path and variability of the agulhas return current the combined analysis of hydrographic kinematic and dynamic data collected during the 1997 1999 kapex cape of good hope experiments reveals a quasi stationary meandering pattern of the agulhas retroflection current east and upstream of the southwest indian ridge the current meanders between 38 s and 40 s in a spatially and temporally continuous fashion and has a core width of approximately full size image 1 k with an associated transport of full size image 1 k in the upper full size image 1 k peak surface velocities decrease from full size image 1 k near the agulhas retroflection to full size image 1 k around 32 e meander troughs northward extremes are found predominantly near 26 8 e 32 6 e and 38 9 e while crests southward extremes are located with high probability near 29 7 e 35 5 e and 42 9 e resulting in a typical wavelength of full size image 1 k cold eddies are shed along the northern boundary of the current from meander troughs into the recirculation regime between the agulhas current proper and the agulhas return current strongest cyclonic eddies are preferably shed in austral autumn the cyclonic eddies so formed propagate westward at an average phase speed of full size image 1 k with however a variability of at least the same magnitude subsequently the cyclones are absorbed by the next meander trough located upstream and to the west of the shedding trough

physical controls on biogeochemical zonation in the southern ocean the primary control on the n s zonation of the southern ocean is the wind induced transport of the antarctic circumpolar current acc the acc divides the southern ocean into three major zones the subantarctic zone saz north of the acc the acc transport zone and the zone south of the acc saccz the zone of acc transport is most often subdivided into two zones the polar frontal zone pfz and the antarctic zone aaz but it may be appropriate to define more subzones or indeed only one at some longitudes to maintain geostrophic balance isopycnals must slope upwards to the south across the acc thus raising nutrient rich deep water closer to the surface as one goes polewards in addition silicate concentrations increase polewards along isopycnals because of diapycnic mixing with silicate rich bottom water surface silicate concentrations therefore decrease northwards from high levels in the saccz to low levels in the saz within the saz and pfz and even in the northern part of the aaz silicate levels may drop to limiting levels for siliceous phytoplankton production during summer nitrate concentrations also decrease northwards but only become limiting in the subtropical zone north of the saz the second circumpolar control is the changing balance of stratification with temperature dominating near surface stratification in the saz and salinity dominating further south because of fresh water input to the surface from melting ice this results in circumpolar features such as the subsurface 2 c temperature minimum and the subduction of the salinity minimum of antarctic intermediate water which are often but not always associated with frontal jets and large transports the transport of the acc is dynamically constrained into narrow bands the number and latitudinal location of which are controlled by the bathymetry and so vary with longitude thus it is not the fronts that are circumpolar but the total acc transport and scalar properties of the salinity and temperature fields evidence of summer silicate and nitrate uptake in all zones saz pfz and aaz shows that there is productivity despite their high nutrient low chlorophyll status blooms covering large areas say 400 km across in the pfz and aaz are found in the vicinity of submarine plateaux which suggest benthic iron fertilizatio

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

the large scale horizontal structure of the antarctic circumpolar current from fgge drifters the last decade of research in the southern ocean has shown that the antarctic circumpolar current acc is a complex system composed of narrow high speed currents separated by broad quiescent zones the circumpolar nature of this structure was examined using position and velocity data obtained from approximately 300 surface drifting buoys deployed in the southern ocean during the first garp global experiment fgge the distribution of buoys on 1 1 squares shows that in some regions most notably south of australia the buoys form three coherent bands of high buoy density which are separated by regions of low buoy density the latitudes of these bands coincide with those of the subtropical front subantarctic front and polar front to further examine the relationship between these fronts and buoy distribution locations of the three fronts determined from historical hydrographic data were used to partition the buoys into zonal bands corresponding to front and nonfront regions a mean buoy density and mean near surface speed were then computed for each zonal band high buoy densities were associated with all three fronts in the region south of australia other regions also showed a tendency although not as pronounced for buoys to accumulate in fronts the mean near surface speeds suggest that the subantarctic and polar fronts are circumpolar moreover the mean near surface speeds associated with the three frontal regions differ speeds within the subantarctic and polar front regions are approximately twice that associated with the subtropical front

