observed fingerprint of a weakening atlantic ocean overturning circulation the atlantic meridional overturning circulation amoc a system of ocean currents in the north atlantic has a major impact on climate yet its evolution during the industrial era is poorly known owing to a lack of direct current measurements here we provide evidence for a weakening of the amoc by about 3 1 sverdrups around 15 per cent since the mid twentieth century this weakening is revealed by a characteristic spatial and seasonal sea surface temperature fingerprint consisting of a pattern of cooling in the subpolar atlantic ocean and warming in the gulf stream region and is calibrated through an ensemble of model simulations from the cmip5 project we find this fingerprint both in a high resolution climate model in response to increasing atmospheric carbon dioxide concentrations and in the temperature trends observed since the late nineteenth century the pattern can be explained by a slowdown in the amoc and reduced northward heat transport as well as an associated northward shift of the gulf stream comparisons with recent direct measurements from the rapid project and several other studies provide a consistent depiction of record low amoc values in recent years

simulated variability of the atlantic meridional overturning circulation to examine the multi annual to decadal scale variability of the atlantic meridional overturning circulation amoc we conducted a four member ensemble with a daily reanalysis forced medium resolution global version of the isopycnic coordinate ocean model micom and a 300 years integration with the fully coupled bergen climate model bcm the simulations of the amoc with both model systems yield a long term mean value of 18 sv and decadal variability with an amplitude of 1 3 sv the power spectrum of the inter annual to decadal scale variability of the amoc in bcm generally follows the theoretical red noise spectrum with indications of increased power near the 20 years period comparison with observational proxy indices for the amoc e g the thickness of the labrador sea water the strength of the baroclinic gyre circulation in the north atlantic ocean and the surface temperature anomalies along the mean path of the gulf stream shows similar trends and phasing of the variability indicating that the simulated amoc variability is robust and real mixing indices have been constructed for the labrador the irminger and the greenland iceland norwegian gin seas while convective mixing in the labrador and the gin seas are in opposite phase and linked to the nao as observations suggest the convective mixing in the irminger sea is in phase with or leads the labrador sea newly formed deep water is seen as a slow anomalous cold and fresh plume flowing southward along the western continental slope of the atlantic ocean with a return flow of warm and saline water on the surface in addition fast travelling topographically trapped waves propagate southward along the continental slope towards equator where they go east and continue along the eastern rim of the atlantic for both types of experiments the northern hemisphere sea level pressure and 2 m temperature anomaly patterns computed based on the difference between climate states with strong and weak amoc yields a nao like pattern with intensified icelandic low and azores high and a warming of 0 25 0 5 c of the central north atlantic sea surface temperature sst the reanalysis forced simulations indicate a coupling between the labrador sea water production rate and an equatorial atlantic sst index in accordance with observations this coupling is not identified in the coupled simulation

surface deformation associated with a dipping hydrofracture crustal deformation associated with hydrofracture is modeled by a dipping rectangular dislocation beneath the surface of an elastic half space the burger s vector is taken normal to the rectangular surface analytical expressions for vertical displacements are obtained from integration of volterra s equation by using mindlin s point force solutions for the elastic halfspace in the limits of 0 and 90 dip the results agree with those obtained by maruyama 1964 for poisson s ratio 0 25 expressions for arbitrary poisson s ratio are given for elongate dislocations the displacement field agrees with curves given by pollard and holzhausen 1979 for two dimensional near surface fractures at various dips at dip 0 a deep square dislocation gives displacements indistinguishable from those of sun s 1969 model of a horizontal penny shaped crack beneath the surface of an elastic half space modifying his result for shallow depths shows that they correspond to a dislocation of semi elliptical cross section with the surface displacement field asymptotically approaching a semi elliptical shape as depth decreases to zero similarly the surface displacement field of the square dislocation approaches that of its rectangular cross section nonlinear inversion of surface tilt fields associated with hydrofracture of deep boreholes gives estimates of fracture geometry position and orientation along with their uncertainties two examples are presented one of a near horizontal and one of a near vertical fracture

recent slowing of atlantic overturning circulation as a recovery from earlier strengthening the atlantic meridional overturning circulation has weakened over the past decade examination of a global reanalysis that matches independent observations shows that the decline is consistent with recovery from an earlier invigoration

simulated variability of the circulation in the north atlantic from 1953 to 2003 abstract the variability of the circulation in the north atlantic and its link with atmospheric variability are investigated in a realistic hindcast simulation from 1953 to 2003 the interannual to decadal variability of the subpolar gyre circulation and the meridional overturning circulation moc is mostly influenced by the north atlantic oscillation nao both circulations intensified from the early 1970s to the mid 1990s and then decreased the monthly variability of both circulations reflects the fast barotropic adjustment to nao related ekman pumping anomalies while the interannual to decadal variability is due to the baroclinic adjustment to ekman pumping buoyancy forcing and dense water formation consistent with previous studies an original characteristic of the oceanic response to nao is presented that relates to the spatial patterns of buoyancy and wind forcing over the north atlantic anomalous ekman pumping associated with a positive nao phase first induces a decrease of the southern sub

