a self consistent model of melting magma migration and buoyancy driven circulation beneath mid ocean ridges numerical modeling and analysis are used to investigate the processes leading to the eruption of mantle derived magma at mid ocean ridges our model includes the following effects melting due to decompression magma migration by percolation and circulation of the mantle driven by both the oceanic plates and the distribution of buoyancy beneath the ridge the distribution of buoyancy is due to both the low density of the liquid and the difference in the density of the residual solids relative to unmelted mantle material the calculation of densities is based on a simple petrological model in which garnet spinel lherzolite melts to form a basaltic liquid and a harzburgite residue we find that the system spontaneously evolves to a state in which a rapid upwelling beneath the ridge axis faster than the plate velocity is confined laterally by stably stratified residual material beneath the newly formed plates this effect is exaggerated if a modest decrease in the shear viscosity of the solid upon melting is included our results provide a simple explanation for the narrowness of the zone of crustal formation at mid ocean ridges the model also predicts a transition from steady state to episodic crustal formation as the spreading velocity is reduced perhaps giving rise to along axis variations in the character of seafloor spreading the narrow rapid upwelling gives rise to substantial porosities at depths that are a large fraction of the depth to the solidus this may allow the liquid at depth to segregate into macroscopic channels which would account for the consensus from experimental petrology that the liquids parental to morb are derived from well below the base of the crust

a mechanism for magmatic accretion under spreading centres the most active volcanic zone on earth occurs along the global mid ocean ridge system where the lithosphere is pulled apart and new oceanic crust is created at rates between 10 and 200 km myr 1 this volcanism is segmented and is concentrated along the axial valleys and highs of slow and fast spreading ridges respectively with transform fault zones separating and generally offsetting each active ridge segment volcanic activity may be nearly absent at the ridge transform intersection where a slow spreading ridge is offset and greatly reduced where a fast spreading ridge is offset implying thinner crust in these regions1 the source of magmas erupted along ocean ridges is the underlying mantle which undergoes decompression melting when rising to fill the gap between the spreading plates the resulting magma aggregates in the upper mantle and ascends to the crust due to its low density compared with the parent mantle rock the melt is believed to pool in crustal magma chambers whence it periodically erupts to the surface typically the formation ascent and aggregation of magmas along the mid ocean ridges is regarded two dimensionally and modelled in a section through the crust and mantle across the strike of the ridges see ref 2 here however we consider the problem with a three dimensional model for which we provide supporting geological evidence in which the magmas rise as a result of a gravitational instability modified rayleigh taylor in the underlying partial melt zone

slow ridge hotspot interactions from global gravity seismic tomography and 87sr 86sr isotope data summary among the mantle hotspots present under oceanic areas a large number are located on or close to active oceanic ridges this is especially true in the slow spreading atlantic and indian oceans the recent availability of worldwide gravity grids and the increasing coverage of geochemical data sets along active spreading centres allow a fruitful comparison of these data with global geoid and seismic tomography models and allow one to study interactions between mantle plumes and active slow spreading ridges the observed correlations allow us to draw preliminary conclusions on the general links between surficial processes which shape the detailed morphology of the ridge axes and deeper processes active in the upper mantle below the ridge axial domains as a whole the interactions are first studied at the scale of the atlantic the mid atlantic ridge from iceland to bouvet island from the correlation between the zero age free air gravity anomaly which reflects the zero age depth of the ridge axis and sr isotopic ratios of ridge axis basalts the study is then extended to a more global scale the slow ridges from iceland to the gulf of aden by including geoid and uppermantle tomography models the interactions appear complex ranging from the evect of large and very productive plumes almost totally overprinting the long wavelength segmentation pattern of the ridge to that of weaker hotspots barely marking some of the observables in the ridge axial domain intermediate cases are observed in which hotspots of medium activity or whose activity has gradually decreased located at some distance from the ridge axis produce geophysical or geochemical signals whose variation along the axis can be correlated with the geometry of the plume head in the upper mantle such observations tend to preclude the use of a single hotspot ridge interaction model and stress the need for additional observations in various plume ridge configurations

guide for estimating riverbed degradation the rate and extent of riverbed degradation resulting from sediment interruption are determined by means of computer based numerical experiments momentum and continuity equations for water and sediment flows in a wide prismatic channel are solved numerically using a recently verified model for the bed armoring process the bed profiles for different times are found to be similar and are coalesced to a single normalized curve the changes in bed levels and mean sediment size and the length of degradation determined from the numerical experiments are correlated by multiple regression analysis to the independent dimensionless variables the depth of degradation depends on the froude number the geometric standard deviation and the normalized median size of the bed material the proposed relationship for bed degradation is verified with the field data of missouri river the computation procedure is illustrated by an example

the structure of three dimensional convection beneath oceanic spreading centres summary buoyant flow beneath oceanic spreading centres can be driven by density gradients due to thermal expansion and compositional gradients arising from the extraction of melt numerical experiments are used to examine the pattern of mantle flow and melting produced by these two sources of buoyancy compositional buoyancy leads to three dimensional flow beneath the spreading axis with along axis variations in upwelling and melt production thermal buoyancy drives axis parallel circulation and convective rolls aligned perpendicular to the spreading axis that originate at a distance off axis controlled by the spreading rate and mantle viscosity hie distance from the axis to the onset of observed sea floor gravity lineations may show a similar dependence on spreading rate compositional density stratification suppresses the formation of thermally driven rolls near the axis and when it is large causes the thermal rolls to become unstable leading to time dependent behaviour the amount of crust produced is less sensitive to spreading rate than in passive flow models however it has a maximum at intermediate spreading rates due to the presence of thermally driven upwelling in the melting region along axis variations in upwelling and melt production suggest that buoyant flow may have an important effect on the segmentation of spreading centres particularly at slow spreading rates

