mechanisms by which gulf of guinea and eastern north atlantic sea surface temperature anomalies can influence african rainfall abstract the sensitivity of precipitation over west africa to sea surface temperature anomalies sstas in the gulf of guinea and the eastern north atlantic is studied using a gcm results from nine perpetual july simulations with various imposed sstas are presented and analyzed to reveal associations between the precipitation and sst fields via large scale circulation and atmospheric moisture anomalies rainfall increases over the guinean coast and decreases over the congo basin when warm sstas are present in the gulf of guinea these precipitation perturbations are related to the forcing of a kelvin and a rossby wave the former is associated with a weakening of the walker circulation while the latter strengthens the west african monsoon rainfall over west africa is less sensitive to cold sstas than to warm anomalies three contributing factors are identified as follows 1 latitude of the sst forcing 2 background flow and 3 nonlinearity of the clausius clapeyron equation no more than a 20 effec

the seasonal cycle in the lower troposphere over west africa from sounding observations in situ sounding observations from selected sites in west and central africa were used to document the seasonal cycle in the lower troposphere associated with the annual migration of precipitation the main purpose of this exercise is to provide a description of the seasonal cycle free of the errors and biases of numerical models which can be substantial in this region many features in the circulation typically associated with the west african monsoon wam exist through almost the entire seasonal cycle in the region the low level easterly jet known as the african easterly jet during the monsoon season remains prominent and closely related to the rain band in other phases of the seasonal cycle even when the rain band is located near the equator the surface southerly monsoon onshore flow penetrates through the rain band in the entire season cycle the depth of the southerly surface monsoon flow undergoes a seasonal variation and is greatest during the peak season of the monsoon a unique feature is a low level 700 hpa northerly flow penetrating through the rain band in various phases of the seasonal cycle risking possible biases of land bound sounding data these observations are synthesized into a schematic describing the salient features in the low level circulation associated with the rain band in different phases of the seasonal cycle this synthesized circulation pattern in west africa is in contrast to the traditional conceptual circulation pattern associated with the marine itcz and other monsoon systems the results from this study call for more research attention to the low level meridional circulation for a full understanding of the dynamics of the wam this study also advocates the study of the wam to be conducted in a broad context of the seasonal cycle in the region and in comparison to the marine itcz copyright 2006 royal meteorological society

an 1800 year archeological experiment in remagnetization limestone masonry from monuments dating from 300ad to 1940 remagnetized partly and progressively with age parallel to the present earth s magnetic field the remagnetization was probably due to viscous remanent magnetization vrm neel s theory predicts that natural vrm acquired at low temperature ta over a long time ta disappears at a higher temperature td in a short period td in the laboratory i calibrated td for the postconstruction remagnetization against the ages td of masonry used in well dated historic buildings in eastern england empirically ta m td b relates acquisition age tayears to demagnetization temperature td c lincoln limestone remagnetizes more quickly m 1 298 10 6 and b 3 598 over 1800 years than speeton chalk m 4298 10 6 and b 1 973 over 350 years magnetite with an effective pseudo single domain or small multidomain size carries the remanence

viscous magnetization archaeology and bayesian statistics of small samples from israel and england 1 certain limestones remagnetize viscously and noticeably over archaeological time intervals after their reorientation into monuments the laboratory demagnetization temperatures tub for the vrm increase with the installation age with rates of 0 07 c0 year for israel chalk and 0 1c0 year for english chalk the empirical relationship may be used to date enigmatic buildings or geomorphological features e g land slips such correlations also give some insight into the viscous remagnetization process over time intervals 4000 years which are unobtainable in laboratory studies the tub age relationship for the viscous remagnetization appears to follow a power law linearized as log10 b log10 tub different pelagic limestones follow different curves and whereas conventional regression estimates the power law exponent b the small sample size recommends a bayesian statistical approach from sites constructed with pelagic chalk from eastern england precise prior information b 0 761 is compared with less precise information for much more ancient sites in northern israel b 0 873 the collective posterior correlation shows a generalized power law exponent b 0 849 that regression explains 84 9 of the collective variance in age r2 0 849 of course site specific calibration is required for archaeological age determinations

elastic deformation of the australian continent induced by seasonal water cycles and the 2010 2011 la nina determined using gps and grace the solid earth deforms elastically in response to changes in atmospheric water and ice mass load i present geodetic gravity recovery and climate experiment and gps observations of continental deformation at seasonal and interannual scales responding to water and atmospheric cycles in australia in the southern summer the central part of the australian continent rises by a few millimeter and the north south perimeter lengthens by 1 mm induced by atmospheric unloading in winter the continent subsides and the perimeter shortens due to atmospheric loading in autumn increased soil moisture and groundwater result in 7 mm of subsidence in northern australia and produce a positive slope trending southward this trend reverses in spring the la nina precipitation in 2010 2011 produced widespread subsidence of 10 mm followed by gradual uplift of 10 mm over the next 3 4 years as water storage depletes slowly through evapotranspiration the geodetic measurements find significant imbalance in the water cycle budget in australia over 2010 2015

