on the origin of the warm inflow to the nordic seas abstract warm and saline waters enter the nordic seas from the south as part of the warm to cold water transformation of the thermohaline circulation of the northern north atlantic one explanation for the origin of the nordic seas inflow is a shallow source hypothesis under which the inflow waters are a modification of upper ocean subtropical waters warm waters from the subtropical gyre are carried to the eastern north atlantic by the north atlantic current and branch northwards joined by poleward upper thermocline flow along the upper continental slope to provide the nordic seas inflow along this pathway the upper water column is progressively cooled and freshened by winter convection the subpolar mode water transformation process and this sets the inflow characteristics a deep source hypothesis provides an alternative explanation for the characteristics of the nordic seas inflow and the pathway delivering the waters to the inflow under this hypothesis inflow waters originate from the core of the mediterranean overflow waters in the gulf of cadiz carried northward at mid depth by the eastern boundary undercurrent in the subtropics continuing into the subpolar gyre along the eastern boundary and rising from depths near 1200m in the rockall trough to less than 600m to cross the wyville thomson ridge into the faroe shetland channel and thence to the nordic seas the deep source hypothesis focus is on lower thermocline source waters beneath the sill depth for the nordic seas inflow in contrast to the shallow source hypothesis focus of transformation of upper thermocline waters above the sill depth on the basis of regional water mass distributions geostrophic shear and direct current measurements we reject the deep source hypothesis in favor of the shallow source hypothesis rather than a flow of deep mediterranean overflow water along the eastern boundary rising from depth to feed the nordic seas the inflow is supplied directly by transformed north atlantic current waters from the same depth range as the inflow mediterranean overflow water is a constituent of the inflow but only through its contribution to defining the temperature salinity relationship of the interior of the subtropical gyre from which the north atlantic current draws its water rather than by direct northward advection of mediterranean outflow water via the eastern boundary undercurrent crossing from the subtropics into the subpolar domain instead the eastern boundary undercurrent wholly expels its transport of mediterranean overflow water into the eastern edge of the subtropical gyre creating the mid depth westward extending subtropical salinity plume for the lower thermocline isopycnal range of the core of the mediterranean overflow water measurements show that at the wyville thomson ridge the flow is southward and descending rather than northward and rising representing cold spillover from the faroese channels entraining warm upper ocean waters as the spillover plume descends and warms into the northern rockall trough no evidence is found in rockall trough northward of 54 n porcupine bank for direct advection of mediterranean overflow waters via an eastern boundary undercurrent extension and the very dilute mediterranean overflow waters in the trough reflect the interaction of the wyville thomson ridge plume descending into the trough from the north and lower thermocline waters delivered to the trough from the south

a numerical study of interannual variability in the north icelandic irminger current author posting american geophysical union 2018 this article is posted here by permission of american geophysical union for personal use not for redistribution the definitive version was published in journal of geophysical research oceans 123 12 2018 8994 9009 doi 10 1029 2018jc013800

variability of sea surface height and circulation in the north atlantic forcing mechanisms and linkages abstract simulations with a coarse resolution global ocean model during 1958 2004 are analyzed to understand the inter annual and decadal variability of the north atlantic analyses of empirical orthogonal functions eofs suggest relationships among basin scale variations of sea surface height ssh and depth integrated circulation and the winter north atlantic oscillation nao or the east atlantic pattern eap indices the linkages between the atmospheric indices and ocean variables are shown to be related to the different roles played by surface momentum and heat fluxes in driving ocean variability in the subpolar region variations of the gyre strength ssh in the central labrador sea and the nao index are highly correlated surface heat flux is important in driving variations of ssh and circulation in the upper ocean and decadal variations of the atlantic meridional overturning circulation amoc surface momentum flux drives a significant barotropic component of flow and makes a noticeable contribution to the amoc in the subtropical region momentum flux plays a dominant role in driving variations of the gyre circulation and amoc there is a strong correlation between gyre strength and ssh at bermuda

decadal changes in the mid depth water mass dynamic of the northeastern atlantic margin bay of biscay the lithium magnesium li mg molar ratios radiocarbon measurements d 14 c and nd isotopic composition end of the aragonite skeleton of a branching cold water coral cwc species madrepora oculata collected alive in the bay of biscay at 691 m water depth were investigated to reconstruct a robust record of the mid depth water mass dynamics between 1950 and 1990 ad temperature estimates based on the skeletons li mg molar ratios reveal small decadal changes of about 1 1 ca t thermocline depth synchronous to and of similar amplitude as surface temperature anomalies d 14 c

