on the linkage between antarctic surface water stratification and global deep water temperature the suggestion is advanced that the remarkably low static stability of antarctic surface waters may arise from a feedback loop involving global deep water temperatures if deep water temperatures are too warm this promotes antarctic convection thereby strengthening the inflow of antarctic bottom water into the ocean interior and cooling the deep ocean if deep waters are too cold this promotes antarctic stratification allowing the deep ocean to warm because of the input of north atlantic deep water a steady state deep water temperature is achieved such that the antarctic surface can barely undergo convection a two box model is used to illustrate this feedback loop in its simplest expression and to develop basic concepts such as the bounds on the operation of this loop the model illustrates the possible dominating influence of antarctic upwelling rate and antarctic freshwater balance on global deep water temperatures

the response of an idealized ocean basin to variable buoyancy forcing the response of an idealized ocean basin to variable buoyancy forcing is examined a general circulation model that employs a gent mcwilliams mixing parameterization is forced by a zonally constant restoring surface temperature profile which varies with latitude and time over a period p in each experiment 17 different values of p are studied ranging from 6 months to 32 000 yr the model s meridional overturning circulation moc exhibits a very strong response on all time scales greater than 15 yr up to and including the longest forcing time scales examined the peak to peak values of the moc oscillations reach up to 125 of the steady state maximum moc and exhibit resonance like behavior with a maximum at centennial to millennial forcing periods depending on the vertical diffusivity this resonance like behavior stems from the existence of two adjustment time scales one of which is set by the vertical diffusion and the other of which is set by the basin width furthermore the linearity of the response as well as its lag with the forcing varies with the forcing period the considerable deviation from the quasi equilibrium response at all time scales above 15 yr is surprising and suggests a potentially important role of the ocean circulation for climate even at milankovich time scales

global thermohaline circulation part i sensitivity to atmospheric moisture transport a global ocean general circulation model of idealized geometry combined with an atmospheric model based on observed transports of heat momentum and moisture is used to explore the sensitivity of the global conveyor belt circulation to the surface freshwater fluxes in particular the effects of meridional atmospheric moisture transports the numerical results indicate that the equilibrium strength of the north atlantic deep water nadw formation increases as the global freshwater transports increase however the global deep water formation that is the sum of the nadw and the southern ocean deep water formation rates is relatively insensitive to changes of the freshwater flux perturbations to the meridional moisture transports of each hemisphere identify equatorially asymmetric effects of the freshwater fluxes the results are consistent with box model results that the equilibrium nadw formation is primarily controlled by the magnitude of the southern hemisphere freshwater flux however the results show that the northern hemisphere freshwater flux has a strong impact on the transient behavior of the north atlantic overturning increasing this flux leads to a collapse of the conveyor belt circulation but the collapse is delayed if the southern hemisphere flux also increases the perturbation experiments also illustrate that the rapidity of collapse is affected by random fluctuations in the wind stress field

simulating the impact of freshwater inputs and deep draft icebergs formed during a mis 6 barents ice sheet collapse 1 an intermediate complexity climate model is used to simulate the collapse of the barents ice sheet during marine isotope stage 6 mis 6 140 ka b p with the purpose of investigating whether a mass input of freshwater from the collapse could have affected the convection and deep water formation in the north atlantic ocean further experiments used a coupled dynamic and thermodynamic iceberg model to determine the effects of deep draft icebergs rather than freshwater alone on the ocean circulation the results predict that the collapse of the barents ice sheet had a significant impact on the meridional overturning circulation in both the atlantic and pacific oceans freshwater fluxes have more of an impact on the atlantic overturning circulation during the actual release period compared to icebergs but the bergs induce effects over longer time scales even after the pulse is removed freshwater fluxes of 0 15 sverdrup sv and iceberg surges of 0 1 sv trigger significant changes in the global patterns particularly in the north pacific where there is strengthening of the overturning circulation at the expense of that in the north atlantic and associated increases in pacific sea surface temperatures these results highlight the importance of simulating not only the correct flux but also the form of the freshwater input from ice sheet collapses appropriately

metal partitioning in acid hypersaline sediments lake tyrrell victoria australia abstract oxic metal rich low ph groundwater enters a playa lake system at lake tyrrell partitioning of metals in the playa sediments was studied to determine the fate of the dissolved metals analysis of leachates from sequential extractions of sediment within the top 70 cm of the lake floor indicates that most of the cu pb zn fe and mn are adsorbed onto feoxides cu and to a lesser extent mn are bound to organics or sulfides in discrete thin layers while pb is deposited on exchangeable sites in upper layers possibly as sulfates metal concentrations are not enriched in the sediments which may be due to low ph s and or limited sulfate reduction in the system the lake tyrrell system may represent an ore forming system before extensive precipitation of metal sulfides

