hydrological and climatological controls on radiocarbon concentrations in a tropical stalagmite precisely dated stalagmites are increasingly important archives for the reconstruction of terrestrial paleoclimate at very high temporal resolution in depth understanding of local conditions at the cave site and of the processes driving stalagmite deposition is of paramount importance for interpreting proxy signals incorporated in stalagmite carbonate here we present a sub decadally resolved dead carbon fraction dcf record for a stalagmite from yok balum cave southern belize the record is coupled to parallel stable carbon isotope 13c and u ca measurements as well as radiocarbon 14c measurements from soils overlying the cave system using a karst carbon cycle model we disentangle the importance of soil and karst processes on stalagmite dcf incorporation revealing a dominant host rock dissolution control on total dcf covariation between dcf 13c and u ca indicates that karst processes are a common driver of all three parameters suggesting possible use of 13c and trace element ratios to independently quantify dcf variability a statistically significant multi decadal lag of variable length exists between dcf and reconstructed solar activity suggesting that solar activity influenced regional precipitation in mesoamerica over the past 1500 years but that the relationship was non static although the precise nature of the observed lag is unclear solar induced changes in north atlantic oceanic and atmospheric dynamics may play a role

interannual to multidecadal euro atlantic blocking variability during winter and its relationship with extreme low temperatures in europe the dominant modes of blocking frequency variability in the atlantic european region are evaluated for the 1871 2010 period an empirical orthogonal function eof analysis of a two dimensional blocking indicator field reveals three dominant eofs describing about 35 of interannual to multidecadal blocking variability the first eof captures an out of phase blocking frequency anomaly over greenland and western europe regions the corresponding principal component time series is strongly correlated with the north atlantic oscillation index but shows also significant correlations with indices of the east atlantic scandinavian and east atlantic western russia patterns the second eof shows a dominant center over the north sea region as well as a less pronounced center with anomalies of the same sign over southeastern greenland the multidecadal variations of this mode of blocking variability are related with a basin wide north atlantic sea surface temperature anomaly which projects partly on the atlantic multidecadal oscillation amo the third mode is an east west dipole of blocking frequency anomalies from scandinavian and southern greenland regions and shows enhanced variability at 20 year time scales the coherent variations of the time coefficients of this pattern with open solar flux suggest a possible solar influence on blocking variability at these time scales furthermore the dominant patterns of blocking variability are related with distinct anomaly patterns in the occurrence of extreme low temperature events over europe at interannual to multidecadal time scales amo as well as the solar signals was detected also in the corresponding extreme low temperature blocking patterns we argue that multivariate analysis of blocking indicators gives additional information about blocking and related extreme climate phenomena variability and predictability comparative with classical sectorial approach

atmospheric volcanic loading derived from bipolar ice cores accounting for the spatial distribution of volcanic deposition 1 previous studies have used small numbers of ice core records of past volcanism to represent hemispheric or global radiative forcing from volcanic stratospheric aerosols with the largest ever assembly of volcanic ice core records and state of the art climate model simulations of volcanic deposition we now have a unique opportunity to investigate the effects of spatial variations on sulfate deposition and on estimates of atmospheric loading we have combined 44 ice core records 25 from the arctic and 19 from antarctica and goddard institute for space studies modele simulations to study the spatial distribution of volcanic sulfate aerosols in the polar ice sheets we extracted volcanic deposition signals by applying a high pass loess filter to the time series and examining peaks that exceed twice the 31 year running median absolute deviation our results suggest that the distribution of volcanic sulfate aerosol follows the general precipitation pattern in both regions indicating the important role precipitation has played in affecting the deposition pattern of volcanic aerosols we found a similar distribution pattern for sulfate aerosols from the 1783 1784 laki and 1815 tambora eruptions as well as for the total b activity after the 1952 1954 low latitude northern hemisphere and 1961 1962 high latitude northern hemisphere atmospheric nuclear weapon tests this confirms the previous assumption that the transport and deposition of nuclear bomb test debris resemble those of volcanic aerosols we compare three techniques for estimating stratospheric aerosol loading from ice core data radioactive deposition from nuclear bomb tests pinatubo sulfate deposition in eight antarctic ice cores and climate model simulations of volcanic sulfate transport and deposition following the 1783 laki 1815 tambora 1912 katmai and 1991 pinatubo eruptions by applying the above calibration factors to the 44 ice core records we have estimated the stratospheric sulfate aerosol loadings for the largest volcanic eruptions during the last millennium these loadings agree fairly well with estimates based on radiation petrology and model simulations we also estimate the relative magnitude of sulfate deposition compared with the mean for greenland and antarctica for each ice core record which provides a guideline to evaluate the stratospheric volcanic sulfate aerosol loading calculated from a single or a few ice core records

an abrupt weakening of the subpolar gyre as trigger of little ice age type episodes we investigate the mechanism of a decadal scale weakening shift in the strength of the subpolar gyre spg that is found in one among three last millennium simulations with a state of the art earth system model the spg shift triggers multicentennial anomalies in the north atlantic climate driven by long lasting internal feedbacks relating anomalous oceanic and atmospheric circulation sea ice extent and upper ocean salinity in the labrador sea yet changes throughout or after the shift are not associated with a persistent weakening of the atlantic meridional overturning circulation or shifts in the north atlantic oscillation the anomalous climate state of the north atlantic simulated after the shift agrees well with climate reconstructions from within the area which describe a transition between a stronger and weaker spg during the relatively warm medieval climate and the cold little ice age respectively however model and data differ in the timing of the onset the simulated spg shift is caused by a rapid increase in the freshwater export from the arctic and associated freshening in the upper labrador sea such freshwater anomaly relates to prominent thickening of the arctic sea ice following the cluster of relatively small magnitude volcanic eruptions by 1600 ce sensitivity experiments without volcanic forcing can nonetheless produce similar abrupt events a necessary causal link between the volcanic cluster and the spg shift can therefore be excluded instead preconditioning by internal variability explains discrepancies in the timing between the simulated spg shift and the reconstructed estimates for the little ice age onset

annually resolved atlantic sea surface temperature variability over the past 2 900 y global warming due to anthropogenic factors can be amplified or dampened by natural climate oscillations especially those involving sea surface temperatures ssts in the north atlantic which vary on a multidecadal scale atlantic multidecadal variability amv because the instrumental record of amv is short long term behavior of amv is unknown but climatic teleconnections to regions beyond the north atlantic offer the prospect of reconstructing amv from high resolution records elsewhere annually resolved titanium from an annually laminated sedimentary record from ellesmere island canada shows that the record is strongly influenced by amv via atmospheric circulation anomalies significant correlations between this high arctic proxy and other highly resolved atlantic sst proxies demonstrate that it shares the multidecadal variability seen in the atlantic our record provides a reconstruction of amv for the past 3 millennia at an unprecedented time resolution indicating north atlantic ssts were coldest from 1400 1800 ce while current ssts are the warmest in the past 2 900 y

a study of cleaning procedures used for foraminiferal mg ca paleothermometry the various cleaning steps required for preparation of foraminiferal samples for mg ca and sr ca analysis are evaluated for their relative importance and effects on measured elemental ratios it is shown that the removal of silicate contamination is the most important step for the measurement of mg ca ratios in an example bulk sample mg ca decreases from 10 5 to 2 5 mmol mol 1 during clay removal oxidation of organic material causes a lowering of sample mg ca in the order of 10 or approximately 1 c when converted to temperature use of dilute acid leaching to remove adsorbed contaminants causes partial dissolution of the sample carbonate and a corresponding decrease in mg ca reductive treatment also causes dissolution of the sample and a decrease in the mg ca ratio of up to 10 15 sample preparation for sr ca analysis does not require the same degree of rigor as is necessary for mg ca work the within run reproducibility of the method described here for g ruber in a core top sample from the arabian sea was 1 8 mean sample ratio was 4 72 mmol mol 1 when converted to temperature this becomes 28 0 2 c the equivalent result for sr ca was 0 5 mean ratio 1 44 mmol mol 1

