drift dynamics in a coupled model initialized for decadal forecasts drifts are always present in models when initialized from observed conditions because of intrinsic model errors those potentially affect any type of climate predictions based on numerical experiments model drifts are usually removed through more or less sophisticated techniques for skill assessment but they are rarely analysed in this study we provide a detailed physical and dynamical description of the drifts in the cnrm cm5 coupled model using a set of decadal retrospective forecasts produced within cmip5 the scope of the paper is to give some physical insights and lines of approach to on one hand implement more appropriate techniques of initialisation that minimize the drift in forecast mode and on the other hand eventually reduce the systematic biases of the models we first document a novel protocol for ocean initialization adopted by the cnrm cerfacs group for forecasting purpose in cmip5 initial states for starting dates of the predictions are obtained from a preliminary integration of the coupled model where full field ocean surface temperature and salinity are restored everywhere to observations through flux derivative terms and full field subsurface fields below the prognostic ocean mixed layer are nudged towards nemovar reanalyses nudging is applied only outside the 15 s 15 n band allowing for dynamical balance between the depth and tilt of the tropical thermocline and the model intrinsic biased wind a sensitivity experiment to the latitudinal extension of no nudging zone 1 s 1 n instead of 15 hereafter referred to as noeq has been carried out in this paper we concentrate our analyses on two specific regions the tropical pacific and the north atlantic basins in the pacific we show that the first year of the forecasts is characterized by a quasi systematic excitation of el nino southern oscillation enso warm events whatever the starting dates this through ocean to atmosphere heat transfer materialized by diabatic heating can be viewed for the coupled model as an efficient way to rapidly adjust to its own biased climate mean state weak cold enso events tend to occur the second year of the forecast due to the so called discharge recharge mechanism while the spurious oscillatory behavior is progressively damped the latter mechanism is much more pronounced in retrospective forecasts initialized from the noeq configuration for which the enso flip flop is still detectable at leadtime 4 year associated atmospheric teleconnections interfere worldwide with regional drifts especially in the north pacific and more remotely in the north atlantic in the latter basin the drift can be interpreted as the model response to intrinsic atmospheric circulation biases found in the stand alone atmosphere component of the model which project onto the negative phase of the north atlantic oscillation a fast adjustment up to 5 year leadtime occurs leading to a rapid slackening of both the vertical atlantic meridional overturning circulation and horizontal circulations especially in the subpolar gyre slower adjustment of the entire water masses distribution in the north atlantic then takes over involving several mechanisms we show that a weak feedback is locally present between the atmospheric circulation and the ocean drift that controls the timescale of the setting of the coupled model biases

venus topography a harmonic analysis a model of venusian global topography has been obtained by fitting an eighteenth degree harmonic series to pioneer venus orbiter radar altimeter data the mean radius is 6051 45 0 04 km the corresponding mean density is 5244 8 0 5 kg m 3 the center of figure is displaced from the center of mass by 0 339 0 088 km towards 6 6 10 1 n 148 8 7 7 e the figure of venus is distinctly triaxial but the orientation and magnitudes of the principal topographic axes correlate rather poorly with the gravitational principal axes however the higher degree harmonics of topography and gravity are significantly correlated the topographic variance spectrum of venus is very similar in form to those of the moon mars and especially earth we suggest that this spectral similarity simply reflects a statistical balance between constructional and degradational geomorphic processes venus and earth are particularly similar and differ from the moon and mars in that the larger bodies both exhibit a significant low degree deficit relative to the extrapolated trend of the higher harmonics

dynamics of decadal variability in the atlantic subpolar gyre a stochastically forced oscillator internal variability of the atlantic subpolar gyre is investigated in a 600 years control simulation of a comprehensive coupled climate model the subpolar gyre shows irregular oscillations of decadal time scale with most spectral power between 15 and 20 years positive and negative feedback mechanisms act successively on the circulation leading to an internal oscillation this involves periodically enhanced deep convection in the subpolar gyre center and intermittently enhanced air sea thermal coupling as a result anomalies of the large scale atmospheric circulation can be transferred to the ocean on the ocean s intrinsic time scale exciting the oscillator stochastically a detailed understanding of oscillatory mechanisms of the ocean and their sensitivity to atmospheric forcing holds considerable potential for decadal predictions as well as for the interpretation of proxy data records

inter hemispheric asymmetry in the early pleistocene pacific warm pool the position of the southern boundary of the pacific warm pool is shown to have been stable since the early pleistocene based upon a planktic foraminiferal mg ca derived reconstruction of subtropical sea surface temperature in the coral sea this contrasts with previous reconstructions showing warm pool contraction from the north and east and means that the early pleistocene warm pool was more hemispherically asymmetric than its present configuration the latter was not established until 1ma supporting a strengthening of the northern hadley cell which was not replicated in its southern counterpart prior to the mid pleistocene transition copyright 2010 by the american geophysical union

high resolution gravity model of venus the anomalous gravity field of venus shows high correlation with surface features revealed by radar we extract gravity models from the doppler tracking data from the pioneer venus orbiter pvo by means of a two step process in the first step we solve the nonlinear spacecraft state estimation problem using a kalman filter smoother the kalman filter has been evaluated through simulations this evaluation and some unusual features of the filter are discussed in the second step we perform a geophysical inversion using a linear bayesian estimator to allow an unbiased comparison between gravity and topography we use a simulation technique to smooth and distort the radar topographic data so as to yield maps having the same characteristics as our gravity maps the maps presented cover 2 3 of the surface of venus and display the strong topography gravity correlation previously reported the topography gravity scatter plots show two distinct trends

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

pacific thermocline bridge revisited abstract the coupling on decadal time scales of the mid latitude and tropical pacific via an oceanic bridge in the thermocline is investigated using ocean general circulation model hindcasts and a coupled ocean atmosphere model results indicate that in the tropics decadal anomalies of isopycnal depth are forced by ekman pumping and are largely independent of the arrival of subducted anomalies in the thermocline that originate in the mid latitudes of either hemisphere in the coupled model temperature anomalies on isopycnals show little coupling from the tropics to the northern hemisphere but are lag correlated between southern hemisphere mid and low latitudes however anomaly magnitudes on the equator are small these results suggest that the oceanic bridge to the northern hemisphere explains only a small part of the observed decadal variance in the equatorial pacific coupling to the southern mid latitudes via temperature anomalies on isopycnals remains an intriguing possibility

low frequency interaction between horizontal and overturning gyres in the ocean 1 low frequency variability of the horizontal circulation in an idealized eddy permitting numerical model drives the dominant mode of low frequency variability in the meridional overturning circulation this coupling takes place through the influence of lateral advection in the cyclonic high latitude boundary current on the mixed layer depth along the boundary the mean and low frequency variability of the meridional overturning circulation are well predicted by a diagnostic estimate that assumes the downwelling is controlled by the thermal wind shear within the mixed layer along the boundary which is in turn determined by a simple balance between lateral advection and surface cooling the more general result is the demonstration that the mean and low frequency variability of the meridional overturning streamfunction are controlled by the baroclinic pressure gradient within the mixed layer along the boundary which may be influenced by numerous factors such as low frequency variability in lateral advection wind stress surface buoyancy fluxes or ice melt and freshwater runoff

constraints on the venusian crustal thickness variations in the isostatic stagnant lid approximation 1 the isostatic stagnant lid isl approximation is applied to the long wavelength venusian topography and geoid to place constraints on the thickness of the crust and lithosphere previous studies showed that a purely thermal isostasy model of the lithosphere the stagnant lid accounts for a significant portion of the topography and geoid anomalies here we assume that the remaining discrepancy between the observed and model geoid is due to crustal thickness variations and that the crust the lithosphere and the mantle are in a state of double isostatic equilibrium the lithosphere is isostatically floating on top of the mantle and the crust is isostatically floating on top of the lithosphere the addition of another variable the crustal thickness allows an exact fit to the observed topography and geoid this also makes the solution non unique an exact fit to topography and geoid can be achieved in a broad range of average lithospheric and crustal thicknesses additional constraints such as the gabbro eclogite phase transition and the existence of relatively young volcanism limit this range to some extent the inferred variations in the crustal thickness are less sensitive to model assumptions and generally agree with the results obtained in previous studies