combined laboratory and numerical studies of the interaction between buoyant and plate driven upwelling beneath segmented spreading centers a combination of laboratory and numerical models are used to examine the mantle flow beneath a segmented ridge generated by the interaction of a linear buoyant upwelling source with plate driven flow in the absence of plate spreading the linear buoyant source creates a very narrow across axis two dimensional upwelling pattern the plate driven flow consists of a quasi linear sheet like upwelling that cuts beneath ridge transform inside corners and is not centered beneath the spreading segments when buoyant and plate driven flows are combined material rises beneath the inside corners and flows away from the axis asymmetrically near the ends of segments this results in a geometrical misfit between the center of mantle upwelling and the ridge axis if a similar pattern of mantle flow occurs beneath a segmented mid ocean ridge the result will be a thinner crust toward segment ends and possibly a negative correlation between extent of mantle melting and average depth of melting these results indicate that even with an essentially two dimensional source in cases where it is oblique to the actual spreading segments the upwelling beneath a segmented ridge will appear to be three dimensional along axis since slow spreading ridges are generally more segmented than fast spreading ridges this effect is likely to be more important at slow spreading ridges

the fluid dynamics of plume ridge and plume plate interactions an experimental investigation abstract a number of hotspots occur at or near mid ocean ridges e g iceland causing elevated bathymetry gravity anomalies and geochemical enrichment for hundreds of kilometers along otherwise normal ridge axes this is commonly presumed to be the result of hot plume material from the deep mantle flowing outward from a narrow plume conduit here we explore the dynmics of plume ridge and plume plate interaction using laboratory experiments we model chemically buoyant plumes as opposed to thermally buoyant plumes so we ignore the effects of thermal diffusion while concentrating on the strictly fluid dynamical aspects of the interaction we find that the spreading of a continuously fed plume beneath a fixed plate is described by a simple expression derived from a balance between plume buoyancy and viscous resistance to spreading for plumes spreading beneath ridges we find two remarkably distinct phases of flow at first the plume spreads approximately as it would beneath a fixed plate then a rapid transition occurs to a steady state condition in which the plume width along the ridge waist is constant this width is found to be proportional to a natural length scale that increases with the square root of the plume volumetric flux and the ratio of the mantle plume viscosities and decreases with the square root of the plate spreading velocity although our experimental conditions with uniform mantle viscosity and no thermal diffusion are perhaps oversimplified compared to the real earth the results scale reasonably well to observation our model suggests that sufficient time has elapsed 60 myr for the iceland plume to reach steady state in its interaction with the mid atlantic ridge using independent estimates for the plume volumetric flux at the iceland hotspot and assuming a viscosity ratio of one the model predicts a minimum waist width of about 850 km in close agreement with the observed value of about 920 km data from other hotspots jan mayen azores galapagos are also consistent with the experimental results for average mantle plume viscosity ratios of 1 3 using estimated plume fluxes with a simple conservation of mass equation yields reasonable estimates of plume thickness ranging from 70 to 180 km for these hotspots thus the experimental model appears to be reasonably consistent with observation given that thermal diffusion has been disregarded

on the geometry of mantle flow beneath drifting lithospheric plates summary the convective flow pattern beneath the oceanic lithosphere is strongly influenced by the movement of the lithospheric plate on top assuming upper mantle convection the wavelengths geometric structure and time of onset for ridge perpendicular rolls as functions of the rayleigh number and the plate velocity have been investigated by numerical modelling the related gravity anomalies have been estimated and compared to observations from the se pacific which show ridge perpendicular anomalies with wavelengths of 150 220 km and amplitudes of about 10 mgal for constant viscosity the type of secondary circulation perpendicular to the diverging ridge depends on the relation between imposed plate velocity vs and maximum velocity in the free convection case vhfree if vs is less than vhfree the flow is dominated by off ridge up and downwellings if vs is about 10 times larger than vhfree the flow remains 2 d if vs is between these extrema a chain of ridge plumes develops from the lower thermal boundary layer with adjacent ridge perpendicular walls that lead to roll like circulation richter rolls throughout the model box the wavelength of these rolls is a function of the rayleigh number and the plate velocity for parameters realistic for the earth rolls with considerable amplitude first develop after more than 50 myr with wavelengths of at least 250 km for temperature and pressure depth dependent viscosity the wavelengths of the evolving rolls are determined by the rayleigh number related to the maximum viscosity in the interior of the convecting system and not to the average viscosity for models with a strong decrease in viscosity to values of about 1019 pa s in a 100 km wide asthenospheric channel below the lithosphere ridge perpendicular rolls which only fill the low viscosity layer appear after a few million years after a transitional stage of some tens of millions of years rolls in the whole upper mantle also evolve the asthenospheric rolls develop from instabilities of the upper thermal boundary layer and have wavelengths of about 220 280 km the time needed for alignment of these rolls to the direction of plate movement is a few million years and the distance from the ridge to the point of onset of the rolls is about 200 500 km both are functions of the plate velocity while the formation time for rolls in the whole upper mantle is quite large and their existence is related largely to two layered convection this type of flow might not be very relevant for the earth although it has been used to account for 3 d structures at oceanic ridges to explain the observed gravity anomalies in the se pacific roll like circulation in a low viscosity asthenospheric layer is the preferred candidate where the modelled amplitudes and wavelengths of gravity anomalies fit the observations

the influence of transient upper level barotropic growth on the development of baroclinic waves in a previous study of two cyclones within the fronts and atlantic storm track experiment iop11 and iop17 it was shown that upper level barotropic growth dominated the initial phase of the development the cyclones later developed due to baroclinic energy conversion these cyclones resemble petterssen s type b paradigm this contribution presents the dynamics of archetypical type b cyclogenesis by addressing the consequences of upper level barotropic growth before baroclinic cyclone development within the framework of idealized primitive equation simulations localized and finite amplitude upper level pressure perturbations have been designed to represent a variety of amplitudes and horizontal tilts of a localized barotropic trough structure these are added to a zonal jet basic state and it is shown that favourable initial situations similar to the observed cyclones lead to much stronger and faster cyclonic development compared with unfavourable ones the most effective barotropic initial situation without a vertical tilt and considering growth over a time scale of 48 hours is one with a horizontal tilt against the shear of an upper level trough on the cyclonic side of a jet associated with a baroclinic zone

