surface climate variations over the north atlantic ocean during winter 1900 1989 abstract the low frequency variability of the surface climate over the north atlantic during winter is described using 90 years of weather observations from the comprehensive ocean atmosphere data set results are based on empirical orthogonal function analysis of four components of the climate system sea surface temperature sst air temperature wind and sea level pressure an important mode of variability of the wintertime surface climate over the north atlantic during this century is characterized by a dipole pattern in ssts and surface air temperatures with anomalies of one sign cast of newfoundland and anomalies of the opposite polarity off the southeast coast of the united states wind fluctuations occur locally over the regions of large surface temperature anomalies with stronger than normal winds overlying cooler than normal ssts this mode exhibits variability on quasi decadal and biennial time scales the decadal fluctuations are irregular in length averaging 9 years before 1945 and 1

modelling twentieth century global ocean circulation and iceberg flux at 48 n implications for west greenland iceberg discharge abstract we have used a coupled ocean iceberg model to study the variation in global ocean circulation and north atlantic iceberg flux from 1900 to 2008 the latter component of the study focused particularly on greenland icebergs feeding into the labrador current and past newfoundland the model was forced with daily heat freshwater and wind fluxes from the twentieth century reanalysis the reanalysis heat fluxes were shown to be offset from the shorter ncep reanalysis and a grid point correction was applied to this component of the forcing the model produces a generally realistic ocean circulation although with an enhanced atlantic meridional overturning largely due to the forcing the modelled iceberg flux at 48 n is well correlated with the long term observed flux when using a modelled iceberg discharge that varies in a similar fashion to the highly variable observed flux at 48 n from this model we infer changes in the spatial and temporal variability of iceberg calving from western greenland during the first third of the twentieth century the majority of modelled icebergs reaching 48 n derive from southern greenland while only after 1930 is the traditional perspective of a majority of such icebergs originating from baffin bay consistent with model results decadal scale changes in the dominant regional sources are found with oscillations between western greenland and northern baffin bay the latter origin was modelled to be most important in the last third of the twentieth century although west greenland sources have increased in importance in recent years the model correctly reproduces the pronounced late spring peak in flux at 48 n for southern greenland icebergs but has an approximately six month offset for icebergs from baffin bay most likely due to resolution issues leading to model icebergs not being delayed in shallow coastal waters whereas in reality they may be grounded for some time or trapped in coastal sea ice

upper ocean t s variations in the greenland sea and their association to climatic conditions 1 hydrography from the nordic seas from 1951 to 2003 was analyzed focusing on the upper ocean conditions during known periods of deep mixing the analysis used data from noaa national oceanographic data center and from international council for the exploration of the sea oceanographic database because of the sparseness of the wintertime data only summertime data are analyzed primarily at 200 m which retains memory of previous winter s effects below summer mixed layer it is found that the salinity variability in the central greenland gyre follows closely the sea level pressure slp fluctuations found along the greenland coast e g at angmagssalik corresponding large scale slp field resembles north atlantic oscillation nao in its negative index phase the dissimilarity between the central gyre salinity fluctuations and the incoming atlantic water salinity fluctuations suggests that mixing with other water masses arctic origin waters and recirculating atlantic waters present in the central gyre is important in modifying the incoming atlantic water salinity at interannual timescales however the largest freshwater addition to the atlantic waters takes place in the norwegian sea or even further upstream in the north atlantic the variability of this freshwater intake resembles the nao index on multidecadal timescales

causes and consequences of arctic freshwater routing into the nordic seas during late marine isotope stage 5 sediment core ps1904 reveals continuous records of planktic and benthic foraminiferal stable isotopes 18o 13c from the north eastern greenland continental margin the data show good comparability with other records from the nordic seas allowing the stratigraphic range of ps1904 to be dated to marine isotope stage mis 6 focusing on mis 5 reveals light 18o values during mis 5a compared to the last interglacial peak mis 5e which indicates that surface and bottom water layers were strongly affected by freshwater during the former event we present two possible scenarios explaining the origin and routing of the freshwater i drainage of a eurasian proglacial lake coupled with the collapse of the kara sea ice sheet at the mis 5b a boundary and ii destabilization and melting of the nearby greenland ice sheet although both scenarios could have acted simultaneously sediment records from the eurasian sector of the arctic ocean hint at the proglacial lake system in north western siberia as the largest freshwater source regardless of the actual source the freshwater lowered the surface ocean salinity causing water column stratification and sea ice expansion increased sea ice abundance led to a higher albedo and probably contributed to the cooling and global ice sheet growth that occurred subsequently during mis 4

descent and modification of the overflow plume in the denmark strait bulk properties of the denmark strait overflow dso plume observed in velocity and hydrography surveys undertaken in 1997 and 1998 are described despite the presence of considerable short term variability it is found that the pathway and evolution of the plume density anomaly are remarkably steady bottom stress measurements show that the pathway of the plume core matches well with a rate of descent controlled by friction the estimated entrainment rate diagnosed from the rate of plume dilution with distance shows a marked increase in entrainment at approximately 125 km from the sill leading to a net dilution consistent with previous reports of a doubling of overflow transport measured by current meter arrays the entrainment rate increase is likely related to the increased topographic slopes in the region compounded by a decrease in interface stratification as the plume is diluted and enters a denser background

open ocean convection in the irminger sea open ocean deep convection is known to occur in a very few locations in the present climate convection is important for the ventilation of the oceans and for the operation of the meridional overturning circulation using data from ships and profiling floats we present evidence for the occurrence of convection in the irminger sea of the north atlantic south east of greenland confirmation of this convective site in the north atlantic will influence our understanding of the connection of the atmosphere to the ocean depths and of the mechanisms of climate variability

the north atlantic oscillation signature in deuterium and deuterium excess signals in the greenland ice sheet project 2 ice core 1840 1970 the greenland ice sheet project 2 gisp2 core can enhance our understanding of the relationship between parameters measured in the ice in central greenland and variability in the ocean atmosphere and cryosphere of the north atlantic ocean and adjacent land masses seasonal summer winter to annual responses of d and deuterium excess isotopic signals in the gisp2 core to the seesaw in winter temperatures between west greenland and northern europe from a d 1840 to 1970 are investigated this seesaw represents extreme modes of the north atlantic oscillation which also influences sea surface temperatures ssts atmospheric pressures geostrophic wind strength and sea ice extents beyond the winter season temperature excursions inferred from the d record during seesaw extreme nao mode years move in the same direction as the west greenland side of the seesaw symmetry with the west greenland side of the seesaw suggests a possible mechanism for damping in the ice core record of the lowest decadal temperatures experienced in europe from a d 1500 to 1700 seasonal and annual deuterium excess excursions during seesaw years show negative correlation with d this suggests an isotopic response to a sst land temperature seesaw the isotopic record from gisp2 may therefore give information on both ice sheet and sea surface temperature variability cross plots of d and d show a tendency for data to be grouped according to the prevailing mode of the seesaw but do not provide unambiguous identification of individual seesaw years a combination of ice core and tree ring data sets may allow more confident identification of ga and gb extreme nao mode years prior to 1840

coupled north atlantic slope water forcing on gulf of maine temperatures over the past millennium to investigate ocean variability during the last millennium in the western gulf of maine gom we collected a 142 year old living bivalve arctica islandica l in 2004 and three fossil a islandica shells calibrated 14cams 1030 78 ad 1320 45 ad 1357 40 ad for stable isotope and growth increment analysis a statistically significant relationship exists between modern gom temperature records shell isotope derived 30 m r 0 79 p 0 007 prince 5 50 m r 0 72 p 0 019 boothbay harbor sst r 0 76 p 0 011 and labrador current lc transport data from the eastern newfoundland slope during 1993 2003 in all cases as lc transport increased gom water temperatures decreased the following year decadal trends in the north atlantic oscillation nao and the atlantic multidecadal oscillation amo influence gom water temperatures in the most recent period with water temperatures decreasing during nao and amo negative modes most likely linked to lc transport and gulf stream interaction mean shell derived isotopic changes 18oc during the last 1 000 years were 0 47 and likely reflect a 1 2 c cooling from 1000 ad to present based on these results we suggest that observed cooling in the gom during the last millennium was due to increased transport and or cooling of the lc and decreased gulf stream influence on the gom

the summer hydrography and surface circulation of the east siberian shelf sea during the ice free summer season in 1995 the authors deployed and subsequently tracked 39 surface drifters to test the hypothesis that the discharge from the kolyma river forces a buoyancy driven coastal current from the east siberian sea toward bering strait the observed mean flow is statistically significant at the 95 level of confidence but its direction contradicts their initial hypothesis instead of a coastally trapped eastward flow the authors find a laterally sheared westward flow with maximum velocities offshore that correlate only weakly with the local winds at a daily wind dominated timescale the drifter data reveal spatially coherent flows of up to 0 5 m s21 the lagrangian autocorrelation scale is about 3 days and the lagrangian eddy length scale reaches 40 km this spatial scale exceeds the nearshore internal deformation radius by a factor of 3 however it more closely corresponds to the internal deformation radius associated with the offshore ice edge bulk estimates of the horizontal mixing coefficient resemble typical values of isotropic open ocean dispersion at midlatitudes hydrographic observations and oxygen isotope ratios of seawater indicate a low proportion of riverine freshwater relative to sea ice melt in most areas of the east siberian sea except close to the kolyma delta the observations require a reevaluation of the conceptual view of the summer surface circulation of the east siberian sea eastward buoyancy driven coastal currents do not always form on this shelf despite large river discharge instead ice melt waters of a retreating ice edge act as a line source of buoyancy that in 1995 forced a westward surface flow in the east siberian sea

