vertical motion in the gulf stream near 68 w abstract the authors compute and compare vertical motions from three different data sources within a 300 km square domain centered in the gulf stream near 38 n 68 w and show that vertical motions inferred from all three independent data sources and different analysis methods give similar results the time derivatives of rafos float pressures on isopycnal surfaces are used to determine directly observed vertical motion wraf second vertical motions are inferred from the heat equation using measured temperature changes and the backing and veering of currents observed by a stack of current meters to produce wcm third vertical motions are calculated from the quasigeostrophic vorticity equation using geostrophic streamfunctions from inverted echo sounder measurements to produce wies wcm agrees well with wraf from all floats that pass within 10 km of a current meter mooring daily fields of wcm show very good coherence with time series of wcm for periods longer than 16 d but noise domination for periods

mean stream coordinates structure of the subantarctic front temperature salinity and absolute velocity 1 the mean synoptic structure of the northern strongest branch of the antarctic circumpolar current southwest of tasmania at the subantarctic front saf is estimated by a stream coordinates analysis of data from overlapping arrays of inverted echo sounders iess and horizontal electric field recorders deployed during the 1995 1997 sub antarctic flux and dynamics experiment the stream coordinates are derived from a daily objective mapping of the temperature field obtained from combining the ies travel time measurements with an empirical look up table constructed from the extensive hydrography acquired during woce full water column stream coordinates sections of temperature salinity and absolute velocity are presented and compared with prior observations the along stream current has a single peak with surface velocities reaching about 50 cm s 1 the vertical structure of the along stream velocity is roughly consistent with a combined external and first internal normal mode description that is adapted to the buoyancy frequency as it varies across the front although there are some significant differences the cross stream structure of along stream velocity is very nearly symmetric about the jet axis but the lateral shear magnitude is slightly larger on the cold side of the saf separating the baroclinic and barotropic currents reveals that the saf currents are diffluent primarily baroclinically in the cross stream direction baroclinic cross stream diffluence of approximately 0 23 sv per km sv 106 m3 s 1 or about 16 sv per degree of longitude at 51 s the 2 year mean total saf transport is 75 sv for a 220 km width the barotropic contribution is small 8 sv but not negligible

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

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

direct observations of the antarctic circumpolar current transport on the northern flank of the kerguelen plateau 1 the standing meander in the antarctic circumpolar current acc found on the northern flank of the kerguelen plateau was investigated during the southern ocean finestructure cruise in november december 2008 an 18 year time series of surface geostrophic currents from satellite altimetry shows that the meander as observed during this survey is typical of the region hydrographic stations were occupied between 65 75 e and 43 48 s on the shelf 200 m depth and slope into the deep ocean to the north of kerguelen 4700 m providing the most detailed survey of this region to date geostrophic shears are referenced to lowered acoustic doppler current profiler velocities to give the first estimate of the total volume transport in this region and the transport budget is closed around the survey box the subtropical front subantarctic front and a northern branch of the polar front together have an associated transport of 174 22 sv eastward while 174 sv is large compared with typical drake passage transports it is reconciled with other estimates of the total transport with the additional 15 sv of the indonesian throughflow baroclinic transport referenced to the deepest common level between station pairs is 119 sv consistent with other estimates of the baroclinic transport in this area at this longitude the fronts of the acc are exceptionally close together we discuss the exchange of properties across the fronts

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

repeat ridge jumps and microcontinent separation insights from ne arabian sea abstract microcontinents separate due to ridge jumps associate either asymmetric sea floor spreading or plume ridge interactions india separated from seychelles at 64 ma by asymmetric sea floor spreading initially when the spreading centre in the mascarene basin jumped towards the indian sub continent between magnetic chrons c29 and c28 the subsequent tectonics is difficult to comprehend since laxmi ridge another microcontinent formed during the later phase most of the studies considered the laxmi ridge as a sliver others considered it to be oceanic crust high resolution deep 25 km seismic data reveals that i the ridge possesses 15 km deep sea ward dipping reflector sdr packages ii normal faulted rift valleys devoid of syn rift sedimentary packages and iii axial magma chambers 5 7 km beneath the ridge top additionally from 2d forward gravity models we deduce that the ridge most possibly comprises of high density oceanic crust we conclude the laxmi ridge to be indeed composed of oceanic crust and a fossil spreading centre we thus identified the ridge jumps and their relation to the seychelles microcontinent separation previous numerical models suggest that the time required for a ridge jump is controlled by magmatic heating spreading rate at the ridge and plate ages for repeated ridge jumps the additional factor is the dynamic relation between the plume and lithosphere in terms of melt transfer and heating we find that the medium spreading rates and high magmatic heating due to the reunion plume and young plates favoured rapid and repeated ridge jumps towards the plume

a precambrian microcontinent in the indian ocean ridges of thick raised crust on the indian ocean floor were thought to be mostly volcanic seamounts formed above the reunion mantle plume dating of zircon minerals in mauritian lavas however indicates that fragments of an ancient microcontinent may be preserved beneath the seamounts contributing to the thickened crust

the circulation and water masses of the antarctic shelf and continental slope between 30 and 80 e abstract the circulation and water masses from the antarctic continental shelf to 62 s between 30 and 80 e are described using hydrographic data collected on seven hydrographic sections during the baseline research on oceanography krill and the environment west broke west experiment the eastern limb of the weddell gyre dominates circulation between 30 and 40 e and is significantly cooler and fresher than the region to the east the antarctic circumpolar current acc extends from the north into the survey region east of 40 e reaching as far south as 65 5 s at 60 e this results in increasing observed maximum temperature and salinities progressively towards the east peaking at 80 e due to the intrusion of the southern acc front saccf to 63 s this southward extension is steered by the southern end of kerguelen plateau causing a horizontal shear of over 0 15 m s 1 between the eastward acc and westward flowing antarctic slope current asc the asc is observed at all six meridional sections immediately north of the shelf break it is strongly barotropic and transports a total of 15 8 7 4 sv westwards while the bottom referenced baroclinic component only contributes 1 3 0 3 sv at each section this current intensifies to a narrow westward jet with absolute velocities up to 0 3 m s 1 over the steepest shelf slope gradients at 70 e a v shape is observed in the asf this and the nearby presence of denser shelf water and ice shelf water is characteristic of antarctic bottom water aabw formation but no new aabw is found on this section instead significantly warmer saltier and less oxygenated aabw to the east and newly formed aabw high on the continental slope immediately to the west suggest a formation region just west of 70 e this newly formed aabw progressively becomes warmer and saltier west of 60 e and is observed extending offshore and moving westward below eastward flowing water masses acc frontal positions are found to be 1 2 farther north in the survey region than suggested by historical climatology