the spacing of rift axis highs dependence upon diapiric processes in the underlying asthenosphere abstract detailed bathymetric mapping has revealed a near harmonic distribution of intermediate wavelength morphologic highs on slow to fast spreading centers one model suggests that the highs may result from the thermal instabilities generated by the interaction of cold fracture zone walls with the axial asthenosphere this may force a deepening of the isotherms near the fracture zone and an uplift of the isotherms midway between the fracture zones a preferred model based on newly accumulated data suggests that the highs may be a result of upwelling within the asthenosphere underlying the ridge axis where a linear region of instability develops at the base of a partial melting zone less dense material at depths of 40 80 km may penetrate the overlying material at discrete and ideally evenly spaced points harmonically spaced upwellings should generate excess magma at discrete points on the ridge axis creating volcanic highs the spacing between highs may depend upon the continuity and thickness of the deep axial instability layer the temperature and viscosity ratios within the magma and the distance between major transform faults instability of the diapirs may stimulate the migration of the highs through time

the east pacific rise near 21 n 13 n and 20 s inferences for along strike variability of axial processes of the mid ocean ridge abstract data obtained along various segments of the mid ocean ridge mor are used to construct an idealized model for crustal accretion the model seeks to predict the topographic volcanic tectonic and hydrothermal characteristics of any given spreading segment of the mor as a function of distance away from the bounding transform faults this model is based on a series of detailed mapping efforts carried out on segments of the mor having a broad range of spreading rates between 2 and 16 cm yr this paper includes the results of the french searise program carried out during the summer of 1980 aboard the n o jean charcot and two american cruises conducted in 1981 aboard the r v melville

experiments on the interaction of thermal convection and compositional layering at the base of the mantle accumulation and survival of compositional heterogeneity at the base of the convecting mantle is investigated using laboratory experiments with temperature dependent viscous fluids two models of heterogeneity in the d layer are considered 1 a reservoir of dense material above the core mantle boundary and 2 continuous recharge of the d layer by subduction of cold negatively buoyant high viscosity slabs of oceanic lithosphere survival of a dense reservoir depends on rp the ratio of stabilizing compositional buoyancy to destabilizing thermal buoyancy rapid mixing occurs for rp 1 for rp 1 mixing occurs by slow entrainment only the reservoir model predicts d layer heterogeneity is concentrated beneath lower mantle upwellings differentiation of composite slabs consisting of dense crustal and light depleted components occurs in a hot thermal boundary layer composite slabs pile up in the basal thermal boundary layer and upon heating the light component is ejected by rayleigh taylor instabilities the dense component accumulates in the boundary layer arrayed in a pattern of interconnected spokes and is slowly entrained into thermal plumes according to these experiments the time required for conversion of slabs to plumes in d is 1 2 gyr the correlation between lateral variations in d layer thickness and lateral variations in lower mantle seismic velocity suggests that d may consist of dense material perhaps former oceanic lithosphere which has sunk through the lower mantle

plume capture by divergent plate motions implications for the distribution of hotspots geochemistry of mid ocean ridge basalts and estimates of the heat flux at the core mantle boundary abstract the coexistence of stationary mantle plumes with plate scale flow is problematic in geodynamics we present results from laboratory experiments aimed at understanding the effects of an imposed large scale circulation on thermal convection at high rayleigh number 106 ra 109 in a fluid with a temperature dependent viscosity in a large tank a layer of corn syrup is heated from below while being stirred by large scale flow due to the opposing motions of a pair of conveyor belts immersed in the syrup at the top of the tank three regimes are observed depending on the ratio v of the imposed horizontal flow velocity to the rise velocity of plumes ascending from the hot boundary and on the ratio of the viscosity of the interior fluid to the viscosity of the hottest fluid in contact with the bottom boundary when v 1 and 1 large scale circulation has a negligible effect on convection and the heat flux is due to the formation and rise of randomly spaced plumes when v 10 and 100 plume formation is suppressed entirely and the heat flux is carried by a sheet like upwelling located in the center of the tank at intermediate v and depending on established plume conduits are advected along the bottom boundary and ascending plumes are focused towards the central upwelling heat transfer across the layer occurs through a combination of ascending plumes and large scale flow scaling analyses show that the bottom boundary layer thickness and in turn the basal heat flux q depend on the peclet number pe and when 10 q pe1 2 and when 1 q pe 1 3 consistent with classical scalings when applied to the earth our results suggest that plate driven mantle flow focuses ascending plumes towards upwellings in the central pacific and africa as well as into mid ocean ridges furthermore plumes may be captured by strong upwelling flow beneath fast spreading ridges this behavior may explain why hotspots are more abundant near slow spreading ridges than fast spreading ridges and may also explain some observed variations of mid ocean ridge basalt morb geochemistry with spreading rate moreover our results suggest that a potentially significant fraction of the core heat flux is due to plumes that are drawn into upwelling flows beneath ridges and not observed as hotspots

the influence of the 1997 99 el niňo southern oscillation on extratropical baroclinic life cycles over the eastern north pacific the el nino southern oscillation enso strongly influences the interannual and seasonal atmospheric circulation over the north pacific the present study shows that the meridional displacement of the time mean upper level jet associated with enso modulates the time mean barotropic meridional wind shear over the central and eastern north pacific storm track earlier theoretical and observational studies established the influence of barotropic wind shear on the life cycles of extratropical cyclones and their upper level potential vorticity pv waves the present study suggests that differences in the time mean flow associated with the 1997 99 enso cycle had a similar impact on tropopause pv structure meridional eddy fluxes of momentum and temperature and predictability

