modelling the climate evolution from the last interglacial to the start of the last glaciation the role of arctic ocean freshwater budget 1 the earth s climate has alternated between glacial and interglacial stages but we still do not understand how the changes in insolation have been amplified by the climate system to lead to such a large climatic response here we use a climate model of intermediate complexity to show that the orbitally induced shift from an interglacial to a pre glacial climate is highly sensitive to the high northern latitudes moisture budget using the insolation forcing alone the model produces a first order response of sea surface temperatures and thermohaline circulation evolution which agrees with the available geological data under the same gradual insolation evolution adding a small freshwater input into the arctic ocean induces a rapid climate switch from an interglacial to a pre glacial mode these results suggest a simple connection between orbital forcing and the thermohaline circulation through a freshwater threshold within the ocean atmosphere sea ice system

open system coral ages reveal persistent suborbital sea level cycles sea level is a sensitive index of global climate that has been linked to earth s orbital variations with a minimum periodicity of about 21 000 years although there is ample evidence for climate oscillations that are too frequent to be explained by orbital forcing suborbital frequency sea level change has been difficult to resolve primarily because of problems with uranium thorium coral dating here we use a new approach that corrects coral ages for the frequently observed open system behavior of uranium series nuclides substantially improving the resolution of sea level reconstruction this curve reveals persistent sea level oscillations that are too frequent to be explained exclusively by orbital forcing

atlantic meridional overturning circulation during the last glacial maximum the circulation of the deep atlantic ocean during the height of the last ice age appears to have been quite different from today we review observations implying that atlantic meridional overturning circulation during the last glacial maximum was neither extremely sluggish nor an enhanced version of present day circulation the distribution of the decay products of uranium in sediments is consistent with a residence time for deep waters in the atlantic only slightly greater than today however evidence from multiple water mass tracers supports a different distribution of deep water properties including density which is dynamically linked to circulation

basinwide integrated volume transports in an eddy filled ocean the temporal evolution of the strength of the atlantic meridional overturning circulation amoc in the subtropical north atlantic is affected by both remotely forced basin scale meridionally coherent climate relevant transport anomalies such as changes in high latitude deep water formation rates and locally forced transport anomalies such as eddies or rossby waves possibly associated with small meridional coherence scales which can be considered as noise the focus of this paper is on the extent to which local eddies and rossby waves when impinging on the western boundary of the atlantic affect the temporal variability of the amoc at 26 5 degrees n continuous estimates of the amoc at this latitude have been made since april 2004 by combining the florida current ekman and midocean transports with the latter obtained from continuous density measurements between the coasts of the bahamas and morocco representing respectively the western and eastern boundaries of the atlantic at this latitude within 100 km of the western boundary there is a threefold decrease in sea surface height variability toward the boundary observed in both dynamic heights from in situ density measurements and altimetric heights as a consequence the basinwide zonally integrated upper midocean transport shallower than 1000 m as observed continuously between april 2004 and october 2006 varies by only 3 0 sv 1 sv 10 6 m 3 s 1 rms instead upper midocean transports integrated from western boundary stations 16 40 and 500 km offshore to the eastern boundary vary by 3 6 6 0 and 10 7 sv rms respectively the reduction in eddy energy toward the western boundary is reproduced in a nonlinear reduced gravity model suggesting that boundary trapped waves may account for the observed decline in variability in the coastal zone because they provide a mechanism for the fast equatorward export of transport anomalies associated with eddies impinging on the western boundary an analytical model of linear rossby waves suggests a simple scaling for the reduction in thermocline thickness variability toward the boundary physically the reduction in amplitude is understood as along boundary pressure gradients accelerating the fluid and rapidly propagating pressure anomalies along the boundary the results suggest that the local eddy field does not dominate upper midocean transport or amoc variability at 26 5 degrees n on interannual to decadal time scales

insolation values for the climate of the last 10 million years new values for the astronomical parameters of the earth s orbit and rotation eccentricity obliquity and precession are proposed for paleoclimatic research related to the late miocene the pliocene and the quaternary they have been obtained from a numerical solution of the lagrangian system of the planetary point masses and from an analytical solution of the poisson equations of the earth moon system the analytical expansion allows the direct determination of the main frequencies with their phase and amplitude numerical and analytical comparisons with the former astronomical solution ber78 are performed so that the accuracy and the interval of time over which the new solution is valid can be estimated the corresponding insolation values have also been computed this analysis leads to the conclusion that the new values are expected to be reliable over the last 5 ma in the time domain and at least over the last 10 ma in the frequency domain

