circulation mixing and production of antarctic bottom water abstract the antarctic source of bottom water to the abyssal layer of the world ocean is examined as well as its large scale flow pattern and ultimate entrainment rate into the deep water above we make use of the available high quality station data in the southern ocean to construct bottom maps of neutral density and mean property maps including chlorofluorocarbon cfc for the abyssal layer underneath a selected neutral density surface the maximum density at the sill depth of drake passage is used to distinguish between the voluminous deep water mass that is a continuous component of the antarctic circumpolar current from the relatively denser bottom water originated along the antarctic continental margins based on water density antarctic bottom water aabw is defined here generically to include all volumes of non circumpolar water of antarctic origin over the shelf regime multiple localized sources of specific aabw types contribute to the abyssal layer of the adjacent antarctic basins characteristics of these dense bottom waters reflect closely those observed on the parent shelf water mass spreading paths of newly formed deep and bottom waters over the slope regime and their subsequent oceanic circulation patterns are analyzed on the basis of global property maps for the aabw layer interior mixing and interbasin exchanges of aabw are deduced from mean characteristic curves following the southern streamline of the antarctic circumpolar current outflow and mixing of aabw from the weddell sea to the argentine basin is depicted using density and cfc distributions of two zonal hydrographic lines recirculation and mixing of deep and bottom waters within the weddell gyre are also detailed using a meridional section along the greenwich meridian the strength of all localized sources of aabw combined is estimated by two independent approaches an estimate of the total production rate of aabw is calculated based on the oceanic cfc budget for the aabw layer offshore of the 2500 m isobath the sum of all downslope inputs of well ventilated bottom water types underneath the top isopycnal must account for the measured cfc content in the bottom layer the resulting total aabw production rate is about 8 sv which is a conservative figure that neglects the loss of cfc bearing waters across the top isopycnal in recent years whereas about 9 5 sv is calculated assuming a well mixed bottom layer making use of their transient nature cfc distributions at the top of the aabw layer indicate that more direct and rapid entrainment of cfc rich bottom waters below occurs over localized areas with relatively strong upwelling rates and enhanced vertical mixing a second more ad hoc but independent oceanic mass budget of the bottom layer is also constructed a typical basin wide rate of deep upwelling of 3 10 7 m s 1 requires 10 sv 1 sv 106 m3 s 1 of newly formed aabw to sink down the slope around antarctica we have also formulated a spatial distribution of deep upwelling on the isopycnal at the top of the aabw it is expressed as a combination of wind topographic and turbulent components which in turn are functions of the isopycnal depth bottom depth and bottom layer thickness this non uniform upwelling field yields about 12 sv of aabw exported across the top isopycnal fortuitously the overall average of upward speed at the top isopycnal 3 7 10 7 m s 1 compares well with previous estimates of deep upwelling in the northern basins a series of likely sites for strong vertical entrainment of aabw are clearly identified in the modeled distribution of deep upwelling consonant with the observed cfc distributions on the top isopycnal altogether regions with relatively high upwelling rates 5 10 7 m s 1 occupy only a quarter of the total areal extent of the top isopycnal but they account for as much as 45 of the total vertical transport

detecting holocene changes in thermohaline circulation throughout the last glacial cycle reorganizations of deep ocean water masses were coincident with rapid millennial scale changes in climate climate changes have been less severe during the present interglacial but evidence for concurrent deep ocean circulation change is ambiguous

holocene rainfall variability in southern chile a marine record of latitudinal shifts of the southern westerlies geochemical and clay mineral parameters of a high accumulation marine sediment core from the chilean continental slope 41 s provide a 7700 yr record of rainfall variability in southern chile related to the position of the southern westerlies we especially use the iron content measured with a time resolution of ca 10 yr on average of 14 caccelerator mass spectrometry dated marine sediments as a proxy for the relative input of iron poor coastal range and iron rich andean source rocks variations in this input are most likely induced by rainfall changes in the continental hinterland of the core position based on these interpretations we find a pronounced rainfall variability on multicentennial to millennial time scales superimposed on generally more arid conditions during the middle holocene 7700 to 4000 cal yr b p compared to the late holocene 4000 to present this variability and thus changes in the position of the southern westerlies are first compared to regional terrestrial paleoclimate data sets from central and southern chile in order to derive possible wider implications and forcing mechanisms of the holocene latitudinal shifts of the southern westerlies we then compare our data to ice core records from both tropical south america and coastal antarctica these records show similar bands of variability centered at ca 900 and 1500 yr comparisons of band pass filters suggest a close connection of shifts of the southern westerlies to changes within the tropical climate system the correlation to climate conditions in coastal antarctica shows a more complicated picture with a phase shift at the beginning of the late holocene coinciding with the onset of the modern state of el nin o southern oscillation system the presented data provide further evidence that the well known millennial scale climate variability during the last glacial continued throughout the holocene fl 2001 elsevier science b v all rights reserved

