downward control of the mean meridional circulation and temperature distribution of the polar winter stratosphere abstract according to the downward control principle the extratropical mean vertical velocity on a given pressure level is approximately proportional to the meridional gradient of the vertically integrated zonal force per unit mass exerted by waves above that level in this paper a simple numerical model that includes parameterizations of both planetary and gravity wave breaking is used to explore the influence of gravity wave breaking in the mesosphere on the mean meridional circulation and temperature distribution at lower levels in the polar winter stratosphere the results of these calculations suggest that gravity wave drag in the mesosphere can affect the state of the polar winter stratosphere down to altitudes below 30 km the effect is most important when planetary wave driving is relatively weak that is during southern winter and in early northern winter in southern winter downwelling weakens by a factor of 2 near the stratopause and by 20 at 30 km when gravity wave drag is not included

a gravity wave drag matrix for complex terrain abstractwe propose a simplified scheme to predict mountain wave drag over complex terrain using only the regional average low level wind components u and v the scheme is tuned and tested on data f

chaotic subduction like downflows in a spherical model of convection in the earth s mantle numerical models of three dimensional nonlinear thermal convection of very viscous fluids have recently been developed in rectangular1 4 and spherical5 12 geometries as analogues of convection in the earth s mantle here we describe model calculations for a compressible fluid in a three dimensional spherical shell with 80 of the surface heat flow generated within the model mantle in agreement with estimates for the earth s mantle13 the rayleigh number and the numerical resolution for these calculations are greater than those of our previous studies9 10 of internally heated convection in a spherical shell our numerical solutions are strongly chaotic with surface planforms dominated by long curvilinear downflows reminiscent of the descending slabs in the earth s mantle although analogy to the earth s mantle is necessarily imperfect owing to for example the absence of variable viscosity and rheology and hence of lithospheric plates our results suggest that descending slabs play an important part in driving mantle convection and that their chaotic evolution may influence the spatial and temporal behaviour of plates and thus the dispersal and aggregation of continents

secondary gravity wave generation over new zealand during the deepwave campaign multiple events during the deep propagating gravity wave experiment measurement program revealed mountain wave mw breaking at multiple altitudes over the southern island of new zealand these events were measured during several research flights from the national science foundation national center for atmospheric research gulfstream v aircraft utilizing a rayleigh lidar an na lidar and an advanced mesospheric temperature mapper simultaneously a flight on 29 june 2014 observed mws with horizontal wavelengths of 80 120 km breaking in the stratosphere from 10 to 50 km altitude a flight on 13 july 2014 observed a horizontal wavelength of 200 240 km mw extending from 20 to 90 km in altitude before breaking data from these flights show evidence for secondary gravity wave sgw generation near the breaking regions the horizontal wavelengths of these sgws are smaller than those of the breaking mws indicating a nonlinear generation mechanism these observations reveal some of the complexities associated with mw breaking and the implications this can have on momentum fluxes accompanying sgws over mw breaking regions

the proximity of hotspots to convergent and divergent plate boundaries an analysis of four different hotspot distributions ranging from morgan s 1972 original list of 19 to vogt s 1981 list of 117 reveals that the hotspots are preferentially located near divergent plate boundaries the probability of this proximity occurring by chance alone is quite remote less than 0 01 for all four hotspot distributions the same analysis also reveals that the hotspots are preferentially excluded from regions near convergent plate boundaries the probability of this exclusion occurring by chance alone is 0 1 or less for three out of the four distributions examined we interpret this behavior as being a consequence of the effects of large scale convective circulation on ascending mantle plumes mantle thermal plumes the most probable source of hotspots arise from instabilities in a basal thermal boundary layer plumes are suppressed from regions beneath convergent boundaries by descending flow and are entrained into the upwelling flow beneath spreading centers plate scale convective circulation driven by subduction may also advect mantle thermal plumes toward spreading centers

regional to global evolution of impacts of parameterized mountain wave drag in the lower stratosphere abstractmountain ranges are regional features on earth as are the regions of mountain wave drag mwd exerted by dissipating atmospheric gravity waves generated by flow over them however these r

