evidence for large scale eddy driven gyres in the north atlantic analysis of a recent climatological database for the north atlantic has detailed an extensive large scale recirculation system in the intermediate waters of the north atlantic basin the pressure fields that define this recirculation coupled with the potential vorticity fields associated with the recirculating flow provide observational evidence for basin scale eddy driven flow in the global ocean the recirculations are intimately tied to the waters carried southward by the deep western boundary current and are therefore likely to affect the distribution of climatic anomalies in the north atlantic

a simple model of the large scale circulation of mediterranean water and labrador sea water abstract a steady 2 1 2 layer planetary geostrophic model is used to study the circulation and mixing of mediterranean water mw and labrador sea water lsw the model includes parameterizions of salt fingering mesoscale variability and meddies a close relationship between the vertical density ratio and expected strength of salt fingering and the potential vorticity anomaly of the upper salt tongue is identified and used to parameterize salt fingering as a spatially nonhomogeneous diapycnal mass flux a balance between baroclinic rossby wave propagation and salt fingering produces an asymmetric tongue of potential vorticity extending into the interior from the eastern boundary consistent with the observed mediterranean salt tongue the resulting internal pressure gradients give rise to a large scale anticyclonic circulation in the upper layer of the salt tongue with northward flow at mid latitudes consistent with geostrophic flow estimates based on hydrographic data salt fingering that attains maximum strength at mid latitudes forces a large scale circulation in the lower mw lsw depth range that advects water from the western boundary into the eastern basin near 50 n where it then flows to the south under the upper mw and finally returns to the western boundary this pattern is generally consistent with a variety of estimates for the circulation of lsw and suggests that the eastward flow of lsw off the western boundary may be at least in part forced by salt fingering in the interior the addition of mixing due to mesoscale eddies modifies the details of the potential vorticity distribution but does not change the qualitative behavior representing meddies as a distributed source of mass to the upper layer results in a further westward extension of the salt tongue signature and enhances the strength of the deep recirculation

deep flow along the western boundary south of the blake bahama outer ridge abstract in june july 1990 hydrographic chlorofluorocarbon cfc and velocity observations were taken along the western boundary of the north atlantic south of the blake bahama outer ridge from 30 to 24 n between the northern bahamas and 71 w the deep flow in the region associated with the deep western boundary current forms a pattern of strong narrow currents and cyclonic gyres close to the continental slope with broad slower southward flow offshore the cfcs reveal that the most recently ventilated water i e having the highest cfc concentrations due to more recent contact with the atmosphere in the northern north atlantic is found along the western boundary in two distinct cores between potential temperatures 4 6 c and 1 9 2 4 c geostrophic transport streamlines were constructed for the deep flow referenced using direct velocity observations at 26 5 n and assuming mass conservation between closed areas bounded by the hydrographic sections the tracers and transports are used together

a test of geostrophy in the gulf stream july in 1982 two detailed sections of density and absolute velocity were taken across the gulf stream northeast of cape hatteras to conduct an accurate test of geostrophy in a strong current the sections which were taken about 4 days apart were each completed within 48 hours using one ship to make closely spaced 12 km conductivity temperature depth measurements to 2000 m and a second ship to simultaneously take pegasus absolute velocity profiles to the ocean bottom the gulf stream path and curvature were also surveyed the dynamically inferred velocity profiles were made absolute by matching their velocities to the pegasus profiles at 2000 m the geostrophic method properly referenced underestimated the observed velocities by 10 25 cm s 1 in the core of the current above 500 m where speeds exceeded 150 cm s 1 the difference is a factor of 2 larger than the sampling and measurement errors in corresponding parts of the current estimated to be 5 10 cm s 1 in the strong current and 2 5 cm s 1 elsewhere we can account for the supergeostrophic currents quite effectively by including the centripetal acceleration from the path curvature in the momentum equation in this case the differences in the current core decrease to less than 5 10 cm s 1 well within the uncertainties of the method the transport above 2000 m is less sensitive to curvature effects it agrees to within 3 sv before and 1 sv after correcting for curvature or approximately 2 of the total 93 sv transport the deep velocity field below 1000 m had significant changes 10 20 cm s 1 in structure and even reversals in the 4 days between the sections with cross stream scales of 50 100 km this deep variability attributed to topographic rossby waves introduces more than 10 sv uncertainty in defining the total volume transport of the gulf stream

