the annual cycle in equatorial convection and sea surface temperature abstract the coupled atmosphere ocean system in the equatorial eastern pacific and atlantic exhibits a distinct annual cycle that is reflected in contrasting conditions at the times of the two equinoxes the contrasts are so strong that they dominate the annual march of zonally averaged outgoing longwave radiation for the equatorial belt the march equinox corresponds to the warm season when the equatorial cold tongues in the eastern pacific and atlantic are absent with the onset of summer monsoon convection over colombia central america and west africa in may june northward surface winds strengthen over the eastern pacific and atlantic the equatorial cold tongues reappear and the marine convection shifts from the equatorial belt to the intertropical convergence zones itczs along 8 n as the northern summer program the itczs remain strong and shift northward to new 10 n while sea surface temperature sst continues to drop over the cold tongues and the southern tropics perhaps in response to the

on the interpretation of caribbean paleo temperature reconstructions during the younger dryas 1 a conundrum exists regarding whether the sea surface temperatures decreased or increased over the southern caribbean and the western tropical atlantic region during the younger dryas when the north atlantic cooled substantially and the atlantic thermohaline circulation was weakened significantly despite the proximity of core locations some proxy reconstructions record a surface cooling while others indicate a warming we suggest that this seemingly paradoxical finding may at least partially be attributed to the competing physical processes that result in opposing signs of temperature change in the region in response to weakened north atlantic meridional overturning circulation our coupled ocean atmosphere model experiments indicate that the temperature response over the southern caribbean and western tropical atlantic regions is complex and can vary considerably in small spatial scales depending on the nature of physical processes that dominate

deformations and ground temperatures at a road embankment in northern canada the paper examines the behaviour of a highway embankment in an area of discontinuous permafrost about 18 km northwest of thompson manitoba frequent maintenance has been required research involve

multidecadal variability of the north brazil current and its connection to the atlantic meridional overturning circulation 1 the north brazil current nbc connects the north and south atlantic and is the major pathway for the surface return flow of the atlantic meridional overturning circulation amoc here we calculate the nbc geostrophic transport time series based on 5 decades of observations near the western boundary off the coast of brazil results reveal a multidecadal nbc variability that lags labrador sea deep convection by a few years the nbc transport time series is coherent with the atlantic multidecadal oscillation in sea surface temperature which also has been widely linked to amoc fluctuations in previous modeling studies our results thus suggest that the observed multidecadal nbc transport variability is a useful indicator for amoc variations the suggested connection between the nbc and amoc is assessed in a 700 year control simulation of the geophysical fluid dynamics laboratory s cm2 1 coupled climate model the model results are in agreement with observations and further demonstrate that the variability of nbc transport is a good index for tracking amoc variations concerning the debate about whether a slowdown of amoc has already occurred under global warming the observed nbc transport time series suggests strong multidecadal variability but no significant trend

tropical oceanic response to extratropical thermal forcing in a coupled climate model a comparison between the atlantic and pacific oceans abstractthe tropical oceanic response to the extratropical thermal forcing is quantitatively estimated in a coupled climate model this work focuses on comparison of the responses between the tropical atlantic and pacific under the same extratropical forcing the tropical sea surface temperature responses are comparable however the responses in the tropical subsurface in the two oceans are distinct the tropical subsurface response in the atlantic can be twice of that in the pacific the maximum subsurface temperature change in the tropical pacific occurs in the eastern lower thermocline while that in the tropical atlantic occurs in the west and well below the lower thermocline the different responses in the tropical atlantic and pacific are closely related to the different changes in the meridional overturning circulations the pacific shallow overturning circulation or the subtropical cell tends to slow down speed up in response to the extratropical warming cooling forcing the changes in the

the timing of mediterranean sapropel deposition relative to insolation sea level and african monsoon changes the mediterranean basin is sensitive to global sea level changes and african monsoon variability on orbital timescales both of these processes are thought to be important to the deposition of organic rich sediment layers or sapropels throughout the eastern mediterranean yet their relative influences remain ambiguous a related issue is that an assumed 3 kyr lag between boreal insolation maxima and sapropel mid points remains to be tested here we present new geochemical and ice volume corrected planktonic foraminiferal stable isotope records for sapropels s1 holocene s3 s4 and s5 marine isotope stage 5 in core lc21 from the southern aegean sea the records have a radiometrically constrained chronology that has already been synchronised with the red sea relative sea level record and this allows detailed examination of the timing of sapropel deposition relative to insolation sea level and african monsoon changes we find that sapropel onset was near synchronous with monsoon run off into the eastern mediterranean but that insolation sapropel monsoon phasings were not systematic through the last glacial cycle these latter phasings instead appear to relate to sea level changes we propose that persistent meltwater discharges into the north atlantic e g at glacial terminations modified the timing of sapropel deposition by delaying the timing of peak african monsoon run off these observations may reconcile apparent model data offsets with respect to the orbital pacing of the african monsoon our observations also imply that the previous assumption of a systematic 3 kyr lag between insolation maxima and sapropel midpoints may lead to overestimated insolation sapropel phasings finally we surmise that both sea level rise and monsoon run off contributed to surface water buoyancy changes at times of sapropel deposition and their relative influences differed per sapropel case depending on their magnitudes sea level rise was clearly important for sapropel s1 whereas monsoon forcing was more important for sapropels s3 s4 and s5

sixteenth canadian geotechnical colloquium frost heave in soils concepts and engineering frost heave in soils is analysed from a fundamental point of view to predict the development of periodic ice lens formation that is observed in frozen soils a model of simultaneous heat and mass transfer associated with three new concepts is described and was found to exhibit many of the frost heave features observed in laboratory tests an efficient frost heave parameter termed the segregation potential sp is also described and found to be an adequate input to a general formulation of heat and mass transfer for the prediction of frost heave in soils for both laboratory and field conditions representative values of sp depend on the stress and thermal histories of the soil deposit the sp based approach can be used for solving two dimensional frost heave problems and yields stress temperature and ice content distributions with time key words frost heave soils segregation potential simulation thermodynamics

bipolar modulation of millennial scale west african monsoon variability during the last glacial 75 000 25 000 years ago abstract time series of planktonic foraminiferal 18 o and ba ca based sea surface salinity sss estimates from the eastern gulf of guinea eastern equatorial atlantic indicate changes in runoff that reflect variability of spatially integrated precipitation over the equatorial west african monsoon area millennial scale and recurring runoff induced sss rises and declines in the range of 1 5 and 2 psu practical salinity unit reveal rapid oscillation between dry and wet phases the timing of decreased runoff coincides with oscillation of dansgaard oeschger stadials and heinrich events the most severe monsoon weakening correlating with the latter 18 o residual time series derived by removing temperature ice volume and salinity components from the foraminiferal 18 o record suggest that weak monsoon precipitation during stadials and heinrich events was accompanied by significant shifts in 18 o precipitation toward higher values furthermore 18 o analysis of individual tests of globigerinoides ruber pink 18 o indiv during dry episodes show a total range and variance of 2 3 and 0 25 n 121 indicating that seasonal contrast of sea surface freshening was significantly reduced during heinrich events relative to that of interstadials which show a total range and variance of 3 35 and 0 42 n 140 on the basis of the timing and magnitude of changes in the monsoon record it is evident that northern high latitude climate was the most dominant control on the west african monsoon variability however a southern high latitude imprint is also apparent during some episodes this centennially resolved climate record demonstrates that the equatorial west african monsoon experienced profound changes in the amount seasonal contrast and moisture source of summer monsoon precipitation during the last glacial the most plausible mechanism is a large scale southward displacement of the monsoon trough most likely initiated by large scale reorganization of atmospheric circulation in response to northern high cooling and southern high latitude warmth

