toward a boot strap hypothesis of plate tectonics feedbacks between plates the asthenosphere and the wavelength of mantle convection abstract the solid earth system is characterized by plate tectonics a low viscosity zone beneath plates the asthenosphere and long wavelength flow in the convecting mantle we use suites of numerical experiments to show 1 how long wavelength flow and the operation of plate tectonics can generate and maintain an asthenosphere and 2 how an asthenosphere can maintain long wavelength flow and plate tectonics plate subduction generates a sub adiabatic temperature gradient in the mantle which together with temperature dependent viscosity leads to a viscosity increase from the upper to the lower mantle this allows mantle flow to channelize in a low viscosity region beneath plates an asthenosphere forms dynamically flow channelization in turn stabilizes long wavelength convection the degree of dynamic viscosity variations from the upper to the lower mantle increases with the wavelength of convection and drops toward zero if the system transitions from plate tectonics to a single plate planet the plate margin strength needed to initiate that transition increases for long wavelength cells long wavelength flow allows plate tectonics to exist over a wider range of plate margin strength the coupled feedbacks allow for a linked causality between plates the asthenosphere and the wavelength of mantle flow with none being more fundamental than the others and the existence of each depending on the others under this hypothesis the asthenosphere is defined by an active process plate tectonics which maintains it and is maintained by it and plate tectonics is part of an emergent self sustaining flow system that bootstraps itself into existence

the 1992 2009 transport variability of the east greenland irminger current at 60 n the east greenland irminger current egic decadal transport variability likely influences deep convection intensity in the labrador and irminger seas but is poorly known yet the egic transport west of the 2000 m isobath was estimated for the first time between 1992 and 2009 by combining surface geostrophic velocities derived from altimetry with an estimate of the vertical structure of the transport variability statistically determined from a moored array deployed in 2004 2006 the reconstructed 17 year time series of the egic transport was then validated against independent estimates confirming that indeed the vertical distribution of the egic variability has not changed significantly over the last two decades the 1992 2009 mean transport is 19 5 sv with a standard error of 0 3 sv 1 sv 10 6 m 3 s 1 in 1992 1996 the egic transport was close to the average over the following decade 1997 2005 the egic transport declined by 3 sv 15 so that the 2004 2006 mean transport inferred from the moored array is 2 2 sv 10 less than the 1992 2009 mean it was followed by a period of higher transport the seasonal to interannual transport variability is coherent with the variability of the windstress curl at the center of the irminger sea

thermal convection in a volumetrically heated infinite prandtl number fluid with strongly temperature dependent viscosity implications for planetary thermal evolution parameterized models of the thermal evolution of planets are usually based on the assumption that the lithosphere convecting mantle boundary can be defined by an isotherm at a temperature below which viscosity is infinite on geologic timescales recent experimental results argue against this assumption we have investigated both the definition of the lithosphere convecting mantle boundary and the power law relation describing convecting heat transfer based on numerical experiments of thermal convection in a volumetrically heated fluid with temperature dependent viscosity other recent studies have treated only the heating from below but volumetric heating is likely to be the dominant mode of heating in planetary mantles either as a consequence of radioactive heating or as a proxy for secular cooling convection can occur either in the whole box or be located under a stagnant lid in the lid regime convection is driven by a temperature contrast depending on the rheology of the fluid and the interior temperature this result in agreement with experimental studies indicates that boundary between the stagnant lid and the convecting layer similar to the lithosphere convecting mantle boundary cannot be defined as a fixed isotherm during thermal evolution of planets the viscosity contrast in the convecting mantle remains constant not the temperature at the bottom of the lithosphere we present an example showing that the evolution of planets is strongly dependent on the criterion chosen to define the lithosphere convecting mantle boundary for reasonable values of the activation energy for thermally activated creep the temperature defining the lithosphere convecting mantle boundary the mantle temperature and the thickness of the lithosphere could be larger than expected from previous models which treat the base of the lithosphere as a fixed isotherm

geological consequences of super sized earths 1 the discovery of terrestrial scale extrasolar planets and their calculated abundance in the galaxy has prompted speculation on their surface conditions and thermal structure both are dependent on the tectonic regime of a planet which is itself a function of the balance between driving forces and the resistive strength of the lithosphere here we use mantle convection simulations to show that simply increasing planetary radius acts to decrease the ratio of driving to resisting stresses and thus super sized earths are likely to be in an episodic or stagnant lid regime this effect is robust when associated increases in gravity are included as the more dominant effect is increased fault strength rather than greater buoyancy forces the thermo tectonic evolution of large terrestrial planets is more complex than often assumed and this has implications for the surface and conditions habitability of such worlds

deep water formation and meridional overturning in a high resolution model of the north atlantic the authors use different versions of the model of the wind and thermohaline driven circulation in the north and equatorial atlantic developed under the woce community modeling effort to investigate the mean flow pattern and deep water formation in the subpolar region and the corresponding structure of the basin scale meridional overturning circulation transport a suite of model experiments has been carded out in recent years differing in horizontal resolution 1 1 2 1 3 0 4 1 6 0 2 thermohaline boundary conditions and parameterization of small scale mixing the mass transport in the subpolar gyre and the production of north atlantic deep water nadw appears to be essentially controlled by the outflow of dense water from the greenland and norwegian seas in the present model simulated by restoring conditions in a buffer zone adjacent to the boundary near the greenland scotland ridge deep winter convection homogenizes the water column in the center of the labrador sea to about 2000 m the water mass properties potential temperature about 3 c salinity about 34 9 psu and the volume 1 1 1053 km3 of the homogenized water are in fair agreement with observations the convective mixing has only little effect on the net sinking of upper layer water in the subpolar gyre sensitivity experiments show that the export of nadw from the subpolar north atlantic is more strongly affected by changes in the overflow conditions than by changes in the surface buoyancy fluxes over the labrador and irminger seas even if these suppress the deep convection completely the host of sensitivity experiments demonstrates that realistic meridional overturning and heat transport distributions for the north atlantic with a maximum of 1 pw can be obtained with nadw production rates of 15 16 sv provided the spurious upwelling of deep water that characterizes many model solutions in the gulf stream regime is avoided by adequate horizontal resolution add mixing parameterization