toroidal poloidal energy partitioning and global lithospheric rotation during cenozoic time the motion of surficial plates can be split into two terms the toroidal field related to shear between plates or to spin around their centres and the poloidal field associated with horizontal divergence at ridges and subductions recent papers have suggested that the present day plate motion minimizes the ratio of toroidal to poloidal energies 1 and that the existence of a global lithospheric rotation results from viscosity variations between oceans and continents 2 this paper tests and confirms these two hypotheses using plate reconstructions during cenozoic time the partition of energy between the two modes always favours the poloidal velocities the global rotation computed using a viscosity one order of magnitude lower below oceans than below continents agrees with observations the existence of a non linear rheology in a uniform asthenosphere leads to a satisfactory location of the global lithospheric rotation but with too small an amplitude

hotspots mantle plumes flood basalts and true polar wander persistent long lived stationary sites of excessive mantle melting are called hotspots hotspots leave volcanic trails on lithospheric plates passing across them the global constellation of fixed hotspots thus forms a convenient frame of reference for plate motions through the orientations and age distributions of volcanic trails left by these melting anomalies hotspots appear to be maintained by whole mantle convection in the form of upward flow through narrow plumes evidence suggests that plumes are deflected little by horizontal flow of the upper mantle mantle plumes are largely thermal features and arise from a thermal boundary layer most likely the mantle layer just above the core mantle boundary experiments and theory show that gravitational instability drives flow beginning with the formation of diapirs such a diapir will grow as it rises fed by flow through the trailing conduit and entrainment of surrounding mantle the structure thus develops a large spherical plume head and a long narrow tail on arrival at the base of the lithosphere the plume head flattens and melts by decompression producing enormous quantities of magma which erupt in a short period these are flood basalt events that have occurred on continents and in ocean basins and that signal the beginning of major hotspot tracks the plume supported hotspot reference frame is fixed in the steady state convective flow of the mantle and is independent of the core generated axial dipole paleomagnetic reference frame comparison of plate motions measured in the two frames reveals small but systematic differences that indicate whole mantle motion relative to the earth s spin axis this is termed true polar wander and has amounted to some 12 since early tertiary time the direction and magnitude of true polar wander have varied sporadically through the mesozoic probably in response to major changes in plate motions particularly subduction zone location that change the planet s moments of inertia

metamorphism and diachronous cooling in a contractional orogen the strandja massif nw turkey the southern part of the strandja massif northern thrace turkey comprises a basement of various gneisses micaschists and rare amphibolite and a cover of metaconglomerate and metasandstone separated from each other by a pre metamorphic unconformity metamorphic grade decreases from the epidote amphibolite facies in the south to the albite epidote amphibolite greenschist facies transition in the north estimated p t conditions are 485 530 c and 0 60 0 80 gpa in the epidote amphibolite facies domain and decrease towards the transitional domain between greenschist and epidote amphibolite facies rb sr muscovite ages range from 162 9 1 6 ma to 149 1 2 1 ma and are significantly older 279 296 ma in the northernmost part of the study area the rb sr biotite ages decrease from 153 9 1 5 ma in the south to 134 4 1 3 ma in the north these age values in conjunction with the attained temperatures suggest that the peak metamorphism occurred at around 160 ma and cooling happened diachronously and rb sr muscovite ages were not reset during the metamorphism in the northernmost part structural features such as i consistent s dipping foliation and sw to se plunging stretching lineation ii top to the n shear sense and iii n vergent ductile shear zones and brittle thrusts suggest a n vergent compressional deformation coupled with exhumation we tentatively ascribe this metamorphism and subsequent diachronous cooling to the northward propagation of a thrust slice the compressional events in the strandja massif were most probably related to the coeval n vergent subduction collision system in the southerly lying rhodope massif

three dimensional spherical models of convection in the earth s mantle three dimensional spherical models of mantle convection in the earth reveal that upwelling cylindrical plumes and downwelling planar sheets are the primary features of mantle circulation thus subduction zones and descending sheetlike slabs in the mantle are fundamental characteristics of thermal convection in a spherical shell and are not merely the consequences of the rigidity of the slabs which are cooler than the surrounding mantle cylindrical mantle plumes that cause hotspots such as hawaii are probably the only form of active upwelling and are therefore not just secondary convective currents separate from the large scale mantle circulation active sheetlike upwellings that could be associated with mid ocean ridges did not develop in the model simulations a result that is in agreement with evidence suggesting that ridges are passive phenomena resulting from the tearing of surface plates by the pull of descending slabs