climate records covering the last deglaciation the oxygen 18 oxygen 16 ratio of molecular oxygen trapped in ice cores provides a time stratigraphic marker for transferring the absolute chronology for the greenland ice sheet project gisp ii ice core to the vostok and byrd ice cores in antarctica comparison of the climate records from these cores suggests that near the beginning of the last deglaciation warming in antarctica began approximately 3000 years before the onset of the warm bolling period in greenland atmospheric carbon dioxide and methane concentrations began to rise 2000 to 3000 years before the warming began in greenland and must have contributed to deglaciation and warming of temperate and boreal regions in the northern hemisphere

persistent solar influence on north atlantic climate during the holocene surface winds and surface ocean hydrography in the subpolar north atlantic appear to have been influenced by variations in solar output through the entire holocene the evidence comes from a close correlation between inferred changes in production rates of the cosmogenic nuclides carbon 14 and beryllium 10 and centennial to millennial time scale changes in proxies of drift ice measured in deep sea sediment cores a solar forcing mechanism therefore may underlie at least the holocene segment of the north atlantic9s 1500 year cycle the surface hydrographic changes may have affected production of north atlantic deep water potentially providing an additional mechanism for amplifying the solar signals and transmitting them globally

atlantic thermohaline circulation in a coupled general circulation model unforced variations versus forced changes a 1200 yr unforced control run and future climate change simulations using the parallel climate model pcm a coupled atmosphere ocean land sea ice global model with no flux adjustments and relatively high resolution 2 8 for the atmosphere and 2 3 for the oceans are analyzed for changes in atlantic ocean circulations for the forced simulations historical greenhouse gas and sulfate forcing of the twentieth century and projected forcing for the next two centuries are used the atlantic thermohaline circulation thc shows large multidecadal 15 40 yr variations with mean peak amplitudes of 1 5 3 0 sv 1 sv 10 6 m 3 s 1 and a sharp peak of power around a 24 yr period in the control run associated with the thc oscillations there are large variations in north atlantic ocean heat transport sea surface temperature sst and salinity sss sea ice fraction and net surface water and energy fluxes which all lag the variations in thc strength by 2 3 yr however the net effect of the sst and sss variations on upper ocean density in the midlatitude north atlantic leads the thc variations by about 6 yr which results in the 24 yr period the simulated sst and sea ice spatial patterns associated with the thc oscillations resemble those in observed sst and sea ice concentrations that are associated with the north atlantic oscillation nao the results suggest a dominant role of the advective mechanism and strong coupling between the thc and the nao whose index also shows a sharp peak around the 24 yr time scale in the control run in the forced simulations the thc weakens by 12 in the twenty first century and continues to weaken by an additional 10 in the twenty second century if co2 keeps rising but the thc stabilizes if co2 levels off the thc weakening results from stabilizing temperature increases that are larger in the upper and northern atlantic ocean than in the deep and southern parts of the basin in both the control and forced simulations as the thc gains loses strength and depth the separated gulf stream gs moves southward northward while the subpolar gyre centered at the labrador sea contracts from expands to the east with the north atlantic current nac being shifted westward eastward these horizontal circulation changes which are dynamically linked to the thc changes induce large temperature and salinity variations around the gs and nac paths

deepwater source variations during the last climatic cycle and their impact on the global deepwater circulation the degree of similarity of the 13c records of the planktonic foraminiferal species n pachyderma and of the benthic foraminiferal genus cibicides in the high latitude basins of the world ocean is used as an indicator of the presence of deepwater sources during the last climatic cycle whereas continuous formation of deep water is recognized in the southern ocean the norwegian sea stopped acting as a sink for surface water during isotope stage 4 and the remainder of the last glaciation however deep water formed in the north atlantic south of the norwegian sea during the last climatic cycle as early as isotope substage 5d and this area was also the only active northern source during stages 4 2 a detailed reconstruction of the geographic distribution of 13c in benthic foraminifera in the atlantic ocean during the last glacial maximum shows that the most important deepwater mass originated from the southern ocean whereas the glacial north atlantic deep water cannot be traced south of 40 n at shallower depth an oxygenated 13c rich intermediate water mass extended from 45 n to 15 s in the pacific ocean a ventilation higher than the modern one was also found in open ocean in the depth range 700 2600 m and is best explained by stronger formation of intermediate water in high northern latitudes

