suppression of eddy diffusivity across jets in the southern ocean abstract geostrophic eddies control the meridional mixing of heat carbon and other climatically important tracers in the southern ocean the rate of eddy mixing is typically quantified through an eddy diffusivity there is an ongoing debate as to whether eddy mixing in enhanced in the core of the antarctic circumpolar current or on its flanks a simple expression is derived that predicts the rate of eddy mixing that is the eddy diffusivity as a function of eddy and mean current statistics this novel expression predicts suppression of the cross jet eddy diffusivity in the core of the antarctic circumpolar current despite enhanced values of eddy kinetic energy the expression is qualitatively and quantitatively validated by independent estimates of eddy mixing from altimetry observations this work suggests that the meridional eddy diffusivity across the antarctic circumpolar current is weaker than presently assumed because of the suppression of eddy mixing by the strong zonal current

major trace and rare earth element geochemistry of the archaean maggo gneisses southern nain province labrador the early middle archaean maggo gneisses of the southern nain province labrador form the southwest portion of the once contiguous north atlantic nutak craton nac the gneisses and their late middle archaean metamorphosed and migmatized equivalents are typical of grey gneiss terranes exposed worldwide geochemically the gneisses exhibit a continuous range of composition from 53 7 to 78 4 wt sio2 and straddle the boundary between low and high al trondhjemites major element distributions are comparable to those of other archaean aged nac gneisses amitsoq uivak and nuk gneisses however the na2o and k2o contents are scattered the gneisses are depleted in k rb and ba are enriched in sr and have high field strength element distributions similar to those for nac gneisses rare earth element ree patterns of maggo gneisses can be subdivided on the basis of the nature of the eu anomaly into two groups of samples i with negative eu anomalies and higher ree contents and ii with positive

eddies observed by argo floats part ii form stress and streamline length in the southern ocean abstractargo floats measure horizontal current velocities at the parking depth and vertical profiles of temperature and salinity these data were used to study the roles that eddies play in the dynamics of the antarctic circumpolar current acc in the southern ocean a zonal momentum budget was quantified in a box spanning the latitudes of the drake passage and bounded by the sea surface and the 1000 dbar depth the input of eastward zonal momentum from the wind 17 1 1011 n was approximately twice the downward transfer of eastward momentum across the isopycnal whose mean depth was 1000 dbar which was mediated via form stress carried by eddies 8 1 1 9 1011 n the zonal momentum budget was closed to within uncertainty meaning that the momentum not accounted for by eddies was explained by the coriolis term associated with meridional transport the form stress was spatially concentrated near meridional ridges particularly on their eastern flanks the localization was extreme 7 of the total a

atlantic meridional overturning circulation during the last glacial maximum the circulation of the deep atlantic ocean during the height of the last ice age appears to have been quite different from today we review observations implying that atlantic meridional overturning circulation during the last glacial maximum was neither extremely sluggish nor an enhanced version of present day circulation the distribution of the decay products of uranium in sediments is consistent with a residence time for deep waters in the atlantic only slightly greater than today however evidence from multiple water mass tracers supports a different distribution of deep water properties including density which is dynamically linked to circulation

internal waves and turbulence in the antarctic circumpolar current abstractthis study reports on observations of turbulent dissipation and internal wave scale flow properties in a standing meander of the antarctic circumpolar current acc north of the kerguelen plateau the authors characterize the intensity and spatial distribution of the observed turbulent dissipation and the derived turbulent mixing and consider underpinning mechanisms in the context of the internal wave field and the processes governing the waves generation and evolution the turbulent dissipation rate and the derived diapycnal diffusivity are highly variable with systematic depth dependence the dissipation rate is generally enhanced in the upper 1000 1500 m of the water column and both the dissipation rate and diapycnal diffusivity are enhanced in some places near the seafloor commonly in regions of rough topography and in the vicinity of strong bottom flows associated with the acc jets turbulent dissipation is high in regions where internal wave energy is high consistent with the idea that i

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

ocean and atmosphere storm tracks the role of eddy vorticity forcing abstract signatures of eddy variability and vorticity forcing are diagnosed in the atmosphere and ocean from weather center reanalysis and altimetric data broadly covering the same period 1992 2002 in the atmosphere there are localized regions of eddy variability referred to as storm tracks at the entrance of the storm track the eddies grow providing a downgradient heat flux and accelerating the mean flow eastward at the exit and downstream of the storm track the eddies decay and instead provide a westward acceleration in the ocean there are similar regions of enhanced eddy variability along the extension of midlatitude boundary currents and the antarctic circumpolar current within these regions of high eddy kinetic energy there are more localized signals of high eady growth rate and downgradient eddy heat fluxes as in the atmosphere there are localized regions in the southern ocean where ocean eddies provide statistically significant vorticity forcing which acts to accelerate the mean flow

mean antarctic circumpolar current transport measured in drake passage the antarctic circumpolar current is an important component of the global climate system connecting the major ocean basins as it flows eastward around antarctica yet due to the paucity of data it remains unclear how much water is transported by the current between 2007 and 2011 flow through drake passage was continuously monitored with a line of moored instrumentation with unprecedented horizontal and temporal resolution annual mean near bottom currents are remarkably stable from year to year the mean depth independent or barotropic transport determined from the near bottom current meter records was 45 6 sv with an uncertainty of 8 9 sv summing the mean barotropic transport with the mean baroclinic transport relative to zero at the seafloor of 127 7 sv gives a total transport through drake passage of 173 3 sv this new measurement is 30 larger than the canonical value often used as the benchmark for global circulation and climate models

storm track dynamics this paper reviews the current state of observational theoretical and modeling knowledge of the midlatitude storm tracks of the northern hemisphere cool season observed storm track structures and variations form the first part of the review the climatological storm track structure is described and the seasonal interannual and interdecadal storm track variations are discussed in particular the observation that the pacific storm track exhibits a marked minimum during midwinter when the background baroclinicity is strongest and a new finding that storm tracks exhibit notable variations in their intensity on decadal timescales are highlighted as challenges that any comprehensive storm track theory or model has to be able to address physical processes important to storm track dynamics make up the second part of the review the roles played by baroclinic processes linear instability downstream development barotropic modulation and diabatic heating are discussed understanding of these processes forms the core of our current theoretical knowledge of storm track dynamics and provides a context within which both observational and modeling results can be interpreted the eddy energy budget is presented to show that all of these processes are important in the maintenance of the storm tracks the final part of the review deals with the ability to model storm tracks the success as well as remaining problems in idealized storm track modeling which is based on a linearized dynamical system are discussed perhaps on a more pragmatic side it is pointed out that while the current generation of atmospheric general circulation models faithfully reproduce the climatological storm track structure and to a certain extent the seasonal and enso related interannual variations of storm tracks in depth comparisons between observed and modeled storm track variations are still lacking

barotropic and baroclinic contributions to along stream and across stream transport in the antarctic circumpolar current the southern ocean s ability to store and transport heat and tracers as well as to dissipate momentum and energy are intimately related to the vertical structure of the antarctic circumpolar current acc here the partition between barotropic and baroclinic flow in the time mean acc is investigated in a southern ocean state estimate the zonal geostrophic transport is predominantly baroclinic with at most 25 of the transport at any longitude carried by the barotropic component following surface streamlines changes in vertical shear and near bottom velocity are large and result in changes in the local partition of barotropic baroclinic vertically integrated transport from 10 90 in the center of the basins to 50 50 near complex topography the velocity at depth is not aligned with the surface velocity this nonequivalent barotropic flow supports significant cross stream transports barotropic and baroclinic mass transport across the acc is on average in opposite directions with the net barotropic cross stream transport being poleward and the net baroclinic equatorward the sum partially cancels out leaving a net geostrophic poleward transport across the different fronts between 5 and 20 sv temperature is also transported across the fronts by the nonequivalent barotropic part of the acc with maximum values across the northern acc fronts equivalent to 0 2 pw the sign and magnitude of these transports are not sensitive to the choice of stream coordinate these cross stream volume and temperature transports are variable in space and dependent on the interactions between deep flow and bathymetry thus difficult to infer from surface and hydrographic observations alone