an altimetry based gravest empirical mode south of africa 1 development and validation 1 hydrographic transects of the antarctic circumpolar current acc south of africa are projected into baroclinic stream function space parameterized by pressure and dynamic height this produces a two dimensional gravest empirical mode gem that captures more than 97 of the total density and temperature variance in the acc domain weekly maps of absolute dynamic topography data derived from satellite altimetry are combined with the gem to obtain a 16 year time series of temperature and salinity fields the time series of thermohaline fields are compared with independent in situ observations the residuals decrease sharply below the thermocline and through the entire water column the mean root mean square rms error is 0 15c 0 02 and 0 02 kg m 3 for temperature salinity and density respectively the positions of acc fronts are followed in time using satellite altimetry data these locations correspond to both the observed and gem based positions the available temperature and salinity information allow one to calculate the baroclinic zonal velocity field between the surface and 2500 dbar this is compared with velocity measurements from repeat hydrographic transects at the goodhope line the net accumulated transports of the acc derived from these different methods are within 1 3 sv of each other similarly gem produced cross sectional velocities at 300 dbar compare closely to the observed data with the rms difference not exceeding 0 03 m s 1 the continuous time series of thermohaline fields described here are further exploited to understand the dynamic nature of the acc fronts in the region and which is given by swart and speich 2010

location and dynamics of the antarctic polar front from satellite sea surface temperature data the location of the antarctic polar front pf was mapped over a 7 year period 1987 1993 within images of satellite derived sea surface temperature the mean path of the pf is strongly steered by the topographic features of the southern ocean the topography places vorticity constraints on the dynamics of the pf that strongly affect spatial and temporal variability over the deep ocean basins the surface expression of the pf is weakened and the pf meanders over a wide latitudinal range near large topographic features width and temperature change across the front increase and large scale meandering is inhibited elevated mesoscale variability is seen within and downstream of these areas and may be the result of baroclinic instabilities initiated where the pf encounters large topographic features the strong correlations between topography and pf dynamics can be understood in the context of the planetary potential vorticity ppv or f h field mean ppv at the pf varies by more than a factor of 2 along its circumpolar path however at the mesoscale the pf remains within a relatively narrow range of ppv values around the local mean away from large topographic features the pf returns to a preferred ppv value of 25 10 9 m 1 s 1 despite large latitudinal shifts the mean paths of the surface and subsurface expressions of the pf are closely coupled over much of the southern ocean

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

circulation stratification and seamounts in the southwest indian ocean abstract circulation in the vicinity of six seamounts along the southwest indian ridge was studied as part of a multidisciplinary survey in november 2009 examination of altimetric data shows that several of the seamounts lie in the area of slow mean westward flow between the southern tip of madagascar 25 s and the agulhas return current arc flowing eastward between 37 s and 40 s the mean westward drift of mesoscale features was 4 1 0 9 cm s 1 integrated between madagascar and 37 s this westward drift can account for 50 sv 1 sv 106 m3 s 1 which added to 25 sv of southward flow past madagascar is sufficient to account for the total agulhas current transport of 70 21 sv the transport of the arc was also measured at two longitudes down to 2000 m combined with earlier crossings of the arc in 1986 and 1995 the full depth transport of the arc is estimated at 71 85 sv at longitudes 40 50 e indicating that the agulhas current then arc transport continues unreduced as far as 50 e before beginning to recirculate in the southwest indian ocean subtropical gyre the primary control on the circulation near each seamount was its position relative to any mesoscale eddy at the time of the survey melville lay on the flank of a cyclonic eddy that had broken off the arc and was propagating west before remerging with the next meander of the arc nearby sapmer on the other hand was in the centre of an anticyclonic eddy resulting in very weak stratification over the seamount at the time of the survey middle of what lies most often on the northern flank of the arc in strong currents but was at the time of the survey near the edge of the same eddy as sapmer coral in the subtropical front south of the arc was in waters much colder fresher denser and more oxygenated than all the other seamounts walter was close to the path of eddies propagating southwest from east of madagascar while atlantis the furthest east and north seamount experienced the weakest eddy currents

the south atlantic current in this paper we use the historical hydrographic data base for the south atlantic ocean to investigate i the hydrographic boundary between the subtropical gyre and the antarctic circumpolar current acc the sub tropical front stf and ii the southern current band of the gyre which is called the south atlantic current sac the stf begins in the west in the brazil falkland malvinas confluence zone but at locations at and west of 45 w this front is often coincident with the brazil current front east of 45 w the stf appears to be a distinct feature to at least the region south of africa whereupon it continues into the indian ocean the associated current band of increased zonal speed is the sac which except for one instance is found at or north of the surface stf until indian ocean water from the agulhas retroflection is reached a reversal of baroclinicity in the stf is observed south of a highly saline agulhas ring causing the sac to separate from the stf and turn north into the benguela current zonal flow south of the stf is generally weak and serves to separate the south atlantic and circumpolar currents in the argentine basin the sac has a typical volume transport of 30 sv 1 sv 106m3s 1 in the upper 1000 m relative to a deep potential density surface 4 45 87 kg m 3 and can be as high as 37 sv it is thus comparable to or stronger than the brazil current in the cape basin the transport of the sac is reduced to about 15 sy before it turns north to feed the benguela current in late 1983 this flow was joined by about 8 sv of water from the agulhas current