on the relationships between features of the depth latitude meridional overturning streamfunctions across global coupled climate models a comparative analysis of the state and response of the latitude depth meridional overturning streamfunctions in the climate model inter comparison project 3 cmip3 model set is presented simulated overturning strengths of the north atlantic cell tend to converge towards observational estimates the models whose simulations of the north atlantic cell are closest to observational estimates indicate a 29 5 13 decrease in the maximum intensity of that cell by 2 100 in contrast agreement with regard to the state and the response to anthropogenic radiative forcing of the global southern ocean abyssal cell is poor among the models a weak relationship between the mean state and the response of the abyssal cell can be used to constrain the reduction of the southern abyssal cell by 2 100 to 29 3 20 7 in rough agreement with the decrease predicted in the northern cell the biases across the cmip3 models in the northern deep cell and southern abyssal cell cannot be related dynamically by a buoyancy based seesaw like argument the absence or presence of characteristic relationships between the state and evolution of different features of the overturning streamfunction indicate that the main reasons for across model spread are how each model deals with subgrid scale processes and viscosity this highlights the fact that subgrid scale parameterizations and resolution improvements should be a priority of model development these factors are able to explain qualitatively the inter model differences between the northern overturning cells of the different models across model differences in the winds over the southern ocean are responsible for much of the disparity in the overturning circulation cells of the southern ocean

abrupt increase in greenland snow accumulation at the end of the younger dryas event the warming at the end of the last glaciation was characterized by a series of abrupt returns to glacial climate the best known of which is the younger dryas event1 despite much study of the causes of this event and the mechanisms by which it ended many questions remain unresolved1 oxygen isotope data from greenland ice cores2 4 suggest that the younger dryas ended abruptly over a period of about 50 years dust concentrations2 4 in these cores show an even more rapid transition 20 years this extremely short timescale places severe constraints on the mechanisms underlying the transition but dust concentrations can reflect subtle changes in atmospheric circulation which need not be associated with a large change in climate here we present results from a new greenland ice core gisp2 showing that snow accumulation doubled rapidly from the younger dryas event to the subsequent preboreal interval possibly in one to three years we also find that the accumulation rate change from the oldest dryas to the bo11ing allerod warm period was large and abrupt the extreme rapidity of these changes in a variable that directly represents regional climate implies that the events at the end of the last glaciation may have been responses to some kind of threshold or trigger in the north atlantic climate system

antarctic bottom water warming and freshening contributions to sea level rise ocean freshwater budgets and global heat gain abstractfreshening and warming of antarctic bottom water aabw between the 1980s and 2000s are quantified assessing the relative contributions of water mass changes and isotherm heave the analysis uses highly accurate full depth ship based conductivity temperature depth measurements taken along repeated oceanographic sections around the southern ocean fresher varieties of aabw are present within the south pacific and south indian oceans in the 2000s compared to the 1990s with the strongest freshening in the newest waters adjacent to the antarctic continental slope and rise indicating a recent shift in the salinity of aabw produced in this region bottom waters in the weddell sea exhibit significantly less water mass freshening than those in the other two southern basins however a decrease in the volume of the coldest deepest waters is observed throughout the entire southern ocean this isotherm heave causes a salinification and warming on isobaths from the bottom up to the shallow potential tem

was a change in thermohaline circulation responsible for the little ice age long term 800 year and short term 20 year estimates of the rate of deep water formation in the southern ocean are in serious disagreement this disagreement suggests to me that the rate of deep water formation may have been larger during the little ice age lia than it is now