circulation and stirring in the southeast pacific ocean and the scotia sea sectors of the antarctic circumpolar currenta abstractthe large scale middepth circulation and eddy diffusivities in the southeast pacific ocean and scotia sea sectors between 110 and 45 w of the antarctic circumpolar current acc are described based on a subsurface quasi isobaric rafos float based lagrangian dataset these rafos float data were collected during the diapycnal and isopycnal mixing experiment in the southern ocean dimes the mean flow adjusted to a common 1400 m depth shows the presence of jets in the time averaged sense with speeds of 6 cm s 1 in the southeast pacific ocean and upward of 13 cm s 1 in the scotia sea these jets appear to be locked to topography in the scotia sea but aside from negotiating a seamount chain are mostly free of local topographic constraints in the southeast pacific ocean the eddy kinetic energy eke is higher than the mean kinetic energy everywhere in the sampled domain by about 50 the magnitude of the eke increases drastically by a factor of 2 or more as the current crosses over the hero and

acc meanders energy transfer and mixed barotropic baroclinic instability along stream variations in the dynamics of the antarctic circumpolar current acc impact heat and tracer transport regulate interbasin exchange and influence closure of the overturning circulation topography is primarily responsible for generating deviations from zonal mean properties mainly through standing meanders associated with regions of high eddy kinetic energy here an idealized channel model is used to explore the spatial distribution of energy exchange and its relationship to eddy geometry as characterized by both eddy momentum and eddy buoyancy fluxes variations in energy exchange properties occur not only between standing meander and quasi zonal jet regions but throughout the meander itself both barotropic and baroclinic stability properties as well as the magnitude of energy exchange terms undergo abrupt changes along the path of the acc these transitions are captured by diagnosing eddy fluxes of energy and by adopting the eddy geometry framework the latter typically applied to barotropic stability properties is applied here in the depth along stream plane to include information about both barotropic and baroclinic stability properties of the flow these simulations reveal that eddy momentum fluxes and thus barotropic instability play a leading role in the energy budget within a standing meander this result suggests that baroclinic instability alone cannot capture the dynamics of acc standing meanders a challenge for models where eddy fluxes are parameterized

measurements and analysis of setting velocities of natural quartz sand grains abstract settling velocities in water were measured for natural quartz sand grains in the sieve size range 0 75 to 1 50 0 35 1 68 mm obtained from beach sand samples the grains were selected so as to provide a wide range of roundness from very angular to very well rounded r 0 6 5 1 in a first set of experiments only two axial diameters were measured under a microscope the longest axial diameter d1 and the intermediate diameter di in a second data set the smallest axial diameter ds was measured as well settling velocities were measured in a 6 meter long settling tube it is found that the intermediate grain diameter is on average equal to the nominal diameter computed as dn dsdid1 1 3 this permits a simpler analysis of grain settling through microscopic measurements of di alone but does not allow for corrections of grain shape effects the measured settling velocity wm is then compared with the settling velocity of a sphere ws calculated using di as the sphere diameter and the relationship wm 0 977w0 913s is found to be consistent with the data an alternative approach is provided where ws is calculated directly from the sieve diameter and then corrected to the actual settling velocity wm for that mean grain size the results thus provide graphs and empirical relationships which permit the evaluation of actual grain settling rates rather than settling velocities of spheres from the sieve diameter or from microscopic measurements of di for individual grains these procedures provide a mean settling velocity that takes into consideration the average degree of grain irregularities found in natural sands a more detailed analysis is then conducted to examine the effects of roundness and sphericity on the settling of individual grains it is found that grain roundness has no measurable effect on the settling rate the settling velocity is shown to be a function of grain sphericity either expressed as the corey shape factor or e shape factor grain asymmetries ust also be important accounting for much of the scatter in the results

eddy forcing of the mean flow in the southern ocean we examine the force or acceleration exerted by eddies on the mean flow of the southern ocean using mapped sea surface height anomalies derived from satellite altimeter measurements we compare this with the position of jets in the mean flow indicated by regions of increased gradient in the mean sea surface temperature the eddy force itself is not of primary importance rather the curl of that force affects the mean flow so the irrotational force due to the gradient of eddy kinetic energy can be ignored leaving a remainder that can be interpreted in terms of the eddy vorticity flux unlike previous modeling and altimetric studies we find that the eddies act to decelerate some of the strongest jets and act as vorticity sources for others the strongest jets are clearly not accelerated by eddies a problem of interpretation does remain however since it is not clear how sea surface temperatures are related to the mean flow this ambiguity can only be completely resolved by measurement of the mean flow requiring a geoid that is accurate at length scales of order 100 km

understanding the structure of changes in the southern ocean eddy field the southern ocean is riddled with mesoscale eddies although just a few km in size these loops and vortices are key parts of the climate system and are important in controlling how ocean circulation responds to changes in forcing observations reveal that changes in the intensity of these eddies vary significantly around the southern ocean this contrasts with the nature of the atmospheric forcing which is more zonally symmetric recent progress using high resolution modeling has pinpointed where intrinsic variability dominates over wind driven variability and hence the areas where future responses to climatic changes in forcing are likely to be clearest

upper ocean eddy heat flux across the antarctic circumpolar current in drake passage from observations time mean and seasonal variability abstracteddy heat flux plays a fundamental role in the southern ocean meridional overturning circulation providing the only mechanism for poleward heat transport above the topography and below the

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

forced and intrinsic variability in the response to increased wind stress of an idealized southern ocean we use ensemble runs of a three layer quasigeostrophic idealized southern ocean model to explore the roles of forced and intrinsic variability in response to a linear increase of wind stress imposed over a 30 year period we find no increase of eastward circumpolar volume transport in response to the increased wind stress a large part of the resulting time series can be explained by a response in which the eddy kinetic energy is linearly proportional to the wind stress with a possible time lag but no statistically significant lag is found however this simple relationship is not the whole story several intrinsic timescales also influence the response we find an e folding timescale for growth of small perturbations of 1 2 weeks the energy budget for intrinsic variability at periods shorter than a year is dominated by exchange between kinetic and potential energy at longer timescales we find an intrinsic mode with period in the region of 15 years which is dominated by changes in potential energy and frictional dissipation in a manner consistent with that seen by hogg and blundell 2006 a similar mode influences the response to changing wind stress this influence robust to perturbations is different from the supposed linear relationship between wind stress and eddy kinetic energy and persists for 5 10 years in this model suggestive of a forced oscillatory mode with period of around 15 years if present in the real ocean such a mode would imply a degree of predictability of southern ocean dynamics on multi year timescales