descent and modification of the overflow plume in the denmark strait bulk properties of the denmark strait overflow dso plume observed in velocity and hydrography surveys undertaken in 1997 and 1998 are described despite the presence of considerable short term variability it is found that the pathway and evolution of the plume density anomaly are remarkably steady bottom stress measurements show that the pathway of the plume core matches well with a rate of descent controlled by friction the estimated entrainment rate diagnosed from the rate of plume dilution with distance shows a marked increase in entrainment at approximately 125 km from the sill leading to a net dilution consistent with previous reports of a doubling of overflow transport measured by current meter arrays the entrainment rate increase is likely related to the increased topographic slopes in the region compounded by a decrease in interface stratification as the plume is diluted and enters a denser background

antarctic sea ice climatology variability and late twentieth century change in ccsm4 abstracta preindustrial control run and an ensemble of twentieth century integrations of the community climate system model version 4 ccsm4 are evaluated for antarctic sea ice climatology modes of variability trends and covariance with related physical variables such as surface temperature and sea level pressure compared to observations the mean ice cover is too extensive in all months this is in part related to excessively strong westerly winds over 50 60 s which drive a large equatorward meridional ice transport and enhanced ice growth near the continent and also connected with a cold bias in the southern ocean in spite of these biases in the climatology the model s sea ice variability compares well to observations the leading mode of austral winter sea ice concentration exhibits a dipole structure with anomalies of opposite sign in the atlantic and pacific sectors both the el nino southern oscillation and the southern annular mode sam project onto this mode in twentieth century inte

shifting surface currents in the northern north atlantic ocean 1 analysis of surface drifter tracks in the north atlantic ocean from the time period 1990 to 2007 provides evidence that warm subtropical waters have recently increased their penetration toward the nordic seas prior to 2000 the warm water branches of the north atlantic current fed by the gulf stream turned southeastward in the eastern north atlantic since 2001 these paths have shifted toward the rockall trough through which the most saline north atlantic waters pass to the nordic seas these surface drifters are able to overcome the ekman drift which would force them southward under the westerly winds dominating the subpolar atlantic yet the changes in path cannot be accounted for by changes in ekman drift eddy kinetic energy from satellite altimetry shows increased energy along the shifted drifter pathways across the mid atlantic ridge since 2001 these near surface changes have occurred during continual weakening of the north atlantic subpolar gyre as seen by altimetry they are also consistent with the observed increase in temperature and salinity of the waters flowing northward into the nordic seas these findings suggest the changes in the vertical structure of the northern north atlantic ocean its dynamics and exchanges with the higher latitudes wind stress and its curl changes are discussed as a possible forcing of the changes in the pathways of the subtropical waters

on the definition of seasons in paleoclimate simulations with orbital forcing 1 orbital forcing is a major driver of climate variability on timescales of 10 000 to 100 000 years changes in the orbital parameters cause variations in the length of the seasons by several days consequently models using a fixed present day calendar result in biased paleoseasons especially in boreal autumn when the vernal equinox is used as an anchor point the bias is estimated for temperatures and precipitation in a transient model simulation over the last 21 000 years and an accelerated simulation over the last 129 000 years the largest differences of up to 4 k occur over the continents in high latitudes precipitation estimates are mostly affected in the low latitudes the time dependent bias is large enough to modify the temporal characteristics of temperature and precipitation indices it is discussed to what extent the bias in one season is distorting comparisons between models and paleoproxies the bias has minor implications for proxy model comparisons in general however proxies of monsoon activity should be compared with fixed angular seasons for process studies and climate sensitivity studies the use of fixed angular seasons is imperative

timing of meltwater pulse 1a and climate responses to meltwater injections the temporal relationship between meltwater pulse 1a mwp 1a and the climate history of the last deglaciation remains a subject of debate by combining the greenland ice core project d18o ice core record on the new greenland ice core chronology 2005 timescale with the u th dated barbados coral record we conclusively derive that mwp 1a did not coincide with the sharp bolling warming but instead with the abrupt cooling of the older dryas to evaluate whether there is a relationship between meltwater injections north atlantic deep water nadw formation and climate change we present a high resolution record of nadw flow intensity from eirik drift through the last deglaciation it indicates only a relatively minor 200 year weakening of nadw flow coincident with mwp 1a our compilation of records also indicates that during heinrich event 1 and the younger dryas there were no discernible sea level rises and yet these periods were characterized by intense nadw slowdowns shutdowns clearly deepwater formation and climate are not simply controlled by the magnitude or rate of meltwater addition instead our results emphasize that the location of meltwater pulses may be more important with nadw formation being particularly sensitive to surface freshening in the arctic nordic seas

sensitivity studies of northern hemisphere glaciation using an atmospheric general circulation model abstract the u k university global atmospheric modeling programme gcm is used to investigate whether the growth of northern hemisphere ice sheets could have been initiated by changes of orbital parameters and sea surface temperature two different orbital configurations corresponding to the present day and 115 kyr bp are used the reduced summer solar insulation in the northern hemisphere results in a decrease of the surface temperature by 4 to 10 c in the northern continents and to perennial snow in some high latitude regions therefore the model results support the hypothesis that a deficit of summer insulation can create conditions favorable for initiation of ice sheet growth in the northern hemisphere a decreased sea surface temperature northward of 65 n during the northern hemisphere summer may contribute to the maintenance of ice sheets a simple mixed layer ocean model coupled to the gcm indicates that the changes of sea surface temperature and extension of sea ice due to insulation changes pl