assessing the role of differential frost heave in the origin of non sorted circles abstract a l washburn famously proposed and reviewed 19 hypotheses for the origin of patterned ground in periglacial environments over 50 years ago washburn 1956 of these 19 mechanisms only a few have been analyzed starting from a fundamental description of the physics to assess their potential contribution to the initiation of patterned ground here differential frost heave dfh is investigated as the origin of non sorted circles in periglacial landscapes model results indicating that dfh can spontaneously lead to patterned ground are compared to measurements of patterned ground in the canadian arctic archipelago the characteristic size of the predicted emerging pattern depends strongly on the maximum depth of freezing but is only weakly dependent on the soil composition the predicted emerging patterns may also be dramatically different in size with a small change in active layer when a relatively thin 10 cm insulating snow covers the ground during freezing the model predicted trends agree with field observations of pattern size and active layer depth at two distinct sites although two data points cannot conclusively indicate a trend this correlation gives support for the possibility of determining some subsurface properties using remote sensing images of surface patterned ground

ice sheet modulation of deglacial north american monsoon intensification the north american monsoon the dominant source of rainfall for much of the arid us southwest remains one of the least understood monsoon systems the late pleistocene evolution of this monsoon is poorly constrained largely because glacial changes in winter rainfall obscure summer monsoon signatures in many regional proxy records here we develop deglacial records of monsoon strength from isotopic analyses of leaf wax biomarkers in marine sediment cores reconstructions indicate a regional decrease in monsoon rainfall during the last glacial maximum and that the deglacial trajectory of the north american monsoon closely tracks changes in north american ice cover in climate model simulations north american ice cover shifts the westerlies southwards favouring the mixing of cold dry air into the us southwest this process known as ventilation weakens the monsoon by diluting the energy fluxes required for convection as the ice sheet retreats northwards the monsoon strengthens and local ocean conditions may play a larger role in regulating its intensity we conclude that on glacial interglacial timescales ice sheet induced reorganizations of atmospheric circulation have a dominant influence on the north american monsoon the intensity of the north american summer monsoon was modified by changes in the extent of the laurentide ice sheet during the last glacial maximum and subsequent deglaciation according to isotope records and numerical simulations

influence of the meridional overturning circulation on tropical atlantic climate and variability abstract the influence of the meridional overturning circulation on tropical atlantic climate and variability has been investigated using the atmosphere ocean coupled model speedy micom miami isopycnic coordinate ocean model in the ocean model micom the strength of the meridional overturning cell can be regulated by specifying the lateral boundary conditions in case of a collapse of the basinwide meridional overturning cell the sst response in the atlantic is characterized by a dipole with a cooling in the north atlantic and a warming in the tropical and south atlantic the cooling in the north atlantic is due to the decrease in the strength of the western boundary currents which reduces the northward advection of heat the warming in the tropical atlantic is caused by a reduced ventilation of water originating from the south atlantic this effect is most prominent in the eastern tropical atlantic during boreal summer when the mixed layer attains its minimum depth as a consequence the seasonal cycle

causes of tropical atlantic paleo salinity variation during periods of reduced amoc 1 during periods of reduced atlantic meridional overturning circulation amoc associated with a freshening of northern north atlantic surface water paleo proxy records indicate a corresponding surface salinity increase over the entire tropical atlantic although latitudinal shifts in the mean position of the atlantic intertropical convergence zone itcz can explain certain features of the paleo salinity reconstructions this mechanism does not offer an explanation for the reconstructed basin wide paleo salinity response to amoc change here we present new results from general circulation model simulations that suggest the sea surface salinity sss increase in the tropical north atlantic during periods of weakened amoc is mainly controlled by the atmospheric response to the north atlantic cooling while the oceanic teleconnection contributes to increased sss over the equatorial and south tropical atlantic ocean

abrupt onset and termination of the african humid period rapid climate responses to gradual insolation forcing a detailed ca 100 yr resolution and well dated 18 ams 14 c dates to 23 cal ka bp record of latest pleistocene holocene variations in terrigenous eolian sediment deposition at odp site 658c off cap blanc mauritania documents very abrupt large scale changes in subtropical north african climate the terrigenous record exhibits a well defined period of low influx between 14 8 and 5 5 cal ka bp associated with the african humid period when the sahara was nearly completely vegetated and supported numerous perennial lakes an arid interval corresponding to the younger dryas chronozone punctuates this humid period the african humid period has been attributed to a strengthening of the african monsoon due to gradual orbital increases in summer season insolation however the onset and termination of this humid period were very abrupt occurring within decades to centuries both transitions occurred when summer season insolation crossed a nearly identical threshold value which was 4 2 greater than present these abrupt climate responses to gradual insolation forcing require strongly non linear feedback processes and current coupled climate model studies invoke vegetation and ocean temperature feedbacks as candidate mechanisms for the non linear climate sensitivity the african monsoon climate system is thus a low latitude corollary to the bi stable behavior of high latitude deep ocean thermohaline circulation which is similarly capable of rapid and large amplitude climate transitions

coupled thermal and hydrological evolution of tropical africa over the last deglaciation we analyzed the distribution of branched tetraether membrane lipids derived from soil bacteria in a marine sediment record that was recovered close to the congo river outflow and the results enabled us to reconstruct large scale continental temperature changes in tropical africa that span the past 25 000 years tropical african temperatures gradually increased from 21 to 25 c over the last deglaciation which is a larger warming than estimated for the tropical atlantic ocean a direct comparison with sea surface temperature estimates from the same core revealed that the land sea temperature difference was through the thermal pressure gradient an important control on central african precipitation patterns