stratospheric gravity wave fluxes and scales during deepwave abstractduring the deep propagating gravity wave experiment deepwave project in june and july 2014 the gulfstream v research aircraft flew 97 legs over the southern alps of new zealand and 150 legs over the tasman sea and southern ocean mostly in the low stratosphere at 12 1 km altitude improved instrument calibration redundant sensors longer flight legs energy flux estimation and scale analysis revealed several new gravity wave properties over the sea flight level wave fluxes mostly fell below the detection threshold over terrain disturbances had characteristic mountain wave attributes of positive vertical energy flux efz negative zonal momentum flux and upwind horizontal energy flux in some cases the fluxes changed rapidly within an 8 h flight even though environmental conditions were nearly unchanged the largest observed zonal momentum and vertical energy fluxes were mfx 550 mpa and efz 22 w m 2 respectively a wide variety of disturbance scales were found at flight level over

hotspots and sunspots surface tracers of deep mantle convection in the earth and sun abstract the pattern of new appearances of hotspots on the earth is investigated using available age data the worldwide total of cenozoic and mesozoic hotspots predicted by extrapolation from the observed number of continental flood basalts is 40 in agreement with crough s counts there are found to be no true antipodal pairs in one interpretation of the data new appearances of hotspots occur first at high latitudes in both hemispheres and then migrate toward the equator shortly before the migration cycle is finished the next cycle begins a relative deficiency of hotspots however is observed in very low equatorial and very high polar regions in addition hotspots occupy two large hemispherical groups in longitude that slowly drift in the same direction as the axial body rotation an observed magnetic superchron seems to end when a new hotspot migration cycle in latitude begins but consecutive superchrons show opposite polarity although the hotspot data are rather sparse their suggested patterns resemble the well known patterns of complexes of active regions on the sun as exemplified by proxy data such as sunspots both the earth and the sun possess large convecting rotating and magnetized mantles in which the characteristic surface tracers are believed to reflect the patterns of convective phenomena very deep in the mantle even though the tracers themselves hotspots and sunspots are certainly not analogs of each other the present study supports existing theoretical ideas that the large scale patterns of deep mantle convection in the earth and sun may fundamentally resemble each other despite the enormous difference in their molecular viscosities magnetic polarity reversal histories also show close similarities in the two bodies suggesting the operation of basically similar convective dynamos however the sun has displayed no known analog of the earth s magnetically disturbed intervals

on the origins of mesospheric gravity waves 1 using hourly data from a three year simulation based on a gravity wave resolving general circulation model we have first inferred a global view of gravity wave sources and propagation affecting significantly the momentum balance in the mesosphere the meridional cross section of momentum fluxes suggests that there are a few dominant propagation paths originating from the subtropics in summer and the middle to high latitudes in winter these gravity waves are focused into the mesospheric jets in their respective seasons acting effectively to decelerate the jets the difference in the source latitudes likely contributes to the hemispheric asymmetries of the jets the horizontal distribution of the momentum fluxes indicates that the dominant sources are steep mountains and tropospheric westerly jets in winter and vigorous monsoon convection in summer the monsoon regions are the most important window to the middle atmosphere in summer because of the easterlies associated with the monsoon circulation

recent developments in gravity wave effects in climate models and the global distribution of gravity wave momentum flux from observations and models recent observational and theoretical studies of the global properties of small scale atmospheric gravity waves have highlighted the global effects of these waves on the circulation from the surface to the middle atmosphere the effects of gravity waves on the large scale circulation have long been treated via parametrizations in both climate and weather forecasting applications in these parametrizations key parameters describe the global distributions of gravity wave momentum flux wavelengths and frequencies until recently global observations could not define the required parameters because the waves are small in scale and intermittent in occurrence recent satellite and other global datasets with improved resolution along with innovative analysis methods are now providing constraints for the parametrizations that can improve the treatment of these waves in climate prediction models research using very high resolution global models has also recently demonstrated the capability to resolve gravity waves and their circulation effects and when tested against observations these models show some very realistic properties here we review recent studies on gravity wave effects in stratosphere resolving climate models recent observations and analysis methods that reveal global patterns in gravity wave momentum fluxes and results of very high resolution model studies and we outline some future research requirements to improve the treatment of these waves in climate simulations copyright 2010 royal meteorological society and crown in the right of canada

