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

residual mean solutions for the antarctic circumpolar current and its associated overturning circulation abstract residual mean theory is applied to the streamwise averaged antarctic circumpolar current to arrive at a concise description of the processes that set up its stratification and meridional overturning circulation on an f plane simple solutions are found in which transfer by geostrophic eddies colludes with applied winds and buoyancy fluxes to determine the depth and stratification of the thermocline and the pattern of associated residual meridional overturning circulation

the adiabatic pole to pole overturning circulation abstractthe adiabatic pole to pole cell of the residual overturning circulation roc is studied in a two hemisphere semienclosed basin with a zonally reentrant channel occupying the southernmost eighth of the domain three different models of increasing complexity are used a simple analytically tractable zonally averaged model a coarse resolution numerical model with parameterized eddies and an eddy resolving general circulation model two elements are found to be necessary for the existence of an adiabatic pole to pole cell 1 a thermally indirect wind driven overturning circulation in the zonally reentrant channel analogous to the deacon cell in the antarctic circumpolar current acc region and 2 a set of outcropping isopycnals shared between the channel and the semienclosed region of the northern hemisphere these points are supported by several computations varying the domain geometry the surface buoyancy distribution and the wind forcing all three models give results that are qualitat

global ocean meridional overturning abstract a decade mean global ocean circulation is estimated using inverse techniques incorporating air sea fluxes of heat and freshwater recent hydrographic sections and direct current measurements this information is used to determine mass heat freshwater and other chemical transports and to constrain boundary currents and dense overflows the 18 boxes defined by these sections are divided into 45 isopycnal neutral density layers diapycnal transfers within the boxes are allowed representing advective fluxes and mixing processes air sea fluxes at the surface produce transfers between outcropping layers the model obtains a global overturning circulation consistent with the various observations revealing two global scale meridional circulation cells an upper cell with sinking in the arctic and subarctic regions and upwelling in the southern ocean and a lower cell with sinking around the antarctic continent and abyssal upwelling mainly below the crests of the major bathymetric ridges

control of lower limb overturning circulation in the southern ocean by diapycnal mixing and mesoscale eddy transfer a simple model is developed of the lower limb of the meridional overturning circulation in the southern ocean based on residual mean theory it is hypothesized that the strength of the lower limb overturning is strongly controlled by the magnitude of abyssal diapycnal mixing and that of mesoscale eddy transfer k in particular it is argued that k the scaling and associated theory find support in a suite of sensitivity experiments with an idealized ocean general circulation model this study shows that intense diapycnal mixing is required to close the buoyancy budget of the lower limb overturning circulation in contrast to the upper limb where air sea buoyancy fluxes can provide the required diabatic forcing

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

the injection of zonal momentum by buoyancy forcing in a southern ocean model abstractan overturning circulation driven by prescribed buoyancy forcing is used to set a zonal volume transport in a reentrant channel ocean model with three isopycnal layers the channel is designed to represent the southern ocean such that the forced overturning resembles the lower limb of the meridional overturning circulation moc the relative contributions of wind and buoyancy forcing to the zonal circulation are examined it is found that the zonal volume transport is strongly dependent on the buoyancy forcing and that the eddy kinetic energy is primarily set by wind stress forcing the zonal momentum budget integrated over each layer is considered in the buoyancy forced wind forced and combined forcing case at equilibrium sources and sinks of momentum are balanced but the transient spinup reveals the source of momentum for the current in the buoyancy forced case the forcing creates a baroclinic shear with westward flow in the lower layer allowing topographic form stress and bottom fric

solution of a model for the oceanic pycnocline depth scaling of overturning strength and meridional pressure difference 1 we present an analysis of the model by gnanadesikan 1999 for the pycnocline depth in the ocean an analytic solution for the overturning strength as a function of the meridional pressure difference is derived and used to discuss their mutual scaling we show that scaling occurs only in two unphysical regimes of the model in the absence of the southern ocean so processes i e for a northern overturning cell the volume transport is proportional to the square root of the pressure difference linear scaling is seen when the overturning is restricted entirely to the so i e when no northern downwelling exists for comparison we present simulations with the coupled climate model climber 3 which show linear scaling over a large regime of pressure differences in the north atlantic na we conclude that the pycnocline model is not able to reproducethelinearscalingbetweenitstwocentralvariables pressure and volume transport index terms 4532 oceanography physical general circulation 4203 oceanography general analytical modeling 4267 oceanography general paleoceanography citation levermann a and a griesel 2004 solution of a model for the oceanic pycnocline depth scalingofoverturningstrengthandmeridionalpressuredifference

