snow grain size profiles deduced from microwave snow emissivities in antarctica spaceborne microwave radiometers are an attractive tool for observing antarctic climate because their measurements are related to the snow temperature however the conversion from microwave emission to snow temperature is not simple and strongly depends on the emissivity through snow properties this difficulty in predicting the snow property profile for antarctic conditions is the main bottleneck in the retrieval of accurate climate information from microwave radiometers we attempt to explain the vertically polarized emissivity at 19 3 and 37 ghz derived from brightness temperatures acquired by the special sensor microwave imager ssm i and physical temperature from the era 40 re analysis in antarctica the snow emissivities at 19 3 and 37 ghz are nearly equal although a decrease with frequency is expected to explain this we consider various profiles of snow grain size and density and predict their emissivity using a dense medium radiative transfer dmrt model the results show that the emissivities cannot be explained by constant profiles of grain size and density heterogeneous snowpacks need to be considered we first test random variations of snow density and grain radius with depth and then monotonic and continuous variations in the snow grain radius in both cases we show that an overall increase of the snow grain radius with depth is required to match the observed emissivity in antarctica in addition two parameters characterizing the snow grain profiles are retrieved and compared with 1 in situ measurements of grain size at various locations in east antarctica 2 grain size estimated using visible spaceborne radiometers and 3 a semi empirical relationship for grain growth

antarctic snow accumulation mapped using polarization of 4 3 cm wavelength microwave emission different parts of antarctica receive different amounts of snowfall each year in this paper we map the variations of the mean annual snow accumulation across the ice sheet we also quantify the uncertainty in our estimates more objectively than has been possible for earlier maps the new map is produced using observations from satellites and ground based measurements after a logarithmic transformation these are combined using the geostatistical method of continuous part universal kriging to give an estimate of the snow accumulation within each cell of a rectangular grid covering antarctica we also derive spatial averages over the major drainage systems of the ice sheet along with their confidence intervals we obtain a value of 143 4 kg m 2 a 1 for the average rate of snow accumulation upon the grounded ice sheet of antarctica

successive aeolian saltation studies of idealized collisions as observed by bagnold and experimentally reconfirmed by other workers the impact angles of saltating grains are remarkably constant over a wide range of conditions lying between 10 and 16 it can be shown that successive saltation contains a mechanism which very effectively confines impact angles to that range this control mechanism is most effective at windspeeds less than about 15 30 m s 1 depending on grain diameter and mass the control mechanism is evaluated from model calculations of grain populations saltating over a level bed consisting of a layer of loose grains the grains are assumed to be spherical and uniform in size and mass also rigid and perfectly elastic the model also describes distributions of maximum height of grain paths and of lift off angles compared to other processes involved in aeolian saltation successive saltation is the only process with a high probability of transferring energy from horizontal into vertical grain movement this fact together with the calculations presented strongly suggests that successive saltation plays a major role in saltation in air successive saltation of uniform grains is theoretically impossible if the ground over which saltation occurs is tilted by about 15 against wind direction values of tilt angles in this range are observed in nature as stoss side angles of dunes and ripples leading to the concept that stoss sides are tilted up by deposition until successive saltation is subdued

simulations of northern hemisphere ice sheet retreat sensitivity to physical mechanisms involved during the last deglaciation a 3 dimensional thermomechanical ice sheet model is used to simulate the evolution of the geometry of northern hemisphere ice sheets through the last deglaciation the ice sheet model is forced by a time evolving climatology provided by the linear interpolation through time of climate snapshots simulated by the lmd5 3 atmospheric general circulation model agcm at different periods of the last deglaciation 21 15 9 6 and 0 kyr bp the agcm is driven by insolation atmospheric co2 content ice sheet configuration and sea surface temperatures although our approach is able to produce the complete continental ice retreat our simulated deglaciation presents a phase lag with reconstructions based on observational evidences this suggests that physical mechanisms related to climate forcing and or ice sheet internal dynamics are not properly represented the influence of millennial scale forcing feedback mechanisms between ice sheet elevation and surface mass balance and parameterization of the ice flow is also tested through a set of sensitivity experiments the rapid variability has a strong impact on the evolution of the ice volume because of nonlinear effects in temperature mass balance relationships fennoscandia appears to be strongly sensitive to the small scale ice sheet instability both ice sheets are to some extent sensitive to an increased basal sliding