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

north atlantic climate and deep ocean flow speed changes during the last 230 years variations in the near bottom flow speed of iceland scotland overflow water isow are documented in a 230 year long deep sea sediment record from the eastern flank of reykjanes ridge in the subpolar north atlantic at sub decadal time scales for recent decades the isow palaeocurrent reconstructions show similarities with observational hydrographic data furthermore recent isow flow changes fall mostly within the range of its variability of the past 230 years the record also reveals a hitherto unrecognized coupling of deep flow speeds in the subpolar north atlantic with the north atlantic oscillation nao index with more less vigorous isow flow during negative positive phases of the nao our results suggest that the changes in isow vigor are largely controlled by the transport and characteristics of labrador sea water rather than variations in the overflow itself with implications for the meridional overturning of the atlantic ocean and climate

late holocene paleoenvironments of northwest iceland from lake sediments abstract frozen sediment cores from two lakes in northwest iceland vatnsdalsvatn and thi riksvallavatn provide a nearly 1000 year decadal scale record of past environmental changes based on environmental magnetism inductively coupled plasma optical emission spectroscopy geochemistry carbon content and c n analyses accelerator mass spectrometry 14 c and 210 pb results provide chronologies for both sites covering the late 20th century to 960 bp at vatnsdalsvatn and to 480 bp at thi riksvallavatn these analyses reveal higher levels of sedimentary carbon during the little ice age between 600 and 50 bp at vatnsdalsvatn c n analyses suggest that some of this carbon enrichment came from terrestrial plants higher erosion rates during the little ice age may also have increased the influx of limiting nutrients enhancing algal productivity erosion is the dominant mechanism for sedimentary changes at both sites and seems to occur at predictable intervals geochemical indicators of erosion oscillate with a distinct 80 100 yr periodicity for nine centuries at vatnsdalsvatn and for four centuries at thi riksvallavatn these erosion cycles are consistent with long term stability in the positive mode of the north atlantic oscillation but may also be attributed to local climatic effects associated with great salinity anomalies

arctic sea ice variability on a timescale of weeks and its relation to atmospheric forcing abstract northern hemisphere sea ice concentration 500 hpa height sea level pressure and 1000 500 hpa thickness at 7 day intervals are examined for the period 1972 1989 with emphasis on the winter season the temporal variability of sea ice concentration is largest along the climatological mean ice edge where its frequency distribution is strongly bimodal with ice free and ice covered conditions being observed much more frequently than partial ice cover these results confirm impressions based on visual inspection of satellite imagery that most of the variability in these regions is associated with the advance and retreat of the ice edge relationships between large scale patterns of atmospheric variability and sea ice variability are investigated making use of singular value decomposition of the temporal covariance matrix the analysis is conducted separately for the atlantic and pacific sectors in agreement with earlier studies based upon monthly mean data on sea ice concentration the strongest

thermohaline circulation at three key sections in the north atlantic over 1985 2002 efforts are presently underway to monitor the thermohaline circulation thc in the north atlantic a measuring strategy has been designed to monitor both the meridional overturning circulation moc in the subtropics and dense outflows at higher latitudes to provide a historical context for these new observations we diagnose an eddy permitting ocean model simulation of the period 1985 2002 we present time series of the thc moc and heat transport at key hydrographic sections in the subtropics the northeast atlantic and the labrador sea the simulated thc compares well with observations we find considerable variability in the thc on each section most strikingly in the labrador sea during the early 1990 s consistent with observed changes overturning in the northeast atlantic declines by 20 over the 1990 s coincident with an increase in the subtropics we speculate that moc weakening may soon be detected in the subtropics if the decline continues in mid latitudes

labrador sea water in the eastern north atlantic part i a synoptic circulation inferred from a minimum in potential vorticity abstract a synoptic distribution of labrador sea water lsw in the eastern north atlantic is determined from a regularly sampled but sparse 3 resolution survey covering 39 to 54 n 11 to 34 w during spring 1991 the core of lsw can be defined by a minimum in potential vorticity pv using property values at this minimum the authors infer the circulation of lsw in addition using a known source function for salinities at the core of lsw estimates are able to be made of lsw vintages the authors then compared the synoptic circulation to historical data youngest 1986 vintage lsw crosses the mid atlantic ridge to the eastern basin between 48 and 51 n at 34 w this water then flows north to the iceland basin and eastward to the rockall trough where it was found to be of 1978 vintage tongues of low salinity low temperature and high oxygen extend southward on the eastern side of the mid atlantic ridge indicating that lsw also flows southward in the eastern basin at the southern edge of the s

engineered damage zone sealing during a water injection test at the tournemire url low permeability clay formations provide good candidate host rocks for geological disposal of radioactive waste because there is expected to be limited movement of gas or water through the formation however when constructing tunnels the stress state in the formation around the tunnel will change which can lead to damage to the formation changing the bulk hydraulic properties of the formation close to the tunnel there is the potential for this damaged zone to act as a preferential pathway for fluid flow and radionuclide transport a water injection experiment is ongoing at the tournemire underground rock laboratory to investigate the hydraulic properties of the toarcian argillite in which the laboratory is constructed water is injected into the formation at the end of a sealed borehole and moves preferentially in the damaged zone along the borehole walls the rate of water injection into the rock changed over the first year of the experiment and the causes of this change are investigated in this paper by numerical modelling the study demonstrates that the change in water injection rate into the argillite can be explained by the evolving hydraulic properties of the damaged zone around the borehole the findings of the modelling study are discussed in the context of long term radioactive waste disposal

applying uv cameras for so2 detection to distant or optically thick volcanic plumes abstract ultraviolet uv camera systems represent an exciting new technology for measuring two dimensional sulfur dioxide so 2 distributions in volcanic plumes the high frame rate of the cameras allows the retrieval of so 2 emission rates at time scales of 1 hz or higher thus allowing the investigation of high frequency signals and making integrated and comparative studies with other high data rate volcano monitoring techniques possible one drawback of the technique however is the limited spectral information recorded by the imaging systems here a framework for simulating the sensitivity of uv cameras to various so 2 distributions is introduced both the wavelength dependent transmittance of the optical imaging system and the radiative transfer in the atmosphere are modeled the framework is then applied to study the behavior of different optical setups and used to simulate the response of these instruments to volcanic plumes containing varying so 2 and aerosol abundances located at various distances from the sensor results show that uv radiative transfer in and around distant and or optically thick plumes typically leads to a lower sensitivity to so 2 than expected when assuming a standard beer lambert absorption model furthermore camera response is often non linear in so 2 and dependent on distance to the plume and plume aerosol optical thickness and single scatter albedo the model results are compared with camera measurements made at kilauea volcano hawaii and a method for integrating moderate resolution differential optical absorption spectroscopy data with uv imagery to retrieve improved so 2 column densities is discussed