variability in temperature and geometry of the norwegian current over the past 600 yr stable isotope and grain size evidence from the norwegian margin core p1 003mc was retrieved from 851 m water depth on the southern norwegian continental margin close to the boundary between the norwegian current nc and the underlying cold norwegian sea deep water the core chronology was established by using 210pb measurements and 14c dates suggesting a sampling resolution of between 2 and 9 yr sea surface temperature sst variations in the nc are reconstructed from stable oxygen isotope measurements in two planktonic foraminifera species neogloboquadrina pachyderma d and globigerina bulloides the high temporal resolution of the sst proxy records allows direct comparison with instrumental ocean temperature measurements from ocean weather ship ows mike in the norwegian sea and an air temperature record from the coastal island ona western norway the comparison of the instrumental and the proxy sst data suggests that n pachyderma d calcify during summer whereas g bulloides calcify during spring the 18o records of both species suggest that the past 70 yr have been the warmest throughout the past 600 yr the spring and summer proxy temperature data suggest differences in the duration of the cold period of the little ice age the spring temperature was 1 3 c colder throughout most of the period between ca ad 1400 and 1700 and the summer temperature was 1 2 c colder throughout most of the period between ca ad 1400 and 1920 fluctuations in the depth of the lower boundary of the nc have been investigated by examining grain size data and benthic foraminiferal assemblages the data show that the transition depth of the lower boundary of the nc was deeper between ca ad 1400 and 1650 than after ca ad 1750 until present copyright 2003 john wiley sons ltd

cooling and ventilating the abyssal ocean the abyssal ocean is filled with cold dense waters that sink along the antarctic continental slope and overflow sills that lie south of the nordic seas recent inte grations of chlorofluorocarbon 11 cfc measurements are similar in antarctic bottom water aabw and in lower north atlantic deep water nadw but antarctic inputs are 2 c colder than their northern counterparts this indicates comparable ventilation rates from both polar re gions and accounts for the southern ocean dominance over abyssal cooling the decadal cfc based estimates of recent ventilation are consistent with other hydrographic observa tions and with longer term radiocarbon data but not with hypotheses of a 20 th centuryslowdown in the rate of aabw formation significant variabilityis not precluded bythe available ocean measurements however and interannual to decadal changes are increasinglyevident at high latitudes

flow of bottom water in the northwestern weddell sea the weddell sea is known to feed recently formed deep and bottom water into the antarctic circumpolar water belt from whence it spreads into the basins of the world ocean the rates are still a matter of debate to quantify the flow of bottom water in the northwestern weddell sea data obtained during five cruises with r v polarstern between october 1989 and may 1998 were used during the cruises in the weddell sea five hydrographic surveys were carried out to measure water mass properties and moored instruments were deployed over a time period of 8 5 years to obtain quasi continuous time series the average flow in the bottom water plume in the northwestern weddell sea deduced from the combined conductivity temperature depth and moored observations is 1 3 0 4 sv intensive fluctuations of a wide range of timescales including annual and interannual variations are superimposed the variations are partly induced by fluctuations in the formation rates and partly by current velocity fluctuations related to the large scale circulation taking into account entrainment of modified warm deep water and weddell sea deep water during the descent of the plume along the slope between 0 5 sv and 1 3 sv of surface ventilated water is supplied to the deep sea this is significantly less than the widely accepted ventilation rates of the deep sea if there are no other significant sources of newly ventilated water in the weddell sea either the dominant role of weddell sea bottom water in the southern ocean or the global ventilation rates have to be reconsidered

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

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

constraints on deep water formation from the oceanic distributions of 10be 1 an analysis of the oceanic distribution of cosmogenic 10be shows that deep waters are formed by descending of cold dense surface waters at a rate of 18 sv 106 m3 s 1 in the northern north atlantic and 21 sv of which 17 sv enters the pacific in the antarctic over the last mixing cycle of 800 years lending support to the estimates based on radiocarbon and nutrient distributions it also indicates that deep water production in the northern north atlantic is 50 higher averaging over the recent 100 years than over the last mixing cycle suggesting a possible increase in the thermohaline ventilation of the north atlantic during the past century

deuterium excess record from central greenland over the last millennium hints of a north atlantic signal during the little ice age a stacked water isotope record for summit central greenland has been established covering approximately the last 900 years measurements of 18o and d in the grip deep ice core and the 230 m core s93 allow the reconstruction of a millennial record of the deuterium excess in a near annual resolution a short period of particularly high values of the deuterium excess at the beginning of the fourteenth century may be associated with the medieval warm period mwp the little ice age lia might be represented by a 100 year period of very low excess values in the sixteenth and seventeenth centuries using a simple isotope model 18o and deuterium excess are interpreted in terms of surface temperature variations over central greenland and over the subtropical north atlantic greenland s principal vapor source region an estimated cooling of 0 7 c of subtropical sea surface temperatures ssts during the little ice age and a warming of 0 6 c during the short warm period in the medieval is in agreement with previous studies over periods of about 100 years an antiphasing between gradually decreasing 18o and increasing deuterium excess is observed interannual to decadal scale variability associated to the north atlantic oscillation may be responsible for this anti phase relationship an alternative explication is a north south oscillation in north atlantic sea surface temperatures associated with short term changes in the thermohaline circulation