mantle convection with a brittle lithosphere thoughts on the global tectonic styles of the earth and venus summary plates are an integral part of the convection system in the fluid mantle but plate boundaries are the product of brittle faulting and plate motions are strongly influenced by the existence of such faults the conditions for plate tectonics are studied by considering brittle behaviour using byerlee s law to limit the maximum stress in the lithosphere in a mantle convection model with temperature dependent viscosity when the yield stress is high convection is confined below a thick stagnant lithosphere at low yield stress brittle deformation mobilizes the lithosphere which becomes a part of the overall circulation surface deformation occurs in localized regions close to upwellings and downwellings in the system at intermediate levels of the yield stress there is a cycling between these two states thick lithosphere episodically mobilizes and collapses into the interior before reforming the mobile lid regime resembles convection of a fluid with temperature dependent viscosity and the boundary layer scalings are found to be analogous this regime has a well defined nusselt number rayleigh number relationship which is in good agreement with scaling theory the surface velocity is nearly independent of the yield stress indicating that the plate motion is resisted by viscous stresses in the mantle analysis suggests that mobilization of the earth s lithosphere can occur if the friction coeycient in the lithosphere is less than 0 03 0 13 lower than laboratory values but consistent with seismic field studies on venus the friction coeycient may be high as a result of the dry conditions and brittle mobilization of the lithosphere would then be episodic and catastrophic

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

seasonal overturning of the labrador sea as observed by argo floats abstractargo floats are used to investigate labrador sea overturning and its variability on seasonal time scales this is the first application of argo floats to estimate overturning in a deep water formation region in the north atlantic unlike hydrographic measurements which are typically confined to the summer season floats offer the advantage of collecting data in all seasons seasonal composite potential density and absolute geostrophic velocity sections across the mouth of the labrador sea assembled from float profiles and trajectories at 1000 m are used to calculate the horizontal and overturning circulations the overturning exhibits a pronounced seasonal cycle in depth space the overturning doubles throughout the course of the year and in density space it triples the largest overturning 1 2 sv 1 sv 106 m3 s 1 in depth space and 3 9 sv in density space occurs in spring and corresponds to the outflow of recently formed labrador sea water the overturning decreases through summer and reac

what is the thermohaline circulation the term thermohaline circulation is widely used in the context of past and future climate change in his perspective wunsch argues that there are many different and inconsistent definitions for this term he defines ocean circulation in terms of mass fluxes which are largely driven by surface forces such as wind and tides and argues that thermohaline circulation should only refer to the separate transport of heat and salinity in the ocean

convection scaling and subduction on earth and super earths super earths are the smallest class of discovered extra solar planets owing to their relatively small mass some might resemble earth and perhaps be habitable because of the connection to habitability through thermal evolution we investigate the tectonic regime of massive terrestrial planets two independent studies o neill c lenardic a oct 2007 geological consequences of super sized earths grl 34 l19204 valencia d o connell r j sasselov d d nov 2007a inevitability of plate tectonics on super earths apjl 670 l45 l48 have reached opposing conclusions about the likelihood of plate tectonics here we offer possible reasons for the discordant findings and address three key aspects of sustaining plate tectonics deformation on faults negative buoyancy and energy dissipation during subduction we show that in general the ratio of driving force to plate resistance increases with planetary mass this is a consequence of increasing convective stresses thinning plates and similar plate structure we conclude that even though the strength of dry and wet of faults increases for massive terrestrial planets the convective stress increases even more allowing deformation to take place also despite shorter timescales for plate cooling rocky super earths achieve negative buoyancy at subduction zones finally by investigating the effects of energy dissipation during subduction we find that massive terrestrial planets dissipate less energy during subduction and hence provide a positive feedback to sustain active lid tectonics in conclusion rocky super earths have more favorable conditions than earth for the subduction of plates and hence for sustaining plate tectonics

the role of the heating mode of the mantle in intermittent reorganization of the plate velocity field summary the geological record indicates that stages of relatively steady plate motion have been punctuated by comparatively brief periods in which plate velocities have reorganized the distribution of buoyancy sources in the mantle has generally been regarded as evolving too slowly to explain these rapid transitions in plate velocity we investigate the feedback between mantle convection and plate velocity using 2 d and 3 d mantle convection models that incorporate mobile dynamic plates we focus on the influence of internal heating in the mantle and consider the effect of mantle viscosity stratification and different plate geometries on the plate velocity time dependence as either the rayleigh number or the internal heating rate is increased to magnitudes approaching mantle values the record of the plate motion from our calculations becomes characterized by intermittent changes in direction this behaviour is a result of the influence of plates on heat loss from the inherently unsteady internally heated convecting system plate motion instills a pattern of organization on the underlying convection that reflects the plate geometry and results in the formation of sheet like downwelling structures at convergent plate boundaries in both 2 d and 3 d calculations in contrast upwellings in 3 d models are not sheet like the role of the sheet like downwellings is critical in the observed episodic reorganization of the plate velocities warm material below the plates is entrained by plate motion into regions enveloping the downwelling sheets during periods of fairly steady plate motion buoyancy associated with the build up of heat around the downwelling sheets leads to the creation of an unstable convection pattern this build up of heat is dramatic in calculations with mantle like internal heating rates and resists continued long term plate motion towards mature downwellings when there are limitations on the degree of freedom of the direction of plate movement such as in 2 d models these effects become even more pronounced accordingly the effect of plates on mean global thermal quantities is more dramatic in 2 d calculations than it is in 3 d calculations nevertheless 3 d calculations incorporating plates of different sizes do exhibit rapid reorganizations in their convection patterns as the pull of young slablike features supersedes the pull of mature downwelling sheets we compare the timing and frequency of the reorganization events in our calculations with the general characteristics of plate motions determined from plate reconstruction studies