evaluation of labrador sea water formation in a global finite element sea ice ocean model setup based on a comparison with observational data the deep water formation in the labrador sea is simulated with the finite element sea ice ocean model fesom in a regionally focused but globally covered model setup the model has a regional resolution of up to 7 km and the simulations cover the time period 1958 2009 we evaluate the capability of the model setup to reproduce a realistic deep water formation in the labrador sea two classes of modeled labrador sea water lsw the lighter upper lsw ulsw and the denser deep lsw dlsw are analyzed their layer thicknesses are compared to ulsw and dlsw layer thicknesses derived from observations in the formation region for the time interval 1988 2009 the results indicate a suitable agreement between the modeled and observational derived ulsw and dlsw layer thicknesses except for the period 2003 2007 where deviations in the modeled and observational derived layer thicknesses could be linked to discrepancies in the atmospheric forcing of the model it is shown that the model is able to reproduce four phases in the temporal evolution of the potential density temperature and salinity since the late 1980s which are known in observational data these four phases are characterized by a significantly different lsw formation the first phase from 1988 to 1990 is characterized in the model by a fast increase in the convection depth of up to 2000 m accompanied by an increased spring production of deep labrador sea water dlsw in the second phase 1991 1994 the dlsw layer thickness remains on a high level for several years while the third phase 1995 1998 features a gradual decrease in the deep ventilation and the renewal of the deep ocean layers the fourth phase from 1999 to 2009 is characterized by a slowly continuing decrease of the dlsw layer thickness on a deeper depth level by applying a composite map analysis between an index of dlsw and sea level pressure over the entire simulation period from 1958 to 2009 it is shown that a pattern which resembles the structure of the north atlantic oscillation nao is one of the main triggers for the variability of lsw formation our model results indicate that the process of dlsw formation can act as a low pass filter to the atmospheric forcing so that only persistent nao events have an effect whether ulsw or dlsw is formed based on composite maps of the thermal and haline contributions to the surface density flux we can demonstrate that the central labrador sea in the model is dominated by the thermal contributions of the surface density flux while the haline contributions are stronger over the branch of the labrador sea boundary current system lsbcs where they are dominated by the haline contributions of sea ice melting and formation our model results feature a shielding of the central labrador sea from the haline contributions by the lsbcs which only allows a minor haline interaction with the central labrador sea by lateral mixing based on the comparison of the simulated and measured lsw layer thicknesses as well as vertical profiles of potential density temperature and salinity it is shown that the fesom model is a suitable tool to study the regional dynamics of lsw formation and its impact on a global not regional restricted scale

a shell derived time history of bomb 14c on georges bank and its labrador sea implications bomb produced radiocarbon has been used in the past as an important tracer of ocean circulation and as a valuable tool for calculating co2 air sea exchange however previous studies of the ocean s time varying bomb 14c record have been confined exclusively to analyzing banded corals and thus their application has been limited to the lower latitudes the first time history of bomb 14c from the high latitude north atlantic ocean is obtained from a 54 year old mollusc specimen bivalvia arctica islandica which was collected live from georges bank 41 n in 1990 the annual growth bands of its shell were analyzed for 14c using accelerator mass spectrometry producing a 14c time history from 1939 to 1990 the depleted condition of the georges bank bomb 14c signal relative to two coral derived north atlantic 14c time histories suggests a significant deepwater source for the waters on georges bank supported by previous work linking the origin of waters on georges bank to the labrador sea the 14c budget on georges bank is modeled as labrador sea water which largely becomes confined to the shelf and partially equilibrates with the atmosphere during a 1 year transit time from the labrador sea to georges bank this model is also used to estimate a time history of bomb 14c for the labrador sea prebomb 14c values calculated for the surface labrador sea suggest that a greater inventory of bomb 14c has accumulated here than has previously been reported however the estimated prebomb average 14c 70 6 for this period is nearly identical to the 70 previously calculated for the prebomb source of north atlantic deep water and is in agreement with transient tracers in the ocean subsurface tritium data from the central labrador sea deduced variations in the ventilation and or 14co2 uptake rates in the labrador sea correspond with observed changes in surface salinity of the labrador sea suggesting a reduction in deepwater formation during the late 1960s and 1970s

cooling and freshening of the subpolar north atlantic ocean since the 1960s little is known of the interdecadal variability in the thermohaline circulation of the world s oceans yet such knowledge is essential as a background to studies of the effects of natural and anthropogenic climate change the subpolar north atlantic is an area of extensive water mass modification by heat loss to the atmosphere lying as it does at the northern limit of the global thermohaline conveyor belt 12 changes in this region may ultimately have global consequences here we report that in august 1991 the waters between greenland and the united kingdom were on average 0 08 c and 0 15 c colder than in 1962 and 1981 respectively and slightly less saline than in 1962 the cause appears to be renewed formation of intermediate water in the labrador sea from cooler and fresher source waters and the spreading of this water mass from the west variations in the source characteristics of labrador sea water can be traced across the north atlantic with a circulation time of 18 19 years between the labrador sea and rockall trough more recently formed labrador sea water with even lower temperature and salinity should cool and freshen the north atlantic still further as it circulates around the ocean in the coming decade

late holocene variability of upper north atlantic deep water temperature and salinity 1 magnesium calcium ratios in benthic foraminifera cibicidoides pachyderma from a sediment core on the laurentian slope 1854 m exhibit strong millennial scale fluctuations during the past 4000 years we convert these data to seawater paleotemperatures using a new monospecific linear equation results suggest that the temperature of upper north atlantic deep water dominated by labrador seawater today has varied by at least 2 c during the late holocene millennial scale coolings coincide with previously identified periods of increased drift ice and regional glacier advances including the little ice age paired oxygen isotope measurements indicate that salinity and perhaps density were reduced during the cold periods we discuss possible mechanisms for transmitting this cold fresh signal from surface waters to intermediate depths our reconstructed late holocene ranges in upper north atlantic deep water properties greatly exceed those of the instrumental record and imply that large changes may be yet to come

classification seasonality and persistence of low frequency atmospheric circulation patterns abstract orthogonally rotated principle component analysis rpca of northern hemisphere 1 month mean 700 mb heights is used to identify and describe the seasonality and persistence of the major modes of interannual variability the analysis is detailed and comprehensive in that 1 a high resolution approximately equal area 358 point grid is used for the virtually maximum possible 35 year period of record 2 a positive bias in the nmc data base in the early 1950s in the subtropics is largely eliminated for the first time and 3 homogeneous separate analyses of each month of the year are carried out detailing the mouth to month changes in the dominant circulation patterns winter results are similar to those of other recent rpca and teleconnection studies except that some less obvious patterns are identified and further detail of the better known patterns is provided two north south dipole patterns are found over the pacific ocean west pacific oscillation and east pacific pattern and over the atla

a twentieth century reanalysis forced ocean model to reconstruct the north atlantic climate variation during the 1920s the observed north atlantic multi decadal variability for the period 1872 2009 is reconstructed with the max planck institute ocean model which is forced with an ensemble of the atmospheric twentieth century reanalysis special emphasis is put on the early part of the experiments which includes a prominent climate variation during the 1920s the experiments are in agreement with selected hydrographic records indicating a transition from cold and fresh north atlantic water properties prior to the 1920 climate variation towards warm and saline waters afterwards examining the variation reveals that sea level pressure slp anomalies prior to the 1900s resemble a negative phase of north atlantic oscillation and associated weak winds result in a weak north atlantic current nac and sub polar gyre spg this leads to a reduced transport of warm and saline waters into the higher latitudes simultaneously arctic freshwater release results in the accumulation of cold and fresh water properties which cover the upper layers in the labrador sea and subsequently suppress convection from the 1910s the arctic freshwater export is reduced and nac and spg are strengthened as a result of an increased slp gradient over the north atlantic concurrently labrador sea convection and atlantic meridional overturning circulation amoc increase the intensified nac spg and amoc redistribute sub tropical water into the north atlantic and nordic seas thereby increasing observed and modelled temperature and salinity during the 1920s

the contribution of the greenland and barents seas to the deep water of the arctic ocean the deep waters of the arctic ocean are traditionally held to be fed by an influx of norwegian sea deep water nsdw via the northward flowing west spitsbergen current discrete sample and ctd observations obtained from the greenland spitsbergen passage in august 1981 during the transient tracers in the ocean tto north atlantic expedition showed a 100 m thick layer of modified greenland sea deep water gsdw colder and fresher than nsdw at 2500 m spreading northward along the bottom of a deep unimpeded channel underneath the nsdw since the available data indicate that arctic ocean deep water aodw has a higher salinity than nsdw mixing of nsdw and gsdw can not produce aodw therefore other sources such as the peripheral arctic shelf seas must contribute dense saline water to the arctic ocean concentrations of 137cs and 90sr observed in aodw are greater than those observed in gsdw and nsdw the concentrations of these radionuclides on the barents sea shelf are sufficiently high and in the correct relative proportions to support this proposition

simulated variability of the circulation in the north atlantic from 1953 to 2003 abstract the variability of the circulation in the north atlantic and its link with atmospheric variability are investigated in a realistic hindcast simulation from 1953 to 2003 the interannual to decadal variability of the subpolar gyre circulation and the meridional overturning circulation moc is mostly influenced by the north atlantic oscillation nao both circulations intensified from the early 1970s to the mid 1990s and then decreased the monthly variability of both circulations reflects the fast barotropic adjustment to nao related ekman pumping anomalies while the interannual to decadal variability is due to the baroclinic adjustment to ekman pumping buoyancy forcing and dense water formation consistent with previous studies an original characteristic of the oceanic response to nao is presented that relates to the spatial patterns of buoyancy and wind forcing over the north atlantic anomalous ekman pumping associated with a positive nao phase first induces a decrease of the southern sub

