rheology of till beneath storglaciaren sweden in order to study in situ the rheology of a deforming subglacial till various instruments were emplaced in till beneath storglaciaren sweden boreholes were used to gain access to the till beneath about 100 m of ice tiltmeters provided an estimate of the shear strain rate in the till two other instruments yielded measures of till strength in addition water pressures were recorded in boreholes and in the till a computer controlled distance meter provided an effectively continuous record of the surface velocity and data from frequent surveys of a stake network were used to estimate the mean basal drag based on a force balance calculation tilt rates varied directly with effective pressure so decreases in water pressure apparently increased the coupling between the glacier and the bed surface speed was either out of phase with tilt or varied independently of tilt thus increases in speed were apparently a consequence either of longitudinal coupling or of reduced coupling between the glacier and the bed they were not a result of till deformation till strength varied directly with effective pressure which is consistent with it being a mohr coulomb or frictional material the devices measuring till strength are presumed to have been pulled through the till at a speed that varied in phase with the surface speed but till strength did not vary systematically with surface speed this implies that the residual strength of the till is insensitive to strain rate thus the appropriate constitutive equation for till rheology may be of the form e e k where k is a constant this is consistent with experimental data reported in the geotechnical literature

the instability theory of drumlin formation and its explanation of their varied composition and internal structure despite their importance in understanding glaciological processes and constraining large scale flow patterns in palaeo glaciology there is little consensus as to how drumlins are formed attempts to solve the drumlin problem often fail to address how they are created from an initially flat surface in the absence of obvious cores or obstacles this is a key strength of the instability theory which has been described in a suite of physically based mathematical models and proposes that the coupled flow of ice and till causes spontaneous formation of relief in the till surface encouragingly model predictions of bedform height and length are consistent with observations and furthermore the theory has been applied to a range of subglacial bedforms and not just drumlins however it has yet to confront the myriad observations relating to the composition and internal structure of drumlins and this could be seen as a major deficiency this paper is a first attempt to assess whether the instability theory is compatible with the incredible diversity of sediments and structures found within drumlins we summarise the underlying principles of the theory and then describe and attempt to explain the main types of drumlin composition e g bedrock till glaciofluvial sediments and combinations thereof contrary to a view which suggests that the presence of some sedimentary sequences e g horizontally stratified cores is inconsistent with the theory we suggest that one would actually expect a diverse range of constituents depending on the inheritance of sediments that pre date drumlin formation the duration and variability of ice flow and the balance between erosion and deposition till continuity at the ice bed interface we conclude that the instability theory is compatible with and potentially strengthened by what is known about drumlin composition and as such offers the most complete and promising solution to the drumlin problem to date

sensitivity of the extended ugamp general circulation model to the specification of gravity wave phase speeds two simulations made using the extended ugamp general circulation model eugcm are compared the first uses a gravity wave parametrization scheme that only includes orographically forced gravity waves with zero phase speeds whilst the second uses a similar scheme except that it includes a gravity wave with a phase speed of 20 ms 1 results from the first simulation show that wintertime westerlies are reasonably estimated but summertime and equatorial easterlies are excessive owing to an absence of gravity wave drag in these region this absence of gravity wave drag also means that the summertime mean meridional circulation is too weak and summer mesospheric model temperatures are accordingly warmer than observations by up to 35 degc inclusion of gravity waves with phase speeds of 20 ms 1 reduces summer and equatorial easterlies and decreases summer mesophere temperatures because unlike zero phase speed waves these waves can propagate into the summer and equatorial middle atmosphere and thus of the 20 ms 1 gravity wave also improves the simulation of the semiannual oscillation both at the stratopause and at the mesopause

drumlins carved by deforming till streams below the laurentide ice sheet the peterborough drumlin field 5000 km 2 is the largest in central canada and was formed during readvance of the laurentide ice sheet shortly after 13 000 b p morphometric analysis of 998 drumlins combined with study of 6921 borehole logs and downhole geophysical data has established the distribution of drumlin types and their stratigraphy along a 70 km flow line through the drumlin field in the north drumlins are spindle shaped composed of till resting directly on bedrock and were formed by deformation of till into low pressure zones in the lee of bedrock scarps eskers record subglacial drainage by channeled meltwaters flowing on bedrock to the south drift thickness increases and eskers are absent drumlins are larger and more ovoid and are composed of overridden proglacial and glaciolacustrine sediments overlain erosively by deformation till till is thin over drumlin crests and thick in interdrumlin swales and records subglacial cannibalization and mixing of overridden sediments the drumlin form is the product of erosion by streams of deforming till flowing in swales separating cores composed of older sediments systematic down field changes in drumlin shape and size can be related to decreased duration of subglacial deformation toward the limit of the readvance and increased thickness of preexisting sediments available for dissection by subglacial till streams absence of eskers in the central and southern parts of the drumlin field suggests that the ice base was drained by mass flow of the deforming substrate the substantial subsurface data available from the peterborough area indicate that drumlins are predominantly erosional in origin and are the geomorphological expression of a deforming subglacial bed below the margin of the laurentide ice sheet

the sensitivity of subglacial bedform size and distribution to substrate lithological control the varied nature of a glacial substrate has often been invoked to account for the varied distribution arrangement and morphological expression of subglacial bedforms the ease and rate of sediment production from the bedrock substrate and the rheological mechanical and hydraulic properties of the resultant subglacial sediment layer have been argued in certain samples or settings to account for the local to regional distribution of bedforms and bedform types and varying morphological expression such as size shape elongation or spatial arrangement from locally coherent observations and relationships it is tempting to extrapolate such patterns to express a more fundamental lithological control on subglacial bedform properties and formation here we use a large systematically compiled dataset 30 000 subglacial bedforms inscribed by the irish ice sheet to explore potential lithological controls upon subglacial bedform distribution and morphological expression we expect any lithological control upon form size or distribution to be expressed through a spatial correlation between bedform properties and those of the bed lithology we extract bedform morphometric and distribution statistics and underlying lithology data from datasets held in a geographic information system for comparative analyses at ice sheet scale i e considering our whole bedform population we find no apparent relationship between the lithology of either bedrock or till substrate and bedform occurrence density of arrangement or size regional to local examples do in contrast exhibit some coincident changes in substrate and bedform expression these are typically manifest as an abrupt shortening or lengthening of drumlins at a lithological boundary superimposed upon more gradual regional trends however not all local variability observed in the bedform population can be attributed to a lithological driver and where it can be inferred lithological control is often in tandem with or subordinate to other drivers such as topographic setting or ice dynamics we propose a hierarchy of controls upon bedform form and distribution strongly coupled to spatial scale lithological differences can modulate local scale form and expression but bedform incidence and properties are primarily governed by ice sheet scale glaciological patterns and drivers only under certain unknown conditions at a local scale is the bedforming process sensitive to the lithological properties of the bed

early jurassic magmatism on the antarctic peninsula and potential correlation with the subcordilleran plutonic belt of patagonia early jurassic silicic volcanic rocks of the chon aike province v1 187 182 ma are recognized from many localities in the southern antarctic peninsula and ne patagonia and are essentially coeval with the extensive karoo 182 ma and ferrar 183 ma large igneous provinces of pre breakup gondwana until recently plutonic rocks of this age were considered either rare in or absent from the antarctic peninsula batholith which was thought to have been mainly constructed during the middle jurassic and the mid cretaceous new u pb zircon geochronology from the antarctic peninsula and recently published u pb ages from elsewhere in the peninsula and patagonia are used to demonstrate the more widespread nature of early jurassic plutonism eight samples are dated here from the central and southern antarctic peninsula they are all moderately to strongly foliated granitoids tonalite granite granodiorite and locally represent the crystalline basement they yield ages in the range 188 181 ma which overlap with published ages of 185 180 ma from granitoids from elsewhere on the antarctic peninsula and from the subcordilleran plutonic belt of patagonia 185 181 ma whereas early jurassic plutons of the subcordilleran plutonic belt of patagonia are directly related to subduction processes along the proto pacific margin of gondwana coeval volcanic rocks of the chon aike province are interpreted to be directly associated with extension and plume activity during the initial stages of gondwana breakup this indicates that subduction was continuing when chon aike province volcanism started the early jurassic plutonism on the antarctic peninsula is transitional between subduction related and breakup related magmatism supplementary material cathodoluminescence images of analysed zircon grains from sites on the antarctic peninsula are available at https doi org 10 6084 m9 figshare c 3521973

