planktonic foraminiferal mg ca as a proxy for past oceanic temperatures a methodological overview and data compilation for the last glacial maximum as part of the multi proxy approach for the reconstruction of the glacial ocean margo incentive published and unpublished temperature reconstructions for the last glacial maximum lgm based on planktonic foraminiferal mg ca ratios have been synthesised and made available in an online database development and applications of mg ca thermometry are described in order to illustrate the current state of the method various attempts to calibrate foraminiferal mg ca ratios with temperature including culture trap and core top approaches have given very consistent results although differences in methodological techniques can produce offsets between laboratories which need to be assessed and accounted for where possible dissolution of foraminiferal calcite at the sea floor generally causes a lowering of mg ca ratios this effect requires further study in order to account and potentially correct for it if dissolution has occurred mg ca thermometry has advantages over other paleotemperature proxies including its use to investigate changes in the oxygen isotopic composition of seawater and the ability to reconstruct changes in the thermal structure of the water column by use of multiple species from different depth and or seasonal habitats presently available data are somewhat limited to low latitudes where they give fairly consistent values for the temperature difference between late holocene and the lgm 2 3 5 c data from higher latitudes are more sparse and suggest there may be complicating factors when comparing between multi proxy reconstructions

an 15 000 year record of el nino driven alluviation in southwestern ecuador debris flows have deposited inorganic laminae in an alpine lake that is 75 kilometers east of the pacific ocean in ecuador these storm induced events were dated by radiocarbon and the age of laminae that are less than 200 years old matches the historic record of el nino events from about 15 000 to about 7000 calendar years before the present the periodicity of clastic deposition is greater than or equal to 15 years thereafter there is a progressive increase in frequency to periodicities of 2 to 8 5 years this is the modern el nino periodicity which was established about 5000 calendar years before the present this may reflect the onset of a steeper zonal sea surface temperature gradient which was driven by enhanced trade winds

the salinity temperature and δ18o of the glacial deep ocean we use pore fluid measurements of the chloride concentration and the oxygen isotopic composition from ocean drilling program cores to reconstruct salinity and temperature of the deep ocean during the last glacial maximum lgm our data show that the temperatures of the deep pacific southern and atlantic oceans during the lgm were relatively homogeneous and within error of the freezing point of seawater at the ocean s surface our chloride data show that the glacial stratification was dominated by salinity variations in contrast with the modern ocean for which temperature plays a primary role during the lgm the southern ocean contained the saltiest water in the deep ocean this reversal of the modern salinity contrast between the north and south atlantic implies that the freshwater budget at the poles must have been quite different a strict conversion of mean salinity at the lgm to equivalent sea level change yields a value in excess of 140 meters however the storage of fresh water in ice shelves and or groundwater reserves implies that glacial salinity is a poor predictor of mean sea level

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

tropical cyclone genesis factors in simulations of the last glacial maximum abstractlarge scale environmental factors that favor tropical cyclogenesis are calculated and examined in simulations of the last glacial maximum lgm from the paleoclimate modelling intercomparison project phase 2 pmip2 despite universally colder conditions at the lgm values of tropical cyclone potential intensity which both serves as an upper bound on thermodynamically achievable intensity and indicates regions supportive of the deep convection required are broadly similar in magnitude to those in preindustrial era control simulation some regions including large areas of the central and western north pacific feature higher potential intensities at the lgm than they do in the control runs while other regions including much of the atlantic and indian oceans are lower changes in potential intensity are strongly correlated with the degree of surface cooling during the lgm additionally two thermodynamic parameters one that measures midtropospheric entropy deficits relevant for tropical cyclogen

enso variability and teleconnections during glacial climates abstract understanding the origin of sub milankovitch and millennial scale abrupt climate change still poses a major challenge an important role of communicating the orbital scale forcing into abrupt climate change on millennial and centennial timescales has been hypothesized for the tropics in earlier studies based on the assumption of stationary teleconnections this paper presents a modelling study for the last glacial maximum and marine isotope stage 3 using the comprehensive climate model ccsm3 glacial boundary conditions and freshwater hosing at high latitudes of the north atlantic are imposed to mimic a heinrich stadial and dansgaard oeschger stadials and interstadials it is shown that glacial enso variability is strongly enhanced by a substantial slowdown of the atlantic meridional overturning circulation the simulations also suggest that glacial boundary conditions induce major modifications to enso teleconnections and that the blueprint of modern enso teleconnections should only be applied with caution to glacial climate periods

florida straits density structure and transport over the last 8000 years 1 the density structure across the florida straits is reconstructed for the last 8000 years from oxygen isotope measurements on foraminifera in sediment cores the oxygen isotope measurements suggest that the density contrast across the florida current increased over this time period the magnitude of this change corresponds to an increase in the geostrophic transport referenced to 800 m water depth of 4 sverdrups sv over the last 8000 years the spatial and seasonal distribution of incoming solar radiation due to changes in the earth s orbit has caused systematic changes in the atmospheric circulation including a southward migration of the intertropical convergence zone over the last 8000 years these changes in atmospheric circulation and the associated wind driven currents of the upper ocean could readily account for a 4 sv increase in the strength of the florida current we see no evidence in our data for dramatic changes in the strength of the atlantic meridional overturning circulation over this time period

the influence of changing orbital parameters and surface boundary conditions on climate simulations for the past 18 000 years abstract general circulation model experiments at 3000 year intervals for the past 18 000 years were made to estimate the magnitude timing and pattern of the climatic response to prescribed changes of orbital parameters date of perihelion axial tilt eccentricity and glacial age lower boundary conditions ice sheets land albedo sea ice and sea surface temperature the experiments used the community climate model ccm of the national center for atmospheric research ncar the response of monsoon circulations and tropical precipitation to the orbitally produced solar radiation changes was much larger than the response to changes of glacial age boundary conditions the continental interior of eurasia was 2 4 k warmer in summer and summer monsoon precipitation of north africa south asia was increased by 10 20 between 12 000 and 6000 yr bp before present when perihelion occurred during northern summer rather than in winter as now and the earth s axial tilt was larger than now southern hemisphe

carbon isotopic changes in benthic foraminifera from the western south atlantic reconstruction of glacial abyssal circulation patterns oxygen and carbon isotopic analyses have been performed on the benthic foraminifer planulina wuellerstorfi in seven late quaternary cores from the vema channel rio grande rise region the cores are distributed over the water depth interval of 2340 to 3939 m which includes the present transition from north atlantic deep water nadw to antarctic bottom water aabw the carbon isotopic records in the cores vary as a function of water depth the shallowest and deepest cores show no significant glacial interglacial difference in 13c four of the five cores presently located in the nadw have benthic foraminiferal 13c that is lower during glacial isotopic stages based on bathymetric gradients in 13c we conclude that like today there were two water masses present in the vema channel during glacial intervals a water mass enriched in 13c overlying another water mass depleted in 13c the largest gradient of change of 13c with depth however occurred at 2 7 km 1 km shallower than the present position of this gradient on the basis of paleontologic and sedimentologic evidence we consider it unlikely that the nadw aabw transition shallowed to this level reduced carbon isotopic gradients between the deep basins of the north atlantic and pacific oceans during the last glaciation suggest that production of nadw was reduced lower production of nadw may have modified the local abyssal circulation pattern in the vema channel region

global distribution of koppen geiger climate types during the last glacial maximum mid holocene and present abstract this research presents koppen geiger k g climate type maps of the last glacial maximum lgm of 21 000 years ago 21 ka the mid holocene mh of 6 ka and the present 1976 2005 data used to calculate and map the k g climate type are year monthly mean 2 m air temperature and precipitation from the national center for atmospheric research ncar community climate system model version 4 ccsm4 community atmosphere model version 4 cam4 as part of the coupled model intercomparison project phase 5 cmip5 results suggest that the model depicts similar modern climate distributions as in data based research excepting a larger distribution of tropical wet dry and humid subtropical types and a smaller distribution of hot arid and hot semi arid types the modeled modern k g climate types cover similar percentages of earth s terrestrial and marine surface as during the mh with slight enlargement of the tropical wet dry and decreased area of mesothermal monsoonal hot summer type in the modern period at the lgm the k g polar e climate was far more extensive than in modern times with concurrent decreases in area of the tropical a climates because koppen type classifications are intended to mimic the biome distributions these results may be helpful in identifying biotic realms from the geologic past and the potential vegetative response to future climate changes

causes of reduced north atlantic storm activity in a cam3 simulation of the last glacial maximum abstract the aim of this paper is to determine how an atmosphere with enhanced mean state baroclinity can support weaker baroclinic wave activity than an atmosphere with weak mean state baroclinity as a case study a last glacial maximum lgm model simulation previously documented to have reduced baroclinic storm activity relative to the modern day climate simulated by the same model despite having an enhanced midlatitude temperature gradient is considered several candidate mechanisms are evaluated to explain this apparent paradox a linear stability analysis is first performed on the jet in the modern day and the lgm simulation the latter has relatively strong barotropic velocity shear it was found that the lgm mean state is more unstable to baroclinic disturbances than the modern day mean state although the three dimensional jet structure does stabilize the lgm jet relative to the eady growth rate next feature tracking was used to assess the storm track seeding and temporal growth of distur

reduced north atlantic deep water flux to the glacial southern ocean inferred from neodymium isotope ratios the global circulation of the oceans and the atmosphere transports heat around the earth broecker and denton1 suggested that changes in the global ocean circulation might have triggered or enhanced the glacial interglacial cycles but proxy data for past circulation taken from sediment cores in the south atlantic ocean have yielded conflicting interpretations of ocean circulation in glacial times 13c variations in benthic foraminifera2 3 4 5 6 support the idea of a glacial weakening or shutdown of north atlantic deep water production whereas other proxies such as cd ca ba ca and 231pa 230th ratios show little change from the last glacial maximum to the holocene epoch7 8 9 here we report neodymium isotope ratios from the dispersed fe mn oxide component of two southeast atlantic sediment cores both cores show variations that tend towards north atlantic signatures during the warm marine isotope stages 1 and 3 whereas for the full glacial stages 2 and 4 they are closer to pacific ocean signatures we conclude that the export of north atlantic deep water to the southern ocean has resembled present day conditions during the warm climate intervals but was reduced during the cold stages an increase in biological productivity may explain the various proxy data during the times of reduced north atlantic deep water export

