mechanisms controlling the sensitivity of the atlantic thermohaline circulation to the parameterization of eddy transports in ocean gcms abstract the authors identify and describe the important dynamical mechanisms that explain the significant sensitivity of the atlantic thermohaline circulation to the parameterization of heat and salt transports by mesoscale eddies in numerical models in particular the effects of the gent mcwilliams gm scheme which has a strong flattening effect on isopycnals and a simple horizontal diffusion scheme are considered and compared two control runs one with each scheme exhibit very different circulations and density structures to analyze the dynamical reasons for the differences between the control runs a number of numerical experiments with regionally varying diffusion coefficients are carried out emphasizing the effects of different schemes in key regions the main effect of eddies in the southern ocean in nature is to shoal the subsurface isopycnal surfaces thus increasing the density of the northward inflow of relatively dense intermediate waters into the atlantic as will be seen this is more

topographic enhancement of eddy efficiency in baroclinic equilibration abstractthe processes that determine the depth of the southern ocean thermocline are considered in existing conceptual frameworks the thermocline depth is determined by competition between the mean and eddy heat transport with a contribution from the interaction with the stratification in the enclosed portion of the ocean using numerical simulations this study examines the equilibration of an idealized circumpolar current with and without topography the authors find that eddies are much more efficient when topography is present leading to a shallower thermocline than in the flat case a simple quasigeostrophic analytical model shows that the topographically induced standing wave increases the effective eddy diffusivity by increasing the local buoyancy gradients and lengthening the buoyancy contours across which the eddies transport heat in addition to this local heat flux intensification transient eddy heat fluxes are suppressed away from the topography especially upstream indicating that local

a barotropic model of eddy saturation abstracteddy saturation refers to a regime in which the total volume transport of an oceanic current is insensitive to the wind stress strength baroclinicity is currently believed to be the key to

hydrographic biases in global coupled climate models and their relation to the meridional overturning circulation comparison of the volumetric s distribution of models participating in the climate model intercomparison project 3 cmip3 indicates that these models differ widely in their ability to represent the thermohaline properties of water masses relationships between features of the quasi equilibrium hydrographic mean state of these models and aspects of their overturning circulations are investigated this is achieved quantitatively with the help of seven diagnostic hydrographic stations these few stations were specifically selected to provide a minimalist schematic of the global water mass system relationships between hydrographic conditions in the north atlantic measured with a subset of these stations suggest that hydrographic properties in the subpolar north atlantic are set by the circulation field of each model pointing towards deficiencies in the models ability to resolve the gulf stream north atlantic current system as a major limitation since diapycnal mixing and viscosity parameterizations differ across cmip3 models and exert a strong control on the overturning it is likely that these architectural differences ultimately explain the main across model differences in overturning circulation temperature and salinity in the north atlantic the analysis of properties across the quasi equilibrium states of the cmip3 models agrees with previously reported relationships between meridional steric height gradients or horizontal density contrasts at depth and the strength of the deep water cell robust relationships are also found in the southern ocean linking measures of vertical stratification with the strength of the abyssal circulations across the cmip3 models consistent correlations between aspects of the quasi equilibrium hydrography in the southern ocean and the sensitivity of the abyssal cell to increasing radiative forcing by 2100 were found using these relations in conjunction with modern hydrographic observations to interpolate the fate of the abyssal cell suggests that the southern abyssal cell may decrease by roughly 20 by the end of the century similar systematic relationships between the quasi equilibrium hydrographic states of the models and the sensitivity of their atlantic deep water cell could not be found

on the transport and angular momentum balance of channel models of the antarctic circumpolar current abstract angular momentum balances are discussed both in general as well as in the context of simple channel models of the antarctic circumpolar current acc particular emphasis is placed on the close relationship between the angular momentum balance and the meridional circulation it is found that topographic form drag is established very early in the integration whereas interfacial form drag can take much longer to develop restrictions on the geostrophic portion of the meridional circulation imposed by zonally reconnecting potential vorticity contours in the upper ocean allow derivation of an estimate for the steady state transport the estimate assumes there to be little or no circumpolar flow at great depth an assumption that stems from the belief that the band of zonally reconnecting geostrophic contours in the southern ocean does not extend to the ocean floor the predicted transport is proportional to the strength of the stratification and compares favorably with numerical results in the lite

