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

assessing the roles of three eddy types in restratifying the labrador sea after deep convection abstractrestratification after deep convection is one of the key factors in determining the temporal variability of dense water formation in the labrador sea in the subsurface it is primarily governed by lateral buoyancy fluxes during early spring the roles of three different eddy types in this process are assessed using an idealized model of the labrador sea that simulates the restratification season the first eddy type warm core irminger rings is shed from the boundary current along the west coast of greenland all along the coastline the boundary current forms boundary current eddies the third type convective eddies arises directly around the convection area in the model the latter two eddy types are together responsible for replenishing 30 of the winter heat loss within 6 months irminger rings add another 45 to this number the authors results thus confirm that the presence of irminger rings is essential for a realistic amount of restratification in this area the model results are com

flow paths and variability of the north atlantic current a comparison of observations and a high resolution model the north atlantic current nac is subject to variability on multiannual to decadal time scales influencing the transport of volume heat and freshwater from the subtropical to the eastern subpolar north atlantic na current observational time series are either too short or too episodic to study the processes involved here we compare the observed continuous nac transport time series at the western flank of the mid atlantic ridge mar and repeat hydrographic measurements at the ovide line in the eastern atlantic with the nac transport and circulation in the high resolution 1 20 ocean model configuration viking20 1960 2008 the modeled baroclinic nac transport relative to 3400 m 24 5 7 1 sv at the mar is only slightly lower than the observed baroclinic mean of 27 4 4 7 sv from 1993 to 2008 and extends further north by about 0 5 in the eastern atlantic the western nac wnac carries the bulk of the transport in the model while transport estimates based on hydrographic measurements from five repeated sections point to a preference for the eastern nac enac the model is able to simulate the main features of the subpolar na providing confidence to use the model output to analyze the influence of the north atlantic oscillation nao model based velocity composites reveal an enhanced nac transport across the mar of up to 6 7 sv during positive nao phases most of that signal 5 4 sv is added to the enac transport while the transport of the wnac was independent of the nao

further intensification of deep convection in the labrador sea in 2016 there has been a progressive deepening of winter convection in the labrador sea since 2012 with the individual profile maximum depth exceeding 1800 m since 2014 and reaching 2100 m in 2016 this increase during repeated positive phases of the winter north atlantic oscillation nao resembles that during the formation of the record depth 2500 m labrador sea water lsw class in 1987 1994 attributed to repeated positive nao forcing having provided critical preconditioning the 2012 2016 lsw class is one of the deepest and most persistent ever observed back to 1938 year round observations from profiling argo floats since 2002 complemented by annual surveys are providing novel information on the seasonal to decadal evolution of lsw such as its variable density the recent multiyear preconditioning and its 2016 density being the highest since the mid 1990s these findings should help international observation programs and numerical model studies investigating lsw influences on the subpolar north atlantic and atlantic meridional overturning circulation

mean full depth summer circulation and transports at the northern periphery of the atlantic ocean in the 2000s a mean state of the full depth summer circulation in the atlantic ocean in the region in between cape farewell greenland scotland and the greenland scotland ridge gsr is assessed by combining 2002 2008 yearly hydrographic measurements at 59 5 n mean dynamic topography satellite altimetry data and available estimates of the atlantic nordic seas exchange the mean absolute transports by the upper ocean mid depth and deep currents and the meridional overturning circulation moc 16 5 2 2 sv at 0 27 55 at 59 5 n are quantified in the density space inter basin and diapycnal volume fluxes in between the 59 5 n section and the gsr are then estimated from a box model the dominant components of the meridional exchange across 59 5 n are the north atlantic current nac 15 5 0 8 sv 0 27 55 east of the reykjanes ridge the northward irminger current ic 12 0 3 0 sv and southward western boundary current wbc 32 1 5 9 sv in the irminger sea and the deep water export from the northern iceland basin 3 7 0 8 sv 0 27 80 about 60 12 7 1 4 sv of waters carried in the moc upper limb 0 27 55 by the nac ic across 59 5 n 21 1 1 0 sv recirculates westward south of the gsr and feeds the wbc 80 10 2 1 7 sv of the recirculating nac ic derived upper ocean waters gains density of 0 27 55 and contributes to the moc lower limb accordingly the contribution of light to dense water conversion south of the gsr 10 sv to the moc lower limb at 59 5 n is one and a half times larger than the contribution of dense water production in the nordic seas 6 sv