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

a wind induced thermohaline circulation hysteresis and millennial variability regimes abstract the multiple equilibria of the thermohaline circulation thc used here in the sense of the meridional overturning circulation as function of the surface freshwater flux has been studied intensively following a stommel paper from 1961 it is shown here that multistability and hysteresis of the thc also exist when the wind stress amplitude is varied as a control parameter both the massachusetts institute of technology ocean general circulation model mitgcm and a simple three box model are used to study and explain different dynamical regimes of the thc and thc variability as a function of the wind stress amplitude starting with active winds and a thermally dominant thermohaline circulation state the wind stress amplitude is slowly reduced to zero over a time period of 40 000 yr 40 kyr and then increased again to its initial value over another 40 kyr it is found that during the decreasing wind stress phase the thc remains thermally dominant until very low wind stress amplitude at which

thermohaline circulation hysteresis a model intercomparison we present results from an intercomparison of 11 different climate models of intermediate complexity in which the north atlantic ocean was subjected to slowly varying changes in freshwater input all models show a characteristic hysteresis response of the thermohaline circulation to the freshwater forcing which can be explained by stommel s salt advection feedback the width of the hysteresis curves varies between 0 2 and 0 5 sv in the models major differences are found in the location of present day climate on the hysteresis diagram in seven of the models present day climate for standard parameter choices is found in the bi stable regime in four models this climate is in the mono stable regime the proximity of the present day climate to the stommel bifurcation point beyond which north atlantic deep water formation cannot be sustained varies from less than 0 1 sv to over 0 5 sv

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

a diagnostic indicator of the stability of the atlantic meridional overturning circulation in ccsm3 a diagnostic indicator dmov is proposed in this paper to monitor the stability of the atlantic meridional overturning circulation amoc the dmov is a diagnostic for a basinwide salt advection feedback and defined as the difference between the freshwater transport induced by the amoc across the southern border of the atlantic ocean and the overturning liquid freshwater transport from the arctic ocean to the north atlantic as validated in the community climate system model version 3 ccsm3 for an amoc in the conveyor state a positive dmov freshwater convergence in the atlantic basin indicates a monostable amoc and a negative dmov freshwater divergence indicates a bistable amoc based on dmov the authors investigate the amoc stability in the last glacial maximum lgm and analyze the modulation of the amoc stability by an open closed bering strait moreover the authors estimate that the real amoc is likely to be bistable in the present day since some observations suggest a negative dmov freshwater divergence is currently in the atlantic basin however this estimation is very sensitive to the choice of the observational data

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

statistical parameterization of heterogeneous oceanic convection abstract a statistical convective adjustment scheme is proposed that attempts to account for the effects of mesoscale and submesoscale variability of temperature and salinity typically observed in the oceanic convective regions temperature and salinity in each model grid box are defined in terms of their mean variance and mutual correlations subgrid scale instabilities lead to partial mixing between different layers in the water column this allows for a smooth transition between the only two states convection on and convection off allowed in standard convective adjustment schemes the advantage of the statistical parameterization is that possible instabilities associated with the sharp transition between the two states which are known to occasionally affect the large scale model solution are eliminated the procedure also predicts the generation of correlations between temperature and salinity and the presence of convectively induced upgradient fluxes that have been obtained in numerical simulati

observations of gulf stream induced and wind driven upwelling in the georgia bight using ocean color and infrared imagery ocean color and infrared imagery from aircraft and satellite sensors are combined with in situ measurements of currents chlorophyll temperature salinity coastal winds and sea level to study upwelling events in the georgia bight where the gulf stream and continental shelf waters strongly interact these interactions were observed over an 8 day period in april 1980 during which five independent upwellings occurred the upwellings included a nearshore wind driven upwelling enhanced by topographic effects three gulf stream filament induced upwellings and what appears to have been a gulf stream meander induced event the chlorophyll distributions as inferred from the imagery are found to be reliable tracers which indicate filamentary circulation and propagation and advective routes by which nearshore water moves offshore the propagation of two of the filaments and the meander are studied in detail using the imagery and mooring array temperature time series both techniques yield nearly identical results for the phase speeds of each event there is evidence that one of the filaments accelerated as it entered into the charleston bump area where the gulf stream deflects offshore the other filament exhibited a steady deceleration as it moved past cape canaveral and eventually lost any semblance of wave motion also includedmore are data from april 1979 which offer corroborating evidence of filamentary structure and circulation intercomparisons of field measurements and the nimbus 7 coastal zone color scanner estimates of surface pigment concentrations show excellent agreement over the range of 0 1 7 0 mg m sup 3 54 references 22 figures 6 tables less

oceanic responses to gradual transitions of equator to pole temperature gradients summary responses of an ocean model to gradual transitions of equator to pole gradients show that several quasiequilibrium states within the thermally driven regime are generated under identical forcing conditions these different states are associated with different patterns the location and the penetration depth of oceanic convection small changes in the thermal forcing conditions can push the system into an oscillatory regime once in this regime oscillations appear to proceed as internal processes one important implication of these results is that the presentday ocean climate may not recover fully if the atmospheric coz level after doubling is lowered to the present day level

on the role of topography and wind stress on the stability of the thermohaline circulation a frictional geostrophic model is used to examine how the stability of the thermohaline circulation is affected by idealized topographic variations and the presence or absence of wind stress if the flow exhibits collapses the authors consider how topography and wind stress affect the ensuing oscillations large scale slope up toward the north or the west can significantly destabilize the circulation by modifying the barotropic flow and reducing the depth of convection wind stress stabilizes the circulation by deepening the thermocline in the subtropical gyre wind driving can also radically reduce the period of oscillations by destabilizing the northern halocline in the collapsed phase the overall period of the oscillation is usually governed by the time taken for diffusive warming to destabilize the deep ocean