subduction over the southern indian ocean in a high resolution atmosphere ocean coupled model this study examines the subduction of the subantarctic mode water in the indian ocean in an ocean atmosphere coupled model in which the ocean component is eddy permitting the purpose is to assess how sensitive the simulated mode water is to the horizontal resolution in the ocean by comparing with a coarse resolution ocean coupled model subduction of water mass is principally set by the depth of the winter mixed layer it is found that the path of the agulhas current system in the model with an eddy permitting ocean is different from that with a coarse resolution ocean this results in a greater surface heat loss over the agulhas return current and a deeper winter mixed layer downstream in the eddy permitting ocean coupled model the winter mixed layer depth in the eddy permitting ocean compares well to the observations whereas the winter mixed layer depth in the coarse resolution ocean coupled model is too shallow and has the wrong spatial structure to quantify the impacts of different winter mixed depths on the subduction a way to diagnose local subduction is proposed that includes eddy subduction it shows that the subduction in the eddy permitting model is closer to the observations in terms of the magnitudes and the locations eddies in the eddy permitting ocean are found to 1 increase stratification and thus oppose the densification by northward ekman flow and 2 increase subduction locally these effects of eddies are not well reproduced by the eddy parameterization in the coarse resolution ocean coupled model

understanding the regional variability of eddy diffusivity in the pacific sector of the southern ocean a diagnostic framework is presented based on the nakamura effective diffusivity to investigate the regional variation in eddy diffusivity comparison of three different diffusivity calculations enables the effects of locally enhanced tracer diffusion to be distinguished from the streamwise average it also enables the distinction to be made between locally generated complexity in the tracer structure and that advected into a particular domain the technique is applied to the pacific sector of the southern ocean the results highlight the important role that the mean flow plays in determining eddy diffusivity the effective diffusivity is not simply related to the eddy kinetic energy in regions of a strong mean flow the eddy diffusivity can be suppressed even in the presence of moderately strong eddy activity conversely in a region of weak mean flow the eddy diffusivity can be enhanced even in the presence of only weak eddy activity this casts doubt on the ability of parameterizations based solely on the eddy kinetic energy to adequately characterize the eddy diffusivity in regions of strongly varying mean flow such as the southern ocean the results are however consistent with the eddy transport and mixing variability predicted by potential vorticity based arguments

eddy saturation of equilibrated circumpolar currents abstractthis study uses a sector configuration of an ocean general circulation model to examine the sensitivity of circumpolar transport and meridional overturning to changes in southern ocean wind stress and global diapycnal mixing at eddy permitting and finer resolution the sensitivity of circumpolar transport to forcing magnitude is drastically reduced at sufficiently high resolution there is little or no sensitivity of circumpolar transport to wind stress even in the limit of no wind in contrast the meridional overturning circulation continues to vary with southern ocean wind stress but with reduced sensitivity in the limit of high wind stress both the circumpolar transport and meridional overturning continue to vary with diapycnal diffusivity at all model resolutions the circumpolar transport becomes less sensitive to changes in diapycnal diffusivity at higher resolution although sensitivity always remains in contrast the overturning circulation is more sensitive to change in diapycnal

topographic influences on wind driven stratified flow in a β plane channel an idealized model for the antarctic circumpolar current abstract topographic influences are examined in an eddy resolving model of oceanic channel flow forced by steady zonal winds with small explicit lateral friction transient eddies generated by the baroclinic instability of the mean flow transfer momentum downward to the bottom layer in the flat bottom case bottom friction is the only efficient sink of eastward momentum when bottom topography is present the topographic form stress can replace the bottom friction sink in the momentum budget and a large decrease of the zonal transport results large wale topography of the scale of the forcing provides the largest form stress topographic effects decay with height as suggested by the prandit scaling and therefore only topographic scales larger than the rossby radius can affect the whole water column in that case the interfaces are deformed by standing eddies on topographic length scales and standing eddies replace transient eddies in transferring momentum downward the bottom layer mean streamfunc

on the dynamics of wind driven circumpolar currents the factors controlling the transport of the antarctic circumpolar current acc have recently been a topic of heated debate at the latitudes of drake passage potential vorticity contours are uninterrupted by coastlines and large amplitude flows are possible even with weak forcing and dissipation the relationship between the dynamics of circumpolar currents and inertial recirculations in closed basins is discussed in previous studies sverdrup balance and baroclinic adjustment theories have both been proposed as theories of the acc transport these theories predict the circumpolar transport as various simple functions of the surface wind stress a series of experiments is performed with a simple channel model with different wind strengths and different idealized basin geometries to investigate the relationship between wind strength and circumpolar transport the results show that baroclinic adjustment theories do predict transport in the special case of a periodic channel with no topographic variations or when the wind forcing is very weak more generally the transport is determined by a complex interplay between wind forcing eddy fluxes and topographic effects there is no support for the idea that sverdrup balance determines the transport through drake passage