the role of north brazil current transport in the paleoclimate of the brazilian nordeste margin and paleoceanography of the western tropical atlantic during the late quaternary abstract reconstructions of surface paleoceanographic conditions of the western equatorial atlantic and past climates of the adjacent northeast brazilian the nordeste continental margin were undertaken by analyzing sediments from a piston core and associated gravity and box cores recovered from 3107 meter water depth at 0 20 n on the equatorial brazilian continental slope the record is dated by radiocarbon analysis and oxygen isotopic stratigraphy of planktonic foraminifers and spans from near modern to approximately 110 ka high resolution xrf analysis provides insight into the paleoclimate history of the nordeste during the last glacial interval several large amplitude and abrupt peaks are observed in the time series of ti ca and are usually accompanied by peaks of fe k together these record periods of increased precipitation and intense weathering on the adjacent continent and increased terrestrial sediment discharge from nordeste rivers into the atlantic within the limits of dating accuracy most ti ca peaks correlate with heinrich events in the north atlantic this record thus corroborates and extends back in time the previous record of arz et al 1998 determined on sediment cores from farther southeast along the nordeste margin stable oxygen isotopic analysis and mg ca paleothermometry on the near surface dwelling planktonic foraminiferal species globierinoides ruber find that mean sea surface temperature sst during glacial time 20 55 ka n 97 was 23 89 0 79 c and the mean sst during the late holocene 0 5 ka n 14 was 26 89 0 33 c ssts were 0 5 2 c higher and inferred sea surface salinities were lower during most of the periods of elevated ti ca thus as observed in previous studies the western equatorial atlantic was warm at least locally and the adjacent southern tropical continent was wet at the same time that the high latitude north atlantic was cold using the syntrace ccsm3 fully coupled climate model with transient forcing for the period 22 ka to present we find that decreased transport of the north brazil current co occurs with reduced atlantic meridional overturning circulation and colder than normal ssts in the north atlantic region these simulated conditions are invariably associated with significantly increased precipitation in the nordeste region

wind effect on the atlantic meridional overturning circulation via sea ice and vertical diffusion effects of wind and fresh water on the atlantic meridional overturning circulation amoc are investigated using a fully coupled climate model the amoc can change significantly when perturbed by either wind stress or freshwater flux in the north atlantic this study focuses on wind stress effect our model results show that the wind forcing is crucial in maintaining the amoc reducing wind forcing over the ocean can cause immediately weakening of the vertical salinity diffusion and convection in the mid high latitudes atlantic resulting in an enhancement of vertical salinity stratification that restrains the deep water formation there triggering a slowdown of the thermohaline circulation as the thermohaline circulation weakens the sea ice expands southward and melts providing the upper ocean with fresh water that weakens the thermohaline circulation further the wind perturbation experiments suggest a positive feedback between sea ice and thermohaline circulation strength which can eventually result in a complete shutdown of the amoc this study also suggests that sea ice variability may be also important to the natural amoc variability on decadal and longer timescales

discrete ice lens theory for frost heave in soils the existing segregation potential sp method for frost heave prediction in soils is semiempirical in nature and does not explicitly predict the relationship between heave rate temperature gradient and other more fundamental soil properties the sp method assumes that the heave rate is directly related to the temperature gradient at the frost front but acknowledges that the sp parameter is dependent on pressure suction at the frost front cooling rate soil type and so forth this paper extends and modifies an approximate analytical technique of gilpin and accounts for the effects of distributed phase change within the freezing fringe in both the head and mass transfer components of the formulation the approach requires as input a relationship between frozen hydraulic conductivity and temperature and predicts the discrete location of each ice lens within the freezing soil the solution can be carried out quickly on a microcomputer to obtain the heave suction at the frost front ice lens temperatur

pipelines in permafrost geotechnical issues and lessons 12010 r m hardy address 63rd canadian geotechnical conference geotechnical input to the design construction and operations of pipelines in permafrost may differ significantly from that for pipelines in temperate terrain the general remoteness and terrain f

wet phases in the sahara sahel region and human migration patterns in north africa the carbon isotopic composition of individual plant leaf waxes a proxy for c3 vs c4 vegetation in a marine sediment core collected from beneath the plume of sahara derived dust in northwest africa reveals three periods during the past 192 000 years when the central sahara sahel contained c3 plants likely trees indicating substantially wetter conditions than at present our data suggest that variability in the strength of atlantic meridional overturning circulation amoc is a main control on vegetation distribution in central north africa and we note expansions of c3 vegetation during the african humid period early holocene and within marine isotope stage mis 3 50 45 ka and mis 5 120 110 ka the wet periods within mis 3 and 5 coincide with major human migration events out of sub saharan africa our results thus suggest that changes in amoc influenced north african climate and at times contributed to amenable conditions in the central sahara sahel allowing humans to cross this otherwise inhospitable region

cooling performance and deformation behavior of crushed rock embankments on the qinghai tibet railway in permafrost regions abstract the structural functionality of roads and railways in warm permafrost regions is influenced by both physical and mechanical processes at work within the underlying subgrade this paper uses ten years worth of monitoring data to examine the thermal regimes and deformation behaviors of three different types of crushed rock embankments employed in the construction of the qinghai tibet railway which is located in a warm permafrost region of the tibetan plateau china more specifically the efficiency of u shaped crushed rock crushed rock revetment and crushed rock basement embankments in stabilizing permafrost temperature is evaluated with consideration to the permafrost table ground temperature and mean annual ground temperature then deformation laws in the three embankments were analyzed finally deformation characteristics and sources for general subgrade are discussed and summarized on the basis of monitoring data and previous research results show the permafrost table of each of the three crushed rock embankments to be higher than that of the natural ground at all times and that the underlying permafrost warms with time regardless of the type of embankment employed the deformation characteristics of general subgrade in permafrost regions are determined to be non uniform and persistent furthermore u shaped crushed rock embankment out performed both crushed rock revetment embankment and crushed rock basement embankment in terms of cooling capacity and ability to weaken the sunny shady slope effect these findings stand to provide important guidance for understanding the deformation mechanisms of subgrade in warm permafrost regions as well as for improving qinghai tibet railway embankment quality and operational safety

palaeoclimate africa on the edge saharan humidity has varied dramatically throughout the pleistocene era a new deep sea sediment record reveals large and rapid hydrological shifts that are linked to the competing influences of low and high latitude climate processes

origin of increased terrigenous supply to the ne south american continental margin during heinrich stadial 1 and the younger dryas we investigate the redistribution of terrigenous materials in the northeastern ne south american continental margin during slowdown events of the atlantic meridional overturning circulation amoc the compilation of stratigraphic data from 108 marine sediment cores collected across the western tropical atlantic shows an extreme rise in sedimentation rates off the parnaiba river mouth about 2 s during heinrich stadial 1 hs1 18 15 ka sediment core geob16206 1 raised offshore the parnaiba river mouth documents relatively constant 143nd 144nd values expressed as end 0 throughout the last 30 ka whereas the homogeneous end 0 data support the input of fluvial sediments by the parnaiba river from the same source area directly onshore the increases in fe ca al si and rb sr during hs1 indicate a marked intensification of fluvial erosion in the parnaiba river drainage basin in contrast the end 0 values from sediment core geob16224 1 collected off french guiana about 7 n suggest amazon sourced materials within the last 30 ka we attribute the extremely high volume of terrigenous sediments deposited offshore the parnaiba river mouth during hs1 to i an enhanced precipitation in the catchment region and ii a reduced north brazil current which are both associated with a weakened amoc

