permeability anisotropy of natural clays as a function of strain seven series of tests have been performed on specimens from five natural soft clay sites to determine the variation of the vertical and horizontal permeabilities when the soil is unidimensionally compressed the results show that the permeability anisotropy does not increase significantly with compression up to strains of 25 for marine clays in varved clays mostly because the clay laminae are more compressible than the silty laminae the anisotropy increases with vertical compression key words permeability clay anisotropy laboratory compression

hydraulic conductivity of naturally sedimented and reconstituted clays interpreted from consolidation tests abstract the hydraulic conductivity of naturally sedimented and reconstituted clays is often interpreted using the settlement time curves measured from consolidation tests understanding the accuracy of the interpreted hydraulic conductivity is essential in geological and geotechnical engineering to the best knowledge of the authors such a work has been little reported up to now this study aims at investigating the errors of the interpreted hydraulic conductivity via the comparison between the interpreted values and the measurements the ratios of the interpreted values of hydraulic conductivity over the measurements are examined based on the data from the consolidation hydraulic conductivity tests performed in this study in conjunction with those published by our research team the investigated clays cover wide geological origins and depositional states the geological origins and depositional states are linked with the errors of the interpreted hydraulic conductivity for understanding the roles of these factors in the uncertainty of the interpreted hydraulic conductivity the causes responsible for the discrepancy in hydraulic conductivity between the interpretations and the measurements are identified the probability distribution of the errors of the interpreted hydraulic conductivity is also investigated

potential sea level rise from antarctic ice sheet instability constrained by observations large parts of the antarctic ice sheet lying on bedrock below sea level may be vulnerable to marine ice sheet instability misi a self sustaining retreat of the grounding line triggered by oceanic or atmospheric changes there is growing evidence that misi may be underway throughout the amundsen sea embayment ase which contains ice equivalent to more than a metre of global sea level rise if triggered in other regions the centennial to millennial contribution could be several metres physically plausible projections are challenging numerical models with sufficient spatial resolution to simulate grounding line processes have been too computationally expensive to generate large ensembles for uncertainty assessment and lower resolution model projections rely on parameterizations that are only loosely constrained by present day changes here we project that the antarctic ice sheet will contribute up to 30 cm sea level equivalent by 2100 and 72 cm by 2200 95 quantiles where the ase dominates our process based statistical approach gives skewed and complex probability distributions single mode 10 cm at 2100 two modes 49 cm and 6 cm at 2200 the dependence of sliding on basal friction is a key unknown nonlinear relationships favour higher contributions results are conditional on assessments of misi risk on the basis of projected triggers under the climate scenario a1b ref 9 although sensitivity to these is limited by theoretical and topographical constraints on the rate and extent of ice loss we find that contributions are restricted by a combination of these constraints calibration with success in simulating observed ase losses and low assessed risk in some basins our assessment suggests that upper bound estimates from low resolution models and physical arguments up to a metre by 2100 and around one and a half by 2200 are implausible under current understanding of physical mechanisms and potential triggers

dewatering induced subsidence during excavation in a shanghai soft deposit foundation dewatering has become a major cause of land subsidence in shanghai the burial depth of foundations in relation to geotechnical construction works is less than 75 m and the corresponding groundwater includes phreatic low pressure artesian and the first confined aquifers based on the geological and hydrogeological conditions beneath shanghai methods of dewatering may be divided into three modes and further five patterns according to the insertion depth of the dewatering retaining system the most common dewatering mode aims to reduce the water pressure in the confined aquifer by setting the dewatering wells inside the pit whilst the retaining walls are buried in the confined aquifer and partially cut off the confined aquifer layer to predict the settlement due to foundation dewatering numerical models are generally adopted which are similar to those used to predict land subsidence induced by regional groundwater withdrawal however since foundation dewatering is conducted along with the setting of retaining walls and foundation pit excavation which differs from regional groundwater withdrawal interactions between the retaining wall dewatering well the dewatering excavation and dewatering recharge are important factors affecting the analytical model since the grading of the shallow soil layers is different stratified settlement characteristics of the shallow soil strata and seepage erosion which results in additional deformation need to be given particular consideration

shear resistance and continuity of subglacial till hydrology rules the field observations of g s boulton stimulated widespread interest in deformable beds shear resistance of till in its critical state is insensitive to strain rate and increases linearly with effective pressure during unsteady deformation pseudo viscous shear resistance can be caused by dilation of consolidated tills and resultant pore pressure decline this effect is probably uncommon however because susceptible tills of low hydraulic diffusivity are also those least likely to consolidate significantly during effective pressure transients stick slip motion at whillans ice stream antarctica indicates that its basal till must weaken during rapid slip and strengthen during longer periods of slower slip recurrence intervals for rapid slip episodes there 6 18 hours indicate that till strength variations if driven by changes in pore pressure either related or unrelated to basal freezing are focused in the uppermost several centimeters of the bed ploughing of grains at the bed surface and associated excess pore pressures in adjacent till can account for rate weakening during rapid slip with pore pressure decay causing strengthening between slip episodes by promoting shallow sluggish subglacial water flow and low effective pressure soft beds may help sustain themselves by slowing their own transport soft bed shear resistance kinematics and continuity are problems rooted in subglacial hydrology

instability of the thermohaline circulation with respect to mixed boundary conditions is it really a problem for realistic models abstract this comment discusses two issues raised by a recent study of the stability of the thermohaline circulation tziperman et al a numerical problem is pointed out that may have affected the results of the reported calculations in addition it is argued that the state transition of the model circulation found in the above study is triggered by a convective instability mechanism not the advective mechanism discussed by its authors should this be the case then the conclusion of tziperman et al that the ocean is close to a stability transition point must be questioned

elastic viscoplastic consolidation modelling and interpretation of pore water pressure responses in clay underneath tarsiut island it has been reported that the excess pore water pressure in clay underneath tarsiut caisson retained island increased for many months following completion of construction with vertical load unchanged it was thought that this increase might be caused by the creep behaviour of the soft clay however the phenomenon of excess pore water pressure increase under constant loading has not been simulated successfully at the tarsiut island site in this paper a newly developed elastic viscoplastic evp model is implemented in a finite element fe program for consolidation analysis this fe program is used to model the consolidation behaviour of the clay under tarsiut island loading the phenomenon of pore water pressure increase with time following completion of construction of the island has been successfully simulated using the fe program with the new evp model the mechanism of the pore water pressure increase is explained it is found that the creep compression nature of the clay is the main internal fac

