the mutual interaction between continental scale ice sheets and atmospheric stationary waves the great continental ice sheets of the pleistocene represented significant obstacles to the northern hemisphere midlatitude westerlies they must therefore have forced large changes in the atmospheric circulation and consequently also in the patterns of accumulation and melting over the ice sheets themselves a simplified threedimensional coupled ice sheet stationary wave model is developed in order to understand the ice sheet s response to the circulation changes that it induces consistent with ice age climate simulations the ice sheet topography induces an anticyclonic circulation over the ice sheet causing a slight warming over the western slopes and a stronger cooling over the remainder the modeled feedbacks significantly affect the ice sheet configuration with the most important influences being the patterns of summer temperature and the topographically induced precipitation field the time evolution of the ice sheet is also changed by the atmospheric feedbacks and the results suggest the possibility of multiple equilibrium solutions

the sub himalayan fold thrust belt in the 1905 kangra earthquake zone a critical taper model perspective for seismic hazard analysis 1 we investigate the broader epicentral area of the m 7 8 1905 kangra earthquake s north india affecting the sub himalayan hills the tectonics of the area is characterized by two major rentrants kangra and dehradun interposed by the nahan salient the first order topography between the himalayan frontal thrust and the main boundary thrust shows a marked lateral variation along strike of the mean gradient characterized by a very small mean slope angle 1 in correspondence with the reentrants and higher values 3 in the salient these tectonic and topographic features also show a good correspondence with the peculiar macroseismic field of the 1905 event s which is characterized by two distinct intensity maxima separated by a distance of 150 km clearly overlapping the two major tectonic reentrants in this paper based on available geological and geophysical information and a critical analysis of the general mechanical constraints the seismogenic volume of the external sector of the chain is investigated in terms of critical taper model attempting to clarify the possible correlations between tectonics topography and seismicity in the sub himalayan belt based on different assumptions three possible seismotectonic scenarios are explored in order to constrain their likelihood and therefore to suggest a potential seismic gap in the area corresponding to the nahan salient which may experience an event of significant magnitude in the future

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

the influence of continental ice atmospheric co2 and land albedo on the climate of the last glacial maximum the contributions of expanded continental ice reduced atm9spheric co2 and changes in land albedo to the maintenance of the climate of the last glacial maximum lgm are examined a series of experiments is perform ed using an atmosphere mixed layer ocean model in which these changes in boundary conditions are incorporated either singly or in combination the model used has been shown to produce a reasonably reajistic simulation of the reduced temperature of the lgm manabe and broccoli 1985 by comparing the results from pairs of ex periments the effects of each of these envirornnental changes can be determined expanded continental ice and reduced atmospheric co2 are found to have a substantial impact on global mean temperature the ice sheet effect is confined almost ex clusively to the northern hemisphere while lowered co2 cools both hemispheres changes in land albedo over ice free areas have only a minor thermal effect on a global basis the reduction of co2 content in the atmosphere is the primary contributor to the cooling of the southern hemisphere the model sensitivity to both the ice sheet and co2 effects is characterized by a high latitude amplification and a late autumn and early winter maximum

links between rossby wave breaking and the north atlantic oscillation arctic oscillation in present day and last glacial maximum climate simulations upper tropospheric rossby wave breaking processes are examined in coupled ocean atmosphere simulations of the last glacial maximum lgm and of the modern era lgm statistics of the northern hemisphere in winter computed from the paleoclimate modeling intercomparison project phase ii pmip2 dataset are compared with those from preindustrial simulations and from the 40 yr ecmwf re analysis era 40 particular attention is given to the role of wave breaking events in the north atlantic oscillation nao for each simulation anticyclonic awb and cyclonic cwb wave breaking events during lgm are shown to be less and more frequent respectively than in the preindustrial climate especially in the pacific this is consistent with the slight equatorward shift of the eddy driven jets in the lgm runs the most remarkable feature of the simulated lgm climate is that it presents much weaker latitudinal fluctuations of the eddy driven jets this is accompanied by less dispersion in the wave breaking events a physical interpretation is provided in terms of the fluctuations of the low level baroclinicity at the entrance of the storm tracks the nao in the preindustrial simulations and in era 40 is characterized by strong latitudinal fluctuations of the atlantic eddy driven jet as well as by significant changes in the nature of the wave breaking during the positive phase the eddy driven jet moves to the north with more awb events than usual and is well separated from the subtropical african jet the negative phase exhibits a more equatorward atlantic jet and more cwb events in contrast the lgm nao is less well marked by the latitudinal vacillation of the atlantic jet and for some models this property disappears entirely the lgm nao corresponds more to acceleration deceleration or extension retraction of the atlantic jet the hemispheric point of view of the arctic oscillation exhibits similar changes

north atlantic eddy driven jet in interglacial and glacial winter climates the atmospheric westerly flow in the north atlantic na sector is dominated by atmospheric waves or eddies generating via momentum flux convergence the so called eddy driven jet the position of this jet is variable and shows for the present day winter climate three preferred latitudinal states a northern central andsouthernpositioninthena here theauthorsanalyzethebehavioroftheeddy drivenjetunderdifferent glacial and interglacial boundary conditions using atmosphere land only simulations with the ccsm4 climate model as state of the art climate models tend to underestimate the trimodality of the jet latitude the authors apply a bias correction and successfully extract the trimodal behavior of the jet within ccsm4 the analysis shows that during interglacial times i e the early holocene and the eemian the preferred jet positions are rather stable and the observed multimodality is the typical interglacial character of the jet during glacial times the jet is strongly enhanced its position is shifted southward and the trimodal behavior vanishes this is mainly due to the presence of the laurentide ice sheet lis the lis enhances stationary waves downstream thereby accelerating and displacing the na eddy driven jet by anomalous stationary momentum flux convergence additionally changes in the transient eddy activity caused by topography changes as well as other glacial boundary conditions lead to an acceleration of the westerly winds over the southernnaattheexpenseofmorenorthernareas consequently bothstationaryandtransienteddiesfoster the southward shift of the na eddy driven jet during glacial winter times

influence of the antarctic ice sheet on southern high latitude climate during the cenozoic albedo vs topography effect sensitivity of high latitude temperature distribution to the variation of the antarctic ice sheet is investigated using an atmospheric general circulation model coupled with a mixed layer ocean results suggest that cooling effect of the ice sheet is not negligible compared to the effect of atmospheric co2 decrease such as during the cenozoic cooling effect of high surface albedo of the ice sheet is enhanced over the ice sheet and slightly suppressed over the circumpolar ocean by the high topography of the ice sheet high surface albedo and high topography of the ice sheet each causes increase and decrease of southward atmospheric heat transport respectively whose contribution to maintaining the heat balance anomaly of atmosphere ocean system is comparable to that of local adjustment of radiation balance

role of southern ocean stratification in glacial atmospheric co2 reduction evaluated by a three dimensional ocean general circulation model atmospheric carbon dioxide co2 concentration during glacial periods is known to be considerably lower than during interglacial periods however previous studies using an ocean general circulation model ogcm fail to reproduce this paleoclimate proxy data of the last glacial maximum indicate high salinity 37 0 psu and long water mass residence time 3 000 years in the southern ocean suggesting that salinity stratification was enhanced and more carbon was stored there reproducibility of salinity and water mass age is considered insufficient in previous ogcms which might affect the reproducibility of atmospheric co2 concentration this study investigated the role of increased stratification of the southern ocean in the glacial co2 variation using an ogcm we found that deep water formation in east antarctica is required to explain high salinity in the south atlantic saltier deep southern ocean resulted in increased atmospheric co2 concentration against previous estimates this is partly due to increased volume transport of antarctic bottom water and associated decrease in the water mass age of the deep pacific ocean on the other hand weakening of vertical mixing contributed to increase of the vertical gradient of dissolved inorganic carbon and decrease of atmospheric co2 concentration however we show that it is unable to explain all of the glacial co2 variations by the contribution of the southern ocean our findings indicate that detailed understanding of the impact of enhanced stratification in the southern ocean on the pacific ocean might be crucial to understanding the mechanisms behind the glacial interglacial ocean carbon cycle variations

changes in the atmospheric ch4 gradient between greenland and antarctica during the last glacial and the transition to the holocene significant variations in the interpolar difference of atmospheric ch4 concentration over the holocene period were observed by chappellaz et al 1997 here we extend this study to the last glacial and the transition to the holocene we observe a gradient of 3 4 parts per billion by volume ppbv during the last glacial maximum it increases to 26 10 ppbv during the bolling allerod and remains at 26 9 ppbv during the younger dryas cold period on average we find an interpolar difference of 14 4 ppbv during the cold phases and of 37 10 ppbv during the warm periods of the last glacial with a three box model we derive from the measured gradients the contributions of methane from the tropics and the mid to high latitudes of the northern hemisphere the tropics have been the largest source in all glacial epochs the contribution by the northern latitudes have been very small during the last glacial maximum but surprisingly large during the earlier part of the glacial epoch the model result suggests completely unexpected that the higher atmospheric ch4 concentration during the warm dansgaard oeschger events are caused by a higher source strength of the northern latitudes and not of the tropics

surface wind stress threshold for glacial atlantic overturning 1 using a coupled model of intermediate complexity the sensitivity of the last glacial maximum lgm atlantic meridional overturning circulation amoc to the strength of surface wind stress is investigated a threshold is found below which north atlantic deep water formation dwf takes place south of greenland and the amoc is relatively weak above this threshold dwf occurs north of the greenland scotland ridge leading to a vigorous amoc this nonlinear behavior is explained through enhanced salt transport by the wind driven gyre circulation and the overturning itself both pattern and magnitude of the nordic sea s temperature difference between strong and weak amoc states are consistent with those reconstructed for abrupt climate changes of the last glacial period our results thus point to a potentially relevant role of surface winds in these phenomena