climate and marine carbon cycle response to changes in the strength of the southern hemispheric westerlies 1 it has been previously suggested that changes in the strength and position of the southern hemisphere westerlies could be a key contributor to glacial interglacial atmospheric co2 variations to test this hypothesis we perform a series of sensitivity experiments using an earth system model of intermediate complexity a strengthening of the climatological mean surface winds over the southern ocean induces stronger upwelling and increases the formation of antarctic bottom water enhanced ekman pumping brings more dissolved inorganic carbon dic rich waters to the surface however the stronger upwelling also supplies more nutrients to the surface thereby enhancing marine export production in the southern hemisphere and decreasing the dic content in the euphotic zone the net response is a small atmospheric co2 increase 5 ppmv compared to the full glacial interglacial co2 amplitude of 90 ppmv roughly the opposite results are obtained for a weakening of the southern hemisphere westerly winds

triple water isotopologue record from wais divide antarctica controls on glacial interglacial changes in 17oexcess of precipitation measurements of the 17oexcess of h2o were obtained from ice cores in west and east antarctica combined with previously published results from east antarctica the new data provide the most complete spatial and temporal view of antarctic 17oexcess to date there is a steep spatial gradient of 17oexcess in present day precipitation across antarctica with higher values in marine influenced regions and lower values in the east antarctic interior there is also a spatial pattern to the change in 17oexcess between the last glacial maximum lgm and holocene periods at coastal locations there is no significant change in 17oexcess at both the west antarctic ice sheet divide site and at vostok east antarctica the lgm to early holocene change in 17oexcess is about 20 per meg atmospheric general circulation model gcm experiments show that both the observed spatial gradient of 17oexcess in modern precipitation and the spatial pattern of lgm to early holocene change can be explained by kinetic isotope effects during snow formation under supersaturated conditions requiring a high sensitivity of supersaturation to temperature the results suggest that fractionation during snow formation is the primary control on 17oexcess in antarctic precipitation variations in moisture source relative humidity play a negligible role in determining the glacial interglacial 17oexcess changes observed in antarctic ice cores additional gcm experiments show that sea ice expansion increases the area over which supersaturating conditions occur amplifying the effect of colder temperatures temperature and sea ice changes alone are sufficient to explain the observed 17oexcess glacial interglacial changes across antarctica

mid holocene east asian summer climate as simulated by the pmip2 models abstract in the present study datasets derived from twelve coupled ocean atmosphere general circulation models oagcms in the paleoclimate modelling intercomparison project phase two were used to analyze the east asian summer climate during the mid holocene about 6000 calendar years ago on the whole the oagcms reproduced warmer and wetter summer climate conditions in east asia during the mid holocene the multi model ensemble showed that in east asia the regionally averaged summer surface air temperature sat increased by 0 89 c summer precipitation was 5 8 higher and an obviously strengthened southerly wind corresponded to a strong summer monsoon in the mid holocene when compared to preindustrial levels the data model comparison in china reveals a good agreement between the oagcms results and the reconstructed changes in the summer sat in east china during the mid holocene in north china the simulated sat anomalies are 0 5 c lower overall than reconstruction in contrast the oagcms fail to capture the strongest warming in the southern qinghai tibetan plateau on the other hand the simulated precipitation agrees well with proxy data except for in the central parts of china where the simulated summer precipitation disagrees in sign with reconstruction in addition there is a large spread among the simulations particularly over and around the qinghai tibetan plateau and inter model discrepancies are larger for precipitation than for sat as a whole

the carolina sandhills quaternary eolian sand sheets and dunes along the updip margin of the atlantic coastal plain province southeastern united states the carolina sandhills is a physiographic region of the atlantic coastal plain province in the southeastern united states in chesterfield county south carolina the surficial sand of this region is the pinehurst formation which is interpreted as eolian sand derived from the underlying cretaceous middendorf formation this sand has yielded three clusters of optically stimulated luminescence ages 1 75 to 37 thousand years ago ka coincident with growth of the laurentide ice sheet 2 28 to 18 ka coincident with the last glacial maximum lgm and 3 12 to 6 ka mostly coincident with the younger dryas through final collapse of the laurentide ice sheet relict dune morphologies are consistent with winds from the west or northwest coincident with modern and inferred lgm january wind directions sand sheets are more common than dunes because of effects of coarse grain size mean range 0 35 0 59 mm and vegetation the coarse grain size would have required lgm wind velocities of at least 4 6 m sec accounting for effects of colder air temperatures on eolian sand transport the eolian interpretation of the carolina sandhills is consistent with other evidence for eolian activity in the southeastern united states during the last glaciation

how to survive the glacial apocalypse hominin mobility strategies in late pleistocene central asia abstract previous research concerning the biogeography of hominin populations in central asia indicates persistence across interglacial glacial sequences hominin groups are present on the landscape during the coldest episodes of the last glacial period moreover the inner asian mountain corridor iamc likely served as a geographic conduit for human groups that found refuge in the foothill regions of the altai mountains as well as those of the southwestern horn of the tien shan this conduit can be construed as the stage upon which hominin admixture occurred the present study broadens the geographic focus of previous work to include the steppe and steppe desert zones immediately adjacent to the biologically productive foothills of the iamc using an ecological threshold model four abiotic variables that best predict hominin site locations are analyzed to examine differences in fundamental niche structure when the iamc foothills are compared to the adjacent steppic zones our null hypothesis is that the foothills and adjacent steppe present similar abiotic profiles our results however indicate significant differences between these regions suggesting the foothills would have presented hominins with a more attractive landscape in both glacial and interglacial time periods than the steppe counterintuitively these differences are actually more extreme during interglacial time periods this preliminary model of hominin environment interactions serves as a useful example for the ways by which mid scale hominin dispersal trajectories are mapped and interpreted

enhanced ventilation of the north atlantic subtropical gyre thermocline during the last glaciation the ocean s intermediate1 6 and deep7 8 circulation are both known to have differed during the last glaciation from those of today but little is known about the history of the subtropical gyres variations in gyre processes should be closely linked to variations in global climate as gyre circulation is driven by air sea interactions and reflects the climate at the ocean surface the gyres are also a significant reservoir of carbon and nutrients so that gyre processes affect the distribution of carbon and nutrients in the oceans and thus atmospheric co2 here we use measurements of 13c and 18o in foraminifera from the bahamas to produce a detailed reconstruction of nutrient and temperature profiles in the thermocline during the last glaciation the thermocline of the glacial north atlantic subtropical gyre lacked the oxygen minimum that is characteristic of the modern ocean and was depleted in nutrients indicating greater more uniform thermocline ventilation at that time thus not only intermediate waters1 5 but the entire upper water column within the glacial north atlantic was depleted of nutrients moreover glacial thermocline waters were cooler had a steeper temperature gradient and a shallower base both the ventilation and the thermal structure of the glacial thermocline are consistent with what is known of the glacial climate9 14

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

connecting antarctic sea ice to deep ocean circulation in modern and glacial climate simulations antarctic sea ice formation plays a key role in shaping the abyssal overturning circulation and stratification in all ocean basins by driving surface buoyancy loss through the associated brine rejection changes in antarctic sea ice have therefore been suggested as drivers of major glacial interglacial ocean circulation rearrangements here the relationship between antarctic sea ice buoyancy loss deep ocean stratification and overturning circulation is investigated in last glacial maximum and preindustrial simulations from the paleoclimate modelling intercomparison project pmip the simulations show substantial inter model differences in their representation of the glacial deep ocean state and circulation which is often at odds with the geological evidence we argue that these apparent inconsistencies can largely be attributed to differing and likely insufficient antarctic sea ice formation discrepancies can be further amplified by short integration times deep ocean equilibration and sea ice representation should therefore be carefully evaluated in the forthcoming pmip4 simulations

rapid and early deglaciation in the central brooks range arctic alaska alpine style glaciation was rare in the arctic during the last glaciation because ice sheets occupied most of the glaciated high latitudes due to the tight coupling of alpine glacier fluctuations with climate the geomorphic evidence of such fluctuations in the brooks range alaska usa presents a unique opportunity to study past climate changes in this portion of the arctic we use cosmogenic 10 be exposure dating to directly date last glacial maximum lgm terminal moraines and deglaciation in the central brooks range 10 be ages from moraine boulders indicate that the lgm culminated at ca 21 ka and was followed by substantial retreat upvalley prior to a second moraine building episode culminating at ca 17 ka subsequent rapid deglaciation occurred between ca 16 ka and 15 ka when glaciers receded to within their neoglacial limits initial deglaciation after the lgm was likely caused by ice sheet induced atmospheric circulation changes and increasing insolation brooks range glaciers largely disappeared during heinrich stadial 1 prior to significant warming in the north atlantic region during the bolling allerod but coincident with global co 2 rise glacier fluctuations during the late glacial period if any were restricted to within their neoglacial extents this new chronology suggests that ice sheet modulated atmospheric circulation and global co 2 dominate glacial climate forcings in arctic alaska

influence of the sea ice thickness distribution on polar climate in ccsm3 abstract the sea ice simulation of the community climate system model version 3 ccsm3 t42 gx1 and t85 gx1 control simulations is presented and the influence of the parameterized sea ice thickness distribution itd on polar climate conditions is examined this includes an analysis of the change in mean climate conditions and simulated sea ice feedbacks when an itd is included it is found that including a representation of the subgrid scale itd results in larger ice growth rates and thicker sea ice these larger growth rates represent a higher heat loss from the ocean ice column to the atmosphere resulting in warmer surface conditions ocean circulation most notably in the southern hemisphere is also modified by the itd because of the influence of enhanced high latitude ice formation on the ocean buoyancy flux and resulting deep water formation changes in atmospheric circulation also result again most notably in the southern hemisphere there are indications that the itd also modifies simulated sea