a linear equivalent barotropic model of the antarctic circumpolar current with realistic coastlines and bottom topography abstract a linear equivalent barotropic eb model is applied to study the effects of the bottom topography h and baroclinicity on the total transport and the position of the antarctic circumpolar current acc the model is based on the observation of killworth that the time mean velocity field of the fram model is self similar in the vertical a realistic large scale topography h is constructed by filtering 5 minute resolution data with an appropriate smoothing kernel it is shown that the asymptotic behavior of the solution of the barotropic model a particular case of the eb model in the limit of very small bottom friction depends on subtle details of topography and basin geometry given the uncertainties of the smoothing procedure the authors conclude that the barotropic model is not robust with respect to possible variations of model topography the authors found that the eb model with a vertical profile function similar to that of killworth reproduces the major features of the time and depth ave

meridional transport across a zonal channel topographic localization experiments are performed using a two layer isopycnic numerical model in a zonal channel with a large meridional topographic ridge in the lower layer the model is forced only by a steady meridional volume transport in the upper layer and develops a current structure similar to the antarctic circumpolar current meridional volume flux across time mean geostrophic streamlines is found to be due to a combination of the geostrophic eddy bolus flux and the lateral reynolds stress the proportion of each depends on the strength of the forcing the reynolds stress increases with the forcing while the bolus flux is relatively constant topography localizes the eddy fluxes at and downstream of the topography where eddy energies are greatest the strength of the zonal transport is governed by the onset of baroclinic instability and so is relatively insensitive to the strength of the meridional transport

what sets the surface eddy mass flux in the southern ocean abstract the ocean circulation and climate advanced modelling occam global eddy permitting ocean general circulation model has been used to investigate the surface eddy mass flux in the southern ocean the isopycnal eddy mass flux in the surface layer is almost uniformly poleward and scales well with the local ekman transport this seems at odds with other models and observations suggesting topographic localization of the eddy fluxes with locally large rotational components integrated over the thermocline depth the eddy fluxes do show such topographic localization the surface eddy mass flux is mainly a consequence of the intermittent deepening of the mixed layer with the seasonal cycle which redistributes the ekman transport over the stack of layers that eventually become ventilated baroclinic instability gives rise to much smaller eddy induced transports independent of the framework in which the residual mean flow is analyzed isopycnal or geometric the eddy induced transport that opposes the

connection between the decadal variability in the southern ocean circulation and the southern annular mode previous studies demonstrated the remarkable upward trend of the southern annular mode sam and southern ocean wind stress in association with anthropogenic forcing an oceanic reanalysis data set is used to investigate the response of the circulation in the southern ocean to the decadal variability of sam our results indicate the strengthening and the poleward shift of the northward ekman velocity as well as the ekman pumping rate which led to a corresponding strengthening trend in the deacon cell this strengthening in turn intensified the meridional density gradient and the tilting of the isopycnal surfaces on the interannual time scale the antarctic circumpolar currents acc transport exhibits a positive correlation with sam index as seen separately in observations however there is no significant trend in the total transport of acc possible reasons are discussed

southern ocean velocity and geostrophic transport fields estimated by combining jason altimetry and argo data zonal geostrophic velocity fields above 1975 dbar have been estimated for the southern ocean from 2004 into 2011 based on sea surface topography observed by jason altimetry and temperature salinity measured by argo autonomous floats the velocity at 1000 dbar estimated with the method has been compared to argo drift trajectory at the same pressure level available from the asia pacific data research center apdrc the inferred velocities agree with those from the argo drift within the estimated sampling error of the latter but have fewer gaps in space and time the velocity has also been integrated from depth to surface to determine the mean and time variable zonal geostrophic transport in the southern ocean between 29 5 s and 58 5 s primarily in the south atlantic and south indian ocean basins due to limitations in coverage of argo analysis shows errors can be reduced by 70 by averaging gridded results over wide areas zonal transport averaged over the entire indian ocean basin shows a significant correlation with the antarctic oscillation aao at low frequencies transport is higher than normal during a positive phase of the aao and lower during the negative phase

potential vorticity constraints on the dynamics and hydrography of the southern ocean abstract constraints on the hydrography and geostrophic velocity shear of the southern ocean implicit in its potential vorticity field are discussed and illustrated by diagnostic study of observed and modeled potential vorticity fields a stress driven thermodynamically inactive eddy resolving quasigeostrophic model of the southern ocean suggests that through the systematic erosion of potential vorticity gradients by geostrophic eddies the large scale flow equilibrates toward a state in which interior potential vorticity gradients are small observations of the large scale isopycnal distribution of potential vorticity ipv deduced from climatological hydrographic data reveal a much richer structure the most striking feature of the ipv field is the presence of large near surface gradients of ipv a vortocline coinciding with the axis of the antarctic circumpolar current acc to the south of this front potential vorticity pv is large to its north pv has low values and relative to those foun