strength and geometry of the glacial atlantic meridional overturning circulation during the last glacial maximum the pattern of atlantic meridional overturning circulation was different from today a combination of sediment chemistry and a scavenging model suggests that the glacial circulation was shallower and at least as vigorous as today

open ocean convection observations theory and models we review what is known about the convective process in the open ocean in which the properties of large volumes of water are changed by intermittent deep reaching convection triggered by winter storms observational laboratory and modeling studies reveal a fascinating and complex interplay of convective and geostrophic scales the large scale circulation of the ocean and the prevailing meteorology two aspects make ocean convection interesting from a theoretical point of view first the timescales of the convective process in the ocean are sufficiently long that it may be modified by the earth s rotation second the convective process is localized in space so that vertical buoyancy transfer by upright convection can give way to slantwise transfer by baroclinic instability moreover the convective and geostrophic scales are not very disparate from one another detailed observations of the process in the labrador greenland and mediterranean seas are described which were made possible by new observing technology when interpreted in terms of underlying dynamics and theory and the context provided by laboratory and numerical experiments of rotating convection great progress in our description and understanding of the processes at work is being made

glacial atlantic overturning increased by wind stress in climate models previous paleoclimate model intercomparison project pmip simulations of the last glacial maximum lgm atlantic meridional overturning circulation amoc showed dissimilar results on transports and structure here we analyze the most recent pmip3 models which show a consistent increase on average by 41 26 and deepening 663 550 m of the amoc with respect to preindustrial simulations in contrast to some reconstructions from proxy data simulations run with the university of victoria uvic ocean circulation model suggest that this is caused by changes in the northern hemisphere wind stress brought about by the presence of ice sheets over north america in the lgm when forced with lgm wind stress anomalies from pmip3 models the uvic model responds with an increase of the northward salt transport in the north atlantic which strengthens north atlantic deep water formation and the amoc these results improve our understanding of the lgm amoc s driving forces and suggest that some ocean mechanisms may not be correctly represented in pmip3 models or some proxy data may need reinterpretation

dynamics of the north american ice sheet complex during its inception and build up to the last glacial maximum the north american ice sheet complex played a major role in global sea level fluctuations during the late quaternary but our knowledge of its dynamics is based mostly on its demise from the last glacial maximum lgm a period characterised by non linear behaviour in the form of punctuated ice margin recession episodic ice streaming and major shifts in the location of ice divides in comparison knowledge of the pre lgm ice complex is poorly constrained largely because of the fragmentary nature of the evidence relating to ice sheet build up in this paper we explore the inception and growth of ice 120 20 ka using a glacial systems model which has been calibrated against a large and diverse set of data relating to the deglacial interval we make use of calibration data prior to the lgm but its scarcity introduces greater uncertainty which is partly alleviated by our large ensemble analysis results suggest that following the last interglaciation oxygen isotope stage ois 5e the ice complex initiated over the north eastern canadian arctic and in the cordillera within a few thousand years it then underwent rapid growth to an ois 5 maximum at 110 ka 5d and covered 70 of the area occupied by the lgm ice cover although only 30 by volume an ois 5 minimum is modelled at 80 ka 5a before a second phase of rapid growth at the start of ois 4 which culminated in a large ice complex at 65 ka almost as large as at the lgm subsequent deglaciation was rapid maximum modelled sea level contribution of 16 cm per century and resulted in an ois 3 minimum between ca 55 60 ka thereafter the ice complex grew towards its lgm configuration interrupted by several phases of successively less significant mass loss our results support and extend previous inferences based on geological evidence and reinforce the notion of a highly dynamic pre lgm ice complex e g with episodes of 10 s m of eustatic sea level equivalent in 50 but even the thin ois 5 ice sheets exhibit fast flow features several 1000 m a 1 in major topographic troughs notwithstanding the severe limitations imposed by the use of the shallow ice approximation we note that most fast flow features generated prior to the lgm correspond to the location of known ice streams during deglaciation i e in major topographic troughs and over soft sediments at the southern and western margins moreover the modelled flux of these ice streams sensu lato appears to be non linearly scaled to ice sheet volume i e there is no evidence that decay phases were associated with significantly increased ice stream activity this hypothesis requires testing using a model with higher order physics and future modelling would also benefit from additional pre lgm constraints e g dated ice free margin positions to help reduce and quantify uncertainties

strong and deep atlantic meridional overturning circulation during the last glacial cycle extreme abrupt northern hemisphere climate oscillations during the last glacial cycle 140 000 years ago to present were modulated by changes in ocean circulation and atmospheric forcing1 however the variability of the atlantic meridional overturning circulation amoc which has a role in controlling heat transport from low to high latitudes and in ocean co2 storage is still poorly constrained beyond the last glacial maximum2 3 4 here we show that a deep and vigorous overturning circulation mode has persisted for most of the last glacial cycle dominating ocean circulation in the atlantic whereas a shallower glacial mode with southern sourced waters filling the deep western north atlantic prevailed during glacial maxima3 5 our results are based on a reconstruction of both the strength and the direction of the amoc during the last glacial cycle from a highly resolved marine sedimentary record in the deep western north atlantic parallel measurements of two independent chemical water tracers the isotope ratios of 231pa 230th and 143nd 144nd 6 7 8 which are not directly affected by changes in the global cycle reveal consistent responses of the amoc during the last two glacial terminations any significant deviations from this configuration resulting in slowdowns of the amoc were restricted to centennial scale excursions during catastrophic iceberg discharges of the heinrich stadials severe and multicentennial weakening of north atlantic deep water formation occurred only during heinrich stadials close to glacial maxima with increased ice coverage probably as a result of increased fresh water input in contrast the amoc was relatively insensitive to submillennial meltwater pulses during warmer climate states and an active amoc prevailed during dansgaard oeschger interstadials greenland warm periods

last glacial maximum ocean thermohaline circulation pmip2 model intercomparisons and data constraints the ocean thermohaline circulation is important for transports of heat and the carbon cycle we present results from pmip2 coupled atmosphere ocean simulations with four climate models that are also being used for future assessments these models give very different glacial thermohaline circulations even with comparable circulations for present an integrated approach using results from these simulations for last glacial maximum lgm with proxies of the state of the glacial surface and deep atlantic supports the interpretation from nutrient tracers that the boundary between north atlantic deep water and antarctic bottom water was much shallower during this period there is less constraint from this integrated reconstruction regarding the strength of the lgm north atlantic overturning circulation although together they suggest that it was neither appreciably stronger nor weaker than modern two model simulations identify a role for sea ice in both hemispheres in driving the ocean response to glacial forcing

the influence of continental ice sheets on the climate of an ice age the climate influence of the land ice that existed 18 000 years before present 18k b p is investigated by use of a general circulation model of the atmosphere coupled with a static mixed layer ocean simulated climates are obtained from two versions of the model one with the land ice distribution of the present and the other with that of 18k b p in the northern hemisphere the tropospheric flow field is strongly influenced by the laurentide ice sheet and features a split flow straddling the ice sheet with a strong jet stream forming the southern branch the northern branch of the flow brings very cold air over the north atlantic ocean where thick sea ice is maintained the distribution of sea surface temperature sst difference between the two experiments in the northern hemisphere resembles the difference between the sst at 18k b p and at present as estimated by the climap project 1981 the 18k b p ice sheets have very little influence upon atmospheric temperature and sst in the southern hemisphere this is because the interhemispheric heat transport hardly changes as the loss of heat energy due to the reflection of solar radiation by continental ice sheets in the northern hemisphere is almost completely counterbalanced by the in situ reduction of upward terrestrial radiation hydrologic changes in the model climate are also found with statistically significant decreases in soil moisture occurring in a zone located to the south of the ice sheets in north america and eurasia these findings are consistent with some geological evidence of regionally drier climates from the last glacial maximum

interactions between stationary waves and ice sheets linear versus nonlinear atmospheric response this study examines the mutual interaction between topographically forced atmospheric stationary waves and continental scale ice sheets using a thermomechanical ice sheet model coupled to a linear

atmospheric co2 forcing on glacial thermohaline circulation and climate 1 a coupled climate model study indicates the paleoclimate record of glacial thermohaline circulation thc and reversed deep sea temperature salinity t s distribution in the atlantic can be explained largely by lower glacial atmospheric co2 alone the reduced co2 leads to increased southern ocean wintertime sea ice cover and salinity increased production of dense antarctic bottom water aabw enhanced cold and saline aabw penetration into the deep north atlantic increased oceanic vertical stability and reduced north atlantic deep water nadw circulation the dominant role of co2 forcing during the glacial implies a positive feedback between the southern ocean regulated thc and the glacial global carbon cycle