mechanisms of decadal north atlantic climate variability and implications for the recent cold anomaly decadal sea surface temperature sst fluctuations in the north atlantic ocean influence climate over adjacent land areas and are a major source of skill in climate predictions however the mechanisms underlying decadal sst variability remain to be fully understood this study isolates the mechanisms driving north atlantic sst variability on decadal time scales using low frequency component analysis which identifies the spatial and temporal structure of low frequency variability based on observations large ensemble historical simulations and pre industrial control simulations we identify a decadal mode of atmosphere ocean variability in the north atlantic with a dominant time scale of 13 18 years large scale atmospheric circulation anomalies drive sst anomalies both through contemporaneous air sea heat fluxes and through delayed ocean circulation changes the latter involving both the meridional overturning circulation and the horizontal gyre circulation the decadal sst anomalies alter the atmospheric meridional temperature gradient leading to a reversal of the initial atmospheric circulation anomaly the time scale of variability is consistent with westward propagation of baroclinic rossby waves across the subtropical north atlantic the temporal development and spatial pattern of observed decadal sst variability are consistent with the recent observed cooling in the subpolar north atlantic this suggests that the recent cold anomaly in the subpolar north atlantic is in part a result of decadal sst variability

anomalously weak labrador sea convection and atlantic overturning during the past 150 years the atlantic meridional overturning circulation amoc is a system of ocean currents that has an essential role in earth s climate redistributing heat and influencing the carbon cycle1 2 the amoc has been shown to be weakening in recent years 1 this decline may reflect decadal scale variability in convection in the labrador sea but short observational datasets preclude a longer term perspective on the modern state and variability of labrador sea convection and the amoc1 3 5 here we provide several lines of palaeo oceanographic evidence that labrador sea deep convection and the amoc have been anomalously weak over the past 150 years or so since the end of the little ice age lia approximately ad 1850 compared with the preceding 1 500 years our palaeoclimate reconstructions indicate that the transition occurred either as a predominantly abrupt shift towards the end of the lia or as a more gradual continued decline over the past 150 years this ambiguity probably arises from non amoc influences on the various proxies or from the different sensitivities of these proxies to individual components of the amoc we suggest that enhanced freshwater fluxes from the arctic and nordic seas towards the end of the lia sourced from melting glaciers and thickened sea ice that developed earlier in the lia weakened labrador sea convection and the amoc the lack of a subsequent recovery may have resulted from hysteresis or from twentieth century melting of the greenland ice sheet 6 our results suggest that recent decadal variability in labrador sea convection and the amoc has occurred during an atypical weak background state future work should aim to constrain the roles of internal climate variability and early anthropogenic forcing in the amoc weakening described here

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

a multimodel study of sea surface temperature and subsurface density fingerprints of the atlantic meridional overturning circulation abstractthe atlantic meridional overturning circulation amoc is an important component of the north atlantic climate system here simulations from 10 coupled climate models are used to calculate patterns of sea surface temperature sst and subsurface density change associated with decadal amoc variability the models are evaluated using observational constraints and it is shown that all 10 models suffer from north atlantic deep water transports that are too shallow although the biases are least severe in the community climate system model version 4 ccsm4 in the models that best compare with observations positive amoc anomalies are associated with reduced labrador sea stratification and increased midocean 800 1800 m densities in the subpolar gyre maximum correlations occur when amoc anomalies lag labrador sea stratification and subsurface density anomalies by 2 6 yr and 0 3 yr respectively in all 10 models north atlantic warming follows positive amoc anomalies but the patterns and magnitud

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

the origins of late twentieth century variations in the large scale north atlantic circulation abstractsurface forcing perturbation experiments are examined to identify the key forcing elements associated with late twentieth century interannual to decadal atlantic circulation variability as simulated in an ocean sea ice hindcast configuration of the community earth system model version 1 cesm1 buoyancy forcing accounts for most of the decadal variability in both the atlantic meridional overturning circulation amoc and the subpolar gyre circulation and the key drivers of these basin scale circulation changes are found to be the turbulent buoyancy fluxes evaporation as well as the latent and sensible heat fluxes these three fluxes account for almost all of the decadal amoc variability in the north atlantic even when applied only over the labrador sea region year to year changes in surface momentum forcing explain most of the interannual amoc variability at all latitudes as well as most of the decadal variability south of the equator the observed strengthening of southern ocean westerly wi

