mid to late holocene mediterranean climate variability contribution of multi proxy and multi sequence comparison using wavelet analysis in the northwestern mediterranean basin abstract forcings and mechanisms underlying holocene climate variability still remain poorly this work review already published paleoclimatic time series and proposes an alternative way to compare them using spectral analysis such an approach may emphasize joint features between different signals and lead us closer to the causes of climate changes ten paleoclimatic proxy records from 5 sequences from the gulf of lions and surrounding areas were compiled these paleoclimate time series were supplemented with proxies of the north atlantic oscillations nao el nino southern oscillations enso and the intertropical convergence zone itcz variability a comparison of their frequency content is proposed using wavelet analysis for unevenly sampled time series a new algorithm is used in order to propagate the age model errors within wavelet power spectra three main groups of shared features specific to the mid and late holocene after 7000 yrs cal bp can be defined on the basis of the results of these analyses an atlantic cyclic period solar cyclic periods and tropical cyclic periods the atlantic cyclic period is probably related to fluctuations of the atlantic thermohaline circulation which would induce changes in the storm track extension and position thereby impacting upon precipitation and storminess over a millennial scale the centennial scale solar variability might induce a nao like variability of the atmospheric circulation thereby influencing storminess in western europe and mediterranean finally tropical cyclic periods are possibly registered in of the gulf of lions and atlantic climate proxies potentially highlighting the influence of enso variability over the western mediterranean

spatial fingerprint and magnitude of changes in the atlantic meridional overturning circulation during marine isotope stage 3 pronounced millennial scale climate variability during marine isotope stage 3 mis3 is considered to be linked to changes in the state of the atlantic meridional overturning circulation amoc i e a warm interstadial cold stadial state corresponds to a strong weak amoc based on a series of freshwater hosing extraction experiments with the state of the art community climate system model version 3 we construct a global spatial fingerprint of oceanic temperature anomalies in response to amoc changes under mis3 boundary conditions highest sensitivity to amoc changes especially in summer is found in northeastern north atlantic sea surface temperature but a characteristic temperature fingerprint is also found at subsurface levels after testing significance of the linear sea surface temperature sst amoc regressions the model results are combined with paleo sst records to estimate the magnitude of millennial scale dansgaard oeschger amoc variations during mis3 the results suggest a mean difference in amoc strength between interstadial and non heinrich stadial states of 9 2 1 2 sv 1

exploring the oxygen isotope fingerprint of dansgaard oeschger variability and heinrich events abstract we present the first transient simulations of marine isotope stage 3 mis 3 performed with an oxygen isotope enabled climate model our simulations span several dansgaard oeschger cycles and three heinrich stadials and are directly compared with oxygen isotope records from 13 sediment and 2 ice cores our results are consistent with a 30 50 weakening of the atlantic meridional overturning circulation during dansgaard oeschger stadials and a complete shutdown during heinrich stadials we find that the simulated 18 o anomalies differ significantly between heinrich stadials and non heinrich stadials this difference is mainly due to different responses in ocean circulation and therefore climate impacting oceanic 18 o while the volume of 18 o depleted meltwater plays a secondary role

destabilization of glacial climate by the radiative impact of atlantic meridional overturning circulation disruptions during each of the dramatic global warmings that ended the pleistocene ice ages the atlantic meridional overturning circulation amoc was disrupted it is not clear whether this was a contributing cause or simply an effect of deglaciation here we show that in an ensemble of simulations with a global climate model amoc disruption causes a consistent and sustained positive radiative imbalance of 0 4wm2 the imbalance is accommodated by heat accumulation in the ocean interior representing an overall planetary warming subsequently released by deep convection in the north atlantic when the amoc resumes the results suggest a means by which amoc disruptions could have helped to tip the planet out of stable glaciated states however the fact that amoc disruptions occurred during prior heinrich stadials without causing deglaciation shows that other factors such as ice sheet dynamics or controls on co2 were also key for deglaciation

episodic speleothem deposition tracks the terrestrial impact of millennial scale last glacial climate variability in sw ireland abstract eighty four new u th ages are presented for twenty randomly selected broken displaced and reworked calcite speleothems retrieved from clastic sedimentary fill and from isolated bedding plane shelves in crag cave sw ireland the dated pre holocene samples span much of the last glacial ranging in age from 85 15 0 60 to 23 45 0 17 ka speleothem deposition requires the presence of liquid water and because crag cave is a shallow system deposition is considered likely only when mean annual air temperatures maat exceed the freezing point of water deposition at this mid latitude ocean marginal site occurred episodically during mis5a through to mis2 synchronously within dating uncertainties with the timing of greenland interstadials gi in the latter part of marine isotope stage 3 mis3 deposition was particularly intense consistent with regional scale climate amelioration inferred previously from radiocarbon ages for sparse mis3 organic and freshwater surficial deposits in n ireland a brief episode of speleothem deposition at c 23 40 0 22 ka coincides with gi 2 demonstrating the sensitivity of the site to brief climate amelioration episodes in greenland during mis2 conditions favourable for speleothem deposition occurred periodically during the last glacial indicating temperature changes of at least 10 c between stadials and interstadials at this mid latitude site deposition ceased during greenland stadials gs including during periods of ice rafting in the adjacent n atlantic ocean heinrich events oxygen and carbon isotope ratios of the last glacial speleothems are generally elevated reflecting non equilibrium isotope fractionation effects however establishment of low 13c values often occurred within a few decades of climate amelioration indicating that biogenic co2 production resumed rapidly at this site particularly during mis3 speleothem 18o variability was driven largely by long term changes in the 18o value of the adjacent north atlantic surface water in turn largely reflecting changes in global ice volume in common with published speleothem datasets warming episodes in ireland associated with gi events typically pre date their timing in the gicc05 and gicc05modelext time scales but lie comfortably within the maximum counting uncertainties of these ice core age models

interhemispheric air temperature phase relationships in the nonlinear dansgaard oeschger oscillation it has previously been suggested that the southern ocean might act as a thermal reservoir in mediating the relationship between the northern and southern hemisphere air temperature signals during a dansgaard oeschger cycle on the basis of a climate model based analysis we demonstrate on the contrary that it is equatorial and subtropical north atlantic thermocline waters that act so as to integrate and damp the northern hemisphere signal to fix the amplitude and phase of the southern hemisphere counterpart to the more intense north atlantic component of the oscillation the dynamics of the southern ocean component of the atlantic meridional overturning circulation are critical in explaining the evolution of the interhemispheric temperature phase relationships european project for ice coring in antarctica dronning maud land inferred air temperature variations are in phase with sea surface temperatures north of the antarctic sea ice edge suggesting that it is via a fast atmospheric pathway that this temperature signal is transmitted into the antarctic ice core record