distribution and circulation of labrador sea water abstract labrador sea water is the final product of the cyclonic circulation of subpolar mode water in the open northern north atlantic mccartney and talley 1982 the temperature and salinity of the convectively formed subpolar mode water decrease from 14 7 c 36 08 to 3 4 c 34 88 on account of the cumulative effects of excess precipitation and cooling the coldest mode water is labrador sea water which spreads at mid depths and is found throughout the north atlantic ocean north of 40 n and along its western boundary to 18 n a vertical minimum in potential vorticity is used as the primary tracer for labrador sea water labrador sea water is advected in three main directions out of the labrador sea 1 northeastward into the irminger sea 2 southeastward across the atlantic beneath the north atlantic current and 3 southward past newfoundland with the labrador current and thence westward into the slope water region crossing under the gulf stream off cape hatteras the labrador sea water core is

comparison between instrumental observational and high resolution proxy sedimentary records of late holocene climatic change a discussion of possibilities abstract variability of the north atlantic region is a key factor in the workings of global climate changes related to orbital factors and to the thermohaline circulation thc occur on timescales ranging from 1000 to 100 000 years whereas subdecadal decadal and multidecadal trends are associated with changes in atmospheric circulation sea surface temperature sst and salinity long term changes are known only from proxies whereas shorter periodicities are identified in instrumental and other observational data between lies a range of less well known centennial periodicities too long to be recorded observationally and too short for all but the highest resolution proxy records such as tree rings atmospheric 14c and varved lake sediments numerous investigations of such periodicities involve comparisons between the proxy and the observational record however results depend on the length of the datasets being analysed the relative strength of the periodicities present whether data are stationary or non stationary the degree to which they have been smoothed in order to remove particular fluctuations and the methods of analysis employed ideally comparisons should be between datasets of equal length and of similar properties but these criteria are not always easy to fulfill a search for periodicities in a range of well known multidecadal datasets reveals that whilst some frequencies previously identified in the central england temperature cet record and in the koch sea ice index are statistically significant those found by several authors in the index of the north atlantic oscillation nao are not as the index approximates to white noise instead decadal periodicity of the nao described by several authors appears to be result of smoothing of the raw data periodicities of 6 1 33 9 and 213 years identified in a proxy record from laminated sediments from loch ness scotland appear respectively to be related to a subdecadal variations in sst and salinity in the north atlantic ocean b multidecadal interactions between the nao and the north atlantic thc and c a well known centennial solar periodicity

the thermohaline circulation of the arctic ocean and the greenland sea the thermohaline circulation of the arctic ocean and the greenland sea is conditioned by the harsh high latitude climate and by bathymetry warm atlantic water loses its heat and also becomes less saline by added river run off in the arctic ocean this leads to rapid cooling of the surface water and to ice formation brine released by freezing increases the density of the surface layer but the ice cover also insulates the ocean and reduces heat loss this limits density increase and in the central arctic ocean a low salinity surface layer and a permanent ice cover are maintained only over the shallow shelves where the entire water column is cooled to freezing can dense water form and accumulate to eventually sink down the continental slope into the deep ocean the part of the atlantic water which enters the arctic ocean is thus separated into a low density surface layer and a denser deep circulation these two loops exit through fram strait the waters are partly rehomogenized in the greenland sea the main current is confined to the greenland continental slope but polar surface water and ice are injected into the central gyre and create a low density lid allowing for ice formation in winter this leads to a density increase sufficient to trigger convection upwelling and subsequent ice melt the convection maintains the weak stratification of the gyre and also reinforces the deep circulation loop as the transformed waters return to the north atlantic the low salinity upper water of the east greenland current enters the labrador sea and influences the formation of labrador sea deep water the dense loop passes through denmark strait and the faroe shetland channel and sinks to contribute to the north atlantic deep water changes in the forcing conditions might alter the relative strength of the two loops this could affect the oceanic thermohaline circulation on a global scale

an isopycnic model study of the north atlantic part ii interdecadal variability of the subtropical gyre abstract in a companion paper a spinup integration of the north atlantic ocean with the miami isopycnic coordinate model was presented the wintertime mixed layer in the central north atlantic was subject to relatively little change in salinity or depth but cooled markedly most probably because of heat loss associated with a partial surface relaxation to climatological sea surface temperatures in a region in which the gulf stream was too far to the north this mixed layer cooling caused the isopycnic layers in the model ventilated subtropical gyre to rise and surprisingly to warm while the experiment was not an attempt to simulate changes in the real atlantic ocean it nevertheless appears from observations that in recent decades the mixed layer in nature has undergone a change similar to that exhibited by the model mixed layer since it is expected that changes in the ventilated subtropical gyre will be governed largely by changes in the mixed layer in the central north atlantic from where the ve

saline outflow from the arctic ocean its contribution to the deep waters of the greenland norwegian and iceland seas abstractsince 1985 various investigators have proposed that norwegian sea deep water nsdw is formed by mixing of warm and saline deep water from the arctic ocean with the much colder and fresher deep water formed by convection in the greenland sea gsdw we here report on new observations which suggest significant modification and expansion of this conceptual model we find that saline outflows from the arctic ocean result in several distinct intermediate and deep salinity maxima within the greenland sea the southward transport of the two most saline modes is probably near 2 sv mixing of gsdw and the main outflow core found over the greenland slope derived from about 1700 m in the arctic ocean cannot by itself account for the properties of nsdw instead the formation of nsdw must at least in part involve a source which in the arctic ocean is found below 2000 m the mixing of various saline outflows is diapycnal while significant nsdw production appears to occur in northern fram strait large amounts of saline arctic ocean outflow also traverse the western greenland sea without mixing and enter the iceland sea during the past decade deep convection in the greenland sea has been greatly reduced while deep outflow from the arctic ocean appears to have continued resulting in a markedly warmer slightly more saline and less dense deep regime in the greenland sea american geophysical union 1991

a study of the interaction of the north atlantic oscillation with ocean circulation observed patterns of wind stress curl and air sea heat flux associated with the north atlantic oscillation nao are used to discuss the response of ocean gyres and thermohaline circulation to nao forcing and their possible feedback on the nao the observations motivate and are interpreted in the framework of a simple mathematical model that couples ekman layers ocean gyres and thermohaline circulation to the atmospheric jet stream meridional shifts in the zero wind stress curl line are invoked to drive anomalies in ocean gyres and north south dipoles in air sea flux drive anomalous thermohaline circulation both gyres and thermohaline circulation play a role in modulating sea surface temperature anomalies and hence through air sea interaction the overlying jet stream the model which can be expressed in the form of a delayed oscillator with ocean gyres and or thermohaline circulation providing the delay identifies key nondimensional parameters that control whether the ocean responds passively to nao forcing or actively couples it suggests that both thermohaline circulation and ocean gyres can play a role in coupled interactions on decadal timescales

the formation of labrador sea water part i large scale processes abstract data obtained in the western labrador sea during march 1976 by hudson are analysed to show that new labrador sea water was being formed at this time on the basis of hydrographic and moored current meter data it is hypothesized that a 200 km scale cyclonic gyre forms in winter in the western labrador sea and that this gyre retains the developing deep mixed layers in this general area long enough for the transformation to labrador sea water to take place using a model it is demonstrated that water columns found along the western boundary of the labrador sea can be modified by cooling evaporation and mixing to form deep mixed layers with the properties of labrador sea water approximately 105 km3 of new labrador sea water was formed in 1976 an estimate that is consistent with earlier estimates of mean annual production rates this water 2 9 c 34 84 is some 0 6 c cooler and 0 06 fresher than that defined by lazier 1973 from his data collected in 1966 the variation of labrador sea water

the greenland sea in water 1993 and 1994 preconditioning for deep convection abstract we present here a detailed analysis of the time dependent large scale and mesoscale features observed in the greenland sea during winter and spring 1993 and 1994 based on a comprehensive data set this study gives particular emphasis to the data from hydrology and rafos vcm floats in both years the basin underwent an overturning to 1000 and 800 m depth respectively with extensive horizontal variability these inhomogeneities are shown to be closely linked with mesoscale eddies identified at a late stage during the preconditioning phase to deep convection it is suggested that these eddies are remnants of previous convective events one of them found at 500 m depth in 1994 was thoroughly studied the dynamical coupling between the eddy and the modified atlantic water maw advected over it from the northwest led to the deepest convected water column found within the basin during this particular year sea ice absent in the gyre in winter 1994 but partly present in the colder 1993 winter does not appear to be a strong prerequisite for deep convection heat loss at the surface combined with salt input by the maw and remnant subsurface mesoscale eddies of past convective events are the most likely agents that determine the depth reached by convection and the characteristics of the new greenland arctic intermediate water formed in the greenland gyre in 1993 and 1994

