the devonian lacustrine sediments of orkney scotland implications for climate cyclicity basin structure and maturation history the orcadian basin of northern scotland largely of middle devonian age is a major lacustrine rift basin with the widely correlateable achanarras sandwick lake bed extending for at least 800 km along the rift above this lake bed there are about 540 m of cyclic lacustrine sediments with lake sedimentation terminated by alluvial fan progadation the upper stromness flagstone formation and the rousay flagstone formation making up this lacustrine sequence are given revised and precise definitions within four reference sections from the orkney islands the sequence consists of about 45 first order cycles averaging about 12 m in thickness which resulted from long term climatic fluctuations in rainfall with a cycle time scale of about 25 000 years longer climatic fluctuations of about 100000 and 400000 years are also present these periods correspond to the precession and two eccentricity orbital cycles detailed knowledge of the stratigraphy constrains the structural and maturation history of the basin the basin formed in a series of extensional half graben and the orkney area suffered later inversion the lake sediments first reached the oil window at relatively shallow depths c 2000 m probably in the early carboniferous implying very high geothermal gradients during the early part of the basin history as corroborated by the presence of devonian volcanics in the region in the orkney area uplift probably started in the late carboniferous

dynamics of decadal variability in the atlantic subpolar gyre a stochastically forced oscillator internal variability of the atlantic subpolar gyre is investigated in a 600 years control simulation of a comprehensive coupled climate model the subpolar gyre shows irregular oscillations of decadal time scale with most spectral power between 15 and 20 years positive and negative feedback mechanisms act successively on the circulation leading to an internal oscillation this involves periodically enhanced deep convection in the subpolar gyre center and intermittently enhanced air sea thermal coupling as a result anomalies of the large scale atmospheric circulation can be transferred to the ocean on the ocean s intrinsic time scale exciting the oscillator stochastically a detailed understanding of oscillatory mechanisms of the ocean and their sensitivity to atmospheric forcing holds considerable potential for decadal predictions as well as for the interpretation of proxy data records

decadal variability of subpolar gyre transport and its reverberation in the north atlantic overturning 1 analyses of sea surface height ssh records based on satellite altimeter data and hydrographic properties have suggested a considerable weakening of the north atlantic subpolar gyre during the 1990s here we report hindcast simulations with high resolution ocean circulation models that demonstrate a close correspondence of the ssh changes with the volume transport of the boundary current system in the labrador sea the 1990s decline of about 15 of the long term mean appears as part of a decadal variability of the gyre transport driven by changes in both heat flux and wind stress associated with the north atlantic oscillation nao the changes in the subpolar gyre as manifested in the deep western boundary current off labrador reverberate in the strength of the meridional overturning circulation moc in the subtropical north atlantic suggesting the potential of a subpolar transport index as an element of a moc monitoring system

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

deep water formation and meridional overturning in a high resolution model of the north atlantic the authors use different versions of the model of the wind and thermohaline driven circulation in the north and equatorial atlantic developed under the woce community modeling effort to investigate the mean flow pattern and deep water formation in the subpolar region and the corresponding structure of the basin scale meridional overturning circulation transport a suite of model experiments has been carded out in recent years differing in horizontal resolution 1 1 2 1 3 0 4 1 6 0 2 thermohaline boundary conditions and parameterization of small scale mixing the mass transport in the subpolar gyre and the production of north atlantic deep water nadw appears to be essentially controlled by the outflow of dense water from the greenland and norwegian seas in the present model simulated by restoring conditions in a buffer zone adjacent to the boundary near the greenland scotland ridge deep winter convection homogenizes the water column in the center of the labrador sea to about 2000 m the water mass properties potential temperature about 3 c salinity about 34 9 psu and the volume 1 1 1053 km3 of the homogenized water are in fair agreement with observations the convective mixing has only little effect on the net sinking of upper layer water in the subpolar gyre sensitivity experiments show that the export of nadw from the subpolar north atlantic is more strongly affected by changes in the overflow conditions than by changes in the surface buoyancy fluxes over the labrador and irminger seas even if these suppress the deep convection completely the host of sensitivity experiments demonstrates that realistic meridional overturning and heat transport distributions for the north atlantic with a maximum of 1 pw can be obtained with nadw production rates of 15 16 sv provided the spurious upwelling of deep water that characterizes many model solutions in the gulf stream regime is avoided by adequate horizontal resolution add mixing parameterization

the 8 2 ka event abrupt transition of the subpolar gyre toward a modern north atlantic circulation 1 climate model simulations of the 8 2 ka event show an abrupt strengthening of the atlantic subpolar gyre that allows us to connect two major but apparently contradictory climate events of the early holocene the freshwater outburst from proglacial lakes and the onset of labrador sea water formation the 8 2 ka event is the largest climatic signal of our present interglacial with a widespread cooling in the north atlantic region about 8200 years before present it coincides with a meltwater outburst from north american proglacial lakes that is believed to have weakened the atlantic meridional overturning circulation and northward heat transport followed by a recovery of the deep ocean circulation and rising temperatures after a few centuries marine proxy data however date the onset of deep water formation in labrador sea to the same time the subsequent strengthening of the slope current system created a regional signal recorded as an abrupt and persistent surface temperature decrease although similarities in timing are compelling a mechanism to reconcile these apparently contradictory events was missing our simulations show that an abrupt and persistent strengthening of the atlantic subpolar gyre provides a plausible explanation the intense freshwater pulse triggered a transition of the gyre circulation into a different mode of operation stabilized by internal feedbacks and persistent after the cessation of the perturbation as a direct consequence deep water formation around its center intensifies this corresponds to the modern flow regime and stabilizes the meridional overturning circulation possibly contributing to the holocene s climatic stability