southern ocean overturning compensation in an eddy resolving climate simulation the southern ocean s antarctic circumpolar current acc and meridional overturning circulation moc response to increasing zonal wind stress is for the first time analyzed in a high resolution 0 1 ocean and 0 25 atmosphere fully coupled global climate simulation using the community earth system model results from a 20 yr wind perturbation experiment where the southern hemisphere zonal wind stress is increased by 50 south of 30 s show only marginal changes in the mean acc transport through drake passage an increase of 6 136 144 sverdrups sv 1 sv 10 6 m 3 s 1 in the perturbation experiment compared with the control however the upper and lower circulation cells of the moc do change the lower cell is more affected than the upper cell with a maximum increase of 64 versus 39 respectively changes in the moc are directly linked to changes in water mass transformation from shifting surface isopycnals and sea ice melt giving rise to changes in surface buoyancy forcing the increase in transport of the lower cell leads to upwelling of warm and salty circumpolar deep water and subsequent melting of sea ice surrounding antarctica the moc is commonly supposed to be the sum of two opposing components a wind and transient eddy overturning cell here the transient eddy overturning is virtually unchanged and consistent with a large scale cancellation of localized regions of both enhancement and suppression of eddy kinetic energy along the mean path of the acc however decomposing the time mean overturning into a time and zonal mean component and a standing eddy component reveals partial compensation between wind driven and standing eddy components of the circulation

on the importance of surface forcing in conceptual models of the deep ocean in the major ocean basins diapycnal mixing upwells dense antarctic bottom water which returns southward and closes the deepest cell of the meridional overturning circulation moc this cell ventilates the deep ocean and regulates the partitioning of co2 between the atmosphere and the ocean the oceanographic community s conceptual understanding of the deep stratification and moc has evolved from classic abyssal recipes arguments to a more recent appreciation of along isopycnal upwelling in the southern ocean consistent with a weakly mixed ocean interior both the deep stratification and the deep moc are shown here to be sensitive to the form of the surface buoyancy forcing in a two dimensional model that includes a circumpolar channel and northern basin for a fixed surface buoyancy condition the deep stratification is essentially prescribed whereas for a fixed surface buoyancy flux the deep stratification varies by orders of magnitude over the range of diapycnal diffusivity observed in the ocean these cases also produce different scalings for the deep moc with in both weak and strong regimes in addition these scalings are shown to be sensitive not only to the type of surface boundary condition but also to the latitudinal structure of the surface fluxes this latter point is crucial as buoyancy budgets and dynamical features of the circulation are poorly constrained along the antarctic margins this study emphasizes the need for caution in the interpretation of simple conceptual models that while useful may not include all mechanisms that contribute to the moc s strength and structure

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

antarctic circumpolar current transport and barotropic transition at macquarie ridge theory and numerical simulations suggest that topographic interactions are central to the dynamics of the antarctic circumpolar current acc but few observations are available to test these ideas we use direct velocity measurements satellite altimetry and an ocean state estimate to investigate the interaction of the acc with the macquarie ridge satellite altimeter data show that the subantarctic front crosses the ridge through a gap immediately north of macquarie island yearlong current meter records reveal strong deep mean flow 0 2 m s 1 at 3000 m and substantial transport 52 8 106 m3 s 1 in the 50 km wide gap the acc becomes much more barotropic at the ridge acceleration of the deep jet is balanced by the ageostrophic along gap pressure gradient convergence of zonal momentum by the mean vertical velocity and dissipation the study helps explain how the acc negotiates large topographic obstacles and highlights the role of local nonlinear processes in the dynamical balance of the acc

the global influence of localized dynamics in the southern ocean the circulation of the southern ocean connects ocean basins links the deep and shallow layers of the ocean and has a strong influence on global ocean circulation climate biogeochemical cycles and the antarctic ice sheet processes that act on local and regional scales which are often mediated by the interaction of the flow with topography are fundamental in shaping the large scale three dimensional circulation of the southern ocean recent advances provide insight into the response of the southern ocean to future change and the implications for climate the carbon cycle and sea level rise

jets and topography jet transitions and the impact on transport in the antarctic circumpolar current the southern ocean s antarctic circumpolar current acc naturally lends itself to interpretations using a zonally averaged framework yet navigation around steep and complicated bathymetric obstacles suggests that local dynamics may be far removed from those described by zonally symmetric models in this study both observational and numerical results indicate that zonal asymmetries in the form of topography impact global flow structure and transport properties the conclusions are based on a suite of more than 1 5 million virtual drifter trajectories advected using a satellite altimetry derived surface velocity field spanning 17 years the focus is on sites of cross front transport as defined by movement across selected sea surface height contours that correspond to jets along most of the acc cross front exchange is localized in the lee of bathymetric features with more than 75 of crossing events occurring in regions corresponding to only 20 of the acc s zonal extent these observations motivate a series of numerical experiments using a two layer quasigeostrophic model with simple zonally asymmetric topography which often produces transitions in the front structure along the channel significantly regimes occur where the equilibrated number of coherent jets is a function of longitude and transport barriers are not periodic jet reorganization is carried out by eddy flux divergences acting to both accelerate and decelerate the mean flow of the jets eddy kinetic energy is amplified downstream of topography due to increased baroclinicity related to topographic steering the combination of high eddy kinetic energy and recirculation features enhances particle exchange these results stress the complications in developing consistent circumpolar definitions of the acc fronts

small scale topographic form stress and local dynamics of the southern ocean the contribution of small scale abyssal hill topography to the topographic form stress and local dynamics of the southern ocean is investigated using a high resolution model of the sector of the southern ocean south of tasmania and new zealand the results of two simulations with and without small o 1 100 km scale topography confirm that the effects of small scale topography are exerted through the generation of strong topographic form stress leading to transient eddy dissipation and changes in flow meanders small scale topographic form stress is comparable in magnitude to that generated by large scale topography but with a pairwise distribution of positive and negative stress values upstream and downstream of the macquarie ridge consistent with the meandering of the flow in the experiment without small scale topography the bottom mean flow speed increases while the surface mean speed slightly decreases making the mean flow more barotropic eddy kinetic energy also greatly enhances throughout the water column after removing small scale topography our results suggest that small scale topography has strong impact on transient eddies and plays an important role for setting the vertical structure of the flow and the equilibration and position of flow meanders

topographic form stress in the southern ocean state estimate we diagnose the southern ocean momentum balance in a six year eddy permitting state estimate of the southern ocean we find that 95 of the zonal momentum input via wind stress at the surface is balanced by topographic form stress across ocean ridges while the remaining 5 is balanced via bottom friction and momentum flux divergences at the northern and southern boundaries of the analysis domain while the time mean zonal wind stress field exhibits a relatively uniform spatial distribution time mean topographic form stress concentrates at shallow ridges and across the continents that lie within the antarctic circumpolar current acc latitudes nearly 40 of topographic form stress occurs across south america while the remaining 60 occurs across the major submerged ridges that underlie the acc topographic form stress can be divided into shallow and deep regimes the shallow regime contributes most of the westward form stress that serves as a momentum sink for the acc system while the deep regime consists of strong eastward and westward form stresses that largely cancel in the zonal integral the time varying form stress signal integrated longitudinally and over the acc latitudes tracks closely with the wind stress signal integrated over the same domain at zero lag 88 of the variance in the six year form stress time series can be explained by the wind stress signal suggesting that changes in the integrated wind stress signal are communicated via rapid barotropic response down to the level of bottom topography this article is protected by copyright all rights reserved

role of suction dynamics in evolution of intertidal sandy flats field evidence experiments and theoretical model 1 in this paper we present and discuss the role of the dynamics of suction that is negative pore water pressure relative to atmospheric air pressure in the evolution of intertidal sandy flats this is done through the combined use of field evidence laboratory experiments and a theoretical model field observations were performed in the estuary of obitsu river located in the east coast of tokyo bay japan laboratory experiments were performed in a calibration chamber as well as in a laterally scaled model flat subjected to water groundwater level variations the present study demonstrates that the suction dynamics associated with the tide induced groundwater level variations play a substantial role in the temporospatial evolutions of voids stiffness and surface shear strength in cyclically exposed and submerged soil the suction induced void state changes are a consequence of cyclic elastoplastic contraction of the soil and are accompanied by distinct morphological changes despite the lack of significant surface shear stresses acting on the soil such soil behavior occurs in regions where suction develops above the groundwater level during low tides and strongly depends on the way in which such suction develops discussions are made on how these suction dynamics effects could contribute via feedback involved in surface transport processes to the overall morphological evolution of cross shore intertidal flat soils