antipodal hotspot pairs on the earth the results of statistical analyses performed on three published hotspot distributions suggest that significantly more hotspots occur as nearly antipodal pairs than is anticipated from a random distribution or from their association with geoid highs and divergent plate margins the observed number of antipodal hotspot pairs depends on the maximum allowable deviation from exact antipodality at a maximum deviation of 700 km 26 to 37 of hotspots form antipodal pairs in the published lists examined here significantly more than would be expected from the general hotspot distribution two possible mechanisms that might create such a distribution include 1 symmetry in the generation of mantle plumes and 2 melting related to antipodal focusing of seismic energy from large body impacts

madagascar basalts tracking oceanic and continental sources extensive upper cretaceous volcanism in southern madagascar was fed in part by mantle sources resembling those expressed today in the indian ocean at marion and prince edward islands and on the central southwest indian ridge in addition very low end t to 17 4 high 87sr 86sr t to 0 72126 tholeiites in southwestern madagascar were variably but highly contaminated by ancient continental material broadly like that affecting the bushe and poladpur formations of the later deccan traps in india alkalic dikes in southwestern madagascar have a rough analogue in the mahabaleshwar formation of the deccan in that they document the influence of a low 206pb 204pb negative end relatively low 87sr 86sr reservoir a very similar reservoir is manifested at present in mid ocean ridge basalts on the central southwest indian ridge near 40 e the original location of this end member appears likely to have been in the madagascan lithospheric mantle a portion of which may have been removed in the middle cretaceous by the action of the marion hotspot or the rifting of indo madagascar an origin within the hotspot itself also may be possible however recent products of the hotspot appear to lack completely the necessary low 206pb 204pb low end signatures

paleomagnetism and the nature of the geodynamo records of direct observations of the earth s magnetic field cover less than a ten millionth ofthe known lifetime of the field thus our knowledge of several geomagnetic phenomena critical to our understanding of the geodynamo must come firom the paleomagnetic record a combination of substantial advances during the past decade or so both in dynamo theory previously the domain of the mathematician and in paleomagnetism previously the domain of the geologist has led to provocative models of the earth s magnetic field and a better understanding of the geodynamo

the midlatitude lower stratospheric mountain wave valve layer abstractthe vertical propagation and attenuation of mountain waves launched by new zealand terrain during the deep propagating gravity wave experiment deepwave field campaign are investigated new zealand mountain waves were frequently attenuated in a lower stratospheric weak wind layer between z 15 and 25 km this layer is termed a valve layer as conditions within this layer primarily minimum wind speed control mountain wave momentum flux through it analogous to a valve controlling mass flux through a pipe this valve layer is a climatological feature in the wintertime midlatitude lower stratosphere above the subtropical jet mountain wave dynamics within this valve layer are studied using realistic weather research and forecasting wrf model simulations that were extensively validated against research aircraft radiosonde and satellite observations locally wave attenuation is horizontally and vertically inhomogeneous evidenced by numerous regions with wave induced low richardson numbers an

seasonal cycle of orographic gravity wave occurrence above small islands in the southern hemisphere implications for effects on the general circulation 1 orographic gravity waves generated by flow over the topography of small islands in the southern oceans have been observed from orbit with the atmospheric infrared sounder on the aqua satellite we examine the occurrence frequencies of these waves in the stratosphere at 40km above 14 islands and examine geographical and seasonal changes our results show that these small island mountain waves occur commonly in the stratosphere in the may september season though not every day differing seasonal variations are evident at different islands and the seasonal variations are closely related to latitude and prevailing wind patterns we also examine interannual variability in 2years of data and the relationships between occurrence frequencies momentum fluxes and stratospheric and surface winds the results suggest that stratospheric winds have a first order limiting effect on the observations of these island mountain waves in atmospheric infrared sounder airs data surface wind direction and island orographic relief have an additional but secondary influence on the island mountain wave occurrence frequencies in airs data the implications are that these wave events are extremely common and that on many days when the waves are not observed in airs data they have likely dissipated and induced a drag force on the atmosphere below the 40km observation level observations of momentum flux during these wave events also permit a first estimate of their importance to the general circulation of the southern hemisphere