surface changes in the eastern labrador sea around the onset of the little ice age despite the relative climate stability of the present interglacial it has been punctuated by several centennial time scale climatic oscillations the latest of which are often colloquially referred to as the medieval climatic anomaly mca and the little ice age lia the most favored explanation for the cause of these anomalies is that they were triggered by variability in solar irradiance and or volcanic activity and amplified by ocean atmosphere sea ice feedbacks as such changes in the strength of the atlantic meridional overturning circulation amoc are widely believed to have been involved in the amplification of such climatic oscillations the labrador sea is a key area of deep water formation the waters produced here contribute approximately one third of the volume transport of the deep limb of the amoc and drive changes in the north atlantic surface hydrography and subpolar gyre circulation in this study we present multi proxy reconstructions from a high resolution marine sediment core located south of greenland that suggest an increase in the influence of polar waters reaching the labrador sea close to mca lia transition changes in freshwater forcing may have reduced the formation of labrador sea water and contributed towards the onset of the lia cooling

interaction between warm atlantic sourced waters and the east greenland current in northern denmark strait 68 n during the last 10 600 cal a bp changes in the holocene interaction of the i cold fresh east greenland current egc and ii warm saline irminger current ic in northern denmark strait have been reconstructed from benthic and planktic foraminifera assemblages ice rafted debris grain size analyses and quantitative x ray diffraction during the time from c 10 600 to 8000 cal a bp palaeoceanographic reconstructions reveal waning deglacial influence from the receding greenland ice sheet and presence of a strong egc caused low surface water productivity from c 8000 cal a bp a predominant influence of atlantic sourced ic waters on subsurface water conditions became established in northern denmark strait which accompanied low surface water productivity relatively warm surface and subsurface water conditions i e reduced egc and strong ic influence are found from c 6500 to 4500 cal a bp representing holocene optimum like conditions a mid to late holocene egc strengthening caused increased stratification and formation of a distinct halocline however we recognize millennial scale periods of reduced stratification by an enhanced influence of atlantic sourced ic water on surface water conditions i at c 2500 1400 cal a bp the time of the roman warm period and ii at c 300 cal a bp the later part of the little ice age these periods of oceanic warming probably relate to changes in the subpolar gyre dynamics that led to enhanced entrainment of atlantic sourced ic water into northern denmark strait

globorotalia truncatulinoides in central western mediterranean sea during the little ice age abstract globorotalia truncatulinoides oscillations have been recorded from different marine sediment cores collected in the central and western mediterranean sea the abundances of this species over the last 500 yrs demonstrates its potential value as bio indicator of particular oceanographic condition during the maunder minimum mm event of the little ice age lia the comparison between the g truncatulinoides abundance patterns of the balearic basin central and south tyrrhenian sea and central and eastern sicily channel allows to highlight a similar response of this species during the mm event in the central western mediterranean sea the ecological meanings of this species and its peculiar high abundance percentages in the total assemblages suggest the development of enhanced vertical mixing conditions during mm winter season with a strong advection of nutrients from the nutrient rich deeper layers and enhances the productivity levels in the mixed layer the intensified vertical mixing could be linked to persistence of an atmospheric blocking event recorded by several authors during the mm

climate changes reconstructed from a glacial lake in high central asia over the past two millennia abstract climatic changes in arid central asia aca over the past two millennia have been widely concerned however less attention has been paid to those in the high central asia hca where the asian water tower nurtures the numerous oases by glacier and or snow melt here we present a new reconstruction of the temperature and precipitation change over the past two millennia based on grain size of a well dated glacial lake sediment core in the central of southern tianshan mountains the results show that the glacial lake catchment has experienced cold wet climate conditions during the dark age cold period 300 600 ad dacp and the little ice age 1300 1870 ad lia whereas warm dry conditions during the medieval warm period 700 1270 ad mwp integration of our results with those of previously published lake sediment records stalagmite 18o records ice core net accumulation rates tree ring based temperature reconstructions and mountain glacier activities suggest that there has a broadly similar hydroclimatic pattern over the hca areas on centennial time scale during the past two millennia comparison between hydroclimatic pattern of the hca and that of the aca areas suggests a prevailing warm dry and cold wet hydroclimatic pattern over the whole westerlies dominated central asia areas during the past two millennia we argue that the position and intensity of the westerlies which are closely related to the phase of the north atlantic oscillation nao and the strength of the siberian high pressure sh could have jointly modulated the late holocene central asia hydroclimatic changes

subpolar north atlantic sea surface temperature since 6 ka bp indications of anomalous ocean atmosphere interactions at 4 2 ka bp abstract atmospheric circulation may change with future climate change in response to modification of meridional temperature gradients but the potential influence on ocean circulation is as yet unclear over the mid late holocene atmospheric circulation in the north atlantic region has fluctuated on millennial timescales therefore the ocean response to these changes can be investigated using the paleoceanographic records that have been developed in the north eastern subpolar north atlantic here we present a diatom based sea surface temperature reconstruction from the iceland basin south of iceland the reconstruction shows the warmest temperatures of the record at 6 1 4 ka bp cooler temperatures at 4 2 ka bp and warmer temperatures thereafter inter record comparisons indicate that the cold period at c 4 2 ka bp may have resulted from a strengthened east greenland current and or melting of the greenland ice sheet in response to a negative north atlantic oscillation the findings highlight that atmospheric circulation changes are likely to cause pronounced variations in the latitudinal exchange of heat which may have consequences for deep water formation and global ocean circulation

a shift towards wetter and windier conditions in southern sweden around the prominent solar minimum 2750 cal a bp a variety of palaeoclimatic records show a shift towards cooler wetter and windier conditions in europe around 2800 cal a bp the shift broadly coincides with an increase of the atmospheric c 14 concentration suggesting a connection between solar activity variations and climate change here we investigate a peat record from undarsmosse in southern sweden in a previous study based on a low resolution chronology this record showed increased aeolian sand influx and sphagnum spore content around 2800 cal a bp indicating high storm activity and wetter conditions we applied the c 14 wiggle match dating technique on the same record to construct a robust chronology to evaluate the temporal relationship to solar forcing in addition we performed plant macrofossil analysis to determine local vegetation changes based on the new chronology a shift to a sphagnum dominated bog representing wetter conditions and the onset of a period with increased storminess occurred around 2700 cal a bp these changes are within age model uncertainties synchronous with climatic changes inferred from other sites in europe suggesting a shift in the larger scale atmospheric circulation possibly triggered by decreased solar activity less

reconstructing north atlantic marine climate variability using an absolutely dated sclerochronological network reconstructing regional to hemispheric scale climate variability requires the application of spatially representative and climatically sensitive proxy archives large spatial networks of dendrochronologies have facilitated the reconstruction of atmospheric variability and inferred variability in the atlantic ocean system however the marine environment has hitherto lacked the direct application of the spatial network approach because of the small number of individual absolutely dated marine archives in this study we present the first analyses of a network of absolutely dated annually resolved growth increment width chronologies from the marine bivalves glycymeris glycymeris and arctica islandica the network contains eight chronologies spanning 500 km along the western british continental shelf from the southern irish sea to north west scotland correlation analysis of the individual chronologies and a suite of climate indices including the atlantic multidecadal oscillation amo central england surface air temperature cet northeast atlantic sea surface temperatures sst s and the winter north atlantic oscillation wnao demonstrates that despite the large geographical distances been sites and the heterogeneous nature of the marine environment the increment width variability in these series contains an element of coherence likely driven by a common response to changing environmental forcing a nested principal component analysis pca was used to construct five composite series which explain between 31 and 74 of the variance across the individual chronologies linear regression analyses indicate that the composite series explain up to 41 of the variance in northeast atlantic ssts over the calibration period 1975 2000 calibration verification reduction of error re and coefficient of efficiency ce statistics indicate that the composite series contains significant skill at reconstructing multi decadal northeast atlantic sst variability over the past two centuries 1805 2010 these data suggest that composite series derived from sclerochronology networks can facilitate the robust reconstruction of marine climate over past centuries to millennia providing invaluable baseline records of natural oceanographic variability

