mechanisms governing the development of the north atlantic warming hole in the cesm le future climate simulations abstracta warming deficit in north atlantic sea surface temperatures is a striking feature in global climate model future projections this north atlantic warming hole has been related to a slowing

latitudinal structure of the meridional overturning circulation variability on interannual to decadal time scales in the north atlantic ocean abstractthe latitudinal structure of the atlantic meridional overturning circulation amoc variability in the north atlantic is investigated using numerical results from three ocean circulation si

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

the relationship between u s east coast sea level and the atlantic meridional overturning circulation a review scientific and societal interest in the relationship between the atlantic meridional overturning circulation amoc and u s east coast sea level has intensified over the past decade largely due to 1 projected and potentially ongoing enhancement of sea level rise associated with amoc weakening and 2 the potential for observations of u s east coast sea level to inform reconstructions of north atlantic circulation and climate these implications have inspired a wealth of model and observation based analyses here we review this research finding consistent support in numerical models for an antiphase relationship between amoc strength and dynamic sea level however simulations exhibit substantial along coast and intermodel differences in the amplitude of amoc associated dynamic sea level variability observational analyses focusing on shorter generally less than decadal timescales show robust relationships between some components of the north atlantic large scale circulation and coastal sea level variability but the causal relationships between different observational metrics amoc and sea level are often unclear we highlight the importance of existing and future research seeking to understand relationships between amoc and its component currents the role of ageostrophic processes near the coast and the interplay of local and remote forcing such research will help reconcile the results of different numerical simulations with each other and with observations inform the physical origins of covariability and reveal the sensitivity of scaling relationships to forcing timescale and model representation this information will in turn provide a more complete characterization of uncertainty in relevant relationships leading to more robust reconstructions and projections

changes in the pool of labrador sea water in the subpolar north atlantic 1 measurements of chlorofluorocarbon inventories during 1997 2003 allow the detection and quantification of significant changes in the formation rates of two modes of labrador sea water lsw upper ulsw and deep lsw both here defined in fixed density intervals both modes contribute to the cold limb of the meridional overturning circulation results reveal that the lighter ulsw formed since the mid 1990s has started to replace the large pool of the deep lsw stored in the western north atlantic formation of deep lsw was absent in 1997 2003 formation of ulsw compensated for this absence during 1998 99 7 9 sv but afterwards significantly declined to 2 5 sv the decrease of the overall lsw formation throughout 1997 2003 correlates with a declining eastward baroclinic mass transport between the centers of the subpolar and subtropical gyres since 1997 a warming of lsw and a gradually decreasing north atlantic oscillation index after 1999

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

connecting large scale atmospheric circulation river flow and groundwater levels in a chalk catchment in southern england summary groundwater is an important water resource and globally it represents the largest distributed store of freshwater in southern england groundwater is a major source for public water supply and many aquifers have recently experienced both extreme low and high groundwater levels in this paper we use observations of precipitation river discharge and groundwater levels 1964 2010 and an atmospheric reanalysis to explore the large scale climate patterns preceding the nine highest and lowest march river discharge and groundwater levels in the chalk catchment of the river lambourn berkshire downs southern england peak monthly precipitation is shown to occur from october to january while the highest river discharge and groundwater levels are found from february to april for high discharge groundwater levels composite anomaly patterns of the mean sea level pressure show a stronger than average pressure gradient across the north atlantic ocean with enhanced water vapour transport across southern england for the lowest discharge groundwater levels a blocking high pressure system is found across the british isles deflecting storms and precipitation to the north significantly the intra composite variability suggests that different sequences of atmospheric states may lead to high and low discharge groundwater events