shifting surface currents in the northern north atlantic ocean 1 analysis of surface drifter tracks in the north atlantic ocean from the time period 1990 to 2007 provides evidence that warm subtropical waters have recently increased their penetration toward the nordic seas prior to 2000 the warm water branches of the north atlantic current fed by the gulf stream turned southeastward in the eastern north atlantic since 2001 these paths have shifted toward the rockall trough through which the most saline north atlantic waters pass to the nordic seas these surface drifters are able to overcome the ekman drift which would force them southward under the westerly winds dominating the subpolar atlantic yet the changes in path cannot be accounted for by changes in ekman drift eddy kinetic energy from satellite altimetry shows increased energy along the shifted drifter pathways across the mid atlantic ridge since 2001 these near surface changes have occurred during continual weakening of the north atlantic subpolar gyre as seen by altimetry they are also consistent with the observed increase in temperature and salinity of the waters flowing northward into the nordic seas these findings suggest the changes in the vertical structure of the northern north atlantic ocean its dynamics and exchanges with the higher latitudes wind stress and its curl changes are discussed as a possible forcing of the changes in the pathways of the subtropical waters

variability along the atlantic water pathway in the forced norwegian earth system model the growing attention on mechanisms that can provide predictability on interannual to decadal time scales makes it necessary to identify how well climate models represent such mechanisms in this study we use a high 0 25 horizontal grid and a medium 1 resolution version of a forced global ocean sea ice model utilising the norwegian earth system model to assess the impact of increased ocean resolution our target is the simulation of temperature and salinity anomalies along the pathway of warm atlantic water in the subpolar north atlantic and the nordic seas although the high resolution version has larger biases in general at the ocean surface the poleward propagation of thermohaline anomalies is better resolved in this version i e the time for an anomaly to travel northward is more similar to observation based estimates the extent of these anomalies can be rather large in both model versions as also seen in observations e g stretching from scotland to northern norway the easternmost branch into the nordic and barents seas carrying warm atlantic water is also improved by higher resolution both in terms of mean heat transport and variability in thermohaline properties a more detailed assessment of the link between the north atlantic ocean circulation and the thermohaline anomalies at the entrance of the nordic seas reveals that the high resolution is more consistent with mechanisms that are previously published this suggests better dynamics and variability in the subpolar region and the nordic seas in the high resolution compared to the medium resolution this is most likely due a better representation of the mean circulation in the studied region when using higher resolution as the poleward propagation of ocean heat anomalies is considered to be a key source of climate predictability we recommend that similar methodology presented herein should be performed on coupled climate models that are used for climate prediction

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

response of north atlantic ocean circulation to atmospheric weather regimes a new framework is proposed for investigating the atmospheric forcing of north atlantic ocean circulation instead of using classical modes of variability such as the north atlantic oscillation nao or the east atlantic pattern the weather regimes paradigm was used using this framework helped avoid problems associated with the assumptions of orthogonality and symmetry that are particular to modal analysis and known to be unsuitable for the nao using ocean only historical and sensitivity experiments the impacts of the four winter weather regimes on horizontal and overturning circulations were investigated the results suggest that the atlantic ridge ar negative nao nao and positive nao nao regimes induce a fast monthly to interannual time scales adjustment of the gyres via topographic sverdrup dynamics and of the meridional overturning circulation via anomalous ekman transport the wind anomalies associated with the scandinavian blocking regime sbl are ineffective in driving a fast wind driven oceanic adjustment the response of both gyre and overturning circulations to persistent regime conditions was also estimated ar causes a strong wind driven reduction in the strengths of the subtropical and subpolar gyres while nao causes a strengthening of the subtropical gyre via wind stress curl anomalies and of the subpolar gyre via heat flux anomalies nao induces a southward shift of the gyres through the southward displacement of the wind stress curl the sbl is found to impact the subpolar gyre only via anomalous heat fluxes the overturning circulation is shown to spin up following persistent sbl and nao and to spin down following persistent ar and nao conditions these responses are driven by changes in deep water formation in the labrador sea

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

observations inferences and mechanisms of the atlantic meridional overturning circulation a review this is a review about the atlantic meridional overturning circulation amoc its mean structure temporal variability controlling mechanisms and role in the coupled climate system the amoc plays a central role in climate through its heat and freshwater transports northward ocean heat transport achieved by the amoc is responsible for the relative warmth of the northern hemisphere compared to the southern hemisphere and is thought to play a role in setting the mean position of the intertropical convergence zone north of the equator the amoc is a key means by which heat anomalies are sequestered into the ocean s interior and thus modulates the trajectory of climate change fluctuations in the amoc have been linked to low frequency variability of atlantic sea surface temperatures with a host of implications for climate variability over surrounding landmasses on intra annual timescales variability in amoc is large and primarily reflects the response to local wind forcing meridional coherence of anomalies is limited to that of the wind field on interannual to decadal timescales amoc changes are primarily geostrophic and related to buoyancy anomalies on the western boundary a pacemaker region for decadal amoc changes is located in a western transition zone along the boundary between the subtropical and subpolar gyres decadal amoc anomalies are communicated meridionally from this region amoc observations as well as the expanded ocean observational network provided by the argo array and satellite altimetry are inspiring efforts to develop decadal predictability systems using coupled atmosphere ocean models initialized by ocean data

