propagation of the great salinity anomaly of the 1990s around the northern north atlantic 1 time series of t and s extending through 2001 are used to describe propagation of the great salinity anomaly of the 1990s gsa 90s comparison of the distance time relations for the gsa 70s 80s and 90s reveals a substantial intensification of the large scale circulation in the northern north atlantic especially in the subarctic gyre between newfoundland and the faroes the advection rate of the gsa 70s 80s and 90s between newfoundland and the faroe shetland channel is conservatively estimated to have been 3 5 10 and 10 cm s respectively the circulation intensification apparently occurred within a decade between the gsa 70s and 80s during the next decade the advection rate increased from 10 to 13 cm s between newfoundland and iceland basin the gsa 90s was advected towards the faroe shetland channel by the northern iceland basin s branch of the north atlantic current whereas the contribution of the southern branch via the rockall trough was minimal

a 2000 year record of atlantic water temperature variability from the malangen fjord northeastern north atlantic a high resolution sedimentary record from the subarctic malangen fjord in northern norway northeastern north atlantic has been investigated in order to reconstruct variations in influx of atlantic water for the last 2000 years the fjord provides a regional oceanographic climatic signal reflecting changes in the north atlantic heat flux at this latitude because of its deep sill and the relatively narrow adjoining continental shelf the reconstructions are based on oxygen and carbon isotopic studies of benthic foraminifera from a high accumulation basin in the malangen fjord providing subdecadal time resolution a comparison between instrumental measurements of bottom water temperatures at the core location and the reconstructed temperatures from benthic foraminiferal 18o for the same time period demonstrates that the stable isotope values reflect the bottom water temperatures very well the reconstructed temperature record shows an overall decline in temperature of c 1 c from c 40 bc to ad 1350 this

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

application of tephrochronology to the timing and correlation of palaeoceanographic events recorded in holocene and late glacial shelf sediments off north iceland abstract the north icelandic shelf is partly distinguished by the tjornes fracture zone featuring numerous active basins in a mud dominated shelf environment late glacial and holocene high resolution sedimentary records from this area have been studied with tephrochronology as the main tool for correlation and for exact timing of palaeoceanographic events in the area data from three new piston cores from the shelf demonstrate the importance of tephra markers for the first chronological evaluation and correlation of the cores the correlation is extended with lithological logs and with magnetic susceptibility records a detailed multidisciplinary study including biostratigraphy and tephrochronology of a late holocene record of predominantly muddy sediments at the same location demonstrates that marked variations in the distribution of water masses occurred repeatedly through the last 4500 cal yr of special interest is the exact timing of a marked drop in sea surface temperature in the area indicated by ice rafting debris concentration to about 50 years before the hekla 3 eruption which occurred at 2980 cal yr bp this appears to predate most records of a general cooling event in nw europe by a couple of centuries two different possible age models one based on 14c dates combined with tephra markers and one based on tephra markers alone are discussed in context with the problem of different marine reservoir ages of the water masses in the area

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

amplified inception of european little ice age by sea ice ocean atmosphere feedbacks abstractthe inception of the little ice age 1400 1700 ad is believed to have been driven by an interplay of external forcing and climate system internal variability while the hemispheric signal seems to have been dominated by solar irradiance and volcanic eruptions the understanding of mechanisms shaping the climate on a continental scale is less robust in an ensemble of transient model simulations and a new type of sensitivity experiments with artificial sea ice growth the authors identify a sea ice ocean atmosphere feedback mechanism that amplifies the little ice age cooling in the north atlantic european region and produces the temperature pattern suggested by paleoclimatic reconstructions initiated by increasing negative forcing the arctic sea ice substantially expands at the beginning of the little ice age the excess of sea ice is exported to the subpolar north atlantic where it melts thereby weakening convection of the ocean consequently northward ocean heat transport is reduced reinf

holocene lowering of the laurentide ice sheet affects north atlantic gyre circulation and climate the laurentide ice sheet which covered canada during glacial periods had a major influence on atmospheric circulation and surface climate but its role in climate during the early holocene 9 7 ka when it was thinner and confined around hudson bay is unclear it has been suggested that the demise of the ice sheet played a role in the 8 2 ka event an abrupt 1 3 c northern hemisphere cooling lasting 160 years through the influence of changing topography on atmospheric circulation to test this hypothesis and to investigate the broader implications of changing ice sheet topography for climate we analyse a set of equilibrium climate simulations with ice sheet topographies taken at 500 year intervals from 9 5 to 8 0 ka between 9 5 and 8 0 ka our simulations show a 2 c cooling south of iceland and a 1 c warming between 40 and 50 n in the north atlantic these surface temperature changes are associated with a weakening of the subtropical and subpolar gyres caused by a decreasing wind stress curl over the mid north atlantic as the ice sheet lowers the climate response is strongest during the period of peak ice volume change 9 5 8 5 ka but becomes negligible after 8 5 ka the climatic effects of the laurentide ice sheet lowering during the holocene are restricted to the north atlantic sector thus topographic forcing is unlikely to have played a major role in the 8 2 ka event and had only a small effect on holocene climate change compared to the effects of changes in greenhouse gases insolation and ice sheet meltwater