size and shape characteristics of drumlins derived from a large sample and associated scaling laws abstract ice sheets flowing across a sedimentary bed usually produce a landscape of blister like landforms streamlined in the direction of the ice flow and with each bump of the order of 102 to 103 m in length and 101 m in relief such landforms known as drumlins have mystified investigators for over a hundred years a satisfactory explanation for their formation and thus an appreciation of their glaciological significance has remained elusive a recent advance has been in numerical modelling of the land forming process in anticipation of future modelling endeavours this paper is motivated by the requirement for robust data on drumlin size and shape for model testing from a systematic programme of drumlin mapping from digital elevation models and satellite images of britain and ireland we used a geographic information system to compile a range of statistics on length l width w and elongation ratio e where e l w for a large sample mean l is found to be 629 m n 58 983 mean w is 209 m and mean e is 2 9 n 37 043 most drumlins are between 250 and 1000 metres in length between 120 and 300 metres in width and between 1 7 and 4 1 times as long as they are wide analysis of such data and plots of drumlin width against length reveals some new insights all frequency distributions are unimodal from which we infer that the geomorphological label of drumlin is fair in that this is a true single population of landforms rather than an amalgam of different landform types drumlin size shows a clear minimum bound of around 100 m horizontal maybe drumlins are generated at many scales and this is the minimum or this value may be an indication of the fundamental scale of bump generation proto drumlins prior to them growing and elongating a relationship between drumlin width and length is found with r2 0 48 and that is approximately w 7 l 1 2 when measured in metres a surprising and sharply defined line bounds the data cloud plotted in e w l space and records a scale dependent maximum elongation limit approximated by emax l1 3 when l measured in metres for a given length for some reason as yet unknown drumlins do not exceed the elongation ratio defined by this scaling law we also report and compare our statistics to an amalgamated sample 25 907 drumlins of measures derived from around 50 published investigations any theory must be able to explain the drumlin statistics and fundamental scaling properties reported herein and they thus provide powerful tests for drumlin modelling

stratospheric circulation features deduced from sams constituent data abstract stratospheric circulation is investigated by further analyses of three years of stratospheric and mesospheric sounder sams data eddy effects on constituent transport are investigated with two transport formulations the transformed eulerian mean formulation and the effective transport formulation the transformed eulerian mean formulation together with calculated residual mean winds v w is used to delineate regions or times of significant eddy contributions to constituent transport significant regions are found in the stratosphere in all seasons not only in the northern hemisphere nh winter high latitudes where contributions from nonlinear and nonsteady perturbations in sudden and final warming events are expected but also in the midlatitude middle stratosphere in autumn and at winter summer low latitudes near the stratopause questionably large calculated 0 1 m magnitudes near the stratopause during solstice suggest that the use of residual winds calculated here may be i

rogen ribbed moraine identification and possible origin abstract it is emphasized that rogen moraine is a morphological term applied to a special kind of transverse moraine characteristically these ridges show transitions to drumlins it is inferred that this morphology is created in at least two steps including remoulding of pre existing landforms

ring shear studies of till deformation coulomb plastic behavior and distributed strain in glacier beds a ring shear device was used to study the factors that control the ultimate steady strength of till at high shear strains tests at a steady strain rate and at different stresses normal to the shearing direction yielded ultimate friction angles of 26 3 and 18 6 for tills containing 4 and 30 clay sized particles respectively other tests at steady normal stresses and variable shear strain rates indicated a tendency for both tills to weaken slightly with increasing strain rate this weakening may be due to small increases in till porosity these results provide no evidence of viscous behavior and suggest that a coulomb plastic idealization is reasonable for till deformation however viscous behavior has often been suggested on the basis of distributed shear strain observed in subglacial till we hypothesize that deformation may become distributed in till that is deformed cyclically in response to fluctuations in basal water pressure during a deformation event transient dilation ofdiscrete shear zones should cause a reduction in internal pore water pressure that should strengthen these zones relative to the surrounding till a process called dilatant hardening consequent changes in shear zone position when integrated over time may yield the observed distributed strain

discrete element modeling of subglacial sediment deformation 1 the discrete element method dem is used in this study to explore the highly nonlinear dynamics of a granular bed when exposed to stress conditions comparable to those at the bed of warm based glaciers complementary to analog experiments the numerical approach allows a detailed analysis of the material dynamics and the shear zone development during progressive shear strain the geometry of the heterogeneous stress network is visible in the form of force carrying grain bridges and adjacent volumetrically dominant inactive zones we demonstrate how the shear zone thickness and dilation depend on the level of normal overburden stress and we show how high normal stress can mobilize material to great depths the particle rotational axes tend to align with progressive shear strain with rotations both along and reverse to the shear direction the results from successive laboratory ring shear experiments on simple granular materials are compared to results from similar numerical experiments the simulated dem material and all tested laboratory materials deform by an elastoplastic rheology under the applied effective normal stress these results demonstrate that the dem is a viable alternative to continuum models for small scale analysis of sediment deformation it can be used to simulate the macromechanical behavior of simple granular sediments and it provides an opportunity to study how microstructures in subglacial sediments are formed during progressive shear strain

the stratopause semiannual oscillation in the berlin troposphere stratosphere mesosphere gcm abstract the tropical stratopause semiannual oscillation sao in the berlin troposphere stratosphere mesosphere general circulation model tsm gcm is investigated the model is able to produce a semiannual oscillation that is properly located in time and space in comparison with observations however the westerly phase is 10 15 m s 1 too weak while the second easterly phase of the year is about the same amount too strong a case study is performed to examine the westerly forcing of the sao in detail at 1 hpa in one particular march when the strongest westerly acceleration of the year takes place only 18 of the forcing is due to the dissipation of kelvin waves whereas 72 of the forcing is due to the dissipation of other slow eastward propagating planetary scale waves and 10 is due to the dissipation of eastward propagating inertio gravity waves throughout the year the kelvin wave contribution to the total body force is below 50 the modeled kelvin waves are dissipated at higher altitudes than o

looking through drumlins testing the application of ground penetrating radar ground penetrating radar gpr is becoming a commonly applied technique in geomorphology however its use in the study of subglacial bedforms has yet to be fully explored and exploited this paper presents the results of a gpr feasibility study conducted on a drumlinized terrain in cumbria uk where five drumlins were investigated using multiple radar antenna frequencies the site was selected for the presence of nearby bedrock outcrops suggesting a shallow drumlinized diamict bedrock contact and a permeable lithology despite the clayey sediment and unfavourable weather conditions a considerable penetration depth of 12m was achieved when using a 50mhz antenna with a separation of 1m trace spacing of 1m and 128 fold vertical stack results indicate that the drumlinized diamict is in direct erosional contact with the bedrock while the internal drumlin geometry is generally chaotic on the stoss side evidence of layering dipping downflow at an angle greater than the drumlin surface profile was found on the lee side the inter drumlin areas comprise 4m of infill sediment that masks part of the original drumlin profile overall this study indicates that gpr can be deployed successfully in the study of glacial bedform sedimentary architecture