dynamic millennial scale climate changes in the northwestern pacific over the past 40 000 years 1 ice core records of polar temperatures and greenhouse gases document abrupt millennial scale oscillations that suggest the reduction or shutdown of thermohaline circulation thc in the north atlantic ocean may induce the abrupt cooling in the northern hemisphere it remains unknown however whether the sea surface temperature sst is cooling or warming in the kuroshio of the northwestern pacific during the cooling event here we present an ams 14c dated foraminiferal mg ca sst record from the central okinawa trough and document that the sst variations exhibit two steps of warming since 21 ka at 14 7 ka and 12 8 ka and a cooling 1 5 c during the interval of the younger dryas by contrast we observed no sst change or oceanic warming 1 5 2 c during the episodes of northern hemisphere cooling between 21 40 ka we therefore suggest that the antarctic like timing and amplitude of millennial scale sst variations in the subtropical northwestern pacific between 20 40 ka may have been determined by rapid ocean adjustment processes in response to abrupt wind stress and meridional temperature gradient changes in the north pacific

effect of freeze thaw cycles on the freezing characteristics of a clayey silt at various overconsolidation ratios repeated freezing and thawing affect the structure of clayey silts over a wide range of consolidation ratios while the overall void ratio of the thawed soil either decreased as in lightly overconsolidated soils or increased as in heavily overconsolidated samples freezing and thawing caused a reduction in segregation potential after each freeze thaw event all the changes occurred during the first three cycles the maximum reduction in segregation potential was about 50 in all samples the implications for frost heave predictions in the field are discussed key words freeze thaw clayey silt segregation potential frost heave

fast teleconnections to the tropical atlantic sector from atlantic thermohaline adjustment 1 we examine the first decade of adjustment of the north atlantic climate following an abrupt freshening of the high north atlantic and resulting slowdown of the atlantic meridional overturning circulation amoc in a coupled model with reference to previously proposed teleconnection mechanisms after an initial ocean driven cooling at the subtropical subpolar gyre boundary subsequent equatorward progression is dominated by an atmosphere surface ocean mechanism driving evaporative cooling in the tropical north atlantic and resulting in an anomalous southward itcz displacement by year 2 the tropical cooling is countered by fast ocean baroclinic adjustment to the reduced overturning that increases the volume of tropical surface waters as well as ocean circulation changes to the increased northeasterly trades however it is the atmosphere surface ocean teleconnection that explains the hemispheric asymmetric nature of the tropical atlantic surface climate impact to amoc slowdown underlining its crucial importance in high to low latitude teleconnections

spatio temporal evolution of the west african monsoon during the last deglaciation on the basis of a multi proxy data set from the gulf of guinea eastern equatorial atlantic we reconstruct the spatio temporal evolution of the west african monsoon wam and present evidence for a decoupling between latitudinal shifts of the rain belt and wam intensification the onset of deglacial monsoon invigoration at 16 600 years before present lagged northward migration of a weak rainfall zone by 2800 years conversely during the younger dryas yd time interval wam precipitation was severely reduced but we find no evidence for a large scale retreat of the rainfall front this observation is not in agreement with the hypothesis of a large scale shift of the intertropical convergence zone south of the tropical wam region during the yd our results can be better reconciled with the newly emerging concept of a strong influence of tropical easterly and african easterly jets on modern wam

frost susceptibility related to soil index properties the analysis of frost heave data on several soils confirmed that segregation potential hence frost susceptibility of saturated soils was best related to the average size of the fines fraction the specific surface area of the fines fraction and the ratio of the material s water content to its liquid limit the influence of overburden pressure can also be accounted for by an empirical relationship between the segregation potential the average size of the fines fraction and the compressibility index of the soil the segregation potential was also proportional to the relative fines content in soils where the fines do not completely fill the voids of the coarser fraction this study led to the development of a new frost susceptibility assessment methodology based on simple geotechnical routine soil index testing that was validated on a highway site on frost susceptible subgrade till key words frost heave index properties criteria soil segregation potential

glacial interglacial variability in atlantic meridional overturning circulation and thermocline adjustments in the tropical north atlantic article i nfo k 37 tex86 atlantic meridional overturning circulation thermocline changes in the strength of atlantic meridional overturning circulation amoc are known to have profound impacts on global climate coupled modelling studies have suggested that on annual to multi decadal time scales a slowdown of amoc causes a deepening of the thermocline in the tropical atlantic however this process has been poorly constrained by sedimentary geochemical records here we reconstruct surface u k 37 index and thermocline tex h 86 water temperatures from the guinea plateau margin eastern tropical atlantic over the last two glacial interglacial cycles 192 kyr these paleotemperature records show that periods of reduced amoc as indicated by the 13 c benthic foraminiferal record from the same core coincide with a reduction in the near surface vertical temperature gradient demonstrating for the first time that amoc induced tropical atlantic thermocline adjustment exists on longer millennial time scales modelling results support the interpretation of the geochemical records and show that thermocline adjustment is particularly pronounced in the eastern tropical atlantic thus variations in amoc strength appear to be an important driver of the thermocline structure in the tropical atlantic from annual to multi millennial time scales

intermediate depth warming in the tropical atlantic related to weakened thermohaline circulation combining paleoclimate data and modeling results for the last deglaciation 1 benthic foraminiferal oxygen isotope ratios from two sediment cores recovered at 426 and 1299 m water depth in the eastern and western tropical atlantic show that a slowdown of the thermohaline circulation thc during heinrich event h1 and the younger dryas was accompanied by rapid and intense warming of intermediate depth waters millennial scale covariations of low paleosalinities in the subpolar north atlantic with decreased benthic oxygen isotope ratios in the eastern tropical atlantic throughout the past 10 000 years suggest that thc weakening might be related to middepth warming during the holocene period as well climate model experiments simulating a strong reduction of the thc in the atlantic ocean under present day and glacial conditions reveal that the increase of temperature in the middepth tropical and south atlantic is a common feature for both climatic states caused by a reduced ventilation of cold intermediate and deep waters in conjunction with downward mixing of heat from the thermocline from the similarity of the paleoclimatic records with the model simulations we infer that the characteristic pattern of temperature change in the atlantic ocean related to weakened thermohaline circulation can serve as an indicator of present day and future thc slowdown

interannual to decadal changes in the western boundary circulation in the atlantic at 11 s the western boundary current system off brazil is a key region for diagnosing variations of the atlantic meridional overturning circulation amoc and the southern subtropical cell in july 2013 a mooring array was installed off the coast at 11 s similar to an array installed between 2000 and 2004 at the same location here we present results from two research cruises and the first 10 5 months of moored observations in comparison to the observations a decade ago average transports of the north brazil undercurrent and the deep western boundary current dwbc have not changed between the observational periods dwbc eddies that are predicted to disappear with a weakening amoc are still present upper layer changes in salinity and oxygen within the last decade are consistent with an increased agulhas leakage while at depths water mass changes are likely related to changes in the north atlantic as well as tropical circulation changes