sea ice induced changes in ocean circulation during the eemian we argue that arctic sea ice played an important role during early stages of the last glacial inception two simulations of the institut pierre simon laplace coupled model 4 are analyzed one for the time of maximum high latitude summer insolation during the last interglacial the eemian and a second one for the subsequent summer insolation minimum at the last glacial inception during the inception increased arctic freshwater export by sea ice shuts down labrador sea convection and weakens overturning circulation and oceanic heat transport by 27 and 15 respectively a positive feedback of the atlantic subpolar gyre enhances the initial freshening by sea ice the reorganization of the subpolar surface circulation however makes the atlantic inflow more saline and thereby maintains deep convection in the nordic seas these results highlight the importance of an accurate representation of dynamic sea ice for the study of past and future climate changes

on the structure and origin of major glaciation cycles 1 linear responses to milankovitch forcing time series of ocean properties provide a measure of global ice volume and monitor key features of the wind driven and density driven circulations over the past 400 000 years cycles with periods near 23 000 41 000 and 100 000 years dominate this climatic narrative when the narrative is examined in a geographic array of time series the phase of each climatic oscillation is seen to progress through the system in essentially the same geographic sequence in all three cycles we argue that the 23 000 and 41 000 year cycles of glaciation are continuous linear responses to orbitally driven changes in the arctic radiation budget and we use the phase progression in each climatic cycle to identify the main pathways along which the initial local responses to radiation are propagated by the atmosphere and ocean early in this progression deep waters of the southern ocean appear to act as a carbon trap to stimulate new observations and modeling efforts we offer a process model that gives a synoptic view of climate at the four end member states needed to describe the system s evolution and we propose a dynamic system model that explains the phase progression along causal pathways by specifying inertial constants in a chain of four subsystems solutions to problems involving systems of such complexity are not born full grown like athena from the head of zeus rather they evolve slowly in stages each of which requires a pause to examine data at great lengths in order to guarantee a sure footing and to properly choose the next step victor p starr

digital imaging technology and experimental archeology a methodological framework for the identification and interpretation of fire modified rock fmr abstract this paper presents the results of a series of experiments for the identification and analysis of fire modified rock fmr fmr is a common but frequently overlooked artifact type experiments were conducted simulating the effects of different hypothetical burning scenarios on rocks similar to those found in a south african middle stone age site a digital imaging method was then used to quantify fmr color values designed to limit intra analyst bias statistical tests and a blind test suggest that unburned rocks and experimental fmr can be separated statistically based on physical appearance two burning scenario models based on measured experimental data were applied to archaeological fmr from a south african middle stone age site named pinnacle point 5 6 pp5 6 and show that the archaeological samples are not statistically different from a simulated campfire and possibly a lithic raw material heat treatment fire

the impact of convection on enso from a delayed oscillator to a series of events abstract the ncar community climate system model version 3 ccsm3 exhibits persistent errors in its simulation of the el nino southern oscillation enso mode of coupled variability the amplitude of the oscillation is too strong the dominant 2 yr period too regular and the width of the sea surface temperature response in the pacific too narrow with positive anomalies extending too far into the western pacific two changes in the parameterization of deep convection result in a significant improvement to many aspects of the enso simulation the inclusion of convective momentum transport cmt and a dilution approximation for the calculation of convective available potential energy cape are used in development integrations and a striking improvement in enso characteristics is seen an increase in the periodicity of enso is achieved by a reduction in the strength of the existing short circuited delayed oscillator mode the off equatorial response is weaker and less tropically confined largely as a

meridional overturning circulation in the south atlantic at the last glacial maximum the geostrophic shear associated with the meridional overturning circulation is reflected in the difference in density between the eastern and western margins of the ocean basin here we examine how the density difference across 30 s in the upper 2 km of the atlantic ocean and thus the magnitude of the shear associated with the overturning circulation has changed between the last glacial maximum and the present we use oxygen isotope measurements on benthic foraminifera to reconstruct density today the density in upper and intermediate waters along the eastern margin in the south atlantic is greater than along the western margin reflecting the vertical shear associated with the northward flow of surface and intermediate waters and the southward flowing north atlantic deep waters below the greater density along the eastern margin is reflected in the higher 18o values for surface sediment benthic foraminifera than those found on the western margin for the upper 2 km for the last glacial maximum the available data indicate that the eastern margin foraminifera had similar 18o to those on the western margin between 1 and 2 km and that the gradient was reversed relative to today with the higher 18o values in the western margin benthic foraminifera above 1 km if this reversal in benthic foraminifera 18o gradient reflects a reversal in seawater density gradient these data are not consistent with a vigorous but shallower overturning cell in which surface waters entering the atlantic basin are balanced by the southward export of glacial north atlantic intermediate water