reversal of the 1960s to 1990s freshening trend in the northeast north atlantic and nordic seas hydrographic time series in the northeast north atlantic and nordic seas show that the freshening trend of the 1960s 1990s has completely reversed in the upper ocean since the 1990s temperature and salinity have rapidly increased in the atlantic inflow from the eastern subpolar gyre to the fram strait in 2003 2006 salinity values reached the previous maximum last observed around 1960 and temperature values exceeded records the mean properties of the atlantic inflow decrease northwards but variations seen in the eastern subpolar gyre at 57 n persist with the same amplitude and pattern along the pathways to fram strait time series correlations and extreme events suggest a time lag of 3 4 years over that distance this estimate allows predictions to be made the temperature of atlantic water in the fram strait may start to decline in 2007 or 2008 salinity a year later but both will remain high at least until 2010

old radiocarbon ages in the southwest pacific ocean during the last glacial period and deglaciation marine radiocarbon 14c dates are widely used for dating oceanic events and as tracers of ocean circulation essential components for understanding ocean climate interactions past ocean ventilation rates have been determined by the difference between radiocarbon ages of deep water and surface water reservoirs but the apparent age of surface waters currently approximately 400 years in the tropics and approximately 1 200 years in antarctic waters might not be constant through time as has been assumed in radiocarbon chronologies and palaeoclimate studies here we present independent estimates of surface water and deep water reservoir ages in the new zealand region since the last glacial period using volcanic ejecta tephras deposited in both marine and terrestrial sediments as stratigraphic markers compared to present day values surface reservoir ages from 11 900 14c years ago were twice as large 800 years and during glacial times were five times as large 2 000 years contradicting the assumption of constant surface age furthermore the ages of glacial deep water reservoirs were much older 3 000 5 000 years the increase in surface to deep water age differences in the glacial southern ocean suggests that there was decreased ocean ventilation during this period

a simulation of thermohaline effects of a great salinity anomaly abstract model simulations of an idealistic great salinity anomaly gsa demonstrate that variability in the sea ice export from the arctic when concentrated to short pulses can have a large influence on the meridional heat transport and can lead to an altered overturning state one single freshwater disturbance resulting from excess ice export as in 1968 can disrupt the deep mixing process the critical condition for a large oceanic response is defined by the intensity duration and timing of the ice pulse in particular as it exits through the denmark strait a recovery from this event takes several years for advection and diffusion to remove the salinity anomaly concurrently the influence of the gsa propagates to the subtropics via the boundary currents and baroclinic adjustment as a result of this adjustment there are large up to 20 changes in the strength of the overturning cell and in the meridional heat transport in the subtropics and subpolar areas simulations show a temperature sali

testing the robustness of a precipitation proxy based north atlantic oscillation reconstruction abstract the reconstruction of past atmospheric circulation is crucial for the understanding of natural climate change and its driving factors a recent reconstruction suggests that during medieval times the european region was dominated by a persistent positive phase of the north atlantic oscillation nao followed by a shift to a more oscillatory behavior we test this hypothesis and the concept underlying the reconstruction in a pseudo proxy approach using instrumental records reanalysis data sets and millennial simulations with four different climate models while a shift from a more positive to a more negative phase of the nao seems to be likely the amplitude and persistence of the reconstructed positive phase cannot be reproduced by models the analysis further reveals that proxy locations that were used in the reconstruction are not always sufficient to describe the nao this is reflected in a failure of the reconstruction to verify against instrumental records of the nao in the 19th century by adding complementary proxies the robustness of an nao reconstruction can be improved to the degree that it would withstand the tests presented here

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

gas hydrate crystals may help build reefs during a recent cruise in the porcupine basin off southwest ireland we discovered two extensive and hitherto largely unsuspected deep water reef provinces including a giant cluster of hundreds of buried mounds the ring shapes of many reefs suggest that they are caused by an axial fluid expulsion at the sea bed a transient flow well confined in space and time we are exploring various hypotheses but a stimulating avenue for research is opened by a glacially controlled growth pulse and subsequent decay of a shallow layer of gas hydrates as a methane buffer and probably indirectly as a ground for overlying biological communities

simulated decadal oscillations of the atlantic meridional overturning circulation in a cold climate state the significance of the atlantic meridional overturning circulation moc for regional and hemispheric climate change requires a complete understanding using fully coupled climate models here we present a persistent decadal oscillation in a coupled atmosphere ocean general circulation model while the present study is limited by the lack of comparisons with paleo proxy records the purpose is to reveal a new theoretically interesting solution found in the fully coupled climate model the model exhibits two multi century long stable states with one dominated by decadal moc oscillations the oscillations involve an interaction between anomalous advective transport of salt and surface density in the north atlantic subpolar gyre their time scale is fundamentally determined by the advection in addition there is a link between the moc oscillations and north atlantic oscillation nao like sea level pressure anomalies the analysis suggests an interaction between the nao and an anomalous subpolar gyre circulation in which sea ice near and south of the labrador sea plays an important role in generating a large local thermal anomaly and a meridional temperature gradient the latter induces a positive feedback via synoptic eddy activity in the atmosphere in addition the oscillation only appears when the nordic sea is completely covered by sea ice in winter and deep convection is active only near the irminger sea such conditions are provided by a substantially colder north atlantic climate than today