freshening of the labrador sea surface waters in the 1990s another great salinity anomaly 1 the observed labrador sea surface salinities for 1951 2001 in region 55w 40w 45n 55n show freshening in the 1990s which may rival the great salinity anomaly gsa of the 1970s dickson et al 1988 the 1990s freshening is distinct in the lowest range of salinity values associated with the coastal waters the variability of the highest salinity values provided by the north atlantic current entering the labrador sea shows that the high salinity atlantic waters were not widely present in the 1970 s and 1990s this can be interpreted as weakened transport of saline waters to the area this explanation is consistent with an arctic north atlantic model results forced by ncep ncar reanalysis data for 1958 2001 which suggest that these periods are associated with a reduced meridional overturning the model also associates the coastal freshening with southward current anomalies which are well correlated with the number of icebergs crossing 48n

a 250 year decadally resolved radiocarbon time history in the gulf of maine reveals a hydrographic regime shift at the end of the little ice age

centennial scale surface hydrology off portugal during marine isotope stage 3 insights from planktonic foraminiferal fauna variability 1 the marine isotopic stage 3 mis3 at ocean drilling program odp site 1060 gulf stream shows both sharp onset and end of interstadials the existence of very short lived warm events during stadials and points to differences in detail between the sea surface temperature sst record from the western north atlantic and the atmospheric temperature record inferred from 18o in greenland ice investigating mis3 and obtaining comparable data from other locations appears crucial the eastern atlantic provides well documented records of climate changes we have selected a core from off portugal and use it to examine dansgaard oeschger events d o at centennial scale resolution 139 years on average between two data points we have obtained a faunal data set for core md01 2444 37 n 10 w 2600 m water depth and use a group of species globigerina bulloides globigerinita glutinata as a proxy of upwelling intensity driven by trade winds intensity changes we tentatively relate the variation of this group to a north atlantic oscillation like phenomenon nao off portugal we observe that it resembles the rainfall index in the caribbean as recorded at odp site 1002 cariaco basin which traces the intertropical convergence zone itcz location through changes of terrigenous inputs the driest intervals itzc to the south at site 1002 correspond to intervals of increased upwelling in md01 2444 as well as the driest periods identified during stadials on similar cores in the area because the itzc to the south is consistent with an el nino southern oscillation enso situation our study suggests a positive correlation between enso like conditions and nao like conditions at a millennial timescale during interstadial intervals when increased wetness over cariaco is recorded itcz to the north and the upwelling in md01 2444 is decreased we see from both ssts and faunal tropical indicators that md01 2444 site is warm in addition interstadials are equally warm through each so called bond cycle this contrasts with the greenland ice core project grip record where interstadial peaks are successively cooler through each bond cycle this record confirms a link between tropical climate linked to itcz position and the climate of southern europe at millennial timescales in spite of showing a very good correlation with polar latitudes grip through 18o on globigerina bulloides in addition because the warmest ssts and the 18o on g bulloides are so remarkably different our work points to changes in seasonality as a strong control over the climatic pattern of the north atlantic area and the marked influence of winter conditions

possible north atlantic origin for changes in enso properties during the 1970s the most intense el nino episodes in more than a century occurred after the 1970s climate shift previous studies show that the characteristics of the el nino southern oscillation enso phenomenon changed synchronously with the shift but the associated causes are not fully understood an analysis of the observed tropical pacific sea surface temperature sst anomalies shows that their increase in the eastern part of the basin after the 1970s is not related to the canonical enso pattern but to the tropical pacific meridional mode tpmm we present observational evidence which supports the hypothesis that the change in the tpmm was triggered by the great salinity anomaly gsa which manifested in the north atlantic during the late 1960s the gsa induced a weak labrador convection and a sst dipole south of greenland the associated atmospheric structure includes a north pacific oscillation sea level pressure dipole in the pacific sector this excites the tpmm which contributes to the intense el nino events and to the enhanced enso s asymmetry observed after the shift our results imply that if the gsa has not an anthropic origin as was suggested then the tropical pacific climate shift has a natural origin this is supported by the end of the north atlantic regime in the 1990s and by the rebound of the tropical pacific after 1998

river runoff sea ice meltwater and pacific water distribution and mean residence times in the arctic ocean hydrographic and tracer data collected during ark iv 3 fs polarstern in 1987 arctic91 ib oden and aos94 ccgs louis s st laurent expeditions reveal the evolution of the near surface waters in the arctic ocean during the late 1980s and early 1990s salinity nutrients dissolved oxygen and 18 o data are used to quantify the components of arctic freshwater river runoff sea ice meltwater and pacific water the calculated river runoff fractions suggest that in 1994 a large portion of water from the pechora ob yenisey kotuy and lena rivers did not flow off the shelf closest to their river deltas but remained on the shelf and traveled via cyclonic circulation into the laptev and east siberian seas river runoff flowed off the shelf at the lomonosov ridge and most left the shelf at the mendeleyev ridge arctic91 and aos94 pacific water fraction estimates of upper halocline water the traditionally defined core of the pacific water mass document a decrease in extent compared to historical data the front between atlantic water and pacific water shifted from the lomonosov ridge location in 1991 to the mendeleyev ridge in 1994 the relative age structure of the upper waters is described by using the 3 h 3 he age the mean 3 h 3 he age measured in the halocline within the salinity surface of 33 1 0 3 is 4 3 1 7 years and that for the 34 2 0 2 salinity surface is 9 6 4 6 years lateral variations in the relative age structure within the halocline and atlantic water support the well known cyclonic boundary current circulation

geohydromechanical processes in the excavation damaged zone in crystalline rock rock salt and indurated and plastic clays in the context of radioactive waste disposal abstract the creation of an excavation disturbed zone edz or excavation damaged zone edz is expected around all man made openings in geologic formations macro and micro fracturing and in general a redistribution of in situ stresses and rearrangement of rock structures will occur in this zone resulting in drastic changes of permeability to flow mainly through the fractures and cracks induced by excavation the edz may have significant implications for the operation and long term performance of an underground nuclear waste repository various issues of concern need to be evaluated such as processes creating fractures in the edz the degree of permeability increase and the potential for sealing or healing with permeability reduction in the zone in recent years efforts along these lines have been made for a potential repository in four rock types crystalline rock salt indurated clay and plastic clay and these efforts have involved field laboratory and theoretical studies the present work involves a synthesis of the ideas and issues that emerged from presentations and discussions on edz in these four rock types at a cluster conference and workshop held in luxembourg in november 2003 first definitions of excavation disturbed and edzs that may be applicable to all four rock types are proposed then an approach is suggested for the synthesis and intercomparison of geohydromechanical processes in the edz for crystalline rock salt indurated clay and plastic clay comparison tables of relevant processes associated factors and modelling and testing techniques are developed a discussion of the general state of the art and outstanding issues is also presented a substantial bibliography of relevant papers on the subject is supplied at the end of the paper

how to identify winter convection in the greenland sea from hydrographic summer data 1 the identification of winter convection depths from later field observations is an often required but not trivial task we show from field measurements that the effects of winter convection in the greenland sea can vary enormously including such opposite changes as an increase or decrease of temperature salinity or stability also small scale fluctuations in the vertical can grow larger or cease commonly used indicators for past convection in the greenland sea like overall vertical homogeneity or a freshwater input to deeper layers alone are therefore not reliable to construct a time series of winter convection we explain this variety of effects with the occurrence of two rather different ventilation scenarios possible in the greenland sea they are exemplified by two winters using mainly hydrographic observations of the adjoining summers the observed differences are substantiated by the results of a one dimensional model out of this a multicriteria catalogue is developed which can be used to determine unambiguously convection depths in the greenland sea the fulfillment of one of the criteria is sufficient to evidence convection but the suitable criterion may differ from winter to winter a straightforward indicator proving the absence of winter convection does not exist

the long lived mollusc arctica islandica a new paleoceanographic tool for the reconstruction of bottom temperatures for the continental shelves of the northern north atlantic ocean the carbonate shell of the bivalve arctica islandica has been recognized for more than a decade as a potentially important marine geochemical biorecorder owing to this species great longevity 200 years and wide geographic distribution throughout the northern north atlantic ocean a region vital to global climate and ocean circulation however until now this potential has not been realized owing to the difficulty of precisely sampling the shell of this slow growing species using newly available automated microsampling techniques combined with micromass stable isotope mass spectrometry a stable oxygen isotope record 1956 1957 and 1961 1970 has been obtained from a live captured 38 year old a islandica specimen collected near the former position of the nantucket shoals lightship 41 n 69 w the shell s 18o signal is compared with an expected signal derived from ambient bottom temperature and salinity data recorded at the lightship for the same period the results show that a islandica s 18o record 1 is in phase with its growth banding confirming the annual periodicity of this species growth bands 2 is in oxygen isotopic equilibrium with the ambient seawater 3 shows a consistent shell growth shutdown temperature of 6 c which translates into an 8 month may december shell growth period at this location and 4 records the ambient bottom temperature with a precision of 1 2 c these results add important information on the life history of this commercially important shellfish species and demonstrate that a islandica shells can be used to reconstruct inter and intra annual records of the continental shelf bottom temperature