two paradigms of baroclinic wave life cycle behaviour two idealized baroclinic wave 6 life cycles examined here suggest a framework of opposite extremes a in which to view the behaviour of real synoptic scale disturbances in middle latitudes and b in which to interrelate the synoptic and wave theoretic viewpoints using the saturation propagation saturation sps picture of wave 6 life cycle behaviour the two life cycles dented by lc1 and lc2 are higher resolution versions of the simmons hoskins basic and anomalous cases showing strong and weak late decay of eddy kinetic energy eke they illustrate in varying degrees two extreme types of behaviour here disignated anticyclonic and cyclonic and epitomized by strongly contrasting upper air trough behaviour anticyclonic behaviour dominates the late stages of lc1 and is characterized by backward tilted thinning troughs being advected anticyclonically and equatorward as in the commoner cases of planetary scale mid stratospheric rossby wave breaking cyclonic behaviour dominates lc2 and is characterized by forward tilted broadening troughs wrapping themselves up cyclonically and poleward producing major cut off cyclones in high latitudes these morphologies are visualized by upper air maps of potential temperature on the nominal tropopause defined as a constant potential vorticity surface some atmospheric mid latitude disturbances examined here using the same visualization applied to operational analyses show the same two extreme types of trough behaviour together with intermediate cases the sps picture is re examined using eliassen palm and refractive index cross sections it is shown in particular by reference to a wave activity theorem of haynes that the late stages of lc2 can be looked upon as a remarkably clear and morphologically novel large amplitude counterpart of the nonlinear reflection scenario of rossby wave critical layer theory the late stages of lc1 by constrast look more akin to an nonlinear critical layer absorption scenario lc2 exhibits region of largely undular pv contours adjacent to a nonlinear of irreversiby deformen pv contous in the latter region pv rearrangement and hence absorption of rossby wave activity has largely ceased this accounts for the more persistent ekee in the lc2 case

axial depth anomalies from 10 to 50 north along the mid atlantic ridge correlation with other mantle properties abstract we present an analysis of intermediate to long wavelength a few hundred to a few thousand kilometers axial depth anomalies along the mid atlantic ridge between 10 and 50 n the maximum depth of the rift valley is chosen as the elevation datum for oceanic crust of zero age the large depth anomalies are correlated for short wavelengths less than a few hundred kilometers with some fracture zones irrespective of their offset and for intermediate to long wavelengths with mantle properties expressed in 1 excess elevation near triple junctions 2 trace element and isotope geochemistry of the basaltic liquids emplaced at the ridge axis and 3 anomalies in the earth s gravity field we suggest that the correlations may be explained in two ways the depth anomalies of intermediate to long wavelength may represent the sites of upwelling and downwelling of the deep mantle alternatively the depth anomalies could be due to a regionalization in temperature or mineralogy in the asthenosphere

the relative importance of plate driven and buoyancy driven flow at mid ocean ridges the dynamical interaction between three dimensional 3 d buoyant flow and plate driven mantle flow beneath a mid ocean ridge is examined using a combination of laboratory and numerical experiments in a unique laboratory setup a layer of strongly temperature dependent viscous fluid is heated from below and cooled from above to drive thermal convection forced plate driven flow is modeled by dragging mylar sheeting in opposite directions across the fluid surface uniform viscosity 3 d numerical models have been designed to simulate the laboratory runs to provide additional information on the flow and to attempt to isolate the effects of variable viscosity in one type of experiment we modeled buoyancy on the scale of the partial melting region with a linear heat source beneath the spreading axis in a second set of experiments the entire base of the tank is heated in order to investigate the interaction between plate driven flow and larger scale upper mantle convection the pattern of segment scale 100 150 km convection beneath a spreading center is found to be a strong function of the spreading rate and the rayleigh number ra of the buoyant flow purely two dimensional 2 d flow exists only in the case of low ra 105 and faster spreading rates 4 6 cm yr half rate three dimensionality is strongly enhanced during transient pulses of upwelling convection on the scale of the upper mantle can contribute significant long wavelength spatial 600 1000 km and temporal 20 40 m y variability in upwelling rates and temperatures at spreading centers and may provide an alternative model for plume ridge interaction for a given ra the upwelling near the spreading axis can essentially be described by three regimes weakly 3 d at the slow spreading rates strongly 3 d at slow to intermediate rates and 2 d at fast spreading rates the regime boundaries shift toward higher plate velocities with increasing ra comparison of the laboratory and numerical experiments indicates that temperature dependent viscosity may strengthen the position of focused upwelling centers and cause a sharper transition between 2 d and 3 d upwelling patterns taken together these results suggest that buoyancy driven dynamic flow is an important element in the geodynamics of mid ocean ridges

the dynamics of off axis plume ridge interaction in the uppermost mantle two decades of geochemical and geophysical observations have led to the plume channel model whereby buoyant off axis mantle plumes feed and interact with diverging plates at mid ocean ridge axes which act as sinks of upper mantle material here we present results from two dimensional 2 d numerical experiments which incorporate the essential physics and fluid dynamic aspects of the plume ridge upper mantle system in order to test the feasibility of this plume ridge interaction model specifically experiments test the relative importance of physical effects such as strong viscosity variations and thermal and chemical buoyancy forcing in plume ridge dynamics results indicate that both transient and steady state connections may be established between off axis plumes and ridges for a range of realistic mantle conditions the presence of a strongly sloping rheological boundary layer rbl is a necessary condition for long term communication between an off axis buoyant upwelling and a spreading ridge the flux of buoyant material to the ridge is also shown to increase with increasing plume to mantle density contrast decreasing plume viscosity and smaller plume ridge separation distances plume ridge interaction regimes are defined based on the competing effects of plate driven and buoyancy driven flow thermal erosion of the viscous lithosphere strongly inhibits long term plume ridge interaction by enhancing the plates ability to deflect the plume both head and conduit away from the ridge axis

dynamical characterization of the fastex cyclogenesis cases summary a series of diagnostics based on the automatic tracking of cyclones is applied to the 25 life cycles captured during the fastex field experiment the tracking enables a setting of the various cases into a common frame of reference that moves with the system time filtering is used to further separate the events into a basic state and perturbations information is obtained on environmental properties such as strain or baroclinicity combining these diagnostics it is shown that instability mechanisms based on the existence of potential vorticity strips or the control of these instabilities by deformation may be involved in the initial genesis of a number of cases but do not seem to characterize specific types nor to determine a systematic behaviour on the other hand it is shown that rapid deepening phases leading eastern atlantic systems to their maximum amplitude happens suddenly and at the same time as crossing the jet stream even in the absence of a clear additional upper level feature for cold air systems development means meeting a diffluent zone this is unlike the american east coast systems that are associated with an exit entrance complex another remarkable feature is the generation of a frontal waveas a re sult of the parent vorticity maximum being apparently stretched and split this has occurred in 25 of cases a number of other properties of the fastex cases are shown such as several occurrences of large