configuration of a southern ocean storm track abstractdiagnostics of ocean variability that reflect and influence local transport properties of heat and chemical species vary by an order of magnitude along the southern ocean s antarctic circumpolar current acc topographic hotspots are important regions of localized transport anomalies this study uses a primitive equation channel model to investigate the structure of eddy kinetic energy eke one measure of variability in an oceanic regime a storm track approach emphasizes the importance of stationary eddies which result from flow interactions with topography on setting eke distributions the influence of these interactions extends far downstream of the topography and impacts eke patterns through localized convergence and divergence of heat unlike for zonal averages local contributions to the stationary fluxes from terms that integrate to zero in a zonal average are important the simulations show a strong sensitivity of the zonal structure as well as the distribution and amplitude of sta

an exploration of the role of transient eddies in determining the transport of a zonally reentrant current the meridional ekman transport in a zonally reentrant channel may be balanced by diabatic circulations standing eddies associated with topography or by lagrangian mean eddy mass fluxes a simple model is used to explore the interaction between these mechanisms a key assumption of this study is that diabatic forcing in the poleward edge of the channel acts to create lighter fluid as is the case with net freshwater fluxes into the southern ocean for weak wind forcing or strong diabatic constraint a simple scaling argument accurately predicts the level of baroclinic shear however given our understanding of the relative magnitudes of ekman flux and deep upwelling this is not the appropriate parameter range for the antarctic circumpolar current with stronger wind stresses eddies are prominent with baroclinic instability initially developing in the vicinity of large topography arguments have been advanced by a number of authors that baroclinic instability should limit the velocity shear leading to a stiff upper limit on the transport of the current however in the simulations presented here baroclinic instability is largely confined to the region of topographic highs and the approach to a current that is independent of the wind stress occurs gradually several recent parameterizations of transient eddy fluxes do not reproduce key features of the observed behavior

scales of spatial and temporal variability in the southern ocean spatial and temporal variability in geosat altimeter data is used to consider whether the antarctic circumpolar current acc responds on the large scale to global changes in wind forcing or whether its variability is primarily mesoscale the geosat data indicate a spatial decorrelation scale of 85 km and a temporal e folding scale of 34 days using these scales to define autocovariance functions the sea surface height variability is objectively mapped the resulting maps indicate substantial evidence of mesoscale eddy activity over 17 day time intervals meanders of the polar front and subantarctic front appear to elongate break off as rings and propagate statistical analysis of acc variability from altimeter data is conducted using empirical orthogonal functions eofs the first mode eof describes 16 of the variance in total sea surface height across the acc reducing the domain into basin scales does not significantly increase the variance represented by the first eof suggesting that the scales of motion are relatively short and may be determined by local instability mechanisms rather than larger basin scale processes likewise neither complex nor extended eofs indicate statistically significant traveling wave modes

the dynamics of the antarctic circumpolar current abstract the dynamics of the antarctic circumpolar current acc in a near eddy resolving model of the southern ocean fram are investigated a streamwise coordinate system is used rather than a more conventional approach of considering zonally averaged quantities the motivation for this approach is the large deviation from a purely zonal flow made by the current comparisons are made with a zonal mean analysis of the same model it is found that the topographic form drag is the main sink of the momentum that is input by the wind however in contrast to a zonal mean analysis other terms namely horizontal mixing bottom friction and advection of momentum are no longer negligible the total effect of transient eddies is to produce a drag on the mean flow again in contrast to the zonally averaged case the vertical penetration of stress is considered a generalized formula is derived for the interfacial form stress averaged along a convoluted path and that includes nonquasigeostrophic effects the i

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

the role of mesoscale eddies in the remote oceanic response to altered southern hemisphere winds abstract it has been suggested that a strengthening of the southern hemisphere winds would induce a more vigorous overturning through an increased northward ekman flux bringing more light waters into the oceanic basins and enhancing the upwelling of north atlantic deep water in the southern ocean thereby increasing ocean ventilation simulations from a coarse and a fine resolution version of a coupled model subject to idealized wind stress changes in the southern ocean are presented in the fine resolution eddy permitting model changes in poleward eddy fluxes largely compensate for the enhanced equatorward ekman transport in the southern ocean as a consequence northward transport of light waters pycnocline depth northern hemisphere overturning and southern ocean upwelling anomalies are much reduced compared with simulations in the coarse resolution model with parameterized eddies these results suggest a relatively weak sensitivity of present day global ocean overturning circulation to the proj