sediment deformation beneath glaciers rheology and geological consequences experiments beneath breidamerkurjokull in iceland have led to development of flow laws for the subglacial till relating strain rate to shear stress and effective pressure and assuming either bingham fluid or nonlinearly viscous fluid behavior water pressures in the till are less than ice pressures and it is suggested that this may lead to infiltration of ice into the sediment which inhibits sliding at the ice sediment interface where water pressures are equal or near to ice pressures infiltration does not occur and sliding may result a one dimensional theory of subglacial deformation is developed in which the empirical flow law is coupled with a model of subglacial hydrology and consolidation this predicts stable states in which subglacial sediment either does not deform or a dilatant deforming horizon forms with positive effective pressures at the ice bed interface or unstable states where zero or negative effective pressures are predicted time dependent analyses show that response times following perturbations of the system may be of the order of 103 years and thus that unsteady behavior may be normal on glaciers flowing over unlithified sediment beds it is suggested that the natural variability of material properties in subglacial sediment beds leads to the development of drumlins on the glacier bed it is suggested that unstable deformation at zero or negative effective stress leads to piping in subglacial sediments at the glacier terminus and the growth of sediment floored subglacial tunnels their frequency is that which is sufficient to draw down subglacial water pressures so as to prevent unstable deformation where they discharge large water volumes subglacial sediments flow laterally toward them producing tunnel valleys this sediment is then removed by water flowing along the axial tunnel tunnel valleys can be regarded as the equivalent in soft sediment areas of eskers in bedrock areas

the planar shape of drumlins abstract the asymmetry of the planar shape of drumlins is an established paradigm in the literature and characterizes drumlins as resembling tear drops with a blunt bullet shaped stoss end and a tapering pointed lee end it is widely cited and never been seriously questioned in this paper the planar shape of 44 500 drumlins mapped in various regional settings from drumlin fields in north america and northern europe were objectively analysed by means of geographic information system tools two parameters were considered the first denoted here as as pl focuses on the relative position of the point of intersection between the axes of the maximum length and the maximum width it is defined as the distance between the upstream i e beginning of the drumlin and the intersection point measured along the longitudinal axis divided by the entire length of the long axis results indicate that the intersection point of the majority of drumlins 64 is very close to the longitudinal midpoint 0 33 as pl as pl a is defined as the ratio between the area of the upstream half of the drumlin to that of the entire drumlin results show that for most drumlins 81 the upper half area is almost as large as the down half 0 45 as pl a

an analysis of the structure and forcing of the equatorial semiannual oscillation in zonal wind the low frequency variability in the equatorial middle atmosphere is dominated by the semiannual oscillation sao and quasi biennial qbo oscillation although the general characteristics and forcing mechanisms of these oscillations are thought to be known these oscillations are still not well reproduced in modeling experiments of the middle atmosphere the relatively long period 1992 1996 of upper atmosphere research satellite uars measurements and u k meteorological office ukmo assimilated model data have been used to examine the zonal wind sao in the upper stratosphere the sao in zonal wind is found to be latitudinally asymmetric about the equator with strongest amplitudes in the southern hemisphere subtropics the latitudinal asymmetry is diagnosed by using the tem momentum equation and ukmo data the asymmetry is mostly due to the different phasing of the peak advective forcing by the mean meridional circulation in each hemisphere the contribution of gravity waves to the zonal wind sao are also explored by using the forcing from a linear gravity wave model gravity waves are found to be the primary source of westerly momentum in each hemisphere during fall and contribute to the latitudinal asymmetry of the sao by having more easterly forcing in the southern hemisphere during summer compared to in the northern hemisphere during summer

pressure dependent viscosity and interfacial instability in coupled ice sediment flow we study an interfacial instability in the coupled flow of ice and subglacial sediment both modelled as viscous media unlike other interfacial instabilities in coupled viscous flows at zero reynolds number the mechanism considered here does not rely on buoyancy or the effect of an upper free surface but on the pressure dependence of the sediment viscosity specifically the instability relies on sediment rheology being such that when sediment flows in simple shear sediment flux increases with compressive normal stress at the ice sediment interface when the velocity of the interface is kept constant when ice moves over a shallow bump in the interface it generates a higher compressive stress on the bump s upstream side than in its lee if in addition the effective sediment viscosity is low compared with that of ice interfacial velocity remains approximately constant and this then implies that more sediment flows into the bump than out of it causing it to grow modelling ice as a newtonian material we show that this mechanism works for a wide range of sediment rheologies including the highly nonlinear shear thinning ones typically thought most appropriate for the description of nearly plastic sediment the instabilities predicted are essentially two dimensional with infinite transverse wavelength and a nonlinear model shows that growth is unbounded until cavitation occurs in the lee of evolving bumps on the interface the instability mechanism does not seem to predict the formation of common glacial landforms but may explain the formation of water filled cavities on deformable glacier beds

the composition and internal structure of drumlins complexity commonality and implications for a unifying theory of their formation investigation of drumlins is significant to both glaciology and palaeoglaciology but the sheer diversity of their composition and internal structure is often cited as a major obstacle towards a satisfactory unifying explanation of their formation this paper presents the first systematic survey of the vast literature on this subject with the aim of concisely summarising observations and identifying any emergent patterns or commonality that theories of drumlin formation should be able to explain results confirm that investigations are often limited by availability of suitable sediment exposures 40 of studies report data from 5 drumlins and 44 do not specify sample size although borehole data and geophysical techniques can alleviate this problem however it is clear that the constituents of drumlins are incredibly diverse in terms of their composition e g a range of lithologies clast shapes sizes and fabrics structure e g sediments that are sorted homogeneous surface conformable unconformable and evidence of deformation e g ranging from pervasive to non pervasive limited to absent despite this diversity our review leads us to suggest that drumlin composition can be simplified to five basic types i mainly bedrock ii part bedrock part till iii mainly till iv part till part sorted sediments and v mainly sorted sediments this is a potentially significant step in that it reduces the oft cited complexity of drumlin composition and provides a more realistic goal for theories or numerical models of drumlin formation to target the various types can occur within the same drumlin field which leaves us with two possible implications for drumlin formation 1 different types of drumlin are formed seeded by different processes despite being morphologically similar equifinality investigation of drumlin composition may therefore reveal diagnostic processes explanations for these different types of drumlin and we argue that bedrock drumlins are an example 2 a single process occurs across large parts of the ice bed interface to create drumlinised terrain in a variety of sediments investigation of drumlin composition may in this case simply reflect pre existing sediments but importantly the way in which the drumlin forming mechanism modifies is modified by them we argue that the latter simpler explanation applies to all other types of drumlin excluding purely bedrock forms and conclude that the diversity in drumlin composition is no obstacle to a single unifying theory

the use of integrated sedimentological and geophysical methods in drumlin research a case study of pigeon point clew bay northwest ireland despite significant advances over the past decades our understanding of drumlin formation and associated ice bed processes is still incomplete in this paper we present the integrated use of geomorphological sedimentological and geophysical techniques as a powerful means to force a breakthrough towards solving the drumlin enigma we report on investigations of the anatomy of the pigeon point drumlin clew bay ireland we found that the bulk of the landform which displays a classical drumlin shape consists of silty clayey diamicton showing evidence of deformation hydrofracturing and comminution the unit is interpreted as a sub glacial traction till comminution till the thin unit overlying this basal till consists of silty sandy diamicton and is interpreted as a para glacially modified melt out till the partly cemented third unit consists of stratified massive to graded sands and gravels its contact with the sub glacial traction till consists of a series of concave shapes which suggests that it was deposited in meltwater channels that flowed in sub glacial cavities and that cut laterally into the drumlin we propose that highs in the undulating rockhead relief as shown in the seismic profile have provided nuclei which initiated drumlin formation this idea is supported by the observation of local detached bedrock slabs that grade upwards into a comminution till in the long profile very high normalized induced polarization ip values form a wedge shape which is interpreted as a set of conjugate thrusts or a pop up structure the structure is positioned directly above one of the undulations in the bedrock suggesting a direct relationship the high values are thought to reflect the presence of pre existing clays which were sheared into the till thus forming linings in the thrust features it is concluded that glacitectonic processes notably differential bedrock weathering and thrusting have played a key role in the formation of this drumlin

