internal processes within the african easterly wave system the internal processes within an african easterly wave aew system involving mass dynamic and water vapour fields are investigated using era i reanalysis in order to highlight the interactions between convection and aews the budgets of heat moisture and momentum are analysed during the different phases of aews detected using synoptic scale precipitable water anomalies as proposed by poan et al 2013 the strong climatological meridional gradient of moisture present in the sahel impacts the shape of the apparent heat source and humidity sink aew events over the sahel are associated with a meridional shift of the intertropical convergence zone itcz large exchanges of momentum by small scale convective transport are also highlighted between the low and mid levels contributing to the reinforcement of the aew circulation at 600 hpa and the damping of the monsoon flow this also appears as a possible mechanism for the vertical tilt of the meridional wind associated with aews heat budget computation in the southern flank of the west african heat low hl region where such aews occur reveals that the heating anomalies are mainly driven by the horizontal advections the vertical circulation acts as a precursor which initiates the heat transport in the lower troposphere however weaker turbulent mixing also participates in the development of these anomalies especially in the boundary layer these budgets ultimately allow the distinct contributions of diabatic and adiabatic processes to be determined

oxygen isotope constraints on the petrogenesis of volcanic arc magmas from martinique lesser antilles abstract volcanic rocks from the island of martinique in the lesser antilles island arc are highly diverse isotopically values of 18o smow in lavas erupted over the last 5 ma from five stratigraphically discrete and geochemically distinct volcanic centers are morne jacob 7 5to 7 9 pitons du carbet 8 0to 8 9 diamant 6 3to 7 4 piton mt conil 7 2to 7 3 andmt pelee 6 5to 7 4 13 of the 27 samples have been corrected for variable amounts of secondary alteration and low temperature hydration the least18o rich andesites on martinique have 18o values similar to those of the northern lesser antilles 18o 6 0 to 7 3 and other intra oceanic arcs like the marianas 18o 5 5 to 6 8 o isotope ratios are compositionally dependent correlations between 18o and radiogenic isotope ratios 87sr 86sr 143nd 144nd 206pb 204pb and indices of fractionation show that modification of isotopic compositions occurred during magmatic differentiation rather than at source a two stage model is proposed for the origin and differentiation of magmas on martinique primary basaltic melts which are broadly similar in composition to primitive island arc magmas elsewhere e g marianas aleutians south sandwich islands are generated from the subduction modified asthenospheric mantle wedge these magmas ascend into the arc crust and differentiate on martinique crystal fractionation is accompanied by contamination with isotopically distinct crust including a component of terrigenous sediments thus subduction zone processes at the lesser antilles and other island arcs are similar involving only a small albeit significant isotopically volumetric contribution from subducted sediment

oxygen and strontium isotopic investigations of subduction zone volcanism the case of the volcano arc and the marianas island arc abstract recent fresh volcanic rocks of the intra oceanic mariana and volcano arcs were analyzed for o and sr isotopic compositions in order to determine the source of these magmas fresh non arc volcanic rocks from the regions surrounding the mariana volcano arcs and some dsdp sediments were also analyzed for comparison the oxygen isotopic ratios of the arc lavas 5 5 6 8 exhibited a small inter island variation that cannot be entirely explained by fractional crystallization the sr isotopic composition of the arc lavas is remarkably uniform 0 70332 0 70394 for the marianas three models are considered in order to explain the observed isotopic characteristics 1 bulk mixing and melting of morb type mantle with a subducted sediments and b subducted oceanic crust excluding sediments 2 melting of a mixture of sediment derived fluids and morb type mantle and 3 melting of a mixture of sediment derived fluids and oceanic island or hot spot type mantle the last model fits the data best the conclusion that very small and variable amounts of sediment derived fluid 1 are required to explain the observed inter island o isotopic variation is consistent with that of other workers who used different isotopic and trace element methods the generation of magmas in the mariana volcano arcs involves very little sediment and the source region of mariana lavas is isotopically indistinguishable from that of hot spot basalts

storm track dynamics this paper reviews the current state of observational theoretical and modeling knowledge of the midlatitude storm tracks of the northern hemisphere cool season observed storm track structures and variations form the first part of the review the climatological storm track structure is described and the seasonal interannual and interdecadal storm track variations are discussed in particular the observation that the pacific storm track exhibits a marked minimum during midwinter when the background baroclinicity is strongest and a new finding that storm tracks exhibit notable variations in their intensity on decadal timescales are highlighted as challenges that any comprehensive storm track theory or model has to be able to address physical processes important to storm track dynamics make up the second part of the review the roles played by baroclinic processes linear instability downstream development barotropic modulation and diabatic heating are discussed understanding of these processes forms the core of our current theoretical knowledge of storm track dynamics and provides a context within which both observational and modeling results can be interpreted the eddy energy budget is presented to show that all of these processes are important in the maintenance of the storm tracks the final part of the review deals with the ability to model storm tracks the success as well as remaining problems in idealized storm track modeling which is based on a linearized dynamical system are discussed perhaps on a more pragmatic side it is pointed out that while the current generation of atmospheric general circulation models faithfully reproduce the climatological storm track structure and to a certain extent the seasonal and enso related interannual variations of storm tracks in depth comparisons between observed and modeled storm track variations are still lacking

18o 16o and chemical relationships in k rich volcanic rocks from australia east africa antarctica and san venanzo cupaello italy 18o 16o analyses were made on a set of leucite bearing igneous rocks from a variety of localities around the world the samples chosen for study all have relatively primitive characteristics such as low sio2 contents 42 52 wt whole rock 18o values from the 10 15 m y old australian leucitites range from 7 1 to 11 0 however none of these values represents the original 18o of the erupted lava as every sample has undergone some subsolidus18o enrichment the 18o values of the primary magmas 6 5 can be accurately calculated from the 18o of clinopyroxene mineral separates however the leucite mineral separates pure stoichiometric ka1si2o6 have 18o 8 8to 10 6 and clearly have not retained their primary igneous oxygen isotopic compositions the whole rock 18o values exhibit a positive correlation with h2o contents and extrapolation to a plausible water content for such a subaerially erupted lava 0 5 wt h2o gives a range of primary 18o values of 5 8 to 6 8 the much less hydrated bufumbira africa and gaussberg antarctica samples have 18o 6 7to 7 9and 6 1to 7 0 respectively these have probably undergone only minor subsolidus18o enrichments the san venanzo italy samples are even less hydrated so their extremely high 18o values of 10 8 to 12 0 are definitely magmatic values those data confirm our earlier conclusions that the leucite bearing magmas erupted in north central italy are unique in having much higher 18o values than potassic rocks from southern italy or anywhere else in the world the northward increase in 18o in the leucite bearing rocks of italy is thus clearly a manifestation of progressively greater interaction between mantle derived magmas 18o 5 5to 7 5 and the high 18o sedimentary and metasedimentary rocks of the italian continental crust 18o 15to 25 which were strongly heated during the episode of tuscan anatectic granitic magmatism

