedimentation mechanisms including not only aggregation but also gravitational convective instabilities of cloud base hydrometeor formation and destruction and entrainment of small particles by larger ones finally we show that half of the deposit by mass in the medial area was deposited by the co blast plume and that a significant proportion of the mount st helens fallout deposit is nonjuvenile which has implications for the magmatic budget of this eruption

influence of antarctic ice sheet lowering on the southern hemisphere climate modeling experiments mimicking the mid miocene a coupled global atmosphere ocean model is used to study the influence of the antarctica ice sheet in a configuration that mimics that of the early miocene on the atmospheric and oceanic circulations based on different climate simulations of the present day ctr and conducted with distinct antarctic ice sheet topography ais exp it is found that the reduction of the antarctic ice sheet topography ais induces warming of the southern hemisphere and reduces the meridional thermal gradient consequently the atmospheric transient low level eddy heat flux overline v prime t prime and the eddy momentum flux overline u prime v prime are reduced causing the reduced transport of heat from the mid latitudes to the pole the stationary flow and transient wave anomalies generate changes in the ssts which modify the rate of deep water formation strengthening the formation of the antarctic bottom water substantial changes are predicted to occur in the atmospheric and oceanic heat transport and a comparison between the total heat transport of the atmosphere ocean system as simulated by the ais exp and the ctr runs shows that the reduction of the ais height leads to reduced southern hemisphere poleward and increased equatorward heat transport these results are in agreement with reduced storm track activities and baroclinicity

spatial patterns of variability in antarctic surface temperature connections to the southern hemisphere annular mode and the southern oscillation 1 the 17 year 1982 1998 trend in surface temperature shows a general cooling over the antarctic continent warming of the sea ice zone with moderate changes over the oceans warming of the peripheral seas is associated with negative trends in the regional sea ice extent effects of the southern hemisphere annular mode sam and the extrapolar southern oscillation so on surface temperature are quantified through regression analysis positive polarities of the sam are associated with cold anomalies over most of antarctica with the most notable exception of the antarctic peninsula positive temperature anomalies and ice edge retreat in the pacific sector are associated with el nino episodes over the past two decades the drift towards high polarity in the sam and negative polarity in the so indices couple to produce a spatial pattern with warmer temperatures in the antarctic peninsula and peripheral seas and cooler temperatures over much of east antarctica

surface melting derived from microwave radiometers a climatic indicator in antarctica this paper aims at presenting a new dataset including the melt events derived from microwave remote sensing occurring in antarctica from summer 1979 80 to 2005 06 the method for detecting melt events and sources of error is presented and then trends in melt duration for every pixel are extracted from the dataset mapped and analyzed the analysis focuses on two particular cases and the main results show that 1 the trends over the period 1980 2006 in the antarctic peninsula match with lengthening of the melt season on the ice shelves and surprisingly shortening of the melt season in the mountainous area of the peninsula and 2 the trends over the period 1996 2006 on the entire continent show a dipolar pattern with the western regions experiencing decreasing melt duration whereas east antarctica and the ross ice shelf experience increasing melt duration this pattern closely mirrors the temperature pattern expected when the southern annular mode is in a decreasing trend as it is over the period 1996 2006 for further analysis and validation the dataset has been made available at http www lgge obs ujf grenoble fr picard melting

the evolving instability of the remnant larsen b ice shelf and its tributary glaciers 2015 elsevier b v following the 2002 disintegration of the northern and central parts of the larsen b ice shelf the tributary glaciers of the southern surviving part initially appeared relatively unchanged and hence assumed to be buttressed sufficiently by the remnant ice shelf here we modify this perception with observations from icebridge altimetry and insar inferred ice flow speeds our analyses show that the surfaces of leppard and flask glaciers directly upstream from their grounding lines lowered by 15 to 20 m in the period 2002 2011 the thinning appears to be dynamic as the flow of both glaciers and the remnant ice shelf accelerated in the same period flask glacier started accelerating even before the 2002 disintegration increasing its flow speed by 55 between 1997 and 2012 starbuck glacier meanwhile did not change much we hypothesize that the different evolutions of the three glaciers are related to their dissimilar bed topographies and degrees of grounding we apply numerical modeling and data assimilation that show these changes to be accompanied by a reduction in the buttressing afforded by the remnant ice shelf a weakening of the shear zones between its flow units and an increase in its fracture the fast flowing northwestern part of the remnant ice shelf exhibits increasing fragmentation while the stagnant southeastern part seems to be prone to the formation of large rifts some of which we show have delimited successive calving events a large rift only 12 km downstream from the grounding line is currently traversing the stagnant part of the ice shelf defining the likely front of the next large calving event we propose that the flow acceleration ice front retreat and enhanced fracture of the remnant larsen b ice shelf presage its approaching demise

a doubling in snow accumulation in the western antarctic peninsula since 1850 1 we present results from a new medium depth 136 metres ice core drilled in a high accumulation site 73 59 s 70 36 w on the south western antarctic peninsula during 2007 the gomez record reveals a doubling of accumulation since the 1850s from a decadal average of 0 49 mweq y 1 in 1855 1864 to 1 10 mweq y 1 in 1997 2006 with acceleration in recent decades comparison with published accumulation records indicates that this rapid increase is the largest observed across the region evaluation of the relationships between gomez accumulation and the primary modes of atmospheric circulation variability reveals a strong temporally stable and positive relationship with the southern annular mode sam furthermore the sam is demonstrated to be a primary factor in governing decadal variability of accumulation at the core site r 0 66 the association between gomez accumulation and enso is complex while sometimes statistically significant the relationship is not temporally stable thus at decadal scales we can utilise the gomez accumulation as a suitable proxy for sam variability but not for enso