circulation and deep water export at the western exit of the subpolar north atlantic the current system east of the grand banks was intensely observed by world ocean circulation experiment woce array acm 6 during 1993 95 with eight moorings reaching about 500 km out from the shelf edge and covering the water column from about 400 m depth to the bottom more recently a reduced array by the institut fur meerskunde ifm at kiel germany of four moorings was deployed during 1999 2001 focusing on the deep water flow near the western continental slope both sets of moored time series each about 22 months long are combined here for a mean current boundary section and both arrays are analyzed for the variability of currents and transports a mean hydrographic section is derived from seven ship surveys and is used for geostrophic upper layer extrapolation and isopycnal subdivision of the mean transports into deep water classes the offshore part of the combined section is dominated by the deep reaching north atlantic current nac with currents still at 10 cm s 1 near the bottom and a total northward transport of about 140 sv sv 106 m3 s 1 with the details depending on the method of surface extrapolation used the mean flow along the western boundary was southward with the section mean north atlantic deep water outflow determined to be 12 sv below the 27 74 kg m 3 isopycnal however east of the deep western boundary current dwbc the deep nac carries a transport of 51 sv northward below 27 74 kg m 3 resulting in a large net northward flow in the western part of the basin from watermass signatures it is concluded that the deep nac is not a direct recirculation of dwbc water masses transport time series for the dwbc variability are derived for both arrays the variance is concentrated in the period range from 2 weeks to 2 months but there are also variations at interannual and longer periods with much of the dwbc variability being related to fluctuations and meandering of the nac a significant annual cycle is not recognizable in the combined current and transport time series of both arrays the moored array results are compared with other evidence on deep outflow and recirculation including recent models of different types and complexity

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

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

geochemistry of precambrian mafic magmatic rocks of the western himalaya india petrogenetic and tectonic implications abstract precambrian sequences of the higher himalayan crystallines vaikrita group and the lesser himalaya chail jutogh and jaunsar groups in the garhwal and himachal regions of the western himalaya include abundant metamorphosed mafic lavas and dykes amphibolites a gabbroic body within the chail group has been dated at 1907 91 ma initial 87 sr 86 sr ratio of 0 7022 0 0008 by the whole rock rb sr method this age is consistent with several age data 1800 2000 ma whole rock rb sr method assigned to the granitoids and gneisses of this region these amphibolites and gabbros exhibit low ti tholeiite characteristics the lesser himalayan samples are enriched in light rare earth elements lree and large ion lithophile element lile with distinct negative nb sr p and ti anomalies conversely the vaikrita group samples are characterized by less enriched lree lile and absence of the above anomalies but have distinct positive sr anomalies the chemical characteristics of the lesser himalayan samples are remarkably similar to the basal aravalli volcanics of the nw indian shield inferred to reflect significant components from enriched mantle sources and crustal contamination samples of the vaikrita group appear to have been influenced by an asthenospheric mantle and are less contaminated as indicated by the absence of negative nb p and ti anomalies these distinct geochemical characteristics are used to demarcate the main central thrust along the vaikrita thrust the chail and jutogh groups are affiliated with the lesser himalaya and the chail group gabbro body may represent portion of one among the numerous magma chambers that fed a large precambrian magmatic province in the south of the main central thrust the similar emplacement ages 1800 2000 ma for the mafic and felsic magmatic rocks suggest that this region experienced a major episode of crustal generation and evolution in a rift environment

hydrological responses of the chalk aquifer to the regional climatic signal piezometric variability was investigated using monthly time series from 40 piezometers in different areas across the upper normandy region over the 1968 2009 period the piezometers were classified according to their hydrological behaviour this paper aimed at characterising the dominant modes explaining piezometric variability and at investigating the causes of this variability significant components in the piezometric signals were identified the annual cycle and an interannual mode 4 12 years with different variances for each observation borehole group the influence of the morphostructural organisation of the upper normandy chalk plateau on the modalities of the water level variability in the chalk aquifer was defined the relative contributions and standard deviations of each component found in the signal time series of the groundwater level was estimated the highest values were recorded in the northern study area and clay with flints and aquifer thicknesses influence the transfer on an annual scale while the loess thickness influences the transfer on a multi year scale

impact of labrador sea convection on the north atlantic meridional overturning circulation abstract the overturning and horizontal circulations of the labrador sea are deduced from a composite vertical section across the basin the data come from the late spring early summer occupations of the world ocean circulation experiment woce ar7w line during the years 1990 97 this time period was chosen because it corresponded to intense wintertime convection the deepest and densest in the historical record suggesting that the north atlantic meridional overturning circulation moc would be maximally impacted the composite geostrophic velocity section was referenced using a mean lateral velocity profile from float data and then subsequently adjusted to balance mass the analysis was done in depth space to determine the net sinking that results from convection and in density space to determine the diapycnal mass flux i e the transformation of light water to labrador sea water the mean overturning cell is calculated to be 1 sv 1 sv 106 m3 s 1 as compared with a horizontal gyre of 18 sv the