response of a comprehensive climate model to a broad range of external forcings relevance for deep ocean ventilation and the development of late cenozoic ice ages over the past few million years the earth descended from the relatively warm and stable climate of the pliocene into the increasingly dramatic ice age cycles of the pleistocene the influences of orbital forcing and atmospheric co2 on land based ice sheets have long been considered as the key drivers of the ice ages but less attention has been paid to their direct influences on the circulation of the deep ocean here we provide a broad view on the influences of co2 orbital forcing and ice sheet size according to a comprehensive earth system model by integrating the model to equilibrium under 40 different combinations of the three external forcings we find that the volume contribution of antarctic aabw vs north atlantic nadw waters to the deep ocean varies widely among the simulations and can be predicted from the difference between the surface densities at aabw and nadw deep water formation sites minima of both the aabw nadw density difference and the aabw volume occur near interglacial co2 270 400 ppm at low co2 abundant formation and northward export of sea ice in the southern ocean contributes to very salty and dense antarctic waters that dominate the global deep ocean furthermore when the earth is cold low obliquity i e a reduced tilt of earth s rotational axis enhances the antarctic water volume by expanding sea ice further at high co2 aabw dominance is favoured due to relatively warm subpolar north atlantic waters with more dependence on precession meanwhile a large laurentide ice sheet steers atmospheric circulation as to strengthen the atlantic meridional overturning circulation but cools the southern ocean remotely enhancing antarctic sea ice export and leading to very salty and expanded aabw together these results suggest that a sweet spot of low co2 low obliquity and relatively small ice sheets would have poised the amoc for interruption promoting dansgaard oeschger type abrupt change the deep ocean temperature and salinity simulated under the most representative glacial state agree very well with reconstructions from the last glacial maximum lgm which lends confidence in the ability of the model to estimate large scale changes in water mass geometry the model also simulates a circulation driven increase of preformed radiocarbon reservoir age which could explain most of the reconstructed lgm preindustrial ocean radiocarbon change however the radiocarbon content of the simulated glacial ocean is still higher than reconstructed for the lgm and the model does not reproduce reconstructed lgm deep ocean oxygen depletions these ventilation related disagreements probably reflect unresolved physical aspects of ventilation and ecosystem processes but also raise the possibility that the lgm ocean circulation was not in equilibrium finally the simulations display an increased sensitivity of both surface air temperature and aabw volume to orbital forcing under low co2 we suggest that this enhanced orbital sensitivity contributed to the development of the ice age cycles by amplifying the responses of climate and the carbon cycle to orbital forcing following a gradual downward trend of co2

sensitivity of the subpolar atlantic climate to local winds 1 the mild climate of northern europe is thought to be in part maintained by the deep water formation in the northern north atlantic and the associated meridional overturning circulation it has been argued that this circulation is controlled by the wind stress in the southern ocean using a coupled climate model it is shown that the subpolar atlantic wind stress also plays an important role a partial or a complete suppression of this wind stress results in the low salinity arctic waters filling the subpolar atlantic interior as a result the regions of deep water sinking shift southward and the associated overturning circulation convective activity and oceanic heat loss in the northern north atlantic strongly weaken the sea ice advances southward and the north atlantic climate including over much of europe becomes colder

last glacial maximum ice sheet impacts on north atlantic climate variability the importance of the sea ice lid 1 last glacial maximum lgm ice 5g vm2 ice 6g vm5a and paleoclimate modelling intercomparison project phase 3 ice sheet reconstructions are employed in high resolution coupled climate model simulations to investigate the changes they induce in north atlantic climate variability an initial ice 5g vm2 experiment develops a rapid increase of sea ice extent once a thermal threshold is exceeded in this multimillennial simulation subpolar sea ice concentration and thickness are found to be strongly impacted by topographically induced downstream thermal effects from the laurentide ice sheet in each of the reconstructions however in the two additional lgm perturbation experiments the modeled changes in sea ice area are sufficiently similar to the lgm ice 5g vm2 experiment to lead to an equilibrium atlantic meridional overturning circulation strength that is also reduced by the same 40 from preindustrial even though we find significant variation among the models in the deep convection regions of the subpolar glacial north atlantic model predicted sea ice concentrations in this critical region exceed those based upon multiproxy reconstructions and we trace these significant differences to the intensity of the interannual variability of sea ice cover predictions

the southern westerlies during the last glacial maximum in pmip2 simulations the southern hemisphere westerly winds are an important component of the climate system at hemispheric and global scales variations in their intensity and latitudinal position through an ice age cycle have been proposed as important drivers of global climate change due to their influence on deep ocean circulation and changes in atmospheric co2 the position intensity and associated climatology of the southern westerlies during the last glacial maximum lgm however is still poorly understood from empirical and modelling standpoints here we analyse the behaviour of the southern westerlies during the lgm using four coupled ocean atmosphere simulations carried out by the palaeoclimate modelling intercomparison project phase 2 pmip2 we analysed the atmospheric circulation by direct inspection of the winds and by using a cyclone tracking software to indicate storm tracks the models suggest that changes were most significant during winter and over the pacific ocean for this season and region three out four models indicate decreased wind intensities at the near surface as well as in the upper troposphere although the lgm atmosphere is colder and the equator to pole surface temperature gradient generally increases the tropospheric temperature gradients actually decrease explaining the weaker circulation we evaluated the atmospheric influence on the southern ocean by examining the effect of wind stress on the ekman pumping again three of the models indicate decreased upwelling in a latitudinal band over the southern ocean all models indicate a drier lgm than at present with a clear decrease in precipitation south of 40 s over the oceans we identify important differences in precipitation anomalies over the land masses at regional scale including a drier climate over new zealand and wetter over nw patagonia

rain driven by receding ice sheets as a cause of past climate change 1 the younger dryas cold period which interrupted the transition from the last ice age to modern conditions in greenland is one of the most dramatic incidents of abrupt climate change reconstructed from paleoclimate proxy records changes in the atlantic ocean overturning circulation in response to freshwater fluxes from melting ice are frequently invoked to explain this and other past climate changes here we propose an alternative mechanism in which the receding glacial ice sheets cause the atmospheric circulation to enter a regime with greater net precipitation in the north atlantic region this leads to a significant reduction in ocean overturning circulation causing an increase in sea ice extent and hence colder temperatures positive feedbacks associated with sea ice amplify the cooling we support the proposed mechanism with the results of a state of the art global climate model our results suggest that the atmospheric precipitation response to receding glacial ice sheets could have contributed to the younger dryas cooling as well as to other past climate changes involving the ocean overturning circulation

planetary waves kinematically forced by himalayan orography abstract an analysis is made of the planetary scale response of the atmosphere to the kinematic effects of orographic forcing by in particular the tibetan plateau himalayan mountain complex theoretical scaling arguments are used to deduce a critical mountain height hc beyond which the component of flow around will dominate that over the orography the hc is proportional to the meridional scale of the orography and depends on latitude for north south scales appropriate for the himalayas hc 1 5 km which is much less than the actual height of 3706 m when resolved with tour zonal planetary waves with the implication that the around component will dominate a steady state planetary wave model which has a full kinematic nonlinear lower boundary condition is used to simulate the response to the eastern orography whose height has been multiplied by factors ranging from 0 1 to 2 0 although the mountain configuration was fixed the locations of the simulated perturbation highs and lows change substantiall

the thermal threshold of the atlantic meridional overturning circulation and its control by wind stress forcing during glacial climate 1 paleo proxy data suggest that the atlantic meridional overturning circulation amoc was shallower and weaker at the last glacial maximum lgm than at present in this study we have identified the existence of a thermal threshold of the amoc which may explain why many coupled climate models fail to simulate the weaker amoc during the lgm by using results obtained from a coupled climate model and conducting sensitivity simulations with an ocean general circulation model we found that the sudden transition from the present day amoc to the weaker glacial amoc occurs when we gradually change the degree of surface cooling from present day to glacial conditions this result is related to response of deep convection in the northern north atlantic ocean moderate cooling enhances deep convection whereas sufficient cooling results in total covering of sea ice there and suppression of deep convection the findings indicate the existence of a thermal threshold controlling the amoc where the present day type amoc suddenly shifts to the weaker glacial amoc once the surface cooling exceeds this threshold we also demonstrate that wind stress forcing plays a critical role in controlling the value of the thermal threshold our study suggests that slight differences in the degree of surface cooling or wind stress forcing for lgm simulations could lead to the very different response of the amoc during the lgm as reported in previous lgm simulations

last glacial maximum temperatures over the north atlantic europe and western siberia a comparison between pmip models margo sea surface temperatures and pollen based reconstructions abstract evaluating the ability of models to simulate climates different from the modern one is important for climate prediction here we present a first comparison between results from simulations of the last glacial maximum climate and continental and surface ocean reconstructions for the north atlantic europe and western siberia the simulations include prescribed sea surface temperature sst and slab ocean atmospheric general circulation model runs performed within the pmip1 project and atmosphere ocean fully coupled runs performed after pmip1 and within the pmip2 project the surface ocean reconstructions are from the margo project continental reconstructions are based on pollen data over the north atlantic most models simulate the strengthening of the sst meridional gradient suggested by the reconstructions but most do not reproduce the lgm control sst anomaly at the right location nor with the right amplitude over western siberia the model results are much improved when a new ice sheet reconstruction ice 5g is used to force the models the main discrepancy remains for western europe winter temperatures for which lgm control anomalies are significantly underestimated by all models all models indicate that this region during the lgm experienced significantly higher interannual variability in coldest month temperatures compared to the control runs this increased variability could have conspired to bias the apparently extremely cold pollen based temperature reconstructions

centennial scale climate change from decadally paced explosive volcanism a coupled sea ice ocean mechanism northern hemisphere summer cooling through the holocene is largely driven by the steady decrease in summer insolation tied to the precession of the equinoxes however centennial scale climate departures such as the little ice age must be caused by other forcings most likely explosive volcanism and changes in solar irradiance stratospheric volcanic aerosols have the stronger forcing but their short residence time likely precludes a lasting climate impact from a single eruption decadally paced explosive volcanism may produce a greater climate impact because the long response time of ocean surface waters allows for a cumulative decrease in sea surface temperatures that exceeds that of any single eruption here we use a global climate model to evaluate the potential long term climate impacts from four decadally paced large tropical eruptions direct forcing results in a rapid expansion of arctic ocean sea ice that persists throughout the eruption period the expanded sea ice increases the flux of sea ice exported to the northern north atlantic long enough that it reduces the convective warming of surface waters in the subpolar north atlantic in two of our four simulations the cooler surface waters being advected into the arctic ocean reduced the rate of basal sea ice melt in the atlantic sector of the arctic ocean allowing sea ice to remain in an expanded state for 100 model years after volcanic aerosols were removed from the stratosphere in these simulations the coupled sea ice ocean mechanism maintains the strong positive feedbacks of an expanded arctic ocean sea ice cover allowing the initial cooling related to the direct effect of volcanic aerosols to be perpetuated potentially resulting in a centennial scale or longer change of state in arctic climate the fact that the sea ice ocean mechanism was not established in two of our four simulations suggests that a long term sea ice response to volcanic forcing is sensitive to the stability of the seawater column wind and ocean currents in the north atlantic during the eruptions