seasonal and interannual variability of the west greenland current system in the labrador sea in 1993 2008 the west greenland current system wgcs transports heat and freshwater into the labrador sea influencing the formation of labrador sea water a key component of the atlantic meridional overturning circulation notwithstanding its importance relatively little is known about the structure and transport of this current system and its seasonal and interannual variability here we use historical hydrographic data from 1992 to 2008 combined with aviso satellite altimetry to diagnose the mean properties as well as seasonal and interannual variability of the boundary current system we find that while the surface fresh cold west greenland current is amplified in summer the subsurface warm salty irminger current has maximum transport in winter when its waters are also warmer and saltier seasonal changes in the total transport are thus mostly due to changes in the baroclinic structure of the current by contrast we find a trend toward warmer saltier waters and a slowdown of the wgcs within the period studied the latter is attributed to changes in the barotropic component of the current superimposed on this trend warm and salty anomalies transit through the system in 1997 and 2003 and are associated with a rapid increase in the transport of the boundary current due to changes in the baroclinic component the boundary current changes precede similar changes in the interior with a 1 2 year lag indicating that anomalies advected into the region by the boundary current can play an important role in the modulation of convection in the labrador sea

mantle convection with reversing mobile plates a benchmark study 1 recent investigations of mantle convection with mobile plates have documented a mode of convection with periodic and or aperiodic plate reversals 2 d or reorganizations 3 d we show that it is possible to calibrate plate formulations so that three different modeling methods reproduce the global flow diagnostics to within 3 in 2 d calculations plate reversals are independent of the method of plate generation once the calculations are tuned to reproduce the same physical characteristics e g nusselt number and plate velocity the periods of oscillation of the flow reversals from three different plate methods agree to within 4

temporal variations in the convective style of planetary mantles investigations of mantle convection with temperature and strain rate dependent viscosity have shown the existence of fundamentally different convective styles by varying e g the rayleigh number the viscosity contrast or the strain rate dependency of viscosity the planform of convection in the asymptotic stationary state changes from the so called stagnant lid regime to an episodic behaviour and further to a state characterised by a permanently mobilised surface our studies suggest that this transition may not only be induced by a change of parameters but also occurs temporally for fixed parameters we have in fact observed convective systems in the stagnant lid regime that show isolated events of surface mobilisation occurring out of a thermally equilibrated state we use a 3d numerical mantle convection model to investigate mantle convection and surface dynamics as a coupled fluid dynamical system our studies focus on the existence of a transitional regime in which temporal variations between the stagnant lid and the episodic regime occur we were able to deduce a mobilisation criterion that describes the stability of the stagnant surface thus allowing for a quantitative analysis of the transition to a temporarily mobilised surface this criterion is also suitable to predict the occurrence of surface mobilisation events

recent variability of the north atlantic thermohaline circulation inferred from surface heat and freshwater fluxes abstract an annual mean surface forced component of the north atlantic thermohaline circulation thc in density space surf is diagnosed from observed surface heat and freshwater fluxes the climatological mean of surf over 1980 97 indicates steady overturning rates for those water masses that are principally formed through surface buoyancy forcing 8 sv of greenland sea deep water gsdw 4 sv of labrador sea water lsw and 4 sv of 18 water stmw obtained for each year over 1980 97 surf reveals interannual to decadal variability in the renewal and overturning of these water masses the total surface forced overturning rate varies in the range 10 20 sv reaching peak values in 1989 90 and generally decreasing over the period 1990 97 more dramatic changes are apparent in the overturning of different water masses the overturning of stmw exhibits large amplitude interannual variability a background positive trend over 1980 97 is dominated by variance of 13 4 sv by contrast the overtur

on the connection between dense water formation overturning and poleward heat transport in a convective basin an isopycnal two layer idealized model for a convective basin is proposed consisting of a convecting interior region and a surrounding boundary current buoyancy and wind driven parameterized eddy fluxes govern the exchange between the two to balance the interior buoyancy loss the boundary current becomes denser as it flows around the basin geostrophy imposes that this densification be accompanied by sinking in the boundary current and hence by an overturning circulation the poleward heat transport associated with convection in the basin can thus be viewed as a result of both an overturning and a horizontal circulation when adapted to the labrador sea the model is able to reproduce the bulk features of the mean state the seasonal cycle and even the shutdown of convection from 1969 to 1972 according to the model only 40 of the poleward heat buoyancy transport of the labrador sea is associated with the overturning circulation an exact solution is presented for the linearized equations when changes in the boundary current are small numerical solutions are calculated for variations in the amount of convection and for changes in the remotely forced circulation around the basin these results highlight how the overturning circulation is not simply related to the amount of dense water formed a speeding up of the circulation around the basin due to wind forcing for example will decrease the intensity of the overturning circulation while the dense water formation remains unvaried in general it is shown that the fraction of poleward buoyancy or heat transport carried by the overturning circulation is not an intrinsic property of the basin but can vary as a result of a number of factors