implications of the deep circulation and ventilation of the indian ocean on the renewal mechanism of north atlantic deep water in the indian ocean sector of the great ocean conveyor scheme the north atlantic deep water nadw replacement is realized by the transformation of the sinking circumpolar deep water cdw from the southern ocean to a prevailingly upward flow in the northern indian ocean this water mass transformation scenario has been studied by using hydrographie data including potential temperature salinity dissolved oxygen and nutrients in a water mass mixing model and by calculation of dianeutral velocity the model comprises a mixing system of three major deep water masses nadw cdw and north indian deep water this third water mass is introduced as a virtual water mass to represent the aged cdw the assumption is feasible as the artificial water mass can be eliminated on the deepest n 28 12 neutral surface characterized by isopycnal mixing the experiment is fulfilled when a conservative variable initial phosphate po40 is introduced and all conservative parameters are used in the mixing scheme the n 28 12 surface provides a major isopycnal transition path of cdw to the northern indian ocean and thus cdw is transformed to an upward flow dominant dianeutral up welling is detected on all four neutral surfaces north of 30 s showing increasingly strong dianeutral velocity toward shallower surfaces with the water mass mixing pattern and spreading path dianeutral circulation and dynamical information of the acceleration potential 10 m2 s 2 mapped on neutral surfaces a schematic of deep water circulation and ventilation of the indian ocean emerges from this study the contribution of nadw is largely limited to the southwest indian ocean the northward cdw is found mainly in two paths one in the western indian ocean apparent between 2000 and 3000 m through the western crozet and madagascar basins and farther north into the somali basin and arabian basin and another in the central indian basin apparent below 3000 m where ridge blocking becomes effective in the west and nadw salinity reaches a maximum while the major path switches from west to the central basin in the deeper layer 3000 3500 m the flow in the western indian ocean reverses southward north of the southeast indian ridge the deep path bifurcates from which point the eastern branch feeds the deep western boundary flow in the west australian basin except for part of the transformed cdw which is contributed to the upward flow in the north the return flow of cdw is found on the eastern side of these basins

cross frontal exchange of antarctic intermediate water and antarctic bottom water in the crozet basin abstract the hydrographic survey made across the antarctic circumpolar current acc in the southwestern crozet basin during the april may 1993 antares 1 cruise revealed unprecedented strong cross frontal injections of newly formed antarctic intermediate water aaiw into the subtropical zone north of the acc the acc in this area is mostly associated with the subantarctic front saf and subtropical front stf which are tightly merged into one structure bordered to the north with the eastward extension of the agulhas return current the oxygen and salinity extrema of the newly injected aaiw at a depth of 1200 m well north of the stf reach as high as 6 17 ml l 1 and 34 15 these values coincide almost exactly with those in the aaiw source found at a depth of 500 m just south of the saf but surpass considerably oxygen by 1 35 ml l 1 and salinity by 0 23 psu those values corresponding to the old aaiw at the nearby subtropical stations such spectacular cross frontal injections of new aaiw were not continuous in space and time but were found to be rather local intermittent and impulsive this was closely correlated with the highly perturbed meandering frontal zone due to the frequent passages of strong atmospheric depressions during the cruise there is also clear evidence of the northward penetration of antarctic bottom water aabw through the crozet kerguelen gap and its northward flow along the deep western boundary of the crozet basin supporting previous observations moreover our observations indicate more precisely the bottom water pathway which is concentrated against the continental rise west of 56 e following roughly the 4000 m isobath it is concluded that the crozet basin frontal zone acts as a barrier only for the upper layer water masses and that it is dynamically transparent for the cross frontal exchange of intermediate and deeper layer water masses of potential density anomalies greater than 27 1 including aaiw and aabw

a mean synoptic view of the subantarctic front south of australia abstract a mean synoptic view of the subantarctic front saf is obtained from current meter and hydrographic data by averaging absolute and baroclinic velocity measurements in bins defined by a cross stream coordinate that moves with the current the cross stream coordinate is derived from current meter measurements of vertical shear of the horizontal velocity and is expressed in terms of specific volume anomaly at 780 dbar 780 by averaging absolute and baroclinic velocity measurements in stream coordinates the spatial smoothing that results from eulerian averaging of measurements of a meandering current is avoided the mean saf velocity profile is composed of one central peak and several smaller peaks in the central peak the 2 yr mean absolute velocity from current meters reaches 52 cm s 1 at 420 dbar the mean baroclinic velocity from six ctd sections reaches 34 cm s 1 the saf flow is coherent at all levels reaches the seafloor and is at least 220 km wide the cross stream structure of barocl