the dissolved chemical and isotopic signature downflow the confluence of two large rivers the case of the parana and paraguay rivers summary the parana river basin is one of the largest hydrological systems in south america 2 6 10 6 km 2 downflow the confluence of tributaries most large rivers exhibit transverse and longitudinal inhomogeneities that can be detected for tens or even hundreds of kilometers concordantly a noticeable cross sectional chemical asymmetry in the dissolved load was distinguished in the middle parana river after the confluence of its main tributaries i e the paraguay and upper parana rivers water chemistry and isotopic signature in three cross sections along the middle parana river as well as from main and minor tributaries and some deep 105 m bs and shallow boreholes 15 m bs located near both river banks were analyzed in order to define the extent of mixing and identify possible contributions from groundwater discharges downflow the confluence of the upper parana and paraguay rivers a chemical and isotopic asymmetry was observed mainly through the values of ec major ions ca 2 na mg 2 cl and so 4 2 some trace elements fe u th ba sr as and ree and stable isotopes 18 o and 2 h toward its western margin higher elemental concentrations which resembled that of the paraguay river were measured whereas at the eastern border waters were more diluted and preserved the chemical signature of the upper parana river this variability remained detectable at least until 225 km downflow the confluence where differences between western and eastern margins were less evident at 580 km downflow the confluence a slight inversion in the transverse chemical asymmetry was observed this trend switch can be the result of the input of solutes from minor tributaries that reach the main channel from the east and or may be due to higher groundwater discharges from the east bank a mass balance model was applied as a first approach to estimate the groundwater inflow using the geochemical tracer 222 rn the results indicate that groundwater contributions represent between 0 5 and 6 of the total water inputs to the middle parana river under baseflow conditions this implies that the chemical asymmetry in the middle parana river is mostly due to the incomplete mixing of the main tributaries though the influence of groundwater is not a determining factor in the chemical variability of the river it may partially explain the higher concentrations of some trace elements found in the eastern margin 580 km downflow the confluence

climatic variability in the northwestern alps france as evidenced by 600 years of terrigenous sedimentation in lake le bourget cores recovered from periglacial lake le bourget deep basin northwestern alps were investigated to examine the ine uence of the little ice age lia on terrigenous lacustrine sedimentation growing glaciers in the regional watershed induced catastrophic rhone river e oods and major undere ow deposits in the deep basin during the early e fteenth the sixteenth and the mid eighteenth centuries the lia is characterized by a decrease in deposition from intere ows from ad 1550 to 1740 and an increase in deposition from undere ows from ad 1550 to 1800 on one hand spectral analyses of the laminations in intere ow deposits reveal 4 5 years cyclicities from ad 1440 to 1550 as well as 7 8 and 13 14 years cyclicities from ad 1740 to 1870 on the other hand spectral analyses of a clay mineral ratio ree ecting undere ow deposits highlight 45 50 years cyclicities from ad 1550 to 1800 these pluriannual decadal and pluridecadal periods are typical of the north atlantic oscillation nao a nao like period in our data would be a consequence of periodical variations in rainfall and snow accumulation during late autumn and winter over lake le bourget s watershed

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

a possible 20th century slowdown of southern ocean deep water formation chlorofluorocarbon 11 inventories for the deep southern ocean appear to confirm physical oceanographic and geochemical studies in the southern ocean which suggest that no more than 5 x 10 6 cubic meters per second of ventilated deep water is currently being produced this result conflicts with conclusions based on the distributions of the carbon 14 carbon ratio and a quasi conservative property po 4 in the deep sea which seem to require an average of about 15 x 10 6 cubic meters per second of southern ocean deep ventilation over about the past 800 years a major reduction in southern ocean deep water production during the 20th century from high rates during the little ice age may explain this apparent discordance if this is true a seesawing of deep water production between the northern atlantic and southern oceans may lie at the heart of the 1500 year ice rafting cycle

