stommel s box model of thermohaline circulation revisited the role of mechanical energy supporting mixing and the wind driven gyration the classical two box model of stommel is extended in two directions replacing the buoyancy constraint with an energy constraint and including the wind driven gyre stommel postulated a buoyancy constraint for the thermohaline circulation and his basic idea has evolved into the dominating theory of thermohaline circulation however recently it is argued that the thermohaline circulation is maintained by mechanical energy from wind stress and tides the major difference between these two types of models is the bifurcation structure the stommel like model has two thermal modes one stable and another one unstable and one stable haline mode whereas the energy constraint model has one stable thermal mode and two saline modes one stable and another one unstable adding the wind driven gyre changes the threshold value of thermohaline bifurcation greatly thus the inclusion of the wind driven gyre is a vital step in completely modeling the physical processes related to thermohaline circulation

an analytical model of the response of the meridional overturning circulation to changes in wind and buoyancy forcing abstractan analytical model of the full depth ocean stratification and meridional overturning circulation for an idealized atlantic basin with a circumpolar channel is presented the model explicitly describes the ocean response to both southern ocean winds and the global pattern and strength of prescribed surface buoyancy fluxes the construction of three layers defined by the two isopycnals of overturning extrema allows the description of circulation and stratification in both the upper and abyssal ocean the system is fully solved in the adiabatic limit to yield scales for the surface layer thickness buoyancies of each layer and overturning magnitudes the analytical model also allows scaling of the antarctic circumpolar current acc transport the veracity of the three layer framework and derived scales is confirmed by applying the analytical model to an idealized geometry eddy permitting ocean general circulation model consistent with previous results the abyssal overturning is found to scale

atlantic dominance of the meridional overturning circulation north atlantic na deep water formation and the resulting atlantic meridional overturning cell is generally regarded as the primary feature of the global overturning circulation and is believed to be a result of the geometry of the continents here instead the overturning is viewed as a global energy driven system and the robustness of na dominance is investigated within this framework using an idealized geometry ocean general circulation model coupled to an energy moisture balance model various climatic forcings are tested for their effect on the strength and structure of the overturning circulation without winds or a high vertical diffusivity the ocean does not support deep convection a supply of mechanical energy through winds or mixing purposefully included or due to numerical diffusion starts the deep water formation once deep convection and overturning set in the distribution of convection centers is determined by the relative strength of the thermal and haline buoyancy forcing in the most thermally dominant state i e negligible salinity gradients strong convection is shared among the na north pacific np and southern ocean so while near the haline limit convection is restricted to the na the effect of a more vigorous hydrological cycle is to produce stronger salinity gradients favoring the haline state of na dominance in contrast a higher mean ocean temperature will increase the importance of temperature gradients because the thermal expansion coefficient is higher in a warm ocean leading to the thermally dominated state an increase in so winds or global winds tends to weaken the salinity gradients also pushing the ocean to the thermal state paleoobservations of more distributed sinking in warmer climates in the past suggest that mean ocean temperature and winds play a more important role than the hydrological cycle in the overturning circulation over long time scales

where does dense water sink a subpolar gyre example it is proposed that a dominant component of the downwelling limb of the thermohaline circulation takes place in regions where convective mixing is found adjacent to steep topography a simple theoretical estimate of the overturning forced by such boundary convection is derived that depends only on the properties of the oceanic mixed layer along the boundary scaling estimates indicate that sinking forced by boundary convection is an order of magnitude greater than sinking in the open ocean resulting from large scale dynamics or baroclinic instability of deep convective sites recent hydrographic observations in the labrador sea are used to estimate the downwelling due to these different mechanisms and support the notion that boundary sinking dominates the theory compares well with the overturning rates diagnosed in a noneddy resolving general circulation model over a wide range of parameters as a direct consequence of these dynamics the high latitude hydrography and overturning circulation in the model are very sensitive to the presence of cyclonic rim currents lateral density advection by the rim currents in the subpolar gyre increases the stratification and limits the mixing near the boundaries thus reducing the maximum downwelling as a result most of the high latitude meridional heat transport is carried by the horizontal circulation instead of the overturning circulation such rim currents are found in different configurations of the model including 1 a continental slope and standard diffusion parameters and 2 zero horizontal diffusion and a flat bottom