a 20 year coupled ocean sea ice atmosphere variability mode in the north atlantic in an aogcm in order to understand potential predictability of the ocean and climate at the decadal time scales it is crucial to improve our understanding of internal variability at this time scale here we describe a 20 year mode of variability found in the north atlantic in a 1 000 year pre industrial simulation of the ipsl cm5a lr climate model this mode involves the propagation of near surface temperature and salinity anomalies along the southern branch of the subpolar gyre leading to anomalous sea ice melting in the nordic seas which then forces sea level pressure anomalies through anomalous surface atmospheric temperatures the wind stress associated to this atmospheric structure influences the strength of the east greenland current across the denmark strait which in turn induces near surface temperature and salinity anomalies of opposite sign at the entrance of the labrador sea this starts the second half of the cycle after approximatively 10 years the time scale of the cycle is thus essentially set by advection of tracers along the southern branch of the subpolar gyre and by the time needed for anomalous east greenland current to accumulate heat and freshwater anomalies at the entrance of the labrador sea the atlantic meridional overturning circulation amoc does not play a dominant role in the mode that is confined in the subpolar north atlantic but it also has a 20 year preferred timescale this is due to the influence of the propagating salinity anomalies on the oceanic deep convection the existence of this preferred timescale has important implications in terms of potential predictability of the north atlantic climate in the model although its realism remains questionable and is discussed

geostrophic dynamics of meridional transport variability in the subpolar north atlantic the north atlantic meridional overturning 6 circulation moc is believed to play an important role in regulating the earth s climate yet there is still much uncertainty regarding the dynamics of the moc and its variability it is well established however that through geostrophy the zonally integrated meridional transport at a particular latitude and depth can be determined from the east west bottom pressure difference across the basin therefore rather than consider the moc as a large scale system this paper focuses on the dynamics of this geostrophic relationship in two numerical ocean models at a single latitude 50 14 n in the sub polar atlantic first it is shown that the bottom pressure on the western boundary is sufficient to recover with high fidelity the interannual meridional transport variability at 50 17 n over a one hundred year period in the climate model hadcm3 it is found that the variability of western boundary pressure is closely associated with density changes over the continental slope these changes lead to a large zonal gradient in potential energy and an unfeasible depth mean velocity over the slope the western boundary pressure from which the meridional transport can be recovered is generated as a compensation to this and limits the depth mean flow this demonstrates that in numerical ocean models at least meridional transport variability is generated as a local response to density changes on the western slope whether this is a true representation of actual ocean variability is uncertain but if it were then meridional transport variability could largely be determined using only the density field on the western slope

where does dense water sink a subpolar gyre example it is proposed that a dominant component of the downwelling limb of the thermohaline circulation takes place in regions where convective mixing is found adjacent to steep topography a simple theoretical estimate of the overturning forced by such boundary convection is derived that depends only on the properties of the oceanic mixed layer along the boundary scaling estimates indicate that sinking forced by boundary convection is an order of magnitude greater than sinking in the open ocean resulting from large scale dynamics or baroclinic instability of deep convective sites recent hydrographic observations in the labrador sea are used to estimate the downwelling due to these different mechanisms and support the notion that boundary sinking dominates the theory compares well with the overturning rates diagnosed in a noneddy resolving general circulation model over a wide range of parameters as a direct consequence of these dynamics the high latitude hydrography and overturning circulation in the model are very sensitive to the presence of cyclonic rim currents lateral density advection by the rim currents in the subpolar gyre increases the stratification and limits the mixing near the boundaries thus reducing the maximum downwelling as a result most of the high latitude meridional heat transport is carried by the horizontal circulation instead of the overturning circulation such rim currents are found in different configurations of the model including 1 a continental slope and standard diffusion parameters and 2 zero horizontal diffusion and a flat bottom

mantle downwelling and crustal convergence a model for ishtar terra venus the ishtar terra region contains the highest topography known on venus over 10 km above mean planetary radius as well as abundant tectonic features many of apparently compressional origin these characteristics suggest that ishtar is a crustal convergence zone overlying downwelling mantle in order to explore quantitatively the implications of this hypothesis for ishtar s origin we present models of viscous crustal flow driven by gradients in topography assuming a free slip surface boundary condition we find that if the crustal convergence hypothesis is correct then the crustal thickness in the plains surrounding ishtar can be no more than about 25 km thick this upper bound assumes a cold 10 k km 1 geotherm and the stiffest available diabase flow law if the geothermal gradient is larger or the rheology is weaker the crust must be even thinner for net crustal convergence to be possible this upper bound is in good agreement with several independent estimates of crustal thickness of 15 30 km in the plains of venus based on modeling of the spacing of tectonic features and of impact crater relaxation if the surface layer of venus provides a no slip boundary then our models allow the crustal thickness in the plains to be up to 50 km but the likely existence of faults that cut through the crust makes a no slip surface layer unlikely our upper bound on crustal thickness is much less than that derived from an airy isostasy model of ishtar s gravity anomaly much of the observed gravity anomaly must be due to density anomalies in the mantle beneath ishtar although we treat ishtar as a crustal convergence zone our crustal flow model shows that under some circumstances near surface material may actually flow away from ishtar providing a possible explanation for grabenlike structures in fortuna tessera

a refined gravity model from lageos gem l2 a refined gravity field model goddard earth model gem l2 has been derived using the lageos orbital data yielding better baseline measurements for the analysis of tectonic plate motion this field also contributes to an improved understanding of long wavelength features such as the sea slope across broad ocean basins through its significant improvement of the long wavelength geoid through degree and order 4 the geoid for these terms has an accuracy estimated at 8 cm in gem l2 gem l2 as in all recent goddard earth models relies heavily on the precise near earth satellite laser ranging data in this case provided by nasa s crustal dynamics program two and a half years of lageos laser data acquired from over 20 well distributed stations were combined with the existing data from the best satellite derived model gem 9 to develop the new lageos model testing shows that the lageos gravity field error at long wavelengths is less than half that for gem 9 independent tests using well determined longitude accelerations of 24 hour satellites have verified the improved accuracy of the new model a comparison of global laser base stations from independent data sets of alternating 15 day data segments over the 2 1 2 years of lageos show total inter station positioning to 1 8 cm when using this new field the same comparison using the 1979 versus the 1980 lageos data yields 5 2 cm this difference in agreement reflects the change in data distribution and other systematic errors along with the tectonic motion which has occurred between these chronologically distinguishable data sets five day average polar motion values with a precision of 10 cm and change in length of day values accurate to better than 5 msec have been derived in the solution the adjustment of these parameters are necessary to achieve the accurate stations and geopotential results in gem l2

assimilation of rapid array observations into an ocean model experiments assimilating the rapid dataset of deep temperature and salinity profiles at 26 5 n on the western and eastern atlantic boundaries into a 1 global nemo ocean model have been performed the meridional overturning circulation moc is then assessed against the transports calculated directly from observations the best initialization found for this short period was obtained by assimilating the en3 upper ocean hydrography database prior to 2004 after which different methods of assimilating 5 day average rapid profiles at the western boundary were tested the model moc is strengthened by 2 sv giving closer agreement with the rapid array transports when the western boundary profiles are assimilated only below 900 m the approximate depth of the florida straits which are not well resolved and when the t s observations are spread meridionally from 10 to 35 n along the deep western boundary the use of boundary focused covariances has the largest impact on the assimilation results otherwise using more conventional gaussian covariances has a very local impact on the moc at 26 n with strong adverse impacts on the moc stream function at higher and lower latitudes even using boundary focused covariances only enables the moc to be strengthened for 2 years after which the increased transport of warm waters leads to a negative feedback on water formation in the subpolar gyre which then reduces the moc this negative feedback can be mitigated if en3 hydrography data continue to be assimilated along with the rapid array boundary data copyright 2012 royal meteorological society and crown in the right of canada

geological correlations with the interior density structure of venus a model has been constructed by stipulating that venusian long wavelength topography and geoid elevations are explained by the sum of density anomalies associated with mantle convection and lithospheric compensation mechanisms for convenience the latter are represented by a single airy surface because individual lithospheric components cannot be resolved at the wavelengths considered in this paper mantle convection is represented at a single depth by mass perturbations that drive viscous flow we invert spherical harmonic representations of the topography and the geoid to create global representations of the mantle convection pattern and near surface density anomalies the mantle convection pattern matches well with previous numerical simulations of mantle convection that show a pattern of isolated upwellings amidst an interconnected network of downwellings the near surface density anomalies show no particular pattern the inversion results have been compared with a global synthesis of magellan data impact crater density does not correlate with the mantle convection pattern or the near surface density anomalies large volcanic structures are associated with upwellings planitiae are associated with downwellings coronae are anticorrelated with high amplitude upwellings and downwellings and apparently are passive features associated with rifting that is related to mantle convection in a complex fashion tesserae are associated with negative lithospheric density anomalies interpreted as areas of thickened crust tesserae show no definitive relationship to the present mantle convection pattern