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

observed and modeled pathways of the iceland scotland overflow water in the eastern north atlantic abstract the spreading of iceland scotland overflow water isow in the eastern north atlantic has largely been studied in an eulerian frame using numerical models or with observations limited to a few locations no study to date has provided a comprehensive description of the isow spreading pathways from both eulerian and lagrangian perspectives in this paper we use a combination of previously unreported current meter data hydrographic data rafos float data and a high resolution 1 12 numerical ocean model to study the spreading pathways of isow from both of these perspectives we identify three isow transport cores in the central iceland basin 59 n with the major core along the eastern boundary of the reykjanes ridge rr and the other two in the basin interior based on trajectories of observed and or numerical floats seeded along 59 n we also describe the isow spreading pathways and quantify their relative importance within 10 years 7 11 of isow from 59 n escapes into the irminger sea via gaps in the rr north of the charlie gibbs fracture zone cgfz the water that moves through these gaps principally originates from the shallower isow layer along the rr eastern boundary 10 13 travels further southward until the cgfz where it crosses westward into the western subpolar gyre 18 21 of isow spreads southward along the eastern flank of the mid atlantic ridge into the western european basin web most of the remaining water stays in the iceland basin over the 10 year period a model based investigation provides a first look at the temporal variability of these isow pathways we find that the fraction of southward water exported into the web is anti correlated with the export through the cgfz a result assumed to reflect these pathways interactions with the north atlantic current in magnitude and or position shift

strontium isotopes as tracers of groundwater induced floods the somme case study france major ions trace elements and o h and sr isotopes have been determined for surface water and groundwater in the somme catchment northern france the present study using a coupled hydrological and geochemical stable and sr isotopes approach focuses on the interactions between ground and surface water stable water isotopes 18o 2h show that all waters have a purely local origin from precipitation without significant evaporation or water rock interaction as all points plot close to the general meteoric water line water chemistry in the different catchments shows large variations in major element contents plotting na vs cl contents and mg2 no3 k so42 sr2 concentrations reveals a slight variations in groundwater composition b agricultural input into surface waters and c some similarities between ground and surface waters suggesting the former to be responsible for the chemical signature of the dissolved river load strontium isotope ratios measured in groundwater range from 0 70769 to 0 70808 clearly higher than that of chalk matrix water 0 707337 variations of sr isotope signatures in groundwater from the chalk aquifer are significant and help identifying different groundwater bodies the different signatures in groundwater are better explained by matrix heterogeneity which can present lateral facies variations than by a strong anthropogenic influence the sr isotope compositional range in surface waters is very wide varying from 0 70775 to 0 70816 and defining a trend towards a more radiogenic sr isotope signature with high sr contents and high mg sr ratios the relationship between 87sr 86sr and the mg sr ratios allows different end members to be defined the rivers are influenced by anthropogenic disturbance even during flood periods and river water has geochemical characteristics very similar to those of groundwater indicating a strong influence from groundwater feeding the rivers

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

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

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

dynamics of river aquifer interactions along a chalk stream the river lambourn uk this paper describes the spatial and temporal pattern of groundwater flow accretion to the river lambourn a 234 km2 chalk catchment of the west berkshire downs uk which has been largely unaffected by groundwater abstraction variations in the discharge measured at four fixed gauges in the catchment coupled with information on the length of flowing channel over the period 1983 2001 are used to describe regional patterns in flow accretion mean catchment accretion generally exceeds 0 15 m3 s 1 km 1 but there are significant differences between perennial reaches indicating how the combination of local structural controls and seasonal changes in the drainage net affect flow accretion data from current meter surveys were used to determine the spatial variability in flow accretion 505 paired observations along 12 reaches between 1 and 2 95 km in length indicated a consistent spatial trend in accretion accretion was high in upstream and downstream channel reaches and in middle reaches where dry valleys intersected the main valley a flow accretion index was developed to describe the relationship of flow accretion in each of the 12 study reaches to catchment discharge the relationship varied from a strong positive correlation with catchment discharge two reaches a weak positive correlation three reaches a strong negative correlation two reaches to no relationship to catchment discharge four reaches the results highlight the need to reconsider the usual assumption of uniform or uniformly increasing flow accretion in chalk catchments moreover they emphasize the importance of catchment topography and illustrate how flow accretion in individual reaches may vary between high and low groundwater levels