the transient response of the southern ocean pycnocline to changing atmospheric winds 1 the vertical density structure of the southern ocean is dynamically linked to wind stress at the surface but the nature of this coupling is not fully understood observations from the last several decades show a significant increase in the strength of westerly winds over the southern ocean but an appreciable change in the tilt of constant density surfaces isopycnals has not yet been detected there using a combination of theory and idealized numerical simulations we show that the response of the density structure occurs on centennial timescales making it difficult to detect significant changes with a few decades of hydrographic observations dynamic coupling between the circumpolar current and northern basins regulates the slow adjustment of the density structure our results provide a new interpretation for recent observations and highlight the importance of the interaction between regional southern ocean dynamics and global ocean circulation citation jones d c t ito andn s lovenduski 2011 thetransientresponseofthe southern ocean pycnocline to changing atmospheric winds geophys res lett 38 l15604 doi 10 1029 2011gl048145

hydrochemical assessment of the rochefort marsh role of surface and groundwater in the hydrological functioning of the wetland abstract the interaction between a wetland the rochefort agricultural marsh and its underlying aquifers was studied using both geochemical and isotopic approaches water from the various components of the hydrosystem water from ditches shallow bri groundwater silty clay deposits deep bri groundwater and groundwater in the kimmeridgian aquifer confined and unconfined was sampled and analysed for major and trace elements sr br stable water isotopes 18 o and 2 h and sr isotopes the salinity of groundwater in the marsh cl up to 15 5 g l can be attributed to seawater trapped in the clay sediments during the flandrian 12 000 6400 bp the groundwater has since undergone more or less major chemical evolution due to water rock interactions and mixing with fresh water rainwater ditch water and kimmeridgian groundwater from the edge of the marsh the chemical characteristics of the groundwater in the confined kimmeridgian underlying the bri formation exclude the possibility of a salt wedge the combined use of 18 o 2 h cl br and sr contents and 87 sr 86 sr ratios enabled exchanges between surface water in marsh canals and ditches and groundwater in the shallow bri aquifer 7 m and in the confined kimmeridgian 18 m aquifers to be defined and quantified during the summer the ditches recharge the shallow bri aquifer by sub irrigation gravity flow by up to 26 the presence of a very compact layer of clay 1 2 m deep prevents the rapid recharge of the deep bri aquifer by rainfall and direct hydraulic connections between the ditches and the deep bri aquifer on the scale of the marsh the confined kimmeridgian groundwater recharges the bri aquifer by upward flow maximum contribution estimated to be 76 13 this flow being relatively limited and preferentially on the edge of the marsh the deep bri groundwater contributes very locally to recharging the confined kimmeridgian aquifer maximum contribution estimated to be 61 36 this study demonstrates the interest in combining hydrological and geochemical tools in order to identify and quantify the hydraulic interconnections between a wetland and its groundwater components a prior understanding of how wetlands function is crucial for the sustainable management of such ecosystems

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

time dependent adjustment in a simple model of the mid depth meridional overturning cell abstracta recently proposed reduced gravity model of the warm water branch of the middepth meridional overturning circulation in a rectangular basin with a circumpolar connection is extended to include time dependence the model describes the balance between gain of warm water through northward ekman advection across the circumpolar current loss of warm water through eddy fluxes southward across the current net gain or loss of warm water through diabatic processes north of the current and changes in the thickness of the warm water layer the steady solutions are the same as those found previously when the previous parameterization of diabatic fluxes is used time dependent solutions are considered for the approach of the solution to a new equilibrium when the forcing or parameters are abruptly changed and then held fixed an initial adjustment occurs through a combination of boundary and equatorial adjustment followed by planetary wave propagation the longer term adjustment to equilibrium consists p

the connection between southern ocean winds the atlantic meridional overturning circulation and indo pacific upwelling abstractcoupled gcm simulations are analyzed to quantify the dynamic effect of southern ocean so winds on transports in the ocean it is found that the closure for skew diffusivity in the non eddy resolving ocean model does not allow for a realistic eddy saturation of the zonal transports in the so in response to the wind changes and that eddy compensation of the meridional transports in the so is underestimated too despite this underestimated eddy compensation in the so however and in contrast to previous suggestions the atlantic meridional overturning circulation amoc strength is almost insensitive to so winds in the limit of weak so winds the amoc waters upwell not in the so but rather in the tropical indo pacific through their effect on sea ice weaker so winds also lead to less production of antarctic bottom water and therefore a deeper and stronger amoc