a laboratory study of particle ploughing and pore pressure feedback a velocity weakening mechanism for soft glacier beds if basal water discharge and pressure are sufficiently high a soft bedded glacier will slip over its bed by ploughing the process in which particles that span the ice bed interface are dragged across the bed surface results of laboratory experiments indicate that resistance to ploughing can decrease with increasing ploughing velocity velocity weakening during ploughing at various velocities 15 400 m a 1 till was compacted in front of idealized particles causing pore pressures there that were orders of magnitude higher than the ambient value this excess pore pressure locally weakened the till in shear thereby decreasing ploughing resistance by a factor of 3 0 6 6 with a six fold increase in ploughing velocity characteristic timescales of pore pressure diffusion and compaction down glacier from ploughing particles depend on till diffusivity ploughing velocity and sizes of ploughing particles these timescales accurately predict the ranges of these variables over which excess pore pressure and velocity weakening occurred existing ploughing models do not account for velocity weakening a new ploughing model with no adjustable parameters predicts ploughing resistance to no worse than 38 but requires that excess pore pressures be measured velocity weakening by this mechanism may affect fast glacier flow sediment transport by bed deformation and basal seismicity

observations inferences and mechanisms of the atlantic meridional overturning circulation a review this is a review about the atlantic meridional overturning circulation amoc its mean structure temporal variability controlling mechanisms and role in the coupled climate system the amoc plays a central role in climate through its heat and freshwater transports northward ocean heat transport achieved by the amoc is responsible for the relative warmth of the northern hemisphere compared to the southern hemisphere and is thought to play a role in setting the mean position of the intertropical convergence zone north of the equator the amoc is a key means by which heat anomalies are sequestered into the ocean s interior and thus modulates the trajectory of climate change fluctuations in the amoc have been linked to low frequency variability of atlantic sea surface temperatures with a host of implications for climate variability over surrounding landmasses on intra annual timescales variability in amoc is large and primarily reflects the response to local wind forcing meridional coherence of anomalies is limited to that of the wind field on interannual to decadal timescales amoc changes are primarily geostrophic and related to buoyancy anomalies on the western boundary a pacemaker region for decadal amoc changes is located in a western transition zone along the boundary between the subtropical and subpolar gyres decadal amoc anomalies are communicated meridionally from this region amoc observations as well as the expanded ocean observational network provided by the argo array and satellite altimetry are inspiring efforts to develop decadal predictability systems using coupled atmosphere ocean models initialized by ocean data

predicting atlantic meridional overturning circulation amoc variations using subsurface and surface fingerprints abstract recent studies have suggested that the leading modes of north atlantic subsurface temperature tsub and sea surface height ssh anomalies are induced by atlantic meridional overturning circulation amoc variations and can be used as fingerprints of amoc variability based on these fingerprints of the amoc in the gfdl cm2 1 coupled climate model a linear statistical predictive model of observed fingerprints of amoc variability is developed in this study the statistical model predicts a weakening of amoc strength in a few years after its peak around 2005 here we show that in the gfdl coupled climate model assimilated with observed subsurface temperature data including recent argo network data 2003 2008 the leading mode of the north atlantic tsub anomalies is similar to that found with the objectively analyzed tsub data and highly correlated with the leading mode of altimetry ssh anomalies for the period 1993 2008 a statistical auto regressive ar model is fit to the time series of the leading mode of objectively analyzed detrended north atlantic tsub anomalies 1955 2003 and is applied to assimilated tsub and altimetry ssh anomalies to make predictions a similar statistical ar model fit to the time series of the leading mode of modeled tsub anomalies from the 1000 year gfdl cm2 1 control simulation is applied to predict modeled tsub ssh and amoc anomalies the two ar models show comparable skills in predicting observed tsub and modeled tsub ssh and amoc variations

does the atlantic multidecadal oscillation get its predictability from the atlantic meridional overturning circulation abstractthis paper investigates the predictive relation between the atlantic multidecadal oscillation amo and atlantic meridional overturning circulation across different climate models three overturning patterns that are significantly coupled to the amo on interannual time scales across all climate models are identified using a statistical optimization technique including these structures in an autoregressive model extends amo predictability by 2 9 years relative to an autoregressive model without these structures

historical variability of sea ice edge position in the nordic seas 1 historical ice observations in the nordic seas from april through august are used to construct time series of ice edge position anomalies spanning the period 1750 2002 while analysis showed that interannual variability remained almost constant throughout this period evidence was found of oscillations in ice cover with periods of about 60 to 80 years and 20 to 30 years superimposed on a continuous negative trend the lower frequency oscillations are more prominent in the greenland sea while higher frequency oscillations are dominant in the barents the analysis suggests that the recent well documented retreat of ice cover can partly be attributed to a manifestation of the positive phase of the 60 80 year variability associated with the warming of the subpolar north atlantic and the arctic the continuous retreat of ice edge position observed since the second half of the 19th century may be a recovery after significant cooling in the study area that occurred as early as the second half of the 18th century

two decades of the atlantic meridional overturning circulation anatomy variations extremes prediction and overcoming its limitations the zonally integratedmeridional volumetransport in the north atlantic atlantic meridionaloverturning circulation amoc is described in a 19 yr long ocean state estimate one consistent with a diverse global dataset apart from a weak increasing trend at high northern latitudes the amoc appears statistically stable over the last 19yr with fluctuations indistinguishable from those of a stationary gaussian stochastic process this characterization makes it possible to study using highly developed tools extreme values predictability and the statistical significance ofapparenttrends gaussianbehaviorisconsistentwith the central limittheorem for a process arising from numerous independent disturbances in this case generators include internal instabilities changes in wind and buoyancy forcing fields boundary waves the gulf stream and deep western boundary current transports the interior fraction in sverdrup balance and all similar phenomena arriving as summation effects from long distances and times as a zonal integral through the sum of the large variety of physical processes in the three dimensional ocean circulation understanding of the amoc if it is of central climate importance requires breaking it down into its unintegrated components over the entire basin

sensitivity of the eddy momentum flux to meridional resolution in atmospheric gcms abstract gcm experiments with zonally symmetric climates are used to demonstrate that the increase in the meridional eddy momentum fluxes and zonal surface winds that occurs when resolution is increased is primarily due to the increase in meridional rather than zonal resolution it is argued that the sensitivity to meridional resolution reflects the need to resolve the small scales generated in the rossby wave field as waves radiate from the midlatitude baroclinic eddy source region into regions with small mean winds some additional experiments highlight the sensitivity of surface winds and eddy momentum fluxes to the subgrid scale horizontal mixing formulation in low revolution models

increased effective moisture in northern vietnam during the little ice age abstract we present a 650 yr record of hydroclimate inferred from 18o values of lacustrine carbonate from ao tien fairy pond in northern vietnam ao tien occupies a collapsed karst basin that formed during the worst drought of the last millennium i e the angkor i drought which marks a dry transitional period between the medieval climate anomaly and the little ice age a multi centennial trend of increasing effective moisture spans the little ice age ending with aridity in the late 18th century that accompanied great societal turmoil across southeast asia after 1850 ce the ao tien record exhibits a drying trend that continues to the present over the last 650 years the 18o inferred hydroclimate is significantly correlated with the reconstructed southern oscillation index of precipitation soipr suggesting that a prolonged la nina like state existed during the little ice age generating overall wetter conditions in vietnam decadal scale droughts and pluvials from the ao tien record agree with a robust tree ring hydroclimate reconstruction from southern vietnam that identified the angkor i and ii droughts of the late 13th early 14th centuries and significant droughts during the early and late 17th century and latter half of the 18th century the decrease in effective moisture during the last 30 years is also evident in the tree rings and is consistent with a documented weakening of the pacific walker circulation major pluvials are indicated for the period between the two angkor droughts and the wettest period of the record is during the early 18th century comparison of the ao tien data with records from indonesia to china indicates regionally coherent climate during the little ice age however the overall wet conditions in easterly sites are difficult to ascribe to shifts in the itcz rather the pattern suggests that pacific walker circulation may have been the overriding control on precipitation