gravity wave forcing in the stratosphere observational constraints from the upper atmosphere research satellite and implications for parameterization in global models 1 global models that include parameterized gravity wave effects have an excessively broad range of tuning parameters to choose from that allow a very broad range of gravity wave momentum forcing distributions we derive a set of constraints on gravity wave characteristics near the tropopause from databased estimates of gravity wave effects in the stratosphere we use 5 6 years of upper atmosphere research satellite data and uk met office analyzed wind and temperature fields to first derive estimates of the gravity wave mean flow forcing in the stratosphere we then compare these estimates to a suite of gravity wave model calculations to infer the constraints on the gravity wave characteristics near the tropopause these constraints apply to the portion of the gravity wave spectrum that dissipates in the stratosphere and does not include those that dissipate and drive the mesosphere we focus on equatorial and extratropical regions separately and find substantially different wave characteristics between the two we also find a seasonal variation in gravity wave characteristics in the extratropics near the equator gravity waves assist in driving the quasibiennial and semiannual oscillations and our constraints on gravity waves likely include a blend of characteristics of both ordinary gravity waves and some kelvin wave modes in the extratropics gravity wave forcing primarily assists in the annual winter to summer transition from westward to eastward winds

size shape and spatial arrangement of mega scale glacial lineations from a large and diverse dataset mega scale glacial lineations msgls are a characteristic landform on ice stream beds solving the puzzle of their formation is key to understanding how ice interacts with its bed and how this in turn influences the dynamics of ice streams however a comprehensive and detailed characterization of this landform s size shape and spatial arrangement which might serve to test and refine formational theories is largely lacking this paper presents a detailed morphometric analysis and comparison of 4043 msgls from eight palaeo ice stream settings three offshore norway and antarctica four onshore canada and one from under a modern ice stream in west antarctica the length of msgls is lower than previously suggested mode 1000 2000 m median 2892 m and they initiate and terminate at various locations on an ice stream bed their spatial arrangement reveals a pattern that is characterized by an exceptional parallel conformity 80 of all mapped msgls have an azimuth within 5 from the mean values and a fairly constant lateral spacing mode 200 300 m median 330 m which we interpret as an indication that msgls are a spatially self organized phenomenon results show that size shape and spatial arrangement of msgls are consistent both within and also generally between different ice stream beds we suggest this results from a common mechanism of formation which is largely insensitive to local factors although the elongation of msgls mode 6 8 median 12 2 is typically higher than features described as drumlins these values and those of their width mode 100 200 m median 268 m overlap which suggests the two landforms are part of a morphological continuum and may share a similar origin we compare their morphometry with explicit predictions made by the groove ploughing and rilling instability theories of msgl formation although the latter was most compatible neither is fully supported by observations 2014 the authors earth surface processes and landforms published by john wiley sons ltd

the global residual mean circulation in the middle atmosphere for the northern winter period abstract the residual mean circulation rmc has been calculated from the transformed thermodynamic equation using lims limb infrared monitor of the stratosphere data for the 100 0 1 mb region for discussion it has been divided into two components the diabatic circulation associated with the diabatic heating and the transient circulation more directly connected to eddy activity the slowly varying diabatic circulation reveals an equator to pole circulation at lower levels in the stratosphere usually overlain by a summer to winter pole circulation however there are strong seasonal variations so that the pole to pole circulation fills the entire region at the december solstice while the equator to pole circulation extends to above 0 1 mb at the equinoxes the transient circulation is characterized by rapid variations and small vertical and horizontal scales though generally smaller than the diabatic circulation it can dominate in the lower stratosphere during disturbed conditions this circul

qbo sao and tropical waves in the berlin tsm gcm sensitivity to radiation vertical resolution and convection the sensitivity of tropical waves in a general circulation model gcm to radiation vertical resolution and deep convection is investigated the implications of this for the simulation of the quasi biennial oscillation qbo and the stratopause seminannual oscillation sao are examined varying the vertical resolution produced the greatest impact increasing the resolution leads to a more realistic kelvin wave dissipation and the westerly phase of a qbo starts to develop but is ended early by the advection of unrealistically strong easterly winds from the southern summer hemisphere the dissipation of kelvin waves at lower altitudes leads to a weakening of the westerly phase of the sao only small not statisically significant differences have been found between the integrations using the kuo and the betts miller convection schemes there is slightly more westerly momentum in the integration with the kuo parameterization the mass flux convection scheme of tiedtke is unsuitable for use in the berlin troposphere stratosphere mesosphere gcm it simulates unrealistic latent heat distributions and favors instabilities which generate artifical kelvin waves the radiation scheme has little impact on the dissipation of the tropical waves but replacing the newtonian cooling approach allows more physically consistent integrations

sliding of till over bedrock scratching polishing comminution and kinematic wave theory the observed ratio of removal of material by i scratches gouged by large clasts and ii polishing by debris suggests that the latter is the dominant form of bedrock erosion we examine this with a multi stage breakdown model of glacial debris which includes active erosion by fines these models suggest that a mother clast can through the action of its daughter debris erode several times its own volume of bedrock till sliding over bedrock is likely to be a potent source of scouring by proposing a till bedrock sliding law we investigate the kinematics of sliding till bodies ice flow naturally tends to thin and extend till cover even in the absence of longitudinal gradients in the applied stress thicker till cover has an increased effective pressure at its base a lower sliding velocity and for larger thicknesses a decrease in sediment flux with thickness this implies backward moving kinematic waves and shocks it is suggested that this is related to the blunt upstream faces of drumlins and that drumlinization can be a consequence of debris sliding over bedrock

on the rheology of till the deformation of subglacial till is instrumental in causing certain glacier surges the motion of rapid ice streams in ice sheets and ice sheet surges which are associated with heinrich events and consequent rapid climatic shifts during the last ice age it may also be the means whereby drumlins are formed and these in turn may act as a brake on large scale ice flow it is therefore important in building models to understand the rate at which till deforms and how this controls the basal ice velocity in recent years two paradigms have emerged on the one hand theoreticians have tended to use a viscous rheology though this lacks quantitative support on the other hand field and laboratory studies suggest that till behaves plastically in this paper i will examine some of the dynamic consequences of this latter assumption and show how the dichotomy between viscous and plastic may be less clearcut than previously thought

hairpin erosional marks horseshoe vortices and subglacial erosion abstract crescentic erosional marks wrapped around obstacles and extending into straight grooves are termed hairpin erosional marks they are u shaped to semi circular in cross section are deepest around the upstream shoulders of the obstacles and range over seven orders of magnitude in scale some are asymmetrical with one groove better developed than the other the straight grooves may broaden and shallow rapidly in the downflow direction upstream grooves merge with rather than cross downstream crescentic marks grooves are distinct from glacial abrasion forms and many are not striated hairpin erosional marks are interpreted as being products of scour by horseshoe vortices in turbulent fluids this interpretation is supported by the close similarity of these marks to scours produced experimentally and in nature by horseshoe vortices estimated reynolds numbers show that such vortices are probable around obstacles in wind and flowing water but are extremely unlikely in ice aeolian hairpin erosional marks related to yardangs are illustrated to support this conclusion the surface texture cross sectional shapes and spatial patterns of hairpin erosional marks also discount formation by ice a model for the evolution of isolated erosional drumlins from an initial field of hairpin erosional marks extends earlier research on a meltwater hypothesis for drumlins