impact of the north american ice sheet orography on the last glacial maximum eddies and snowfall the present work evaluates the influence of the north american ice sheet orography on the last glacial maximum lgm 21 000 years ago atmospheric circulation and snowfall in the northern mid latitudes focusing on the north atlantic sector three atmospheric general circulation model experiments are analysed a control and an lgm climate simulation and an lgm run with flat ice sheets over northern north america this ice sheet orography affects lee cyclogenesis over north america and forces differences in stationary waves and therefore in the baroclinicity of the mean flow as a result the atlantic storm track is reinforced in the flat ice sheet experiment compared to the lgm one this in turn has a profound impact on snowfall over northern europe implying a coupling between the two ice sheets

linking taiwan s subcritical hsuehshan range topography and foreland basin architecture of 10 1 kilometers and 10 4 10 5 years causal links between the structural and synorogenic stratigraphic architecture of the foreland basin and coincidence of the puli topographic embayment provide a valuable case study of the effects of changing boundary conditions e g variable erodibility or strength of rocks along strike controlling the evolution of critically tapered thin skinned orogens deeper incision of river networks into a thicker sequence of unconsolidated synorogenic sediments in the central western foreland may affect the onset of a topographically subcritical state

along strike growth of the ostler fault new zealand consequences for drainage deflection above active thrusts 1 rarely are geologic records available to constrain the spatial and temporal evolution of thrust fault growth as slip accumulates during repeated earthquake events here we utilize multiple generations of dated and deformed fluvial terraces to explore two key aspects of the along strike kinematic development of the ostler fault zone in southern new zealand over the past 100 k y accumulation of fault slip through space and time and fixed length thrust growth that results in patterns of drainage diversion suggestive of laterally propagating faults along the ostler fault surface deformation patterns revealed by topographic surveying of terrace profiles in nine transverse drainages define systematic variations in fault geometry and suggest deformation over both listric and planar thrust ramps kinematic modeling of folded terrace profiles and 100 fault scarp surveys along major fault sections reveals remarkably similar slip distributions for multiple successions of geomorphic surfaces spanning 100 k y spatially abrupt and temporally sustained displacement gradients across zones of fault section overlap suggest that either persistent barriers to fault propagation or interference between overlapping faults dominate the interactions of fault tips from the scale of individual scarps to the entire fault zone deformed terrace surfaces dated using optically stimulated luminescence and cosmogenic radionuclides indicate steady maximum rates of fault slip of 1 9 mm yr during the late quaternary slip data synthesized along the central ostler fault zone imply that displacement accumulated at approximately constant fault lengths over the past 100 k y a northward temporal progression of abandoned wind gaps along this section thus reflects lateral tilting in response to amplification of displacement rather than simple fault lengthening or lateral propagation oscillations of climate at 104 yr time scales modulate the formation and incision of geomorphic surfaces during successive glacial stages superimposed on apparently steadier rates of fault slip such climate dependent surfaces contribute to a pattern of progressive drainage deflection along the central ostler fault zone that is largely independent of fault propagation

northern hemisphere storm tracks in present day and last glacial maximum climate simulations a comparison of the european pmip models abstract extratropical weather systems are an essential feature of the midlatitude climate and global circulation at the last glacial maximum lgm the formation of regions of high transient activity referred to as storm tracks is strongly affected by the presence of large ice sheets over northern america and scandinavia and by differences in sea surface temperature sst distributions in the framework of the palaeoclimate modelling intercomparison project simulations of the lgm climate have been run with a wide range of atmospheric general circulation models agcms using the same set of boundary conditions allowing a valuable comparison between simulations of a climate very different from the present one in this study the authors focus on the storm track representation in the models and its relationship with the surface temperatures the mean flow and the precipitation storm tracks are described using transient eddy diagnostics such as mean sea level pressure variance and three dimensional

analogue sandbox models of thrust wedges with variable basal frictions abstract scaled analogue sandbox models have been used to simulate the growth of coulomb thrust wedges in isotropic cohesionless and anisotropic cohesionless materials variations in the basal coefficient of friction by using different materials as basal detachments combination of basal detachments and the anisotropy of the layered system have been investigated imbricate fans of dominantly foreland vergent thrust systems are developed in the way similar to those developed in accretionary prisms and in foreland fold and thrust belts critical taper wedges close to theoretically predicted geometries are developed for intermediate values of basal friction mb 0 47 whereas for the lower value of basal friction mb 0 37 low taper wedges are formed with slopes less than the theoretically predicted supra critical wedges are formed when the high basal friction mb 0 55 is either equal to or greater than the coefficient of friction of the deforming material in the wedges then the wedges have a high taper angle close to the angle of rest for the modelling material similar geometries developed when the experiments were carried out on combination of two materials with basal frictions as intermediate low low intermediate and low high the spacing of thrust faults increases with increased basal friction higher basal friction or anisotropy within the layered systems favours displacement along foreland vergent thrusts and eventually checks backward breaking thrusting

a lateglacial climatic reversion in hokkaido northeast asia inferred from the larix pollen record abstract at present larix gmelini is a main component of taiga developed in north sakhalin northeast china and east siberia during the interval from 11 8 to 12 4 ka bp pollen assemblages from north hokkaido japan are dominated by larix pollen assemblages of this lateglacial climatic reversal known as the kenbuchi stadial are similar to pollen assemblages from the last glacial maximum in north hokkaido vegetation of hokkaido inferred from these pollen assemblages open taiga composed of larix gmelini pinus pumila and picea jezoensis and or picea glehnii may have been like that of north sakhalin today comparison of these fossil pollen assemblages from hokkaido with pollen assemblages from modern surface samples in sakhalin suggests that 11 8 to 12 4 ka bp temperatures relative to today were at least 9 c lower in january and 7 c lower in august and that annual precipitation was at least 735 mm lower than present

is the wind stress forcing essential for the meridional overturning circulation 1 we use a global couple d atm osphere ocean sea ice model of interm ediate complexity to demonstrate that wind forcing i s a crucial element to sustain meridional overturning flow i n t he atlantic neglecting wind stress in our multi century lon g simulations leads to a complete shutdown of the conveyor belt ci r culation this r esult may have tremendous impacts f or an assessment of the sensitivity of 2 d clima t e models which typicall y do not capture wind driven gyres it is argued that wind effects may be a key element in determining the f ate and length of a collapsed thc state possible paleo implications will be discussed

late cenozoic reorganization of the arabia eurasia collision and the comparison of short term and long term deformation rates the arabia eurasia collision deforms an area of 3 000 000 km2 of continental crust making it one of the largest regions of convergent deformation on earth there are now estimates for the active slip rates total convergence and timing of collision related deformation of regions from western turkey to eastern iran this paper shows that extrapolating the present day slip rates of many active fault systems for 3 7 million years accounts for their total displacement this result means that the present kinematics of the arabia eurasia collision are unlikely be the same as at its start which was probably in the early miocene 16 23 ma or earlier in some but not all active fault systems short term 10 year and long term 5 million year average deformation rates are consistent there is little active thickening across the turkish iranian plateau and possibly the interior of the greater caucasus these are two areas where present shortening rates would need more than 7 million years to account for the total crustal thickening and where there are structural and or stratigraphic data for pre late miocene deformation we suggest that once thick crust up to 60 km built up in the turkish iranian plateau and the greater caucasus convergence took place more easily by crustal shortening in less elevated regions such as the zagros simple folded zone the south caspian region and foothills of the greater caucasus or in other ways such as westward transport of turkey between the north and east anatolian faults the time and duration of this changeover are not known for certain and are likely be diachronous although deformation started or intensified in many of the currently active fault systems at 5 2 ma

active tectonics of the turkish iranian plateau 1 this paper examines how active faulting in the turkey iran caucasus region accommodates the arabia eurasia collision and the velocity field observed by gps the overall shortening across the zone is in general spatially separated partitioned into right lateral strike slip in the turkish iranian plateau and thrusting in the greater caucasus a band of counterclockwise rotating nw se right lateral strike slip faults accommodates a nw se gradient in ne directed velocity relative to eurasia between the black and caspian seas a nnw sse band of previously unrecognized oblique normal faults is present on the turkey iran border we estimate the offsets on faults from geomorphological features and show that these offsets can be achieved in 5 2 ma at present rates this implies a reorganization of deformation in the collision zone at that time after the initial collision at 12 ma probably in response to mantle induced dynamic uplift

the geomorphologic responses to hinge migration in the fault related folds in the southern tunisian atlas in kinematic models of fault related folds hinges must be mobile during fold growth yet hinge migration has been rarely demonstrated in nature several complementary and independent geomorphological responses are presented here that indicate fold hinge migration in the southern tunisian atlas the deformation of the overlying quaternary pediment suggests recent migration of the forelimb synclinal hinge as predicted by the rafini and mercier model the uplift of the first order transverse drainage network the poor maturation and shallow incision features of limb drainage networks toward the base of the limb slope suggest that new and un eroded material was regularly uplifted from the down slope flat plains as predicted by the fault propagation fold model