suction effects in deep boom clay block samples extensive investigations have been and are being carried out on a stiff clay from an underground research laboratory located at mol belgium called boom clay in the context of research into deep nuclear waste disposal suction effects in deep boom clay block samples were investigated through the characterisation of the water retention and of the swelling properties of the clay the data obtained allowed an estimation of the sample initial suction that was reasonably compatible with the in situ state of stress at a depth of 223 m the relationship between suction and stress changes during loading and unloading sequences were also examined by running oedometer tests with suction measurements a rather wide range of the ratio s v being s the suction and v the effective vertical stress was obtained 0 61 1 different from that proposed by bishop et al 1974 finally the effect of suction release under an isotropic stress close to the estimated sample suction was investigated a slight swelling 1 7 was observed and further compression provided a satisfactory value of the overconsolidation ratio confirming the suggestions of taking some precautions before putting a swelling sample in contact with water as suggested by graham et al 1987 the various experimental data gathered in this study finally evidenced a relatively good state of conservation of the block sample used

interdecadal variations in north atlantic sea surface temperature and associated atmospheric conditions abstract evidence is presented for a distinct pattern of ocean atmosphere relationship associated with interdecadal variability in the north atlantic region using a century of surface marine observations it is shown that middle and high latitude sea surface temperature sst display a long term fluctuation with negative anomalies before 1920 and during the 1970s and 1980s positive sst conditions prevailed from about 1930 to 1960 the pattern of interdecadal sst variability is constructed by subtracting the average field during 15 cold years from that during a similar interval of warm years the early century warming and the more recent cooling display a similar spatial pattern in both cases the pattern is basin scale and largely of one polarity with maxima in the vicinity of iceland in the labrador sea and northeast of bermuda the corresponding differences in surface atmospheric conditions are determined by averaging and subtracting in the same manner fields of sea level pressure slp and surf

coupling between a glacier and a soft bed ii model results the relation between the local effective pressure and shear stress on till beneath storglaciaren sweden discussed in iverson and others 1999 provides an empirical basis for studying the processes that control the strength of the ice bed coupling particles in the bed that protrude into the glacier sole support shear stresses that are limited by either ploughing or the traditional sliding mechanisms model calculations based on studies of cone penetration through fine grained sediment and sliding theory agree with the observed relation between shear stress and effective pressure if the water layer at the ice bed interface is assumed to thicken rapidly as the effective pressure approaches zero studies of the hydraulics of linked cavities provide support for this assumption if the mean thickness of the water layer reflects the extent of microcavity development at the interface comparison of the calculated shear stress with the ultimate strength of till suggests that bed deformation limits the shear stress on till beneath storglaciaren only at intermediate effective pressures at very low effective pressures like those inferred at the site of the tiltmeter discussed in iverson and others 1999 and at sufficiently high effective pressures ploughing and sliding should focus motion near the glacier sole a calculation using parameter values appropriate for ice stream b west antarctica suggests that ploughing may occur there at shear stresses not sufficient to deform the bed at depth this conclusion is reinforced by the likelihood that pore pressures in excess of hydrostatic should develop down glacier from ploughing particles thereby weakening the bed near the glacier sole however given the apparent sensitivity of the ice bed coupling to basal conditions that may be highly variable any blanket assumption regarding the flow mechanism of ice masses on soft beds should probably be viewed with skepticism

simulation of abrupt climate change induced by freshwater input to the north atlantic ocean temperature records from greenland ice cores1 2 suggest that large and abrupt changes of north atlantic climate occurred frequently during both glacial and postglacial periods one example is the younger dryas cold event broecker3 speculated that these changes result from rapid changes in the thermohaline circulation of the atlantic ocean which were caused by the release of large amounts of melt water from continental ice sheets here we describe an attempt to explore this intriguing phenomenon using a coupled ocean atmosphere model in response to a massive surface flux of fresh water to the northern north atlantic of the model the thermohaline circulation weakens abruptly intensifies and weakens again followed by a gradual recovery generating episodes that resemble the abrupt changes of the ocean atmosphere system recorded in ice and deep sea cores4 the associated change of surface air temperature is particularly large in the northern north atlantic ocean and its neighbourhood but is relatively small in the rest of the world

a numerical model study of the effects of interannual time scale wave propagation on the predictability of the atlantic meridional overturning circulation we investigate processes leading to uncertainty in forecasts of the atlantic meridional overturning circulation amoc a climate model is used to supply initial conditions for ensemble simulations in which members initially have identical ocean states but perturbed atmosphere states baroclinic transports diverge on interannual timescales even though the ocean is not eddy permitting interannual fluctuations of the model amoc in the subtropical gyre are caused by westward propagating rossby waves divergence of the predicted amoc with time occurs because the waves develop different phases in different ensemble members predominantly due to differences in eastern boundary windstress curl these windstress fluctuations communicate with interior ocean transports via modifications to the vertical velocity and the vortex stretching term dw dz consequently errors propagate westwards resulting in longer predictability times in the interior ocean compared with the eastern boundary another source of divergence is transport anomalies propagating along the gulf stream and other boundary currents the propagation mechanism seems to be predominantly advection by mean currents and we show that the arrival of westward propagating waves can trigger development of these anomalies the mean state of the amoc has a small effect on interannual predictability in the subtropical gyre most likely because eastern boundary windstress curl predictability is not strongly dependent on the state of the amoc in the subtropics eastern boundary windstress curl was more predictable at 45 degree sign n when the amoc was in a strongly decreasing state but unlike at 30 degree sign n no mechanism was found linking windstress curl fluctuations with deep transports

contribution of alaskan glaciers to sea level rise derived from satellite imagery over the last 50 years retreating glaciers and ice caps gic contributed 0 5 mm yr to sea level rises slr and one third is believed to originate from ice masses bordering the gulf of alaska however these estimates of ice wastage in alaska are based on methods that measure a limited number of glaciers and extrapolate the results to estimate ice loss for the many thousands of others how these methods capture the complex pattern of decadal elevation changes at the scale of individual glacier and mountain range is unclear here combining a comprehensive glacier inventory with elevation changes derived from sequential digital elevation models dems we found that between 1962 and 2006 alaskan glaciers lost 41 9 8 6 km 3 yr water equivalent w e and contributed 0 12 0 02 mm yr to slr our ice loss is 34 lower than previous estimates reasons for our lower values include the higher spatial resolution of our glacier inventory and the reduction of ice thinning under debris and at the glacier margins which were not resolved in earlier work estimates of mass loss from gic in other mountain regions could be subject to similar revisions