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

an equivalent barotropic mode in the fine resolution antarctic model abstract both the 6 year time mean flow and the eddy kinetic energy in the fine resolution antarctic model are found to be approximately self similar in the vertical with the velocity showing a rapid decay with depth a simple argument is given as to why this would be expected to occur in this and other numerical models and an analysis is given as to the degree of steering of the mean flow by topography

eddy variability and energetics from direct current measurements in the antarctic circumpolar current south of australia two year time series measurements of current velocity and temperature in the subantarctic front saf south of australia from 1993 to 1995 provide estimates of eddy fluxes of heat and momentum across the antarctic circumpolar current acc and further insight into the variability of the acc the saf was found to be an energetic meandering jet with vertically coherent fluctuations varying on a timescale of 20 days with typical amplitude 30 cm s21 at 1150 dbar a daily varying coordinate frame that follows the direction of flow allowed mesoscale variability of the saf to be isolated from variability due to meandering of the front and proved very successful for examining eddy fluxes vertically averaged cross stream eddy heat flux was 11 3 kw m 22 poleward and significantly different from zero at 95 for fluctuations in the 2 90 day band zonally integrated this eddy heat flux 50 9 pw is more than large enough to balance heat lost south of the polar front and is as large as cross saf fluxes found in drake passage cross stream eddy momentum fluxes were small not significantly different from zero and of indeterminate sign but tending to decelerate the mean flow a relationship between vertical motion and meander phase identified in the gulf stream was found to hold for the acc eddy kinetic energy levels were similar to those in drake passage and southeast of new zealand eddy potential energy was up to an order of magnitude larger than at other acc sites most likely because meandering of the front is more common here baroclinic conversion was found to be the dominant mechanism by which eddies grow south of australia typical eddy growth time is estimated to be 30 days approximately twice as fast as in drake passage consistent with eddy energy growing rapidly downstream

observation of a large lee wave in the drake passage abstractlee waves are thought to play a prominent role in southern ocean dynamics facilitating a transfer of energy from the jets of the antarctic circumpolar current to microscale turbulent motions important in water mass transformations two em apex profiling floats deployed in the drake passage during the diapycnal and isopycnal mixing experiment dimes independently measured a 120 20 m vertical amplitude lee wave over the shackleton fracture zone a model for steady em apex motion is developed to calculate absolute vertical water velocity augmenting the horizontal velocity measurements made by the floats the wave exhibits fluctuations in all three velocity components of over 15 cm s 1 and an intrinsic frequency close to the local buoyancy frequency the wave is observed to transport energy and horizontal momentum vertically at respective peak rates of 1 3 0 2 w m 2 and 8 1 n m 2 the rate of turbulent kinetic energy dissipation is estimated using both thorpe scales and a method that isolate

circumpolar structure and distribution of the antarctic circumpolar current fronts 2 variability and relationship to sea surface height 1 in part 1 of this study we showed that the antarctic circumpolar current acc consisted of multiple fronts each of which was consistently associated with a particular contour of sea surface height ssh or approximate streamline in part 2 we have used maps of ssh to examine the variability of the acc fronts between 1992 and 2007 the ssh label associated with each frontal branch is nearly constant around the circumpolar belt the front labels are also nearly constant in time the bands of enhanced ssh gradient i e fronts occur along the same streamlines throughout the 15 year period of observations both short and long period changes of the ssh frontal labels of the acc are small based on a tight relationship between dynamic height and cumulative baroclinic transport of the acc the baroclinic transport variability of the individual branches of the acc is also expected to be small the major change in the total acc baroclinic transport occurs in the drake passage the streamline associated with the northern branch of the saf saf n does not pass through drake passage and the waters carried by this branch are not observed there instead the transport of the saf n turns north in the pacific to supply the export of water to the indian ocean north and south of australia strong eddy activity in the southeast pacific acts to dissipate the hydrographic signature of the saf n there in the atlantic the saf n reappears as the same streamline is again associated with enhanced ssh gradients to the east of the brazil malvinas confluence zone while the large changes in ssh have occurred in the southern ocean between 1992 and 2007 there are strong regional differences because the acc fronts are robustly associated with particular ssh contours the changes in ssh reflect shifts in the position of the acc fronts in the circumpolar average each of the acc fronts has shifted to the south by about 60 km the changes in ssh in the southern ocean are largely due to changes in ocean circulation rather than warming and freshening by atmospheric fluxes much larger changes in ssh are observed in some locations of the southern ocean particularly where the fronts interact with large scale topography the northern branch of the pf pf n near the kerguelen plateau is an extreme example where the pf n followed a path around the northern end of kerguelen plateau between 1992 and 1997 passed through the fawn trough after 2003 and oscillated between the two paths between 1997 and 2003

modal decomposition of the low frequency currents and baroclinic instability at drake passage abstract recently collected hydrographic data show that each of the four water mass zones previously identified in drake passage is characterized by a distinctly different vertical profile of brunt vaisala frequency stratification in drake passage is rather weak resulting in small values of the first baroclinic radius of deformation varying from 17 3 km for the subantarctic zone in the north to 7 7 km for the continental zone in the south using current meter records collected in 1979 at nine moorings the vertical structure of the low frequency currents is described in terms of dynamic normal and empirical modes dominance of the barotropic and first baroclinic modes was evident irrespective of mooring location accounting for typically 83 98 of record variance the first empirical mode which explains more than 90 of record variance at most moorings is surface intensified and appears to be a superposition of the barotropic and first baroclinic modes due to the observed modal coupling between the b

strong bottom currents and cyclogenesis in drake passage 1 observations from 38 bottom moored current and pressure recording inverted echo sounders cpies deployed in drake passage during the 2007 2008 international polar year provide unprecedented coverage of near bottom currents and pressures spanning the entire antarctic circumpolar current year long mean currents exceed 10 cm s 1 north of the polar front and mean directions are not in general aligned with the surface fronts topographic steering is most evident at the continental margins deep eddy kinetic energy eke is maximum at about 200 cm2 s 2 between the subantarctic and polar fronts coinciding with the location but about one quarter of the value of a maximum in surface eke multiple high speed current events with peak speeds of 60 70 cm s 1 and lasting 30 to 70 days are coherent across sites separated by 45 km the observed spinup of eddies coinciding with meanders in the surface fronts is consistent with deep cyclogenesis