orbital and millennial antarctic climate variability over the past 800 000 years a high resolution deuterium profile is now available along the entire european project for ice coring in antarctica dome c ice core extending this climate record back to marine isotope stage 20 2 800 000 years ago experiments performed with an atmospheric general circulation model including water isotopes support its temperature interpretation we assessed the general correspondence between dansgaard oeschger events and their smoothed antarctic counterparts for this dome c record which reveals the presence of such features with similar amplitudes during previous glacial periods we suggest that the interplay between obliquity and precession accounts for the variable intensity of interglacial periods in ice core records

rapid climatic variations during marine isotopic stage 3 magnetic analysis of sediments from nordic seas and north atlantic abstract the bulk magnetic parameters of seven deep sea cores distributed from the nordic seas 67 n to the north atlantic as far south as the bermuda rise 33 n exhibit short term variations which correlate with rapid climatic changes during marine isotopic stage 3 mis3 the magnetic mineralogy is uniformly dominated by well sorted low ti content magnetites indicating that these variations are due to variations in the relative amount of magnetic minerals because the magnetic minerals predominantly originate from one common source area the nordic basaltic province these changes arise from changes in the efficiency of the transport of the magnetic particles by deep currents from the source to the site of deposition these results therefore show that the fast climatic changes are related to coeval fast changes in the strength of the deep sea circulation the latter was active reduced during the interstadials stadials and heinrich events transporting the magnetic particles from the norwegian sea into the north atlantic ocean along a path similar to the present path of the nadw it is tentatively suggested that the faeroe shetland channel and the denmark strait were the only two active paths for the overflow water during mis3 the presence of magnetic oscillations in the bermuda rise core in phase with those from the north atlantic indicates that the activity of the southern newfoundland basin gyre was linked to that of the nadw during mis3

orbital forcing of arctic climate mechanisms of climate response and implications for continental glaciation progress in understanding how terrestrial ice volume is linked to earths orbital configuration has been impeded by the cost of simulating climate system processes relevant to glaciation over orbital time scales 10 3 10 5 years a compromise is usually made to rep resent the climate system by models that are averaged over one or more spatial dimensions or by three dimensional models that are limited to simulating par ticular snapshots in time we take advantage of the short equilibration time 310 years of a climate model consisting of a three dimensional atmosphere coupled to a simple slab ocean to derive the equilibrium climate response to accelerated variations in earths orbital configuration over the past 165 000 years prominent decreases in ice melt and increases in snowfall are sim ulated during three time intervals near 26 73 and 117 thousand years ago ka when aphelion was in late spring and obliquity was low there were also significant decreases in ice melt and increases in snowfall near 97 and 142 ka when eccentricity was relatively large aph elion was in late spring and obliquity was high or near its long term mean these glaciation friendly time intervals correspond to prominent and secondary phases of terrestrial ice growth seen within the marine d 18 o record both dynamical and thermal effects contribute to the increases in snowfall during these periods through increases in storm activity and the fraction of precipi tation falling as snow the majority of the mid to high latitude response to orbital forcing is organized by the properties of sea ice through its influence on radiative feedbacks that nearly double the size of the orbital forcing as well as its influence on the seasonal evolution of the latitudinal temperature gradient

synchronising edml and northgrip ice cores using δ18o of atmospheric oxygen δ18oatm and ch4 measurements over mis5 80 123 kyr abstract water isotope records from the epica dronning maud land edml and the northgrip ice cores have revealed a one to one coupling between antarctic isotope maxima aim and greenland dansgaard oeschger do events back to 50 kyr in order to explore if this north south coupling is persistent over marine isotopic stage 5 mis 5 a common timescale must first be constructed here we present new records of 18o of o2 18oatm and methane ch4 measured in the air trapped in ice from the edml 68 147 kyr and northgrip 70 123 kyr ice cores we demonstrate that through the period of interest ch4 records alone are not sufficient to construct a common gas timescale between the two cores millennial scale variations of 18oatm are evidenced over mis 5 both on the antarctic and greenland ice cores and are coupled to ch4 profiles to synchronise the northgrip and edml records they are shown to be a precious tool for ice core synchronisation with this new dating strategy we produce the first continuous and accurate sequence of the north south climatic dynamics on a common ice timescale for the last glacial inception and the first do events of mis 5 reducing relative dating uncertainties to an accuracy of a few centuries at the onset of do events 24 to 20 this edml northgrip synchronisation provides new firm evidence that the bipolar seesaw is a pervasive pattern from the beginning of the glacial period the relationship between antarctic warming amplitudes and their concurrent greenland stadial duration highlights the particularity of do event 21 and its antarctic counterpart our results suggest a smaller southern ocean warming rate for this long do event compared to do events of mis 3