climate variables along a traverse line in dronning maud land east antarctica temperature density and accumulation data were obtained from shallow firn cores drilled during an overland traverse through a previously unknown part of dronning maud land east antarctica the traverse area is characterised by high mountains that obstruct the ice flow resulting in a sudden transition from the polar plateau to the coastal region the spatial variations of potential temperature near surface firn density and accumulation suggest that katabatic winds are active in this region proxy wind data derived from firn density profiles confirm that annual mean wind speed is strongly related to the magnitude of the surface slope the high elevation of the ice sheet south of the mountains makes for a dry cold climate in which mass loss owing to sublimation is small and erosion of snow by the wind has a potentially large impact on the surface mass balance a simple katabatic wind model is used to explain the variations of accumulation along the traverse line in terms of divergence convergence of the local transport of drifting snow the resulting wind and snowdrift patterns are closely connected to the topography of the ice sheet ridges are especially sensitive to erosion while ice streams and other depressions act as collectors of drifting snow

antarctic temperatures over the past two centuries from ice cores 1 we present a reconstruction of antarctic mean surface temperatures over the past two centuries based on water stable isotope records from high resolution precisely dated ice cores both instrumental and reconstructed temperatures indicate large interannual to decadal scale variability with the dominant pattern being anti phase anomalies between the main antarctic continent and the antarctic peninsula region comparative analysis of the instrumental southern hemisphere sh mean temperature record and the reconstruction suggests that at longer timescales temperatures over the antarctic continent vary in phase with the sh mean our reconstruction suggests that antarctic temperatures have increased by about 0 2 c since the late nineteenth century the variability and the long term trends are strongly modulated by the sh annular mode in the atmospheric circulation

factors controlling the near surface wind field in antarctica abstract using data from the regional atmospheric climate model racmo ant1 the momentum budget of the antarctic atmospheric surface layer sl taken as the lowest model layer located at 6 7 m above the surface is presented in july winter the katabatic pressure gradient force pgf dominates the sl momentum budget over the steep coastal slopes which results in strong 10 15 m s 1 and directionally constant katabatic winds farther inland where surface slopes are more gentle the large scale pgf may become equally important with the circumpolar pressure trough north of the antarctic coastline the large scale pgf acts along with the katabatic pgf in the downslope direction in wilkes land dronning maud land and on the western ross ice shelf the large scale pgf causes equivalent geostrophic winds in excess of 10 m s 1 thermal wind effects that oppose the downslope acceleration are especially strong in areas where large scale forcing is weak which allows cold air to pile up over the flat ice she

annular modes in the extratropical circulation part i month to month variability the leading modes of variability of the extratropical circulation in both hemispheres are characterized by deep zonally symmetric or annular structures with geopotential height perturbations of opposing signs in the polar cap region and in the surrounding zonal ring centered near 458 latitude the structure and dynamics of the southern hemisphere sh annular mode have been extensively documented whereas the existence of a northern hemisphere nh mode herein referred to as the arctic oscillation ao has only recently been recognized like the sh mode the ao can be defined as the leading empirical orthogonal function of the sea level pressure field or of the zonally symmetric geopotential height or zonal wind fields in this paper the structure and seasonality of the nh and sh modes are compared based on data from the national centers for environmental prediction national center for atmospheric research reanalysis and supplementary datasets the structures of the nh and sh annular modes are shown to be remarkably similar not only in the zonally averaged geopotential height and zonal wind fields but in the mean meridional circulations as well both exist year round in the troposphere but they amplify with height upward into the stratosphere during those seasons in which the strength of the zonal flow is conducive to strong planetary wave mean flow interaction midwinter in the nh and late spring in the sh during these active seasons the annular modes modulate the strength of the lagrangian mean circulation in the lower stratosphere total column ozone and tropopause height over mid and high latitudes and the strength of the trade winds of their respective hemispheres the nh mode also contains an embedded planetary wave signature with expressions in surface air temperature precipitation total column ozone and tropopause height it is argued that the horizontal temperature advection by the perturbed zonal mean zonal wind field in the lower troposphere is instrumental in forcing this pattern a companion paper documents the striking resemblance between the structure of the annular modes and observed climate trends over the past few decades

southern hemisphere circulation anomalies associated with extreme antarctic peninsula winter temperatures the southern hemisphere reveals markedly different circulation patterns associated with extreme warm and cold antarctic peninsula ap winter temperatures warm winters are associated with negative 500 hpa height anomalies in the amundsen sea bellingshausen sea as bs and positive anomalies in the south pacific sp and scotia sea with opposing anomalies existent in cold winters furthermore a switch in the relative strength of the two arms of the new zealand split jet the subtropical jet stj and polar front jet pfj occurs with the pfj stj strengthened and the stj pfj weakened in warm cold years leading to increased cyclonic activity in the as bs sp and a corresponding decrease in the sp as bs these hemispheric anomaly patterns bear a strong resemblance to those associated with el nino southern oscillation enso events and their origins can be ascribed to tropical sea surface temperatures sst changes however the correspondence between warm cold enso events and cold warm winters is not perfect potential contributors to this non linearity include intraseasonal tropical sst variations not necessarily represented in the usual filtered enso indices and the persistence of local sea ice anomalies west of the peninsula

small scale spatio temporal characteristics of accumulation rates in western dronning maud land antarctica spatio temporal variations of the recently determined accumulation rate are investigated using ground penetrating radar gpr measurements and firn core studies the study area is located on ritscherflya in western dronning maud land antarctica at an elevation range 1400 1560 m accumu lation rates are derived from internal reflection horizons irhs tracked with gpr which are connected to a dated firn core gpr derived internal layer depths show small relief along a 22 km profile on an ice flowline average accumulation rates are about 190 kg m 2 a 1 1980 2005 with spatial variability 1 of 5 along the gpr profile the interannual variability obtained from four dated firn cores is one order of magnitude higher showing 1 standard deviations around 30 mean temporal variations of gpr derived accumulation rates are of the same magnitude or even higher than spatial variations temporal differences between 1980 90 and 1990 2005 obtained from two dated irhs along the gpr profile indicate temporally non stationary processes linked to spatial variations comparison with similarly obtained accumulation data from another coastal area in central dronning maud land confirms this observation our results contribute to understanding spatio temporal variations of the accumulation processes necessary for the validation of satellite data e g altimetry studies and gravity missions such as gravity recovery and climate experiment grace