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

advances in modelling groundwater behaviour in chalk catchments groundwater in chalk catchments is a major resource that also helps support internationally important habitats and ecosystems its dual porosity and dual permeability properties coupled with large scale structural features such as hard rock layers and marls produce a highly complex hydrogeological system recent impacts from groundwater flooding as well as vulnerability to drought have raised questions over the ability of traditional approaches to model these aquifers current work on near surface hydrological processes has highlighted the importance of the soil and weathered zone for controlling recharge rates in addition karst like features sedimentary deposits and valley bottom processes govern stream aquifer interaction and present a challenge in their representation in any modelling system methods that have and are being developed to incorporate these features and their use in modelling chalk catchments are described these are required in order to address major challenges such as groundwater flooding and drought impacts both of which could become more frequent and intense as a result of climate change

river flow teleconnections across the northern north atlantic region 1 correlation of monthly inter regional river flow and composite analysis of large scale climatic patterns associated with high and low monthly regional river flow are presented for the northern north atlantic domain for 1968 1997 hydrological regions are defined using cluster analysis adjacent hydrological regions generally display year round positive correlation of river flow for the first time inverse river flow relationships are identified between north america and europe in autumn and between northern and southern regions of northern europe from april may and july december composite analysis shows linkages between northern european river flow and the icelandic low azores high and intervening pressure gradient river flow in northeastern north america is more closely associated with the east coast pressure trough trans atlantic hydrological teleconnections appear related to the north atlantic oscillation with the inverse meridional relationship in northern europe linked in part to the scandinavian teleconnection pattern

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

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

palaeomagnetic estimates of crustal shortening in the himalayan thrusts and zangbo suture this article reports new palaeomagnetic results from precisely dated late palaeocene limestones from the former northern margin of the indian subcontinent now in tibet between the major himalayan thrusts and the india tibet suture zone convergence in the suture and intracontinental thrusting can be estimated separately and are respectively found to be of the order of 550 km 650 km and 400 km 400 km comparison with kinematic models derived from magnetic anomalies constrains the age of the collision at about 50 myr

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

the effect of temperature on metal mobility in subseafloor hydrothermal systems constraints from basalt alteration experiments abstract crystalline basalt was reacted with a na ca k cl fluid of seawater chlorinity at 400 350 and 300 c 400 bar and a fluid rock mass ratio of 1 to assess the effect of temperature on metal mobility in ridge crest hydrothermal systems basalt alteration at 400 c yielded a fluid chemistry characterized by high zn cu fe mn sio 2 h 2 s and h 2 concentrations lowering temperature to 350 and then to 300 c caused dissolved metal concentrations to decrease drastically black smoker vent fluids generally have measured temperatures 355 c the dissolved concentrations of zn cu fe and mn observed in these fluids however could only be achieved or exceeded during the 400 c phase of the experiment indicating that hot spring fluids have lost heat and possibly dissolved fe and base metals on ascent to the seafloor from a high temperature reaction zone the relative rates at which zn cu fe and mn achieve new steady state concentrations in response to temperature reduction during basalt alteration are cu fe zn mn thus dissolved zn and especially mn concentrations are more likely to record accurately temperatures characterizing subseafloor reaction zones while fe and cu which respond rapidly to changes in temperature are likely to precipitate along fluid flow paths especially in near seafloor locations this trend is confirmed by the relative abundance of dissolved metals in hot spring fluids variably influenced by heat loss processes calculations suggest that hot spring fluids may lose heat by conduction through sulfide mounds and chimney walls at flow rates observed for many vent systems since heat loss processes influence the chemistry especially dissolved metals as well as measured temperatures of axial hot springs fluids these effects must be taken into account to model accurately phase relations and fluid mineral equilibria in deeply seated subseafloor reaction zones