intersecting intracontinental tertiary transform fault systems in the north american cordillera in central british columbia north trending dextral strike slip faults that cut late eocene granite also truncate northwest trending dextral strike slip faults the northwest trending strike slip faults bound the wolverine metamorphic complex wolverine complex which has been uplifted primarily by northwest southeast eocene crustal extension and somewhat by late eocene northerly extension the crustal extension is indicated by shallow dipping extensions faults dyke complexes and stretching lineations the wolverine complex and its bounding faults define a crustal pull apart in an en echelon dextral transform the northwest and north trending dextral strike slip faults in central british columbia are the continuations of faults that transect the interior of the north american cordillera and they represent at least two distinct plate boundaries intermittently active during the early to middle eocene and the late eocene to early oligocene each of these systems consists of en echelon strike slip fault

impact of precipitation seasonality changes on isotopic signals in polar ice cores a multi model analysis abstract for central greenland water isotope analysis indicates a temperature difference of about 10 c since the last glacial maximum lgm however borehole thermometry and gas diffusion thermometry indicate that lgm surface temperatures were about 20 c colder than today two general circulation model studies have shown that changes in the seasonal precipitation timing in central greenland might have caused a warm bias in the lgm water isotope proxy temperatures and that this bias could explain the difference in the estimated paleotemperatures here we present an analysis of a number of atmospheric general circulation model simulations mostly done within the framework of the paleoclimate modeling intercomparison project the models suggest that the seasonal cycle of precipitation and surface mass balance over central greenland at the lgm might have been very different from today this supports the idea that the accuracy of the water isotope thermometry at the lgm in greenland might be compromised as a result of a modified surface mass balance seasonality however the models disagree on the amplitude and sign of the bias for central east antarctica a strong seasonality effect on the lgm isotopic signal is not simulated by any of the analyzed models for the mid holocene 6 kyr bp the models suggest relatively weak isotope paleothermometry biases linked to changes in the surface mass balance seasonality over both ice sheets

eight glacial cycles from an antarctic ice core the antarctic vostok ice core provided compelling evidence of the nature of climate and of climate feedbacks over the past 420 000 years marine records suggest that the amplitude of climate variability was smaller before that time but such records are often poorly resolved moreover it is not possible to infer the abundance of greenhouse gases in the atmosphere from marine records here we report the recovery of a deep ice core from dome c antarctica that provides a climate record for the past 740 000 years for the four most recent glacial cycles the data agree well with the record from vostok the earlier period between 740 000 and 430 000 years ago was characterized by less pronounced warmth in interglacial periods in antarctica but a higher proportion of each cycle was spent in the warm mode the transition from glacial to interglacial conditions about 430 000 years ago termination v resembles the transition into the present interglacial period in terms of the magnitude of change in temperatures and greenhouse gases but there are significant differences in the patterns of change the interglacial stage following termination v was exceptionally long 28 000 years compared to for example the 12 000 years recorded so far in the present interglacial period given the similarities between this earlier warm period and today our results may imply that without human intervention a climate similar to the present one would extend well into the future

projection of climate change onto modes of atmospheric variability two possible interpretations of forced climate change view it as projecting either linearly or nonlinearly onto the dominant modes of variability of the climate system an evaluation of these two interpretations is performed using annual mean sea level pressure slp and surface air temperature sat fields obtained from integrations of the geophysical fluid dynamics laboratory coupled general circulation model forced with varying concentrations of greenhouse gases the dominant modes of slp both represent much of the total variability and remain important in warmer climates with sat however the dominant modes are often related to variations in the sea ice edge and so do not remain important once the ice has retreated those unrelated to sea ice remain dominant in the warmer climates but represent smaller fractions of the total variability in general climate change tends to project most strongly onto the more dominant modes the change in slp projects partially onto the top two modes in the northern hemisphere reflecting both an overall decrease in hemispheric slp as well as the pattern of change in the southern hemisphere the change projects negligibly onto the dominant patterns between equilibrium climates but very strongly onto the antarctic oscillation like mode in the transient integrations changes in sat project partially onto the dominant modes but relate more to the mean warming rather than the pattern of change in general the change projects most strongly onto the more dominant modes in all slp domains the projection of climate change overwhelmingly manifests itself as a linear translation in the mode consistent with the linear interpretation in sat domains related to sea ice variability the projection reflects an increased tendency toward ice free regimes consistent with the nonlinear perspective however this nonlinear projection represents only a small portion of the overall climate change