ice core evidence for significant 100 year regional warming on the antarctic peninsula we present a new 150 year high resolution stable isotope record delta o 18 from the gomez ice core drilled on the data sparse south western antarctic peninsula revealing a similar to 2 7 degrees c rise in surface temperatures since the 1950s the record is highly correlated with satellite derived temperature reconstructions and instrumental records from faraday station on the north west coast thus making it a robust proxy for local and regional temperatures since the 1850s we conclude that the exceptional 50 year warming previously only observed in the northern peninsula is not just a local phenomena but part of a statistically significant 100 year regional warming trend that began around 1900 a suite of coupled climate models are employed to demonstrate that the 50 and 100 year temperature trends are outside of the expected range of variability from pre industrial control runs indicating that the warming is likely the result of external climate forcing citation thomas e r p f dennis t j bracegirdle and c franzke 2009 ice core evidence for significant 100 year regional warming on the antarctic peninsula geophys res lett 36 l20704 doi 10 1029 2009gl040104

variability of subantarctic mode water and antarctic intermediate water in the drake passage during the late twentieth and early twenty first centuries a time series of the physical and biogeochemical properties of subantarctic mode water samw and antarctic intermediate water aaiw in the drake passage between 1969 and 2005 is constructed using 24 transects of measurements across the passage both water masses have experienced substantial variability on interannual to interdecadal time scales samw is formed by winter overturning on the equatorward flank of the antarctic circumpolar current acc in and to the west of the drake passage its interannual variability is primarily driven by variations in wintertime air sea turbulent heat fluxes and net evaporation modulated by the el nino southern oscillation enso despite their spatial proximity the aaiw in the drake passage has a very different source than that of the samw because it is ventilated by the northward subduction of winter water originating in the bellingshausen sea changes in aaiw are mainly forced by variability in winter water properties resulting from fluctuations in wintertime air sea turbulent heat fluxes and spring sea ice melting both of which are linked to predominantly enso driven variations in the intensity of meridional winds to the west of the antarctic peninsula a prominent exception to the prevalent modes of samw and aaiw formation occurred in 1998 when strong wind forcing associated with constructive interference between enso and the southern annular mode sam triggered a transitory shift to an ekman dominated mode of samw ventilation and a 1 2 yr shutdown of aaiw production the interdecadal evolutions of samw and aaiw in the drake passage are distinct and driven by different processes samw warmed by 0 3 c and salinified by 0 04 during the 1970s and experienced the reverse trends between 1990 and 2005 when the coldest and freshest samw on record was observed in contrast aaiw underwent a net freshening by 0 05 between the 1970s and the twenty first century although the reversing changes in samw were chiefly forced by a 30 yr oscillation in regional air sea turbulent heat fluxes and precipitation associated with the interdecadal pacific oscillation with a sam driven intensification of the ekman supply of antarctic surface waters from the south contributing significantly too the freshening of aaiw was linked to the extreme climate change that occurred to the west of the antarctic peninsula in recent decades there a freshening of the winter water ventilating aaiw was brought about by increased precipitation and a retreat of the winter sea ice edge which were seemingly forced by an interdecadal trend in the sam and regional positive feedbacks in the air sea ice coupled climate system all in all these findings highlight the role of the major modes of southern hemisphere climate variability in driving the evolution of samw and aaiw in the drake passage region and the wider south atlantic and suggest that these modes may have contributed significantly to the hemispheric scale changes undergone by those waters in recent decades

the relationship between the southern hemisphere annular mode and antarctic peninsula summer temperatures analysis of a high resolution model climatology abstract the large regional summer warming on the east coast of the northern antarctic peninsula ap which has taken place since the mid 1960s has previously been proposed to be caused by a trend in the southern hemisphere annular mode sam the authors utilize a high resolution regional atmospheric model climatology 14 km grid spacing to study the mechanisms that determine the response of the near surface temperature to an increase in the sam t sam month to month variations in near surface temperature and surface pressure are well represented by the model it is found that north of 68 s t sam is much larger on the eastern lee side than on the western windward side of the barrier this is because of the enhanced westerly flow of relatively warm air over the barrier which warms and dries further as it descends down the lee slope the downward motion on the eastern side of the barrier causes a decrease in surface mass balance and cloud cover south of 68 s vertical deflection across

downward particle fluxes wind and a phytoplankton bloom over a polar continental shelf a stormy impulse for the biological pump abstract downward particle fluxes were collected on two sites on the antarctic continental shelf 407 m to 491 m depth during 30 days in the austral spring and summer 2003 2004 the sampling period included the onset and demise of a phytoplankton bloom and the occurrence of a storm the calm period previous to the storm enabled the phytoplankton to flourish and set conditions for a bloom the storm enhanced mixing production and particle aggregation that increased the transport of organic matter to the seabed the wind induced particle pulse represented 53 306 mg oc m 2 and 826 mg bsi m 2 of the total mass flux collected during the study period at midwater column 179 m depth close to the seabed 28 m above the seafloor the sample corresponding to the period of the highest flux in the upper trap was lost however high oc and bsi fluxes 279 mg oc m 2 and 901 mg bsi m 2 were still collected three days after these fluxes were 42 and 30 of the respective fluxes collected at this depth throughout the study diatom aggregates and faecal pellets were the main vehicles for the transport of organic matter faecal pellets and foraminifera tests were ubiquitous in the traps however ellipsoidal faecal pellets were only present in the deeper traps where in some periods were more abundant than the faecal strings comparatively smaller particle fluxes than in other antarctic settings during the same season were found and the differences attributed to the denser sea ice coverage 60 observed during the present study the chemical quality and the magnitude of the particle fluxes suggest that the phytoplankton bloom and the storm developed at least over a 75 km long area the coincidence of these events produced in three days organic carbon and biogenic silica pulses over an area of at least 805 km2 of the continental shelf that may have amounted 85 to 225 tons and 308 to 725 tons respectively the occurrence of storms 15 m s 1 during the spring summer season in the last 20 years averaged 12 events per season suggesting that the action of the wind could be very important in the development of organic matter rich green mats on the sea floor