tectonics and evolution of venus the global tectonics of venus differs significantly from that of earth most markedly in that the surface is covered predominately by gently rolling terrain there apparently are no features like ocean rises the gravity is positively correlated with topography at all wavelengths and the few highlands are estimated to be supported or compensated at a depth of approximately 100 kilometers the surface of venus appears to be covered mainly by an ancient crust the high surface temperature making subduction difficult it seems likely that well over 1 billion years ago water was destabilized at the surface and soon after plate tectonics ceased the highlands appear to be actively supported presumably as manifestations of long enduring hot spots

rapid and synchronous ice dynamic changes in east greenland two major outlet glaciers in east greenland have suddenly begun to accelerate and retreat the speeds of kangerdlugssuaq and helheim remained steady during the 1990s despite progressive and substantial thinning but have abruptly increased within the last two years more than doubling ice flux to the ocean had it been an isolated example the comparable 1998 speed up of jakobshavn isbra in west greenland might have been explained simply by its chance retreat past a pinning point now that two further greenland outlets have exhibited similar behavior a common process seems likely a remarkable correspondence in the inter annual patterns of speed and ice front variation between kangerdlugssuaq and helheim implies a significant sensitivity to regional environmental factors the period of continued warming and thinning appears to have primed these glaciers for a step change in dynamics not included in current models we should expect further greenland outlet glaciers to follow suit copyright 2006 by the american geophysical union

long term rotation and mantle dynamics of the earth mars and venus in response to internal and surface tectonic processes the terrestrial planets have the ability to displace the axis of rotation with respect to the mantle this behavior is quantified by means of a nondimensional number defined here as the rotational number ro that allows classification of the planets into two categories the first containing mars and the earth where true polar wander is a feasible mechanism and the second to which venus belongs where rotation equilibrium is attained by means of mega wobbles driven by internal mass anomalies the number ro is related to the timescale characterizing the readjustment of the equatorial bulge during long term polar motion and to the length of the sidereal day if these two timescales are well separated the planet experiences true polar wander nonlinear liouville equations for stratified viscoelastic bodies with linear maxwell rheology are solved in order to assess the relevance of surface and mantle processes in driving long term rotation instabilities in the terrestrial planets the amount of true polar wander estimated for the earth and mars is reproduced correctly by our modeling with mantle viscosities and lithospheric thickness consistent with other studies the major difference between the earth and mars is the driving mechanism subduction for the earth and lithospheric loading for mars when mantle viscosities similar to those of the earth are considered for venus the most updated estimate for the offset of about 0 5 between the rotation axis and the axis of maximum inertia is well reproduced during the mega wobbles induced by internal mass redistribution we show that the degree 2 topography of these three planets can be affected by their rotation which is responsible for the dominance of the sectorial component on the earth and mars and for the zonal component on venus

regional topographic rises on venus geology of western eistla regio and comparison to beta regio and atla regio regional topographic rises in the equatorial region of venus western eistla regio beta regio and atla regio form a unique class of structures which on the basis of geologic mapping and geophysical data are interpreted to be sites of mantle upwelling or plumes the geologically least complex of these regions is western eistla regio an elongated domical rise that contains the volcanoes sif and gula montes sif mons is a large volcano diameter 100 km whose local stratigraphy indicates that it has undergone a history marked by initial radial fracturing widespread outpouring of lava additional fracturing along its northern flank and extrusion of lava that is either extremely fluid or was erupted at a high rate of effusion sif contains a 40 km diameter summit caldera within which are nested smaller calderas and pit structures theoretical considerations suggest multiple levels of magma emplacement within the volcano with the primary reservoir residing in the crust below the edifice gula mons is made up of multiple volcanic source regions and is associated with guor linea a zone of lithospheric stretching faulting and rifting the geologically complex highland of beta regio is a tectonic junction with three arms of devana chasma intersecting at theia mons magellan data reveal 1 the details of rifting and volcanism concentrated at theia mons 2 that rhea mons is highly deformed and possesses characteristics of tesserae and whose origin as a volcano is placed in question 3 that tesserae are a major unit arrayed in a pattern around theia from east to west starting on the eastern flank of beta and continuing westward to the northern part of asteria regio 4 that the northern periphery of the highland contains coronae connected by fractures forming chains and 5 the presence of a deformed crater within devana chasma split and extended in an east west direction indicating that part of the rift has undergone as much as 20 30 extension like beta atla regio is a complex area of converging rifts five separate arms that are centered on the volcano ozza mons the magellan images show that volcanism and rifting have occurred contemporaneously at ozza with lava flows both covering and being crosscut by faults a second volcano maat mons contributes to the infilling of the rift dali chasma and does not appear to be deformed by faulting two of the rifts at atla contain chains of coronae suggesting that upwelling along linear zones of extension has occurred beta atla and western eistla are similar in that they are broad regional rises are associated with major rift zones have large apparent depths of isostatic compensation and contain large volcanoes their associations with features mapped as coronae and tesserae are quite variable suggesting that the detailed characteristics of the individual highlands are most likely linked to each areas local geologic environment

aspherical earth structure from fundamental spheroidal mode data free oscillation data reveal heterogeneity in the earth s mantle whose geographical pattern is dominated by spherical harmonics of angular degree two and correlates well with the hydrostatically referenced geoid the heterogeneity can be modelled as localized in the transition zone 420 670 km depth and may be related to a large scale component of mantle convection

variations in effective compensation depth across aphrodite terra venus aphrodite terra is the largest elevated terrain on venus and it serves as a focal point in current discussions of global tectonic style using the topography and gravity data acquired by the pioneer venus orbiter pvo the authors have estimated an effective depth of airy compensation for each of 75 orbital arcs that provide fairly uniform areal coverage of the entire province the most pronounced pattern that emerges is a general increase in compensation depth to the east the most rapid change occurs near 135 degree the average west of there is 70 km while the average to the east is 230 km superimposed on this larger trend are five distinctive regional patterns four well defined peaks and one interval of widely scattered and poorly constrained depths the maxima in compensation depth are well correlated with regional topographic highs while these observations are easily reconciled with the general notion that aphrodite is a region of crustal divergence the great depth of compensation is difficult to accord with the more specific suggestion that aphrodite is a terrestrial type divergent plate margin the alternative suggestion that aphrodite and the other equatorial highlands on venus are surface manifestations of hot upwelling mantle plumes ismore consistent both with the great depths of compensation and with the pattern of regional peaks and intervening troughs the broader scale increase in effective depth of compensation from west to east is more enigmatic but it might reflect an age progression of the plumes less

fundamental issues in the geology and geophysics of venus a number of important and currently unresolved issues in the global geology and geophysics of venus will be addressable with the radar imaging altimetry and gravity measurements now forthcoming from the magellan mission among these are the global volcanic flux and the rate of formation of new crust the global heat flux and its regional variations the relative importance of localized hot spots and linear centers of crustal spreading to crustal formation and tectonics and the planform of mantle convection on venus and the nature of the interactions among interior convective flow near surface deformation and magmatism

gravity anomalies compensation mechanisms and the geodynamics of western ishtar terra venus pioneer venus line of sight orbital accelerations have been used to calculate the geoid and vertical gravity anomalies for western ishtar terra on various planes of altitude z0 the apparent depth of isostatic compensation at z0 1400 km is 180 20 km based on the usual method of minimum variance in the isostatic anomaly we seek to explain this observation as well as the regional elevation peripheral mountain belts and inferred age of western ishtar terra in terms of one of three broad geodynamic models under local mantle upwelling western ishtar is the surface expression of a hotspot which produces topography both dynamically and by volcanic construction this model holds that the mountain belts form as a result of incipient downward mantle return flow an upwelling with its centroid at 160 km depth and buoyancy stress 100 mpa can satisfy the observed long wavelength geoid and topography provided that the crust mantle density contrast does not control the response of the crust so that late time subsidence does not occur and that the mantle viscosity does not increase markedly with depth mantle upwelling produces a combination of strike slip motion and azimuthal extension on the uplift itself radial compression is restricted to the surrounding plains under local mantle downwelling cold sinking mantle beneath western ishtar induces shear tractions on the crust which thicken and elevate it volcanism is potentially the result of basal crustal remelting the formation of mountain belts is assumed to be a by product of crustal thickening a deeply seated downwelling 300 700 km buoyancy stress 200 300 mpa can satisfy the geoid and central topography provided that the crust can respond separately to mantle flow and that a more viscous lower mantle exists the predicted surface strains during intermediate stages of the uplift are in qualitative agreement with observations azimuthally oriented thrusts at the uplift margin incorporation of the effective viscosities of diabase and olivine suggests that uplift above a plume will be rapid but the thermal gradient over downgoing flow must be at least as high as the expected mean value for venus 20 k km in order to attain substantial uplift within several hundred million years a third scenario regional compression may be distinguished from the second in that western ishtar is simply a locus of strain accumulation and crustal thickening from regional stress fields neither broad downward mantle flow nor lithospheric subduction need occur however both the source of stress and geoid anomaly are problematic if the topography of western ishtar is assumed to be airy compensated then the geoid signature must arise from unrelated mantle density anomalies in this case the lithosphere must be decoupled from mantle flow or else the mantle must be effectively rigid in order to suppress dynamic topography a more complex model than those analyzed in detail here is needed to produce both the correct surface strains and apparent compensation depth we favor upwelling models in which a mantle plume has flattened out and the surface has been subsequently deformed western ishtar terra must be distinguished from other paradigm hotspots on venus by different environmental conditions or by being in a unique stage of hotspot evolution further quantitative investigations of the interaction of mantle shear tractions with a laterally heterogeneous lithosphere are required