surprising return of deep convection to the subpolar north atlantic ocean in winter 2007 2008 in the process of open ocean convection in the subpolar north atlantic ocean surface water sinks to depth as a distinct water mass the characteristics of which affect the meridional overturning circulation and oceanic heat flux in addition carbon is sequestered from the atmosphere in the process in recent years this convection has been shallow or non existent which could be construed as a consequence of a warmer climate here we document the return of deep convection to the subpolar gyre in both the labrador and irminger seas in the winter of 2007 2008 we use profiling float data from the argo programme to document deep mixing analysis of a variety of in situ satellite and reanalysis data shows that contrary to expectations the transition to a convective state took place abruptly without going through a phase of preconditioning changes in hemispheric air temperature storm tracks the flux of fresh water to the labrador sea and the distribution of pack ice all contributed to an enhanced flux of heat from the sea to the air making the surface water sufficiently cold and dense to initiate deep convection given this complexity we conclude that it will be difficult to predict when deep mixing may occur again

intra seasonal variability of the dwbc in the western subpolar north atlantic highlights a joint analysis of deep current meter records in the western north atlantic intra seasonal variability dominates the deep boundary current topographic waves near 10d periods trapped over steep topography basin centers are showing longer periods 50d caused by the eddy field observed variability characteristics compared to high resolution model simulation abstract the deep western boundary current dwbc along the western margin of the subpolar north atlantic is an important component of the deep limb of the meridional overturning near its northern origins a network of moored arrays from denmark strait to the tail of the grand banks has been installed for almost two decades to observe the boundary currents and transports of north atlantic deep water as part of an internationally coordinated observatory for the atlantic meridional overturning circulation the dominant variability in all of the moored velocity time series is in the week to month period range while the temporal characteristics of this variability change only gradually between denmark strait and flemish cap a broad band of longer term variability is present farther along the path of the dwbc at the grand banks and in the interior basins labrador and irminger seas the vigorous intra seasonal variability may well mask possible interannual to decadal variability that is typically an order of magnitude smaller than the high frequency fluctuations here the intra seasonal variability is quantified at key positions along the dwbc path using both observations and high resolution model data the results are used to evaluate the model circulation and in turn the model is used to relate the discrete measurements to the overall pattern of the subpolar circulation topographic waves are found to be trapped by the steep topography all around the western basins the labrador and irminger seas in the labrador sea the high intra seasonal variability of the boundary current regime is separated by a region of extremely low variability in narrow recirculation cells from the basin interior there the variability is also on intra seasonal timescales but at much longer periods around 50 days

spreading of denmark strait overflow water in the western subpolar north atlantic insights from eddy resolving simulations with a passive tracer abstractthe oceanic deep circulation is shared between concentrated deep western boundary currents dwbcs and broader interior pathways a process that is sensitive to seafloor topography this study investigates the spreading and deepening of denmark strait overflow water dsow in the western subpolar north atlantic using two eddy resolving atlantic simulations including a passive tracer injected into the dsow the deepest layers of dsow transit from a narrow dwbc in the southern irminger sea into widespread westward flow across the central labrador sea which remerges along the labrador coast this abyssal circulation in contrast to the upper levels of overflow water that remain as a boundary current blankets the deep labrador sea with dsow farther downstream after being steered around the abrupt topography of orphan knoll dsow again leaves the boundary forming cyclonic recirculation cells in the deep newfoundland basin the deep recirculation mostly driven by the meandering pathway of the up

decline of subpolar north atlantic circulation during the 1990s observations of sea surface height reveal that substantial changes have occurred over the past decade in the mid to high latitude north atlantic ocean topex poseidon altimeter data show that subpolar sea surface height increased during the 1990s and the geostrophic velocity derived from altimeter data exhibits declining subpolar gyre circulation combining the data from earlier satellites we find that subpolar circulation may have been weaker in the late 1990s than in the late 1970s and 1980s direct current meter observations in the boundary current of the labrador sea support the weakening circulation trend of the 1990s and together with hydrographic data show that the mid to late 1990s decline extends deep in the water column analysis of the local surface forcing suggests that the 1990s buoyancy forcing has a dynamic effect consistent with altimetric and hydrographic observations a weak thermohaline forcing allows the decay of the domed structure of subpolar isopycnals and weakening of circulation