magnetic study of large apollo samples possible evidence for an ancient centered dipolar field on the moon abstract we present new rock magnetic and paleomagnetic results obtained from seventeen apollo samples measurements of the magnetic properties of the samples confirm that the magnetic properties of lunar rocks are dominated by about 0 1 wt of multidomain feni grains the ferromagnetic mineral assemblage is to the first order the same in all lunar lithologies with higher concentration in basalts than in anorthosites some of which are diamagnetic impact processing leads to an increase of the ferromagnetic content out of the seventeen samples eleven show at least one stable component of natural remanent magnetization in some cases this magnetization may be a magnetic contamination however five basalt samples have a component of magnetization that may have recorded a lunar paleofield acquired on the moon for these samples plus two similar samples from a previous study using the rock magnetic fabric as a paleohorizontal proxy paleoinclinations can be estimated from the paleomagnetic data these paleoinclinations are best explained with a dipolar field and a magnetic paleopole located at 75 n therefore our results suggest the existence on the moon at least between 3 8 and 3 3 ga of a dipolar field whose axis is close to the present day rotation axis of the moon the estimated surface field intensities of several tens of t support previous result that the moon had a dynamo field in the past and the proposed paleofield geometry suggests that this dynamo was centered on the lunar present day rotation axis

identifying types of remanent magnetization in igneous contact zones abstract the magnetization of host rock adjacent to an igneous intrusion e g dyke will be completely reset near the contact partially reset in a zone of hybrid directions farther away and unaffected at even greater distances if the intrusion related direction and the host direction are known the hybrid zone magnetization can be analysed in terms of ptrm vrm and crm overprints of the host magnetization using orthogonal component plots of thermal demagnetization results straight line segments representing the host and the intrusion induced magnetization show a sharp intersection in cases of 1 a pure ptrm overprint 2 a vptrm overprint of one magnetic carrier 3 two magnetic minerals with a vptrm partial overprint of the high blocking temperature mineral and a complete vptrm overprint of the low blocking temperature mineral and 4 a crm overprint where its blocking temperature range is entirely distinct from that of the host component a gradual transition between both components during thermal demagnetization is expected for a two magnetic minerals both of which have a vptrm partial overprint b a vptrm partial overprint of a continuous range of magnetic mineral compositions and c a crm overprint in which the blocking temperature range partially overlaps that of the host component swings from one direction to the other and back are observed if the blocking temperature of one of the components completely overlaps that of the other one of them being due to crm

magnetism in rocks rock magnetism is the study of induced and remanent magnetization of ferrimagnetic mineral grains in rocks sediments soils and organisms its applications include environmental magnetism magnetic anisotropy sources of continental and oceanic magnetic anomalies records of geomagnetic field variations and polarity reversals and the paleomagnetic record of plate motions and the wilson cycle this paper reviews the beginnings of rock magnetism and then traces the development of six particularly interesting areas pseudo single domain behavior magnetic domains and micromagnetic structures diagnostic tests of the type and stability of remanent magnetization magnetic microanalysis thermoviscous remagnetization and chemical remanent magnetization other areas including sediment and soil magnetism are covered in a companion paper by verosub and roberts on environmental magnetism

a revised picture of the structure of the monsoon and land itcz over west africa this article presents an overview of the land itcz intertropical convergence zone over west africa based on analysis of ncar ncep reanalysis data the picture that emerges is much different than the classic one the most important feature is that the itcz is effectively independent of the system that produces most of the rainfall rainfall linked directly to this zone of surface convergence generally affects only the southern sahara and the northern most sahel and only in abnormally wet years in the region a second feature is that the rainbelt normally assumed to represent the itcz is instead produced by a large core of ascent lying between the african easterly jet and the tropical easterly jet this region corresponds to the southern track of african easterly waves which distribute the rainfall this finding underscores the need to distinguish between the itcz and the feature better termed the tropical rainbelt the latter is conventionally but improperly used in remote sensing studies to denote the surface itcz over west africa the new picture also suggests that the moisture available for convection is strongly coupled to the strength of the uplift which in turn is controlled by the characteristics of the african easterly jet and tropical easterly jet rather than by moisture convergence this new picture also includes a circulation feature not generally considered in most analyses of the region this feature a low level westerly jet termed the african westerly jet plays a significant role in interannual and multidecadal variability in the sahel region of west africa included are discussions of the how this new view relates to other aspects of west africa meteorology such as moisture sources rainfall production and forecasting desertification climate monitoring hurricanes and interannual variability the west african monsoon is also related to a new paradigm for examining the interannual variability of rainfall over west africa one that relates changes in annual rainfall to changes in either the intensity of the rainbelt or north south displacements of this feature the new view presented here is consistent with a plethora of research on the synoptic and dynamic aspects of the african easterly waves the disturbances that are linked to rainfall over west africa and spawn hurricanes over the atlantic and with our knowledge of the prevailing synoptic and dynamic features this article demonstrate a new aspect of the west africa monsoon a bimodal state with one mode linked to dry conditions in the sahel and the other linked to wet conditions the switch between modes appears to be linked to an inertial instability mechanism with the cross equatorial pressure gradient being a critical factor the biomodal state has been shown for the month of august only but this month contributes most of the interannual variability this new picture of the monsoon and interannual variability shown here appears to be relevant not only to interannual variability but also to the multidecadal variability evidenced in the region between the 1950s and 1980s