a link between reduced barents kara sea ice and cold winter extremes over northern continents the recent overall northern hemisphere warming was accompanied by several severe northern continental winters as for example extremely cold winter 2005 2006 in europe and northern asia here we show that anomalous decrease of wintertime sea ice concentration in the barents kara b k seas could bring about extreme cold events like winter 2005 2006 our simulations with the echam5 general circulation model demonstrate that lower troposphere heating over the b k seas in the eastern arctic caused by the sea ice reduction may result in strong anti cyclonic anomaly over the polar ocean and anomalous easterly advection over northern continents this causes a continental scale winter cooling reaching 1 5 c with more than three times increased probability of cold winter extremes over large areas including europe our results imply that several recent severe winters do not conflict the global warming picture but rather supplement it being in qualitative agreement with the simulated large scale atmospheric circulation realignment furthermore our results suggest that high latitude atmospheric circulation response to the b k sea ice decrease is highly nonlinear and characterized by transition from anomalous cyclonic circulation to anticyclonic one and then again back to cyclonic type of circulation as the b k sea ice concentration gradually reduces from 100 to ice free conditions we present a conceptual model which may explain the nonlinear local atmospheric response in the b k seas region by counter play between convection over the surface heat source and baroclinic effect due to modified temperature gradients in the vicinity of the heating area

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

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

out of the woods driftwood insights into holocene pan arctic sea ice dynamics the collation of 913 driftwood samples from across the western arctic with spatiotemporal distribution and available provenance data enabled the production of a high resolution proxy based reconstruction of holocene arctic ocean surface current and sea ice dynamics regionally bounded driftwood based sea ice reconstructions studies suggest spatiotemporally complex past arctic sea ice extent and movement however a large scale compilation of holocene arctic driftwood has not previously been developed sparse driftwood in the early holocene 8 2 cal ka b p deglacial period was followed by increased driftwood deposition in the warmer mid holocene 8 2 4 2 cal ka b p characterized by an enhanced transpolar drift tpd 7 cal ka b p leading to sea ice loss through the fram strait driftwood incursion peaks show spatial e w progression from the eurasian archipelagos to greenland and the canadian arctic archipelago suggesting a progressive shift in the orientation of the tpd on centennial millennial time scales and intermediate phases in the arctic oscillation late holocene cooling 4 2 cal ka b p is indicated by increased influx of probably north american picea via a strengthened beaufort gyre bg which enhanced sea ice recirculation starting in the western arctic and progressing eastward in recent millennia 2 cal ka b p a more variable driftwood record alternates between bg and tpd dominance on centennial time scales to further constrain a spatiotemporal reconstruction of variations in holocene ocean current and sea ice dynamics a more definitive determination of driftwood provenance is recommended to build upon the current framework such as through radiogenic isotope tracing and adna analysis

late glacial and holocene palaeoceanography of the north icelandic shelf high resolution gravity cores and box cores from the north icelandic shelf have been studied for palaeoceanographic history based on lithological and biostratigraphical foramin iferal data results from two outer shelf cores covering the last 13 6 k 14 c yr bp are presented in this paper the sediments accumulated in north south trending basins on each side of the kolbeinsey ridge at water depths of ca 400 m sedimentation rates up to 1 5 m kyr 1 are observed during the late glacial and holocene the vedde and saksunarvatn tephras are present in the cores as well as the hekla 1104 a new tephra kol gs 2 has been identified and dated to 13 4 k 14 c yr bp and another tephra geochemically identical to the borrobol tephra has been found at the same level at present the oceanographic polar front is located on the north icelandic shelf which experiences sharp oceanographic surface boundaries between the cold east icelandic current and the warmer irminger current past changes in sedimentological and biological processes in the study area are assumed to be related to fluctuations of the polar front the area was deglaciated before ca 14 kyr bp but there is evidence of ice rafting up to the end of the gs 1 greenland stadial 1 younger dryas period increasing again towards the end of the holocene foraminiferal studies show a relatively strong gs 2 pre 13 kyr bp palaeo irminger current followed by severe cooling and then by unstable conditions during the remainder of the gi 1 greenland interstadial 1 bolling allerod and gs 1 younger dryas another cooling event occurred during the preboreal before the holocene current system was established at about 9 kyr bp after a climatic optimum between 9 and 6 kyr bp the climate began to deteriorate and fluctuate copyright 2000 john wiley sons ltd

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

deglacial changes in ocean circulation from an extended radiocarbon calibration temporal variations in the atmospheric concentration of radiocarbon sometimes result in radiocarbon based age estimates of biogenic material that do not agree with true calendar age this problem is particularly severe beyond the limit of the high resolution radiocarbon calibration based on tree ring data which stretches back only to1 2 about 11 8 kyr before present bp near the termination of the younger dryas cold period if a wide range of palaeoclimate records are to be exploited for better understanding the rates and patterns of environmental change during the last deglaciation extending the well calibrated radiocarbon timescale back further in time is crucial several studies attempting such an extension using uranium thorium dated corals3 4 5 and laminae counts in varved sediments6 7 8 9 show conflicting results here we use radiocarbon data from varved sediments in the cariaco basin in the southern caribbean sea to construct an accurate and continuous radiocarbon calibration for the period 9 to 14 5 kyr bp nearly 3 000 years beyond the tree ring based calibration a simple model compared to the calculated atmospheric radiocarbon concentration and palaeoclimate data from the same sediment core suggests that north atlantic deep water formation shut down during the younger dryas period but was gradually replaced by an alternative mode of convection possibly via the formation of north atlantic intermediate water