weakening of the equatorial atlantic cold tongue over the past six decades seasonal and interannual variations of the equatorial cold tongue are defining features of the tropical atlantic ocean an analysis of bias corrected observations suggests that cold tongue variability has weakened over the past six decades

decadal change of the south atlantic ocean angola benguela frontal zone since 1980 high resolution simulations with a regional atmospheric model coupled to an intermediate level mixed layer ocean model along with multiple atmospheric and oceanic reanalyses are analyzed to understand how and why the angola benguela frontal zone abfz has changed since 1980 a southward shift of 0 05 0 55 latitude decade 1 in the annual mean abfz position accompanied by an intensification of 0 05 to 0 13 k 100 km decade 1 has occurred as ocean mixed layer temperatures have warmed cooled equatorward poleward of the front over the 1980 2014 period these changes are captured in a 35 year model integration the oceanic warming north of the abfz is associated with a weakening of vertical entrainment reduced cooling associated with vertical diffusion and a deepening of the mixed layer along the angola coast these changes coincide with a steady weakening of the onshore atmospheric flow as the zonal pressure gradient between the eastern equatorial atlantic and the congo basin weakens oceanic cooling poleward of the abfz is primarily due to enhanced advection of cooler water from the south and east increased cooling by vertical diffusion and shoaling of the mixed layer depth in the atmosphere these changes are related to an intensification and poleward shift of the south atlantic sub tropical anticyclone as surface winds hence the westward mixed layer ocean currents intensify in the benguela upwelling region along the namibian coast with a few caveats these findings demonstrate that air sea interactions play a prominent role in influencing the observed decadal variability of the abfz over the southeastern atlantic since 1980

observation of oceanic origin of sahel precipitation from space abstract the west african monsoon region is of great concern due to the negative ecological economical and societal impacts of the persistent drought our results support the established postulation that the onset of summer rain in the southern region is associated with the migration of the inter tropical convergence zone and in phase with the meridional moisture transport from the gulf of guinea we present recent satellite measurements that illustrate the phase shift of the precipitation in the northern region the sahel with respect to the region to the south these satellite data and associated analysis techniques let us combine the effect of the onshore flow at the surface and offshore transport aloft to demonstrate with sufficient clarity that the depth integrated moisture advection across the west coast from the open atlantic ocean into the sahel region is in phase with the annual and inter annual variations of sahel rainfall but out of phase with rainfall in the southern region around the gulf of guinea the peak sahel rainfall is associated with the short lived net moisture influx from the atlantic which may interact with local instability related to the african easterly waves the data show two precipitation regimes in western africa monsoon region associated with moisture input from two different regions of the ocean

estimation of the segregation potential of fine grained soils using the frost heave response of two reference soils the frost heave response of quarry fines from several locations in the province of quebec was studied in the laboratory using one dimensional step freezing tests with free access to water comparis

response of eastern tropical atlantic central waters to atlantic meridional overturning circulation changes during the last glacial maximum and heinrich stadial 1 shows a mg ca temperature sensitivity of 0 19 mmol mol perc estimates of the lgm and hs1 thermocline temperatures are comparable to the present day values between 200 and 400 m water depth but were 1 2 1 5 c warmer at 550 570 m depth the hs1 thermocline waters 200 570 m depth did not show any warming relative to the lgm this is in contrast to previous climate model studies which concluded that tropical atlantic thermocline waters warmed significantly when atlantic meridional overturning circulation was reduced however our results suggest that thermocline temperatures of the northeastern tropical atlantic show no pronounced sensitivity to changes in the thermohaline circulation during glacial periods in contrast we find a significant increase in thermocline water salinity during the lgm 200 550 m depth and hs1 200 400 m depth with respect to the present day which we relate to changes in the wind driven circulation we infer that the lgm thermocline 200 550 m depth and the hs1 upper thermocline 200 400 m depth in the northeastern tropical atlantic was ventilated by surface waters from the north atlantic rather than the southern sourced waters this suggests that the frontal zone between the modern south atlantic and north atlantic central waters was probably shifted southward during the lgm and hs1

strontium geochemical evidence for the origin of the barite deposits from sardinia italy sr content of 137 samples of barite 81 samples of fluorite and 65 samples of calcite from 27 post cambrian karst and hydrothermal deposits karst calcites have an average sr content of 26 ppm quite similar to that of the hydrothermal calcites hydrothermal fluorites show an average content of 56 ppm forty five percent of the karst barites have an sr content over 6 000 ppm while as much as 83 8 percent of the hydrothermal barites contain more than 6 000 ppm sr the sr isotope composition of the cambrian stratiform barites 0 70867 or 3 is similar to that of cambrian seawater suggesting that the bulk of the sr was provided by this source the sr isotope ratios of the karst barite average 87 sr 86 sr 0 70947 or 1 indicate that sr was not only derived by recycling of the stratiform barite but was also provided by the noncarbonate fraction of the cambrian wall rocks the sr isotope composition of the hydrothermal barites ranges from 0 70990 or 4 to 0 71837 or 4 suggesting that the sr derived mainly from a high 87 sr 86 sr ratio source modified journal abstract

southern ocean origin of multidecadal variability in the north brazil current analysis of model data from a long 200 years simulation of a high resolution version of the parallel ocean program indicates a connection between a mode of multidecadal variability in the southern ocean the so called southern ocean mode and multidecadal variability in the north brazil current the multidecadal sea surface height variability in the southern ocean propagates northward and submerges at about 40 s northward propagating anomalies in ocean heat content are found between 5 and 40 s at depths down to 1 km and affect the north brazil current similar variability and connections between southern ocean and north brazil current are also found in a 200 years simulation of a high resolution global version of the community earth system model the results provide a new mechanism for the low frequency variability of the north brazil current

sensitivity of the atlantic intertropical convergence zone to last glacial maximum boundary conditions 1 recent paleoproxy records suggest that the mean latitude of the atlantic intertropical convergence zone itcz varied synchronously with north atlantic climate over a range of timescales throughout the holocene and last glacial maximum we show that the present day meridional mode of atmosphere ocean variability in the tropical atlantic is a potentially useful model for understanding these paleoclimate changes the tropical atlantic in a coupled atmospheric general circulation and slab ocean model responds to last glacial maximum conditions with a southward displacement of the itcz this response arises primarily through the land ice sheet that forces increased north atlantic trades analogous to the forcing on the present day meridional mode changes to sea ice coverage and to ocean heat transport associated with a weakened atlantic thermohaline circulation also cause a meridional mode response though through different mechanisms our results highlight the potential for tropical atlantic paleoclimate to be driven from the high latitude influences in particular land ice on glacial interglacial timescales