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

breaking the linkage between labrador sea water production and its advective export to the subtropical gyre abstractdeep water formation in the northern north atlantic has been of long standing interest because the resultant water masses along with those that flow over the greenland scotland ridge constitute the lower limb of the atlantic meridional overturning circulation amoc which carries these cold deep waters southward to the subtropical region and beyond it has long been assumed that an increase in deep water formation would result in a larger southward export of newly formed deep water masses however recent observations of lagrangian floats have raised questions about this linkage motivated by these observations the relationship between convective activity in the labrador sea and the export of newly formed labrador sea water lsw the shallowest component of the deep amoc to the subtropics is explored this study uses simulated lagrangian pathways of synthetic floats produced with output from a global ocean sea ice model it is shown that substantial recirculation of newly formed lsw in the

from interannual to decadal 17 years of boundary current transports at the exit of the labrador sea over the past 17 years the western boundary current system of the labrador sea has been closely observed by maintaining the 53 n observatory moorings and shipboard station data measuring the top to bottom flow field offshore from the labrador shelf break volume transports for the north atlantic deep water nadw components were calculated using different methods including gap filling procedures for deployment periods with suboptimal instrument coverage on average the deep western boundary current dwbc carries 30 2 6 6 sv of nadw southward which are almost equally partitioned between labrador sea water lsw 14 9 3 9 sv and lower north atlantic deep water lnadw 15 3 3 8 sv the transport variability ranges from days to decades with the most prominent multiyear fluctuations at interannual to near decadal time scales 5 sv in the lnadw overflow water mass these long term fluctuations appear to be in phase with the nao modulated wind fluctuations the boundary current system off labrador occurs as a conglomerate of nearly independent components namely the shallow labrador current the weakly sheared lsw range and the deep baroclinic bottom intensified current core of the lnadw all of which are part of the cyclonic labrador sea circulation this structure is relatively stable over time and the 120 km wide boundary current is constrained seaward by a weak counterflow which reduces the deep water export by 10 15

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

atlantic meridional overturning circulation and the prediction of north atlantic sea surface temperature highlights north atlantic sea surface temperature exhibits high decadal predictability potential model bias hinders exploiting the decadal predictability potential an innovative method was developed to overcome some of the bias problem north atlantic sea surface temperature will stay anomalously warm until about 2030 abstract the atlantic meridional overturning circulation amoc a major current system in the atlantic ocean is thought to be an important driver of climate variability both regionally and globally and on a large range of time scales from decadal to centennial and even longer measurements to monitor the amoc strength have only started in 2004 which is too short to investigate its link to long term climate variability here the surface heat flux driven part of the amoc during 1900 2010 is reconstructed from the history of the north atlantic oscillation the most energetic mode of internal atmospheric variability in the atlantic sector the decadal variations of the amoc obtained in that way are shown to precede the observed decadal variations in basin wide north atlantic sea surface temperature sst known as the atlantic multidecadal oscillation amo which strongly impacts societally important quantities such as atlantic hurricane activity and sahel rainfall the future evolution of the amo is forecast using the amoc reconstructed up to 2010 the present warm phase of the amo is predicted to continue until the end of the next decade but with a negative tendency

studies of labrador sea water formation and variability in the subpolar north atlantic in the light of international partnership and collaboration abstract labrador sea water lsw the lightest contribution to north atlantic deep water nadw and one of the most prominent water masses of the subpolar north atlantic has seen remarkable changes over the past century lsw originates in the labrador sea where it is formed through wintertime ocean convection of varying intensity depth and spatial extent formation of lsw followed by its respective injection into the mid depth circulation system is mandatory for ventilating and renewing water layers of the interior ocean indispensably important for unraveling the history of variability in formation and properties of lsw as well as for mapping its large scale spreading and export are sustained physical and chemical observations from the deep ocean these observations started at the beginning of the 20th century from occasional mostly national surveys and today constitute large scale multi national collaborative efforts including a vast arsenal of sophisticated instrumentation in a historical context we revisit major milestones over the past 100 years which have established and are constantly adding to shaping today s knowledge on lsw and present first details on the latest vintage of lsw generated during the strong winter of 2013 2014 respective argo data reveal mixed layer depths greater than 1700 m marking formation of a new cold and fresh anomaly that has spread since then over the subpolar north atlantic we further summarize the on going observational efforts in the subpolar north atlantic and present a compilation of hydrographic standard lines that serve to provide top to bottom information on nadw components