on the role of wind driven sea ice motion on ocean ventilation abstract simulations with a coupled ocean atmosphere sea ice model are used to investigate the role of wind driven sea ice motion on ocean ventilation two model experiments are analyzed in detail one including and the other excluding wind driven sea ice transport model simulated concentrations of chlorofluorocarbons cfcs are compared with observations from the weddell sea the southeastern pacific and the north atlantic the authors show that the buoyancy fluxes associated with sea ice divergence control the sites and rates of deep and intermediate water formation in the southern ocean divergence of sea ice along the antarctic perimeter facilitates bottom water formation in the weddell and ross seas neglecting wind driven sea ice transport results in unrealistic bottom water formation in drake passage and too strong convection along the southern ocean sea ice margin whereas convection in the weddell and ross seas is suppressed the freshwater fluxes implicitly associated with sea ice export also

control of synorogenic sedimentation on back and out of sequence thrusting insights from analog modeling of an orogenic front outer carpathians southern poland 1 scaled sandbox models based on field and seismic data were used to simulate miocene evolution of the carpathian orogenic front the experiments investigated the influence of synorogenic deposition on the geometry and kinematics of growing tectonic structures the deposition introduced at the foreland of an active fold thrust wedge resting on top of a weak ductile detachment horizon generally results in transferring the deformation to the rear part of the model depending on the rate and across strike extent of synorogenic sedimentation and on the moment it starts a single major back thrust above the rear edge of the ductile layer can form or out of sequence foreland verging thrusts can be initiated alternatively older thrust faults can be reactivated at the rear part of the model thrust wedge leading to destruction of the earlier formed structures our experiments explain the origin and evolution of three different structural geometries known from the carpathian orogenic front involving major back thrusts related to triangle zones and out of sequence breaching forethrusts that substantially reorganized deformation patterns in a growing thrust wedge the development of these geometries was controlled by massive synorogenic sedimentation in the foreland from the modeling and natural examples we infer a number of typical structural situations that can occur at an orogenic front underlain by a weak ductile horizon depending on parameters controlling synorogenic sedimentation and tectonic push exerted by an approaching thrust wedge

reduced atlantic storminess during last glacial maximum evidence from a coupled climate model the last glacial maximum lgm 21 000 yr before present was the time of maximum land ice extent during the last ice age a recent simulation of the lgm climate by a state of the art fully coupled global climate model is shown to exhibit strong steady atmospheric jets and weak transient eddy activity in the atlantic sector compared to today s climate in contrast previous work based on uncoupled atmospheric model simulations has shown that the lgm jets and eddy activity in the atlantic sector are similar to those observed today with the main difference being a northeastward extension of their maxima the coupled model simulation is shown to agree more with paleoclimate proxy records and thus is taken as the more reliable representation of lgm climate the existence of this altered atmospheric circulation state during lgm in the model has implications for understanding the stability of glacial climates for the possibility of multiple atmospheric circulation regimes and for the interpretation of paleoclimate proxy records

reversed north atlantic gyre dynamics in present and glacial climates the dynamics of the north atlantic subpolar gyre spg are assessed under present and glacial boundary conditions by investigating the spg sensitivity to surface wind stress changes in a coupled climate model to this end the gyre transport is decomposed in ekman thermohaline and bottom transports surface wind stress variations are found to play an important indirect role in spg dynamics through their effect on water mass densities our results suggest the existence of two dynamically distinct regimes of the spg depending on the absence or presence of deep water formation dwf in the nordic seas and a vigorous greenland scotland ridge gsr overflow in the first regime the gsr overflow is weak and the spg strength increases with wind stress as a result of enhanced outcropping of isopycnals in the centre of the spg as soon as a vigorous gsr overflow is established its associated positive density anomalies on the southern gsr slope reduce the spg strength this has implications for past glacial abrupt climate changes insofar as these can be explained through latitudinal shifts in north atlantic dwf sites and strengthening of the north atlantic current regardless of the ultimate trigger an abrupt shift of dwf into the nordic seas could result both in a drastic reduction of the spg strength and a sudden reversal in its sensitivity to wind stress variations our results could provide insight into changes in the horizontal ocean circulation during abrupt glacial climate changes which have been largely neglected up to now in model studies

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

abrupt glacial climate shifts controlled by ice sheet changes the volume of the northern hemisphere ice sheet controlled abrupt millennial scale climate changes during the last glacial dansgaard oeschger events are millennial scale warm periods interrupting cold glacial climates although they are perhaps the most prominent example of abrupt change observed in the ice core record of the past 800 000 years their underlying physical controls remain debated xu zhang and colleagues now use a fully coupled climate model to show that dansgaard oeschger events tend to occur when ice sheets are at an intermediate height a condition in which small perturbations to the coupled atmosphere ocean sea ice system can lead to rapid nonlinear feedbacks and abrupt changes in climate during glacial periods of the late pleistocene an abundance of proxy data demonstrates the existence of large and repeated millennial scale warming episodes known as dansgaard oeschger do events1 this ubiquitous feature of rapid glacial climate change can be extended back as far as 800 000 years before present bp in the ice core record2 and has drawn broad attention within the science and policy making communities alike3 many studies have been dedicated to investigating the underlying causes of these changes but no coherent mechanism has yet been identified3 4 5 6 7 8 9 10 11 12 13 14 15 here we show by using a comprehensive fully coupled model16 that gradual changes in the height of the northern hemisphere ice sheets nhiss can alter the coupled atmosphere ocean system and cause rapid glacial climate shifts closely resembling do events the simulated global climate responses including abrupt warming in the north atlantic a northward shift of the tropical rainbelts and southern hemisphere cooling related to the bipolar seesaw are generally consistent with empirical evidence1 3 17 as a result of the coexistence of two glacial ocean circulation states at intermediate heights of the ice sheets minor changes in the height of the nhiss and the amount of atmospheric co2 can trigger the rapid climate transitions via a local positive atmosphere ocean sea ice feedback in the north atlantic our results although based on a single model thus provide a coherent concept for understanding the recorded millennial scale variability and abrupt climate changes in the coupled atmosphere ocean system as well as their linkages to the volume of the intermediate ice sheets during glacials

southern ocean sea ice and radiocarbon ages of glacial bottom waters abstract analyzing simulations of glacial ocean circulation and radiocarbon distribution i show that increased sea ice cover over the southern ocean reduces ventilation and radiocarbon content of the deep ocean two simulations one present day and one glacial tuned to have similar rates of north atlantic deep water formation are used insulation from air sea gas exchange due to more extended sea ice cover increases southern ocean radiocarbon ages by more than 100 yr higher rates of sea ice formation and export from high southern latitudes lead to a salinification of antarctic bottom water aabw making it the most saline deep water mass of the glacial world oceans this increases the density of aabw and hence its rate of formation mass and radiocarbon fluxes to the deep ocean are thus decoupled both older southern ocean waters and a stronger flux of aabw increase radiocarbon ages of glacial bottom waters by up to 300 yr available reconstructions of glacial bottom water properties are broadly consistent with the simulation these results question previous inferences from radiocarbon distributions on glacial deep water formation rates

stabilization of thermohaline circulation by wind driven and vertical diffusive salt transport the stability of the thermohaline circulation is investigated using an ocean general circulation model coupled to a simple atmospheric model the atmospheric model is so developed that it represents the wind stress and the freshwater flux more realistically than existing energy balance models the coupled model can reproduce the realistic deep ocean circulation without any flux adjustment effects of the wind stress and the vertical diffusion on the thermohaline circulation are studied by conducting various experiments with the coupled model the ekman upwelling between 60 n and 90 n brings up salt to the sea surface while the compensation flow of the ekman transport and the wind driven gyre circulation between 30 n and 60 n carry salt horizontally to the high latitudes by carrying out experiments where the wind stress is completely or partly removed it is demonstrated that either of the vertical or the horizontal salt transport prevents the halocline formation at high latitudes and maintains the thermohaline circulation for an experiment in which the vertical diffusivity is enhanced at high latitudes it is shown that the vertical diffusion at high latitudes also prevents the halocline formation and stabilizes the thermohaline circulation it is also shown that the value of the vertical diffusivity at high latitude affects the existence of the multiple equilibria of the thermohaline circulation

northern hemisphere storm tracks during the last glacial maximum in the pmip2 ocean atmosphere coupled models energetic study seasonal cycle precipitation mid latitude eddies are an important component of the climatic system due to their role in transporting heat moisture and momentum from the tropics to the poles and also for the precipitation associated with their fronts especially in winter we study northern hemisphere storm tracks at the last glacial maximum lgm and their influence on precipitation using ocean atmosphere general circulation model oagcm simulations from the second phase of the paleoclimate modelling intercomparison project pmip2 the difference with pmip1 results in terms of sea surface temperature forcing fundamental for storm track dynamics is large especially in the eastern north atlantic where sea ice extends less to the south in oagcms compared to atmospheric only gcms our analyses of the physics of the eddies are based on the equations of eddy energetics all models simulate a consistent southeastward shift of the north pacific storm track in winter related to a similar displacement of the jet stream partly forced by the eddies themselves precipitation anomalies are consistent with storm track changes with a southeastward displacement of the north pacific precipitation pattern the common features of north atlantic changes in the lgm simulations consist of a thinning of the storm track in its western part and an amplification of synoptic activity to the southeast in the region between the azores islands and the iberian peninsula which reflects on precipitation this southeastward extension is related to a similar displacement of the jet partly forced by the eddies in the western north atlantic the synoptic activity anomalies are at first order related to baroclinic generation term anomalies but the mean flow baroclinicity increase due to the presence of the laurentide ice sheet is partly balanced by a loss of eddy efficiency to convert energy from the mean flow moisture availability in this region is greatly reduced due to more advection of dry polar air by stationary waves leading to less synoptic scale latent heat release and hence less precipitation also in terms of seasonality the stormy season is shifted later in the year by a few days to a month depending on the season and the model considered this shift does not directly reflect on the first order seasonal cycle of precipitation which also depends on other mechanisms especially in summer