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

long term observations of north atlantic current transport at the gateway between western and eastern atlantic in the western north atlantic warm and saline water is brought by the north atlantic current nac from the subtropics into the subpolar gyre four inverted echo sounders with high precision pressure sensors pies were moored between 47 40 n and 52 30 n to study the main pathways of the nac from the western into the eastern basin the array configuration that forms three segments northern central and southern allows partitioning of the nac and some assessment of nac flow paths through the different mid atlantic ridge fracture zones we exploit the correlation between the nac transport measured between 2006 and 2010 and the geostrophic velocity from altimeter data to extend the time series of nac transports to the period from 1992 to 2013 the mean nac transport over the entire 21 years is 27 5 sv consisting of 60 warm water of subtropical origin and 40 subpolar water we did not find a significant trend in the total transport time series but individual segments had opposing trends leading to a more focused nac in the central subsection and decreasing transports in the southern and northern segments the spectral analysis exhibits several significant peaks the two most prominent are around 120 days identified as the time scale of meanders and eddies and at 4 9 years most likely related to the nao transport composites for the years of highest and lowest nao indices showed a significantly higher transport 2 9 sv during strong nao years mainly in the southern segment

distribution and circulation of labrador sea water abstract labrador sea water is the final product of the cyclonic circulation of subpolar mode water in the open northern north atlantic mccartney and talley 1982 the temperature and salinity of the convectively formed subpolar mode water decrease from 14 7 c 36 08 to 3 4 c 34 88 on account of the cumulative effects of excess precipitation and cooling the coldest mode water is labrador sea water which spreads at mid depths and is found throughout the north atlantic ocean north of 40 n and along its western boundary to 18 n a vertical minimum in potential vorticity is used as the primary tracer for labrador sea water labrador sea water is advected in three main directions out of the labrador sea 1 northeastward into the irminger sea 2 southeastward across the atlantic beneath the north atlantic current and 3 southward past newfoundland with the labrador current and thence westward into the slope water region crossing under the gulf stream off cape hatteras the labrador sea water core is

latitudinal dependence of atlantic meridional overturning circulation amoc variations 1 amoc variations are often thought to propagate with the kelvin wave speed resulting in a short time lead between high and low latitudes amoc variations however as shown in this paper using a coupled climate model gfdl cm2 1 with the existence of interior pathways of north atlantic deep water nadw from flemish cap to cape hatteras as that observed recently amoc variations estimated in density space propagate with the advection speed in this region resulting in a much longer time lead several years between subpolar and subtropical amoc variations and providing a more useful predictability the results suggest that amoc variations have significant meridional coherence in density space and monitoring amoc variations in density space at higher latitudes might reveal a stronger signal with a several year time lead

how does the deep western boundary current cross the gulf stream abstract the manner in which the deep western boundary current dwbc crosses the gulf stream is investigated using data from a hydrographic survey conducted in 1990 absolute geostrophic velocity vectors are computed using in situ float data to obtain the reference level three density layers are considered in detail two mid depth layers which together make up the shallowest water mass component of the dwbc 500 1200 m and a deep layer consisting of the norwegian greenland overflow water 2500 3500 m the shallowest layer does not make it through the crossover and is completely entrained by the gulf stream however the resulting drop in equatorward transport is almost completely replenished by offshore entrainment just south of the crossover in the intermediate layer which is denser than the gulf stream coming off the shelf part of the dwbc recirculates to the northeast while the onshoremost portion continues equatorward in the deep layer only a small amount of recirculation occurs the lateral

intense mixing of lower thermocline water on the crest of the mid atlantic ridge the mixing of warm upper ocean water with the colder water beneath is an essential component of global ocean circulation because it increases the buoyancy of deep water but it is not clear where this mixing takes place louis st laurent and andreas thurnherr have measured the mixing that occurs as water flows through a narrow passage on the crest of the mid atlantic ridge the rates they observe are unusually high if similar mixing rates occur in the other narrow passages on the mid atlantic ridge in the north atlantic basin these sites could account for as much buoyancy flux at the depth of the passages as the rest of the basin combined so mixing in narrow passages on mid ocean ridges sites that have previously been overlooked may make a significant contribution to buoyancy flux at the global scale observations from the crest of the mid atlantic ridge suggest that passages in rift valleys and ridge flank canyons provide the most energetic sites for oceanic turbulence measurements show that large diffusivities characterize the mixing downstream of a sill in a well stratified boundary layer with mixing levels remaining of the order of 10 4m2 s 1 at the base of the main thermocline buoyancy exchange between the deep and the upper ocean which is essential for maintaining global ocean circulation mainly occurs through turbulent mixing1 2 this mixing is thought to result primarily from instability of the oceanic internal wave field3 but internal waves tend to radiate energy away from the regions in which they are generated rather than dissipate it locally as turbulence4 and the resulting distribution of turbulent mixing remains unknown another more direct mixing mechanism involves the generation of turbulence as strong flows pass through narrow passages in topography but the amount of turbulence generated at such locations remains poorly quantified owing to a lack of direct measurements here we present observations from the crest of the mid atlantic ridge in the subtropical north atlantic ocean that suggest that passages in rift valleys and ridge flank canyons provide the most energetic sites for oceanic turbulence our measurements show that diffusivities as large as 0 03 m2 s 1 characterize the mixing downstream of a sill in a well stratified boundary layer with mixing levels remaining of the order of 10 4 m2 s 1 at the base of the main thermocline these mixing rates are significantly higher than the diffusivities of the order of 10 5 m2 s 1 that characterize much of the global thermocline and the abyssal ocean5 our estimates suggest that overflows associated with narrow passages on the mid atlantic ridge in the north atlantic ocean produce as much buoyancy flux as has previously been estimated for the entire romanche fracture zone6 7 a large strait in the mid atlantic ridge that connects the north and south atlantic basins this flux is equivalent to the interior mixing that occurs in the entire north atlantic basin at the depth of the passages suggesting that turbulence generated in narrow passages on mid ocean ridges may be important for buoyancy flux at the global scale