models for the hotspot distribution published hotspot catalogues all show a hemispheric concentration beyond what can be expected by chance cumulative distributions about the center of concentration are described by a power law with a fractal dimension closer to 1 than 2 random sets of the corresponding sizes do not show this effect a simple shift of the random sets away from a point would produce distributions similar to those of hotspot sets the possible relation of the hotspots to the locations of ridges and subduction zones is tested using large sets of randomly generated points to estimate areas within given distances of the plate boundaries the probability of finding the observed number of hotspots within 10 of the ridges is about what is expected

alleviation of a systematic westerly bias in general circulation and numerical weather prediction models through an orographic gravity wave drag parametrization systematic westerly biases in the northern hemisphere wintertime flow of the meteorological office 15 layer operational model and 11 layer general circulation model are described evidence that the failure to parametrize subgrid scale orographic gravity wave drag may account for such biases is presented this evidence is taken from aircraft studies surface pressure drag measurements and studies of the zonally averaged momentum budget a parametrization scheme is described in which the surface stress is proportional to the near surface wind speed and static stability and to the variance of subgrid scale orography the stress is absorbed in the vertical by considering the influence of such gravity wave activity on static stability and vertical wind shear a richardson number dependent wave breaking formulation is devised and the vertical stress profile determined by a saturation hypothesis whereby the breaking waves are maintained at marginal stability it is shown that wave breaking preferentially occurs in the boundary layer and in the lower stratosphere results from a simple zonally symmetric model show how the adjustment to thermal wind balance with a wave drag in the stratosphere warms polar regions by adiabatic descent and decelerates the mean westerlies in the troposphere the influence of the parametrization scheme on integrations of the 11 layer model is described and found to be generally beneficial in a discussion of the reasons why this problem has only recently emerged it is suggested that the satisfactory northern hemisphere winter circulations of previous coarser general circulation models were due to a compensation implied by underestimating both the surface drag and the horizontal flux of momentum by explicitly resolved large scale eddies

revised and synthetic apparent polar wander paths of the african eurasian north american and indian plates and true polar wander since 200 ma we have reviewed paleomagnetic data available for the eurasian african north american and indian plates over the last 200 ma selection criteria are those generally accepted with an emphasis on evidence for lack of remagnetization accurate dating and proper structural analysis this results in 23 35 51 and 2 poles for eurasia africa north america and india respectively we believe that this limited set of higher quality data is more likely to reveal key features of apparent polar wander apw paths than averaging of larger data sets involving less stringent selection criteria we propose and describe revised apw paths but more importantly we next use relative motion models to transfer all data in a common reference frame we find good agreement between transferred data when they are averaged separately for each plate in independent 20 ma windows this is a check on consistency of paleomagnetic data kinematic models and of the geocentric dipole hypothesis transferred data from all plates are averaged in 20 ma windows to generate synthetic apw paths for all plates studied these synthetic paths are in agreement with the original revised apw paths that use only data from a single plate moreover both geographic and time resolution are improved spatial confidence intervals are of the order of 5 the synthetic paths display interesting features such as a previously ill recognized apw loop for eurasia paleomagnetic and hotspot apw are next compared and a determination of true polar wander tpw is derived we find significant tpw amounting to over 20 in the last 200 ma tpw appears to be episodic with a standstill between 180 and 110 ma there is general agreement with a previous study of livermore et al 1984 the tpw standstill appears to correlate with a time of decreasing reversal frequency ending with the cretaceous long normal superchron other periods of fast tpw would seem to correspond to increasing reversal frequency however it is suggested that the major tpw hairpin at 50 ma might correspond to the collision of india with eurasia tentative correlations with core or mantle indicators can be understood as a result of couplings between the core mantle and lithosphere we believe primarily related to episodic evolution of the d layer