simulating the 128 ka antarctic climate response to northern hemisphere ice sheet melting using the isotope enabled hadcm3 warmer than present antarctic and southern ocean temperatures during the last interglacial approximately 128 000 years ago have been attributed to changes in north south ocean heat transport causing opposing hemispheric temperature anomalies we investigate the magnitude of antarctic warming and antarctic ice core isotopic enrichment in response to northern hemisphere meltwater input during the early last interglacial a 1 600 year hadcm3 simulation driven by 0 25 sv of meltwater input reproduces 50 60 of the peak southern ocean summer sea surface temperature anomaly sea ice retreat and ice core isotope enrichment we also find a robust increase in the proportion of cold season precipitation during the last interglacial leading to lower isotopic values at the antarctic ice core sites these results suggest that a hadcm3 simulation including 0 25 sv for 3 000 4 000 years would reconcile the last interglacial observations providing a potential solution for the last interglacial missing heat problem

space geodetic and water level gauge constraints on continental uplift and tilting over north america regional convergence of the ice 6g_c vm5a vm6 models

ocean circulation ice shelf and sea ice interactions explain dansgaard oeschger cycles the last glacial interval experienced abrupt climatic changes called dansgaard oeschger do events these events manifest themselves as rapid increases followed by slow decreases of oxygen isotope ratios in greenland ice core records despite promising advances a comprehensive theory of the do cycles with their repeated ups and downs of isotope ratios is still lacking here based on earlier hypotheses we introduce a dynamical model that explains the do variability by rapid retreat and slow regrowth of thick ice shelves and thin sea ice in conjunction with changing subsurface water temperatures due to insulation by the ice cover our model successfully reproduces observed features of the records such as the sawtooth shape of the do cycles waiting times between do events across the last glacial and the shifted antiphase relationship between greenland and antarctic ice cores our results show that these features can be obtained via internal feedbacks alone warming subsurface waters could have also contributed to the triggering of heinrich events our model thus offers a unified framework for explaining major features of multimillennial climate variability during glacial intervals

tropical sea surface temperatures for the past four centuries reconstructed from coral archives most annually resolved climate reconstructions of the common era are based on terrestrial data making it a challenge to independently assess how recent climate changes have affected the oceans here as part of the past global changes ocean2k project we present four regionally calibrated and validated reconstructions of sea surface temperatures in the tropics based on 57 published and publicly archived marine paleoclimate data sets derived exclusively from tropical coral archives validation exercises suggest that our reconstructions are interpretable for much of the past 400 years depending on the availability of paleoclimate data within and the reconstruction validation statistics for each target region analysis of the trends in the data suggests that the indian western pacific and western atlantic ocean regions were cooling until modern warming began around the 1830s the early 1800s were an exceptionally cool period in the indo pacific region likely due to multiple large tropical volcanic eruptions occurring in the early nineteenth century decadal scale variability is a quasi persistent feature of all basins twentieth century warming associated with greenhouse gas emissions is apparent in the indian west pacific and western atlantic oceans but we find no evidence that either natural or anthropogenic forcings have altered el nino southern oscillation related variance in tropical sea surface temperatures our marine based regional paleoclimate reconstructions serve as benchmarks against which terrestrial reconstructions as well as climate model simulations can be compared and as a basis for studying the processes by which the tropical oceans mediate climate variability and change

origin and consequences of cyclic ice rafting in the northeast atlantic ocean during the past 130 000 years deep sea sediment cores recovered from the northeast atlantic ocean were examined in order to elucidate the influence of the earth s orbital parameters on major ice rafting analyses of coarse grained ice rafted debris and planktonic foraminifers revealed a strong reaction to the precession signal since 130 000 yr b p dropstone layers have been deposited each half period of a precessional cycle 11 000 1000 yr ice rafting occurs during times of winter minimum summer maximum insolation and summer minimum winter maximum insolation in the first case high summer insolation forces meltwater discharge from the ice sheets into the polar seas which subsequently enhances formation of sea ice during the winter in the second case growth of continental ice enhances iceberg production which also leads to a salinity reduction of surface seawater both situations result in a southward penetration of polar water thus the marine record of dropstones documents ice rafting not only during weichselian stades but also during cold events within interstades the regularity of ice rafting yields a useful framework to calibrate and elucidate climatic changes not only in the region of the north atlantic ocean but also in remote areas such as the pacific ocean and the antarctic

isotope calibrated greenland temperature record over marine isotope stage 3 and its relation to ch4 large temperature variations on millennial time scales in greenland characterised the last ice age abrupt warmings known as dansgaard oeschger do events can be traced in the 18oice record of greenland ice cores however it has been shown that 18oice is not a direct temperature proxy measurements of the isotopic composition of gases trapped in the ice can be used to calibrate the paleothermometer here we present a continuous temperature record based on high resolution 15n measurements and firn model studies it covers a sequence of 9 do events 9 17 during the time period from 38 to 64 kyr bp for which temperature changes of 8 to 15 c were estimated the difference between the modern and the glacial 18oice t relationship can be explained by a combination of source temperature changes and changes in the annual distribution of precipitation a detailed comparison of the temperature evolution with reconstructions of the atmospheric methane ch4 concentration shows that ch4 rises lag temperature increases at the onset of do events by 25 to 70 yr within data resolution the strong correlation between greenland temperature and ch4 on millennial and submillennial time scales suggests that variations on these time scales were probably of hemispheric extent

beyond the bipolar seesaw toward a process understanding of interhemispheric coupling abstract the thermal bipolar ocean seesaw hypothesis was advanced by stocker and johnsen 2003 as the simplest possible thermodynamic model to explain the time relationship between dansgaard oeschger do and antarctic isotope maxima aim events in this review we combine palaeoclimate observations theory and general circulation model experiments to advance from the conceptual model toward a process understanding of interhemispheric coupling and the forcing of aim events we present four main results 1 changes in atlantic heat transport invoked by the thermal seesaw are partially compensated by opposing changes in heat transport by the global atmosphere and pacific ocean this compensation is an integral part of interhemispheric coupling with a major influence on the global pattern of climate anomalies 2 we support the role of a heat reservoir in interhemispheric coupling but argue that its location is the global interior ocean to the north of the antarctic circumpolar current acc not the commonly assumed southern ocean 3 energy budget analysis indicates that the process driving antarctic warming during aim events is an increase in poleward atmospheric heat and moisture transport following sea ice retreat and surface warming over the southern ocean 4 the antarctic sea ice retreat is itself driven by eddy heat fluxes across the acc amplified by sea ice albedo feedbacks the lag of antarctic warming after amoc collapse reflects the time required for heat to accumulate in the ocean interior north of the acc predominantly the upper 1500 m before it can be mixed across this dynamic barrier by eddies

