three eurasian teleconnection patterns spatial structures temporal variability and associated winter climate anomalies the eurasian eu pattern is a distinct teleconnection pattern observed in boreal winter since the eu pattern was first identified three types have been reported in the literature the conventional eu pattern the type 1 eu pattern or scandinavian scand pattern and the type 2 eu pattern or east atlantic west russia eatl wrus pattern based on several reanalysis and observational datasets the three eu patterns are extracted using the rotated empirical orthogonal function method in order to provide a further distinction and understanding of the three eu patterns a comprehensive side by side comparison is performed among them including their temporal variability horizontal and vertical structure related stationary rossby wave activity impact on climate and possible driving factors associated with external forcing the results reveal that all three eu patterns are characterised by a clear quasi barotropic wave train structure but each has a distinct source and centre of action accordingly their impacts on the precipitation and surface air temperature also differ from one other further evidence suggests that the conventional eu pattern is likely driven by anomalous sea surface temperatures sst over the north atlantic in which process the transient eddies are actively involved the scand pattern is partly maintained by the vorticity source over western europe which arises from the anomalous convergence divergence over the mediterranean and is efficiently driven by the tropical and southern indian ocean sst via divergent circulation the eatl wrus pattern shows some linkage to the north american snow cover and the involved process remains unclear and needs further investigation

a thermodynamically self consistent damage equation for grain size evolution during dynamic recrystallization summary we employ basic non equilibrium thermodynamics to propose a general equation for the mean grain size evolution in a deforming medium under the assumption that the whole grain size distribution remains self similar we show that the grain size reduction is controlled by the rate of mechanical dissipation in agreement with recent findings our formalism is self consistent with mass and energy conservation laws and allows a mixed rheology as an example we consider the case where the grain size distribution is lognormal as is often experimentally observed this distribution can be used to compute both the kinetics of diffusion between grains and of dynamic recrystallization the experimentally deduced kinetics of grain size coarsening indicates that large grains grow faster than what is assumed in classical normal grain growth theory we discuss the implications of this model for a mineral that can be deformed under both dislocation creep and grain size sensitive diffusion creep using experimental data of olivine our predictions of the piezometric equilibrium in the dislocation creep regime are in very good agreement with the observations for this major mantle forming mineral we show that grain size reduction occurs even when the average grain size is in diffusion creep because the largest grains of the grain size distribution can still undergo recrystallization the resulting rheology that we predict for olivine is time dependent and more non linear than in dislocation creep as the deformation rate remains an increasing function of the deviatoric stress this rheology is not localizing

decadal precipitation variability over europe and its relation with surface atmospheric circulation and sea surface temperature the relationships between decadal 5 year period observed winter precipitation pp variability over europe and atmospheric circulation and sea surface temperature sst during the period 1950 1995 are investigated more than 48 of the decadal precipitation variability is described by two modes a meridional dipolar pattern high pp over northern europe and low over southern europe the mediterranean and most of central europe explaining about 32 of the variance and an alternating pattern in latitude over europe high in central europe and low in far north western europe and the mediterranean explaining about 16 of the variance the first mode is strongly related to the north atlantic oscillation a canonical correlation analysis between regional precipitation and sea level pressure fields shows that a pp pattern similar to the first decadal pp mode is associated with a north westerly circulation anomaly pattern while a pp pattern similar to the second decadal pp mode is associated with a monopolar circulation anomaly pattern over most parts of europe the correlation map between the time series of expansion coefficients of the first pp mode and sea surface temperature emphasizes a global pattern which resembles the linear trend coefficients pattern of sst for the analysed period the second pp mode is associated with an sst anomaly pattern similar to that of the sst anomalies characterizing decadal el ni o southern oscillation sst pattern composite maps of global 500 mb geopotential height based on the time series of expansion coefficients of pp modes emphasize global patterns compatible with sst patterns after removing the linear trend from the data the first pp pattern remains almost unchanged but it emphasizes mainly regional connections the second spatial pattern of the second pp mode and its connections are not affected significantly by removing the trend

abrupt tectonics and rapid slab detachment with grain damage a simple model for necking and detachment of subducting slabs is developed to include the coupling between grain sensitive rheology and grain size evolution with damage necking is triggered by thickened buoyant crust entrained into a subduction zone in which case grain damage accelerates necking and allows for relatively rapid slab detachment i e within 1 my depending on the size of the crustal plug thick continental crustal plugs can cause rapid necking while smaller plugs characteristic of ocean plateaux cause slower necking oceanic lithosphere with normal or slightly thickened crust subducts without necking the model potentially explains how large plateaux or continental crust drawn into subduction zones can cause slab loss and rapid changes in plate motion and or induce abrupt continental rebound

constitution of the mantle 3 density elastic properties and the mineralogy of the 400 km discontinuity abstract on the basis of phase diagrams of the feo mgo sio 2 system and equations of state for minerals we have calculated internally consistent thermodynamic profiles of adiabatic bulk modulus seismic parameter and density for a series of petrological models it is shown that the sequences of phase assemblage changes across the 400 km discontinuity and the thickness of divariant transitions depend strongly on the iron content the ol px ratio and the temperature at the transition depth calculations show that the iron rich model is inadequate to described the seismic properties in the uppermost mantle and through the 400 km discontinuity the distribution of physical properties in the iron rich model at the depths of 360 400 km is of a gradient type which does not agree with the seismic evidence an important constraint on the upper mantle chemistry is an fe fe mg ration less than 0 17 comparisons of calculated thermodynamic properties for various temperature and bulk composition models indicate that thermodynamic profiles of the olivine and pyroxene models do not match seismic observations through the 400 km discontinuity hence the sio 2 content in the isochemical upper mantle should be in excess of 36 4 mol and less than 44 4 mol from all the petrological models considered only a pyrolite model sio 2 40 mol fe fe mg 0 12 ol px 1 may satisfy available seismological constraints the results obtained indicate that seismic properties across the 400 km discontinuity might be consistent with an isochemical pyrolite composition and do not require chemical stratification the pyrolite models considered show that the phase transitions occur over a range of 8 15 km at a depth near 400 km it is proposed that in the different tectonic regions of the earth a 400 km discontinuity might be considered as a combination of first and second order discontinuities with a phase transition sequence px px px px within a thickness range of around 15 km low temperature pyrolite model or as pseudo first order discontinuity through a narrow divariant loop px about 8 km wide high temperature pyrolite model

tectonic plate generation and two phase damage void growth versus grain size reduction 1 the generation of plate tectonics from mantle convection requires shear localization in order to yield narrow weak plate boundaries separating broad strong plates a plausible shear localizing mechanism involves damage e g distributed microcracking one theoretical model of which involves two phase mechanics two phase damage theory employs a nonequilibrium relation between interfacial surface energy pressure and viscous deformation thereby providing a description of void generation and microcracking and hence weakening failure and shear localization here we examine the application of this theory to the problem of generating plate like behavior from convective type divergent convergent poloidal motion through a source sink formulation we extend the previous damage theory to consider two possible damage effects 1 growth and nucleation of voids associated with dilation of the host matrix and 2 increasing fineness i e reducing coarseness of the mixture by for example grain size reduction void generating damage is found to be poor at plate generation because of the predominance of dilational motion that is adverse to the development of plate like flow fineness generating damage is found to be very efficient at generating plate like behavior if we assume that the matrix viscosity is a simple function of grain void size as is typical for diffusion creep the implied grain size reduction mechanism is different from that of dynamic recrystallization and appears to be more capable of generating the requisite shear localization for forming tectonic plates from mantle flow

global patterns of decadal scale variability observed in sea surface temperature and lower tropospheric circulation fields the global patterns associated with decadal scale variability dsv are examined by a lag correlation technique based on local anomaly indices using the fields of measured sea surface temperature sst and 850 hpa geopotential height for the last 50 years the three dominant patterns are identified and the variability is examined the first spreads over the entire pacific which is concurrent with the decadal scale modulation of the el nino southern oscillation des variability the second is confined to the local midlatitude north pacific lnp variability and the third extends over the north atlantic with the decadal north atlantic oscillation nao dna variability the global sst pattern of des variability exhibits large scale equatorial symmetry in the pacific which is similar to that of the pacific decadal oscillation but is distinguished by the signals in the subtropical frontal zones these sst anomalies are accompanied by anomalous subtropical highs that appear prior to the anomalous depression around australia the lnp variability which is related with the arctic oscillation is characterized by the sst anomalies along the north pacific subarctic frontal zone moving eastward accompanied by the anomalous aleutian low this variability develops decays without with coherent variability in the tropics it shows the 6 year quadrature phase relationship with the des variability indicative of an interdecadal variability with a period of 24 years the dna variability is featured by the atmospheric nao and by the sst anomalies in four zonal bands that spread in the north atlantic from the tropics to high latitudes this variability is independent of either the des or lnp variability

evolution of a polymineralic mantle shear zone and the role of second phases in the localization of deformation 1 the influence of second phases e g pyroxenes on olivine grain size was studied by quantitative microfabric analyses of samples of the hilti massif mantle shear zone semail ophiolite oman the microstructures range from porphyroclastic tectonites to ultramylonites from outside to the center of the shear zone starting at conditions of ridge related flow they formed under continuous cooling leading to progressive strain localization the dependence of the average olivine grain size on the second phase content can be split into a second phase controlled and a dynamic recrystallization controlled field in the former the olivine grain size is related to the ratio between the second phase grain size and volume fraction zener parameter in the latter dynamic recrystallization manifested by a balance between grain growth and grain size reduction processes yields a stable olivine grain size in both fields the average olivine and second phase grain size decreases with decreasing temperature combining the microstructural information with deformation mechanism maps suggests that the porphyroclastic tectonites 1100 c and mylonites 800 c formed under the predominance of dislocation creep since olivine rich layers are intercalated with layer parallel polymineralic bands in the mylonites nearly equiviscous conditions can be assumed in the ultramylonites diffusion creep represents the major deformation mechanism in the polymineralic layers it is this switch in deformation mechanism from dislocation creep to diffusion creep that forces strain to localize in the fine grained polymineralic domains at low temperatures 700 c underlining the role of the second phases on strain localization in cooling mantle rocks

