distribution of marine organisms and its geological significance in the modern reef complex of the ryukyu islands abstract extensive investigations of biota in the reef complex around the ryukyu islands have revealed ecologic specificity of many benthic organisms and have shown that characteristic assemblages are found in each of the topographic zones and sub areas the moat is divisible into a nearshore seagrass bed and an offshore sand bottom both inner reef flat and outer reef flat are characterized by abundant occurrences of hermatypic corals and nonarticulated coralline algae however the former is dominated by branching and foliaceous forms of corals and various large fleshy erect forms of algae whereas the latter is dominated by encrusting and tabular forms of corals lacking these algae corals and coralline algae are not present on the reef crest which is covered by rubble and gravel where algal turf and sargassum are spreading encrusting and tabular forms of corals flourish on the shallower part of the reef slope with high coverage while with increasing depth the coverage decreases and the dominating coral forms change with hemispherical and encrusting forms on the middle part of reef slope and foliaceous and encrusting forms on the deeper part of reef slope nonarticulated coralline algae are distributed throughout the reef slope the composition of coral and coralline algal assemblages changes dramatically with increasing depth foraminiferal algal nodules rhodoliths are the most abundant constituent on the island shelf commonly with cycloclypeus carpenteri there are likely to be two types of shelves in tropical to subtropical regions nutrient rich halimeda dominant and nutrient poor rhodolith dominant sediments abundant in bryozoan skeletons occur occasionally on the shelf

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

rotating horizontal convection horizontal convection hc is the generic name for the flow resulting from a buoyancy variation imposed along a horizontal boundary of a fluid we study the effects of rotation on three dimensional hc numerically in two stages first when baroclinic instability is suppressed and second when it ensues and baroclinic eddies are formed we concentrate on changes to the thickness of the near surface boundary layer the stratification at depth the overturning circulation and the flow energetics during each of these stages our results show that for moderate flux rayleigh numbers o 1 0 11 rapid rotation greatly alters the steady state solution of hc when the flow is constrained to be uniform in the transverse direction rapidly rotating solutions do not support a boundary layer exhibit weaker overturning circulation and greater stratification at all depths in this case diffusion is the dominant mechanism for lateral buoyancy flux and the consequent buildup of available potential energy leads to baroclinically unstable solutions when these rapidly rotating flows are perturbed baroclinic instability develops and baroclinic eddies dominate both the lateral and vertical buoyancy fluxes the resulting statistically steady solution supports a boundary layer larger values of deep stratification and multiple overturning cells compared with non rotating hc a transformed eulerian mean approach shows that the residual circulation is dominated by the quasi geostrophic eddy streamfunction and that the eddy buoyancy flux has a non negligible interior diabatic component the kinetic and available potential energies are greater than in the non rotating case and the mixing efficiency drops from sim 0 7 to sim 0 17 the eddies play an important role in the formation of the thermal boundary layer and together with the negatively buoyant plume help establish deep stratification these baroclinically active solutions have characteristics of geostrophic turbulence

eastern boundary contribution to the residual and meridional overturning circulations a model of the thermocline linearized around a specified stratification and the barotropic linear winddriven stommel solution is constructed the forcings are both mechanical the surface wind stress and thermodynamical the surface buoyancy boundarycondition the effects of diapycnal diffusivity and of eddy fluxesofbuoyancy parameterizedintermsofthelarge scalebuoyancygradient areincluded theeddyfluxes of buoyancy are especially important near the boundaries where they mediate the transport in and out of the narrow ageostrophic down upwelling layers the dynamics of these narrow layers can be replaced by effective boundary conditions on the geostrophically balanced flow the effective boundary conditions state that the residual flow normal to the effective coast vanishes the separate eulerian and eddy induced components may be nonzero this formulation conserves the total mass and the total buoyancy while permitting an exchangebetweentheeulerianandeddytransportofbuoyancy withinthedown upwellinglayers inturn this exchange allows buoyancy gradients along all solid boundaries including the eastern one a special focus is on the buoyancy along the eastern and western walls since east west buoyancy difference determines the meridional overturning circulation the inclusion of advection of buoyancy by the barotropic flow allows a meaningful distinction between the meridional and the residual overturning circulations while retaining the simplicity of a linear model the residual flow in both meridional and zonal directions reveals how the subsurface buoyancy distribution is established and in particular how the meridional buoyancy gradient is reversed at depth in turn the horizontal buoyancy gradient maintains stacked counterrotating cells in the meridional and residual overturning circulations quantitative scaling arguments are given for each of these cells which show how the buoyancy forcing the wind stress and the diapycnal and eddy diffusivities as well as the other imposed parameters affect the strength of the overturn