covariance of initial 87sr 86sr ratios δ18o and sio2 in continental flood basalt suites the role of contamination and alteration when mutual positive correlations occur between delta sup 18 o sup 87 sr sup 86 sr initial ratio r sub o and sio sub 2 in continental flood basalt suites they are generally accepted as being due to crustal contamination in continental flood basalt suites that have undergone coupled assimilation fractional crystallization in which the contaminant is granitic continental crust r sub o reflects the degree of contamination and will correlate positively with sio sub 2 which reflects the degree of differentiation posteruptive alteration of a suite having a range of sio sub 2 values should result in a positive correlation between sio sub 2 and delta sup 18 o because the ability of a volcanic rock to concentrate sup 18 o depends in part on silica content suites that have undergone assimilation fractional crystallization followed by alteration after eruption may there for exhibit a positive correlation between delta sup 18 o sio sub 2 and r sub o the delta sup 18 o data in such suites may consequently suggest erroneously high degrees of contamination

temporal association of arc continent collision progressive magma contamination in arc volcanism and formation of gold rich massive sulphide deposits on wetar island banda arc whole rock 87sr 86sr and 18o analyses of volcanic rocks and 3he 4he analyses of sulphides and sulphates from mineralized rocks on wetar indonesia indicate a variable contribution of assimilated crustal material or sediment sourced from the subducted australian craton to the south these new data support the idea of progressive source contamination with precisely dated events showing that wetar island hosts the most extreme examples of crustal assimilation in the region the increased continental contamination occurs during the pliocene zanclian to piacenzian during distinct magmatic events between 5 and 4 ma and at 2 4 ma when 87sr 86sr ratios in unaltered lavas with whole rock 18o values between 5 7 and 9 6 increase from 0 707484 to extreme radiogenic values of 0 711656 the earlier of these magmatic events is important in the generation of the hydrothermal systems responsible for the mineralization recorded on wetar samples from this yield radiogenic 3he 4he ratios between 0 5 and 1 4 r ra similar to the data from volcanic rocks on nearby romang the later magmatic event coincides with the arrival of the australian continental margin at the subduction zone along the banda arc progressive incorporation of continental sourced components into the source region below the wetar island edifice coincides with the formation of gold rich volcanogenic massive sulphide deposits hosted within the contaminated volcanic pile

intermittent african easterly wave activity in a dry atmospheric model influence of the extratropics abstracta dynamical model is constructed of the northern summertime global circulation maintained by empirically derived forcing based on the same dynamical code that has recently been used to study african easterly waves aews as convectively triggered perturbations thorncroft et al leroux and hall in the configuration used here the model faithfully simulates the observed mean distributions of jets and transient disturbances and explicitly represents the interactions between them this simple gcm is used to investigate the origin and intraseasonal intermittency of aews in an artificially dry no convection context a long integration of the model produces a summertime climatology that includes a realistic african easterly jet and westward propagating 3 5 day disturbances over west africa these simulated waves display intraseasonal intermittency as the observed aews also do further experiments designed to discern the source of this intermittency in the model show that the simulated waves are

evolution of high ca high sr c series basalts from grenada lesser antilles the effects of intra crustal contamination basaltic lavas from grenada lesser antilles may be divided into two series on the basis of cao mgo relations the c series roughly corresponds to the ankaramites or high sr series of the earlier literature and may be distinguished from the m series basalts by higher cao at a given mgo and by their strongly augite phyric nature compositional variation within the c series is dominated by fractional crystallization of augite and plagioclase but this was accompanied by assimilation of sedimentary material from the island arc crust the assimilation fractional crystallization process is well illustrated by an increase in 206pb 204pb from 18 6 to 19 4 with fractionation and correlated increases in nd sr zr sm and rb ba the most primitive members of the c series can not be generated from erupted m series near primary basalts but instead show evidence for an origin by olivine fractionation from high mg basalts never seen at the surface these appear to have been derived from a source similar to that of morb but enriched in 87sr and other hydrophile elements by dehydration of subducted lithosphere and in ree and radiogenic pb by a small component of subducted sediment

stable isotope compositions and water contents of boninite series volcanic rocks from chichi jima bonin islands japan measurements of stable isotope compositions and water contents of boninite series volcanic rocks from the island of chichi jima bonin islands japan confirm that a large amount 1 6 2 4 wt of primary water was present in these unusual magmas an enrichment of 0 6 in18o during differentiation is explained by crystallization of18o depleted mafic phases silicic glasses have elevated 18o values and relatively low d values indicating that they were modified by low temperature alteration and hydration processes mafic glasses on the other hand have for the most part retained their primary isotopic signatures since eocene time primary d values of 53 for boninite glasses are higher than those of morb and suggest that the water was derived from subducted oceanic lithosphere

generation of african easterly wave disturbances relationship to the african easterly jet abstract the relationship between african easterly waves and the background climatology in which they form is studied using a regional climate model the surface and lateral boundary conditions in the model are manipulated to modify the background climatology especially the african easterly jet and the itcz and the behavior of the waves in these different settings is evaluated three climate simulations are presented with monthly mean lateral and surface boundary conditions one has a strong jet and a strong itcz the second has a strong jet and a weak itcz and the third has a weak jet and a strong itcz in these simulations the presence of wave activity is more closely associated with the concentration of the itcz than the strength of the african easterly jet in particular the simulation with a strong jet accompanied by a weak itcz does not produce significant wave activity but a weak jet with a strong itcz has realistic wave disturbances both the charney stern and the fjortoft necessary conditi

on the dynamics of a storm track abstract an idealized primitive equation model is used to determine the factors controlling the dynamics and maintenance of eddy activity in a storm track the results show that localized regions of enhanced baroclinicity do not necessarily lead to localization of eddy activity by studying the energetics of the storm track it is shown that while baroclinic conversion does indeed correlate with the region of maximum baroclinicity it is the downstream radiation of energy through the ageostrophic geopotential fluxes which acts as a trigger for the development and maintenance of eddy activity over less baroclinic regions extending the region of eddy activity much further downstream from the region of high baroclinicity examples of eddy life cycles are given that show that convergence and divergence of ageostrophic fluxes can dominate baroclinic and barotropic conversion especially in regions with weak baroclinicity factors that may limit the zonal extent of a storm track are discussed evidence of down