low frequency interaction between horizontal and overturning gyres in the ocean 1 low frequency variability of the horizontal circulation in an idealized eddy permitting numerical model drives the dominant mode of low frequency variability in the meridional overturning circulation this coupling takes place through the influence of lateral advection in the cyclonic high latitude boundary current on the mixed layer depth along the boundary the mean and low frequency variability of the meridional overturning circulation are well predicted by a diagnostic estimate that assumes the downwelling is controlled by the thermal wind shear within the mixed layer along the boundary which is in turn determined by a simple balance between lateral advection and surface cooling the more general result is the demonstration that the mean and low frequency variability of the meridional overturning streamfunction are controlled by the baroclinic pressure gradient within the mixed layer along the boundary which may be influenced by numerous factors such as low frequency variability in lateral advection wind stress surface buoyancy fluxes or ice melt and freshwater runoff

response of the north atlantic dynamic sea level and circulation to greenland meltwater and climate change in an eddy permitting ocean model the response of the north atlantic dynamic sea surface height ssh and ocean circulation to greenland ice sheet gris meltwater fluxes is investigated using a high resolution model the model is forced with either present day like or projected warmer climate conditions in general the impact of meltwater on the north atlantic ssh and ocean circulation depends on the surface climate in the two major regions of deep water formation the labrador sea and the nordic seas the basin mean ssh increases with the increase of the gris meltwater flux this ssh increase correlates with the decline of the atlantic meridional overturning circulation amoc however while in the labrador sea the warming forcing and gris meltwater input lead to sea level rise in the nordic seas these two forcings have an opposite influence on the convective mixing and basin mean ssh relative to the global mean the warming leads to less sea ice cover in the nordic seas which favours stronger surface heat loss and deep mixing lowering the ssh and generally increasing the transport of the east greenland current in the labrador sea the increased ssh and weaker deep convection are reflected in the decreased transport of the labrador current lc which closes the subpolar gyre in the west among the two major components of the lc transport the thermohaline and bottom transports the former is less sensitive to the gris meltwater fluxes under the warmer climate the ssh difference across the lc which is a component of the bottom velocity correlates with the long term mean amoc rate

surprising return of deep convection to the subpolar north atlantic ocean in winter 2007 2008 in the process of open ocean convection in the subpolar north atlantic ocean surface water sinks to depth as a distinct water mass the characteristics of which affect the meridional overturning circulation and oceanic heat flux in addition carbon is sequestered from the atmosphere in the process in recent years this convection has been shallow or non existent which could be construed as a consequence of a warmer climate here we document the return of deep convection to the subpolar gyre in both the labrador and irminger seas in the winter of 2007 2008 we use profiling float data from the argo programme to document deep mixing analysis of a variety of in situ satellite and reanalysis data shows that contrary to expectations the transition to a convective state took place abruptly without going through a phase of preconditioning changes in hemispheric air temperature storm tracks the flux of fresh water to the labrador sea and the distribution of pack ice all contributed to an enhanced flux of heat from the sea to the air making the surface water sufficiently cold and dense to initiate deep convection given this complexity we conclude that it will be difficult to predict when deep mixing may occur again

hysteresis behavior of the atlantic ocean circulation identified in observational data abstract on the basis of the multidecadal component of a fram strait sea ice export reconstruction and on a sea surface temperature proxy for changes in the ocean circulation a bifurcation diagram for two cycles of the atlantic ocean multidecadal oscillation is constructed it suggests a hysteresis behavior that is consistent with nonlinear convective adjustment to changes in deep water formation in the north atlantic and with the meridional overturning circulation bistability associated with two distinct configurations of the north atlantic subpolar gyre simulated in response to idealized carbon dioxide increase the nonlinear dynamics of the atlantic ocean circulation emphasized here on the basis of observational data adds to the hysteresis behavior shown in theoretical and numerical studies

decline of subpolar north atlantic circulation during the 1990s observations of sea surface height reveal that substantial changes have occurred over the past decade in the mid to high latitude north atlantic ocean topex poseidon altimeter data show that subpolar sea surface height increased during the 1990s and the geostrophic velocity derived from altimeter data exhibits declining subpolar gyre circulation combining the data from earlier satellites we find that subpolar circulation may have been weaker in the late 1990s than in the late 1970s and 1980s direct current meter observations in the boundary current of the labrador sea support the weakening circulation trend of the 1990s and together with hydrographic data show that the mid to late 1990s decline extends deep in the water column analysis of the local surface forcing suggests that the 1990s buoyancy forcing has a dynamic effect consistent with altimetric and hydrographic observations a weak thermohaline forcing allows the decay of the domed structure of subpolar isopycnals and weakening of circulation