a simple model of plate generation from mantle flow summary a simple model of non newtoniancreeping ow is used to evaluate classes of rheologieswhichallow viscous mantle ow to becomeplate like the model describes shallow layerlithosphericmotion driven by sources and sinks the sources represent spreading ridges while the sinks rep resent subduction zones the sources and sinks thus also prescribe the poloidal component ofthe surface ow eld the toroidal strike slip component of the ow eld is found via thesolution of the stokes equation with non newtonian rheology as a rst basic investigation ofthe model the horizontal divergence from the two dimensional rectangular velocity eld ofolson bercovici 1991 is used for the source sink eld the degree to which the induced uid ow reproducesthe rectangularplate is used to measure the success of differentrheologiesin generating plate like ows results indicate that power law rheologies even in the limit ofvery high power law index can only produce modest plate like ow for example the ratioof toroidal to poloidal kinetic energy for a source sink eld derived from a square plate is atbest 0 65 whereas a perfect square plate has a ratio of 1 0 moreover the power law rheologyappears to reach an asymptotic limit in its ability to produce plate like behavior this impliesthat plate tectonics is unlikely to arise from a power law rheology even in the limit of very high a class of rheologies that yield signi cantly more promisi ng results arise from the carreaupseudo plastic rheology with the power law index taken to be 0 one rheology in thisclass is the continuum model for stick slip earthquake behavior of whitehead gans 1974 which is essentially the carreau equation with 1 this class of rheologies referred to asthe stick slip rheologies induces a toroidal to poloidal kinetic energy ratio for a square plates ssource sink function which can be as high as0 9 the viscosity or strength distribution forthis class of rheologies also appears more plate like showing fairly uniform high viscosity re gions pseudo plates and sharply de ned low viscosity zones pseudo margins in contrast even the most nonlinear power law rheology produces spatially varying high viscosity regionsand relatively smooth low viscosity margins the greater success of the stick slip rheologiesin producing plates is attributed to a self lubricating mechanism in which the transfer of mo mentum from regions of high shear to low shear is inhibited in contrast even in the limit ofin nite power law index a power law rheology can retard but never prohibit momentum trans fer this feature is essential to the sharpening of velocity pro les into plate like pro les whichis illustrated with a simple boundary layertheory key words plate tectonics mantle convection non newtonian ow toroidal poloidal cou pling

rheology of two phase systems a microphysical and observational approach abstract ductile shear zones commonly contain distinctive bands of high strain rock characterized by intimately mixed fine grained two phase or polyphase material these ultramylonite bands are weaker than the surrounding material and may play a critical role in strain localization how such zones develop how the phases become evenly dispersed the bulk rheology and the controls on grain size are all unclear the following generic scenario may resolve some of these questions 1 dislocation creep and dynamic recrystallization cause grain size reduction commonly the recrystallized grain sizes of the two phases differ 2 grain size reduction causes a switch to grain boundary diffusion creep which requires grain boundary sliding diffusion allows one phase to fill spaces that open between grains of the other this will happen most rapidly in the finer grained phase the grain size of the resulting mixture is therefore controlled by that of the finer grained phase this leads to mixing and dispersion of the two phases producing a fine grained evenly dispersed two phase aggregate 3 the bulk rheology will be controlled by grain boundary diffusion creep of the two phases with the grain size controlled by the finer grained phase bulk flow laws can be developed for quartz feldspar and olivine orthopyroxene ultramylonites based on these concepts using appropriate mixing laws

ultrahigh pressure phase relations in the system mg4si4o12mg3al2si3o12 high pressure phase relations in the system mg4si4o12 mg3al2si3o12 were investigated at pressures from 10 to 22 gpa and at a temperature of 1000 c the experimental results indicate that the maximum solubility of enstatite mg4si4o12 in pyrope mg3al2si3o12 is 80 mol enstatite at 15 gpa at pressures between 15 and 19 gpa the solubility is insensitive to pressure at 19 gpa garnet ilmenite transformation first occurs in enstatite composition at pressures above 19 gpa the compositional range of the ilmenite solid solution expands toward pyrope composition with increasing pressure based on the present experiments if pyrolite mantle is assumed the pyroxene garnet transformation appears at 350 400 km depth and garnet modified spinel and garnet spinel phase assemblages become stable successively up to 580 km depth at greater depths garnet decomposes to ilmenite garnet and hence the pyrolite mantle consists of ilmenite garnet spinel assemblage

grain growth kinetics in olivine aggregates abstract grain growth kinetics were studied in hot pressed fine grained olivine aggregates experimental conditions include pressures of 0 1 mpa 300 mpa and 1 gpa and temperatures of 1473 1573 and 1673 k with or without the presence of water at the initial stage of grain growth growth of larger grains occurs rapidly consuming smaller grains which results in significant pore entrapment almost homogeneous grain size distribution is established in this process after this stage grain size distribution remains almost homogeneous in most cases it was found that the presence of water enhances the grain growth kinetics when the amount of water is limited however when the amount of water is large the water containing pores significantly inhibit the grain boundary migration therefore the enhancement of grain growth due to the presence of water is due to the role of dissolved water in olivine rather than the mass transport through water abnormal grain growth was found in some samples in these cases normal grain growth was inhibited by the presence of a significant amount of water filled pores or grains with significantly larger size than the rest occurred in the starting materials in 0 1 mpa runs significant porosity developed and the grain growth rates were significantly lower than in those at high confining pressures these results show that both water and pores have important effects on grain growth kinetics in olivine when the effects of pores or secondary phases in inhibiting grain growth are unimportant the grain growth rate in olivine is very fast growth to 100 m size will occur in 102 to 103 hours at 1500 k thus fine grain size in olivine will be maintained only for a short geological time unless the grain boundaries are effectively pinned by secondary particles

mechanisms of shear localization in the continental lithosphere inference from the deformation microstructures of peridotites from the ivrea zone northwestern italy the ultramafic massif of balmuccia northwestern italy shows a variety of deformation fabrics including some localized shear zones that resulted in nearly complete melting pseudotachylyte a series of peridotite specimens were collected near one of the pseudotachylyte fault veins to investigate the mechanisms of shear localization the microstructural analyses show at least three deformation stages the first stage i is nearly homogeneous deformation at low stress 3 mpa and high temperature 1300 1500 k followed by localized deformation stage ii at a higher stress 60 mpa and moderate temperature 1000 1150 k and finally semi brittle deformation stage iii at a very high stress 400 mpa that resulted in the formation of pseudotachylyte the stage ii deformation resulted in relatively small strains in most areas but significant localized deformation leading to dynamic recrystallization occurred the degree of which increases systematically toward the fault zone based on this observation together with the observation that the brittle deformation post dated dynamic recrystallization we conclude that the shear localization in this locality occurred as a result of ductile deformation rather than brittle deformation various mechanisms of shear localization in the ductile regime are examined and we conclude that the grain size reduction due to dynamic recrystallization at relatively high stresses and moderate temperatures is a probable mechanism of shear localization the intermediate regime between dislocation and diffusion creep where rheology is grain size sensitive yet continuous recrystallization occurs due to dislocation creep is suggested to play an important role in shear localization in the upper mantle the conditions of shear localization due to this mechanism are examined based on the laboratory data on creep and dynamic recrystallization it is shown that shear localization occurs at relatively low temperatures and or high stresses localized deformation in the continental lithosphere will occur well below the brittle ductile transition 10 km depth down to 40 80 km depth depending upon the geothermal gradient and water fugacity

olivine spinel transitions experimental and thermodynamic constraints and implications for the nature of the 400 km seismic discontinuity the sequence of high pressure phase transitions in olivine is traditionally used as a model for seismic velocity variations in the 200 to 650 km depth interval in a mantle of peridotitic bulk composition it has been proposed that the observed seismic velocity increase at 400 km depth is too sharp and of too small a magnitude to be attributable to the phase change and that the upper mantle must thus be chemically stratified with the 400 km discontinuity being due either to a combination of phase changes in a layer of pyroxene garnet rich piclogite composition or to a chemical boundary between such a piclogite layer and an overlying peridotitic layer using available calorimetric thermoelastic and synthesis data and their associated experimental uncertainties we have derived internally consistent high pressure phase relations for the mg2sio4 fe2sio4 join we find that the divariant transition which is generally regarded as occurring over a broad depth interval for mantle olivine compositions is in fact extremely sharp the seismic discontinuity corresponding to the transition in mg0 9fe0 1 2sio4 should occur over a depth interval isothermal of about 6 km at a depth of approximately 400 km the sharpness of this transition is quite insensitive to uncertainties in the constraining calorimetric thermoelastic and synthesis data in addition we have computed seismic velocity profiles for a model mantle consisting of pure olivine of mg0 9fe0 1 2sio4 composition comparison of these computed profiles to those derived from recent seismic studies indicates that the magnitude of the observed velocity increase at 400 km depth is consistent with a mantle transition zone composed of about 60 70 olivine we conclude that there is no need to infer the existence of pyroxene garnet rich compositions such as eclogite or piclogite in the transition zone since an upper mantle of homogeneous olivine rich peridotitic composition is consistent with the available seismic velocity data

teleconnection pattern influence on sea wave climate in the bay of biscay the potential modification of hydrodynamic factors such as waves is a source of concern for many coastal communities because of its potential effect on shoreline evolution in the northern atlantic swell is created by storm winds that cross the atlantic following west east tracks these tracks are shifted more southward or northward depending on the season and on recurring large scale atmospheric pressure anomalies also called teleconnection patterns this study investigates the trends of sea wave patterns in the bay of biscay and relates their interannual variability to teleconnection patterns sea wave parameter time series from the 40 yr european centre for medium range weather forecasts