linking african easterly wave activity with equatorial waves and the influence of rossby waves from the southern hemisphere abstracta connection is found between african easterly waves aews equatorial westward moving mixed rossby gravity wmrg waves and equivalent barotropic rossby waves rws from the southern hemisphere sh the amplitude and phase of equatorial waves is calculated by projection of broadband filtered era interim data onto a horizontal structure basis obtained from equatorial wave theory mechanisms enabling interaction between the wave types are identified aews are dominated by a vorticity wave that tilts eastward below the african easterly jet and westward above the tilt necessary for baroclinic wave growth however a strong relationship is identified between amplifying vorticity centers within aews and equatorial wmrg waves although the waves do not phase lock positive vorticity centers amplify whenever the cross equatorial motion of the wmrg wave lies at the same longitude in the upper troposphere southward flow and east of this in the lower troposphere northward flow two mechanisms could

case study of an intense african easterly wave abstract the life cycle of an intense african easterly wave aew over the african continent is examined using european centre for medium range weather forecasts ecmwf operational analyses meteosat satellite images and synoptic observations this system the strongest aew of 2000 can be tracked from central north africa into the eastern atlantic ocean where it is associated with the genesis of hurricane alberto synoptic analysis of the kinematic and thermodynamic fields is supplemented by analysis of potential vorticity pv allowing exploration at the role of multiple scales in the evolution of this aew the authors analysis promotes the division of the aew life cycle into three distinctive phases i initiation the aew development is preceded by a large convective event composed of several mesoscale convective systems over elevated terrain in sudan this convection provides a forcing on the baroclinically and barotropically unstable state that exists over tropical north africa ii baroclini

a study of the energetics of african easterly waves using a regional climate model abstract the evolution and spatial distribution of the energetics of african waves are studied complete eddy energy equations for an open system are derived for the computation of energy transformations during wave generation and dissipation it is found that baroclinic overturning is the dominant energy source although barotropic conversions can be almost equally important when there is concentrated moist convection south of the jet or shallow cumulus convection beneath the jet the generation of active waves usually results from the nearly in phase evolution of baroclinic and barotropic conversions which are associated with significant rainfall over africa significant barotropic instability associated with the horizontal shear is usually induced by concentrated deep convection on the southern flank of the jet barotropic conversions associated with the vertical wind shear may attain even greater magnitudes than that associated with the horizontal shear when shallow cumulus convection beneath the jet

rossby waves in zonally opposing mean flow behavior in northwest pacific summer monsoon abstract the interactions between monsoon circulations and tropical disturbances in the northwest pacific where the low level mean flow is westerly in the west and easterly in the east are studied with a barotropic model the authors model results suggest that the scale contraction by the confluent background flow the nonlinear dynamics the effect and the large scale convergence are important for the energy and enstrophy accumulation near the region where the zonal flow reverses the energy enstrophy accumulation can be maintained with a continuous rossby wave emanation upstream the largest accumulation occurs when the emanating zonal wavelength is around 2000 km longer rossby waves experience less scale contraction and nonlinear effects while shorter rossby waves cannot hold a coherent structure against dispersive effects the nonlinear energy enstrophy accumulation mechanism is significantly different from previous linear energy accumulation theories in the linear theories this is primarily a

mechanisms of contamination in lesser antilles island arc magmas from radiogenic and oxygen isotope relationships abstract oxygen sr and nd isotopic compositions for 29 volcanic samples from the lesser antilles are reported these span a considerable range in 18o from values similar to those of mantle derived lavas 18o 6 6 to 18o 14 0 although the highest 18o results are obtained from evolved andesites and dacites there is no clear correlation between 18o and sio2 an attempt is made to eliminate the effects of varying magma chemistry on 18o through differentiation by normalising to the chemical index of garlick 1 this enables the effects on 18o of processes other than magmatic fractionation to be more clearly assessed it is concluded that the range in 18o values measured cannot result from hydrothermal interaction or weathering alone although limited alteration of a few samples may be responsible for some scatter in the oxygen isotope data when 18o is plotted against radiogenic isotopic compositions well correlated trends are observed 18o increases with increasing87sr 86sr and decreasing143nd 144nd such behaviour is consistent with the contamination of mantle derived magmas by variable amounts of an ancient crustal component the shapes of mixing curves defining this process suggest that contamination occurs largely within the arc crust constraints from geochemical and geophysical data suggest that a terrigenous sediment intercalated within the arc crust is assimilated however subsequent fractional crystallisation at more shallow levels in the crust has obscured any chemical trends which might have resulted from combined assimilation fractional crystallisation furthermore it is suggested that the mantle derived magma has already undergone source contamination by the introduction of a slab derived fluid into the overlying mantle wedge

characteristics of african easterly waves depicted by ecmwf reanalyses for 1991 2000 abstract several interesting characteristics of african easterly waves aews were observed and investigated by previous studies two separate propagation paths genesis mechanisms restriction of vertical development and the interaction with the african easterly jet aej however some aspects of these characteristics have been neglected the contrast of the aew population along the two propagation paths the aew genesis mechanism over the saharan thermal low and the role played by the low level north african circulation in this mechanism the dynamical mechanism restricting the vertical development of aews and the synoptic relationship and interaction between the aej and the aews along the two propagation paths the ecmwf reanalyses for the 1991 2000 period supplemented with those of 1979 were analyzed to explore these aew features major findings of this effort are the following the population of aews along the propagation path north of the aej aewn is approximately 2 5 times of that along the pr

is there hidden potential in carajas insights through the geophysical signature of cristalino deposit abstract geophysical methods are fundamental to exploration of iocg deposits because they are associated with magnetite hematite and sulfides which can produce different and significant geophysical responses because of the frequent occurrence of ferromagnetic minerals magnetic data have been used extensively in exploration for iocg deposits the geophysical data at the cristalino iocg deposit in carajas brazil provide two important insights 1 magnetic rocks are not fully mapped by total gradient data when these rock units have the same strike direction as the declination of the earth s magnetic field in low latitude areas and 2 deposits that are mainly composed of brecciated to massive sulfide minerals are only weakly magnetic or even non magnetic these characteristics are important for exploration programs because qualitative analysis of magnetic data especially by the commonly used total gradient calculated from total field anomaly is the main tool for selecting prospects in low latitude regions and important targets could be undetectable by this method gravity data have been extensively applied recently to exploration programs in carajas but similarly to magnetic data the core of the brecciated to massive chalcopyrite mineralization can have lower densities than the surrounding alteration with hematite and magnetite and therefore be associated with low gravity anomalies that are not usually targeted in mineral exploration the electromagnetic response will increase with the increasing connection between sulfides varying from moderate in vein systems to moderate to high in stringer and stockwork systems to high anomalies in massive systems to better understand the impact of these features on exploration programs we show in this study that the magnetic gravity and electromagnetic responses vary with mineralization end members from a vein system formed at greater depths that produce high magnetic and gravity anomalies associated with iron oxides from alteration and moderate electromagnetic anomaly poor connection between sulfides to stringer stockwork and massive systems formed at shallower depths that exhibit lower magnetic and gravity anomalies but a high electromagnetic anomaly our analyses indicate that it is possible that the deposits mostly associated with massive sulfide could have been missed and there could be significant hidden potential in the carajas region we illustrate that the detection of such potential depends on the quantitative integration of magnetic data with gravity and electromagnetic data through the inversion and multiphysics approach