circulation and water properties of the southwest indian ocean spring 1987 abstract the results obtrained during a cruise in the south western indian ocean from south africa to mauritius in september 1987 are presented temperature salinity oxygen and nutrients were measured at 70 ctd station 20 nautical miles apart the data perspective is enhanced by noaa satellite infrared imagery and geosat altimetry the major features encountered on the cruise were two intense cyclonic eddies in the vicinity of the mozambique ridge and the mozambique basin and which seemed to form part of the broader western boundary region of the southern indian ocean evidence was found of a significant equatoward flux at the madagascar ridge which provided input of water from higher latitudes the passage of north atlantic deep water was constrained by the bottom topography south of the madagascar ridge and this led to a reduction of its salinity and oxygen concentration the positive correlation between the ctd results and the altimetry enabled the identification tracking and description of a number of eddies in the region the transition zone of these eddies from east of madagascar to the african continent was identified and so was the apparent closure of the southern entrance of the mozambique channel to migrating eddies

ocean currents and gradients of surface layer properties in the vicinity of the madagascar ridge including seamounts in the south west indian ocean abstract this work is part of the madridge project special issue which aims to describe pelagic ecosystems in the vicinity of three prominent shallow seamounts in the south west indian ocean one here named mad ridge 240 m below the surface plus walters shoal 18 m on the madagascar ridge and la perouse 60 m on the abyssal plain east of madagascar the three span latitudes 20 s and 33 s some 1500 km the study provides the background oceanography for the once off multidisciplinary snapshot cruise studies around the seamounts as life on seamounts is determined by factors such as summit depth proximity to the light layers of the ocean and the ambient circulation a first description of regional spatial field climatologies 16 22 years and monthly along ridge gradients of surface wind driving force water column properties of sea surface temperature mixed layer depth chlorophyll a and eddy kinetic energy plus ocean currents is provided being relevant to many applications in the study domain these properties in particular reveal contrasting environments along the madagascar ridge and between the three seamounts that should drive biological differences relative to the other two seamounts mad ridge is in the more extreme situation being at the end of the east madagascar current where it experiences sturdy albeit variable currents and the frequent passing of mesoscale eddies

an appraisal of the velocity reversal hypothesis for stable pool riffle sequences in the river severn england the velocity reversal hypothesis is the linchpin for a number of recent conceptual models pertaining to sediment sorting and the maintenance of pool riffle sequences in gravel bedded streams the literature in support of the hypothesis together with published adverse criticism is reviewed it is concluded that convincing evidence for the ubiquitous occurrence of such a reversal in a range of channel geometries is currently unavailable continuity considerations indicate that riffles need to be considerably wider than pools for a reversal in the mean velocity to occur under conditions of subcritical flow high stage and stable morphology these observations are substantiated by a detailed study of the hydraulic geometry of stable pool riffle sequences in the river severn england neither the sectionally averaged velocity nor the near bed shear velocity is sensibly greater in the pools than over the riffles during bankfull or near bankfull flow instead a tendency towards equalization of the values of average hydraulic variables is noted as discharge increases a detailed investigation of the three dimensional character of the flow is required to demonstrate whether the entrainment forces within pools can locally exceed those over neighbouring riffles unusual behaviour of the energy gradient over riffles during moderate discharges is related to backwater effects as mediated by the spacing of the riffles the hydraulic data are used to comment on the stability of the test reaches in the context of the development of the river severn in the vicinity of shrewsbury

deep inflow into the mozambique basin more than 200 conductivity temperature depth ctd stations were worked around the southwest indian ridge and del cano rise as part of the world ocean circulation experiment a selection of these data provides information about the inflow of bottom water into the mozambique basin the basin is closed below 3000 m yet the inflow is significantly large of order 1 sv 1 sv 106 m3 s 1 estimates of the basin scale upwelling at 4000 m suggest that the vertical velocity is also large 10 10 5 cm s 1 or more an order of magnitude greater than global ocean estimates examination of the characteristics of the bottom water in the mozambique and agulhas basins and the prince edward fracture zone shows that bottom water enters the mozambique basin from the agulhas basin and also directly from the enderby basin most of the transport enters the mozambique basin via the agulhas basin where two regions of northward flow below 4000 m are found the major flow on the eastern flank of the mozambique ridge is through and above the deep extending 5900 m trench that connects the agulhas and mozambique basins the second weaker flow enters the transkei basin along the deep eastern flank of the agulhas plateau then turning east into the mozambique basin the only source of bottom water to the agulhas basin is the enderby basin but a more direct route between the enderby and mozambique basins exists via the prince edward fracture which extends deeper than 4000 m throughout its length and links the two basins directly across the southwest indian ridge full depth ctd stations trace the changing characteristics of the deep and bottom water in the fracture and moored current meter data show the strength and persistence of the throughflow strong mixing with the overlying deep water elevates the salt content of the bottom water by comparison with the other water in the mozambique basin thus two distinct bottom waters of the mozambique basin originate in the same place the enderby basin and their different characteristics are solely a function of the routes they have taken and the processes encountered along the different pathways