correlation of the nappe stack in the ibero armorican arc across the bay of biscay a joint french spanish project a correlation between allochthonous units exposed in the nw iberian massif and the southern armorican massif is carried out based on lithological associations structural position age and geochemistry of protoliths and tectonometamorphic evolution the units on both sides of the bay of biscay are grouped into upper middle and lower allochthons whereas an underlying allochthonous thrust sheet identified in both massifs is referred to as the parautochthon the lower allochthon represents a fragment of the outermost edge of gondwana that underwent continental subduction shortly after the closure of a palaeozoic ocean which in turn is represented by the middle allochthon the latter consists of supra subduction ophiolites and metasedimentary sequences alternating with basic mid ocean ridge basalt morb type volcanics with inheritances suggesting the proximity of a continental domain seafloor spreading began at the cambro ordovician boundary and oceanic crust was still formed during the late devonian covering the lifetime of the rheic ocean which is possibly represented by the middle allochthon the opening of the oceanic domain was related to pulling apart the peri gondwanan continental magmatic arc which is represented by the upper allochthon

modeling the effect of freshwater pulses on the early holocene climate the influence of high frequency climate variability freshwater 4 67 10 14 m 3 into the labrador sea at three different constant rates 1 5 sv 1 sv 1 10 6 m 3 s 1 i n 10 years 0 75 sv in 20 years and 0 3 sv in 50 years for each rate five ensemble experiments have been performed varying in initial conditions the freshwater pulses produce a weakening of the thermohaline circulation the perturbed state is in agreement with proxy evidence for the 8 2 ka event two types of recovery of the thermohaline circulation occurred differing in time scale 1 200 years and 2 200 years in the experiments with 10 year and 20 year pulses both types of recovery were observed this suggests that the model response is unpredictable in the range of parameters studied here it is hypothesized that the unpredictability is associated with annual to decadal climate variability our results demonstrate that several types of recovery may exist with the same kind of perturbation the interpretation of events observed in proxy data may be thus more complex than realized until now since the magnitude and duration of climatic events caused by freshwater pulses is likely to depend strongly on nonlinear dynamics inside the coupled atmosphere sea ice ocean system index terms 3344 meteorology and atmospheric dynamics paleoclimatology 4255 oceanography general numerical modeling 4532 oceanography physical general circulation 4215 oceanography general climate and interannual variability 3309 9325 information related to geographic region atlantic ocean keywords holocene thermohaline circulation predictability meltwater coupled climate model

multiple equilibria in ocean models as a side effect of convective adjustment abstract multiple equilibria under identical surface forcing have been found in a model for the thermohaline circulation of the ocean thc the equilibria are characterized by different convective patterns with the help of numerical continuation techniques various regions in parameter space are identified where multiple states coexist for the same salinity flux robust to variations in surface forcing or to a reduction of the model from fully thermohaline to purely thermal this multiplicity is related to convective mixing this finding is in agreement with results from other model studies of the thc the manifestation of the multiple states depends strongly on grid resolution and details of the parameterization of convective adjustment the differences between multiple equilibria tend to become smaller for higher vertical resolution indicating convergence problems this makes it unlikely that this specific multiplicity represents a real physical property of thermohaline flows rather it appears that

effects of the gulf stream on ocean waves in the present study a third generation numerical wave model is used to study effects of a straight gulf stream and a gulf stream ring on ocean waves in swell and storm conditions the model accounts for all relevant processes of propagation generation and dissipation of the waves including current effects without imposing a priori restraints on the spectral development of the waves the dominating mechanism affecting the waves appears to be current induced refraction even though the short crestedness of the incoming waves tends to mask its effects also in swell conditions depending on wind and wave conditions refraction may trap locally generated waves in the straight gulf stream or it may reflect wave energy back to the open ocean in the gulf stream ring refraction induces a considerable variation in significant wave height and short crestedness but it hardly affects the mean wave direction in storm conditions the processes of generation and dissipation are considerably enhanced in countercurrent situations and reduced following current situations

wave refraction in an ocean front side looking airborne radar near cape hatteras north carolina recorded images of ocean waves wind streaks and intermediate water bodies within the gulf stream front in the enclosed waters of pamlico sound the radar images showed good accuracy in defining directions and distinguished waves from wind streaks at overlapping frequencies in the spectrum in passing through the cyclonic shear in the gulf stream front waves from both sides turned to their left and so specified their directions of propagation differences between spectra obtained across the front are in accord with current refraction theory an inverse wave refraction calculation furnished current shear values from which horizontal profiles of water velocity were compiled the total calculated velocity change of 1 5 m s is similar to previous measurements a secondary front near shore contained a velocity difference of 0 25 m s velocity changes across the gulf stream front were correlated with temperature changes the radar there showed bands of high reflectivity at the edges of intermediate water bodies whose temperature differences implied very small velocity steps increased roughness at the intermediate water boundaries was attributed to current interaction with waves

