multidecadal variability of potential temperature salinity and transport in the eastern subpolar north atlantic the extended ellett line eel hydrographic section extends from scotland to iceland crossing the rockall trough hatton rockall basin and iceland basin with 61 full depth stations at a horizontal resolution of 10 to 50 km the eel samples the upper limb of the atlantic meridional overturning circulation flowing across the iceland scotland ridge into the nordic seas the rockall trough has been sampled nearly four times per year from 1975 to 1996 and the full section annually since 1996 the eel is an exceptionally long timeseries of deep ocean temperatures and salinities this study extends prior work in the rockall trough and examines for the first time 18 year records in the iceland and hatton rockall basins we quantify errors in the timeseries from two sources observational errors and aliasing the data quality and annual sampling are suitable for observing interannual to decadal variability because the variability exceeds our error estimates the upper waters of all 3 basins are cooler fresher from 1997 to 2001 warmer more saline 2001 to 2006 and cooler fresher from 2006 to 2014 a reference level for geostrophic shear is developed heuristically and by comparison with sea surface altimetry the mean northward transport in the upper waters is 6 7 3 7 sv and there is a 6 1 2 5 sv southward flow below the thermocline although the magnitude of the iceland basin overturning circulation 4 3 1 9 sv is greater than in the rockall trough 3 0 3 7 sv the variability is greater in the rockall trough we discuss the results in the context of our understanding of drivers of variability this article is protected by copyright all rights reserved

decadal variability of subpolar gyre transport and its reverberation in the north atlantic overturning 1 analyses of sea surface height ssh records based on satellite altimeter data and hydrographic properties have suggested a considerable weakening of the north atlantic subpolar gyre during the 1990s here we report hindcast simulations with high resolution ocean circulation models that demonstrate a close correspondence of the ssh changes with the volume transport of the boundary current system in the labrador sea the 1990s decline of about 15 of the long term mean appears as part of a decadal variability of the gyre transport driven by changes in both heat flux and wind stress associated with the north atlantic oscillation nao the changes in the subpolar gyre as manifested in the deep western boundary current off labrador reverberate in the strength of the meridional overturning circulation moc in the subtropical north atlantic suggesting the potential of a subpolar transport index as an element of a moc monitoring system

reversal of the 1960s to 1990s freshening trend in the northeast north atlantic and nordic seas hydrographic time series in the northeast north atlantic and nordic seas show that the freshening trend of the 1960s 1990s has completely reversed in the upper ocean since the 1990s temperature and salinity have rapidly increased in the atlantic inflow from the eastern subpolar gyre to the fram strait in 2003 2006 salinity values reached the previous maximum last observed around 1960 and temperature values exceeded records the mean properties of the atlantic inflow decrease northwards but variations seen in the eastern subpolar gyre at 57 n persist with the same amplitude and pattern along the pathways to fram strait time series correlations and extreme events suggest a time lag of 3 4 years over that distance this estimate allows predictions to be made the temperature of atlantic water in the fram strait may start to decline in 2007 or 2008 salinity a year later but both will remain high at least until 2010

transformation of the labrador sea water in the subpolar north atlantic development spreading and decay of the thermohaline properties of two labrador sea water lsw classes are described during the development phase a specific lsw class repeatedly mixed by winter convection in the labrador sea becomes colder denser thicker and deeper once convection weakens the lsw class becomes isolated from the upper layer and starts to decay rapidly losing its volume while retaining the same density due to isopycnal mixing with the neighbouring warm saline intermediate waters a similar pattern in temperature salinity and density is seen in the other basins with different time lags from about two years in the irminger sea to ten years in the northern iceland basin and rockall trough regions the influence of lsw on the thermohaline properties of other north atlantic water masses is also discussed