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

a theory of the interhemispheric meridional overturning circulation and associated stratification abstracta quantitative theoretical model of the meridional overturning circulation and associated deep stratification in an interhemispheric single basin ocean with a circumpolar channel is presented the theory includes the effects of wind eddies and diapycnal mixing and predicts the deep stratification and overturning streamfunction in terms of the surface forcing and other parameters of the problem it relies on a matching among three regions the circumpolar channel at high southern latitudes a region of isopycnal outcrop at high northern latitudes and the ocean basin between the theory describes both the middepth and abyssal cells of a circulation representing north atlantic deep water and antarctic bottom water it suggests that although the strength of the middepth overturning cell is primarily set by the wind stress in the circumpolar channel middepth stratification results from a balance between the wind driven upwelling in the channel and deep water formation at high northern latitudes d

interhemispheric influence of surface buoyancy conditions on a circumpolar current 1 this study shows that the surface buoyancy conditions in the northern hemisphere may influence the stratification and transport of the antarctic circumpolar current acc we use a course resolution ocean general circulation model ogcm in an idealized single basin configuration with a circumpolar channel a decrease in the magnitude of the surface temperature meridional gradient in the northern hemisphere reduces production of the deep water affecting the interhemispheric meridional overturning circulation moc and deepening the thermocline in both hemispheres the induced change of stratification in the southern hemisphere circumpolar region increases the zonal volume transport of circumpolar current because of an increase in the local meridional density gradient and the associated thermal wind shear which is the dominant baroclinic component of the total volume transport the result is robust to variations in the background vertical mixing and the parameterization scheme for mesoscale eddies

a simple dynamical model of the warm water branch of the middepth meridional overturning cell a reduced gravity model is presented of the warm water branch of the middepth meridional overturning circulation in a rectangular basin with a circumpolar connection the model describes the balance between production of warm water by ekman advection across the circumpolar current dissipation of warm water by eddy fluxes southward across the current and the net production or dissipation of warm water by diabatic processes north of the current the results emphasize the role of the eastern boundary condition in setting the thermocline structure northofthe current andthe nonlinear interactions betweenwind forcing eddyfluxes and diabatic mixing which together control the structure and amplitude of the model meridional overturning circulation solutionsareshowntoexistinwhichthenorthwardekmantransportacrossthecircumpolarcurrent is completely compensated by southward eddy fluxes and the meridional overturning north of the current is entirely driven by diabatic forcing and interior upwelling through the base of the layer other solutions are shown to exist in which the interior upwelling into the warm layer at midlatitudes is negligible and the meridional overturning circulation consists of a continuous cell that carried the fluid delivered by the northward ekman transport across the circumpolar current through midlatitudes to the northern hemisphere subpolar gyre where it cools and returns todepth the results emphasize that the coupled elements ofwind driving eddy fluxes and diabatic processes are inextricably intertwined in the middepth meridional overturning circulation

circumpolar response of southern ocean eddy activity to a change in the southern annular mode 1 analysis of satellite altimeter data reveals anomalously high eddy kinetic energy eke in the antarctic circumpolar current acc during the period 2000 2002 around 2 3 years earlier 1998 the circumpolar eastward wind stress as quantified by the southern annular mode sam showed a significant positive peak and we have shown previously that the acc peaked around 1998 in response an eddy resolving ocean model is used to investigate the delay between wind forcing and the eddy response and demonstrates that the lag is due to the time taken to influence the deep circulation of the acc winds over the southern ocean have shown a strong climatic increase over the past few decades if this increase in winds is also reflected as an increase in eddy activity as our analysis suggests it might then the increased poleward heat flux may have played a significant role in the observed warming of the southern ocean citation meredith m p and a m hogg 2006 circumpolar response of southern ocean eddy activity to a change in the southern annular mode geophys res lett 33 l16608 doi 10 1029 2006gl026499

a theory of deep stratification and overturning circulation in the ocean abstract a simple theoretical model of the deep stratification and meridional overturning circulation in an idealized single basin ocean with a circumpolar channel is presented the theory includes the effects of wind eddies and diapycnal mixing predicts the deep stratification in terms of the surface forcing and other problem parameters makes no assumption of zero residual circulation and consistently accounts for the interaction between the circumpolar channel and the rest of the ocean the theory shows that dynamics of the overturning circulation can be characterized by two limiting regimes corresponding to weak and strong diapycnal mixing the transition between the two regimes is described by a nondimensional number characterizing the strength of the diffusion driven compared to the wind driven overturning circulation in the limit of weak diapycnal mixing deep stratification throughout the ocean is produced by the effects of wind and eddies in a circumpolar channel and maintained even in the