grain damage hysteresis and plate tectonic states abstract shear localization in the lithosphere is an essential ingredient for understanding how and why plate tectonics is generated from mantle convection on terrestrial planets the theoretical model for grain damage and pinning in two phase polycrystalline rocks provides a frame work for understanding lithospheric shear weakening and plate generation and is consistent with laboratory and field observations of mylonites grain size evolves through the competition between coarsening which drives grain growth and damage which drives grain reduction the interface between crystalline phases controls zener pinning which impedes grain growth damage to the interface enhances the zener pinning effect which then reduces grain size forcing the rheology into the grain size dependent diffusion creep regime this process thus allows damage and rheological weakening to co exist providing a necessary positive self weakening feedback moreover because pinning inhibits grain growth it promotes shear zone longevity and plate boundary inheritance however the suppression of interface damage at low interface curvature wherein inter grain mixing is inefficient and other energy sinks of deformational work are potentially more facile causes a hysteresis effect in which three possible equilibrium grain sizes for a given stress coexist 1 a stable large grain weakly deforming state 2 a stable small grain rapidly deforming state analogous to ultramylonites and 3 an unstable intermediate grain size state perhaps comparable to protomylonites a comparison of the model to field data suggests that shear localized zones of small grain mylonites and ultra mylonites exist at a lower stress than the co existing large grain porphyroclasts rather than as predicted by paleopiezometers or paleowattmeters at a much higher stress this interpretation of field data thus allows localization to relieve instead of accumulate stress the model also predicts that a lithosphere that deforms at a given stress can acquire two stable deformation regimes indicative of plate like flows i e it permits the coexistence of both slowly deforming plate interiors and rapidly deforming plate boundaries earth seems to exist squarely inside the hysteresis loop and thus can have coexisting deformation states while venus appears to straddle the end of the loop where only the weakly deforming branch exists

relationships between dynamically recrystallized grain size and deformation conditions in experimentally deformed olivine rocks new microstructural data on experimentally deformed wet and dry natural olivine rocks anita bay and aheim dunite together with the other reliable experimental data indicate that the experimental stress recrystallized grain size relationship in olivine rocks is largely independent of water content and temperature and is only slightly dependent on the flow properties of the material the experimental data cover a stress range of 30 300 mpa water contents from 30 ppm to 300 ppm and temperatures in the range 1100 1650 c local melt contents of up to 10 volume cannot be demonstrated to have a significant effect on the stress grain size relationship

plate generation and two phase damage theory in a model of mantle convection summary the formation of narrow rapidly deforming plate boundaries separating strong plate interiors are integral components of the generation of plate tectonics from mantle convection the development of narrow plate boundaries requires the interaction of a non linear rheology and convection one such non linear rheology is two phase damage theory which employs a nonequilibrium relation between interfacial surface energy pressure and viscous deformation thereby forming a theoretical model for void generation two phase damage theory was recently extended to allow for deformational work to increase the fineness reduce the grain size of the matrix phase we present results testing two phase damage theory in a 2 d convectively driven system where we allow for 1 pure void generating damage 2 pure fineness generating damage and 3 combined void and fineness generating damage pure void generating damage is found to be unsuccessful at producing plate like features finenessgenerating damage is successful at inducing plate like behaviour in certain circumstances including increasing viscosity sensitivity to fineness and certain regimes of damage input and healing rate cases with combined void and fineness generating damage produce significantly more localization than the end members due to the apparent increase of deformational work input into fineness generation the interaction of microcracks and grain size reduction in two phase damage theory suggests a rheological model for shear localization necessary for the formation of plate tectonic boundaries

collapse of passive margins by lithospheric damage and plunging grain size abstract the collapse of passive margins has been proposed as a possible mechanism for the spontaneous initiation of subduction in order for a new trench to form at the junction between oceanic and continental plates the cold and stiff oceanic lithosphere must be weakened sufficiently to deform at tectonic rates such rates are especially hard to attain in the cold ductile portion of the lithosphere at which the mantle lithosphere reaches peak strength the amount of weakening required for the lithosphere to deform in this tectonic setting is dictated by the available stress stress in a cooling passive margin increases with time e g due to ridge push and is augmented by stresses present in the lithosphere at the onset of rifting e g due to drag from underlying mantle flow increasing stress has the potential to weaken the ductile portion of the lithosphere by dislocation creep or by decreasing grain size in conjunction with a grain size sensitive rheology like diffusion creep while the increasing stress acts to weaken the lithosphere the decreasing temperature acts to stiffen it and the dominance of one effect or the other determines whether the margin might weaken and collapse here we present a model of the thermal and mechanical evolution of a passive margin wherein we predict formation of a weak shear zone that spans a significant depth range of the ductile portion of the lithosphere stiffening due to cooling is offset by weakening due to grain size reduction driven by the combination of imposed stresses and grain damage weakening via grain damage is modest when ridge push is the only source of stress in the lithosphere making the collapse of a passive margin unlikely in this scenario however adding even a small stress contribution from mantle drag results in damage and weakening of a significantly larger portion of the lithosphere we posit that rapid grain size reduction in the ductile portion of the lithosphere can enable or at least significantly facilitate the collapse of a passive margin and initiate a new subduction zone we use this model to estimate the conditions for passive margin collapse for modern and ancient earth as well as for venus

teleconnections in the geopotential height field during the northern hemisphere winter abstract contemporaneous correlations between geopotential heights on a given pressure surface at widely separated points on earth referred to as teleconnections in this paper are studied in an attempt to identify and document recurrent spatial patterns which might be indicative of standing oscillations in the planetary waves during the northern hemisphere winter with time scales on the order of a month or longer a review of existing literature on the subject reveals the existence of at least four such patterns the north atlantic and north pacific oscillations identified by walker and bliss 1932 a zonally symmetric seesaw between sea level pressures in polar and temperature latitudes first noted by lorenz 1951 and what we will refer to as the pacific north american pattern which has been known to operational long range forecasters in this country since the 1950 s a data set consisting of nmc monthly mean sea level pressure and 500 mb height analyses for a 15 year period is used as a basis fo

classification seasonality and persistence of low frequency atmospheric circulation patterns abstract orthogonally rotated principle component analysis rpca of northern hemisphere 1 month mean 700 mb heights is used to identify and describe the seasonality and persistence of the major modes of interannual variability the analysis is detailed and comprehensive in that 1 a high resolution approximately equal area 358 point grid is used for the virtually maximum possible 35 year period of record 2 a positive bias in the nmc data base in the early 1950s in the subtropics is largely eliminated for the first time and 3 homogeneous separate analyses of each month of the year are carried out detailing the mouth to month changes in the dominant circulation patterns winter results are similar to those of other recent rpca and teleconnection studies except that some less obvious patterns are identified and further detail of the better known patterns is provided two north south dipole patterns are found over the pacific ocean west pacific oscillation and east pacific pattern and over the atla

low level potential vorticity and cyclogenesis to the lee of the alps abstract high resolution numerical model simulations over the alpine region are presented that reveal the presence of low level elongated bands of potential vorticity pv downstream of high topography these pv streamers or pv banners occur when the synoptic scale wind turns into a direction across the alps individual pairs of banners with anomalously positive and negative values of pv can be attributed to flow splitting either on the scale of the whole of the alps primary banners or on that of individual massifs and peaks of the model topography secondary banners the pv bands have amplitudes of up to 2 5 and 5 pvu a width of 50 150 km can attain a length of up to 1500 km and extend in the vertical from the surface up to the 500 hpa level on occasions the pv banners are associated with zones of enhanced horizontal wind shear the analysis of daily output from the operational nwp model run of the swiss meteorological institute also demonstrates that such pv streamers are a frequent feature

interdecadal variations in the relationship between the winter north atlantic oscillation and temperature in south central china abstractinterdecadal variations in the relationship between the winter north atlantic oscillation nao and surface air temperature in china are investigated using observational and reanalysis data focus is on south central china in which temperature variability is strongly related to the nao it is revealed that the relationship shows clear interdecadal variations in midwinter during 1951 2015 a relatively weak in phase relationship occurs before the early 1970s p1 but a significant out of phase relationship dominates in the last two decades of the twentieth century p2 though it is clearly weaker from the late 1990s onward observational evidence shows that such interdecadal variations are related mainly to variations in the spatial pattern and amplitude of the nao the northern center of the nao shifted eastward over the second half of the twentieth century in addition the amplitude of the center strengthened from p1 to p2 resulting in a perturbation in the atmospheric circulation response p

generation of plate tectonics with two phase grain damage and pinning source sink model and toroidal flow abstract the grain damage and pinning mechanism of bercovici and ricard 2012 for lithospheric shear localization is employed in two dimensional flow calculations to test its ability to generate toroidal strike slip motion and influence plate evolution this mechanism posits that damage to the interface between phases in a polycrystalline material like peridotite composed primarily of olivine and pyroxene increases the number of small zener pinning surfaces which then constrain mineral grains to ever smaller sizes regardless of creep mechanism this effect allows a self softening feedback in which damage and grain reduction can co exist with a grain size dependent diffusion creep rheology moreover grain growth and weak zone healing are greatly impeded by zener pinning thereby leading to long lived relic weak zones the fluid dynamical calculations employ source sink driven flow as a proxy for convective poloidal flow upwelling downwelling and divergent convergent motion and the coupling of this flow with non linear rheological mechanisms excites toroidal or strike slip motion the numerical experiments show that pure dislocation creep rheology and grain damage without zener pinning as occurs in a single phase assemblages permit only weak localization and toroidal flow however the full grain damage with pinning readily allows focussed localization and intense plate like toroidal motion and strike slip deformation rapid plate motion changes are also tested with abrupt rotations of the source sink field after a plate like configuration is developed the post rotation flow and material property fields retain memory of the original configuration for extensive periods leading to suboptimally aligned plate boundaries e g strike slip margins non parallel to plate motion oblique subduction and highly localized weak and long lived acute plate boundary junctions such as at what is observed at the aleutian kurile intersection the grain damage and pinning theory therefore readily satisfies key plate tectonic metrics of localized toroidal motion and plate boundary inheritance and thus provides a predictive theory for the generation of plate tectonics on earth and other planets

mechanisms for the generation of plate tectonics by two phase grain damage and pinning abstract weakening and shear localization in the lithosphere are essential ingredients for understanding how and whether plate tectonics is generated from mantle convection on terrestrial planets we present a new theoretical model for the mechanism of lithospheric weakening and shear localization and hence plate generation through damage grain evolution and zener pinning in two phase polycrystalline lithospheric rocks grain size evolves through the competition of coarsening which drives grain growth with damage which drives grain reduction however in a two phase medium the interface between phases induces zener pinning which impedes grain growth and facilitates damage the size of the pinning surfaces is given by the roughness of the interface and damage to the interface causes smaller pinning surfaces which in turn drive down the grain size forcing the rheology into the grain size dependent diffusion creep regime this process allows damage and rheological weakening to co exist which is normally considered impossible in single phase assemblages moreover pinning greatly inhibits grain growth and shear zone healing which is much faster in single phase materials hence the resulting shear localization is rapid less than 1 myr but the healing time for a dormant weak zone is very slow greater than 100 myrs these effects therefore permit rapidly forming and long lived plate boundaries the model therefore provides a key ingredient and predictive theory for the generation of plate tectonics on earth and other planets

wave breaking characteristics of midlatitude blocking in this article northern hemisphere winter midlatitude blocking is analysed through its wave breaking characteristics rossby wave breaking is identified as a key process in blocking occurrence as it provides the mechanism for the meridional reversal pattern typical of blocking two indices are designed to detect the major properties of wave breaking i e the orientation cyclonic anticyclonic direction of breaking or db index and the relative contribution of air masses warm cold relative intensity or ri index the use of the db index differentiates between the anticyclonic cases over europe and asia and the cyclonic events over the oceanic basins one of the three regions displaying cyclonic type was found over the atlantic ocean the other two being over the pacific ocean the first of these is located over the western side of the pacific and is dominated by warm air extrusions whereas the second is placed northward of the exit region of the jet stream where the meridional gradient is much weaker two european blocking types have been detected using the ri index which separates out the cases dominated by warm and cold air masses the latter cases in particular exhibited a well structured dipole with associated strong anomalies in both temperature and precipitation copyright 2011 royal meteorological society