climatology of sign reversals of the meridional potential vorticity gradient over africa and australia abstract a 10 yr climatology 1986 95 was performed using ecmwf gridded analyses on isentropic surfaces to identify regions where the lower tropospheric meridional potential vorticity pv gradient changes sign across africa and australia during their respective summer seasons while an african sign reversal has been documented no similar study has been performed for the australian region which also has desert on the poleward side of open ocean in each hemisphere a northward decrease of pv is sufficient to produce a sign reversal it was found that pv decreases northward in the lower troposphere across northern australia with the maximum reversal on the 315 k surface it had comparable magnitude but smaller zonal extent 3000 km than that on the 320 k surface in africa 5000 km in each region the sign reversal was associated with cyclonic pv anomalies on the equatorward side and anticyclonic anomalies on the poleward side olr was used as a proxy for deep convective heating in order to evaluate

on the instability of the african easterly jet and the generation of african waves reversals of the potential vorticity gradient abstract the relationship between the generation of african easterly waves and instability growing in regions with reversed potential vorticity gradients is studied using a regional climate model results indicate that the convective generation of potential vorticity pv due to the meridional and vertical gradients of diabatic heating in the upper and lower troposphere causes a vertically elongated pv anomaly on the southern flank of the african easterly jet this pv maximum at 9 n in the midtroposphere together with a pv minimum near 15 n at lower levels because of dry convection over the sahara reverses the meridional pv gradient between 9 and 15 n which suggests that the zonal flow may be unstable in this region analysis of the seasonal mean eliassen palm flux for african waves indicates that wave energy generated convectively through baroclinic overturning in the upper troposphere propagates downward and triggers barotropic conversions south of the jet and baroclinic conversions below and north

geochemical evolution of st kitts and montserrat lesser antilles the volcanic rocks of st kitts have uniform 87 sr 86 sr ratios c 0 7037 and high 143 nd 144 nd ratios c 0 5130 in a suite which ranges from basalt to rhyolite trace element abundances and the flat chondrite normalized ree patterns resemble those of island arc tholeiitic suites in comparison the montserrat lavas have higher concentrations of alkaline elements and are lree enriched geochemical differences may be related to partial displacement of the axis of the volcanic arc about 9 ma this would have led to the development of the st kitts suite in an essentially new and immature island arc environment being closer to the junction with the former arc axis the composition of the montserrat lavas may reflect previous subduction related mantle enrichment and or the influence of a sedimentary component isotopic ratios and most trace elements in the redonda basalt indicate island arc tholeiite or in some respects morb affinities but it has a high sr content and is lree enriched despite its distinctive composition redonda is situated on the main axis of the volcanic arc and its lavas were evidently contemporaneous with the younger eruptions of st kitts and montserrat

a climatological study of transient mean flow interactions over west africa a statistical study is made of the relationship between intraseasonal variations in mid tropospheric flow over west africa and transient eddy activity during the june september monsoon season in order to investigate the interaction between the african easterly jet aej african easterly waves aews and convection ncep2 reanalyses are used together with outgoing long wave radiation olr data from june september 1979 2007 intraseasonal variability in the 600 hpa zonal wind is isolated using a 10 120 day filter the leading empirical orthogonal function eof describes north south displacements of the jet axis episodes of aew activity over the continent are evaluated in terms of the perturbation kinetic energy pke of the less than six day high pass filtered wind at 700 and 850 hpa pke and olr are also filtered for the intraseasonal signal lag covariance analyses reveal a two way interaction between the aej and synoptic scale transients prior to episodes of enhanced transient activity the aej is strengthened in the jet entrance region during and following these episodes the aej is strengthened on the northern flank of the jet exit region e vector analysis shows that this displacement of the aej is consistent with forcing by transients that have an aew like structure analysis of olr shows that periods of enhanced transient activity over west africa are associated with enhanced convection over the region at intraseasonal time scales and are correlated with prior episodes of enhanced convection to the east in the convective trigger region identified by thorncroft et al copyright 2009 royal meteorological society

hydrogen isotope systematics of submarine basalts the d h ratios and water contents in fresh submarine basalts from the mid atlantic ridge the east pacific rise and hawaii indicate that the primary d h ratios of many submarine lavas have been altered by processes including 1 outgassing 2 addition of seawater at magmatic temperature and 3 low temperature hydration of glass decreases in d and h2o from exteriors to interiors of pillows are explained by outgassing of water whereas inverse relations between d and h2o in basalts from the galapagos rise and the famous area are attributed to outgassing of ch4 and h2 a good correlation between d values and h2o is observed in a suite of submarine tholeiites dredged from the kilauea east rift zone where seawater added directly to the magma affected only the isotopic compositions of hydrogen and argon analyses of some glassy rims indicate that the outer millimeter of the glass can undergo lowtemperature hydration by hydroxyl groups having d values as low as 100 d values vary with h2o contents of subaerial transitional basalts from molokai hawaii and subaerial alkali basalts from the society islands indicating that the primary d values were similar to those of submarine lavas extrapolations to possible unaltered d values and h2o contents indicate that the primary d values of most thoteiite and alkali basalts are near 80 5 the weight percentages of water are variable 0 15 0 35 for mor tholeiites about 0 25 for hawaiian tholeiites and up to 1 1 for alkali basalts the primary d values of 80 for most basalts are comparable to those measured for deep seated phlogopites these results indicate that hydrogen in marked contrast to other elements such as sr nd pb and o has a uniform isotopic composition in the mantle this uniformity is best explained by the presence of a homogeneous reservoir of hydrogen that has existed in the mantle since the very early history of the earth

segmentation of the lesser antilles subduction zone abstract precise hypocentral locations of earthquakes recorded on local seismic networks from 1978 to 1984 have established the configuration of the benioff zone beneath the lesser antilles island arc there are two distinct segments to the zone one to the north of martinique which trends nnw and one to the south of st lucia which trends nne the northern zone dips at 60 50 the southern zone has a dip of 50 45 in the north but is vertical in the south pleistocene volcanoes are aligned in distinct segments which closely mirror the kink in the benioff zone mid miocene swarms of dykes in martinique and st lucia have nw and ne azimuthal modes respectively we interpret this to mean that this kink in the subduction zone existed as long ago as the miocene the kink may represent the deformation of a single subducting plate or two separate north and south american plates subducting beneath the caribbean plate with a triple junction in the region of martinique st lucia