the role of sea ice and other fresh water in the arctic circulation salinity stratification is critical to the vertical circulation of the high latitude ocean we here examine the control of the vertical circulation in the northern seas and the potential for altering it by considering the budgets and storage of fresh water in the arctic ocean and in the convective regions to the south we find that the present day greenland and iceland seas and probably also the labrador sea are rather delicately poised with respect to their ability to sustain convection small variations in the fresh water supplied to the convective gyres from the arctic ocean via the east greenland current can alter or stop the convection in what may be a modern analog to the halocline catastrophes proposed for the distant past the north atlantic salinity anomaly of the 1960s and 1970s is a recent example it must have had its origin in an increased fresh water discharge from the arctic ocean similarly the freshening and cooling of the deep north atlantic in recent years is a likely manifestation of the increased transfer of fresh water from the arctic ocean into the convective gyres finally we note that because of the temperature dependence of compressibility a slight salinity stratification in the convective gyres is required to efficiently ventilate the deep ocean

the interannual variability of the surface eddy kinetic energy in the labrador sea abstract the variability of the surface eddy kinetic energy eke in the labrador sea is investigated with a suite of numerical integrations using a regional ocean model simulations are performed over the period 1980 2001 and are compared to satellite observations over the last 9 years the surface eke pattern in the basin is dominated by a region along the west coast of greenland where eddies mainly anticyclonic are formed by instability of the main currents flowing over the continental slope consistent with previous idealized results here the interannual changes are linked to the shear of the incoming boundary current system imposed as boundary condition to the model domain the highly variable strength of the east greenland current at the northeast boundary derived from the simple ocean data assimilation soda reanalysis strongly influences the vortex formation in the center of the labrador sea where deep convection occurs a statistically significant portion of the modeled interannual surface eke variability is correlated with the local atmospheric forcing and both heat and wind fluxes play an important role and can be adopted as predictors at a lag of 2 3 months the arctic oscillation index can also be used as a remote indicator of the atmospheric fluxes but with lower skill than local measurements in contrast the north atlantic oscillation index does not correlate significantly with the surface eke at intraseasonal and interannual scales the analysis of altimeter data over the 1993 2001 supports the existence of this asymmetry between the regime locally forced by the atmosphere in the central basin and the regime remotely forced by the incoming boundary current along the west greenland coast those results have important implications for monitoring and predicting the surface eddy kinetic energy variability in the labrador sea

labrador sea water in the eastern north atlantic part ii mixing dynamics and the advective diffusive balance abstract deep wintertime convection in the labrador basin produces labrador sea water lsw which spreads throughout the north atlantic the lsw is not formed with constant characteristics but varies on decadal timescale these changes in the source of lsw propagate throughout the north atlantic having a dramatic impact on middepth properties in regions far removed from the labrador basin here by means of a property variance correlation analysis on isopycnal surfaces through the core of lsw the authors show that the boundary anomalies of lsw survive the effects of mixing over the length and timescales taken for lsw to reach the eastern north atlantic typically the anomaly amplitude is reduced by a factor of 5 during transit and from a simple estimate the integrated timescale for this reduction is seven years uncertainties in the oxygen anomaly analysis exclude specific interpretation but constrain the boundary condition variability to about 5 the lateral effect of unresolved processes resembl

hydrography of the labrador sea during active convection the hydrographic structure of the labrador sea during wintertime convection is described the cruise part of the deep convection experiment took place in february march 1997 amidst an extended period of strong forcing in an otherwise moderate winter because the water column was preconditioned by previous strong winters the limited forcing was enough to cause convection to approximately 1500 m the change in heat storage along a transbasin section relative to an occupation done the previous october gives an average heat loss that is consistent with calibrated national centers for environmental prediction surface heat fluxes over that time period 200 wm 22 deep overturning was observed both seaward of the western continental slope which was expected as well as within the western boundary current itself something that had not been directly observed previously these two geographical regions separated by roughly the 3000 m isobath produce separate water mass products the offshore water mass is the familiar cold fresh dense classical labrador sea water lsw the boundary current water mass is a somewhat warmer saltier lighter vintage of classical lsw though in the far field it would be difficult to distinguish these products the offshore product was formed within the cyclonic recirculating gyre measured by lavender et al in a region that is limited to the north most likely by an eddy flux of buoyant water from the eastern boundary current the velocity measurements taken during the cruise provide a transport estimate of the boundary current throughput and offshore recirculation finally the overall trends in stratification of the observed mixed layers are described

changes in the properties and distribution of the intermediate and deep waters in the fram strait abstract the fram strait is the only deep connection between the arctic ocean and the nordic seas greenland iceland and norwegian seas with a sill depth of approximately 2600 m consequently observations from this area reflect changes in the deep waters of the arctic mediterranean possible changes in the properties and distribution of the intermediate and deep water masses passing through the fram strait have been studied for the period from 1982 to 2008 at a zonal section at 79 n with special emphasis on the period from 1997 to 2008 the temperature of the arctic intermediate water shows large interannual variability where the period from 2002 to 2004 was especially cold indicating a strong winter convection in the nordic seas prior to those years the deep water originating from the norwegian sea has become warmer by about 0 10 c during the study period the changes in the properties of the norwegian sea deep water are caused by changes in its composition i e an increasing fraction of the eurasian basin deep water and a decreasing fraction of the greenland sea deep water each year if this trend continues the norwegian sea deep water will have similar characteristics as the eurasian basin deep water in about 15 years a prominent feature seen in the s diagrams from the deep fram strait is the disappearance of water with the characteristics of greenland sea deep water this very cold water temperature below 1 1 c was not observed after 1997 and the minimum temperature in the strait increased by 0 24 c between 1982 and 2008 in the fram strait the fraction of intermediate waters has increased while the fraction of deep waters from the nordic seas has decreased the fraction of deep waters originating from the arctic ocean shows no trend

isow spreading and mixing as revealed by deep argo floats launched in the charlie gibbs fracture zone to improve our understanding of deep circulation we deployed five deep argo floats 0 4 000 m in the charlie gibbs fracture zone cgfz which channels the flow of iceland scotland overflow water isow a dense water mass of the north atlantic ocean the floats were programed to drift at 2 750 dbar in the isow layer the floats mainly moved westward in the cgfz although some of them followed different routes for few cycles depending on northward intrusions of the north atlantic current over the cgfz one float revealed a direct route for isow from cgfz to the deep western boundary current at flemish cap in the cgfz oxygen data acquired by the floats revealed that the isow layer characterized by salinity higher than 34 94 and density greater than 27 8 kg m was mainly composed of the highly oxygenated isow and the less oxygenated north east atlantic deep water neadw a complex water mass from the east atlantic in the isow layer the relative contribution of isow was generally larger in the northern valley than in the southern valley of cgfz northward intrusions of the north atlantic current above the cgfz increased the relative contribution of neadw in the northern valley and favors mixing between isow and neadw the isow neadw signal flowing westward from the cgfz toward the deep western boundary current was progressively diluted by labrador sea water and denmark strait overflow water oxygen measurements from deep argo floats are essential for a better understanding and characterization of the mixing and spreading of deep water masses

synoptic sections of the denmark strait overflow we report on a rapid high resolution survey of the denmark strait overflow dso as it crosses the sill the first such program to incorporate full water column velocity profiles in addition to conventional hydrographic measurements seven transects with expendable profilers over the course of one week are used to estimate volume transport as a function of density our observations reveal the presence of a strongly barotropic flow associated with the nearly vertical front dividing the arctic and atlantic waters the seven section mean transport of water denser than 27 8 is 2 7 0 6sv while the mean transport of water colder than 2 0 c is 3 8 0 8 sv although this is larger than the 2 9 sv of 2 c water measured by a 1973 current meter array we find that a sampling of our sections equivalent to the extent of that array also measures 2 9sv of cold water both the structure and magnitude of the measured flow are reproduced well by a high resolution numerical model of buoyancy driven exchange with realistic topography