influence of syntectonic sedimentation on thrust wedges in analogue models scaled analogue sandbox models have been used to simulate the growth of coulomb thrust wedges that include syntectonic sedimentation during thrusting increases in the syncontractional sedimentation rate dramatically change the shape of the coulomb thrust wedge decreasing the number of thrust slices the internal shortening and the critical taper of the wedge thrust trajectories change from ramp flat to sigmoidal with increased syncontractional sedimentation the analogue models show many similarities to thrust fault geometries in accretionary prisms and foreland fold and thrust belts in which thrust slices prograded into basins with active sedimentation

a numerical study on the atmospheric circulation over the midlatitude north pacific during the last glacial maximum abstract the dynamics of the atmospheric circulation change over the midlatitude north pacific under the boundary conditions during the last glacial maximum lgm have been studied by atmospheric general circulation models gcms with different ocean feedbacks three boundary conditions in the lgm were different from those of the present day pd ice sheet with elevated topography and high albedo atmospheric co2 concentration and insolation the ocean component was treated as follows a full circulation ocean with dynamical and thermal ocean feedback coupled general circulation model cgcm a slab ocean only with thermal feedback used to calculate the surface heat balance slab ocean gcm sgcm and no ocean feedback by fixing sea surface temperature sst with pure atmospheric dynamics agcm both cgcm and sgcm simulated a weakened pacific high pressure system in boreal summer during the lgm compared to the pd and an intensified aleutian low pressure system in winter both in summer and winter t

high latitude climate sensitivity to ice sheet forcing over the last 120 kyr abstract interpretation of ice core records is currently limited by paucity of modelling at adequate temporal and spatial resolutions several key questions relate to mechanisms of polar amplification and inter hemispheric coupling on glacial interglacial timescales here we present the first results from a large set of global ocean atmosphere climate model snap shot simulations covering the last 120 000 years using the hadley centre climate model hadcm3 at up to 1 kyr temporal resolution two sets of simulations were performed in order to examine the roles of orbit and greenhouse gases versus ice sheet forcing of orbital scale climate change a series of idealised heinrich events were also simulated but no changes to aerosols or vegetation were prescribed this paper focuses on high latitudes and inter hemispheric linkages the simulations reproduce polar temperature trends well compared to ice core reconstructions although the magnitude is underestimated polar amplification varies with obliquity but this variability is dampened by including variations in land ice coverage while the overall amplification factor increases the relatively constant amplification of antarctic temperatures with ice sheet forcing included suggests it is possible to use antarctic temperature reconstructions to estimate global changes which are roughly half the magnitude atlantic ocean overturning circulation varies considerably only with the introduction of northern hemisphere ice sheets but only weakens in the north atlantic in the deep glacial when ocean sea ice feedbacks result in the movement of the region of deep convection to lower latitudes and with the introduction of freshwater to the surface north atlantic in order to simulate heinrich events

higher laurentide and greenland ice sheets strengthen the north atlantic ocean circulation during the last glacial interglacial cycle changes in the large scale north atlantic ocean circulation occurred and at the same time topography of the laurentide and greenland ice sheets also varied in this study we focus on detecting the changes of the north atlantic gyres western boundary current and the atlantic meridional overturning circulation amoc corresponding to different laurentide and greenland ice sheet topographies using an earth system model we conducted simulations for five climate states with different ice sheet topographies pre industrial mid holocene last glacial maximum 32 kilo years before present and eemian interglacial our simulation results indicate that higher topographies of the laurentide and greenland ice sheets strengthen surface wind stress curl over the north atlantic ocean intensifying the subtropical and subpolar gyres and the western boundary currents the corresponding decrease in sea surface height from subtropical to subpolar favors a stronger amoc an offshore shift of the gulf stream is also identified during the glacial periods relative to that during the pre industrial due to lower sea levels explaining a weaker glacial gulf stream detected in proxy data meanwhile the north atlantic gyres and amoc demonstrate a positively correlated relation under each of the climate conditions with higher ice sheets

the force balance of sea ice in a numerical model of the arctic ocean the balance of forces in the sea ice model of hibler 1979 is examined the model predicts that internal stress gradients are an important force in much of the arctic ocean except in summer when they are significant only off the northern coasts of greenland and the canadian archipelago a partition of the internal stress gradient between the pressure gradient and the viscous terms reveals that both are significant although they operate on very different timescales the acceleration term is generally negligible while the sum of coriolis plus sea surface tilt is small thus the seasonal average force balance in fall winter and spring is mostly between three terms of roughly equal magnitudes air drag water drag and internal stress gradients this is also true for the monthly average force balance however we find that there is a transition around the weekly timescale and that on a daily basis the force balance at a particular location and time is often between only two terms either between air drag and water drag or between air drag and internal stress gradients the model is in agreement with the observations of thorndike and colony 1982 in that the correlation between geostrophic wind forcing and the model s ice velocity field is high this result is discussed in the context of the force balance we show that the presence of significant internal stress gradients does not preclude high wind ice correlation a breakdown of the internal stress gradient into component parts reveals that the shear viscous force is far from negligible which casts strong doubt on the theoretical validity of the cavitating fluid approximation in which this component is neglected finally the role of ice pressure is examined by varying the parameter p we find a strong sensitivity in terms of the force balance as well as ice thickness and velocity

insolation driven 100 000 year glacial cycles and hysteresis of ice sheet volume comprehensive climate and ice sheet models show that insolation and internal feedbacks between the climate the ice sheets and the lithosphere asthenosphere system explain the 100 000 year period on which the northern hemisphere ice sheets grow and shrink for the past million years earth s climate has been dominated by a 100 000 year cycle of glaciation involving the advance and retreat of the northern hemisphere ice sheets variations in insolation the solar radiation reaching earth s surface caused by constant changes in earth s orbit and orientation to the sun are closely associated with glaciations but the physical mechanisms driving the cycle have remained unclear now ayako abe ouchi and colleagues demonstrate that insolation variations interact with the size of the ice sheets and the solid earth to control the 100 000 year cycles their model shows that internal feedbacks between insolation climate and the elevation of the land beneath the ice sheet allows ice sheets to grow gradually until a maximum size is reached at that point the lowered ice sheet elevation caused by the delayed rebound of the solid earth means that a small increase in insolation can destroy the ice sheet in just a few thousand years the growth and reduction of northern hemisphere ice sheets over the past million years is dominated by an approximately 100 000 year periodicity and a sawtooth pattern1 2 gradual growth and fast termination milankovitch theory proposes that summer insolation at high northern latitudes drives the glacial cycles3 and statistical tests have demonstrated that the glacial cycles are indeed linked to eccentricity obliquity and precession cycles4 5 yet insolation alone cannot explain the strong 100 000 year cycle suggesting that internal climatic feedbacks may also be at work4 5 6 7 earlier conceptual models for example showed that glacial terminations are associated with the build up of northern hemisphere excess ice 5 8 9 10 but the physical mechanisms underpinning the 100 000 year cycle remain unclear here we show using comprehensive climate and ice sheet models that insolation and internal feedbacks between the climate the ice sheets and the lithosphere asthenosphere system explain the 100 000 year periodicity the responses of equilibrium states of ice sheets to summer insolation show hysteresis11 12 13 with the shape and position of the hysteresis loop playing a key part in determining the periodicities of glacial cycles the hysteresis loop of the north american ice sheet is such that after inception of the ice sheet its mass balance remains mostly positive through several precession cycles whose amplitudes decrease towards an eccentricity minimum the larger the ice sheet grows and extends towards lower latitudes the smaller is the insolation required to make the mass balance negative therefore once a large ice sheet is established a moderate increase in insolation is sufficient to trigger a negative mass balance leading to an almost complete retreat of the ice sheet within several thousand years this fast retreat is governed mainly by rapid ablation due to the lowered surface elevation resulting from delayed isostatic rebound14 15 16 which is the lithosphere asthenosphere response carbon dioxide is involved but is not determinative in the evolution of the 100 000 year glacial cycles

the last glacial cycle transient simulations with an aogcm a number of transient climate runs simulating the last 120 kyr have been carried out using famous a fast atmosphere ocean general circulation model aogcm this is the first time such experiments have been done with a full aogcm providing a three dimensional simulation of both atmosphere and ocean over this period our simulation thus includes internally generated temporal variability over periods from days to millennia and physical detailed representations of important processes such as clouds and precipitation although the model is fast computational restrictions mean that the rate of change of the forcings has been increased by a factor of 10 making each experiment 12 kyr long atmospheric greenhouse gases ghgs northern hemisphere ice sheets and variations in solar radiation arising from changes in the earth s orbit are treated as forcing factors and are applied either separately or combined in different experiments the long term temperature changes on antarctica match well with reconstructions derived from ice core data as does variability on timescales longer than 10 kyr last glacial maximum lgm cooling on greenland is reasonably well simulated although our simulations which lack ice sheet meltwater forcing do not reproduce the abrupt millennial scale climate shifts seen in northern hemisphere climate proxies or their slower southern hemisphere counterparts the spatial pattern of sea surface cooling at the lgm matches proxy reconstructions reasonably well there is significant anti correlated variability in the strengths of the atlantic meridional overturning circulation amoc and the antarctic circumpolar current acc on timescales greater than 10 kyr in our experiments we find that ghg forcing weakens the amoc and strengthens the acc whilst the presence of northern hemisphere ice sheets strengthens the amoc and weakens the acc the structure of the amoc at the lgm is found to be sensitive to the details of the ice sheet reconstruction used the precessional component of the orbital forcing induces 20 kyr oscillations in the amoc and acc whose amplitude is mediated by changes in the eccentricity of the earth s orbit these forcing influences combine to first order in a linear fashion to produce the mean climate and ocean variability seen in the run with all forcings