comparison of gravity waves in the southern hemisphere derived from balloon observations and the ecmwf analyses abstractthe increase of spatial resolution allows the ecmwf operational model to explicitly resolve a significant portion of the atmospheric gravity wave gw field but the realism of the simulated gw field in the ecmwf analyses still needs to be precisely evaluated here the authors use data collected during the concordiasi stratospheric balloon campaign to assess the representation of gws in the ecmwf analyses over antarctica and the southern ocean in spring 2010 the authors first compare the balloonborne gw momentum fluxes with those in ecmwf analyses throughout the campaign and find a correct agreement of the geographical and seasonal patterns however the authors also note that ecmwf analyses generally underestimate the balloon fluxes by a factor of 5 which may be essentially due to the spatial truncation of the ecmwf model intermittency of wave activity in the analyses and observations are found comparable these results are confirmed on two case studies dealing with orographic and nonorographi

on the applicability of thermal conduction models to mid plate volcanism comments on a paper by gass et al the proposition that mid plate hot spot magmatism is strongly discouraged by a combination of thick lithosphere and rapid plate motion is examined empirically on a global basis and found to be inadequate although hot spots perhaps tend to avoid cratonic areas the nonrandom worldwide distribution of hot spots appears primarily to reflect the state of the sublithospheric mantle plate thickness or speed is of lesser importance the high level of african mid plate volcanism since 25 m y b p appears to be part of a global pattern controlled by events in the mantle rather than a result of the africa plate coming to rest over sublithospheric thermal sources about 40 m y b p

latitude and depth variation of solar rotation measurements of the frequencies of various modes of trapped acoustic waves provide information about rotation and structure within the sun previous work dealt with observations of wave modes confined near the solar equator which provided some information about the depth variation of rotation without resolving a possible latitude variation1 2 recent work extended measurements to modes covering various latitude ranges from which the variation with latitude of solar rotation can be studied3 5 since these measurements were restricted to modes with spherical harmonic degrees less than 50 they provide averages of rotation over great depth ranges that do not resolve the convective envelope we now present new results for degrees up to 98 which allow the convective envelope to be isolated for degrees between 20 and 98 we find no evidence that internal rotation differs significantly with depth or latitude from the rotation of surface magnetic field patterns modes covering a wide latitude range have systematically lower frequencies than those confined near the equator indicating the existence of a structural asymmetry within the sun

monsoon speeds up indian plate motion article i nfo short term plate motion variations on the order of a few myr are a powerful probe into the nature of plate boundary forces as mantle related buoyancies evolve on longer time scales new reconstructions of the ocean floor spreading record reveal an increasing number of such variations but the dynamic mechanisms producing them are still unclear here we show quantitatively that climate changes may impact the short term evolution of plate motion by linking explicitly the observed counter clockwise rotation of the indian plate since 10 ma to increased erosion and reduced elevation along the eastern himalayas due to temporal variations in monsoon intensity by assimilating observations into empirical relations for the competing contributions of erosion and mountain building we estimate the first order decrease in elevation along the eastern himalayas since initial strengthening of the monsoon furthermore we show with global geodynamic models of the coupled mantle lithosphere system that the inferred reduction in elevation is consistent with the indian plate motion record over the same period of time and that lowered gravitational potential energy in the eastern himalayas following stronger erosion is a key factor to foster plate convergence in this region our study implicates lateral variations in plate coupling and their temporal changes as an efficient source to induce an uncommon form of plate motion where the euler pole falls within its associated plate

angular symmetries of hotspot distributions the symmetries in the distribution of hotspots are investigated by an analogy of the angular autocorrelation of scalar fields it has been applied to two lists of hotspot locations revealing the axes as well as the angles of symmetries the existence of angular symmetries suggests that the hotspot distribution observable on the earth s surface is probably influenced by symmetries of deep geodynamic processes

parameterization of directional absorption of orographic gravity waves and its impact on the atmospheric general circulation simulated by the weather research and forecasting model abstractin this work a new parameterization scheme is developed to account for the directional absorption of orographic gravity waves ogws using elliptical mountain wave theory the vertical mom