simulation of the last glacial inception and rapid ice sheet growth in the mcgill paleoclimate model 1 a two dimensional 2 d dynamic ice sheet model coupled to the mcgill paleoclimate model mpm under milankovitch forcing and vostok derived atmospheric co2 levels is used to investigate the last glacial inception and subsequent rapid ice sheet growth in the northern hemisphere nh the impacts on ice sheet growth of the elevation effect of orography and the freezing of rain refreezing of meltwater are evaluated the results show that while milankovitch forcing only is sufficient to initiate the formation of permanent north american and eurasian ice at around 120 kyr bp rapid ice sheet growth during the next 10 kyr only occurs when the above two processes and an active ocean component are included the modelled ice volume equivalent drop in sea level during this growth period is estimated to be about two thirds of that found from sea level reconstructions finally the ice sheet thermohaline circulation interactions and ice sheet thickness distribution are also investigated

changes in the hydrological cycle ocean circulation and carbon nutrient cycling during the last interglacial and glacial transition 1 a complex earth system model has been forced by insolation changes for the last interglacial lig for which geological records evidence a climate cooling that culminated in the last glacial inception during an early warm period 127 125 ka b p the arctic north atlantic drainage basin north of 30 n receives 27 000 m3 s more fresh water mainly imported across the watershed of north america compared to a cooler period centered around 115 ka b p together with a strong surface warming by up to 2 k this results in a weaker north atlantic overturning reduced by 20 sea ice cover is reduced by 3 106 km2 in the northern hemisphere stimulating a stronger productivity in the barents and kara seas in the north pacific a lowered salinity and a surface warming of more than 3 k increase the stratification the eastern pacific becomes nutrient poorer which leads to a reduction in the export production in areas with vigorous upwelling the climate cooling in the course of the lig induces a carbon release by the terrestrial biosphere of 308 gt which is counteracted by an oceanic sedimentary uptake of 290 gt the rest remains in the atmosphere increasing the pco2 by 9 ppm the large oceanic co2 uptake corresponds to a rise of the calcite lysocline of about 300 m in the midlatitude atlantic the chemically interactive carbonate sediment pool in the atlantic is reduced by 10 for nearly the entire north atlantic our results are contradictory to the common interpretation of carbonate preservation proxies as a reliable tool for monitoring changes in deep water circulation

prehistoric maritime migration in the pacific islands an hypothesis of enso forcing long distance human migration across the pacific ocean occurred during the late holocene and originated almost entirely in the west as prevailing tradewinds blow from the east the mechanisms of p

glacial interglacial circulation changes inferred from 231pa 230th sedimentary record in the north atlantic region 1 studies from the subtropical western and eastern atlantic ocean using the 231pa 230th ratio as a kinematic proxy for deep water circulation provided compelling evidence for a strong link between climate and the rate of meridional overturning circulation moc over the last deglaciation in this study we present a compilation of existing and new sedimentary 231pa 230th records from north atlantic cores between 1710 and 4550 m water depth comparing sedimentary 231pa 230th from different depths provides new insights into the evolution of the geometry and rate of deep water formation in the north atlantic during the last 20 000 years the 231pa 230th ratio measured in upper holocene sediments indicates slow water renewal above 2500 m and rapid flushing below consistent with our understanding of modern circulation in contrast during the last glacial maximum lgm glacial north atlantic intermediate water gnaiw drove a rapid overturning circulation to a depth of at least 3000 m depth below 4000 m water renewal was much slower than today at the onset of heinrich event 1 transport by the overturning circulation declined at all depths gnaiw shoaled above 3000 m and significantly weakened but did not totally shut down during the bolling allerod ba that followed water renewal rates further decreased above 2000 m but increased below our results suggest for the first time that ocean circulation during that period was quite distinct from the modern circulation mode with a comparatively higher renewal rate above 3000 m and a lower renewal rate below in a pattern similar to the lgm but less accentuated moc during the younger dryas appears very similar to ba down to 2000 m and slightly slower below