influences of tropical extratropical interaction on the multidecadal amoc variability in the ncep climate forecast system we have examined the mechanisms of a multidecadal oscillation of the atlantic meridional overturning circulation amoc in a 335 year simulation of the climate forecast system cfs the climate prediction model developed at the national centers for environmental prediction ncep both the mean and seasonal cycle of the amoc in the cfs are generally consistent with observation based estimates with a maximum northward volume transport of 16 sv 106 m3 s near 35 n at 1 2 km the annual mean amoc shows an intermittent quasi 30 year oscillation its dominant structure includes a deep anomalous overturning cell referred to as the anomalous amoc with amplitude of 0 6 sv near 35 n and an anomalous subtropical cell stc of shallow overturning spanning across the equator the mechanism for the oscillation includes a positive feedback between the anomalous amoc and surface wind stress anomalies in mid latitudes and a negative feedback between the anomalous stc and amoc a strong amoc is associated with warm sea surface temperature anomaly ssta centered near 45 n which generates an anticyclonic easterly surface wind anomaly this anticyclonic wind anomaly enhances the regional downwelling and reinforces the anomalous amoc in the mean time a wind evaporation sst wes feedback extends the warm ssta to the tropics and induces a cyclonic wind stress anomaly there which drives a tropical upwelling and weakens the stc north of the equator the stc anomaly in turn drives a cold upper ocean heat content anomaly hca in the northern tropical atlantic and weakens the meridional heat transport from the tropics to the mid latitude through an anomalous southward western boundary current the anomalous stc transports cold hca from the subtropics to the mid latitudes weakening the mid latitude deep overturning

hydraulic conductivity of kaolinite silt mixtures subjected to closed system freezing and thaw consolidation kaolinite silt mixtures at different clay contents were subjected to closed system freezing and thaw consolidation to obtain data on the hydraulic conductivity of thawed samples a simplified void ratio model was developed based on the compressibility characteristics of the clay aggregates and the freezing induced suctions the close agreement between predictions and observations suggests that the assumptions of the model are reasonable especially that maximum freezing induced suctions developed in the clay aggregates are related to the temperature at which no significant change in unfrozen water content occurs rather than the actual freezing temperature furthermore the compression characteristics of thawed mixtures is essentially controlled by the recompression of the clay aggregates in the overconsolidated domain the void ratio model is linked to a permeability model that suggests that the hydraulic conductivity of thawed mixtures can be defined by a reference value of the unfrozen soil and a parame

effect of atlantic meridional overturning circulation changes on tropical atlantic sea surface temperature variability a 2 layer reduced gravity ocean model study abstract previous coupled climate model simulations reveal that a dipole like sst pattern with cooler warmer temperature over the north south tropical atlantic emerges in response to a slowdown of the atlantic meridional overturning circulation amoc using a 2 layer reduced gravity ocean model a systematic investigation into oceanic processes controlling the tropical atlantic sea surface temperature sst response to amoc changes by varying the strength of northward mass transport at the open boundaries was conducted it is found that the north brazil current nbc reverses its direction in response to a shutdown of the amoc such a circulation change causes a decrease in upper equatorial ocean stratification and warming in the gulf of guinea and off the coast of africa these findings point to the importance of oceanic dynamics in the equatorial sst response to amoc changes sensitivity experiments further show that the sst response relates nonlinearly to amoc changes the strength of the sst res

mechanisms for millennial scale global synchronization during the last glacial period global climate during the last glacial period was punctuated by abrupt warmings and occasional pulses of freshwater into the north atlantic that disrupted deepwater production these massive freshwater pulses known as heinrich events arose in part from instabilities within the laurentide ice sheet paleoevidence from the north atlantic suggests that these events altered the production of deep water and changed downstream climate throughout the northern hemisphere in the tropical western pacific sea surface temperatures and salinity varied together with ocean and climate changes at high latitudes here we present results from coupled modeling experiments that shed light on a possible dynamical link between the north atlantic ocean and the western tropical pacific this link involves a global oceanic standing wave pattern brought about by millennial scale glacial density variations in the north atlantic atmospheric teleconnections triggered by meridional sea surface temperature gradients and local air sea interactions furthermore our modeling results are compared with hydrological records from the cariaco basin the indian ocean the sulu sea and northern australia

tropical north atlantic subsurface warming events as a fingerprint for amoc variability during marine isotope stage 3 the role of atlantic meridional overturning circulation amoc as the driver of dansgaard oeschger do variability that characterized marine isotope stage 3 mis 3 has long been hypothesized although there is ample proxy evidence suggesting that do events were robust features of glacial climate there is little data supporting a link with amoc recently modeling studies and subsurface temperature reconstructions have suggested that subsurface warming across the tropical north atlantic can be used to fingerprint a weakened amoc during the deglacial because a reduction in the strength of the western boundary current allows warm salinity maximum water of the subtropical gyre to enter the deep tropics to determine if amoc variability played a role during the do cycles of mis 3 we present new high resolution mg ca and 18o records spanning 24 52 kyr from the near surface dwelling planktonic foraminifera globigerinoides ruber and the lower thermocline dwelling planktonic foraminifera globorotalia truncatulinoides in southern caribbean core vm12 107 11 33 n 66 63 w 1079 m depth our subsurface mg ca record reveals abrupt increases in mg ca ratios the largest equal to a 4 c warming during the interstadial stadial transition of most do events during this period this change is consistent with reconstructions of subsurface warming events associated with cold events across the deglacial using the same core additionally our data support the conclusion reached by a recently published study from the florida straits that amoc did not undergo significant reductions during heinrich events 2 and 3 this record presents some of the first high resolution marine sediment derived evidence for variable amoc during mis 3

relative impacts of insolation changes meltwater fluxes and ice sheets on african and asian monsoons during the holocene in order to better understand the evolution of the afro asian monsoon in the early holocene we investigate the impact on boreal summer monsoon characteristics of 1 a freshwater flux in the north atlantic from the surrounding melting ice sheets and 2 a remnant ice sheet over north america and europe sensitivity experiments run with the ipsl cm4 model show that both the meltwater flux and the remnant ice sheets induce a cooling of similar amplitude of the north atlantic leading to a southward shift of the inter tropical convergence zone over the tropical atlantic and to a reduction of the african monsoon the two perturbations have different impacts in the asian sector the meltwater flux results in a weakening of the indian monsoon and no change in the east asian monsoon whereas the remnant ice sheets induce a strengthening of the indian monsoon and a strong weakening of the east asian monsoon despite the similar coolings in the atlantic ocean the ocean heat transport is reduced only in the meltwater flux experiment which induces slight differences between the two experiments in the role of the surface latent heat flux in the tropical energetics in the meltwater experiment the southward shift of the subtropical jet acts to cool the upper atmosphere over the tibetan plateau and hence to weaken the indian monsoon in the ice sheet experiment this effect is overwhelmed by the changes in extratropical stationary waves induced by the ice sheets which are associated with a larger cooling over the eurasian continent than in the meltwater experiment however these sensitivity experiments suggest that insolation is the dominant factor explaining the relative changes of the african indian and east asian monsoons from the early to the mid holocene

