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

a wind driven isopycnic coordinate model of the north and equatorial atlantic ocean 1 model development and supporting experiments numerical approximations to the dynamic equations are given which allow basin size ocean circulation models formulated in isopycnic coordinates to accommodate variable bottom topography and irregular coastlines emphasis is placed on computational economy through the use of a split explicit time integration scheme on the proper formulation of the advection and coriolis terms in the momentum equation in case of strongly varying layer thickness and on the correct estimation of the horizontal pressure gradient force in grid boxes truncated by steep bottom slopes the algorithms are tested in a series of two and three layer double gyre experiments in cases of steady forcing leading to a steady circulation we are able to reproduce the expected motionless final state in coordinate layers that are below the direct influence of the wind forcing this includes layers intersecting topographic obstacles a long term 25 30 year vacillation tentatively associated with the outcropping of isopycnals along the edge of the cyclonic gyre in a steadily forced model is documented the paper forms the basis for a subsequent study in which circulation features obtained in a realistic north atlantic setting will be discussed

eastward flow through the mid atlantic ridge at 11 n and its influence on the abyss of the eastern basin abstract the dilute antarctic bottom water of the north atlantic eastern trough is supplied from the western trough through fractures in the mid atlantic ridge in particular the influence on eastern trough property distributions of flow through the romanche and vema fracture zones near the equator and 11 n respectively has been noted previously here new observations are reported that document the abyssal circulation of the northeastern atlantic basins gambia abyssal plain south canary basin and north canary basin in particular the dominance of vema influence the absence of romanche influence and the existence of a system of deep western boundary currents and estimated transport deep isopycnals slope steeply across the vema s eastern end near 39 w corresponding to a geostrophic transport through the vema of 2 1 to 2 3 106 m3s 1 colder than 2 0 c this is half or more of the estimated bottom water that flows north across the equator into the subtropical western north atlantic this transp

anthropogenic carbon accumulation in the subtropical north atlantic 1 recent data suggest the accumulation of anthropogenic carbon dioxide canth in the subtropical north atlantic is not occurring at a steady rate throughout the water column carbon measurements from three transatlantic cruises along 24 5 n in 1992 1998 and 2004 were investigated for changes in canth using both a back calculation shortcut technique and extended multiple linear regression for three time periods 1992 1998 1998 2004 and 1992 2004 we observed spatial and vertical changes in canth storage along with a general increase in total concentration in the surface layers total dissolved inorganic carbon tco2 and canth concentrations increased in line with atmospheric co2 levels tco2 8 8 0 5 mol kg 1 for 1992 1998 and 8 6 0 5 mol kg 1 for 1998 2004 and canth 8 0 0 2 mol kg 1 for 1992 1998 and 6 8 0 3 mol kg 1 for 1998 2004 in deeper waters canth was significantly different than zero for all depths above 5000 dbar between 1992 and 2004 while on a subdecadal timescale significant variability was observed for canth at a depth range of 800 1000 dbar evidence is presented for the arrival at 24 5 n at depth of freshly ventilated labrador sea water from the subpolar north atlantic between 1992 and 1998 as well as consistent smaller canth signals alongside the mid atlantic ridge this is in addition to low level stable increases identified in the deep eastern basin between 1992 and 2004 the first time that canth has been detected and confirmed by new measurements of carbon tetrachloride and cfc 11 from 2004 these results highlight the importance of the subtropics as a site for long term canth storage away from the surface