density compensated overturning in the labrador sea the atlantic meridional overturning circulation a key constituent of the climate system is projected to slow down in the twenty first century due to a weakening of the labrador sea convection itself a response to greenhouse gas warming and or enhanced freshwater flux from the arctic however the first observations from the overturning in the subpolar north atlantic program reveal a minimal response of the meridional overturning circulation to the strong labrador sea convection during the winters of 2015 2016 from an analysis of the observational and reanalysis data we show here that this weak response can be explained by a strong density compensation in the labrador sea although convection induces important changes of temperature and salinity in the basin interior the export of the thermal and haline anomalies to the boundary current largely takes place along density surfaces as a result the transformation across density surfaces that is the imprint on the overturning circulation is relatively small this finding highlights the critical relationship between temperature and salinity in determining the overturning strength in the labrador sea and underlines the necessity of accurate freshwater flux estimates for improved meridional overturning circulation predictions compensation of the effects of salt and temperature changes on water density in the labrador sea can explain the minimal response of the atlantic overturning circulation to convection according to analyses of observations and reanalysis data

glacial reduction of amoc strength and long term transition in weathering inputs into the southern ocean since the mid miocene evidence from radiogenic nd and hf isotopes combined seawater radiogenic hafnium hf and neodymium nd isotope compositions were extracted from bulk sediment leachates and foraminifera of site 1088 odp leg 177 2082 m water depth on the agulhas ridge the new data provide a continuous reconstruction of long and short term changes in ocean circulation and continental weathering inputs since the mid miocene due to its intermediate water depth the sediments of this core sensitively recorded changes in admixture of north atlantic deep water nadw to the antarctic circumpolar current acc as a function of the strength of the atlantic meridional overturning circulation amoc nd isotope compositions end range from 7 to 11 with glacial values generally 1 to 3 units more radiogenic than during the interglacials of the quaternary the data reveal episodes of significantly increased amoc strength during late miocene and pliocene warm periods whereas peak radiogenic end values mark a strongly diminished amoc during the major intensification of northern hemisphere glaciation near 2 8 ma and in the pleistocene after 1 5 ma in contrast the hf isotope compositions ehf show an essentially continuous evolution from highly radiogenic values of up to 11 during the miocene to less radiogenic present day values 2 to 4 during the late quaternary the data document a long term transition in dominant weathering inputs where inputs from the south america are replaced by those from southern africa moreover radiogenic peaks provide evidence for the supply of radiogenic hf originating from patagonian rocks to the atlantic sector of the southern ocean via dust inputs

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

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

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

deformation of a uniform half space due to a long inclined tensile fault summary the calculation of the deformation due to shear and tensile faults in a half space is a fundamental tool for the investigation of seismic and volcanic sources the solution of the two dimensional problem of a long inclined shear fault is well known the purpose of the present paper is to present closed form analytical expressions for the subsurface stresses and displacements caused by a long inclined tensile fault buried in a homogeneous isotropic half space the expression for the airy stress function satisfying appropriate boundary conditions at the surface of the half space is obtained this airy stress function is used to derive the expressions for the displacements and stresses at an arbitrary point of the half space the variation of the displacement field with the horizontal distance from the fault is studied numerically the effect of the depth of the upper edge of the fault and the dip angle on the deformation field is also examined

atmosphere drives recent interannual variability of the atlantic meridional overturning circulation at 26 5 n the atlantic meridional overturning circulation amoc has been observed at 26 5 n since 2004 during 2009 2010 there was a transient 30 weakening of the amoc driven by anomalies in geostrophic and ekman transports here we use simulations based on the met office forecast ocean assimilation model foam to diagnose the relative importance of atmospheric forcings and internal ocean dynamics in driving the anomalous geostrophic circulation of 2009 2010 data assimilating experiments with foam accurately reproduce the mean strength and depth of the amoc at 26 5 n in addition agreement between simulated and observed stream functions in the deep ocean is improved when we calculate the amoc using a method that approximates the observing array at 26 5 n the main features of the geostrophic circulation anomaly are captured by an ensemble of simulations without data assimilation these model results suggest that the atmosphere played a dominant role in driving recent interannual variability of the amoc