observational evidence for flow between the subtropical and tropical atlantic the atlantic subtropical cells abstract this study determines the mean pathways and volume transports in the pycnocline and surface layer for water flowing between the subtropical and tropical atlantic ocean using potential vorticity salinity geostrophic flow maps on isopycnal surfaces and surface drifter velocities in both hemispheres subducted salinity maximum waters flow into the tropics in the pycnocline along both interior and western boundary pathways the north atlantic ventilating trajectories are confined to densities between about 23 2 and 26 0 and only about 2 sv sv 106 m3 s 1 of water reaches the tropics through the interior pathway whereas the western boundary contributes about 3 sv to the equatorward thermocline flow flow on shallower surfaces of this density range originates from the central atlantic near 40 w between 12 and 16 n whereas flow on the deeper surfaces originates from near 20 w just off the coast of africa at higher latitudes the pathways skirt around the potential vorticity barrier located

deglacial sea surface temperature and salinity increase in the western tropical atlantic in synchrony with high latitude climate instabilities a sediment core from the western tropical atlantic covering the last 21 000 yr has been analysed for centennial scale reconstruction of sea surface temperature sst and ice volume corrected oxygen isotopic composition of sea water delta 18 o ivc sw using mg ca and delta 18 o of the shallow dwelling planktonic foraminifer globigerinoides ruber white at a period between 15 5 and 17 5 kyrbp the mg ca sstand delta 18 o ivc sw a proxy for sea surface salinity sss reveals a warming of around 2 5 degrees c along with all increase in salinity a second period of pronounced warming and sss increase occurred between 11 6 and 13 5 kyr bp within age model uncertainties both warming intervals were synchronous with air temperature increase over antarctica and ice retreat in the southern south atlantic and terminated with abrupt centennial scale sss decrease and slight sst cooling in conjunction with interglacial reactivation of the meridional overturning circulation moc we suggest that during these warm intervals production of saline and warm water of the north brazil current resulted in pronounced heat and salt accumulation and was associated with warming in the southern atlantic southward displacement of the intertropical convergence zone and weakened moc at the termination of the younger dryas and heinrich event 1 intensification of cross equatorial heat and salt transport caused centennial scale cooling and freshening of the western tropical atlantic surface water this study shows that the western tropical atlantic served as a heat and salt reservoir during deglaciation the sudden release of accumulated heat and salt at the end of younger drays and heinrich event 1 may have contributed to the rapid reinvigoration of the atlantic moc c 2005 elsevier b v all rights reserved

location of segregated ice in frost susceptible soil abstract x ray photography has been used to locate the position of a growing ice lens in soil under laboratory conditions in order to establish the temperature of the actively growing face by means of its position in the thermal gradient field it can be shown that in a general way the phase change temperatures are predictable from the clapeyron equation the structure of the ice phase also appears to follow the predictions of the heave rate equation proposed by penner and ueda 1978 when heaving occurs at low overburden pressures there is a tendency for the ice lens to be very discrete and essentially soil free at higher pressures and under similar thermal gradients it tends to develop in a more diffuse band and over a much wider temperature range

oceanic link between abrupt changes in the north atlantic ocean and the african monsoon abrupt changes in the african monsoon which have been recorded throughout the late pleistocene and early holocene epochs tend to coincide with changes in north atlantic thermohaline circulation a numerical simulation shows that the interaction between thermohaline circulation in the north atlantic ocean and wind driven currents in the topical atlantic ocean contributes to the rapidity of african monsoon transitions during abrupt climate change events

distinct roles of the southern ocean and north atlantic in the deglacial atmospheric radiocarbon decline in the context of the atmospheric co214c c catm14 changes since the last ice age two episodes of sharp catm14 decline have been related to either the venting of deeply sequestered low 14c co2 through the southern ocean surface or the abrupt onset of north atlantic deep water nadw formation in model simulations using an improved reconstruction of 14c production atlantic circulation change and southern ocean co2 release both contribute to the overall deglacial catm14 decline but only the onset of nadw can reproduce the sharp catm14 declines to fully simulate catm14 data requires an additional process that immediately precedes the onsets of nadw we hypothesize that these early catm14 declines record the thickening of the ocean s thermocline in response to reconstructed transient shutdown of nadw and or changes in the southern hemisphere westerly winds such thermocline thickening may have played a role in triggering the nadw onsets

experimental study on unfrozen water content and soil matric potential of qinghai tibetan silty clay a new soil moisture content sensor coupled with a new matric potential sensor that can operate in the subfreezing environment was used to measure the moisture content and soil matric potential dynamics of qinghai tibetan silty clay combined with nuclear magnetic resonance nmr technique and thermal resistor temperature probe the characteristics of unfrozen water content and soil matric potential and their relationships with temperature were analyzed the results show that initial water content has an impact on the freezing point and unfrozen water content the decrease in the initial water content results in a depression in the freezing point the qinghai tibetan silty clay has more similar unfrozen water content characteristic to clay than to silt there is approximately 3 of unfrozen water content retained when the soil temperature drops to 15 c the change of soil matric potential with temperature is similar to that of the unfrozen water content the matric potential value of the saturated silty clay is approximately 200 kpa when the soil temperature drops to 20 c the measured matric potentials are significantly lower than the calculated theoretical values based on the freezing point depression moisture migration experiment indicates that soil matric potential controls the direction of moisture movement and moisture redistribution including ice and liquid water during the soil freezing

decadal scale rainfall variability in ethiopia recorded in an annually laminated holocene age stalagmite an annually laminated uranium series dated holocene stalagmite from southeast ethiopia has been analysed for growth rate and 13c and 18o variations at annual to biennial resolution in order to provide the first long duration proxy record of decadal scale rainfall variability in this climatically sensitive region our study site 10 n is climatically influenced by both summer june august and spring march may rainfall caused by the annual movement of the inter tropical convergence zone itcz and modulated by large scale anomalies in the atmospheric circulation and in ocean temperatures here we show that stalagmite growth episodic throughout the last 7800 years demonstrates decadal scale 8 25 yr variability in both growth rate and 18o a hydrological model was employed and indicates that this decadal variability is due to variations in the relative amounts of rainfall in the two rain seasons our record unique in its combination of length a total of 1000 years annual chronology and high resolution 18o shows for the first time that such decadal scale variability in rainfall in this region has occurred through the holocene which implies persistent decadal scale variability for the large scale atmospheric and oceanic driving factors

links between tropical rainfall and north atlantic climate during the last glacial period the last glacial period was marked by dramatic climate fluctuations sediment records from the cariaco basin and the arabian sea suggest that cooling in the north atlantic region was tightly coupled with a southward displacement of the intertropical convergence zone and a weakening of the indian summer monsoon

tropical atlantic climate response to low latitude and extratropical sea surface temperature a little ice age perspective 1 proxy reconstructions and model simulations suggest that steeper interhemispheric sea surface temperature sst gradients lead to southerly intertropical convergence zone itcz migrations during periods of north atlantic cooling the most recent of which was the little ice age lia 100 450 ybp evidence suggesting low latitude atlantic cooling during the lia was relatively small 1 c raises the possibility that the itcz may have responded to a hemispheric sst gradient originating in the extratropics we use an atmospheric general circulation model agcm to investigate the relative influence of low latitude and extratropical ssts on the meridional position of the itcz our results suggest that the itcz responds primarily to local low latitude sst anomalies and that small cool anomalies 0 5 c can reproduce the lia precipitation pattern suggested by paleoclimate proxies conversely even large extratropical cooling does not significantly impact low latitude hydrology in the absence of ocean atmosphere interaction