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

plate motions and the viscosity structure of the mantle insights from numerical modelling abstract the viscosity structure of the earth s mantle is likely to play an important role with respect to the motion of the lithospheric plates a zone of low viscosity in the upper mantle as postulated by seismological studies and also by investigations of postglacial uplift has been proposed to facilitate plate motion another inferred feature of the viscosity profile in the mantle is a significant increase at greater depth we have employed a three dimensional mantle convection model to explore the relation between the appearance of plate motion and the viscosity structure of the mantle beneath the model allows for a complex fluid rheology strong temperature pressure and stress dependence of the viscosity and can so account for the self consistent formation of plates at the surface of the convecting mantle plate like motion appears only in a limited parameter range and we demonstrate that this appearance is closely linked to a specific viscosity structure the viscosity is characterized by a local minimum at shallow depth low viscosity zone and by a local maximum at greater depth self consistent plate motion is only observed if such a viscosity structure exists in order to determine the functional relationship between the viscosity structure and plate motion we also employ an approach in which a certain viscosity profile is prescribed these studies again reveal the importance of the local extrema in the viscosity profile taking into account an increased interior viscosity also obtained by the pressure dependence we find stable plate motion as a general result we observe that the balance of the temperature and stress dependence of the viscosity is necessary to obtain plate motion at the same time we find a viscosity profile showing a minimum at shallow depth and a maximum at greater depth

using argo data to investigate the meridional overturning circulation in the north atlantic abstract using a variety of oceanographic data including direct volume transports in the florida strait and argo float profiles and drift velocities at 24 n and 36 n in the north atlantic inverse calculations are presented in which the net meridional transport down to a depth of approximately 1600 m is estimated at both latitudes for a 5 year period 2003 2007 the upper ocean is divided into seven layers using neutral density and mass conservation constraints have been applied to a closed box bounded by these latitudes including the florida strait ekman layer transports have been included in the top most layer and the inverse calculation has solved for changes from the initial reference velocities ekman and florida strait transports given a priori estimates on the accuracy of each of these quantities solutions with and without transformations due to mediterranean water mw formation are made our results indicate that 1 time averaged transport estimates derived from argo have significant less eddy noise than individual hydrographic sections 2 argo drift velocities provide information to the inverse solution for the ocean interior and 3 comparison of the total integrated interior mass transports in the thermocline waters for the period 2003 2007 with the previous estimates based on trans ocean hydrographic sections shows that within the errors of our estimation the upper limb of the atlantic meridional overturning circulation has not significantly changed since 1957

rapid export of waters formed by convection near the irminger sea s western boundary the standard view of the overturning circulation emphasizes the role of convection yet for waters to contribute to overturning they must not only be transformed to higher densities but also exported equatorward from novel mooring observations in the irminger sea 2014 2016 we describe two water masses that are formed by convection and show that they have different rates of export in the western boundary current upper irminger sea intermediate water appears to form near the boundary current and is exported rapidly within 3 months of its formation deep irminger sea intermediate water forms in the basin interior and is exported on longer time scales the subduction of these waters into the boundary current is consistent with an eddy transport mechanism our results suggest that light intermediate waters can contribute to overturning as much as waters formed by deeper convection and that the export time scales of both project onto overturning variability

interdecadal variations of the thermohaline circulation in a coupled ocean atmosphere model abstract a fully coupled ocean atmosphere model is shown to have irregular oscillations of the thermohaline circulation in the north atlantic ocean with a time scale of approximately 50 years the irregular oscillation appears to be driven by density anomalies in the sinking region of the thermohaline circulation approximately 52 n to 72 n combined with much smaller density anomalies of opposite sign in the broad rising region the spatial pattern of see surface temperature anomalies associated with this irregular oscillation bears an encouraging resemblance to a pattern of observed interdecadal variability in the north atlantic the anomalies of sea surface temperature induce model surface air temperature anomalies over the northern north atlantic arctic and northwestern europe

rheology of the upper mantle a synthesis rheological properties of the upper mantle of the earth play an important role in the dynamics of the lithosphere and asthenosphere however such fundamental issues as the dominant mechanisms of flow have not been well resolved a synthesis of laboratory studies and geophysical and geological observations shows that transitions between diffusion and dislocation creep likely occur in the earth s upper mantle the hot and shallow upper mantle flows by dislocation creep whereas cold and shallow or deep upper mantle may flow by diffusion creep when the stress increases grain size is reduced and the upper mantle near the transition between these two regimes is weakened consequently deformation is localized and the upper mantle is decoupled mechanically near these depths

self consistent generation of tectonic plates in time dependent three dimensional mantle convection simulations 1 presented here are self consistent three dimensional simulations of mantle convection some of which display an approximation of plate tectonic behavior that is continuous in space and time plate behavior arises through a reasonable material description of silicate deformation with a simple yield stress being sufficient to give first order plate like behavior however the required yield strength or fault frictional coefficient is much less than experimentally determined values toroidal poloidal ratios are within geologically observed limits the sensitivity of the system to yield strength and the form of strength envelope is systematically investigated optimum plate character is obtained in a narrow range of yield strength below which diffuse boundaries and above which episodic behavior and eventually a rigid lid are observed models with mobile lids develop very long wavelength horizontal structure the longest wavelength possible in the domain two dimensional models display much greater time dependence than three dimensional models

multiproxy geochemical analysis of a panthalassic margin record of the early toarcian oceanic anoxic event toyora area japan abstract the early toarcian oceanic anoxic event oae was a significant palaeoenvironmental perturbation that led to marked changes in ocean chemistry and climate and which also had a long lasting impact on marine ecosystems the global significance of the event has been recognised from the widespread occurrence of a 3 7 negative excursion in the carbon isotope 13 c composition of marine organic and inorganic matter and terrestrial plant material this feature of the event is indicative of a pronounced perturbation to the global carbon cycle an inference further supported by widespread evidence for seawater deoxygenation and elevated rates of organic carbon burial nevertheless the precise palaeoenvironmental impacts of this event from sections outside of the boreal and tethyan realms are uncertain here we present the results of a multiproxy geochemical study of an expanded record of the early toarcian event from the northwest panthalassa ocean margin exposed in southwest japan toyora area yamaguchi prefecture our results indicate that in the studied succession organic matter enrichment persisted through the early toarcian event but elemental redox proxies do not support persistent seawater anoxia analyses of terrigenously derived major and trace element abundances coupled with sedimentological observations reveal an increase in coarse grained sediment close to the onset of a 4 excursion in 13 c org and coincident with an inferred increase in terrestrial organic matter flux these observations are consistent with previously published evidence for a marked strengthening of hydrological cycling and increased runoff that occurred contemporaneously with abrupt warming at the onset of the carbon isotope event