temporal and spatial variation of stable isotope ratios and accumulation rates in the hinterland of neumayer station east antarctica four firn cores were retrieved in 2007 at two ridges in the area of the ekstrom ice shelf dronning maud land coastal east antarctica in order to investigate the recent regional climate variability and the potential for future extraction of an intermediate depth core stable water isotope analysis tritium content and electrical conductivity were used to date the cores for the period 1981 2006 a strong and significant correlation between the stable isotope composition of firn cores in the hinterland and mean monthly air temperatures at neumayer station was r 0 54 0 71 no atmospheric warming or cooling trend is inferred from our stable isotope data for the period 1962 2006 the stable isotope record of the ice firn cores could expand well beyond the meteoro logical record of the region no significant temporal variation of accumulation rates was detected however decreasing accumulation rates were found from coast to hinterland as well as from east halvfarryggen to west sorasen the deuterium excess d exhibits similar differences higher d at sorasen lower d at halvfarryggen with a weak negative temporal trend on halvfarryggen 0 04 a 1 probably implying increasing oceanic input we conclude that halvfarryggen acts as a natural barrier for moisture carrying air masses circulating in the region from east to west

changes in antarctic temperature wind and precipitation in response to the antarctic oscillation output of a 14 year integration with a high resolution 55 km 55 km regional atmospheric climate model is used to study the response of antarctic near surface climate to the antarctic oscillation aao the periodical strengthening and weakening of the circumpolar vortex in the southern hemisphere in spite of the relatively short record wind temperature and precipitation show widespread and significant aao related signals when the vortex is strong high aao index northwesterly flow anomalies cause warming over the antarctic peninsula ap and adjacent regions in west antarctica and the weddell sea in contrast cooling occurs in east antarctica the eastern ross ice shelf and parts of marie byrd land most of the annual temperature signal stems from the months march august the spatial distribution of the precipitation response to changes in the aao does not mirror temperature changes but is in first order determined by the direction of flow anomalies with respect to the antarctic topography when the vortex is strong high aao index the western ap becomes wetter while the ross ice shelf parts of west antarctica and the lambert glacier basin east antarctica become drier

momentum budget of the east antarctic atmospheric boundary layer results of a regional climate model output of a regional atmospheric climate model is used to quantify the average january and july momentum budget of the atmospheric boundary layer abl over the east antarctic ice sheet and the surrounding oceans results are binned in nine elevation intervals over the ice sheet and six distance intervals over the ocean in january when surface cooling is weak the large scale pressure gradient force dominates the abl momentum budget in july under conditions of strong surface cooling a shallow katabatic jet develops over the gentle slopes of the interior ice sheet and a strong deep jet over the steep coastal slopes in the coastal regions the abl thickens considerably caused by the piling up of cold air over the adjacent sea ice and ice shelves this represents the main opposing force for the katabatic winds horizontal and vertical advection are generally small in the cross slope direction the momentum budget represents a simple balance between surface drag and coriolis turning intraseasonal variability of the large scale wind field in the abl can be explained in terms of the strength of the polar vortex the background baroclinicity and the topography of the ice sheet subsidence is found over the interior ice sheet and rising motion in the coastal zone reflecting the acceleration and deceleration of the katabatic circulation however vertical velocities are generally small because the downslope mass flux in the abl is confined to a shallow layer below the wind speed maximum

improving on mass flow rate estimates of volcanic eruptions 1 we introduce a novel analytical expression that allows for fast assessment of mass flow rate of both vertically rising and bent over volcanic plumes as a function of their height while first order physical insight is maintained this relationship is compared with a one dimensional plume model to demonstrate its flexibility and then validated with observations of the 1980 mount st helens and of the 2010 eyjafjallajokull eruptions the influence of wind on the dynamics of volcanic plumes is quantified by a new dimensionless parameter and it is shown how even vertically rising plumes such as the one associated with the mount st helens 1980 eruption can be significantly affected by strong wind comparison between a one dimensional model and the analytical equation gives anr2 value of 0 88 while existing expressions give negativer2 values due to their inability to adapt to different source and atmospheric conditions therefore this new expression has important implications both for current strategies of real time forecasting of ash transport in the atmosphere and for the characterization of explosive eruptions based on the study of tephra deposits in addition this work provides a framework for the application of more complete three dimensional numerical models as it greatly reduces the parameter space that needs to be explored

holocene accumulation and ice sheet dynamics in central west antarctica 1 we derive depth age relationships across the ice divide between the ross and amundsen seas by tracking radar detected layers from the byrd ice core and a dated 105 m core near the divide the depth age relationships and an ice flow model are used to establish histories of accumulation and ice sheet dynamics over the past 8000 years results show that accumulation was approximately 30 higher than today from 5000 to 3000 years ago antarctic climate variability today is dominated by periodic fluctuations in strength of the circumpolar vortex which raises the possibility that the vortex was systematically weaker during the period of high accumulation accumulation today decreases almost linearly across the divide it is unlikely that this pattern has changed through the holocene the radar detected stratigraphy shows no evidence of the arched layers that are expected beneath a stable divide that is frozen to its bed implying that the divide has also been migrating and or the basal ice has been sliding through the holocene we cannot rule out the possibility of sliding because the basal ice is near its pressure melting point other evidence indicates that divide migration is likely the ross sea sector is now near steady state but it had a strong negative imbalance 200 years ago when kamb ice stream was active in contrast recent speedups of pine island and thwaites glaciers have likely caused the mass balance of the amundsen sea sector to become negative the divide is likely migrating toward the ross sea today