subglacial bedforms reveal an exponential size frequency distribution subglacial bedforms preserved in deglaciated landscapes record characteristics of past ice sediment flow regimes providing insight into subglacial processes and ice sheet dynamics individual forms vary considerably but they can often be grouped into coherent fields typically called flow sets that reflect discrete episodes of ice flow within these bedform size frequency distributions predominantly height width and length are currently described by several statistics e g mean median and standard deviation that arguably do not best capture the defining characteristics of these populations this paper seeks to create a better description based upon semi log plots which reveal that the frequency distributions of bedform dimensions drumlin mega scale glacial lineation and ribbed moraine plot as straight lines above the mode this indicates by definition an exponential distribution for which a simple and easily calculated yet statistically rigorous description is designed three descriptive parameters are proposed gradient the exponent characterising bedforms likely least affected by non glacial factors area normalised y intercept 0 quantifying spatial density and the mode below small features are less prevalent due to i measurement data sampling and mapping fidelity ii possible post glacial degradation or iii genesis not being created sub glacially this new description has the benefit of being insensitive to the impact of potentially unmapped or degraded smaller features and better captures properties relating to ice flow importantly using flow sets can now be more usefully compared with each other across all deglaciated regions and with the output of numerical ice sheet models applications may also exist for analogous fluvial and aeolian bedforms identifying the characteristic exponential and that it is typical of emergent subglacial bedforms is a new and potentially powerful constraint on their genesis perhaps indicating that ice sediment interaction is fundamentally stochastic in nature

rheological nonlinearity and flow instability in the deforming bed mechanism of ice stream motion contrary to what has recently been assumed in modeling the proposed deforming bed mechanism for the rapid motion of antarctic ice streams the rheology of water saturated till is probably highly nonlinear according to information from soil mechanics and preliminary experiments on till from the base of ice stream b the equivalent flow law exponent n is probably as high as 100 and the nonlinearities of the shear stress and effective pressure dependences are closely linked the high nonlinearity has important consequences for the deforming bed mechanism a flow system operating by this mechanism can be unstable as a result of feedback from the generation of basal water by shear heating of basal till the short term feedback effect is analyzed for a perturbation in a model ice stream in which the basal meltwater is transported through a distributed system of narrow gap conduits at the ice till interface although the analysis is approximate and some of the system parameters are poorly known the results suggest that the deforming bed mechanism is unstable for n 20 the apparent lack of such an instability in the currently active ice streams implies that their motion is controlled not by the deforming bed mechanism but by some other as yet unidentified mechanism

streamlined subglacial bedforms on the narke plain south central sweden areal distribution morphometrics internal architecture and formation abstract a flow set of close to 1000 drumlins has been mapped by means of lidar derived digital elevation models and investigated by trenching the area is situated on the sw part of the narke plain and its surrounding uplands in south central sweden which was deglaciated in the early preboreal in a glacioaquatic setting we find that there is considerable morphological difference in drumlin distribution patterns over crystalline basement areas compared to streamlined terrain over palaeozoic sedimentary rock basement the former area is characterized by thin quaternary drift and the drumlins are all of the rock cored type built due to active deposition of sediment around obstacles to glacier flow the latter area is characterized by deep quaternary drift and the drumlins are more elongate and also larger in all dimensions as compared to rock cored drumlins irrespective of these geomorphological differences on local landscape scale we find that drumlin morphometric values remain part of a morphological continuum at the regional scale based on the internal sediment architecture as revealed in two cross drumlin sections we find that the soft cored drumlins were formed by compressional constructive deformation along with excavational deformation along the flanks of the emerging drumlins which shaped the separating troughs intermediate type drumlins are those that demonstrate a coupling between underlying palaeozoic sediment strata in areas of shallow drift sheet these are the result of differing rheological response between incorporated sedimentary rock and a deforming bed below the ice bed interface an overall conclusion is that we find geomorphic and architectural compositional differences between the drumlins and the flowset they form we can closely relate these differences to contextual geological variations with respect to basement type and drift depth we argue that drumlin formation is better explained not by one single unifying process but rather a set of processes erosion deformation deposition by which the subglacial system finds an efficiency equilibrium whereby an obstacle is shaped so that it enhances flow with a minimum of drag i e the typical streamlined form is the result of a positive feedback cycle that tends towards efficiency this approach argues that equifinality is playing a significant role in drumlin formation with a plurality of initial conditions and processes which when combined with a uniformity of specific overriding processes and mechanical properties results in a diversity of morphology and core type at the meso scale and a continuum of landforms at the macro scale

drumlin formation related to inverted melt water erosional marks drumlin forms are described from maps and air photographs of a part of the athabasca plains northern saskatchewan three major forms spindle parabolic and transverse asymmetrical are recognized these forms which may show superimposed minor elements depart from classical descriptions of drumlins but are similar to moulds of erosional marks created by separated fluid flows assemblages of drumlins also show characteristics similar to those of erosional marks the form analogy between drumlins and moulds of erosional marks is carried to a conclusion that drumlins may be formed by the infilling of erosional marks created on the under side of glaciers by separated subglacial melt water flows estimates of specific discharge are obtained by means of an expected range of reynolds number geomorphological evidence is given for large scale erosion by subglacial melt water a discussion of the sedimentology stratigraphy and deformational structure of the interiors of drumlins shows that they may be explained by the erosional mark hypothesis this paper emphasizes the importance of melt water as a geomorphic agent and may have broad implications for ice sheet dynamics and profiles rates of deglaciation and the occurrence of bedrock thrusting by ice

drumlins and catastrophic subglacial floods shaw j kvill d and rains b 1989 drumlins and catastrophic subglacial floods in j menzies and j rose editors subglacial bedforms drumlins rogen moraine and associated subglacial bedforms sediment geol 62 177 202 a number of lines of evidence are shown to be consistent with drumlin formation by catastrophic meltwater floods drumlins are associated with landforms related to such floods and landscapes produced by outburst floods in extragiacial areas contain drumlin like landforms some drumlins which contain mainly residual materials are compared to similar forms covering a wide range of scales and are interpreted to be scour remnant ridges produced by the action of horseshoe vortices on subgiacial materials these vortices are a product of flow separation in turbulent fluids and this interpretation implies drumlin formation by meltwater other drumlin calied cavity fill drumlins have distinctive shapes that are analogous with infilled erosional marks produced by turbulent separated flows these drumlins are interpreted as the infills of cavities eroded upwards into glacier beds by broad subgiacial meltwater sheetflows the sedimentology and internal structure of the cavity fill drumlins indicate rapid deposition from hyperconcentrated flow clast shape surface texture and lithology are consistent with the short distance of giaciofhivial transport predicted by the cavity fill hypothesis estimates of flow discharge 0 6 6 x 107 m3 s total volume of flow 84 1 103 km3 and flow duration 16 162 days are given for the catastrophic flood proposed to have produced the livingstone like saskatchewan drumlin field subgiacially derived floods are used to explain the sudden drainage of extraglacial lakes and major depositional events in proglacial lakes and it is suggested that they might have contributed to the lid of cold freshwater over the north atlantic held to have caused a late glacial cooling event the flood hypothesis requires extensive subgiacial lakes to have occurred beneath the laurentide and fennoscandian ice sheets which if they existed must have influenced the form and behaviour of these major bodies of ice

a glaciofluvial origin for drumlins of the livingstone lake area saskatchewan the livingstone lake drumlin field of northern saskatchewan is on the athabasca plains it is draped over high ground underlain by the precambrian athabasca sandstone the drumlins occur in spindle parabolic and transverse asymmetrical forms a map of individual drumlins shows that features of similar form occur together the drumlins are related on a regional scale to even larger landforms there is also an arrangement of drumlins in streams parallel to the assumed flow direction these patterns illustrate spatial organisation of the drumlin forming flows the livingstone lake drumlins are closely related to tunnel valleys large crescentic scours and eskers exposed bedrock reveals widespread scalloping and sichelwannen an argument is made that drumlins are related to these glaciofluvial forms but were created at different times and by different flows than those forming small scale flutings and other lodgment features sediments within the drumlins are mainly primary water sorted deposits some of wh