the breakup of east gondwana assimilating constraints from cretaceous ocean basins around india into a best fit tectonic model published models for the cretaceous sea oor spreading history of east gondwana result in unlikely tectonic scenarios for at least one of the plate boundaries involved and or violate particular constraints from at least one of the associated ocean basins we link east gondwana spreading corridors by integrating magnetic and gravity anomaly data from the enderby basin off east antarctica within a regional plate kinematic framework to identify a conjugate series of east west trending magnetic anomalies m4 to m0 126 7 120 4 ma the mid ocean ridge that separated greater india from australia antarctica propagated from north to south starting at 136ma northwest of australia and reached the southern tip of india at 126 ma sea oor spreading in the enderby basin was abandoned at 115 ma when a ridge jump transferred the elan bank and south kerguelen plateau to the antarctic plate our revised plate kinematic model helps resolve the problem of successive two way strike slip motion between madagascar and india seen in many previously published reconstructions and also suggests that sea oor spreading between them progressed from south to north from 94 to 84 ma this timing is essential for tectonic ow lines to match the curved fracture zones of the wharton and enderby basins as greater india gradually began to unzip from madagascar from 100 ma in our model the 85 east ridge and kerguelen fracture zone formed as conjugate anks of a leaky transform fault following the 100ma spreading reorganization our model also identi es the afanasy nikitin seamounts as products of the conrad rise hotspot

nature of the crust in the laxmi basin 14 20 n western continental margin of india 1 the nature of the crust in the laxmi basin western margin of india is an uncertain issue more importantly this has implications on paleogeographic reconstructions of the western indian ocean we have analyzed three geophysical data sets and modeled gravity and magnetic anomalies for determining nature of the crust basement of the laxmi basin includes numerous highs which make the basement uneven and shallower compared to the western basin the laxmi basin is characterized by a broad gravity high and a narrower prominent gravity low within it while within the basin the broad anomaly gradually increases toward north the panikkar ridge is associated with the gravity low which is comparable at least in sign to known negative gravity anomaly of the laxmi ridge intrusive structures mapped in the laxmi basin coincide with significant magnetic anomalies which were earlier interpreted as seafloor spreading anomalies model studies reveal that the laxmi basin consists of 14 km thick stretched continental crust in which magmatic bodies have been emplaced whereas the panikkar ridge remains less altered stretched continental crust the crust of the laxmi basin is mostly thinner than crust under the laxmi ridge and continental margin in addition to the rift drift related stretching of the continental margin the laxmi basin possibly has undergone extra stretching in e w direction during the pre tertiary period at 68 ma deccan volcanism on western india may have disrupted the initial conditions that were leading to onset of spreading in the basin subsequently the reunion hot spot had emplaced the volcanic material within the stretched thinned continental crust we interpret the laxmi basin as a failed rift undergone stretching following intraplate kinematics prior to deccan volcanism

time mean flow as the prevailing contribution to the poleward heat flux across the southern flank of the antarctic circumpolar current a case study in the fawn trough kerguelen plateau the major mechanisms of the oceanic poleward heat flux in the southern ocean are still in debate the long standing belief stipulates that the poleward heat flux across the antarctic circumpolar current acc is mainly due to mesoscale transient eddies and the cross stream heat flux by time mean flow is insignificant this belief has recently been challenged by several numerical modeling studies which stress the importance of mean flow for the meridional heat flux in the southern ocean here this study analyzes moored current meter data obtained recently in the fawn trough kerguelen plateau to estimate the cross stream heat flux caused by the time mean flow and transient eddies it is shown that the poleward eddy heat flux in this southern part of the acc is negligible while that from the mean flow is overwhelming by two orders of magnitude this is due to the unusual anticlockwise turning of currents with decreasing depth which is associated with significant bottom upwelling engendered by strong bottom currents flowing over the sloping topography of the trough the circumpolar implications of these local observations are discussed in terms of the depth integrated linear vorticity budget which suggests that the six topographic features along the southern flank of the acc equivalent to the fawn trough case would yield sufficient poleward heat flux to balance the oceanic heat loss in the subpolar region as eddy activity on the southern flank of the acc is too weak to transport sufficient heat poleward the nonequivalent barotropic structure of the mean flow in several topographically constricted passages should accomplish the required task

vertical motion in the gulf stream and its relation to meanders vertical motion at 700 m in the gulf stream main thermocline is computed using the adiabatic heat equation and a combination of data from inverted echo sounders and moored current meter measurements of t and u v at 400 700 1000 and 3500 m daily maps of all data on a regular grid were created using optimal estimation from these maps the tendency and advection terms in the heat equation were calculated the mapped vertical motions w agree well with w observed contemporaneously along isopycnal rafos float tracks strong events occur frequently within the gulf stream diagnosed vertical motions greater than 2 mm s 1 that persist for days are common occurring roughly every week within the 250 km mapped region upwelling and downwelling regions have horizontal scales of about 100 km both along and across stream case studies and animation elucidate the relationship between meander properties and the structure phasing and amplitude of the w fields typically the largest w is positioned near the steepest thermocline slope downwelling upwelling occurs entering exiting troughs strongest vertical motions occur in slowly propagating features and vertical motions are usually weaker for rapidly propagating features this observed dependence upon phase speed is opposite to predictions of recent kinematic and conceptual models the main contribution to observed w and the factor missing in those models is the existence of strong abyssal flow serving as a barotropic reference for the entire water column with a component which may flow orthogonal to the baroclinic front along sloping isopycnals the strongest vertical and cross frontal components are usually associated with slowly moving or stalled steep meanders associated with which abyssal cyclones or anticyclones have developed the vertical motion which is forced in these cases can significantly alter the vertical vorticity and or frontal strength through tilting of the strong vertical shear and of the strong vertical stratification vertical and cross frontal transports of 5 sv occur in steep meanders