engineering properties of loess in algeria loess in north africa has been investigated using samples from algeria the specific gravity atterberg limits grain size distribution and dry density were determined the hydro collapsibility properties due to wetting under different stress levels were measured in single oedometer tests the results of this investigation indicate that the properties of algerian loess are similar to those of loess from many parts of the world such as iowa and libya they can be classified as silty loess

a new interpolation method to measure delta evolution and sediment flux application to the late holocene coastal plain of the argens river in the western mediterranean abstract rapid environmental changes along the mediterranean coasts influenced the sedimentary dynamics shoreline position and human settlements in deltaic areas over the last millennia an innovative and multiproxy approach using geostatistical modelling was developed to estimate geomorphic evolution and sediment fluxes in deltaic areas while palaeoecological conditions and environmental changes were assessed from geochemistry and cluster analyses of molluscan fauna this method was applied to the coastal plain of the argens river in southern france depositional environments of prodelta delta front river channel floodplain marsh and abandoned channel were identified from the study of facies associations the late holocene sediment flux in the argens river bayhead delta ranged from 15 800 2300 to 52 000 8500 m3 yr 1 the highest rate of sediment deposition between 2500 and 2000 cal yr bp was associated with increased river flooding in southeastern france a general decrease in subaqueous sediment flux over the past 2500 years was primarily controlled by a decrease in accommodation space and delta slope from 500 to 0 cal yr bp a sharp increase in subaerial sediment flux probably due to hydroclimatic change was contemporaneous with a period of high flood frequency during the little ice age a southward shift of the north atlantic westerlies combined with either a negative phase of the east atlantic west russian pattern or a negative phase of the east atlantic pattern coupled to a northward migration of the intertropical convergence zone could have led to increased fluvial activity in the lower argens valley during the late holocene

enhanced signature of solar variability in eurasian winter climate 1 we demonstrate that open solar flux fs derivable from geomagnetic data exhibits stronger correlations with atmospheric circulation variations than conventionally used measures of solar activity the circulation anomalies are particularly enhanced over the north atlantic eurasian sector where there are large changes in the occurrence of blocking and the winter mean surface temperature differs by several degrees between high and low solar terciles the relationship is stronger and simpler for fs being more linear between high and low solar winters while the circulation anomalies strongly resemble the north atlantic oscillation they also extend deeper into eurasia especially in high solar conditions this distinct signature may be useful for the detection and attribution of observed changes and also the identification of dynamical mechanisms

mg ca and sr ca ratios in planktonic foraminifera proxies for upper water column temperature reconstruction reliable temperature estimates from both surface and subsurface ocean waters are needed to reconstructpast upper water column temperature gradients and past oceanic heat content this work examines therelationships between trace element ratios in fossil shells and seawater temperature for surface dwellingforaminifera species globigerinoides ruber white and globigerina bulloides and deep dwelling species globorotalia inflata globorotalia truncatulinoides dextral and sinistral and pulleniatina obliquiloculata mg ca and sr ca ratios in shells picked in 29 modern core tops from the north atlantic ocean are calibratedusing calculated isotopic temperatures mg ca ratios on g ruber and g bulloides agree with published dataand relationships for deep dwelling species mg ca calibration follows the equation mg ca 0 78 0 04 exp 0 051 0 003 t with a significant correlation coefficient of r

last millennium climate and its variability in ccsm4 abstractan overview of a simulation referred to as the last millennium lm simulation of the community climate system model version 4 ccsm4 is presented the ccsm4 lm simulation reproduces many large scale climate patterns suggested by historical and proxy data records with northern hemisphere nh and southern hemisphere sh surface temperatures cooling to the early 1800s common era by 0 5 c nh and 0 3 c sh followed by warming to the present high latitudes of both hemispheres show polar amplification of the cooling from the medieval climate anomaly mca to the little ice age lia associated with sea ice increases the lm simulation does not reproduce la nina like cooling in the eastern pacific ocean during the mca relative to the lia as has been suggested by proxy reconstructions still dry medieval conditions over the southwestern and central united states are simulated in agreement with proxy indicators for these regions strong global cooling is associated with large volcanic erup

persistent positive north atlantic oscillation mode dominated the medieval climate anomaly the medieval climate anomaly mca was the most recent pre industrial era warm interval of european climate yet its driving mechanisms remain uncertain we present here a 947 year long multidecadal north atlantic oscillation nao reconstruction and find a persistent positive nao during the mca supplementary reconstructions based on climate model results and proxy data indicate a clear shift to weaker nao conditions into the little ice age lia globally distributed proxy data suggest that this nao shift is one aspect of a global mca lia climate transition that probably was coupled to prevailing la nina like conditions amplified by an intensified atlantic meridional overturning circulation during the mca

controls of strength and rate of downstream fining above a river base level we report sensitivity tests on a numerical model of the development of downstream fining by size selective transport and deposition above a base level in a gravelbed river with no lateral inputs of water or sediment strong fining requires deposition mainly to the surface otherwise its amount and streamwise distribution are not highly sensitive to any single factor but vary somewhat with hiding factor excess shear stress and bed sorting low initial concavity inhibits fining but different higher concavities generate fairly similar fining profiles lower sediment supply leads to a coarser bed with less fining but there is no simple relationship between fining and aggradation rate the rate of development of fining is sensitive to all these factors but is always rapid initially followed by a far slower approach towards final equilibrium surface grain size adjustment is potentially an important degree of freedom in river response to any environmental change or human impact which causes overloading

is the gulf stream responsible for europe s mild winters is the transport of heat northward by the gulf stream and north atlantic drift and its subsequent release into the midlatitude westerlies the reason why europe s winters are so much milder than those of eastern north america and other places at the same latitude here it is shown that the principal cause of this temperature difference is advection by the mean winds south westerlies bring warm maritime air into europe and north westerlies bring frigid continental air into north eastern north america further analysis of the ocean surface heat budget shows that the majority of the heat released during winter from the ocean to the atmosphere is accounted for by the seasonal release of heat previously absorbed and not by ocean heat flux convergence therefore the existence of the winter temperature contrast between western europe and eastern north america does not require a dynamical ocean two experiments with an atmospheric general circulation model coupled to an ocean mixed layer confirm this conclusion the difference in winter temperatures across the north atlantic and the difference between western europe and western north america is essentially the same in these models whether or not the movement of heat by the ocean is accounted for in an additional experiment with no mountains the flow across the ocean is more zonal western europe is cooled the trough east of the rockies is weakened and the cold of north eastern north america is ameliorated in all experiments the west coast of europe is warmer than the west coast of north america at the same latitude whether or not ocean heat transport is accounted for in summary the deviations from zonal symmetry of winter temperatures in the northern hemisphere are fundamentally caused by the atmospheric circulation interacting with the oceanic mixed layer copyright 2002 royal meteorological society