large scale oceanographic constraints on the distribution of melting and freezing under ice shelves previous studies suggest that ice shelves experience asymmetric melting and freezing topography may constrain oceanic circulation and thus basal melt freeze patterns through its influence on the potential vorticity pv field however melting and freezing induce a local circulation that may modify locations of heat transport to the ice shelf this paper investigates the influence of buoyancy fluxes on locations of melting and freezing under different bathymetric conditions an idealized set of numerical simulations the decoupled simulations employs spatially and temporally fixed diapycnal fluxes these experiments in combination with scaling considerations indicate that while flow in the interior is governed by large scale topographic gradients recirculation plumes dominate near buoyancy fluxes thermodynamically decoupled models are then compared to those in which ice ocean heat and freshwater fluxes are driven by the interior flow the coupled simulations near the southern boundary strong cyclonic flow forced by melt induced upwelling drives inflow and melting to the east recirculation is less evident in the upper water column as shoaling of meltwater freshened layers dissipates the dynamic influence of buoyancy forcing yet freezing remains intensified in the west in coupled simulations the flow throughout the cavity is relatively insensitive to bathymetry stratification the slope of the ice shelf and strong meridionally distributed buoyancy fluxes weaken its influence

eddy saturation of the southern ocean a baroclinic versus barotropic perspective eddy saturation is the regime in which the total time mean volume transport of an oceanic current is relatively insensitive to the wind stress forcing and is often invoked as a dynamical descript

jet topography interactions affect energy pathways to the deep southern ocean abstractin the southern ocean strong eastward ocean jets interact with large topographic features generating eddies that feed back onto the mean flow deep reaching eddies interact with topography where turbulent dissipation and generation of internal lee waves play an important role in the ocean s energy budget however eddy effects in the deep ocean are difficult to observe and poorly characterized this study investigates the energy contained in eddies at depth when an ocean jet encounters topography this study uses a two layer ocean model in which an imposed unstable jet encounters a topographic obstacle either a seamount or a meridional ridge in a configuration relevant to an antarctic circumpolar current frontal jet the authors find that the presence of topography increases the eddy kinetic energy eke at depth but that the dominant processes generating this deep eke depend on the shape and height of the obstacle as well as on the baroclinicity of the jet before it encounters topography i

wind driven spreading of fresh surface water beneath ice shelves in the eastern weddell sea solar heated fresh antarctic surface water asw is a permanent feature along the eastern weddell sea ews coast in summer down to a depth of roughly 200 m recently asw has been observed beneath the fimbul ice shelf suggesting that it might play an important role in basal melting we propose that wind driven coastal downwelling is the main mechanism that spreads asw beneath the ice shelf in this sector of antarctica we validate this hypothesis with observations scaling analyses and numerical modeling along three principle lines i data analyses of about 1500 salinity profiles collected by instrumented seals indicate that the observed freshening of the coastal water column is likely explained by the on shore ekman transport and subsequent downwelling of asw ii an analytical model of the coastal momentum balance indicates that wind driven downwelling is capable of depressing the buoyant surface water to a depth similar to the ice shelf draft and iii simulations from both idealized and regional eddy resolving numerical ice shelf ocean models support our proposition our main conclusion is that wind driven spreading of asw beneath the ice shelf occurs when downwelling exceeds the depth of the ice shelf base furthermore our study adds to the understanding of the oceanic processes at the antarctic slope front in the ews with possible implications for other sectors of antarctica

baroclinic adjustment in drake passage driven by tropical pacific forcing the time series of high quality hydrographic measurements of the acc in drake passage is revisited to investigate the extent of baroclinic adjustment in the region on time scales of up to two decades we find that substantial adjustment of the upper kilometer of northern drake passage has occurred on interannual to decadal time scales driven primarily by tropical pacific forcing via the poleward propagation of boundary waves the decadal scale signal consists of a marked deepening of isopycnals by 200m between the early 1990s and the late 2000s and reflects enso forced wind driven baroclinic changes over an extensive region of the eastern south pacific 2012 american geophysical union all rights reserved

wind driven flow over topography in a zonal β plane channel a quasi geostrophic model of the antarctic circumpolar current abstract the paper gives a detailed account of the dynamical balance of a wind driven zonally unbounded flow over topography the problem is investigated with a quasi geostrophic plane channel with two layers and eddy resolution the channel has a width of 1500 km and a zonal periodicity of 4000 km apart from the dimensions the model structure is similar to the one used by mcwilliams et al the experiments with this model address the problem of the relative role of transient and standing eddies as well as bottom friction and topographic form stress in the balance of a current driven by a steady surface windstress the response of the system is investigated for different values of the friction parameter and various locations of topographic obstacles in the bottom layer of the channel the principal momentum balance emerging from these experiments supports the concept of munk and palmen for the dynamics of the antarctic circumpolar current which proposes that the momentum input by windstress is transfe