similar rates of modern and last glacial ocean thermohaline circulation inferred from radiochemical data today the ocean thermohaline circulation transports half of the 231pa produced by radioactive decay in the atlantic ocean water column to the southern ocean this export respectively imparts low and high 231pa 230th ratios to the surface sediments of these oceans ocean sediments from the last glacial maximum bear a similar isotopic fingerprint implying that advection of north atlantic deep intermediate water into the circumpolar deep water of the southern ocean occurred at a similar or slightly higher rate during the last glacial period

reconstruction of sea surface conditions at middle to high latitudes of the northern hemisphere during the last glacial maximum lgm based on dinoflagellate cyst assemblages abstract a new calibration database of census counts of organic walled dinoflagellate cyst dinocyst assemblages has been developed from the analyses of surface sediment samples collected at middle to high latitudes of the northern hemisphere after standardisation of taxonomy and laboratory procedures the database comprises 940 reference data points from the north atlantic arctic and north pacific oceans and their adjacent seas including the mediterranean sea as well as epicontinental environments such as the estuary and gulf of st lawrence the bering sea and the hudson bay the relative abundance of taxa was analysed to describe the distribution of assemblages the best analogue technique was used for the reconstruction of last glacial maximum lgm sea surface temperature and salinity during summer and winter in addition to sea ice cover extent at sites from the north atlantic n 63 mediterranean sea n 1 and eastern north pacific n 1 three of the north atlantic cores from the continental margin of eastern canada revealed a barren lgm interval probably because of quasi permanent sea ice six other cores from the greenland and norwegian seas were excluded from the compilation because of too sparse assemblages and poor analogue situation at the remaining sites n 54 relatively close modern analogues were found for most lgm samples which allowed reconstructions the new lgm results are consistent with previous reconstructions based on dinocyst data which show much cooler conditions than at present along the continental margins of canada and europe but sharp gradients of increasing temperature offshore the results also suggest low salinity and larger than present contrasts in seasonal temperatures with colder winters and more extensive sea ice cover whereas relatively warm conditions may have prevailed offshore in summer from these data we hypothesise low thermal inertia in a shallow and low density surface water layer

late quaternary climate precipitation δ18o and indian monsoon variations over the tibetan plateau the himalaya tibet orogen contains one of the largest modern topographic and climate gradients on earth proxy data from the region provide a basis for understanding tibetan plateau paleo climate and paleo elevation reconstructions paleo climate model comparisons to proxy data compliment sparsely located data and can improve climate reconstructions this study investigates temporal changes in precipitation temperature and precipitation o18 op18 over the himalaya tibet from the last glacial maximum lgm to present we conduct a series of atmospheric general circulation model gcm echam5 wiso experiments at discrete time slices including a pre industrial pi pre 1850 ad mid holocene mh 6 ka bp and lgm 21 ka bp simulations model predictions are compared with existing proxy records model results show muted climate changes across the plateau during the mh and larger changes occurring during the lgm during the lgm surface temperatures are 2 0 4 0 c lower across the himalaya and tibet and 5 0 c lower at the northwest and northeast edge of the tibetan plateau lgm mean annual precipitation is 200 600 mm yr lower over on the tibetan plateau model and proxy data comparison shows a good agreement for the lgm but large differences for the mh large differences are also present between mh proxy studies near each other the precipitation weighted annual mean 18op lapse rate at the himalaya is about 0 4 km0 4 km larger during the mh and 0 2 km0 2 km smaller during the lgm than during the pi finally rainfall associated with the continental indian monsoon between 70 e 110 e and 10 n 30 n is about 44 less in the lgm than during pi times the lgm monsoon period is about one month shorter than in pi times taken together these results document significant spatial and temporal changes in temperature precipitation and 18op over the last 21 ka these changes are large enough to impact interpretations of proxy data and the intensity of the indian monsoon

simulation of the last 21 000 years using accelerated transient boundary conditions abstract the earth system model of intermediate complexity ecbilt clio has been used for transient simulations of the last deglaciation and the holocene the forcing effects of the ice sheets greenhouse gas concentrations and orbital configurations are prescribed as time varying boundary conditions in this study two key aspects of the transient simulations are investigated which are of broader relevance for long term transient paleoclimate modeling the effect of using accelerated boundary conditions and of uncertainties in the initial state simulations with nonaccelerated boundary conditions and an acceleration factor 10 were integrated these simulations show that the acceleration can have a significant impact on the local climate history in the outcropping regions of the high southern latitudes and the convective regions in the north atlantic the acceleration leads to damped and delayed temperature response to the boundary conditions furthermore uncertainties in the initial state can strongly

foraminiferal faunal estimates of paleotemperature circumventing the no analog problem yields cool ice age tropics the sensitivity of the tropics to climate change particularly the amplitude of glacial to interglacial changes in sea surface temperature sst is one of the great controversies in paleoclimatology here we reassess faunal estimates of ice age ssts focusing on the problem of no analog planktonic foraminiferal assemblages in the equatorial oceans that confounds both classical transfer function and modern analog methods a new calibration strategy developed here which uses past variability of species to define robust faunal assemblages solves the no analog problem and reveals ice age cooling of 5 o to 6oc in the equatorial current systems of the atlantic and eastern pacific oceans classical transfer functions underestimated temperature changes in some areas of the tropical oceans because core top assemblages misrepresented the ice age faunal assemblages our finding is consistent with some geochemical estimates and model predictions of greater ice age cooling in the tropics than was inferred by climate long range investigation mapping and prediction climap 1981 and thus may help to resolve a long standing controversy our new foraminiferal transfer function suggests that such cooling was limited to the equatorial current systems however and supports climap s inference of stability of the subtropical gyre centers

large sensitivity to freshwater forcing location in 8 2 ka simulations the 8 2 ka event is a key test case for simulating the coupled climate response to changes in the atlantic meridional overturning circulation amoc recent advances in quantifying freshwater fluxes at 8 2 ka from the proxy record have improved the realism of the forcing magnitude in model simulations yet this forcing is still generally applied in an unrealistic geographic manner across most of the labrador sea rather than just along the labrador coast previous simulations with eddy resolving ocean models have come to conflicting conclusions regarding the ability of such a coastally confined flow to impact the amoc these simulations have also not incorporated full atmosphere models nor have they used the new meltwater forcing values for 8 2 ka we use the community climate system model version 3 with an ocean model resolution only slightly coarser than that used in previous eddy resolving simulations to test the sensitivity to freshwater forcing location when revised freshwater forcing is applied across the labrador sea the amoc is reduced by 40 and climate anomalies compare well with proxy records of the 8 2 ka event in terms of magnitude and duration when the forcing is added just along the labrador coast however most meltwater joins the subtropical gyre and travels to the subtropics with minor impact to the amoc 10 decrease it is likely that model biases in the placement of the north atlantic current remain an important limitation for correctly simulating the 8 2 ka event

paired benthic foraminiferal cd ca and zn ca evidence for a greatly increased presence of southern ocean water in the glacial north atlantic benthic foraminiferal 13c and cd ca studies suggest that deep atlantic circulation during the last glacial maximum was very different from today with high nutrient low 13c high cd deep southern ocean water sow penetrating far into the north atlantic however if some glacial 13c values are biased by productivity artifacts and or air sea exchange processes then the existing 13c data may be consistent with the continual dominance of north atlantic deep water nadw cibicidoides wuellerstorfi cd ca results presented here indicate that the glacial north atlantic was strongly enriched in dissolved cd below 2500 m depth if nadw formation was still vigorous relative to sow formation these data could be explained by either increased preformed nutrient levels in the high latitude north atlantic or by increased organic matter remineralization within lower nadw high glacial zn ca values in the same samples however are best explained by a substantially increased mixing with zn rich sow the cause was most likely a partial replacement of nadw by less dense glacial north atlantic intermediate water this reorganization also lowered deep north atlantic co32 concentrations by perhaps 10 to 15 mol kg 1

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

global ocean response to orbital forcing in the holocene 1 global upper ocean evolution in the holocene is studied in two coupled ocean atmosphere models under orbital forcing conditions at 3 6 8 and 11 ka the annual mean sea surface temperature sst changes in the early to mid holocene are found to be forced mainly by the annual mean insolation forcing with an overall symmetric response of colder equator 1 c in the arctic this sst change is consistent with a synthesis of mid holocene paleo sst records in contrast the temperature response in the thermocline is dominated by an antisymmetric pattern with a cooling warming in the northern southern hemisphere midlatitudes the thermocline response is determined predominantly by surface water subduction and ultimately the insolation forcing in local late winter