a high resolution salinity time series 1993 2012 in the north atlantic from argo and altimeter data the study of salinity changes has been hampered by the lack of temporal and spatial resolution of the observations in order to improve the spatial and temporal distribution of salinity observations we used the gravest empirical mode gem technique to calculate high resolution salinity distributions as a function of dynamic height for the period 1993 2012 this technique combined argo and altimeter data to exploit the relationship between t s profiles and dynamic height in the north atlantic the method was valid in the upper 700 m mainly at and near the pathways of the north atlantic current nac but failed in regions with weak stratification or with ambiguities in the t s relationships coherent multiannual large scale variability was observed with many features present in all regions albeit with weaker amplitudes in the eastern basins some of the interannual features in the northeastern atlantic basins were unrelated to the variability further south and west pointing to an occasional advection of subtropical water in the eastern atlantic origin and advection of salinity anomalies with the nac from the north american basin into the western subpolar north atlantic are correlated with the state of the north atlantic oscillation nao and dampened by the surface freshwater fluxes other mechanisms influencing the salinity pattern are the changing location of the subpolar front also related to the nao the large multiyear variability in the 20 year time series obscured any potential trends caused by global warming only the rockall trough showed a salinity increase of 0 03 per decade

dynamics of the atlantic meridional overturning circulation part 2 forcing by winds and buoyancy abstract recently schloesser et al 2012 explored the dynamics of the descending branch of meridional overturning circulations mocs by obtaining analytic solutions to a variable density 2 layer model vlom forced only by a surface buoyancy flux key processes involved are the poleward thickening of the upper layer along the eastern boundary due to kelvin wave adjustments the westward propagation of that coastal structure by rossby waves and their damping by mixing the resulting zonal pressure gradient causes the surface moc branch to converge into the northern basin near the eastern boundary in this paper we extend the schloesser et al 2012 study to include forcing by a zonal wind stress x y much of the paper is devoted to the derivation and analysis of analytic solutions to vlom for validation we also report corresponding numerical solutions to an ocean general circulation model ogcm solutions are obtained in a flat bottom rectangular basin confined to the northern hemisphere the buoyancy forcing relaxes upper ocean density to a prescribed profile y that increases polewards until it becomes as large as the deep ocean density at latitude y2 north of y2 then the ocean is homogeneous a 1 layer system the wind stress x drives subtropical and subpolar gyres and in our standard solution the latter extends north of y2 vertical diffusion is not included in vlom minimized in the ogcm consequently the moc is not closed by upwelling associated with interior diffusion but rather by flow through the southern boundary of the basin into a southern boundary sponge layer in the ogcm and solutions are uniquely determined by specifying the strength of that flow or the thermocline depth along the tropical eastern boundary solutions forced by x and differ markedly from those forced only by because water flows across y2 throughout the interior of the subpolar gyre not just near the eastern boundary in some of our solutions the strength of the moc s descending branch is determined entirely by this wind driven mechanism whereas in others it is also affected by rossby wave damping near the eastern boundary upwelling can occur in the interior of the subpolar gyre and in the western boundary layer providing shortcuts for the overturning circulation consequently there are different rates for the convergence of upper layer water near y 2 m n and the export of deep water south of the subpolar gyre m the latter being a better measure of large scale moc strength when western boundary upwelling occurs in our solutions m is independent of the diapycnal processes in the subpolar ocean

detection and monitoring of ongoing aseismic slip in the tokai region central japan analysis of global positioning system data shows that the rate of crustal deformations in the tokai region of japan a seismic gap area changed over the past 18 months kalman filtering analysis shows aseismic slip on the plate boundary in the western tokai region centered on lake hamana adjacent to the anticipated tokai earthquake source area the cumulative moment magnitude reaches 6 7 in june 2002 with a relative slip increase northeast of lake haman from january 2002 an existence of aseismic slip in the western tokai supports the hypothesis of a silent event as the cause of uplifting several days before the 1944 tonankai earthquake

gulf stream subtropical gyre properties across two dansgaard oeschger cycles salinity increase in the subtropical gyre system may have pre conditioned the north atlantic ocean for a rapid return to stronger overturning circulation and high latitude warming following meltwater events during the last glacial period here we investigate the gulf stream subtropical gyre system properties over dansgaard oeschger do cycles 14 to 12 including heinrich ice rafting event 5 during the holocene and last glacial maximum a positive gradient in surface dwelling planktonic foraminifera 18o globigerinoides ruber can be observed between the gulf stream and subtropical gyre due to decreasing temperature increasing salinity and a change from summer to year round occurrence of g ruber we assess whether this gradient was a common feature during stadial interstadial climate oscillations of marine isotope stage 3 by comparing existing g ruber 18o from odp site 1060 subtropical gyre location and new data from odp site 1056 gulf stream location between 54 and 46 ka our results suggest that this gradient was largely absent during the period studied during the major warm do interstadials 14 and 12 we infer a more zonal and wider gulf stream influencing both odp sites 1056 and 1060 a gulf stream presence during these major interstadials is also suggested by the large vertical 18o gradient between shallow dwelling planktonic foraminifera species especially g ruber and the deep dwelling species globorotalia inflata at site 1056 which we associate with strong summer stratification and gulf stream presence a major reduction in this vertical 18o gradient from 51 ka until the end of heinrich event 5 at 48 5 ka suggests site 1056 was situated within the subtropical gyre in this mainly cold period from which we infer a migration of the gulf stream to a position nearer to the continental shelf indicative of a narrower gulf stream with possibly reduced transport