a zonally averaged ocean model for the thermohaline circulation part i model development and flow dynamics abstract a two dimensional latitude depth ocean model is developed on the basis of the zonally averaged balance equations of mass momentum heat and salt its purpose is to investigate the dynamics and variability of the buoyancy forced thermohaline circulation for the time scales of interest an annually averaged model is selected and the momentum balance is taken to be diagnostic the east west pressure gradient which arises upon zonally averaging the momentum equations is parameterized in terms of the meridional pressure gradient the thermohaline circulation is driven by mixed surface boundary conditions i e temperatures are relaxed to prescribed values while the salt flux is held constant the dynamics of the flow is investigated in hemispheric and global geometries for both short and long time integrations the latter extending over many thousands of years as has been noted by previous investigators it is possible to perturb a steady state such that a diffusively dominated regime results

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

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

cretaceous black shales in angola basin of south atlantic ocean abstract conditions favorable to deposition of black shales in the deep ocean occurred several times during the cretaceous anoxic sediments were laid down in the south atlantic during two distinct periods the aptian early albian and the late albian coniacian at site 530 of dsdp leg 75 in the angola basin black shales averaging 5 1 and containing up to 16 5 organic carbon were found in a late albian coniacian turbidite sequence shipboard analysis shows most of the organic matter to be of marine origin although several layers contain some terrigenous material based upon rock eval pyrolysis and the absence of significant amounts of light hydrocarbons all of this organic matter is classified as immature thicknesses of the black shale layers are generally several centimeters or less and they are separated by fine grained turbidites containing less than 0 3 organic carbon these aptian coniacian shales seem to be the result of downslope density flows of shelf edge sediments rich in organic matter preservation of organic matter was a result of rapid burial not by stagnant basin waters furthermore this mode of preservation appears to be responsible for the cretaceous black shales throughout the south atlantic end of article last page 958

convective mixing and the thermohaline circulation abstract an idealized three dimensional model of buoyancy driven flow in a single hemisphere is used to investigate the relationship between the meridional overturning and the efficiency by which convective mixing eliminates static instability in the fast limit mixing timescale hours to weeks the meridional overturning is not rate limited by the efficiency of convective mixing if convective mixing is made less efficient the model s meridional overturning strength increases moreover the dominant downwelling occurs not at the highest surface density hence the deep ocean is relatively buoyant the numerical results are explained by the different influences of convective mixing and downward advection on the deep ocean heat budget they underscore the fundamentally three dimensional nature of the meridional overturning in addition the narrowness of deep downwelling is related to the geostrophic dynamics of deep temperature anomalies near the eastern wall the model results presented here are in con

meridional density gradients do not control the atlantic overturning circulation a wide body of modeling and theoretical scaling studies support the concept that changes to the atlantic meridional overturning circulation amoc whether forced by winds or buoyancy fluxes can be understood in terms of a simple causative relation between the amoc and an appropriately defined meridional density gradient mdg the mdg is supposed to translate directly into a meridional pressure gradient here two sets of experiments are performed using a modular ocean model coupled to an energy moisture balance model in which the positive amoc mdg relation breaks down in the first suite of seven model integrations it is found that increasing winds in the southern ocean cause an increase in overturning while the surface density difference between the equator and north atlantic drops in the second suite of eight model integrations the equation of state is manipulated so that the density is calculated at the model temperature plus an artificial increment dt that ranges from 238 to 98c an increase in dt results in increased sensitivity of density to temperature gradients the amoc in these model integrations drops as the mdg increases regardless of whether the density difference is computed at the surface or averaged over the upper ocean traditional scaling analysis can only produce this weaker amoc if the scale depth decreases enough to compensate for the stronger mdg five estimates of the depth scale are evaluated and it is found that the changes in the amoc can be derived from scaling analysis when using the depth of the maximum overturning circulation or estimates thereof but not from the pycnocline depth these two depth scales are commonly assumed to be the same in theoretical models of the amoc it is suggested that the correlation between the mdg and amoc breaks down in these model integrations because the depth and strength of the amoc is influenced strongly by remote forcing such as southern ocean winds and antarctic bottom water formation