quantifying the influence of atlantic heat on barents sea ice variability and retreat abstractthe recent arctic winter sea ice retreat is most pronounced in the barents sea using available observations of the atlantic inflow to the barents sea and results from a regional ice ocean model the authors assess and quantify the role of inflowing heat anomalies on sea ice variability the interannual variability and longer term decrease in sea ice area reflect the variability of the atlantic inflow both in observations and model simulations during the last decade 1998 2008 the reduction in annual july june sea ice area was 218 103 km2 or close to 50 this reduction has occurred concurrent with an increase in observed atlantic heat transport due to both strengthening and warming of the inflow modeled interannual variations in sea ice area between 1948 and 2007 are associated with anomalous heat transport r 0 63 with a 70 103 km2 decrease per 10 tw input of heat based on the simulated ocean heat budget it is found that the heat transport into the western barents sea sets the bou

potential causes of abrupt climate events a numerical study with a three dimensional climate model 1 a multi millennia simulation performed with a three dimensional climate model under constant forcing shows abrupt climate events lasting for several centuries caused by a spontaneous transition to an infrequently visited state of the oceanic thermohaline circulation this state is characterized by a more southern location of the main area of deep ocean convection in the north atlantic and implies a large cooling in the mid and high latitudes of the northern hemisphere this transition of the thermohaline circulation occurs spontaneously less than once in 5000 years in the model but such transitions can also be triggered by a reduction of the solar irradiance

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

holocene history of landscape instability in iceland can we deconvolve the impacts of climate volcanism and human activity abstract biogeochemical proxy records from icelandic lake sediment track large scale shifts in north atlantic holocene climate and highlight the impact that north atlantic ocean and atmospheric circulation has on iceland s climate and environment following early holocene warmth centennial scale climate change is superimposed on millennial scale cooling culminating in the transition to the little ice age 1300 1900 ce although the long term cooling trend is presumably driven by variations in earth s orbit and the concomitant decline in northern hemisphere nh summer insolation the centennial scale variability has been linked to the strength of the atlantic meridional overturning circulation amoc volcanism coupled with sea ice ocean related feedbacks internal modes of atmospheric variability and plausibly variations in solar irradiance one manifestation of these regional climate changes on iceland is the intensification of soil erosion resulting in the degradation of ecosystems and landscape in recent millennia persistent and severe soil erosion has also been linked to human impact on the environment following the settlement 870 ce rapid population growth introduction of livestock and the poorly consolidated nature of tephra dominated soils lake proxy composite records suggest that although event dominated landscape instability and soil erosion from the early to middle holocene were likely triggered by large volcanic eruptions the landscape was capable of recovering however a threshold was reached 5 ka bp resulting in a state change whereby the icelandic landscape could no longer fully recover from cold events and or tephra fall landscape sensitivity to climate further intensified at 1 5 ka bp as identified by regime shift analysis hence widespread and irreversible soil erosion began several centuries before the acknowledged settlement of iceland with a second acceleration 1250 ce a 2 ka fully coupled climate transient simulation using cesm1 1 shows a 0 5 c reduction in summer temperature around iceland in the first millennium ce consistent with increased landscape instability and soil erosion in iceland a second phase of persistent summer cooling in the model occurs after 1150 ce with stronger cooling after 1450 ce reaching a maximum shortly after 1850 ce 1 c lower than at the start of the simulation our results suggest that natural variations in regional climate and volcanism are likely responsible for soil erosion prior to human impact with intensification of these processes following settlement particularly during the cooling associated with the little ice age given that the conclusions drawn in this review diverge from the standard paradigm of human induced soil erosion history in iceland research should continue to focus on this complex question from multiple disciplines in particular a combination of emerging biogeochemical techniques e g lipid biomarkers and ancient dna may be best poised to test and quantify the relative roles of natural environmental variables and human settlement in the history of soil erosion on iceland

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

transient simulation of last deglaciation with a new mechanism for bolling allerod warming we conducted the first synchronously coupled atmosphere ocean general circulation model simulation from the last glacial maximum to the bolling allerod ba warming our model reproduces several major features of the deglacial climate evolution suggesting a good agreement in climate sensitivity between the model and observations in particular our model simulates the abrupt ba warming as a transient response of the atlantic meridional overturning circulation amoc to a sudden termination of freshwater discharge to the north atlantic before the ba in contrast to previous mechanisms that invoke amoc multiple equilibrium and southern hemisphere climate forcing we propose that the ba transition is caused by the superposition of climatic responses to the transient co 2 forcing the amoc recovery from heinrich event 1 and an amoc overshoot