a model of the zonally averaged stratification and overturning in the southern ocean the ocean area south of the antarctic circumpolar current acc frontal system is a region of major watermass modification influx of north atlantic deep water nadw small scale mixing eddy transport and diffusion as well as the fluxes of momentum and buoyancy at the sea surface combine in a complex array of processes to generate the unique stratification of the southern ocean with its southward uprising isopycnals and northward flux of antarctic intermediate water aaiw and antarctic bottom water comprehensive analytical models of this scenario are rare the authors develop and apply a model based on zonally and temporally averaged theory to explain the conversion of nadw into aaiw with all of the aforementioned processes contained in an extremely simplified way eddies appear via a transformed eulerian mean tem approach with a conventional downgradient parameterization of the meridional density flux the structure of the eddy coefficient is estimated from hydrographic and wind stress data by a simple inverse approach mixing is limited to a near surface layer and is treated in a most simple entrainment form the model determines the zonal mean density stratification in the southern ocean and the baroclinic transport of the acc from the applied wind stress and the surface density flux and unravels the role and importance of the different processes responsible for shaping the stratification ekman and eddy induced advection and pumping mixing surface buoyancy flux and eddy induced diffusion all of these processes must be present to yield an agreement between the simulated stratification and the observed one but details of their parameterization might not be too critical the acc transport is shown to have a contribution forced by the local wind stress as well as another contribution relating to the nonlocal forcing by wind stress and density flux over the entire antarctic zone

the dependence of southern ocean meridional overturning on wind stress an eddy resolving numerical model of a zonal flow meant to resemble the antarctic circumpolar current is described andanalyzedusing the framework of j marshallandt radko in additionto windand buoyancy forcing at the surface the model contains a sponge layer at the northern boundary that permits a residual meridional overturning circulation moc to exist at depth the strength of the residual moc is diagnosed for different strengths of surface wind stress it is found that the eddy circulation largely compensates for the changes in ekman circulation the extent of the compensation and thus the sensitivity of the moc to the winds depend on the surface boundary condition a fixed heat flux surface boundary severely limits the ability of the moc to change an interactive heat flux leads to greater sensitivity to explain the moc sensitivity to the wind strength under the interactive heat flux transformed eulerian mean theory is applied in which the eddy diffusivity plays a central role in determining the eddy response a scaling theory for the eddy diffusivity based on the mechanical energy balance is developed and tested the average magnitude of the diffusivity is found to be proportional to the square root of the wind stress the moc sensitivity to the winds based on this scaling is compared with the true sensitivity diagnosed from the experiments

southern ocean upwelling earth s obliquity and glacial interglacial atmospheric co2 change previous studies have suggested that during the late pleistocene ice ages surface deep exchange was somehow weakened in the southern ocean s antarctic zone which reduced the leakage of deeply sequestered carbon dioxide and thus contributed to the lower atmospheric carbon dioxide levels of the ice ages here high resolution diatom bound nitrogen isotope measurements from the indian sector of the antarctic zone reveal three modes of change in southern westerly wind driven upwelling each affecting atmospheric carbon dioxide two modes related to global climate and the bipolar seesaw have been proposed previously the third mode which arises from the meridional temperature gradient as affected by earth s obliquity axial tilt can explain the lag of atmospheric carbon dioxide behind climate during glacial inception and deglaciation this obliquity induced lag in turn makes carbon dioxide a delayed climate amplifier in the late pleistocene glacial cycles