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

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

on the flow of atlantic water and temperature anomalies in the nordic seas toward the arctic ocean the climatic conditions over the arctic ocean are strongly influenced by the inflow of warm atlantic water conveyed by the norwegian atlantic slope current nwasc based on sea surface height ssh data from altimetry we develop a simple dynamical measure of the nwasc transport to diagnose its spatio temporal variability this supports a dynamical division of the nwasc into two flow regimes the svinoy branch svb in the southern norwegian sea and the fram strait branch fsb west of spitsbergen the svb transport is well correlated with the ssh and atmospheric variability within the nordic seas factors that also affect the inflow to the barents sea in contrast the fsb is influenced by regional atmospheric conditions around svalbard and northern barents sea using a composite analysis we further relate anomalous strong svb flow events to temperature fluctuations along the core of atlantic water a warm composite anomaly is found to propagate northward with a tendency to amplify enroute after these events a roughly 12 months delayed temperature signal is identified in the fsb however also in the lofoten basin interior a delayed temperature signal is found which appears to originate from the nwasc this study suggests that hydrographic anomalies both upstream from the north atlantic and locally generated in the norwegian sea are important for the oceanic heat and salt transport that eventually enters into the arctic

a dipole pattern of the sea surface height anomaly in the north atlantic 1990s 2000s 1 despite a long term trend of sea level rise continuing into the 2000s in the subpolar north atlantic variations in the sea surface height have behaved differently in both spatial and temporal domains a dipole pattern centered between the northern atlantic subpolar region and the region near the gulf stream was observed in the linear trends of the sea surface height anomaly ssha by applying the ensemble empirical mode decomposition eemd we found that this dipole pattern is mainly associated with the interannual to decadal ssha oscillations of the two regions which are 180 out of phase with each other over the time span of this study the low frequency variations of the ssha in the subpolar region are strongly inversely correlated with the cumulative north atlantic oscillation nao index r 0 84 in contrast with the gulf stream region which is positively correlated r 0 22 this therefore reveals an asymmetric response of the regional ssha to the cumulative nao forcing in which the subpolar variability leads that of the gulf stream region by 29 months moreover there is a remarkable reversal of the ssha trends from the 1990s to the 2000s which is unexpected given a weak and fluctuating nao behavior since mid 1990s such ssha variations in the 2000s might be related to the lagged variations of the atlantic meridional overturning circulation amoc

subpolar mode water in the northeastern atlantic 2 origin and transformation 1 the processes that lead to the transformation and origin of the eastern north atlantic subpolar mode waters spmw are investigated from observational data using an extended walin framework air sea flux data from the national oceanography center southampton nocs and hydrographic data from the a24 cruise collected during the world ocean circulation experiment woce are used to estimate the contribution of diapycnal and isopycnal fluxes to the density classes that include spmw surface diapycnal volume flux is the dominant source of waters in the spmw density in the north atlantic subpolar gyre the diapycnal volume flux occurs along the main branches of the north atlantic current nac and it has an average transport of 14 6 5 sv with a maximum of 21 5 sv across the 27 35sq isopycnal the regional distribution of the diapycnal flux on isopycnal surfaces is computed to identify the areas with the largest diapycnal flux these regions coincide with the location of spmw based on potential vorticity the surface diapycnal flux is associated with obduction and subduction through the permanent pycnocline therefore the water involved in the transformation of spmws is continuously exchanged with the ocean interior in addition we suggest that subduction is not associated with smooth advection from the mixed layer to the ocean interior but is water mass loss entrainment into the deep overflows of the subpolar gyre the isopycnal component of the spmw throughput is estimated from the geostrophic transport across the a24 section from greenland to scotland and is 10 to 40 of the diapycnal flux

