antarctic bottom and lower circumpolar deep water circulation in the eastern indian ocean 1 net northward transport below n 28 1 kg m 3 3200 m into the perth basin of between 4 4 and 5 8 sv is estimated from a year long current meter mooring array between the broken and naturaliste plateaus northward transport of between 2 0 and 2 5 sv of antarctic bottom water n 28 2 kg m 3 that must upwell within the southern region of the perth basin results in an area averaged diapycnal velocity and diffusivity of w 2 5 3 1 10 6 m s 1 and 13 15 10 4 m2 s 1 respectively diffusivity estimates for the perth basin are several times larger than area averaged mixing estimates for the abyssal subtropical south atlantic and pacific oceans however the dissipation of turbulent kinetic energy required to maintain the abyssal mixing in the perth basin e o 10 9 w kg 1 is similar to that required in the south atlantic ocean the area averaged diffusivity in the perth basin does not require unreasonable energy dissipation rates as this ocean basin is only weakly stratified the abyssal diffusivity of the perth basin results from intense mixing at the basin boundary and in the basin interior over rough topography the complex bathymetry and low abyssal stratification suggests that the indian ocean for a given energy dissipation may support a larger meridional overturning circulation than other subtropical basins

application of an inverse method to interpret 231pa 230th observations from marine sediments constraints then sediment 231 pa 230 th data for each time interval are combined with an advection scavenging model in order to determine their in consistency with the modern circulation estimate we find that the majority of sediment 231 pa 230 th data for the holocene lgm or h1 can be brought into consistency with the modern circulation if plausible assumptions are made about the large scale distribution of 231 pa and about model uncertainties moreover the adjustments in the data needed to reach compatibility with a hypothetical state of no flow no advection are positively biased for each time interval suggesting that the 231 pa 230 th data including that for h1 are more consistent with a persistence of some circulation than with no circulation our study does not imply that earlier claims of a circulation change during the lgm or h1 are inaccurate but that these claims cannot be given a rigorous basis given the current uncertainties involved in the analysis of the 231 pa 230 th data

direct measurement of the deep circulation within the brazil basin abstract in support of the deep basin experiment part of the world ocean circulation experiment a large number of neutrally buoyant floats were released within the brazil basin during the 1990s in an attempt to measure directly the circulation in the deep ocean interior three levels corresponding to the three major subthermocline water masses were selected and results from the deeper two north atlantic deep water nadw and antarctic bottom water aabw are described at this writing processing of acquired tracking data is incomplete hence this paper reports on the progress of the observational program and gives our initial conclusions it appears that the flow in the deep brazil basin is unlike previous conjectures in which the circulation patterns can be characterized as being primarily meridional both along the western boundary and in the interior the existence of a deep western boundary current dwbc is quite clear in the float data at the nadw level but less prominent in the aabw and the interior flow is dominantly zonal with unexpectedly small meridional space scales integral time scales are long of order 20 30 days and eddy kinetic energy levels are low of order 1 cm2 s2 in spite of the low energy levels a surprising number of our floats became caught up in vortices a line of seamounts extending offshore near 20 s known as the vitoria trindade seamounts interrupts the dwbcs and is the location for eddy formation and apparent flow away from the boundary into the interior although it has been speculated that this could feed a narrow zonal current of nadw the namib col current our float trajectories suggest a return to the western boundary rather than a continuation to the east

the flux and mixing rates of antarctic bottom water within the north atlantic calculation was made of the northward flow of antarctic bottom water into the western north atlantic basin through a passageway approximately 300 km wide between the ceara rise and the mid atlantic ridge at about 4 n two moorings carrying four current meters each were placed in the region where currents were expected to be the greatest and were left for 360 days the current was not steady but exhibited large surges with approximately a 60 day time scale whose cause is unknown current meters in the fastest flowing region all exhibited a net northward flow it is estimated that approximately 0 8 106 m3 s 1 of water colder than 1 9 c potential temperature flowed on the average into the north atlantic with this number possibly being too small by 0 3 106 m3 s 1 or too big by 0 1 106 m3 s 1 depending upon estimates of the width of the region which the current meters sample however these numbers differ from geostrophic estimates of 1 98 106 m3 s 1 the disagreement coming almost entirely from the fact that the geostrophic estimates give a sizeable flux of water colder than 1 2 c while the current meters do not based upon the two different flux rates residence times and mixing coefficients are calculated for antarctic bottom water for each 0 1 c potential temperature interval from 1 0 c to 1 9 c finally some comments on the dynamics of the northward flow are made