climate sensitivity implications for the response of geomorphological systems to future climate change climate sensitivity is defined as the equilibrium temperature response of the climate system to a doubling of atmospheric co2 levels from pre industrial levels despite three decades of debate in the climatological literature on the estimation and significance of climate sensitivity very little appears in the geomorphological literature on the implications of this for geomorphological systems this paper reviews the concept of climate sensitivity and applies its findings to an assessment of future landscape change in cold regions it is concluded that paraglacial processes will become the dominant mechanism of sediment transfer in currently glaciated catchments and that this period of sediment mobilization will be the last episode of major sediment movement for geological time periods there is a scientific consensus oreskes 2004 that the mean surface temperature of the earth has increased in recent decades and that the warming amounts to around 0 8 8c since the beginning of the twentieth century ipcc 2007 from this nasa goddard institute of space studies giss estimates that 2005 was the warmest year to date since reliable instrumental measurements become available although the world meteorological office wmo and climatic research unit cru place it just behind 1998 detection and attribution studies show that there is high probability at least 90 that this warming is largely the result of anthropogenic emissions of greenhouse gases ghg mainly co2 in the troposphere and that the amount and rate of warming lie outside of the range of natural variation and are unprecedented within the context of the holocene the last 11 000 years continued future warming is expected to have important consequences for a range of earth systems including the atmosphere cryosphere oceans hydrological systems and the biosphere and there are compelling reasons to expect increases in the magnitude and frequency of some natural hazards such as floods huntington 2006 droughts mason goddard 2001 and landslides e g fischer et al 2006 and increases in the intensity of tropical cyclones emanuel 2005 there are also concerns about the stability of several of the large ice sheets on earth e g overpeck et al 2006 as break up of these has the ability to impact upon global sea levels and regulate ocean currents given that climate is a major driver of many earth systems the rate nature and extent of future climate changes are important variables for understanding the future behaviour of geomorphological processes and the likely evolution of geomorphological systems as a result it is surprising that little appears in the geomorphological literature assessing climate sensitivity as a factor in landform and landscape evolution it is therefore the purpose of this paper to synthesize current understanding of climate sensitivity defined later and discuss briefly the implications of this for assessing the response of mountain geomorphological systems to future temperature rises metrics for climate change metrics are of interest to geomorphologists and other earth scientists modelling the response of earth systems to changes in climate as they provide a baseline measurement from which to calculate change the most widely used metric is the global mean surface temperature which is estimated by the hadcru hadley climatic research unit data set which excludes the polar regions or by the nasa giss assessment which extrapolates to the poles the warming of mean surface temperature is related theoretically and empirically to the idea of radiative forcing at the top of the atmosphere radiative forcing is measured in w m and can be defined as a measure of the influence a factor has in altering the balance of incoming and outgoing energy in the earth atmosphere system and is an index of the importance of the factor as a potential climate change mechanism ipcc 2001 despite this metric having been used by the ipcc assessment reports and adopted by from knight j harrison s eds periglacial and paraglacial processes and environments the geological society london special publications 320 257 265 doi 10 1144 sp320 16 0305 8719 09 15 00 the geological society publishing house 2009 policy makers there have been a number of challenges to its use the most credible challenge comes from the suggestion that a global heat budget measures the warming of the earth in a more complete way than the global mean surface temperature e g pielke 2003 in addition there are important uncertainties and limitations associated with the concept national academies of sciences 2005 especially as radiative forcing fails to account adequately with regional forcings such as by aerosols and land use changes that may have significant climatic effects at the local scale

soil feedback drives the mid holocene north african monsoon northward in fully coupled ccsm2 simulations with a dynamic vegetation model we explore climate vegetation interactions in mid holocene north africa with a suite of community climate system model ccsm2 simulations the ccsm includes synchronously coupled atmosphere ocean sea ice land and vegetation models the ccsm s present day precipitation for north africa compares well with simulations of other models and observations mid holocene data reveal a wetter and greener sahara compared to the present the ccsm exhibits a greater closer to the expected precipitation increase than other models and in response grasses advance from 18 75 to 22 5 n in much of north africa precipitation is enhanced locally by the northward advance of grasses but suppressed regionally mainly due to an insufficient albedo decrease with the expansion of vegetation prior studies have always lowered the surface albedo with the expansion of vegetation in north africa in the ccsm s mid holocene simulations the albedo decreases more because wetter soils are simulated darker than drier soils than due to expanding vegetation these results isolate albedo as the key ingredient in obtaining a positive precipitation vegetation feedback in north africa two additional simulations support this conclusion in the first simulation the desert s sandy soil textures are changed to loam to represent increased organic matter soil water retention and grass cover increase albedo decreases somewhat precipitation responds with a small yet widespread increase in the second simulation a darker soil color is prescribed for this region now the monsoon advances north about 4 these results illustrate a north african monsoon highly sensitive to changes in surface albedo and less sensitive to changes in evapotranspiration

high resolution climate simulation of the last glacial maximum the climate of the last glacial maximum lgm is simulated with a high resolution atmospheric general circulation model the ncar ccm3 at spectral truncation of t170 corresponding to a grid cell size of roughly 75 km the purpose of the study is to assess whether there are significant benefits from the higher resolution simulation compared to the lower resolution simulation associated with the role of topography the lgm simulations were forced with modified climap sea ice distribution and sea surface temperatures sst reduced by 1 c ice sheet topography reduced co2 and 21 000 bp orbital parameters the high resolution model captures modern climate reasonably well in particular the distribution of heavy precipitation in the tropical pacific for the ice age case surface temperature simulated by the high resolution model agrees better with those of proxy estimates than does the low resolution model despite the fact that tropical ssts were only 2 1 c less than the control run there are many lowland tropical land areas 4 6 c colder than present comparison of t170 model results with the best constrained proxy temperature estimates noble gas concentrations in groundwater now yield no significant differences between model and observations there are also significant upland temperature changes in the best resolved tropical mountain belt the andes we provisionally attribute this result in part as resulting from decreased lateral mixing between ocean and land in a model with more model grid cells a longstanding model data discrepancy therefore appears to be resolved without invoking any unusual model physics the response of the asian summer monsoon can also be more clearly linked to local geography in the high resolution model than in the low resolution model this distinction should enable more confident validation of climate proxy data with the high resolution model elsewhere an inferred salinity increase in the subtropical north atlantic may have significant implications for ocean circulation changes during the lgm a large part of the amazon and congo basins are simulated to be substantially drier in the ice age consistent with many but not all paleo data these results suggest that there are considerable benefits derived from high resolution model regarding regional climate responses and that observationalists can now compare their results with models that resolve geography at a resolution comparable to that which the proxy data represent

past extent of sea ice in the northern north atlantic inferred from foraminiferal paleotemperature estimates 1 using 150 core top samples we developed a conservative but fairly robust new measure to reconstruct past changes in maximum seasonal sea ice distribution in the northern north atlantic hitherto a major unknown the proxy is based on similarity maximum modern analog technique simmax estimates of threshold temperatures near the sea surface sst today almost 100 of all sites with sst 2 5 c during summer 0 4 for winters 1978 1987 and 0 75 c for little ice age winters lie seaward of the sea ice margin when applied to 60 sediment records of the last glacial maximum this proxy shows that peak glacial sea ice was far more restricted than in the classic climap project members 1981 reconstruction during glacial summer sea ice only covered the arctic ocean and western fram strait the northern north atlantic and nordic seas were largely ice free and thus formed a high latitude moisture source for the continued buildup of continental ice sheets in contrast sea ice spread far south across the iceland faeroe ridge during glacial winter with an inferred patch of sea ice also in the central east atlantic near the center of the azores high a broad ice free channel extended from 50 to 60 n forming an ideal site for large scale convection of glacial upper north atlantic deep water the extreme seasonality in glacial sea ice formation and melt in the nordic seas implies a major consumption of the regional energy income

last glacial maximum and holocene climate in ccsm3 abstract the climate sensitivity of the community climate system model version 3 ccsm3 is studied for two past climate forcings the last glacial maximum lgm and the mid holocene the lgm approximately 21 000 yr ago is a glacial period with large changes in the greenhouse gases sea level and ice sheets the mid holocene approximately 6000 yr ago occurred during the current interglacial with primary changes in the seasonal solar irradiance the lgm ccsm3 simulation has a global cooling of 4 5 c compared to preindustrial pi conditions with amplification of this cooling at high latitudes and over the continental ice sheets present at lgm tropical sea surface temperature sst cools by 1 7 c and tropical land temperature cools by 2 6 c on average simulations with the ccsm3 slab ocean model suggest that about half of the global cooling is explained by the reduced lgm concentration of atmospheric co2 50 of present day concentrations there is an increase in the antarctic circumpolar current an

glacial north atlantic sea surface conditions reconstructed by glamap 2000 the response of the tropical ocean to global climate change and the extent of sea ice in the glacial nordic seas belong to the great controversies in paleoclimatology our new reconstruction of peak glacial sea surface temperatures ssts in the atlantic is based on census counts of planktic foraminifera using the maximum similarity technique version 28 simmax 28 modern analog technique with 947 modern analog samples and 119 well dated sediment cores our study compares two slightly different scenarios of the last glacial maximum lgm the environmental processes of the ice age land oceans glaciers epilog and glacial atlantic ocean mapping glamap 2000 time slices the comparison shows that the maximum lgm cooling in the southern hemisphere slightly preceeded that in the north in both time slices sea ice was restricted to the north western margin of the nordic seas during glacial northern summer while the central and eastern parts were ice free during northern glacial winter sea ice advanced to the south of iceland and faeroe in the central northern north atlantic an anticyclonic gyre formed between 45degrees and 60degreesn with a cool water mass centered west of ireland where glacial cooling reached a maximum of 12degreesc in the subtropical ocean gyres the new reconstruction supports the glacial to interglacial stability of sst as shown by climap project members climap 1981 the zonal belt of minimum sst seasonality between 2degrees and 6degreesn suggests that the lgm caloric equator occupied the same latitude as today in contrast to the climap reconstruction the glacial cooling of the tropical east atlantic upwelling belt reached up to 6degrees 8degreesc during northern hemisphere summer differences between these simmax based and published u37 k and mg ca based equatorial sst records are ascribed to strong sst seasonalities and sst signals that were produced by different planktic species groups during different seasons

tropical pacific variability in the ncar climate system model a 300 yr simulation with the ncar climate system model csm version 1 captured only 60 of the observed enso signal and exaggerated the interannual variability of sst in the western tropical pacific here a simulation with a new version of the csm which significantly improves the spatial and temporal patterns of tropical pacific variability is described maximum sst variability is shifted to the central and eastern pacific a better simulation of the equatorial pacific thermocline structure results in nino 3 and nino 4 statistics com parable to the observed estimates for the last century the evolution of sst and subsurface temperature anomalies is in excellent agreement with observed events the majority of events evolve as a standing mode with weak sst anomalies occurring in the northern spring in the eastern tropical pacific and maximum anomalies covering the eastern tropical pacific ocean to the date line by the following northern winter at the same time subsurface temperature anomalies spread eastward and upward along the tropical thermocline the delayed oscillator and wyrtki s buildup hypothesis are consistent with aspects of the csm simulation on the equator a westerly wind stress anomaly in the central pacific forces off equatorial upwelling anomalies which propagate westward reaching the western boundary about one half year later this upwelling signal then propagates eastward along the equator arriving 2 months before cooling in the eastern pacific basin the tropical pacific atmospheric response to warm oceanic events also agrees with observational analyses with a negative southern oscillation pattern in sea level pressure wind stress anomalies and low level convergence to the west of the maximum sst anomalies and enhanced deep convection and precipitation in the central and eastern tropical pacific