local and remote impacts of a tropical atlantic salinity anomaly the climatic impacts of an enhanced evaporation prescribed during 50 years in the tropical atlantic are investigated in a coupled ocean atmosphere general circulation model locally the salinity increase leads to a rapid deepening and cooling of the surface mixed layer this induces a deepening of the equatorial undercurrent and an intensification of the south equatorial current a remote atmospheric response to the tropical atlantic perturbation is detected in the north atlantic sector after ten years it has the form of a robust wave like tropospheric perturbation seemingly excited by the weakening of atmospheric deep convection over the amazonian basin meanwhile the salt anomaly is carried northward by the mean oceanic circulation it is traced up to the convection sites and then on its return path at depth towards lower latitudes consistent with the density increase deep convection is enhanced after the arrival of the salt anomaly and the atlantic meridional overturning circulation amoc intensifies about 20 years after the beginning of the perturbation the adjustment of the tropical atlantic to the amoc intensification then modifies its initial response to the freshwater forcing leading to a weaker cooling in the northern tropical atlantic than in the southern tropical atlantic a slight northward shift of the tropical atlantic precipitation pattern and an intensification of the north brazil current on the other hand no significant anomalous precipitations are found in the pacific the initial remote atmospheric response is also modulated by an nao like response to the amoc intensification

localization of gravity and topography constraints on the tectonics and mantle dynamics of venus summary we develop a method for spatio spectral localization of harmonic data on a sphere and use it to interpret recent high resolution global estimates of the gravity and topography of venus in the context of geodynamical models our approach applies equally to the simple spatial windowing of harmonic data and to variable length scale analyses which are analogous to a wavelet transform in the cartesian domain using the variable length scale approach we calculate the localized rms amplitudes of gravity and topography as well as the spectral admittance between the two fields as functions of position and wavelength the observed admittances over 10 per cent of the surface of venus highland plateaus and tessera regions are consistent with isostatic compensation of topography by variations in crustal thickness while admittances over the remaining 90 per cent of the surface rises plains and lowlands indicate that longwavelength topography is dominantly the result of vertical convective tractions at the base of the lithosphere the global average crustal thickness is less than 30 km but can reach values as large as 40 km beneath tesserae and highland plateaus we also note that an earth like radial viscosity structure cannot be rejected by the gravity and topography data and that without a mechanical model of the lithosphere admittance values cannot constrain the thickness of the thermal boundary layer of venus modelling the lithosphere as a thin elastic plate indicates that at the time of formation of relief in highland plateaus and tesserae the effective elastic plate thickness k was less than 20 km estimates of t at highland rises are consistently less than 30 km our inability to find regions with t 30 km is inconsistent with predictions made by a class of catastrophic resurfacing models

the modern geoid and ancient plate boundaries abstract the non hydrostatic geoid is dominated by three large anomalies an area of high gravity potential in the equatorial pacific another stretching from greenland through africa to the southwest indian ocean and a semi continuous low region passing from hudson s bay through siberia to india and on to antarctica none of these three high amplitude greater than 60 m and long wavelength anomalies corresponds to present day plate boundaries however if the modern geoid is plotted over the positions of continents and plate boundaries at 125 ma b p reconstructed relative to hotspots a strong correlation emerges the modern geoidal low corresponds in position to the areas of subduction surrounding the pacific 125 ma ago the geoidal high now centered on africa is entirely contained within ancient pangaea and the equatorial pacific high overlies the location of the spreading centers preserved in the magnetic anomalies of the central pacific the most plausible cause of the large geoidal undulations is lower mantle convection only weakly coupled to plate motions the correspondence between modern geoid and ancient plate boundaries implies either that the coupling was much more intimate in the past or that there is a lag of at least 100 ma in response of the lower mantle to upper mantle conditions

gravitational and topographic isotropy of the earth moon mars and venus the internal structures and surface shapes of the terrestrial planets are expected to be dominantiy isotropic we attempt to test this hypothesis by developing a criterion for recognizing statistical isotropy of spherical scalar fields and applying that test to the nonhydrostatic components of the gravitational and topographic fields of the earth moon mars and venus the criterion for topographic or geoidal surfaces is that the directional variance should be independent of azimuth that is the mean square east west slopes should be equal to the mean square north south slopes all four planets generally pass the criteria with a few minor exceptions most of the exceptions are likely to be artifacts of the current generation of planetary gravity and topography models rather than real features of the planets of the several broad band departures from isotropy which we found the only one likely to be real is the tendency for the earth s gravity and topography to have only 80 as much variance in a north south direction as in an east west direction

on the ratio of dynamic topography and gravity anomalies in a dynamic earth growing evidence from a variety of geologic indicators points to significant topography maintained convectively by viscous stresses in the mantle however while gravity is sensitive to dynamically supported topography there are only small free air gravity anomalies 30mgal associated with earth s long wavelength topography this has been used to suggest that surface heights computed assuming a complete isostatic equilibrium provide a good approximation to observed topography here we show that the apparent paradox is resolved by the well established formalism of global self gravitating viscously stratified earth models the models predict a complex relation between dynamictopography mass and gravity anomalies that is not summarized by a constant admittancei e ratio of gravity anomalies to surface deflectionsas one would infer from analytic flow solutions formulated in a half space our results suggest that sizable dynamic topography may exist without a corresponding gravity signal

reconstructing the subsurface ocean decadal variability using surface nudging in a perfect model framework initialising the ocean internal variability for decadal predictability studies is a new area of research and a variety of ad hoc methods are currently proposed in this study we explore how nudging with sea surface temperature sst and salinity sss can reconstruct the three dimensional variability of the ocean in a perfect model framework this approach builds on the hypothesis that oceanic processes themselves will transport the surface information into the ocean interior as seen in ocean only simulations five nudged simulations are designed to reconstruct a 150 years target simulation defined as a portion of a long control simulation the nudged simulations differ by the variables restored to sst or sst sss and by the area where the nudging is applied the strength of the heat flux feedback is diagnosed from observations and the restoring coefficients for sss use the same time scale we observed that this choice prevents spurious convection at high latitudes and near sea ice border when nudging both sst and sss in the tropics nudging the sst is enough to reconstruct the tropical atmosphere circulation and the associated dynamical and thermodynamical impacts on the underlying ocean in the tropical pacific ocean the profiles for temperature show a significant correlation from the surface down to 2 000 m due to dynamical adjustment of the isopycnals at mid to high latitudes sss nudging is required to reconstruct both the temperature and the salinity below the seasonal thermocline this is particularly true in the north atlantic where adding sss nudging enables to reconstruct the deep convection regions of the target by initiating a previously documented 20 year cycle of the model the sst sss nudging is also able to reproduce most of the amoc variations a key source of decadal predictability reconstruction at depth does not significantly improve with amount of time spent nudging and the efficiency of the surface nudging rather depends on the period events considered the joint sst sss nudging applied everywhere is the most efficient approach it ensures that the right water masses are formed at the right surface density the subsequent circulation subduction and deep convection further transporting them at depth the results of this study underline the potential key role of sss for decadal predictability and further make the case for sustained large scale observations of this field