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

eddy momentum flux and its contribution to the southern ocean momentum balance a large amount of momentum is transferred to the southern ocean by strong westerly winds analytical and numerical models have suggested that transient eddies may be important in transporting this momentum away from the region of wind forcing either horizontally1or vertically downwards where it is balanced by bottom topographic drag2 5 there are however few long term in situ observations of horizontal eddy momentum flux6 8 and no large scale measurements of vertical eddy fluxes to test these models as a result the momentum balance of the antarctic circumpolar current acc remains uncertain and the role of eddies controversial here we use geosat satellite altimeter data to resolve directional eddy kinetic energy and horizontal eddy momentum flux in the acc on fine spatial and temporal scales the complex spatial distribution of surface eddy momentum flux is strongly influenced by bottom topography the horizontal eddy momentum flux tends generally to concentrate the mean flow although some regions of divergence are observed our results show that the zonally averaged horizontal eddy momentum flux from transient eddies is an order of magnitude too small and in the wrong direction to directly balance the eastward momentum input from wind1

re examination of geophysical data off northwest india implications to the late cretaceous plate tectonics between india and africa abstract the gop and laxmi basins lying off northwest india have been assigned ambiguous crustal types and evolution mechanisms the chagos laccadive ridge clr complex lying along the southwest coast of india has been attributed to different evolutionary processes late cretaceous seafloor spreading between india and africa formed the mascarene basin and the plate reconstruction models depict unequal crustal accretion in this basin re interpretation of magnetic data in the gop and laxmi basins suggests that the underlying oceanic crust was accreted contemporaneously from 79 ma at slow half spreading rates 0 6 to 1 5 cm yr separating the seychelles laxmi ridge complex from india the spreading ridge became extinct at 71 ma in the central region between 19 20 n and 65 5 67 e and at 68 7 ma in the gop basin extinction progressed southwards with time until 64 1 ma at 14 5 n in the laxmi basin this spreading probably limited the seafloor spreading in the northern mascarene basin while normal to fast spreading continued in the south the fracture zone fz2 acted as a major boundary between the northern and southern mascarene basin and may have also restricted the southward propagation of the laxmi basin spreading ridge plate reconstruction models generated in this study accommodate several microplates between madagascar and india and trace their movements over time within the india africa spreading corridor furthermore these models suggest the formation of the seychelles laxmi ridge microplate by 79 ma in a complex three plate system this microplate may have existed up to 64 1 ma and experienced an anticlockwise rotation of about 50 under the influence of the deccan hotspot activity the laxmi basin spreading ridge jumped southwestward splitting the seychelles laxmi ridge complex and initiating the carlsberg ridge the presence of about 160 km of oceanic crust in the gop and laxmi basins partially reduces the space problem faced in the earlier plate reconstruction models

absolute geostrophic velocity within the subantarctic front in the pacific ocean velocity measurements from a shipboard acoustic doppler current profiler adcp are used as a reference for geostrophic current calculations on six sections across the subantarctic front saf in the pacific ocean the resulting cross track velocity estimates near the bottom range from 4 to 10 cm s 1 to the east in the eastward jet at the saf in adjacent regions of westward surface flow the near bottom velocity is usually to the west on one section where simultaneous lowered adcp velocity profiles are available they confirm the results from the shipboard adcp annual mean velocity sections from the parallel ocean program numerical model also show near bottom velocities exceeding 5 cm s 1 with the same tendency for the zonal velocity component near the bottom to match the direction of the surface jets transport across the entire antarctic circumpolar current acc cannot be estimated accurately from adcp referenced geostrophic sections because even a very small cross track bias integrates to a large error a preliminary look at the 1992 model transport stream function shows that the effect of bottom referencing varies from section to section it can cause 40 sv recirculations to be missed and can cause net transport to be underestimated or overestimated by o 30 sv

effects of bottom topography in the large scale circulation of the southern ocean abstract the characteristics of an unforced stratified f plane geostrophic flow over topography are described and scaling arguments are made to justify the use of such a flow as a first order approximation to a real large scale circulation consideration of integral constraints then provides an insight into the ways in which second order processes must balance the wind forcing the importance of bottom pressure in this model is used to test the scalings and theory on a dataset taken from the fine resolution antarctic model two plots of bottom pressure each with depth dependence filtered out in a different way confirm the scalings producing the following conclusions the effect of topography on the bottom boundary condition no flow through the boundary is important to the first order f plane geostrophic circulation the turning of horizontal velocities with depth is limited especially in regions of strong flow and a picture of bottom pressure appropriately filtered for depth dependence contai

role of mesoscale eddies in cross frontal transport of heat and biogeochemical tracers in the southern ocean abstractthis study examines the role of processes transporting tracers across the polar front pf in the depth interval between the surface and major topographic sills which this study refers to as the pf core a preindustrial control simulation of an eddying climate model coupled to a biogeochemical model gfdl climate model version 2 6 cm2 6 simplified version of the biogeochemistry with light iron nutrients and gas minibling 0 1 ocean model is used to investigate the transport of heat carbon oxygen and phosphate across the pf core with a particular focus on the role of mesoscale eddies the authors find that the total transport across the pf core results from a ubiquitous ekman transport that drives the upwelled tracers to the north and a localized opposing eddy transport that induces tracer leakages to the south at major topographic obstacles in the ekman layer the southward eddy transport only partially compensates the northward ekman transport while below the ekman layer the sout

somali basin chain ridge and origin of the northern somali basin gravity and geoid low the northern somali basin located between chain ridge and the horn of africa north of 4 n is characterized by a distinct 5 m geoid low and by large negative gravity anomalies the boundaries of the basin are marked by steep gradients in both gravity and geoid basement in northern somali basin is 1 2 km deeper than on the carlsberg ridge flank to the southeast or on the sheba ridge flank to the north with a sharp discontinuity across the boundary of the basin the western somali basin to the south was created in the late jurassic and early cretaceous by the movement of madagascar away from africa reinterpretation of the magnetic anomalies in the western somali basin shows that they record both limbs of a mid ocean ridge that was active by m22 time kimmeridgian and died soon after mo aptian magnetic and gravity data allow the relict ridge crest to be traced from davie ridge near the african coast to the dhow vlcc ars fracture zone complex at 50 e davie ridge is a transform fault connecting the western somali basin spreading center with a similar age spreading center in the mozambique basin the dhow vlcc ars complex can be shown to continue north of the 4 n bathymetric high separating the northern and western somali basins and to intersect the african margin near 7 n the northern somali basin thus appears to be the third of a series of oceanic basins separated by long transform faults created during movement between east and west gondwanaland the original northern somali basin was split apart by the northward motion of india in the late cretaceous and chain ridge formed along the new boundary thermal and gravity modeling shows that the flexure resulting from differential subsidence across chain ridge combined with the difference in lithospheric thermal structure late jurassic vs early tertiary on either side of it explains well the amplitude and shape of the observed geoid step and gravity anomalies across chain ridge the geoid step up from the basin up to the african coast can be modeled as an edge effect between the juxtaposed jurassic oceanic and african continental crust thus the geoid and gravity low over the northern somali basin results from the superposition of a continental edge effect anomaly and the fracture zone edge effect anomaly