influence of oceanic boundary conditions in simulations of antarctic climate and surface mass balance change during the coming century abstract this article reports on high resolution 60 km atmospheric general circulation model simulations of the antarctic climate for the periods 1981 2000 and 2081 2100 the analysis focuses on the surface mass balance change one of the components of the total ice sheet mass balance and its impact on global eustatic sea level contrary to previous simulations in which the authors directly used sea surface boundary conditions produced by a coupled ocean atmosphere model for the last decades of both centuries an anomaly method was applied here in which the present day simulations use observed sea surface conditions while the simulations for the end of the twenty first century use the change in sea surface conditions taken from the coupled simulations superimposed on the present day observations it is shown that the use of observed oceanic boundary conditions clearly improves the simulation of the present day antarctic climate compared to model runs using boundary conditions from a coupled climate

integrating ice flow history geochronology geology and geophysics to trace mineralized glacial erratics to their bedrock source an example from south central british columbia1this article is one of a series of papers published in this special issue on the theme of new insights in cordilleran intermontane geoscience reducing exploration risk in the mountain pine beetle affected area british columbia 2earth sciences sector contribution number 20100079 this study demonstrates how ice flow history geochronology geology and geophysics may be integrated to en hance the effectiveness of boulder tracing in glaciated regions affected by multiple ice flow events mineralized felsic gran itoid boulders erratics were discovered 18 years ago on a claim block located 10 km northwest of little fort in the bonaparte lake map area nts 092p in south central british columbia although the boulders have yielded significant gold concentrations up to 4 15 g t their bedrock source is not known the till near the boulders contains up to 1382 gold grains per 15 kg of bulk material with 75 of the grains having pristine morphology suggesting a short distance of glacial transport a u pb zircon crystallization age of 198 1 0 5 ma on one mineralized boulder indicates derivation from an early jurassic intrusion using a vector addition model based on regional ice flow patterns the most recent and detailed bedrock geology map and recently acquired airborne radiometrics and magnetic data the northeast sector of the thuya batholith 195 205 ma is interpreted as the most likely bedrock source of the mineralized boulders resume la presente etude demontre comment l historique de l ecoulement glaciaire la geochronologie la geologie et la geophysique peuvent etre integres pour rehausser l efficacite de detection de blocs mineralises dans des regions ayant subi la glaciation et touchees par des evenements d ecoulement glaciaires multiples des blocs granitoides felsiques mineralises erratiques ont ete decouverts il y a 18 ans sur une concession miniere situee a 10 km au nord ouest de little fort dans la region de la carte bonaparte lake snrc 092p dans le centre sud de la colombie britannique bien que les blocs aient donne des concentrations d or importantes atteignant 4 15 g t leur source dans le socle n est pas connue le till a proxi mite des blocs contient jusqu a 1382 grains d or par 15 kg de materiel en vrac de plus 75 des grains ont une morpholo gie intacte suggerant une courte distance de transport glacial un ge de cristallisation u pb de 198 1 0 5 ma determine sur des grains de zircon d un bloc mineralise indique que ce bloc proviendrait d une intrusion datant du jurassique precoce en utilisant un modele de somme vectorielle base sur des patrons regionaux d ecoulement glaciaire les cartes geologiques du socle les plus recentes et les plus detaillees ainsi que des donnees radiometriques et magnetiques nouvellement acquises le secteur nord est du batholite de thuya 195 205 ma est interprete comme etant la source la plus probable des blocs mi neralises traduit par la redaction