long period changes in the bottom water flowing through vema channel further details of the warming of bottom water flowing through the vema channel first reported by zenk and hogg 1996 are given because cross channel gradients of temperature are large and the channel is narrow careful analysis is required to determine whether or not the bottom water temperature minimum was adequately sampled by a particular station or cruise during a period from 1972 to 1996 at least 14 visits with quality hydrographic measurements have been made to the region of these 11 were judged successful and their data indicate an abrupt rise in potential temperature by 0 03 c from 0 18 c to 0 15 c in the early 1990s that has remained until the latest observation in the spring of 1996 although this observation is consistent with the report of warming of antarctic bottom water in the argentine basin by coles et al 1996 their associated conclusion that this water mass has also freshened by 0 008 psu on potential density surfaces is in contradiction with our finding of no measurable change

slowdown of circumpolar deepwater flow during the late neogene evidence from a mudwave field at the argentine continental slope geochemical evidence from boreholes suggests enhanced transport of northern component water ncw to southern latitudes from about 6 ma onward however information on how this change in transport influenced the intensity and position of current systems is sparse here we use seismic reflection profiles interpreted together with bathymetric data to investigate current derived deposits at the central argentine margin upslope migrating mudwaves overlying a late miocene erosional unconformity provide evidence that circumpolar deepwater flow slowed down with the onset of ncw inflow during the last 3 ma changes in dimensions and migration rates of the waves are small indicating continuous bottom current flow conditions similar to today with only minor variations in flow speed suggesting that the deep western boundary current in the western south atlantic as observed today has been a pervasive feature of the global thermohaline circulation system during the plio pleistocene

east australian current volume transports at 30 s estimates from the world ocean circulation experiment hydrographic sections pr11 p6 and the pcm3 current meter array between september 1991 and march 1994 the oceanic region off the east coast of australia at 30 s was the subject of an intense observational program part of the world ocean circulation experiment the objective was to improve our understanding of the dynamics of the east australian current eac and to measure its volume and energy flux the two main components were the pacific current meter array 3 pcm3 with six moorings spanning a total distance of 120 km and repeat high resolution hydrographic surveys the long term average frompcm3 shows the eac as a narrow swift and highly variable current centered at 40 km from the coast with strong shear in the upper 1000 m and mean southward velocities of 0 6 m s 1 in the core the measurements also revealed a thick countercurrent underneath the poleward flow the mean volume transport for the entire section covered by the array between the coast and 154 4 e was 22 1 4 6 sv toward the south with an rms variability of 30 sv combining the data from the pcm3 array with the hydrographic sections generated nine detailed snapshots of the absolute geostrophic current field the snapshots were used to evaluate the reliability of the pcm3 data for determining volume transports the rms difference between transports derived from direct current observations and from hydrographie data was 3 8 sv indicating that the pcm3 array can be used to describe the current variability despite the sparse spatial distribution of velocity measurements transport variability was dominated by periods between 90 and 140 days which are attributed to eddy shedding in the region occasional intense northward flow events were observed where total transports reached up to 50 sv northward on those occasions the southward boundary current changed from being surface intensified and mainly baroclinic to northward and more barotropic related to the northward migration past the array of the eac coastal separation point

flow through discovery gap abstract a narrow gap discovery gap in the east azores fracture zone at 37 n in the eastern atlantic provides a channel for the exchange of bottom water between the madeira and iberian abyssal basins a detailed survey defines its length 150 km width 10 50 km depth of sills near 4800 m and sediment thickness 0 500 m year long measurements of deep flow are made from six moorings and ten current meters mean flows are about 5 cm s 1 these data are supplemented by 10 days of float tracking near 4700 m and numerous density profiles within and around the gap a persistent southwest northeast flow of dense water is found in discovery gap and the flux of water colder than a potential temperature of 2 05 c is measured to be 0 21 04 106 m3 s 1 this discharge spreads over an area of about 1011 m2 beyond the gap exit where it is warmed both by geothermal heating and by mixing with overlying water an estimate of 1 5 to 4 cm2 s 1 is derived for the diapycnal diffusivity similar to values dete