reconciling tracer and float observations of the export pathways of labrador sea water for more than fifty years it has been generally accepted by oceanographers that the deep western boundary current dwbc is the principal conduit of recently convected labrador sea water lsw exported from the high latitude north atlantic to the equator supporting this supposition is observational evidence that the waters of the dwbc have consistently greater equatorward velocities higher concentrations of passive tracers and younger ages compared to ocean interior waters however recent observations and simulations of floats launched in the dwbc in the labrador sea show that most water parcels are quickly ejected from the dwbc and follow instead interior pathways to the subtropics here we show that tracer observations from the last three decades are compatible with the existence of both dwbc and basin interior export pathways from analyses of observational data and model output we find that equatorward transport in the basin interior is consistent with the large scale vorticity balance at mid depth furthermore from the modeling analysis we show that despite higher localized concentrations of tracer and particles in the dwbc only 5 of particles released in the labrador sea are transported from the subpolar to subtropical gyre via a continuous dwbc pathway thus the interior pathway is a significant contributor to lsw export highlights lagrangian observations of labrador sea water match eulerian observations there is deep equatorward flow in the basin interior this interior pathway is significant compared to the pathway along the boundary

the climatology of the north atlantic abstract data from approximately 144 000 hydrographic stations in the north atlantic have been retrieved from the national oceanic data centre and analysed to produce maps of mean pressure temperature salinity and oxygen on selected potential density surfaces for the domain bounded by 0 85 w and 0 65 n the data span the period from 1904 to 1990 with the majority of the data from the last four decades the data set for this region is 60 larger than that used in the production of levitus climatological world atlas this increase in stations coupled with smoothing scales specific to the north atlantic rather than the global ocean considerably improves the resolution of the basin s features the mean property fields and their associated standard deviations are resolved on a one degree grid with little smoothing contrasting with the levitus atlas where properties although presented on a one degree grid have been smoothed on the order of 1000km another important feature of this database is the process by which irregularly spaced data are averaged onto a regular grid in a significant departure from the levitus analysis which averaged on depth surfaces these data were averaged on potential density surfaces thus eliminating an artificial mixing of water mass properties the database is used to describe the baroclinicity of well known features such as the gulf stream the north atlantic current and the deep western boundary current and to elucidate the recirculations associated with these currents it additionally resolves several new features in the intermediate and deep north atlantic these features include the signature of a large scale deep recirculation that extends southwestward from the eastward extension of the north atlantic current to the separation point of the gulf stream near cape hatteras this recirculation which spans approximately 2000m of the water column encompasses more local recirculations and potentially mixes subpolar and subtropical waters furthermore in the upper thermocline this database reveals a coherent azores current that stretches from the gulf stream system south of the tail of the grand banks to madeira this flow is marked by divergences to the south and convergences from the north such that its downstream transport is not much changed

deciphering the pathways for the deep limb of the meridional overturning circulation for the past 50 years it has been assumed that the principal pathway for the deep limb of the atlantic meridional overturning circulation amoc is the deep western boundary current dwbc however recent observations of lagrangian floats have shown that the dwbc is not necessarily a unique dominant or continuous pathway for these deep waters a significant portion of the deep water export from the subpolar to the subtropical gyres follows a pathway through the interior of the newfoundland and subtropical basins which is constrained by the western boundary and the western flank of the mid atlantic ridge the hypothesis that deep eddy driven recirculation gyres are a mechanism for partitioning the deep limb of the amoc into the dwbc and this interior pathway is investigated here eulerian and lagrangian analyses of the output of ocean general circulation models at eddy resolving eddy permitting and non eddy permitting resolutions are used to test this hypothesis eddy driven recirculation gyres simulated in the eddy resolving and eddy permitting models and similar to recirculations inferred from hydrographic data are shown to shape the export pathways of deep water from the subpolar to the subtropical gyres