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

reversed north atlantic gyre dynamics in present and glacial climates the dynamics of the north atlantic subpolar gyre spg are assessed under present and glacial boundary conditions by investigating the spg sensitivity to surface wind stress changes in a coupled climate model to this end the gyre transport is decomposed in ekman thermohaline and bottom transports surface wind stress variations are found to play an important indirect role in spg dynamics through their effect on water mass densities our results suggest the existence of two dynamically distinct regimes of the spg depending on the absence or presence of deep water formation dwf in the nordic seas and a vigorous greenland scotland ridge gsr overflow in the first regime the gsr overflow is weak and the spg strength increases with wind stress as a result of enhanced outcropping of isopycnals in the centre of the spg as soon as a vigorous gsr overflow is established its associated positive density anomalies on the southern gsr slope reduce the spg strength this has implications for past glacial abrupt climate changes insofar as these can be explained through latitudinal shifts in north atlantic dwf sites and strengthening of the north atlantic current regardless of the ultimate trigger an abrupt shift of dwf into the nordic seas could result both in a drastic reduction of the spg strength and a sudden reversal in its sensitivity to wind stress variations our results could provide insight into changes in the horizontal ocean circulation during abrupt glacial climate changes which have been largely neglected up to now in model studies

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

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

on the connection between dense water formation overturning and poleward heat transport in a convective basin an isopycnal two layer idealized model for a convective basin is proposed consisting of a convecting interior region and a surrounding boundary current buoyancy and wind driven parameterized eddy fluxes govern the exchange between the two to balance the interior buoyancy loss the boundary current becomes denser as it flows around the basin geostrophy imposes that this densification be accompanied by sinking in the boundary current and hence by an overturning circulation the poleward heat transport associated with convection in the basin can thus be viewed as a result of both an overturning and a horizontal circulation when adapted to the labrador sea the model is able to reproduce the bulk features of the mean state the seasonal cycle and even the shutdown of convection from 1969 to 1972 according to the model only 40 of the poleward heat buoyancy transport of the labrador sea is associated with the overturning circulation an exact solution is presented for the linearized equations when changes in the boundary current are small numerical solutions are calculated for variations in the amount of convection and for changes in the remotely forced circulation around the basin these results highlight how the overturning circulation is not simply related to the amount of dense water formed a speeding up of the circulation around the basin due to wind forcing for example will decrease the intensity of the overturning circulation while the dense water formation remains unvaried in general it is shown that the fraction of poleward buoyancy or heat transport carried by the overturning circulation is not an intrinsic property of the basin but can vary as a result of a number of factors

bistability of the atlantic subpolar gyre in a coarse resolution climate model 1 high resolution models indicate that the dynamics of the nordic seas and the subpolar gyre is crucial for deep water formation and overturning circulation in the atlantic during the last decades significant changes in ocean properties have been observed in this region we show that large scale dynamics as captured by coarse resolution climate models allows for a bistability of the atlantic subpolar gyre we suggest three positive feedbacks which yield the necessary nonlinearity for this behavior transitions between the two states can be triggered by small fluctuations in surface freshwater flux to the nordic seas only slightly larger than natural variability presently observed in the region

the sensitivity of the greenland scotland ridge overflow to forcing changes processes that influence the volume and heat transport across the greenland scotland ridge system are investigated in a numerical model with horizontal resolution the focus is on the sensitivity of cross ridge transports and the reaction of the subpolar north atlantic ocean circulation to changes in wind stress and buoyancy forcing on seasonal to interannual timescales a general relation between changes in wind stress or cross ridge density contrasts and the overturning transport of greenland iceland norwegian seas source water is established from a series of idealized experiments the relation is used subsequently to interpret changes in an experiment over the years 1992 97 with realistic forcing on seasonal and interannual timescales there is a clear correlation between heat flux and wind stress curl variability the realistic model suggests a steady decrease in the strength of the cyclonic subpolar gyre of the north atlantic with a corresponding decrease in heat transport during the 1990s

on the role of eddies and surface forcing in the heat transport and overturning circulation in marginal seas the factors that determine the heat transport and overturning circulation in marginal seas subject to wind forcing and heat loss to the atmosphere are explored using a combination of a high resolution ocean circulation model and a simple conceptual model the study is motivated by the exchange between the subpolar north atlantic ocean and the nordic seas a region that is of central importance to the oceanic thermohaline circulation it is shown that mesoscale eddies formed in the marginal sea play a major role in determining the mean meridional heat transport and meridional overturning circulation across the sill the balance between theoceaniceddyheatfluxandatmosphericcooling ascharacterizedbyanondimensionalnumber isshownto be the primary factor in determining the properties of the exchange results from a series of eddy resolving primitive equation model calculations for the meridional heat transport overturning circulation density of convective waters and density of exported waters compare well with predictions from the conceptual model over a wide range of parameter space scaling and model results indicate that wind effects are small and the mean exchange is primarily buoyancy forced these results imply that one must accurately resolve or parameterize eddy fluxes in order to properly represent the mean exchange between the north atlantic and the nordic seas and thus between the nordic seas and the atmosphere in climate models