on the interannual variability of surface salinity in the atlantic the mechanisms controlling the interannual variability of sea surface salinity sss in the atlantic are investigated using a simulation with the echam4 opa8 coupled model and for comparison the ncep reanalysis and an observed sss climatology anomalous ekman advection is found to be as important as the freshwater flux in generating sss anomalies in contrast to sea surface temperature sst anomalies which are primarily caused by surface heat flux fluctuations since the surface heat flux feedback does not damp the sss anomalies but generally damps existing sst anomalies sss anomalies have a larger characteristic time scale as a result they are more influenced by the mean currents and the geostrophic variability which dominate the sss changes at low frequency over much of the basin the link between sss anomalies and the dominant patterns of atmospheric variability in the north atlantic sector is also discussed it is shown that the north atlantic oscillation generates sss anomalies much more by ekman advection than by freshwater exchanges at least in the coupled model there is little one to one correspondence between the main atmospheric and sss anomaly patterns unlike what is found for sst anomalies

observational evidence of alternating zonal jets in the world ocean 1 multiple zonal jets with the east west velocity direction alternating with latitude are discovered in satellite altimetry data the time varying jets are shown to populate every part of the world ocean and its marginal seas and are best seen in the anomaly of geostrophic vorticity at midlatitudes the jets have a meridional wavelength of about 300 kms with r m s sea level velocity and vorticity values of 2 4 cm 6 9 cm s and 1 5 10 6 s 1 correspondingly realistic data from the high resolution ogcm run on the earth simulator are used to justify high vertical coherence of the jets structure and relevance of the jets to an evolving mesoscale eddy field strong coupling between the jets and mesoscale eddies is hypothesized

geoid anomalies and dynamic topography from convection in cylindrical geometry applications to mantle plumes on earth and venus summary a variety of evidence suggests that at least some hotspots are formed by quasi cylindrical mantle plumes upwelling from deep in the mantle we model such plumes in cylindrical axisymmetric geometry with depth dependent newtonian viscosity cylindrical and sheet like cartesian upwellings have significantly different geoid and topography signatures however rayleigh number nusselt number systematics in the two geometries are quite similar the geoid anomaly and topographic uplift over a plume are insensitive to the viscosity of the surface layer provided that it is at least 1000 times the interior viscosity increasing the rayleigh number or including a low viscosity asthenosphere decreases the geoid anomaly and the topographic uplift associated with an upwelling plume increasing the aspect ratio increases both the geoid anomaly and the topographic uplift of a plume the nusselt number is a weak function of the aspect ratio with its maximum value occurring at an aspect ratio of slightly less than 1

volcanism and tectonics on venus we review recent developments in the study of volcanism and tectonics on venus venus s crust is basaltic dry and probably about 30 km thick the mantle convects giving rise to plumes and has a similar composition and mean temperature 1300 c but a higher viscosity 10 20 pa s than that of the earth inferred melt generation rates constrain the lithospheric thickness to between 80 and 200 km the elastic thickness of the lithosphere is about 30 km on average the present day lack of plate tectonics may be due to strong faults and the high viscosity of the mantle most of the differences between earth and venus processes can be explained by the absence of water venus underwent a global resurfacing event 300 600 ma ago the cause and nature of which remains uncertain the present day surface heat flux on venus is about half the likely radiogenic heat generation rate which suggests that venus has been heating up since the resurfacing event

subducted slabs and the geoid constraints on mantle rheology and flow geoid anomalies are primarily the result of the density contrasts driving mantle convection and plate motion the total geoid anomaly resulting from a given density contrast in a convecting earth is affected by the mass anomalies associated with the flow induced deformation of the upper surface and internal compositional boundaries as well as by the density contrast itself these boundary deformations and hence the total gravity field depend on the radial distribution of effective viscosity if the internal density contrasts can be estimated as is the case for subducted slabs useful constraints can be placed on the depth and on the variation of viscosity with depth of the convecting system the degree 4 9 components of the observed long wavelength geoid are highly correlated with those predicted by a density model for seismically active subducted slabs the positive sign of the correlation requires that the effective viscosity increases with depth by a factor of 30 or more the amplitude of the correlation cannot be explained by the density contrasts associated with just the seismically active parts of subducted slabs however the amplitude can be explained if the density contrasts associated with subduction extend into the lower mantle or if old lithosphere is piled up at the base of the upper mantle beneath subduction zones to a thickness in excess of 350 km over horizontal distances of thousands of kilometers mantlewide convection in a mantle that has a viscosity increasing with depth provides a simple explanation of the long wavelength geoid anomalies over subduction zones

reconstructing extreme amoc events through nudging of the ocean surface a perfect model approach while the atlantic meridional overturning circulation amoc is thought to be a crucial component of the north atlantic climate past changes in its strength are challenging to quantify and only limited information is available in this study we use a perfect model approach with the ipsl cm5a lr model to assess the performance of several surface nudging techniques in reconstructing the variability of the amoc special attention is given to the reproducibility of an extreme positive amoc peak from a preindustrial control simulation nudging includes standard relaxation techniques towards the sea surface temperature and salinity anomalies of this target control simulation and or the prescription of the wind stress fields surface nudging approaches using standard fixed restoring terms succeed in reproducing most of the target amoc variability including the timing of the extreme event but systematically underestimate its amplitude a detailed analysis of the amoc variability mechanisms reveals that the underestimation of the extreme amoc maximum comes from a deficit in the formation of the dense water masses in the main convection region located south of iceland in the model this issue is largely corrected after introducing a novel surface nudging approach which uses a varying restoring coefficient that is proportional to the simulated mixed layer depth which in essence keeps the restoring time scale constant this new technique substantially improves water mass transformation in the regions of convection and in particular the formation of the densest waters which are key for the representation of the amoc extreme it is therefore a promising strategy that may help to better constrain the amoc variability and other ocean features in the models as this restoring technique only uses surface data for which better and longer observations are available it opens up opportunities for improved reconstructions of the amoc over the last few decades

multidecadal variability of the overturning circulation in presence of eddy turbulence at low resolution idealized ocean circulation models forced by prescribed differential surface heat fluxes show spontaneous multidecadal variability depending critically on eddy diffusivity coefficients the existence of this critical threshold in the range of observational estimates legitimates some doubt on the relevance of such intrinsic oscillations in the real ocean through a series of numerical simulations with increasing resolution up to eddy resolving ones 10 km and various diapycnal diffusivities this multidecadal variability proves a generic ubiquitous feature at least in model versions with a flat bottom the mean circulation largely changes in the process of refining the horizontal grid along with the associated implicit viscosity and diffusivity and the spatial structure of the variability is largely modified but there is no clear influence of the resolution on the main oscillation period the interdecadal variability appears even more robust to low vertical diffusivity and overturning when mesoscale eddies are resolved the mechanism previously proposed for these oscillations involving westward propagating baroclinically unstable rossby waves in the subpolar region and its feedback on the mean circulation appears unaffected by mesoscale turbulence and is simply displaced following the polar front

effect of surface restoring on subsurface variability in a climate model during 1949 2005 initializing the ocean for decadal predictability studies is a challenge as it requires reconstructing the little observed subsurface trajectory of ocean variability in this study we explore to what extent surface nudging using well observed sea surface temperature sst can reconstruct the deeper ocean variations for the 1949 2005 period an ensemble made with a nudged version of the ipslcm5a model and compared to ocean reanalyses and reconstructed datasets the sst is restored to observations using a physically based relaxation coefficient in contrast to earlier studies which use a much larger value the assessment is restricted to the regions where the ocean reanalyses agree i e in the upper 500 m of the ocean although this can be latitude and basin dependent significant reconstruction of the subsurface is achieved in specific regions namely region of subduction in the subtropical atlantic below the thermocline in the equatorial pacific and in some cases in the north atlantic deep convection regions beyond the mean correlations ocean integrals are used to explore the time evolution of the correlation over 20 year windows classical fixed depth heat content diagnostics do not exhibit any significant reconstruction between the different existing observation based references and can therefore not be used to assess global average time varying correlations in the nudged simulations using the physically based average temperature above an isotherm 14 c alleviates this issue in the tropics and subtropics and shows significant reconstruction of these quantities in the nudged simulations for several decades this skill is attributed to the wind stress reconstruction in the tropics as already demonstrated in a perfect model study using the same model thus we also show here the robustness of this result in an historical and observational context