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

a simple model of mass driven abyssal circulation over a general bottom topography abstract the classic stommel arons problem is revisited in the context of a basin with a general bottom topography containing an equator topography is taken to be smoothly varying and as such there are no vertical side walls in the problem the perimeter of the abyssal basin is thus defined as the curve along which the layer depth vanishes because of this it is not required that the component of horizontal velocity perpendicular to the boundary curve vanish on the boundary planetary geostrophic dynamics leads to a characteristic equation for the interface height field in which characteristics typically originate from a single point located on the eastern edge of the basin at the equator for a simple choice of topography it is possible to solve the problem analytically in the linear limit of weak forcing the solution exhibits an intensified flow on the western edge of the basin this flow is pan of the interior solution and is thus not a traditional dissipative western boundary current when the

dynamical pathways of antarctic bottom water in the atlantic abstract a reduced gravity model is developed to represent the flow of antarctic bottom water aabw over realistic bathymetry in an atlantic domain the dynamics are based on the steady planetary geostrophic shallow water equations including a linear bottom friction and a uniform diapycnal upwelling through the top of the model layer the model solutions are broadly consistent with observations of the distribution and transport of aabw the flows occur predominantly along potential vorticity contours which are in turn broadly oriented along bathymetric contours the characteristic weak flow across potential vorticity contours of the stommel arons model is present as a small addition to this stronger forced mode along potential vorticity contours as a consequence mass balance is maintained not by hypothesized western boundary currents as in the stommel arons model but by the interplay between topographic slope currents and interior recirculations in particular a transposition is found in the flow

topography and grounding in a simple bottom layer model a reduced gravity model for a dense bottom layer under a motionless deep layer is used to investigate bottom layer flow with topography the two competing effects in the model are topography which introduces a topographic beta effect and strong layer thickness variation which allows for the possibility of zero layer thickness or grounding the goal is to arrive at a simple observationally relevant characterization of the model which can help to distinguish different dynamical mechanisms operating in the ocean several regimes are identified including situations for which the layer depth goes to zero with and without topography ways of distinguishing observationally between competing effects are suggested the study is motivated by the observed shift of antarctic bottom water in the western north atlantic ocean from the west to the east this phenomenon is used to discriminate key elements of the model

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

transequatorial flow of antarctic bottom water in the western atlantic ocean abyssal geostrophy at the equator abstract in its general northward flow along the western trough of the atlantic antarctic bottom water aabw must pass over several sills separating the various abyssal basins at the equator the western trough is deformed by major east west offsets of the mid atlantic ridge and the continental margin of brazil forming a newly zonal abyssal channel about 250 km wide centered at the equator and extending approximately 1000 km along its axis in which the aabw is confined thus the general northward flow of aabw is topographically constrained to be westward as it crosses the equator a hydrographic section across this channel at 37 w shows the aabw isopycnals to be bowl shaped within and beneath the level of the channel walls the equatorial geostrophic relation permits us to compute a zonal velocity from the well defined parabolic distribution of dynamic height relative to a reference level at the transition between aarw and the overlying deep water here 4 3 106 m3 s 1 is estimated for the wes

mantle phase transitions and postglacial rebound the short term response of a univariant or divariant isochemical phase boundary to a pressure change is studied the pressure change can be caused by vertical displacement of material in response to surface loading the aim is to determine if the density discontinuity is nonadiabatic i e will affect fluid flow or adiabatic for the basic state both a quasi static mantle with a smooth thermal structure in the transition region and the case of a steady convection current through the boundary are considered in the latter case the peculiar temperature distribution near the boundary affects significantly the response to additional loading a univariant transition behaves fully nonadiabatic at low convective velocities below the verhoogen limit but increasingly adiabatic above a divariant transition behaves partly adiabatic already in a static mantle and the adiabatic character is strongly enhanced with increasing velocity of convection with realistic combinations of the relevant parameters for a divariant mantle phase change the response will be hardly more than 10 nonadiabatic and probably less than 1 this result affects especially the interpretation of the gravity anomaly above formerly glaciated areas in terms of mantle viscosity and layering of mantle convection