marine ice sheet profiles and stability under coulomb basal conditions the behavior of marine terminating ice sheets such as the west antarctic ice sheet is of interest due to the possibility of rapid grounding line retreat and consequent catastrophic loss of ice critical to modeling this behavior is a choice of basal rheology where the most popular approach is to relate the ice sheet velocity to a power law function of basal stress recent experiments however suggest that near grounding line tills exhibit coulomb friction behavior here we address how coulomb conditions modify ice sheet profiles and stability criteria the basal rheology necessarily transitions to coulomb friction near the grounding line due to low effective stresses leading to changes in ice sheet properties within a narrow boundary layer ice sheet profiles taper off towards a flatter upper surface compared with the power law case and basal stresses vanish at the grounding line consistent with observations in the coulomb case the grounding line ice flux also depends more strongly on flotation ice thickness which implies that ice sheets are more sensitive to climate perturbations furthermore with coulomb friction the ice sheet grounds stably in shallower water than with a power law rheology this implies that smaller perturbations are required to push the grounding line into regions of negative bed slope where it would become unstable these results have important implications for ice sheet stability in a warming climate

marine ice sheet instability amplifies and skews uncertainty in projections of future sea level rise sea level rise may accelerate significantly if marine ice sheets become unstable if such instability occurs there would be considerable uncertainty in future sea level rise projections due to imperfectly modeled ice sheet processes and unpredictable climate variability in this study we use mathematical and computational approaches to identify the ice sheet processes that drive uncertainty in sea level projections using stochastic perturbation theory from statistical physics as a tool we show mathematically that the marine ice sheet instability greatly amplifies and skews uncertainty in sea level projections with worst case scenarios of rapid sea level rise being more likely than best case scenarios of slower sea level rise we also perform large ensemble simulations with a state of the art ice sheet model of thwaites glacier a marine terminating glacier in west antarctica that is thought to be unstable these ensemble simulations indicate that the uncertainty solely related to internal climate variability can be a large fraction of the total ice loss expected from thwaites glacier we conclude that internal climate variability alone can be responsible for significant uncertainty in projections of sea level rise and that large ensembles are a necessary tool for quantifying the upper bounds of this uncertainty

oceanic wave dynamics and interdecadal variability in a climate system model western boundary disturbances detected in the first multicentury integration of the national center for atmospheric research climate system model are analyzed and the role they play in the model interdecadal variability is investigated the boundary signals propagate southwestward along the coast of north america at depth they reach the equator and propagate along the equator but with decreasing amplitude away from the western boundary their southwestward propagation is associated with the evolution of the western boundary current system the gulf stream system in the upper part of the water column and the deep western boundary current at depth from weaker than average to stronger than average conditions thus the boundary disturbances seem to play an important role in the model thermohaline circulation variability at interdecadal timescales they show similarities with the boundary waves found in ocean only models exhibiting interdecadal variability and their role in the model s response to changes in the rate of sinking appears to be in agreement with the theory of the spin up of the deep ocean circulation the main result of this study is that the similarities with ocean only results and theories support the idea that interdecadal timescales of variability originate from the ocean dynamics

ice flow of the antarctic ice sheet we present a reference comprehensive high resolution digital mosaic of ice motion in antarctica assembled from multiple satellite interferometric synthetic aperture radar data acquired during the international polar year 2007 to 2009 the data reveal widespread patterned enhanced flow with tributary glaciers reaching hundreds to thousands of kilometers inland over the entire continent this view of ice sheet motion emphasizes the importance of basal slip dominated tributary flow over deformation dominated ice sheet flow redefines our understanding of ice sheet dynamics and has far reaching implications for the reconstruction and prediction of ice sheet evolution

global sst influence on twentieth century nao variability recent studies have suggested that sea surface temperature sst is an important source of variability of the north atlantic oscillation nao here we deal with four basic aspects contributing to this issue 1 we investigate the characteristic time scales of this oceanic influence 2 quantify the scale dependent hindcast potential of the nao during the twentieth century as derived from sst driven atmospheric general circulation model agcm ensembles 3 the relevant oceanic regions are identified corresponding sst indices are defined and their relationship to the nao are evaluated by means of cross spectral analysis and 4 our results are compared with long term coupled control experiments with different ocean models in order to ensure whether the spectral relationship between the sst regions and the nao is an intrinsic mode of the coupled climate system involving the deep ocean circulation rather than an artefact of the unilateral sst forcing the observed year to year nao fluctuations are barely influenced by the sst on the decadal time scales the major swings of the observed nao are well reproduced by various ensembles from the middle of the twentieth century onward including the negative state in the 1960s and part of the positive trend afterwards a six member echam4 t42 ensemble reveals that the sst boundary condition affects 25 of total decadal mean and interdecadal trend nao variability throughout the twentieth century the most coherent nao related sst feature is the well known north atlantic tripole additional contributions may arise from the southern pacific and the low latitude indian ocean the coupled climate model control runs suggest only the north atlantic sst nao relationship as being a true characteristic of the coupled climate system the coherence and phase spectra of observations and coupled simulations are in excellent agreement confirming the robustness of this decadal scale north atlantic air sea coupled mode

estimating the sliding velocity of a pleistocene ice sheet from plowing structures in the geologic record 1 as an ice sheet slides over its sediment bed some clasts partly embedded in the glacier sole plow through the bed surface the size distribution of such clasts if it can be characterized from structures in the geologic record can be used to estimate the sliding velocity of a past ice sheet by combining a theory of glacier sliding with a geotechnical theory of cone penetration sliding velocity can be calculated in terms of clast size parameters a fluidity parameter for ice and the thermodynamic properties of ice and clasts if frictional properties of the bed are measured the effective normal stress on the bed and bed shear strength during glaciation can also be calculated we used this approach to estimate the sliding velocity of an illinoian ice sheet that left plowing structures in cemented outwash near peoria illinois fluidity parameters for normal and basal temperate ice yielded sliding velocities of 140 168 m yr 1 and 60 72 m yr 1 respectively these are overestimates if solutes impeded regelation of ice past clasts or if friction between debris laden ice and clasts retarded slip preconsolidation stresses determined in laboratory tests on silt from the bed agree with effective normal stresses calculated using clast size parameters the high shear strength of the bed 145 kpa and primary structures preserved within it indicate that additional movement due to pervasive shear of the bed was unlikely application of this method elsewhere would provide basal velocity data that are otherwise unavailable for testing and tuning of ice sheet models

a review of north atlantic modes of natural variability and their driving mechanisms 1 this paper reviews three modes of natural variability that have been identified in the north atlantic ocean namely the north atlantic oscillation nao the atlantic multidecadal oscillation amo and the atlantic meridional mode amm this manuscript focuses on the multidecadal fluctuations of these three modes a range of different mechanisms to initiate phase reversals in these modes on multidecadal timescales has been suggested previously we propose a systematic grouping of these mechanisms into three types that involve respectively 1 the dependency of the atlantic thermohaline circulation thc on salinity 2 the sensitivity of the thc to changes in ocean heat transport and 3 the dependency of the nao to changes in the atlantic meridional temperature gradient some new density data is also provided demonstrating physical links between the thc and the amo