advection of north atlantic deep water from the labrador sea to the southern hemisphere recently formed labrador seawater lsw and overflow water from denmark strait dsow are main components of the atlantic meridional overturning circulation both exhibit a distinct chlorofluorocarbon cfc maximum here we use 25 years of cfc observations in the atlantic to study the main features of the circulation of lsw and dsow from the cfc data the age and fraction of young deep water are inferred due to the superior spatial data resolution compared to former attempts regional differences in the spreading velocity and pathways of young deep water become evident dependent on the regional circulation the observed distributions of young lsw and dsow showed that the dwbc is the fastest pathway to reach the southern hemisphere the downstream decrease of the fractions of young lsw in the dwbc is slower compared to model studies from 47 n to 42 n dwbc transports of young lsw and dsow decrease by 44 and 49 respectively at 26 n the dwbc transport of young water is still 39 of the lsw formation rate and 44 of the dsow overflow transport interior pathways also exist especially in the subpolar north atlantic and in the transition zone between the subpolar and subtropical gyre compared to dsow the distributions indicate a higher tendency for lsw to follow additional interior pathways north of 45 n the major part of lsw is younger than 20 years the general weakening of new lsw formation since the 1990s worked toward a homogenization between the lsw in the western and the eastern subpolar north atlantic

multiple equilibria as a possible mechanism for decadal variability in the north atlantic ocean decadal climate variability in the north atlantic has received increased attention in recent years because modeling results suggest predictability of heat content and circulation indices several years ahead however determining the applicability of these results in the real world is challenging because of an incomplete understanding of the underlying mechanisms here the authors show that recent attempts to reconstruct the decadal variations in one of the dominant circulation systems of the region the subpolar gyre spg are not always consistent a coherent picture is partly recovered by a simple conceptual model solely forced by reanalyzed surface air temperatures this confirms that surface heat flux indeed plays a leading role for this typeofvariability ashasbeensuggestedinpreviousstudies theresultsfurthersuggestthatlargevariationsin the spg correspond to the crossing of a bifurcation point that is predicted from idealized experiments and an analytical solution of the model used herein performance of this conceptual model is tested against a statistical stochastic model hysteresis and the existence of two stable modes of the spg circulation shape its response to forcing by atmospheric temperatures the identification of the essential dynamics and the reduction to a minimal model of spg variability provide a quantifiable basis and a framework for future studies on decadal climate variability and predictability

interannual transport variability of upper labrador sea water at flemish cap the transport of upper labrador sea water ulsw at flemish cap 47 n 45 w is investigated in the period 1960 2009 using the output from an 8 km resolution numerical ocean model the average model transport of ulsw decreases southward from 6 7 sv at 53 n to 4 5 sv at 45 n due to interior pathways the largest fraction of the total ulsw volume transport goes around flemish cap within the deep western boundary current dwbc 72 but a significant part goes through flemish pass 20 at interannual timescales the variability at flemish pass shows a distinct behavior when compared to the variability in the dwbc and to the upstream fluctuations a running correlation method is applied to obtain the connection of the transport variability at flemish pass with several quantities representative for different physical mechanisms 1 the north atlantic oscillation index 2 the ekman transport 3 the rate of ulsw formation in the labrador sea 4 the position of the north atlantic current nac relative to the slope and 5 the averaged transport in the subpolar gyre weakened or strengthened transport of ulsw through flemish pass coincides with changes of the atmospheric forcing or with changes of the nac s position strong meandering of the nac close to the dwbc reduces the transport off flemish cap and the ulsw flow is redirected into the flemish pass enhancing the transport there in contrast the transport variability in the dwbc is mainly caused by upstream fluctuations and changes according to the rate of ulsw formation