dust sources and deposition during the last glacial maximum and current climate a comparison of model results with paleodata from ice cores and marine sediments mineral dust aerosols in the atmosphere have the potential to affect the global climate by influencing the radiative balance of the atmosphere and the supply of micronutrients to the ocean ice and marine sediment cores indicate that dust deposition from the atmosphere was at some locations 2 20 times greater during glacial periods raising the possibility that mineral aerosols might have contributed to climate change on glacial interglacial time scales to address this question we have used linked terrestrial biosphere dust source and atmospheric transport models to simulate the dust cycle in the atmosphere for current and last glacial maximum lgm climates we obtain a 2 5 fold higher dust loading in the entire atmosphere and a twenty fold higher loading in high latitudes in lgm relative to present comparisons to a compilation of atmospheric dust deposition flux estimates for lgm and present in marine sediment and ice cores show that the simulated flux ratios are broadly in agreement with observations differences suggest where further improvements in the simple dust model could be made the simulated increase in high latitude dustiness depends on the expansion of unvegetated areas especially in the high latitudes and in central asia caused by a combination of increased aridity and low atmospheric co2 the existence of these dust source areas at the lgm is supported by pollen data and loess distribution in the northern continents these results point to a role for vegetation feedbacks including climate effects and physiological effects of low co2 in modulating the atmospheric distribution of dust

investigating the sensitivity of glaciers to climate variability since the mis 2 in the upper ganga catchment saraswati valley central himalaya abstract moraines outwash gravel terraces fluvial drapes and lacustrine sequences are used to infer the pattern of glacial fluctuations in the saraswati valley upper ganga catchment located in transitional climatic zone between dry steppe of the tibetan plateau in the north and sub humid higher himalaya in the south the saraswati valley has preserved evidence of four glacier advances these are identified as the saraswati glacial stage sgs 1 oldest to sgs 4 youngest based on the relative dating and luminescence ages obtained on younger advances the sgs 1 is ascribed to pre marine isotopic stage mis 2 the sgs 2 is dated to the mis 2 24 5 2 8 21 2 2 0 ka the sgs 3 is speculatively ascribed to the younger dryas yd and the sgs 4 is suggested to be of the mid holocene 6 ka age the bifurcated moraine ridges of sgs 2 represent standstill conditions during the post last glacial maximum lgm period the deglaciation is represented by outwash gravel terraces impounded sedimentary lacustrine sequences and fluvial drapes overlying and abutting the moraines are dated to early mid 11 9 0 9 7 5 0 6 ka and late holocene 3 3 0 2 1 7 0 3 ka intensified moderate indian summer monsoon considering the timing of glacial advances and stand still condition it is proposed that across the orographic barrier rain shadow valleys glaciers responded sensitively to the intensified anticyclonic flow of the cooler mid latitude westerlies implying that in a monsoon dominated transient climatic zone even the rain shadow valleys responded sensitively to temperature changes

the de correlation of westerly winds and westerly wind stress over the southern ocean during the last glacial maximum motivated by indications from paleo evidence this paper investigates the changes of the southern westerly winds sww and westerly wind stress between the last glacial maximum lgm and pre industrial in the pmip3 cmip5 simulations highlighting the role of antarctic sea ice in modulating the wind effect on ocean particularly a de correlation occurs between the changes in sww and westerly wind stress caused primarily by an equatorward expansion of winter antarctic sea ice that undermines the efficacy of wind in generating stress over the liquid ocean such de correlation may reflect the lgm condition in reality in view of the fact that the model which simulates this condition has most fidelity in simulating modern sww and antarctic sea ice therein two models stand out for their agreements with paleo evidence regarding the change of sww and the westerly wind stress they simulate strengthened and poleward migrated lgm sww in the atmosphere consistent with the indications from dust records whilst in the ocean they well capture an equatorward shifted pattern of the observed oceanic front shift with most pronounced equatorward shifted westerly wind stress during the lgm

fast mechanisms for the formation of new plate boundaries abstract we demonstrate the important role played by non linear feedback in promoting the fast breakup of the continental lithosphere and the formation of new plate boundaries we compare the difference in fluid dynamic and elasto visco plastic approaches and emphasize the advection of stresses in finite element models with large strain lagrangian formulation two types of instabilities were found at multiple time length scales ranging from 100 years and km scale to 0 1 myr and 100 km scale one mechanism relies on the thermo mechanical elasto plastic energy conversion and the other stems from the void volatile interaction elasticity takes on a guiding role in the propagation of shear zones closely followed by a thermal wave which is again followed by a wave of volatiles this last mechanism consolidates the fault zones and establishes the plate boundary for a geological time scale using a fully coupled thermal mechanical approach we obtain for the first time a two dimensional solution which shows that the constant velocity boundary conditions can lead to episodic avalanche type tectonic phases within

a coupled climate model simulation of the last glacial maximum part 2 approach to equilibrium the climate of the last glacial maximum lgm is simulated with a coupled climate model the simulated climate undergoes a rapid adjustment during the first several decades after imposition of lgm boundary conditions as described in part 1 and then evolves toward equilibrium over 900 model years the climate simulated by the coupled model at this period is compared with observationally based lgm reconstructions and with lgm results obtained with an atmosphere mixed layer slab ocean version of the model in order to investigate the role of ocean dynamics in the lgm climate global mean surface air temperature and sea surface temperature sst decrease by about 10 c and 5 6 c in the coupled model which includes ocean dynamics compared to decreases of 6 3 and 3 8 c in slab ocean case the coupled model simulates a cooling of about 6 5 c over the tropics which is larger than that of the climap reconstruction 1 7 c and larger than that of the slab ocean simulation 3 3 c but which is in reasonable agreement with some recent proxy estimates the ocean dynamics of the coupled model captures features found in the climap reconstructions such as a relative maximum of ocean cooling over the tropical pacific associated with a mean la nina like response and lead to a more realistic sst pattern than in the slab model case the reduction in global mean precipitation simulated in the coupled model is larger 15 than that simulated with the slab ocean model 10 in conjunction with the enhanced cooling some regions such as the usa and the mediterranean region experience increased precipitation in accord with proxy paleoclimate evidence the overall much drier climate over the ocean leads to higher sea surface salinity sss in most ocean basins except for the north atlantic where sss is considerably lower due to an increase in the supply of fresh water from the mississippi and amazon rivers and presumably a decrease in salt transport by the weakened north atlantic overturning circulation the north atlantic overturning stream function weakens to less than half of the control run value the overturning is limited to a shallower depth less than 1000 m and its outflow is confined to the northern hemisphere in the southern ocean convection is much stronger than in the control run leading to a stronger overturning stream function associated with enhanced antarctic bottom water formation as a result southern ocean water masses fill the entire deep ocean the antarctic circumpolar current acc transport through the drake passage increases by about 25 the acc transport despite weaker zonal winds is enhanced due to changes in bottom pressure torque the weakening of the overturning circulation in the north atlantic and the accompanying 30 decrease in the poleward ocean heat transport contrasts with the strengthening of the overturning circulation in the southern ocean and a 40 increase in heat transport as a result sea ice coverage and thickness are affected in opposite senses in the two hemispheres the lgm climate simulated by the coupled model is in reasonable agreement with paleoclimate proxy evidence the dynamical response of the ocean in the coupled model plays an important role in determining the simulated and undoubtedly the actual lgm climate

modelling of heat transfer with the random walk method part 1 application to thermal energy storage in porous aquifers abstract this study examines the application of the random walk method of resolution in the modelling of thermal energy storage in fractured aquifers the media under consideration have a high degree of fracturing but a low porosity the aperture of the fracture is small in relation to its overall extent the customary method of resolution developed to investigate the transport phenomenon in such fractured media is first presented briefly next the stochastic equations of the energy displacement in fractured aquifers are developed depending on the statistical location of the energy particles in the media a stochastic approach is used to determine the flow velocity field the method is applied to different configurations of fracture networks for a single well system of aquifer thermal energy storage the temperature fields corresponding to a fractured aquifer and its equivalent porous configuration are compared an equivalent porous medium is not an adequate substitute for a fractured aquifer in such modelling the effect of the porosity on the energy storage is obvious the temperature of the extracted water and the energy recovery factor increase with the fracture porosity except in a highly anisotropic medium the orientation distribution of the fractures has little effect on the heat transfer in an aquifer without regional flow in the presence of a natural hydraulic gradient the energy recovery factor tends towards zero and the production temperature towards the original ambient temperature of the aquifer in this case a charge and discharge well pair proves to be a suitable alternative to the single well system

modeling the surface mass balance response of the laurentide ice sheet to bolling warming and its contribution to meltwater pulse 1a meltwater pulse mwp 1a occurred 14 5 14 ka and is the largest abrupt rise in sea level 10 20 m of sea level rise of the last deglaciation the timing of mwp 1a is coincident with or shortly follows the abrupt warming of the north atlantic region into the bolling warm period which could have triggered a large laurentide ice sheet lis contribution to mwp 1a given that outside of the arctic lis iceberg discharge probably did not increase during the bolling much of the lis mwp 1a contribution likely occurred through surface ablation here we test the response of lis surface mass balance to bolling warming by forcing a lis energy mass balance model with climate from an atmosphere ocean general circulation model our modeling approach neglects changes in lis mass from dynamics and iceberg calving allowing us to isolate the surface mass balance response model results suggest that lis surface ablation can explain much of the sea level rise just prior to mwp 1a lis surface mass balance becomes more negative in response to the bolling warming contributing an additional 2 9 1 0 m of sea level rise in 500 yr in addition to the background contribution of 4 0 0 8 m the modeled lis mwp 1a contribution is less than previous assumptions but agrees with geochemical runoff and lis area volume estimates the fraction of mwp 1a attributable to other ice sheets particularly antarctica depends on the total sea level rise that occurred during this mwp

modeling evidence for enhanced el nino southern oscillation amplitude during the last glacial maximum we present a numerical eigenmode analysis of an intermediate el nin o southern oscillation enso model which is driven by present day observed background conditions as well as by simulated background conditions for the last glacial maximum lgm about 21 000 years ago the background conditions are obtained from two lgm simulations which were performed with the national center for atmospheric research climate system model csm1 4 and an earth system model of intermediate complexity ecbilt clio our analysis clearly shows that the leading present day unstable recharge discharge mode changes its stability as well as its frequency during lgm conditions simulated lgm background conditions were favorable to support large amplitude self sustained interannual enso variations in the tropical pacific our analysis indicates that off equatorial climate conditions as well as a shoaling of the thermocline play a crucial role in amplifying the lgm enso mode