multi year predictability in a coupled general circulation model multi year to decadal variability in a 100 year integration of a bmrc coupled atmosphere ocean general circulation model cgcm is examined the fractional contribution made by the decadal component generally increases with depth and latitude away from surface waters in the equatorial indo pacific ocean the relative importance of decadal variability is enhanced in off equatorial wings in the subtropical eastern pacific the model and observations exhibit enso like decadal patterns analytic results are derived which show that the patterns can in theory occur in the absence of any predictability beyond enso time scales in practice however modification to this stochastic view is needed to account for robust differences between enso like decadal patterns and their interannual counterparts an analysis of variability in the cgcm a wind forced shallow water model and a simple mixed layer model together with existing and new theoretical results are used to improve upon this stochastic paradigm and to provide a new theory for the origin of decadal enso like patterns like the interdecadal pacific oscillation and pacific decadal oscillation in this theory enso driven wind stress variability forces internal equatorially trapped kelvin waves that propagate towards the eastern boundary kelvin waves can excite reflected internal westward propagating equatorially trapped rossby waves rws and coastally trapped waves ctws ctws have no impact on the off equatorial sub surface ocean outside the coastal wave guide whereas the rws do if the frequency of the incident wave is too high then only ctws are excited at lower frequencies both ctws and rws can be excited the lower the frequency the greater the fraction of energy transmitted to rws this lowers the characteristic frequency reddens the spectrum of variability off the equator relative to its equatorial counterpart at low frequencies dissipation acts as an additional low pass filter that becomes more effective as latitude increases at the same time enso driven off equatorial surface heating anomalies drive mixed layer temperature responses in both hemispheres both the eastern boundary interactions and the accumulation of surface heat fluxes by the surface mixed layer act to low pass filter the enso forcing the resulting off equatorial variability is therefore more coherent with low pass filtered decadal enso indices e g nino3 sea surface temperature sst than with unfiltered enso indices consequently large correlations between variability and nino3 extend further poleward on decadal time scales than they do on interannual time scales this explains why decadal enso like patterns have a broader meridional structure than their interannual counterparts this difference in appearance can occur even if enso indices do not have any predictability beyond interannual time scales the wings around 15 20 s and sub surface variability at many other locations are predictable on interannual and multi year time scales this includes westward propagating internal rws within about 25 of the equator the slowest of these take up to 4 years to reach the western boundary this sub surface predictability has significant oceanographic interest however it is linked to only low levels of sst variability consequently extrapolation of delayed action oscillator theory to decadal time scales might not be justified

laboratory evaluation of smear zone and correlation between permeability and moisture content in this study the extent of the smear zone and the reduction of permeability and water content within the smear zone were investigated using a large scale consolidometer the installation of vertical drains by means of a mandrel causes significant disturbance of the subsoil surrounding the mandrel resulting in a smear zone the extent of the smear zone for moruya clay new south wales australia was estimated on the basis of normalized permeability and the reduction of water content by taking undisturbed samples horizontally and vertically at different locations this study reveals that a significant reduction in water content and horizontal permeability takes place towards the drain whereas the variation in the vertical permeability is negligible the smear zone for moruya clay was found to be 2 5 times the equivalent radius of the mandrel with the horizontal permeability varying from 1 09 to 1 64 an average of 1 34 times smaller than that of the undisturbed zone finally a correlation between the permeability decrease and water content reduction within smear zone is proposed

a slip law for glaciers on deformable beds slip of marine terminating ice streams over beds of deformable till is responsible for most of the contribution of the west antarctic ice sheet to sea level rise flow models of the ice sheet and till bedded glaciers elsewhere require a law that relates slip resistance slip velocity and water pressure at the bed we present results of experiments in which pressurized ice at its melting temperature is slid over a water saturated till bed steady state slip resistance increases with slip velocity owing to sliding of ice across the bed but above a threshold velocity till shears at its rate independent coulomb strength these results motivate a generalized slip law for glacier flow models that combines processes of hard bedded sliding and bed deformation

hydraulic conductivity of homogenized tailings from hard rock mines tailings produced by milling for one extraction from hard rock mines which range in size from clay to fine sand have relatively low hydraulic conductivity k the value of k must be known with a certain degree of certainty to analyze consolidation and seepage conditions in and around tailings ponds in this paper the authors present the results of a laboratory investigation on hydraulic conductivity of homogenized tailings from hard rock mines after describing some of the basic properties of four different materials including mineralogy grain size atterberg limits compaction characteristics and consolidation curves permeability test results are given the hydraulic conductivity value usually varies between 10 4 and 10 5 cm s the effect of various factors on this value including void ratio and grain size is then discussed in relation to predictive models it is shown that a modified version of the kozeny carman equation in which a tortuosity factor and a grain size distribution function are i

decadal windstorm activity in the north atlantic european sector and its relationship to the meridional overturning circulation in an ensemble of simulations with a coupled climate model the relationship between decadal variations in the north atlantic meridional overturning circulation moc and north atlantic western european windstorm activity during the extended winter season is studied according to an ensemble of three 240 year long simulations performed with the echam5 mpiom model periods of high decadal windstorm activity frequently occur in the years following a phase of weak moc i e when the moc starts to recover these periods are characterised by a distinctive pattern in the mixed layer ocean heat content ohc a positive anomaly is located in the region 45 n 52 n 35 w 16 w west of france negative anomalies are located to the north and south the signal can be detected both in the heat content of the oceanic mixed layer and in the sea surface temperatures its structure is consistent with anomalously enhanced baroclinic instability in the region with the strong negative ohc gradient 30 w 10 w 45 n 60 n which eventually produces a higher probability of windstorms

on the influence of dsow in a numerical model of the north atlantic general circulation abstract two numerical experiments regarding the north atlantic circulation are compared the two experiments are initialized with climatological temperatures and salinities and are integrated for ten years with different surface boundary conditions in the lceland sea one case climatological wind stress sst and surface salinity to force the model it reproduces the atlantic circulation found in similar studies a change in surface boundary values in the iceland sea in order to improve the denmark strait overflow water properties in the second case results in major changes in circulation and hydrography the meridional overturning more than doubles and horizontal gyres are intensified recirculation cells occur north and south of the gulf stream the gulf stream separates from the north american coast in a more realistic manner and its further course is in better agreement with observations than in the reference run the subsurface temperature distribution differs by several kelvin in the subpolar and