comments on u th he dating of fluorite application to the la azul fluorspar deposit in the taxco mining district mexico by pi et al mineralium deposita 39 976 982

simulated pathways of the overflow waters in the north atlantic subpolar to subtropical export motivated by previous observational and modeling studies that demonstrate interior pathways for the export of labrador sea water lsw into the subtropical north atlantic we analyze whether the deeper denser overflow water ow pathways are similarly distributed in the basin for this study we rely on the simulation of ow pathways in the context of an ocean general circulation model from our analysis we suggest that ow in the north atlantic is not restricted to the dwbc in its equatorward transport thus a measure of ow transport variability within the dwbc downstream of the subpolar gyre cannot be safely equated to transport variability of the lower limb of the amoc since this study of ow export pathways relies solely on simulated pathways the applicability of our results to the north atlantic rests on the representativeness of the model s deep flow field

seasonal and interannual variations of irminger ring formation and boundary interior heat exchange in flame abstractthe contribution of warm core anticyclones shed by the irminger current off west greenland known as irminger rings to the restratification of the upper layers of the labrador sea is investigated in the 1 12 family of linked atlantic models experiment flame model the model output covering the 1990 2004 period shows strong similarities to observations of the irminger current as well as ring observations at a mooring located offshore of the eddy formation region in 2007 09 an analysis of fluxes in the model shows that while the majority of heat exchange with the interior indeed occurs at the site of the irminger current instability the contribution of the coherent irminger rings is modest 18 heat is provided to the convective region mainly through noncoherent anomalies and enhanced local mixing by the rings facilitating further exchange between the boundary and interior the time variability of the eddy kinetic energy and the boundary to interior heat flux in the model are strongly corr

origin of rhyolitic lavas in the mesa central mexico by crustal melting related to extension the emplacement of a voluminous sequence of rhyolitic lava flows and domes characterizes oligocene volcanism in the mesa central mc of mexico its dominant effusive style of emplacement contrasts deeply with the predominantly explosive volcanism of the sierra madre occidental volcanic province toward the west whole rock geochemical major and trace elements and sr nd isotopic data of the mc oligocene rhyolitic lavas document a marked change in magma composition at around 30 ma allowing us to distinguish between a lower and an upper sequence lavas from the lower sequence are geochemically similar to the high k rhyolitic rocks of the eastern sierra madre occidental major and trace element variations are characteristic of mantle derived magmas evolving through fractional crystallization the initial 87 sr 86 sr and e nd values are nearly constant 0 70644 0 70770 and 1 2 to 2 1 respectively and indicate some contribution from crustal material lavas from the upper sequence are high silica peraluminous rhyolites with strong enrichment in fluorine and in some incompatible lithophile elements rb la sm yb y th u nb ta and strong depletion in the feldspar compatible elements sr ba eu initial 87 sr 86 sr ratios of the upper sequence lavas are high and variable 0 70812 0 72190 and decrease as silica content increases whereas the e nd values are relatively constant 1 4 to 2 8 the trace element behavior indicates an origin by variable degrees of non modal partial melting of granulitic low crustal rocks and chemical disequilibrium during melting processes the high and variable sr isotopic ratios could also be related to isotope disequilibrium melting processes if the isotopic heterogeneities between individual mineral phases were preserved during heating of the source rocks the changes in geochemical compositions are related to the onset of crustal extension at high strain rates documented for the mc crustal extension promoted crustal melting at high melting rates high melt segregation rates rapid ascent of low viscosity fluorine rich magmas and inhibited melt stagnation in magma chambers such conditions favored the effusive volcanic style and support the possibility of melting under disequilibrium conditions