ventilation and transformation of labrador sea water and its rapid export in the deep labrador current a model of the subpolar north atlantic ocean is used to study different aspects of ventilation and water mass transformation during a year with moderate convection intensity in the labrador sea the model realistically describes the salient features of the observed hydrographic structure and current system including boundary currents and recirculations ventilation and transformation rates are defined and compared the transformation rate of labrador sea water lsw defined in analogy to several observational studies is 6 3 sv sv 106 m3 s 1 in the model using an idealized ventilation tracer mimicking analyses based on chlorofluorocarbon inventories an lsw ventilation rate of 10 sv is found differences between both rates are particularly significant for those water masses that are partially transformed into denser water masses during winter the main export route of the ventilated lsw is the deep labrador current lc backward calculation of particle trajectories demonstrates that about one half of the lsw leaving the labrador sea within the deep lc originates in the mixed layer during that same year near the offshore flank of the deep lc at about 55 w the transformation of lsw begins in january and is at a maximum in february march while the export of transformed lsw out of the central labrador sea continues for several months lsw generated near the boundary current is exported more rapidly with maximum transport rates during march april within the deep lc

the effect of rheological parameters on plate behaviour in a self consistent model of mantle convection abstract in a three dimensional numerical model of mantle convection we have explored the self consistent formation and evolution of plates this was done by investigating the influence of temperature stress and pressure dependence of the viscosity on plate like behaviour further the role of a depth dependent thermal expansivity and of internal heating was examined first order self consistent plate tectonics can be obtained for temperature and stress dependent viscosity we observe virtually undeformed moving plates and the phenomenon of trench migration however plate motion only appears during short intervals which are interrupted by long phases characterised by an immobile lid an improvement is achieved by adding pressure dependence of the viscosity and of the thermal expansivity to the model this leads to extended plates with continuous plate motion and the appearance of a low viscosity zone internal heating accelerates subduction and gives an increase in plate velocity the plates are otherwise left intact we find that the surface velocity is largely determined by subduction rather than by upwellings plate like behaviour is restricted to a narrow parameter window the window is bordered by an episodic or a stagnant lid type of flow on the one side and by a domain in which convection is akin to constant viscosity flow on the other side with these different convective regimes the behaviour of various terrestrial planets can be described self consistently occasionally even a change from the plate regime to the stagnant lid regime occurs such a type of transition has possibly changed the style of tectonics on some terrestrial planets such as mars

the pressure weakening effect in super earths consequences of a decrease in lower mantle viscosity on surface dynamics 1 the quest to identify habitable planets has raised interest in the surface dynamics of terrestrial bodies in this context super earths a new class of exoplanets have become of special interest in the past decade scalings to super earth sizes when compared to the earth suggest changes to convective stresses and mantle temperatures which can cause either an increase in surface mobility or in plate resistance mantle viscosity which depends on temperature stress and pressure plays a critical role in both cases new mineralogical assumptions suggest that the viscosity in super earths acts differently than in the earth and what had been assumed for super earths in planets larger than the earth pressure will become so high that pressure weakening and a decrease of viscosity in the lowermost mantle results we present a numerical convection study featuring this viscosity decrease and find that this leads to a reduction in surface mobility

convection in three dimensions with surface plates generation of toroidal flow this work presents numerical calculations of mantle convection that incorporate some of the basic observational constraints imposed by plate tectonics the model is three dimensional and includes surface plates it allows plate velocity to change dynamically according to the forces which result from convection we show that plates are an effective means of introducing a toroidal component into the flow field after initial transients the plate motion is nearly parallel to transform faults and in the direction that tends to minimizes the toroidal flow field the toroidal field decays with depth from its value at the surface the poloidal field is relatively constant throughout the layer but falls off slightly at the top and bottom boundaries layered viscosity increasing with depth causes the toroidal field to decay more rapidly effectively confining it to the upper low viscosity layer the effect of viscosity layering on the poloidal field is relatively small which we attribute to its generation by temperature variations distributed throughout the system the generation of toroidal flow by surface plates would seem to account for the observed nearly equal energy of toroidal and poloidal fields of plate motions on the earth a low viscosity region in the upper mantle will cause the toroidal flow to decay significantly before reaching the lower mantle the resulting concentration of toroidal flow in the upper mantle may result in more thorough mixing there and account for some of the geochemical and isotopic differences proposed to exist between the upper and lower mantles

the formation of labrador sea water part i large scale processes abstract data obtained in the western labrador sea during march 1976 by hudson are analysed to show that new labrador sea water was being formed at this time on the basis of hydrographic and moored current meter data it is hypothesized that a 200 km scale cyclonic gyre forms in winter in the western labrador sea and that this gyre retains the developing deep mixed layers in this general area long enough for the transformation to labrador sea water to take place using a model it is demonstrated that water columns found along the western boundary of the labrador sea can be modified by cooling evaporation and mixing to form deep mixed layers with the properties of labrador sea water approximately 105 km3 of new labrador sea water was formed in 1976 an estimate that is consistent with earlier estimates of mean annual production rates this water 2 9 c 34 84 is some 0 6 c cooler and 0 06 fresher than that defined by lazier 1973 from his data collected in 1966 the variation of labrador sea water