assimilating the ice 6g_c reconstruction of the latest quaternary ice age cycle into numerical simulations of the laurentide and fennoscandian ice sheets we analyze the effects of nudging 100 kyr numerical simulations of the laurentide and fennoscandian ice sheets towards the glacial isostatic adjustment based gia based ice 6g c reconstruction of the most recent ice age cycle starting with the ice physics approximations of the pism ice sheet model and the searise simulation protocols we incorporate nudging at characteristic time scales f through anomalous mass balance terms in the ice mass conservation equation as should be expected these mass balances exhibit physically unrealistic details arising from pure gia based reconstruction geometry when nudging is very strong f 20 yr for north america while weakly nudged f 1000 yr solutions deviate from ice 6g c sufficiently to degrade its observational fit quality for reasonable intermediate time scales f 100 yr and 200 yr we perturbatively analyze nudged ice dynamics as a superposition of leading order smoothing that diffuses ice 6g c in a physically and observationally consistent manner and higher order deviations arising for instance from biases in the time dependence of surface climate boundary conditions based upon the relative deviations between respective nudged simulations in which these biases follow surface temperature from ice cores and eustatic sea level from marine sediment cores we compute ice core climate adjustments that suggest how local paleoclimate observations may be applied to the systematic refinement of ice 6g c our results are consistent with a growing body of evidence suggesting that the geographical origins of meltwater pulse 1b mwp1b may lie primarily in north america as opposed to antarctica as reconstructed in ice 6g c

thermohaline instability and the formation of glacial north atlantic super polynyas at the onset of dansgaard oeschger warming events late quaternary rapid warming events inferred on the basis of oxygen isotopic data from greenland ice cores are the most prominent characteristic of millennial scale dansgaard oeschger oscillations in a coupled climate model simulation which has accurately reproduced this oscillatory behavior for the first time we show that formation of a glacial north atlantic super polynya characterizes the initial stage of transition from cold stadial to warm interstadial conditions the winter polynya forms within the otherwise sea ice covered north atlantic as a consequence of the onset of a thermohaline convective instability beneath an extensive stadial sea ice lid early in the stadial period the tendency for thermal convective instability of an extensive warm pool beneath the sea ice lid is strongly inhibited by a stabilizing vertical salinity gradient which gradually diminishes until a thermohaline convective instability occurs that leads to polynya formation and the rapid retreat of north atlantic sea ice cover

a salt oscillator in the glacial atlantic 2 a scale analysis model a proposal has been made by broecker et al 1990 that rapid changes on a time scale of a thousand years or so seen over much of the last major glacial in the greenland ice core record represent significant climate changes and are caused by a salt oscillator in the glacial atlantic this proposal is examined in terms of a rudimentary quantitative model scale analysis asserts that heat transported to the high latitude atmosphere when the thermohaline circulation is turned on is large enough to produce the melting rates found by fairbanks 1989 for the time interval around that of the younger dryas event and that these melting rates are of the same order of magnitude as the mass flux associated with water vapor flux to the pacific ocean estimated by broecker 1989 scale analysis also suggests that the salinity fluxes associated with 1 the water vapor flux mechanism 2 the rapid melting episodes of fairbanks 3 possibly ice sheet growth events 4 net transport by the thermohaline circulation and 5 net transport by turbulent eddy mixing are roughly of the same order of magnitude and therefore may be important mechanisms for producing salinity oscillations on a time scale of a few thousands of years see broecker 1990 by integration of simple salt conservation equations it is found that model oscillations with a period of a few thousand years occur over a significant range of salinity fluxes estimated fluxes fall well within the range for which oscillations exist the model also suggests that there may exist close coupling between the european scandinavian ice sheets and the bimodal response of the oscillator that is significant increases or decreases in continental ice volume may accompany each complete cycle of the oscillator in addition it appears that continental ice may be required for the salt oscillator to function a crucial element which cannot adequately be investigated with the present model concerns the local effect of salinity source sinks associated with melt water production the proximity of these source regions on the neighboring ice sheets to the local regions where production of deep water occurs may play a critical role in the functioning of the proposed salt oscillator in addition further treatment of thermodynamics is needed to investigate the feasibility of a salinity driven oscillator

the antarctic dipole and its predictability this study investigates the nature of interan nualvariability ofantarcticseaiceanditsrelationshipwith the tropical climate we nd that the dominant interan nual variance structure in the sea ice edge and surface air temperature elds is organized as a quasi stationary wave which we call the antarctic dipole adp it is charac terized by an out of phase relationship between the ice and temperature anomalies in the central eastern pacic and atlantic sectors of the antarctic the dipole consists of a strong standing mode and a weaker propagating motion within each basin s ice eld it has the same wavelength as the antarctic circumpolar wave acw and dominates the acw variance the dipole is clearly associated with tropical enso events it can be predicted with moderate skill using linear regression involving surface temperature two to four months ahead the prediction performs better in extreme warm cold years and best in la ni na years

heinrich events massive late pleistocene detritus layers of the north atlantic and their global climate imprint 1 millennial climate oscillations of the glacial interval are interrupted by extreme events the so called heinrich events of the north atlantic their near global footprint is a testament to coherent interactions among earth s atmosphere oceans and cryosphere on millennial timescales heinrich detritus appears to have been derived from the region around hudson strait it was deposited over approximately 500 250 years several mechanisms have been proposed for the origin of the layers binge purge cycle of the laurentide ice sheet jokulhlaup activity from a hudson bay lake and an ice shelf buildup collapse fed by hudson strait to determine the origin of the heinrich events i recommend 1 further studies of the timing and duration of the events 2 further sedimentology study near the hudson strait and 3 greater spatial and temporal resolution studies of the layers as well as their precursory intervals studies of previous glacial intervals may also provide important constraints

glacial ocean overturning intensified by tidal mixing in a global circulation model due to lower sea levels during the last glacial maximum lgm tidal energy dissipation was shifted from the shallow margins into the deep ocean here using a high resolution tide model we estimate that global energy fluxes below 200 m depth were almost quadrupled during the lgm applying the energy fluxes to a consistent tidal mixing parameterization of a global climate model results in a large intensification of mixing global mean vertical diffusivity increases by more than a factor of 3 and consequently the simulated meridional overturning circulation accelerates by 21 46 in the model these effects are at least as important as those from changes in surface boundary conditions our findings contrast with the prevailing view that the abyssal lgm circulation was more sluggish we conclude that changes in tidal mixing are an important mechanism that may have strongly increased the glacial deep ocean circulation and should no longer be neglected in paleoclimate simulations