eocene tectonics in the omineca belt northern british columbia canada field 40ar 39ar and fission track data from the horseranch range a field and geochronometric study of the horseranch range north central british columbia was carried out to determine the cooling and uplift history of the area and to assess the possibility that the range may be a metamorphic core complex 40 ar 39 ar cooling ages from hornblende muscovite and biotite in a variety of rock types and structural positions and apatite fission track data indicate rapid cooling from about 525 to 50 c or less in the middle to late eocene epoch uplift and cooling took place by oblique normal dextral strike slip along a northwest striking moderately to steeply southwest dipping mylonite zone associated with regional mid cretaceous to oligocene northwest striking dextral strike slip faulting uplift by oblique normal slip differs from dip slip displacement reported for metamorphic core complexes in the southern omineca belt the peak of regional metamorphism at 650 c and 5 to 7 kb was late early cretaceous or younger in age and predated mylonitization there is no evidence of resetting of older minerals this is in contrast to the nearby swannell ranges which implies to us that differential uplift took place in the northern omineca belt probably due to control of bounding faults

retention of greenland runoff by refreezing implications for projected future sea level change melting produced at the surface of subfreezing permeable firn does not freely percolate but instead refreezes near its point of origin raising the firn temperature and decreasing the pore space if sufficient meltwater is introduced the firn will warm to 0 c and subsequent water will fill the remaining pore space without freezing only after the residual water content of the firn is exceeded will water flow through the firn and possibly escape as runoff this process prevents summer melt on high arctic glaciers and ice caps from escaping in its entirety and it must be accounted for in modeling glacier runoff on the basis of surface energy balance a model is presented here which describes in a simple way the transient process of infiltration refreezing and runoff in a future warming climate the model is applied to greenland for which predictions of runoff induced sea level rise that do not consider the refreezing process could be as much as 5 0 cm too high over 150 years

crustal structure and lithology of the northern canadian cordillera alternative interpretations of snorcle seismic reflection lines 2a and 2b seismic reflectors at shallow crustal levels recorded in snorcle slave northern cordillera lithospheric evolution lines 2a and 2b in the northern canadian cordillera can be reconciled with many stratigraphic and structural elements known from geological mapping clearly evident examples are the crustal penetrating northern rocky mountain trench nrmt fault the several kilometres thin slices of slide mountain sylvester allochthon and cache creek terranes and the northward dipping hotailuh fault interpretations of lithological successions and structures at deeper crustal levels are more speculative of fundamental significance is a southwesterly tapering wedge of continental crust overlain in its distal part by the allochthonous terrane stikinia and northeast of thibert fault by a thick probably structurally thickened sequence of sedimentary rocks and the extensive granitic cassiar batholith the top of the wedge is interpreted to be a tectonic accretion surface resulting from collision of sti

spatial and temporal variability of snow accumulation in east antarctica from traverse data recent snow accumulation rate is a key quantity for ice core and mass balance studies several accumulation measurement methods stake farm fin core snow radar profiling surface morphology remote sensing were used compared and integrated at eight sites along a transect from terra nova bay to dome c east antarctica to provide information about the spatial and temporal variability of snow accumulation thirty nine cores were dated by identifying tritium b marker levels 1965 66 and non sea salt nss so4 2 spikes of the tambora indonesia volcanic event 1816 in order to provide information on temporal variability cores were linked by snow radar and global positioning system surveys to provide detailed information on spatial variability in snow accumulation stake farm and ice core accumulation rates are observed to differ significantly but isochrones snow radar correlate well with ice core derived accumulation the accumulation ablation pattern from stake measurements suggests that the annual local noise metre scale in snow accumulation can approach 2 years of ablation and more than four times the average annual accumulation with no accumulation or ablation for a 5 year period in up to 40 of cases the spatial variability of snow accumulation at the kilometre scale is one order of magnitude higher than temporal variability at the multi decadal secular scale stake measurements and firn cores at dome c confirm an approximate 30 increase in accumulation over the last two centuries with respect to the average over the last 5000 years

surface melt induced acceleration of greenland ice sheet flow ice flow at a location in the equilibrium zone of the west central greenland ice sheet accelerates above the midwinter average rate during periods of summer melting the near coincidence of the ice acceleration with the duration of surface melting followed by deceleration after the melting ceases indicates that glacial sliding is enhanced by rapid migration of surface meltwater to the ice bedrock interface interannual variations in the ice acceleration are correlated with variations in the intensity of the surface melting with larger increases accompanying higher amounts of summer melting the indicated coupling between surface melting and ice sheet flow provides a mechanism for rapid large scale dynamic responses of ice sheets to climate warming