the internal activity and thermal evolution of earth like planets abstract we model the internal thermal evolution of planets with earth like composition and masses ranging from 0 1 to 10 earth masses over a period of 10 billion years we also characterize the internal activity of the planets by the velocity of putative tectonic plates the rate at which mantle material is processed through melting zones and the time taken to process one mantle mass the more massive the planet the larger its processing rate which scales approximately as m 0 8 1 0 the processing times for all the planets increase with time as they cool and become less active as would be expected the surface heat flow scales with planet mass all planets have similar declines in mantle temperature except for the largest in which pressure effects cause a larger decline the larger planets have higher mantle temperatures over all times the less massive the planet the larger the decrease in core temperature with time the core heat flow is also found to decrease more rapidly for smaller planet masses finally rough predictions are made for the time required to generate an atmosphere from estimates of the time to degas water and carbon dioxide in mantle melting zones the degassing times depend strongly on the initial temperature of the planet but for the temperatures used in our model all the planets degas within 32 ma after their formation

boundary currents and watermass transformation in marginal seas the properties of watermass transformation and the thermohaline circulation in marginal seas with topography and subject to a spatially uniform net surface cooling are discussed the net heat loss within the marginal sea is ultimately balanced by lateral advection from the open ocean in a narrow boundary current that flows cyclonically around the basin heat loss in the interior is offset by lateral eddy fluxes originating in the boundary current the objectives of this study are to understand better what controls the density of waters formed within the marginal sea the temperature of the outflowing waters the amount of downwelling and the mechanisms of heat transport within the marginal sea the approach combines heat budgets with linear stability theory for a baroclinic flow over a sloping bottom to provide simple theoretical estimates of each of these quantities in terms of the basic parameters of the system the theory compares well to a series of eddy resolving primitive equation model calculations the downwelling is concentrated within the boundary current in both a diffusive boundary layer near topography and an eddy driven region on the offshore edge of the boundary current for most high latitude regions the horizontal gyre is expected to transport more heat than does the overturning gyre

open ocean convection observations theory and models we review what is known about the convective process in the open ocean in which the properties of large volumes of water are changed by intermittent deep reaching convection triggered by winter storms observational laboratory and modeling studies reveal a fascinating and complex interplay of convective and geostrophic scales the large scale circulation of the ocean and the prevailing meteorology two aspects make ocean convection interesting from a theoretical point of view first the timescales of the convective process in the ocean are sufficiently long that it may be modified by the earth s rotation second the convective process is localized in space so that vertical buoyancy transfer by upright convection can give way to slantwise transfer by baroclinic instability moreover the convective and geostrophic scales are not very disparate from one another detailed observations of the process in the labrador greenland and mediterranean seas are described which were made possible by new observing technology when interpreted in terms of underlying dynamics and theory and the context provided by laboratory and numerical experiments of rotating convection great progress in our description and understanding of the processes at work is being made

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

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

monitoring the atlantic meridional overturning circulation the rapid climate change programme rapid has established a prototype system to continuously observe the strength and structure of the atlantic meridional overturning circulation moc at 26 5 n here we provide a detailed description of the rapid moc monitoring array and how it has evolved during the first four deployment years as well as an overview of the main findings so far the rapid moc monitoring array measures 1 gulf stream transport through florida strait by cable and repeat direct velocity measurements 2 ekman transports by satellite scatterometer measurements 3 deep western boundary currents by direct velocity measurements 4 the basin wide interior baroclinic circulation from moorings measuring vertical profiles of density at the boundaries and on either side of the mid atlantic ridge and 5 barotropic fluctuations using bottom pressure recorders the array became operational in late march 2004 and is expected to continue until at least 2014 the first 4 years of observations april 2004 april 2008 have provided an unprecedented insight into the moc structure and variability we show that the zonally integrated meridional flow tends to conserve mass with the fluctuations of the different transport components largely compensating at periods longer than 10 days we take this as experimental confirmation of the monitoring strategy which was initially tested in numerical models the moc at 26 5 n is characterised by a large variability even on timescales as short as weeks to months the mean maximum moc transport for the first 4 years of observations is 18 7 sv with a standard deviation of 4 8 sv the mechanisms causing the moc variability are not yet fully understood part of the observed moc variability consists of a seasonal cycle which can be linked to the seasonal variability of the wind stress curl close to the african coast close to the western boundary fluctuations in the gulf stream and in the north atlantic deep water nadw coincide with bottom pressure variations at the western margin thus suggesting a barotropic compensation ongoing and future research will put these local transport variations into a wider spatial and climatic context

the impact of rayleigh number on assessing the significance of supercontinent insulation several processes unfold during the supercontinent cycle more than one of which might result in an elevation in subcontinental mantle temperatures thus multiple interpretations of the concept of continental insulation exist although a consensus seems to have formed that subcontinental mantle upwellings appear below large continents extensively ringed by subduction zones there are differing views on what role continental insulation plays in the production of elevated mantle temperatures here we investigate how the heating mode of the mantle can change the influence of the thermal blanket effect we present 2 d and 3 d cartesian geometry mantle convection simulations with thermally and mechanically distinct oceanic and continental plates the evolution of mantle thermal structure is examined after continental accretion at subduction zones e g the formation of pangea for a variety of different mantle heating modes our results show that in low rayleigh number models the impact of the role of continental insulation on subcontinental temperatures increases when compared to models with higher convective vigor broad hot upper mantle features generated in low rayleigh number models due in part to the thermal blanket effect are absent at higher rayleigh numbers we find that subcontinental heating in a high rayleigh number flow occurs almost entirely as a consequence of the influence of subduction initiation at the continental margin rather than the influence of continental insulation in our models featuring earth like convective vigor we find that it is difficult to obtain subcontinental temperatures in significant excess of suboceanic temperatures over timescales relevant to supercontinent aggregation