what sets the strength of the middepth stratification and overturning circulation in eddying ocean models abstract the processes maintaining stratification in the oceanic middepth between approximately 1000 and 3000 m are explored using an eddy resolving general circulation model composed of a two hemisphere semienclosed basin with a zonal reentrant channel in the southernmost eighth of the domain the middepth region lies below the wind driven main thermocline but above the diffusively driven abyssal ocean here it is argued that middepth stratification is determined primarily in the model s antarctic circumpolar current competition between mean and eddy overturning in the channel leads to steeper isotherms and thus deeper stratification throughout the basin than would exist without the channel isotherms that outcrop only in the channel are nearly horizontal in the semienclosed portion of the domain whereas isotherms that also outcrop in the northern hemisphere deviate from horizontal and are accompanied by geostrophically balanced meridional transport a northern source of deep water water with temp

reconstructed summer degree days in central alaska and northwestern canada since 1524 abstract three time series based on precisely dated annual tree ring widths have been used to reconstruct june plus july degree days for the central alaska and northwestern canada region the time series are the longest recently developed chronologies for the area and represent 57 core samples from 27 trees the degree day reconstruction extending back to a d 1524 exhibits much year to year variation and extended warming and cooling trends including a general warming trend from about 1840 to 1960 the reconstruction is in agreement with some subaretic glacial information and with data of percentage melting from arctic ice cores this and similar reconstructions can provide quantitative information to compare with general circulation and energy budget models for longer time periods than are available in recorded meteorological data

observations inferences and mechanisms of the atlantic meridional overturning circulation a review this is a review about the atlantic meridional overturning circulation amoc its mean structure temporal variability controlling mechanisms and role in the coupled climate system the amoc plays a central role in climate through its heat and freshwater transports northward ocean heat transport achieved by the amoc is responsible for the relative warmth of the northern hemisphere compared to the southern hemisphere and is thought to play a role in setting the mean position of the intertropical convergence zone north of the equator the amoc is a key means by which heat anomalies are sequestered into the ocean s interior and thus modulates the trajectory of climate change fluctuations in the amoc have been linked to low frequency variability of atlantic sea surface temperatures with a host of implications for climate variability over surrounding landmasses on intra annual timescales variability in amoc is large and primarily reflects the response to local wind forcing meridional coherence of anomalies is limited to that of the wind field on interannual to decadal timescales amoc changes are primarily geostrophic and related to buoyancy anomalies on the western boundary a pacemaker region for decadal amoc changes is located in a western transition zone along the boundary between the subtropical and subpolar gyres decadal amoc anomalies are communicated meridionally from this region amoc observations as well as the expanded ocean observational network provided by the argo array and satellite altimetry are inspiring efforts to develop decadal predictability systems using coupled atmosphere ocean models initialized by ocean data

homogenization of potential vorticity in planetary gyres the mean circulation of planetary fluids tends to develop uniform potential vorticity q in regions where closed time mean streamlines or closed isolines of mean potential vorticity exist this state is established in statistically steady flows by geostrophic turbulence or by wave induced potential vorticity flux at the outer edge of the closed contours the expelled gradients of q are concentrated beyond this transition lies motionless fluid or a different flow regime in which the planetary gradient of q may be dominant the homogenized regions occur where direct forcing by external stress or heating within the closed isoline is negligible upon the potential density surface under consideration in the stably stratified ocean such regions are found at depths greater than those of direct wind induced stress or penetrative cooling in channel models of the atmosphere we again find constant q when mesoscale eddies cause the dominant potential vorticity flux in the real atmosphere the results here can apply only where internal heating is negligible the derivations given here build upon the prandtl batchelor theorem which applies to non rotating steady two dimensional flow supporting evidence is found in numerical circulation models and oceanic observations