late holocene sea level changes in eastern quebec and potential drivers abstract late holocene sea level changes can be reconstructed from salt marsh sediments with decimetre scale precision and decadal scale resolution these records of relative sea level changes comprise the net sea level contributions from mechanisms that act across local regional and global scales recent efforts help to constrain the relative significance of these mechanisms that include sediment dynamics and isostasy which cause relative sea level changes via vertical land motion ocean atmosphere processes that influence regional scale ocean mass redistribution and ocean cryosphere and steric interactions that drive global scale ocean volume changes there remains a paucity of high resolution late holocene sea level data from eastern canada this precludes an interrogation of the mechanisms that define sea level changes over recent centuries and millennia in a region sensitive to oceanic atlantic multidecadal variability atlantic meridional overturning circulation atmospheric north atlantic oscillation arctic oscillation and cryospheric ice mass balance changes we present new relative sea level data that span the past three millennia from baie des chaleurs in the gulf of st lawrence generated using salt marsh foraminifera supported with plant macrofossil analyses the accompanying chronology is based on radiocarbon and radionuclide analyses which are independently verified using trace metal and microcharcoal records relative sea level has risen at a mean rate of 0 93 1 25 mm yr 1 over the past 1500 years residual structure within the reconstruction internal variability has contributed up to an additional 0 61 0 46 mm yr 1 of short lived rsl rise prior to 1800 ce following a sea level low stand during the little ice age acceleration in relative sea level rise is identified between 1800 and 1900 ce within the estimates of internal variability and from 1950 ce to present in both the secular and residual trends phases of relative sea level changes in the gulf of st lawrence are concomitant with periods of glacier mass loss following the little ice age phase periods of the north atlantic oscillation and the atlantic meridional overturning circulation and northern hemisphere warming quantifying the individual effects of these different mechanisms is important for understanding how ocean atmosphere processes redistribute ocean mass upon larger scale background ocean volume changes

measured volume heat and salt fluxes from the atlantic to the arctic mediterranean 1 the flow of warm and saline atlantic water towards the arctic crosses the greenland scotland ridge in three current branches since the mid 1990 s extensive monitoring with quasi permanent moorings and regular ctd cruises has been in operation on three sections crossing the branches averaged over the years 1999 to 2001 values of volume heat relative to 0 c and salt flux due to the total atlantic inflow across the greenland scotland ridge into the nordic seas are estimated as 8 5 sv 1 sv 106 m3 s 1 313 1012 w and 303 106 kg s 1 in this period the average temperature and salinity of the atlantic inflow were 8 5 c and 35 25 respectively within the observational uncertainty we do not find any significant seasonal variation of the volume flux but a negative correlation between the inflow flux through the faroe shetland channel and through the other two gaps was indicated

anomalously weak labrador sea convection and atlantic overturning during the past 150 years the atlantic meridional overturning circulation amoc is a system of ocean currents that has an essential role in earth s climate redistributing heat and influencing the carbon cycle1 2 the amoc has been shown to be weakening in recent years 1 this decline may reflect decadal scale variability in convection in the labrador sea but short observational datasets preclude a longer term perspective on the modern state and variability of labrador sea convection and the amoc1 3 5 here we provide several lines of palaeo oceanographic evidence that labrador sea deep convection and the amoc have been anomalously weak over the past 150 years or so since the end of the little ice age lia approximately ad 1850 compared with the preceding 1 500 years our palaeoclimate reconstructions indicate that the transition occurred either as a predominantly abrupt shift towards the end of the lia or as a more gradual continued decline over the past 150 years this ambiguity probably arises from non amoc influences on the various proxies or from the different sensitivities of these proxies to individual components of the amoc we suggest that enhanced freshwater fluxes from the arctic and nordic seas towards the end of the lia sourced from melting glaciers and thickened sea ice that developed earlier in the lia weakened labrador sea convection and the amoc the lack of a subsequent recovery may have resulted from hysteresis or from twentieth century melting of the greenland ice sheet 6 our results suggest that recent decadal variability in labrador sea convection and the amoc has occurred during an atypical weak background state future work should aim to constrain the roles of internal climate variability and early anthropogenic forcing in the amoc weakening described here

reconstruction of sea level pressure fields over the eastern north atlantic and europe back to 1500 spatially and temporally high resolution estimates of past natural climate variability are important to assess recent significant climate trends the mid latitude atmospheric circulation is the dominant factor for regional changes in temperature rainfall and other climatic variables here we present reconstructions of gridded monthly sea level pressure slp fields back to 1659 and seasonal reconstructions from 1500 1658 for the eastern north atlantic european region 30 w to 40 e 30 n to 70 n these were developed using principal component regression analysis based on the combination of early instrumental station series pressure temperature and precipitation and documentary proxy data from eurasian sites the relationships were derived over the 1901 1960 calibration period and verified over 1961 1990 under the assumption of stationarity in the statistical relationships a transfer function derived over the 1901 1990 period was used to reconstruct the 500 year large scale slp fields systematic quality testing indicated reliable winter reconstructions throughout the entire period lower skill was obtained for the other seasons although meaningful monthly reconstructions were available from around 1700 onwards when station pressure series became available the quality and the reconstructed slp fields for two exceptionally cold years 1573 1740 are discussed and climatologically interpreted an eof analysis of the 1500 1999 winter slp revealed firstly a zonal flow pattern with pronounced decadal to centenial time scale variations secondly a monopole pattern over northwest europe and thirdly a pattern modulating the meridional flow component over europe these 500 year slp reconstructions should be useful for modelling studies particulary for analyses of low frequency atmospheric variability and for circulation dynamics

solar forcing of north atlantic surface temperature and salinity over the past millennium there were several centennial scale fluctuations in the climate and oceanography of the north atlantic region over the past 1 000 years including a period of relative cooling from about ad 1450 to 1850 known as the little ice age1 these variations may be linked to changes in solar irradiance amplified through feedbacks including the atlantic meridional overturning circulation2 changes in the return limb of the atlantic meridional overturning circulation are reflected in water properties at the base of the mixed layer south of iceland here we reconstruct thermocline temperature and salinity in this region from ad 818 to 1780 using paired 18o and mg ca ratio measurements of foraminifer shells from a subdecadally resolved marine sediment core the reconstructed centennial scale variations in hydrography correlate with variability in total solar irradiance we find a similar correlation in a simulation of climate over the past 1 000 years we infer that the hydrographic changes probably reflect variability in the strength of the subpolar gyre associated with changes in atmospheric circulation specifically in the simulation low solar irradiance promotes the development of frequent and persistent atmospheric blocking events in which a quasi stationary high pressure system in the eastern north atlantic modifies the flow of the westerly winds we conclude that this process could have contributed to the consecutive cold winters documented in europe during the little ice age

multiple equilibria as a possible mechanism for decadal variability in the north atlantic ocean decadal climate variability in the north atlantic has received increased attention in recent years because modeling results suggest predictability of heat content and circulation indices several years ahead however determining the applicability of these results in the real world is challenging because of an incomplete understanding of the underlying mechanisms here the authors show that recent attempts to reconstruct the decadal variations in one of the dominant circulation systems of the region the subpolar gyre spg are not always consistent a coherent picture is partly recovered by a simple conceptual model solely forced by reanalyzed surface air temperatures this confirms that surface heat flux indeed plays a leading role for this typeofvariability ashasbeensuggestedinpreviousstudies theresultsfurthersuggestthatlargevariationsin the spg correspond to the crossing of a bifurcation point that is predicted from idealized experiments and an analytical solution of the model used herein performance of this conceptual model is tested against a statistical stochastic model hysteresis and the existence of two stable modes of the spg circulation shape its response to forcing by atmospheric temperatures the identification of the essential dynamics and the reduction to a minimal model of spg variability provide a quantifiable basis and a framework for future studies on decadal climate variability and predictability

solar forcing as an important trigger for west greenland sea ice variability over the last millennium abstract arctic sea ice represents an important component of the climate system and the present reduction of sea ice in the arctic is of major concern despite its importance little is known about past changes in sea ice cover and the underlying forcing mechanisms here we use diatom assemblages from a marine sediment core collected from the west greenland shelf to reconstruct changes in sea ice cover over the last millennium the proxy based reconstruction demonstrates a generally strong link between changes in sea ice cover and solar variability during the last millennium weaker or stronger solar forcing may result in the increase or decrease in sea ice cover west of greenland in addition model simulations show that variations in solar activity not only affect local sea ice formation but also control the sea ice transport from the arctic ocean through a sea ice ocean atmosphere feedback mechanism the role of solar forcing however appears to have been more ambiguous during an interval around ad 1500 after the transition from the medieval climate anomaly to the little ice age likely to be driven by a range of factors