crustal control of ridge segmentation inferred from observations of the reykjanes ridge large amplitude variations in topography and inferred crustal thickness along the axes of mid ocean ridges often referred to as segmentation1 are mainly observed at slow spreading ridges2 4 this observation has led to the suggestion that mantle processes give rise to segmentation only when spreading rates are low5 6 here we make the alternative proposal that the development of segmentation is controlled by the temperature of the crust segmentation cannot develop when the lower crust is hot enough to undergo rapid ductile flow thermal models predict that thick crust at a slow spreading ridge may be as hot as normal thickness crust along fast spreading ridges we accordingly test our hypothesis at a slow spreading ridge characterized by thick crust the reykjanes ridge topography and gravity data along the reykjanes ridge axis indeed show an absence of segmentation suggesting that the thermal state of the crust rather than any mantle process controls the development of this structure

buoyancy effects on mantle flow under mid ocean ridges results of a series of two dimensional numerical experiments of mantle flow melting and melt migration under a spreading center are reported the model predicts the distribution of melt in the subridge mantle the width over which most melt is delivered to the crust and the thickness of crust the sources of buoyancy considered are thermal expansion compositional variation caused by melt extraction and the phase change of solid to melt we infer that the steady state average viscosity of the mantle below a ridge cannot be much less than about 1019 pa s for a lower average viscosity thermal convection causes rapid cooling of a large region under a ridge raising the viscosity results imply that transient increases in mantle temperature should lead to larger increases in the oceanic crustal thickness for slow spreading ridges than for fast spreading ridges we assume that the viscosity is proportional to exp 0 where is melt fraction we parameterize the permeability in terms of the reference velocity for percolation of the melt vr where the relative velocity of melt to solid is vr analytic approximations are used to extrapolate the model results to large values of permeability and small values of 0 if vr is less than 1 m yr then more melt would be retained in the subridge mantle than is estimated from analysis of topography and gravity data at fast spreading centers for vr greater than about 100 m yr so little melt would be retained in the mantle that it is difficult to explain the gravity data and the low shear wave velocity structure close to the east pacific rise estimated from seismic surface waves buoyancy effects can lead to a region of mantle upwelling and melting that is as narrow as the observed zone of oceanic crustal accretion for most melt to be added to the crust within a few kilometers of a fast spreading center requires that 0 be less than 0 015 if vr equals 1 m yr and less than 0 003 if vr is 100 m yr

three dimensional mantle convection beneath a segmented spreading center implications for along axis variations in crustal thickness and gravity segmentation and along axis variations within individual segments indicate the inherently three dimensional nature of mantle up welling and melting beneath oceanic spreading centers numerical convection experiments are used to explore the effects of local buoyancy forces on upwelling and melt production beneath a segmented spreading center the experiments are conducted in a region consisting of a thermally defined rigid lithosphere and a uniform viscosity asthenosphere overlying a higher viscosity mantle half space a periodic plate boundary geometry is imposed consisting of spreading segments and transform offsets buoyancy forces are caused by thermal expansion and the compositional density reduction due to the extraction of partial melt the relative magnitudes of the buoyant and plate driven components of mantle flow are controlled by the spreading rate and mantle viscosity with buoyant flow more important at lower spreading rates and viscosities buoyant flow beneath the spreading axis amplifies along axis variations in upwelling near a ridge transform intersection and distributes the variations along the entire spreading axis buoyant flow may thus be responsible for the more three dimensional character of slow spreading centers away from the spreading axis thermal buoyancy drives convective rolls that align with the direction of plate motion and which have an along axis wavelength controlled by the prescribed thickness of the asthenosphere however the position and stability of rolls are influenced by the segmentation geometry in cases where the spreading center geometry does not allow a stable configuration of rolls the flow is time dependent along axis variations in upwelling cause variations in melt production which imply large variations in crustal thickness that dominate the surface gravity signal the crustal thickness distributions implied by these numerical experiments produce bulls eye shaped negative mantle bouguer anomalies centered over spreading segments as observed at several spreading centers the amplitude of the anomaly increases with decreasing spreading rate

dynamical consequences of compositional and thermal density stratification beneath spreading centers extraction of partial melt to form oceanic crust reduces the density of upwelling mantle beneath spreading centers this compositional buoyancy can significantly affect both the rate of upwelling and the pattern of mantle flow the relative importance of buoyant upwelling and upwelling due to plate spreading depends on the mantle viscosity for a low enough mantle viscosity buoyant upwelling can significantly enhance the thickness of the crust relative to that which would be produced by plate spreading alone for a mantle viscosity of 1022 pa s upwelling due to plate spreading is dominant and crustal thickness is predicted to be a strong function of spreading rate however for a mantle viscosity of 1019 pa s buoyancy dominates mantle upwelling and the calculated crustal thickness like the oceanic crust is nearly independent of spreading rate thermal and compositional density variations result in opposing buoyancy forces that can cause time dependent upwelling for a mantle viscosity of 1020 pa s and a low 1 cm yr spreading rate this behavior takes the form of undamped crustal thickness oscillations with a single well defined period for even slightly lower mantle viscosity the frequency increases and crustal thickness oscillations become more complex with two or more frequencies present while numerical experiments have thus far been carried out only for viscosities significantly greater than that likely to characterize the mantle beneath spreading centers we speculate that these crustal thickness oscillations may explain important aspects of seafloor structure