three dimensional structure and dynamics of african easterly waves part i observations the mean structure of african easterly waves aews over west africa and the adjacent atlantic is isolated by projecting dynamical fields from reanalysis and radiosonde data onto space time filtered satellite derived outgoing longwave radiation these results are compared with previous studies and an idealized modeling study in a companion paper which provides evidence that the waves bear a close structural resemblance to the fastest growing linear normal mode of the summertime basic state flow over africa there is a significant evolution in the three dimensional structure of aews as they propagate along 10 n across west africa at this latitude convection occurs in northerly flow to the east of the greenwich meridian then shifts into the wave trough and finally into southerly flow as the waves propagate offshore into the atlantic itcz in contrast to the north of the african easterly jet along 15 n convection remains in southerly flow throughout the waves trajectory along 10 n over west africa the location of convection is consistent with the adiabatic dynamical forcing implied by the advection of perturbation vorticity by the mean thermal wind in the zonal direction as in the companion paper offshore and along 15 n the relationship between the convection and dynamics is more complex and not as easily explained in terms of dynamical forcing alone

nd sr o isotopic and chemical evidence for a two stage contamination history of mantle magma in the central alpine bergell intrusion two different contamination processes have been identified as having been operative in the genesis of a plutonic suite initial contamination of a mantle source and subsequent crustal contamination of uprising partial melts from the mantle these processes are indicated by a detailed analyses of nd sr and oxygen isotopes together with major and trace elements of the 32 30 ma calc alkaline bergell intrusion this intrusion is located at the suture of the alpine continental collision zone and contains rock types capable of discriminating between mantle and intracrustal processes a range from basaltic andesitic dykes in the surrounding country rocks cumulitic hornblendites gabbros tonalite granodiorite and lamprophyres to pegmatites and aplites is exposed in this single intrusion the results of ree modelling and isotopic compositions of the basic members suggest that the cumulates were fractionated from a picrobasaltic liquid originating by partial melting of enriched subcontinental mantle end 4 increases in 87sr 86sr 0 7055 and 18o 6 7 in these samples relative to the mantle array and compositions of other periadriatic intrusions are most likely the result of an initial contamination of the mantle source by dehydration or partial melting of altered subducted oceanic crust slight differentiation of such a picrobasaltic liquid produced the basaltic andesitic dykes simultaneous fractional crystallization and contamination of the uprising magma by continental crust produced crustal isotopic signatures which increase with acidity to values of end 7 6 87sr 86sr 0 716 and 18o 10 the crustal imprint and lree enrichment in the dominating tonalite increase with decreasing crystallization depth which indicates that the tonalites were emplaced in several distinct batches with different degrees of contamination shoshonitic lamprophyres which intruded into the partly solidified granodiorite were generated in a deep strongly contaminated mantle source the posttectonic 26 ma novate leucogranite is not cogenetic with the main bergell body but rather formed from a predominantly crustal source if the described features are indeed due to mantle source contamination processes which are well known for volcanic arcs it must be concluded that these may also play a significant role in the genesis of calcalkaline plutonic suites

geochemistry of basalts from central and southern new hebrides arc implication for their source rock composition abstract the basaltic rocks from the central and southern islands of the new hebrides aneityum tanna erromango efate emae tongoa and epi have geochemical features typical of island arc volcanics they are enriched in lile and depleted in zr hf nb and ta compared to n type morb the rocks were derived from a similar upper mantle source as n type morb but with a higher degree of partial melting in addition their source was enriched in lile k rb sr ba and lree probably by migrating hydrous fluids released during the dehydration of the subducted oceanic slab the basalts from futuna island which is located farther from the trench display characteristics typical of calc alkaline rocks the futuna basalts were generated from a different lile enriched upper mantle source it seems that this upper mantle source was modified by interaction with partial melts from the subducted oceanic lithosphere

the genesis of african easterly waves by upstream development abstracta genesis mechanism for african easterly waves aews is proposed in the same manner that new troughs and ridges in the midlatitudes form downstream of existing ones through a mechanism known as downstream development it is proposed that new aews can be generated upstream of existing aews a local eddy kinetic energy budget of the aew that ultimately became hurricane alberto 2000 demonstrates that upstream development explains its genesis more convincingly than previous theories of aew genesis the energetics and ageostrophic secondary circulation of a composite aew are consistent with a new aew forming as a result of this mechanism some strengths and weaknesses of upstream development as a paradigm for aew genesis are discussed with respect to other potential mechanisms

are multiple tropical cyclone events similar among basins abstractthe present study intercompares multiple tropical cyclone event mtce characteristics among each global tropical cyclone tc basin using best track data specific focus is placed on examining the number of mtces and tcs during mtces the zonal distance between tcs during mtces and the spatiotemporal separation between genesis events during mtces the results suggest that the ratio of mtces relative to single tcs is substantially higher in the eastern north pacific enp western north pacific wnp and south indian ocean si basins compared to the north atlantic na and south pacific sp the prolific nature of enp wnp and si mtce activity results in approximately half of tcs occurring during mtces during new tc genesis the majority of preexisting tcs are generally located westward at a consistent zonal distance from new tc genesis for mtces within each basin with median values between 1620 and 1961 km tc induced rossby wave dispersion may set this zonal length scale as implied by it

three dimensional structure and dynamics of african easterly waves part iii genesis this paper promotes the view that african easterly waves aews are triggered by localized forcing most likely associated with latent heating upstream of the region of observed aew growth a primitive equation model is used to show that aews can be triggered by finite amplitude transient and localized latent heating on a zonally varying basic state that is linearly stable heating close to the entrance region of the african easterly jet aej is shown to initiate aews downstream the heating leads to an initial trough that reaches the west african coast about 5 7 days later depending on the nature of the heating profile after this a structure that projects strongly onto the leading linear normal mode of the basic state becomes established characterized by a number of westward propagating disturbances that strongly resemble aews the sensitivity of the forced aews to the nature and location of the heating profile is examined aews are most efficiently triggered by heating profiles that establish lower tropospheric circulations close to the entrance region of the aej in the present study this was best achieved by lower tropospheric heating from shallow convection or upper level heating and lower level cooling from a stratiform precipitation profile both profiles have significant heating gradients in the vertical in the mid to lower troposphere this triggering paradigm for the genesis of aews has consequences for the variability and predictability of aews at weather and climate time scales in addition to the nature of the aej often emphasized it is crucial to consider the nature and variability of upstream heating triggers