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

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

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

stratosphere troposphere coupling at inter decadal time scales implications for the north atlantic ocean received 24 december 2011 revised 31 january 2012 accepted 1 february 2012 published 1 march 2012 1 evidence of stratosphere troposphere coupling at interdecadal time scales is searched for in a 260 year simulation performed with a climate model including a state of the art stratosphere the boundary conditions of the simulation are specified according to preindustrial conditions and are kept constant from year to year it is shown that long lasting 20 years positive and negative anomalies of the northern winter stratospheric polar vortex exist in the simulation given that there are no externally imposed low frequency time variations these persistent variations are due to internal dynamical processes of the modeled coupled atmosphere ocean system by composite analysis it is shown that the long lasting stratospheric vortex anomalies are connected through the troposphere to mean sea level pressure surface temperature and sea ice cover anomalies these connections are reminiscent of intra seasonal stratosphere troposphere coupling over the ocean the surface temperature and sea ice cover anomalies are indicative of the delayed atlantic meridional overturning circulation response to atmospheric forcing the latter is indeed found to be anomalously strong weak during the long lasting positive negative stratospheric vortex anomalies providing evidence for a potential role of the stratosphere in decadal prediction citation manzini e c cagnazzo p g fogli a bellucci and w a muller 2012 stratosphere troposphere coupling at inter decadal time scales implications for the north atlantic ocean geophys res lett 39 l05801 doi 10 1029 2011gl050771

persistent anomaly forcing in a two level global circulation model abstract perpetual january data from a simple global two level model are used to diagnose the forcing of the model s low frequency lf large scale flow the forcing is considered within the context of a hypothetical low order model that resolves only the slowly varying planetary scale modes thus the forcing is partitioned into resolved components consisting of lf planetary scale i e autonomous processes and unresolved components that involve high frequency hf transient eddy and lf synoptic scale effects spatial patterns of the net tendencies of low pass filtered planetary scale barotropic streamfunction and vertically averaged temperature are compared with corresponding patterns of resolved and unresolved forcing during the onset maintenance and decay stages of composited persistent anomalies growth stages of positive and negative persistent anomalies that are found in both the atlantic atl and pacific pac regions are dominated by autonomous processes for positive anomalies the n

can north atlantic sea ice anomalies account for dansgaard oeschger climate signals abstract north atlantic sea ice anomalies are thought to play an important role in the abrupt dansgaard oeschger d o cycles of the last glacial period this model study investigates the impacts of changes in north atlantic sea ice extent in glacial climates to help provide geographical constraints on their involvement in d o cycles based on a coupled climate model simulation of the last glacial maximum 21 ka the nordic seas and western north atlantic broadly south of greenland are identified as two plausible regions for large and persistent displacements of the sea ice edge in the glacial north atlantic sea ice retreat scenarios targeting these regions are designed to represent ice cover changes associated with the cold to warm stadial to interstadial transitions of d o cycles the atmospheric responses to sea ice retreat in the nordic seas and in the western north atlantic are tested individually and together using an atmospheric general circulation model the nordic seas ice retreat causes 1

variability in drift ice export from the arctic ocean to the north icelandic shelf over the last 8000 years a multi proxy evaluation abstract north iceland represents a climatically sensitive region in part due to its location at the confluence of southward flowing and drift ice laden polar waters from the arctic ocean delivered by the east greenland current and the relatively warm and saline irminger current a northerly flowing branch of the north atlantic current despite its pivotal location there is a paucity of high resolution and long term sea ice records for the region with some disparities in certain previous investigations here the identification of the biomarker ip 25 as a reliable proxy for drift ice for north iceland has been confirmed by measuring its abundance in surface sediments from the region and comparison of outcomes with documentary records of sea ice and other proxy data by analysing ip 25 in a well dated marine sediment core from the north icelandic shelf nis md99 2269 we also provide a high resolution ca 25 yr record of drift sea ice for the region and complement this with a lower resolution record ca 100 yr obtained from a second core site located further east jr51 gc35 statistical treatment of equi spaced time series reveals strong linear correlations between ip 25 and a further drift ice proxy quartz in each core thus linear regression analysis between both proxies gave correlation coefficients r 2 of 0 74 and 0 66 for md99 2269 25 yr and jr51 gc35 100 yr respectively further the individual proxies were well correlated between the two cores with r 0 91 and 0 77 for ip 25 and quartz respectively the ip 25 based sea ice record for md99 2269 combined with other new biomarker and foraminifera data and previously published proxy data for primary productivity and sea surface temperature suggest that the paleoceanographic evolution for the nis over the last 8 ka can be classified into three main intervals the early mid holocene ca 8 6 2 cal ka bp was characterized by relatively low or absent drift ice low primary productivity and relatively high ssts during the mid holocene ca 6 2 3 3 cal ka bp drift ice increased concomitant with decreasing ssts although primary productivity was somewhat enhanced during this interval ip 25 first reached its mean value for the entire record at ca 5 cal ka bp before increasing continuously ca 4 3 cal ka bp broadly in line with the onset of neoglaciation as seen in some other proxy records further increases in drift ice were evident during the late holocene ca 3 3 cal ka bp to present culminating in maximum sea ice during the little ice age in addition the ip 25 record from md99 2269 shows some positive regime shifts from the general trend especially at ca 3 8 2 7 1 5 0 7 and 0 4 cal ka bp that have analogs in some other paleoceanographic reconstructions influenced by the east greenland current the abrupt increases in ip 25 at ca 1 5 and 0 7 cal ka bp are coincident with rapid cooling identified previously in an icelandic lacustrine temperature record suggesting significant coupling between the marine and terrestrial systems the contribution of sea ice to the broader climate system is further evidenced through the identification of statistically significant periodicities ca 1000 yr and ca 200 230 yr in the drift ice proxy data that have counterparts in previous studies concerning atmospheric and oceanic variability and solar forcing mechanisms