mechanisms of multidecadal atlantic meridional overturning circulation variability diagnosed in depth versus density space abstractmultidecadal variability of the atlantic meridional overturning circulation amoc is examined based on a comparison of the amoc streamfunctions in depth and in density space in a 700 yr present day control integration of the fully coupled community climate system model version 3 the commonly used depth coordinate amoc primarily exhibits the variability associated with the deep equatorward transport that follows the changes in the labrador sea deep water formation on the other hand the density based amoc emphasizes the variability associated with the subpolar gyre circulation in the upper ocean leading to the changes in the labrador sea convection combining the two representations indicates that the 20 yr periodicity of the amoc variability in the first half of the simulation is primarily due to an ocean only mode resulting from the coupling of the deep equatorward flow and the upper ocean gyre circulation near the gulf stream and north atlantic current in addition the density based amoc

influences of time dependent precipitation on water mass transformation heat fluxes and deep convection in marginal seas abstractinfluences of time dependent precipitation on water mass transformation and heat budgets in an idealized marginal sea are examined using theoretical and numerical models the equations proposed by spall in 2012 are extended to cases with time dependent precipitation whose form is either a step function or a sinusoidal function the theory predicts the differences in temperature and salinity between the convective water and the boundary current as well as the magnitudes of heat fluxes into the marginal sea and across the sea surface moreover the theory reveals that there are three inherent time scales relaxation time scales for temperature and salinity and a precipitation time scale the relaxation time scales are determined by a steady solution of the theoretical model with steady precipitation the relaxation time scale for temperature is always smaller than that for salinity as a result of not only the difference in the form of fluxes at the surface but also the variation in the eddy transpor

interdecadal variations of the thermohaline circulation in a coupled ocean atmosphere model abstract a fully coupled ocean atmosphere model is shown to have irregular oscillations of the thermohaline circulation in the north atlantic ocean with a time scale of approximately 50 years the irregular oscillation appears to be driven by density anomalies in the sinking region of the thermohaline circulation approximately 52 n to 72 n combined with much smaller density anomalies of opposite sign in the broad rising region the spatial pattern of see surface temperature anomalies associated with this irregular oscillation bears an encouraging resemblance to a pattern of observed interdecadal variability in the north atlantic the anomalies of sea surface temperature induce model surface air temperature anomalies over the northern north atlantic arctic and northwestern europe

late eemian warming in the nordic seas as seen in proxy data and climate models 1 we analyze a transient simulation of the last glacial inception in a climate model of intermediate complexity focusing on sea ice ocean circulation dynamics in the north atlantic and nordic seas as northern high latitude summer insolation decreases toward the end of the eemian interglacial arctic sea ice export to the north atlantic increases this surface fresh water transport weakens deep water formation in the north atlantic and the near surface circulation of the subpolar gyre as a consequence the relative contribution of subpolar gyre waters to the atlantic inflow into the nordic seas is reduced giving way to more warm and saline subtropical waters from the north atlantic current we thus find an episode of relatively high heat and salt transport into the nordic seas during the last glacial inception between 119 000 and 115 000 years before present this stabilizes deep ocean convection in the region and warms scandinavia during a phase of low insolation these findings are in good agreement with proxy data from the nordic seas and north atlantic at the end of the warm interval sea surface temperature drops by about 3 c marking the onset of large scale glacier growth over scandinavia

influences of precipitation on water mass transformation and deep convection abstractthe influences of precipitation on water mass transformation and the strength of the meridional overturning circulation in marginal seas are studied using theoretical and idealized numerical models nondimensional equations are developed for the temperature and salinity anomalies of deep convective water masses making explicit their dependence on both geometric parameters such as basin area sill depth and latitude as well as on the strength of atmospheric forcing in addition to the properties of the convective water the theory also predicts the magnitude of precipitation required to shut down deep convection and switch the circulation into the haline mode high resolution numerical model calculations compare well with the theory for the properties of the convective water mass the strength of the meridional overturning circulation and also the shutdown of deep convection however the numerical model also shows that for precipitation levels that exceed this critical threshold the circulat

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

circulation and transport in the western boundary currents at cape farewell greenland the circulation and volume transports in the western boundary currents around cape farewell greenland are derived from full depth hydrographic and velocity measurements from august september 2005 the western boundary currents from surface to seafloor transport 40 5 8 1 sv southwards in the irminger sea and 53 8 10 8 sv northwards in the labrador sea the deep western boundary current dwbc defined as water with potential density greater than 27 80 kg m 3 transports 12 3 2 5 sv southwards in the irminger sea the deep water transport is reduced south of cape farewell where it changes flow direction from southwards to northwards the south corner at a section over the eirik ridge a bathymetric feature extending southwest of cape farewell the dwbc transports 8 7 1 7 sv westwards the reduction in transport at the south corner is associated with decreased velocities within the deepest layers and the volumetric loss of the most saline deep water types the observations suggest that the part of the shallow and deep western boundary currents diverge at the south corner downstream in the eastern labrador sea the deep water transport is increased to 19 7 3 9 sv northwards with the addition of recirculating denser deep waters the representativeness of the results from the semi synoptic survey are discussed with reference to companion current meter measurements of the dwbc