modes of climate variability synthesis and review of proxy based reconstructions through the holocene abstract modes of climate variability affect global and regional climates on different spatio temporal scales and they have important impacts on human activities and ecosystems as these modes are a useful tool for simplifying the understanding of the climate system it is crucial that we gain improved knowledge of their long term past evolution and interactions over time to contextualise their present and future behaviour we review the literature focused on proxy based reconstructions of modes of climate variability during the holocene i e the last 11 7 thousand years with a special emphasis on i proxy based reconstruction methods ii available proxy based reconstructions of the main modes of variability i e el nino southern oscillation pacific decadal variability atlantic multidecadal variability the north atlantic oscillation the southern annular mode and the indian ocean dipole iii major interactions between these modes and iv external forcing mechanisms related to the evolution of these modes this review shows that modes of variability can be reconstructed using proxy based records from a wide range of natural archives but these reconstructions are scarce beyond the last millennium partly due to the lack of robust chronologies with reduced dating uncertainties technical issues related to proxy calibration and difficulty elucidating their stationary impact or not on regional climates over time while for each mode the available reconstructions tend to agree at mutidecadal timescales they show notable disagreement on shorter timescales beyond the instrumental period the reviewed evidence suggests that the intrinsic variability of modes can be modulated by external forcing such as orbital solar volcanic and anthropogenic forcing the review also highlights some modes experience higher variability over the instrumental period which is partly ascribed to anthropogenic forcing these features stress the paramount importance of further studying their past variations using long climate proxy records for the progress of climate science

a late holocene record of solar forced atmospheric blocking variability over northern europe inferred from varved lake sediments of lake kuninkaisenlampi abstract this study presents a new varved lake sediment sequence from lake kuninkaisenlampi eastern finland the record is constituted by alternations of clastic and biogenic laminae and provides a precise chronology extending back to 3607 94 varve yrs bp the seasonality of the boreal climatic zone with cold winters and mild summers is reflected in the varve structure as a succession of three laminae from bottom to top i a coarse to fine grained detrital lamina marked by detrital catchment material transported by spring floods ii a biogenic lamina with diatoms plant and insect remnants reflecting biological productivity during the season of lake productivity and iii a very fine amorphous organic lamina deposited during the winter stratification the thickness of the detrital lamina in the lake reflects changes in the rate of spring snow melt in the catchment and is therefore considered a proxy for winter conditions hence the record allows reconstructing local climate and environmental conditions on inter annual to the multi centennial timescales we find that minerogenic accumulation reflected in the detrital lamina exhibits a high multi decadal to centennial scale spectral coherency with proxies for solar activity such as 14 c and total solar irradiance suggesting a strong link between solar variability and sediment transport to the lake basin increased catchment erosion is observed during periods of low solar activity which we ascribe to the development of more frequent atmospheric winter blocking circulation induced by solar forced changes in the stratosphere we suggest that soil frost in the catchment of lake kuninkaisenlampi related to more frequent winter blocking led to increased surface run off and ultimately to increased catchment erosion during spring we conclude that during the past ca 3600 years solar forcing may have modulated multi decadal to centennial variations in sedimentation regimes in lakes from eastern finland and potentially in other north european lakes

was common era glacier expansion in the arctic atlantic region triggered by unforced atmospheric cooling abstract the timing and causes of common era ce glacier growth in the arctic atlantic region remain elusive there is mounting evidence of advances that predate the little ice age 1250 1850 ce this challenges the view that 13th century volcanic eruptions triggered change by spurring sea ice expansion recent climate model simulations indicate this response does not require external forcing under contemporaneous pre industrial boundary conditions here we try to reconcile these new insights by combining regional proxy evidence of glacier and sea ice change with a climate model experiment collated recently published reconstructions demonstrate that regional climate shifted towards a colder mean state around 650 950 ce a period marked by low radiative forcing unforced model simulations reproduce the time transgressive evolution of this response which emerged east of greenland and progressed towards svalbard the inferred pattern is associated with sea ice feedbacks triggered by stochastic atmospheric cooling we argue that this mechanism may explain the timing and pattern of ce glacier growth in the region

holocene climatic change in the alaskan arctic as inferred from oxygen isotope and lake sediment analyses at wahoo lake despite recent progress in understanding high latitude climate variability paleoclimate records are scarce from the alaskan arctic we conducted isotopic and sediment analyses at wahoo lake to infer holocene climate variability in northeastern alaska water 18o and d values from the lake and its inlet outlet streams suggest that winter precipitation dominates modern water inputs and that evaporation has limited influence on the lake s hydrological budget the isotopic composition of pisidium exhibits marked variations during the past 11 500 years with 18o ranging between 18 7 and 16 2 and 13c between 7 1 and 2 3 vienna pee dee belemnite vpdb elevated 18o and sediment composition from 11 5 to 8 9 kcal bp suggest evaporative 18o enrichment and arid conditions rising lake levels are evidenced by the disappearance of pisidium and a transition to low carbonate gyttja ca 6 3 kcal bp and by the onset of sediment deposition on an adjacent shelf by 5 3 kcal bp these changes coincided with

atlantic forcing of western mediterranean winter rain minima during the last 12 000 years abstract the limited availability of high resolution continuous archives insufficient chronological control and complex hydro climatic forcing mechanisms lead to many uncertainties in palaeo hydrological reconstructions for the western mediterranean in this study we present a newly recovered 19 63 m long core from lake sidi ali in the north african middle atlas a transition zone of atlantic western mediterranean and saharan air mass trajectories with a multi proxy approach based on magnetic susceptibility carbonate and total organic c content core scanning and quantitative xrf stable isotopes of ostracod shells charcoal counts cedrus pollen abundance and a first set of diatom data we reconstruct western mediterranean hydro climatic variability seasonality and forcing mechanisms during the last 12 000 yr a robust chronological model based on ams 14 c dated pollen concentrates supports our high resolution multi proxy study long term trends reveal low lake levels at the end of the younger dryas during the mid holocene interval 6 6 to 5 4 cal ka bp and during the last 3000 years in contrast lake levels are mostly high during the early and mid holocene the record also shows sub millennial to centennial scale decreases in western mediterranean winter rain at 11 4 10 3 9 2 8 2 7 2 6 6 6 0 5 4 5 0 4 4 3 5 2 9 2 2 1 9 1 7 1 5 1 0 0 7 and 0 2 cal ka bp early holocene winter rain minima are in phase with cooling events and millennial scale meltwater discharges in the sub polar north atlantic our proxy parameters do not show so far a clear impact of saharan air masses on mediterranean hydro climate in north africa however a significant hydro climatic shift at the end of the african humid period 5 ka indicates a change in climate forcing mechanisms the late holocene climate variability in the middle atlas features a multi centennial scale nao type pattern with atlantic cooling and western mediterranean winter rain maxima generally associated with solar minima

coupled modes of north atlantic ocean atmosphere variability and the onset of the little ice age hydroclimate extremes in north america europe and the mediterranean are linked to ocean and atmospheric circulation anomalies in the atlantic but the limited length of the instrumental record prevents complete identification and characterization of these patterns of covariability especially at decadal to centennial timescales here we analyze the coupled patterns of drought variability on either side of the north atlantic ocean basin using independent climate field reconstructions spanning the last millennium in order to detect and attribute epochs of coherent basin wide moisture anomalies to ocean and atmosphere processes a leading mode of broad scale moisture variability is characterized by distinct patterns of north atlantic atmosphere circulation and sea surface temperatures we infer a negative phase of the north atlantic oscillation and colder atlantic sea surface temperatures in the middle of the 15th century coincident with weaker solar irradiance and prior to strong volcanic forcing associated with the early little ice age