coral provides way to age deep water we propose a new method for computing the ratio of the isotopes carbon 14 to carbon 12 in deep water from the past and for testing the results derived from the normal method of age difference between benthic and planktic foraminifera in deep sea sediments1

euro atlantic atmospheric circulation during the late maunder minimum abstractthis paper presents observational evidence of the atmospheric circulation during the late maunder minimum lmm 1685 1715 based on daily wind direction observations from ships in the english channel four wind directional indices and 8 point wind roses are derived at monthly scales to characterize the lmm the results indicate that the lmm was characterized by a pronounced meridional circulation and a marked reduction in the frequency of westerly days all year round as compared to the present 1981 2010 the winter circulation contributed the most to the cold conditions nevertheless findings indicate that the lmm in europe was more heterogeneous than previously thought displaying contrasting spatial patterns in both circulation and temperature as well as large decadal variability in particular there was an increase of northerly winds favoring colder winters in the first half of the lmm but enhanced southerlies contributing to milder conditions in the second half of the lmm the analysis

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

eastern north atlantic deep sea corals tracing upper intermediate water δ14c during the holocene abstract paired 230th u and 14c dating were performed on deep sea corals lophelia pertusa and madrepora oculata from the northeastern north atlantic at 730 m bsl to investigate past changes of the thermohaline circulation these were estimated using the 14c value of the upper intermediate waters based on the 14c ages of the top and base of each coral where possible and the 230th u dating the reliability of these estimates was checked by dating two very young corals of the species l pertusa one of these corals collected alive in 1999 ad gave a 230th u age of 1995 4 ad after correction for non radiogenic 230th another coral the top of which dated to 1969 6 ad recorded the atmospheric 14c 12c increase due to the nuclear tests in the early 1960s the calculated 14c values from these two corals agree with those measured at geosecs station 23 in 1972 1973 ostlund et al earth planet sci lett 23 1974 69 86 and 1991 1992 nydal and gisfelos radiocarbon 38 1996 389 406 this together with the 100 aragonite content and the 234u and 230th 232th values of all the dated corals indicates that none of the corals behaved as open systems with respect to their u series nuclides and that they closely represent the water mass properties in which they lived the pre anthropogenic 14c value of the north atlantic intermediate waters was estimated at 69 4 the reservoir age varies from 400 years to 600 years and this variation is due to atmospheric 14c 12c changes a reservoir age of 610 80 years close to the pre anthropogenic value was determined from one coral dated at 10 430 120 cal yr bp when the global sea level was approximately at 35 m bard et al nature 382 1996 241 244 this suggests a modern like pattern of the oceanic circulation prevailed in the northeast atlantic ocean at this time although the deglaciation was not completely achieved

dating open systems by the 238 u 234 u 230 th method application to quaternary reef terraces abstract the sedimentation conditions and diagenetic processes of biogenic and inorganic carbonates are such that the assumptions of a closed system evolution and the absence of initial th invoked in classical 238 u 234 u 230 th dating methods are very often not fulfilled for such material this problem is of interest for all studies requiring accurate dating of corals or aragonite rich sediments as in palaeoclimatology or neotectonics extending the approach of henderson and slowey nature 404 2000 61 66 we propose a consistent model that takes into account possible initial 230 th excess and where continuous selective redistribution gain or loss of 234 u 234 th and 230 th is controlled by recoil processes this model applied to quaternary marine terraces in barbados marie galante lesser antilles arc and pacific islands is able to explain the large scatter in isotopic compositions observed in the series of cogenetic corals or sediments we show that recoil processes generally tend to increase the 234 u 238 u and 230 th 234 u ratios with time leading to an overestimation of the apparent ages calculated using classical methods on the basis of simple assumptions identical marine initial 234 u 238 u 0 and 230 th 232 th 0 ratios we propose a method for calculating a best estimate of the age of cogenetic series our model offers an alternative way to date old reef terraces where corals are almost systematically altered and where classical dating methods do not apply it may also explain the apparent variations of the 234 u 238 u seawater initial ratio estimated from corals formed in the last 300 ka and the apparent uncoupling between sea level highstands and high latitude northern hemisphere summer insolation our set of equations can be applied to all weathering processes where the mobility of u series isotopes is controlled by recoil

the great salinity anomaly in the northern north atlantic 1968 1982 the widespread freshening of the upper 500 800m layer of the northern north atlantic which this paper describes represents one of the most persistent and extreme variations in global ocean climate yet observed in this century though a range of explanations have been advanced to explain this event including in situ changes in the surface moisture flux this paper describes the great salinity anomaly as largely though not entirely an advective event traceable around the atlantic subpolar gyre for over 14 years from its origins north of iceland in the mid to late 1960s until its return to the greenland sea in 1981 1982 the overall propagation speed around this subpolar gyre is estimated at about 3cm s 1 of the total salt deficit associated with the anomaly as it passed south along the labrador coast in 1971 1973 about 72 109 tonnes a deficit equivalent to about two thirds of this figure 47 109 tonnes ultimately passed through the faroe shetland channel to the barents sea arctic ocean and greenland sea during the mid 1970s possible effects on deep water formation and on the representativeness of historical section data are discussed