transport of the north atlantic deep western boundary current about 39 n 70 w 2004 2008 abstract begun in spring 2004 a sustained measurement program line w is returning high resolution observations of the north atlantic s deep western boundary current dwbc southeast of new england the study focuses on the cold limb of the atlantic meridional overturning circulation near the boundary between the subpolar and subtropical gyres the field study consists of a 6 element continental slope spanning moored array on a line underlying an altimeter satellite ground track and periodic reoccupations of a full depth hydrographic section along the line extending from the continental shelf towards bermuda here data from the first 4 years of the array may 10 2004 april 9 2008 are analyzed along with 9 realizations of the section the array a mix of moored profiler and discrete fixed depth instrument moorings returned temperature salinity and horizontal velocity data with various temporal and depth resolutions after averaging to filter inertial tidal and other high frequency motions the combined moored data set was binned to the lowest common temporal resolution of 5 days the nominal burst sample interval of the moored profilers and interpolated to 2 dbar vertical resolution temperature salinity dissolved oxygen tracer chemical concentrations and direct velocity data were acquired on the hydrographic cruises the present work focuses on the 4 year mean and time varying meridional transport in 4 layers bounded by neutral density surfaces upper and classical labrador sea waters iceland scotland overflow waters and denmark strait overflow waters the 5 d 4 layer summed meridional transport estimates range between 3 5 and 79 9 sv with a record mean average transport of 25 1 sv and standard deviation of 12 5 sv bias adjustment to account for the finite width of the moored array increases the 4 layer mean transport estimate to 28 7 sv at time scales longer than about 1 month the variations in equatorward dwbc transport appear correlated with meridional position of the gulf stream north wall with stronger transport observed when the stream is displaced south

rapid freshening of the deep north atlantic ocean over the past four decades the overflow and descent of cold dense water from the sills of the denmark strait and the faroe shetland channel into the north atlantic ocean is the principal means of ventilating the deep oceans and is therefore a key element of the global thermohaline circulation most computer simulations of the ocean system in a climate with increasing atmospheric greenhouse gas concentrations predict a weakening thermohaline circulation in the north atlantic as the subpolar seas become fresher and warmer1 2 3 and it is assumed that this signal will be transferred to the deep ocean by the two overflows from observations it has not been possible to detect whether the ocean s overturning circulation is changing but recent evidence suggests that the transport over the sills may be slackening4 here we show through the analysis of long hydrographic records that the system of overflow and entrainment that ventilates the deep atlantic has steadily changed over the past four decades we find that these changes have already led to sustained and widespread freshening of the deep ocean

the origin of norwegian sea deep water a nearly homogeneous water mass the norwegian sea deep water is found below 2000 m depth in the norwegian and lofoten basins of the norwegian sea recent observations indicate that this water is a mixture of relatively cold and fresh greenland sea deep water with warmer saltier eurasian basin deep water from the arctic ocean we have found this mixture along the western and southern periphery of the greenland sea near the level where the pressure compensated densities of the parent water masses are equal the along isopycnal mixing produces a remarkably uniform water mass which can be traced only a short distance away from its entry into the norwegian sea through gaps in the mid ocean ridge north of jan mayen island direct measurements of flow through these gaps confirm motion in the proper sense to accomplish this connection

analysis of flow path around the sealing section hg a experiment in the mont terri rock laboratory as part of the hg a experiment in the mont terri rock laboratory large scale in situ water gas injection experiments was conducted in a microtunnel this research work focuses on the numerical analysis of the experimental data and the in situ observations concerning a temporary change of the hydromechanical properties of opalinus clay during experimental operations three phases were numerically interpreted these included the generation of excavation damaged zone during tunnel excavation in which highly permeable flow paths around the tunnel have been formed the self sealing effect during water tests and the pressure evolution during a long term gas injection test a coupled two phase flow and mechanics model taking into account the strong anisotropic properties of opalinus clay was developed to interpret the measured data the hydraulic anisotropy was described by a transversely isotropic permeability tensor an elasto plastic model was established to consider both stiffness anisotropy and strength anisotropy anisotropic plasticity was studied using the microstructure tensor method

long term coordinated changes in the convective activity of the north atlantic the north atlantic is a peculiarly convective ocean the convective renewal of intermediate and deep waters in the labrador sea and greenland iceland sea both contribute significantly to the production and export of north atlantic deep water thus helping to drive the global thermohaline circulation while the formation and spreading of 18 degree water at shallow to intermediate depths off the us eastern seaboard is a major element in the circulation and hydrographic character of the west atlantic for as long as time series of adequate precision have been available to us it has been apparent that the intensity of convection at each of these sites and the hydrographic character of their products have been subject to major interannual change as shown by aagaard 1968 clarke et al 1990 and meincke et al 1992 for the greenland sea in the ows bravo record from the labrador sea eg lazier 1980 et seq and at the panulirus hydrostation s site in the northern sargasso off bermuda eg jenkins 1982 talley and raymer 1982 this paper reviews the recent history of these changes showing that the major convective centres of the greenland and labrador seas are currently at opposite convective extrema in our postwar record with vertical exchange at the former site limited to 1000 m or so but with labrador sea convection reaching deeper than previously observed to over 2300 m as a result greenland sea deep water has become progressively warmer and more saline since the early 70 s due to increased horizontal exchange with the arctic ocean through fram strait while the labrador sea water has become progressively colder and fresher over the same period through increased vertical exchange most recently convection has become deep enough there to reach into the more saline nadw which underlies it so that cooler but now saltier and denser lsw has resulted

eddies of newly formed upper labrador sea water a new type of submesoscale eddy has been observed south of the labrador sea during late winter embedded in equatorward flow along the western boundary the eddy radius is 20 km with a weak dynamic signature swirl speed of 0 5 cm s the center of the eddy is characterized by weak stratification elevated concentrations of oxygen and anthropogenic tracers and low tracer ages all indicative of newly ventilated water strong lateral intrusions distort the shape of the feature the water mass contained in the eddy is not classical labrador sea water from the central labrador sea but is significantly fresher and hence lighter it is of the correct density to be the source of the high chlorofluorocarbon layer of the shallow deep western boundary current observed further south and hence is termed upper labrador sea water using a combination of hydrographic data sets along the western boundary to implement a simple lateral diffusion model it is shown that such eddies decay of the order of several months and are difficult to observe equatorward of the grand banks of newfoundland this is in contrast to deeper lenses of classical labrador sea water which persist further equatorward tracer derived ages of the upper labrador sea water eddy range from 3 to 5 years much older than the lifetime deduced from their lateral diffusion a simple convection model of tracer age shows that this age discrepancy is caused by gas exchange being unable to maintain equilibrium between the deep convecting mixed layer and the atmosphere during formation

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

recirculating components to the deep boundary current of the northern north atlantic abstract the meridional overturning system of the north atlantic is conventionally thought of as transporting warm water 4 c to high latitudes and covd water to low latitudes the northern cold water sources for this system are dense overflow from the nordic seas and less dense water from the labrador sea lsw the overflow components are carried westward by a deep northern boundary current dnbc that is shown here actually to begin southeast of the overflow regions the dnbc lies along the southern side of a system of islands and submarine ridges that divides the nordic seas from the subpolar basins the overflows act to increase the transport of the dnbc and this transport is further increased through the entrainment of warm waters once inside the labrador sea the dnbc is joined by lsw before continuing south to low latitudes as a cold deep western boundary current dwbc however published transport estimates are larger than can be explained by warm water entrainment alone thus indicating entrainment also of recirculating cold components the subject of this paper their sources are shown to be the lsw and cold abyssal waters originating from the southern hemisphere the lsw is entrained downward into the denser part of the dnbc and laterally into the upper part of the dwbc whereas the cold abyssal waters are supplied by an eastern intensified northward flow in the eastern atlantic that serves as the initial source of the dnbc and by a similar northward flow in the western atlantic the meridional overturning system described includes recirculations and poleward transport of cold water in addition tothe components described by the conventional system the cold abyssal waters in both the eastern and western basins have relatively low levels of oxygen and high concentrations of silicate reflecting their southern origin but their influence in the northwestern atlantic is somewhat obscurred by strong recirculating cyclonic gyres in the newfoundland and labrador basins this influence is detected by a deep silicate maximum extending poleward from the mid latitude western atlantic through the two gyres with eastern concentration and recirculating back diluted toward the south in the dwbc within the respective basins such a maximum also extends northward through the irminger basin into the dnbc but there is an ambiguity as to whether this is a direct extension of the signal from the south in the western basin or if it comes from the mid latitude eastern basin by way of westward flow through the charlie gibbs fracture zone it is perhaps consequence of both estimates of the volume transports of the various recirculating cold components indicate that they supply about as much water to the deep boundary currents as do the combined cold water sources in the north