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

formation of lithospheric shear zones effect of temperature on two phase grain damage abstract shear localization in the lithosphere is a characteristic feature of plate tectonic boundaries and is evident in the presence of small grain mylonites localization and mylonitization in the ductile portion of the lithosphere can arise when its polymineralic material deforms by a grain size sensitive rheology in combination with zener pinning which can impede or possibly even reverse grain growth and thus promotes a self softening feedback mechanism however the efficacy of this mechanism is not ubiquitous and depends on lithospheric conditions such as temperature and stress therefore we explore the conditions under which self weakening takes place and in particular the effect of temperature and deformation state stress or strain rate on these conditions in our model the lithosphere like polymineralic material is deformed in a two dimensional simple shear driven by constant stress or strain rate the mineral grains evolve to a stable size which is obtained when the rate of coarsening by normal grain growth and the rate of grain size reduction by damage are in balance damage involves processes by which some of the deformational energy gets transferred into surface energy this can happen by i dynamic recrystallization grain damage and ii stretching deforming and stirring the material interface interface damage the influence of temperature enters through rheological laws which govern the rate of work and damage grain growth kinetics and the damage partitioning fraction which is the fraction of deformational work that goes into creating new surface energy we demonstrate that a two phase damage model in which the partitioning fraction depends on both temperature and roughness of the interface between the phases can successfully match the field data including the reported correlation of grain size and temperature the increasing dominance of dislocation creep at higher temperatures and a large range of grain sizes observed across the depth of a single shear zone

patterns of wintertime jet stream variability and their relation to the storm tracks abstract a new approach is put forward for defining extratropical teleconnection patterns the zonal wind field at 250 hpa is analyzed separately in the north atlantic and north pacific ocean sectors during the winter season december march teleconnectivity of this field is found to be particularly strong eof analysis of the zonal wind field yields patterns that i are robust with respect to the range of frequencies included in the data ii relate clearly to the position of the climatological mean jets and iii are broadly consistent with their traditionally defined counterparts in terms of climatic impacts the patterns are characterized by a north south shifting or an extension retraction of the eddy driven jet in its exit region and similar changes at the entrance region of the subtropical jet the patterns also reflect the degree of separation between the subtropical and eddy driven jets atlantic eofs 1 and 2 are counterparts of the north atlantic oscillation nao and eastern atlantic patter

mountains the global frictional torque and the circulation over the pacific north american region abstract the global mountain m and frictional f torques are lag correlated within the intraseasonal band with f leading m the correlation accounts for 20 45 of their variance two basic feedbacks contribute to the relationship first the mountain torque forces global atmospheric angular momentum aam anomalies and the frictional torque damps them thus d f dt m second frictional torque anomalies are associated with high latitude sea level pressure slp anomalies which contribute to subsequent mountain torque anomalies thus d m dt f these feedbacks help determine the growth and decay of global aam anomalies on intraseasonal timescales the low frequency intraseasonal aspect of the relationship is studied for northern winter through lag regressions on f the linear madden julian oscillation signal is first removed from f to focus the analysis on midlatitude dynamical processes the decorrelation timescale of f is similar to that of teleconnection patterns and zonal index cy

paleowattmeters a scaling relation for dynamically recrystallized grain size during dislocation creep mineral grains often evolve to a stable size dictated by the deformation conditions we suggest that grain size evolution during deformation is determined by the rate of mechanical work provided that other elements of microstructure have achieved steady state and that the dissipation rate is roughly constant then changes in internal energy will be proportional to changes in grain boundary area if normal grain growth and dynamic grain size reduction occur simultaneously then the steady state grain size is determined by the balance of those rates a scaling model using these assumptions and published grain growth and mechanical relations matches stress grain size relations for quartz and olivine rocks with no fitting for marbles the model also explains scatter not rationalized by assuming that recrystallized grain size is a function of stress alone when extrapolated to conditions typical for natural mylonites the model is consistent with field constraints on stresses and strain rates

modes of climate variability synthesis and review of proxy based reconstructions through the holocene abstract modes of climate variability affect global and regional climates on different spatio temporal scales and they have important impacts on human activities and ecosystems as these modes are a useful tool for simplifying the understanding of the climate system it is crucial that we gain improved knowledge of their long term past evolution and interactions over time to contextualise their present and future behaviour we review the literature focused on proxy based reconstructions of modes of climate variability during the holocene i e the last 11 7 thousand years with a special emphasis on i proxy based reconstruction methods ii available proxy based reconstructions of the main modes of variability i e el nino southern oscillation pacific decadal variability atlantic multidecadal variability the north atlantic oscillation the southern annular mode and the indian ocean dipole iii major interactions between these modes and iv external forcing mechanisms related to the evolution of these modes this review shows that modes of variability can be reconstructed using proxy based records from a wide range of natural archives but these reconstructions are scarce beyond the last millennium partly due to the lack of robust chronologies with reduced dating uncertainties technical issues related to proxy calibration and difficulty elucidating their stationary impact or not on regional climates over time while for each mode the available reconstructions tend to agree at mutidecadal timescales they show notable disagreement on shorter timescales beyond the instrumental period the reviewed evidence suggests that the intrinsic variability of modes can be modulated by external forcing such as orbital solar volcanic and anthropogenic forcing the review also highlights some modes experience higher variability over the instrumental period which is partly ascribed to anthropogenic forcing these features stress the paramount importance of further studying their past variations using long climate proxy records for the progress of climate science

climatology of cyclogenesis mechanisms in the mediterranean abstract a general climatology of the main mechanisms involved in mediterranean cyclogenesis is presented a diagnostic study of both composite means and case studies is performed to analyze processes occurring in different seasons and in different cyclogenetic regions within the same season it is shown that cyclones that developed over the three most active areas in winter the gulf of genoa the aegean sea and the black sea are essentially subsynoptic lows triggered by the major north atlantic synoptic systems being affected by local orography and or low level baroclinicity over the northern mediterranean coast it is also suggested that cyclones in two or all three of these regions often occur consecutively linked to the same synoptic system in spring and summer thermally induced lows become progressively more important despite the existence of other factors such as the atlas mountains contributing to lee cyclogenesis in northern africa or the extension of the asian monsoon into the eastern

fabric development as the key for forming ductile shear zones and enabling plate tectonics abstract lithospheric deformation on earth is localized under both brittle and ductile deformation conditions as high temperature ductile rheologies are fundamentally strain rate hardening the formation of localized ductile shear zones must involve a structural or rheological change or a change in deformation conditions such as an increase in temperature in this contribution i develop a localization potential that quantifies the weakening associated with these changes the localization potential corresponds to the increase in strain rate resulting from that change under constant stress conditions i provide analytical expressions for the localization potential associated with a temperature increase grain size reduction an increase in water fugacity melt content or the abundance of a weak mineral phase i show that these processes cannot localize deformation from a mantle convection scale 103 km to a ductile shear zone scale 1 km to achieve this is it necessary to invoke a structural transition whereby the weak phase in a rock forms interconnected layers this process is efficient only if one phase is much weaker than the others or if the weakest phase has a highly non linear rheology micas melt and fine grained aggregates unless dry rheologies are used have the necessary characteristics as none of these phases is expected to be present in the dry lithosphere of venus this concept can explain why venus unlike the earth does not display a global network of plate boundaries the diffuse plate boundary in the central indian ocean may be as yet non localized because serpentinization has not reached the ductile levels of the lithosphere

a vorticity based analysis of the spatial and temporal characteristics of the beaufort anticyclone 1 the beaufort anticyclone is the dominant pressure feature over the arctic ocean in all seasons and has a large influence on the surface wind regime and sea ice motion sea level pressure slp from the national center for atmospheric research national centers for environmental prediction reanalysis is used to create a vorticity metric to investigate the spatial and temporal characteristics of the beaufort anticyclone from 1948 to 2008 correlations between vorticity averaged over the beaufort anticyclone region and northern hemisphere slp are strongest south of alaska and north of siberia these two areas are also present in slp composite maps based on extremes of the vorticity metric temporal characteristics are investigated using running means and spectral analysis in which the only predominant peak is the annual cycle if 30 year time periods are compared little to no trend in the vorticity metric is found except in october december where the more recent time period has slightly larger vorticity magnitudes the beaufort anticyclone vorticity metric is correlated with teleconnection index values the pacific ocean patterns show a larger influence than the atlantic patterns including the arctic oscillation which has been shown in recent studies to have little correspondence to changes in sea ice in the early 21st century a significant correlation is found with the pacific north american pattern in all seasons except summer the el nino southern oscillation shows a significant correlation with the vorticity metric in winter and the pacific decadal oscillation shows a significant correlation in winter and spring