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

high sensitivity of north iceland trollaskagi debris free glaciers to climatic change from the little ice age to the present the trollaskagi peninsula is located in northern iceland between meridian 19 30 w and 18 10 w jutting out into the north atlantic to latitude 66 12 n the aim of this research is to study recent

mid to late holocene climate change an overview the last 6000 years are of particular interest to the understanding of the earth system because the boundary conditions of the climate system did not change dramatically in comparison to larger glacial interglacial changes and because abundant detailed regional palaeoclimatic proxy records cover this period we use selected proxy based reconstructions of different climate variables together with state of the art time series of natural forcings orbital variations solar activity variations large tropical volcanic eruptions land cover and greenhouse gases underpinned by results from general circulation models gcms and earth system models of intermediate complexity emics to establish a comprehensive explanatory framework for climate changes from the mid holocene mh to pre industrial time the redistribution of solar energy due to orbital forcing on a millennial timescale was the cause of a progressive southward shift of the northern hemisphere nh summer position of the intertropical convergence zone itcz this was accompanied by a pronounced weakening of the monsoon systems in africa and asia and increasing dryness and desertification on both continents the associated summertime cooling of the nh combined with changing temperature gradients in the world oceans likely led to an increasing amplitude of the el nin o southern oscillation enso and possibly increasingly negative north atlantic oscillation nao indices up to the beginning of the last millennium on decadal to multi century timescales a worldwide coincidence between solar irradiance minima tropical volcanic eruptions and decadal to multi century scale cooling events was not found however reconstructions show that widespread decadal to multi century scale cooling events accompanied by advances of mountain glaciers occurred in the nh e g in scandinavia and the european alps this occurred namely during the little ice age lia between ad w1350 and 1850 when the lower summer insolation in the nh due to orbital forcing coincided with solar activity minima and several strong tropical volcanic eruptions the role of orbital forcing in the nh cooling the southward itcz shift and the desertifi cation of the sahara are supported by numerous model simulations other simulations have suggested that the fingerprint of solar activity variations should be strongest in the tropics but there is also evidence that changes in the ocean heat transport took place during the lia at high northern latitudes with possible additional implications for climates of the southern hemisphere sh

on the radiocarbon record in banded corals exchange parameters and net transport of 14co2 between atmosphere and surface ocean we have made radiocarbon measurements of banded hermatypic corals from florida belize and the galapagos islands interpretation is presented here of these previously reported results these measurements represent the 14c 12c ratios in dissolved inorganic carbon dioc in the surface ocean waters of the gulf stream and the peru current at the time of coral ring formation a depletion in radiocarbon concentration was observed in coral rings that grew from a d 1900 1952 it was caused by dilution of existing 14c levels with dead co2 from fossil fuel burning the suess effect or se the observed depletion of radiocarbon was greater in corals from the gulf stream 11 than in corals from the peru current 6 a similar trend was observed in the distribution of bomb produced 14c in corals that had grown during the years following a d 1952 the concentration of bomb produced radiocarbon was much higher in corals from temperate regions florida belize hawaiian islands than in corals from tropical regions galapagos islands and canton island a linear relationship appears to exist between the preanthropogenic 14c values and the se values measured in the individual corals because the 14c in corals is derived from two different carbon reservoirs 1 the atmosphere and 2 the subsurface ocean a linear relationship is also observed between the preanthropogenic 14c values and the concentration of bomb produced 14c in the individual corals during a d 1973 the apparent radiocarbon ages of the surface waters in temperate and tropical oceans during the preanthropogenic period range from about 280 to 520 years b p 40 to 69 at all investigated locations it is likely that waters at subsurface depths have the same apparent radiocarbon age of about 670 years b p from the change of oceanic 14c in the surface ocean during post bomb times the approximate annual rate of net input of 14co2 to the ocean waters is calculated to be about 8 of the prevailing 14c difference between atmosphere and ocean from this input and from preanthropogenic 14c values found at each location it can be seen that vertical mixing of water in the peru current is about 3 times greater than that in the gulf stream

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

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

oceanographic changes through the last millennium off north iceland temperature and salinity reconstructions based on foraminifera and stable isotopes abstract temperature and salinity reconstructions for two 1000 year high resolution sedimentary records located at the boundary between atlantic and arctic surface waters on the north icelandic shelf are based on transfer functions and oxygen isotopes for planktonic and benthic foraminifera there is a general increase of arctic water indicator species at the transition from the medieval warm period into the little ice age lia and a subsequent return of atlantic water indicator species towards the end of the lia and in the 20th century the timing of the reconstructed temperature changes both at the beginning and at the end of the lia appears to be slightly different for the different water masses the earliest temperature change is seen in the bottom and subsurface waters where a cooling is reconstructed as early as ad 1150 1200 at both locations whereas previously published diatom based and alkenone based sea surface temperature reconstructions show a change at ad 1300 coinciding with the air temperature shift in the area our results show the need of a thorough understanding of the oceanography in the study area as well as the different living habitat for the biological proxies used for the temperature estimates