atlantic origin of observed and modelled freshwater anomalies in the nordic seas between 1965 and 1990 the waters of the nordic seas and the subpolar basins of the north atlantic ocean freshened substantially1 the arctic ocean also became less saline over this time as a consequence of increasing runoff1 2 3 4 but it is not clear whether flow from the arctic ocean was the main source of the nordic seas salinity anomaly as a region of deep water formation the nordic seas are central to the atlantic meridional overturning circulation but this process is inhibited if the surface salinity is too low2 here we use the instrumental record of nordic seas hydrography along with a global ocean sea ice model hindcast simulation to identify the sources and magnitude of freshwater that has accumulated in the nordic seas since 1950 we find that the freshwater anomalies within the nordic seas can mostly be explained by less salt entering the southern part of the basin with the relatively saline atlantic inflow with seemingly little contribution from the arctic ocean we conclude that hydrographic changes in the nordic seas are primarily related to changes in the atlantic ocean we infer that if the atlantic inflow and nordic seas both freshen similarly this would render the atlantic meridional overturning circulation relatively insensitive to nordic seas freshwater content

north atlantic nordic seas exchanges abstract the northeastern part of the north atlantic is unique in the sense that it is much warmer in the surface than other ocean areas at similar latitudes the main reason for this is the large northward transport of heat that extends to high latitudes and crosses the greenland scotland ridge to enter the nordic seas and the arctic there the warm atlantic water is converted to colder water masses that return southwards over the ridge partly as surface outflows and partly as overflows through the deep passages across the ridge in this paper the state of knowledge on the exchanges especially across the eastern part of the greenland scotland ridge is reviewed based on results from the ices nansen north atlantic norwegian sea exchanges project from the nordic woce project and from other sources the accumulated evidence allows us to describe the exchanges in fair detail the origins of the waters the patterns of their flow towards and over the ridge and their ultimate fate there is also increasing information on temporal variations of the exchanges although dynamical changes are still not well understood quantitative estimates for the volume transport of most of the overflow branches seem reasonably well established and transport measurements of the atlantic inflows to the nordic seas are approaching acceptable levels of confidence which allows preliminary budgets to be presented the deep overflows are driven by pressure gradients set up by the formation of deep and intermediate water the dominance of deep overflows over surface outflows in the water budget argues that this thermohaline forcing also dominates over direct wind stress and estuarine forcing in driving the atlantic water inflow across the greenland scotland ridge while wind stress seems to influence the characteristics and distribution of the atlantic water north of the ridge

on the temporally varying northward penetration of mediterranean overflow water and eastward penetration of labrador sea water abstract historical hydrographic data in the eastern north atlantic are used to suggest a connection between the northward penetration of mediterranean overflow water mow and the location of the subpolar front the latter of which is shown to vary with the north atlantic oscillation nao during persistent high nao periods when the subpolar front moves eastward waters in the subpolar gyre essentially block the northward flowing mow preventing its entry into the subpolar gyre conversely during low nao periods the subpolar front moves westward allowing mow to penetrate past porcupine bank into the subpolar gyre the impacts of an intermittent penetration of mow into the subpolar gyre including the possible effect on water mass transformations remain to be investigated

warm and saline events embedded in the meridional circulation of the northern north atlantic 1 ocean state estimates from 1958 to 2005 from the simple ocean assimilation system soda system are analyzed to understand circulation between subtropical and subpolar atlantic and their connection with atmospheric forcing this analysis shows three periods 1960s around 1980 and 2000s with enhanced warm saline waters reaching high latitudes alternating with freshwater events originating at high latitudes it complements surface drifter and altimetry data showing the subtropical subpolar exchange leading to a significant temperature and salinity increase in the northeast atlantic after 2001 the warm water limb of the atlantic meridional overturning cell represented by soda expanded in density salinity space during these warm events tracer simulations using soda velocities also show decadal variation of the gulf stream waters reaching the subpolar gyre and nordic seas the negative phase of the north atlantic oscillation index usually invoked in such variability fails to predict the warming and salinization in the early 2000s with salinities not seen since the 1960s wind stress curl variability provided a linkage to this subtropical subpolar gyre exchange as illustrated using an idealized two layer circulation model the ocean response to the modulation of the climatological wind stress curl pattern was found to be such that the northward penetration of subtropical tracers is enhanced when amplitude of the wind stress curl is weaker than normal in this case both the subtropical and subpolar gyres weaken and the subpolar density surfaces relax hence the polar front moves westward opening an enhanced northward access of the subtropical waters in the eastern boundary current