rapid interglacial climate fluctuations driven by north atlantic ocean circulation recent data from the grip ice core1 3 in greenland suggest that the climate of the last eemian interglacial period was much less stable than that of the present interglacial rapid transitions between warm and cold periods were found to occur on timescales of just a few decades the north atlantic climate during the eemian period was also shown to be characterized by three states respectively warmer than similar to and colder than today1 2 recent data from the nearby gisp2 ice core have revealed some discrepancies with these findings which remain to be resolved4 5 here we present simulations using an idealized global ocean model which suggest that the north atlantic ocean has three distinct circulation modes each of which corresponds to a distinct climate state we find that adding a simple random component to the mean freshwater flux which forces circulation can induce rapid transitions between these three modes we suggest that increased variability in the hydrological cycle associated with the warmer eemian climate could have caused transition between these distinct modes in the north atlantic circulation which may in turn account for the apparent rapid variability of the eemian climate

north atlantic deep water and antarctic bottom water their interaction and influence on the variability of the global ocean circulation 1 interhemispheric signal transmission in the atlantic ocean connects the deep water production regions of both hemispheres the nature of these interactions and large scale responses to perturbations on timescales of years to millennia have been investigated using a global three dimensional ocean general circulation model coupled to a dynamic thermodynamic sea ice model the coupled model reproduces many aspects of today s oceanic circulation a set of experiments shows the sensitivity to changes in different surface boundary conditions buoyancy changes in the weddell and labrador seas exert a direct effect on the overturning cells of the respective hemisphere they influence the density structure of the deep ocean and thereby lead to alterations in the strength of the acc changing the wind stress south of 30 s influences the magnitude of the deep water production of both hemispheres the interhemispheric effect in these experiments cannot be explained solely by advective mechanisms switching off the wind stress over the latitude band of the drake passage leads to a slow gradual decrease of the water mass transport in the acc resulting in an almost complete cessation the model results prove the necessity to continue integrations over thousands of years until new equilibria are established

the evolution of tides and tidal dissipation over the past 21 000 years the 120 m sea level drop during the last glacial maximum lgm 18 22 kyr bp had a profound impact on the global tides and lead to an increased tidal dissipation rate especially in the north atlantic here we present new simulations of the evolution of the global tides from the lgm to present for the dominating diurnal and semidiurnal constituents the simulations are undertaken in time slices spanning 500 1000 years due to uncertainties in the location of the grounding line of the antarctic ice sheets during the last glacial simulations are carried out for two different grounding line scenarios our results replicate previously reported enhancements in dissipation and amplitudes of the semidiurnal tide during lgm and subsequent deglaciation and they provide a detailed picture of the large global changes in m2 tidal dynamics occurring over the deglaciation period we show that antarctic ice dynamics and the associated grounding line location have a large influence on global semidiurnal tides whereas the diurnal tides mainly experience regional changes and are not impacted by grounding line shifts in antarctica

a decadal scale holocene sea surface temperature record from the subpolar north atlantic constructed using diatoms and statistics and its relation to other climate parameters 1 a sediment core from reykjanes ridge has been studied at 10 to 50 year time resolution to document variability of holocene surface water conditions in the western north atlantic and to evaluate effects of holocene ice rafting episodes diatom assemblages are converted to quantitative sea surface temperatures sst using three different transfer functions spectral and scale space methods are also applied on the records to explore variability at different timescales diatom assemblage and sst records clearly show that decaying remnants of the laurentide ice sheet strongly influenced early holocene climate in the western north atlantic this overrode the predominance of milankovitch forcing which played a key role in the development of holocene climate in the eastern north atlantic and nordic seas superimposed on general holocene climate change is high frequency sst variability on the order of 1 3 c the record also documents climatic oscillations with 600 to 1000 1500 and 2500 year periodicities with a time dependent dominance of different periodicities through the holocene a clear change in variability occurred about 5 ka bp the sst record also provides evidence for holocene cooling events hce that in some cases correlate to documented southward intrusions of ice into the north atlantic

mechanisms behind the temporary shutdown of deep convection in the labrador sea lessons from the great salinity anomaly years 1968 71 abstractfrom 1969 to 1971 convection in the labrador sea shut down thus interrupting the formation of the intermediate dense water masses the shutdown has been attributed to the surface freshening induced by the great salinity anomaly gsa a freshwater anomaly in the subpolar north atlantic the abrupt resumption of convection in 1972 in contrast is attributed to the extreme atmospheric forcing of that winter here oceanic and atmospheric data collected in the labrador sea at ocean weather station bravo and a one dimensional mixed layer model are used to examine the causes of the shutdown and resumption of convection in detail these results highlight the tight coupling of the ocean and atmosphere in convection regions and the need to resolve both components to correctly represent convective processes in the ocean they are also relevant to present day conditions given the increased ice melt in the arctic ocean and from the greenland ice sheet the analysis herein shows that the shutdown was initiat

trace metal concentrations in modern marine sabkhas abstract trace metal data from a number of hydrologically different types of modern marine sabkhas are presented indicating that these environments do not have elevated trace metal concentrations compared to recent near shore marine sediments and therefore are not active loci of trace metal enrichment today