paleocean circulation during the last deglaciation a bipolar seesaw hughen et al 1998 have documented that during the first 200 years of younger dryas time the 14c content of atmospheric co2 increased by 50 and that during the remainder of this 1200 year duration cold event it steadily declined the initial increase in 14c c was likely the result of a reduction in the atlantic s conveyor circulation however were the subsequent radiocarbon decline due to the rejuvenation of this potent heat pump then it is difficult to understand why the climate conditions in the northern atlantic basin remained cold throughout the younger dryas modeling exercises by stocker and wright 1996 mikolajewicz 1998 and schiller et al 1998 show that if the conveyor is terminated the transfer of radiocarbon into the deep sea shifts to the southern ocean thereby stabilizing the atmospheric 14c c ratio paleoclimatic evidence from the antarctic continent suggests that this model based scenario might have been played out in the real world while the younger dryas cooling has been documented in many places around the world including new zealand denton and hendy 1994 sowers and bender 1995 using their 18o in o2 based correlation between the ice core 18o in ice records for antarctica and greenland have demonstrated that in antarctica the younger dryas was a time of maximum warming the point of this paper is that the steep rise in 18o rise in antarctic ice which commenced close to the onset of the younger dryas might have been caused by heat released to the atmosphere in response to an increase in deep sea ventilation in the southern ocean

true to milankovitch glacial inception in the new community climate system model the equilibrium solution of a fully coupled general circulation model with present day orbital forcing is compared to the solution of the same model with the orbital forcing from 115 000 years ago the difference in snow accumulation between these two simulations has a pattern and a magnitude comparable to the ones inferred from reconstructions for the last glacial inception this is a major improvement over previous similar studies and the increased realism is attributed to the higher spatial resolution in the atmospheric model which allows for a more accurate representation of the orography of northern canada and siberia the analysis of the atmospheric heat budget reveals that as postulated by milankovitch s hypothesis the only necessary positive feedback is the snow albedo feedback which is initiated by reduced melting of snow and sea ice in the summer however this positive feedback is almost fully compensated by an increased meridionalheat transportin theatmosphereandareducedconcentrationoflow arcticclouds in contrasttosimilar previous studies the ocean heat transport remains largely unchanged this stability of the northern north atlanticcirculation is explainedby the regulatingeffect of thefreshwaterimportthroughthe naresstrait and northwest passage and the spiciness import by the north atlantic current

sensitivity of simulated climate to model resolution abstract dynamical measures of the climate e g winds eddy fluxes simulated by a general circulation model are compared at different horizontal and vertical resolutions for the december january and february period the simulations of the troposphere are found to improve significantly as the horizontal resolution increases in the range of spectral truncations from t21 to t63 little sensitivity is found to changes in vertical resolution between about 2 8 km and 0 7 km vertical grid spacing the improvements in the southern hemisphere troposphere are greater than in the northern hemisphere as the horizontal resolution increases the eddy momentum fluxes and kinetic energies in both hemispheres increase monotonically with horizontal resolution at t63 the southern hemisphere winds eddy fluxes and eddy kinetic energies agree favorably with observations while serious discrepancies are present at lower resolutions in the northern hemisphere the eddy momentum flux at t63 is slightly larger than obser

dependence of multiple climate states on ocean mixing parameters multiple equilibria of the climate system inferred from paleo reconstructions have also been observed in both ocean only and coupled ocean atmosphere general circulation models these multiple states are thought to be associated with different modes of operation of the meridional overturning circulation in the north atlantic it has recently been suggested that the stability of these states depends on the amount of vertical mixing in ocean models here we investigate the dependence of the hysteresis behaviour of the thermohaline circulation to sub gridscale mixing processes in the ocean using a simplified coupled ocean atmosphere model we find that both vertical and horizontal diffusivities have considerable influence on the stability of the different circulation modes they also change the transition points from one circulation pattern to another larger vertical diffusivities lead to higher values of additional precipitation into the north atlantic being necessary to stop the formation of deep water however for slightly increased evaporation the state without deep water formation becomes increasingly unstable for stronger vertical diffusion larger values of horizontal mixing lead to a narrowing of the phase space for which two equilibria are stable these results suggest that it is currently not possible given the large uncertainty in ocean mixing to quantitatively determine possible thresholds for the transition of the north atlantic thermohaline circulation between on and off modes this presents a policy predicament as it makes it extremely difficult to assign a probability for the future occurence of such nonlinear climate transition

pleistocene raised marine deposits on wrangel island northeast siberia and implications for the presence of an east siberian ice sheet two previously undocumented pleistocene marine transgressions on wrangel island northeastern siberia question the presence of an east siberian or beringian ice sheet during the last glacial maximum lgm the tundrovayan transgression 459 000 780 000 yr b p is represented by raised marine deposits and landforms 15 41 m asl located up to 18 km inland the presence of high sea level 64 000 73 000 yr ago the krasny flagian transgression is preserved in deposits and landforms 4 7 m asl in the krasny flag valley these deposits and landforms were mapped dated and described using amino acid geochronology radiocarbon optically stimulated luminescence electron spin resonance oxygen isotopes micropaleontology paleomagnetism and grain sizes the marine deposits are eustatic and not isostatic in origin all marine deposits on wrangel island predate the lgm indicating that neither wrangel island nor the east siberian or chukchi seas experienced extensive glaciation over the last 64 000 yr