a new dinosaur ichnofauna from tunisia implications for the palaeobiogeography of peri adriatic carbonate platforms in the mid cretaceous abstract new dinosaur tracks discovered at the chenini site near tataouine southern tunisia are described isolated tracks as well as manus pes couples of a quadrupedal dinosaur are referred to a hadrosaurian tracemaker and assigned to cf apulosauripus federicianus nicosia et al 2000b an ichnospecies originally described in the upper cretaceous carbonate platform deposits of southern italy this discovery thus represents the first report of hadrosaurian tracks in africa the occurrence of tracks similar to a federicianus in tunisia also represents the first evidence of the presence of comparable ichnofauna in the upper cretaceous of both northern africa and the carbonate platforms of the mediterranean area supporting these new data the cenomanian theropod ichnofauna recently described from southern tunisia is here compared with coeval theropod tracks from the peri adriatic carbonate platforms this comparison documents a number of shared morphological features suggesting possible affinities of the trackmakers and supporting the hypothesis of a subaerial connection linking the carbonate platforms of the mediterranean area to the northern margins of africa in the mid cretaceous

subpolar mode water in the northeastern atlantic 1 averaged properties and mean circulation 1 subpolar mode waters spmw in the eastern north atlantic subpolar gyre are investigated with hydrographic and lagrangian data surface drifters and isopycnal floats historical hydrographic data show that spmws are surface water masses with nearly uniform properties confined between the ocean surface and the permanent pycnocline spmws represented by densities 27 3 27 4 and 27 5 are present in the eastern subpolar gyre and are influenced by the topography and the regional circulation construction of an absolute surface stream function from surface drifters shows that spmws are found along the mean path of each of the several branches of the north atlantic current nac and their density increases gradually downstream the rockall trough branch of the nac carries 27 3 27 4 and 27 5 spmw toward the iceland faroe front in the iceland basin the subarctic front along the western flank of the rockall plateau carries a similar sequence of spmw the western side of the central iceland basin branch of the nac on the other hand veers westward and joins the east reykjanes ridge current feeding the 27 5 spmw on the reykjanes ridge the separation among the various nac branches most likely explains the different properties that characterize the 27 5 spmw found on the reykjanes ridge and on the iceland faroe ridge since the branches of the nac have a dominant northeastward direction the newly observed distribution of spmw combined with the new stream function calculation modify the original hypothesis of mccartney and talley 1982 of a smooth cyclonic pathway for spmw advection and density increase around the subpolar gyre

direct velocity observations of volume flux between iceland and the shetland islands atlantic waters flowing northward into the nordic seas are important for their role as an early indicator of changes to deepwater formation as such this requires a fundamental understanding of the pathways and volume fluxes through the primary passageways from the atlantic into the nordic seas a mean annual volume transport of 6 1 0 3 sv was observed flowing in above the t 27 8 isopycnal a proxy for the lower limit of atlantic water depth through the faroe shetland channel fsc and over the iceland faroes ridge ifr from march 2008 to june 2012 using repeat velocity sections obtained from a vessel mounted acoustic doppler current profiler adcp a new vessel route has expanded the spatial coverage of fsc observations and reveals a difference in average inflow transport which most likely results from an interannual variation in the total transport through the fsc which in turn is tied to a weakening of the southerly flow over the western slope of the channel this interannual variability has increased the mean transport through the fsc from 0 9 sv observed over the first 2 years of this program by rossby and flagg 2012 to a 4 5 year mean of 1 7 0 2 sv which emphasizes the importance of knowing the flow along the faroese shelf interannual fluctuations in transport observed over the ifr are related to the width of the inflow over the faroese half of the ridge