assimilation of sediments embedded in the oceanic arc crust myth or reality abstract arc magmas are commonly assumed to form by melting of sub arc mantle that has been variably enriched by a component from the subducted slab although most magmas that reach the surface are not primitive the impact of assimilation of the arc crust is often ignored with the consequence that trace element and isotopic compositions are commonly attributed only to varying contributions from different components present in the mantle this jeopardises the integrity of mass balance recycling calculations here we use sr and o isotope data in minerals from a suite of volcanic rocks from st lucia lesser antilles arc to show that assimilation of oceanic arc basement can be significant analysis of 87sr 86sr in single plagioclase phenocrysts from four soufriere volcanic complex svc st lucia hand samples with similar composition 87sr 86sr 0 7089 0 7091 reveals crystal isotopic heterogeneity among hand samples ranging from 0 7083 to 0 7094 with up to 0 0008 difference within a single hand sample o 18 measurements in the svc crystals show extreme variation beyond the mantle range with 7 5 to 11 1 for plagioclase n 19 10 6 to 11 8 for quartz n 10 9 4 to 9 8 for amphibole n 2 and 9 to 9 5 for pyroxene n 3 while older lavas pre soufriere volcanic complex with less radiogenic whole rock sr composition 87sr 86sr 0 7041 0 7062 display values closer to mantle range 6 4 to 7 9 for plagioclase n 4 and 6 to 6 8 for pyroxene n 5 we argue that the 87sr 86sr isotope disequilibrium and extreme o 18 values provide compelling evidence for assimilation of material located within the arc crust positive correlations between mineral o 18 and whole rock 87sr 86sr 143nd 144nd and 206 207 208pb 204pb shows that assimilation seems to be responsible not only for the isotopic heterogeneity observed in st lucia but also in the whole lesser antilles since st lucia encompasses almost the whole arc range of isotopic compositions this highlights the need for detailed mineral scale investigation of oceanic arc suites to quantify assimilation that could otherwise lead to misinterpretation of source composition and subduction processes

maintenance of the african easterly jet the maintenance of the african easterly jet aej has been examined using a zonally symmetric general circulation model with simple parametrizations it is shown that the aej is maintained in association with two diabatically forced meridional circulations one associated with surface fluxes and dry convection in the saharan heat low region and one associated with deep moist convection in the intertropical convergence zone equatorward of this the heat low heating which reaches the height of the aej around 700 mb is particularly important in maintaining the aej and its associated meridional gradients in potential vorticity it is concluded that the mean observed aej results from a combination of the diabatically forced meridional circulations which maintain it and easterly waves which weaken it

piecewise tendency diagnosis of dynamical processes governing the development of an upper tropospheric mobile trough abstract the intensification and evolution of midlatitude upper tropospheric mobile troughs may be viewed in terms of the isentropic advection and deformation of the tropopause potential vorticity gradient the potential vorticity viewpoint allows one to qualitatively assess observed events in the context of existing theories of mobile trough genesis such as baroclinic instability or downstream development in order to quantitatively determine the role of distinct dynamical process the method of piecewise tendency diagnosis or ptd is developed ptd is an extension of piecewise potential vorticity inversion applied to height tendencies with the forcing terms in the quasigeostrophic height tendency equation partitioned into potential vorticity advection associated with distinct dynamical processes a particular case of mobile trough genesis which occurred during 1 4 december 1980 over north america is diagnosed using ptd although about 20 of the intensification of the trough was due to superposition

the madden julian oscillation s influence on african easterly waves and downstream tropical cyclogenesis abstractthe influence of the madden julian oscillation mjo over tropical africa and atlantic is explored during the northern hemisphere summer months the mjo is assessed by using real time multivariate mjo rmm indices these indices divide the active convective signal of the mjo into 8 phases convection associated with the mjo is enhanced over tropical africa during rmm phases 8 1 and 2 convection becomes suppressed over tropical africa during the subsequent rmm phases phases 3 7 african convective signals are associated with westward propagating equatorial rossby waves the mjo modulates african easterly wave aew activity aew activity is locally enhanced during rmm phases 1 3 and suppressed during rmm phases 6 8 enhanced aew activity occurs during periods of enhanced convection over tropical africa consistent with stronger or more frequent triggering of aews as well as more growth associated with latent heat release enhanced aew activity occurs during the low level westerly wind phase of

the low level structure of african easterly waves in 1995 abstract the existence of african easterly waves aews north of the african easterly jet aej core with maximum amplitude at low levels has been confirmed and clarified using radiosonde data and the u k meteorological office global model analysis from the hurricane season of 1995 at bamako 12 5 n 8 0 w the aews were characterized mainly by maximum amplitudes at the level of the aej around 700 mb whereas at dakar 14 7 n 17 5 w the waves were characterized by maxima between 850 and 950 mb the low level waves to the north of the aej arise in association with baroclinic interactions between the negative meridional potential vorticity pv gradients in the jet core and the positive low level gradient of potential temperature enhanced by the presence of low static stability air north of the aej these waves follow the positive meridional gradients over northern africa in contrast to the jet level aews that follow the meridional pv gradients at the level of the aej cross correlation analysis

lead isotopic compositions of south sandwich island volcanic rocks and their bearing on magmagenesis in intra oceanic island arcs pb isotope ratios have been measured in 12 volcanic rocks from the south sandwich islands the ranges are 206pb204pb 18 51 18 66 207pb204pb 15 55 15 64 208pb204pb 38 42 38 64 in 207pb204pb 206pb204pb and 208pb204pb 206pb204pb correlation diagrams the south sandwich data plot distinctly above the fields for ocean ridge basalts and yield trends showing apparent mixing with a sedimentary end member similar to south atlantic pelagic sediments as reported by chow and patterson 1962 and this study armstrong and cooper 1971 have likewise shown that volcanics from the lesser antilles show mixing trends with north atlantic sediments in pb isotope correlation diagrams the north atlantic sediments have distinctly higher 206pb204pb and 208pb204pb ratios compared to the south atlantic sediments the parallel relationships between sediments and volcanic island arc rocks of the north and south atlantic provide strong evidence for a component of pb from subducted sediments in the lavas of the west atlantic basin in contrast to these data lavas from the mariana arc in the western pacific show little or no component of pb from pelagic sediments the reason for the different behaviors in the two settings is speculative

analysis of convection and its association with african easterly waves abstract the association between convection and african easterly wave aew activity over tropical africa and the tropical atlantic during the boreal summer is examined using satellite brightness temperature tb and ecmwf reanalysis datasets spectral analysis using 18 yr of tb data shows significant variance in the 2 6 day range across most of the region within the regions of deep convection this time scale accounts for about 25 35 of the total variance the 2 6 day convective variance has similar amplitudes over western and eastern africa while dynamic measures of aew activity show stronger amplitudes in the west this study suggests that weak aew activity in the east is consistent with initial wave development there and indicates that convection triggered on the western side of the mountains over central and eastern africa near darfur western sudan and ethiopia has a role in initiating aews westward the subsequent development and growth of aews in west africa is associated with stronger cohe