is there a continuous subtropical front south of africa a 14 3 year series of weekly absolute sea surface height ssh and associated geostrophic velocities is used for a study of the subtropical to subantarctic frontal system in the region south of africa detecting the fronts from surface velocity maxima confirms a two stepped transition in both the southeastern atlantic ocean the northern and southern subtropical fronts nstf sstf and southwestern indian ocean the agulhas front and sstf as proposed previously from hydrographic data an additional front associated with westward flow north of the nstf is indicative of a partial eastern closure of the south atlantic subtropical gyre between 11 e and 17 5 e the role of northwestward propagating agulhas rings in connecting the two fronts explains the varying location the sstf which marks the southern limit of subtropical waters is found continuous from the atlantic to the indian ocean in the time averaged ssh field in the weekly fields however the velocity maximum criterion defining the front is often met at the southern flanks of agulhas rings or agulhas eddies at 12 e 23 e indicating front disruptions such discontinuities suggest that no south atlantic central water is directly advected into the indian ocean instead the eastward limb of the indo atlantic super gyre encompassing the subtropical gyres of both oceans would rest at these longitudes on diffusive processes at the upper levels and possibly be enhanced at depth a schematic diagram of the fronts and their relations to agulhas rings and eddies is proposed

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

eddy heat flux across the antarctic circumpolar current estimated from sea surface height standard deviation eddy heat flux ehf is a predominant mechanism for heat transport across the zonally unbounded mean flow of the antarctic circumpolar current acc observations of dynamically relevant divergent four year mean ehf in drake passage from the cdrake project as well as previous studies of atmospheric and oceanic storm tracks motivates the use of sea surface height ssh standard deviation h as a proxy for depth integrated downgradient time mean ehf ehf in the acc statistics from the southern ocean state estimate corroborate this choice and validate throughout the acc the spatial agreement between h and ehf seen locally in drake passage eight regions of elevated ehf are identified from nearly 23 5 years of satellite altimetry data elevated cross front exchange usually does not span the full latitudinal width of the acc in each region implying a hand off of heat between acc fronts and frontal zones as they encounter the different ehf hot spots along their circumpolar path integrated along circumpolar streamlines defined by mean ssh contours there is a convergence of ehf in the acc 1 06 pw enters from the north and 0 02 pw exits to the south temporal trends in low frequency ehf are calculated in a running mean sense using h from overlapping 4 year subsets of ssh significant increases in downgradient ehf magnitude have occurred since 1993 at kerguelen plateau southeast indian ridge and the brazil malvinas confluence whereas the other five ehf hot spots have insignificant trends of varying sign

measuring the in situ erosion shear stress of intertidal sediments with the cohesive strength meter csm the shear resistance of muddy sediments directly governs the susceptibility of the sediment to erosion by tidal and wave induced currents measurements of the natural erosion shear stress are important in modelling estuarine systems and in determining the possible impact of human disturbance a second generation erosion instrument the cohesive strength meter csm designed to measure the critical erosion shear stress ocr of intertidal sediments in situ is described the csm allows measurements of small scale spatial and temporal variation of sediment stability to be made on a scale and rapidity previously unachievable new features of specialized hardware are described and the instrument is empirically calibrated in terms of an equivalent horizontal bed shear stress using quartz sand some example results from recent field trials on the konigshafen sylt romo bight are presented the erosion characteristics of this intertidal flat varied on a cm scale due to the presence of patchy diatom biofilms areas with a diatom biofilm were more stable than those without giving a biostabilization index of 6 2

vertical vorticity balance in meanders downstream the agulhas retroflection abstract the agulhas current flows poleward along the western boundary of the southeastern indian ocean where at the southernmost latitude of the african continent it executes a dramatic anticyclonic turn or retroflection to the east since 1978 a large number of drifting buoys have passed through this eastward flowing agulhas return current arc or the zonal frontal boundary between subtropical and subpolar waters of the south indian ocean the spatial distribution of the ensemble averaged near surface velocity along the arc axis reveals a series of steady state meanders of 700 km wavelength and amplitudes that decrease from 170 km in the first meander to 50 km in the following four meanders here an analysis of vorticity balance of the meandering arc speed axis is presented that demonstrates a balance between the term and advection of curvature vorticity this balance implies that the arc axis or frontal region is horizontally nondivergent in agreement with the other observations of flow in t

abrupt transitions in submesoscale structure in southern drake passage glider observations and model results abstractenhanced vertical velocities associated with submesoscale motions may rapidly modify mixed layer depths and increase exchange between the mixed layer and the ocean interior these dynamics are of particular importance in the southern ocean where the ventilation of many density classes occurs here we present results from an observational field program in southern drake passage a region preconditioned for submesoscale instability due to its strong mesoscale eddy field persistent fronts strong down front winds and weak vertical stratification two gliders sampled from december 2014 through march 2015 upstream and downstream of shackleton fracture zone sfz the acquired time series of mixed layer depths and buoyancy gradients enabled calculations of potential vorticity and classifications of submesoscale instabilities the regions flanking the sfz displayed remarkably different characteristics despite similar surface forcing mixed layer depths were nearly twice as deep and horizontal buoyancy

the effect of the kerguelen plateau on the ocean circulation abstractthe kerguelen plateau is a major topographic feature in the southern ocean located in the indian sector and spanning nearly 2000 km in the meridional direction from the polar to the subantarctic region it deflects the eastward flowing antarctic circumpolar current and influences the physical circulation and biogeochemistry of the southern ocean the kerguelen plateau is known to govern the local dynamics but its impact on the large scale ocean circulation has not been explored by comparing global ocean numerical simulations with and without the kerguelen plateau this study identifies two major kerguelen plateau effects 1 the plateau supports a local pressure field that pushes the antarctic circumpolar current northward this process reduces the warm water transport from the indian to the atlantic ocean 2 the plateau generated pressure field shields the weddell gyre from the influence of the warmer subantarctic and subtropical waters the first effect influences the strength of the antarct

acceleration of the antarctic circumpolar current by wind stress along the coast of antarctica the influence of wind forcing on variability of the antarctic circumpolar current acc is investigated using a series of eddy permitting ocean sea ice models at interannual and decadal time scales the acc transport is sensitive to both the mean strength of westerly winds along the acc circumpolar path consistent with zonal momentum balance theories and sensitive to the wind stresses along the coast of antarctica consistent with the free mode theory of hughes et al a linear combination of the two factors explains differences in acc transport across 11 regional quasi equilibrium experiments repeated single year global experiments show that the acc can be robustly accelerated by both processes across an ensemble of simulations with realistic forcing over the second half of the twentieth century interannual acc transport variability owing to the free mode mechanism exceeds that due to the zonal momentum balance mechanism by a factor of between 3 5 and 5 to one while the acc transport may not accelerate significantly owing to projected increases in along acc winds in future decades significant changes in transport could still occur because of changes in the stress along the coast of antarctica