dilution of the northern north atlantic ocean in recent decades declining salinities signify that large amounts of fresh water have been added to the northern north atlantic ocean since the mid 1960s we estimate that the nordic seas and subpolar basins were diluted by an extra 19 000 5000 cubic kilometers of freshwater input between 1965 and 1995 fully half of that additional fresh water about 10 000 cubic kilometers infiltrated the system in the late 1960s at an approximate rate of 2000 cubic kilometers per year patterns of freshwater accumulation observed in the nordic seas suggest a century time scale to reach freshening thresholds critical to that portion of the atlantic meridional overturning circulation

rapid switches in subpolar north atlantic hydrography and climate during the last interglacial mis 5e plateauing just below early mis 5e values a planktonic d 18 o minimum during the cooling event indicates that marked freshening of the surface waters accompanied the cooling we suggest that switches in the subpolar gyre hydrography occurred during a warmer climate involving regional changes in freshwater fluxes balance and east greenland current influence in the study area the nature of these hydrographic transitions suggests that they are most likely related to large scale circulation dynamics potentially amplified by gis meltwater influences

boundary currents and watermass transformation in marginal seas the properties of watermass transformation and the thermohaline circulation in marginal seas with topography and subject to a spatially uniform net surface cooling are discussed the net heat loss within the marginal sea is ultimately balanced by lateral advection from the open ocean in a narrow boundary current that flows cyclonically around the basin heat loss in the interior is offset by lateral eddy fluxes originating in the boundary current the objectives of this study are to understand better what controls the density of waters formed within the marginal sea the temperature of the outflowing waters the amount of downwelling and the mechanisms of heat transport within the marginal sea the approach combines heat budgets with linear stability theory for a baroclinic flow over a sloping bottom to provide simple theoretical estimates of each of these quantities in terms of the basic parameters of the system the theory compares well to a series of eddy resolving primitive equation model calculations the downwelling is concentrated within the boundary current in both a diffusive boundary layer near topography and an eddy driven region on the offshore edge of the boundary current for most high latitude regions the horizontal gyre is expected to transport more heat than does the overturning gyre

open ocean convection observations theory and models we review what is known about the convective process in the open ocean in which the properties of large volumes of water are changed by intermittent deep reaching convection triggered by winter storms observational laboratory and modeling studies reveal a fascinating and complex interplay of convective and geostrophic scales the large scale circulation of the ocean and the prevailing meteorology two aspects make ocean convection interesting from a theoretical point of view first the timescales of the convective process in the ocean are sufficiently long that it may be modified by the earth s rotation second the convective process is localized in space so that vertical buoyancy transfer by upright convection can give way to slantwise transfer by baroclinic instability moreover the convective and geostrophic scales are not very disparate from one another detailed observations of the process in the labrador greenland and mediterranean seas are described which were made possible by new observing technology when interpreted in terms of underlying dynamics and theory and the context provided by laboratory and numerical experiments of rotating convection great progress in our description and understanding of the processes at work is being made

a decadal prediction case study late twentieth century north atlantic ocean heat content an ensemble of initialized decadal prediction dp experiments using the community climate system model version 4 ccsm4 shows considerable skill at forecasting changes in north atlantic upper ocean heat content and surface temperature up to a decade in advance coupled model ensembles were integrated forward from each of 10 different start dates spanning from 1961 to 2006 with ocean and sea ice initial conditions obtained from a forced historical experiment a coordinated ocean ice reference experiment with interannual forcing core ia which exhibits good correspondence with late twentieth century ocean observations from the north atlantic subpolar gyre spg region north atlantic heat content anomalies from the dp ensemble correlate highly with those from the core ia simulation after correcting for a drift bias in particular theobservedlarge rapid rise in spgheatcontentin themid 1990sis successfullypredicted in the ensemble initialized in january of 1991 a budget of spg heat content from the core ia experiment sheds light on the origins of the 1990s regime shift and it demonstrates the extent to which low frequency changes in ocean heat advection related to the atlantic meridional overturning circulation dominate temperature tendencies in this region similar budgets from the dp ensembles reveal varying degrees of predictive skill in the individual heat budget terms with large advective heat flux anomalies from the south exhibiting the highest correlation with core ia the skill of the dp in this region is thus tied to correct initialization of ocean circulation anomalies while external forcing is found to contribute negligibly and for incorrect reasons to predictive skill in this region over this time period

a possible mechanism for the strong weakening of the north atlantic subpolar gyre in the mid 1990s 1 the extent and strength of the north atlantic subpolar gyre spg changed rapidly in the mid 1990s going from large and strong in 1995 to substantially weakened in the following years the abrupt change in the intensity of the spg is commonly linked to the reversal of the north atlantic oscillation nao index changing from strong positive to negative values in the winter 1995 96 in this study we investigate the impact of the initial spg state on the subsequent behavior of the spg by means of an ocean general circulation model driven by ncep ncar reanalysis fields our sensitivity integrations suggest that the weakening of the spg cannot be explained by the change in the atmospheric forcing alone rather for the time period around 1995 the spg was about to weaken irrespective of the actual atmospheric forcing due to the ocean state governed by the persistently strong positive nao during the preceding seven years 1989 1995 our analysis indicates that it was this preconditioning of the ocean in combination with the sudden drop in the nao in 1995 96 that lead to the strong and rapid weakening of the spg in the second half of the 1990s this hypothesis explains the diverging evolution of the strength of the spg and the atmospheric forcing winter nao after 1995 as has been suggested recently