variability of labrador sea water transported through flemish pass during 1993 2013 flemish pass located at the western subpolar margin is a passage sill depth 1200 m that is constrained by the grand banks and the underwater plateau flemish cap in addition to the deep western boundary current dwbc pathway offshore of flemish cap flemish pass represents another southward transport pathway for two modes of labrador sea water lsw the lightest component of north atlantic deep water carried with the dwbc this pathway avoids potential stirring regions east of flemish cap and deflection into the interior north atlantic ship based velocity measurements between 2009 and 2013 at 47 n in flemish pass and in the dwbc east of flemish cap revealed a considerable southward transport of upper lsw through flemish pass 15 27 1 0 to 1 5 sv about 98 of the denser deep lsw were carried around flemish cap as flemish pass is too shallow for considerable transport of deep lsw hydrographic time series from ship based measurements show a significant warming of 0 3 c decade and a salinification of 0 03 decade of the upper lsw in flemish pass between 1993 and 2013 almost identical trends were found for the evolution in the labrador sea and in the dwbc east of flemish cap this indicates that the long term hydrographic variability of upper lsw in flemish pass as well as in the dwbc at 47 n is dominated by changes in the labrador sea which are advected southward fifty years of numerical ocean model simulations in flemish pass suggest that these trends are part of a multidecadal cycle

variability of the directly observed middepth subpolar north atlantic circulation satellite views of the ocean have suggested a decline of the subpolar north atlantic surface circulation during the 1990s and 2000s this was a period of unprecedented observational capacity in the basin thanks to the presence of many hundreds of profiling floats we use more than 40 000 subsurface displacements of these floats to characterize the circulation at 1000 m depth and its evolution from 1997 2013 we show a statistically significant slowdown in the labrador sea boundary currents of 0 8 cm s 1 per decade 95 confidence interval of 1 4 to 0 15 cm s 1 per decade a conservative estimate of the uncertainty otherwise the middepth circulation field was largely stable our analysis of the location where the north atlantic current crosses the mid atlantic ridge shows that profiling floats can reveal steering by bathymetric features but do not reveal of decadal variability in the position where the current crosses the ridge

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

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

the deep western boundary current in the labrador sea from observations and a high resolution model long term observations from a 17 year long mooring array at the exit of the labrador sea at 53 n are compared to the output of a high resolution model viking20 both are analyzed to define robust integral properties on basin and regional scale which can be determined and evaluated equally well while both the observations and the model show a narrow dwbc cyclonically engulfing the labrador sea the model s boundary current system is more barotropic than in the observations and spectral analysis indicates stronger monthly to interannual transport variability compared to the model the observations show a stronger density gradient hence a stronger baroclinicity from center to boundary despite this the observed temporal evolution of the temperature in the central labrador sea is reproduced the model results yield a mean export of north atlantic deep water nadw 33 0 5 7 sv which is comparable to the observed transport 31 2 5 5 sv at 53 n the results also include a comparable spatial pattern and march mixed layer depth in the central labrador sea maximum depth 2000 m during periods containing enhanced deep convection 1990 s our analyses show increased correlation between lsw and lnadw model transport at 53 n our results indicate that the transport variability in lsw and lnadw at 53 n is a result of a complex modulation of wind stress and buoyancy forcing on regional and basin wide scale

tracking labrador sea water property signals along the deep western boundary current observations of the deep western boundary current dwbc at line w on the western north atlantic continental slope southeast of cape cod from 1995 to 2014 reveal water mass changes that are consistent with changes in source water properties upstream in the labrador sea this is most evident in the cold dense and deep class of labrador sea water dlsw that was created and progressively replenished and deepened by recurring winter convection during the severe winters of 1987 1994 the arrival of this record cold fresh and low potential vorticity anomaly at line w lags its formation in the labrador sea by 3 7 years complementary observations along the path of the dwbc provide further evidence that this anomaly is advected along the boundary and indicate that stirring between the boundary and the interior intensifies south of the flemish cap finally the consistency of the data with realistic advective and mixing time scales is assessed using the waugh and hall 2005 model framework the data are found to be best represented by a mean transit time of 5 years from the labrador sea to line w with a leading order role for both advection by the dwbc and mixing between the boundary flow and interior waters