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 role of closed gyres in setting the zonal transport of the antarctic circumpolar current abstracteddy permitting simulations are used to show that basinlike gyres can be observed in the large scale barotropic flow of a wind driven channel with a meridional topographic ridge this is confirmed using both two layer quasigeostrophic and 25 level primitive equation models at high horizontal resolution comparing results from simulations with and without the topographic ridge it is shown that the zonal baroclinic transport in the channel increases with increasing wind stress when the bottom topography is flat but not when there is a meridional ridge the saturation of transport for increasing wind occurs in conjunction with the development of recirculating gyres in the large scale barotropic streamfunction this suggests that the total circulation can be thought of as a superposition of a gyre mode which has zero circumpolar transport and a free circumpolar mode which contains all of the transport basinlike gyres arise in the channel because the topography steers the barotropic streamlines a

may the shields theory be extended to cohesive and adhesive benthic sediments 1 a threshold criterion for incipient motion of cohesive adhesive sediments based on moment balance and dimensional considerations is here developed the criterion discriminates between single particles and flocs critical conditions and induces a modification to the traditional shields curve commonly adopted for noncohesive particles this modification is particularly effective for the smaller size particles but tending to vanish for the larger ones the proposed approach is validated on the basis of experiments performed on sediment cores sampled at different depths from seven lakes with different trophic conditions and organic matter content the incipient motion conditions are evaluated by means of an original procedure based on image analysis techniques which enables the entrainment between single particles and flocs to be distinguished the adhesion force which is mainly due to the biological activity at the sediment water interface is estimated following the proposed criterion and it is found to depend on the organic matter content in particular the cores sampled in the littoral photic zone are characterized by a bell shaped dependence between adhesion coefficient and organic matter content which is typical for indicators of biological activity of the littoral ecosystems

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

downstream self destruction of storm tracks the northern hemisphere storm tracks have maximum intensity over the pacific and atlantic basins their intensity is reduced over the continents downstream here simulations with an idealized aquaplanet general circulation model are used to demonstrate that even without continents storm tracks have a self determined longitudinal length scale their length is controlled primarily by the planetary rotation rate and is similar to thatofearth sstormtracksforearth srotationrate downstream stormtracksself destruct thedownstream eddy kinetic energy is lower than it would be without the zonal asymmetries that cause localized storm tracks likely involved in the downstreamself destruction ofstormtracks arethe energyfluxesassociatedwiththem the zonalasymmetries that causelocalized stormtracksenhancetheenergytransport throughthe generation of stationary eddies and this leads to a reduced baroclinicity that persists far downstream of the eddy kinetic energy maxima

the polar front in drake passage a composite mean stream coordinate view the polar front pf is studied using 4 years of data collected by a line of current and pressure recording inverted echo sounders in drake passage complemented with satellite altimetry the location of the pf is bimodal in latitude a northern and southern pf exist at separate times separated geographically by a seafloor ridge the shackleton fracture zone and hydrographically by 17 cm of geopotential height expressed in stream coordinates vertical structures of buoyancy are determined with a gravest empirical mode analysis baroclinic velocity referenced to zero at 3500 dbar width and full transport about 70 sv of the jets are statistically indistinguishable the two jets alternate carrying the baroclinic transport rather than coexisting influences of local bathymetry and deep cyclogenesis manifest as differences in deep reference velocity structures downstream reference velocities of the pf n and pf s reach maximum speeds of 0 09 and 0 06 m s 1 respectively buoyancy fields are indicative of upwelling and poleward residual circulation at the pf based on potential vorticity and mixing lengths the northern and southern pf both act as a barrier to cross frontal exchange while remaining susceptible to baroclinic instability

eddy generation and jet formation via dense water outflows across the antarctic continental slope along various stretches of the antarctic margins dense antarctic bottom water aabw escapes its formation sites and descends the continental slope this export necessarily raises the isopycnals associated with lighter density classes over the continental slope resulting in density surfaces that connect the near freezing waters of the continental shelf to the much warmer circumpolar deep water cdw at middepth offshore in this article an eddy resolving process model is used to explore the possibility that aabw export enhances shoreward heat transport by creating a pathway for cdw to access the continental shelf without doing any work against buoyancy forces in the absence of a net alongshore pressure gradient the shoreward cdw transport is effected entirely by mesoscale and submesoscale eddy transfer eddies are generated partly by instabilities at the pycnocline sourcing their energy from the alongshore wind stress but primarily by instabilities at the cdw aabw interface sourcing their energy from buoyancy loss on the continental shelf this combination of processes induces a vertical convergence of eddy kinetic energy and alongshore momentum into the middepth cdw layer sustaining a local maximum in the eddy kinetic energy over the slope and balancing the coriolis force associated with the shoreward cdw transport the resulting slope turbulence self organizes into a series of alternating along slope jets with strongly asymmetrical contributions to the slope energy and momentum budgets cross shore variations in the potential vorticity gradient cause the jets to drift continuously offshore suggesting that fronts observed in regions of aabw down slope flow may in fact be transient features

day night variation of cohesive sediment stability surface sediment properties related to cohesive sediment stability were measured over 8 consecutive day and night time emersion periods at three upper intertidal sites on a mudflat in august 2003 during the transition from spring to neap tides significant differences between day and night time critical erosion shear stress c and chlorophyll a were found a high degree of temporal and spatial variability existed between the sediment properties during the first half of the study period a rhythmic day night variation occurred between c chl a colloidal s and edta extracted carbohydrate during the second part of the study the magnitude of variation of these parameters diminished results showed that sediments were more stable during the day than at night differences between day and night time sediment stability were related not only to diatom migration but also to wave energy during preceding immersion periods no significant relationships existed between c and either chl a or colloidal s or edta extracted carbohydrate sediment content it is suggested that tidal phasing in terms of both the time during the day at which low water spring and neap tides occur as well as the duration of the emersion period control the biomass dynamics the tidal phasing effect is expected to be more pronounced on a cohesive intertidal flat where low water spring tides occur at noon and midnight the results of this study will be of use in time dependent estuarine models

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

the formation of nonzonal jets over sloped topography coherentjetsareubiquitousfeaturesoftheocean scirculation andtheircharacteristics suchasorientation and energetics may be influenced by topography in this study the authors introduce a large scale topographic slope with an arbitrary orientation into quasigeostrophic doubly periodic barotropic and baroclinic systems in both systems the flow organizes itself into coherent tilted nonzonal jets that are aligned perpendicular to the barotropic potential vorticity pv gradient in the two layer system the upper layer the lower layer and the barotropic pv gradients all have different orientations and therefore the jets cross the layer wise pv gradients the fact that the jets cross layer wise pv gradients and the requirement of conservation of pv for fluid parcels together results in the drift of the tilted jets across the domain like their zonalcounterparts thetiltedjetsexhibitstrongtransportanisotropy thedynamicalresponsetojetdeflection is very strong in the two layer baroclinic case with eddy energy production increasing by orders of magnitude as the topographic slope becomes more zonal this increase in eddy energy is also reflected in an increase in jet spacing and a reduction in strength of the across jet transport barriers shown using an effective diffusivity diagnostic the dynamics identified here while formally valid within the constraints of quasigeostrophic scalings provide important insight into the sensitive relationship between flow orientation and flow stability in regions with broad topographic slopes

estimates and implications of surface eddy diffusivity in the southern ocean derived from tracer transport abstract near surface effective diffusivities associated with geostrophic eddies in the southern ocean are estimated by numerically monitoring the lengthening of idealized tracer contours as they are strained by surface geostrophic flow observed by satellite altimetry the resulting surface diffusivities show considerable spatial variability and are large 2000 m2 s 1 on the equatorward flank of the antarctic circumpolar current and are small 500 m2 s 1 at the jet axis regions of high and low effective diffusivity are shown to be collocated with regions of respectively weak and strong isentropic potential vorticity gradients the maps of diffusivity are used along with climatological estimates of surface wind stress and air sea buoyancy flux to estimate surface meridional residual flows and the relative importance of eulerian and eddy induced circulation in the streamwise averaged dynamics of the antarctic circumpolar current