weaker gulf stream in the florida straits during the last glacial maximum as it passes through the florida straits the gulf stream consists of two main components the western boundary flow of the wind driven subtropical gyre and the northward flowing surface and intermediate waters which are part of the global conveyor belt compensating for the deep water that is exported from the north atlantic ocean1 the mean flow through the straits is largely in geostrophic balance and is thus reflected in the contrast in seawater density across the straits2 here we use oxygen isotope ratios of benthic foraminifera which lived along the ocean margins on the boundaries of the florida current during the last glacial maximum to determine the density structure in the water and thereby reconstruct transport through the straits using the geostrophic method a technique which has been used successfully for estimating present day flow3 our data suggest that during the last glacial maximum the density contrast across the florida straits was reduced with the geostrophic flow referenced to the bottom of the channel at only about two thirds of the modern value if the wind driven western boundary flow was not lower during the last glacial maximum than today these results indicate a significantly weaker conveyor belt component of the gulf stream compared to present day values whereas previous studies based on tracers suggested that deep waters of north atlantic origin were not widespread during glacial times indicating either a relatively weak or a shallow overturning cell our results provide evidence that the overturning cell was indeed weaker during glacial times

simulating the amplification of orbital forcing by ocean feedbacks in the last glaciation according to milankovitch theory the lower summer insolation at high latitudes about 115 000 years ago allowed winter snow to persist throughout summer leading to ice sheet build up and glaciation1 but attempts to simulate the last glaciation using global atmospheric models have failed to produce this outcome when forced by insolation changes only2 3 4 5 these results point towards the importance of feedback effects for example through changes in vegetation or the ocean circulation for the amplification of solar forcing6 7 8 9 here we present a fully coupled ocean atmosphere model of the last glaciation that produces a build up of perennial snow cover at known locations of ice sheets during this period we show that ocean feedbacks lead to a cooling of the high northern latitudes along with an increase in atmospheric moisture transport from the equator to the poles these changes agree with available geological data10 11 12 13 14 15 and together they lead to an increased delivery of snow to high northern latitudes the mechanism we present explains the onset of glaciation which would be amplified by changes in vegetation in response to weak orbital forcing

quantifying ice sheet feedbacks during the last glacial inception 1 the last glacial inception 116 ky ago has long been used to test the sensitivity of climate models to insolation from these simulations atmospheric oceanic and vegetation feedbacks have been shown to amplify the initial insolation signal into a rapid growth of ice sheets over the northern hemisphere however due to the lack of comprehensive atmosphere ocean vegetation northern hemisphere ice sheet models the impact of all these feedbacks acting concurrently has not yet been evaluated here we present the results from such a model which simulates significant ice sheet growth over north america but none over eurasia our analyses focus on the different behaviours over these regions and the quantification of the ice sheet feedbacks on climate

influence of the atlantic subpolar gyre on the thermohaline circulation during the past decade record high salinities have been observed in the atlantic inflow to the nordic seas and the arctic ocean which feeds the north atlantic thermohaline circulation thc this may counteract the observed long term increase in freshwater supply to the area and tend to stabilize the north atlantic thc here we show that the salinity of the atlantic inflow is tightly linked to the dynamics of the north atlantic subpolar gyre circulation therefore when assessing the future of the north atlantic thc it is essential that the dynamics of the subpolar gyre and its influence on the salinity are taken into account

late slowdown of the atlantic meridional overturning circulation during the last glacial inception new constraints from sedimentary 231 pa 230 th our study gives new constraints on the response of atlantic meridional overturning circulation amoc export to various forcings during the last glacial inception the decay corrected excess sedimentary 231 pa 230 th activity ratio hereafter referred to as pa th has been measured over that period in two deep cores from the western su90 11 44 degrees 04 n 40 degrees 01 w 3645 m and eastern md01 2446 39 degrees 03 n 12 degrees 37 w 3547 m basins of the north atlantic both records display significant changes despite the relatively short half life of 231 pa similar to 32 kyr compared to the period we investigate the pa th variability does not correlate to changes in local opal flux normalized to 230 th moreover the pa th profiles display a high degree of coherency with indirect proxies of amoc activity such as the benthic foraminifera delta 13 c and the mid latitude summer sea surface temperature in nearby reference cores these additional pieces of evidence support our interpretation of the pa th as reflecting amoc export the pa th repeatedly underwent rapid changes during the last glacial inception associated with the extension of ice rafted detritus in the north atlantic highlighting the control of ice sheet dynamics through freshwater forcing on amoc export amoc export remains large during periods of ice sheet growth and its decreases lag the northern hemisphere summer insolation forcing amoc modulation appears driven by ice sheet dynamics itself driven by the seasonal insolation gradient between low and high northern hemisphere latitudes and the associated intensity of the meridional oceanic and atmospheric circulation c 2009 elsevier b v all rights reserved