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

temperature trends during the present and last interglacial periods a multi model data comparison though primarily driven by insolation changes associated with well known variations in earth s astronomical parameters the response of the climate system during interglacials includes a diversity of feedbacks involving the atmosphere ocean sea ice vegetation and land ice a thorough multi model data comparison is essential to assess the ability of climate models to resolve interglacial temperature trends and to help in understanding the recorded climatic signal and the underlying climate dynamics we present the first multi model data comparison of transient millennial scale temperature changes through two intervals of the present interglacial pig 8 1 2 ka and the last interglacial lig 123 116 2 ka periods we include temperature trends simulated by 9 different climate models alkenone based temperature reconstructions from 117 globally distributed locations about 45 of them within the lig and 12 ice core based temperature trends from greenland and antarctica 50 of them within the lig the definitions of these specific interglacial intervals enable a consistent inter comparison of the two intervals because both are characterised by minor changes in atmospheric greenhouse gas concentrations and more importantly by insolation trends that show clear similarities our analysis shows that in general the reconstructed pig and lig northern hemisphere mid to high latitude cooling compares well with multi model mean temperature trends for the warmest months and that these cooling trends reflect a linear response to the warmest month insolation decrease over the interglacial intervals the most notable exception is the strong lig cooling trend reconstructed from greenland ice cores that is not simulated by any of the models a striking model data mismatch is found for both the pig and the lig over large parts of the mid to high latitudes of the southern hemisphere where the data depicts negative temperature trends that are not in agreement with near zero trends in the simulations in this area the positive local summer insolation trend is counteracted in climate models by an enhancement of the southern ocean summer sea ice cover and or an increase in southern ocean upwelling if the general picture emerging from reconstructions is realistic then the model data mismatch in mid and high southern hemisphere latitudes implies that none of the models is able to resolve the correct balance of these feedbacks or alternatively that interglacial southern hemisphere temperature trends are driven by mechanisms which are not included in the transient simulations such as changes in the antarctic ice sheet or meltwater induced changes in the overturning circulation

climate change and arctic ecosystems 1 vegetation changes north of 55 n between the last glacial maximum mid holocene and present a unified scheme to assign pollen samples to vegetation types was used to reconstruct vegetation patterns north of 55 n at the last glacial maximum lgm and mid holocene 6000 years b p the pollen data set assembled for this purpose represents a comprehensive compilation based on the work of many projects and research groups five tundra types cushion forb tundra graminoid and forb tundra prostrate dwarf shrub tundra erect dwarf shrub tundra and low and high shrub tundra were distinguished and mapped on the basis of modern pollen surface samples the tundra forest boundary and the distributions of boreal and temperate forest types today were realistically reconstructed during the mid holocene the tundra forest boundary was north of its present position in some regions but the pattern of this shift was strongly asymmetrical around the pole with the largest northward shift in central siberia 200 km little change in beringia and a southward shift in keewatin and labrador 200 km low and high shrub tundra extended farther north than today at the lgm forests were absent from high latitudes graminoid and forb tundra abutted on temperate steppe in northwestern eurasia while prostrate dwarf shrub erect dwarf shrub and graminoid and forb tundra formed a mosaic in beringia graminoid and forb tundra is restricted today and does not form a large continuous biome but the pollen data show that it was far more extensive at the lgm while low and high shrub tundra were greatly reduced illustrating the potential for climate change to dramatically alter the relative areas occupied by different vegetation types

comparing proxies for the reconstruction of lgm sea surface conditions in the northern north atlantic abstract within the frame of the multiproxy approach for the reconstruction of the glacial ocean margo project sea surface conditions of the last glacial maximum lgm 23 19 ka were reconstructed using different proxies which were calibrated to a standardized modern hydrography in the north atlantic the revised lgm margo data set provides a comprehensive coverage including the nordic seas the data set includes reconstruction based on planktonic foraminifer assemblages dinocyst assemblages alkenone coccolithophorid biomarkers and mg ca ratios in planktonic foraminifers several hydrological features of the lgm north atlantic can be identified that are supported by all four proxies they include an extensive perennial sea ice cover along the eastern canadian and greenland margins seasonally ice free central north atlantic and norwegian sea and the winter sea ice limit being located at about 55 n all proxies also suggest significantly colder than modern sea surface conditions in the southeastern part of the north atlantic and a more zonal temperature pattern than at present with a steep sst gradient of the order of 10 c between 40 n and 45 n however in the nordic seas large discrepancies remain well above the level of uncertainty of the sst reconstructions these discrepancies might be related to stratification of the upper water mass linked to large annual amplitude of temperature with contrasting winter and summer conditions high interannual interdecadal variability or taphonomic processes affecting some of the proxies the average lgm sst in the nordic seas cannot at present be assessed with confidence however the existing evidence suggests that highly variable sea surface conditions and at least occasional advection of north atlantic waters may have characterised the glacial nordic seas

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

tropical pacific climate at the mid holocene and the last glacial maximum simulated by a coupled ocean atmosphere general circulation model 1 simulations for the mid holocene 6000 years before present 6 ka and the last glacial maximum lgm 21 ka have been performed by a global ocean atmosphere coupled general circulation model gcm after the initial spin up periods both runs were integrated for about 200 years for 6 ka the model shows an enhanced seasonal variation in surface temperature and a northward shift of the african and the indian summer monsoon rain area overall circulation features in the tropics correspond to a strong walker circulation state with negative sea surface temperature sst and precipitation anomalies in the central pacific and positive precipitation anomalies over the indian and australian monsoon regions it is noted that there is about a 0 35 c cooling of the global mean sst in contrast to the 6 ka result the simulated tropical climate anomaly at 21 ka corresponds to a weak walker circulation state the simulated lgm sst decrease is about 2 c over the tropical western pacific it is larger about 5 c over the caribbean sea this sst anomaly is in broad agreement with the observed proxy data interestingly the model simulates a warmer than present sst over the subtropical pacific this may be related to a weaker subtropical anticyclone due to weakened monsoon circulation at the lgm

an oceanic cold reversal during the last deglaciation a detailed deuterium excess profile measured along the dome c epica european project for ice coring in antarctica core reveals the timing and strength of the sea surface temperature changes at the source regions for dome c precipitation we infer that an oceanic cold reversal took place in the southern indian ocean 800 years after the antarctic cold reversal the temperature gradient between the oceanic moisture source and antarctica is similar to the dome c sodium profile during the deglaciation illustrating the strong link between this gradient and the strength of the atmospheric circulation

reduced enso variability at the lgm revealed by an isotope enabled earth system model studying the el nino southern oscillation enso in the past can help us better understand its dynamics and improve its future projections however both paleoclimate reconstructions and model simulations of enso strength at the last glacial maximum lgm 21 ka bp have led to contradicting results here we perform model simulations using the recently developed water isotope enabled community earth system model icesm for the first time model simulated oxygen isotopes are directly compared with those from enso reconstructions using the individual foraminifera analysis ifa we find that the lgm enso is most likely weaker comparing with the preindustrial the icesm suggests that total variance of the ifa records may only reflect changes in the annual cycle instead of enso variability as previously assumed furthermore the interpretation of subsurface ifa records can be substantially complicated by the habitat depth of thermocline dwelling foraminifera and their vertical migration with a temporally varying thermocline

deep circulation of the north atlantic over the last 200 000 years geochemical evidence variations in the cadmium calcium ratio of north atlantic deep water are recorded in the fossil shells of benthic foraminifera the oceanic distribution of cadmium is similar to that of the nutrients hence the cadmium calcium ratio in shells records temporal variations in nutrient distributions data from a north atlantic sediment core show that over the past 200 000 years there has been a continuous supply of nutrient depleted waters into the deep north atlantic the intensity of this source relative to nutrient enriched southern waters diminished by about a factor of 2 during severe glaciations this evidence combined with carbon isotope data indicates that the continental carbon inventory may have been less variable than previously suggested

what sets the mean transport through drake passage twelve experiments with two coarse resolutions of a global ocean model using a variety of surface forcings are analyzed to address the question of what sets the mean transport through drake passage seven of the experiments do not have an active sea ice model but the remaining five do previous theories have suggested that the drake passage transport is governed by the cape horn sverdrup transport or alternatively is proportional to the square root of the meridional ekman transport at the latitude of drake passage the results presented here do not support either of these theories the drake passage transport depends quite strongly on the isopycnal diffusivity parameter in the model and less strongly on the background vertical diffusivity and horizontal viscosity parameters however when the magnitudes of these parameters are fixed the results show a very strong correlation between drake passage transport and both the strength of the meridional ekman transport at the latitude of drake passage and the thermohaline circulation off the antarctic shelf the relationships are monotonic but not linear the best estimate is that the meridional ekman transport drives 100 sv of drake passage transport while the remaining 30 sv are driven by the global thermohaline circulation