freshening drives contraction of antarctic bottom water in the australian antarctic basin antarctic bottom water aabw in the australian antarctic basin has become fresher and lighter since the late 1960s with largest changes observed near the sources of aabw the volume of aabw with neutral density n 28 30 kg m 3 decreased by 50 between 1969 1971 and 2008 2012 thinning at a rate of 100 m per decade contraction of dense aabw was balanced primarily by expansion of water lighter than n 28 15 kg m 3 prior to 1995 and by inflation of less dense classes of aabw after 1995 oxygen concentrations remained unchanged in the densest aabw and increased on neutral surfaces for aabw with n 28 31 kg m 3 between 1995 and 2011 2012 consistent with continued ventilation over this time period the observed changes in potential temperature salinity density volume and oxygen of aabw can be accounted for by freshening of the source waters but cannot be explained by changes in formation rate alone key points freshening drives contraction of antarctic bottom water aabw aabw has contracted by about 50 and thinned more than 100 m per decade since 1970 small increase in oxygen on isopycnals suggests continued ventilation of aabw 2014 american geophysical union all rights reserved

relative sea level in the western mediterranean basin a regional test of the ice 7g_na vm7 model and a constraint on late holocene antarctic deglaciation abstract the mediterranean basin is a region of special interest in the study of past and present relative sea level evolution given its location south of the ice sheets that covered large fractions of northern europe during the last glaciation the large number of biological geological and archaeological sea level indicators that have been retrieved from its coastal regions as well as its high density of modern coastal infrastructure models of the glacial isostatic adjustment gia process provide reconstructions of past relative sea level evolution and can be tested for validity against past sea level indicators from the region it is demonstrated herein that the latest ice 7g na vm7 model of the gia process the north american component of which was refined using a full suite of geophysical observables is able to reconcile the vast majority of uniformly analyzed relative sea level constraints available for the western part of the mediterranean basin a region to which it was not tuned we also revisit herein the previously published interpretations of relative sea level information obtained from roman era coastal mediterranean fish tanks analyze the far field influence of the rate of late holocene antarctic ice sheet melting history on the exceptionally detailed relative sea level history available from southern tunisia and extend the analysis to complementary constraints on the history of antarctic ice sheet melting available from islands in the equatorial pacific ocean the analyses reported herein provide strong support for the global exportability of the ice 7g na vm7 model a result that speaks directly to the ability of spherically symmetric models of the internal viscoelastic structure to explain globally distributed observations while also identifying isolated regions of remaining misfit which will benefit from further study

a salt oscillator in the glacial atlantic 1 the concept as shown by the work of dansgaard and his colleagues climate oscillations of one or so millennia duration punctuate much of glacial section of the greenland ice cores these oscillations are characterized by 5 c air temperature changes severalfold dust content changes and 50 ppm co2 changes both the temperature and co2 change are best explained by changes in the mode of operation of the ocean in this paper we provide evidence which suggests that oscillations in surface water conditions of similar duration are present in the record from a deep sea core at 50 n based on this finding we suggest that the greenland climate changes are driven by oscillations in the salinity of the atlantic ocean which modulate the strength of the atlantic s conveyor circulation

abrupt north atlantic circulation changes in response to gradual co2 forcing in a glacial climate state glacial climate is marked by abrupt millennial scale climate changes known as dansgaard oeschger cycles the most pronounced stadial coolings heinrich events are associated with massive iceberg discharges to the north atlantic these events have been linked to variations in the strength of the atlantic meridional overturning circulation however the factors that lead to abrupt transitions between strong and weak circulation regimes remain unclear here we show that in a fully coupled atmosphere ocean model gradual changes in atmospheric co2 concentrations can trigger abrupt climate changes associated with a regime of bi stability of the atlantic meridional overturning circulation under intermediate glacial conditions we find that changes in atmospheric co2 concentrations alter the transport of atmospheric moisture across central america which modulates the freshwater budget of the north atlantic and hence deep water formation in our simulations a change in atmospheric co2 levels of about 15 ppmv comparable to variations during dansgaard oeschger cycles containing heinrich events is sufficient to cause transitions between a weak stadial and a strong interstadial circulation mode because changes in the atlantic meridional overturning circulation are thought to alter atmospheric co2 levels we infer that atmospheric co2 may serve as a negative feedback to transitions between strong and weak circulation modes

dansgaard oeschger oscillations predicted in a comprehensive model of glacial climate a kicked salt oscillator in the atlantic during the period from 60 000 to 35 000 years ago summit greenland ice core records of the oxygen isotopic ratio 18o 16o exhibit intense millennium time scale oscillations these dansgaard oeschger oscillations have been interpreted to represent the variations in north atlantic air temperature caused by correlative changes in the strength of north atlantic deep water production we apply a comprehensive model of glacial climate to unambiguously identify the mechanism responsible for this phenomenon this is shown to involve a salt oscillation of relaxation oscillator form this nonlinear oscillation does not require the existence of feedback due to freshwater release from grounded ice on the continents during the warm phase of the cycle

asymmetric response of the subtropical western south atlantic thermocline to the dansgaard oeschger events of marine isotope stages 5 and 3 abstract the last glacial period 116 11 7 ka bp was an interval characterized by a sequence of abrupt millennial scale events well documented in greenland and antarctica ice cores throughout this period greenland cold stadials were accompanied by warm conditions in the thermocline to intermediate waters of the atlantic ocean that may have played a role in both the basal melting of ice shelves and the rapid atmospheric warming during the onset of warm interstadials climate model simulations indicated an accentuated response of the subtropical western south atlantic thermocline to the disturbances in the atlantic circulation such works encourage investigations upon thermocline deep dwelling planktic foraminifera in this region however a study with this aim was not performed here we present a paleoceanographic reconstruction from the subtropical western south atlantic based on the thermocline planktic foraminifera globorotalia inflata our high resolution 18o record for the last glacial period presents a millennial scale variability that strongly resembles the structure of the greenland dansgaard oeschger cycles during marine isotope stage mis 5 during mis 3 this millennial scale variability is absent or considerably dampened mg ca derived temperature and seawater 18o corrected for ice volume for the mis 5 interval demonstrate that the region was warmer and saltier colder and fresher during early glacial stadials interstadials our data suggest a reorganization of the northward heat transport throughout the last glacial in which regions as far south as 24 s acted as heat reservoirs in periods of weakened atlantic meridional overturning circulation during mis 5 but not necessarily or only marginally during mis 3