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

how much has the north atlantic ocean overturning circulation changed in the last 50 years abstractvolume transports from six ocean reanalyses are compared with four sets of in situ observations across the greenland scotland ridge gsr in the labrador sea boundary current in the deep western boundary current at 43 n and in the atlantic meridional overturning circulation amoc at 26 n in the north atlantic the higher resolution reanalyses on the order of are better at reproducing the circulation pattern in the subpolar gyre than those with lower resolution on the order of 1 simple ocean data assimilation soda and estimating the circulation and climate of the ocean ecco jet propulsion laboratory jpl produce transports at 26 n that are close to those observed 17 sv 1 sv 106 m3 s 1 ecco version 2 and soda produce northward transports across the gsr observed transport of 8 2 sv that are 22 and 29 too big respectively by contrast the low resolution reanalyses have transports that are either too small by 31 for ecco jpl and 49 for ocean reanalysis system

an estimate of the eddy induced circulation in the labrador sea regions of oceanic deep convection such as the labrador sea are prone to baroclinic instability the resulting geostrophic eddies play a crucial role in the post convection adjustment process which involves both rearrangement of mass so as to release available potential energy and exchange of heat and salt with the boundaries in this study it is proposed that the slumping of isopycnals associated with baroclinic instability drives an eddy induced overturning circulation consisting of a surface intensified flow transporting low salinity water from the boundary currents into the interior sinking motion in the interior and an outflow at depth transporting newly ventilated labrador sea water towards the boundaries typical eddy induced velocities estimated from hydrographic data are roughly 0 5 cm s for the surface inflow 1 m day for the vertical motion and 0 1 cm s for the deeper outflow in close agreement with those calculated in a numerical model

detecting labrador sea water formation from space 1 in situ monitoring of deep water formation in the labrador sea is severely hampered by the harsh winter conditions in this area furthermore the ongoing monitoring programs do not cover the entire labrador sea and are often summer observations the network of satellite altimeters does not suffer from these limitations and could therefore give valuable additional information altimeters can in theory detect deep water formation because the water column becomes denser during convection and therefore the sea surface becomes lower this signal is small compared to variability in sea surface height induced by other sources but when properly filtered and appropriately averaged in time and space all four winters with labrador sea water formation or renewal in the 1994 2009 period 1994 1995 2000 and 2008 have a clear large negative anomaly the magnitude of this anomaly compares favorably with the range predicted by theory and in situ data analysis out of 16 winters only one winter 2006 would be falsely identified as a deep convection winter based on its sea surface height anomaly signal while the method did not miss a single deep convection winter for most deep water formation winters even the spatial structure of the mixed layer depth distribution can be inferred

on the linkage between labrador sea water volume and overturning circulation in the labrador sea a case study on proxies abstractalthough proxies have generally been used to study deep ocean convection and overturning circulation in the labrador sea their efficacy has not been explicitly evaluated because observations that directly measure those variables are scarce in this study the volume of newly formed labrador sea water lsw and the overturning circulation in the labrador sea are estimated using observational data and output from a high resolution ocean model and then compared to proxies used to represent those variables the comparisons reveal the limitations of proxies highlighting the desirability of robust estimates derived from direct monitoring in the region i e from argo and overturning in the subpolar north atlantic program osnap a linkage among lsw formation overturning circulation in the labrador sea and the export of lsw from the basin on interannual time scales is found in the model

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

assessing variability in the size and strength of the north atlantic subpolar gyre recent studies on the size and strength of the north atlantic subpolar gyre spg offer contrasting assessments of the gyre s temporal variability studies that use empirical orthogonal function eof analyses of satellite sea surface height ssh report a rapid decline in spg size and strength since 1992 20 per decade while concurrent in situ observations report either no trend or a slight decline here we investigate this discrepancy by analyzing the size and strength of the spg with satellite ssh from 1993 to 2015 with two separate methods indirectly via eof analysis and more directly through measurements of the gyre center and boundary we define the boundary of the gyre as the largest closed contour of ssh the center as the minimum ssh and the strength as the difference between the ssh at the boundary and the center we identify a linear decline over the study period in the spg strength 5 1 per decade but find no statistically significant trend in the spg area the trend in the gyre strength is weaker than the eof based trend and is most likely below the level of detection of the in situ measurements we conclude that the variability previously identified as a sharp decline in spg circulation can be more appropriately attributed to basin wide sea level rise during the satellite altimetry period in addition we find that the properties of the eastern spg do not co vary with the spg size suggesting that spg dynamics do not control the strength of the inter gyre throughput