millennial scale variations of the holocene north atlantic mid depth gyre inferred from radiocarbon and neodymium isotopes in cold water corals abstract variations in north atlantic ocean mid depth circulation during the holocene are poorly understood it is believed that they had a significant influence on the properties of water entering the nordic sea by redistributing heat and freshwater potentially affecting deep water formation and climate to improve our knowledge of the ne atlantic mid depth circulation radiocarbon and neodymium isotope analyses have been then carried out on precisely dated u th l pertusa and m oculata coral fragments from two sediment cores taken at 750 m water depth on the sw rockall trough margin cold water coral cwc end values vary between 12 2 0 3 and 16 6 0 4 and result from variable contributions of unradiogenic mid depth subpolar gyre mid spg water 15 and more radiogenic eastern north atlantic water enaw 11 which is transported northward to the rockall trough by boundary currents along the european margin increased coral end reflects a westward contraction of the mid spg water and a higher proportion of enaw the mid holocene from 8 8 to 6 8 ka bp is marked by unradiogenic coral end from 16 6 0 4 to 14 6 0 5 indicating a greater eastward extension of the mid spg this is followed by a shift from 6 8 to 5 ka bp toward more radiogenic end values from 15 4 0 3 to 13 3 0 2 suggesting a westward contraction of the mid spg and a higher proportion of enaw the mid holocene long term change in end is characterized by millennial variations of up to 2 5 epsilon units well marked during the late holocene indicating that eastward extension of the mid spg coeval with warm periods in northern europe e g the medieval climatic anomaly and the roman warm period most of the cwc derived 14c values match the global ocean values indicating that the water masses bathing the corals were generally well ventilated the exceptions are a few short intervals of eastward extension of the mid spg which are characterized by lower 14c during the late holocene we propose that these minor 14c fluctuations in the rockall trough may be related to local changes in the mixed layer depth or to variability in the advection of water from the labrador sea where deep convection gives rise to thermocline waters that are relatively depleted in terms of 14c the eastward extension of the mid spg between 8 8 and 6 8 ka bp is associated with the holocene climatic optimum concurrent with a maximum strength of the iceland scotland overflow water isow which is indicative of an increase in the atlantic meridional overturning circulation amoc this period is followed by a transition in the north atlantic circulation which occurred around 6 8 ka bp and coincides with the onset of labrador sea water formation a decrease of the isow strength and a contraction of the mid spg leading to a greater intrusion of saline subtropical water into the subpolar atlantic

decadal water mass variations along 20 w in the northeastern atlantic ocean abstract water mass variations in the northeastern atlantic ocean along 20 w are analyzed with pentadal resolution over the past 15 years using data from four repeat occupations of a meridional hydrographic section running south from iceland the section was sampled in 1988 1993 1998 and 2003 the results are interpreted in the context of changes in air sea forcing ocean circulation and water properties associated with the north atlantic oscillation nao the nao index oscillated around zero from 1984 to 1988 was strongly positive from 1989 to 1995 after which it shifted to lower positive and occasionally negative values from 1996 to 2003 previously published studies suggest that after the 1995 1996 shift of the nao the subpolar gyre largely retreated to the northwest in the northeastern atlantic ocean resulting in an increasingly southeastern character of local water masses with time water property changes extending from the subpolar mode water spmw just below the seasonal pycnocline through the density range shared by mediterranean outflow water and subarctic intermediate water saiw along 20 w are consistent with changes in wind driven ocean circulation and air sea heat flux associated with shifts in the nao especially after accounting for ocean memory after periods of lower nao index the spmw is warmer saltier and lighter at these same times large increases of apparent oxygen utilization aou and potential vorticity are found at the spmw base consistent with spmw ventilation to lighter densities during lower nao index periods deeper and denser in the water column the cold fresh and dense saiw signature within the permanent pycnocline that was most strongly present in 1993 near the culmination of a period of high nao index is much reduced in 1988 and 1998 in 2003 after a prolonged period of lower nao index increasing influence of warmer saltier subtropical waters is clear within the permanent pycnocline the deep penetration of the changes implies that they are caused primarily by circulation changes resulting from nao associated wind shifts but changes in air sea heat flux could also have played a role

mechanisms underlying recent decadal changes in subpolar north atlantic ocean heat content the subpolar north atlantic spna is subject to strong decadal variability with implications for surface climate and its predictability in 2004 2005 spna decadal upper ocean and sea surface temperature trends reversed from warming during 1994 2004 to cooling over 2005 2015 this recent decadal trend reversal in spna ocean heat content ohc is studied using a physically consistent observationally constrained global ocean state estimate covering 1992 2015 the estimate s physical consistency facilitates quantitative causal attribution of ocean variations closed heat budget diagnostics reveal that the spna ohc trend reversal is the result of heat advection by midlatitude ocean circulation kinematic decompositions reveal that changes in the deep and intermediate vertical overturning circulation cannot account for the trend reversal but rather ocean heat transports by horizontal gyre circulations render the primary contributions the shift in horizontal gyre advection reflects anomalous circulation acting on the mean temperature gradients maximum covariance analysis mca reveals strong covariation between the anomalous horizontal gyre circulation and variations in the local wind stress curl suggestive of a sverdrup response results have implications for decadal predictability