northern hemisphere climate control on the environmental dynamics in the glacial black sea lake abstract the marine isotope stage mis 3 stands out due to its abrupt changes from cold and dry stadials to warm and humid interstadials the so called dansgaard oeschger cycles that also affected temperature and rainfall in the black sea region this study is based on a gravity core from the southeastern se black sea that covers the last glacial lake stage from 64 to 20 ka bp by using the composition of major and trace elements in the sediments terrestrial plant derived n alkane flux and sr ca from benthic ostracods we reconstruct the variability of riverine and aeolian input salinity and productivity in the se black sea region in response to the northern hemisphere climate oscillations during colder and drier stadials the aeolian input increased relative to the riverine discharge potentially due to southward shifted and or stronger westerly winds and due to changes in the vegetation cover an evaporation exceeding freshwater supply by rainfall and rivers possibly caused higher salinity and a lower lake level the environmental status during mis 4 and 2 is very much comparable with the stadial conditions during mis 3 during warmer and more humid interstadials lower salinity and presumably positive lake level changes most likely resulted from increased precipitation and river discharge this likely increased primary productivity through an augmented nutrient supply lowest average salinities are suggested for the middle part of mis 3 in response to enhanced meltwater from the disintegrating fennoscandian ice sheet and or by generally more humid conditions

coherent resonant millennial scale climate oscillations triggered by massive meltwater pulses the role of mean and stochastic freshwater forcing on the generation of millennial scale climate variability in the north atlantic is studied using a low order coupled atmosphere ocean sea ice model it is shown that millennial scale oscillations can be excited stochastically when the north atlantic ocean is fresh enough this finding is used in order to interpret the aftermath of massive iceberg surges heinrich events in the glacial north atlantic which are characterized by an excitation of dansgaard oeschger events based on model results it is hypothesized that heinrich events trigger dansgaard oeschger cycles and that furthermore the occurrence of heinrich events is dependent on the accumulated climatic effect of a series of dansgaard oeschger events this scenario leads to a coupled ocean ice sheet oscillation that shares many similarities with the bond cycle further sensitivity experiments reveal that the timescale of the oscillations can be decomposed into stochastic linear and nonlinear deterministic components a schematic bifurcation diagram is used to compare theoretical results with paleoclimatic data

high resolution record of northern hemisphere climate extending into the last interglacial period high resolution record of northern hemisphere climate extending into the last interglacial period

the antarctica component of postglacial rebound model ice 6g_c vm5a based on gps positioning exposure age dating of ice thicknesses and relative sea level histories

space geodesy constrains ice age terminal deglaciation the global ice 6g_c vm5a model a new model of the last deglaciation event of the late quaternary ice age is here described and denoted as ice 6g c vm5a it differs from previously published models in this sequence in that it has been explicitly refined by applying all of the available global positioning system gps measurements of vertical motion of the crust that may be brought to bear to constrain the thickness of local ice cover as well as the timing of its removal additional space geodetic constraints have also been applied to specify the reference frame within which the gps data are described the focus of the paper is upon the three main regions of last glacial maximum ice cover namely north america northwestern europe eurasia and antarctica although greenland and the british isles will also be included if peripherally in the discussion in each of the three major regions the model predictions of the time rate of change of the gravitational field are also compared to that being measured by the gravity recovery and climate experiment satellites as an independent means of verifying the improvement of the model achieved by applying the gps constraints several aspects of the global characteristics of this new model are also discussed including the nature of relative sea level history predictions at far field locations in particular the caribbean island of barbados from which especially high quality records of postglacial sea level change are available but which records were not employed in the development of the model although ice 6g c vm5a is a significant improvement insofar as the most recently available gps observations are concerned comparison of model predictions with such far field relative sea level histories enables us to identify a series of additional improvements that should follow from a further stage of model iteration

dansgaard oeschger cycles interactions between ocean and sea ice intrinsic to the nordic seas dansgaard oeschger d o cycles are the most dramatic frequent and wide reaching abrupt climate changes in the geologic record on greenland d o cycles are characterized by an abrupt warming of 10 5 c from a cold stadial to a warm interstadial phase followed by gradual cooling before a rapid return to stadial conditions the mechanisms responsible for these millennial cycles are not fully understood but are widely thought to involve abrupt changes in atlantic meridional overturning circulation due to freshwater perturbations here we present a new high resolution multiproxy marine sediment core monitoring changes in the warm atlantic inflow to the nordic seas as well as in local sea ice cover and influx of ice rafted debris in contrast to previous studies the freshwater input is found to be coincident with warm interstadials on greenland and has a fennoscandian rather than laurentide source furthermore the data suggest a different thermohaline structure for the nordic seas during cold stadials in which relatively warm atlantic water circulates beneath a fresh surface layer and the presence of sea ice is inferred from benthic oxygen isotopes this implies a delicate balance between the warm subsurface atlantic water and fresh surface layer with the possibility of abrupt changes in sea ice cover and suggests a novel mechanism for the abrupt d o events observed in greenland ice cores

impact of abrupt sea ice loss on greenland water isotopes during the last glacial period greenland ice cores provide excellent evidence of past abrupt climate changes however there is no universally accepted theory of how and why these dansgaard oeschger do events occur several mechanisms have been proposed to explain do events including sea ice ice shelf buildup ice sheets atmospheric circulation and meltwater changes do event temperature reconstructions depend on the stable water isotope formula see text o and nitrogen isotope measurements from greenland ice cores interpretation of these measurements holds the key to understanding the nature of do events here we demonstrate the primary importance of sea ice as a control on greenland ice core formula see text o 95 of the variability in formula see text o in southern greenland is explained by do event sea ice changes our suite of do events simulated using a general circulation model accurately captures the amplitude of formula see text o enrichment during the abrupt do event onsets simulated geographical variability is broadly consistent with available ice core evidence we find an hitherto unknown sensitivity of the formula see text o paleothermometer to the magnitude of do event temperature increase the change in formula see text o per kelvin temperature increase reduces with do event amplitude we show that this effect is controlled by precipitation seasonality

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

climate change in the western antarctic peninsula since 1945 observations and possible causes temperature records from stations on the west coast of the antarctic peninsula show a very high level of interannual variability and over the last 50 years larger warming trends than are seen elsewhere in antarctica in this paper we investigate the role of atmospheric circulation variability and sea ice extent variations in driving these changes owing to a lack of independent data the reliability of antarctic atmospheric analyses produced in the 1950s and 1960s cannot be readily established but examination of the available data suggests that there has been an increase in the northerly component of the circulation over the peninsula since the late 1950s few observations of sea ice extent are available prior to 1973 but the limited data available indicate that the ice edge to the west of the peninsula lay to the north of recently observed extremes during the very cold conditions prevailing in the late 1950s the ultimate cause of the atmospheric circulation changes remains to be determined and may lie outside the antarctic region