roles of sea ice surface wind feedback in maintaining the glacial atlantic meridional overturning circulation and climate abstractduring glacial periods climate varies between two contrasting modes the interstadials and stadials these climate changes are often associated with drastic reorganizations of the atlantic

the north american cordillera an impediment to growing the continent wide laurentide ice sheet abstractthis study examines the evolution of a continental scale ice sheet on a triangular representation of north america with and without the influence of the cordilleran region simulations are conducted using a comprehensive atmospheric general circulation model asynchronously coupled to a three dimensional thermomechanical ice sheet model the atmospheric state is updated for every 2 106 km3 increase in ice volume and the coupled model is integrated to steady state in the first experiment a flat continent with no background topography is used the ice sheet evolves fairly zonally symmetric and the equilibrium state is continent wide and has the highest point in the center of the continent this equilibrium ice sheet forces an anticyclonic circulation that results in relatively warmer cooler summer surface temperatures in the northwest southeast owing to warm cold air advection and radiative heating due to reduced cloudiness the second experiment includes a simplified representation of t

arctic oscillation during the mid holocene and last glacial maximum from pmip2 coupled model simulations abstract changes in the arctic oscillation ao during the mid holocene and the last glacial maximum were compared to preindustrial pi simulations using four coupled ocean atmosphere models i e community climate system model ccsm third climate configuration of the met office unified model hadcm3 met office surface exchanges scheme version 2 moses2 l institut pierre simon laplace ipsl and model for interdisciplinary research on climate 3 2 miroc3 2 from the second phase of the paleoclimate modeling intercomparison project results show that the amplitude of the simulated ao during the mid holocene is a little smaller than that of the preindustrial simulation although the ao pattern and vertical structures are similar to those in the preindustrial simulation the polar westerlies are slightly weakened and displaced downward to the lower stratosphere accompanied by weakening of the polar vortex and warming of the cold polar cap region during the last glacial maximum when the northern

influence of glacial ice sheets on the atlantic meridional overturning circulation through surface wind change coupled modeling studies have recently shown that the existence of the glacial ice sheets intensifies the atlantic meridional overturning circulation amoc however most models show a strong amoc in their simulations of the last glacial maximum lgm which is biased compared to reconstructions that indicate both a weaker and stronger amoc during the lgm therefore a detailed investigation of the mechanism behind this intensification of the amoc is important for a better understanding of the glacial climate and the lgm amoc here various numerical simulations are conducted to focus on the effect of wind changes due to glacial ice sheets on the amoc and the crucial region where the wind modifies the amoc first from atmospheric general circulation model experiments the effect of glacial ice sheets on the surface wind is evaluated second from ocean general circulation model experiments the influence of the wind stress change on the amoc is evaluated by applying wind stress anomalies regionally or at different magnitudes as a boundary condition these experiments demonstrate that glacial ice sheets intensify the amoc through an increase in the wind stress at the north atlantic mid latitudes which is induced by the north american ice sheet this intensification of the amoc is caused by the increased oceanic horizontal and vertical transport of salt while the change in sea ice transport has an opposite though minor effect experiments further show that the eurasian ice sheet intensifies the amoc by directly affecting the deep water formation in the norwegian sea

the mechanisms that determine the response of the northern hemisphere s stationary waves to north american ice sheets abstractstationary waves describe the persistent meanders in the west east flow of the extratropical atmosphere here changes in stationary waves caused by ice sheets over north america are examin

maintenance of the sea ice edge abstract a coupled global climate model is used to evaluate processes that determine the equilibrium location of the sea ice edge and its climatological annual cycle the extent to which the wintertime ice edge departs from a symmetric ring around either pole depends primarily on coastlines ice motion and the melt rate at the ice ocean interface at any location the principal drivers of the oceanic heat flux that melts sea ice are absorbed solar radiation and the convergence of heat transported by ocean currents the distance between the ice edge and the pole and the magnitude of the ocean heat flux convergence at the ice edge are inversely related the chief exception to this rule is in the east greenland current where the ocean heat flux convergence just east of the ice edge is relatively high but ice survives due to its swift southward motion and the protection of the cold southward flowing surface water in regions where the ice edge extends relatively far equatorward absorbed solar radiation is t

ice age paleotopography a gravitationally self consistent theory of postglacial relative sea level change is used to infer the variation of surface ice and water cover since the last glacial maximum lgm the results show that lgm ice volume was approximately 35 percent lower than suggested by the climap reconstruction and the maximum heights of the main laurentian and fennoscandian ice complexes are inferred to have been commensurately lower with respect to sea level use of these ice age boundary conditions in atmospheric general circulation models will yield climates that differ significantly from those previously inferred on the basis of the climap data set

a one dimensional model for the interaction between continental scale ice sheets and atmospheric stationary waves the great continental ice sheets of the pleistocene represented a significant topographic obstacle to the westerly winds in northern midlatitudes this work explores how consequent changes in the atmospheric stationary wave pattern might have affected the shape and growth of the ice sheets themselves a one dimensional 1 d model is developed which permits an examination of the types and magnitudes of the feedbacks that might be expected when plausible temperature perturbations are introduced at the ice sheet margin which are proportional to the stationary wave amplitude the equilibrium shape of the ice sheet is significantly altered and depends on the sign of the perturbation the proposed feedback also affects the response of the ice sheet to time varying climate forcing the results suggest that the evolution of a continental scale ice sheet with a land based margin may be significantly determined by the changes it induces in the atmospheric circulation

linear weakening of the amoc in response to receding glacial ice sheets in ccsm3 the transient response of the atlantic meridional overturning circulation amoc to a deglacial ice sheet retreat is studied using the community climate system model version 3 ccsm3 with a focus on orographic effects rather than meltwater discharge it is found that the amoc weakens significantly 41 in response to the deglacial ice sheet retreat the amoc weakening follows the decrease of the northern hemisphere ice sheet volume linearly with no evidence of abrupt thresholds a wind driven mechanism is proposed to explain the weakening of the amoc lowering the northern hemisphere ice sheets induces a northward shift of the westerlies which causes a rapid eastward sea ice transport and expanded sea ice cover over the subpolar north atlantic this expanded sea ice insulates the ocean from heat loss and leads to suppressed deep convection and a weakened amoc a sea ice ocean positive feedback could be further established between the amoc decrease and sea ice expansion

structural geometries and magnitude of shortening in the eastern kura fold thrust belt azerbaijan implications for the development of the greater caucasus mountains 1 the greater caucasus are the northernmost extent of the arabia eurasia collision and are thought to represent the main locus of shortening within the central portion of the collision zone between 40 and 48 e recent work suggests that in detail since the plio pleistocene much of the shortening in the eastern portion of the caucasus system has been focused within the kura fold thrust belt along the southeastern margin of the greater caucasus here we present new field mapping and stratigraphic investigations of the eastern termination of the kura fold thrust belt in azerbaijan to better constrain the structural geometries magnitude of shortening and initiation age for this portion of the fold thrust belt our work suggests that this area of the fold thrust belt exhibits significant along strike variations in structural style and evolution and can effectively be divided into two distinct domains at 48 e the western domain is characterized by a subcritical median surface slope and isolated folds and thrusts propagating out of sequence whereas the eastern domain is dominated by a single duplex structure and a history of in sequence development in a critically tapered wedge we hypothesize that these variations result from changes in relative rates of syn tectonic sedimentation erosion and convergence velocity along strike we find that within the western domain the fold thrust belt has accommodated 12 km of total shortening an unconformity within the western domain brackets the initiation age of this portion of the fold thrust belt to between 1 8 and 0 88 ma yielding permissible average shortening rates of between 6 7 and 13 6 mm yr comparison of these average shortening rates to the geodetically measured shortening rate of 8 mm yr indicates that since initiation the fold thrust belt has accommodated 83 100 of convergence between the greater and lesser caucasus at this longitude

an initial intercomparison of atmospheric and oceanic climatology for the ice 5g and ice 4g models of lgm paleotopography abstract this paper investigates the impact of the new ice 5g paleotopography dataset for last glacial maximum lgm conditions on a coupled model simulation of the thermal and dynamical state of the glacial atmosphere and on both land surface and sea surface conditions the study is based upon coupled climate simulations performed with the ocean atmosphere sea ice model of intermediate complexity climate de bilt coupled large scale ice ocean ecbilt clio model four simulations focusing on the last glacial maximum 21 000 calendar years before present bp have been analyzed a first simulation lgm 4g that employed the original ice 4g ice sheet topography and albedo and a second simulation lgm 5g that employed the newly constructed ice sheet topography denoted ice 5g and its respective albedo intercomparison of the results obtained in these experiments demonstrates that the lgm 5g simulation delivers significantly enhanced cooling over canada compared to the lgm 4g simulation whereas positive t