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

direct velocity measurements in the subantarctic front and over campbell plateau southeast of new zealand 1 flow variability in the subantarctic front saf and over the campbell plateau southeast of new zealand is examined using data from a current meter mooring array along with conductivity temperature depth ctd and satellite altimetry data these data are part of an ongoing study of ocean climate variability in the region contrasting flow regimes were found with very energetic and highly variable flows in the saf flowing along the subantarctic slope and much weaker flows over the campbell plateau currents up to 75 cm s 1 were observed in the core of the saf while flows over the plateau were typically less than 10 cm s 1 estimates of the total bottom referenced transport from three ctd sections across the saf varied between 48 and 68 sv transports were also estimated using topex poseidon altimetry sea truthed with the ctd data the resulting 8 year time series exhibited high variability resulting from current meanders and eddy activity one event in the record appeared to be the result of a cyclonic eddy passing through the mooring array this event was modeled using an idealized geostrophic eddy the simulation reproduces the main features observed in the current meter records and the surface topography from the topex poseidon altimeter inspection of the topex poseidon data suggests that large cyclonic and anticyclonic eddies occur in the frontal region with a frequency of around 9 yr 1

transport of bottom water in the romanche fracture zone and the chain fracture zone abstract two moored arrays deployed in the romanche fracture zone and chain fracture zone in the equatorial atlantic ocean provide two year long time series of current and temperature in the lower north atlantic deep water and the antarctic bottom water total time averaged transport of antarctic bottom water potential temperature 1 9 c across the mid atlantic ridge amounts to 1 22 106 m3 s 1 eastward with a standard deviation of 0 25 106 m3 s 1 a time averaged transport of 0 36 0 23 106 m3 s 1 eastward is found for the lower north atlantic deep water in the 1 9 2 1 c temperature range but this may represent only a fraction of the total flow of this water mass across the ridge contributions of the romanche fracture zone and chain fracture zone to the antarctic bottom water transport are similar while the chain fracture zone has the greater share of lower north atlantic deep water transport semiannual and annual periods are detected in the transport time series and together expl

evolution of the deep western boundary current of antarctic bottom water in the brazil basin abstract a synthesis of woce and other hydrographic data shows that the deep western boundary current of antarctic bottom water has a double core structure and that it is differentially modified during its northward transit through the brazil basin at 25 s it descends about 200 m while continuing equatorward as a paired double core current over the relatively broad sloping topography of the western region the downslope core is faster 4 6 cm s 1 versus 3 cm s 1 and is coincident with the high density core equatorward of 19 s the cores begin to diverge and by 15 s the velocity cores have separated and the high speed pattern has inverted between 13 and 11 s the colder slower flowing branch around 3 cm s 1 deepens by 350 m with a noticeable decrease in the vertical stratification of overlying water characteristics subsequently it continues to the equator toward the romanche chain fracture zones the warmer faster flowing branch around 8 cm s 1 proceeds to the equator along the continent

antarctic bottom water flow through the hunter channel the rio grande rise acts as a natural barrier for the equatorward flow of antarctic bottom water in the subtropical south atlantic in addition to the vema channel the hunter channel cuts through this obstacle and offers a separate route for bottom water import into the southern brazil basin on the occasion of the deep basin experiment a component of the world ocean circulation experiment woce the expected deep flow through the hunter channel was directly observed for the first time by an array of moored current meters and thermistor chains from december 1992 to may 1994 main results are i the hunter channel is in fact a conduit for bottom water flow into the brazil basin our new mean transport from moored current meters 2 92 1 24 106 m3 s 1 is significantly higher than earlier estimates that were based on geostrophic calculations ii during the woce observational period a tendency toward increased bottom water temperatures was observed this observation from the hunter channel is consistent with findings from the vema channel iii the overflow through the hunter channel is highly variable and puts in perspective earlier synoptic geostrophic transport estimates