the role of mantle depletion and melt retention buoyancy in spreading center segmentation abstract numerical experiments are used to examine the structure of mantle flow beneath the axis of a spreading center buoyancy results from the depletion of residual mantle in iron relative to magnesium as melt is extracted mantle depletion buoyancy and from the presence of low density melt melt retention buoyancy 3 d buoyant mantle flow arises spontaneously from an initially 2 d solution for low mantle viscosity and low spreading rate at high viscosity and high spreading rate initially 2 d solutions remain 2 d this may explain the fundamentally different structure of fast and slow spreading centers in a uniform viscosity halfspace the along axis wavelength of 3 d buoyant upwelling scales with the maximum depth of melting the only length scale in this system for reasonable maximum depths of melting 60 90 km along axis wavelengths of 200 300 km are preferred longer than the 50 100 km segmentation length of slow spreading centers in a viscosity layered halfspace the thickness of the asthenosphere introduces another length scale a wavelength of segmentation comparable to the asthenosphere thickness also develops in our numerical experiments this suggests the possibility that a wavelength corresponding to the spacing between gravity lows may be controlled by the asthenosphere thickness while the spacing of major fracture zones corresponds to the longer wavelength 3 times the maximum depth of melting intrinsic to the melting region the along axis structure of 3 d flow varies from narrow focused upwellings at low spreading rates and mantle viscosities to broad regions of upwelling at high spreading rates and mantle viscosity to allow an along axis crustal thickness variation no larger than that which is observed 3 4 km the highly focused upwelling and crustal production predicted at slow spreading rates requires appreciable along axis transport of melt

the paradox of the axial profile isostatic compensation along the axis of the mid atlantic ridge mantle bouguer gravity anomalies mba and bathymetry on three profiles covering more than 1000 km along the axis of the mid atlantic ridge mar are highly correlated suggesting that along axis topographic relief is locally compensated by variations in crustal thickness and or mantle density structure the quantitative relationship between topography and gravity on these profiles could be explained by the flexure of a thin narrow elastic strip representing the response to isostatic loads of an inner rift valley isolated from the rest of the plate by weak bounding normal faults the paradox is that across axis profiles show that the median valley is an uncompensated feature apparently created by a dynamic mechanism new extensive off axis coverage of the mar from 31 to 36 s shows that the high correlation does not persist outside the axial zone we suggest that the on axis correlation exists because the mechanism creating the median valley is controlled by the mantle thermal structure and along axis variations in crustal thickness both of which contribute to the mba if the mechanism is extension of a brittle ductile lithosphere the critical parameter controlling topographic relief is the thickness of the relatively stiff mantle layer immediately beneath the crust at the ridge axis a three dimensional 3 d thermal model incorporating passive mantle flow hydrothermal circulation the plate boundary geometry and variable magmatic heating associated with observed variations in crustal thickness predicts variations in the thickness of the stiff mantle layer that correlate with the observed axial topography we model the expected topography using cross axis sections of the 3 d thermal model and a 2 d finite element model of an extending lithosphere that incorporates temperature and strain dependent rheology as well as the flexural response of a thickening plate the predicted topographic signal produced by the combined dynamic and isostatic effects matches the amplitude of the observed axial bathymetry at the 18 mm yr 1 spreading half rate in our south atlantic survey the 2 d models also reproduce the observed rapid transition along the axis between rift valley no rift valley morphology we conclude that extensional forces acting on segmented oceanic lithosphere with varying rates of crustal production produce the highly variable morphology of the southern mar and the along axis correlation between mba and bathymetry

a 6 year amsu based climatology of upper level troughs and associated precipitation distribution in the mediterranean region 1 we present a novel climatology of upper tropospheric troughs and associated precipitation in the mediterranean region using 6 years 2001 2006 of advanced microwave sounding unit data troughs are more frequent or persistent during november through march in the whole of the mediterranean basin with up to 25 of days in the northwestern part of the basin the interannual variability of number of intrusions is stronger from september through june during summer the variability is reduced except in the nw region and no troughs were observed in the se region moderate precipitation occurs in at least 60 of the cases when an upper level trough is present with weak seasonal dependency in the southern mediterranean moderate precipitation is an ubiquitous feature and presents small year to year fluctuations the occurrence of deep convective rain associated with trough is lower around 50 and reduces to 20 in the northern part of the basin during winter this occurrence varies a lot with individual years especially in the southern mediterranean upper level troughs have more often a n s orientation but while no particular dependency of frequency of precipitation whether moderate or convective on their inclination was found the location of precipitation is generally favored downstream the north atlantic oscillation and scandinavia patterns were found to be the most important large scale circulation modes influencing the variability of number of intrusions but the east atlantic west russia and east atlantic patterns are also important in the southeast region

gravity anomalies isostasy and mantle flow at the east pacific rise crest bathymetry and gravity data obtained during a detailed hydrosweep survey of the southern east pacific rise from 7 s to 9 s are used to investigate isostasy at the axis of a fast spreading mid ocean ridge in particular we examine the manner in which the bathymetric crestal high is supported and how this support varies along the axis within the 160 km long 7 12 s 8 38 s ridge segment the crestal high stands about 400 m above the adjacent ridge flanks and has a nearly constant minimum axial depth for a distance of 140 km the summit is broad and flat and an axial summit caldera is present for the entire length of the ridge segment however the width and cross sectional shape of the crestal high vary systematically along the axis it is broad with gentle slopes in the center of the segment but becomes progressively narrower and steeper toward the ends of the ridge segment the ridge crest is marked by a free air gravity anomaly high about 15 20 km wide with an amplitude of 10 15 mgal relative to the ridge flanks mantle bouguer anomalies vary systematically along the axis with minimum values found near the center of the segment the axial mantle bouguer anomalies thus do not reflect the axial depth but are correlated with changes in the cross sectional area of the crestal bathymetric high the effects of cooling and subsidence away from the axis were removed from the bathymetry and free air anomalies to isolate residual topographic and gravity anomalies associated with the ridge crest the residual crestal bathymetric high was modeled as a flexural feature resulting from the upward buoyant load of a region of low density material centered beneath the axis the lithosphere was treated as a broken plate either with a constant flexural rigidity or an effective elastic thickness te which grows at a rate proportional to the square root of distance from the axis the best fitting values of te for the constant rigidity case are in the range of 0 3 0 6 km for the growing plate model te increases at a rate of 0 2 0 3 km the gravity constrains the mass deficiency to extend to a depth of 20 30 km for both lithospheric models we interpret this low density material as a region of partial melt feeding magma to the ridge axis the best fitting density anomalies imply that a 4 9 melt fraction is present beneath the crestal high upwelling of melt to the axis is thus confined to a narrow zone within about 10 km of the axis the mass deficiency and thus the upwelling partial melt are not distributed evenly along the ridge axis but rather are concentrated in the central portion of the ridge segment it thus appears that differences between the along axis gravity and depth patterns observed at slow spreading and at fast spreading ridges are not the result of a change from three dimensional focused upwelling at slow spreading ridges to two dimensional sheetlike upwelling at fast spreading ridges rather the differences in axial gravity and depth between fast and slow spreading ridges reflect differences in the efficiency of the shallow along axis magma distribution