source water distribution and quantification of north atlantic deep water and antarctic bottom water in the atlantic ocean abstract the distribution and quantification of the source waters of the north atlantic deep water nadw and the antarctic bottom water aabw in the atlantic ocean were investigated using 40 years of climatology data 1973 2014 constructed from the dataset available in the 2013 world ocean database the quasi interdecadal variability of nadw and aabw source waters spanned the late 1980s until early 2010 in the analysis of three world ocean circulation experiment sections woce a05 a10 and a16 the study was performed by applying optimum multiparameter analysis to quantify the mixture of six nadw and aabw source waters four for nadw and two for aabw the nadw source waters decrease their contributions from north to south except for the region between 30 s and 45 s where the labrador sea water lsw contribution suggests an upper nadw deepening in addition iceland scotland overflow water isow and lsw contribute to the southernmost boundary of nadw in the southern ocean the aabw source waters were observed as far as 45 n with a 15 contribution with significant recirculation in equatorial regions and higher contributions in argentine and brazil basins the denmark strait overflow water dsow and isow showed the highest temporal variability 20 not only near their formation region but also in the south atlantic the aabw source waters did not present high temporal variability although temporal changes were found near their formation region based on their spatial distribution the argentine and brazil basins were noted as the main locations to use for investigating this issue the results provide new insights into the mixing inside the deep atlantic ocean and the global circulation cell the results also specify the source water masses that present higher temporal variability and the suitable locations to observe these changes

propagation pathways of classical labrador sea water from its source region to 26 n 1 more than two decades of hydrography on the abaco line east of the bahamas at 26 n reveals decadal variability in the salinity of classical labrador sea water clsw despite the long distance from its source region in the north atlantic ocean hydrographic time series from the labrador sea and from the abaco line show a pronounced step like decrease in salinity between 1985 and 1995 in the labrador sea and between 1995 and 2010 at the abaco line suggesting a time lag between the two locations of approximately 9 years the amplitude of the anomaly at the abaco line is 50 of the amplitude in the labrador sea a similar time lag and reduction of amplitude is found in the high resolution ofes model in which salinity anomalies can be observed propagating through the deep western boundary current as well as through a broad interior pathway on its way south to the abaco line the clsw becomes 8 standard deviations saltier due to isopycnal mixing with mediterranean outflow water mow climatological data in the north atlantic suggests that the mixing ratio of mow to clsw at the abaco line is 1 4 and that no variability in mow is required to explain the observed variability at the abaco line the data studied here suggest that decadal clsw anomalies stay relatively coherent while getting advected despite the important role of interior pathways

the east greenland spill jet high resolution hydrographic and velocity measurements across the east greenland shelf break south of denmark strait have revealed an intense narrow current banked against the upper continental slope this is believed to be the result of dense water cascading over the shelf edge and entraining ambient water the current has been named the east greenland spill jet it resides beneath the east greenland irminger current and transports roughly 2 sverdrups of water equatorward strong vertical mixing occurs during the spilling although the entrainment farther downstream is minimal a vorticity analysis reveals that the increase in cyclonic relative vorticity within the jet is partly balanced by tilting vorticity resulting in a sharp front in potential vorticity reminiscent of the gulf stream the other components of the irminger sea boundary current system are described including a presentation of absolute transports