global positioning system an overview and program static and geodynamic positioning iag special study group 1 140 relative static positioning with the global positioning system basic technical considerations keynote presentation present capabilities of gps for high precision regional surveys keynote presentation continuously monitoring gps networks for deformation measurements keynote presentation gps data preprocessing for cycle slip detection on preprocessing of gps data a comparison of several techniques to reduce signal multipath from the global positioning system multipath effects in static and kinematic gps surveying gps observations in a local network covering big height differences interim status report on donav repeatable measurements of baseline length by global positioning system in central japan some first experiences with the wm102 gps receiver monitoring displacements by gps a calibration test some results in the preliminary data analyses of the fixed point gps baseline determination network in central japan the resolution of mean sea level anomalies along the new south wales coastline by gps orbit determination iag special study group 2 106 cstg subcommission high accuracy global positioning system orbit determination progress and prospects keynote presentation the development of a global gps tracking system in support of space and ground based gps programs keynote presentation gps orbits and baseline experiments mini mac ti comparisons orbit determination for gps satellites high precision gps positioning by fiducial techniques gps broadcast orbits versus precise orbits a comparison study optimization and design iag special study group 4 117 optimization of gps theory techniques and operational systems progress and prospects keynote presentation strain analysis of systematic distortions of gps networks optimizing the movement of gps receivers optimum positions for gps points and supporting fix points in geodetic networks logistic optimization for gps satellite networks by generalized s o d algorithms gps vertical surveying a discussion of some special considerations statistical testing and quality analysis of 3 d networks part i theory statistical testing and quality analysis of 3 d networks part ii application to gps dynamic kinematic gps ins iag special study group 1 105 kinematic applications of gps and gps ins algorithms procedures and equipment trends keynote presentation a discussion of models for gps ins integration german canadian experiment in airborne ins gps integration for photogrammetric applications the use of phase for kinematic positioning by gps high precision kinematic positioning using gps at the ign recent results radio tracking systems iag special study group 2 54 geoscience from gps tracking by earth satellites keynote presentation mapping the earth s gravity field with orbiting gps receivers keynote presentation the doris satellite radio tracking system status and plans prare status and prospects methods of realisation of coherent signals in satellite systems appendix 1 presidential address mueller ivan i 125 years of international cooperation in geodesy appendix 2 list of participants author index

a century of climate variability in central dronning maud land east antarctica and its relation to southern annular mode and el nino southern oscillation stable isotope records of oxygen and hydrogen were studied from a 65 m long ice core retrieved from the central dronning maud land east antarctica in order to reconstruct the coastal antarctic climate variability during the last century and its relation to the southern annular mode sam and el nino southern oscillation enso the 18 o records showed a significant relation to the sam with a dominant 4 years variability except during specific periods 1918 1927 19381947 1989 2005 when enso teleconnection was established through the in phase relation between sam and southern oscillation index soi the combined influence of enso and sam was seen on surface air temperatures in this region mainly during the austral summer season from 1989 to 2005 further a significant relationship between 18 o and sam was observed on a decadal scale which overrides the intermittent influence of enso major shifts in the deuterium excess record were observed during periods of enso teleconnections which support a shift in moisture source regions during the periods of stronger enso teleconnections surface air temperatures estimated using the 18 o t spatial slope for this region depicted a significant warming of 1 c for the past century the study reveals that throughout the last century sam was the dominant mode of climatic variability in the coastal region of central dronning maud land on a decadal scale

central west antarctica among the most rapidly warming regions on earth the west antarctic ice sheet is contributing to sea level rise but temperature trends in the region have remained uncertain a complete temperature record for byrd station in central west antarctica spanning from 1958 to 2010 establishes west antarctica as one of the fastest warming regions globally

spatial and temporal variability in snow accumulation at the west antarctic ice sheet divide over recent centuries mean annual accumulation rates of 0 200 0 204 and 0 221 mweq a 1 respectively small scale spatial variability ssv was determined using an analysis of variance of accumulation in the ice core array thereby quantifying the uncertainty in individual accumulation records results indicate that the spatial variability was 0 030 mweq a 1 o r approximately 15 of the average annual accumulation an accumulation record representative of the wais divide local area over recent centuries was developed using a principal component analysis to identify the coherent accumulation signal the wais divide local record exhibited 14 interannual variability 1 standard deviation of the mean with the ssv reduced to 0 017 mweq a 1 correlations of the wais divide local accumulation record with atmospheric indices e g antarctic oscillation exhibited periods when the records oscillate in and out of phase thus reconstructing local and global atmospheric indices from wais divide accumulation records over recent centuries may prove problematic

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

a damage mechanics assessment of the larsen b ice shelf prior to collapse toward a physically based calving law calving is a primary process of mass ablation for glaciers and ice sheets though it still eludes a general physical law here we propose a calving framework based on continuum damage mechanics coupled with the equations of viscous deformation of glacier ice we introduce a scalar damage variable that quantifies the loss of load bearing surface area due to fractures and that feeds back with ice viscosity to represent fracture induced softening the calving law is a standard failure criterion for viscous damaging materials and represents a macroscopic brittle instability quantified by a critical or threshold damage we constrain this threshold using the ice sheet system model issm by inverting for damage on the larsen b ice shelf prior to its 2002 collapse by analyzing the damage distribution in areas that subsequently calved we conclude that calving occurs after fractures have reduced the load bearing capacity of the ice by 60 10 2012 american geophysical union all rights reserved

an antarctic circumpolar wave in surface pressure wind temperature and sea ice extent the southern ocean is the only oceanic domain encircling the globe it contains the strong eastward flow of the antarctic circumpolar current and is the unifying link for exchanges of water masses at all depths between the world s major ocean basins1 as these exchanges are an important control on mean global climate the southern ocean is expected to play an important role in transmitting climate anomalies around the globe interannual variability has been often observed at high southern latitudes and observations of sea ice extent suggest that such features propagate eastwards around the southern ocean2 3 here we use data from a variety of observational techniques to identify significant interannual variations in the atmospheric pressure at sea level wind stress sea surface temperature and sea ice extent over the southern ocean these anomalies propagate eastward with the circumpolar flow with a period of 4 5 years and taking 8 10 years to encircle the pole this system of coupled anomalies which we call the antarctic circumpolar wave is likely to play an important role in climate regulation and dynamics both within and beyond the southern ocean

spatial and temporal variability of antarctic ice sheet microwave brightness temperatures 1 annual and interannual variability of 21 years of passive microwave brightness temperatures on the antarctic ice sheet is documented through principal component analysis the leading modes show that brightness temperatures are dominantly forced by the annual temperature cycle with surface melting signals explaining about 5 of the total variance of the data analysis of the data with the annual cycle and melting signals removed results in two significant interannual modes the first is consistent with the antarctic oscillation and its influence on antarctic surface temperatures while the second reflects regional scale variability that appears to be linked to sea ice anomalies and the el nino southern oscillation the expression of these characteristic patterns of southern hemisphere climate variability on the antarctic continent provides a framework for interpreting modern climate records as well as a basis for reconstructing past climate variability with ice cores