modeling time series of microwave brightness temperature in antarctica this paper aims to interpret the temporal variations of microwave brightness temperature at 19 and 37 ghz and at vertical and horizontal polarizations in antarctica using a physically based snow dynamic and emission model sdem sdem predicts time series of top of atmosphere brightness temperature from widely available surface meteorological data era 40 re analysis to do so it successively computes the heat flux incoming the snowpack the snow temperature profile the microwaves emitted by the snow and finally the propagation of the microwaves through the atmosphere up to the satellite since the model contains several parameters whose value is variable and uncertain across the continent the parameter values are optimized for every 50 km 50 km pixel simulation results show that the model is inadequate in the melt zone where surface melting occurs on at least a few days a year because the snowpack structure and its temporal variations are too complex in contrast the accuracy is reasonably good in the dry zone and varies between 2 and 4 k depending on the frequency and polarization of observations and on the location at the antarctic scale the error is larger where wind is usually stronger suggesting either that meteorological data are less accurate in windy regions or that some neglected processes e g windpumping surface scouring are important at dome c in calm conditions a detailed analysis shows that most of the error is due to inaccuracy of the era 40 air temperature 2 k finally the paper discusses the values of the optimized parameters and their spatial variations across the antarctic

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

warm climate isotopic simulations what do we learn about interglacial signals in greenland ice cores measurements of last interglacial stable water isotopes in ice cores show that central greenland 18o increased by at least 3 compared to present day attempting to quantify the greenland interglacial temperature change from these ice core measurements rests on our ability to interpret the stable water isotope content of greenland snow current orbitally driven interglacial simulations do not show 18o or temperature rises of the correct magnitude leading to difficulty in using only these experiments to inform our understanding of higher interglacial 18o here analysis of greenhouse gas warmed simulations from two isotope enabled general circulation models in conjunction with a set of last interglacial sea surface observations indicates a possible explanation for the interglacial 18o rise a reduction in the winter time sea ice concentration around the northern half of greenland together with an increase in sea surface temperatures over the same region is found to be sufficient to drive a 3 interglacial enrichment in central greenland snow warm climate 18o and d in precipitation falling on greenland are shown to be strongly influenced by local sea surface condition changes local sea surface warming and a shrunken sea ice extent increase the proportion of water vapour from local isotopically enriched sources compared to that from distal isotopically depleted sources precipitation intermittency changes under warmer conditions leads to geographical variability in the 18o against temperature gradients across greenland little sea surface warming around the northern areas of greenland leads to low 18o against temperature gradients 0 1 0 3 per c whilst large sea surface warmings in these regions leads to higher gradients 0 3 0 7 per c these gradients imply a wide possible range of present day to interglacial temperature increases 4 to 10 c thus we find that uncertainty about local interglacial sea surface conditions rather than precipitation intermittency changes may lead to the largest uncertainties in interpreting temperature from greenland ice cores we find that interglacial sea surface change observational records are currently insufficient to enable discrimination between these different 18o against temperature gradients in conclusion further information on interglacial sea surface temperatures and sea ice changes around northern greenland should indicate whether 5 c during the last interglacial is sufficient to drive the observed ice core 18o increase or whether a larger temperature increases or ice sheet changes are also required to explain the ice core observations

strain partitioning in an obliquely convergent orogen plutonism and synorogenic collapse coast mountains batholith british columbia canada 1 we describe the crustal structure of the coast mountains batholith between 54 and 55 n within the canadian cordillera with emphasis on emplacement of the 7 km thick kasiks sill complex ksc kinematic patterns that developed during emplacement of the ksc are the result of interactions between magma transport magma accumulation and regional deformation the sills were emplaced during nw directed normal shearing and flattening of country rocks that host the ksc a 2 km thick shallowly ne dipping mylonite zone cuts the eastern side of the ksc kinematic indicators within the mylonite zone record top to the east normal displacements structural analysis shows that mylonite formation occurred during subvertical shortening and east northeast subhorizontal extension u pb zircon age dates show that ene directed normal shearing along the eastern side of the ksc and wnw directed normal shearing within the ksc occurred contemporaneously between 54 and 51 ma indicating strong strain partitioning between the mylonite and the ksc this pattern of strain partitioning is interpreted to have been driven by return flow of melt laden crust in response to tectonic denudation of the upper crust seismic profiling shows that many of these structures extend to mid and lower crustal depths comparison of our results with other regions within the canadian cordillera indicates that orogen scale right lateral strike slip faults deformed synchronously with wide spread magmatism and formation of extensional gneiss domes thus the crustal structure of the coast mountains batholith was the result of early tertiary batholith construction during dextral oblique convergence and synorogenic collapse