transport of the north atlantic deep western boundary current about 39 n 70 w 2004 2008 abstract begun in spring 2004 a sustained measurement program line w is returning high resolution observations of the north atlantic s deep western boundary current dwbc southeast of new england the study focuses on the cold limb of the atlantic meridional overturning circulation near the boundary between the subpolar and subtropical gyres the field study consists of a 6 element continental slope spanning moored array on a line underlying an altimeter satellite ground track and periodic reoccupations of a full depth hydrographic section along the line extending from the continental shelf towards bermuda here data from the first 4 years of the array may 10 2004 april 9 2008 are analyzed along with 9 realizations of the section the array a mix of moored profiler and discrete fixed depth instrument moorings returned temperature salinity and horizontal velocity data with various temporal and depth resolutions after averaging to filter inertial tidal and other high frequency motions the combined moored data set was binned to the lowest common temporal resolution of 5 days the nominal burst sample interval of the moored profilers and interpolated to 2 dbar vertical resolution temperature salinity dissolved oxygen tracer chemical concentrations and direct velocity data were acquired on the hydrographic cruises the present work focuses on the 4 year mean and time varying meridional transport in 4 layers bounded by neutral density surfaces upper and classical labrador sea waters iceland scotland overflow waters and denmark strait overflow waters the 5 d 4 layer summed meridional transport estimates range between 3 5 and 79 9 sv with a record mean average transport of 25 1 sv and standard deviation of 12 5 sv bias adjustment to account for the finite width of the moored array increases the 4 layer mean transport estimate to 28 7 sv at time scales longer than about 1 month the variations in equatorward dwbc transport appear correlated with meridional position of the gulf stream north wall with stronger transport observed when the stream is displaced south

boundary circulation at the exit of the labrador sea the deep circulation and related transports of the southern labrador sea are determined from direct current observations from ship surveys and a moored current meter array the measurements covered a time span from summer 1997 to 1999 and show a well defined deep boundary current extending approximately out to the 3300 m depth contour and weak reverse currents farther offshore the flow has a strong barotropic component and significant baroclinic flow is only found in the shallow labrador current at the shelf break and associated with a deep core of denmark strait overflow water the total deep water transport below 27 74 kg m 3 was 26 5 sv sv 106 m3 s 1 comprising labrador sea water lsw gibbs fracture zone water gfzw and denmark strait overflow water dsow intraseasonal variability of the flow and transport was high ranging from 15 to 35 sv and the annual means differed by 17 a seasonal cycle is confined to the shallow labrador current in its deeper part where the mean flow is still strong no obvious seasonality could be detected the transport of the interior anticyclonic recirculation was estimated from lowered acoustic doppler current profiler stations and geostrophy yielding about 9 sv thus the net deep water outflow from the labrador sea was about 17 sv the baroclinic transport of gfzw and dsow referenced to the depth of the isopycnal 27 80 kg m 3 is only about one third of the total transport in these layers longer term variations of the total transports are not represented well by the baroclinic contribution

spreading of labrador sea water in the eastern north atlantic the spreading of labrador sea water lsw in the eastern north atlantic south of 55 n is described on the basis of recent high resolution hydrographic lines and using an inverse model of that basin general circulation the isopycnic salinity anomaly relative to a standard s curve is used to detect the main branches and limits of dominant influence of the intermediate water masses the lsw crosses the mid atlantic ridge at latitudes around 50 n with parts spreading farther eastward and southward one southward pathway along the eastern side of the mid atlantic ridge stands out as an eastern basin counterpart of the western atlantic deep western boundary current an entry into the bay of biscay in its northern part is also evidenced with striking similitudes between two data sets from 1974 and 1989 1990 at the southern limit of the lsw extension in the vicinity of the azores biscay rise a marked front exists basinwide around 1800 m with the more saline deep mediterranean water dmw the inverse model velocity field is consistent with most features of the tracer distributions and allows estimation of lsw transports 6 3 1 2 sv of lsw with salinities lower than 34 94 cross the 35 w meridian eastward between 44 n and 54 n one fourth of which recirculates southwestward in the western basin while another fourth turns northward across 54 n and the remaining half proceeds southward in the eastern basin the model circulation suggests that dmw results from mixing between eastern basin lsw and the overlying mediterranean outflow water most probably through double diffusion we finally describe several mesoscale structures of anomalously high lsw influence south of the lsw dmw front notably one sampled by a rafos float the velocity and potential vorticity fields suggest that these lsw eddies are formed through baroclinic instability at the lsw dmw front

why does the deep western boundary current leak around flemish cap abstractthe southward flowing deep limb of the atlantic meridional overturning circulation is comprised of both the deep western boundary current dwbc and interior pathways the latter are fed by

mean atlantic meridional overturning circulation across 26 5 n from eddy resolving simulations compared to observations 1 observations along 26 5 n are used to examine the time mean structure of the atlantic meridional overturning circulation amoc in eddy resolving simulations with the hybrid coordinate ocean model hycom the model results yield a 5 year mean amoc transport of 18 2 sv compared to 18 4 sv based on data the modeled northward limb of the amoc has a vertical structure similar to observations the southward limb is shallower than observed but deeper than other ocean general circulation models and includes a secondary transport maximum near 4000 m corresponding to nordic seas overflow water the modeled flow through the florida strait and the deep western boundary current dwbc east of abaco bahamas are also approximately consistent with observations the model results are used to clarify the sources of the northward amoc transport and to explore the circulation pattern of the southward transport in the western subtropical north atlantic in the range 18 33 n about 14 1 sv of the modeled northward amoc transport is through the florida strait and the remainder through the mid ocean primarily in the ekman layer but also below 600 m the modeled amoc transport is about 2 3 surface water and 1 3 antarctic intermediate water with no contribution from the thermocline water in between in the western subtropical north atlantic the model results depict a complicated deep circulation pattern associated with the complex bathymetry the dwbc flows southward then eastward in both the upper and lower north atlantic deep water nadw layers but with different offshore recirculation pathways and there exists a second more northern branch of eastward flow in the lower nadw layer