frozen bed fennoscandian and laurentide ice sheets during the last glacial maximum the areal extents of the laurentide and fennoscandian ice sheets during the last glacial maximum about 20 000 years ago are well known1 but thickness estimates range widely from high domed2 to thin3 with large implications for our reconstruction of the climate system regarding for example northern hemisphere atmospheric circulation and global sea levels this uncertainty stems from difficulties in determining the basal temperatures of the ice sheets and the shear strength of subglacial materials4 a knowledge of which would better constrain reconstructions of ice sheet thickness here we show that in the absence of direct data the occurrence of ribbed moraines in modern landscapes can be used to determine the former spatial distribution of frozen and thawed bed conditions we argue that ribbed moraines were formed by brittle fracture of subglacial sediments induced by the excessive stress at the boundary between frozen and thawed bed conditions resulting from the across boundary difference in basal ice velocity maps of glacial landforms from aerial photographs of canada and scandinavia reveal a concentration of ribbed moraines around the ice sheet retreat centres of quebec keewatin newfoundland and west central fennoscandia together with the evidence from relict landscapes that mark glacial areas with frozen bed conditions the distribution of ribbed moraines on both continents suggest that a large area of the laurentide and fennoscandian ice sheets was frozen based and therefore high domed and stable during the last glacial maximum

alleviation of a systematic westerly bias in general circulation and numerical weather prediction models through an orographic gravity wave drag parametrization systematic westerly biases in the northern hemisphere wintertime flow of the meteorological office 15 layer operational model and 11 layer general circulation model are described evidence that the failure to parametrize subgrid scale orographic gravity wave drag may account for such biases is presented this evidence is taken from aircraft studies surface pressure drag measurements and studies of the zonally averaged momentum budget a parametrization scheme is described in which the surface stress is proportional to the near surface wind speed and static stability and to the variance of subgrid scale orography the stress is absorbed in the vertical by considering the influence of such gravity wave activity on static stability and vertical wind shear a richardson number dependent wave breaking formulation is devised and the vertical stress profile determined by a saturation hypothesis whereby the breaking waves are maintained at marginal stability it is shown that wave breaking preferentially occurs in the boundary layer and in the lower stratosphere results from a simple zonally symmetric model show how the adjustment to thermal wind balance with a wave drag in the stratosphere warms polar regions by adiabatic descent and decelerates the mean westerlies in the troposphere the influence of the parametrization scheme on integrations of the 11 layer model is described and found to be generally beneficial in a discussion of the reasons why this problem has only recently emerged it is suggested that the satisfactory northern hemisphere winter circulations of previous coarser general circulation models were due to a compensation implied by underestimating both the surface drag and the horizontal flux of momentum by explicitly resolved large scale eddies

impact of the doppler spread parameterization on the simulation of the middle atmosphere circulation using the ma echam4 general circulation model the zonal mean circulation of the middle atmosphere simulated by a general circulation model that includes the doppler spread parameterization of gravity wave momentum deposition is presented to illustrate the impact of the parameterization this simulation is compared to another obtained by using rayleigh friction instead it is found that the doppler spread parameterization improves the extratropical zonal mean circulation in the mesosphere by reducing the strength of the zonal winds above the jet maximum the parameterization also improves the simulation of the equatorial semiannual oscillation in the zonal mean zonal wind at the stratopause by enhancing the eastward phase and its downward propagation

glacial curvilineations new glacial landforms produced by longitudinal vortices in subglacial meltwater flows abstract the glacial landscape of the dobrzyn plateau in central poland contains a complex suite of enigmatic bedforms consisting of elongate and sinuous sediment ridges occurring within an anabranched network of tunnel channels terminating at a former ice margin marked by extensive glaciofluvial fans the term glacial curvilineations is proposed to describe these enigmatic bedforms that occur as fields of parallel sinuous ridges separated by troughs the regional pattern of glacial curvilineations replicates the morphology and pattern of tunnel channel margins and suggests a common genesis tunnel channels and glacial curvilineations are eroded in the interbedded diamictons and glaciofluvial sediments of the plateau we propose that tunnel channels and glacial curvilineations are the products of subglacial meltwater erosion curvilineations are erosional remnants produced by longitudinal vortices within the tunnel channel forming flows parallelism of curvilineations reflects spacing of turbulent structures within the subglacial flow tunnel channel margins also control the sinuosity of curvilineations by dictating both the path and curvature of longitudinal vortices arguments for meltwater erosion are strengthened by the presence of extensive glaciofluvial fans at tunnel channel termini they are the depositional counterparts to the extensive subglacial fluvial dissection of the plateau understanding curvilineation genesis may shed light on glacier hydrologic processes tunnel channels develop abruptly in the plateau surface suggesting localized point sources of meltwater such as drainage of subglacial and or supraglacial lakes the extent of tunnel channels and lateral continuity of glacial curvilineations suggests broad sheet like flows as wide as a few kilometres

subglacial bedforms of the zemgale ice lobe south eastern baltic abstract presented in this paper are the results of the mapping of 6600 subglacial bedforms of the zemgale ice lobe zil their composition and internal structure in the south eastern baltic region topographic maps at scale 1 10 000 were mainly used to map bedforms in latvia while a digital elevation model with cell size of 5 m was used for north central lithuania the zil operated during the deglaciation of the late weichselian scandinavian ice sheet sis at least in the middle lithuanian and north lithuanian glacial phases and created a subglacial landform assemblage consisting of glacial lineations such as drumlins mega flutings mega scale glacial lineations msgls and transversal morphologies such as ribbed moraines it is observed that lineations of different scales co exist suggesting subglacial bedform transition and possibly a continuum of bedforms highly elongated lineations with a length of up to 24 km and an elongation ratio of up to 1 50 interpreted as msgls are observed in the ne part of the middle lithuanian plain in the so called zemgale drumlin field 20 of lineations have elongation ratios 10 1 indicating the fastest ice flow in the central part of the main body of the lobe based on test surveys and investigated outcrops the drumlin cores consist of sorted sediments with different levels and depths of glaciotectonic deformation overlain by subglacial till the main drumlin forming ice stress was mostly parallel to the crests of drumlins while some ice stress from inter drumlin depressions acted during the final episodes of their formation the observed sediment structures indicate multiple episodes of the basal ice bed coupling and decoupling which can best be explained by the mosaic ice bed deformation model ribbed moraines are characterized by a complicated structure of multiple till units interbedded with sorted sediments that have been formed by repeated subglacial overthrusting

geometry and dynamics of an east antarctic ice sheet outlet glacier under past and present climates 1 the behavior of east antarctic ice sheet eais outlet glaciers both at present and during the recent geological past is of considerable interest both from the point of view of understanding the mechanics of contemporary glacier dynamics and also with regard to epoch scale ice sheet stability during plio pleistocene climate transitions here we use a glacier flowline model that incorporates the effects of longitudinal stresses to numerically simulate ferrar glacier first under present day environmental conditions and subsequently under both colder and warmer climate regimes representing the last glacial maximum lgm and mid pliocene peak warmth respectively using airborne radar profiles insar derived surface velocities ice core and geological data for empirical constraint we present a diagnostic simulation that uses an iterative method to closely reproduce observed dynamics our model suggests that the glacier is largely cold based under present conditions flows predominantly by way of internal deformation and cascades over bedrock ridges due to the combined action of changes in cross sectional valley geometry local steepening of the glacier surface and the non local effects of longitudinal coupling time dependent evolutionary simulation of a lower profile glacier under a colder drier lgm climate predicts flow velocities lower than present with minimal bedrock erosion or basal till flux conversely the warmer than present mid pliocene climate produces a more dynamic glacier that is warm based and sliding along much of its bed we propose that eais outlet glaciers such as the ferrar respond dynamically along their length in response to changing environmental forcings with most significant changes taking place in their lower reaches adjustment to perturbations in upper catchments is more muted