initialisation and predictability of the amoc over the last 50 years in a climate model the mechanisms involved in atlantic meridional overturning circulation amoc decadal variability and predictability over the last 50 years are analysed in the ipsl cm5a lr model using historical and initialised simulations the initialisation procedure only uses nudging towards sea surface temperature anomalies with a physically based restoring coefficient when compared to two independent amoc reconstructions both the historical and nudged ensemble simulations exhibit skill at reproducing amoc variations from 1977 onwards and in particular two maxima occurring respectively around 1978 and 1997 we argue that one source of skill is related to the large mount agung volcanic eruption starting in 1963 which reset an internal 20 year variability cycle in the north atlantic in the model this cycle involves the east greenland current intensity and advection of active tracers along the subpolar gyre which leads to an amoc maximum around 15 years after the mount agung eruption the 1997 maximum occurs approximately 20 years after the former one the nudged simulations better reproduce this second maximum than the historical simulations this is due to the initialisation of a cooling of the convection sites in the 1980s under the effect of a persistent north atlantic oscillation nao positive phase a feature not captured in the historical simulations hence we argue that the 20 year cycle excited by the 1963 mount agung eruption together with the nao forcing both contributed to the 1990s amoc maximum these results support the existence of a 20 year cycle in the north atlantic in the observations hindcasts following the cmip5 protocol are launched from a nudged simulation every 5 years for the 1960 2005 period they exhibit significant correlation skill score as compared to an independent reconstruction of the amoc from 4 year lead time average this encouraging result is accompanied by increased correlation skills in reproducing the observed 2 m air temperature in the bordering regions of the north atlantic as compared to non initialized simulations to a lesser extent predicted precipitation tends to correlate with the nudged simulation in the tropical atlantic we argue that this skill is due to the initialisation and predictability of the amoc in the present prediction system the mechanisms evidenced here support the idea of volcanic eruptions as a pacemaker for internal variability of the amoc together with the existence of a 20 year cycle in the north atlantic they propose a novel and complementary explanation for the amoc variations over the last 50 years

a dynamic model of venus s gravity field unlike earth long wavelength gravity anomalies and topography correlate well on venus venus s admittance curve from spherical harmonic degree 2 to 18 is inconsistent with either airy or pratt isostasy but is consistent with dynamic support from mantle convection a model using whole mantle flow and a high viscosity near surface layer overlying a constant viscosity mantle reproduces this admittance curve on earth the effective viscosity deduced from geoid modeling increases by a factor of 300 from the asthenosphere to the lower mantle these viscosity estimates may be biased by the neglect of lateral variations in mantle viscosity associated with hot plumes and cold subducted slabs the different effective viscosity profiles for earth and venus may reflect their convective styles with tectonism and mantle heat transport dominated by hot plumes on venus and by subducted slabs on earth convection at degree 2 appears much stronger on earth than on venus a degree 2 convective structure may be unstable on venus but may have been stabilized on earth by the insulating effects of the pangean supercontinental assemblage

stagnant lid convection on venus the effect of strongly temperature dependent viscosity on convection in the interior of venus is studied systematically with the help of finite element numerical models for viscosity contrasts satisfying experimental constraints on the rheology of rocks venus is likely to be in the regime of stagnant lid convection this regime is characterized by the formation of a slowly creeping very viscous lid on top of the mantle venusian lithosphere and is in agreement with the tectonic style observed on venus stagnant lid convection explains large geoid to topography ratios on venus by the thermal thinning of a thick lithosphere the thickness of the lithosphere can be as large as 400 550 km for beta regio and 200 400 km on average geoid and topography data and experimental data on the rheology of rocks provide constraints on the viscosity of the mantle 10 20 10 21 pa s the convective stresses in the interior 0 2 0 5 mpa the stresses in the lid 100 200 mpa the velocity in the interior 0 5 3 cm yr 1 and the heat flux beneath the lithosphere 8 16 mw m 2 parameterized convection calculations of thermal history of venus are difficult to reconcile with a thick present day lithosphere however a sufficiently thick lithosphere can be formed if a convective regime with mobile plates was replaced by stagnant lid convection around 0 5 b y ago one of the possible explanations for the cessation of venusian plate tectonics is that during the evolution of venus stresses in the lid dropped below the yield strength of the lithosphere this model predicts a drastic drop in the heat flux thickening of the lithosphere and suppression of melting and is consistent with the hypothesis of cessation of resurfacing on venus around 0 5 b y ago

a conceptual model for the relationship between coronae and large scale mantle dynamics on venus 1 among the most enigmatic features on venus are the more than 500 quasi circular structures collectively referred to as coronae associated volcanism and tectonism suggest that coronae result from small transient mantle plumes although only about 20 have clearly associated positive gravity anomalies indicative of current activity coronae are concentrated within a region defined by the dynamically active highland areas of beta atla and themis regiones bat coronae avoid the lowest or highest topography and geoid subsets of the corona population indicate that the spatial distribution of coronae has evolved from a previously less concentrated global distribution to the current bat concentration recent laboratory experiments on thermal convection at high rayleigh number using fluids with temperature dependent viscosity suggest a straightforward interpretation of these relationships small scale upwellings from a hot lower thermal boundary layer tend to be excluded by longer wavelength downwelling structures from the cold upper boundary layer the relative paucity of coronae in the broad lowland plains regions is consistent with the inference of mantle downwellings beneath the plains coronae are largely excluded from compensated highlands perhaps because the thickened crust is not easily disrupted in these regions laboratory experiments indicate that small transient boundary layer instabilities can coexist with and be focused toward large scale upwellings possibly explaining the spatial concentration of coronae in the bat region thus coronae provide important insights into interacting scales of convective flow in venus s mantle and into the evolution of venus s mantle dynamics direct tests of our model are possible through geological mapping

reconciling two alternative mechanisms behind bi decadal variability in the north atlantic understanding the preferential timescales of variability in the north atlantic usually associated with the atlantic meridional overturning circulation amoc is essential for the prospects for decadal prediction however the wide variety of mechanisms proposed from the analysis of climate simulations potentially dependent on the models themselves has stimulated the debate of which processes take place in reality one mechanism receiving increasing attention identified both in idealized models and observations is a westward propagation of subsurface buoyancy anomalies that impact the amoc through a basin scale intensification of the zonal density gradient enhancing the northward transport via thermal wind balance in this study we revisit a control simulation from the institut pierre simon laplace coupled model 5a ipsl cm5a characterized by a strong amoc periodicity at 20 years previously explained by an upper ocean atmosphere sea ice coupled mode driving convection activity south of iceland our study shows that this mechanism interacts constructively with the basin wide propagation in the subsurface this constructive feedback may explain why bi decadal variability is so intense in this coupled model as compared to others

global isostatic geoid anomalies for plate and boundary layer models of the lithosphere abstract isostatic geoid anomalies are usually interpreted using a flat earth one dimensional idealization isostatic anomalies on the spherical self gravitating earth differ from this idealization because 1 degree one terms in the spherical harmonic expansion vanish 2 each term in the spherical harmonic expansion is multiplied by l 2 l 0 5 relative to the flat earth case 3 mass in cones rather than straight sided columns is constant and 4 further deformation of the earth is induced by the gravitational attraction of the deformation caused by the isostatic potential anomaly when the effect of each of these is quantified the second third and fourth nearly cancel leaving the degree one over the horizon effect providing the most important difference calculations of model isostatic geoid anomalies for the spherical analogues developed here of the plate and boundary layer thermal models show that this effect can bias estimates of geoid slopes by over 20 although the effect is usually less than 5 the geoid anomalies for these two models are quite different over old ocean basins but they are unlikely to be distinguishable on the basis of global geoid observations owing to the presence of other larger perturbations in the geoid stripping the effects of plate aging and a hypothetical uniform 35 km thick isostatically compensated continental crust from the observed geoid emphasizes that the largest amplitude geoid anomaly is the geoid low of almost 120 m over west antarctica this anomaly is a factor of two greater in amplitude than the low of 60 m over sri lanka

tests of crustal divergence models for aphrodite terra venus aphrodite terra the largest highland region of venus is a likely site of mantle upwelling active volcanism and extensional tectonics we examine three alternative kinematic models for the interaction of mantle convection and the surface one in which little horizontal surface displacement results from mantle flow vertical tectonics one in which shear strain from large horizontal displacements is accommodated only in narrow zones of deformation plate divergence and one in which strain from large horizontal motions is broadly accommodated distributed deformation in support of a divergent plate model for aphrodite head and crumpler have cited the presence of an organized system of lineaments held to be analogues to oceanic fracture zones small scale bilateral symmetry of topographic elements hypothesized to be rifted and separated relief analogous to some terrestrial oceanic plateaus and subsidence of topography proportional to the square root of distance consistent with that expected for a divergent thermal boundary layer we undertake quantitative tests of each of these assertions and we compare the results with those from similar tests of the mid atlantic ridge we find that apart from the long wavelength symmetry of broadly elevated regions there is no convincing evidence for regional bilateral symmetry of features several hundred kilometers in size on either planet the fit of the topography of aphrodite to that of a thermal boundary layer is in general much poorer than for the earth and so other mechanisms such as dynamic uplift or crustal thickness variations must dominate the topography the broad saddle shaped region between thetis and atla regiones shows the best fit to the root distance relation yielding apparent spreading half rates of a few centimeters per year but the goodness of fit and apparent spreading rates are quite sensitive to the distance range analyzed calculation of a single pole of relative motion for the entire postulated system of transform faults shows that the inferred fracture zone traces are not consistent with a simple two plate model regardless of past pole motions a multiple plate geometry is therefore called for and one or more of the lineaments must act as a plate boundary along nearly its full length if the plate divergence hypothesis is correct such a boundary would be distinguishable from other lineaments by geological evidence for recent nontransform motion in forthcoming magellan radar images in the absence of such evidence as long as the existence and distribution of the lineaments are verified then broad disruption of a thin lithosphere is favored in such a model lineaments may be surface manifestations of mantle convective flow a model dominated by primarily vertical tectonics is tenable only if the lineaments are not confirmed by magellan