stochastic atmospheric forcing as a cause of greenland climate transitions abstractan unforced simulation of the community climate system model version 4 ccsm4 is found to have greenland warming and cooling events that resemble dansgaard oeschger cycles in pattern and magnitude with the caveat that only three transitions were available to be analyzed it is found that the transitions are triggered by stochastic atmospheric forcing the atmospheric anomalies change the strength of the subpolar gyre leading to a change in labrador sea sea ice concentration and meridional heat transport the changed climate state is maintained over centuries through the feedback between sea ice and sea level pressure in the north atlantic indications that the initial atmospheric pressure anomalies are preceded by precipitation anomalies in the western pacific warm pool are discussed the full evolution of the anomalous climate state depends crucially on the climatic background state

a minimum thermodynamic model for the bipolar seesaw 1 the simplest possible model is proposed to explain a large fraction of the millennial climate variability measured in the isotopic composition of antarctic ice cores the model results from the classic bipolar seesaw by coupling it to a heat reservoir in this thermal bipolar seesaw the heat reservoir convolves northern time signals with a characteristic timescale applying the model to the data of grip and byrd we demonstrate that maximum correlation can be obtained using a timescale of about 1000 1500 years higher correlations are obtained by first filtering out the long term variability which is due to astronomical and greenhouse gas forcing and not part of the thermal bipolar seesaw the model resolves the apparent confusion whether northern and southern climate records are in or out of phase synchronous or time lagged

century scale perspectives on observed and simulated southern ocean sea ice trends from proxy reconstructions since 1979 when continuous satellite observations began southern ocean sea ice cover has increased whilst global coupled climate models simulate a decrease over the same period it is uncertain whether the observed trends are anthropogenically forced or due to internal variability or whether the apparent discrepancy between models and observations can be explained by internal variability the shortness of the satellite record is one source of this uncertainty and a possible solution is to use proxy reconstructions which extend the analysis period but at the expense of higher observational uncertainty in this work we evaluate the utility for change detection of 20th century southern ocean sea ice proxies we find that there are reliable proxies for the east antarctic amundsen bellingshausen and weddell sectors in late winter and for the weddell sea in late autumn models and reconstructions agree that sea ice extent in the east antarctic amundsen and bellingshausen seas has decreased since the early 1970s consistent with an anthropogenic response however the decrease is small compared to internal variability and the change is not robustly detectable we also find that optimal fingerprinting filters out much of the uncertainty in proxy reconstructions the ross sea is a confounding factor with a significant increase in sea ice since 1979 that is not captured by climate models however existing proxy reconstructions of this region are not yet sufficiently reliable for formal change detection

a new characterization of the turbulent diapycnal diffusivities of mass and momentum in the ocean the diapycnal diffusivity of mass supported by turbulent events in the ocean interior plays a fundamental role in controlling the global overturning circulation the conventional representation of this diffusivity due to osborn 1980 assumes a constant mixing efficiency we replace this methodology by a generalized osborn formula which involves a mixing efficiency that varies nonmonotonically with at least two nondimensional variables using these two variables we propose dynamic parameterizations for mixing efficiency and turbulent prandtl number the latter quantifies the ratio of momentum to mass diapycnal diffusivities based on the first synthesis of an extensive direct numerical simulation of inhomogeneously stratified shear induced turbulence data from argo floats are employed to demonstrate the extent of the spatial and statistical variability to be expected in both the diapycnal diffusivities of mass and momentum we therefore suggest that previous estimates of these important characteristics of the global ocean require reconsideration

relative timing of deglacial climate events in antarctica and greenland the last deglaciation was marked by large hemispheric millennial scale climate variations the bolling allerod and younger dryas periods in the north and the antarctic cold reversal in the south a chronology from the high accumulation law dome east antarctic ice core constrains the relative timing of these two events and provides strong evidence that the cooling at the start of the antarctic cold reversal did not follow the abrupt warming during the northern bolling transition around 14 500 years ago this result suggests that southern changes are not a direct response to abrupt changes in north atlantic thermohaline circulation as is assumed in the conventional picture of a hemispheric temperature seesaw

the role of oceans and sea ice in abrupt transitions between multiple climate states the coupled climate dynamics underlying large rapid and potentially irreversible changes in ice cover are studied a global atmosphere ocean sea ice general circulation model with idealized aquaplanet geometry is forced by gradual multi millennial variations in solar luminosity the model traverses a hysteresis loop between warm ice free conditions and cold glacial conditions in response to 6 5wm 22 variations in global annual mean insolation comparison of several model configurations confirms the importance of polar ocean processes in setting the sensitivity and time scales of the transitions a sawtooth character is found with fasterwarmingandslowercooling reflectingtheopposingeffectsofsurfaceheatingandcoolingonupper ocean buoyancy and thus effective heat capacity the transition from a glacial to warm equable climate occurs in about 200 years in contrast to the freshwater hosing scenario transitions are driven by radiative forcing and sea ice feedbacks the ocean circulation and notably the meridional overturning circulation moc does not drive the climate change the moc and associated heat transport collapses poleward of the advancing ice edge but this is a purely passive response to cooling and ice expansion the moc does however play a key role in setting the time scales of the transition and contributes to the asymmetry between warming and cooling

instability of the atlantic overturning circulation during marine isotope stage 3 variations in the strength of the atlantic meridional overturning circulation amoc were involved in the occurrences of dansgaard oeschger d o events during marine isotope stage 3 mis3 the stability of the amoc to north atlantic freshwater perturbations is studied using a comprehensive climate model under mis3 boundary conditions an amoc stability diagram constructed from a series of equilibrium freshwater perturbation experiments reveals a highly nonlinear dependence of amoc strength on freshwater forcing the mis3 baseline state is remarkably unstable with respect to minor perturbations the global climate signal associated with a change in amoc strength is consistent with a transition from an interstadial to a stadial state including an annual mean surface air temperature drop of 8 k in central greenland we suggest that minor freshwater perturbations in the hydrologic cycle e g related to ice sheet processes had the potential to trigger d o type climate shifts associated with a threshold in the atmosphere ocean system