sensitivity of antarctic circumpolar current transport and eddy activity to wind patterns in the southern ocean abstractthe southern hemisphere westerly winds have intensified in recent decades associated with a positive trend in the southern annular mode sam however the response of the antarctic circumpolar current acc transport and eddy field to wind forcing remains a topic of debate this study uses global eddy permitting ocean circulation models driven with both idealized and realistic wind forcing to explore the response to interannual wind strengthening the response of the barotropic and baroclinic transports and eddy field of the acc is found to depend on the spatial pattern of the changes in wind forcing in isolation an enhancement of the westerlies over the acc belt leads to an increase of both barotropic and baroclinic transport within the acc envelope with lagged enhancement of the eddy kinetic energy eke in contrast an increase in wind forcing near antarctica drives a largely barotropic change in transport along closed f h contours free mode with little change in eddy activity under

robustness of an effective diffusivity diagnostic in oceanic flows the robustness of nakamura s effective diffusivity diagnostic for quantifying eddy diffusivities of tracers within a streamwise average framework is carefully examined in an oceanographic context and the limits of its applicability are detailed the near surface southern ocean geostrophic flow obtained from altimetric observations is chosen for particular examination since it exhibits strong eddy activity careful analysis indicates that the effective diffusivity calculation is not strongly sensitive to the resolution of the velocity field or to the tracer initial conditions after a spinup time of 3 months and that a numerical diffusivity of 50 m 2 s 21 is appropriate to resolve the batchelor scale at a computational resolution of 1 208 the spatial variability of the effective diffusivity is examined and shown to be related to the eddy kinetic energy eke mean zonal current u and phase speed of the eddies c following the streamwise averge of eke u 2 c 2 in agreement with linear baroclinic instability theory the temporal variability of the effective diffusivity over the period 1996 2001 indicates significant variability on seasonal time scales with a magnitude of 610 215 interannual variations are investigated and discussed in the context of changes in the wind stress associated with the southern annular mode

comparing idealized and complex topographies in quasigeostrophic simulations of an antarctic circumpolar current abstractthe circumpolar transport of a wind driven quasigeostrophic antarctic circumpolar current is considered simple theory suggests transport in a strongly forced regime the focus of this study is largely determined by a partitioning of the southward sverdrup flux into drake passage latitudes some streamlines feed a basin contribution to the circumpolar transport and others feed a large scale recirculation gyre simulations assuming an idealized scotia ridge topography are considered to test for sensitivity to resolution considerable sensitivity to both vertical and horizontal resolution is found and associated with this is a tight stationary eddy trapped on the western flank of the ridge that is this eddy is sensitive to resolution and exerts an influence that acts to reduce the circumpolar transport simulations using the scotia ridge like topography are also compared to others using more realistic topography in the idealized ridge topography experiments there is only a single ridge again

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

implementation of a geometrically informed and energetically constrained mesoscale eddy parameterization in an ocean circulation model abstractthe global stratification and circulation as well as their sensitivities to changes in forcing depend crucially on the representation of the mesoscale eddy field in a numerical ocean circ

southern ocean response to strengthening winds in an eddy permitting global climate model abstract a global climate model with horizontal resolutions in the ocean ranging from relatively coarse to eddy permitting is used to investigate the resolution dependence of the southern ocean response to poleward intensifying winds through the past and present centuries the higher resolution simulations show poleward migration of distinct ocean fronts associated with a more highly localized near surface temperature response than in the lower resolution simulations the higher resolution simulations also show increasing southward eddy heat transport less high latitude cooling and greater sea ice loss than the lower resolution simulations for all resolutions from relatively coarse to eddy permitting there is poleward migration of the antarctic circumpolar current in the atlantic and the western half of the indian basin finally zonal transports associated with the antarctic circumpolar current are shown to be sensitive to resolution and this is discussed in the context of recent observed change

eemian tropical and subtropical african moisture transport an isotope modelling study during the last interglacial insolation maximum eemian mis 5e the tropical and subtropical african hydrological cycle was enhanced during boreal summer months the climate anomalies are examined with a general circulation model echam4 that is equipped with a module for the direct simulation of 18o and deuterium h218o and hdo respectively in all components of the hydrological cycle a mechanism is proposed to explain the physical processes that lead to the modelled anomalies differential surface heating due to anomalies in orbital insolation forcing induce a zonal flow which results in enhanced moisture advection and precipitation increased cloud cover reduces incoming short wave radiation and induces a cooling between 10 n and 20 n the isotopic composition of rainfall at these latitudes is therefore significantly altered increased amount of precipitation and stronger advection of moisture from the atlantic result in isotopically more depleted rainfall in the eemian east african subtropics compared to pre industrial climate the east west gradient of the isotopic rainfall composition reverses in the eemian simulation towards depleted values in the east compared to more depleted western african rainfall in the pre industrial simulation the modelled re distribution of 18o and d is the result of a change in the forcing of the zonal flow anomaly we conclude that the orbitally induced forcing for african monsoon maxima extends further eastward over the continent and leaves a distinct isotopic signal that can be tested against proxy archives such as lake sediment cores from the ethiopian region