determining paleoceanographic circulations with emphasis on the last glacial maximum the inference from radiocarbon and other data that the north atlantic ocean circulation during the last glacial maximum was reduced is examined because glacial periods are thought to be windier than non glacial ones a reduced circulation is difficult to rationalize a first step is to note that the circulation of mass is unlikely to correspond simply to the circulation of heat or freshwater salt by merely permitting an enhanced input of southern hemisphere water into the north atlantic one can accommodate significantly increased radiocarbon ages such an input is equally interpretable as owing to an increased mass circulation rather than a decreased one inferred estimates from paleo shear estimates in the florida current are re interpreted as producing an indeterminate net flow there determination of the paleocirculation rates is not simpler than the problem in the modern ocean it is fully three dimensional and requires an adequate data base to determine the mass flux rates

linkages between salinity and brine channel distribution in young sea ice a high resolution study of bulk salinity was undertaken on laboratory grown sea ice to determine the extent of spatial variability of salinity and whether this was associated with brine channel structures ice samples at two different stages of development were compared with respect to physical state brine distribution and brine channel structures cold growing sea ice is shown to have steep bulk salinity gradients and a highly variable brine distribution that is closely linked to the location and morphology of brine channels areas of high bulk salinity are found to correspond directly to the positions of the brine channels and these areas are surrounded by ice that shows significant brine depletion it is observed that redistribution of the brine occurs as the ice cover goes through warming and melting phases the redistribution is attributed to an increase in porosity and pore connectivity permitting migration of brine through the ice the distribution of brine in warm ice is shown to be more homogeneous and independent of brine channel structures a number of mechanisms exploiting the enhanced mobility of brine in warm sea ice are used to explain the changes in the salinity profiles and brine distribution

stalagmite records of hydroclimate in central california during termination 1 abstract the last deglaciation is marked by large and abrupt hydroclimatic changes in the great basin and the american southwest however comparatively little is known about how hydroclimate varied on the western side of the sierra nevada we present new evidence for abrupt changes in precipitation amount in the central sierra nevada during the last deglaciation our new record from mclean s cave overlaps with a previously published record from nearby moaning cave oster et al 2009 and extends the record of hydroclimatic change in the region through heinrich stadial 1 mclean s cave speleothem 18o 13c mg ca and sr ca indicate a shift to drier conditions at the beginning of heinrich stadial 1 from 16 1 to 17 5 ka followed by wetter conditions at the end of heinrich stadial 1 when the majority of great basin lakes reached their deglacial highstands during the last deglaciation coincident shifts in the moaning and mclean s cave proxy records indicate drier conditions in the western sierra nevada during millennial scale intervals of warming at high latitudes such as the bolling and allerod and wetter conditions during cooler intervals such as the older and younger dryas thus the sierra nevada speleothem records are consistent with other regional records that document increased winter rainfall during millennial scale cold periods of the last deglaciation however regional differences exist in the hydroclimatic response to the younger dryas with central californian and interior southwestern sites indicating an increase in winter storms that is not apparent in southern california dynamic simulations of atmospheric circulation indicate that wetter conditions during the younger dryas relative to the bolling at mclean s cave may have resulted from a stronger storm track in the north pacific at the latitude of northern california

mid holocene climate in new caledonia southwest pacific coral and pmip models monthly resolved results abstract the mid holocene climate is characterized by an insolation seasonality decrease in the southern hemisphere but measurements of its actual impact on monthly resolved sea surface temperature sst in the southwest pacific region are still insufficient a new caledonian 5 5 ka cal bp coral provides a 20 year long seasonally resolved record of ocean surface conditions as inferred from coral sr ca and ba ca results were compared to monthly series of paleoclimate modeling intercomparison project phase 2 pmip2 model simulations anomalous stable isotope and u ca values are observed in a restricted area of the skeleton related to dissolution features the mid holocene sst seasonal amplitude mean and variability are higher than presently in new caledonia the increased seasonal amplitude in the mid holocene being most probably due to the occurrence of colder winters other southwest pacific mid holocene coral data showed also such an increased seasonal amplitude this could mean that the south pacific convergence zone spcz was weaker or reached locations more northerly than at present which could fit with northward shifts of the inter tropical convergence zone itcz during south hemisphere winter inversely strong rainfalls during the summer deduced from the ba ca signal and reconstructed sea surface salinity sss were interpreted as reflecting pronounced southwestward shifts of the spcz in summer as those occurring today during la nina events none of the six pmip2 models used reproduce the proxy based mid holocene increase of sst seasonal amplitude model maps show a less intense spcz in winter that would be consistent with higher sst seasonal amplitude finally we stress the need for more seasonally resolved data to validate this enhanced mid holocene sst seasonal amplitude in the southwest pacific region and to better understand the underlying mechanisms

modeling el nino and its tropical teleconnections during the last glacial interglacial cycle 1 simulations with the ncar climate system model csm a global coupled ocean atmosphere sea ice model for the last glacial interglacial cycle reproduce recent estimates based on alkenones and mg ca ratios of sea surface temperature sst changes and gradients in the tropical pacific and predict weaker el ninos la ninas compared to present for the holocene and stronger el ninos la ninas for the last glacial maximum lgm changes for the lgm holocene are traced to a weakening strengthening of the tropical pacific zonal sst gradient wind stresses and upwelling and a sharpening weakening of the tropical thermocline results suggest that proxy evidence of weaker precipitation variability in new guinea and ecuador are explained not only by changes in el nino la nina but also changes in the atmospheric circulation and hydrologic cycle index terms 3344 meteorology and atmospheric dynamics paleoclimatology 4267 oceanography general paleoceanography 4522 oceanography physical el nino 3337 meteorology and atmospheric dynamics numerical modeling and data assimilation 9604 information related to geologic time cenozoic citation otto bliesner b l e c brady s i shin z liu and c shields modeling el nino and its tropical teleconnections during the last glacial interglacial cycle geophys res lett 30 23 2198 doi 10 1029 2003gl018553 2003

simulated changes in atmospheric dust in response to a heinrich stadial 1 heinrich stadials hs thought to be triggered by increased ice discharge in the high latitude north atlantic at glacial periods resulted in large freshwater forcing that weakened the atlantic meridional overturning circulation amoc these events are strongly expressed in paleoclimate records throughout the atlantic basin and much of the tropics compared to the last glacial maximum recent proxy data suggest hs were much drier and dustier in large parts of the tropics we use a global climate model coupled to a prognostic dust model to examine the response of dust to changes in climate during hs despite some significant changes in regional precipitation patterns in response to the simulated shift in the intertropical convergence zone we find that changes in winds have a larger effect on dust mobilization and deposition patterns than soil moisture or vegetation changes associated with a weaker amoc although europe was colder and drier during hs the annual mean glaciogenic dust emission rate is lower because a southward shift in the northern hemisphere jet stream leads to weaker winds and less dust mobilization the proximity of the westerly wind anomaly associated with the jet stream shift increases gustiness and dust mobilization over northwestern africa and shifts the african easterly jet southward resulting in less african dust transport and deposition over the north tropical atlantic drier conditions over the sahel region of north africa however do not lead to increased dust mobilization in our model when we perturb sahelian soil characteristics our results are in better agreement with proxies

a global perspective on last glacial maximum to holocene climate change while the abrupt climate events of the last deglaciation are well defined in ice core records from the polar regions of both hemispheres their manifestation elsewhere is less well constrained here we compile 104 high resolution paleoclimate records to characterize the timing and spatial pattern of climate change during the last deglaciation this compilation indicates relatively concurrent timing of the last glacial maximum lgm peak glacial conditions and the altithermal peak interglacial conditions in the northern 22 1 4 3 ka and 8 0 3 2 ka and southern 22 3 3 6 ka and 7 4 3 7 ka hemispheres suggesting the hemispheres were synchronized by greenhouse gases local insolation and or northern hemisphere induced ocean circulation changes the magnitude of the glacialeinterglacial temperature change increases with latitude reflecting the polar amplification of climate change with a likely minimum global mean cooling of ww4 9 c during the lgm relative to the altithermal empirical orthogonal function eof analysis of 71 records spanning 19e11 ka indicates that two modes explain 72 of deglacial climate variability eof1 61 of variance shows a globally near uniform pattern with its principal component pc1 strongly correlated with changes in atmospheric co2 e of2 11 of variance exhibits a bipolar seesaw pattern between the hemispheres with its principal component pc2 resembling changes in atlantic meridional overturning circulation strength eof analysis of 90 records from 15 to 11 ka indicates that northern and southern modes of climate variability characterize the younger dryas bolling allerod interval these modes dominate at the higher latitudes of each hemisphere and exhibit a complex interaction in the tropics the magnitude of the younger dryas climate anomaly cooler drier increases with latitude in the northern hemisphere with an opposite pattern warmer wetter in the southern hemisphere reflecting a general bipolar seesaw climate response global mean temperature decreased by w0 6 c during the younger dryas therefore our analysis supports the paradigm that while the younger dryas was a period of global climate change it was not a major global cooling event but rather a manifestation of the bipolar seesaw driven by a reduction in atlantic meridional overturning circulation strength

the effect of ocean dynamics in a coupled gcm simulation of the last glacial maximum general circulation models gcms of the climate system are powerful tools for understanding and predicting climate and climate change the last glacial maximum lgm provides an extreme test of the model s ability to simulate a change of climate and allows us to increase our understanding of mechanisms of climate change we have used a coupled high resolution ocean atmosphere gcm hadcm3 to simulate the equilibrium climate at the lgm the effect of ocean dynamics is investigated by carrying out a parallel experiment replacing the dynamic three dimensional ocean gcm with a static thermodynamic mixed layer ocean model changes to the ocean circulation and feedbacks between the ocean atmosphere and sea ice have an important influence on the surface response and are discussed the coupled model produces an intensified thermohaline circulation and an increase in the amount of heat transported northward by the atlantic ocean equatorward of 55 n which is at odds with the interpretation of some proxy records such changes which the thermodynamic mixed layer ocean model cannot produce have a large impact around the north atlantic region and are discussed in the study