mixing variability in the southern ocean a key remaining challenge in oceanography is the understanding and parameterization of small scale mixing evidence suggests that topographic features play a significant role in enhancing mixing in the southern ocean this study uses 914 high resolution hydrographic profiles from novel em apex profiling floats to investigate turbulent mixing north of the kerguelen plateau a major topographic feature in the southern ocean a shear strain finescale parameterization is applied to estimate diapycnal diffusivity in the upper 1600m of the ocean the indirectestimates of mixing match direct microstructure profiler observations made simultaneously it is found that mixing intensities have strong spatial and temporal variability ranging from o 10 26 t oo 10 23 m 2 s 21 this study identifies topographic roughness current speed and wind speed as the main factors controlling mixing intensity additionally the authors find strong regional variability in mixing dynamics and enhanced mixing in the antarctic circumpolar current frontal region this enhanced mixing is attributed to dissipating internal waves generated by the interaction of the antarctic circumpolar currentandthetopographyofthekerguelenplateau extendingthemixingobservationsfromthekerguelen region to the entire southern ocean this study infers a large water mass transformation rate of 17 sverdrups sv 1sv 10 6 m 3 s 21 across the boundary of antarctic intermediate water and upper circumpolar deep water in the antarctic circumpolar current this work suggests that the contribution of mixing to the southern ocean overturning circulation budget is particularly significant in fronts

horizontal and vertical structure of the gulf stream velocity field at 68 w abstract a curent meter mooring instrumented from the bottom into the thermocline was deployed in the gulf stream at 68 w for a year data from the uppermost instrument indicate the gulf stream moved back and forth across the mooring site so that the horizontal as well as vertical structure of the stream may be deduced the two key points to the success of the analysis are 1 the well defined relationship between temperature and cross stream distance in the thermocline enabling the use of the former as a horizontal coordinate and 2 a daily changing definition of gulf stream flow direction based on the shear between the thermocline and 2000 m depth time series of daily rotated velocities may be used to calculate empirical orthogonal functions for the along and cross stream vertical structures which are decoupled and are respectively baroclinic and barotropic using the inferred horizontal coordinate one can estimate masss momentum and kinetic energy fluxes agree well with historical data bryden

structure and early evolution of the arabian sea and east somali basin summary the laxmi ridge is a large scale basement high buried beneath the sediments of the indus fan the location of the ocean continent transition oct on this margin has previously been proposed at either the southern edge of the laxmi ridge or beyond it towards the india pakistan shelf the former explains the margin parallel laxmi basin as thinned continental crust the latter as a failed rift of earlier seafloor spreading to examine the structure of this margin a reassessment of marine magnetic data has detailed seafloor spreading magnetic anomalies prior to anomaly 24 in both the arabian and east somali basins the previously identified anomaly 28 is not interpreted as a seafloor spreading anomaly but as a magnetized basement feature adjacent to and merging with the ridge the laxmi spur new gravity models across the laxmi ridge and adjacent margin using ship and satellite data corroborate the existence of underplated crust beneath the laxmi ridge and basin and the location of the oct at the southern edge of the ridge the results are not compatible with the existence of a pre anomaly 28 phase of seafloor spreading although large scale intrusions may be the origin of some of the basement features in the laxmi basin the models also identify the laxmi spur as a low density feature with a natural remanent magnetization nrm compatible with serpentinization the laxmi ridge is mapped to the southeast where it appears to terminate at a point coinciding with the appearance of e w magnetic lineations and gravity anomalies at 15 5 n thereafter it becomes indistinct this is interpreted as necessary in the reconstruction to the mascarene plateau to avoid continental overlap

comparison of synoptic and climatologically mapped sections in the south pacific ocean abstract to understand the extent to which oceanic climate shifts could be detected a south pacific climatology has been used to create pseudosections of temperature salinity and other tracers along a zonal and meridional lines at 15 s and 90 w respectively interpolations from the climatology were made using combined empirical orthogonal functions and objective mapping comparisons are made with independent measurements taken in 1987 of temperature and salinity at 15 s temperature and salinity fields between the surface and 300 db along the 15 s section are predicted with an uncertainty sufficiently small to display significant differenccs in temperature and salinity related to el nino of 1987 the 90 w pseudosection is a forecast of a synoptic section to be obtained as part of woce in 1992 explicit values for the smallest temperature shift with depth that could be detected are produced

detecting and characterizing ekman currents in the southern ocean abstractthis study presents a unique array of velocity profiles from electromagnetic autonomous profiling explorer em apex profiling floats in the antarctic circumpolar current acc north of kerguelen the authors use these profiles to examine the nature of ekman spirals formed by the action of the wind on the ocean s surface in light of ekman s classical linear theory and more recent enhancements vertical decay scales of the ekman spirals were estimated independently from current amplitude and rotation assuming a vertically uniform geostrophic current decay scales from the ekman current heading were twice as large as those from the current speed decay indicating a compressed spiral consistent with prior observations and violating the classical theory however if geostrophic shear is accurately removed the observed ekman spiral is as predicted by classical theory and decay scales estimated from amplitude decay and rotation converge toward a common value no statistically robust relationship is