radium isotopes in the lower mississippi river radium isotopes have been measured in mississippi river water at several locations between st francisville and venice louisiana measured activities averaged 11 1 dpm 100 l disintegrations per minute per 100 l for dissolved 226ra with a 228ra 226ra activity ratio of 1 04 and a 224ra 228ra activity ratio of 1 06 for the section of river above a phosphogypsum waste discharge input down river from this site the dissolved 226ra activity averaged 22 4 dpm 100 l with a 228ra 226ra activity ratio of 0 55 and a 224ra 228ra activity ratio of 0 87 chemical and isotopic reequilibration of the radium in the waste with that in the river water and suspended sediments occurs very rapidly fluxes of 226ra are calculated to be 2 44 1014 dpm yr disintegrations per minute per year for the main stem of the mississippi during phosphogypsum disposal compared to 0 98 1014 dpm yr when no phosphogypsum disposal is occurring 28ra flux is calculated to be 1 06 1014 dpm yr during disposal and 1 01 1014 dpm yr in the absence of disposal the radium flux is estimated to be 45 in the dissolved state and 55 in the adsorbed state

circulation and transports in the newfoundland basin western subpolar north atlantic the southwestern part of the subpolar north atlantic east of the grand banks of newfoundland and flemish cap is a crucial area for the atlantic meridional overturning circulation here the exchange between subpolar and subtropical gyre takes place southward flowing cold and fresh water is replaced by northward flowing warm and salty water within the north atlantic current nac as part of a long term experiment the circulation east of flemish cap has been studied by seven repeat hydrographic sections along 47 degrees n 2003 2011 a 2 year time series of current velocities at the continental slope 2009 2011 19 years of sea surface height and 47 years of output from an eddy resolving ocean circulation model the structure of the flow field in the measurements and the model shows a deep reaching nac with adjacent recirculation and two distinct cores of southward flow in the deep western boundary current dwbc one core above the continental slope with maximum velocities at mid depth and the second farther east with bottom intensified velocities the western core of the dwbc is rather stable while the offshore core shows high temporal variability that in the model is correlated with the nac strength about 30 sv of deep water flow southward below a density of sigma 27 68 kg m 3 in the dwbc the nac transports about 110 sv northward approximately 15 sv originating from the dwbc and 75 sv recirculating locally east of the nac leaving 20 sv to be supplied by the nac from the south

deep water formation the subpolar gyre and the meridional overturning circulation in the subpolar north atlantic on interannual to decadal times scales model simulations suggest a strong relationship between anomalies in the deep water formation rate the strength of the subpolar gyre and the meridional overturning circulation in the north atlantic whether this is valid can only be confirmed by continuous long observational time series several measurement components are already in place but crucial arrays to obtain time series of the meridional volume and heat transport in the subpolar north atlantic are still missing here we summarize the recent developments of the deep water formation rates and the subpolar gyre transports we discuss how existing observational components in the subpolar north atlantic could be supplemented to provide long term monitoring of the meridional heat and volume transport through a combined analysis of observations and model results the temporal and spatial scales that had to be covered with instruments are discussed together with the key regions with the highest variability in the velocity and temperature fields

moored observations of the deep western boundary current in the nw atlantic 2004 2014 a moored array spanning the continental slope southeast of cape cod sampled the equatorward flowing deep western boundary current dwbc for a 10 year period may 2004 may 2014 daily profiles of subinertial velocity temperature salinity and neutral density are constructed for each mooring site and cross line dwbc transport time series are derived for specified water mass layers time averaged transports based on daily estimates of the flow and density fields in stream coordinates are contrasted with those derived from the eulerian mean flow field modes of dwbc transport variability are investigated through compositing and comparisons are made to transport estimates for other latitudes integrating the daily velocity estimates over the neutral density range of 27 8 28 125 kg m3 encompassing labrador sea and overflow water layers a mean equatorward dwbc transport of 22 8 x 106 m3 s 1 9 x 106 m3 s is obtained notably a statistically significant trend of decreasing equatorward transport is observed in several of the dwbc components as well as the current as a whole the largest linear change a 4 decrease per year is seen in the layer of labrador sea water that was renewed by deep convection in the early 1990s whose transport fell from 9 0 x 106 m3 s at the beginning of the field program to 5 8 x 106 m3 s at its end the corresponding linear fit to the combined labrador sea and overflow water dwbc transport decreases from 26 4 x 106 m3 s to 19 1 x 106 m3 s in contrast no long term trend is observed in upper ocean slope water transport these trends are discussed in the context of decadal observations of the north atlantic circulation and subpolar air sea interaction water mass transformation