new radiocarbon dates for the vedde ash and the saksunarvatn ash from western norway abstract the vedde ash bed mid younger dryas and the saksunarvatn ash early holocene are important regional stratigraphic event markers in the north atlantic the norwegian sea and the adjacent land area it is thus essential to date them as precisely as possible the occurrence of the saksunarvatn ash is reported for the first time from western norway and both tephras are dated precisely by ams analyses of terrestrial plant material and lake sediment at krakenes the vedde ash has been previously dated at sites in western norway to about 10 600 yr b p it is obvious in the younger dryas sediments at krakenes and its identity is confirmed geochemically the mean of four ams dates of samples ofsalix herbacealeaves adjacent to the tephra is 10 310 50 yr b p the saksunarvatn ash is not visible in the early holocene lake sediment at krakenes after removal of organic material and diatoms the identity of the tephra particles was confirmed geochemically and their stratigraphic concentration was estimated from curve matching of a series of seven ams dates of terrestrial plant macrofossils and whole sediment the radiocarbon age of the ash is 8930 9060 yr b p corresponding to an age of 9930 10 010 cal yr b p 7980 8060 cal yr b c

the influence of sea ice on ocean heat uptake in response to increasing co2 two significant changes in ocean heat uptake that occur in the vicinity of sea ice cover in response to increasing co2 are investigated with community climate system model version 3 ccsm3 a deep warming below 500 m and extending down several kilometers in the southern ocean and warming in a 200 m layer just below the surface in the arctic ocean ocean heat uptake caused by sea ice retreat is isolated by running the model with the sea ice albedo reduced artificially alone this integration has a climate response with strong ocean heat uptake in the southern ocean and modest ocean heat uptake in the subsurface arctic ocean the arctic ocean warming results from enhanced ocean heat transport from the northern north atlantic at the time of co2 doubling about 1 3 of the heat transport anomaly results from advection of anomalously warm water and 2 3 results from strengthened inflow at the same time the overturning circulation is strengthened in the northern north atlantic and arctic oceans wind stress changes cannot explain the circulation changes which instead appear related to strengthened convection along the siberian shelves deep ocean warming in the southern ocean is initiated by weakened convection which is mainly a result of surface freshening through altered sea ice and ocean freshwater transport below about 500 m changes in convection reduce the vertical and meridional temperature gradients in the southern ocean which significantly reduce isopycnal diffusion of heat upward around antarctica the geometry of the sea ice cover and its influence on convection have a strong influence on ocean temperature gradients making sea ice an important player in deep ocean heat uptake in the southern ocean

synchronous radiocarbon and climate shifts during the last deglaciation radiocarbon data from the cariaco basin provide calibration of the carbon 14 time scale across the period of deglaciation 15 000 to 10 000 years ago with resolution available previously only from holocene tree rings reconstructed changes in atmospheric carbon 14 are larger than previously thought with the largest change occurring simultaneously with the sudden climatic cooling of the younger dryas event carbon 14 and published beryllium 10 data together suggest that concurrent climate and carbon 14 changes were predominantly the result of abrupt shifts in deep ocean ventilation

multidecadal variability and late medieval cooling of near coastal sea surface temperatures in the eastern tropical north atlantic 1 multidecadal variations in atlantic sea surface temperatures sst influence the climate of the northern hemisphere however prior to the instrumental time period information on multidecadal climate variability becomes limited and there is a particular scarcity of sufficiently resolved sst reconstructions here we present an eastern tropical north atlantic reconstruction of ssts based on foraminiferal mg ca ratios that resolves multidecadal variability over the past 1700 years spectral power in the multidecadal band 50 to 70 years period is significant over several time intervals suggesting that the atlantic multidecadal oscillation amo has been influencing local sst since our data exhibit high scatter the absence of multidecadal variability in the remaining record does not exclude the possibility that sst variations on this time scale might have been present without being detected in our data cooling by 0 5 c takes place between about ad 1250 and ad 1500 while this corresponds to the inception of the little ice age lia the end of the lia is not reflected in our record and sst remains relatively low this transition to cooler ssts parallels the previously reconstructed shift in the north atlantic oscillation toward a low pre 20th century mean state and possibly reflects common solar forcing

bi decadal variability excited in the coupled ocean atmosphere system by strong tropical volcanic eruptions decadal and bi decadal climate responses to tropical strong volcanic eruptions sves are inspected in an ensemble simulation covering the last millennium based on the max planck institute earth system model an unprecedentedly large collection of pre industrial sves up to 45 producing a peak annual average top of atmosphere radiative perturbation larger than 1 5 wm 2 is investigated by composite analysis post eruption oceanic and atmospheric anomalies coherently describe a fluctuation in the coupled ocean atmosphere system with an average length of 20 25 years the study provides a new physically consistent theoretical framework to interpret decadal northern hemisphere nh regional winter climates variability during the last millennium the fluctuation particularly involves interactions between the atlantic meridional overturning circulation and the north atlantic gyre circulation closely linked to the state of the winter north atlantic oscillation it is characterized by major distinctive details among them the most prominent are a a strong signal amplification in the arctic region which allows for a sustained strengthened teleconnection between the north pacific and the north atlantic during the first post eruption decade and which entails important implications from oceanic heat transport and from post eruption sea ice dynamics and b an anomalous surface winter warming emerging over the scandinavian western russian region around 10 12 years after a major eruption the simulated long term climate response to sves depends to some extent on background conditions consequently ensemble simulations spanning different phases of background multidecadal and longer climate variability are necessary to constrain the range of possible post eruption decadal evolution of nh regional winter climates