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

differing source water inputs moderated by evaporative enrichment determine the contrasting δ18ocellulose signals in maritime antarctic moss peat banks 1 oxygen isotope palaeoclimate records preserved in moss tissue cellulose are complicated by environmental influences on the relationships between source water inputs and evaporative conditions we carried out stable isotope analyses of precipitation collected from the maritime antarctic and cellulose extracted from co located chorisodontium aciphyllum dominated moss peat bank deposits accumulated since 1870 a d analyses of stable oxygen and hydrogen isotope composition of summer precipitation on signy island 60 7 s 45 6 w established a local meteoric water line lmwl similar to both the global mwl and other lmwls and almost identical to the hadam3 isotope enabled global circulation model output the oxygen isotopic composition of cellulose 18oc revealed little temporal variation between four moss peat banks on signy island since 1870 however 18oc followed two patterns with sites a and d consistently 3 enriched relative to 18oc values from sites b and c the growing moss surfaces at sites a and d are likely to have been hydrated by isotopically heavier summer precipitation whilst at sites b and c the moss banks are regularly saturated by the isotopically depleted snow melt streams laboratory experiments revealed that evaporative enrichment of c aciphyllum moss leaf water by 5 occurred rapidly following saturation ecologically equivalent to post rainfall or snow melt periods in addition to the recognized source water cellulose fractionation extent of 27 3 such a shift would account for the 32 difference measured between 18o of signy island precipitation and cellulose

linking variable explosion style and magma textures during 2002 at stromboli volcano italy microtextural characteristics of fresh ejecta from stromboli volcano were examined from three periods of differing eruption style and intensity in 2002 activity shifted from relatively weak and infrequent ash charged explosions during january through may into two broad cycles of waxing activity in june through late september and late september through december followed by the onset on 28 december of the 2002 2003 effusive eruption analyzed sets of lapilli from may september october and 28 december show contrasts in the physical properties of magma resident in the shallow conduit during this range of activity three distinct textures are observed among the analyzed pyroclasts low density ld with an abundance of subspherical bubbles the presence of large irregularly shaped bubbles and a light to transparent glass matrix transitional texture tt with an intermediate number of subspherical bubbles a high frequency of large irregularly shaped bubbles and a honey colored glass matrix and high density hd with sparse relatively small bubbles conspicuous large irregular bubbles and a dark glass matrix observational and quantitative data density vesicle size indicate that these textures are linked through variable residence time in stromboli s shallow conduit with an ongoing evolution from ld to hd magma calculations suggest that residual ld magma will evolve to hd texture in a period of hours to days contrasting amounts of the ld tt and hd magmas are present in each sample with the most tt in may the most ld in september october and the most hd in december this implies that the shallow magma had a different rheology at each collection period the viscosity of ld and hd magmas are calculated to be in the range of 2 000 to 2 600 and 3 000 to 5 000 pa s respectively which with their changing proportions must have implications for rates of bubble slug ascent and processes of fragmentation this study suggests that an increasing maturity of magma in stromboli s shallow conduit with resultant increase in viscosity feeds back to reduce the intensity of explosions whereas a steady flux of ld magma favors more powerful explosions

the influence of conduit processes on changes in style of basaltic plinian eruptions tarawera 1886 and etna 122 bc basaltic volcanism is most typically thought to produce effusion of lava with the most explosive manifestations ranging from mild strombolian activity to more energetic fire fountain eruptions however some basaltic eruptions are now recognized as extremely violent i e generating widespread phreatomagmatic subplinian and plinian fall deposits we focus here on the influence of conduit processes especially partial open system degassing in triggering abrupt changes in style and intensity that occurred during two examples of basaltic plinian volcanism we use the 1886 eruption of tarawera new zealand the youngest known basaltic plinian eruption and the only one for which there are detailed written eyewitness accounts and the well documented 122 bc eruption of mount etna italy and present new grain size and vesicularity data from the proximal deposits these data show that even during extremely powerful basaltic eruptions conduit processes play a critical role in modifying the form of the eruptions even with very high discharge and presumably ascent rates partial open system behaviour of basaltic melts becomes a critical factor that leads to development of domains of largely stagnant and outgassed melt that restricts the effective radius of the conduit the exact path taken in the waning stages of the eruptions varied in response to factors which included conduit geometry efficiency and extent of outgassing and availability of ground water but a relatively abrupt cessation to sustained high intensity discharge was an inevitable consequence of the degassing processes