sensitivity of grounding line dynamics to the choice of the friction law basal slip accounts for a large part of the flow of ice streams draining ice from antarctica and greenland into the ocean therefore an appropriate representation of basal slip in ice flow models is a prerequisite for accurate sea level rise projections various friction laws have been proposed to describe basal slip in models here we compare the influence on grounding line gl dynamics of four friction laws the traditional weertman law and three effective pressure dependent laws namely the schoof tsai and budd laws it turns out that even when they are tuned to a common initial reference state the weertman budd and schoof laws lead to thoroughly different steady state positions although the schoof and tsai laws lead to much the same result in particular under certain circumstances it is possible to obtain a steady gl located on a reverse slope area using the weertman law furthermore the predicted transient evolution of the gl as well as the projected contributions to sea level rise over a 100 year time horizon vary significantly depending on the friction law we conclude on the importance of choosing an appropriate law for reliable sea level rise projections and emphasise the need for a coupling between ice flow models and physically based subglacial hydrological models

south pacific decadal climate variability and potential predictability the south pacific decadal oscillation spdo characterizes the southern hemisphere contribution to the pacific wide interdecadal pacific oscillation ipo and is analogous to the pacific decadal oscillation pdo centered in the north pacific in this study upper ocean variability and potential predictability of the spdo is examined in hadisst data and an atmosphere forced ocean general circulation model the potential predictability of the ipo related variability is investigated in terms of both the fractional contribution made by the decadal component in the south tropical and north pacific oceans and in terms of a doubly integrated first order autoregressive ar1 model despite explaining a smaller fraction of the total variance we find larger potential predictability of the spdo relative to the pdo we identify distinct local drivers in the western subtropical south pacific where nonlinear baroclinic rossby wave topographic interactions act to low pass filter decadal variability in particular we show that the kermadec ridge in the southwest pacific enhances the decadal signature more prominently than anywhere else in the pacific basin applying the doubly integrated ar1 model we demonstrate that variability associated with the pacific south american pattern is a critically important atmospheric driver of the spdo via a reddening process analogous to the relationship between the aleutian low and pdo in the north pacific albeit that the relationship in the south pacific appears to be even stronger our results point to the largely unrecognized importance of south pacific processes as a key source of decadal variability and predictability

study on the hydraulic conductivity of boom clay the hydraulic conductivity is a parameter in geotechnical engineering practice that is key in seepage and consolidation analysis experimental results show that the hydraulic conductivity is mainly governed by the soil porosity and correlations with void ratio are usually proposed the validity of these correlations has been verified for soft clays and sands however few studies have included stiff clays in this work the hydraulic conductivity of boom clay a stiff clay taken from the essen site in belgium was determined from both consolidation and constant head percolation tests the data obtained was then analyzed to evaluate the existing correlations to predict the hydraulic conductivity in addition as these correlations usually require a referred hydraulic conductivity at a known void ratio the existing methods are often difficult to use in practice thus a new method was developed allowing the prediction of hydraulic conductivity without the referred value which was based on two existing co

rate weakening drag during glacier sliding accurately specifying the relationship between basal drag on a hard rough glacier bed and sliding speed is a long standing and central challenge in glaciology drag on a rigid bed consisting of steps with linear treads inclined upglacier a good idealization for the bedrock morphology of some hard bedded glaciers has been considered in sliding theories but never studied empirically balancing forces parallel to step treads indicates that drag should be independent of sliding speed and cavity size and set by the limit equilibrium condition sometimes called iken s bound in this study we used a large ring shear device to slide ice at its pressure melting temperature across a stepped bed over a range of steady sliding speeds 29 348 m yr 1 and under a steady effective pressure 500 kpa contrary to expectation drag decreased 42 with increasing sliding speed and cavity size experimental deviations from theory cannot explain this decrease in drag with increasing sliding speed i e rate weakening we suggest that stress bridging in ice between ice bed contact zones and cavities causes stress gradients that require viscous deformation of ice to sustain stress equilibrium so that contact zones can be at shear stresses below limit equilibrium values a parameter linearly dependent on sliding speed that scales the extent of ice deformation to areas of ice bed contact allows the experimental drag relationship to be fitted with a simple sliding model rate weakening drag has now been observed for two contrasting bed morphologies stepped and sinusoidal highlighting the need to consider such behavior in glacier flow models

a predictability study of simulated north atlantic multidecadal variability the north atlantic is one of the few places on the globe where the atmosphere is linked to the deep ocean through air sea interaction while the internal variability of the atmosphere by itself is only predictable over a period of one to two weeks climate variations are potentially predictable for much longer periods of months or even years because of coupling with the ocean this work presents details from the first study to quantify the predictability for simulated multidecadal climate variability over the north atlantic the model used for this purpose is the gfdl coupled ocean atmosphere climate model used extensively for studies of global warming and natural climate variability this model contains fluctuations of the north atlantic and high latitude oceanic circulation with variability concentrated in the 40 60 year range oceanic predictability is quantified through analysis of the time dependent behavior of large scale empirical orthogonal function eof patterns for the meridional stream function dynamic topography 170 m temperature surface temperature and surface salinity the results indicate that predictability in the north atlantic depends on three main physical mechanisms the first involves the oceanic deep convection in the subpolar region which acts to integrate atmospheric fluctuations thus providing for a red noise oceanic response as elaborated by hasselmann the second involves the large scale dynamics of the thermohaline circulation which can cause the oceanic variations to have an oscillatory character on the multidecadal time scale the third involves nonlocal effects on the north atlantic arising from periodic anomalous fresh water transport advecting southward from the polar regions in the east greenland current when the multidecadal oscillatory variations of the thermohaline circulation are active the first and second eof patterns for the north atlantic dynamic topography have predictability time scales on the order of 10 20 y whereas eof 1 of sst has predictability time scales of 5 7 y when the thermohaline variability has weak multidecadal power the hasselmann mechanism is dominant and the predictability is reduced by at least a factor of two when the third mechanism is in an extreme phase the north atlantic dynamic topography patterns realize a 10 20 year predictability time scale additional analysis of sst in the greenland sea in a region associated with the southward propagating fresh water anomalies indicates the potential for decadal scale predictability for this high latitude region as well the model calculations also allow insight into regional variations of predictability which might be useful information for the design of a monitoring system for the north atlantic predictability appears to break down most rapidly in regions of active convection in the high latitude regions of the north atlantic

dynamic patterns of ice stream flow in a 3 d higher order ice sheet model with plastic bed and simplified hydrology 1 predicting ice sheet mass balance is challenging because of the complex flow of ice streams to address this issue we have coupled a three dimensional higher order ice sheet model to a basal processes model where subglacial till has a plastic rheology and evolving yield stress the model was tested for its sensitivity to regional water availability first with an assumed undrained bed the ice stream oscillates between active and stagnant phases solely as a result of thermodynamic feedbacks occurring at the ice till interface however the velocity amplitude decreases over time as insufficient basal meltwater causes the ice stream to gradually thicken and enter a slow flowing ice sheet mode second we assume that the till is able to assimilate water from a hypothetical regional hydrological system this leads to significantly different long term behavior as a continuously oscillating ice stream mode is maintained the extra water incorporated in the till leads to higher velocities triggering stronger thermodynamic feedbacks between the ice and till layer results also suggest that fast flowing ice streams may be modulated by till properties as a result of the duration of thermal conditions during the preceding stagnant phase similarly till properties beneath stagnant ice streams are influenced by basal conditions during the preceding fast flow phase our findings support the inference that ice streams are strongly influenced by the presence of a regional hydrological system underscoring the need to accurately describe the coupling between ice dynamics basal conditions and regional subglacial hydrology in ice sheet models