the instability of the thermohaline circulation in a low order model abstract although the instability of the thermohaline circulation has been widely observed in numerical ocean models theoretical advances have been hindered by the nonlinearity of heat and salt transports a circulation governed by lateral temperature and salinity gradients because the instability occurs initially in polar waters through the formation of haloclines and the halt of convection any explanatory model must have at least a surface and a deep layer the model proposed here two surface boxes above a deep one reduces to a 2 degrees of freedom dynamical system when convection is active and 3 degrees when it is interrupted the instability that is induced by a negative freshwater perturbation in polar waters has three stages the first stage is a rapid 5 yr adjustment to a transient thermal attractor that results from an approximate balance between heat advection and air sea heat fluxes the second stage is a slow evolution that self organizes near this attractor which preconditions the insta

wave current interaction near the gulf stream during the surface wave dynamics experiment this paper presents a case study on the wave current interaction near the local curvature of a gulf stream meander the wave data were obtained from in situ measurements by a pitch roll discus buoy during the surface wave dynamics experiment swade conducted off wallops island virginia from october 1990 to march 1991 owing to the advection of the gulf stream by the semidiurnal tide the discus buoy was alternately located outside and inside the gulf stream the directional wave measurements from the buoy show the changes in wave direction wave energy and directional spreading when waves encountered the current in the gulf stream meanders a wave refraction model using the ray tracing method with an estimated gulf stream velocity field and meandering condition was used to simulate wave refraction patterns and to estimate wave parameters at relative locations corresponding to buoy measurements the numerical simulation shows that a focusing zone of wave rays was formed near the boundary and behind the crest of a simulated gulf stream meander the focusing of wave rays causes changes in wave direction increases in wave energy and decreases in wave directional spreading which are in good agreement with the results from the buoy measurements

response of the northern north atlantic and arctic oceans to a sudden change of the north atlantic oscillation 1 the influence of a switch from a long time high north atlantic oscillation nao state to an enduring nao situation is investigated with a coupled sea ice ocean model we compare the response to a sustained shift to nao conditions with that to a short lived transition to nao and back to nao forcing observed changes in sea ice and oceanic variables between high and low nao states are well captured in the model response the ocean circulation adjustment includes a fast barotropic anomaly accompanied by an enhancement of the meridional overturning and the northward heat transport at 48 n the slow response contains a substantial decrease of the northward heat transport which is caused by a reduction of the transport of subpolar and subtropical gyres after a delay of around 2 years the oceanic heat transport reacts to counteract the meridional temperature gradient imposed by the surface forcing the propagation time of baroclinic rossby waves is irrelevant for the delay between the forcing and the oceanic response rather the strength of the subpolar gyres is determined by the rapid spin up of a topographic sverdrup circulation and the subsequent slower changes through the jebar joint effect of baroclinicity and relief in an experiment where only the wind stress is changed to nao conditions the adjustment is dampened strongly by the nao thermohaline forcing the gyres decrease slower than in the nao case comparison with an experiment where only the wind stress is changed to nao conditions indicates that the thermohaline forcing is most important the heat fluxes counteract the wind stress forcing such that the meridional overturning and the northward heat transport decrease only slightly compared to the control run

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

proximity of the present day thermohaline circulation to an instability threshold the relation between the mean state of the thermohaline circulation thc and its stability is examined using a realistic geometry primitive equation coupled ocean atmosphere ice global general circulation model the main finding is that a thermohaline circulation that is 25 weaker and less dominated by thermal forcing than that of today s ocean is unstable within this coupled gcm unstable initial ocean climates lead in the coupled model to an increase of the thc to strong oscillations or to a thc collapse the existence of an unstable range of weak states of the thc provides a natural explanation for largeamplitude thc variability seen in the paleo record prior to the past 10 000 years a weakening of the thc due to an external forcing e g ice melting and freshening of the north atlantic may push it into the unstable regime once in this regime the thc strongly oscillates due to the inherent instability of a weak thc hence the strong thc variability in this scenario does not result from switches between two or more quasi stable steady states

deglacial meltwater discharge north atlantic deep circulation and abrupt climate change high resolution paleogeochemical data from the north atlantic ocean indicate that in the interval 15 000 to 10 000 14c years before present b p north atlantic deep water nadw production was decreased or eliminated four times at about 14 500 and probably older 13 500 12 000 and 10 500 years b p each of these changes occurred at the same time as abrupt events of meltwater discharge to the surface ocean inferred from oxygen isotope studies of planktonic foraminifera and from glacial geological studies on land in addition each of these times may be associated with brief episodes of cooler climate in the north atlantic region the best example of which is the younger dryas cooling of 10 500 years ago these results support models linking meltwater discharge decreased nadw production and decreased north atlantic heat flux

stability of a cold core eddy in the presence of convection hydrostatic versus nonhydrostatic modeling geostrophic eddies in a stratified liquid are susceptible to baroclinic instabilities in this paper the authors consider these instabilities when such an eddy is simultaneously cooled homogeneously from above as a linear stability analysis shows the developing convection modifies the background stratification the stability boundaries and the patterns of the dominant modes the coupling between the effects of convection and the largescale flow development of the eddy is studied through high resolution numerical simulations using both nonhydrostatic and hydrostatic models in the latter models several forms of convective adjustment are used to model convection both types of models confirm the development of the dominant modes and indicate that their nonlinear interaction leads to localized intense convection by comparing nonhydrostatic and hydrostatic simulations of the flow development carefully it is shown that convective adjustment may lead to erroneous smallscale variability a simple alternative formulation of convective adjustment is able to give a substantial improvement