pelagic communities of the south west indian ocean seamounts r v dr fridtjof nansen cruise 2009 410 the seamounts of the southern indian ocean remain some of the most poorly studied globally and yet have been subject to deep sea fishing for decades and may face new exploitation through mining of seabed massive sulphides in the future as an attempt to redress the knowledge deficit on deep sea benthic and pelagic communities associated mainly with the seamounts of the south west indian ridge two cruises were undertaken to explore the pelagic and benthic ecology in 2009 and 2011 respectively in this volume are presented studies on pelagic ecosystems around six seamounts five on the south west indian ridge including atlantis bank sapmer seamount middle of what seamount melville bank and coral seamount and one un named seamount on the madagascar ridge in this paper existing knowledge on the seamounts of the southwestern indian ocean is presented to provide context for the studies presented in this volume an account of the overall aims approaches and methods used primarily on the 2009 cruise are presented including metadata associated with sampling and some of the limitations of the study sampling during this cruise included physical oceanographic measurements multibeam bathymetry biological acoustics and net sampling of phytoplankton macrozooplankton and micronekton nekton the studies that follow reveal new data on the physical oceanography of this dynamic region of the oceans and the important influence of water masses on the pelagic ecology associated with the seamounts of the south west indian ridge new information on the pelagic fauna of the region fills an important biogeographic gap for the mid to high latitudes of the oceans of the southern hemisphere

fronts transport and weddell gyre at 30 e between africa and antarctica a detailed description of the frontal structure of major currents and estimates of transport between africa and antarctica at 30 e were made on the basis of a finely resolved hydrographic section made during the 1996 civa 2 cruise particular emphasis was put on a refinement of the eastern boundary of the weddell gyre by analyzing also supplementary hydrographic data from the 1993 civa 1 cruise and the best available historical hydrographic data in the weddell and enderby basins contrary to the general belief our results show that the weddell gyre extends far beyond 30 e reaching at least as far east as the enderby land promontory 53 e from this point westward currents along the antarctic continental margin constitute the gyre s southern boundary thus closing the cyclonic circulation of the gyre at its easternmost end as the most convincing evidence for this refined circulation scheme significant westward transport south of 65 s has been identified from the civa 1 acoustic doppler current profiler data we have shown also using an inertial jet model that the blocked feature of the weddell cold regime 57 s to the west of 25 e is due to the topographic control of flow by local bathymetry during the civa 2 cruise 70 of the antarctic circumpolar current acc was concentrated near the polar front 51 s with surface speeds reaching up to 70 cm s 1 the baroclinic current field in the subpolar area south of the weddell front 58 s for civa 2 and 57 s for civa 1 is featureless 3 cm s 1 but yielding a nonnegligible eastward transport of 15 sv 1 sv 106 m3 s 1 this bottom referenced baroclinic transport is entirely compensated by a substantial westward barotropic transport south of 65 s the acc transport across the 30 e section during the civa 2 cruise is estimated to be 160 sv

vorticity and vertical circulation at an ocean front abstract density and velocity data with 4 km horizontal resolution from a survey of a front during fasinex frontal air sea interaction experiment are combined to describe the structure of the top 300 m of the ocean the geostrophic velocity field is derived and is used to examine the relative importance of stratification relative vorticity and twisting terms in ertel s potential vorticity q tenfold isopycnic changes in q are found across a horizontal scale of only 10 km these changes are confined to isopycnals that outcrop from the seasonal thermocline into the mixed layer the ageostrophic velocity field is quantified by solution of the omega equation and vertical velocities of up to 40 m day 1 are found small 40 km surface trapped top 200 m eddies are found to play a crucial role in the transport and effective diffusion of properties across the thermocline out of the mixed layer