stochastic atmospheric forcing as a cause of greenland climate transitions abstractan unforced simulation of the community climate system model version 4 ccsm4 is found to have greenland warming and cooling events that resemble dansgaard oeschger cycles in pattern and magnitude with the caveat that only three transitions were available to be analyzed it is found that the transitions are triggered by stochastic atmospheric forcing the atmospheric anomalies change the strength of the subpolar gyre leading to a change in labrador sea sea ice concentration and meridional heat transport the changed climate state is maintained over centuries through the feedback between sea ice and sea level pressure in the north atlantic indications that the initial atmospheric pressure anomalies are preceded by precipitation anomalies in the western pacific warm pool are discussed the full evolution of the anomalous climate state depends crucially on the climatic background state

last interglacial mis5e hydrographic shifts linked to meltwater discharges from the east greenland margin abstract proximal evidence of the surface ocean response to size reduction of the greenland ice sheet gis during the last interglacial mis5e and preceding glacial termination t2 remains largely elusive using a new sediment record from the western iceland sea the behavior of the northeastern gis and its relation to the subpolar north atlantic surface hydrography is examined extremely light oxygen isotopic 18o values are found off central east greenland during early mis5e and point to enhanced meltwater release potentially from the northeastern sector of the gis data from downstream the cold east greenland current egc and its eastward branches suggest a far reaching effect of this meltwater not only in the nordic seas but also in the se labrador sea in particular whereas an early mis5e warming at 128 5 126 5 ka in the two regions coincided with the relative reduction of meltwater input into the egc the subsequent cooling noted at 126 5 ka followed a renewed major freshwater event off central east greenland our data further indicate persistent freshwater influence from the east greenland margin over the entire mis5e interval and in addition also reveal a late mis5e meltwater event the latter event occurred just prior to the last glacial inception and emphasizes the importance of greenland meltwater as forcing factor on interglacial climates

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

millennial scale holocene hydrological changes in the northeast atlantic new insights from la grande vasiere mid shelf mud belt a mid to late holocene paleohydrological reconstruction from the northeast atlantic is proposed through the study of a high resolution sedimentary record from the northern continental shelf of the

the climate influence on the mid depth northeast atlantic gyres viewed by cold water corals 1 the neodymium nd isotopic composition expressed in epsilon units nd of reef framework forming cold water corals provides unique measures of water mass provenance and mixing within the northeast atlantic today and in the past a reconstruction of near thermocline water nd from cold water corals of the gulf of cadiz and porcupine seabight spanning over the past 300 000 years now revealed that climate cooling during marine isotope stages mis 7 2 and mis 8 9 led to a retraction of the mid depth subpolar gyre mspg to the west conversely northern hemisphere warming and increasing fresh water fluxes to the northwest labrador sea favor a stronger eastward extension of the mspg blocking the northward flow of temperate atlantic water as observed during the early mis 1 and the early stage mis 5 5 these changes are likely the result of large scale south north displacement of the westerlies similar to present day observations that the north atlantic oscillation nao is linked with mid depth ocean circulation based on these observations we hypothesize that further climate warming will also strengthen the mspg leading to a salt and temperature decrease in the northeast atlantic whereas salinity and temperature will increase in the temperate atlantic however the amplitude of such changes on north atlantic overturning remains to be tested

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

interpretations of the jebar term abstract in diagnostic calculations of the ocean s circulation the so called jebar joint effect of baroclinicity and relief term may induce a significant depth average current field as has been noted in the oceanographic literature here we present two consistent interpretations of this term in the equation governing the vorticity of the depth averaged current a topographic vortex stretching term proportional to the dot product of depth averaged velocity and the depth gradient arises we show that jebar corrects this term by removing the contribution of the geostrophic flow referenced to the bottom which cannot generate topographic vortex stretching in the evolution equation for the vorticity of the depth integrated flow a bottom torque term is present we show that the jfbar term discussed above enters here as a contribution to the bottom torque emphasizing the role of jebar as a forcing term we also show that when a diagnostic calculation is formulated completely in terms of the transport str

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

deep water formation the subpolar gyre and the meridional overturning circulation in the subpolar north atlantic on interannual to decadal times scales model simulations suggest a strong relationship between anomalies in the deep water formation rate the strength of the subpolar gyre and the meridional overturning circulation in the north atlantic whether this is valid can only be confirmed by continuous long observational time series several measurement components are already in place but crucial arrays to obtain time series of the meridional volume and heat transport in the subpolar north atlantic are still missing here we summarize the recent developments of the deep water formation rates and the subpolar gyre transports we discuss how existing observational components in the subpolar north atlantic could be supplemented to provide long term monitoring of the meridional heat and volume transport through a combined analysis of observations and model results the temporal and spatial scales that had to be covered with instruments are discussed together with the key regions with the highest variability in the velocity and temperature fields