spatial variability of turbulent mixing in the abyssal ocean ocean microstructure data show that turbulent mixing in the deep brazil basin of the south atlantic ocean is weak at all depths above smooth abyssal plains and the south american continental rise the diapycnal diffusivity there was estimated to be less than or approximately equal to 0 1 x 10 4 meters squared per second in contrast mixing rates are large throughout the water column above the rough mid atlantic ridge and the diffusivity deduced for the bottom most 150 meters exceeds 5 x 10 4 meters squared per second such patterns in vertical mixing imply that abyssal circulations have complex spatial structures that are linked to the underlying bathymetry

the mediterranean sea overturning circulation abstractthe time mean zonal and meridional overturning circulations of the entire mediterranean sea are studied in both the eulerian and residual frameworks the overturning is characterized by cel

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

widespread intense turbulent mixing in the southern ocean observations of internal wave velocity fluctuations show that enhanced turbulent mixing over rough topography in the southern ocean is remarkably intense and widespread mixing rates exceeding background values by a factor of 10 to 1000 are common above complex bathymetry over a distance of 2000 to 3000 kilometers at depths greater than 500 to 1000 meters this suggests that turbulent mixing in the southern ocean may contribute crucially to driving the upward transport of water closing the ocean s meridional overturning circulation and thus needs to be represented in numerical simulations of the global ocean circulation and the spreading of biogeochemical tracers

control of bering strait transport by the meridional overturning circulation author s cessi paola abstract abstractit is well established that the mean transport through bering strait is balanced by a sea level difference between the north pacific and the arctic ocean but no mechanism has been proposed to explain this sea level difference it is argued that the sea level difference across bering strait which geostrophically balances the northward throughflow is associated with the sea level difference between the north pacific and the north atlantic arctic in turn the latter difference is caused by deeper middepth isopycnals in the indo pacific than in the atlantic especially in the northern high latitudes because there is deep water formation in the atlantic but not in the pacific because the depth of the middepth isopycnals is associated with the dynamics of the upper branch of the meridional overturning circulation moc a model is formulated that quantitatively relates the sea level difference between the north pacific and the arctic north atlantic with the wind stress in the antarctic circumpolar region since this forcing powers the moc and with the outcropping isopycnals shared between the northern hemisphere and the antarctic circumpolar region since this controls the location of deep water formation this implies that if the sinking associated with the moc were to occur in the north pacific rather than the north atlantic then the bering strait flow would reverse these predictions formalized in a theoretical box model are confirmed by a series of numerical experiments in a simplified geometry of the world ocean forced by steady surface wind stress temperature and freshwater flux

eddy driven buoyancy gradients on eastern boundaries and their role in the thermocline abstract it is demonstrated that eddy fluxes of buoyancy at the eastern and western boundaries maintain alongshore buoyancy gradients along the coast eddy fluxes arise near the eastern and western boundaries because on both coasts buoyancy gradients normal to the boundary are strong the eddy fluxes are accompanied by mean vertical flows that take place in narrow boundary layers next to the coast where the geostrophic constraint is broken these ageostrophic cells have a velocity component normal to the coast that balances the geostrophic mean velocity it is shown that the dynamics in these thin ageostrophic boundary layers can be replaced by effective boundary conditions for the interior flow relating the eddy flux of buoyancy at the seaward edge of the boundary layers to the buoyancy gradient along the coast these effective boundary conditions are applied to a model of the thermocline linearized around a mean stratification and a state of rest the linear model parameterizes the eddy fluxes of buoya