periodic hotspot distribution and small scale convection in the upper mantle hotspots are commonly attributed to mantle plumes and to a first order the hotspot distribution is correlated with long wavelength geoid anomalies and with higher temperatures in the lower mantle suggesting that the plumes are generated in the lower mantle i show that the second order distribution is periodic with a typical spacing of 800 1000 km such a spacing would be too narrow for plumes to ascend directly from the lower mantle but is close to the thickness of the upper mantle the alignment of convection cells in the upper mantle seems to control the periodicity to reconcile the first a and second order distributions assuming limited mass fluxes across the 670 km boundary a lateral deflection of plumes as they breach the transition zone from below is required

cenozoic global plate motions cenozoic global plate motions relative to the hot spots are investigated and compared to plate motions in a mean lithosphere reference frame plate motions were analyzed over six time intervals divided by ages 10 25 43 48 and 56 ma chosen as much as possible to coincide with key plate reorganizations alternative motion circuits and rotational parameters were considered and evaluated with paleomagnetic data from the pacific and north american plates the circuit found to be in best agreement with the paleomagnetic data is one in which the hot spots in the atlantic region are assumed to be fixed relative to the hot spots in the pacific region throughout the cenozoic the hot spot and mean lithosphere reference frames have been in continual slow relative motion the rate of motion is nonuniform however most of the motion having occurred during the middle cenozoic the net cenozoic rotation of the lithosphere relative to the hot spots is described by a right handed rotation of 7 about a euler pole at 46 s 87 e which yields a 5 displacement of the north poles of the two reference frames this motion is small enough that inferences drawn about plate speeds in one reference frame should be valid in the other analysis of the global motions resulting from our preferred model showed that many characteristics of current plate motions have persisted throughout the cenozoic plate speeds correlate with latitude plates moving faster near the equator than near the poles throughout the cenozoic as at present continental plates except for the indian plate moved slower than oceanic plates throughout the cenozoic even the structure of the velocity fields as revealed in a contour of root mean square velocities in equatorial bands persists throughout the cenozoic the migration of the paleomagnetic axis over time is also compared to the hot spot and mean lithosphere reference frames the paleomagnetic axis has shifted 5 10 relative to the hot spot frame and a lesser amount relative to the mean lithosphere frame

dynamically supported geoid highs over hotspots observation and theory hotspots are associated with long wavelength geoid highs an association that is even stronger when the geoid highs associated with subduction zones are removed we quantify these associations by expanding the hotspot distribution in spherical harmonics and calculating correlation coefficients as a function of harmonic degree the hotspot distribution spectrum is nearly white except for peaks at degrees 2 and 6 it is correlated positively with the slab residual geoid for degrees 2 6 with low seismic velocity in the lower mantle at degree 2 and with low seismic velocity in the upper mantle at degree 6 we test fluid mechanical models for hotspots including lithospheric delamination and hot plumes by calculating their predicted dynamic geoid responses and comparing them to the observations these models include the effects of temperature dependent rheology our preferred hotspot model based on observations of the geoid and seismic tomography has plumes preferentially occurring in regions of large scale background temperature highs in a mantle which has a substantial viscosity increase with depth although other models are possible alternatively strong plumes in the lower mantle may neck down and attenuate in the upper mantle due to changes in plume viscosity the major low density anomalies causing the geoid highs associated with plumes appear to be in the lower mantle we hypothesize that cold lower mantle underlying the complementary geoid lows is due to deeply subducted oceanic lithosphere and that hotspot plumes tend to be excluded from these cold regions of the mantle

the impact of mountain waves on an idealized baroclinically unstable large scale flow abstractthe feedback of mountain waves and low level blocking on an idealized baroclinically unstable wave passing over an isolated ridge is examined through numerical simulation theoretical analy

impact cratering at geologic stage boundaries the largest known cenozoic impact craters with the most accurately measured ages are found to correlate very closely with geologic stage boundaries the level of confidence in this result is 98 99 even under the most pessimistic assumptions concerning dating errors one or more large impacts may have led in at least some cases to the extinctions and first appearances of biotic species that mark many of the geologic stage boundaries