ribbed bedforms on palaeo ice stream beds resemble regular patterns of basal shear stress traction ribs inferred from modern ice streams rapidly flowing ice streams are an important mechanism through which ice sheets lose mass and much work has been focussed on elucidating the processes that increase or decrease their velocity recent work using standard inverse methods has inferred previously unrecognised regular patterns of high basal shear stress sticky spots 200 kpa beneath a number of ice streams in antarctica and greenland termed traction ribs they appear at a scale intermediate between smaller ribbed moraines and much larger mega ribs observed on palaeo ice sheet beds but it is unclear whether they have a topographic expression at the bed here we report observations of rib like bedforms from dems along palaeo ice stream beds in western canada that resemble both the pattern and dimensions of traction ribs their identification suggests that traction ribs may have a topographic expression that lies between and partly overlaps with ribbed moraines and much larger mega ribs these intermediate sized bedforms support the notion of a ribbed bedform continuum their formation remains conjectural but our observations from palaeo ice streams coupled with those from modern ice masses suggest they are related to wave like instabilities occurring in the coupled flow of ice and till and modulated by subglacial meltwater drainage their form and pattern may also involve glaciotectonism of subglacial sediments

simulation of the semi annual oscillation of the equatorial middle atmosphere using the extended ugamp general circulation model the semi annual oscillation sao is examined in two runs of a middle atmosphere general circulation model the first run pgw includes a parametrization of gravity waves with zero phase speed only whilst the second run agw also includes gravity waves with phase speeds of 20 m s 1 run pgw successfully simulates equatorial saos near the stratopause and in the mid to upper mesosphere however the zonal winds at all heights are easterly and the amplitude of the oscillation is too weak by up to 15 m s 1 near the stratopause and too strong by about 45 m s 1 in the upper mesosphere the corresponding saos in run agw however have amplitudes that are within 5 m s 1 of those observed by rocketsondes and have a pattern of easterly and westerly winds that are in reasonable agreement with the observations zonal mean momentum budget studies show that both at the stratopause and in the mid to upper mesosphere the westerly phase of the oscillation in run agw is stronger than in run pgw owing to an increase in total eddy forcing associated with the introduction of the 20 m s 1 phase speed gravity wave the easterly phase of the stratopause oscillation on the summer side of the equator is driven by advection of easterlies across the equator however on the winter side of the equator the net zonal forcing is westerly in may but easterly in november since easterly eddy forcing associated with extratropical planetary waves is stronger in november than in may the easterly phase of the mid to upper mesosphere oscillation is forced by contributions from both the residual circulation and eddy forcing the eddy forcing contribution is more easterly than in other modelling studies owing to weaker kelvin wave activity and to easterly forcing associated with model diurnal tides introduction of the 20 m s 1 gravity waves also improves the simulation of an n2o like tracer in run agw a double peak of tracer is present in april in reasonable agreement with observations but this feature is absent in run pgw

observations of ch4 and n2o by the nimbus 7 sams a comparison with in situ data and two dimensional numerical model calculations monthly mean zonal mean measurements of ch4 and n2o for 1979 made by the stratospheric and mesospheric sounder sams on the nimbus 7 satellite are presented for the first time comparison with in situ and other available data confirms a general pattern of mixing ratios decreasing with height and of maxima at a given pressure level in low latitudes the sams data by virtue of its extensive coverage reveals new features these include during certain months a double peak when along a constant pressure surface mixing ratio maxima are found in low latitudes of both hemispheres with a local minimum at the equator in the upper stratosphere are found regions of weak and strong horizontal gradients comparison with a two dimensional model shows many areas of agreement especially for ch4 and the model is used to interpret atmospheric behavior the model overestimates the observed n2o in the upper stratosphere this is investigated in terms of a possible underestimation of the photochemical sink we believe strongly however that uncertainties in transport representation cannot be ruled out

the stability of a viscous till sheet coupled with ice flow considered at wavelengths less than the ice thickness a turbation method is used to analyse the stability of a thin till layer overlain by a deep ice layer ice is modelled as a linearly viscous fluid while the till viscosity has power law dependence on stress and effective pressure a linearized set of equations yields descriptions of the coupling of the ice flow with the sediment flow and reveals parameter ranges where the till perturbation amplitude can grow this shect flow instability is an essential part of any theory of drumlin formation and shows that viscous models of till have the ability to explain typical deforming bed features this is of great significance for large scale ice shect modelling

evidence for equatorial kelvin modes in nimbus 7 lims abstract eastward propagating disturbances over the equator are diagnosed in two independent nimbus 7 lims limb infrared monitor of the stratosphere data sets they are evident consistently at several pressure levels throughout the stratosphere and account for much of the temperature variance in the tropics the disturbances which can be seen in wavenumbers 1 3 are in phase and symmetric about the equator latitudinally evanescent and have short moderate vertical phase structure 10 40 km which progresses downward wavenumber 1 has spectral components which propagate eastward at periods of 6 7 8 6 days 54 69 m s 1 and 3 5 4 0 days 115 135 m s 1 wavenumber 2 exhibits eastward propagating variance at periods of 6 0 7 5 days 31 39 m s 1 and 3 8 4 3 days 55 62 m s 1 the faster waves appear principally in the upper stratosphere these features are in reasonable agreement with the structure and dispersion characteristics of simple quasi separable kelvin modes with the exception of the slower w

theory of glacial erosion transport and deposition as a consequence of subglacial sediment deformation a theory of erosion transport and deposition of unlithified sediments by glaciers is presented it predicts the large scale areal distribution of zones and rates of erosion and deposition in time and space through a complete glacial cycle together with the resultant intensity of large scale lincations drumlins which will be incised in the landscape the theory also predicts the dispersal patterns of subglacial lithologies together with the form of dispersal trains derived from distinctive sources and the vertical and horizontal distribution of lithologies within a till it predicts major erosional discontinuities within tills and the formation of boulder pavements it suggests that the dominant proportion of the lowland tills produced by pleistocene mid latitude ice sheets was generated by subglacial deformation and explains why they are predominantly fine grained the theory is based on an analysis of glacier dynamic processes and therefore can be used to infer the dynamic behaviour of former ice sheets from the distribution of tills and their lithologic composition

testing techniques to quantify drumlin height and volume synthetic dems as a diagnostic tool glacial bedform height h and volume v likely preserve important information about the behaviour of former ice sheets however large systematic errors exist in the measurement of h and v three semi automated methods to isolate drumlins from other components of the landscape e g trees hills as portrayed by nextmap have recently been devised however it is unclear which is most accurate this paper undertakes the first quantitative comparison of such readily implementable methods illustrating the use of statistically representative synthetic landscapes as a diagnostic tool from this analysis guidelines for quantifying the 3d attributes of drumlins are proposed specifically to avoid obtaining incorrect estimates caused by substantial systematic biases interpreters should currently take three steps declutter the digital elevation model for estimating h but not v remove height data within the drumlin then interpolate across the resultant hole to estimate a basal surface using delaunay triangulation results are demonstrated through analysis of drumlins in an area in western central scotland the guidance arguably represents the best current advice for subglacial bedforms in general highlighting the need for more studies into the quality of mapped data using synthetic landscapes copyright 2014 john wiley sons ltd

drumlinization and drumlin forming instabilities viscous till mechanisms glacially induced flow naturally tends to thin and extended till cover through shock formation even in the absence of longitudinal gradients in the applied stress thicker till cover has an increased effective pressure at its surface and base a lower sliding velocity or deformation rate and above a critical thickness a decrease in wave velocity with thickness leading to reverse facing shocks moving downstream for sliding and for some theologies of internal deformation a decrease in sediment flux with thickness occurs implying backward moving kinematic waves and reverse facing reverse moving shock downstream facing shocks are also formed which move upstream if the till is sliding and downstream if the till is deforming internally eventually shocks coalesce leaving an upstream lacing shock for sliding and a downstream facing shock for internal deformation it is observed that some drumlins have downstream blunt ends only fairly realistic three dimensional drumlin shapes can be produced from symmetric sediment bodies and barchan shapes can be produced from linear forms perpendicular to the ice sheet flow the fact that viscous theories produce drumlinoid forms suggests that on this scale till behaves viscously and the the lower length scale for drumlins represents the plastic viscous transition scale