ice core evidence for secular variability and 200 year dipolar oscillations in atmospheric circulation over east antarctica during the holocene two holocene ice core records from east antarctica vostok and epica dome c were analysed for dust concentration and size distribution at a temporal resolution of 1 sample per 50 years a series of volcanic markers randomly distributed over the common part of the ice cores from 9 8 to 3 5 kyear bp ensures accurate relative dating 33 years dust size records from the two sites display oscillations structured in cycles with sub millennial and secular scale frequencies that are apparently asynchronous the power spectra of the composite sum of the two dust size records display spectral energy mostly for 150 to 500 year periodicities on the other hand the 200 year band is common to both records and the 200 year components of the two sites are out of phase 100 year lead or lag over 5 5 kyear a phenomenon also reflected by a significant 99 conf lev band in the power spectra of the composite difference of the two size records during long range transport mineral dust originating from the southern hemisphere continents is graded to a variable extent depending on the altitude and duration of atmospheric transport relatively coarse dust is associated with air mass penetration from the middle lower troposphere and conversely relatively fine dust with upper troposphere air masses or the influence of subsidence over the antarctic plateau a hypothesis already proposed for the changes that occurred during the last glacial maximum to holocene transition delmonte et al 2004b moreover we assume that the overall fluctuation of air mass advection over antarctica depends on the meridional pressure gradient with respect to low latitudes i e the antarctic oscillation aao we therefore suggest a regional variability in atmospheric circulation over east antarctica the 150 500 year power spectrum of the composite parameter represents the long term variability of the aao imprinted by secular internal oscillations probably related to the southern ocean climatic system on the other hand the dust composite parameter suggests a persistent atmospheric dipole over east antarctica delivering coarser finer dust particles alternatively to vostok and dome c regions with a bi centennial periodicity indeed a seesaw phenomenon in dust size distribution was already observed at three east antarctic sites during the last deglaciation delmonte et al 2004b and was interpreted as a progressive reduction of the eccentricity of the polar vortex with respect to the geographic south pole interestingly the parameter shows a pronounced 200 year oscillation mode throwing new light on the unresolved question of a possible relationship between climate and solar activity

watermass properties of the antarctic slope front a simple model abstract in the antarctic dense shelf water is formed in coastal polynyas and is differentiated from the fresher surface water by the wind induced ice motion that displaces offshore the ice melt from production zones where the shelf water discharges into the deep ocean the antarctic slope front asf is v shaped separating the shelf and surface waters referred to as frontal waters from the circumpolar deep water cdw to elucidate basic constraints on frontal properties a minimal model of homogeneous water masses forced by offshore wind and freshwater input in a perpetual winter is considered with the surface water stirred by and hence aligned with the ice cover there is little leakage of ice or meltwater from the frontal system so ice production and melt merely redistribute heat and salt between frontal waters as such the heat loss to the atmosphere needs to be supplied by entraining cdw which then necessitates the shelf water discharge on account of the mass balance because of the fresh

deep and bottom water of the weddell sea s western rim oceanographic observations from the ice station weddell 1 show that the western rim of the weddell gyre contributes to weddell sea bottom water a thin 300 meters highly oxygenated benthic layer is composed of a low salinity type of bottom water overlying a high salinity component this complex layering disappears near 66 s because of vertical mixing and further inflow from the continental margin the bottom water flowing out of the western rim is a blend of the two types additionally the data show that a narrow band of warmer weddell deep water hugged the continental margin as it flowed into the western rim providing the continental margin with the salt required for bottom water production

bioturbation artifacts in zero age sediments 1 most seafloor sediments are dated with radiocarbon and the sediment is assumed to be zero age modern when the signal of atmospheric testing of nuclear weapons is present fraction modern fm 1 using a simple mass balance we show that even with fm 1 half of the planktonic foraminifera at the seafloor can be centuries old because of bioturbation this calculation and data from four core sites in the western north atlantic indicate that first during some part of the little ice age lia there may have been more antarctic bottom water than today in the deep western north atlantic alternatively bioturbation may have introduced much older benthic foraminifera into surface sediments second paleo based warming of sargasso sea surface waters since the lia must lag the actual warming because of bioturbation of older and colder foraminifera

the little ice age only the latest big chill boston at the spring meeting of the american geophysical union here earlier this month paleoceanographers reported that they had found tiny bits of ice borne rock in north atlantic sediments laid down during the little ice age from about 1400 to 1900 and in older sediments every millennium or two beginning at least 130 000 years ago the finding implies that earth has experienced a long string of little ice ages perhaps driven by variations in the sun or by changes in ocean currents it also suggests that the world will be warming naturally as part of this roughly 1500 year climate cycle on top of any human induced greenhouse effect