miocene subduction related magmatism in southern sardinia sr nd and oxygen isotopic evidence for mantle source enrichment abstract subduction beneath southern sardinia in miocene times gave rise to a suite of basalts basaltic andesites high k andesites and rhyolitic ignimbrites the monte arcuentu volcanic complex is a typical product of this activity although the volcanic rocks display some tholeiitic affinities in terms of k 2 o sio 2 relations they resemble calc alkaline lavas of the aeolian and aegean arcs the most mafic members of the suite have moderately high ni 164 ppm and cr 550 ppm contents and high mgo 10 wt and are therefore mantle derived mantle normalised trace element patterns show high k rb ba and th but low nb and ta characteristic of subduction related magmas sr contents in almost all samples are particularly low 165 265 ppm 87 sr 86 sr i values range from 0 7054 to 0 7113 whilst nd i is between 0 9 and 8 8 the sr nd isotopic field overlaps those of the high k rocks of roccamonfina and the roman volcanic province and extends that of the aeolian islands and the aegean arc correlations of radiogenic isotope ratios with sio 2 values in bulk rocks are much stronger than with mgo conventionally analysed 18 o whole rock values range from 6 4 in basalt to 12 4 in a high k andesite however 18 o values for clinopyroxene phenocrysts analysed by a laser fluorination method range from 6 17 to 7 46 good correlations are seen between 18 o cpx values and both mgo and sio 2 differences between 18 o whole rock and 18 o cpx values are probably due to late stage alteration of the groundmass and cast doubt on the validity of petrogenetic interpretations based solely on whole rock oxygen isotope analysis the sr nd o isotopic variation can be modelled either by contamination of mantle derived magmas by hercynian continental crust or by extensive enrichment of the mantle source by subducted sediments in the 18 o cpx vs 87 sr 86 sr i diagram crustal contamination hyperbolae are concave upward because of the low sr contents of the monte arcuentu magmas so both crustal contamination trends and mantle enrichment trends can generate the same isotopic relationships however the strong correlations of radiogenic isotopic ratios with sio 2 and the weak correlations with mgo are most easily explained by extensive 2 10 mantle enrichment by subducted siliceous sediments the correlation between sio 2 and 87 sr 86 sr i for the monte arcuentu magmas is very unusual within subduction related magmatism whereas that between 18 o and 87 sr 86 sr appears to show that the suite forms an end member composition for many island arc and active continental margin volcanic suites

energy dispersion in african easterly waves the existence of an upstream eastward group velocity for african easterly waves aews is shown based on single point lag regressions using gridded reanalysis data from 1990 to 2010 the eastward energy dispersion is consistent with the direction of ageostrophic geopotential flux vectors a local eddy kinetic energy eke budget reveals that early in the life cycle of aews growth rate due to geopotential flux convergence exceeds baroclinic and barotropic growth rates later in the life cycle eke decay due to geopotential flux divergence cancels or exceeds baroclinic and barotropic growth a potential vorticity pv budget is used to diagnose tendencies related to group propagation although both upstream and downstream group speeds are possible because of the reversal in the mean meridional pv gradient upstream propagation associated with the positive poleward pv gradient dominates wave packet evolution analogous to the concept of downstream development of midlatitude baroclinic waves new aews develop preferentially upstream of the older ones thus providing a mechanism for seeding new waves it is suggested that these results are also relevant to aew intermittency and storm track structure

atlantic tropical systems of 1994 and 1995 a comparison of a quiet season to a near record breaking one abstract totals of 70 and 63 tropical waves also known as african or easterly waves were counted in the atlantic basin during the 1994 and 1995 hurricane seasons these waves led to the formation of 9 of the 12 total number of tropical cyclones in 1994 and 19 of the 21 total number of tropical cyclones in 1995 tropical waves contributed to the formation of 75 of the eastern pacific tropical cyclones in 1994 and 73 in 1995 upper and lower level prevailing wind patterns observed during the below normal season of 1994 and the very active one of 1995 are discussed tropical wave characteristics between the two years are compared

a multi scale analysis of the extreme rain event of ouagadougou in 2009 this study presents a multi scale analysis of an extreme rain event that occurred in burkina faso on 1st september 2009 with an absolute record of 263 mm rainfall observed at ouagadougou this high impact weather system results from the combination of several favourable ingredients at different scales the sea surface temperature anomaly patterns in july august 2009 of both the atlantic cold tongue the tropical atlantic dipole and the mediterranean sea are favourable factors for the northward penetration of the west african monsoon the intense convective activity of the last 10 day period in august is associated with the crossing of a convectively coupled kelvin wave increasing the african easterly wave aew activity and of an equatorial rossby wave at the synoptic scale this event corresponds to the passage of a train of 3 aews with increasing magnitude behind the 1st aew trough axis an intense and deep southerly monsoon burst develops it contributes to the amplification of the 2nd aew and its breaking associated with the formation of an intense meso vortex on the southern flank of the african easterly jet compared to the fast moving squall line the dominant type of precipitating weather system over the sahel the ouagadougou precipitating system appears to be a moist vortex propagating slowly allowing rainfall accumulation without wind gusts or convective cold pools observed at the surface the main precipitation area is located about 2 longitude downshear westward due to the african easterly jet of the centre of this strong meso vortex

on the relationship between african easterly waves and the african easterly jet this idealized modeling study investigates how convectively triggered african easterly waves aews are influenced by the intraseasonal variability of the african easterly jet aej a set of 10 day averaged zonally varying basic states is constructed with the ncep 2 reanalysis 1979 2006 a primitive equation model is used to simulate linear aews on each of these basic states using the same idealized convective heating localized over the darfur mountains as an initial trigger it is shown that the transient response depends strongly on the basic state with the same trigger many configurations of the aej fail to produce a wave disturbance while others produce strong easterly wave structures necessary conditions for the development of strong waves can be characterized by a strong jet a strong vertical shear or a strong and extended potential vorticity reversal in strong wave cases the jet is extended to the south and west and the jet core is aligned with the maximum of surface westerlies maximizing the vertical shear the pattern that is optimal for generating easterly waves also closely resembles the dominant mode of variation of the aej revealed by an empirical orthogonal function eof analysis of the set of basic states

absolute and convective instability of the african easterly jet abstractthe stability of the african easterly jet aej is examined using idealized numerical simulations it is found that a zonally homogeneous representation of the aej can support absolute instability in the form of african easterly waves aews this finding is verified through a local energy budget which demonstrates the presence of both upstream and downstream energy fluxes these energy fluxes allow unstable wave packets to spread upstream and downstream relative to their initial point of excitation this finding is further verified by showing that the ground relative group velocity of these wave packets has both eastward and westward components in contrast with normal mode instability theory which emphasizes wave growth through energy extraction from the basic state the life cycle of the simulated aews is strongly governed by energy fluxes convergent fluxes at the beginning of the aew storm track generate new aews whereas divergent fluxes at the end of the storm track lead to their decay i