observed patterns of month to month storm track variability and their relationship to the background flow month to month storm track variability is investigated via eof analyses performed on era 40 monthlyaveraged high pass filtered daily 850 hpa meridional heat flux and the variances of 300 hpa meridional wind and 500 hpa height the analysis is performed both in hemispheric and sectoral domains of the northern and southern hemispheres patterns characterized as pulsing and latitudinal shifting of the climatologicalmean storm tracks emerge as the leading sectoral patterns of variability based on the analysis presented storm track variability on the spatial scale of the two northern hemisphere sectors appears to be largely but perhaps not completely independent pulsing and latitudinally shifting storm tracks are accompanied by zonal wind anomalies consistent with eddy forced accelerations and geopotential height anomalies that project strongly on the dominant patterns of geopotential height variability the north atlantic oscillation nao northern hemisphere annular mode nam is associated with a pulsing of the atlantic storm track and a meridional displacement of the upper tropospheric jet exit region whereas the eastern atlantic ea pattern is associated with a latitudinally shifting storm track and an extension or retraction of the upper tropospheric jet analogous patterns of stormtrack and upper tropospheric jet variability are associated with the western pacific wp and pacific north america pna patterns wave mean flow relationships shown here are more clearly defined than in previous studies and are shown to extend through the depth of the troposphere the southern hemisphere annular mode sam is associated with a latitudinally shifting storm track over the south atlantic and indian oceans and a pulsing south pacific storm track the patterns of storm track variability are shown to be related to simple distortions of the climatological mean upper tropospheric jet

composite analysis of large scale environments conducive to western pacific polar subtropical jet superposition abstractalthough considerable research attention has been devoted to examination of the northern hemisphere polar and subtropical jet streams relatively little has been directed toward understanding the circumstances that conspire to produce the relatively rare vertical superposition of these usually separate features this study investigates the structure and evolution of large scale environments associated with jet superposition events in the northwest pacific an objective identification scheme using ncep ncar reanalysis 1 data is employed to identify all jet superpositions in the western pacific 30 40 n 135 175 e for boreal winters djf between 1979 80 and 2009 10 the analysis reveals that environments conducive to western pacific jet superposition share several large scale features usually associated with east asian winter monsoon eawm northerly cold surges including the presence of an enhanced hadley cell like circulation within the jet entrance region it is further demonstrated that s

north south precipitation patterns in western north america on interannual to decadal timescales the overall amount of precipitation deposited along the west coast and western cordillera of north america from 258 to 558n varies from year to year and superimposed on this domain average variability are varying north south contrasts on timescales from at least interannual to interdecadal in order to better understand the north south precipitation contrasts their interannual and decadal variations are studied in terms of how much they affect overall precipitation amounts and how they are related to large scale climatic patterns spatial empirical orthogonal functions eofs and spatial moments domain average central latitude and latitudinal spread of zonally averaged precipitation anomalies along the westernmost parts of north america are analyzed and each is correlated with global sea level pressure slp and sea surface temperature series on interannual defined here as 3 7 yr and decadal 7 yr timescales the interannual band considered here corresponds to timescales that are particularly strong in tropical climate variations and thus is expected to contain much p

a few remarks on the kinetics of static grain growth in rocks static grain growth is a relatively simple transformation in which grain size increases under driving forces caused by grain and interphase boundary curvature given the relative simplicity of the protocol for grain growth experiments measurements of grain boundary mobility show surprising variations boundary mobilities during grain growth are affected by solute and impurity chemistry chemical fugacity of trace and major elements pore size and number pore fluid chemistry the presence of melts and the presence of solid second phases as well as temperature and pressure all of these factors may exert influence on grain growth of rocks in natural situations and many are also present during the laboratory experiments provided that the necessary kinetics parameters are known bounds may be placed on the interface mobility when pores partial melts or solutes are present to predict the rate of grain growth in natural situations will require improved laboratory data and careful consideration of the thermodynamic conditions likely to be encountered in nature

a simple model of the 1982 83 californian el nino steric height steric height anomaly sea level and sea level anomaly show that the 1940 41 and the 1982 83 californian el nino events were resonant responses of the california current to the expansion and intensification of the aleutian low and decrease in strength of the north pacific high in both cases the subsurface anomalies were dynamically produced by onshore transport of subarctic water and a downward tilt of the inshore thermocline sea surface temperature anomalies however resulted from thermodynamic as well as dynamic processes most previous californian el nino or anti el nino events probably were resonant or anti resonant responses of the seasonal cycle of the california current to anomalous large scale atmospheric forcing

structure and variability of the west greenland current in summer derived from 6 repeat standard sections abstract six historical sections across the west greenland current are examined three sections have been regularly occupied since the late 1950s while the three southern ones have been taken since 1984 significant variability is observed for the freshwater core of the coastal current on the shelf with salinity varying by over 3 units between years there is also significant variability in the shape and offshore position of the main shelf break front leading to large variability in eulerian velocities significant presence of irminger water is seen during the 1960s and the 2000s being found right across the sections in recent years maximum mean transport relative to the 34 8 isohaline of 5 5 3 9 sv relative to 700 db for 1984 2005 is observed at the cape desolation section transports decrease to the north with the majority of the exchange with the interior of the labrador sea occurring between cape desolation and fylla bank inter decadal transport variability is observed at fylla bank while a decline in transports since peaks in the early 1990s is seen at cape farewell and cape desolation freshwater transport is largest at cape desolation with a mean summer transport of 60 2 20 5 msv freshwater transport increases slightly between cape farewell and cape desolation and we suggest it is related to local discharge by glaciers into juliannehaab bight as well as the melting of sea ice we also find that years of high greenland ice cap melt are consistently associated with years of high freshwater transport at cape desolation suggesting a portion of the freshwater transport of the west greenland current may be associated with melt from the greenland ice sheet finally significantly enhanced freshwater transports 33 msv at cape desolation compared to the long term mean are seen in 2008 probably a signature of the record arctic ocean ice melt and export in 2007

southern hemisphere winter extratropical cyclone characteristics and vertical organization observed with the era 40 data in 1979 2001 abstract the mean characteristics and trends of southern hemisphere sh winter extratropical cyclones occurring at six levels of the troposphere over the period 1979 2001 have been investigated using the 40 yr ecmwf re analysis era 40 data cyclonic systems were identified with the melbourne university cyclone finding and tracking scheme this study shows that mean sea level pressure mslp cyclones are more numerous more intense smaller deeper and slower moving than higher level cyclones the novel vertical tracing scheme devised for this research revealed that about 52 of sh winter mslp cyclones have a vertically well organized structure extending through to the 500 hpa level about 80 of these vertically coherent sh cyclones keep their westward tilt until the surface cyclones reach their maximum depths and the mean distance is 300 km between the surface and the 500 hpa level cyclone centers when the surface cyclones obtain their maturity according to the authors definition of vertical orga

a description of low frequency atmospheric oscillations over the extratropical northern hemisphere using singular spectrum analysis the method of singular spectrum analysis ssa is used to describe low frequency atmospheric variability over the extratropical northern hemisphere nh from 500 hpa geopotential heights analysis reveals significant intraseasonal and interannual oscillations confined principally to eastern sections of the pacific ocean and northern sections of the atlantic ocean generally where earlier studies have identified persistent anomalies from the results it is suggested that persistent anomalies may undergo significant modulation on the quasi biennial time scale

rock strength and texture evolution during deformation in the earth s ductile lithosphere a two phase thermodynamics model building upon the two phase and grain damage theory we propose a new formulation allowing to track the evolution of phase mixing segregation during ductile deformation of a two phased aggregate our model is based on a set of variables characterizing a rock texture the mean grain sizes and the phase proportion during ductile deformation activation of different micro mechanical processes impacts the aggregate texture dislocation and diffusion creep are the two main deformation processes considered we only account for the effect of zener pinning in slowing down grain growth and allow for active grain size reduction mechanisms in the diffusion creep domain for this purpose an equation is proposed to track the phase mixing evolution during ductile deformation numerical application using anorthite rheology shows that any grain reduction mechanisms that could be active in the diffusion creep regime requires a very high partition fraction in order to reach the grain size size predicted by the feldspar piezometer application of this model to gabbroic composition relevant for the ductile crust demonstrates that the strong coupling between phases grain sizes and interface evolution results in steady state grain sizes far below the field boundary this effect is coeval with an important increase of mixing between the two phases in addition accounting for the phase mixing results in a drop of the global aggregate stress during deformation this model allows for further comparison of mylonitized textures evolution with natural shear zones at the local and regional scales

heterogeneities in the mantle transition zone from observations of p to sv converted waves abstract the method of detection of p to sv converted waves from distant earthquakes vinnik 1977 was applied to sets of long period records from a few seismograph stations in europe and the west of north america the results obtained suggest that the converted phases related to the major boundaries in the mantle can be reliably detected and the depths of conversion evaluated with an accuracy of a few kilometres the depth of the olivine spinel transition is close to 400 km and no difference between the estimates for the north of europe and the west of north america is found the depth of the boundary separating the upper and lower mantle is close to 640 km which is 30 km less than in the recent earth reference models fine s velocity stratification of this transition changes laterally from a high gradient layer 50 km thick terminated at the bottom by a sharp discontinuity to a gradient layer 100 km or more thick without the discontinuity a striking anomaly of the mantle transition zone is found in the rio grande rift area where a well pronounced boundary is found at 510 km depth

implications of grain size evolution on the seismic structure of the oceanic upper mantle abstract we construct a 1 d steady state channel flow model for grain size evolution in the oceanic upper mantle using a composite diffusion dislocation creep rheology grain size evolution is calculated assuming that grain size is controlled by a competition between dynamic recrystallization and grain growth applying this grain size evolution model to the oceanic upper mantle we calculate grain size as a function of depth seafloor age and mantle water content the resulting grain size structure is used to predict shear wave velocity v s and seismic quality factor q for a plate age of 60 myr and an olivine water content of 1000 h 10 6 si we find that grain size reaches a minimum of 15 mm at 150 km depth and then increases to 20 30 mm at a depth of 400 km this grain size structure produces a good fit to the low seismic shear wave velocity zone lvz in oceanic upper mantle observed by surface wave studies assuming that the influence of hydrogen on anelastic behavior is similar to that observed for steady state creep further it predicts a viscosity of 10 19 pa s at 150 km depth and dislocation creep to be the dominant deformation mechanism throughout the oceanic upper mantle consistent with geophysical observations we predict larger grain sizes than proposed in recent studies in which the lvz was explained by a dry mantle and a minimum grain size of 1 mm however we show that for a 1 mm grain size diffusion creep is the dominant deformation mechanism above 100 200 km depth inconsistent with abundant observations of seismic anisotropy from surface wave studies we therefore conclude that a combination of grain size evolution and a hydrated upper mantle is the most likely explanation for both the isotropic and anisotropic seismic structure of the oceanic upper mantle our results also suggest that melt extraction from the mantle will be significantly more efficient than predicted in previous modeling studies that assumed grain sizes of 1 mm