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

bifurcations of the thermohaline circulation in a simplified three dimensional model of the world ocean and the effects of inter basin connectivity a coarse resolution global three dimensional idealised model of the world ocean based on the thermocline equations is described it is demonstrated that at suitable resolution such a model can be integrated for hundreds or thousands of millenia and could therefore form a component of an earth system model designed to investigate climate change on these time scales as in other ocean models the thermohaline circulation is found to exhibit multiple stable states the bifurcation structure of the model has been explored as a function of surface saline forcing and is found to depend on the inter basin connectivity the topography and the form of the imposed wind forcing

rapid climate transitions in a coupled ocean atmosphere model recent geochemical data1 2 have challenged the view that rapid climate fluctuations in the north atlantic at the end of the last glacial were caused by the thermohaline circulation of the ocean being switched on or off instead these data suggest that the circulation pattern must have switched between a warm deep mode and a cold shallow mode probably associated with different sites of deep convection3 here i present simulations with a three dimensional ocean model coupled to an idealized atmosphere which show this kind of transition the mechanism for the transition is a rearrangement of convection in the north atlantic triggered by a brief freshwater pulse this results in a drop in sea surface temperature in the north atlantic by up to 5 c within less than 10 years the rate of north atlantic deep water nadw forma tion is the same in the cold climate as in the warm climate but nadw sinks to intermediate depths only while antarctic bottom water pushes north to fill the entire abyssal atlantic

oceanic ridge crest processes oceanic ridge crest processes constitute a vigorous frontier of multidisciplinary research with global ramifications in the earth atmospheric and biologic sciences this deep ocean frontier is gradually opening in response to investigations which are elucidating the role of these processes in the lithosphere hydrosphere atmosphere and biosphere and the application of advanced instrumental and interpretive techniques which are increasing the quality and quantity of data these techniques include crustal drilling with borehole logging deep sea drilling project reports high resolution bathymetry sea beam etc side scan sonar seamarc i and ii gloria etc imagery with video and photography multichannel seismics determination of water column properties ctd with rosette sampler etc and submersible applications

wave current interaction study in the gulf of alaska for detection of eddies by synthetic aperture radar high resolution ers i synthetic aperture radar sar images are used to detect a mesoscale eddy such features limit dispersal of pollock larvae and therefore likely influence recruitment of fish in the gulf of alaska during high sea states and high winds the direct surface signature of the eddy was not clearly visible but the wave refraction in the eddy area was observed the rays of the wave field are traced out directly from the sar image the ray pattern gives information on the refraction pattern and on the relative variation of the wave energy along a ray through wave current interaction these observations are simulated by a ray tracing model which incorporates a surface current field associated with the eddy the numerical results of the model show that the waves are refracted and diverge in the eddy field with energy density decreasing the model data comparison for each ray shows the model predictions are in good agreement with the sar data

on the use of laser profilometry for ocean wave studies airborne laser profilometer measurements of wave skewness and spectra are presented the measurements were made during high wind and sea state conditions in the north atlantic the sea states ranged from 4 5 to 8 7 m and varied from swell to wind wave dominated situations skewness values for the five cases ranged from 0 08 to 0 28 and compare favorably with other existing measurements also the phillips and wallops spectral models show excellent agreement with profilometer spectra of developing seas for one case where the seas were fully developed the pierson moskowitz spectrum provides the best representation

on the mechanism of north atlantic decadal variability abstract north atlantic decadal climate variability is studied with a coupled atmosphere ocean sea ice model ecbilt after having reached an approximate statistical equilibrium in coupled mode without applying flux corrections a subsequent 1000 yr integration is performed and analyzed compared to the current climate the surface temperatures are 2 c warmer in the tropics to almost 8 c warmer in the polar regions the covariability between the atmosphere and ocean is explored by performing a singular value decomposition svd of boreal winter sst anomalies and 800 hpa geopotential height anomalies the first svd pair shows a red variance spectrum in sst and a white spectrum in 800 hpa height the second mode shows a peak in both spectra at a timescale of about 16 18 yr the geopotential height pattern is the model s equivalent of the north atlantic oscillation nao pattern the sst anomaly pattern is a north south oriented dipole additional experiments have revealed that the decadal oscillation in ec

imperfections of the thermohaline circulation latitudinal asymmetry and preferred northern sinking abstract the present atlantic thermohaline circulation is dominated by deep water formation in the north despite the fact that surface buoyancy forcing has relatively modest latitudinal asymmetry many studies have shown that even with buoyancy forcing that is symmetric about the equator spontaneous symmetry breaking can produce a single overturning cell with intense sinking in the north this occurs by salt advection at sufficiently large freshwater forcing in this symmetry breaking case a southern sinking solution and a symmetric solution are also possible a simple coupled ocean atmosphere model of the zonally averaged thermohaline circulation is used to examine the effect of latitudinal asymmetries in the boundary conditions the greater continental area in the northern hemisphere combined with the slight asymmetry in the observed freshwater flux induce a strong preference for the northern sinking solution examining the relation to the solution under symmetric conditions the salt advection mech