multiple equilibria of an asymmetric two basin ocean model abstract an ocean general circulation model is used to examine the role of model geometry and surface buoyancy and wind stress forcing in the asymmetry of the global thermohaline circulation the model domain is a highly idealized atlantic and pacific linked by a circumpolar ocean in the south and the integrations are performed under mixed boundary conditions diagnosed from spinups under various temperature and salinity profiles constructed from the present day climatology the model exhibits a tendency to favor either a conveyor type circulation with sinking in the northern north atlantic and upwelling in the north pacific or a southern sinking state with deep sinking in the antarctic only this bias is not dictated solely by the hydrological cycle nor apparently by the greater northern extension of the atlantic basin but presumably by the overall asymmetry of the geometry equilibria with northern sinking in both basins can appear however when the winds in the southern ocean are reduced or th

the north atlantic oscillation in coupled climate models a cmip1 evaluation this study investigates the north atlantic oscillation nao simulated by 17 global coupled ocean atmosphere models participating in the coupled model intercomparison project cmip robust nao indices are defined by calculating the leading principal components of winter time mean surface temperatures land and sea in the north atlantic region 120 w 60 e 20 80 n encouragingly 13 out of 17 of the models capture the nao surface temperature quadrupole pattern with centres of action over northwest europe the northwest atlantic the southeastern usa and the middle east the northern dipole is better captured than the southern dipole which is often simulated too far eastwards over the atlantic ocean out of the 17 models ten models produce nao indices that vary similar to the observations as stationary weakly red noise with only small correlations between successive winters r 0 3 another five models drift monotonically towards warmer conditions and two models exhibit long term stochastic trends several of the models significantly overestimate the teleconnection between nao and the tropical enso phenomenon

arctic sea ice seasonal prediction by a linear markov model abstracta linear markov model has been developed to predict sea ice concentration sic in the pan arctic region at intraseasonal to seasonal time scales which represents an original effort to use a reduced dimension statistical model in forecasting arctic sea ice year round the model was built to capture covariabilities in the atmosphere ocean sea ice system defined by sic sea surface temperature and surface air temperature multivariate empirical orthogonal functions of these variables served as building blocks of the model a series of model experiments were carried out to determine the model s dimension the predictive skill of the model was evaluated by anomaly correlation and root mean square errors in a cross validated fashion on average the model is superior to the predictions by anomaly persistence damped anomaly persistence and climatology the model shows good skill in predicting sic anomalies within the arctic basin during summer and fall long term trends partially contribute to the

the great salinity anomaly in the northern north atlantic 1968 1982 the widespread freshening of the upper 500 800m layer of the northern north atlantic which this paper describes represents one of the most persistent and extreme variations in global ocean climate yet observed in this century though a range of explanations have been advanced to explain this event including in situ changes in the surface moisture flux this paper describes the great salinity anomaly as largely though not entirely an advective event traceable around the atlantic subpolar gyre for over 14 years from its origins north of iceland in the mid to late 1960s until its return to the greenland sea in 1981 1982 the overall propagation speed around this subpolar gyre is estimated at about 3cm s 1 of the total salt deficit associated with the anomaly as it passed south along the labrador coast in 1971 1973 about 72 109 tonnes a deficit equivalent to about two thirds of this figure 47 109 tonnes ultimately passed through the faroe shetland channel to the barents sea arctic ocean and greenland sea during the mid 1970s possible effects on deep water formation and on the representativeness of historical section data are discussed

permeability and consolidation of normally consolidated soils this paper first presents a unique relationship between permeability and void ratio this relationship is then used for deriving the governing differential equation of consolidation for normally consolidated soils where void ratio vs logarithm of effective stress can be approximated by a straight line the derivation shows that the coefficient of consolidation can be related directly to the applied effective stress

the permeability of natural soft clays part ii permeability characteristics reply the permeability characteristics of a number of intact natural soft clays from quebec the usa and sweden have been investigated in the laboratory the variation of permeability with void ratio is best represented in terms of a linear e vs lg k relation which is generally valid in the range of volumetric strains encountered in engineering practice the permeability at the in situ void ratio is shown to be a complex function of such parameters as the void ratio the clay fraction the plasticity index and the fabric of the clay permeability anisotropy is negligible in marine clays the e vs lg k relationships of the various clays tested are well ordered in terms of an empirical parameter ip clay fraction irrespective of the geological origin of the clay the slope of the e vs lg k relationship referred to as permeability change index ck is simply related to the initial void ratio by ck 0 5eo keywords permeability clays laboratory tests permeability anisotropy permeability void ratio re

rapid soft bed sliding of the puget glacial lobe the puget lobe of the late pleis inferred to be lodgment till based on the tocene cordilleran ice sheet advanced into presence of oriented stones oriented stria western washington over a thick sequence of tions on stones overconsolidation and unlithified sediments the basal drag due to deformation features sliding over this surface was calculated by 1 this paper investigates the rapid motion of idealizing the sediment bed as a rigid planar the puget lobe over its bed of deformable sedi surface scattered with roughness elements cor ment studies of large vanished glaciers com responding to individual sedimentary particles plement basal motion studies of modern glaci and 2 taking the minimum reconstructed slid ers in which the glacier geometry and velocity ing velocity to be 500 m yr 1 the calculated are well constrained but the characteristics of basal drag minimized by choices of parameters the bed can only be inferred from observations is 400 kpa much higher than the reconstructed over very limited areas kamb et al 1985 gravitational driving stress of 40 kpa indi seismic measurements blankenship et al 1986 cating that a rigid bed and low water pressure or modelling lingle and brown 1987 obser are not consistent with the glacierfs rapid vations on glacier beds recently exposed by motion high subglacial water pressure aver glacial retreat can provide information about aging 90 of the ice overburden 10 mpa is conditions during a somewhat greater time span inferred from overconsolidation of subglacial but are restricted to the marginal environ clays the occurrence and deformation of ment in addition inferences about the bed of water deposited sediments within the till and modern glaciers are limited by the short dura the requirement to drain large quantities of tion and seasonality of the observations a water from the interface suggest that water notable benefit of studying the beds of pieis pressure reached 99 of the ice overburden tocene glaciers is that they record basal ploughing of till by ice entrained clasts per conditions for intervals spanning entire glaci vasive shearing and finally ice bed separa ations and accessible far from the glacier tion become possible as water pressure margins increases at the low inferred effective nor after a brief summary of data available from mal stress the basal drag is reduced from the previous reconstructions of the puget lobe the rigid bed case by water layers which decouple basal conditions of the lobe are explored along the smallest particles from the glacier sole two complementary lines we first present cal and ploughing which reduces the resistance culations of basal drag due to glacier sliding offered by the largest particles the limited for different models of till bed roughness that shear strain observed in much of the till indicate that rapid sliding requires very low implies pervasive shearing did not contribute effective stresses and extensive ice bed significantly to ice motion basal motion was decoupling or ploughing second we summarize confined to the ice bed interface or to dis geological and geotechnical data that indicate tinct faults within the substrate