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

response of continental magmatic arcs to regional tectonic changes recorded by synorogenic plutons in the middle crust an example from the coast mountains of british columbia abstract the coast mountains of british columbia record an increase in magmatic activity acceleration in exhumation rate and a change from transpression to extension between 60 and 52 ma structural analysis of fabrics in three mid crustal plutons and country rocks leads to conclusions about pluton emplacement mechanisms and strain partitioning during changing tectonic conditions the quottoon and kitlope plutons 60 ma have steep foliations and lineations consistent with partitioned transpressional deformation the chief matthew s pluton 57 55 ma intruded during the formation of a sub horizontal transposition foliation and has radially distributed lineations consistent with sub vertical flattening during extension the change in orientation of the foliation represents an almost orthogonal rotation of the shortening direction from sub horizontal to sub vertical the chief mathew s pluton is interpreted to intrude initially into gently dipping fractures perpendicular to the steeply dipping foliation these melt filled fractures acted as conduits for melt triggering horizontal flow and eventually formed kilometer scale sills the steeply dipping fabrics of the coast shear zone provided pathways for melt to ascended from lower to middle crustal depths partitioning deformation into three dimensional domains of flattening simple shear and constrictional strain created the space for plutons this pattern of deformation reflects the interaction of regional deformation with magma emplacement

glacier surge after ice shelf collapse the possibility that the west antarctic ice sheet will collapse as a consequence of ice shelf disintegration has been debated for many years this matter is of concern because such an event would imply a sudden increase in sea level evidence is presented here showing drastic dynamic perturbations on former tributary glaciers that fed sections of the larsen ice shelf on the antarctic peninsula before its collapse in 1995 satellite images and airborne surveys allowed unambiguous identification of active surging phases of boydell sjogren edgeworth bombardier and drygalski glaciers this discovery calls for a reconsideration of former hypotheses about the stabilizing role of ice shelves

enhanced ice sheet growth in eurasia owing to adjacent ice dammed lakes large proglacial lakes cool regional summer climate because of their large heat capacity and have been shown to modify precipitation through mesoscale atmospheric feedbacks as in the case of lake agassiz 1 several large ice dammed lakes with a combined area twice that of the caspian sea were formed in northern eurasia about 90 000 years ago during the last glacial period when an ice sheet centred over the barents and kara seas 2 blocked the large northbound russian rivers 3 here we present high resolution simulations with an atmospheric general circulation model that explicitly simulates the surface mass balance of the ice sheet we show that the main influence of the eurasian proglacial lakes was a significant reduction of ice sheet melting at the southern margin of the barents kara ice sheet through strong regional summer cooling over large parts of russia in our simulations the summer melt reduction clearly outweighs lake induced decreases in moisture and hence snowfall such as has been reported earlier for lake agassiz1 we conclude that the summer cooling mechanism from proglacial lakes accelerated ice sheet growth and delayed ice sheet decay in eurasia and probably also in north america

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

aerodynamic entrainment threshold effects of boundary layer flow conditions experimental data are presented demonstrating the influence of boundary layer flow conditions on aerodynamic entrainment of grains in the absence of intersaltation collisions new methods are proposed for 1 the unambiguous determination of aerodynamic threshold for any grain population and 2 approximation of the probability density function pdf distributions of threshold shear velocity for aerodynamic entrainment in wind tunnel experiments the orderly spatial development of flow conditions within a developing boundary layer over the roughened surface of a flat plate constrains the aerodynamic threshold condition in terms of both mean and fluctuating values initial grain dislodgements and subsequent erosion from narrow strips of loose finely fractionated ballotini were recorded photographically as wind speed was increased boundary layer parameters including average threshold shear velocity u t were calculated using the momentum integral method direct observations show that sporadic oscillation of grains preceded dislodgement at slightly higher velocities most grains rolled over their neighbours before entering saltation initial entrainment in spatially semi organized flurries of 50 or more grains was followed by quiescent periods at airflow velocities close to threshold these observations provide strong circumstantial evidence linking both the nature and spatial pattern of initial grain motions to sweep events during the fluid bursting process for each grain fraction values of u t were found to span an unexpectedly wide range and to decrease downwind from the leading edge of the plate as turbulence intensity increased a probabilistic entrainment model is applied to the aerodynamic threshold condition so as to incorporate the effects of changing turbulent flow regimes over the plate analysis of strip erosion curves gives both an objective definition of the threshold condition and usable approximations of the pdf for u t required by the model and for future stochastic treatment of the threshold condition