gravity field of venus at constant altitude and comparison with earth the acceleration gravity anomaly estimates by spacecraft tracking determined from doppler residuals are components of the gravity field directed along the spacecraft earth line of sight los these data constitute a set of vector components of a planet s gravity field the specific component depending upon where the earth happened to be at the time of each measurement and they are at varying altitudes above the planet surface from this data set we have solved for the gravity field using the method of harmonic splines which imposes a smoothness criterion to select a gravity model compatible with the los data maps of geoid and gravity at spacecraft and at constant altitudes are presented for the planet venus for venus the major geoid and gravity anomalies each coincide spatially with the locations of positive topographic features these anomalies at 300 km altitude have significantly greater magnitude than anomalies that on earth are coincident with topographic features another striking difference is that most of the major geoid anomalies on the earth do not coincide with locations of its largest topographic features including continents but are due to deep sources on venus major mass anomalies appear to be restricted principally to the surficial 200 km of the planet a substantially thicker crust on venus 70 80 km compared to the earth s 5 40 km can explain its greater range of gravity anomaly and the positive correlation of gravity anomaly withtopographic height four regions with high gravity to topography response functions are interpreted to be due to crustal loading by recent volcanism perhaps combined with regional elevation of surface topography by thermal expansion from heating at depth

radiating instability of a meridional boundary current a linear stability analysis of a meridional boundary current on the beta plane is presented the boundary current is idealized as a constant speed meridional jet adjacent to a semi infinite motionless far field the far field region can be situated either on the eastern or the western side of the jet representing a western or an eastern boundary current respectively it is found that when unstable the meridional boundary current generates temporally growing propagating waves that transport energy away from the locally unstable region toward the neutral far field this is the so called radiating instability and is found in both barotropic and two layer baroclinic configurations a second but important conclusion concerns the differences in the stability properties of eastern and western boundary currents an eastern boundary current supports a greater number of radiating modes over a wider range of meridional wavenumbers it generates waves with amplitude envelopes that decay slowly with distance from the current the radiating waves tend to have an asymmetrical horizontal structure they are much longer in the zonal direction than in the meridional a consequence of which is that unstable eastern boundary currents unlike western boundary currents have the potential to act as a source of zonal jets for the interior of the ocean

stress analysis of tellus regio venus based on gravity and topography comparison with venera 15 16 radar images the tectonics of the tellus regio highland of venus have been examined using pioneer venus altimetry and gravity data and venera 15 16 radar images a thin elastic shell model has incorporated the pioneer venus data to calculate both global long wavelength and regional shorter wavelength stresses for various assumed values of crust lithosphere and mantle thickness and modes of compensation the effects of these parameters on the calculated stress magnitudes have been examined the model also allows calculation of crustal thickness and mantle density anomalies the resulting calculated stress fields have been compared to the surface morphology observed in the venera 15 16 radar images and interpreted in terms of the stress history the best fitting parameters are consistent with minor amounts of lithospheric flexure being necessary to produce the observed surface features the highland region is underlain by a thick 15 20 km crust which is responsible for most of the topographic support a positive density anomaly exists below this crust the denser mantle may represent the down welling portion of a flow field in which the horizontal flow feeding the downwelling has entrained mobile crustal material dynamically maintaining a thickened crustal root near the stagnation point of the flow below tellus regio this hot crustal material may have intruded the overlying cooler crust causing lithospheric inflation and surface volcanism this scenario is in agreement with the available gravity and radar data and can explain the general surface morphology of tellus regio

poleward propagation of spiciness anomalies in the north atlantic ocean 1 recent modelling results suggest that subsurface salinity anomalies propagating from the tropics can reach and precondition the deep water formation regions thus modulating the thc variability the forcing and propagative aspects of this mechanism are presented in the north atlantic ocean over 1948 2002 using an ogcm density compensated salinity anomalies of 0 1 up to 0 35 psu along s 26 kg m 3 are generated in the salinity maximum region at interannual to decadal frequency the relation between subsurface conditions and late winter sea surface salinity variability supports the subduction mechanism they circulate over isopycnals ranging from 25 6s to 26 2s at current speed between 150 m and 250 m depth toward cape hatteras via the gulf of mexico on a typical 6 year transit although mixing along the pathway reduces the amplitude of salinity anomalies by about 66 they largely determine the subsurface spiciness of the gulf stream up to 30 n upstream of the outcrop region citation laurian a a lazar g reverdin k rodgers and p terray 2006 poleward propagation of spiciness anomalies in the north atlantic ocean geophys res lett 33 l13603

two stable equilibria of the atlantic subpolar gyre the cyclonic circulation of the atlantic subpolar gyre is a key mechanism for north atlantic climate variability on a wide range of time scales it is generally accepted that it is driven by both cyclonic winds and buoyancy forcing yet the individual importance and dynamical interactions of the two contributions remain unclear the authors propose a simplified four box model representing the convective basin of the labrador sea and its shallow and deep boundary current system the western subpolar gyre convective heat loss drives a baroclinicflow of relatively light water around the dense center eddy salt flux from the boundary current to the center increases with a stronger circulation favors the formation of dense waters and thereby sustains a strong baroclinic flow approximately 10 25 of the total in contrast when the baroclinic flow is not active surface waters may be too fresh to convect and a buoyancy driven circulation cannot develop this situation corresponds to a second stable circulation mode a hysteresis is found for variations in surface freshwater flux and the salinity of the near surface boundary current an analytical solution is presented and analyzed

gravity topography and crustal evolution of venus abstract the gravity anomalies of venus although small by comparison with those on mars and the moon are still much larger than those on earth for large features on venus even the low degree spherical harmonic terms for venus gravity field indicate a close association of broad positive gravity anomalies with major topographic highs this is striking contrast to the situation on earth where the broad regional gravity anomalies show little correlation with continental masses or plate tectonic features but instead appear to be caused by deep mass anomalies a method for estimating radial gravity anomalies from line of sight acceleration data their interpolation and use of iteration for improved radial anomaly estimates is outlined a preliminary gravity anomaly map of venus at spacecraft altitude prepared using first estimate values is presented a profile across the western part of aphrodite along longitude 85 e was analyzed using time series techniques an elastic plate model would require a plate thickness of about 180 to 200 km to match the general amplitude of the observed gravity anomaly about 33 mgal a thickness much greater than that found for earth structures and because of high surface temperatures unlikely for venus an airy isostatic model convolved with the topography across aphrodite however provides a better match between the predicted and observed gravity anomalies if the nominal crustal thickness is about 70 to 80 km this thickness is over twice that for continental crust on the earth and considerably greater than that of the earth s basaltic ocean crust only 5 km a different differentiation history for venus than that of the earth thus is anticipated high gravity anomalies 110 mgal occur over beta regio and over the topographic high in eastern aphrodite both highs are associated with regions where detected lightning is clustered and thus the topographic features may be active volcanic constructs the large gravity anomalies at these two sites of volcanic activity require an explanation different than that indicated for western aphrodite

the spin and inertia of venus one of the most remarkable rotational features of venus is the presence of a relatively large offset between spin and maximum inertia axis according to the most recent gravity field models this offset may amount to 0 5 considerably larger than the one which characterizes the earth s chandler wobble 0 3 at present it is not clear whether the offset of venus is the result of the limited resolution of the gravity models or alternatively it may be regarded as an evidence in support to the existence of a wobble purpose of this note is to demonstrate on quantitative grounds that an offset between spin axis and maximum inertia axis is to be expected on the time scales of mantle convection due to the peculiar rotational characteristics of venus we also show that the inertia perturbation associated to the rising of hot thermal plumes from the core mantle boundary to the surface is large enough to maintain offsets ranging between 0 1 and 1 of course this does not preclude that other processes such as venusquakes or periodic atmospheric variabilities may contribute significantly to the observed offset on shorter time scales