snow n ao teleconnection and its modulation by the quasi biennial oscillation abstractthis study explores the wintertime extratropical atmospheric response to siberian snow anomalies in fall using observations and two distinct atmospheric general circulation models the role of the quasi biennial oscillation qbo in modulating this response is discussed by differentiating easterly and westerly qbo years the remote influence of siberian snow anomalies is found to be weak in the models especially in the stratosphere where the holton tan effect of the qbo dominates the simulated snow influence on the polar vortex at the surface discrepancies between composite analyses from observations and model results question the causal relationship between snow and the atmospheric circulation suggesting that the atmosphere might have driven snow anomalies rather than the other way around when both forcings are combined the simulations suggest destructive interference between the response to positive snow anomalies and easterly qbo and vice versa at odds with the hypothesis that the s

the p wave upper mantle structure beneath an active spreading centre the gulf of california summary detailed analysis of short period travel time dt d and wave form data reveals the upper mantle structure beneath an oceanic ridge to depths of 900 km more than 1400 digital seismograms from earthquakes in mexico and central america recorded at scarlet yield 1753 travel times and 58 direct measurements of dt d as well as high quality stable waveforms the 29 events combine to form a continuous record section from 9 to 40 with an average station spacing of less than 5 km first the travel times are inverted using the tau method of bessonova et al the resultant model is adjusted to agree with the experimental p values further constraints arise from the observed relative amplitudes of mantle phases which are modelled by trial and error using wkbj synthetic seismograms chapman wiggins model gca which is appropriate for western mexico north of 20 latitude is similar to existing upper mantle models for shield tectonic continental and arc trench regimes below 400 km but differs significantly in the upper 350 km gca velocities are very low in this depth range the model catches up with the others with a very large velocity gradient from 225 to 390 km this well resolved feature is consistent with the shear wave model tna for western north america of grand helmberger the abundant data from 20 to 30 control the detailed shape of the 660 km discontinuity very large velocity gradients lie both above 620 660 km and below 661 680km a 2 8 per cent velocity change

vb cyclones synchronized with the arctic north atlantic oscillation vb cyclones typically emerge in the western mediterranean and propagate to the northeast into central europe this paper explores the temporal characteristics of vb cyclone occurrence based on cyclone tracks identified at the atmospheric levels of z700 and sea level pressure using jra 55 reanalysis data for the period 1959 2015 the risk of vb occurrence was significantly high in the 1960s and has remained at a lower level since then vb cyclones do not occur fully randomly according to a poisson point process eleven well separated and distinct clusters as well as 11 hiatus periods are identified with average occurrence rates of 21 5 and 5 2 yrea 1 respectively during the event of vb the large scale atmospheric circulation is changed into a state favoring the development of successive vb cyclones clustering is very prominent in the case of genoan vb cyclones in summer as well as those vb cyclones developing over the iberian peninsula or the north african coast in winter superposition of the polar and the subtropical jet stream over the western mediterranean is identified as a main feature at the onset of vb cyclones vb cyclone occurrence appears to be synchronized with the northern atlantic oscillation nao at z500 and arctic oscillation ao at z1000 clusters have occurred when both nao and ao were negative this relation applies to western mediterranean cyclones not following a vb track as well however to a much weaker extent in contrast vb cyclone frequency was particularly low from 1988 to 1997 during a sustained positive phase of both nao and ao

role of orthopyroxene in rheological weakening of the lithosphere via dynamic recrystallization for plate tectonics to operate on a terrestrial planet the surface layer the lithosphere must have a modest strength earth 200 mpa but a standard strength profile based on olivine far exceeds this threshold value consequently it is essential to identify mechanisms that reduce the strength of the lithosphere on earth here we report results of high strain laboratory deformation experiments on a representative olivine orthopyroxene composition that show the addition of orthopyroxene substantially reduces the strength in the ductile regime within a certain temperature window the reduction in strength is associated with the formation of small orthopyroxene and olivine grains our samples show heterogeneous microstructures similar to those observed in natural peridotites in shear zones fine grained regions containing both orthopyroxene and olivine that form interconnected bands where a large fraction of strain is accommodated a model is developed to apply these results to geological conditions such a model combined with our experimental observations suggests that orthopyroxene may play a key role in the plastic deformation of the lithosphere in a critical temperature range leading to long term weakening associated with strain localization in the lithosphere

the role of second phases for controlling microstructural evolution in polymineralic rocks a review abstract we present a state of the art review of the microstructural evolution in rocks under static and deformational conditions first the general concepts and processes are introduced using monomineralic aggregates then they are expanded into the more complex context of polymineralic rocks with a dominant matrix phase the first part of this contribution delivers information on sample strategies to quantify polymineralic microfabrics based on comparisons between microfabrics of monomineralic and polymineralic rocks we use the common knowledge collected over the past decades for monomineralic systems and discuss the differences to polymineralic ones in terms of microstructures modal compositions spatial distribution of phases crystallographic preferred orientations and associated processes the article puts particular emphasis on the effect of coupled grain growth mass transfer processes and deformation mechanisms we speculate on the effect of mineral reactions during the evolution of microstructures and rheology in polymineralic aggregates at different metamorphic conditions at the end of the article we demonstrate the great potential of grain size evolution maps as microstructural tool to unravel the geological history of polymineralic rocks that evolved under a variety of geodynamic situations

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

the link between eddy driven jet variability and weather regimes in the north atlantic european sector this study reconciles two perspectives on wintertime atmospheric variability in the north atlantic european sector the zonal mean framework comprising three preferred locations of the eddy driven jet southern central northern and the weather regime framework comprising four classical north atlantic european regimes atlantic ridge ar zonal zo european scandinavian blocking bl greenland anticyclone ga a k means clustering algorithm is used to characterize the two dimensional variability of the eddy driven jet stream defined by the lower tropospheric zonal wind in the era interim reanalysis the first three clusters capture the central jet and northern jet along with a new mixed jet configuration a fourth cluster is needed to recover the southern jet the mixed cluster represents a split or strongly tilted jet neither of which is well described in the zonal mean framework and has a persistence of about one week similar to the other clusters connections between the preferred jet locations and weather regimes are corroborated southern to ga central to zo and northern to ar in addition the new mixed cluster is found to be linked to european scandinavian blocking whose relation to the eddy driven jet was previously unclear

composition of the upper mantle geophysical tests of two petrological models the elastic properties of candidate mantle phases are used to test the viability of olivine rich pyrolitic and cao al 2o 3 rich eclogitic assemblages for the mantle high temperature adiabats for each phase of interest are constructed and compared to mantle seismic properties both pyrolitic and eclogitic assemblages satisfy the seismic properties between 200 and 400 km between 400 and 670 km depth an eclogitic assemblage yields a superior match to velocities and velocity gradients the 400 km seismic discontinuity may represent a chemical boundary between pyrolite and picritic eclogite piclogite or phase transformations in the olivine orthopyroxene components of a piclogitic assemblage containing about 16 olivine high velocity gradients in the transition zone may be explained by the transformation of ca rich cpx to majorite garnet seismic properties at the top of the lower mantle are consistent with pyrolite piclogite or perovskite implying that the 670 km discontinuity may be a chemical boundary

an experimental investigation of the pyroxene garnet transformation in a pyrolite composition and its bearing on the constitution of the mantle abstract the pyroxene garnet transformation in a model pyrolite composition has been investigated over the pressure interval 4 0 to 17 5 gpa at temperatures of 1200 1400 c the transformation is largely completed in the pressure interval 14 16 gpa and a single phase aluminum deficient garnet majorite becomes stable above 16 gpa the mineral assemblages and the density and seismic velocity distributions for pyrolite are calculated as a function of depth on the basis of this and other recent experimental data density and seismic velocity profiles calculated for pyrolite are consistent with models based on seismic observations to a depth of 550 km if the existence of a high density velocity gradient between 550 km and 650 km is confirmed the presence of further phase transformations in majorite and or some degree of chemical heterogeneity in this depth interval would be indicated

interactions between enso transient circulation and tropical convection over the pacific abstract the interannual variability of transient waves and convection over the central and eastern pacific is examined using 30 northern winters of ncep ncar reanalyses 1968 69 1997 98 and satellite outgoing longwave radiation data starting in 1974 there is a clear signal associated with the el nino southern oscillation such that differences in the seasonal mean basic state lead to statistically significant changes in the behavior of the transients and convection with periods less than 30 days which then feed back onto the basic state during a warm event el nino phase the northern hemisphere subtropical jet is strengthened over the central pacific the region of upper tropospheric mean easterlies over the tropical western pacific expands eastward past the date line and the upper tropospheric mean westerly duct over the tropical eastern pacific is weakened the transients tend to propagate along the almost continuous waveguide of the subtropical jet equatorward propagation into the westerly

constitution of the mantle 2 petrological models of transition zone based on fms phase diagram abstract a detailed picture is presented of phase transformations in the mantle transition zone based upon the study of phase relations and topologies in the feo mgo sio2 fms system the p t x diagram for the composition close to pyrolite sio2 40 mol and fe fe mg 0 12 is used in modelling the structure of the mantle transition zone to connect phase transitions in minerals to the observed seismic discontinuities the mineralogical nature of the first seismic discontinuity at a depth of about 400 km is associated either with a univariant transformation px px or with a sharp divariant transformation px px px which should be closer to 420 km for the fixed composition close to pyrolite at a depth of about 650 km neither univariant nor invariant equilibria are observed in the fms system it has been found that in the fms system two extremely narrow 1 2 km wide stability fields of divariant mineral assemblages pv st and pv mw separated by a trivariant zone pv about 5 km wide might be responsible for the nature of the 650 km discontinuity because of the negative slope of the phase boundaries the second seismic discontinuity in the cold subducting slab is deeper than in the surrounding mantle