large arctic temperature change at the wisconsin holocene glacial transition analysis of borehole temperature and greenland ice sheet project ii ice core isotopic composition reveals that the warming from average glacial conditions to the holocene in central greenland was large approximately 15 c this is at least three times the coincident temperature change in the tropics and mid latitudes the coldest periods of the last glacial were probably 21 c colder than at present over the greenland ice sheet

global regional and local scale factors determining glaciation extent in eastern siberia over the last 140 000 years the aim of this work is to evaluate the effect of variations of greenhouse gas concentrations orbital parameters sea surface conditions vegetation and dust deposition on the extent of east siberian mountain glaciations during the late quaternary an atmosphere only general circulation model is used in a series of 16 sensitivity tests at high spatial resolution over the region of interest to systematically evaluate the relative importance of these different forcing parameters no attempt was made to reproduce in detail the history of late quaternary mountain glaciations in east siberia because given a the temporal and spatial scarcity of available evidence of mountain glaciations in this region and b the large uncertainties concerning the boundary conditions to be prescribed in this model such an exercise must necessarily remain incomplete and partially inconclusive the results of this study suggest that moisture delivery from the atlantic is an important factor determining mountain glacier mass balance in eastern siberia and is very sensitive to the geometry of the west eurasian ice sheet this means that variable moisture blocking by the west eurasian ice sheet during the weichselian is the most important single factor explaining the opposite history of glacier and ice sheet extent in west and east eurasia during the weichselian this work confirms earlier results showing that the large 140 kyr bp west eurasian ice sheet caused regional scale cooling extending towards eastern eurasia nevertheless the simulated response of the regional summer temperature and thus glacier extent because of the strong dependency of glacier mass balance of summer melt rates is to a very large extent directly determined by insolation for the early weichselian this leads to a clear maximum of local glacier extent at 70 kyr bp which is in line with the variations of top of the atmosphere insolation on orbital time scales but to some degree at odds with geological evidence which suggests larger glacier extent at 115 and 90 kyr bp than at 70 kyr bp through snow feedbacks the effects of changes in the prescribed vegetation distribution and dust deposition rate are also substantial in summary it appears that the broad features of late quaternary glaciation history in eastern eurasia can be understood in terms of known forcings

intermittent signature of enso in west antarctic precipitation 1 precipitation data from the new era40 reanalyses and from a 200 year simulation confirm a robust main mode of precipitation variability in west antarctica an intermittently strong enso signature is found in this mode however high correlation with enso indices appears infrequent thus the high correlation found in era40 and previously in other chronologically realistic data in the late 1980s and the 1990s may not be expected to last unlike previously suggested by others the sign of the correlation between enso indices and west antarctic precipitation when significant does not appear to change in time precipitation variability at the enso pace in the bellingshausen weddell ross amunsden region is consistently in phase phase opposition respectively with the southern oscillation index this is consistent with a tropospheric wave train connecting the tropical pacific and west antarctic regions which modulates in phase opposition the advection of air and moisture in the 2 regions

antarctic precipitation and climate change predictions horizontal resolution and margin vs plateau issues all climate models participating in the fourth assessment report of the intergovernmental panel on climate change as made available by the program for climate model diagnosis and intercomparison pcmdi as the coupled model intercomparison project 3 cmip3 archive predict a significant surface warming of antarctica by the end of the 21st century under a moderate sresa1b greenhouse gas scenario all models but one predict a concurrent precipitation increase but with a large scatter of results the models with finer horizontal resolution tend to predict a larger precipitation increase because modeled antarctic surface mass balance is known to be sensitive to horizontal resolution extrapolating predictions from the different models with respect to model resolution may provide simple yet better multi model estimates of antarctic precipitation change than mere averaging or even more complex approaches using such extrapolation a conservative estimate of the predicted precipitation increase at the end of the 21st century is 30 kg m 2 a 1 on the grounded ice sheet corresponding to a 1 mm a 1 sea level rise about three quarters of this rise originates from the marginal regions of the antarctic ice sheet with surface elevation below 2250 m this is where field programs are most urgently needed to better understand and monitor accumulation at the surface of antarctica and to improve and verify prediction models