the last 140 ka in the afro asian arid semi arid transitional zone abstract during the last ten years a great number of geological or other proxy records provided radiocarbon u th or tl dated information on the past climatic oscillations in the arid semi arid zones extending between the atlantic and the pacific comparisons and a synthetic view of these results can now be attempted for the last 140 ka and compared with global changes as registered in oceanic or ice cores and with palaeomonsson models for this purpose wide spatial and time scales have to be used on the whole arid humid alternations roughly fit global changes cold phases corresponding to an extension of the arid areas to the south warm phases corresponding to the shrinking of the same to the north the last interglacial is associated with an increase of precipitation throughout the area considered isotopic stage 4 brings no evidence whatsoever of humid conditions two wetter episodes are registrated during stage 3 a major rainfall decrease is everywhere associated with the last glacial maximum 21 15 ka in most regions the arid or semi arid zones extending several hundred kilometers southwards relative to the present day pattern the two abrupt deglaciation steps and the younger dryas are recorded in all of the most sensitive regions at the margins of the present day monsoonal range during the holocene the precipitation increases everywhere by 100 400 mn relative to the present day values the optimum being at 8 5 6 5 ka a climatic deterioration follows with an irregular pattern of dry wet episodes according to the different geographic conditions the humid phase terminates at 3 5 3 ka in the whole transitional zone the major southward shift of the monsoonal precipitation range since its optimum some 8000 years ago fits the astronomical neoglacial trend and may possibly be correlated with past stage 5d although its rapidity and spatial importance may just reflect one of the sharp successive cold warm variations registered by grip during the whole stage 5

a major advance of tropical andean glaciers during the antarctic cold reversal a moraine chronology determined by surface exposure dating shows that glaciers in the northern tropical andes expanded to a larger extent during the antarctic cold reversal 14 500 to 12 900 years ago than during the younger dryas stadial 12 800 to 11 500 years ago contrary to previous studies as a result previous chronologies and climate interpretations from tropical glaciers may need to be revisited the younger dryas was a cold period immediately prior to the present warm interglacial period there is clear evidence that it occurred throughout much of the northern hemisphere but its global extent remains debatable prior work in the tropical andes suggested extensive glacial advances during the younger dryas vincent favier and colleagues present new cosmogenic dating of glacial deposits and a reassessment of existing datasets from other glaciated andean landscapes to show that instead maximal glacial advances took place in the preceding antarctic cold reversal rather than a major advance the younger dryas now appears to have been associated with modest glacial retreat the younger dryas stadial a cold event spanning 12 800 to 11 500 years ago during the last deglaciation is thought to coincide with the last major glacial re advance in the tropical andes1 this interpretation relies mainly on cosmic ray exposure dating of glacial deposits recent studies however have established new production rates2 3 4 for cosmogenic 10be and 3he which make it necessary to update all chronologies in this region1 5 6 7 8 9 10 11 12 13 14 15 and revise our understanding of cryospheric responses to climate variability here we present a new 10be moraine chronology in colombia showing that glaciers in the northern tropical andes expanded to a larger extent during the antarctic cold reversal 14 500 to 12 900 years ago than during the younger dryas on the basis of a homogenized chronology of all 10be and 3he moraine ages across the tropical andes we show that this behaviour was common to the northern and southern tropical andes transient simulations with a coupled global climate model suggest that the common glacier behaviour was the result of atlantic meridional overturning circulation variability superimposed on a deglacial increase in the atmospheric carbon dioxide concentration during the antarctic cold reversal glaciers advanced primarily in response to cold sea surface temperatures over much of the southern hemisphere during the younger dryas however northern tropical andes glaciers retreated owing to abrupt regional warming in response to reduced precipitation and land surface feedbacks triggered by a weakened atlantic meridional overturning circulation conversely glacier retreat during the younger dryas in the southern tropical andes occurred as a result of progressive warming probably influenced by an increase in atmospheric carbon dioxide considered with evidence from mid latitude andean glaciers16 our results argue for a common glacier response to cold conditions in the antarctic cold reversal exceeding that of the younger dryas

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

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

late quaternary paleoclimate of western alaska inferred from fossil chironomids and its relation to vegetation histories abstract fossil chironomidae assemblages with a few chaoboridae and ceratopogonidae from zagoskin and burial lakes in western alaska provide quantitative reconstructions of mean july air temperatures for periods of the late middle wisconsin 39 000 34 000 cal yr b p to the present inferred temperatures are compared with previously analyzed pollen data from each site summarized here by indirect ordination paleotemperature trends reveal substantial differences in the timing of climatic warming following the late wisconsin at each site although chronological uncertainty exists zagoskin lake shows early warming beginning at about 21 000 cal yr b p whereas warming at burial lake begins 4000 years later summer climates during the last glacial maximum lgm were on average 3 5 c below the modern temperatures at each site major shifts in vegetation occurred from 19 000 to 10 000 cal yr b p at zagoskin lake and from 17 000 to 10 000 cal yr b p at burial lake vegetation shifts followed climatic warming when temperatures neared modern values both sites provide evidence of an early postglacial thermal maximum at 12 300 cal yr b p these chironomid records combined with other insect based climatic reconstructions from beringia indicate that during the lgm 1 greater continentality likely influenced regions adjacent to the bering land bridge and 2 summer climates were at times not dominated by severe cold

a simulation of the last glacial maximum climate using the ncar ccsm the national center for atmospheric research community climate system model ncar ccsm is used in a coupled atmosphere ocean sea ice simulation of the last glacial maximum lgm around 21 000 years ago climate in the tropics the simulation shows a moderate cooling of 3 c over land and 2 c in the ocean in zonal average this cooling is about 1 c cooler than the climap sea surface temperatures ssts but consistent with recent estimates of both land and sea surface temperature changes subtropical waters are cooled by 2 2 5 c also in agreement with recent estimates the simulated oceanic thermohaline circulation at the lgm is not only shallower but also weaker than the modern with a migration of deep water formation site in the north atlantic as suggested by the paleoceanographic evidences the simulated northward flow of antarctic bottom water aabw is enhanced these deep circulation changes are attributable to the increased surface density flux in the southern ocean caused by sea ice expansion at the lgm both the gulf stream and the kuroshio are intensified due to the overall increase of wind stress over the subtropical oceans the intensified zonal wind stress and southward shift of its maximum in the southern ocean effectively enhances the transport of the antarctic circumpolar current acc by more than 50 simulated ssts are lowered by up to 8 c in the midlatitudes simulated conditions in the north atlantic are warmer and with less sea ice than indicated by climap again in agreement with more recent estimates the increased meridional sst gradient at the lgm results in an enhanced hadley circulation and increased midlatitude storm track precipitation the increased baroclinic storm activity also intensifies the meridional atmospheric heat transport a sensitivity experiment shows that about half of the simulated tropical cooling at the lgm originates from reduced atmospheric concentrations of greenhouse gases

monsoonal precipitation in the paleo tethys warm pool during the latest permian abstract simulations of the late permian 251 ma are analyzed with respect to the northern hemispheric pangean monsoon we find that the presence and spatial distribution of the warm pool and not land sea temperature differences are the primary forcing agents for the monsoon the land sea temperature gradient as a monsoonal mechanism is tested by eliminating the cathyasian peninsula and is found to have little impact on the spatial character of the monsoon furthermore the response of the monsoon to the warm pool was tested by removing all paleo tethys equatorial islands which allows the warm pool to expand and migrate westward thus shifting the pattern of monsoonal precipitation given that paleogeography changes through time these results have important implications for the migration and strength of the tropical monsoon over geologic time additionally different co2 regimes are presented in which a 10 fold change in forcing produces diverging climates and therefore different warm pool and monsoon locales tropical and equatorial ocean currents impact the seasonal progression and location of the warm pool and atmospheric mass flux for the monsoonal regime is characterized and shown to change with warm pool movement experiments were conducted using the low resolution version of community climate system model version 3 ccsm3 in both coupled and fixed sea surface temperature sst context for validation purposes a modern control is briefly presented with observations and is shown to represent both the spatial and seasonal progression of the indian monsoon

topography s crucial role in heinrich events heinrich events the abrupt changes in the laurentide ice sheet that cause the appearance of the well observed heinrich layers are thought to have a strong effect on the global climate the focus of most studies that have looked at the climate s response to these events has been the freshwater flux that results from melting icebergs however there is the possibility that the varying height of the ice sheet could force a change in the climate in this study we present results from a newly developed coupled climate ice sheet model to show what effect this topographic change has both on its own and in concert with the flux of freshwater from melting icebergs we show that the topographic forcing can explain a number of the climate changes that are observed during heinrich events such as the warming and wettening in florida and the warm sea surface temperatures in the central north atlantic which freshwater forcing alone cannot we also find regions for example the tropical atlantic where the response is a mixture of the two here observations may help disentangle the relative importance of each mechanism these results suggest that the simple paradigm of a heinrich event causing climate change via freshwater inputs into the north atlantic needs to be revised

does southern ocean surface forcing shape the global ocean overturning circulation author s sun s eisenman i stewart al abstract 2018 american geophysical union all rights reserved paleoclimate proxy data suggest that the atlantic meridional overturning circulation amoc was shallower at the last glacial maximum lgm than its preindustrial pi depth previous studies have suggested that this shoaling necessarily accompanies antarctic sea ice expansion at the lgm here the influence of southern ocean surface forcing on the amoc depth is investigated using ocean only simulations from a state of the art climate model with surface forcing specified from the output of previous coupled pi and lgm simulations in contrast to previous expectations we find that applying lgm surface forcing in the southern ocean and pi surface forcing elsewhere causes the amoc to shoal only about half as much as when lgm surface forcing is applied globally we show that this occurs because diapycnal mixing renders the southern ocean overturning circulation more diabatic than previously assumed which diminishes the influence of southern ocean surface buoyancy forcing on the depth of the amoc