the impact of resolution on the adjustment and decadal variability of the atlantic meridional overturning circulation in a coupled climate model variations in the atlantic meridional overturning circulation moc exert an important influence on climate particularly on decadal time scales simulation of the moc in coupled climate models is compromised to a degree that is unknown by their lack of fidelity in resolving some of the key processes involved there is an overarching need to increase the resolution and fidelity of climate models but also to assess how increases in resolution influence the simulation of key phenomena such as the moc in this study we investigate the impact of significantly increasing the ocean and atmosphere resolution of a coupled climate model on the simulation of moc variability by comparing high and low resolution versions of the same model in both versions decadal variability of the moc is closely linked to density anomalies that propagate from the labrador sea southward along the deep western boundary we demonstrate that the moc adjustment proceeds more rapidly in the higher resolution model due the increased speed of western boundary waves however the response of the atlantic sea surface temperatures to moc variations is relatively robust in pattern if not in magnitude across the two resolutions the moc also excites a coupled ocean atmosphere response in the tropical atlantic in both model versions in the higher resolution model but not the lower resolution model there is evidence of a significant response in the extratropical atmosphere over the north atlantic 6 years after a maximum in the moc in both models there is evidence of a weak negative feedback on deep density anomalies in the labrador sea and hence on the moc with a time scale of approximately ten years our results highlight the need for further work to understand the decadal variability of the moc and its simulation in climate models

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

near field modifications of stress induced by dyke injection at shallow depth summary the deformation and stress fields accompanying dyke injection in rift zones are described in terms of a vertical crack opening in response to internal overpressure in plane strain configuration previous numerical computations of the displacement field induced by shallow dykes are found to be affected in some cases by significant distortion mainly due to the limited extension of the computational domain difficulties in handling the singularities in the proximity of the free surface and an incorrect choice of the reference frame in particular it is found that no subsidence can be ascribed to the mere opening of a tensile crack in a homogeneous elastic half space if analytical constant dislocation models are employed surface displacements compare reasonably well with crack model solutions if the upper tip is not too shallow however constant dislocation solutions present unphysical singularities along the tips which distort significantly the near field stress pattern with respect to crack solutions if the principal stress axes are computed from crack models a broad region is found on both sides of the dyke where the induced stress has a nearly vertical intermediate axis while the compressive axis is normal to the dyke plane strike slip earthquakes are expected to prevail in this region above the upper crack tip a small region is present where the tensile stress is dominant and the intermediate stress is still vertical in the proximity of the free surface typically within a few tens of metres of it induced tensile stresses are greater than the lithostatic pressure open fissures might then develop in cohesionless soil or pre faulted rock the induced pressure in the host rock is found to be negative suction in the proximity of the ground surface and positive at greater depth fluid flow within the aquifers can be significantly altered by this induced overpressure and by the anisotropic modification of the pre existing permeability according to the modified coulomb failure criterion in the short term the shallower region characterized by suction is strengthened while the deeper pressurized region is weakened these results can explain in a straightforward way the abundance of strike slip focal mechanisms in volcanic areas the switch between tensional and compressional axes inferred from focal mechanisms of earthquakes in connection with a dyke injection episode on mount etna en echelon fracture systems observed at piton de la fournaise above the feeding dyke and precursory geochemical anomalies

quaternary sedimentary history of adana cilicia and iskenderun basins northeast mediterranean sea detailed single channel airgun and 3 5 khz profiles from the northeastern mediterranean sea show that the continental shelf is formed by superimposed deltaic successions depositional sequences separated by major erosional unconformities each depositional sequence is composed of a sigmoid prograding package overlain by an oblique prograding package representing respectively delta progradation during high and low sea levels of interglacial and subsequent glacial stages during the glacio eustatic lowstands of sea level deltas prograded seaward the present day shelf break denotes the topset to forest transition at maximum progradation during the last glacial period during post glacial transgressions the deltas initially lost their dynamic equilibrium with the environment and rapidly retreated landward leading to the deposition of a thin veneer of sediments originating from reworking of formerly coastal sediments at the highstand position the deltas were re established in the ancestral adana bay and foreset progradation started chronology suggests that the cilicia and iskenderum basins are subsiding at rates of 0 38 m 1000 yr 1 and 0 33 m 1000 yr 1 respectively seismic reflection profiling shows that delta architecture in the adana cilicia and iskenderun basins is mainly controlled by glacio eustatic sea level fluctuations and continuous basin subsidence

static deformation of a uniform half space due to a long dip slip fault summary closed form expressions for the displacements and stresses at an arbitrary point of a homogeneous isotropic perfectly elastic half space caused by a dip slip line source obtained earlier are integrated analytically to derive the elastic residual field due to a long dip slip fault of finite width the results are valid for an arbitrary dip of the fault and for arbitrary receiver locations inside the medium the variation of the displacement and stress field with the distance from the fault is studied numerically contour maps showing the stress field around a long dip slip fault are obtained permanent surface deformations which occur as a result of faulting can be measured from geodetic surveys carried out before and after an earthquake these observations can be used to obtain the fault parameters dip slip width depth by inversion using the theoretical expressions given in the paper the features of the observational data which are diagnostic of the source properties are the asymmetry of the data the magnitude of various components of deformation and the rate of fall of these components with distance the asymmetry depends mainly on the dip the magnitude on the slip and the rate of fall on the depth of the fault