fractionally modified flow in a deep ocean channel application to the vema channel the modification of the exchange flow in a deep southern hemisphere passage resembling the vema channel by frictionally induced secondary circulation is investigated numerically the hydrostatic primitive equation model is a two dimensional version of the sigma coordinate princeton ocean model the time dependent response of a stratified along channel flow forced by barotropic or baroclinic pressure gradients is examined near the bottom where the along channel flow is retarded there is cross channel ekman flux that is associated with downwelling on the eastern side and upwelling on the western side of the channel in the presence of stratification the cross channel flow rearranges the density structure which in turn acts on the along channel velocity via the thermal wind relation eventually the cross isobath ekman flux is shut down in the case of baroclinically driven flow of antarctic bottom water through the vema channel the model reproduces the observed shape of the deep temperature profiles and their cross channel asymmetry the model offers an explanation that is alternative or supplementary to inviscid multilayer hydraulic theory that was proposed in earlier studies it explains the extremely thick bottom boundary layers in the center and on the western slope of the channel the deep thermocline is spread out in the west and sharpened in the east and the coldest water is found on the eastern side of the deep trough the modified density field reduces the along channel flow near the bottom and focuses it into a narrow jet on the eastern side of the channel

structure and evolution of the abyssal jet in the vema channel of the south atlantic the vema channel represents the only major conduit through which the deepest and coldest 0 2 c potential temperature antarctic bottom water aabw flows from the argentine into the brazil basin from 2003 to 2007 two current meter moorings were present on each side of the vema sill close to the narrowest spot of the vema channel the data from the moorings are compared with earlier current and temperature observations on average the maximum current core lies 100 m above the bottom of the sill with a mean northward speed of 0 3 m s 1 farther up in the water column where lower circumpolar deep water and north atlantic deep water prevail one finds a level of sluggish currents with a southward tendency in the sub centimeter per second range the lower boundary of a layer of no motion was observed at 3700 m depth where the mean potential temperature amounts to 1 5 c the evolution of the abyssal warming phenomenon over the last decades with notable fluctuations at the choke point between the argentine and the brazil basin differs from the more stable attitude of deep horizontal currents starting with ctd observations in 1972 we find a steady increase of temperatures of the coldest aabw in the vema channel this general trend of rising abyssal potential temperatures of almost 2 mkelvin per year is based on mostly annual ctd observations the overall warming trend is fully compatible with our three year moored temperature series in agreement with earlier records with high temporal resolution distinct frequently fluctuating horizontal current shear between the western and eastern sides of the vema sill may be explained by two different catchment areas for aabw at the mouth of the vema channel one pathway originates at the american continental rise and advects bottom water in form of the deep western boundary current a second pathway is supplied by an eastern boundary current along the mid atlantic ridge in the argentine basin both source waters merge at the channel entrance mix and their respective strengths can alternate within the sill area

northward abyssal transport through the samoan passage and adjacent regions a conductivity temperature depth hydrographic survey in january february 1994 and a 17 month deployment of current meter moorings from september 1992 to march 1994 were carried out to determine the volume transport water mass characteristics and diathermal fluxes of northward flowing abyssal waters in the samoan passage and adjacent regions of the south pacific ocean geostrophic calculations relative to 1 2 c potential temperature indicated northward transport of 7 8 sv in the samoan passage 1 1 sv through a gap in robbie ridge and 2 8 sv along the eastern flank of the manihiki plateau all of the total of 11 7 sv of northward geostrophic transport was in waters colder than 1 1 c the northward transport distribution was bimodal in temperature with a cold mode of 3 6 sv in the range 0 65 0 70 c occurring entirely in the samoan passage and a warm mode of 3 0 sv in the range of 0 80 0 85 c occurring mainly along the manihiki plateau within the samoan passage 7 1 sv of the northward transport was below 4000 m where the geostrophic calculation was confirmed by an equal estimate of transport from current meters during the simultaneous 3 day period the 17 month mean transport from the moored array was 6 0 sv 0 5 by using the observed temporally varying flow within the samoan passage together with the hydrographic snapshot across the region an estimate of the total mean northward transport of 10 6 sv 1 7 was obtained estimates of the flow across near bottom potential temperature surfaces indicate extraordinarily high rates of mixing with heating of the abyssal layer up to 20 w m 2 corresponding to diffusivities up to 10 1 m2 s 1