paleosecular variation and gad studies of 0 2 ma flow sequences from the aleutian islands alaska 1 a study was undertaken in the late 1960s to investigate paleosecular variations of the geomagnetic field as recorded in volcanic rocks from the aleutian islands the early results were internally consistent but to be wholly credible by today s standards needed more detailed demagnetization complete thermal demagnetization protocols have been applied to the unmeasured archived samples from the six flow sequences used in the initial study and 40ar 39ar techniques have been used to improve the time resolution the flow sequences have ages ranging from about 50 ka to 2 ma and the number of sequential flows in the individual sequences varies from 8 to 21 after strict selection criteria were applied mad 5 95 5 for both demagnetization data from samples and samples within one flow the number of acceptable flows per flow sequence dropped to between 5 and 15 with the exception of a sequence showing transitional field behavior the between flow dispersion and the 95 values for the other sequences were notably low with respect to secular variation models and their mean directions were very close to the gad field since the time represented by the individual sequences is not well determined the low dispersion could represent very short eruption times in contrast the lack of dispersion with respect to the gad field can be taken to indicate good time averaging since the locations of the sampled flows are at roughly the same latitude about 50 n but are spread over about 10 of longitude the dispersion was calculated for both the locality means and the flow means these data represent the whole 2 myr and give a dispersion which is lower than current secular variation models predict a similar data set published for locations in western canada that are at roughly the same latitude and overlap in age with the aleutian sites gives dispersions that are close to the model predictions at face value this can be interpreted as indicating low secular variation for the aleutian sites

high latitude salinity effects and interhemispheric thermohaline circulations a general circulation model for the ocean is used to investigate the interaction between the global scale thermohaline circulation and the salinity distribution it is shown that an equatorially asymmetric circulation can be maintained even under equatorially symmetric basin geometry and surface forcing multiple equilibrium solutions are obtained for the same forcing by perturbing the high latitude salinity field in an otherwise equatorially symmetric initial condition the timescale of the transition from the symmetric circulation to an asymmetric circulation depends critically on the sign of the initial salinity perturbation

response of air sea carbon fluxes and climate to orbital forcing changes in the community climate system model 1 a global general circulation model coupled to an ocean ecosystem model is used to quantify the response of carbon fluxes and climate to changes in orbital forcing compared to the present day simulation the simulation with the earth s orbital parameters from 115 000 years ago features significantly cooler northern high latitudes but only moderately cooler southern high latitudes this asymmetry is explained by a 30 reduction of the strength of the atlantic meridional overturning circulation that is caused by an increased arctic sea ice export and a resulting freshening of the north atlantic the strong northern high latitude cooling and the direct insolation induced tropical warming lead to global shifts in precipitation and winds to the order of 10 20 these climate shifts lead to regional differences in air sea carbon fluxes of the same order however the differences in global net air sea carbon fluxes are small which is due to several effects two of which stand out first colder sea surface temperature leads to a more effective solubility pump but also to increased sea ice concentration which blocks air sea exchange and second the weakening of southern ocean winds that is predicted by some idealized studies occurs only in part of the basin and is compensated by stronger winds in other parts

on the driving processes of the atlantic meridional overturning circulation because of its relevance for the global climate the atlantic meridional overturning circulation amoc has been a major research focus for many years yet the question of which physical mechanisms ultimately drive the amoc in the sense of providing its energy supply remains a matter of controversy here we review both observational data and model results concerning the two main candidates vertical mixing processes in the ocean s interior and wind induced ekman upwelling in the southern ocean in distinction to the energy source we also discuss the role of surface heat and freshwater fluxes which influence the volume transport of the meridional overturning circulation and shape its spatial circulation pattern without actually supplying energy to the overturning itself in steady state we conclude that both wind driven upwelling and vertical mixing are likely contributing to driving the observed circulation to quantify their respective contributions future research needs to address some open questions which we outline

role of orbitally induced changes in tundra area in the onset of glaciation the link between glacial interglacial cycles and changes in insolation due to variations in the earth s orbital parameters is well established1 4 but of the attempts to simulate incipient glaciation using three dimensional general circulation models gcms driven by orbital forcing alone5 10 only one10 has been successful gcm experiments7 11 show that reduced summer insolation 115 000 years ago during an interglacial to glacial climate shift produces sufficient high latitude cooling to cause expansion of tundra at the expense of boreal forest11 which in turn can induce more cooling11 14 here we show using a global climate model that the increase in surface albedo under snow covered conditions that results from a biome model estimate11 of tundra expansion 115 000 years ago is sufficient to induce glaciation over extreme northeastern canada if the additional cooling from this estimated tundra expansion induces further expansion then widespread glaciation occurs at latitudes above 65 n these results suggest that the climate feedback from high latitude tundra expansion might have contributed to the onset of the most recent glaciation