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

the deacon cell and the other meridional cells of the southern ocean abstract the meridional circulation cells of the southern ocean are investigated using the results from a fine resolution primitive equation model zonal integration along depth levels shows the classical series of meridional cells but integration along density layers shows a number of differences including the virtual disappearance of the deacon cell to investigate the differences the meridional transport is calculated as a function of both density and depth the results show that the deacon cell is associated with systematic changes in the depth of density surfaces between the western boundary current region off south america and the return flow in the interior of the ocean water flowing on each density surface produces a meridional cell with a vertical excursion of a few hundred meters thew cells combine without water crossing density surfaces to produce a single integrated deacon cell extending from the surface to below 2000 m the results also show that at each latitude water on each of the

eddy kinetic energy of the north atlantic subpolar gyre from satellite altimetry the surface eddy kinetic energy field of the north atlantic subpolar gyre 40 65 n 5 60 w is studied using the altimetric satellites geosat 1987 1988 and ers 1 1992 1993 this is on the whole an area of low eddy activity previous studies of the north atlantic have usually concentrated on the highly variable gulf stream system we review what is known of the surface circulation of the region we confirm that there is a close relationship between the eddy kinetic energy and the mean currents and that it is strongly constrained by the topography the eddy field occurs in deep water and follows the contours particularly around the flemish cap high eddy kinetic energy is seen on the flanks of topography and is higher where the slope is steeper this is particularly clear on both east and west flanks of the rockall plateau interannual changes in the currents are revealed by the differences between the geosat and ers 1 eddy kinetic energy maps in some years 1988 and 1992 1993 the northern core of the north atlantic current nac usually assumed to be tied to the charlie gibbs fracture zone 52 n is considerably weaker and may not exist at all while in 1987 there is a distinct core here in 1988 and 1992 1993 the main eddy region shifts farther to the south to about 49 n in 1988 and 50 n in 1992 1993 in 1987 there is also a current core even farther south centered at about 47 n implying that in that year the nac is arranged into two distinct branches

water masses and circulation pathways through the iceland basin during vivaldi 1996 quasi synoptic data from late 1996 spanning the subpolar north atlantic have been used to determine the major pathways of the north atlantic current nac at that time high spatial resolution allows fronts to be accurately positioned on each seasoar section a clearly defined front of the nac the southern branch turns north at around 25 w and continues through the middle of the iceland basin as far as 60 n 20 w a second branch the northern branch or subarctic front turns north around 30 w and retroflects westward north of 54 n to re enter the irminger basin and become part of the irminger current up the western side of the reykjanes ridge a third eastward branch turns sharply northwest at the mouth of the rockall trough to skirt the southwestern margin of hatton bank this branch carries a tongue of saline eastern north atlantic water enaw over hatton bank and in consequence enaw covers the whole of the hatton and rockall banks as well as the rockall trough bounded in the west by the southern branch the most saline water found in rockall trough spills out into the northern iceland basin between rockall and lousy banks this saline weakly stratified tongue can be traced westward to the south of iceland continuing southwestward along the eastern flank of the reykjanes ridge subarctic intermediate water is carried into the iceland basin creating a fresh tongue bounded east and west by the more saline enaw over hatton bank and the eastern flank of the reykjanes ridge respectively

the great salinity anomaly in the northern north atlantic 1968 1982 the widespread freshening of the upper 500 800m layer of the northern north atlantic which this paper describes represents one of the most persistent and extreme variations in global ocean climate yet observed in this century though a range of explanations have been advanced to explain this event including in situ changes in the surface moisture flux this paper describes the great salinity anomaly as largely though not entirely an advective event traceable around the atlantic subpolar gyre for over 14 years from its origins north of iceland in the mid to late 1960s until its return to the greenland sea in 1981 1982 the overall propagation speed around this subpolar gyre is estimated at about 3cm s 1 of the total salt deficit associated with the anomaly as it passed south along the labrador coast in 1971 1973 about 72 109 tonnes a deficit equivalent to about two thirds of this figure 47 109 tonnes ultimately passed through the faroe shetland channel to the barents sea arctic ocean and greenland sea during the mid 1970s possible effects on deep water formation and on the representativeness of historical section data are discussed

on the spatial structure and temporal variability of poleward transport between scotland and greenland 1 the flow north of warm subtropical water though the northeastern atlantic is known to have many pathways that vary over time here we use a combination of upper ocean current measurements betwe