evolution of the zonal mean state in the equatorial middle atmosphere during october 1978 may 1979 abstract the evolution of the zonal mean state in the equatorial middle atmosphere is investigated with the use of daily mapped temperatures derived from the limb infrared monitor of the stratosphere lims experiment these quasi global high vertical resolution data cover the pressure range 100 05 mb and the period 25 october 1978 28 may 1979 the equatorial semiannual oscillation sao in zonal mean temperature derived zonal wind and meridional shear of the zonal wind is described in detail rocket profiles are used to validate features seen in lims data these include ranges in temperature and zonal wind of 20 k and 100 m s 1 and cross equatorial shears of at 3 day 1 consistent with the theory that the wave mean flow interaction is essential to the sao flow acclerations over the equator exhibit strong week to week variations while easterly accelerations are moderate and occur in deep cool layers westerly accelerations are generally stronger and occur in shallow warm layers which descend with time

are drumlins a product of a thermo mechanical instability abstract of numerous theories of drumlin genesis none has been widely accepted it seems evident however that some form of positive feedback process is involved under certain circumstances perturbations are amplified herein we suggest that patchy areas of frozen bed provide the initial perturbation such frozen patches may occur in local areas underlain by material of lower thermal conductivity or on slight topographic highs drag exerted by the frozen patch deflects ice flow into its lee dragging with it mobile till eroded from the thawed area the energy balance is such that this till likely refreezes either producing a topographic perturbation or amplifying an existing one the resulting topography then deflects more of the geothermal heat away from the developing hill and into the adjacent trough resulting in a positive feedback once the thermal perturbation exceeds a critical though as yet undefined level melting may decouple the ice from the bed preventing further entrainment of till from thawed areas and thus limiting the height and length of the drumlin

formation of mega scale glacial lineations on the dubawnt lake ice stream bed 1 size shape and spacing from a large remote sensing dataset abstract mega scale glacial lineations msgls are the largest flow parallel bedforms produced by ice sheets and are formed beneath rapidly flowing ice streams knowledge of their characteristics and genesis is likely to result in an improved understanding of the rate at which ice and sediment are discharged by ice sheets but there is little consensus as to how they are formed and there are few quantitative datasets of their characteristics with which to formulate or test hypotheses this paper presents the results of a remote sensing survey of 46 000 bedforms on the dubawnt lake palaeo ice stream bed focussing on a central transect of 17 038 that includes highly elongate bedforms previously described as msgls within this transect lineations exceed 10 km in length max 20 km and 23 have elongation ratios 10 1 max 149 1 highly elongate features are interspersed with much shorter drumlin like features but longer bedforms are typically narrower suggesting that their length develops more quickly than or at the expense of their width bedforms are broadly symmetrical in plan form and have a preferred lateral spacing of 50 250 m which implies a regular rather than random pattern of corrugations comparison with drumlins reveals that the more attenuated msgls simply extend the tail of the distribution of data rather than plotting as a separate population taken together this supports the idea of a subglacial bedform continuum primarily controlled by ice velocity but existing hypotheses of msgl formation are either not supported or are insufficiently developed to explain our observations rather we conclude that under conditions of rapid ice flow msgls attain their great length relatively quickly decades through a probable combination of subglacial deformation which attenuates ridges and erosional processes that removes material from between them

emergent drumlins and their clones from till dilatancy to flow instabilities subglacial bedforms are a range of landforms 10 105 m in length shaped mostly in glacial sediments and are abundant on ice sheet beds numerous explanations for their generation especially of drumlins have been proposed rather than viewing them as resulting from erosion or deposition directly by ice smalley and unwin 1968 proposed that both might occur simultaneously if beneath the glacier a layer of lubricating sediment existed and flowed conducting geomorphic work erosion deposition shaping of its own this idea appears to have steered the field leading perhaps to final resolution of the centuries old problem of drumlin genesis here i trace how the idea evolved leading to the conclusion that subglacial bedforms are emergent phenomena arising from self organisation in the coupled flow of ice sediment and water the ubiquity and patterning of bedforms appears to be well explained by a naturally arising flow instability in the subglacial system and which has been demonstrated by analytical and numerical modelling a problem for the instability theory is an apparent mismatch between predicted and observed sedimentological properties of bedforms a distinction is made between emergent drumlins drumlin clones and obstacle drumlins which helps explain some apparent contradictions and a conceptual framework is erected that might form a basis for confrontation of the theory with the wealth of observational evidence that exists

the semi annual oscillation and equatorial tracer distributions some interesting low latitude features of tracer distributions revealed by recent satellite experiments have been studied using a two dimensional atmospheric model it is found that the equatorial semi annual oscillation forced in the model by simply prescribing a suitable momentum convergence is important in determining the tracer behaviour in particular the meridional circulation associated with the semi annual oscillation leads to a successful model simulation of the double peak in n2o and ch4 mixing ratios observations of which show at certain times of the year maxima on a fixed pressure surface in the subtropics with a local minimum at the equator

subglacial till a physical framework for its properties and processes the continuum theory of mixtures provides a framework around which present knowledge of subglacial till can be organized the proposed framework is provisional in two senses the constitutive description of till will require improvement and not all potentially significant processes have been included till is treated as a three component mixture of water clasts coarse grained solids and fine grained matrix solids these components obey mass balance equations and their behavior is subject to constitutive relations that describe such processes as darcian water transport fine grained sediment transport consolidation shear deformation dilatancy and comminution five state variables uniquely determine the compressibility permeability shear strength and other physical properties numerical solutions of the state evolutions equations illustrate that the properties of till are highly responsive to changes in the subglacial environment

plastic bed beneath hofsjokull ice cap central iceland and the sensitivity of ice flow to surface meltwater flux author s minchew b simons m bjornsson h palsson f morlighem m seroussi h larour e hensley s abstract the author s 2016 the mechanical properties of glacier beds play a fundamental role in regulating the sensitivity of glaciers to environmental forcing across a wide range of timescales glaciers are commonly underlain by deformable till whose mechanical properties and influence on ice flow are not well understood but are critical for reliable projections of future glacier states using synoptic scale observations of glacier motion in different seasons to constrain numerical ice flow models we study the mechanics of the bed beneath hofsjokull a land terminating ice cap in central iceland our results indicate that the bed deforms plastically and weakens following incipient summertime surface melt combining the inferred basal shear traction fields with a coulomb plastic bed model we estimate the spatially distributed effective basal water pressure and show that changes in basal water pressure and glacier accelerations are non local and non linear these results motivate an idealized physical model relating mean basal water pressure and basal slip rate wherein the sensitivity of glacier flow to changes in basal water pressure is inversely related to the ice surface slope

general circulation model simulation of the semiannual oscillation of the tropical middle atmosphere abstract a study has been made of the evolution of the zonal mean zonal wind and temperature in a multiyear integration of the 40 level 3 3 6 resolution skyhi general circulation model gcm that has been developed at gfdl in the tropical upper stratosphere the mean wind variation is dominated by a strong semiannual oscillation sao the peak sao amplitude in the model is almost 25 m s 1 and occurs near the 1 mb level the phase of the sao near the stratopause is such that maximum westerlies occur shortly after the equinoxes these features are in good agreement with the available observations in addition the meridional width of the stratopause sao in the gcm compares well with observations a diagnostic analysis of the zonal mean momentum balance near the tropical stratopause was performed using the detailed fields archived during the gcm integration it appears that the easterly accelerations in the model sao are provided by a combination of i divergence of the meridional component of the el