reconciling theories of a mechanically driven meridional overturning circulation with thermohaline forcing and multiple equilibria it has recently been suggested that the structure and strength of the meridional overturning circulation in the global ocean is governed by the input of mechanical energy to the system by winds and tides however it is not clear how this suggestion relates to the existence of multiple equilibria of the meridional overturning circulation which depends on thermohaline feedbacks and is more consistent with a buoyancy driven view of the circulation both theories have been illustrated by box models in the past stommel in tellus 13 224 230 1961 gnanadesikan in science 283 2077 2079 1999 here we incorporate these two theories into a single box model in an attempt to reconcile the roles of mechanical and buoyancy forcing in driving the meridional overturning circulation the box model has two equilibrium solutions one with sinking at high northern latitudes as in the present day atlantic and one without the circulation is mechanically driven but the northern sinking can be thought of as a release valve which acts as a sink of potential energy when the surface water at high northern latitudes is dense enough to convect while the source of energy comes from mechanical forcing the presence or otherwise of multiple equilibria is therefore determined by thermohaline feedbacks in some areas of parameter space an oscillation between the model s two circulation regimes occurs reminiscent of a bipolar seesaw

oceanic isopycnal mixing by coordinate rotation abstract current numerical models of oceanic circulation differentiate between the eddy diffusion and viscosity transport along the geopotential horizontal and vertical directions only in order to model the effect of anisotropic turbulence as diffusive transport along and across density surfaces the isopycnal mixing tensor has been transformed from a diagonal second rank tensor in the isopycnal coordinate system to a tensor containing off diagonal elements in the geopotential coordinate system

effect of global ocean temperature change on deep ocean ventilation 1 a growing number of paleoceanographic observations suggest that the ocean s deep ventilation is stronger in warm climates than in cold climates here we use a general ocean circulation model to test the hypothesis that this relation is due to the reduced sensitivity of seawater density to temperature at low mean temperature that is at lower temperatures the surface cooling is not as effective at densifying fresh polar waters and initiating convection in order to isolate this factor from other climate related feedbacks we change the model ocean temperature only where it is used to calculate the density to which we refer below as dynamic temperature change we find that a dynamically cold ocean is globally less ventilated than a dynamically warm ocean with dynamic cooling convection decreases markedly in regions that have strong haloclines i e the southern ocean and the north pacific while overturning increases in the north atlantic where the positive salinity buoyancy is smallest among the polar regions we propose that this opposite behavior of the north atlantic to the southern ocean and north pacific is the result of an energy constrained overturning

a box model test of the freshwater forcing hypothesis of abrupt climate change and the physics governing ocean stability 1 observations and an ocean box model are combined in order to test the adequacy of the freshwater forcing hypothesis to explain abrupt climate change given the uncertainties in the parameterization of vertical buoyancy transport in the ocean the combination is carried out using bayesian stochastic inversion which allows us to infer changes in the mass balance of northern hemisphere nh ice sheets and in the meridional transports of mass and heat in the atlantic ocean that would be required to explain dansgaard oeschger interstadials dois from 30 to 39 kyr b p the mean sea level changes implied by changes in nh ice sheet mass balance agree in amplitude and timing with reconstructions from the geologic record which gives some support to the freshwater forcing hypothesis the inversion suggests that the duration of the dois should be directly related to the growth of land ice our results are unaffected by uncertainties in the representation of vertical buoyancy transport in the ocean however the solutions are sensitive to assumptions about physical processes at polar latitudes