thermal convection and the convective regime diagram in super earths numerical models of bottom heated thermal convection of highly compressible fluid with strongly temperature dependent viscosity are presented to understand how the rayleigh number ra and the temperature dependence of viscosity exert control over the regimes of thermal convection in massive super earths thermodynamic properties of mantle materials are pressure dependent but other material properties including the viscosity are not a stagnant lid develops along the surface of the planet when the viscosity contrast across the mantle due to temperature dependence r exceeds 106 at high rayleigh number relevant to super earths the threshold in r which increases with increasing ra is higher than that expected for the earth from earlier boussinesq models the efficiency of convective heat transport measured by the nusselt number nu is considerably lower than that expected from boussinesq models nu depends on ra and r as nu 59 r 0 23 ra 109 0 27 when r 105 strong adiabatic compression significantly reduces the activity of hot ascending plumes especially at high r at r relevant for super earths hot ascending plumes lose their buoyancy on their way and hardly reach the surface boundary hot spot volcanism due to ascending plumes is probably suppressed on super earths the lithosphere is considerably thicker than that suggested by earlier boussinesq models and is unlikely to show a plate like behavior

sinking of dense north atlantic waters in a global ocean model location and controls we investigate the characteristics of the sinking of dense waters in the north atlantic ocean that constitute the downwelling limb of the atlantic meridional overturning circulation amoc as simulated by two global ocean models an eddy permitting model at 1 4 resolution and its coarser 1 counterpart in line with simple geostrophic considerations it is shown that the sinking predominantly occurs in a narrow region close to the continental boundary in both model simulations that is the regions where convection is deepest do not coincide with regions where most dense waters sink the amount of near boundary sinking that occurs varies regionally for the 1 4 resolution model these variations are in quantitative agreement with a relation based on geostrophy and a thermodynamic balance between buoyancy loss and alongshore advection of density which links the amount of sinking to changes in density along the edge of the north atlantic ocean in the 1 model the amount and location of sinking appears not to be governed by this simple relation possibly due to the large impact of overflows and non negligible cross shore density advection if this poor representation of the processes governing the sinking of dense waters in the north atlantic ocean is a generic feature of such low resolution models the response of the amoc to changes in climate simulated by this type of models needs to be evaluated with care

labrador sea boundary currents and the fate of the irminger sea water abstract the general circulation of the labrador sea is studied with a dataset of 53 surface drifters drogued at 15 m and several hydrographic sections done in may 1997 surface drifters indicate three distinct speed regimes fast boundary currents a slower crossover from greenland to labrador and a slow eddy dominated flow in the basin interior mean eulerian velocity maps show several recirculation cells located offshore of the main currents in addition to the cyclonic circulation of the labrador sea above the northern slope of the basin the surface drifters have two preferential paths one between the 1000 m and 2000 m isobaths and the other close to the 3000 m isobath the vertical shear estimated from ctd data supports the presence of two distinct currents around the basin one current more baroclinic flows between the 1000 m and 2000 m isobaths the other one more barotropic flows above the lower continental slope the irminger sea water carried by the boundary currents is altered as it tr

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

overturning the mediterranean thermohaline circulation for more than five decades the mediterranean sea has been identified as a region of so called thermohaline circulation namely of basin scale overturning driven by surface heat and freshwater exchanges the commonly accepted view is that of an interaction of zonal and meridional conveyor belts that sink at intermediate or deep convection sites however the connection between convection and sinking in the overturning circulation remains unclear here we use a multidecadal eddy permitting numerical simulation and glider transport measurements to diagnose the location and physical drivers of this sinking we find that most of the net sinking occurs within 50 km of the boundary away from open sea convection sites vorticity dynamics provides the physical rationale for this sinking near topography only dissipation at the boundary is able to balance the vortex stretching induced by any net sinking which is hence prevented in the open ocean these findings corroborate previous idealized studies and conceptually replace the historical offshore conveyor belts by boundary sinking rings they challenge the respective roles of convection and sinking in shaping the oceanic overturning circulation and confirm the key role of boundary currents in ventilating the interior ocean

wind driven recirculations and exchange in the labrador and irminger seas it is demonstrated that recently observed cyclonic recirculation gyres in the irminger and labrador seas may be forced by the strong cyclonic wind stress curl that develops each winter seaward of the east coast of greenland idealized analytical and numerical models forced with such variable winds over a sloping bottom reproduce the essential aspects of the observed gyres strength location and horizontal and vertical length scales the communication between the forcing region in the irminger sea and the recirculation to the west is achieved by baroclinic topographic rossby wave propagation along potential vorticity contours the circulation is characterized as a time dependent stratified topographic beta plume for weak stratification as found in the subpolar north atlantic the recirculation strength exhibits only weak seasonal variability consistent with the observations even though the forcing is active only during the winter baroclinic rossby waves that develop when the wind forcing ceases in springtime interact with the bottom to provide a source of cyclonic vorticity that maintains the circulation until the wind strengthens again in the following winter

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

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

temporal variability of the atlantic meridional overturning circulation at 26 5 n the vigor of atlantic meridional overturning circulation moc is thought to be vulnerable to global warming but its short term temporal variability is unknown so changes inferred from sparse observations on the decadal time scale of recent climate change are uncertain we combine continuous measurements of the moc beginning in 2004 using the purposefully designed transatlantic rapid climate change array of moored instruments deployed along 26 5 n with time series of gulf stream transport and surface layer ekman transport to quantify its intra annual variability the year long average overturning is 18 7 5 6 sverdrups sv range 4 0 to 34 9 sv where 1 sv a flow of ocean water of 106 cubic meters per second interannual changes in the overturning can be monitored with a resolution of 1 5 sv