dolomite formation within microbial mats in the coastal sabkha of abu dhabi united arab emirates microbial mediation is the only demonstrated mechanism to precipitate dolomite under earth surface conditions a link between microbial activity and dolomite formation in the sabkha of abu dhabi has until now not been evaluated even though this environment is cited frequently as the type analogue for many ancient evaporitic sequences such an evaluation is the purpose of this study which is based on a geochemical and petrographic investigation of three sites located on the coastal sabkha of abu dhabi along a transect from the intertidal to the supratidal zone this investigation revealed a close association between microbial mats and dolomite suggesting that microbes are involved in the mineralization process observations using scanning electron microscopy equipped with a cryotransfer system indicate that authigenic dolomite precipitates within the exopolymeric substances constituting the microbial mats in current models microbial dolomite precipitation is linked to an active microbial activity that sustains high ph and alkalinity and decreased sulphate concentrations in pore waters such models can be applied to the sabkha environment to explain dolomite formation within microbial mats present at the surface of the intertidal zone by contrast these models cannot be applied to the supratidal zone where abundant dolomite is present within buried mats that no longer show signs of intensive microbial activity as no abiotic mechanism is known to form dolomite at earth surface conditions two different hypotheses can reconcile this result in a first scenario all of the dolomite present in the supratidal zone formed in the past when the mats were active at the surface in a second scenario dolomite formation continues within the buried and inactive mats in order to explain dolomite formation in the absence of active microbial metabolisms a revised microbial model is proposed in which the mineral template properties of exopolymeric substances play a crucial role

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

interannual to decadal variability of outflow from the labrador sea a decade of weak convection in the labrador sea associated with decreasing water mass transformation in combination with advective and eddy fluxes into the convection area caused significant warming of the deep waters in both the central labrador sea and boundary current system along the labrador shelf break the connection to the export of deep water was studied based on moored current meter stations between 1997 and 2009 at the exit of the labrador sea near the shelf break at 5 3n more than 100 year long current meter records spanning the full water column have been analyzed with respect to high frequency variability decaying from the surface to the bottom layer and for the annual mean flow showing intra to interannual variability but no detectable decadal trend in the strength of the deep and near bottom flow out of the labrador sea

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

changes in the deep western boundary current at 53 n abstractsouthward transports in the deep western boundary current across 53 n over 1949 99 are determined from a historical reconstruction long term mean transports for given water masses for net southward transport the southward component of the transport not including recirculation given in parentheses are 4 7 2 3 sv 5 1 2 4 sv sv 106 m3 s 1 for the denmark strait overflow water 6 1 2 7 sv 6 8 1 7 sv for the iceland scotland overflow water 6 5 2 6 sv 7 1 1 8 sv for classical labrador sea water and 2 3 1 9 sv 2 7 3 4 sv for upper labrador sea water the estimates take into account seasonal and interannual variability of the isopycnal positions and suggest the importance of including this factor a strong correlation 0 91 is found between variability of the total and baroclinic transports with the barotropic velocity removed at the annual time scale this correlation drops to 0 32 if the baroclinic transports are instead computed based upon the use of a fixed le

on the currents and transports connected with the atlantic meridional overturning circulation in the subpolar north atlantic results from an interannually forced 0 08 degrees eddy resolving simulation based on the hybrid coordinate ocean model in conjunction with a small but well determined transport database are used to investigate the currents and transports associated with the atlantic meridional overturning circulation amoc in the subpolar north atlantic spna the model results yield a consistent warming in the western spna since the early 1990s along with mean transports similar to those observed for the trans basin amoc across the world ocean circulation experiment hydrographic section ar19 16 4 sv and boundary currents at the exit of the labrador sea near 53 degrees n 39 0 sv and east of the grand banks near 43 degrees n 15 9 sv over a 34 year integration the model determined amoc across the ar19 section and the western boundary current near 53 degrees n both exhibit no systematic trend but some long term interannual and longer variabilities including a decadal transport variation of 3 4 sv from relatively high in the 1990s to low in the 2000s the decadal variability of the model boundary current transport near 53 degrees n lags the observed winter time north atlantic oscillation index by about 2 years and leads the model amoc across the ar19 section by about 1 year the model results also show that the long term variabilities are low compared to those on shorter time scales thus rapid sampling of the current over long time intervals is required to filter out high frequency variabilities in order to determine the lower frequency variabilities of interest