frontal movements and property fluxes contributions to heat and freshwater trends in the southern ocean we examine the synoptic variability of temperature and salinity in the southern ocean using a static gravest empirical mode gem mapping based on historical hydrography and a time evolving projection of the gem created using satellite altimetry satgem the gem and satgem projections allow the separation of observed trends into a component due to a shift of the circumpolar fronts adiabatic using the satgem and a component due to changes in water masses diabatic expressed as a temporal trend in residual between historical hydrography and the static gem the mean southward movement of the antarctic circumpolar current acc fronts drives an adiabatic warming of 1 196 0 090 w m 2 distributed over most of the acc and at all depths this is strongest where the meridional temperature gradient is largest such as in the upper 1000 dbar of the subantarctic front saf there is a weak adiabatic freshening of 6 57 0 18 mm yr 1 m 2 concentrated mainly below the antarctic intermediate water and south of the saf residuals between historical hydrography and the static gem field driven by diabatic changes in the water mass structure have temporal trends in temperature and salinity when these trends are integrated over the whole acc there is a net cooling of 0 628 0 129 w m 2 and strong freshening of 30 27 0 70 mm yr 1 m 2 by combining the diabatic and adiabatic components and integrating over the acc the net increase in heat and freshwater in the region is 0 570 0 099 w m 2 and 36 91 0 72 mm yr 1 m 2 respectively the sum of the adiabatic and diabatic changes are consistent with previous trends inferred from observations

energetic eddies at the northern edge of the antarctic circumpolar current in the southwest pacific abstract two year time series measurements of current and temperature were made at the northern edge of the antarctic circumpolar current in the southwestern pacific from 1978 to 1980 to investigate eddy heat and momentum fluxes in the circumpolar region the measurement site near 49 30 s 170 w is at a transition zone or front between subantarctic and subtropical waters in which meridional temperature and salinity gradients and hence vertical shear in eastward velocity extend down to 2 000 m depth energetic eddies are found to have typical amplitudes of 20 cm s 1 at 1 000 m depth to be vertically coherent over the depth range from 1 000 to 5 000 m to vary over temporal scales of 20 days and horizontal scales of 60 km and to propagate southeastward at about 12 cm s 1 observed eddy kinetic energy of 169 cm2 s at 1 000 m depth is larger than that at comparable depths in the gulf stream and kuroshio extension regions and in central drake passage like drake passage eddies these eddies transport a significant amount of heat poleward in the band of periods between 20 and 50 days but this band does not dominate the spectrum the way it does in drake passage energetic fluctuations with periods longer than 50 days effect an eddy heat flux whose sign is ambiguous so that the net meridional heat flux is not statistically significant eddy momentum fluxes transport eastward momentum northward away from the circumpolar current these fluxes are statistically significant but are only about one fourth of the size required to balance the eastward momentum put into the water column by the wind an estimate of the standing eddy momentum flux due to large spatial scale variations in the circumpolar current is also too small so that pressure forces against bottom topography is likely to be the mechanism by which the wind input is balanced

internal waves and mixing near the kerguelen plateau abstractin the stratified ocean turbulent mixing is primarily attributed to the breaking of internal waves as such internal waves provide a link between large scale forcing and small scale mixing the internal wave field north of the kerguelen plateau is characterized using 914 high resolution hydrographic profiles from novel electromagnetic autonomous profiling explorer em apex floats altogether 46 coherent features are identified in the em apex velocity profiles and interpreted in terms of internal wave kinematics the large number of internal waves analyzed provides a quantitative framework for characterizing spatial variations in the internal wave field and for resolving generation versus propagation dynamics internal waves observed near the kerguelen plateau have a mean vertical wavelength of 200 m a mean horizontal wavelength of 15 km a mean period of 16 h and a mean horizontal group velocity of 3 cm s 1 the internal wave characteristics are dependent on regional dynamics suggesting tha

vertical structure and transport of the antarctic circumpolar current in drake passage from direct velocity observations 1 the structure of the antarctic circumpolar current acc in drake passage is examined using 4 5 years of shipboard acoustic doppler current profiler adcp velocity data the extended 1000 m depth range available from the 38 khz adcp allows us to investigate the vertical structure of the current the mean observed current varies slowly with depth while eddy kinetic energy and shear variance exhibit strong depth dependence objectively mapped streamlines are self similar with depth consistent with an equivalent barotropic structure vertical wavenumber spectra of observed currents and current shear reveal intermediate wavenumber anisotropy and rotation indicative of downward energy propagation above 500 m and upward propagation below 500 m the mean observed transport of the acc in the upper 1000 m is estimated at 95 2 sv or 71 of the canonical total transport of 134 sv mean current speeds in the acc jets remain quite strong at 1000 m 10 20 cm s 1 vertical structure functions to describe the current and extrapolate below 1000 m are explored with the aid of full depth profiles from lowered adcp and a 3 year mean from the southern ocean state estimate sose a number of functions including an exponential are nearly equally good fits to the observations explaining 75 of the variance fits to an exponentially decaying function can be extrapolated to give an estimate of 154 38 sv for the full depth transport

evolution of the carlsberg ridge between 60 and 45 ma ridge propagation spreading asymmetry and the deccan reunion hotspot despite the availability of numerous geophysical data no convincing magnetic anomaly identification and structural map of a major part of the arabian and eastern somali basins have been published to date i propose a new interpretation of the available magnetic data based on the identification of tiny wiggles i e singular second order patterns of the magnetic signal only one fracture zone and a set of propagating rifts existed between the owen fracture zone and the chagos laccadive ridge between anomalies 26 and 20 at anomalies 26 25 at least two eastward propagating rifts are identified while at anomalies 24 20 seven westward propagating rifts are confidently recognized the major consequence of this systematic ridge propagation is a tremendous regional asymmetry between anomalies 26 and 25 about 65 of the crust formed at the carlsberg ridge was accreted to the african plate while at anomalies 24 20 more than 75 benefited to the indian plate i suggest that these asymmetries result from the relative position of the carlsberg ridge and the nearby deccan reunion hotspot the ridge tending to remain near the hotspot