near surface eddy heat and momentum fluxes in the antarctic circumpolar current in drake passage abstractthe authors present new estimates of the eddy momentum and heat fluxes from repeated high resolution upper ocean velocity and temperature observations in drake passage and interpret their role in the regional antarctic circumpolar current acc momentum balance the observations span 7 yr and are compared to eddy fluxes estimated from a 3 yr set of output archived from an eddy resolving global parallel ocean program pop numerical simulation in both pop and the observations the stream averaged cross stream eddy momentum fluxes correspond to forcing consistent with both a potential vorticity flux into the axis of the subantarctic front saf and a sharpening of all three main acc fronts through drake passage further the pop analysis indicates that the mean momentum advection terms reflect the steering of the mean acc fronts and are not fully balanced by the eddy momentum forcing which instead impacts the strength and number of acc fronts the comparison between pop and observed eddy heat fluxe

warming of the global ocean spatial structure and water mass trends abstractthis study investigates the multidecadal warming and interannual to decadal heat content changes in the upper ocean 0 700 m focusing on vertical and horizontal patterns of variability these results support a nearly monotonic warming over much of the world ocean with a shift toward southern hemisphere warming during the well observed past decade this is based on objectively analyzed gridded observational datasets and on a modeled state estimate besides the surface warming a warming climate also has a subsurface effect manifesting as a strong deepening of the midthermocline isopycnals which can be diagnosed directly from hydrographic data this deepening appears to be a result of heat entering via subduction and spreading laterally from the high latitude ventilation regions of subtropical mode waters the basin average multidecadal warming mainly expands the subtropical mode water volume with weak changes in the temperature salinity s relationship known as spice variability howeve

a model of the upper branch of the meridional overturning of the southern ocean a zonal average model of the upper branch of the meridional overturning circulation of the southern ocean is constructed and used to discuss the processes wind buoyancy eddy forcing and boundary conditions that control its strength and sense of circulation the geometry of the thermocline wedge set by the mapping between the vertical spacing of buoyancy surfaces the stratification on the equatorial flank of the antarctic circumpolar current and their outcrop at the sea surface is seen to play a central role by setting the interior large scale potential vorticity distribution it is shown that the action of eddies mixing this potential vorticity field induces a residual flow in the meridional plane much as is observed with upwelling of fluid around antarctica northward surface flow and subduction to form intermediate water

global abyssal mixing inferred from lowered adcp shear and ctd strain profiles abstract internal wave wave interaction theories and observations support a parameterization for the turbulent dissipation rate e and eddy diffusivity k that depends on internal wave shear vz2 and strain z2 variances its latest incarnation is applied to about 3500 lowered adcp ctd profiles from the indian pacific north atlantic and southern oceans inferred diffusivities k are functions of latitude and depth ranging from 0 03 10 4 m2 s 1 within 2 of the equator to 0 4 0 5 10 4 m2 s 1 at 50 70 diffusivities k also increase with depth in tropical and subtropical waters diffusivities below 4500 m depth exhibit a peak of 0 7 10 4 m2 s 1 between 20 and 30 latitudes where semidiurnal parametric subharmonic instability is expected to be active turbulence is highly heterogeneous though the bulk of the vertically integrated dissipation e is contributed from the main pycnocline hotspots in e show some correlation with small scale bottom roughness and near bottom flow at sites where st

isopycnal averaging and the residual mean circulation abstract the zonally averaged circulation in the atmosphere or ocean can be misleading if the averaging is performed at constant height in the ocean there is apparently anomalously large diapycnal motion forming the so called deacon cell the atmospheric equivalents are the ferrel cells there are two zonal averaging techniques commonly used to avoid these spurious cells one involves averaging at constant density and this technique has been used in both fluids the other technique which has so far been applied only in the atmosphere involves taking into account perturbation correlation terms to form the residual mean circulation using a taylor series expansion we show that these apparently dissimilar techniques are formally equivalent at leading order in perturbation amplitude the equivalence is demonstrated using output from the fram southern ocean numerical model