dynamics of isolated convective regions in the ocean abstract an initially resting ocean of stratification n is considered subject to buoyancy loss at its surface of magnitude b0 over a circular region of radius r at a latitude where the coriolis parameter is f initially the buoyancy loss gives rise to upright convection as an ensemble of plumes penetrates the stratified ocean creating a vertically mixed layer however as deepening proceeds horizontal density gradients at the edge of the forcing region support a geostrophic rim current which develops growing meanders through baroclinic instability eventually finite amplitude baroclinic eddies sweep stratified water into the convective region at the surface and transport convected water outward and away below setting up a steady state in which lateral buoyancy flux offsets buoyancy loss at the surface in this final state quasi horizontal baroclinic eddy transfer dominates upright plume convection by using parcel theory to consider the energy transformations taking place it is shown that the de

neodymium isotopic composition of deep sea corals from the ne atlantic implications for past hydrological changes during the holocene the nd isotopic composition of deep sea corals lophelia pertusa and madrepora oculata located on the rockall bank at a depth of 747 m was investigated to reconstruct past hydrological changes of the north atlantic the coral 3nd values ranged from 15 4 to 11 9 and demonstrate strong and systematic changes during the holocene most likely reflecting changes in seawater 3nd the 3nd record displays a general trend with a small decrease from 14 1 to 15 2 during the time interval from 9400 to 6000 yrs followed by an abrupt increase in 3nd towards more radiogenic 3nd values ranging from 13 9 to 11 9 these variations imply strong changes in the relative proportions of upper intermediate waters originating in the subtropical and subpolar atlantic which are characterised today by 3nd of 10 4 11 5 and 15 respectively therefore we suggest that the 3nd of reef frameworks forming deep sea corals reveals strong evidence of systematic mid holocene changes in mid depth subpolar gyre circulation a wider eastward extension of the subpolar gyre induced by stronger westerlies and a less intense deepwater formation in the labrador sea occurred during the early mid holocene supposedly larger amounts of freshwater from the subpolar gyre were carried eastward underneath the nordic seas similar to today a freshening of the subpolar gyre may have been offset by a reinforcement of the nac and higher salinities of water masses contributing to the nordic inflow beyond 6000 yrs a weakening of the subpolar gyre triggered an extension of saline subtropical water masses to the eastern north atlantic increasing the potential for labrador sea deep convection and reducing the contribution of subpolar atlantic waters to the basin scale re circulation at the upper intermediate depth and to the nordic seas inflow such modifications could have affected deep water convective activity in the northern hemisphere through a redistribution of freshwater from the early holocene melting of northern hemisphere ice sheets

hydrographic changes in the labrador sea 1960 2005 abstract the labrador sea has exhibited significant temperature and salinity variations over the past five decades the whole basin was extremely warm and salty between the mid 1960s and early 1970s and fresh and cold between the late 1980s and mid 1990s the full column salinity change observed between these periods is equivalent to mixing a 6 m thick freshwater layer into the water column of the early 1970s the freshening and cooling trends reversed in 1994 starting a new phase of heat and salt accumulation in the labrador sea sustained throughout the subsequent years it took only a decade for the whole water column to lose most of its excessive freshwater reinstate stratification and accumulate enough salt and heat to approach its record high salt and heat contents observed between the late 1960s and the early 1970s if the recent tendencies persist the basin s storages of salt and heat will fairly soon likely by 2008 exceed their historic highs the main process responsible for the net cooling and freshening of the labrador sea between 1987 and 1994 was deep winter convection which during this period progressively developed to its record depths it was caused by the recurrence of severe winters during these years and in its turn produced the deepest densest and most voluminous labrador sea water lsw1987 1994 ever observed the estimated annual production of this water during the period of 1987 1994 is equivalent to the average volume flux of about 4 5 sv with some individual annual rates exceeding 7 0 sv once winter convection had lost its strength in the winter of 1994 1995 the deep lsw1987 1994 layer lost communication with the mixed layer above consequently losing its volume while gaining heat and salt from the intermediate waters outside the labrador sea while the 1000 2000 m layer was steadily becoming warmer and saltier between 1994 and 2005 the upper 1000 m layer experienced another episode of cooling caused by an abrupt increase in the air sea heat fluxes in the winter of 1999 2000 this change in the atmospheric forcing resulted in fairly intense convective mixing sufficient to produce a new prominent lsw class lsw2000 penetrating deeper than 1300 m this layer was steadily sinking or deepening over the years following its production and is presently overlain by even warmer and apparently less dense water mass implying that lsw2000 is likely to follow the fate of its deeper precursor lsw1987 1994 the increasing stratification of the intermediate layer implies intensification in the baroclinic component of the boundary currents around the mid depth perimeter of the labrador sea the near bottom waters originating from the denmark strait overflow exhibit strong interannual variability featuring distinct short term basin scale events or pulses of anomalously cold and fresh water separated by warm and salty overflow modifications regardless of their sign these anomalies pass through the abyss of the labrador sea first appearing at the greenland side and then about a year later at the labrador side and in the central labrador basin the northeast atlantic deep water 2500 3200 m originating from the iceland scotland overflow water reached its historically freshest state in the 2000 2001 period and has been steadily becoming saltier since then it is argued that lsw1987 1994 significantly contributed to the freshening density decrease and volume loss experienced by this water mass between the late 1960s and the mid 1990s via the increased entrainment of freshening lsw the hydrostatic adjustment to expanding lsw or both