structure and dynamics of an alpine potential vorticity banner a detailed analysis is undertaken of the primary alpine shear zone that occurred on 1 october 1999 during the mesoscale alpine programme this shear zone emanates from the south western alpine tip in north westerly flow conditions develops in response to the alpine scale flow splitting and separates the northerly mistral wind to the south west of the alps from the quasi stagnant air within the alpine wake the study is based on in situ flight level and dropsonde data from two research aircraft the data are used for the construction of an objective analysis in a cross section perpendicular to the shear zone and for diagnostic computations of the potential vorticity pv flux between vertically stacked flight legs the observed flow structure is compared with simulation results of the swiss model and the non hydrostatic canadian mesoscale compressible community model mc2 operating at horizontal resolutions of 14 and 3 km respectively immediately downstream of the topography the observed shear zone has a surprisingly quasi steady structure that becomes evident provided a slow westward migration is accounted for further downstream the shear zone shows increased signs of transience near the surface it has an overall width of 150 km but narrows to about 25 km near the top of the mistral inversion at a height of 2 km detailed analysis shows that the shear zone has a complex substructure consisting of at least three positive and three negative pv filaments these shear lines are characterized by a pronounced vertical coherence in the mixed layer and throughout most of the inversion layer one of the banners is also evident in low level tracer constituents diagnostic computations of the pv flux along the banners using the generalized bernoulli theorem reveal that most of the associated pv flux is due to vertically oriented vorticity within the inversion layer within the shear zone there is a spanwise circulation of appreciable strength it consists of a deformation flow with the axes of deformation oriented perpendicular to the shear zone this deformation flow in effect squeezes the pv banners and thereby makes them resilient with respect to dissipation and barotropic instabilities detailed intercomparison with numerical model results shows that models with a grid spacing below 15 km are able to capture the overall structure of the shear zone the mc2 model in addition credibly replicates some of the finer scale sub structure of the shear zone in particular at low levels the high predictability and steadiness of the pv banners supports the idea that individual banners are generated by flow separation and or gravity wave breaking events upstream it is argued that the overall width of the shear zone is thus in effect determined by the geometry of the upstream topography and the dynamical processes that govern the downstream advection of the pv banners copyright 2003 royal meteorological society

intermittent layered convection in a model mantle with an endothermic phase change at 670 km it is not yet known whether convection in the earth s mantle occurs in cells that extend throughout the entire depth of the mantle or if the upper and lower mantle convect in independent layers both possibilities seem to be consistent with geochemical and seismologi cal observations1 4 it is generally accepted however that the seismologically observed boundary between the upper and lower mantle at 670 km depth arises from the endothermic phase change spinel perovskite mgo here we investigate the results of including this phase change in a model of mantle convection for realism and to eliminate scaling problems our calculations use commonly accepted values of geophysical parameters for a clapeyron slope y 2 106 a value close to experimental results and theoretical expectations we observe local intermittent mixing between the upper and lower mantle such behaviour may offer a way to reconcile the existence of geophysical evidence for both whole mantle and layered convection

nondissipative and dissipative momentum deposition by mountain wave events in sheared environments abstractmountain waves mws are generated during episodic cross barrier flow over broad spectrum terrain however most mw drag parameterizations neglect transient broad spectrum dynamics here

atmospheric conditions during the deep propagating gravity wave experiment deepwave abstractthis paper describes the results of a comprehensive analysis of the atmospheric conditions during the deep propagating gravity wave experiment deepwave campaign in austral winter 2014 different datasets and diagnostics are combined to characterize the background atmosphere from the troposphere to the upper mesosphere how weather regimes and the atmospheric state compare to climatological conditions is reported upon and how they relate to the airborne and ground based gravity wave observations is also explored key results of this study are the dominance of tropospheric blocking situations and low level southwesterly flows over new zealand during june august 2014 a varying tropopause inversion layer was found to be connected to varying vertical energy fluxes and is therefore an important feature with respect to wave reflection the subtropical jet was frequently diverted south from its climatological position at 30 s and was most often involved in strong forcing events of mountain waves at t