influence of the atlantic subpolar gyre on the thermohaline circulation during the past decade record high salinities have been observed in the atlantic inflow to the nordic seas and the arctic ocean which feeds the north atlantic thermohaline circulation thc this may counteract the observed long term increase in freshwater supply to the area and tend to stabilize the north atlantic thc here we show that the salinity of the atlantic inflow is tightly linked to the dynamics of the north atlantic subpolar gyre circulation therefore when assessing the future of the north atlantic thc it is essential that the dynamics of the subpolar gyre and its influence on the salinity are taken into account

spreading of the labrador sea water to the irminger and iceland basins 1 labrador sea water lsw is a principal convectively formed water mass of the subpolar north atlantic spna using extensive oceanographic archives we demonstrate striking changes in spna caused by massive lsw production during 1987 1994 and document recent salinification and warming imminently bringing spna to the state last time seen in the 1960s two prominent lsw classes are spreading across spna since the 1980s the first record dense deep and voluminous class has been progressively built by intense winter convection through 1987 1994 even though most of this lsw has left spna its remnants are still present there the second shallower class strengthens in 2000 over subsequent years its core becomes slightly thicker and deepens the anomalous signals acquired by these lsw classes in their formation region arrive in the irminger and iceland basins with the characteristic delays of two and five years for deeper lsw and a year and four years for shallower lsw

on the driving processes of the atlantic meridional overturning circulation because of its relevance for the global climate the atlantic meridional overturning circulation amoc has been a major research focus for many years yet the question of which physical mechanisms ultimately drive the amoc in the sense of providing its energy supply remains a matter of controversy here we review both observational data and model results concerning the two main candidates vertical mixing processes in the ocean s interior and wind induced ekman upwelling in the southern ocean in distinction to the energy source we also discuss the role of surface heat and freshwater fluxes which influence the volume transport of the meridional overturning circulation and shape its spatial circulation pattern without actually supplying energy to the overturning itself in steady state we conclude that both wind driven upwelling and vertical mixing are likely contributing to driving the observed circulation to quantify their respective contributions future research needs to address some open questions which we outline

new cnes cls09 global mean dynamic topography computed from the combination of grace data altimetry and in situ measurements 1 an accurate knowledge of the ocean mean dynamic topography mdt is mandatory for the optimal use of altimetric data including their assimilation into operational ocean forecasting systems a new global 1 4 resolution mdt was computed for the 1993 1999 time period with improved data and methodology compared to the previous rio05 mdt field first a large scale mdt is obtained from the cls01 altimetric mean sea surface and a recent geoid model computed from 4 5 years of grace gravity recovery and climate experiment data altimetric sea level anomalies and in situ measurements are then combined to compute synthetic estimates of the mdt and the corresponding mean currents while the rio05 mdt was based on 10 years of in situ dynamic heights and drifting buoy velocities the new field benefits from an enlarged data set of in situ measurements ranging from 1993 to 2008 and includes all hydrological profiles from the argo array moreover the processing of the in situ data has been updated a new ekman model was developed to extract the geostrophic velocity component from the drifting buoy measurements the handling of hydrologic measurements has also been revisited compared to the previous rio05 solution the new global mdt resolves much stronger gradients in western boundary currents with mean velocities being doubled in some places moreover in comparison to several other recent mdt estimates we find that the new cnes cls09 mdt is in better agreement with independent in situ observations

the flux of dense cold overflow water southeast of iceland abstract from the period commencing midsummer 1990 and lasting for 13 months 13 current meters were deployed on seven moorings at depths between 1000 and 2300 m southeast of iceland 63 09 n 17 18 w to 61 44 n 15 24 w the purpose of the array designated woce acm8 was to measure the overflow of dense water downstream of the source in the faeroe bank channel and downstream of any sources on the iceland faeroe ridge in the depth range 1300 2100 m three of the moorings showed a bottom intensified current that was persistent and directed to the southwest at the other moorings the current was erratic and weak the records were combined to form a transport time series for water denser than 0 27 8 kg m 3 in the direction 240 a mean value of 3 2 0 5 sv was found with considerable variability near a period of seven days but there was no convincing evidence for a seasonal signal the amount of pure norwegian sea water present in the overflow was estimated at 1 6 0 15 sv close to previous determina