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

synchronous timing of abrupt climate changes during the last glacial period abrupt climate changes during the last glacial period have been detected in a global array of palaeoclimate records but our understanding of their absolute timing and regional synchrony is incomplete our compilation of 63 published independently dated speleothem records shows that abrupt warmings in greenland were associated with synchronous climate changes across the asian monsoon south american monsoon and european mediterranean regions that occurred within decades together with the demonstration of bipolar synchrony in atmospheric response this provides independent evidence of synchronous high latitude to tropical coupling of climate changes during these abrupt warmings our results provide a globally coherent framework with which to validate model simulations of abrupt climate change and to constrain ice core chronologies

stochastic resonance in the north atlantic large abrupt changes between warm and cold modes of north atlantic climate exhibit spectral power at 1500 years yet some climatic changes were linked to outburst floods and other events that are unlikely to have been truly periodic we hypothesize that a weak periodic forcing has combined with noise from ice sheet related events to cause the observed mode switches this stochastic resonance hypothesis predicts a recurrence pattern between warmings that is distinct from the predictions of simple periodic and stochastic models and at least some other models the ice isotopic data from central greenland ice cores are consistent with the stochastic resonance hypothesis but not with other models we have tested we thus support arguments for the existence of a periodicity of 1500 years in the north atlantic climate system and for the importance of ice sheet events in forcing north atlantic changes

a heuristic model of dansgaard oeschger cycles part i description results and sensitivity studies abstracta simple model for studying the dansgaard oeschger d o cycles of the last glacial period is presented based on the t dokken et al hypothesis for d o cycles the model is a column model representing the nordic seas and is composed of ocean boxes stacked below a one layer sea ice model with an energy balance atmosphere no changes in the large scale ocean overturning circulation are invoked parameterizations are included for latent heat polynyas and sea ice export from the column the resulting heuristic model was found to cycle between stadial and interstadial states at times scales similar to those seen in the proxy observational data with the presence or absence of perennial sea ice in the nordic seas being the defining characteristic for each of these states the major discrepancy between the modeled oscillations and the proxy record is in the length of the interstadial phase which is shorter than that observed the modeled oscillations were found to be robust to parameter changes inclu

the deuterium excess records of epica dome c and dronning maud land ice cores east antarctica new high resolution deuterium excess d data from the two epica ice cores drilled in dronning maud land edml and dome c edc are presented here the main moisture sources for precipitation at edc and edml are located in the indian ocean and atlantic ocean respectively the more southward moisture origin for edml is reflected in a lower present day d value compared to edc the edml and edc isotopic records 18o and d show the main climate features common to the east antarctic plateau and similar millennial scale climate variability during the last glacial period however quite large 18o and d differences are observed during mis5 5 and the glacial inception with a long term behaviour a possibility for this long term difference could be related to uncertainties in past accumulation rate which are used in the glaciological models regional climate anomalies between the two sites during mis5 5 could also be consistent with the observed edml edc 18o and d gradient anomalies simulations performed with the general circulation model echam4 for different time slices provide a temporal temperature isotope slope for the edml region in fair agreement to the modern spatial slope tsite and tsource records are extracted from both ice cores using a modelling approach after corrections for past 18o seawater and elevation changes a limited impact of d on antarctic temperature reconstruction at both edml and edc has been found with a higher impact only at glacial inception the aim antarctic isotope maximum events in both ice cores are visible also after the source correction suggesting that these are real climate features of the glacial period the different shape of the aim events between edc and edml as well as some climate features in the early holocene points to a slightly different climate evolution at regional scale a comparison of our temperature reconstruction profiles with the aerosol fluxes show a strong coupling of the nssca fluxes with antarctic temperatures during glacial period and a tighter coupling of 18o and tsite with ssna flux at edml compared to edc during the glacial period and mis5 5

evidence for general instability of past climate from a 250 kyr ice core record recent results1 2 from two ice cores drilled in central greenland have revealed large abrupt climate changes of at least regional extent during the late stages of the last glaciation suggesting that climate in the north atlantic region is able to reorganize itself rapidly perhaps even within a few decades here we present a detailed stable isotope record for the full length of the greenland ice core project summit ice core extending over the past 250 kyr according to a calculated timescale we find that climate instability was not confined to the last glaciation but appears also to have been marked during the last interglacial as explored more fully in a companion paper3 and during the previous saale holstein glacial cycle this is in contrast with the extreme stability of the holocene suggesting that recent climate stability may be the exception rather than the rule the last interglacial seems to have lasted longer than is implied by the deep sea specmap record4 in agreement with other land based observations5 6 we suggest that climate instability in the early part of the last interglacial may have delayed the melting of the saalean ice sheets in america and eurasia perhaps accounting for this discrepancy

millennial scale oscillations between sea ice and convective deep water formation during the last ice age there were several quasiperiodic abrupt warming events the climatic effects of the so called dansgaard oeschger d o events were felt globally although the north atlantic experienced the largest and most abrupt temperature anomalies similar but weaker oscillations also took place during the interglacial period this paper proposes an auto oscillatory mechanism between sea ice and convective deep water formation in the north atlantic as the source of the persistent cycles a simple dynamical model is constructed by coupling and slightly modifying two existing models of ocean circulation and sea ice the model exhibits mixed mode oscillations consisting of decadal scale small amplitude oscillations and a large amplitude relaxation fluctuation the decadal oscillations occur due to the insulating effect of sea ice and leads to periodic ventilation of heat from the polar ocean gradually an instability builds up in the polar column and results in an abrupt initiation of convection and polar warming the unstable convective state relaxes back to the small amplitude oscillations from where the process repeats in a self sustained manner freshwater pulses mimicking heinrich events cause the oscillations to be grouped into packets of progressively weaker fluctuations as observed in proxy records modulation of this stable oscillation mechanism by freshwater and insolation variations could account for the distribution and pacing of d o and bond events physical aspects of the system such as sea ice extent and oceanic advective flow rates could determine the characteristic 1500 year time scale of d o events the model results with respect to the structure of the water column in the nordic seas during stadial and interstadial phases are in agreement with paleoproxy observations

ocean ice sheet interaction along the se nordic seas margin from 35 to 15 ka bp abstract sediment cores from the south eastern nordic seas simultaneously archive the variability of the fennoscandian ice sheet fis the british irish ice sheet biis and the regional oceanic conditions this study aims to contribute to our understanding of the marine based section of the fis and the biis between 35 000 and 15 000 years bp by using cores md99 2283 md99 2284 and md99 2289 retrieved along the upper continental slope between the faroe shetland channel and the voring plateau for this we present a revised radiocarbon based bayesian modelled chronological framework and a compilation of new and published sedimentological geochemical and micro paleontological datasets our results show a possibly first weichselian fis biis confluence at ca 25 500 years bp in the central north sea which buttressed the biis to the east potentially leading to a northwards biis deflection via the shetlands the norwegian channel ice stream ncis most likely only reached the shelf edge after 23 300 500 years bp possibly for the first time during the weichselian the ncis onset directly preceded a pronounced influx of warm atlantic water to the northern north sea margin possibly implying forcing through ocean melt we find a highly variable ncis with three 1400 yearlong episodes of increased ice rafted debris flux interrupted by 600 yearlong minima when compared to other sides of the european ice sheet these episodes appear to correlate well suggesting a common forcing mechanism in conclusion our data supports recent suggestions that the last glacial stage of the biis was more extensive in the central north sea and the confluence later than previously thought