measurements of sea state variations across oceanic fronts using laser profilometry abstract as part of the grand banks experiment in may 1979 airborne laser profilometer measurements of the ocean wave field were made across a large cold water extrusion situated over the newfoundland ridge the feature is actually an extension of the labrador current which is bordered on the west side by the gulf stream and on the east side by the north atlantic current star shaped flight patterns were flown over the fronts on each side of the cold water feature a graphic technique was applied to the apparent wavenumber spectra in order to determine the changes in wave energy wavelength and direction of propagation of the dominant wind wave and swell components as they move across the fronts at the western front the sea state increased abruptly and the results indicate that wave current interactions were the most important mechanism for wave modification although boundary layer effects were present and increased wave breaking was observed at the eastern front changes in the swell are compared to

a thermal oscillation under a restorative forcing summary the authors report an interdecadal oscillation in a wind and thermally driven ocean general circulation model ogcm the oscillation is tantalizing in that it occursunder a relatively strong thermal damping 26 3 w m 2k examinations involving a two dimensional ogcm a simple thermal flip flop model and a three dimensional ogcm with and without the nonlinear effect of temperature in the state equation of sea water demonstrate that the oscillation is not driven by mechanisms such as the so called convective oscillator or the advective overshooting oscillator instead the oscillation is associated with the propagation of modelled long rossby waves with westward phase velocity it is found that the north south basin extent of the ogcm is an important factor a larger northsouth basin extent is conducive to the generation of thermal wind flows crucial for initiating the propagation of disturbances the geophysical relevance of these results is discussed

the different nature of the interdecadal variability of the thermohaline circulation under mixed and flux boundary conditions the differences between the interdecadal variability under mixed and constant flux boundary conditions are investigated using a coarse resolution ocean model in an idealized flat bottom single hemisphere basin objective features are determined that allow one type of oscillation to be distinguished versus the other first by performing a linear stability analysis of the steady state obtained under restoring boundary conditions it is shown that the interdecadal variability under constant flux and mixed boundary conditions arises respectively from the instability of a linear mode around the mean stratification and circulation and from departure from the initial state based on the budgets of density variance it is shown next that the two types of oscillations have different energy sources under the constant flux boundary condition the thermal mode the downgradient meridional eddy heat flux in the western boundary current regions sustains interdecadal variability whereas under mixed boundary conditions the salinity mode a positive feedback between convective adjustment and restoring surface heat flux is at the heart of the existence of the decadal oscillation furthermore the positive correlations between temperature and salinity anomalies in the forcing layer are shown to dominate the forcing of density variance in addition the vertical structure of perturbations reveals vertical phase lags at different depths in all tracer fields under constant flux while under mixed boundary conditions only the temperature anomalies show a strong dipolar structure the authors propose that these differences will allow one to identify which type of oscillation if any is at play in the more exhaustive climate models

sudden changes in north atlantic circulation during the last deglaciation sudden changes in the flow of warm atlantic surface waters into the norwegian sea occurred frequently during the last deglaciation typically involving shifts in sea surface temperature of 5 c in fewer than 40 years these led to equally large and rapid changes in atmospheric temperatures and to shifts in atlantic deep thermo haline circulation and ice sheet melting rates

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

on the mechanism of the antarctic circumpolar wave abstract the variability in the subpolar southern hemisphere is studied with a coupled atmosphere ocean sea ice model the ecbilt after having reached an approximate statistical equilibrium in coupled mode without flux corrections a subsequent 1000 yr integration is performed and analyzed a singular value decomposition of austral winter sst anomalies and 800 hpa geopotential height in the antarctic circumpolar current region reveals a mode of covariability that resembles the observed antarctic circumpolar wave subsequent analysis of this mode shows that it is basically an oscillation in the subsurface of the ocean additional experiments suggest that it is generated by the advective resonance mechanism the oscillation is excited by the dominant modes of variability in the atmosphere whereas the timescale is set by the ratio of the horizontal scale of these atmospheric modes and the advection velocity of the mean oceanic currents the atmospheric response mainly consists of a local temperature adjus

millennial scale oscillations between sea ice and convective deep water formation during the last ice age there were several quasiperiodic abrupt warming events the climatic effects of the so called dansgaard oeschger d o events were felt globally although the north atlantic experienced the largest and most abrupt temperature anomalies similar but weaker oscillations also took place during the interglacial period this paper proposes an auto oscillatory mechanism between sea ice and convective deep water formation in the north atlantic as the source of the persistent cycles a simple dynamical model is constructed by coupling and slightly modifying two existing models of ocean circulation and sea ice the model exhibits mixed mode oscillations consisting of decadal scale small amplitude oscillations and a large amplitude relaxation fluctuation the decadal oscillations occur due to the insulating effect of sea ice and leads to periodic ventilation of heat from the polar ocean gradually an instability builds up in the polar column and results in an abrupt initiation of convection and polar warming the unstable convective state relaxes back to the small amplitude oscillations from where the process repeats in a self sustained manner freshwater pulses mimicking heinrich events cause the oscillations to be grouped into packets of progressively weaker fluctuations as observed in proxy records modulation of this stable oscillation mechanism by freshwater and insolation variations could account for the distribution and pacing of d o and bond events physical aspects of the system such as sea ice extent and oceanic advective flow rates could determine the characteristic 1500 year time scale of d o events the model results with respect to the structure of the water column in the nordic seas during stadial and interstadial phases are in agreement with paleoproxy observations