on model differences and skill in predicting sea surface temperature in the nordic and barents seas the nordic seas and the barents sea is the atlantic ocean s gateway to the arctic ocean and the gulf stream s northern extension brings large amounts of heat into this region and modulates climate in northwestern europe we have investigated the predictive skill of initialized hindcast simulations performed with three state of the art climate prediction models within the cmip5 framework focusing on sea surface temperature sst in the nordic seas and barents sea but also on sea ice extent and the subpolar north atlantic upstream the hindcasts are compared with observation based sst for the period 1961 2010 all models have significant predictive skill in specific regions at certain lead times however among the three models there is little consistency concerning which regions that display predictive skill and at what lead times for instance in the eastern nordic seas only one model has significant skill in predicting observed sst variability at longer lead times 7 10 years this region is of particular promise in terms of predictability as observed thermohaline anomalies progress from the subpolar north atlantic to the fram strait within the time frame of a couple of years in the same model predictive skill appears to move northward along a similar route as forecast time progresses we attribute this to the northward advection of sst anomalies contributing to skill at longer lead times in the eastern nordic seas the skill at these lead times in particular beats that of persistence forecast again indicating the potential role of ocean circulation as a source for skill furthermore we discuss possible explanations for the difference in skill among models such as different model resolutions initialization techniques and model climatologies and variance

on the spatial coherence of the atlantic water inflow across the nordic seas the co variability of the atlantic water aw branches in the nordic seas is investigated over the period 1979 2012 using an eddy permitting model a noticeable circulation change is found in the mid 1990s prior to the mid 1990s the leading mode of variability defines a large scale pattern with concomitant variations in the atlantic water aw inflow in the faroe shetland channel fsc the norwegian atlantic slope current nwasc the aw inflow to the barents sea and the west spitsbergen current wsc after the mid 1990s the co variability between the nwasc and the aw inflow in both the fsc and the wsc is lost consequently the northern barents sea circulation anomaly pattern which is present throughout the full period becomes the leading mode of circulation in the northern nordic seas after the mid 1990s the circulation change of the mid 1990s appears to be linked to a weakening of the southwesterly wind anomalies in norwegian sea as the northern center of action of the first mode of sea level pressure nao weakens passive tracer experiments suggest that this circulation change may be accompanied by increased heat transfer from the aw current to the interior nordic seas this in turn may have limited the influence of the recently observed aw warming in the iceland scotland passage on the nwasc downstream

air sea interaction and circulation changes in the northeast atlantic the upper ocean of the rockall trough exhibits coherent interannual variations in temperature and salinity over the past 26 years with highs in the mid 1980s and late 1990s and lows in the late 1970s and early 1990s and with ranges of 0 5 c and 0 05 in salinity the origins of the interannual changes are discussed covering three potential influencing factors the propagation of anomalies developed upstream of the basin the effect of local air sea interaction and the result of changes of regional circulation bringing different water masses into the region the changes in heat and freshwater content of the upper ocean are directly compared to observed variations in air sea heat and freshwater fluxes over the period of the time series it is shown that the role of the atmosphere in locally altering the oceanic properties particularly salinity is relatively small and insufficient to explain the changes two recent hydrographic surveys are analyzed to ascertain how the distribution of water masses to the south of the basin may influence the properties of the northern rockall trough upper ocean and the results are reviewed in the context of historical analyses it is found that the critical factor in determining the properties is the varying amount of relatively cool and fresh north atlantic current water mixing with the dominant water mass the warm saline eastern north atlantic central water at the entrance to the basin variations of inflowing water masses are caused by east west changes in the location of the subpolar front and the relationship of these changes in regional circulation to the north atlantic wind stress field is discussed

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

northern north atlantic sea surface height and ocean heat content variability 1 the evolution of nearly 20 years of altimetric sea surface height ssh is investigated to understand its association with decadal to multidecadal variability of the north atlantic heat content altimetric ssh is dominated by an increase of about 14 cm in the labrador and irminger seas from 1993 to 2011 while the opposite has occurred over the gulf stream region over the same time period during the altimeter period the observed 0 700 m ocean heat content ohc in the subpolar gyre mirrors the increased ssh by its dominantly positive trend over a longer period 1955 2011 fluctuations in the subpolar ohc reflect atlantic multidecadal variability amv and can be attributed to advection driven by the wind stress gyre mode bringing more subtropical waters into the subpolar gyre the extended subpolar warming evident in ssh and ohc during the altimeter period represents transition of the amv from cold to warm phase in addition to the dominant trend the first empirical orthogonal function ssh time series shows an abrupt change 2009 2010 reaching a new minimum in 2010 the change coincides with the change in the meridional overturning circulation at 26 5 n as observed by the rapid rapid climate change project and with extreme behavior of the wind stress gyre mode and of atmospheric blocking while the general relationship between northern warming and atlantic meridional overturning circulation amoc volume transport remains undetermined the meridional heat and salt transport carried by amoc s arteries are rich with decade to century timescale variability