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

the influence of tectonic plates on mantle convection patterns temperature and heat flow summary the dynamic coupling between plate motion and mantle convection is investigated in a suite of cartesian models by systematically varying aspect ratios and plate geometries the aim of the study presented here is to determine to what extent plates affect mantle flow patterns temperature and surface heat flux to this end we compare numerical convection models with free slip boundary conditions to models that incorporate between one and six plates where the geometries of the plates remain fixed while the plate velocities evolve dynamically with the flow we also vary the widths of the plates and the computational domain in order to determine what constraint these parameters place on the mean temperature heat flux and plate velocity of mantle convection models we have investigated the influence of plates for three whole mantle convection cases that differ in their heating modes internally heated and basally heated and rheologies isoviscous and depth dependent viscosity we present a systematic investigation of over 30 models that exhibit increasingly complex behaviour in order to understand highly time dependent systems using the insight gained from simpler models in models with aspect ratios from 0 5 to 12 we find that for the same heating mode variations in temperature can be as much as 40 per cent when comparing calculations with unit width plates to models incorporating plates with widths equal to five times the model depth mean surface heat flux may decrease by 60 per cent over the same range of plate widths we also find that internally heated mantle convection models incorporating plates exhibit novel behaviour that we believe has not been described previously in mantle convection studies specifically in internally heated models plate motion is characterized by episodic reversals in direction driven by changes in the mantle circulation from clockwise to counterclockwise and vice versa these flow reversals occur in internally heated convection and are caused by a build up of heat in the interiors of wide convection cells close to mantle downwellings we find that flow reversals occur rapidly and are present in both single plate and multiple plate models that include internal heating this behaviour offers a possible explanation for why the pacific plate suddenly changed its direction some 43 ma

the production of north atlantic deep water sources rates and pathways updating an earlier account by dickson et al 1990 this paper reviews the initial development phase of north atlantic deep water nadw production from the points where the dense inflows from nordic seas cross the greenland scotland ridge to the point off south greenland where the buildup of new production appears almost complete in particular three long term current meter arrays totaling 91 instruments and set at 160 km intervals south from the denmark strait sill are used to validate earlier short term arrays by others and in combination with these earlier arrays to describe the downstream evolution of mean speed depth and entrainment the variability of the overflow current in space and time and the likely contribution of the other three main constituents of nadw production at densities greater than 27 8 from the points of overflow 5 6 sv the transport within this range increases by entrainment and confluence with other contributory streams to around 13 3 sv at cape farewell while recirculating elements prevent us from determining the net southgoing transport a nadw transport of this order appears consistent with recent estimates of net abyssal flow passing south through the north and south atlantic

plate tectonics on super earths equally or more likely than on earth abstract the discovery of extra solar super earths has prompted interest in their possible mantle dynamics and evolution and in whether their lithospheres are most likely to be undergoing active plate tectonics like on earth or be stagnant lids like on mars and venus the origin of plate tectonics is poorly understood for earth likely involving a complex interplay of rheological compositional melting and thermal effects which makes it challenging to make reliable predictions for other planets nevertheless as a starting point it is common to parameterize the complex processes involved as a simple yield stress that is either constant or has a byerlee s law dependence on pressure because the simplifying assumptions made in developing analytical scalings may not be valid over all parameter ranges numerical simulations are needed one numerical study on super earths finds that plate tectonics is less likely on a larger planet o neill and lenardic 2007 in apparent contradiction of an analytical scaling study valencia et al 2007 to try and understand this we here present new calculations of yielding induced plate tectonics as a function of planet size focusing on the idealized end members of internal heating or basal heating as well as different strength profiles and compared to analytical scalings in the present study we model super earths as simple scaled up versions of earth i e assuming constant physical properties keeping the ratio of core mantle radii constant and applying the same temperature difference between top bottom boundaries and the same internal heating rate effects that originate outside of the planet such as tidal forces meteor impacts and intense surface heating from a nearby star are not considered we find that for internally heated convection plate tectonics is equally likely for terrestrial planets of any size whereas for basally heated convection plate tectonics becomes more likely with increasing planet size this is indicated both by analytical scalings and the presented numerical results which agree with each other when scalings are adjusted to account for increasing mean density with increasing planet size plate tectonics becomes more likely with increasing planet size for all scenarios the influence of the pressure variation of viscosity thermal expansivity and conductivity may however act in the opposite sense and needs to be determined in future studies at least in the simplest case factors other than planet size such as the presence of surface water are likely most important for determining the presence or absence of plate tectonics

two stage dissociation in mgsio3 post perovskite abstract the fate of mgsio 3 post perovskite under tpa pressures is key information for understanding and modeling interiors of super earths type exoplanets and solar giants cores here we report a dissociation of mgsio 3 post perovskite into cscl type mgo and p2 1 c type mgsi 2 o 5 at 0 9 tpa obtained by first principles calculations p2 1 c type mgsi 2 o 5 should dissociate further into cscl type mgo and fe 2 p type sio 2 at 2 1 tpa the first dissociation should occur in all solar giants and heavy super earths while the second one should occur only in jupiter and larger exoplanets both dissociations are endothermic and have large negative clapeyron slopes if the first dissociation should occur in the middle of a silicate mantle it could promote mantle layering we provide essential thermodynamic properties of p2 1 c type mgsi 2 o 5 for modeling interiors of super earths

the influence of mantle internal heating on lithospheric mobility implications for super earths abstract super earths a recently discovered class of exoplanets have been inferred to be of a similar rock and metal composition to the earth as a result the possibility that they are characterised by the presence of plate tectonics has been widely debated however as the super earths have higher masses than earth it is assumed that they will also have higher rayleigh numbers and non dimensional heating rates accordingly we conduct a systematic 2d study to investigate the influence of these parameters on the surface behaviour of mantle convection the main focus of our work considers the response of surface motion to the mantle s internal heating however we also include an analysis of other parameters scaling with planet mass such as viscosity in agreement with the findings of valencia and o connell 2009 and van heck and tackley 2011 we find plate like surface mobilisation for increased rayleigh numbers but increasing the internal heating leads to the formation of a strong stagnant lid because the mantle heating effects thermally activated viscosity additionally viscosity is affected by the increased pressures and temperatures of super earths in total our findings indicate that surface mobility will likely be reduced on super sized earths our numerical models show that the interior temperature of the convecting system is of vital importance in planets with a hotter interior plate tectonics is less likely

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