trains of african easterly waves and their relationship to tropical cyclone genesis in the eastern atlantic abstractin this study the relationship between trains of african easterly waves aews and downstream tropical cyclogenesis is studied based on 19 summer seasons july september from 1990 to 2008 of era interim reanalysis fields and brightness temperature from the cloud user archive the signature of aew troughs and embedded convection are tracked from the west african coast to the central atlantic the tracked systems are separated into four groups i systems originating from the north zone of the midtropospheric african easterly jet aej ii those coming from the south part of aej iii systems that are associated with a downstream trough located around 2000 km westward termed duo systems and iv those that are not associated with such a close downstream trough termed solo systems by monitoring the embedded 700 hpa filtered relative vorticity and 850 hpa wind convergence anomaly associated with these families along their trajectories it is shown that the duo generally have stronger dynam

strontium isotopic and selected trace element variations between two aleutian volcanic centers adak and atka implications for the development of arc volcanic plumbing systems major and trace element concentrations and initial 87sr 86sr ratios of lavas from the aleutian volcanic centers of adak and atka have been used to study the evolution of their respective lithospheric plumbing systems the centers are within 150 km of one another and show similar overall silica ranges 47 67 but adak 40 km3 is smaller than atka 200 km3 adak s lavas are chemically and isotopically heterogeneous 87sr 86sr 0 70285 0 70330 and two units contain lithospheric xenoliths the lavas of the much larger atka on the other hand have much less variability in major and trace elements as well as 87sr 86sr 0 70320 0 70345 we suggest that these characteristics are a measure of the relative maturity and cleanliness of the lithospheric plumbing systems that supply magma to these centers because aleutian volcanic centers often remain fixed for relatively long periods of time 5 m y once established magmatic passageways are repeatedly used young plumbing systems are relatively cool and contain large amounts of wallrock contaminant and ascending magmas undergo contamination as well as concurrent crystallization and fractionation with time however heat and mass transfer between ascending magmas and wallrock produce thermal and chemical boundary layers that insulate subsequent magmas in effect the plumbing system matures the chemical heterogeneity displayed by young dirty systems like adak reflects not only the magma source but also the wallrock encountered during ascent and possibly the effects of extensive crystal fractionation thus it is the petrologic data of mature clean systems like atka that yield the most direct and unambiguous information on the ultimate origin of the lavas and their near surface evolution

oxygen isotope geochemistry of the lassen volcanic center california resolving crustal and mantle contributions to continental arc magmatism this study reports oxygen isotope ratios determined by laser fluorination of mineral separates mainly plagioclase from basaltic andesitic to rhyolitic composition volcanic rocks erupted from the lassen volcanic center lvc northern california plagioclase separates from nearly all rocks have o values 6 1 8 4o higher than expected for production of the magmas by partial melting of little evolved basaltic lavas erupted in the arc front and back arc regions of the southernmost cascades during the late cenozoic most lvc magmas must therefore contain high o crustal material in this regard the o values of the volcanic rocks show strong spatial patterns particularly for young rhyodacitic rocks that best represent unmodified partial melts of the continental crust rhyodacitic magmas erupted from vents located within 3 5 km of the inferred center of the lvc have consistently lower o values average 6 3o 0 1o at given sio2 contents relative to rocks erupted from distal vents 47 0 km average 7 1o 0 1o further magmas erupted from vents situated at transitional distances have intermediate values and span a larger range average 6 8o 0 2o basaltic andesitic to andesitic composition rocks show similar spatial variations although as a group the o values of these rocks are more variable and extend to higher values than the rhyodacitic rocks these features are interpreted to reflect assimilation of heterogeneous lower continental crust by mafic magmas followed by mixing or mingling with silicic magmas formed by partial melting of initially high o continental crust 9 0o increasingly hybridized by lower o 6 0o mantle derived basaltic magmas toward the center of the system mixing calculations using estimated endmember source o values imply that lvc magmas contain on a molar oxygen basis approximately 42 to 4 isotopically heavy continental crust with proportions declining in a broadly regular fashion toward the center of the lvc conversely the o values of the rhyodacitic rocks suggest that the continental crust in the melt generation zones beneath the lvc has been substantially modified by intrusion of mantlederived basaltic magmas with the degree of hybridization ranging on a molar oxygen basis from approximately 60 at distances up to 12 km from the center of the system to 97 directly beneath the focus region these results demonstrate on a relatively small scale the strong influence that intrusion of mantle derived mafic magmas can have on modifying the composition of pre existing continental crust in regions of melt production given this result similar but larger scale regional trends in magma compositions may reflect an analogous but more extensive process wherein the continental crust becomes progressively hybridized beneath frontal arc localities as a result of protracted intrusion of subduction related basaltic magmas

two contrasting african easterly wave behaviors abstractthe dominant structural variability of african easterly waves aews is explored using an empirical orthogonal function eof approach the structure of aews is obtained by projecting the w

a case of downstream baroclinic development over western north america abstract numerical simulations have been made of the initiation of a strong ridge trough system over western north america and the eastern pacific the terminus of the pacific storm track with the objective of determining the extent to which downstream development contributed to its growth and the possible influence of topography on the energetics of the storm while a control simulation demonstrated considerable skill in reproducing the storm a simplified simulation in which topography surface that fluxes and latent heating were removed not only reproduced the primary features of the ridge trough system permitting a clearer interpretation of the factors contributing to its growth but actually generated a stronger system suggesting that these effects as a whole inhibited storm development application of an energy budget that distinguishes between energy generation via baroclinic processes and generation via the convergence of geopotential fluxes revealed that early growth of studies that have sh

the impact of moist processes on the african easterly jet african easterly wave system the african easterly jet easterly wave aej aew system was explored in an idealised model prescribed zonally symmetric surface temperature and moisture profiles determine the aej which becomes established through meridional contrasts in dry and moist convection as in previous studies a realistic aej developed with only dry convection including moist processes increased its development rate but reduced its speed and meridional extent aews grew through barotropic baroclinic conversions negative meridional potential vorticity pv gradients arose in the zonally symmetric state through the intrusion of the low pv saharan boundary layer since moist processes strengthened this significantly through diabatically generated pv in the intertropical convergence zone moist aews were three times stronger larger barotropic conversions and faster aej development increased the moist wave growth rate jet level and northerly low level amplitudes grew but in the moist case the low level amplitudes weakened as the aew interacted with convection consistent with their absence from observations during the peak monsoon striking dependencies between the aej aew and rainfall existed two time scales governed their evolution depending on the transfer coefficients 1 the aej s replenishment rate influenced by heat fluxes and 2 the wave growth rate by damping and the slower jet development rate moist aews were characterized by intermittent growth decay with growth preceded by increased mean rainfall and later weakening aejs these dependencies established an internal 8 10 day variability consistent with intra seasonal observations of 9 day rainy sequences this internal variability offers an alternative explanation to the previously proposed external forcing and a new view of the moist aew life cycle copyright 2009 royal meteorological society