lagrangian observations of the middepth and deep velocity fields of the northeastern atlantic ocean the circulation of the northeastern atlantic ocean at intermediate depths is characterized by watermass transformation processes that involve iceland scotland overflow water isow from the northeast labrador sea water lsw from the west and mediterranean water from the south field observations were carried out with 89 eddy resolving floats rafos and marvor types the data coverage achieved is remarkably high and enables a comprehensive study of the eastern basins between iceland and the azores the trajectories show typical pathways of the water masses involved and the role that the complex bottom topography plays in defining them the isow paths tend to lean against the slopes of the reykjanes ridge and rockall plateau westward escapes through multiple gaps in the ridge are possible superimposed on a sustained southward flow in the eastern basin along the mid atlantic ridge lsw pathways leading to the eastern basins are subject to high variability in flow direction and eddy activity in addition to a selection of characteristic trajectories maps of the horizontal distributions of lagrangian eddy kinetic energy and integral time scales are presented these reveal distinct areas of intensified mixing in the iceland basin as well as the sharp contrast between the subpolar and subtropical dynamics a self contained eddy detection scheme is applied to obtain statistics on individual eddy properties and their abundance it is suggested that much of the intensified mixing can be related to cyclonic activity particularly in the subpolar region

the arrival of recently formed labrador sea water in the deep western boundary current at 26 5 n the deep western boundary current dwbc of the north atlantic is a principal conduit between the formation region for labrador sea water lsw and the oceanic interior to the south time series 1985 1997 of hydrographic properties obtained in the dwbc at 26 5 n show that prior to 1994 temperature salinity and transient tracer properties within the lsw density range showed little indication of recently formed parcels properties characteristic of a newer version of lsw cooler fresher and higher tracer concentrations were observed beginning in 1994 and continuing through 1997 longer time series of temperature and salinity developed from a regional data base show both the 1994 and a 1980 1981 event in the abaco region both events are consistent with anomalies in the labrador sea that occurred some 10 years earlier the 10 year transit time from the labrador sea to 26 5 n is less than the 18 year transit time inferred from earlier studies

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

modelling the effects of climate change and its uncertainty on uk chalk groundwater resources from an ensemble of global climate model projections projections of climate for the 2080s from an ensemble of global climate models gcms run under a medium high a2 emissions scenario are used to simulate changes in groundwater resources of a chalk aquifer in central southern england few studies that have investigated the impacts of climate change on groundwater resources have addressed uncertainty in this paper the uncertainty associated with use of a suite of gcm outputs in catchment scale impact studies is quantified a range of predictions is obtained by applying precipitation and temperature change factors derived from thirteen gcms to a distributed recharge model and a groundwater flow model of the chalk aquifer of the marlborough and berkshire downs and south west chilterns in the uk the ensemble average suggests there will be a 4 9 reduction in annual potential groundwater recharge across the study area although this is not statistically significant at the 95 confidence level the spread of results for simulated changes in annual potential groundwater recharge range from a 26 decrease to a 31 increase by the 2080s with ten predicting a decrease and three an increase whilst annual recharge is not found to change significantly the multi model results suggest that the seasonal variation in the groundwater resource will be greater with higher recharge rates during a reduced period of time in winter the spread of predictions for changes in river baseflow at the bottom of the largest river sub catchment is from 16 to 33 in march and from 68 to 56 in october the effects of climate change are shown to depend significantly on the type of land use it is concluded that further research is required to quantify the effect of different vegetation types on chalk covered by different thicknesses of soil and their response to a changing climate

hydrography of the labrador sea during active convection the hydrographic structure of the labrador sea during wintertime convection is described the cruise part of the deep convection experiment took place in february march 1997 amidst an extended period of strong forcing in an otherwise moderate winter because the water column was preconditioned by previous strong winters the limited forcing was enough to cause convection to approximately 1500 m the change in heat storage along a transbasin section relative to an occupation done the previous october gives an average heat loss that is consistent with calibrated national centers for environmental prediction surface heat fluxes over that time period 200 wm 22 deep overturning was observed both seaward of the western continental slope which was expected as well as within the western boundary current itself something that had not been directly observed previously these two geographical regions separated by roughly the 3000 m isobath produce separate water mass products the offshore water mass is the familiar cold fresh dense classical labrador sea water lsw the boundary current water mass is a somewhat warmer saltier lighter vintage of classical lsw though in the far field it would be difficult to distinguish these products the offshore product was formed within the cyclonic recirculating gyre measured by lavender et al in a region that is limited to the north most likely by an eddy flux of buoyant water from the eastern boundary current the velocity measurements taken during the cruise provide a transport estimate of the boundary current throughput and offshore recirculation finally the overall trends in stratification of the observed mixed layers are described