decadal variability of hydrography in the upper northern north atlantic in 1948 1990 we investigate the variability of the north atlantic subarctic gyre in recent decades from time series of station temperature and salinity decadal variability stronger at the surface is identified which exhibits vertical coherence over a layer deeper than the late winter mixed layer in the northwestern atlantic it corresponds to the layer with a component of water from the arctic ocean or from the canadian arctic the spatial coherence of the signal is investigated an empirical orthogonal function decomposition of lagged time series indicates that a single pattern explains 70 of the variance in upper ocean salt content corresponding to a propagating signal from the west to the northeast in the subarctic gyre the most likely interpretation is that the salinity signal originates in the slope currents of the labrador sea and is diffused advected eastward of the grand banks over the near western atlantic in the northwestern atlantic temperature fluctuations are strongly correlated to salinity fluctuations and are aligned along the average t s characteristics this signal suggests large variations in the outflow of fresh cold water in the slope current and is strongly correlated with ice cover a basin scale atmospheric circulation of weakened westerlies at 55 n weaker northwesterlies west of greenland and weaker southerlies over the central and eastern north atlantic is associated with the high salinity and warm water phase of the first principal component this circulation pattern leads fluctuations in the northeast atlantic and lags those in the northwestern part of the basin the wind indices also suggest that the fluctuations of the fresh water outflow occur during intervals of anomalously northerly winds either east of greenland 1965 1968 1969 or off the canadian archipelago 1983 1984

great salinity anomalies in the north atlantic abstract we revisited the great salinity anomaly of the 1970s gsa 70s dickson et al 1988 and documented the newly identified great salinity anomaly of the 1980s hence termed gsa 80s both propagated around the north atlantic in a similar fashion the advective mechanism initially proposed to explain the observed sequence of low salinity low temperature events during the gsa 70s apparently holds also for the gsa 80s the latter was successively observed in the west greenland current 1982 the labrador current 1983 the flemish pass 1984 near the charlie gibbs fracture zone 1984 1985 in the rockall channel 1985 south of iceland 1985 1988 in the north sea 1986 1987 norwegian sea 1987 1988 barents sea 1988 1989 and iceland sea 1989 1990 the advection speed of the gsa 80s seems to be greater than the one of the gsa 70s the 1980s anomaly reached the barents sea 6 to 7 years after peaking in the west greenland current while the 1970s anomaly traveled the same route in 8 to 10 years these anomalies however seem to be of different origin the gsa 70s was apparently boosted remotely by a freshwater sea ice pulse from the arctic via fram strait consequently the gsa 70s was accompanied by a large sea ice extent anomaly in the greenland and iceland seas which propagated into the labrador sea in contrast the gsa 80s was likely formed locally in the labrador sea baffin bay mainly because of the extremely severe winters of the early 1980s but supplemented with a possible contribution of the arctic freshwater outflow via the canadian archipelago facilitated by strong northerly winds which would have enhanced stability and ice formation this anomaly was also associated with a positive sea ice extent anomaly in the labrador sea baffin bay which however had no upstream precursor in the greenland sea thus the gsas are not necessarily caused solely by an increased export of freshwater and sea ice from the arctic via fram strait these results are corroborated by the early 1990s data when a new fresh cold anomaly was formed in the labrador sea and accompanied by a large positive sea ice extent anomaly the harsh winters of the early 1990s were however confined to the labrador sea baffin bay area while the atmospheric and oceanic conditions in the greenland iceland and irminger seas were normal the labrador sea baffin bay area appears therefore to play a key role in formation of gsas as well as in propagation of the gsas formed upstream a likely contribution of the enhanced canadian archipelago freshwater outflow to the gsa formation also seems to be significant two major modes of the gsa origin are thus identified remote generated by an enhanced arctic ocean freshwater export via either fram strait or the canadian archipelago and local resulting from severe winters in the labrador sea baffin bay both modes should be taken into account to model decadal variability of the coupled ocean ice atmosphere system in the north atlantic arctic basin

heat budget in the north atlantic subpolar gyre impacts of atmospheric weather regimes on the 1995 warming event abstract in the mid 1990s the north atlantic subpolar gyre has shown a dramatic warming event that has been thoroughly investigated from observations and numerical simulations some studies suggest that it was due to an interannual wind driven weakening and shrinking of the gyre that facilitated the penetration of warm atlantic water the weakening of the gyre being attributed to changes in the north atlantic oscillation nao and in the east atlantic pattern which are the two dominant modes of atmospheric variability in the north atlantic however other studies suggest that the warming event was due to a decadal buoyancy driven strengthening of the meridional overturning circulation and subsequent intensification of the poleward heat transport in response to the positive nao conditions of 1988 1995 to reconcile this discrepancy the heat budget in the north atlantic subpolar gyre is reconstructed from four ocean hindcast simulations sharing the same modelling platform but using different settings the novelty of this work is the decomposition of the subpolar gyre into a western and an eastern subregion which is motivated by water mass distribution around reykjanes ridge and by the fact that deep convection only occurs in the western subpolar gyre in the western subpolar gyre the 1995 warming event is the decadal baroclinic ocean response to positive nao conditions from 1988 to 1995 the latter induced increased surface heat loss in the labrador sea that intensified deep convection hence strengthened the meridional overturning circulation and the associated poleward heat transport in the eastern subregion a concomittant warming is induced by an interannual barotropic adjustment of the gyre circulation to an abrupt switch from positive nao conditions in winter 1995 to negative nao conditions in winter 1996 indeed the gyre response to negative nao conditions is a cyclonic intergyre gyre that increases northward volume and heat transports at the southeastern limit of the subpolar gyre therefore the discrepancies found in the literature about the 1995 warming event of the north atlantic subpolar gyre are reconciled in the present work which suggests that the atmospheric drivers the mechanisms at stake and the associated timescales are different to the east and to the west of reykjanes ridge