an introduction to the physical oceanography of six seamounts in the southwest indian ocean exploratory surveys of six seamounts in the southwest indian ocean provide a description of physical processes induced by seamounts along the southwest indian ridge mean currents 15 25 cm s 1 in the vicinity of each seamount were dominated by mesoscale eddies the dominant seamount driven process was the generation of internal tides by the barotropic tide interacting with the seamount crests this led to enhanced shear in the vicinity of the crests resulting in mixing where stratification was weak for example in the core of an anticyclonic mesoscale eddy or where there had been a winter mixed layer tidally driven up and downwelling was observed at the seabed with associated variability in bottom temperature of up to 3 c over a tidal cycle vertical displacement of isopycnals by internal tidal waves reached 200 m peak to trough fluorescence in the surface eutrophic layer could thus extend down to the seamount crest on each tidal cycle apparently spatial variations in short conductivity temperature depth sections across each seamount were probably aliased temporal variations from the strong tidal signal evidence for taylor caps or other potential trapped circulations at the seamount crest was weak most likely because currents associated with mesoscale eddies were too strong to allow their formation

phytoplankton pigment distribution from seawifs data in the subtropical convergence zone south of africa a wavelet analysis the agulhas system agulhas current retroflection agulhas return current arc south of africa constitutes a complex energetic system subject to high frequency mesoscale and low frequency variability using chlorophyll pigment concentration data collected by the seawifs ocean color sensor and sea surface height satellite data we demonstrate the utility of wavelet analysis in jointly analyzing ocean color and altimetric signals two distinct dynamical scenarios of the agulhas system october 1997 and january 1998 yield different dominant wavelengths 620 and 500 km for the arc rossby wave they are simultaneously retrieved from the chlorophyll and sea level anomaly signals longitudinal extension of the power resulting from the wavelet analysis also depends on the dynamical regime

circulation pathways and transports of the southern ocean in the vicinity of the southwest indian ridge circulation pathways and transports of the antarctic circumpolar current acc and agulhas return current arc around the complex bathymetry of the southwest indian ridge and del cano rise have been established from two hydrographic surveys and two year long moorings after crossing the southwest indian ridge in a 110 sv 1 sv 106 m3s 1 flow concentrated at 48 s the acc fragments before being reconcentrated into several branches by the bathymetry in particular 30 40 sv turns north around the eastern flank of the del cano rise turns west as part of anticyclonic flow round the del cano rise finally returning eastward as part of the crozet front eddy variability from both satellite data and moorings is low over the bathymetry of the del cano rise so the strong currents found between the del cano rise and the crozet plateau are believed to be permanent features this largely barotropic time independent transport is the major pathway for the acc to enter the crozet basin the anticyclonic flow also transfers water from the mozambique basin across the southwest indian ridge to the south side of the del cano rise given the evidence for fragmentation of the acc where it crosses the southwest indian ridge and the concentration of several jets into the crozet front from the acc subtropical front and agulhas return current it is misleading to relate standard frontal definitions such as the polar front and subantarctic front to the major transport pathways

mean sea surface height of the antarctic circumpolar current from geosat data method and application the mean sea surface height across the antarctic circumpolar current has been reconstructed from height variability measured by the geosat altimeter without assuming prior knowledge of the geoid for this study an automated technique has been developed to estimate mean sea surface height for each satellite ground track using a meandering gaussian jet model and errors have been estimated using monte carlo simulation the results are objectively mapped to produce a picture of the mean subantarctic and polar fronts which together comprise the major components of the antarctic circumpolar current the meandering jet model explains between 40 and 70 of the height variance along the jet axes the results show that the fronts are substantially steered by topography and that the jets have an average gaussian width of about 44 km in the meridional direction and meander about 75 km to either side of their mean locations the average height difference across the subantarctic front saf is 0 7 m and across the polar front pf 0 6 m the mean widths of the fronts are correlated with the size of the baroclinic rossby radius

the antarctic circumpolar wave in a coupled ocean atmosphere gcm abstract a phenomenon called the antarctic circumpolar wave acw suggested earlier from fragmentary observational evidence has been simulated realistically in an extended integration of a max planck institute coupled general circulation model the acw both in the observations and in the model constitutes a mode of the coupled ocean atmosphere sea ice system that inhabits the high latitudes of the southern hemisphere it is characterized by anomalies of such climate variables as sea surface temperature sea level pressure meridional wind and sea ice that exhibit intricate and evolving spatial phase relations to each other the simulated acw signal in the ocean propagates eastward over most of the high latitude southern ocean mainly advected along in the antarctic circumpolar current on average it completes a circuit entirely around the southern ocean but is strongly dissipated in the south atlantic and in the southern indian ocean just marginally maintaining statistical significance in these areas