geochemistry and petrogenesis of late archaean aravalli volcanics basement enclaves and granitoids rajasthan abstract major trace and rare earth element data are presented for the basal volcanics of the aravalli supracrustal sequence for the mafic enclaves within the banded gneiss complex bgc adjacent to the supracrustals and for granitoids within the bgc and their petrogenesis discussed with regard to the problematic relationship between basement and cover in terms of major elements the aravalli lavas range from magnesian 22 mgo komatiites or picrites to fe rich tholeftes but differ from a typical archaean greenstone volcanic sequence in that their trace element patterns are relatively enriched in incompatible and light rare earth elements moreover the range of patterns observed requires a number of distinct mantle sources as well as different degrees of melting there are two distinct types of mafic enclave within the bgc the larger lenses correspond quite well with the range of variation seen in the aravalli sequence and could be feeders the other type close to the bgc aravalli junction has more alkalic characteristics high nb zr and could represent rift volcanics or tectonically accreted fragments of ocean islands the data suggest that the aravalli sequence is not an accreted oceanic terrane but are more consistent with interaction of a deep mantle plume with rather mature sub continental lithosphere the early archaean age of parts of the bgc the chemistry of the granitoids and the alkaline character of the enclaves are also broadly compatible with this model the character of the aravalli sequence is intermediate between that of an archaean greenstone belt and a phanerozoic flood basalt province

geochemistry and petrogenesis of garhwal volcanics implications for evolution of the north indian lithosphere abstract middle proterozoic magmatism in the garhwal himalayas consists of an extensive sequence of mafic volcanics plus associated mafic dyke sheets geochemical data reveal that the volcanic suite is tholeiitic with the dyke sheets showing considerably more fe enrichment than the lava flows all the rocks are distinctly enriched in incompatible elements and light ree relative to primordial mantle abundances but have an equally distinct continental signature reflected in marked negative nb sr p and ti anomalies in their mantle normalised spidergrams although such geochemical characteristics are commonly thought to indicate contamination of the magmas with a continental crust component the evidence is strongly in favour of the compositional characteristics being inherited from their mantle source other proterozoic volcanics in northern india have closely similar geochemical features as have other classic occurrences of younger continental flood basalt suites in adjacent areas of gondwanaland these geochemical characters are probably imposed upon the subcontinental lithosphere at the time of continent formation but in the case of younger cfb suites this may have been modified to a greater or lesser extent by subsequent additions of plume mantle material

a sea change in our view of overturning in the subpolar north atlantic to provide an observational basis for the intergovernmental panel on climate change projections of a slowing atlantic meridional overturning circulation moc in the 21st century the overturning in the subpolar north atlantic program osnap observing system was launched in the summer of 2014 the first 21 month record reveals a highly variable overturning circulation responsible for the majority of the heat and freshwater transport across the osnap line in a departure from the prevailing view that changes in deep water formation in the labrador sea dominate moc variability these results suggest that the conversion of warm salty shallow atlantic waters into colder fresher deep waters that move southward in the irminger and iceland basins is largely responsible for overturning and its variability in the subpolar basin

transport of nordic seas overflow water into and within the irminger sea an eddy resolving simulation and observations 1 results from a climatologically forced eddy resolving 1 12 atlantic simulation using the hybrid coordinate ocean model help clarify some presently unresolved connections between volume transports of nordic seas overflow water at key locations in the northernmost north atlantic ocean the model results demonstrate that in addition to the known westward flow through the charlie gibbs fracture zone cgfz some iceland scotland overflow water isow flows westward through gaps in the reykjanes ridge north of the cgfz into the irminger sea and some flows southward along the eastern flank of the mid atlantic ridge into the west european basin these results provide insights into the well known inconsistency between observed westward transport of isow through the cgfz 2 4 sv and the transports upstream at southeast of iceland section 3 2 sv and downstream in the western irminger sea 4 5 sv although the portion of the simulated isow that flows through cgfz is about 500 m deeper than observed the model results also show two isow pathways of this flow into the irminger sea one northward along the western flank of the reykjanes ridge and the other westward before turning north eastward on the western side of the irminger basin comparisons with the long term moored instrument database in the irminger sea show that the model based mean circulation is in reasonable agreement with observed volume transports of overflow water and that it gives approximately correct temperature and salinity characteristics