atlantic pycnocline theory scrutinized using a coupled climate model simulations with changed southern ocean wind stress oceanic vertical mixing surface freshwater forcing and global warming confirm the basic equations of gnanadesikan s 1999 theory for the atlantic one vertical scale the pycnocline depth d contributes inversely proportional to low latitudinal upwelling and linearly to southern ocean eddy transport the maximum atlantic overturning is confirmed to be quadratic in d but is also shown to be linear in a meridional density difference our simulations strongly suggest that the theory needs to be complemented by a dynamical equation for since changes in both d and are significant and mutually independent while under global warming d varies strongly and is practically constant the situation is reversed for altered surface freshwater fluxes similarly variations in the meridional length scale of southern ocean outcropping require a dynamical equation in order to capture the fundamental behavior of the atlantic meridional overturning circulation

comparison of thermally driven circulations from a depth coordinate model and an isopycnal layer model part i scaling law sensitivity to vertical diffusivity abstract two different types of numerical ocean circulation models are used in a classical idealized problem the thermally induced circulation in an ocean basin bounded by two meridians to the east and west and by the equator and a line of constant latitude a simple scaling theory exists for predicting poleward heat transport and the strength of meridional overturning as a function of vertical diffusivity and other external factors however previous studies have indicated conflicting results and other scaling laws have been proposed experiments with two widely used types of numerical models one based on depth coordinates and the other based on isopycnal layers provide insight into the discrepancies of previous studies in the numerical experiments vertical diffusivity is varied over a range of 200 the source of the difficulty in previous studies is in part traced to applying a fixed restoring coefficient at the upper boundary and considering the buoyancy forcing at the surface fixed irrespective o

the stability of thermohaline circulation in a two box model abstract in stommel s simple two box model which has provided an insight on the thermohaline circulation and climate instability mechanisms a linear mass transport was used however a scaling law based on geostrophy and advective diffusive heat balance suggests a nonlinear mass transport relation for the oceans by including this nonlinear mass transport relation to stommel s box model it is possible to study the effects of the thermocline which was not considered before on the stability of the thermohaline circulation while keeping the simplicity of stommel s box model the results were compared with those obtained with the traditional model using a linear mass transport relation the thermal mode circulation of the nonlinear model is significantly more stable than that of the linear model suggesting the thermohaline catastrophe is less likely to occur in the present north atlantic if the thermocline is considered in the nonlinear model the circulation removes density anomalies rapidly so that s

influence of vertical mixing on the thermohaline hysteresis analyses of an ogcm the thermohaline hysteresis response to varying north atlantic freshwater forcing is studied by means of a three dimensional global ocean general circulation model ogcm the influence of vertical diffusivity is examined using a wide range of mixing coefficients for sufficiently large vertical diffusivity the model shows a pronounced hysteresis behavior so that two equilibrium states of the thermohaline circulation are found one mode with intense deep water formation in the north atlantic on mode and one mode with strongly reduced convective activity off mode by decreasing the diffusivity the two branches of the hysteresis merge in addition the effect of numerical diffusion is analyzed by applying different advection schemes a positive feedback between overturning strength and the numerical diffusivity of upstream advection is found resulting in a reduced stability of the on mode comparing the results with those from zonally averaged models exhibits substantial differences regarding the stability properties of the thermohaline circulation freshwater transports by horizontal gyres have an important effect on the overturning circulation in the ogcm

remote forcing of the antarctic circumpolar current by diapycnal mixing 1 we show that diapycnal mixing can drive a significant antarctic circumpolar current acc volume transport even when the mixing is located remotely in northern hemisphere ocean basins in the case of remote forcing the globally averaged diapycnal mixing coefficient is the important parameter this result is anticipated from theoretical arguments and demonstrated in a global ocean circulation model the impact of enhanced diapycnal mixing on the acc during glacial periods is discussed