evidence for enhanced mixing over rough topography in the abyssal ocean the overturning circulation of the ocean plays an important role in modulating the earth s climate but whereas the mechanisms for the vertical transport of water into the deep ocean deep water formation at high latitudes and horizontal transport in ocean currents have been largely identified it is not clear how the compensating vertical transport of water from the depths to the surface is accomplished turbulent mixing across surfaces of constant density is the only viable mechanism for reducing the density of the water and enabling it to rise however measurements of the internal wave field the main source of energy for mixing and of turbulent dissipation rates have typically implied diffusivities across surfaces of equal density of only approximately 0 1 cm2 s 1 too small to account for the return flow here we report measurements of tracer dispersion and turbulent energy dissipation in the brazil basin that reveal diffusivities of 2 4 cm2 s 1 at a depth of 500 m above abyssal hills on the flank of the mid atlantic ridge and approximately 10 cm2 s 1 nearer the bottom this amount of mixing probably driven by breaking internal waves that are generated by tidal currents flowing over the rough bathymetry may be large enough to close the buoyancy budget for the brazil basin and suggests a mechanism for closing the global overturning circulation

the deacon cell and the other meridional cells of the southern ocean abstract the meridional circulation cells of the southern ocean are investigated using the results from a fine resolution primitive equation model zonal integration along depth levels shows the classical series of meridional cells but integration along density layers shows a number of differences including the virtual disappearance of the deacon cell to investigate the differences the meridional transport is calculated as a function of both density and depth the results show that the deacon cell is associated with systematic changes in the depth of density surfaces between the western boundary current region off south america and the return flow in the interior of the ocean water flowing on each density surface produces a meridional cell with a vertical excursion of a few hundred meters thew cells combine without water crossing density surfaces to produce a single integrated deacon cell extending from the surface to below 2000 m the results also show that at each latitude water on each of the

simple mechanistic models of middepth meridional overturning abstract two idealized three dimensional analytical models of middepth meridional overturning in a basin with a southern hemisphere circumpolar connection are described in the first the overturning circulation can be understood as a pump and valve system in which the wind forcing at the latitudes of the circumpolar connection is the pump and surface thermodynamic exchange at high northern latitudes is the valve when the valve is on the overturning circulation extends to the extreme northern latitudes of the basin and the middepth thermocline is cold when the valve is off the overturning circulation is short circuited and confined near the circumpolar connection and the middepth thermocline is warm the meridional overturning cell in this first model is not driven by turbulent mixing and the subsurface circulation is adiabatic in contrast the pump that primarily drives the overturning cell in the second model is turbulent mixing at low and midlatitudes in the ocean interior in both model

the influence of southern ocean winds on the north atlantic carbon sink observed and predicted increases in southern ocean winds are thought to upwell deep ocean carbon and increase atmospheric co2 however southern ocean dynamics affect biogeochemistry and circulation pathways on a global scale using idealized massachusetts institute of technology general circulation model mitgcm simulations we demonstrate that an increase in southern ocean winds reduces the carbon sink in the north atlantic subpolar gyre the increase in atmospheric co2 due to the reduction of the north atlantic carbon sink is shown to be of the same magnitude as the increase in atmospheric co2 due to southern ocean outgassing the mechanism can be described as follows the increase in southern ocean winds leads to an increase in upper ocean northward nutrient transport biological productivity is therefore enhanced in the tropics which alters the chemistry of the subthermocline waters that are ultimately upwelled in the subpolar gyre the results demonstrate the influence of southern ocean winds on the north atlantic carbon sink and show that the effect of southern ocean winds on atmospheric co2 is likely twice as large as previously thought in past present and future climates

the impact of turbulence and convection on transport in the southern ocean volume transport in the presence of fully resolved convection and turbulence is investigated in a reentrant channel model of the southern ocean the response of the meridional overturning and zonal transport to variations in wind and buoyancy forcing is quantified our simulations show two overturning cells a buoyancy driven lower cell and a wind driven upper cell the lower overturning cell is much larger in meridional extent and magnitude than the upper cell the mean component of the overturning transport is smaller than the fluctuating component in the lower overturning cell indicating that transport is dominated by eddies and or turbulent convective flow in contrast the upper cell can be primarily described as a mean flow indicating minimal eddy compensation both cells strengthen with increasing winds with the upper cell being more sensitive to increasing wind intensity than the lower cell scaling for the mean upper overturning is also derived and matches previous theories which predict a linear sensitivity of the upper cell to wind zonal transport remains insensitive to increasing wind stress suggesting that the system is eddy saturated when turbulence and eddies are resolved these results suggest that fine scale flows control zonal transport and abyssal meridional overturning in the southern ocean while wind stress drives the upper meridional overturning transport highlighting the importance of accurately characterizing turbulence and convection in large scale models