mass changes of the greenland and antarctic ice sheets and shelves and contributions to sea level rise 1992 2002 changes in ice mass are estimated from elevation changes derived from 10 5 years greenland and 9 years antarctica of satellite radar altimetry data from the european remote sensing satellites ers 1 and 2 for the first time the dh dt values are adjusted for changes in surface elevation resulting from temperature driven variations in the rate of firn compaction the greenland ice sheet is thinning at the margins 42 2g t a 1 below the equilibrium line altitude ela and growing inland 53 2g t a 1 above the ela with a small overall mass gain 11 3g t a 1 0 03 mm a 1 sle sea level equivalent the ice sheet in west antarctica wa is losing mass 47 4g t a 1 and the ice sheet in east antarctica ea shows a small mass gain 16 11 gt a 1 for a combined net change of 31 12 gt a 1 0 08 mm a 1 sle the contribution of the three ice sheets to sea level is 0 05 0 03 mm a 1 t he antarctic ice shelves show corresponding mass changes of 95 11 gt a 1 in wa and 142 10 gt a 1 in ea thinning at the margins of the greenland ice sheet and growth at higher elevations is an expected response to increasing temperatures and precipitation in a warming climate the marked thinnings in the pine island and thwaites glacier basins of wa and the totten glacier basin in ea are probably ice dynamic responses to long term climate change and perhaps past removal of their adjacent ice shelves the ice growth in the southern antarctic peninsula and parts of ea may be due to increasing precipitation during the last century

the grain bed collision in sand transport by wind in the last decade much progress has been made toward the development of a comprehensive model of aeolian sediment transport in which the grain bed collision has been identified as having a significant role the grain bed collision has been studied by both physical experiments and numerical simulation a principal objective of these studies has been to gather sufficient data to characterize the collision in order that it may be represented as an empirical function in numerical models of the sand transport system thus the study of the exact physical mechanism of the collision has to some extent been neglected the transport of larger particles in saltation over a loose surface is known to promote the release of finer particles from that surface into suspension thus the precise physics of the grain bed collision become highly significant with regard to the physics of dust release from a surface this paper re examines previous collision data and compares them with a simple collision model this model proposed that the impinging grain strikes a single surface grain and rebounds an alternative mechanism might be that the impinging grain ploughs through the surface striking a number of bed grains before rebounding the collision data are shown to support the model first because the duration of the collisions observed on the high speed film supports a short contact time with the surface and second because the data fit well with the model

mid holocene greening of the sahara first results of the gaim 6000 year bp experiment with two asynchronously coupled atmosphere biome models the mid holocene green sahara represents the largest anomaly of the atmosphere biosphere system during the last 12 000 years although this anomaly is attributed to precessional forcing leading to a strong enhancement of the african monsoon no climate model so far has been able to simulate the full extent of vegetation in the sahara region 6000 years ago here two atmospheric general circulation models lmd 5 3 and echam 3 are asynchronously coupled to an equilibrium biogeography model to give steady state simulations of climate and vegetation 6000 years ago including biogeophysical feedback the two model results are surprisingly different and neither is fully realistic echam shows a large northward extension of vegetation in the western part of the sahara only lmd shows a much smaller and more zonal vegetation shift these results are unaffected by the choice of green or modern initial conditions the inability of lmd to sustain a green sahara 6000 years ago is linked to the simulated strength of the tropical summer circulation during the northern summer monsoon season the meridional gradient of sea level pressure and subsidence over the western part of northern africa are both much weaker in echam than in lmd in the present as well as the mid holocene these features allow the surface moist air flux to penetrate further into northern africa in echam than in lmd this comparison illustrates the importance of correct simulation of atmospheric circulation features for the sensitivity of climate models to changes in radiative forcing particularly for regional climates where atmospheric changes are amplified by biosphere atmosphere feedbacks