north atlantic thermohaline circulation during the past 20 000 years linked to high latitude surface temperature during a surface cooling event 10 000 to 12 000 years ago higher cd ca and lower 13c 12c ratios are observed in benthic foraminifera shells from rapidly accumulating western north atlantic sediments data from sediment cores show that marked nutrient depletion of intermediate waters occurs in association with reduced glacial north atlantic deep water flux it is proposed that cold high latitude sea surface temperatures enhance intermediate water formation at the expense of deep water formation

variability in the deep and intermediate water circulation of the atlantic ocean during the past 25 000 years northern hemisphere modulation of the southern ocean the relative flux of north atlantic deep water nadw out of the atlantic can be monitored in the southern ocean where high 13c nadw mixes rapidly with low 13c recirculated pacific water before forming antarctic bottom water aabw by using 13c measured in benthic foraminifera as a water mass tracer it is shown that the southern ocean was dominated by pacific water with little nadw contribution during the last glaciation these tracer results document the direct physical connection between insolation controlled northern hemisphere oscillations and changes in the chemical and physical properties of southern ocean water the 13c records from the deep tropical atlantic document the transition from an atlantic dominated by southern ocean water during the last glaciation to an atlantic dominated by nadw during the holocene with more southern ocean water in the deep atlantic during the last glaciation the boundary between nadw and southern ocean water was north of its modern position allowing more southern ocean water to enter the eastern atlantic through the romanche fracture zone carbon isotope records from the caribbean and mediterranean seas suggest that the influence of mediterranean overflow water mow extended to the caribbean during the last glaciation and therefore was a volumetrically much more important water mass in the intermediate depth atlantic than it is today

cadmium and delta13c paleochemical ocean distributions during the stage 2 glacial maximum changes in heat and salt transport by ocean currents have been implicated in ice age climate cycles weyl 1 968 newell 1 974 broecker et al 1 990b in addition to the direct physical effects of the ocean on climate the ocean carbon system controls atmospheric co2 and the co2 fluctuations documented in ice core bubbles barnola et a1 1 987 neftel et a1 1 988 are thought to be a significant factor in climate oscillations hansen et al 1 984 dramatic changes in oceanic circulation patterns and carbon system chemistry have occurred during the late quaternary glacial interglacial global climate cycles boyle keigwin 1 982 curry lohmann 1 983 shackleton pisias 1 984 boyle keigwin 1 987 duplessy et al 1 988 curry et al 1 988 broecker et al i 990a these changes in the ocean system may be significant driving forces in the progression of ice age co2 cycles toggweiler sarmiento 1 985 ennever mcelroy 1 985 wenk siegenthaler 1 985 boyle 1 988a b broecker peng 1 990 the oceanic distributions of c513c cd and other carbon system linked properties are controlled by the interaction between biological uptake and decomposition with the general circulation of the ocean because records of past oceanic cd and c5 1 3c are preserved in shells of planktonic and benthic foraminifera

chronology of the palmer deep site antarctic peninsula a holocene palaeoenvironmental reference for the circum antarctic palmer deep sediment cores are used to produce the first high resolution continuous late pleistocene to holocene time series from the antarctic marine system the sedimentary record is dated using accelerator mass spectrometer radiocarbon methods on acid insoluble organic matter and foraminiferal calcite fifty four radiocarbon analyses are utilized in the dating which provides a calibrated timescale back to 13 ka bp reliability of resultant ages on organic matter is assured because duplicates produce a standard deviation from the surface age of less than laboratory error i e 50 years in addition surface organic matter ages at the site are in excellent agreement with living calcite ages at the accepted reservoir age of 1260 years for the antarctic peninsula spectral analyses of the magnetic susceptibility record against the age model reveal unusually strong periodicity in the 400 200 and 50 70 year frequency bands similar to other high resolution records from the holocene but so far unique f

on the driving processes of the atlantic meridional overturning circulation because of its relevance for the global climate the atlantic meridional overturning circulation amoc has been a major research focus for many years yet the question of which physical mechanisms ultimately drive the amoc in the sense of providing its energy supply remains a matter of controversy here we review both observational data and model results concerning the two main candidates vertical mixing processes in the ocean s interior and wind induced ekman upwelling in the southern ocean in distinction to the energy source we also discuss the role of surface heat and freshwater fluxes which influence the volume transport of the meridional overturning circulation and shape its spatial circulation pattern without actually supplying energy to the overturning itself in steady state we conclude that both wind driven upwelling and vertical mixing are likely contributing to driving the observed circulation to quantify their respective contributions future research needs to address some open questions which we outline