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

coherent response of antarctic intermediate water and atlantic meridional overturning circulation during the last deglaciation reconciling contrasting neodymium isotope reconstructions from the tropical atlantic antarctic intermediate water aaiw plays important roles in the global climate system and the global ocean nutrient and carbon cycles however it is unclear how aaiw responds to global climate changes in particular neodymium isotopic composition end reconstructions from different locations from the tropical atlantic have led to a debate on the relationship between northward penetration of aaiw into the tropical atlantic and the atlantic meridional overturning circulation amoc variability during the last deglaciation we resolve this controversy by studying the transient oceanic evolution during the last deglaciation using a neodymium enabled ocean model our results suggest a coherent response of aaiw and amoc when amoc weakens the northward penetration and transport of aaiw decreases while its depth and thickness increase our study highlights that as part of the return flow of the north atlantic deep water nadw the northward penetration of aaiw in the atlantic is determined predominately by amoc intensity moreover the inconsistency among different tropical atlantic end reconstructions is reconciled by considering their corresponding core locations and depths which were influenced by different water masses in the past the very radiogenic water from the bottom of the gulf of mexico and the caribbean sea which was previously overlooked in the interpretations of deglacial end variability can be transported to shallow layers during active amoc and modulates end in the tropical atlantic changes in the aaiw core depth must also be considered thus interpretation of end reconstructions from the tropical atlantic is more complicated than suggested in previous studies

geostrophic circulation in a rectangular basin with a circumpolar connection abstract a simple theory is presented for steady geostrophic circulation of a stratified fluid in a rectangular basin with a circumpolar connection the interior flow obeys the plane sverdrup vorticity balance and the circulation is closed by geostrophic boundary currents the circulation is forced by surface thermal gradients and wind driven ekman transport near the latitudes of the circumpolar connection a thermal circumpolar current arises in response to imposed surface thermal gradients and northward ekman transport across the gap latitudes the transport of this model circumpolar current depends on the imposed surface thermal gradients and the gap geometry but not on the strength of the wind forcing in contrast the circulation induced in a related reduced gravity model by sverdrup transport into the gap latitudes has zero zonally integrated zonal transport the thermal current arises as a consequence of the geostrophic constraint which requires that the northern region fill with warm fluid un

vertical heat transport in eddying ocean models 1 the effect of mesoscale eddies on the vertical heat transport of the ocean is examined using two eddy resolving numerical models the global heat transport by the mean flow and diffusion are both downwards and are balanced by an upward eddy heat flux mean and eddy advective heat fluxes dominate the subpolar regions while diffusive flux is important primarily in the subtropics in the subtropical abyss the mean advective heat flux is balanced by a combination of eddy and diffusive fluxes and the classical munk type advective diffusive heat balance must be modified the munk and wunsch 1998 expression for the vertical turbulent diffusivity over estimates the diffusivity by as much as a factor of four near the base of the main thermocline this implies that the mixing required to close the meridional overturning circulation determined by munk and wunsch 1998 may be an over estimate due to the neglect of mesoscale eddies

the effects of mesoscale eddies on the stratification and transport of an ocean with a circumpolar channel abstract the effects of eddies in a primitive equation ocean model configured in a single hemisphere domain with circumpolar channels at their poleward ends are investigated in particular two regimes for the mass balance in the channel are investigated with small overlying winds the channel stratification is largely set by diffusion operating in the gyre portion of the domain the depth scale varies with a fractional power of the diffusivity but has little dependence on the wind stress as the winds are increased the depth becomes increasingly controlled by a tendency toward small residual circulation in this limit a scaling theory is derived for the stratification in the channel that predicts the overall depth of the thermocline as a power of the wind stress and that allows the eddy length scale to differ from the channel length scale the predicted depth depends on the details of the closure chosen for the eddy buoyancy flux but in general it varies as some fractional power of the wind stress a