foraminiferal evidence of major environmental changes driven by the sun climate coupling in the western portuguese coast 14th century to present abstract new paleoclimatic data from the casa branca salt marsh in the southwest coast of portugal are compared with records previously published on the northwest coast caminha salt marsh to offer a more comprehensive overview of the environmental evolution of the west iberian margin over the last six centuries these reconstructions are based on the foraminiferal records of two dated sediment cores retrieved from the high marsh settings and supported by geochemical sedimentological data both marshes were originally formed in the ad 1300s between the medieval climatic anomaly mca and the little ice age lia highlighting a major episode of increased sediment supply in lower estuaries linked to climate driven changes in continental runoff afterwards the two marshes evolved under different climatic regimes as reflected by their foraminiferal assemblages the environmental conditions are characterized by higher salinity in casa branca southwest with jadammina macrescens and trochammina inflata as dominant species than in caminha northwest where haplophragmoides spp dominates we suggest that a long term trend of a net gain in evapotranspiration at casa branca inducing a higher marsh salinity baseline may explain this microfaunal contrast trochammina inflata seems to be a good indicator of drier periods in the studied area connected to key events of aeolian large dust input to the southwest coast of portugal the influence of the most important climate drivers was assessed namely external solar and internal north atlantic oscillation nao forcings spectral and wavelet transform coherence analyses were used to detect solar footprints on foraminiferal and climate related time series a main significant quasi periodicity was identified within the range of the secular gleissberg cycle of solar activity modulating the annual nao and regional spring summer simulated temperatures after ad 1700 this stronger solar climate coupling may be related to the known upward secular trend in the total solar irradiance after the maunder minimum

oceanic heat pulses fueling moisture transport towards continental europe across the mid pleistocene transition abstract the mid pleistocene transition mpt approx 1 2 0 7 ma is characterized by growing northern hemisphere ice sheets and the shift from a 41 kyr to a 100 kyr glacial interglacial cyclicity concomitant to the growth of large ice sheets atmospheric and oceanic circulation pattern have changed one key feature of the north atlantic is the wind driven subtropical gyre a major provider of heat and moisture for continental europe here we investigate changes in the strength and spatial configuration of the subtropical gyre during the mpt and its impact on the continental moisture balance to reconstruct subtropical gyre dynamics we conducted paired 18o and mg ca analyses on the deep dwelling foraminifera globorotalia inflata from iberian margin site u1385 yielding thermocline temperature ttherm variability between 1400 and 500 ka at the eastern boundary of the subtropical gyre long term trends of ttherm at site u1385 oppose the north atlantic climatic evolution of progressively intensified glacials during the mpt particularly glacials mis 20 and 18 were marked by warm thermocline waters off iberia we infer that a southward shift of the sub polar front displaced the source region of thermocline waters within the subtropical gyre from high to mid latitudes in addition a strong mediterranean outflow water production during the mpt caused the advection of warm waters to iberia humid conditions during mis 20 and 18 in se europe indicate that atmospheric moisture derived from this warm water might have been advected deep into continental europe and contributed to enhanced growth of alpine glaciers

solar forcing of holocene summer sea surface temperatures in the northern north atlantic mounting evidence from proxy records suggests that variations in solar activity have played a significant role in triggering past climate changes however the mechanisms for sun climate links remain a topic of debate here we present a high resolution summer sea surface temperature sst record covering the past 9300 yr from a site located at the present day boundary between polar and atlantic surface water masses the record is age constrained via the identification of 15 independently dated tephra markers from terrestrial archives circumventing marine reservoir age variability problems our results indicate a close link between solar activity and ssts in the northern north atlantic during the past 4000 yr they suggest that the climate system in this area is more susceptible to the influence of solar variations during cool periods with less vigorous ocean circulation furthermore the high resolution sst record indicates that climate in the north atlantic regions follows solar activity variations on multidecadal to centennial time scales

the isotopic signature of planktonic foraminifera from ne atlantic surface sediments implications for the reconstruction of past oceanic conditions the stable isotope compositions of the planktonic foraminifera globigerina bulloides globigerinoides ruber white and pink varieties globigerinoides trilobus globorotalia inflata and globorotalia truncatulinoides right and left coiling types were examined as recorders of north atlantic surface water properties based on 40 box core surface sediments between 60 and 30 n while g ruber white and pink varieties and g trilobus mainly reflect summer surface water conditions in their oxygen isotope composition g bulloides reflects temperatures of the northward migrating spring bloom february march in the south to may june in the north our data show that g bulloides cannot be regarded as an indicator for summer temperatures as deduced from duplessy et al s data gt inflata and gt truncatulinoides right and left coiling build their shells in the coldest waters compared with the other species and reflect temperatures between 100 and 400 m water depth the difference in oxygen isotope composition between g bulloides and g inflata serves as a proxy for water mass stratification g bulloides is the only species that gives a distinct pattern in its carbon isotopic composition showing a high correlation with the surface water phosphate values along the transect and may serve as a proxy for palaeonutrients and or productivity

internally generated decadal cold events in the northern north atlantic and their possible implications for the demise of the norse settlements in greenland we attribute and describe the governing mechanisms of decadal cold excursions in the subpolar north atlantic of similar amplitude and duration to cold events reconstructed from climate proxies during the last millennium detected in an ensemble of three transient and one unperturbed climate simulation the cold events are attributed to internal regional climate variability with varying external forcing increasing their magnitude and frequency the underlying general mechanism consists of a feedback loop initiated by a weakening of the north atlantic subpolar gyre which induces persistent colder and fresher surface conditions in the labrador sea and eventually a deep convection shutdown we thus exclude a hemispheric climate reorganization or a weak ocean overturning circulation as necessary trigger for such events an associated northeastward atmospheric cold advection over the labrador sea deteriorates local living conditions on south greenland essential for the sustainability of the norse settlements

a record of holocene sea ice variability off west greenland and its potential forcing factors abstract we present a reconstruction of holocene sea ice variability from sediment core ga306 gc3 from the holsteinsborg dyb off west greenland which provides an index of palaeoceanographic and palaeoenvironmental conditions within this climatically sensitive region during the last 6700 yr the reconstructed sea ice record combined with previously published proxy data suggests that relatively warm conditions with reduced sea ice extent prior to 5000 cal yr bp were associated with the holocene thermal maximum subsequent cooling and extensive sea ice cover between ca 5000 and 4000 cal yr bp was followed by even colder conditions and persistent sea ice cover during the neoglacial cooling phase particularly after ca 1500 cal yr bp there is a positive correlation between west greenland sea ice cover and solar activity over the past 5000 yr but the correlation is much weaker prior to 5000 cal yr bp in addition there is a strong link between west greenland sea ice and changes in the abundance of arctic benthic foraminifera species related to different water masses of the west greenland current during the entire interval even prior to 5000 cal yr bp our findings indicate that sea ice variability off west greenland was driven not only by solar activity but also by ocean circulation the strength of cold polar water from the east greenland current and warm atlantic water from the irminger current

the complexity of millennial scale variability in southwestern europe during mis 11 abstract climatic variability of marine isotope stage mis 11 is examined using a new high resolution direct land sea comparison from the sw iberian margin site u1385 this study based on pollen and biomarker analyses documents regional vegetation terrestrial climate and sea surface temperature sst variability suborbital climate variability is revealed by a series of forest decline events suggesting repeated cooling and drying episodes in sw iberia throughout mis 11 only the most severe events on land are coeval with sst decreases under larger ice volume conditions our study shows that the diverse expression magnitude character and duration of the millennial scale cooling events in sw europe relies on atmospheric and oceanic processes whose predominant role likely depends on baseline climate states repeated atmospheric shifts recalling the positive north atlantic oscillation mode inducing dryness in sw iberia without systematical sst changes would prevail during low ice volume conditions in contrast disruption of the atlantic meridional overturning circulation amoc related to iceberg discharges colder sst and increased hydrological regime would be responsible for the coldest and driest episodes of prolonged duration in sw europe