the spreading rate dependence of three dimensional mid ocean ridge gravity structure we analyze over 1300 km of high resolution along axis gravity profiles at ridges with half spreading rates ranging from 1 2 to 5 5 cm yr the results show consistently higher along axis gradients of mantle bouguer anomaly at the slow spreading mid atlantic ridge mar 0 3 1 2 mgal km than at the intermediate to fast spreading cocos nazca ridge and east pacific rise epr 0 1 0 2 mgal km the regional peak to trough amplitude of mantle bouguer anomaly is also greater along the mar 30 60 mgal than the cocos nazca ridge and the epr 10 20 mgal with increasing spreading rate the regional peak to trough amplitude of axial seafloor depth decreases from 1000 1700 m to 200 700 m 3 d numerical experiments suggest that mantle contributions to the gravity can be significant only near large offset transforms at the more commonly observed non transform offsets gravity anomalies will reflect crustal thickness variations the along axis gravity data thus indicate that the amplitude of along axis crustal thickness variation decreases with increasing spreading rate we propose that this spreading rate dependent crustal accretion style may originate in the mantle finite amplitude mantle upwelling is intrinsically plume like 3 d beneath a slow spreading ridge but more sheet like 2 d beneath a fast spreading ridge such a transition in mantle upwelling may occur if the relative importance of passive upwelling over buoyant upwelling increases with increasing spreading rate small amplitude 3 d upwellings may occur at a fast spreading ridge but their effects on crustal thickness variations will be significantly reduced by along axis melt flows along a persistent low viscosity crustal magma chamber in contrast the large crustal thickness variations due to 3 d mantle upwellings will be maintained at a slow spreading ridge because less along axis melt flows can occur in the colder and more rigid crust there

laboratory investigation of the interaction of off axis mantle plumes and spreading centres mantle plumes and mid ocean ridge spreading centres are the dominant phenomena through which mass and heat are transported from the mantle to the earth s surface it now seems that the dispersion of near ridge plumes beneath the lithosphere is modulated strongly by mid ocean ridges in particular geochemical and geophysical observations have suggested that rising plumes are diverted towards and feed nearby ridges 1 7 here we confirm the feasibility of this model with laboratory experiments that incorporate the essential physical and fluid dynamic aspects of a plume ridge upper mantle system our results indicate that an off axis plume may communicate thermally and chemically with a spreading ridge through a narrow sub horizontal conduit instead of a broader radially spreading plume head a necessary condition for this communication is the presence of a lithospheric or rheological boundary layer that thickens away from the ridge axis owing to conductive cooling interestingly we find that for high plume temperatures increasing the plume thermal buoyancy may inhibit rather than enhance plume ridge interaction as a result of increased erosion of the overlying lithosphere

large scale cloud precipitation and upper level features during fronts and atlantic storm track experiment as inferred from tiros n operational vertical sounder observations physical parameters retrieved from the measurements of the tiros n operational vertical sounder aboard the national oceanic and atmospheric administration satellites are used to study the large scale cloud precipitation upper level features during the fronts and atlantic storm track experiment period the cloud top pressure a precipitation index and the temperature of lower stratosphere tls respectively the three dynamically consistent i e temporally and spatially collocated retrievals are able to characterize the weather regimes through their organization in locations expected from the cyclonic activity moreover high level cloud patterns when accompanied by rain can be discriminated with respect to tls field when no or weak warm tls feature can be found upstream such patterns are frontal systems on the contrary a warm tls event present upstream of a rainy cloud feature suggests a baroclinic interaction in this case the high level cloud pattern is associated with a large scale baroclinic cyclone or at least a mature low when examining the most precipitating cases using a composite technique the nondeveloped systems in term of absence of high cloud cover are characterized by the absence of warm tls features grouping mostly blocking regime cases on the other hand the cases that are well developed in terms of cloud cover are characterized by warm tls patterns upstream the precipitation area moreover the well developed cases could be grouped into three families depending on the orientation of the warm tls features zonal anticyclonic and cyclonic only the later family characterized by a cyclonically tilted trough clusters the large scale baroclinic systems that all occurred during the zonal regime this suggests that a cyclonically tilted trough has a positive impact on the development of cyclones

observed variability of north atlantic oceanic precipitating systems during winter 1 the variability of large precipitating weather systems as observed from polar satellites over the north atlantic ocean is investigated using a statistical analysis nine winters from 1987 to 1995 have been considered cloud systems and troughs over the area 40 60 n 70 10 w are automatically detected with retrievals of the cloud top pressure a precipitation index and the temperature of the lower stratosphere a classification of the largest precipitating systems as characterized by these variables leads to eight classes whose occurrence significantly differs over the years it also shows the systematic presence of a trough upstream of the precipitating area covered by high level clouds as expected in the case of baroclinic interaction in order to understand the large interannual variability an attempt to identify systematic differences in cyclone structures during different flow regimes is then performed it shows that the large scale typically 3000 km wide and zonally elongated cloud systems are observed mainly to the south of 45 n when the phase of the arctic oscillation ao is negative which also corresponds to the greenland anticyclone regime conversely zonally elongated cloud systems of smaller scale 1000 km wide associated with frontal waves are favored also to the south but during the positive ao phase particularly during zonal regimes the most tilted weather systems without any preferential ao phase are found mostly during the blocking regime these systems are cyclonically tilted in the northeastern branch of the storm track and anticyclonically tilted in the southwestern branch it must be noted that similar results are obtained when the ao daily index is replaced by the north atlantic oscillation index this analysis gives observational evidence of previous idealized simulations linking the large scale circulation to preferential life cycles of weather systems in addition a weak response to extreme el nino southern oscillation events has been observed in the location of cyclones but not in their average structure