the eddy driven thermocline the role of baroclinic eddies in transferring thermal gradients laterally and thus determining the stratification of the ocean is examined the hypothesis is that the density differences imposed at the surface by differential heating are a source of available potential energy that can be partially released by mesocale eddies with horizontal scales on the order of 100 km eddy fluxes balance the diapycnal mixing of heat and thus determine the vertical scale of penetration of horizontal thermal gradients i e the depth of the thermocline this conjecture is in contrast with the current thinking that the deep stratification is determined by a balance between diapycnal mixing and the large scale thermohaline circulation eddy processes are analyzed in the context of a rapidly rotating primitive equation flow driven by specified surface temperature with isotropic diffusion and viscosity the barotropic component of the eddies is found to be responsible for most of the heat flux and so the eddy transport is horizontal rather than isopycnal this eddy transport takes place in the shallow surface layer where eddies as well as the mean temperature undergo diabatic irreversible mixing scaling laws for the depth of the thermocline as a function of the external parameters are proposed in the classical thermocline theory the depth of the thermocline depends on the diffusivity the rotation rate and the imposed temperature gradient in this study the authors find an additional dependence on the viscosity and on the domain width

the ventilated ocean abstractadiabatic theories of ocean circulation and density structure have a long tradition from the concept of the ventilated thermocline to the notion that deep ocean ventilation is controlled by westerly winds over the southern ocean this study explores these ideas using a recently developed lagrangian ocean model lom which simulates ocean motions by computing trajectories of water parcels a unique feature of the lom is its capacity to model ocean circulations in the adiabatic limit in which water parcels exactly conserve their densities when they are not in contact with the ocean surface the authors take advantage of this property of the lom and consider the circulation and stratification that develop in an ocean with a fully adiabatic interior with both isopycnal and diapycnal diffusivities set to zero the ocean basin in the study mimics that of the atlantic ocean and includes a circumpolar channel the model is forced by zonal wind stress and a density restoring at the surface despite the

the role of eddy transfer in setting the stratification and transport of a circumpolar current abstract high resolution numerical experiments of a circumpolar current are diagnosed to study how lateral and vertical transfer of buoyancy by geostrophic eddies balances advection by a meridional circulation driven by surface wind stresses and buoyancy fluxes a theory is developed in the framework of the residual circulation to relate the vertical and horizontal stratification set up to the transfer properties of eddies and the patterns of imposed wind and buoyancy forcing simple expressions are found for the depth of penetration stratification baroclinic transport and residual circulation of the current finally the ideas are applied to the antarctic circumpolar current acc and yield predictions for how its properties depend on wind and buoyancy forcing

does southern ocean surface forcing shape the global ocean overturning circulation author s sun s eisenman i stewart al abstract 2018 american geophysical union all rights reserved paleoclimate proxy data suggest that the atlantic meridional overturning circulation amoc was shallower at the last glacial maximum lgm than its preindustrial pi depth previous studies have suggested that this shoaling necessarily accompanies antarctic sea ice expansion at the lgm here the influence of southern ocean surface forcing on the amoc depth is investigated using ocean only simulations from a state of the art climate model with surface forcing specified from the output of previous coupled pi and lgm simulations in contrast to previous expectations we find that applying lgm surface forcing in the southern ocean and pi surface forcing elsewhere causes the amoc to shoal only about half as much as when lgm surface forcing is applied globally we show that this occurs because diapycnal mixing renders the southern ocean overturning circulation more diabatic than previously assumed which diminishes the influence of southern ocean surface buoyancy forcing on the depth of the amoc