crossing of the equator by the deep western boundary current in the western atlantic ocean abstract property distributions and geostrophic shear from a hydrographic section near 37 w in the atlantic ocean show the deep western boundary current dwbc in the north atlantic deep water nadw established against the western boundary of the brazil basin immediately south of the equator between 2 and 5 s the dwbc thus has directly crossed the equator to the south atlantic following the east southeast trend of the continental slope isobaths the estimated dwbc transport of nadw is 35 106 m3 s 1 similar to other estimates from the tropics discussed here these large dwbc transports are opposed by flow of deep water to the north atlantic immediately offshore of the dwbc with as much as two thirds of the dwbc transport being returned as these recirculations one recirculation center is the guiana basin north of the equator but extends at least a few hundred kilometers south of the equator another is visible at 11 s in the brazil basin the degree of connection of these two observed recirculatio

bottom water circulation in the western north atlantic abstract antarctic bottom water flows into the western north atlantic across the equator shifting from the western side to the eastern side of the trough between the american continents and the mid atlantic ridge as it continues north this is puzzling because such large scale motion is thought to be controlled by dynamics that disallows an eastern boundary current previous explanations for the transposition involve a necessarily small scale density current that changes sides because of the change in sign of rotation across the equator or a topographic effect that changes the sign of the effective mean vorticity gradient and thus requires an eastern boundary current here an alternative explanation for the overall structure of bottom flow is given a source of mass to a thin bottom layer is assumed to upwell uniformly across its interface into a less dense layer at rest a simple formula for the magnitude of the upwelling and thickness of the layer is derived that depends on the source strength to th

impacts of localized mixing and topography on the stationary abyssal circulation abstract stommel and coworkers calculated the stationary geostrophic circulation in the abyssal ocean driven by prescribed sources representing convective downwelling sites and sinks slow widespread upwelling through the thermocline the applied basin geometries were highly idealized with nearly uniform upwelling and gradual bottom slopes in this paper the classical stommel arons theory for the abyssal circulation is extended by introducing pronounced bathymetry in the form of a midocean ridge and strongly enhanced upwelling in the vicinity of this ridge modeled after direct observations of diapycnal mixing rates in the deep ocean locally enhanced upwelling over a midocean ridge drives a plume circulation that is modified by topographic stretching the dynamics of this abyssal circulation pattern are explained by analyzing the combined impacts of the upwelling pattern and the bathymetry on the stationary circulation building on their well known separate impacts on the western flank of the ri

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

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

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

deep planetary circulation and topography simple models of midocean flows abstract the presence of broad flowing water masses in the deep ocean requires dynamical explanation vertical density diffusion and effect are invoked in classical theories here we show how topographic potential vorticity can control broad baroclinic flows with diffusion playing a secondary role the theory and numerical experiments involve a zonal flow over topography such as one finds in the southern ocean although similar effects occur in source sink flows at middle latitudes a 1 layer model that is one active deep layer beneath a thick upper layer of the deep circulation shows intense interactions with both small scale o 500 km wide and large scale o 2500 km wide seafloor slopes broad gentle slopes can block an initially uniform flow expelling the circulation to an outer rim defined by hyperbolic characteristics where it forms into jets with smaller scale topography planetary scale flow encountering a midocean ridge develops transient distortions of the flow and density fields whi

spinup of source driven circulation in an abyssal basin in the presence of bottom topography abstract spinup of the circulation driven by an inflow into an abyssal basin containing a simple large scale topographic feature is studied using an inverted one and a half layer shallow water model two types of topography one a plateau and the other a depression are considered in both cases the topography gives rise to a region of closed geostrophic contours the character of the adjustment and the resultant flow differ markedly between the closed geostrophic contour region and the region outside long wave processes set up a modified stommel arons flow outside the closed contour region while inside the closed contour region frictional spinup leads to a vigorous recirculation it is shown that the ratio of the velocity of the recirculation to that of the stommet arons flow is equal to half the ratio of the radius of the closed contour region to the stommel boundary layer thickness moreover it is shown that the recirculation inside the closed contour region is always cyclonic whether the topog