how much deep water is formed in the southern ocean three tracers are used to place constraints on the production rate of ventilated deep water in the southern ocean the distribution of the water mass tracer po4 phosphate star in the deep sea suggests that the amount of ventilated deep water produced in the southern ocean is equal to or greater than the outflow of north atlantic deep water from the atlantic radiocarbon distributions yield an export flux of water from the north atlantic which has averaged about 15 sv over the last several hundred years cfc inventories are used as a direct indicator of the current production rate of ventilated deep water in the southern ocean although coverage is as yet sparse it appears that the cfc inventory is not inconsistent with the deep water production rate required by the distributions of po4 and radiocarbon it has been widely accepted that the major part of the deep water production in the southern ocean takes place in the weddell sea however our estimate of the southern ocean ventilated deep water flux is in conflict with previous estimates of the flux of ventilated deep water from the weddell sea which lie in the range 1 5 sv possible reasons for this difference are discussed

eight centuries of volcanic signal and climate change at talos dome east antarctica 1 during the 1996 programma nazionale di ricerche in antartide international trans antarctic scientific expedition traverse two firn cores were retrieved from the talos dome area east antarctica at elevations of 2316 m td 89 m long and 2246 m st556 19 m long cores were dated by using seasonal variations in non sea salt nss so42 concentrations coupled with the recognition of tritium marker level 1965 1966 and nss so42 spikes due to the most important volcanic events in the past pinatubo 1991 agung 1963 krakatoa 1883 tambora 1815 kuwae 1452 unknown 1259 the number of annual layers recognized in the td and st556 cores was 779 and 97 respectively the d record obtained from the td core has been compared with other east antarctic isotope ice core records dome c epica south pole taylor dome these records suggest cooler climate conditions between the middle of 16th and the beginning of 19th centuries which might be related to the little ice age lia cold period because of the high degree of geographical variability the strongest lia cooling was not temporally synchronous over east antarctica and the analyzed records do not provide a coherent picture for east antarctica the accumulation rate record presented for the td core shows a decrease during part of the lia followed by an increment of about 11 in accumulation during the 20th century at the st556 site the accumulation rate observed during the 20th century was quite stable

distribution of carbonate ion in the deep ocean support for a post little ice age change in southern ocean ventilation 1 the distribution of co3 ion concentrations in the deep sea proves difficult to reconcile with a steady state thermohaline circulation the problem is that assuming steady state we cannot come up with any satisfactory explanation for the 28 mol kg deficiency of co3 ion concentration in deep pacific and indian ocean water relative to that for the source water mix however following broecker et al 1999 if we assume that during the little ice age deep water formation in the southern ocean was not only much stronger than now but also had lower po4 and co3 concentrations then this anomaly can be explained

performance of a second order moments advection scheme in an ocean general circulation model 1 the reliability of ocean general circulation models ogcms strongly depends on the quality of their tracer advection schemes for the sake of simplicity and computing time tracer advection schemes most commonly used in large scale ogcms tend to be low order schemes which suffer from spurious numerical diffusion and dispersion that result in distorted solutions the application of high order schemes would reduce numerical errors but at a considerable cost in terms of computing time an alternative to the use of high order methods is the implementation of algorithms that take into account the sub grid distribution of tracers one such method is the second order moments som scheme of prather 1986 which is more accurate than a fourth order scheme but at the time consumption of a second order algorithm this article presents results from coarse resolution global ocean simulations with very low explicit diapycnal mixing in which active and passive tracers were advected with the som method we compare the performance of the method with that of more traditional schemes namely the fct flux corrected transport and quicker quadratic upstream interpolation for convective kinematics schemes in general the use of the som method significantly improves tracer distributions and transports compared to fct and quicker thus leading to a better representation of ocean currents notably boundary currents and frontal systems while model simulations employing the fct and quicker schemes recreate a global overturning circulation with strong upwelling occurring in low latitudes the som simulations admit a circulation pattern closer to that known as the reconfigured conveyor belt toggweiler and samuels 1993 in which the bulk of the global ocean upwelling occurs in the southern ocean