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

changes in east atlantic deepwater circulation over the last 30 000 years eight time slice reconstructions using 95 epibenthic 13c records eight time slices were reconstructed to trace the distribution of east atlantic deepwater and intermediate water masses over the last 30 000 years our results show that there have been three distinct modes of deepwater circulation near the stage 3 2 boundary the origin of north atlantic deep water nadw was similar to today mode 1 however after late stage 3 the source region of the nadw end member shifted from the norwegian greenland sea to areas south of iceland mode 2 a reduced nadw flow persisted during the last glacial maximum with constant preformed 13c values the nutrient content of nadw increased markedly near the azores fracture zone from north to south probably because of the mixing of upwelled antarctic bottom water aabw from below which then advected with much higher flux rates into the northeast atlantic later the spread of glacial meltwater over the north atlantic led to a marked short term ventilation minimum below 1800 m about 13 500 14c years ago mode 3 the formation of nadw recommenced abruptly north of iceland 12 800 12 500 years ago and reached a volume approaching that of the holocene in the younger dryas 10 800 10 350 years b p another short term shutdown of deepwater formation followed between 10 200 and 9 600 years b p linked to a further major meltwater pulse into the atlantic each renewal of deepwater formation led to a marked release of fossil co2 from the ocean the likely cause of the contemporaneous 14c plateaus over the last 9000 years deepwater circulation varied little from today apart from a slight increase in aabw about 7000 14c years ago it is also shown that the oxygenated mediterranean outflow varied largely independent of the variations in deepwater circulation over the last 30 000 years

distribution of carbon isotopes in the glacial ocean a model study 1 a series of simulations are conducted with a global climate model of intermediate complexity mobidic this model includes ocean circulation dynamics including the carbon cycle coupled to a zonally averaged atmosphere oceanic distributions of nutrients apparent oxygen utilization radiocarbon and carbon 13 are discussed for the preindustrial era validation as well as three states of the glacial ocean termed interstadial very active formation of deep water in the north atlantic ocean stadial moderate convection and important flow of antarctic deep water in the atlantic and heinrich no formation of north atlantic deep water the stadial and interstadial states are stable the heinrich state is forced by a continuous freshwater discharge into the north atlantic the model exhibits significant changes in the isotopic composition of the ocean between the three modeled glacial states results for 13c tend to be in qualitative agreement with paleoceanographic data except that the model fails at representing the strong depletion in 13c in the southern ocean the heinrich atlantic ocean is older than the stadial ocean at all depths up to 1500 years the interstadial ocean has younger deep water and older intermediate water than the stadial it is recognized that the simulated changes in intermediate water age are less reliable because of the structure of the model color tracers are used to show that changes in the isotopic composition of atlantic bottom water are mainly related to a redistribution of water masses a simple method is tested by which it is possible to reconstruct the north atlantic water flow from the zonal profiles of salinity and 14c finally dividing artificially the gas exchange rate in the southern ocean by four results in a 0 4 decrease in the 13c of antarctic bottom water changes in new production are comparatively less effective at altering the 13c ratio

a decomposition of the atlantic meridional overturning circulation into physical components using its sensitivity to vertical diffusivity abstract the sensitivity of the atlantic ocean meridional overturning circulation to the vertical diffusion coefficient in the global coupled atmosphere ocean sea ice model climber 3 is investigated an important feature of the three dimensional ocean model is its low diffusive tracer advection scheme the strength mmax of the atlantic overturning is decomposed into three components 1 the flow ms exported southward at 30 s 2 the large scale upward flow that balances vertical diffusion in the atlantic and 3 a wind dependent upwelling flux wbound along the atlantic boundaries that is not due to vertical diffusion the export of water at 30 s varies only weakly with but is strongly correlated with the strength of the overflow over the greenland scotland ridge the location of deep convection is found to be mixing dependent such that a shift from the nordic seas to the irminger sea is detected for high values of the ratio r ms mmax gives a measure of the interhemispheric overturning efficienc

variability and multiple equilibria of the thermohaline circulation associated with deep water formation abstract in this study the variability of the thermohaline circulation on decadal and centennial time scales that is related to the process of deep water formation is investigated this is done within the context of a simple geostrophic three layer ocean model with a rectangular closed basin when slowly varying the atmospheric forcing the model response shows sudden transitions characterized by local changes in convective activity many different equilibria were found within the thermally driven regime in stommel s sense i e downwelling occurs near the poles next the deep water formation process was analyzed with a one dimensional box model in this box model four different regimes can be identified convective nonconvective periodic and a regime where both convection and no convection are possible with the same mixed boundary conditions these regimes were identified in the circulation of the ocean model a region was traced where convection is possible according to the authors analysis but