circulation and variability at the southern boundary of the brazil basin as a contribution to the woce deep basin experiment an array of current meters with individual record lengths exceeding ii years was set across the southern boundary of the brazil basin between early 1991 and early 1996 the array spanned the santos plateau the vema channel and the hunter channel all areas believed to be important for transport of antarctic bottom water between the argentine and brazil basins from the combination of geostrophic velocities computed from hydrographic stations and those directly measured the total transport of bottom water potential temperature below 2 degrees c is estimated to be about 6 9 sv sv 10 6 m 3 s 1 northward with about 4 sv coming through the vema channel and the remainder through the hunter channel properties of the eddy field are also discussed eddy energy levels and their spatial distribution are similar to comparable regimes in the north atlantic integral timescales vary from a few days to several weeks with distance from the brazil current and the western boundary the eddy heat bur is in the same direction as the heat advection by the mean flow but considerably smaller

hydraulie control and flow separation in a multi layered fluid with applications to the vema channel abstract observations from a recent field experiment in the vema channel are briefly described these show a remarkable change in the configuration of isopycnal surfaces within the channel and the development of thick nearly homogeneous regions near the bottom which are capped by sharp vertical gradients contrary to previous speculation that these bottom boundary layer result from enhanced vertical mixing a dynamical mechanism is explored this involves the hydraulic adjustment of an inertial semi geostrophic flow to the channel geometry first an active two layer flow in a rectangular geometry is studied to show that internal flow separation can occur when the flow is accelerated sufficiently by a narrowing channel almost always this separation accompanies hydraulic control the slowest upstream moving kelvin wave is stopped and upstream and downstream states are not symmetric with respect to the channel width an active three layer flow with a variable bottom profile is then presented as a more

in situ monitoring of rock fracturing using shear wave splitting analysis an example from a mining setting summary changing stress conditions are well known to cause rupturing of rock this is well constrained on a small scale from laboratory experiments and inferred on a much larger scale from tectonic earthquakes here we present a study of rock fracturing induced by changes in stress state during block caving operations in an australian mine this intermediate scale study provides further evidence of the scalability of processes involved in rock fracturing and thus helps to link laboratory and seismological observations we analyse the temporal evolution of rock fracturing during a production cycle using the analysis of fracture induced anisotropy fracturingoftherockmassismonitoredusingevidenceofseismicanisotropyfromestimates of shear wave splitting and their subsequent inversion for fracture parameters the data set consists of more than 40 000 three component seismograms recorded by an array of sensors which provides excellent ray coverage we applied a novel automatic quality assessment technique to handle this large data set and find that anisotropy and thus fracturing correlates strongly with the excavation process we then perform a grid search over a series of synthetic models based on rock physics to invert the splitting parameters for fracture orientation and density finally by applying a sliding window on our results we are able to identify production related fracture evolution during production the fracture density increases with horizontal fracture density being stronger than the vertical fracture density this can be explained by the removal of the supporting rock during caving during short intervals of reduced production the horizontal fracture density decreases whereas vertical fracture density increases we relate this to change in stress regime with reducing overburden mass during cavity collapse this scenario is similar to a collapsing caldera or the inverse of an inflating magma chamber