synchronous variability in the southern hemisphere atmosphere sea ice and ocean resulting from the annular mode zonally symmetric fluctuations of the midlatitude westerly winds characterize the primary mode of atmospheric variability in the southern hemisphere during all seasons this is true not only in observations but also in an unforced 15 000 yr integration of a coarse resolution r15 coupled ocean atmosphere model here it is documented how this mode of atmospheric variability known as the southern annular mode sam generates ocean circulation and sea ice variations in the model integration on interannual to centennial timescales that are tightly in phase with the sam the positive phase of the sam is associated with an intensification of the surface westerlies over the circumpolar ocean around 608s and a weakening of the surface westerlies farther north this induces ekman drift to the north at all longitudes of the circumpolar ocean and ekman drift to the south at around 308s through mass continuity the ekman drift generates anomalous upwelling along the margins of the antarctic continent and downwelling around 458s the anomalous flow diverging from the antarctic continent also increases the vertical tilt of the isopycnals in the southern ocean so that a more intense circumpolar current is also closely associated with positive sam in addition the anomalous divergent flow advects sea ice farther north resulting in an increase in sea ice coverage finally positive sam drives increases in poleward heat transport at about 308s while decreases occur in the circumpolar region ocean and sea ice anomalies of the opposite sign occur when the sam is negative the ocean and sea ice fluctuations associated with the sam constitute a significant fraction of simulated ocean variability poleward of 308s year round the robustness of the mechanisms relating the sam to oceanic variability suggests that the sam is likely an important source of large scale variability in the real southern hemisphere ocean

the 1991 eruption of hekla iceland the eruption that started in the hekla volcano in south iceland on 17 january 1991 and came to an end on 11 march produced mainly andesitic lava this lava covers 23 km2 and has an estimated volume of 0 15 km3 this is the third eruption in only 20 years whereas the average repose period since 1104 is 55 years earthquakes as well as a strain pulse recorded by borehole strainmeters occurred less than half an hour before the start of the eruption the initial plinian phase was very short lived producing a total of only 0 02 km3 of tephra the eruption cloud attained 11 5 km in height in only 10 min but it became detached from the volcano a few hours later several fissures were active during the first day of the eruption including a part of the summit fissure by the second day however the activity was already essentially limited to that segment of the principal fissure where the main crater subsequently formed the average effusion rate during the first two days of the eruption was about 800 m3 s 1 after this peak the effusion rate declined rapidly to 10 20 m3 s 1 then more slowly to 1 m3 s 1 and remained at 1 12 m3 s 1 until the end of the eruption site observations near the main crater suggest that the intensity of the volcanic tremor varied directly with the force of the eruption a notable rise in the fluorine concentration of riverwater in the vicinity of the eruptive fissures occurred on the 5th day of the eruption but it levelled off on the 6th day and then remained essentially constant the volume and initial silica content of the lava and tephra the explosivity and effusion rate during the earliest stage of the eruption as well as the magnitude attained by the associated earthquakes support earlier suggestions that these parameters are positively related to the length of the preceeding repose period the chemical difference between the eruptive material of hekla itself and the lavas erupted in its vicinity can be explained in terms of a density stratified magma reservoir located at the bottom of the crust we propose that the shape of this reservoir its location at the west margin of a propagating rift and its association with a crustal weakness all contribute to the high eruption frequency of hekla

tropical pacific high latitude south atlantic teleconnections as seen in δ18o variability in antarctic coastal ice cores 1 we use a network of eight ice cores from coastal dronning maud land dml antarctica to examine the role of the tropical enso el nino southern oscillation in the temporal variability of 18o in annual accumulation the longest record from the s100 ice core covering the period 1737 1999 is used to analyze the teleconnections between the tropical pacific and coastal dml on decadal scales and longer a shorter stacked coastal dml 18o series spanning 1955 1999 is constructed to assess the variability of enso teleconnection on interannual scales results suggest that on typical enso timescales of 2 6 years the strength of the teleconnection varies in time being stronger for years with generally negative phase of the southern annular mode sam on the timescales of approximately two decades bidecadal positive isotope anomalies are associated with oceanic warming and a westward sea surface temperature sst gradient in the equatorial pacific bidecadal variability in sam forced by the tropical pacific is proposed as a critical element in the teleconnection our analysis suggests that a multidecadal positive trend in the annual mean 18o values from the analyzed cores can be indicative of the atmospheric warming that begun in this part of the dml already in the 1910s the trend in 18o quantified in terms of long term surface air temperature sat changes is consistent with the instrumental data yet we speculate that the accurate estimation of sat trends requires an assessment of the potential role of secular sam and sea ice extent changes in shaping the isotopic signal

recent atmospheric warming and retreat of ice shelves on the antarctic peninsula in 1978 mercer1 discussed the probable effects of climate warming on the antarctic ice sheet predicting that one sign of a warming trend in this region would be the retreat of ice shelves on the antarctic peninsula analyses of 50 year meteorological records have since revealed atmospheric warming on the antarctic peninsula2 3 and a number of ice shelves have retreated4 8 here we present time series of observations of the areal extent of nine ice shelves on the antarctic peninsula showing that five northerly ones have retreated dramatically in the past fifty years while those further south show no clear trend comparison with airtemperature data shows that the pattern and magnitude of ice shelf retreat is consistent with the existence of an abrupt thermal limit on iceshelf viability the isotherm associated with this limit having been driven south by the atmospheric warming ice shelves therefore appear to be sensitive indicators of climate change

winter water variability in the indian ocean sector of southern ocean during austral summer abstract winter water ww is a feature in the upper layers of the southern ocean during austral summer the interaction of this cold water with surface and deeper subsurface layers can alter the heat budget as well as the carbon cycle however the structure and characteristics of the ww in the southern ocean is poorly understood due to the lack of in situ data in this study hydrographic data collected from the indian expeditions during 2010 and 2011 and from other expeditions during 2003 2006 and 2007 in the southern ocean have been utilized to understand the ww variability and its characteristics in the region between 40 e and 90 e the analysis shows large zonal variations in the distribution of ww during austral summer east of 60 e the ww shows warmer temperature with weaker gradients than the western side which is observed to have a larger meridional extension the high frequency of eddies surface warming and upwelling of the upper circumpolar deep water due to shallow bathymetric features of the kerguelen plateau situated in the eastern side of the study region is proposed as the major mechanisms for this variability the direct impact of southern annular mode on ww variability is investigated and a short term response is observed