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

high field strength element geochemistry of mafic intrusive rocks from the bhagirathi and yamuna valleys garhwal himalaya india abstract incompatible high field strength element geochemistry divides mafic intrusive rocks of garhwal himalaya exposed in the bhagirathi and yamuna valleys into two distinct types viz older garhwal mafic intrusive rocks gmir1 and younger garhwal mafic intrusive rocks gmir2 gmir1 is mainly associated with the central crystallines whereas gmir2 belongs to the garhwal group they outcrop close to the main central thrust mct zone and extend in a nw se direction following the strike direction both types are metamorphosed to amphibolite facies and classified as sub alkaline high fe tholeiitic basalts they are enriched in large ion lithophile and high field strength elements relative to a primitive mantle source gmir2 has higher concentrations of incompatible high field strength elements than the gmir1 further on multi element spidergrams gmir1 samples do not show any elemental anomaly but gmir2 samples show distinct negative nb and sr anomalies gmir1 multi element and rare earth element trends are similar to n morb whereas gmir2 follows trends observed in the within plate basalts continental flood basalts type discrimination diagrams based on incompatible trace elements corroborate the n morb nature of gmir1 and cfb wpb nature of gmir2 geochemical modeling indicates that these mafic intrusive rocks were derived by close system fractional crystallization of depleted for gmir1 to variably enriched parental basalts for gmir2 probably these two contrasting mafic rock types have been juxtaposed during the himalayan orogeny proterozoic mafic rocks of similar geochemical characteristics have been reported from different parts of the northern indian lithosphere observed composition suggests its genetic association with the sub continental lithosphere rather than effect of crustal contamination

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

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

signal propagation related to the north atlantic overturning 1 changes of the meridional overturning circulation moc due to surface heat flux variability related to the north atlantic oscillation nao are analyzed in various ocean models i e eddying and non eddying cases a prime signature of the forcing is variability of the winter time convection in the labrador sea the associated changes in the strength of the moc near the subpolar front 45 n are closely related to the nao index leading moc anomalies by about 2 3 years in both the eddying and non eddying simulation further south the speed of the meridional signal propagation depends on model resolution with lower resolution non eddying case 4 3 resolution the moc signal propagates equatorward with a mean speed of about 0 6 cm s similar as spreading rates of passive tracer anomalies eddy permitting experiments 1 3 show a significantly faster propagation with speeds corresponding to boundary waves thus leading to an almost in phase variation of the moc transport over the subtropical to subpolar north atlantic

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

locally and remotely forced subtropical amoc variability a matter of time scales mechanisms driving the north atlantic meridional overturning circulation amoc variability at low frequency are of central interest for accurate climate predictions although the subpolar gyre region has been identified as a preferred place for generating climate timescale signals their southward propagation remains under consideration complicating the interpretation of the observed time series provided by the rapid climate change meridional overturning circulation and heatflux array western boundary time series rapid mocha wbts program in this study we aim at disentangling the respective contribution of the local atmospheric forcing from signals of remote origin for the subtropical low frequency amoc variability we analyze for this a set of four ensembles of a regional 208s 558n eddy resolving 1 128 north atlantic oceanic configuration where surface forcing and open boundary conditions are alternatively permuted from fully varying realistic to yearly repeating signals their analysis reveals the predominance of local atmospherically forced signal at interannual time scales 2 10 years whereas signals imposed by the boundaries are responsible for the decadal 10 30 years part of the spectrum due to this marked timescale separation we show that although the intergyre region exhibits peculiarities most of the subtropical amoc variability can be understood as a linear superposition of these two signals finally we find that the decadal scale boundary forced amoc variability has both northern and southern origins although the former dominates over the latter including at the site of the rapid array 26 58n

the observation based application of a regional thermohaline inverse method to diagnose the formation and transformation of water masses north of the osnap array from 2013 to 2015 abstractthe strength of the meridional overturning circulation moc in the north atlantic is dependent upon the formation of dense waters that occurs at high northern latitudes wintertime deep co

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

influence of the atlantic meridional overturning circulation on the northern hemisphere surface temperature response to radiative forcing abstractmany modeling studies have shown that the atlantic meridional overturning circulation amoc will weaken under increased greenhouse gas forcing but the influence of amoc internal variabili