isopycnal mixing in ocean circulation models abstract a subgrid scale form for mesoscale eddy mixing on isopycnal surfaces is proposed for use in non eddy resolving ocean circulation models the mixing is applied in isopycnal coordinates to isopycnal layer thickness or inverse density gradient as well as to passive scalars temperature and salinity the transformation of these mixing forms to physical coordinates is also presented

multiple equilibria of an asymmetric two basin ocean model abstract an ocean general circulation model is used to examine the role of model geometry and surface buoyancy and wind stress forcing in the asymmetry of the global thermohaline circulation the model domain is a highly idealized atlantic and pacific linked by a circumpolar ocean in the south and the integrations are performed under mixed boundary conditions diagnosed from spinups under various temperature and salinity profiles constructed from the present day climatology the model exhibits a tendency to favor either a conveyor type circulation with sinking in the northern north atlantic and upwelling in the north pacific or a southern sinking state with deep sinking in the antarctic only this bias is not dictated solely by the hydrological cycle nor apparently by the greater northern extension of the atlantic basin but presumably by the overall asymmetry of the geometry equilibria with northern sinking in both basins can appear however when the winds in the southern ocean are reduced or th

parameter sensitivity of primitive equation ocean general circulation models abstract experiments with a low resolution primitive equation ocean general circulation model with idealized basin geometry and surface forcing have been carried out in order to identify the processes controlling the climatically important aspects of the circulation emphasis was placed on the sensitivity of the model solutions to the magnitude of the vertical diffusivity scaling arguments suggest and the numerical experiments confirm that the solutions are most sensitive to the magnitudes of the wind stress curl and the vertical diffusivity for small vertical diffusivity wind forcing dominates the solution the vertical scale of the thermocline is set by the strength of the ekman pumping and there is a multiple gyre circulation in the upper layers for large vertical diffusivity diabatic surface forcing dominates the solution vertical diffusion controls the vertical scale of the thermocline and there is a single large anticyclonic gyre in the upper layers both the meridionally and zonally inte

reconstructing global overturning from meridional density gradients despite the complexity of the global ocean system numerous attempts have been made to scale the strength of the meridional overturning circulation moc principally in the north atlantic with large scale basin wide hydrographic properties in particular various approaches to scaling the moc with meridional density gradients have been proposed but the success of these has only been demonstrated under limited conditions here we present a scaling relationship linking overturning to twice vertically integrated meridional density gradients via the hydrostatic equation and a rotated form of the geostrophic equation this provides a meridional overturning streamfunction as a function of depth for each basin using a series of periodically forced experiments in a global coarse resolution configuration of the general circulation model nemo we explore the timescales over which this scaling is temporally valid we find that the scaling holds well in the upper atlantic cell at 1000 m for multi decadal and longer timescales accurately reconstructing the relative magnitude of the response for different frequencies and explaining over 85 of overturning variance on timescales of 64 2048 years despite the highly nonlinear response of the antarctic cell in the abyssal atlantic between 76 and 94 of the observed variability at 4000 m is reconstructed on timescales of 32 years and longer the scaling law is also applied in the indo pacific this analysis is extended to a higher resolution stochastically forced simulation for which correlations of between 0 79 and 0 99 are obtained with upper atlantic moc variability on timescales 25 years these results indicate that meridional density gradients and overturning are linked via meridional pressure gradients and that both the strength and structure of the moc can be reconstructed from hydrography on multi decadal and longer timescales provided that the link is made in this way

modelling a torrential event in a river confluence torrential floods characterized by extremely high discharges and sediment transport rates are common in mediterranean areas few field data is available for such floods due to sudden increase in water discharges and short flood duration this paper presents the field data collected during a major flood event in a 90 confluence of two torrential rivers of the mediterranean area a two dimensional model was used to describe the flow characteristics at the confluence the influence on water levels of the torrential character of both rivers and some morphological processes as the tributary mouth gravel bar has been assessed although channel roughness coefficients and sediment deposits influence final water surface elevations they have lesser effect than the discharge ratio between main and tributary rivers