eddy response to southern ocean climate modes 1 interannual variations in southern ocean eddy kinetic energy eke are investigated using 16 years of altimetric data circumpolar averages show a peak in eke from 2000 to 2002 2 3 years after the peak in the southern annular mode sam index although the sam forcing is in phase around the circumpolar band we find the eke response varies regionally the strongest eke is in the pacific with energy peaks occurring progressively later toward the east we suggest that this is due to the presence of two climate modes sam and enso when strong positive sam events coincide with la nina periods as in 1999 anomalous meridional wind forcing is enhanced in the south pacific ocean contributing to the observed increase in eke 2 3 years later when positive sam events coincide with el nino periods as in 1993 the climate modes are in opposition in the south pacific leading to a weak eke response during the mid 1990s numerical modeling supports these observations by applying different combinations of sam and enso we can reproduce both the elevated pacific eke response to sam as well as an additional amplification suppression of eke during la nina el nino in general we find that the eke response depends on the interplay between wind forcing topography and mean flow and produces a strongly heterogeneous distribution in the southern ocean

day night variation of intertidal flat sediment properties in relation to sediment stability the majority of investigations that have measured sediment properties related to intertidal sediment stability have been undertaken during daylight subaerial exposure periods as a consequence models based upon such data represent only partially the intertidal flat surface conditions within any 24 h period in this contribution a comparison is made between surface sediment properties related to sediment stability measured during six consecutive day night semi diurnal subaerial exposure periods at three stations on an intertidal sand flat in late march 1999 the study site was selected on the basis of its suitability for sampling and data collection at night with special regard to safety and logistics seawater temperatures ranged from 4 1 to 9 6 c and salinities from 33 9 to 34 8 eleven parameters related to intertidal flat sediment stability were measured or derived these variables included the critical erosion shear stress c chlorophyll a phaeopigment and colloidal carbohydrate content mean grain size and settling velocity of the surface 0 1 mm sediment fraction bed elevation was described using an acretion erosion parameter aep west and west 1991 whilst additional physical terms included ambient seawater salinity and temperature as well as tidal range and wind speed during the preceding immersion periods one way anova was used to detect significant differences between day and night time emersion periods similarly principal components analysis pca was applied to detect continuous variation between properties the results show a high degree of temporal and spatial variability between day and night time intertidal flat variables the pca differentiating clearly between day and night conditions surface sediments across the intertidal flat exhibited varying degrees of biostabilisation the maximum biostabilisation coefficient 18 was recorded at night in high microalgal biomass areas the minimum 5 occurred during both day and night in areas with lower microalgal biomass all surface sediment parameters varied rhythmically between day and night time significant differences were found between day and night time biostabilisation coefficients however differences between day and night time c values were not detected it is suggested that sediment stability at night is enhanced in high microalgal biomass areas as a result of degradation products from bound extra cellular polysaccharides eps not easily detected using standard extraction procedures

the stability of fine grained sediments from the fraser river delta the stability and erodibility of benthic fine grained sediments on the foreshore and upper foreslope of the fraser river delta were measured using thein situbenthic flume sea carousel the erosion threshold c 0 which varied between 0 11 and 0 50 pa was inversely related to erosion rate it was also proportional to sediment wet bulk density b in the form c 0 7 10 4 b 0 47 this trend agreed with those obtained from four other sites across canada that encompassed a wide range in sediment densities 1000 b 2000 kg m 3 erosion threshold shields parameter c was positively related to grain reynolds number over the silt clay size range 0 01 re 5 c 2 86re0 58 this was similar to earlier findings for water saturated sediment this trend is also in continuity with published threshold values measured in biostabilized sand 5 re 50 copyright 1997 academic press limited three erosion types were observed type ia erosion was a surface phenomenon caused by the presence of a thin organic fluff layer it occurred at current speeds less than 0 3 m s 1 type 1b erosion asymptotically decaying with time characterized the mid portions of each erosion time series and occurred at current speeds of 0 3 0 9 m s 1 type i ii erosion transitional was largely found on the foreslope and occurred at current speeds in excess of 0 8 m s 1 the erosion process began in all cases under turbulent transitional flows the change from type i to type ii erosion corresponded to the onset of turbulent rough flows 60 re 70 this is supported by video observations which showed type i erosion to be largely the result of entrainment of small aggregates and flocs whereas type ii erosion was the result of enlargement of surface irregularities and subsequent undercutting and release of large aggregates mean erosion rate em was independent of either azimuthal current speed or bed shear stress it was largely constant throughout each deployment and varied between 1 4 and 7 4 10 4 kg m 2 s 1

forcing of the overturning circulation across a circumpolar channel by internal wave breaking the hypothesis that the impingement of mesoscale eddy flows on small scale topography regulates diapycnal mixing and meridional overturning across the deep southern ocean is assessed in an idealised model the model simulates an eddying circumpolar current coupled to a double celled meridional overturning with properties broadly resembling those of the southern ocean circulation and represents lee wave induced diapycnal mixing using an online formulation grounded on wave radiation theory the diapycnal mixing generated by the simulated eddy field is found to play a major role in sustaining the lower overturning cell in the model and to underpin a significant sensitivity of this cell to wind forcing the vertical structure of lower overturning is set by mesoscale eddies which propagate the effects of near bottom diapycnal mixing by displacing isopycnals vertically

structural effects of tidal exposures on mudflats along the french guiana coast wetting and drying cycles on intertidal mudflats vary considerably with altitude modifying the physical characteristics of surface sediment in ways favoring or not plant colonization in this context sediment properties were investigated by means of laboratory experiments and field surveys on a wide range of fluid to desiccated muds from the highly dynamic coastline of french guiana changes in physical parameters such as sediment erodability yield stress water loss and pore water salinity indicated a long term compaction of mudflats as well as fluctuations related to the successive wetting and drying cycles mudcracks constituted a spectacular feature representative of the contractional stress they re opened after a few days of dewatering and re healed during the subsequent wetting from the analysis of field data the trapping of avicennia germinans propagules in ephemeral mudcracks turned out to be responsible of 95 of the sprouting on the coastal fringe thus desiccation process usually considered as a typical feature of erosion revealed herein to be a major mechanism of colonization this mechanism undoubtedly affects the 1600 km long amazon coastal system and is believed to exist in many other tropical environments submitted to important siltation

connecting antarctic cross slope exchange with southern ocean overturning previous idealized investigations of southern ocean overturning have omitted its connection with the antarctic continental shelves leaving the influence of shelf processes on antarctic bottom water aabw export unconsidered in particular the contribution of mesoscale eddies to setting the stratification and overturning circulation in the antarctic circumpolar current acc is well established yet their role in cross shelf exchange of water masses remains unclear this study proposes a residual mean theory that elucidates the connection between antarctic cross shelf exchange and overturning in the acc and the contribution of mesoscale eddies to the export of aabw the authors motivate and verify this theory using an eddy resolving process model of a sector of the southern ocean the strength and pattern of the simulated overturning circulation strongly resemble those of the real ocean and are closely captured by the residualmean theory over the continental slope baroclinic instability is suppressed and so transport by mesoscale eddies is reduced this suppression of the eddy fluxes also gives rise to the steep v shaped isopycnals that characterize the antarctic slope front in aabw forming regions of the continental shelf furthermore to produce water on the continental shelf that is dense enough to sink to the deep ocean the deep overturning cell must be at least comparable in strength to wind driven mean overturning on the continental slope this results in a strong sensitivity of the deep overturning strength to changes in the polar easterly winds