ocean gyre circulation changes associated with the north atlantic oscillation abstract observational evidence is presented for interannual to interdecadal variability in the intensity of the north atlantic gyre circulation related to the atmospheric north atlantic oscillation nao patterns a two point baroclinic pressure difference between the subtropical and subpolar gyre centers an oceanic analogue to the much used sea level pressure slp based atmospheric nao indices is constructed from time series of potential energy anomaly pea derived from hydrographic measurements in the labrador basin and at station s near bermuda representing the upper 2000 db eastward baroclinic mass transport between the two centers the transport index indicates a gulf stream and north atlantic current that gradually weakened during the low nao period of the 1960s and then intensified in the subsequent 25 years of persistently high nao to a record peak in the 1990s the peak to peak amplitude difference was 15 20 megatons per second mt s 1 with a 43 yr mean of about 60 mt s 1 a change of 25 33

friction at the bed does not control fast glacier flow the largest uncertainty in the ice sheet models used to predict future sea level rise originates from our limited understanding of processes at the ice bed interface near glacier termini where basal sliding controls ice flow most predictive ice sheet models use a parameterization of sliding that has been theoretically derived for glacier flow over a hard bed we find that this sliding relation does not apply to the 140 greenland glaciers that we analyzed there is no relationship between basal sliding and frictional stress at the glacier bed contrary to theoretical predictions there is a strong relationship between sliding speed and net pressure at the glacier bed this latter finding is in agreement with earlier observations of mountain glaciers that have been largely overlooked by the glaciological community

goodbye hazen hello kozeny carman the century old hazen formula for predicting the permeability of sand is based only on the d10 particle size whereas the half century old kozeny carman formula is based on the entire particle size distribution the particle shape and the void ratio as a consequence the hazen formula is less accurate than the kozeny carman formula it is recommended that the former be retired and the latter be adopted

equilibrium atmospheric response to north atlantic sst anomalies abstract the equilibrium general circulation model gcm response to sea surface temperature sst anomalies in the western north atlantic region is studied a coarse resolution gcm with realistic lower boundary conditions including topography and climatological sst distribution is integrated in perpetual january and perpetual october modes distinguished from one another by the strength of the midlatitude westerlies an sst anomaly with a maximum of 4 c is added to the climatological sst distribution of the model with both positive and negative polarity these anomaly runs are compared to one another and to a control integration to determine the atmospheric response in all cases warming cooling of the midlatitude ocean surface yields a warming cooling of the atmosphere over and to the east of the sst anomaly center the atmospheric temperature change is largest near the surface and decreases upward consistent with this simple thermal response the geopotential height field displays a baroclinic

reworked coccoliths as runoff proxy for the last 400 years the case of gaeta gulf central tyrrhenian sea central italy abstract we present the results of a high resolution study carried out on a shallow water sediment core recovered in the central tyrrhenian sea to reconstruct the runoff history of the catchment basin of volturno and garigliano rivers gulf of gaeta italy over the past 400 years we compared the abundance distribution pattern of reworked coccoliths to the surface runoff model simulation for the volturno and garigliano hydrographic river basins the global historical climatology network index the palmer drought severity index the tevere river discharge anomaly the average summer rainfall in southern italy and the reconstructed north atlantic oscillation signal this comparison suggested that the biotic signal of the reworked coccoliths may be used to detect climatic events from local to global scale the calcareous nannofossil assemblages as well as their diversity index are modulated by oscillation in solar activity where minima in solar activity correspond to minima calcareous nannofossil diversity and vice versa in particular the antiphase correlation between the abundance of reworked coccoliths and the north atlantic oscillation index which modulates winter precipitation suggests that this biotic index could be used as a reliable proxy to reconstruct the variations in the hydrographic basin runoff of the volturno and garigliano rivers in addition power spectral and wavelet analysis carried out on both signals documented the occurrence of climatic cycles of the duration of about 95 yr from 1900 ad upwards a turnover in the periodicity from 95 yr climatic cycles to 22 26 yr cycles occurred in the reworked coccoliths signal suggesting a strong control of solar forcing hale cycle over the last century

the production of north atlantic deep water sources rates and pathways updating an earlier account by dickson et al 1990 this paper reviews the initial development phase of north atlantic deep water nadw production from the points where the dense inflows from nordic seas cross the greenland scotland ridge to the point off south greenland where the buildup of new production appears almost complete in particular three long term current meter arrays totaling 91 instruments and set at 160 km intervals south from the denmark strait sill are used to validate earlier short term arrays by others and in combination with these earlier arrays to describe the downstream evolution of mean speed depth and entrainment the variability of the overflow current in space and time and the likely contribution of the other three main constituents of nadw production at densities greater than 27 8 from the points of overflow 5 6 sv the transport within this range increases by entrainment and confluence with other contributory streams to around 13 3 sv at cape farewell while recirculating elements prevent us from determining the net southgoing transport a nadw transport of this order appears consistent with recent estimates of net abyssal flow passing south through the north and south atlantic

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

influence of relative compaction on the hydraulic conductivity of completely decomposed granite in hong kong the relative compaction rc parameter is commonly used for assessment of the degree of compaction of existing fill slopes and or for the design and construction of new fill slopes this technica

response of circulation and heat transport in the north atlantic to changes in thermohaline forcing in northern latitudes a model study to avoid an explicit simulation of the overflows across the greenland scotland ridge many models of the large scale ocean circulation seek to include the net effect of the inflowing dense water masses by restoring temperature and salinity near the ridge to observed conditions in this paper the authors examine the effect of different datasets for the northern restoring condition in two versions eddy resolving and non eddy resolving of the model of the north and equatorial atlantic that has been developed in recent years as a community modeling effort for woce it is shown that the use of smoothed climatological fields of temperature and salinity south of the denmark strait leads to strong deficiencies in the simulation of the deep flow field in the basin a switch to actual hydrographic data from the denmark strait ignites a rapid dynamic response throughout the north atlantic affecting the transport and vertical structure of the deep western boundary current and by virtue of the jebar efffect the transport of the horizontal gyres meridional overturning and northward heat transport too weak in the cases with climatological boundary conditions increase to more realistic levels in the subtropical north atlantic the initial response to switches in the high latitude thermohaline forcing is mediated by fast waves along the westurn boundary leading to changes in the deep western boundary current in low latitudes after about two years in the non eddy resolving cast the initial timescale depends on the horizontal grid spacing of the model in the high resolution case the first signal reaches the equator in a few months the adjustment to a new dynamic quasi equilibrium involves kelvin waves along the equator and rossby wave in the interior and is attained in less than two decades throughout the north atlantic it is suggested that these fast dynamic adjustment processes could play an important role in possible fluctuations of the thermohaline circulation or transitions between different equilibrium states of the coupled ocean atmosphere system and may have determined the timescale of the observed climatic transitions before and during the last deglaciation