an episodic hypothesis for venusian tectonics it is suggested that episodic plate tectonics occurs on venus episodes of rapid plate tectonics are separated by periods of surface quiescence for the last 500 200 m y it is postulated that the surface of venus has been a single rigid plate that has been thickening due to conductive cooling a near uniform surface age is consistent with observed crater densities and the relatively small number of craters modified by surface tectonics or embayed by lava flows a lithosphere that has conductively thickened for some 500 m y has a thickness of about 300 km nearly an order of magnitude greater than the thickness associated with steady state conductive heat loss such a thick lithosphere can support the high topography and associated gravity anomalies on venus as well as the unrelaxed craters studies of lithospheric flexure at coronae are also consistent with a thick elastic lithosphere incipient subduction associated with large coronae may represent the onset of a new episode of rapid plate tectonics on the earth 75 90 of mantle heat transport is attributed to the creation of new oceanic lithosphere at ocean ridges this process is not operative on venus this paper suggests that episodic plate tectonics on venus constitutes the primary mechanism for mantle heat transport on that planet

african monsoon teleconnections with tropical ssts validation and evolution in a set of ipcc4 simulations a set of 12 state of the art coupled ocean atmosphere general circulation models oagcms is explored to assess their ability to simulate the main teleconnections between the west african monsoon wam and the tropical sea surface temperatures ssts at the interannual to multi decadal time scales such teleconnections are indeed responsible for the main modes of precipitation variability observed over west africa and represent an interesting benchmark for the models that have contributed to the fourth assessment report of the intergovernmental panel on climate change ipcc4 the evaluation is based on a maximum covariance analysis mca applied on tropical ssts and wam rainfall to distinguish between interannual and multi decadal variability all datasets are partitioned into low frequency lf and high frequency hf components prior to analysis first applied to hf observations the mca reveals two major teleconnections the first mode highlights the strong influence of the el nino southern oscillation enso the second mode reveals a relationship between the sst in the gulf of guinea and the northward migration of the monsoon rainbelt over the west african continent when applied to hf outputs of the twentieth century ipcc4 simulations the mca provides heterogeneous results most simulations show a single dominant pacific teleconnection which is however of the wrong sign for half of the models only one model shows a significant second mode emphasizing the oagcms difficulty in simulating the response of the african rainbelt to atlantic sst anomalies that are not synchronous with pacific anomalies the lf modulation of these hf teleconnections is then explored through running correlations between expansion coefficients ecs for ssts and precipitation the observed time series indicate that both pacific and atlantic teleconnections get stronger during the twentieth century the ipcc4 simulations of the twentieth and twenty first centuries do not show any significant change in the pattern of the teleconnections but the dominant enso teleconnection also exhibits a significant strengthening thereby suggesting that the observed trend could be partly a response to the anthropogenic forcing finally the mca is also applied to the lf data the first observed mode reveals a well known inter hemispheric sst pattern that is strongly related to the multi decadal variability of the wam rainfall dominated by the severe drying trend from the 1950s to the 1980s whereas recent studies suggest that this drying could be partly caused by anthropogenic forcings only 5 among the 12 ipcc4 models capture some features of this lf coupled mode this result suggests the need for a more detailed validation of the wam variability including a dynamical interpretation of the sst rainfall relationships

a mantle plume model for the equatorial highlands of venus the equatorial highlands of venus consist of four main structures atla beta ovda and thetis regiones each has a circular to oval shaped planform and rises 4 5 km above the mean planetary radius these highlands are associated with long wavelength geoid highs with amplitudes ranging from 35 m at ovda to 120 m at atla they also contain topographic valleys interpreted as extensional rift zones and beta is known to contain shield volcanoes these characteristics are all consistent with the equatorial highlands being formed by mantle plumes an alternative model in which ovda and thetis are interpreted as spreading centers analogous to terrestrial mid ocean ridges fails to explain most of the observed geoid anomalies and topography in these regions some smaller highlands such as bell regio eistla regio and the hathor innini ushas region may also be plume related but most coronae are unlikely to be the direct result of plume activity we have modeled plumes using a cylindrical axisymmetric finite element code and a depth dependent newtonian rheology we compare our model results with profiles of geoid and topography across atla beta ovda and thetis our best model fits are for beta and atla assuming whole mantle convection and that earth and venus have similar mantle heat flows venus must lack an earth like low viscosity zone in its upper mantle in order satisfy the observed geoid and topography for these features this conclusion is consistent with the long wavelength admittance spectrum of venus and with the observed differences in the slopes of the geoid spectra for the two planets one explanation for the different viscosity structures of the two planets could be that the mantle of venus is drier than earth s mantle

geoid anomalies in a dynamic earth in order to obtain a dynamically consistent relationship between the geoid and the earth s response to internal buoyancy forces we have calculated potential and surface deformation love numbers for internal loading these quantities depend on the depth and harmonic degree of loading they can be integrated as green functions to obtain the dynamic response due to an arbitrary distribution of internal density contrasts spherically symmetric self gravitating flow models are constructed for a variety of radial newtonian viscosity variations and flow configurations including both whole mantle and layered convection we demonstrate that boundary deformation due to internal loading reaches its equilibrium value on the same time scale as postglacial rebound much less than the time scale for significant change in the convective flow pattern by calculating relaxation times for a series of spherically symmetric viscous earth models for uniform mantle viscosity the geoid signature due to boundary deformations is larger than that due to internal loads resulting in net negative geoid anomalies for positive density contrasts geoid anomalies from intermediate wavelength density contrasts are amplified by up to an order of magnitude geoid anomalies are primarily the result of density contrasts in the interior of convecting layers density contrasts near layer boundaries are almost completely compensated layered mantle convection results in smaller geoid anomalies than mantle wide flow for a given density contrast viscosity stratification leads to more complicated spectral signatures because of the sensitivity of the dynamic response functions to model parameters forward models for the geoid can be used to combine several sources of geophysical data e g subducted slab locations seismic velocity anomalies surface topography to constrain better the structure and viscosity of the mantle

on the robustness of the interdecadal modes of the thermohaline circulation ocean models in box geometry forced by constant surface fluxes of density have been found to spontaneously generate interdecadal oscillations of the thermohaline circulation this paper analyzes the sensitivity of these oscillations to various physical effects including the presence of mesoscale turbulence various thermal surface boundary conditions and the presence of wind forcing or bottom topography the role of unstable long baroclinic waves is also reexamined in an attempt to understand the oscillation period in idealized geometry it is found that the low frequency variability of the thermohaline circulation under quasi constant surface fluxes is a robust feature of the large scale circulation it is not strongly affected by energetic mesoscale turbulence the oscillation period is relatively invariant with respect to varying resolution and momentum and tracer horizontal mixing coefficients although it loses some regularity as shorter and longer periods of variability emerge when the mesoscale activity increases in strength with smaller mixing coefficients the oscillations are also retained as the ocean model is coupled to an interactive atmospheric energy balance model the thermohaline modes are robust to a range of exchange coefficients that widens with the amplitude of the mean circulation the presence of an additional wind forced component generally weakens the oscillation and depending on the relative strength of thermodynamic and dynamic forcings the oscillation may be completely killed a simple interpretation is given highlighting the role of upward ekman pumping in damping density anomalies finally the interaction of these baroclinic modes with bottom topography depends strongly on the relative directions of the mean topographic features and the mean currents and baroclinic waves but usually results in a damping influence

mechanism for low frequency variability and salt flux in the mediterranean salt tongue the role of baroclinic instability in the generation of low frequency variability in the interior of ocean subtropical gyres and its possible importance in the zonal flux of salt and heat within the mediterranean salt tongue are discussed the observed zonal enhancement of the low frequency variability is interpreted as the signature of baroclinic instability of the large scale wind driven flow the zonal orientation results from the relatively weak vertical shear of the large scale upper ocean flow and the stabilizing influence of the planetary vorticity gradient there is a reversal in the vertical shear of velocity at middepths in the eastern basin of the north atlantic that results in a local middepth maximum in the zonal eddy flux of density linear theory predicts a vertical structure of this density flux that is nearly coincident with the observed vertical distribution of the warm salty water of the mediterranean salt tongue a primitive equation model is used to investigate the nonlinear large amplitude regime the zonal eddy density or salt flux averaged over several wavelengths and several cycles of wave growth and breaking indicates that this mechanism may be an important component in the overall salt balance in this region