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

local and downstream relationships between labrador sea water volume and north atlantic meridional overturning circulation variability abstractwhile it has generally been understood that the production of labrador sea water lsw impacts the atlantic meridional overturning circulation moc this relationship has not been explored

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

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

geochemistry and petrogenesis of early proterozoic amphibolites west central colorado u s a abstract early proterozoic supracrustal rocks exposed in west central colorado include amphibolites intimately associated with felsic volcanics and associated volcaniclastic sediments three successions are recognized the dubois succession 1770 1760 ma is comprised chiefly of amphibolites and felsic volcanics the amphibolites exhibit low concentrations of light ree and hfse and are broadly similar in composition to tholeiites from immature island arcs and in some respects to nmorb the cochetopa succession 1745 1730 ma includes large amounts of felsic volcanics and volcaniclastic sediments together with amphibolites that exhibit variable but slight incompatible element enrichment similar to basalts from evolved island arc systems and associated back arc basins the black canyon succession is coeval with the cochetopa succession and is comprised chiefly of volcaniclastic sediments with minor amphibolite these amphibolites exhibit significant incompatible element enrichment similar to basalts from continental margin arc systems geochemical modeling indicates that the amphibolites evolved by closed system fractional crystallization of depleted for the dubois succession to variably enriched parental basalts for the cochetopa and black canyon successions chiefly at shallow depths incompatible element distributions in the amphibolites indicate that mantle sources changed with time from relatively depleted 1770 1760 ma to variable but slightly enriched 1745 1730 ma a provisional model calls upon the dubois succession forming in an island arc system followed by collision with the archean wyoming province at 1750 ma the cochetopa succession was deposited in an intra arc basin developed within a new continental margin arc and the black canyon succession formed in a back arc basin associated with this arc

effects of different groundwater depths on the distribution characteristics of soil tamarix water contents and salinity under saline mineralization conditions abstract the objective of this study was to explore the correlations between tamarix and soil water salinity parameters and groundwater depth under saline mineralization conditions and to reveal the transport rules and exchange effects of salinity in soil tamarix systems with different groundwater depths soil columns planted with tamarix chinensis t soil column grown in the yellow river delta were chosen for modeling six different groundwater depths were simulated in the models we measured and analyzed the water and salinity contents of different soil profiles with different groundwater levels and na contents in new t chinensis branches and leaves the results showed the following 1 under shallow and medium groundwater levels with increasing soil depth the differences in the water contents between the t soil column and the control soil layer gradually decreased under deep groundwater levels the differences increased and then decreased planting t chinensis decreased the soil water content and higher groundwater levels corresponded with a greater decrease in the soil water content of the t soil column with increasing groundwater depth the depth at which the water content of the t soil column suddenly changed increased when the groundwater levels were 0 6 m 0 9 m 1 2 m 1 5 m and 1 8 m the critical depths at which the water contents suddenly changed in the t soil columns and control were 0 3 m 0 5 m 0 7 m 1 0 m and 1 3 m respectively 2 although planting t chinensis clearly decreased soil salinity the amplitude of the decrease increased and then decreased with increasing groundwater depth the maximum decrease was reached at a groundwater level of 1 2 m importantly the groundwater level of 1 2 m was the critical level for increasing the capillary water in the t soil column and was the depth at which the water and salinity significantly changed the water and salinity turning depths when the groundwater depths were 0 6 0 9 1 2 1 5 and 1 8 m the water turning depths in the t soil columns and control were 0 3 0 5 0 7 0 7 and 0 7 m respectively under medium 0 9 m and 1 2 m and deep 1 5 m and 1 8 m groundwater depths the minimum soil salinities in the t soil column occurred at depths of 0 5 m 0 7 m 0 7 m and 1 0 m respectively 3 with increasing groundwater depth the water contents of the t soil column t chinensis leaves and new branches gradually decreased the salinity of the t chinensis leaves decreased and then increased and the salinities of the t soil column and new branches increased and then decreased under different groundwater depths the water contents of the t chinensis leaves were significantly higher than the water contents of the branches and the salinity contents of the leaves were significantly lower than the salinity contents of the branches with increasing groundwater depth the water transported from the branches to leaves of t chinensis significantly increased and the ratio of na that migrated decreased and then increased