western boundary current structure and variability east of abaco bahamas at 26 5 n abstract during april 1987 to june 1988 a four mooring transport resolving current meter array was deployed in a section extending 70 km eastward from abaco bahamas at 26 5 n mean currents in the upper layer 800 m showed a clockwise rotation that appears to be associated with a small scale quasi permanent anticyclonic gyre centered just northeast or the bahamas deep mean currents were persistenly southward and indicated a strong deep jet the deep western boundary current dwbc reaching core speeds of 20 cm s 1at 2500 m depth about 25 km seaward of the western boundary vertically integrated meridional volume transport over the section showed a surprisingly large variability ranging from approximately 20 sv northward to 70 sv southward 1 sv 106m3s 1 the mean meridional volume transport was 30 sv to the south with about 3 sv flowing northward above 800 m which could be produced by a weak antilles current and 33 sv flowing southward below 800 m the deep southward transport of 33 sv i

hydrographic changes in the labrador sea 1960 2005 abstract the labrador sea has exhibited significant temperature and salinity variations over the past five decades the whole basin was extremely warm and salty between the mid 1960s and early 1970s and fresh and cold between the late 1980s and mid 1990s the full column salinity change observed between these periods is equivalent to mixing a 6 m thick freshwater layer into the water column of the early 1970s the freshening and cooling trends reversed in 1994 starting a new phase of heat and salt accumulation in the labrador sea sustained throughout the subsequent years it took only a decade for the whole water column to lose most of its excessive freshwater reinstate stratification and accumulate enough salt and heat to approach its record high salt and heat contents observed between the late 1960s and the early 1970s if the recent tendencies persist the basin s storages of salt and heat will fairly soon likely by 2008 exceed their historic highs the main process responsible for the net cooling and freshening of the labrador sea between 1987 and 1994 was deep winter convection which during this period progressively developed to its record depths it was caused by the recurrence of severe winters during these years and in its turn produced the deepest densest and most voluminous labrador sea water lsw1987 1994 ever observed the estimated annual production of this water during the period of 1987 1994 is equivalent to the average volume flux of about 4 5 sv with some individual annual rates exceeding 7 0 sv once winter convection had lost its strength in the winter of 1994 1995 the deep lsw1987 1994 layer lost communication with the mixed layer above consequently losing its volume while gaining heat and salt from the intermediate waters outside the labrador sea while the 1000 2000 m layer was steadily becoming warmer and saltier between 1994 and 2005 the upper 1000 m layer experienced another episode of cooling caused by an abrupt increase in the air sea heat fluxes in the winter of 1999 2000 this change in the atmospheric forcing resulted in fairly intense convective mixing sufficient to produce a new prominent lsw class lsw2000 penetrating deeper than 1300 m this layer was steadily sinking or deepening over the years following its production and is presently overlain by even warmer and apparently less dense water mass implying that lsw2000 is likely to follow the fate of its deeper precursor lsw1987 1994 the increasing stratification of the intermediate layer implies intensification in the baroclinic component of the boundary currents around the mid depth perimeter of the labrador sea the near bottom waters originating from the denmark strait overflow exhibit strong interannual variability featuring distinct short term basin scale events or pulses of anomalously cold and fresh water separated by warm and salty overflow modifications regardless of their sign these anomalies pass through the abyss of the labrador sea first appearing at the greenland side and then about a year later at the labrador side and in the central labrador basin the northeast atlantic deep water 2500 3200 m originating from the iceland scotland overflow water reached its historically freshest state in the 2000 2001 period and has been steadily becoming saltier since then it is argued that lsw1987 1994 significantly contributed to the freshening density decrease and volume loss experienced by this water mass between the late 1960s and the mid 1990s via the increased entrainment of freshening lsw the hydrostatic adjustment to expanding lsw or both