late holocene climate reconstructions for the russian steppe based on mineralogical and magnetic properties of buried palaeosols abstract insights into past climate changes and corresponding evolution of soils and the environment can be gained by multidisciplinary studies of palaeosols here we focus on palaeosols buried beneath archaeological monuments specifically funerary mounds kurgans in the russian steppe the kurgans were constructed and each of the palaeosols buried over a range of different timesteps from the mid holocene to 600 years before present yr bp integrated magnetic mineralogical and pedological data were used to obtain estimates of past climate especially precipitation changes through both time and space a soil magnetism based climofunction derived previously from modern steppe soils and modern climate was applied to each set of palaeosols to obtain quantitative reconstructions of annual precipitation for the time at which the soils were buried independent soil property data clay mineralogy salt content iron mineralogy from mossbauer analysis and optical and electron microscopy were also obtained in order to test and substantiate the magnetic inferences the data obtained indicate that the climate of the lower volga steppe area has varied from the mid holocene onwards precipitation minima occurred at 5000 3800 and 1600 yr bp with intervals of enhanced precipitation at 1900 yr bp and 600 yr bp these rainfall variations appear to occur synchronously with changes in middle eastern precipitation and lake levels suggesting they are controlled by index changes in the north atlantic oscillation

inferring upper mantle flow from seismic anisotropy an experimental perspective abstract patterns of mantle flow are most directly inferred from observations of seismic anisotropy which is mainly caused by the crystallographic preferred orientation cpo of olivine the most abundant mineral in the upper mantle cpo is generated by high temperature ductile deformation which often yields predictable relationships between the elastic or seismic properties of a material and the kinematics of flow over the last 15 years there has been a wealth of new data describing the how olivine cpo forms and evolves as a function of deformation conditions and strain magnitude in this review we explore the relationships between deformation the evolution of cpo and the development of seismic anisotropy from the perspective of experimental rock mechanics we first review the experimental basis for the study of olivine cpo evolution from the formative studies in the early nineteen sixties through recent advances we then review some emerging complications to the study of cpo evolution such as the long lived transient cpos that arise from changes in deformation kinematics mechanisms and conditions finally we discuss the origins of seismic anisotropy and the challenges of interpreting seismic anisotropy in terms of mantle flow

quantifying the effect of pyroxene on deformation of peridotite in a natural shear zone the influence of pyroxene on upper mantle viscosity remains unclear but may occur either through its difference in strength relative to olivine or through its effect on olivine microstructure to determine the role of pyroxene in moderating peridotite viscosity we analyzed microstructures in paired harzburgites and dunites from a natural shear zone exposed in the josephine peridotite olivine crystallographic textures evolve similarly in harzburgites and dunites with increasing strain indicating the operation of similar deformation mechanisms in olivine the mean olivine grain sizes are 1 5 times larger in dunites than in harzburgites whereas the mean olivine subgrain sizes are a factor of 1 2 smaller in dunites than in harzburgites the average stresses in olivine estimated with a subgrain size piezometer are inversely correlated with pyroxene volume fraction and directly correlated with the mean olivine grain size the calculated ratio of pyroxene viscosity to olivine viscosity for each harzburgite dunite pair varies from 1 2 to 3 3 notably our data indicate that olivine viscosity is non newtonian with a finite grain size sensitivity we suggest that at the conditions of the shear zone pyroxene is more viscous than olivine but that the increase in aggregate strength with larger pyroxene fractions is offset by correspondingly smaller olivine grain sizes thus major rheological weakening associated with pyroxene content may only be possible if 1 temperatures are higher than those characterizing this shear zone increasing the viscosity contrast between olivine and pyroxene or 2 stresses are high enough that grain size reduction promotes a transition to a deformation mechanism with extreme grain size sensitivity such as diffusion creep

strain weakening enables continental plate tectonics much debate exists concerning the strength distribution of the continental lithosphere how it controls lithosphere scale strain localization and hence enables plate tectonics no rheological model proposed to date is comprehensive enough to describe both the weakness of plate boundary and rigid like behaviour of plate interiors here we show that the duality of strength of the lithosphere corresponds to different stages of microstructural evolution geological constraints on lithospheric strength and large strain numerical experiments reveal that the development of layers containing weak minerals and the onset of grain boundary sliding upon grain size reduction in olivine cause strain localisation and reduce strength in the crust and subcontinental mantle respectively the positive feedback between weakening and strain localization leads to the progressive development of weak plate boundaries while plate interiors remain

extremely late onset of the 2018 south china sea summer monsoon following a la nina event effects of triple sst anomaly mode in the north atlantic and a weaker mongolian cyclone

controls on the rheological properties of peridotite at a palaeosubduction interface a transect across the base of the oman uae ophiolite abstract studies of experimentally deformed rocks and small scale natural shear zones have demonstrated that volumetrically minor phases can control strain localisation by limiting grain growth and promoting grain size sensitive deformation mechanisms these small scale studies are often used to infer a critical role for minor phases in the development of plate boundaries however the role of minor phases in strain localisation at an actual plate boundary remains to be tested by direct observation in order to test the hypothesis that minor phases control strain localisation at plate boundaries we conducted microstructural analyses of peridotite samples collected along a 1 km transect across the base of the oman united arab emirates uae ophiolite the base of the ophiolite is marked by the semail thrust which represents the now exhumed contact between subducted oceanic crust and the overlying mantle wedge as such the base of the ophiolite provides the opportunity to directly examine a former plate boundary our results demonstrate that the mean olivine grain size is inversely proportional to the abundance of minor phases primarily orthopyroxene as well as clinopyroxene hornblende and spinel consistent with suppression of grain growth by grain boundary pinning our results also reveal that mean olivine grain size is proportional to cpo strength both of which generally decrease towards the metamorphic sole suggesting that the fraction of strain produced by different deformation mechanisms varied spatially experimentally derived flow laws indicate that under the inferred deformation conditions the viscosity of olivine was grain size sensitive as such grain size and thereby the abundance of minor phases influenced viscosity during subduction related deformation along the base of the mantle wedge we calculate an order of magnitude decrease in the viscosity of olivine towards the base of the ophiolite which suggests strain was localised near the subduction interface our data indicate that this rheological weakening was primarily the result of more abundant minor phases near the base of the ophiolite our interpretations are consistent with those of previous studies on experimentally deformed rocks and smaller scale natural shear zones that indicate minor phases can exert the primary control on strain localisation however our study demonstrates for the first time that minor phases can control strain localisation at the scales relevant to a major plate boundary

large scale circulation classification and its links to observed precipitation in the eastern and central tibetan plateau the relationship between the large scale circulation dynamics and regional precipitation regime in the tibetan plateau tp has so far not been well understood in this study we classify the circulation types using the self organizing maps based on the daily field of 500 hpa geopotential height and link them to the precipitation climatology in the eastern and central tp by virtue of an objective determining method 18 circulation types are quantified the results show that the large amount of precipitation in summer is closely related to the circulation types in which the enhanced and northward shifted subtropical high sh over the northwest pacific and the obvious cyclconic circulation anomaly over the bay of bengal are helpful for the indian summer monsoon and east asian summer monsoon to take abundant low latitude moisture to the eastern and southern tp on the contrary the dry winter in the central and eastern tibet corresponds to the circulation types with divergence over the central and eastern tp and the water vapor transportations of east asian winter monsoon and mid latitude westerly are very weak some circulation types are associated with some well known circulation patterns monsoons influencing the tp e g east atlantic pattern el nino southern oscillation indian summer monsoon and the mid latitude westerly and exhibit an overall good potential for explaining the variability of regional seasonal precipitation moreover the climate shift signals in the late 1970s over the eastern pacific north pacific oceans could also be reflected by both the variability of some circulation types and their correspondingly composite precipitations this study extends our understandings for the large scale atmospheric dynamics and their linkages with regional precipitation and is beneficial for the climate change projection and related adaptation activities in the highest and largest plateau in the world

meridional flow variability over the nordic seas in the arctic oscillation framework an anomalous recurring atmospheric circulation pattern of high relevance for the climate of the nordic seas and siberia is identified it is found as the second empirical orthogonal function eof of monthly winter sea level pressure slp anomalies poleward of 30 n where the leading eof is the arctic oscillation ao the most prominent centre of action of the circulation pattern is located over the barents region this barents oscillation bo is shown to have a high temporal correlation with the sensible heat loss of the nordic seas r 0 76 the bo also correlates to eurasian surface air temperature sat anomalies with r 0 72 after the ao related sat variations are removed by means of a linear regression two sets of slp composites are constructed where one is based on low and high nordic seas heat loss months and the other is based on warm and cold eurasian months patterns reminiscent of the bo emerge in the two composites when ao related variability is removed

an analysis of daily maximum wind speed in northwestern europe using generalized linear models abstract the basic climatological pattern and recent trends in daily maximum wind speed dmws for the region 47 5 65 n and 12 5 w 22 5 e are studied using gamma distributions within a generalized linear model between 1958 and 1998 dmws has increased over the ocean in winter but weakened over continental europe in summer large scale circulation changes such as those of the north atlantic oscillation nao and arctic oscillation ao account for the strengthening wind over the ocean global warming may have impacted the regional wind climate in particular southern hemisphere temperature exhibits a significant effect on the distinct oceanic and continental trends in dmws it is suggested that the steady warming of the southern hemisphere during the last few decades may have forced the north atlantic storm track to shift in such a way that storms are enhanced toward the northwestern oceanic area but weakened throughout most of the european continent

grain boundary sliding in san carlos olivine flow law parameters and crystallographic preferred orientation 1 we performed triaxial compressive creep experiments on aggregates of san carlos olivine to develop a flow law and to examine microstructural development in the dislocation accommodated grain boundary sliding regime gbs each experiment included load and temperature steps to determine both the stress exponent and the activation energy grain boundary maps created with electron backscatter diffraction data were used to quantify grain size distributions for each sample inversion of the resulting data produced the following flow law for gbs gbs 104 8 0 8 2 9 0 3 d0 7 0 1 exp 445 20 kj mol 1 rt with d and gbs in units of mpa m and s 1 respectively although relatively weak crystallographic preferred orientations cpos have 010 maxima parallel to the compression direction along with 100 and 001 girdles perpendicular to the compression direction cpos and subgrain boundary misorientation axes suggest that the 010 100 slip system contributes significantly to deformation we propose that these experimental results are best modeled by a deformation mechanism in which strain is accomplished primarily through grain boundary sliding accommodated by the motion of dislocations extrapolation of our flow laws to mantle conditions suggests that gbs is likely to be the dominant deformation mechanism in both lithospheric shear zones and asthenospheric flow and therefore strong upper mantle seismic anisotropy can not be attributed solely to the dominance of dislocation creep