exceptional ocean surface conditions on the se greenland shelf during the medieval climate anomaly diatom inferred 2900 year long records of august sea surface temperature asst and april sea ice concentration asic are generated from a marine sediment core from the se greenland shelf with a special focus on the interval ca 870 1910 common era c e reconstructed in subdecadal temporal resolution the medieval climate anomaly mca between 1000 and 1200 c e represents the warmest ocean surface conditions of the se greenland shelf over the late holocene 880 b c e before the common era to 1910 c e it was characterized by abrupt decadal to multidecadal changes such as an abrupt warming of 2 4 c in 55 years around 1000 c e temperature changes of these magnitudes are rare on the north atlantic proxy data compared to regional air temperature reconstructions our results indicate a lag of about 50 years in ocean surface warming either due to increased freshwater discharge from the greenland ice sheet or intensified sea ice export from the arctic as a response to atmospheric warming at the beginning of the mca a cool phase from 1200 1890 c e associated with the little ice age ends with the rapid warming of asst and diminished asic in the early twentieth century the results show that the periods of warm asst and asic minima are coupled with solar minima suggesting that solar forcing possibly amplified by atmospheric forcing have been behind the variability of surface conditions on the se greenland over the last millennium the results indicate that the se greenland shelf is a climatologically sensitive area where extremely rapid changes are possible and highlights the importance of the area under the present warming conditions

sea surface temperature variability in the north western mediterranean sea gulf of lion during the common era this study investigates the multidecadal scale variability of sea surface temperatures ssts in the convection region of the gulf of lion nw mediterranean sea over the full past 2000 yr common era using alkenone biomarkers our data show colder ssts by 1 7 c over most of the first millennium 200 800 ad and by 1 3 c during the little ice age lia 1400 1850 ad than the 20th century mean 17 9 c although on average warmer those of the medieval climate anomaly mca 1000 1200 ad were lower by 1 c we found a mean sst warming of 2 c 100 yr over the last century in close agreement with the 0 22 and 0 26 c decade values calculated for the western mediterranean sea from in situ and satellite data respectively our results also reveal strongly fluctuating ssts characterized by cold extremes followed by abrupt warming during the lia we suggest that the coldest decades of the lia were likely caused by prevailing negative ea states and associated anticyclone blocking over the north atlantic resulting in cold continental northeasterly winds to blow over western europe and the mediterranean region

the impact of polar mesoscale storms on northeast atlantic ocean circulation every year thousands of mesoscale storms termed polar lows cross the climatically sensitive subpolar north atlantic ocean high resolution numerical simulations of the ocean circulation taking into account the effect of these storms on deep water formation suggest that polar lows significantly affect the global ocean circulation

reconstructing past seasonal to multi centennial scale variability in the ne atlantic ocean using the long lived marine bivalve mollusc glycymeris glycymeris the lack of long term highly resolved annual to sub annual and absolutely dated baseline records of marine variability extending beyond the instrumental period last 50 100 years hinders our ability to develop a comprehensive understanding of the role the ocean plays in the climate system specifically without such records it remains difficult to fully quantify the range of natural climate variability mediated by the ocean and to robustly attribute recent changes to anthropogenic or natural drivers here we present a 211 year 1799 2010 ce all dates hereafter are common era seawater temperature swt reconstruction from the northeast atlantic ocean derived from absolutely dated annually resolved oxygen isotope ratios recorded in the shell carbonate 18oshell of the long lived marine bivalve mollusc glycymeris glycymeris the annual record was calibrated using sub annually resolved 18oshell values drilled from multiple shells covering the instrumental period calibration verification statistics and spatial correlation analyses indicate that the 18oshell record contains significant skill at reconstructing northeast atlantic ocean mean summer swt variability associated with changes in sub polar gyre spg dynamics and the north atlantic current reconciling differences between the 18oshell data and corresponding growth increment width chronology demonstrates that 68 of the variability in g glycymeris shell growth can be explained by the combined influence of biological productivity and swt variability these data suggest g glycymeris can provide seasonal to multi centennial absolutely dated baseline records of past marine variability that will lead to the development of a quantitative understanding of the role the marine environment plays in the global climate system

biological and climate controls on north atlantic marine carbon dynamics over the last millennium insights from an absolutely dated shell based record from the north icelandic shelf given the rapid increase in atmospheric carbon dioxide concentrations pco2 over the industrial era there is a pressing need to construct long term records of natural carbon cycling prior to this perturbation and to develop a more robust understanding of the role the oceans play in the sequestration of atmospheric carbon here we reconstruct the past biological and climate controls on the carbon isotopic 13cshell composition of the north icelandic shelf waters over the last millennium derived from the shells of the long lived marine bivalve mollusk arctica islandica variability in the annually resolved 13cshell record is dominated by multidecadal variability with a negative trend 0 003 0 002 yr 1 over the industrial era 1800 2000 common era this trend is consistent with the marine suess effect brought about by the sequestration of isotopically light carbon 13c of co2 derived from the burning of fossil fuels comparison of the 13cshell record with contemporaneous proxy archives over the last millennium and instrumental data over the twentieth century highlights that both biological primary production and physical environmental factors such as relative shifts in the proportion of subpolar mode waters and arctic intermediate waters entrained onto the north icelandic shelf atmospheric circulation patterns associated with the winter north atlantic oscillation and sea surface temperature and salinity of the subpolar gyre are the likely mechanisms that contribute to natural variations in seawater 13c variability on the north icelandic shelf contrasting 13c fractionation processes associated with these biological and physical mechanisms likely cause the attenuated marine suess effect signal at this locality

selective sorting and abrasion of river gravel i theory most gravel rivers show a tendency for characteristic grain size to decrease in the downstream direction over scales of tens or hundreds of kilometers it has been surmised that this downstream fining is due to some combination of selective sorting by which finer grains are preferentially transported downstream and abrasion by which individual particles are reduced in size here a framework for the simultaneous treatment of both phenomena is developed the analysis is restricted to bed load transport of gravel only abrasion due to binary collisions between moving bed load and stationary bed particles is considered the formulation includes the possibility of differential basin uplift and subsidence due to tectonism and mixtures of rock types with differing coefficients of abrasion the formulation of selective sorting is based on the concept of hiding as applied to a surface based bed load transport relation abrasion is first treated in terms of a lagrangian formulation for individual grains and t

holocene mountain glacier history in the sukkertoppen iskappe area southwest greenland abstract mountain glaciers and ice caps gic independent of the greenland ice sheet respond rapidly to climate variations and records of their past extent provide information on the natural envelope of climate variability here we use a multi proxy approach that combines proglacial lake sediment analysis cosmogenic nuclide surface exposure dating in situ 10be and 14c and radiocarbon dating of recently ice entombed moss to generate a centennial scale record of holocene gic fluctuations in southwestern greenland following local deglaciation 10 9 ka sediments from proglacial crash lake record a glacier advance at 9 ka that is indistinguishable from nearby ice sheet moraines implying a synchronous response of gic and the greenland ice sheet to a centennial scale climate event following this local glacier advance gic experienced net recession until 4 6 ka radiocarbon ages of in situ moss n 29 and crash lake sediments reveal intervals of glacier expansion at 1 8 1 2 and 0 7 ka that are superimposed on an overall trend of net glacier expansion throughout the late holocene in situ 14c concentrations from bedrock adjacent to radiocarbon dated moss samples further constrain the duration of ice cover through the holocene in this region we find that our glacier size proxy records during the past 4 ka are broadly consistent with relatively lower temperatures recorded in gisp2 and occur during or following intervals of volcanic perturbations thus we speculate that volcanic activity although less frequent and intense than in the early holocene and during the little ice age may have led to centennial scale variability imprinted on net glacier expansion due to decreasing summer insolation through the late holocene

impact of volcanic eruptions on decadal to centennial fluctuations of arctic sea ice extent during the last millennium and on initiation of the little ice age abstractthis study evaluates different hypotheses of the origin of the little ice age focusing on the long term response of arctic sea ice and oceanic circulation to solar and volcanic perturbations the authors analyze the last millennium ensemble of climate model simulations carried out with the community earth system model at the national center for atmospheric research the authors examine the duration and strength of volcanic perturbations and the effects of initial and boundary conditions such as the phase of the atlantic multidecadal oscillation they evaluate the impacts of these factors on decadal to multicentennial perturbations of the cryospheric oceanic and atmospheric components of the climate system the authors show that at least in the last millennium ensemble volcanic eruptions are followed by a decadal scale positive response of the atlantic multidecadal overturning circulation followed by a centennial scale enhancement of the northern hemispheric sea ice extent it is hypothesiz