high aluminum orthopyroxene megacrysts haom in the ahvenisto complex se finland and the polybaric crystallization of massif type anorthosites the occurrence of high aluminum orthopyroxene megacrysts haoms in several massif type proterozoic anorthosite complexes has been used as evidence of their polybaric crystallization here we report such petrographic and geochemical xrf and empa evidence from haoms discovered in the 1 64 ga ahvenisto rapakivi granite massif type anorthosite complex in southeastern finland two different types of haoms were recognized type 1 haoms are individual euhedral to subhedral crystals and up to 15 cm in diameter and type 2 haoms occur in pegmatitic pockets closely associated with megacrystic up to 30 cm long plagioclase the type 1 megacrysts in particular are surrounded by complex corona structures composed of plagioclase low al orthopyroxene iddingsite after olivine and sulfides orthopyroxene crystallization pressure estimates based on an al in opx geobarometer reveal a three stage compositional evolution in both textural haom types the al content decreases significantly from the core regions of the haom 4 4 7 6 wt al2o3 through the rims 1 3 3 6 wt into the host rock 0 5 1 5 wt enstatite compositions overlap but are generally higher in the cores en 60 70 and rims en 50 70 of the haoms than in the host rock en 45 60 orthopyroxenes the highest recorded al abundances in the haom cores correspond to crystallization pressures of up to 1 1 gpa 34 km depth whereas the haom rims have crystallized at lower pressures max 0 5 gpa 20 km depth the highest pressure estimates for the host rock orthopyroxene were 0 2 gpa 7 km depth these observations confirm the polybaric magmatic evolution of the ahvenisto anorthosites and suggest that the entire 1 65 1 55 ga fennoscandian rapakivi suite was emplaced at a relatively shallow level 7 km depth in the upper crust global comparison to similar rock types reveals remarkable similarities in the petrogenetic processes controlling haom composition and evolution of anorthosite parental magmas

variability of the deep western boundary current east of the grand banks the deep western boundary current east of the grand banks has been observed during 1999 2005 by moored current meter stations and shipboard current profiling sections these recent observations can be compared with those of a woce moored array deployed during 1993 95 at the same location overall the observations of deep water currents east of the grand banks reported here do not support suggestions of a basin wide slowdown of the atlantic meridional overturning circulation

seasonal overturning of the labrador sea as observed by argo floats abstractargo floats are used to investigate labrador sea overturning and its variability on seasonal time scales this is the first application of argo floats to estimate overturning in a deep water formation region in the north atlantic unlike hydrographic measurements which are typically confined to the summer season floats offer the advantage of collecting data in all seasons seasonal composite potential density and absolute geostrophic velocity sections across the mouth of the labrador sea assembled from float profiles and trajectories at 1000 m are used to calculate the horizontal and overturning circulations the overturning exhibits a pronounced seasonal cycle in depth space the overturning doubles throughout the course of the year and in density space it triples the largest overturning 1 2 sv 1 sv 106 m3 s 1 in depth space and 3 9 sv in density space occurs in spring and corresponds to the outflow of recently formed labrador sea water the overturning decreases through summer and reac

gulf stream structure transport and recirculation near 68 w an analysis of the structure and transport of the gulf stream is undertaken using direct current meter observations from a 13 mooring array deployed near 68 w from june 1988 to august 1990 the analysis is based on a stream coordinate approach in which velocities are rotated into a local downstream coordinate frame and averaged according to their relative cross stream location within the current the picture so obtained represents the average synoptic structure of the gulf stream rather than the eulerian averaged structure in which the current is weakened and broadened by lateral meandering of the current and adjacent recirculations many familiar features of the gulf stream are reproduced in the analysis including an asymmetric velocity profile with larger shear on the cyclonic shoreward side of the current an offshore displacement of the velocity core with depth and a subsurface velocity maximum on the offshore side of the current westward recirculations are also seen on both sides of the gulf stream maximum downstream speeds at the axis of the gulf stream reach approximately 2 0 m s at the surface and 0 7 m s at 1000 m roughly twice the corresponding eulerian averaged values the analysis also reveals a deep extension of the gulf stream at 3500 m depth with a width of 130 km and average speeds of 3 4 cm s the transport of the gulf stream in the stream coordinate frame is 113 8 sv approximately 30 larger than the eulerian averaged transport of 88 sv on the basis of these results and other recent studies the downstream transport increase of the gulf stream and the inflow structure to the gulf stream are reconsidered it is concluded that approximately 30 sv or over half of the transport increase between cape hatteras and 68 w is fed by inflow from the northern side of the gulf stream and that this inflow is concentrated near cape hatteras and 68 w where the gulf stream flows steeply across isobaths converging from the north