evidence from gravity data for focused magmatic accretion along the mid atlantic ridge gravity data from the mid atlantic ridge between latitudes 27 50 n and 30 40 n show that the accretion of magma at the ridge is focused at discrete centres along the spreading axis large positive gravity anomalies equivalent to reductions of almost 50 in crustal thickness are observed over non transform discontinuities bounding spreading segments

three dimensional mantle flow beneath mid ocean ridges we have studied the development of multiscalar mantle flow beneath mid ocean ridges using a three dimensional numerical technique modeling includes various modes of heating and the shear flow induced by plate motion away from a spreading center with and without transform offsets the computational box consists of two layers differing in their viscosities it is shown than when flow is generated by heat sources located for the most part in the lower layer convective cells have horizontal dimensions proportional to the entire thickness of the box it is only when flow is driven by buoyancy forces in the top layer or by transient cooling along the top interface that transverse rolls are found to be confined within the lop low viscosity layer these rolls remain generally located far from the ridge plane except in the case of a viscosity contrast of 3 orders of magnitude between the bottom and top layers we conclude that segmentation beneath mid ocean ridges requires both strong viscosity contrasts and the lack of deep convective flow marine geophysical observations suggest that mantle upwelling beneath mid ocean ridges is two dimensional sheetlike beneath intermediate fast and hotspot influenced spreading centers and three dimensional plumelike beneath slow spreading centers and ridges overlying cold regions of the mantle such as the australian antarctic discordance we propose that these observations can be explained by variations in the viscosity structure and temperature of the mantle along the mid ocean ridge system models including a transform offset show that the active portion of the transform is a region of mantle downwelling the toroidal flow induced by the offset shifts the plumes away from the ridge crests the effects of the toroidal field are strongly magnified when the return flow cannot penetrate in the lower layer due to a large viscosity contrast stresses generated by the flow partly explains the development of inside corner highs and the evolution of transform faults

gravity anomalies and crustal thickness variations along the mid atlantic ridge between 33 n and 40 n multibeam bathymetry and gravity data have been obtained along an 800 km long section of the mid atlantic ridge from just south of the hayes fracture zone at 33 n to the northern edge of the azores platform near 40 n a three dimensional analysis of these gravity and topography data combined with results from earlier seismic refraction studies in this area reveal two different scales of crustal heterogeneity a systematic along axis segment scale 8 9 km near the middle of spreading segments and the thinnest crust 3 4 km near segment offsets the magnitude of this along axis variation in crustal thickness is proportional to the length of the spreading segment and the size of the adjacent ridge offset there also is a distinct asymmetry in crustal structure across the rift valley near large segment offsets with gravity highs inferred to be thinner crust beneath the inside corner highs adjacent to these offsets this segment scale crustal heterogeneity is similar to that reported from the kane to atlantis section of the mid atlantic ridge and from parts of the intermediate spreading southern mid atlantic ridge it is superimposed on a second longer wavelength variation in gravity and crustal thickness associated with the azores hot spot the most pronounced effect of the azores hot spot on the mid atlantic ridge occurs between 38 n and 40 n where the ridge axis rapidly shoals by more than 1000 m the crust thickens by over 2 km and the rift valley largely disappears the absence of a deep axial rift valley on the azores platform and near the midpoints of some ridge segments along the mid atlantic ridge to the south agrees well with the predictions of recent ridge crest thermal and rheological models that suggest a dependence of axial morphology on both crustal thickness and spreading rate the transition from a rift valley to an axial high morphology at these spreading rates 11 mm yr half rate occurs at a crustal thickness of about 9 1km

spreading rate dependence of three dimensional structure in oceanic spreading centres segmentation by transform faults and other types of along axis discontinuity1 is a well described but poorly explained characteristic of oceanic spreading centres here we use numerical experiments to explore the dynamics of mantle flow and melting beneath a mid ocean ridge buoyant upwelling driven by compositional density variations resulting from the extraction of the melt that forms the ocean crust exhibits a spreading rate dependent transition between two dimensional and three dimensional upwelling structures for low spreading rates and mantle viscosities an initial two dimensional structure transforms into a three dimensional one at high spreading rates an initially two dimensional structure remains two dimensional these results suggest that the origin of spreading centre segmentation may be different at fast and slow spreading rates

upper level barotropic growth as a precursor to cyclogenesis during fastex during the fronts and atlantic storm track experiment fastex two large scale cyclones in intensive observation periods 11 and 17 developed in the north atlantic these can be classified in their initial phase as petterssen type b cyclones initiation of the cyclones follows transient upper level barotropic growth caused by a pre existing upper level trough approaching a jet associated with a strong baroclinic zone the cyclones later developed further due to baroclinic energy conversion a favourable orientation of the trough relative to the baroclinic zone horizontal tilt against the shear is crucial for this transient barotropic growth this scenario is therefore proposed as a general dynamical mechanism leading to type b cyclogenesis

crustal thickness on the mid atlantic ridge bull s eye gravity anomalies and focused accretion spreading segments of the mid atlantic ridge show negative bull9s eye anomalies in the mantle bouguer gravity field seismic refraction results from 33 s indicate that these anomalies can be accounted for by variations in crustal thickness along a segment the crust is thicker in the center and thinner at the end of the spreading segment and these changes are attributable to variations in the thickness of layer 3 the results show that accretion is focused at a slow spreading ridge that axial valley depth reflects the thickness of the underlying crust and that along axis density variations should be considered in the interpretation of gravity data