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

changes in the strength of the nordic seas overflows over the past 3000 years the nordic seas overflows constitute the densest component of the deep limb of the atlantic meridional overturning circulation amoc changes in the vigour of the overflows may have had important climatic effects in the past and may also have in the future yet evidence for multidecadal to millennial changes in the deep limb of the amoc and their potential relationship to north atlantic climate variability during the holocene remains weakly constrained here we present grain size data as a proxy for near bottom current speed from sub decadal to decadally resolved sediment cores located in the direct pathway of the two nordic overflows east and west of iceland the iceland scotland overflow water isow and the denmark strait overflow water dsow respectively the results show no clear relationship between reconstructed changes in the vigour of the nordic overflows and the well known periods of centennial scale climate variability recorded in the north atlantic region however well defined millennial scale trends are found in both of the overflow strength records over the last 3000 years which were possibly related to hydrographic reorganizations in the nordic seas driven by the decrease in northern hemisphere summer insolation over the neoglacial period a comparison between the near bottom flow speed reconstructions from isow and dsow suggests an anti phased relationship between the nordic seas overflows east and west of iceland over the last 3000 years this feature has been observed in climate models potentially as a result of shifts in the deep water formation sites as a response to changes in atmospheric patterns over the nordic seas

abrupt onset of the little ice age triggered by volcanism and sustained by sea ice ocean feedbacks 1 northern hemisphere summer temperatures over the past 8000 years have been paced by the slow decrease in summer insolation resulting from the precession of the equinoxes however the causes of superposed century scale cold summer anomalies of which the little ice age lia is the most extreme remain debated largely because the natural forcings are either weak or in the case of volcanism short lived here we present precisely dated records of ice cap growth from arctic canada and iceland showing that lia summer cold and ice growth began abruptly between 1275 and 1300 ad followed by a substantial intensification 1430 1455 ad intervals of sudden ice growth coincide with two of the most volcanically perturbed half centuries of the past millennium a transient climate model simulation shows that explosive volcanism produces abrupt summer cooling at these times and that cold summers can be maintained by sea ice ocean feedbacks long after volcanic aerosols are removed our results suggest that the onset of the lia can be linked to an unusual 50 year long episode with four large sulfur rich explosive eruptions each with global sulfate loading 60 tg the persistence of cold summers is best explained by consequent sea ice ocean feedbacks during a hemispheric summer insolation minimum large changes in solar irradiance are not required

persistent link between solar activity and greenland climate during the last glacial maximum changes in solar activity have previously been proposed to cause decadal to millennial scale fluctuations in both the modern and holocene climates1 direct observational records of solar activity such as sunspot numbers exist for only the past few hundred years so solar variability for earlier periods is typically reconstructed from measurements of cosmogenic radionuclides such as 10be and 14c from ice cores and tree rings2 3 here we present a high resolution 10be record from the ice core collected from central greenland by the greenland ice core project grip the record spans from 22 500 to 10 000 years ago and is based on new and compiled data4 5 6 using 14c records7 8 to control for climate related influences on 10be deposition we reconstruct centennial changes in solar activity we find that during the last glacial maximum solar minima correlate with more negative 18o values of ice and are accompanied by increased snow accumulation and sea salt input over central greenland we suggest that solar minima could have induced changes in the stratosphere that favour the development of high pressure blocking systems located to the south of greenland as has been found in observations and model simulations for recent climate9 10 we conclude that the mechanism behind solar forcing of regional climate change may have been similar under both modern and last glacial maximum climate conditions

palaeoclimate and palaeoceanographic conditions in the westernmost mediterranean over the last millennium an integrated organic and inorganic approach previous studies have revealed the excellence of the westernmost mediterranean records for reconstructing climate conditions over the last two millennia in particular inorganic and organic proxies have revealed a coherent signal of climate and oceanographic responses during this time here we compare and integrate both proxy records for reconstructing climate and oceanographic conditions with focus on the last millennium the higher temperatures indicated by lipid based proxies during the medieval climate anomaly mca are coincident with drier conditions indicated by inorganic proxies in contrast lower temperatures and humid conditions are indicated during the little ice age lia the industrial period was characterized by increasing humidity in comparison with the previous lia however a progressive aridification occurred since the second half of the twentieth century coexisting with a warming trend and higher contribution of c3 grasses proxy records are also interpreted to show atlantic water inflow at ad 1450 and 1950 and an intensification of the upwelling conditions coinciding with a prolonged positive north atlantic oscillation nao phase in the intervals ad 1000 1450 and 1960 1990 large scale oceanic atmospheric circulation patterns the nao and the atlantic meridional circulation and solar irradiance variations seem to have played the major key role during the last millennium together with anthropogenic contributions in the more recent record

climate reconstruction for the last two millennia in central iberia the role of east atlantic ea north atlantic oscillation nao and their interplay over the iberian peninsula abstract a multi proxy characterization of the uppermost sedimentary infill of an iberian alpine lake cimera 2140 m a s l was performed to establish the climatic and environmental conditions for the iberian central range icr over the last two millennia this multi proxy characterization was used to reconstruct the intense runoff events lake productivity and soil erosion in the lake catchment and interpret these factors in terms of temperature and precipitation variability the roman period rp 200 bce 500 ce beginning was characterized by an alternation between cold and warm periods as indicated by short lived oscillations of intense runoff conditions and soil erosion although warm conditions dominated the end of the period and the early middle age ema 500 900 ce onset in the icr a noticeable decrease in intense runoff events and a progressive decrease in soil erosion during the late ema indicated a shift to colder temperatures in terms of precipitation both the rp and ema climate periods displayed a transition from dry to wet conditions that led to a decrease in lake productivity the medieval climate anomaly mca 900 1300 ce was characterized by warm and dry conditions with frequent intense runoff episodes and increases in lake productivity and soil erosion whereas the little ice age lia 1300 1850 ce showed the opposite characteristics the industrial era 1850 2012 ce presented an increase in lake productivity that likely demonstrates the influence of global warming the spatio temporal integration of the cimera record with other iberian reconstructions has been used to identify the main climate drivers over this region during the rp and ema n s and e w humidity gradients were dominant whereas during the mca and lia these gradients were not evident these differences could be ascribed to interactions between the north atlantic oscillation nao and east atlantic ea phases during the rp the general warm conditions and the e w humidity gradient indicate a dominant interplay between a negative nao phase and a positive ea phase nao ea whereas the opposite conditions during the ema indicate a nao ea interaction the dominant warm and arid conditions during the mca and the cold and wet conditions during the lia indicate the interplay of the nao ea and nao ea respectively furthermore the higher solar irradiance during the rp and mca may support the predominance of the ea phase whereas the opposite scenario during the ema and lia may support the predominance of the ea phase which would favour the occurrence of frequent and persistent blocking events in the atlantic region during these periods