on the nonequivalent barotropic structure of the antarctic circumpolar current an observational perspective we examine the vertical structure of the horizontal flow and diagnose vertical velocities in the antarctic circumpolar current acc near the kerguelen plateau using em apex profiling floats eight floats measured horizontal velocity temperature and salinity profiles to 1600 dbar with a vertical spacing of 3 5 dbar four times per day over a period of approximately 3 months horizontal velocity profiles show a complex vertical structure with strong rotation of the velocity vector through the water column the distribution of rotation angles from 1247 profiles is approximately gaussian and rotations of either sign are equally likely forty percent of profiles with speed greater than 5 cm s 1 have a depth integrated rotation of less than 15 degrees over 1300 dbar while the other 60 demonstrate significantly stronger rotation consequently most profiles do not conform to the equivalent barotropic model deep flow parallel and proportional to the surface flow used in simplified dynamical models and in gravest empirical mode climatologies of the acc nevertheless since we find the mean rotation to be zero an equivalent barotropic assumption is valid to first order vertical velocities inferred using conservation of mass and a gradient wind balance in natural coordinates have magnitudes on the order of 100 m day we find robust patterns of upwelling and downwelling phase locked to meanders in the flow as found in earlier studies with the advent of high resolution observations such as those presented here and high resolution models we can advance to a more complete understanding of the rich variability in acc structure that is neglected in the equivalent barotropic model

regional tectonic framework structure and evolution of the western marginal basins of india abstract the kutch saurashtra cambay and narmada basins are pericontinental rift basins in the western margin of the indian craton these basins were formed by rifting along precambrian tectonic trends interplay of three major precambrian tectonic trends of western india dharwar nnw sse aravalli delhi ne sw and satpura ene wsw controlled the tectonic style of the basins the geological history of the basins indicates that these basins were formed by sequential reactivation of primordial faults the kutch basin opened up first in the early jurassic rifting was initiated in late triassic along the delhi trend followed by the cambay basin in the early cretaceous along the dharwar trend and the narmada basin in late cretaceous time along the satpura trend the evolution of the basins took place in four stages these stages are synchronous with the important events in the evolution of the indian sub continent its breakup from gondwanaland in the late triassic early jurassic its northward drifting during the jurassic cretaceous and collision with the asian continent in the early tertiary the most important tectonic events occurred in late cretaceous time the present style of the continental margins of india evolved during early tertiary time the saurashtra arch the extension of the aravalli range across the western continental shelf subsided along the eastern margin fault of the cambay basin during the early cretaceous it formed an extensive depositional platform continuous with the kutch shelf for the accumulation of thick deltaic sediments a part of the saurashtra arch was uplifted as a horst during the main tectonic phase in the late cretaceous the present high thermal regime of the cambay bombay high region is suggestive of a renewed rifting phase

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

paleogeographic maps of the continents bordering the indian ocean since the early jurassic paleomagnetic data from the african plate from the early jurassic onward are critically re evaluated and updated resulting in a set of 30 paleopoles from which an apparent polar wander path is derived using 20 ma intervals this path is then used in conjunction with kinematic models of the indian and atlantic oceans to place other continents bordering the indian ocean eurasia india islands and plateaus madagascar seychelles mascarene and blocks involved in the collision of india with eurasia south tibet indochina in an absolute paleogeographic frame fixed with respect to africa paleogeographic maps are presented for the early jurassic pre breakup the jurassic cretaceous boundary anomaly ml5 the early cretaceous m2 the late cretaceous a 34 the cretaceous tertiary boundary a 29 the early eocene a 24 the middle eocene a 21 and the oligocene miocene boundary a 6 observed and predicted paleomagnetic directions for landmasses other than africa i e for western europe china korea india and australia which are not used in deriving the maps are found to be in very satisfactory agreement the quality of reconstructions as measured by rms discrepancies between observed and predicted paleolatitudes is of the order of 5 the main features of the paleogeographic maps are discussed with the aim of relating oceanic and on land geodynamic events the constraints provided by tibetan data on the india asia collision and the 20 ma reorganization of geology and plate boundaries in a band extending from the south china sea through tibet to the red sea system are discussed in more detail we emphasize the episodic nature of the breakup of gondwanaland and formation of the indian ocean and subsequent interaction with eurasia in which six major discrete events are identified separated by 30 ma phases of rather uniform plate motion

early tertiary seafloor spreading magnetic anomalies and paleo propagators in the northern arabian sea abstract study of closely spaced new marine magnetic profiles in conjunction with published magnetic data provide an updated identification of linear magnetic anomalies in the northern arabian sea from the owen fracture zone in the west to the laxmi and chagos laccadive ridges in the east for the first time the entire magnetic anomaly sequence 28 through 20 is mapped eastwards from 65 e up to about 70 e further eastward the post anomaly 28 oceanic crust appears to have been overlain by the volcanic material forming the chagos laccadive ridge contrary to earlier interpretations the identified magnetic lineations are observed to have been segmented mainly by oblique offsets representing the pseudofaults associated with 11 major paleo propagating ridges these ridge propagation events commenced around anomaly 28 time 63 ma and continued at least until anomaly 23 time 52 ma and during the process a large amount of crust was transferred from the african plate to the indian plate the pre anomaly 24 period was characterized by ridge reorientation whereas such a phenomenon was not apparent during the post anomaly 24 period almost all these propagating ridges appear to have been propagating westward probably to migrate away from the proximity of the reunion hotspot the accretionary process of the early tertiary oceanic crust in the arabian sea thus appears to have been greatly influenced by the propagating ridges and the nearby hotspot

potential vorticity balances and horizontal divergence along particle trajectories in gulf stream meanders east of cape hatteras abstract trajectories of 37 isopycnal rafos floats launched in the gulf stream off cape hatteras have been analyzed to examine the dynamics of meanders from a lagrangian viewpoint using the float data in conjunction with information on the structure of horizontal velocity shear from the pegasus study halkin and rossby variations in planetary curvature and shear vorticity have been estimated along the float trajectories changes in fluid layer thickness were then inferred assuming potential vorticity is conserved following the floats this analysis shows that curvature vorticity changes are typically 10 20 of f planetary vorticity as fluid parcels travel between meander troughs and crests lateral shear changes on the order of 20 30 of f are common as parcels move laterally relative to the jet axis between meander extrema although changes in these two terms are usually of opposite sign and tend to compensate significant layer thickness changes do occur with some parcels exhibiting 30 change

heat flux carried by the antarctic circumpolar current mean flow 1 a stream function projection of historical hydrographic data is applied to study the heat flux problem in the antarctic circumpolar current acc the acc is defined as a circumpolar band consisting of mean streamlines passing through drake passage its mean path exhibits a globally meandering pattern the calculation of zonal heat transport shows that the acc warms along its equatorward segments south atlantic and indian ocean and cools along its poleward segment south pacific the primary heat sources for the acc system are two western boundary currents the brazil current and the agulhas current the mean baroclinic flow relative to 3000 dbar carries 0 14 pw poleward heat flux across 56 s and 0 08 pw across 60 s