topographic control of basin and channel flows the role of bottom pressure torques and friction the topographical control of western boundary currents within a basin and zonal jets in a channel is investigated in terms of the potential vorticity pv and barotropic vorticity bv the curl of the depth integrated velocity budgets using isopycnic adiabatic wind driven experiments along the western boundary the wind driven transport is returned across latitude lines by the bottom pressure torque while friction is only important in altering the pv within an isopycnic layer and in allowing a closed circulation these contrasting balances constrain the geometry of the flow through integral relationships for the bv and pv for both homogenous and stratified basins with sloping sidewalls the northward subtropical jet separates from the western wall and has opposing frictional torques on either side of the jet which cancel in a zonal integral for bv but alter the pv within a layer streamline in a channel with partial topographic barriers the bottom pressure torque is again important in returning wind driven flows along western boundaries and in transferring bv from neighboring wind driven gyres into a zonal jet the depth integrated flow steered by topography controls where the bottom friction alters the pv which can lead to different pv states being attained for separate subbasins along a channel

the response of southern ocean eddies to increased midlatitude westerlies a non eddy resolving model study 1 the midlatitude westerlies of the southern hemisphere have intensified since the 1970s non eddy resolving general circulation models respond to such wind intensification with steeper isopycnals a faster antarctic circumpolar current acc and a stronger atlantic meridional overturning circulation amoc however hydrographic observations show little change in the slope of the southern ocean isopycnals over the past 40 years this insensitivity seems to result from a compensating mechanism whereby an initial increase in the slope of the isopycnals causes eddy activity to intensify and forces the isopycnal slopes down climate models do not yet resolve ocean eddies and the eddy parameterizations included in them do not capture well the compensation mechanism mentioned above we present simulations with a non eddy resolving model incorporating an eddy parameterization in which eddy compensation is greatly enhanced by the use of a non constant spatially varying thickness diffusivity the sensitivity of the simulated acc and amoc to increased southern hemisphere westerlies is greatly reduced compared to simulations using constant and uniform diffusivities

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

constraints on rift propagation history at the cobb offset juan de fuca ridge from numerical modeling of tectonic fabric abstract rift propagation has been proposed as the mechanism for the reorganization of the juan de fuca ridge during the cenozoic in particular magnetic anomalies in the cobb offset region of this ridge have been interpreted in terms of a complex dueling propagator history in which rift propagation has occurred alternately in opposing directions on adjacent ridge segments although the overall propagation history has been inferred from the magnetic data these data are limited in their resolution for the detailed history by the length of time during which magnetic polarity remains constant we thus examine whether more detailed resolution within polarity chrons can be obtained using seafloor tectonic fabric from acoustic imagery by modeling lineations as isochrons synthetic fabric can be generated for a given rift propagation history and the fit of the model to the data can be compared numerically for the cobb we used a propagation history model with a given timing and direction of propagation and generated synthetic fabric for different propagation rates the fabric is best fit for propagation rates between 400 and 900 mm yr slower 900 mm yr propagation rates yield slightly poorer fits the fabric results are thus in accord with the interpretation based on magnetics alone that the rates of rift propagation significantly exceeded the spreading rates an interesting feature of the model is that the complex tectonic fabric is reasonably well fit on this scale 1 km by simple rift propagation models which do not invoke shear zone behavior and thus deviate from rigid plate tectonics

buoyancy driven circulation in an ocean basin with isopycnals intersecting the sloping boundary abstract the dynamics that govern the spreading of a convectively formed water mass in an ocean with sloping boundaries are examined using an isopycnal model that permits the interface between the layers to intersect the sloping boundaries the simulations presented here use a two layer configuration to demonstrate some of the pronounced differences in a baroclinically forced flow between the response in a basin with a flat bottom and vertical walls and a more realistic basin bounded by a sloping bottom each layer has a directly forced signal that propagates away from the forcing along the potential vorticity pv contours of that layer paired opposed boundary currents are generated by refracted topographic rossby waves rather than kelvin waves it is impossible to decompose the flow into globally independent baroclinic and barotropic modes topography causes the barotropic i e depth averaged response to buoyancy forcing to be just as strong as the baroclinic response because layer pv contours di