influence of external forcings on abrupt millennial scale climate changes a statistical modelling study the last glacial period was punctuated by a series of abrupt climate shifts the so called dansgaard oeschger do events the frequency of do events varied in time supposedly because of changes in background climate conditions here the influence of external forcings on do events is investigated with statistical modelling we assume two types of simple stochastic dynamical systems models double well potential type and oscillator type forced by the northern hemisphere summer insolation change and or the global ice volume change the model parameters are estimated by using the maximum likelihood method with the ngrip hbox ca 2 record the stochastic oscillator model with at least the ice volume forcing reproduces well the sample autocorrelation function of the record and the frequency changes of warming transitions in the last glacial period across miss 2 3 and 4 the model performance is improved with the additional insolation forcing the bic scores also suggest that the ice volume forcing is relatively more important than the insolation forcing though the strength of evidence depends on the model assumption finally we simulate the average number of warming transitions in the past four glacial periods assuming the model can be extended beyond the last glacial and compare the result with an iberian margin sea surface temperature sst record martrat et al in science 317 5837 502 507 2007 the simulation result supports the previous observation that abrupt millennial scale climate changes in the penultimate glacial mis 6 are less frequent than in the last glacial miss 2 4 on the other hand it suggests that the number of abrupt millennial scale climate changes in older glacial periods miss 6 8 and 10 might be larger than inferred from the sst record

timing of millennial scale climate change in antarctica and greenland during the last glacial period a precise relative chronology for greenland and west antarctic paleotemperature is extended to 90 000 years ago based on correlation of atmospheric methane records from the greenland ice sheet project 2 and byrd ice cores over this period the onset of seven major millennial scale warmings in antarctica preceded the onset of greenland warmings by 1500 to 3000 years in general antarctic temperatures increased gradually while greenland temperatures were decreasing or constant and the termination of antarctic warming was apparently coincident with the onset of rapid warming in greenland this pattern provides further evidence for the operation of a bipolar see saw in air temperatures and an oceanic teleconnection between the hemispheres on millennial time scales

dansgaard oeschger oscillations in a coupled atmosphere ocean climate model abstract a reduced model of the global thermohaline circulation has been asynchronously coupled to a simple energy balance climate model in order to investigate the natural variability of the overturning circulation that may have been characteristic of late glacial conditions previous analyses with the ocean only component of the model have suggested that the nature of the internal variability of the thermohaline circulation was a strong function of the surface boundary conditions on temperature and salinity flux when the boundary conditions were altered from those corresponding to modern conditions to those appropriate to full glacial conditions then the internal variability was shown to be radically transformed under modern conditions the ocean only version of the model delivered an overturning circulation that was only weakly time dependent with a characteristic period of centuries under full glacial boundary conditions however the circulation became strongly time dependent with a characteristic

a dynamical systems model of the dansgaard oeschger oscillation and the origin of the bond cycle abstract a low dimensional dynamical systems model of the north atlantic thermohaline circulation has been developed in order to better understand the mechanism underlying the so called dansgaard oeschger oscillation that is so clearly evident during the late glacial period oxygen isotope stage 3 of the most recent ice age cycle the reduced system is designed to describe the evolution of the salinity distribution in this region that has previously been analyzed using both two and three dimensional models of the deep ocean circulation the drastically simplified model described herein is shown to accurately represent the essence of the dansgaard oeschger oscillation as this was previously revealed through detailed analyses performed with a model in which the deep circulation was described using a set of linked interacting two dimensional latitude depth basins the authors analyses with the reduced dynamical system reinforce their previous contention that the dansgaard oeschger oscillation is driven

sea level and deep water temperature changes derived from benthic foraminifera isotopic records we show that robust regressions can be established between relative sea level rsl data and benthic foraminifera oxygen isotopic ratios from the north atlantic and equatorial pacific ocean over the last climatic cycle we then apply these regressions to long benthic isotopic records retrieved at one north atlantic and one equatorial pacific site to build a composite rsl curve as well as the associated confidence interval over the last four climatic cycles our proposed reconstruction of rsl is in good agreement with the sparse rsl data available prior to the last climatic cycle we compute bottom water temperature changes at the two sites and at one southern indian ocean site taking into account potential variations in north atlantic local deep water 18o our results indicate that a last glacial maximum lgm enrichment of the ocean mean oxygen isotopic ratio of 0 95 is the lowest value compatible with unfrozen deep waters in the southern indian ocean if local deep water 18o did not increase during glacials with respect to present such a value of the lgm mean ocean isotopic enrichment would impose a maximum decrease in local bottom water 18o at the north atlantic site of 0 30 during glacials

persistent climatic and oceanographic oscillations in the subpolar north atlantic during the mis 6 glaciation and mis 5 interglacial rapidly accumulating sediments from the bjorn drift deposit south of iceland are studied for comparison of glacial interglacial climate changes related to millennial variability of the subpolar surface and deep ocean circulation in the north atlantic high resolution faunal isotopic and sedimentary analyses reveal a strong multimillennial climatic variability interpreted as oscillations in heat transport westward south of iceland during marine isotope stage 6 mis 6 possibly related to the strength of the subpolar gyre spg the oscillations persisted from mis 6 through the following interglacial mis 5 although with diminished magnitude and were respectively characterized by repeated advances of the polar front south of iceland during mis 6 and southward migrations of the arctic front due to cold surface outflow through the east greenland and east iceland currents during mis 5 incursions of cold fresh surface waters and drifting ice affected the dynamics of the spg episodically causing it to weaken and contract to the northwest during these intervals of diminished spg the northward transport of subtropical heat and salt was strengthened and preferentially conveyed to the northeast past iceland enhancing deep water formation in the nordic seas by contrast when the spg was strong more subtropical water and its associated heat were entrained within the relatively warm irminger current flowing westward south of iceland these oceanographic oscillations were associated with repeated multimillennial cooling and warming episodes during the glacial stage mis 6 equivalent to the dansgaard oeschger cycles of the last glaciation

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

climatic effect of antarctic meltwater overwhelmed by concurrent northern hemispheric melt records indicate that 14 500 years ago sea level rose by 12 22 m in under 340 years however the source of the sea level rise remains contentious partly due to the competing climatic impact of different hemispheric contributions antarctic meltwater could indirectly strengthen the atlantic meridional overturning circulation amoc causing northern warming whereas northern hemisphere ice sheet meltwater has the opposite effect this story has recently become more intriguing due to increasing evidence for sea level contributions from both hemispheres using a coupled climate model with freshwater forcing we demonstrate that the climatic influence of southern sourced meltwater is overridden by northern sources even when the antarctic flux is double the north american contribution this is because the southern ocean is quickly resalinized by antarctic circumpolar water these results imply that the pattern of surface climate changes caused by ice sheet melting cannot be used to fingerprint the hemispheric source of the meltwater