why western boundary currents in realistic oceans are inviscid a link between form stress and bottom pressure torques it is shown that wind stress curl is balanced by bottom pressure torque in a zonal integral over any strip wide enough to smooth out the effect of nonlinear terms typically about 3 of latitude the derivation is completely general as long as the zonal wind stress is balanced by form stress at each latitude as is known to be the case in the ocean this implies that viscous torques are not important in western boundary currents their place being taken by bottom pressure torques the prediction is confirmed in the context of a global eddy permitting numerical ocean model this link between form stress and bottom pressure torques makes it easier to consider southern ocean dynamics and subtropical gyre dynamics in the same conceptual framework with topographic interactions being important in both cases

meridional heat and salt transport across the subantarctic front by cold core eddies mesoscale eddies are ubiquitous in the southern ocean particularly in regions where the antarctic circumpolar current interacts with topography eddies play a critical role in the meridional transport of heat and salt across the antarctic circumpolar current however observations of the heat and salt content of eddies are extremely rare here we present new observations that characterize the three dimensional structure of a cyclonic cold core eddy generated at the subantarctic front south of tasmania automated detection software is used with satellite altimetry to follow the eddy through the subantarctic zone the physical properties at the center of the eddy are substantially modified from those near the formation region indicating a strong transformation during the eddy s lifetime the eddy carried heat and salt content anomalies relative to surrounding subantarctic zone waters of 0 5 0 1 1020 j and 2 1 0 4 1012 kg respectively previous studies have not captured the full heat and salt content of eddies due to limited observations and have underestimated their content by a factor of 2 to 3 south of tasmania applying the observed correlation between eddy elevated volume and eddy heat and salt content to the history of cyclonic eddies in the satellite altimeter record leads us to propose that about 21 of the heat carried across the subantarctic front south of tasmania is achieved by cyclonic eddies entering the subantarctic zone the freshwater contribution to the subantarctic zone by long lived cold core eddies is of the same order of magnitude as the ekman flux in this region

jet jumping low frequency variability in the southern ocean abstractthe authors study intrinsic variability in the position of jets in a plane channel ocean with simple topography using a quasigeostrophic numerical model this study links the variability in jet position with abyssal anticyclones that form as a result of interaction of mesoscale eddies and subsurface topography reminiscent of such flows as the zapiola anticyclone a simple dynamical framework explaining this behavior is developed in this framework this study shows that the topographic anticyclones form closed regions of homogenized yet time varying potential vorticity neighboring topographic anticyclones are coupled by eddy fluxes interaction of a baroclinic jet with these two or more anticyclones can drive variability in local jet strength predictions of the dynamical framework are then compared with the results of the numerical model and it is demonstrated that this model has merit in explaining the observed model variability this study argues that this simple mode of variability has

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

energetics of the southern ocean mode recently multidecadal variability in the southern ocean has been found in a strongly eddying global ocean circulation model in this paper we study the lorenz energy cycle of this so called southern ocean mode som the lorenz energy cycle analysis provides details on the energy pathways associated with the som it shows that ocean eddies and the baroclinic energy pathway together with variations in the kinetic energy input by the wind are crucial aspects of the variability it is also shown how convective mixing which is induced by the som in particular in the weddell gyre is responsible for the large scale multidecadal variability in antarctic bottom water and atlantic meridional overturning circulation

estimates of eddy heat flux crossing the antarctic circumpolar current from observations in drake passage abstractthe 4 yr measurements by current and pressure recording inverted echo sounders in drake passage produced statistically stable eddy heat flux estimates horizontal currents in the antarctic circumpolar current acc turn with depth when a depth independent geostrophic current crosses the upper baroclinic zone the dynamically important divergent component of eddy heat flux is calculated whereas full eddy heat fluxes differ greatly in magnitude and direction at neighboring locations within the local dynamics array lda the divergent eddy heat fluxes are poleward almost everywhere case studies illustrate baroclinic instability events that cause meanders to grow rapidly in the southern passage where eddy variability is weak heat fluxes are weak and not statistically significant vertical profiles of heat flux are surface intensified with 50 above 1000 m and uniformly distributed with depth below summing poleward transient eddy heat transport across the lda of 0 010 0 005 pw with the stat

isopycnal mixing suppression by the antarctic circumpolar current and the southern ocean meridional overturning circulation abstractthe meridional overturning circulation moc in the southern ocean is investigated using hydrographic observations combined with satellite measurements of sea surface height a three dimensional spatial and vertical estimate of the isopycnal eddy diffusivity in the southern ocean is obtained using the theory of ferrari and nikurashin that includes the influence of suppression of the diffusivity by the strong time mean flows it is found that the eddy diffusivity is enhanced at depth reaching a maximum at the critical layer near 1000 m the estimate of diffusivity is used with a simple diffusive parameterization to estimate the meridional eddy volume flux this estimate of eddy volume flux is combined with an estimate of the ekman transport to reconstruct the time mean overturning circulation by comparing the reconstruction with and without suppression of the eddy diffusivity by the mean flow the influence of the suppression on the overturning is illuminated it is shown that the suppressio

bottom pressure torque and the vorticity balance from observations in drake passage the vorticity balance of the antarctic circumpolar current in drake passage is examined using 4 years of observations from current and pressure recording inverted echo sounders the time varying vorticity planetary and relative vorticity advection and bottom pressure torque are calculated in a two dimensional array in the eddy rich polar frontal zone pfz bottom pressure torque is also estimated at sites across drake passage mean and eddy nonlinear relative vorticity advection terms dominate over linear advection in the local 50 km scale vorticity budget in the pfz and are balanced to first order by the divergence of horizontal velocity most of this divergence comes from the ageostrophic gradient flow which also provides a second order adjustment to the geostrophic relative vorticity advection bottom pressure torque is approximately one third the size of the local depth integrated divergence although the cdrake velocity fields exhibit significant turning with depth throughout drake passage even in the mean surface vorticity advection provides a reasonable representation of the depth integrated vorticity balance observed near bottom currents are strongly topographically steered and bottom pressure torques grow large where strong near bottom flows cross steep topography at small angles upslope flow over the northern continental slope dominates the bottom pressure torque in cdrake and the mean across this drake passage transect 3 to 4 10 9 m s 2 exceeds the mean wind stress curl by a factor of 15 20

standing and transient eddies in the response of the southern ocean meridional overturning to the southern annular mode abstractto refine the understanding of how the southern ocean responds to recent intensification of the southern annular mode sam a regional ocean model at two eddy permitting resolutions was forced with two synthetic interannual forcings the first forcing corresponds to homogeneously intensified winds while the second concerns their poleward intensification consistent with positive phases of the sam resulting wind driven responses differ greatly between the nearly insensitive antarctic circumpolar current acc and the more sensitive meridional overturning circulation moc as expected eddies mitigate the response of the acc and moc to poleward intensified winds however transient eddies do not necessarily play an increasing role in meridional transport with increasing resolution as winds and resolution increase meridional transport from standing eddies becomes more efficient at balancing wind enhanced overturning these results question the current paradigms on the role of eddies and present