ocean gyre circulation changes associated with the north atlantic oscillation abstract observational evidence is presented for interannual to interdecadal variability in the intensity of the north atlantic gyre circulation related to the atmospheric north atlantic oscillation nao patterns a two point baroclinic pressure difference between the subtropical and subpolar gyre centers an oceanic analogue to the much used sea level pressure slp based atmospheric nao indices is constructed from time series of potential energy anomaly pea derived from hydrographic measurements in the labrador basin and at station s near bermuda representing the upper 2000 db eastward baroclinic mass transport between the two centers the transport index indicates a gulf stream and north atlantic current that gradually weakened during the low nao period of the 1960s and then intensified in the subsequent 25 years of persistently high nao to a record peak in the 1990s the peak to peak amplitude difference was 15 20 megatons per second mt s 1 with a 43 yr mean of about 60 mt s 1 a change of 25 33

two stable equilibria of the atlantic subpolar gyre the cyclonic circulation of the atlantic subpolar gyre is a key mechanism for north atlantic climate variability on a wide range of time scales it is generally accepted that it is driven by both cyclonic winds and buoyancy forcing yet the individual importance and dynamical interactions of the two contributions remain unclear the authors propose a simplified four box model representing the convective basin of the labrador sea and its shallow and deep boundary current system the western subpolar gyre convective heat loss drives a baroclinicflow of relatively light water around the dense center eddy salt flux from the boundary current to the center increases with a stronger circulation favors the formation of dense waters and thereby sustains a strong baroclinic flow approximately 10 25 of the total in contrast when the baroclinic flow is not active surface waters may be too fresh to convect and a buoyancy driven circulation cannot develop this situation corresponds to a second stable circulation mode a hysteresis is found for variations in surface freshwater flux and the salinity of the near surface boundary current an analytical solution is presented and analyzed

exploring mechanisms of variability and predictability of atlantic meridional overturning circulation in two coupled climate models abstractmultidecadal variability in the atlantic meridional overturning circulation amoc of the ocean is diagnosed in the ncar community climate system model version 3 ccsm3 and the gfdl coupled model cm2 1 common diagnostic approaches are applied to draw out similarities and differences between the two models an index of amoc variability is defined and the manner in which key variables covary with it is determined in both models the following is found i amoc variability is associated with upper ocean top 1 km density anomalies dominated by temperature on the western margin of the basin in the region of the mann eddy with a period of about 20 years these anomalies modulate the trajectory and strength of the north atlantic current the importance of the western margin is a direct consequence of the thermal wind relation and is independent of the mechanisms that create those density anomalies ii density anomalies in this key region are part of a larger scale pattern that propagates ar

north atlantic interdecadal variability oceanic response to the north atlantic oscillation 1865 1997 abstract in contrast to the atmosphere knowledge about interdecadal variability of the north atlantic circulation is relatively restricted it is the objective of this study to contribute to understanding how the north atlantic circulation responds to a forcing by the north atlantic oscillation nao on interdecadal timescales for this purpose the authors analyze observed atmospheric and sea surface temperature sst data along with the response of an ocean general circulation model to a realistic monthly surface flux forcing that is solely associated with the nao for the period 1865 1997 in agreement with previous studies it is shown that the relationship between the local forcing by the nao and observed sst anomalies on interdecadal timescales points toward the importance of oceanic dynamics in generating sst anomalies a comparison between observed and modeled sst anomalies reveals that the model results can be used to assess interdecadal variability of the north atlantic circulation the observed

characteristics of residual and colluvial soils developed on granite and of the associated pre wisconsin landforms in north central new brunswick an area in north central new brunswick of approximately 20 km2 where tors are common was found to be undisrupted by glaciation the soils in this area are developed on grus and appear to be of pr

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

response of the north atlantic subpolar gyre to persistent north atlantic oscillation like forcing the response of the north atlantic subpolar gyre spg to a persistent positive or negative phase of the north atlantic oscillation nao is investigated using an ocean general circulation model forced with idealized atmospheric reanalysis fields the integrations are analyzed with reference to a base line integration for which the model is forced with idealized fields representing a neutral state of the nao in the positive nao case the results suggest that the well known cooling and strengthening of the spg are after about 10 years replaced by a warming and subsequent weakening of the spg the latter changes are caused by the advection of warm water from the subtropical gyre stg region driven by a spin up of the atlantic meridional overturning circulation amoc and the effect of an anomalous wind stress curl in the northeastern north atlantic which counteracts the local buoyancy forcing of the spg in the negative nao case however the spg response does not involve a sign reversal but rather shows a gradual weakening throughout the integration the asymmetric spg response to the sign of persistent nao like forcing and the different time scales involved demonstrate strong non linearity in the north atlantic ocean circulation response to atmospheric forcing the latter finding indicates that analysis based on the arithmetic difference between the two nao states e g nao minus nao may hide important aspects of the ocean response to atmospheric forcing