the flow of antarctic bottom water into the brazil basin the total transport of antarctic bottom water across the rio grande rise including the western boundary the vema channel and the hunter channel is estimated from hydrographic measurements across these pathways the contribution of the vema channel is greatest at 3 9 106 m3 s 1 which is very close to earlier estimates the western boundary current contribution is 2 0 106 m3 s 1 and that of the hunter channel 0 7 106 m3 s 1 the lower values outside the vema channel are offset by the important source of mass they form to the lower density classes of bottom water about 40 of the flow is concentrated in the highest density class representing the source of weddell sea deep water to the brazil basin the flow structure is characterized by horizontal and vertical recirculation

temperature fluctuations and current shear in antarctic bottom water at the vema sill the vema channel acts as a major conduit for the equatorward spreading of antarctic bottom water between the argentine and brazil basins for almost two years the thermal stratification above its saddle depth 4660 m called vema sill was recorded by moored thermistors and current meters the lowest 490 m of the water column was instrumented to monitor the well developed benthic boundary layer of antarctic bottom water the latter can be subdivided into weddell sea deep water on the sea bed and lower circumpolar deep water above it the data show fluctuations on various scales including periods each about 1 2 weeks long when the abyssal stratification virtually disappeared assuming a stable ratio between density and temperature time series of bulk richardson numbers are estimated from temperature and current shear data the results suggest a potential for intermittent episodes of locally generated vertical mixing

antarctic bottom water flux in the equatorial western atlantic a moored array at the equator in the western basin of the atlantic provides a 604 day time series of abyssal currents and temperatures spanning the full breadth of the antarctic bottom water aabw flowing from the brazil basin to the guiana basin mean aabw transport is estimated to be 2 0 sv sv 106 m3 s21 comprising organized westward flow of 2 24 sv and return flow of 0 24 sv the low frequency variability is dominated by a quasi annual transport cycle of amplitude 0 9 sv and a 120 day period of amplitude 0 6 sv maximum transports occur in september october while minimum transports occur in february march allowing for this quasiannual cycle and extrapolating the 604 day record to a full two years adds about 7 to the estimated mean aabw transport the array also provides limited sampling in the overlying lower north atlantic deep water lnadw where a southern boundary intensified flow of lnadw gives the strongest recorded mean speed through the array 9 9 cm s21 into the brazil basin the lnadw records also have a quasi annual cycle with strong lnadw flow episodes occurring in april may time series of temperature indicate that the lnadw aabw transition layer rises and falls in synchrony with the quasi annual aabw transport cycle uplifted transition layer during strong aabw transport periods an observed overall warming trend appears to be accompanied by a decline in aabw transport

direct measurements of western boundary currents off brazil between 20 s and 28 s current measurements from three moored arrays on the brazilian continental slope between 20 degrees s and 28 degrees s are investigated for the existence and strength of western boundary currents from near the surface down to the north atlantic deep water the brazil current is found to deepen southward from 100 m to more than 670 m and to strengthen its volume transport to 16 2 x 10 6 m 3 s antarctic intermediate water is transported in a well developed boundary current southward at 28 degrees s and northward north of cabo frio 24 degrees s this result supports earlier suggestions derived from the analysis of hydrographic data that antarctic intermediate water enters the brazil basin from the east and bifurcates as it meets the continental break off brazil north atlantic deep water is transported southward in a weakly developed boundary current that leads to lower estimates of volume transport than expected from earlier hydrographic data analysis

oceanography recycling bottom waters the formation and flow of deep water around antarctica contributes to the global ocean circulation new measurements report the strongest flow speeds recorded so far in a current below 3 000 m depth and help to document a pathway for antarctic bottom water

first resolution of flow through the hunter channel in the south atlantic dense antarctic bottom water formed around the continent of antarctica spreads northward in the atlantic underneath north atlantic deep water gradually mixing and upwelling into it this antarctic water forms a significant element of the meridional circulation in both directions northward as bottom water and southward as deep water it is important to determine the strength of each component to assess its role in ocean circulation such measurements are useful when made in constricted pathways because any flow is more clearly defined a new set of fine resolution hydrograhic measurements in the hunter channel of the south atlantic ocean has been obtained which allow the geostrophic bottom flow there to be estimated for the first time the northward flow through the hunter channel of water cooler than 2 degrees c is thus estimated to be 0 7 x 10 6 m3 s 1