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

intense warming and salinification of intermediate waters of southern origin in the eastern subpolar north atlantic in the 1990s to mid 2000s 1 recent thermohaline changes in the layer of intermediate waters iw advected into the eastern subpolar north atlantic from lower latitudes are quantified using the data from the repeated transatlantic sections positive trends in temperature and salinity in the iw density class at 53 n 0 049 c a and 0 0088 a 1992 2002 and 60 n 0 044 c a and 0 0085 a 1997 2005 are derived the unexpectedly high rates of the iw warming and salinification cannot be explained solely by the long term and recent decadal changes at the intermediate levels in the midlatitude north atlantic and appear to be a consequence of the northward advance of the source water masses caused by the north atlantic oscillation induced contraction of the subpolar gyre

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

north atlantic oscillation induced changes of the upper layer circulation in the northern north atlantic ocean 1 hydrographic data of the 1990s along world ocean circulation experiment sections a1e greenland ireland and a2 newfoundland france indicate a redistribution of cold low saline subarctic waters and warm saline subtropical waters in the upper layer of the northern north atlantic within about 2 years after the north atlantic oscillation had turned from a period with strong westerlies until 1995 to a period with weak westerlies in 1996 and 1997 in the latter period subarctic waters spread preferably southward along the continental slope east of newfoundland rather than eastward with the north atlantic current nac into the irminger iceland and west european basins as a consequence the subarctic front and the associated nac shifted eastward in the newfoundland basin where subarctic waters accumulate and westward in the iceland basin where a large warm and saline anomaly was found in 1996 and 1997 indicating a contraction of the subpolar gyre in the irminger basin the anomaly occurred in 1998 and 1999 whereas the cyclonic circulation in the irminger and iceland basins weakened it intensified in the newfoundland basin where a strengthened labrador current retroflection joined the nac this and the increased anticyclonic recirculation of subtropical waters in the newfoundland and west european basins caused a considerable reduction of the northward heat transport across 47 n in the upper layer in 1997 the anticyclonic circulation occurring in the west european basin suggested a northward expansion of the subtropical gyre

the uniform k distribution of the mare deposits in the orientale basin insights from chang e 2 gamma ray spectrometer the composition of mare basalt units in the orientale basin are investigated by using the potassium k map derived from chang e 2 gamma ray spectrometer ce 2 grs and feo map derived from clementine uv vis data set together with crater retention ages of the mare basalts from literature data we aim to investigate possible magma sources underneath the orientale basin and their chemical evolution over time analyses of the chemical composition of the resurfaced mare basalts together with the reported eruption ages suggest a unique magma generating process for the resurfaced mare deposits the early mare basalts in the central mare orientale and the later resurfaced mare deposits probably derived from magma generated by heat release due to high radioactive element concentrations based on forward modeling the similar k abundances observed in the small mare deposits of the sw polygon area lacus veris and lacus autumni and those in the central mare orientale imply the same heat source for these lava eruptions the chemical similarities e g k feo and tio2 of these regions suggest that mare basalts within the orientate basin are a result of multiple eruptions from a relatively homogeneous source underneath the basin c 2014 elsevier b v all rights reserved

on mapping the diapycnal water mass transformation of the upper north atlantic ocean abstractdiapycnal water mass transformation is the essence behind the atlantic meridional overturning circulation amoc and the associated heat freshwater transports existing studies have mostly

water mass transformation and the north atlantic current in three multicentury climate model simulations 1 the warm and saline subtropical water carried by the north atlantic current undergoes substantial transformation on its way to higher latitudes predominantly from oceanic heat loss to the atmosphere the geographical distribution of the surface forced water mass transformation is assessed in multicentury climate simulations from three different climate models bcm ipslcm4 and mpi m esm with a particular focus on the eastern subpolar north atlantic ocean a diagnosis originally introduced by walin 1982 estimates the surface water mass transformation from buoyancy forcing while the depth structure of the atlantic meridional overturning circulation amoc is similar in all models their climatological heat and freshwater fluxes are very different consistently the models differ in their mean pathways of the north atlantic current location of upper ocean low salinity waters as well as in sea ice cover in the two models with an excessive sea ice extent in the labrador sea most of the water mass transformation in the subpolar region occurs in the eastern part east of 35 w the variability of the eastern water mass transformation on decadal time scales is related to the variable warm northward flow into the subpolar region the upper branch of amoc where a strengthened flow leads an intensified transformation this relationship seems to disappear with a weak connection between the subtropical and subpolar gyres