evaluating southern ocean sea ice for the last glacial maximum and pre industrial climates pmip 2 models and data evidence sea ice is recognized as an important factor controlling climate conditions at high latitudes it has a major impact in controlling the ocean to atmosphere exchanges e g for co2 as well as surface oceanic conditions and deep water formation in the high latitude oceans being at the interface of two major components of the climate system sea ice is useful to evaluate and intercompare different climate models we here analyze the results of eight coupled climate models from which seven are from the pmip 2 database against available reconstructions for winter and summer sea ice extent around antarctica to evaluate the ability of models in representing different climate states we compare their results for both the pre industrial and the last glacial maximum lgm we find that models generally have identical seasonal ranges for the two climate states analyzed which is not supported by data inferences indeed data indicate that the lgm seasonality was enhanced compared to the pre industrial the shape of the sea ice distribution is also different in models circular around the antarctic continent and in reconstructions oval shaped distribution similar to modern sea ice distribution we show that models underestimating the sea ice extent for the pre industrial are also underestimating the last glacial maximum extent our results thus point to difficulties for climate models in representing the sea ice dynamics in the southern ocean

understanding the seasonality of orographically forced stationary waves interaction between mechanical and thermal forcing abstract idealized simulations of the atmosphere s stationary response to the rockies tibetan plateau and the greenland ice sheet are made using a nonlinear quasigeostrophic model and are compared to observations observational data indicate low level heating cooling occurs above the rockies and tibet in the summer winter low level cooling is found above greenland in both seasons the atmosphere responds to both diabatic heating termed thermal forcing and low level flow being obstructed by the mountain s presence termed mechanical forcing the response to thermal and mechanical forcing together can be very different from the response to either forcing individually the presence of modest low level heating or cooling 1 5 k day 1 causes significant changes to the mechanical forcing and thereby to the stationary wave response for example while the nonlinear response to mechanical forcing and low level heating is characterized by a cyclone over the orography the response to mechanical forc

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

dynamic effect of last glacial maximum ice sheet topography on the east asian summer monsoon the effect of ice sheet topography on the east asian summer monsoon easm during the last glacial maximum is studied using ccsm3 in a hierarchy of model configurations it is found that receding ice sheets result in a weakened easm with the reduced ice sheet thickness playing a major role the lower ice sheet topography weakens the easm through shifting the position of the midlatitude jet and through altering northern hemisphere stationary waves in the jet shifting mechanism the lowering of ice sheets shifts the westerly jet northward and decreases the westerly jet over the subtropics in summer which reduces the advection of dry enthalpy and in turn precipitation over the easm region in the stationary wave mechanism the lowering of ice sheets induces an anomalous stationary wave train along the westerly waveguide that propagates into the easm region generating an equivalent barotropic low response this leads to reduced lower tropospheric southerlies which in turn reduces the dry enthalpy advection into east asia and hence the easm precipitation less

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

southern ocean sea ice control of the glacial north atlantic thermohaline circulation 1 a coupled model simulates a shallower and weaker north atlantic deep water circulation at the last glacial maximum compared to the modern with an enhanced intrusion of antarctic bottom water into the north atlantic these circulation changes are caused by the enhanced antarctic bottom water formation which is triggered by the enhanced equatorward sea ice transport ultimately by increased westerlies in the southern ocean at the last glacial maximum

thrust wedges with decollement levels and syntectonic erosion a view from analog models abstract analog sandbox models have been set up to study the impact of syntectonic erosion on thrust wedges with one and two decollement levels different accretion mechanisms are activated depending on interactions between surface processes and wedge mechanics frontal accretion backthrusting underthrusting and underplating due to decollement induced duplex formation at depth these mechanisms may function simultaneously being located at different parts across the wedge for all the experiments a high friction is imposed at the base of models and the volume of eroded material remains equal to the volume of newly accreted material maintaining a constant surface slope during the shortening erosion limits the forward propagation of thrust wedges and favors the underthrusting of basal layers allowing duplex formation erosion promotes development of major backthrusts in the thrust wedges without or with one decollement but no backthrusts was formed in the wedges with two decollements slow erosion allows lower extent of basal underthrusting in comparison with regular rate erosion variations in the erosion taper lead to changes in duplex geometry and exhumation rate in thrust wedges with one or two decollements the 6 erosion taper promotes formation of antiformal stack at the rear part of thrust wedge high rate of basal underthrusting and high extent of erosional removal the cover layers are nearly completely eroded above the antiformal stack and form the synformal klippe in frontal part of thrust wedges the 8 erosion taper allows development of individual ramp anticlines and active forward thrusting of cover layers above the decollement and low rate of basal underplating below it with consequent low extent of erosional removal the results of our experiments support the observations on structural evolution and erosion in the alberta foothills of the canadian rockies

response in atmospheric circulation and sources of greenland precipitation to glacial boundary conditions the response in northern hemisphere atmospheric circulation and the resulting changes in moisture sources for greenland precipitation to glacial boundary conditions are studied in ncar s ccm3 atmospheric general circulation model fitted with a moisture tracking functionality we employ both the climap sst reconstruction and a modification thereto with reconstructions of glacial ice sheets and land masks the individual components of the boundary conditions are added first one at a time and finally together these steps show the atmospheric circulation to respond approximately linearly to the boundary condition changes and the full glacial change may thus be decomposed into contributions from sst and topography changes respectively we find that using the climap sst reconstruction leads to a shift from atlantic toward pacific source regions not found with the modified reconstruction having cooler tropics and less sea ice the occurrence of such a shift depends chiefly on the sst reconstruction and not on the existence of the large northern hemisphere glacial ice sheets the influence of these circulation changes on important factors for ice core interpretation such as precipitation seasonality condensation temperatures and source temperatures are assessed

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

a comparison of pmip2 model simulations and the margo proxy reconstruction for tropical sea surface temperatures at last glacial maximum results from multiple model simulations are used to understand the tropical sea surface temperature sst response to the reduced greenhouse gas concentrations and large continental ice sheets of the last glacial maximum lgm we present lgm simulations from the paleoclimate modelling intercomparison project phase 2 pmip2 and compare these simulations to proxy data collated and harmonized within the multiproxy approach for the reconstruction of the glacial ocean surface project margo five atmosphere ocean coupled climate models aogcms and one coupled model of intermediate complexity have pmip2 ocean results available for lgm the models give a range of tropical defined for this paper as 15 s 15 n sst cooling of 1 0 2 4 c comparable to the margo estimate of annual cooling of 1 7 1 c the models simulate greater sst cooling in the tropical atlantic than tropical pacific but interbasin and intrabasin variations of cooling are much smaller than those found in the margo reconstruction the simulated tropical coolings are relatively insensitive to season a feature also present in the margo transferred based estimates calculated from planktonic foraminiferal assemblages for the indian and pacific oceans these assemblages indicate seasonality in cooling in the atlantic basin with greater cooling in northern summer than northern winter not captured by the model simulations biases in the simulations of the tropical upwelling and thermocline found in the preindustrial control simulations remain for the lgm simulations and are partly responsible for the more homogeneous spatial and temporal lgm tropical cooling simulated by the models the pmip2 lgm simulations give estimates for the climate sensitivity parameter of 0 67 0 83 c per wm 2 which translates to equilibrium climate sensitivity for doubling of atmospheric co2 of 2 6 3 1 c

global glacial isostasy and the surface of the ice age earth the ice 5g vm2 model and grace abstract the 100 kyr quasiperiodic variation of continental ice cover which has been a persistent feature of climate system evolution throughout the most recent 900 kyr of earth history has occurred as a consequence of changes in the seasonal insolation regime forced by the influence of gravitational n body effects in the solar system on the geometry of earth s orbit around the sun the impacts of the changing surface ice load upon both earth s shape and gravitational field as well as upon sea level history have come to be measurable using a variety of geological and geophysical techniques these observations are invertible to obtain useful information on both the internal viscoelastic structure of the solid earth and on the detailed spatiotemporal characteristics of glaciation history this review focuses upon the most recent advances that have been achieved in each of these areas advances that have proven to be central to the construction of the refined model of the global process of glacial isos

stationary waves of the ice age climate abstract a linearized steady state primitive equation model is used to simulate the climatological zonal asymmetries stationary eddies in the wind and temperature fields of the 18 000 ybp climate during winter we compare these results with the eddies simulated in the ice age experiments of broccoli and manabe who used climap boundary conditions and reduced atmospheric co2 in an atmospheric general circulation model gcm coupled with a static mixed layer ocean model the agreement between the models is good indicating that the linear model can be used to evaluate the relative influences of orography diabatic heating and transient eddy heat and momentum transports in generating stationary waves we find that orographic forcing dominates in the ice age climate the mechanical influence of the continental ice sheets on the atmosphere is responsible for most of the changes between the present day and ice age stationary eddies this concept of the ice age climate is complicated by the sensitivity of t

rapid pliocene exhumation of the central greater caucasus constrained by low temperature thermochronometry 1 constraining the timing of onset and rates of deformation within the greater caucasus mountains is key to understanding their role in accommodating deformation across the arabia eurasia orogen we present new low temperature thermochronometric constraints on the cenozoic thermal evolution of the central greater caucasus that elucidate a three phase cooling history between 50 and 30 ma cooling within the range was negligible in oligocene time cooling rates throughout the range increased to 4 c myr these rates remained constant until the early pliocene time when they increased again reaching 25 c myr along the axial part of the range rates and timing of oligocene exhumation are consistent with previous results from the western greater caucasus and are proposed to result from onset of subduction of the greater caucasus back arc basin rapid exhumation of the greater caucasus beginning in pliocene time contrasts with previously reported thermal histories for other portions of the range pliocene exhumation of the central greater caucasus appears to be tectonically driven and coincides with widespread evidence for a major reorganization of the arabia eurasia plate boundary we hypothesize that this exhumation and regionally observed plate reorganization results from the collision of the lesser caucasus with eurasia completing the subduction of oceanic lithosphere across this segment of the arabia eurasia plate boundary