what sets the mean transport through drake passage twelve experiments with two coarse resolutions of a global ocean model using a variety of surface forcings are analyzed to address the question of what sets the mean transport through drake passage seven of the experiments do not have an active sea ice model but the remaining five do previous theories have suggested that the drake passage transport is governed by the cape horn sverdrup transport or alternatively is proportional to the square root of the meridional ekman transport at the latitude of drake passage the results presented here do not support either of these theories the drake passage transport depends quite strongly on the isopycnal diffusivity parameter in the model and less strongly on the background vertical diffusivity and horizontal viscosity parameters however when the magnitudes of these parameters are fixed the results show a very strong correlation between drake passage transport and both the strength of the meridional ekman transport at the latitude of drake passage and the thermohaline circulation off the antarctic shelf the relationships are monotonic but not linear the best estimate is that the meridional ekman transport drives 100 sv of drake passage transport while the remaining 30 sv are driven by the global thermohaline circulation

evolution of initially axisymmetric buoyancy jets a numerical study baroclinic evolution of coastal currents associated with a surface front over variable topography is investigated numerically employing a two layer frontal geostrophic model in the frontal geostrophic dynamical limit the frontal layer velocity is geostrophic to leading order however the advective terms from the momentum equation are accounted for in the leading order balance and the mass conservation equation includes both the temporal and advective contributions in this study special focus is placed on the role of topography and the development of coherent vortex features perturbed axisymmetric currents in an annular domain are seen to develop typical breaking wave instabilities at the outside edge in general the topography is found to be a destabilizing influence however very steep topography is shown to inhibit cross front motions and thus delay subsequent vortex shedding in simulations involving a continuous source of buoyant fluid the ambient layer exhibits a marked increase in anticyclonic vorticity which prevents eddy pinch off the results are compared with previous laboratory experiments involving surface currents and implications for the stability of fronts at a shelf break are discussed

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

semi adiabatic model of the deep stratification and meridional overturning abstractan analytical model of the atlantic deep stratification and meridional overturning circulation is presented that illustrates the dynamic coupling between the southern ocean and the midlatitude gyres the model expressed here in terms of the two and a half layer framework predicts the stratification and meridional transport as a function of the mechanical and thermodynamic forcing at the sea surface the approach is based on the classical elements of large scale circulation theory ideal thermocline inertial western boundary currents and eddy controlled antarctic circumpolar current acc models which are combined to produce a consistent three dimensional view of the global overturning the analytical tractability is achieved by assuming and subsequently verifying that the pattern of circulation in the model is largely controlled by adiabatic processes the time mean and eddy induced isopycnal advection of buoyancy the mean stratification of the lower thermocline is determined by the surface fo

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

role of residual overturning for the sensitivity of southern ocean isopycnal slopes to changes in wind forcing abstractthe antarctic circumpolar current plays a central role in the ventilation of heat and carbon in the global ocean in particular the isopycnal slopes determine where each water mass outcrop

isopycnal momentum budget of the antarctic circumpolar current in the fine resolution antarctic model abstract the momentum budget of the antarctic circumpolar current acc is analyzed using data from the fine resolution antarctic model fram using density as a vertical coordinate since density is approximately conserved on streamlines this steady budget is balanced volume heat and salt budgets are also computed within density layers although these remain time dependent the leading order momentum balance is of approximate equality of the form drag on the top and bottom surfaces of each undulating density layer but because density layers outcrop at the surface this does not imply that the surface wind stress is simply transferred downward by form drag without change of amplitude restricting attention to the net form drag on a layer this is found to be balanced by coriolis force and if the layer outcrops the amount of wind stress put into the layer where it outcrops between 40 and 60 of the density layers at any latitude outcrop somewhere at the surface so that wind stress can be moved d

the net transport of the antarctic circumpolar current through drake passage abstract estimates of the net transport through drake passage are made for three periods during which the year long drake 79 current meter array spanning the passage was in operation relative geostrophic shears from hydrographic surveys in january 1979 april 1979 and january 1980 were referenced to directed speed measurements to give profiles of net speed direct measurements were averaged in time to make them more compatible with the spatially averaged baroclinic shears the agreement between directly measured and baroclinic shears is generally good except in regions of large bathymetric relief and during periods when current cores were shifting past or between moorings the presence of cold core rings during two of the drake 79 hydrogaphic surveys resulted in intensified flow and increased transport within fronts but did not affect the net transport through the passage the three latest estimates of net transport 117 144 and 134 106 m3 s 1 are in close agreement with a previous estimate of 124