the relation of meridional pressure gradients to north atlantic deep water volume transport in an ocean general circulation model we use a coarse resolution ocean general circulation model to study the relation between meridional pressure and density gradients in the southern ocean and north atlantic and the atlantic meridional overturning circulation in several experiments we artificially modify the meridional density gradients by applying different magnitudes of the gent mcwilliams isopycnal eddy diffusion coefficients in the southern ocean and in the north atlantic and investigate the response of the simulated atlantic meridional overturning to such changes the simulations are carried out close to the limit of no diapycnal mixing with a very small explicit vertical diffusivity and a tracer advection scheme with very low implicit diffusivities our results reveal that changes in eddy diffusivities in the north atlantic affect the maximum of the atlantic meridional overturning but not the outflow of north atlantic deep water into the southern ocean in contrast changes in eddy diffusivities in the southern ocean affect both the south atlantic outflow of north atlantic deep water and the maximum of the atlantic meridional overturning results from these experiments are used to investigate the relation between meridional pressure gradients and the components of the atlantic meridional overturning pressure gradients and overturning are found to be linearly related we show that in our simulations zonally averaged deep pressure gradients are very weak between 20 s and about 30 n and that between 30 n and 60 n the zonally averaged pressure grows approximately linearly with latitude this pressure difference balances a westward geostrophic flow at 30 40 n that feeds the southbound deep atlantic western boundary current we extend our analysis to a large variety of experiments in which surface freshwater forcing vertical mixing and winds are modified in all experiments the pycnocline depth assumed to be the relevant vertical scale for the northward volume transport in the atlantic is found to be approximately constant at least within the coarse vertical resolution of the model the model behaviour hence cannot directly be related to conceptual models in which changes in the pycnocline depth determine the strength of atlantic meridional flow and seems conceptually closer to stommel s box model in all our simulations the atlantic overturning seems to be mainly driven by southern ocean westerlies however the actual strength of the atlantic meridional overturning is not determined solely by the southern ocean wind stress but as well by the density pressure gradients created between the deep water formation regions in the north atlantic and the inflow outflow region in the south atlantic

boundary mixing and the dynamics of three dimensional thermohaline circulations boundary mixing is implemented in an ocean general circulation model such that the vertical mixing coefficient ky is nonzero only near side boundaries and in convection regions the model is used in a highly idealized configuration with no wind forcing and very nearly fixed surface density to investigate the three dimensional dynamics of the thermohaline circulation for ky 5 20 3 1024 m2 s21 and lower the meridional overturning strength to great accuracy is proportional to meridional heat transport is proportional to the circulation 2 3 1 2 kk y y patterns resemble those from runs with uniform vertical mixing but vertical motion is entirely confined to the boundary regions near the western boundary there is upwelling everywhere near the eastern boundary there is a consistent pattern of downwelling above upwelling with downwelling reaching deeper at high latitudes this pattern is explained by convection and vertical advective diffusive balance underneath for ky 5 30 3 1024 m2 s21 and higher no steady solutions have been found the meridional overturning oscillates on a timescale of about 25 years a time averaged thermally direct overturning cell is not supported dynamically because convection extends longitudinally across the entire basin and upwelling near the western boundary does not lead to densities higher than at the eastern boundary assuming uniform upwelling in the west level isopycnals near the equator and level isopycnals along the eastern boundary south of the outcropping latitude permits the analytic determination of convection depth at the eastern wall and hence the density difference between the eastern and western walls this difference is at most one quarter the surface density difference between high and low latitudes and agrees in magnitude and latitudinal dependence with the numerical experiments scaling arguments estimate overturning strength as of the order of 10 3 106 m3 s21 and confirm the 2 3 power dependence onky the derivation also gives a dependence of overturning strength with latitude that agrees qualitatively with the numerical results the scaling for the dependence of meridional heat transport on latitude agrees well with the model results scaling for heat transport amplitude agrees less well but correctly predicts a weaker dependence on ky than maximum overturning