the future of antarctica s surface winds simulated by a high resolution global climate model 2 drivers of 21st century changes antarctica s katabatic winds are among the strongest near surface winds on earth and among the most consistent ones as these winds are primarily due to the strong surface cooling greenhouse warming of the surface may act to reduce the strength of these winds as well as their consistency here we use the atmospheric component of the global climate model ec earth in prescribed sea surface temperature sst simulations of the present day 2002 2006 and future 2094 2098 climates using two model resolutions 1 t159l62 100 km 62 vertical levels and 2 t799l91 20 km 91 vertical levels to investigate changes in antarctica s surface winds and the reasons thereof circumpolar westerlies over the southern ocean strengthen and shift poleward because of the deepening of the circumpolar trough and the associated increase in southern annular mode sam especially in high resolution causing weaker coastal easterlies generally surface wind speeds over the antarctica mainland exhibit a small decrease according to the simulations the temperature deficit or inversion strength and associated katabatic forcing exhibit only minor changes over the continent changes in the surface winds over antarctica s slopes can thus be attributed mainly to changes in the synoptic forcing large scale pressure gradient hence with modeled 21st century changes in the katabatic forcing being small changes in zonal and meridional surface winds in and around antarctica are largely decoupled from those over the southern ocean and are governed by changes in synoptic forcing and large scale pressure gradients as a result these changes are largely independent on model resolution

the opening subplinian phase of the hekla 1991 eruption properties of the tephra fall deposit the 1991 hekla eruption started on 17th of january with an intense 50 min long explosive phase that transitioned into fire fountain activity lasting for 2 days the eruptive plume rose to maximum height in about 10 min and the total mass of tephra deposited from the opening phase was 8 6 109 kg vei 3 event the principal axis of tephra fall is to the nne of hekla and grain size analysis reveals a systematic decrease in grain size away from source majority of sample sites show typically unimodal grain size distributions although a few have bimodal distributions where the secondary mode is a subtle finer peak the calculated total grain size distribution tgsd is bimodal with a coarse primary peak 3 5 to 2 5 and a subordinate fine peak 2 5 to 3 5 the coarse peak is lapilli dominated and was deposited within the first 25 km of transport whereas the fine peak is coarse ash dominated and fits well with the modal grain size of samples deposited 65 km from hekla ascent rate of the magma and conditions for vesiculation in the shallow conduit became increasingly uniform with time through the 1991 opening phase

new reconstruction of antarctic near surface temperatures multidecadal trends and reliability of global reanalyses a reconstruction of antarctic monthly mean near surface temperatures spanning 1958 2012 is presented itsprimarygoalistotakeadvantageofarecentlyrevisedkeytemperaturerecordfromwestantarctica byrd to shed further light on multidecadal temperature changes in this region the spatial interpolation relies on a kriging technique aided by spatiotemporal temperature covariances derived from three global reanalyses the european centre for medium range weather forecasts ecmwf interim re analysis erainterim modern era retrospective analysis for research and applications merra and climate forecast system reanalysis cfsr for 1958 2012 the reconstruction yields statistically significant annual warming in the antarctic peninsula and virtually all of west antarctica but no significant temperature change in east antarctica importantly the warming is of comparable magnitude both in central west antarctica and in most of the peninsula rather than concentrated either in one or the other region as previousreconstructions have suggested the transantarctic mountains act for the temperature trends as a clear dividing line between east and west antarctica reflecting the topographic constraint on warm air advection from the amundsen sea basin the reconstruction also serves to highlight spurious changes in the 1979 2009 time series of the three reanalyses that reduces the reliability of their trends illustrating a longstanding issue in high southern latitudes the study concludes with an examination of the influence of the southern annular mode sam on antarctic temperature trends the results herein suggest that the trend of the sam toward its positive phase in austral summer and fall since the 1950s has had a statistically significant cooling effect not only in east antarctica as already well documented and but also only in fall in west antarctica

sedimentation from strong volcanic plumes 1 we develop descriptions of the key processes influencing tephra dispersal from strong volcanic plumes these are characterized by the development of a subvertical eruption column in the atmosphere that forms a spreading current at a level of neutral buoyancy we describe the propagation of the spreading current due to both gravity and wind advection using scaling arguments and a simplified geometry to model particle sedimentation in a windy as well as a wind free environment new parameterizations are used to describe the wind field below the spreading current particle aggregation and particle density variations we conducted a broad study to investigate the effects of these processes and made comparisons with field observations the greatest variations resulted from wind advection below the spreading current which shifts the plume corner mass accumulation and the position of transitions in fallout regimes downwind particle aggregation strongly depends on the initial grain size distribution and significantly affects deposit thinning depending on the aggregate size and density and on the relative amount of aggregating particles small aggregates can reproduce secondary maxima of mass accumulation when sedimenting in a wind field variation of particle density also affects the resulting thinning trend the model provides acceptable reproduction of observations of the propagation of the spreading current tested with the plinian phase of the 18 may 1980 eruption of mount st helens and of observed thinning of tephra fallout deposits tested with deposits from mount st helens 1980 askja d 1875 quizapu 1932 minoan eruption 3 6 ka b p novarupta a 1912 and hudson 1991

the link between climate warming and break up of ice shelves in the antarctic peninsula a review of in situ and remote sensing data covering the ice shelves of the antarctic peninsula provides a series of characteristics closely associated with rapid shelf retreat deeply embayed ice fronts calving of myriad small elongate bergs in punctuated events increasing flow speed and the presence of melt ponds on the ice shelf surface in the vicinity of the break ups as climate has warmed in the antarctic peninsula region melt season duration and the extent of ponding have increased most break up events have occurred during longer melt seasons suggesting that meltwater itself not just warming is responsible regions that show melting without pond formation are relatively unchanged melt ponds thus appear to be a robust harbinger of ice shelf retreat we use these observations to guide a model of ice shelf flow and the effects of meltwater crevasses present in a region of surface ponding will likely fill to the brim with water we hypothesize building on weertman 1973 hughes 1983 and van der veen 1998 that crevasse propagation by meltwater is the main mechanism by which ice shelves weaken and retreat a thermodynamic finite element model is used to evaluate ice flow and the strain field and simple extensions of this model are used to investigate crack propagation by meltwater the model results support the hypothesis