structure and transport of the antarctic circumpolar current and agulhas return current at 40 e hydrographic data are presented from a near meridional section across the agulhas return current the subtropical front and the antarctic circumpolar current acc in the southwest indian sector of the southern ocean the location and characteristics of the fronts are examined and the water masses are defined the agulhas return current emerges as a strong current as far east as 40 e which forms a marked front that is separate from the subtropical front and can be distinguished by a separate water mass comparisons are made with a similar hydrographic section at the greenwich meridian and several notable differences are found the subantarctic front at the greenwich meridian is a pronounced feature carrying a large proportion of the geostrophic transport of the acc on the southwest indian section however the subantarctic front is merged with the polar front at 48 s where the flow associated with the acc is concentrated geostrophic calculations show a large volume transport 84 to 103 sverdrup 1 sv 106 m3 s 1 associated with the agulhas return current at 40 s this is quite separate from the transport of the acc which totals 138 sv south of the subantarctic zone and the crozet plateau or 150 sv including flow in the subantarctic zone the transport of the acc out of the south atlantic sector appears to be higher than the transport into the south atlantic sector at drake passage with the extra transport most probably carried by upper layer and intermediate waters eddies are seen to be an important feature of the subantarctic zone where they help to modify the water masses antarctic intermediate water appears in two forms a saline 34 4 indian ocean variety and a fresh 34 2 atlantic ocean variety the atlantic variety is freshest at the polar front and occupies the southern half of the subantarctic zone within the eddies in the central subantarctic zone there is interleaving between these two forms of antarctic intermediate water north atlantic deep water is unable to cross the southwest indian ridge but can pass south of the crozet plateau in the form of circumpolar deep water which is slightly more saline 34 75 than at drake passage 34 73

seasonal nutrient depletion and carbon export in the subantarctic and polar frontal zones of the southern ocean south of australia seasonal surface water nitrate and phosphate depletions were calculated for the subantarctic zone saz and polar frontal zone pfz of the southern ocean along the world ocean circulation experiment sr3 repeat section south of tasmania australia and three other transects slightly to the west the depletions were calculated between a single winter july section and eight other sections from october to march in the years 1991 1998 and corrected for frontal movements and changes in water masses between the cruises using observed quasi linear nutrient salinity relationships arranged in a seasonal progression depletion began in approximately october and continued through march in good agreement with satellite biomass based estimates of production nitrate depletion in the saz 510 mmol n m 2 was approximately twice that of the pfz 250 mmol n m 2 but phosphate depletions were similar 29 and 24 mmol p m 2 for the saz and pfz respectively the saz exhibited a redfield n p depletion ratio 15 1 2 3 but the pfz ratio was much lower 8 3 5 4 while the saz exhibits greater seasonal nutrient depletion the relative magnitudes of export in the two zones remain uncertain because supply of nutrients from south of the polar front may lower the observed nutrient depletions in the pfz however comparisons with sediment trap and production measurements for the region south of australia suggest the saz and pfz differences derive from differences in production assuming a redfield c n ratio 6 6 carbon export for the longest period examined july march averaged 3400 430 mmol c m 2 in the saz with a range between 2700 and 3900 mmol c m 2 depending on latitude in the pfz carbon export averaged 1600 350 mmol c m 2 with a range of 1100 to 2000 mmol c m 2 these averages represent minimum annual export production estimates because neither resupply nor export outside the july march observational period is included

water mass and biogeochemical variability in the kerguelen sector of the southern ocean a machine learning approach for a mixing hot spot author s rosso isabella mazloff matthew r talley lynne d purkey sarah g freeman natalie m maze guillaume

major nutrients and dissolved oxygen as indicators of the frontal zones in the atlantic sector of the southern ocean 1 the chemical tracers of the main frontal zones of the atlantic sector of the southern ocean are considered before the beginning of the spring bloom frontal zones are distinguished by lateral gradients of dissolved oxygen phosphate nitrate and silicate during the spring bloom the smoothing of nutrient concentrations on both sides of the fronts weakens lateral gradients of chemical properties the position of surface gradients of nutrients within the subtropical frontal zone stfz does not coincide with the location of temperature and salinity gradients as a result fronts in this region have a stepped character the best chemical indicator of the northern stfz front is the dissolved oxygen gradient which coincides with the temperature and salinity gradients the southern boundary of the stfz is distinguished by the gradient of nitrate the chemical criterion for identifying the subantarctic front is the gradient of oxygen which ranges from 0 5 to 4 0 mol kg 1 per km the polar front is identified by the gradient of silicate 0 56 to 2 78 m per km at the surface the weddell scotia confluence wsc is distinguished not by the temperature and salinity but by chemical parameters the best year round criterion is the lateral gradient of silicate to phosphate atomic ratio which ranges from 25 to 35 other markers of the wsc are the gradients of silicate at the surface oxygen at the upper boundary of the circumpolar deep water and the depths of its location