highly resolved observations and simulations of the ocean response to a hurricane 1 an autonomous profiling float called em apex was developed to provide a quantitative and comprehensive description of the ocean side of hurricane ocean interaction em apex measures temperature salinity and pressure to ctd quality and relative horizontal velocity with an electric field sensor three prototype floats were air deployed into the upper ocean ahead of hurricane frances 2004 all worked properly and returned a highly resolved description of the upper ocean response to a category 4 hurricane at a float launched 55 km to the right of the track the hurricane generated large amplitude inertially rotating velocity in the upper 120 m of the water column coincident with the hurricane passage there was intense vertical mixing that cooled the near surface layer by about 2 2 c we find consistent model simulations of this event provided the wind stress is computed from the observed winds using a high wind speed saturated drag coefficient

nonlinear vorticity balance of the subantarctic front in the southeast pacific 1 direct velocity observations from shipboard and lowered acoustic doppler current profilers are used to examine the velocity and vorticity structure of the subantarctic front saf between the east pacific rise and drake passage from surveys made in 2005 and 2006 the saf is characterized by meanders of horizontal wavelength approximately 250 300 km in this region of relatively smooth topography the depth averaged saf jet is observed to be closely aligned with the flow at 150 m as in an equivalent barotropic flow the barotropic or depth averaged vorticity exhibits a balance between advection of planetary vorticity and relative vorticity as would be seen in a doppler shifted short barotropic rossby wave in a mean flow the implied wave speed is consistent with the observed range of current speeds an exponential fit to the vertical structure of the current consistent with the vorticity balance suggests a vertical decay scale of about 1900 m the vorticity balance at 150 m implies a surface divergence which must be balanced at depth by a divergence of the opposite sign the calculation confirms the tentative conclusions of hughes 2005 for this region which were based on a surface climatology but indicates a larger vertical decay scale and wave speed

topographic influence on submesoscale dynamics in the southern ocean topography influences the circulation in the southern ocean generating stationary meanders in the lee of topographic features triggering hot spots of mesoscale eddy kinetic energy and modifying jets and fronts however the relationship between topography and submesoscale flows with length scales of order 1 10 km has not yet been explored the first submesoscale resolving 1 80 resolution ocean model with realistic topography of the indian sector of the southern ocean is used to investigate this interaction the results show spatial inhomogeneity in submesoscale activity that is correlated with topography topographic influence is primarily indirect topography controls mesoscale flows which in turn generate submesoscale activity mesoscale eddy kinetic energy and strain rate can be used to first order to infer submesoscale vertical velocity implying a possible route to parameterise submesoscale activity in coarser resolution models

synoptic maps of temperature and velocity within the subantarctic front south of australia 1 from april 1995 to march 1997 a 450 km array of inverted echo sounders horizontal electric field recorders and current meters measured the horizontal and vertical structure of the current temperature and salinity fields associated with the subantarctic front saf south of australia synoptic maps of the temperature and velocity fields often show the saf divided into two zonally separated jets which are also found in the mean these jets were in close proximity yet directed toward different azimuths the daily maps also show that the saf often flowed as a single strong jet during which times it underwent vigorous meandering with crest to trough meridional distances 250 km these meanders stalled within the array and contorted to form s shaped paths in one case a cold core ring was formed propagating meanders have periods of 20 70 days wavelengths of 240 420 km and phase speeds of 12 6 km d 1 deep currents reveal strong cyclones and anticyclones propagating primarily eastward through the array beneath the meandering jet thus at times the deep flow is aligned with the upper flow whereas at other times significant barotropic flows cross the upper front occasionally the northern polar front was also found within the array the observed variability illustrates how the interaction of a meandering current with transient features such as meanders and eddies makes the identification of a front difficult when a single isotherm or isopycnal is utilized as the definition

early spreading and continental to oceanic basement transition beneath the indus deep sea fan northeastern arabian sea abstract a seismic survey to study the indus deep sea fan penetrated into basement this enabled the mapping of basement structures and a description of the geodynamical evolution of a part of the northeastern arabian sea the main tectonic structure of the basement is an e w graben gop rift with a prominent central e w horst the palitana horst this rift is cut obliquely by several ne sw faults large e w magnetic anomalies are not the continuation of the magnetic anomaly sequence of the oceanic arabian basin however these anomalies are linked to inverse magnetisation in the crust suggesting that the rift is either underlain by stretched continental crust intruded by linear volcanic bodies related to the deccan traps or by oceanic crust older than chron 28 64 ma as proposed in the laxmi basin southeastwards accompanying these structures ne sw faults with a strike slip pattern may have played the role of transform direction during the separation of the laxmi ridge from the indian continent we propose that rifting and oceanic spreading have occurred around a triple junction connecting the oceanic laxmi basin the narmada son lineament and the gop rift this phase preceded oceanic spreading in the arabian basin this interpretation is in agreement with the previously proposed initial separation of india and the seychelles before chron 28 the laxmi ridge could be a part of the seychelles microplate subsequently left attached to india

observed eddy internal wave interactions in the southern ocean the physical mechanisms that remove energy from the southern ocean s vigorous mesoscale eddy field are not well understood one proposed mechanism is direct energy transfer to the internal wave field in the ocean interior via eddy induced straining and shearing of preexisting internal waves the magnitude vertical structure and temporal variability of the rate of energy transfer between eddies and internal waves is quantified from a 14 month deployment of a mooring cluster in the scotia sea velocity and buoyancy observations are decomposed into wave and eddy components and the energy transfer is estimated using the reynolds averaged energy equation we find that eddies gain energy from the internal wave field at a rate of 2 2 0 6 mw m 2 integrated from the bottom to 566 m below the surface this result can be decomposed into a positive eddy to wave component equal to 0 2 0 1 mw m 2 driven by horizontal straining of internal waves and a negative wave to eddy component equal to 2 5 0 6 mw m 2 driven by vertical shearing of the wave spectrum temporal variability of the transfer rate is much greater than the mean value close to topography large energy transfers are associated with low frequency buoyancy fluxes the underpinning physics of which do not conform to linear wave dynamics and are thereby in need of further research our work suggests that eddy internal wave interactions may play a significant role in the energy balance of the southern ocean mesoscale eddy and internal wave fields