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

pattern and velocity of propagation of the global ocean eddy variability 1 satellite altimeter data are used to study the characteristics of the horizontal propagation of eddy variability of the global oceans decade long time series of sea surface height is analyzed for finding the maximum cross correlation with neighboring time series within a window of space and time lags the space and time lags corresponding to the maximum correlation allow an estimate of the propagation velocity of the eddy variability that dominates the variance of sea surface height anomalies the method cannot distinguish the various forms of eddy variability isolated eddies and fronts the meandering of ocean currents or planetary waves however the results provide at a given location of the global oceans a uniquely determined propagation velocity that represents a time averaged description of the tendency of the movement of the local eddy variability the propagation velocity is highly inhomogeneous in space outside the equatorial zone the zonal propagation is intrinsically westward modified by ocean currents which could reverse the zonal propagation to eastward in regions like the gulf stream and the antarctic circumpolar current at midlatitudes and high latitudes the propagation pattern is highly affected by the path of ocean currents and the shape of bottom topography at tropical latitudes the meridional propagation is convergent toward the equator in the western basins and divergent away from the equator in the eastern basins comparison with the simulations of an eddy permitting ocean general circulation model shows overall agreement especially in the latitudinal variation of the zonal propagation velocity the result suggests that the model has captured the essence of the dynamics governing the propagation of ocean eddy variability

equilibration of the antarctic circumpolar current by standing meanders the insensitivity of the antarctic circumpolar current acc s prominent isopycnal slope to changes in wind stress is thought to stem from the action of mesoscale eddies that counterbalance the wind driven ekman overturning a framework verified in zonally symmetric circumpolar flows substantial zonal variations in eddy characteristics suggest that local dynamics may modify this balance along the path of the acc analysis of an eddy resolving ocean gcm shows that the acc can be broken into broad regions of weak eddy activity where surface winds steepen isopycnals and a small number of standing meanders across which the isopycnals relax meanders are coincident with sites of i strong eddy induced modification of the mean flow and its vertical structure as measured by the divergence of the eliassen palm flux and ii enhancement of deep eddy kinetic energy by up to two orders of magnitude over surrounding regions within meanders the vorticity budget shows a balance between the advection of relative vorticity and horizontal divergence providing a mechanism for the generation of strong vertical velocities and rapid changes in stratification temporal fluctuations in these diagnostics are correlated with variability in both the eliassen palm flux and bottom speed implying a link to dissipative processes at the ocean floor at larger scales bottom pressure torque is spatially correlated with the barotropic advection of planetary vorticity which links to variations in meander structure from these results it is proposed that the flexing of standing meanders provides an alternative mechanism for reducing the sensitivity of the acc s baroclinicity to changes in forcing separate from an acc wide change in transient eddy characteristics

the force balance of the southern ocean meridional overturning circulation the southern ocean so limb of the meridional overturning circulation moc is characterized by three vertically stacked cells each with a transport of about 10 sv sv 10 6 m 3 s 1 the buoyancy transport in the so is dominated by the upper and middle moc cells with the middle cell accounting for most of the buoyancy transport across the antarctic circumpolar current a southern ocean state estimate for the years 2005 and 2006 with 1 6 resolution is used to determine the forces balancing this moc diagnosing the zonal momentum budget in density space allows an exact determination of the adiabatic and diapycnal components balancing the thickness weighted residual meridional transport it is found that to lowest order the transport consists of an eddy component a directly wind driven component and a component in balance with mean pressure gradients nonvanishing time mean pressure gradients arise because isopycnal layers intersect topography or the surface in a circumpolar integral leading to a largely geostrophic moc even in the latitude band of drake passage it is the geostrophic water mass transport in the surface layer where isopycnals outcrop that accomplishes the poleward buoyancy transport

topographic control of southern ocean gyres and the antarctic circumpolar current a barotropic perspective abstractin the southern ocean the antarctic circumpolar current is significantly steered by large topographic features and subpolar gyres form in their lee the geometry of topographic features in

a multibasin residual mean model for the global overturning circulation the ocean s overturning circulation is inherently three dimensional yet modern quantitative estimates of the overturning typically represent the subsurface circulation as a two dimensional two cell streamfunction that varies with latitude and depth only this approach suppresses information about zonal mass and tracer transport in this article the authors extend earlier zonally averaged overturning theory to explore the dynamics of a figure eight circulation that cycles through multiple basins a three dimensional residual mean model of the overturning circulation is derived and then simplified to a multibasin isopycnal box model to explore how stratification and diabatic water mass transformations in each basin depend on the basin widths and on deep and bottom water formation in both hemispheres the idealization to multiple two dimensional basins permits zonal mass transport along isopycnals in a southern ocean like channel while retaining the dynamical framework of residual mean theory the model qualitatively reproduces the deeper isopycnal surfaces in the pacific basin relative to the atlantic this supports a transfer of antarctic bottom water from the atlantic sector to the pacific sector via the southern ocean which subsequently upwells in the northern pacific basin a solution for the full isopycnal structure in the southern ocean reproduces observed stratification differences between atlantic and pacific basins and provides a scaling for the diffusive boundary layer in which the zonal mass transport occurs these results are consistent with observational indications that north atlantic deep water is preferentially transformed into antarctic bottom water which undermines the importance of an adiabatic upper overturning cell in the modern ocean

vertical eddy fluxes in the southern ocean the overturning circulation of the southern ocean has been investigated using eddying coupled ocean sea ice models the circulation is diagnosed in both density latitude coordinates and in depth density coordinates depth density coordinates follow streamlines where the antarctic circumpolar current is equivalent barotropic capture the descent of antarctic bottom water follow density outcrops at the surface and can be interpreted energetically in density latitude coordinates wind driven northward transport of light water and southward transport of dense water are compensated by standing meanders and to a lesser degree by transient eddies consistent with previous results in depth density coordinates however wind driven upwelling of dense water and downwelling of light water are compensated more strongly by transient eddy fluxes than fluxes because of standing meanders model realizations are discussed where the wind pattern of the southern annular mode is amplified in density latitude coordinates meridional fluxes because of transient eddies can increase to counter changes in ekman transport and decrease in response to changes in the standing meanders in depth density coordinates vertical fluxes because of transient eddies directly counter changes in ekman pumping

standing eddies in the meridional overturning circulation abstractthe role of standing eddies for the meridional overturning circulation moc is discussed the time mean isopycnal meridional streamfunction is decomposed into a time and zonal mean part a standing eddy part and a transient eddy part it turns out that the construction of an isopycnal moc with an exactly vanishing standing eddy part has to be performed by zonal integration along depth dependent horizontal isolines of time mean density in contrast zonal integration along time mean geostrophic streamlines generally only leads to an isopycnal moc with a reduced standing eddy part a generalized approach of constructing meridional transport streamfunctions by two tracer fields and the generalized way to neutralize the corresponding standing eddy part is given to summarize the discussion using the results of an idealized southern ocean model it is demonstrated that neglecting the depth dependence of the zonal integration paths by integrating along density contours or geostrophic streamlines of a

use of in situ flume to quantify particle flux biodeposition rates and sediment erosion for an intertidal mudflat in relation to changes in current velocity and benthic macrofauna

an eddy permitting southern ocean state estimate abstract an eddy permitting general circulation model of the southern ocean is fit by constrained least squares to a large observational dataset during 2005 06 data used include argo float profiles ctd synoptic sections southern elephant seals as oceanographic samplers seaos instrument mounted seal profiles xbts altimetric observations envisat geosat jason 1 and ocean topography experiment topex poseidon and infrared and microwave radiometer observed sea surface temperature an adjoint model is used to determine descent directions in minimizing a misfit function each of whose elements has been weighted by an estimate of the observational plus model error the model is brought into near agreement with the data by adjusting its control vector here consisting of initial and meteorological boundary conditions although total consistency has not yet been achieved the existing solution is in good agreement with the great majority of the 2005 and 2006 southern ocean observations and better repr