a mantle heterogeneity in the southwest pacific abstract the occurrence in time and space of tertiary intraplate central volcanoes and associated lava fields in the eastern australia new zealand campbell plateau region of the southwest pacific is more consistent with derivation from a large scale mantle anomaly than from discrete plumes the basalt volcanics from the eastern part of the region show a pronounced geochemical similarity those near the indo australia pacific convergent plate boundary show some evidence of interaction with subduction derived basalts the lithospheric swells expected associated with the eruption of magma to form these volcanoes seem broadly correlative with sedimentary patterns off the east coast of australia and the west coast of new zealand at this time

physical and biological patchiness of an upper ocean transect from south africa to the ice edge near the greenwich meridian an undulating ctd seasoar was towed from the subtropical front at 42 s 12 e to the ice edge at 57 5 s 2 w the instrument measured temperature salinity pressure and light in the top 400 m of the water column fluorescence and particle counts also were recorded fluorescence was calibrated to chlorophylla and particle counts were converted to biovolume to provide estimates of phyto and zooplankton respectively the size of particles counted covered the range 0 25 8 0 mm three major fronts subtropical subantarctic and polar were intersected and identified by their physical characteristics the polar front showed surface and subsurface signatures separated by nearly 300 km considerable small scale structure was observed between the fronts in particular a large 150 km diameter warm core eddy was found in the northern part of the subantarctic zone both fronts and eddy were associated with significant biological signals phyto and zooplankton correlated well in the frontal regions and south of the polar front the polar frontal zone was associated with elevated concentrations of biomass whereas concentrations were low at the subantarctic front and south of 55 s in contrast the warm core eddy in the subantarctic zone contained the highest zooplankton biovolume of the section while chlorophyll concentration was only moderate we hypothesise that such a distribution results from grazing the overall distribution of phyto and zooplankton changed across the subantarctic front the vertical extent of biomass deepened from north to south across the front zooplankton tended to be concentrated in relatively narrow deep extending bands to the north but were more widely spread to the south the small scale hydrographic structure correlated significantly with the distribution of plankton suggesting strong physical controls on both small zooplankton and phytoplankton

oxygen isotope and salinity variations in the indian sector of the southern ocean a combined study of stable oxygen isotopes i 18o and salinity of surface ocean waters collected from the southern indian ocean during the late austral summer and early fall 25th january to 1st april 2006 helped to trace atmospheric and oceanic processes near the equator a very clear signature of the domination of evaporation precipitation process was seen while a precipitation dominated zone was located between two evaporation dominated zones south of the equator the signature of the agulhas front af and the subtropical front stf recognizable by the sharp decrease in salinity and i 18o in the surface waters were identified at 41 s and 44 s in february during the return journey march these two fronts had merged and were located at 41 s south of 45 s the slope of the i 18o salinity relationship was close to zero while north of 62 s could have some precipitation evaporation effect south of 63 s was clearly dominated by freezing melting

the baroclinic transport of the antarctic circumpolar current south of africa five hydrographic transects at nominal longitudes 0 e and 30 e and fourteen expendable bathythermograph xbt sections near the former longitude are used to study the baroclinic transport of the antarctic circumpolar current acc between africa and antarctica the bottom referenced geostrophic transport between the subtropical front and the acc southern boundary is 147 10 sv estimating the transport from the xbts using a technique previously employed south of australia proves delicate because of an irregular bathymetry and water mass variations it nevertheless confirms acc transports around 150 sv gathering these and other estimates from the atlantic sector suggests that while north atlantic deep water is injected in the current west of 35 w a partially compensating loss of deep circumpolar water occurs east of this longitude another transport increase from 0 e to 30 e might reflect southward transfer across the subtropical front south of the agulhas retroflection

the south indian ocean current in this paper the historical hydrographic database for the south indian ocean is used to investigate i the hydrographic boundary between the subtropical gyre and the antarctic circumpolar current acc the subtropical front stf and especially ii the southern current band of the gyre a current band of increased zonal speeds in the upper 1000 m is found just north of the stf in the west near south africa and at the surface stf in the open indian ocean until the waters off the coast of australia are reached as neither any other investigation of this current nor a name for it are known the flow has been called the south indian ocean current sioc this name is anologous to the same current band in the south atlantic ocean the south atlantic current the stf is located in the entire south indian ocean near 40 degrees s the associated current band of increased zonal speeds is the sioc which is found at or north of the stf east of 100 degrees e the sioc separates from the stf and continues to the northeast the zonal flow south of the stf is normally weak and serves to separate the south indian ocean and circumpolar currents near africa the sioc has a typical volume transport of 60 sv 1 sv 10 6 m3 s 1 in the upper 1000 m relative to deep potential density surfaces of sigma 4 45 87 kg m 3 2800 3500 m or sigma 2 36 94 kg m 3 1500 2500 m near western australia the sioc is reduced to about 10 sv as it turns to the northeast