reflection properties of phase transition and compositional change models of the 670 km discontinuity seismic observations of the amplitude ratio amplitude p 670p amplitude p p suggest a reflection coefficient r 7 13 for reflections from the underside of the 670 km discontinuity this can be compared with calculated reflection coefficients to constrain the nature of the mantle transition region velocity and density profiles through the transition region were calculated for olivine pyroxene garnet and olivine garnet model mineral assemblages for models with identical upper and lower mantle compositions the computed reflection coefficients from the phase transitions pertinent to the 670 km discontinuity are less than 0 5 these phase transitions produce their maximum r if they are effectively discontinuous which gives r 2 3 except for the garnet to perovskite transformation which gives r 8 in general given realistic impedance contrasts across a phase transition the change in material properties must occur over a region less than 3 km thick for r to be of the order of 5 a discontinuous change in chemical composition at 670 km depth coupled with a change in phase can also produce a reflection coefficient of the observed magnitude r 2 3 for a first order change in phase at 670 km with different iron concentrations on each side of the discontinuity and r 4 6 for cases with a discontinuous increase in silica content based on our analysis the thermal and chemical boundary layers which would occur across such a chemical discontinuity would probably have negligible effects on the reflection coefficient thus although limited by considerable uncertainties the mineralogical and seismic data suggest a two layered model of the mantle with a discontinuous change in both phase and composition at 670 km

plate tectonics damage and inheritance lithospheric damage combined with transient mantle flow and migrating proto subduction is proposed to explain the apparent emergence of plate tectonics three billion years ago modelling confirms that tectonic plate boundaries and fully formed tectonic plates can arise under conditions characteristic of earth but not of venus plate tectonics the division of the earth s surface into rigid plates separated by linear zones of concentrated deformation is unique among known terrestrial bodies as yet there is no consensus view on how the phenomenon arose here david bercovici and yanick ricard outline an explanation that starts with the microscopic properties of deforming minerals when sufficient lithospheric damage accumulates the theory states shear localization occurs and long lived weak zones develop transient mantle flow and migrating proto subduction then lead to the accumulation of plate boundaries and eventually tectonic plates the authors model this process with a grain evolution and damage mechanism coupled to an idealized model of pressure driven lithospheric flow and show that tectonic plate boundaries and fully formed tectonic plates can form under such conditions the initiation of plate tectonics on earth is a critical event in our planet s history the time lag between the first proto subduction about 4 billion years ago and global tectonics approximately 3 billion years ago suggests that plates and plate boundaries became widespread over a period of 1 billion years the reason for this time lag is unknown but fundamental to understanding the origin of plate tectonics here we suggest that when sufficient lithospheric damage which promotes shear localization and long lived weak zones combines with transient mantle flow and migrating proto subduction it leads to the accumulation of weak plate boundaries and eventually to fully formed tectonic plates driven by subduction alone we simulate this process using a grain evolution and damage mechanism with a composite rheology which is compatible with field and laboratory observations of polycrystalline rocks1 2 coupled to an idealized model of pressure driven lithospheric flow in which a low pressure zone is equivalent to the suction of convective downwellings in the simplest case for earth like conditions a few successive rotations of the driving pressure field yield relic damaged weak zones that are inherited by the lithospheric flow to form a nearly perfect plate with passive spreading and strike slip margins that persist and localize further even though flow is driven only by subduction but for hotter surface conditions such as those on venus accumulation and inheritance of damage is negligible hence only subduction zones survive and plate tectonics does not spread which corresponds to observations after plates have developed continued changes in driving forces combined with inherited damage and weak zones promote increased tectonic complexity such as oblique subduction strike slip boundaries that are subparallel to plate motion and spalling of minor plates

greenland ice sheet melt from modis and associated atmospheric variability daily june july melt fraction variations over the greenland ice sheet gis derived from the moderate resolution imaging spectroradiometer modis 2000 2013 are associated with atmospheric blocking forming an omega shape ridge over the gis at 500 hpa height blocking activity with a range of time scales from synoptic waves breaking poleward 5 days to full fledged blocks 5 days brings warm subtropical air masses over the gis controlling daily surface temperatures and melt the temperature anomaly of these subtropical air mass intrusions is also important for melting based on the years with the greatest melt 2002 and 2012 during the modis era the area average temperature anomaly of 2 standard deviations above the 14 year june july mean results in a melt fraction of 40 or more though the summer of 2007 had the most blocking days atmospheric temperature anomalies were too small to instigate extreme melting

representation of extratropical cyclones blocking anticyclones and alpine circulation types in multiple reanalyses and model simulations abstractatmospheric circulation types blockings and cyclones are central features of the extratropical flow and key to understanding the climate system this study intercompares the representation of these features in 10 reanalyses and in an ensemble of 30 climate model simulations between 1980 and 2005 both modern full input reanalyses and century long surface input reanalyses are examined modern full input reanalyses agree well on key statistics of blockings cyclones and circulation types however the intensity and depth of cyclones vary among them reanalyses with higher horizontal resolution show higher cyclone center densities and more intense cyclones for blockings no strict relationship is found between frequency or intensity and horizontal resolution full input reanalyses contain more intense blocking compared to surface input reanalyses circulation type classifications over central europe show that both versions of the twentieth century reanalysis dataset contain more easterlies and

a unified description of localization for application to large scale tectonics 1 localized regions of deformation such as faults and shear zones are ubiquitous in the earth s lithosphere however we lack a simple unified framework of localization that is independent of the mechanism or scale of localization we address this issue by introducing the effective stress exponent ne a parameter that describes how a material responds to a local perturbation of an internal variable being tested for localization the value of ne is based on micromechanics a localizing regime has a negative ne indicating a weakening behavior and localization is stronger for more negative 1 ne we present expressions for the effective stress exponent associated with several mechanisms that trigger localization at large scale brittle failure with loss of cohesion elastoplasticity rate and state dependent friction shear heating and grain size feedback in ductile rocks in most cases localization does not arise solely from the relation between stress and deformation but instead requires a positive feedback between the rheology and internal variables brittle mechanisms failure and friction are generally described by ne of the order of 100 shear heating requires an already localized forcing which could be provided by a brittle fault at shallower levels of the lithosphere grain size reduction combined with a transition from dislocation to diffusion creep leads to localization only if the grain size departs significantly from its equilibrium value because either large scale flow moves rocks through different thermodynamic environments or new grains are nucleated when shear heating or grain size feedback produce localization 1 ne can be extremely negative and can control lithospheric scale localization

grain size evolution and the rheology of ductile shear zones from laboratory experiments to postseismic creep abstract geologic experimental and geodetic studies indicate that ductile shear zones do not deform in steady state we present a theoretical model for an elastically loaded ductile shear zone deforming by combined dislocation and diffusion creep in which grain size is allowed to evolve toward its equilibrium recrystallized value several grain size evolution laws are considered simulations of laboratory experiments show that grain size evolution can lead to changes in the dominant deformation mechanism that are accompanied by transient strain weakening or strain strengthening episodes reminiscent of laboratory observations ductile shear zones can also produce postseismic creep as observed in geodetic data following large earthquakes non steady state ductile creep is difficult to differentiate from frictional sliding with velocity strengthening friction hence ductile creep may play a role in the transition from velocity weakening to velocity strengthening friction with increasing temperature

el nino la nina and the nonlinearity of their teleconnections the paradigm of an atmospheric system varying linearly with respect to extreme phases of the el nino southern oscillation is questioned it is argued that the global response to tropical pacific sea surface temperature forcing will be inherently nonlinear a physical basis for this intrinsic nonlinearity is the thermodynamic control on deep convection climate statistics for warm and cold events of the tropical pacific are analyzed separately for the northern winter periods during 1950 96 composite analysis of 500 mb heights reveal planetary scale teleconnection patterns as noted in earlier studies a new result is the evidence for an appreciable 358 longitude phase shift between the warm and cold event circulation composites and the two wave trains appear to have different tropical origins a large nonlinear component in north american surface climate anomalies is also found which is consistent with such a phase shift in teleconnections in the tropics rainfall anomalies also show evidence of nonlinear behavior the maximum rain anomalies along the equator are located east of the date line during warm events but west of the date line during cold events the interpretation of this behavior is complicated however by the fact that composite warm event sst anomalies are not the exact inverse of their cold event counterparts idealized atmospheric general circulation model agcm experiments are performed in order to test the question of whether the observed nonlinearity is an intrinsic property of the atmospheric system the model is forced with a composite sst anomaly that undergoes a realistic seasonally varying enso life cycle as described by e rasmusson and t carpenter both positive and negative phases of the sst anomaly are used and a 40 member ensemble of warm and cold event model simulations is conducted a nonlinear climate response in the agcm is found that closely resembles the observed composites including a shift in the equatorial positions of the maxmium rain responses and a phase shift of teleconnection patterns in the upper troposphere barotropic model experiments indicate that the inherent nonlinearity in the tropical rain response may itself be responsible for the phase shift in the extratropical teleconnection patterns

a new rossby wave breaking interpretation of the north atlantic oscillation this paper proposes the hypothesis that the low frequency variability of the north atlantic oscillation nao arises as a result of variations in the occurrence of upper level rossby wave breaking events over the north atlantic these events lead to synoptic situations similar to midlatitude blocking that are referred to as high latitude blocking episodes a positive nao is envisaged as being a description of periods in which these episodes are infrequent and can be considered as a basic unblocked situation a negative nao is a description of periods in which episodes occur frequently a similar but weaker relationship exists between wave breaking over the pacific and the west pacific pattern evidence is given to support this hypothesis by using a two dimensional potential vorticity based index to identify wave breaking at various latitudes this is applied to northern hemisphere winter data from the 40 yr ecmwf re analysis era 40 and the events identified are then related to the nao certain dynamical precursors are identified that appear to increase the likelihood of wave breaking these suggest mechanisms by which variability in the tropical pacific and in the stratosphere could affect the nao