global teleconnections in response to freshening over the antarctic ocean abstract in this paper coupled ocean atmosphere responses to freshening over the antarctic ocean are investigated in a fully coupled model with a series of sensitivity experiments in the model 1 0 sv 1 sv 106 m3 s 1 of freshwater flux is uniformly imposed over the antarctic ocean for 400 yr while the ocean and atmosphere remain fully coupled both locally and elsewhere the model explicitly demonstrates that a freshening of the antarctic ocean can induce a significant local cooling coupled with an intensification of the westerly winds and expansion of sea ice furthermore the cooling can extend to the entire southern extratropical and tropical oceans coupled with an intensification of southeasterly trades and the equatorial trade winds some modest warm anomalies also occur in the northern extratropical oceans forming a sharp interhemispheric sst contrast a series of sensitivity experiments are conducted to understand the mechanisms responsible for transmitting the southern high latitude cooling

hydrological variability in florida straits during marine isotope stage 5 cold events modeling and proxy studies indicate that a reduction of atlantic meridional overturning circulation amoc strength profoundly impacts temperatures and salinities in the sub tropical atlantic especially on subsurface levels while previous studies focused on prominent periods of amoc reduction during the last deglaciation we aim to test whether similar reconfigurations of the subtropical hydrography occurred during the moderate climatic alterations punctuating the last interglacial marine isotope stage mis 5 here we present temperature and salinity records from a florida straits core by combining d18o and mg ca analyses on surface globigerinoides ruber white and deep dwelling globorotalia crassaformis foraminifera covering mis 5 in high resolution the data reveal increasing salinities at intermediate depths during interglacial cooling episodes decoupled from relatively stable surface conditions this probably indicates the spatial expansion of saline subtropical gyre waters due to enhanced ekman downwelling and might also point to a changed density structure and altered geostrophic balance in florida straits notably these oceanographic alterations are not consistently occurring during periods of amoc reduction the data suggest that the expansion of gyre waters into florida straits was impeded by the increasing influence of antarctic intermediate water aaiw from mis 5 5 to 107 kyr bp afterward increasingly positive benthic d13c values imply a recession of aaiw allowing the temporary expansion of gyre waters into florida straits we argue that the inferred transient subtropical salt accumulation and warm pool expansion might have played a pivotal role in reinvigorating meridional overturning and dampen the severity of interglacial cold phases

freezing induced water migration in compacted base course materials ramped freezing tests were conducted on three base course materials with fines contents of less than 7 and compacted at different initial states but always at degrees of saturation near or well below 60 three different quarries were studied the natural fines from crushed gneiss with biotite limestone and monzonite were all frost susceptible frost heave was relatively small but significant water intake occurred in all samples during freezing with access to an external water source regardless of initial saturation level the frost susceptibility of coarse grained soils cannot be solely evaluated with respect to frost heave but needs also to consider the amount of water drawn to the freezing front during the freezing process and the consequences of this water during thaw the normalized heave of the base course layer of pavements is a practical and efficient indicator of the frost susceptibility of the base course aggregates if it is larger than 1 the base course material can be considered as fro

influence of the meridional overturning circulation on tropical subtropical pathways abstract an ocean gcm is used for idealized studies of the atlantic circulation in a square basin the subtropical the tropical and the equatorial gyres are produced by forcing the model with a wind stress profile having only latitudinal dependence the goal is to understand the effect of the meridional overturning circulation moc on the atlantic intergyre exchanges the moc is imposed by prescribing an inflow all along the southern boundary and an outflow at the northern boundary the results indicate that the northward flow of the moc has a crucial effect on the subtropical tropical pathways in this idealized configuration the north atlantic wind field creates a basinwide potential vorticity barrier therefore the water subducted in the north atlantic has to flow to the western boundary before turning equatorward this is shown by the trajectories of floats injected in a band of northern latitudes the warm water return flow of the moc inhibits this pathway and reduces the inflow of north atlantic

influence of fines on frost heave characteristics of a well graded base course material the influence of fines on the frost susceptibility of base course crushed aggregates was established by labo ratory freezing tests simulating closely the thermal conditions in the field the frost susceptibility of the fines was var ied by use of different mixtures of granitic fines and commercially available kaolinite clay a total of 13 samples with fines content of 5 10 and 15 and kaolinite fractions of 10 50 75 and 100 were subjected to four freeze thaw cycles the frost susceptibility of well graded crushed aggregates increases with increasing fines content and increasing kaolinite fraction from a quantitative point of view for a given kaolinite fraction the segregation po tential increases linearly with fines content until the fines create a matrix in which the coarser particles are embedded for the material studied this occurs when the fines content is higher than 15 for a given fines content it was also established that the segregation potential increases linearly with kaolinite fraction indicating the importance of mineral ogy it was also established that appropriate thermal testing conditions need to be adopted to prevent undue pore water extraction from the unfrozen soil close to the frost front during laboratory freezing of unsaturated coarse grained soils

a model for water transport and ice lensing in freezing soils a one dimensional model for water transport and ice lensing in incompressible saturated and solute free soil specimens is proposed for the simulation of small scale frost heave tests in the laboratory the model considers 1 open system freezing in which the variables t and pw are independent 2 an ice lens that continues to grow as long as enough energy is available in the frozen fringe to produce the work required for mass transfer to the ice lens and 3 a new ice lens that forms when the vertical strain in the frozen soil reaches the instantaneous tensile failure strain the proposed frost heave model is amenable to computer simulation procedures which provide predictions of rate of frost heave and rate of pore freezing front penetration as functions of applied load thermal and water flow regimes and soil properties position time of initiation and ultimate thickness of individual ice lenses emerge also as a part of the solution a quantitative comparison of observed and simulated responses for various boundary conditions is shown for a reference soil known as devon silt the model was found to exhibit many of the characteristics observed in frost heave tests on devon silt and predicted the effects of overburden pressure on frost heave rate

synchroneity of tropical and high latitude atlantic temperatures over the last glacial termination a high resolution western tropical atlantic sea surface temperature sst record from the cariaco basin on the northern venezuelan shelf based on mg ca values in surface dwelling planktonic foraminifera reveals that changes in sst over the last glacial termination are synchronous within 30 to 90 years with the greenland ice sheet project 2 air temperature proxy record and atmospheric methane record the most prominent deglacial event in the cariaco record occurred during the younger dryas time interval when ssts dropped by 3 to 4 c a rapid southward shift in the atmospheric intertropical convergence zone could account for the synchroneity of tropical temperature atmospheric methane and high latitude changes during the younger dryas