a 4500 year reconstruction of sea surface temperature variability at decadal time scales off north iceland marine paleo records acquired at high temporal resolution provide critical data for testing numerical climate models and help to understand processes underlying ocean variability this study presents a unique 4500 year reconstruction of sea surface temperature sst obtained from alkenones in the north atlantic polar front area off north iceland at an average temporal resolution of 4 5 years spectral analysis of this signal shows dominant multidecadal oscillations which occurred with a stronger amplitude between 2500 and 4200 years bp hand in hand with fluctuations of bottom currents indicated by paleomagnetic proxies contemporaneous large excursions of the inter tropical convergence zone itcz are also recorded by the distant cariaco titanium time series suggesting a link with low latitude atlantic climate we speculate that the oscillations reflect changes of the meridional overturning circulation moc induced by increased enso activity

labrador current variability over the last 2000 years the ice loaded labrador current lc is an important component of the western north atlantic circulation that influences the position and strength of the northern limb of the north atlantic current nac this flow of cold and fresh polar waters originating from the arctic has a marked impact on the north atlantic climate yet little is known about its variability beyond the instrumental period in this study we present the first sub decadal alkenone based 2000 year long sea surface temperature sst records from the western labrador sea a climatically crucial region at the boundary between the lc and the nac our results show a clear link between the lc strength and the northern annular mode nam with a stronger nam and a more vigorous lc during the medieval climate anomaly mca this suggests enhanced lc activity upon future global warming with implications for the atlantic meridional overturning circulation amoc

variability on the century time scale and regime changes in a stochastically forced zonally averaged ocean atmosphere model the response of a zonally averaged climate model of the atmosphere ocean system to a stochastic freshwater forcing depends on its strength three regimes are found for small perturbations of the system the ocean responds with irregular oscillations around a state which is characterized by a strong atlantic overturning circulation for intermediate to large amplitude forcing the thermohaline circulation in the atlantic ocean exhibits pronounced regular overturning flushing events the third regime is a complete collapse of the atlantic overturning transitions can be induced between the three regimes while for moderate values the transitions are reversible for large forcing amplitudes the atlantic overturning circulation collapses irreversibly to the third regime

spreading velocities and dilution of north atlantic deep water in the tropical atlantic based on cfc time series 1 repeated observations of cfc 11 and cfc 12 in the western tropical atlantic over the period from 1990 to 2002 are used to fit parameters of the age spectra for several north atlantic deep water components the upper labrador sea water ulsw the labrador sea water lsw and the lower north atlantic deep water lnadw the most robust results are the mean spreading velocities u from the formation region in the labrador sea and the denmark strait of 1 5 0 3 cm s for the two lsw water masses and 2 0 0 2 cm s for lnadw respectively these values are equivalent to an age of about 15 22 years for all three components at 16 n and 21 31 years at the equator at 35 w the results are compatible with simpler tracer methods as cfc ratios or effective ages the fractions f of the young component i e the fraction of the water mass ventilated in the last 50 years turned out to be quite small ranging from 25 to less than 5 the downstream decrease of f indicates mixing of young nadw with cfc poor older nadw vintages and or mixing with other cfc free water masses from the southern hemisphere the lowest fractions were found in the lnadw with a sharp downstream decrease from the equator at 35 w to 10 s probably caused by the bifurcation into a zonal flow along 2 s and a southward flow along the brazilian coast unlike the small uncertainties estimated for the inferred velocities isopycnal diffusivities range from 100 to 10000 m2 s the temporal evolution of f in the lsw shows a maximum propagating from 7 n in 1996 downstream to 5 s in the deep western boundary current and along 2 s to 23 w we interpret this signal as the arrival of the intensely ventilated lsw formed in 1972 1976 decreasing fractions of the ulsw and lsw south of the equator between 2000 and 2002 are presumably caused by local dynamics

on the deep and bottom waters of the greenland sea from summer 1989 and 1990 data the deep waters of the central and southern greenland sea are examined using bartlett 1989 and 1990 summer data displayed at very high temperature and salinity resolutions the mixing of arctic ocean deep water aodw with greenland sea deep water gsdw in proportions of about 1 1 to form new norwegian sea deep water nsdw is confirmed however the previously accepted route of this new nsdw through the ridge system into the norwegian sea the jan mayen channel is shown to pass deep water only intermittently a passage through mohn s ridge farther north beginning near 72 06 n 2 w is identified as a more likely route part of the ultimate aodw gsdw mixture is shown to continue on along the greenland continental slope into the iceland sea a peculiar isothermal condition was found in the deeper parts of the gsdw this is postulated to result from upward diffusion of temperature from a layer initially having the adiabatic temperature gradient above the isothermal layer is one of warming and decreasing salinity the result of recent successive deep convections to different depths

year to year reoutcropping of eighteen degree water in an eddy resolving ocean simulation abstractwinter outcropping of the eighteen degree water edw and its subsequent dispersion are studied using a eddy resolving simulation of the family of linked atlantic modeling experiments flame outcropped edw columns in the model simulations are detected in each winter from 1990 to 1999 and particles are deployed in the center of each outcropped edw column subsequently the trajectories of these particles are calculated for the following 5 yr the particles slowly spread away from the outcropping region into the nonoutcropping subducted edw region south of 30 n and eventually to the non edw region in the greater subtropical gyre approximately 30 of the particles are found in non edw waters 1 yr after deployment after 5 yr only 25 of the particles are found within edw the reoutcropping time is defined as the number of years between when a particle is originally deployed in an outcropping edw column and when that particle is next found in an outcropping edw column of the particles 66 ar

patterns of low frequency monthly sea level pressure variability 1899 1986 and associated wave cyclone frequencies abstract spatial patterns of low frequency sea level pressure slp variability are identified by performing rotated principal component analysis rpca on a long term 1899 1986 northern hemisphere gridded dataset the analysis is limited to the region 160 e eastward to 40 e due to missing data early in the century the objective is to identify a comprehensive set of highly recurrent slp teleconnection patterns to examine some aspects of their seasonality and to identify the associated mean winter pressure fields and cyclone frequencies occurring at times of opposite eigenvector polarity the results are further described in the context of the southern oscillation and known midtropospheric teleconnection patterns four low frequency variability patterns are identified over the atlantic european sector including i the north atlantic 0scillation nao and spatial patterns with slp variability centers over ii the eastern atlantic eatl iii southern europe and the northern mediterranean basin s

meridional flow variability over the nordic seas in the arctic oscillation framework an anomalous recurring atmospheric circulation pattern of high relevance for the climate of the nordic seas and siberia is identified it is found as the second empirical orthogonal function eof of monthly winter sea level pressure slp anomalies poleward of 30 n where the leading eof is the arctic oscillation ao the most prominent centre of action of the circulation pattern is located over the barents region this barents oscillation bo is shown to have a high temporal correlation with the sensible heat loss of the nordic seas r 0 76 the bo also correlates to eurasian surface air temperature sat anomalies with r 0 72 after the ao related sat variations are removed by means of a linear regression two sets of slp composites are constructed where one is based on low and high nordic seas heat loss months and the other is based on warm and cold eurasian months patterns reminiscent of the bo emerge in the two composites when ao related variability is removed

the production of north atlantic deep water sources rates and pathways updating an earlier account by dickson et al 1990 this paper reviews the initial development phase of north atlantic deep water nadw production from the points where the dense inflows from nordic seas cross the greenland scotland ridge to the point off south greenland where the buildup of new production appears almost complete in particular three long term current meter arrays totaling 91 instruments and set at 160 km intervals south from the denmark strait sill are used to validate earlier short term arrays by others and in combination with these earlier arrays to describe the downstream evolution of mean speed depth and entrainment the variability of the overflow current in space and time and the likely contribution of the other three main constituents of nadw production at densities greater than 27 8 from the points of overflow 5 6 sv the transport within this range increases by entrainment and confluence with other contributory streams to around 13 3 sv at cape farewell while recirculating elements prevent us from determining the net southgoing transport a nadw transport of this order appears consistent with recent estimates of net abyssal flow passing south through the north and south atlantic

stochastic stability of open ocean deep convection open ocean deep convection is a highly variable and strongly nonlinear process that plays an essential role in the global ocean circulation a new view of its stability is presented here in which variability as parameterized by stochastic forcing is central the use of an idealized deep convection box model allows analytical solutions and straightforward conceptual understanding while retaining the main features of deep convection dynamics in contrast to the generally abrupt stability changes in deterministic systems measures of stochastic stability change smoothly in response to varying forcing parameters these stochastic stability measures depend chiefly on the residence times of the system in different regions of phase space which need not contain a stable steady state in the deterministic sense deep convection can occur frequently even for parameter ranges in which it is deterministically unstable this effect is denoted wandering unimodality the stochastic stability concepts are readily applied to other components of the climate system the results highlight the need to take climate variability into account when analyzing the stability of a climate state