dating of tsunami boulders from ishigaki island japan with a modified viscous remanent magnetization approach abstract reworked boulders are expected to acquire a viscous remanent magnetization vrm approximately parallel to earth s modern magnetic field the magnitude of such a vrm depends on several factors including the time since reworking and ambient temperature for which there are well known theoretical relationships vrm unblocking temperature can therefore be a powerful tool for determining the reworking age of boulders and can be used to assess the timing of geological hazards such as landslides and tsunami events in this study vrm unblocking temperatures for twenty seven samples from four coral tsunami boulders on ishigaki island japan are compared with three candidate time temperature relationships for vrm acquisition for the pullaiah nomogram which is applicable to single domain magnetite particle assemblages nineteen samples from four boulders agree well with the expected unblocking temperature derived from previously reported 14c ages two samples have low unblocking temperatures and six samples appear to have anomalously high vrm unblocking the walton nomogram which is used for lognormal grain size distributions relates acquisition temperatures required to produce equal magnetic paleo intensities and cannot explain the high unblocking temperatures it produces younger predicted ages than the youngest boulder 14c age we find that an alternative time temperature relationship defined by a stretched exponential law has the potential to yield reworking ages for the anomalous boulders that are consistent with the 14c ages we suggest that future vrm dating can be undertaken using a combination of the pullaiah nomogram and the stretched exponential law

thermoviscous remagnetization in some appalachian limestones experimental evidence shows that blocking temperature relaxation time theory for single domain magnetite can grossly underestimate laboratory thermal demagnetization temperatures of a present day viscous remanence in devonian limestones of new york state thermoviscous remagnetization at moderately elevated burial temperatures during the late paleozoic cannot therefore be readily excluded for the origin of the stable secondary component in these rocks more generally these results emphasize that single domain thermal activation theory cannot be assumed a priori to correctly predict temperature of thermoviscous remanence acquisition in all magnetite bearing rocks the prob leto

magnetic properties opaque mineralogy and magnetic anisotropies of serpentinized peridotites from odp hole 670a near the mid atlantic ridge abstract magnetic mineralogy and magnetic properties of 16 samples of serpentinized peridotites harzburgites from odp hole 670a are studied the degree of serpentinization was determined by a new method which combines saturation magnetization and whole rock iron content a unique curie temperature t c of 580 c corresponding to pure magnetite was observed microprobe analyses whole rock metal content and curie temperatures indicate the absence of awaruite ni 3 fe or native metals hysteresis parameters reveal the contribution of pseudo single domain psd grains in samples with a low degree of serpentinization and large multi domain md grains in highly serpentinized samples with j rs j s 0 08 natural remanent magnetization nrm is mainly unstable and possibly composite particularly in the highly serpentinized samples the intensity j a of anhysteretic remanent magnetization arm in the geomagnetic field is much higher than j n intensity of nrm with j a j n averaging 2 4 the koenigsberger ratio q is low and susceptibility k depends on degree of serpentinization ranging from 1 2 to 51 5 10 6 m 3 kg 1 nrm inclinations are scattered with some tendency to lie near the horizontal plane which is close to the foliation plane anisotropies of susceptibility of arm and of saturation remanent magnetization srm were studied and compared with spinel foliation determined for three less serpentinized samples in these samples magnetic anisotropies seem to be mimetic of high temperature deformation before serpentinization when the degree of serpentinization is high later magnetic fabrics probably related to magnetite veins dominate previous anisotropies the temperature dependence of susceptibility and of viscous remanent magnetization vrm were studied the thermal variation of susceptibility shows a shallow hopkinson effect at about 300 400 c time dependence of induced and remanent magnetization increases with temperature reaching maximum values at about 300 400 c based on these observations induced magnetization and vrm of lower crustal and upper mantle rocks and their contribution to marine magnetic anomalies are discussed

diagenetic sources of stable remanence in remagnetized paleozoic cratonic carbonates a rock magnetic study hysteresis measurements lowrie fuller tests cisowski tests and low temperature demagnetization experiments on samples of the knox dolomite trenton limestone and onondaga limestone indicate that the remanence in these remagnetized carbonate units resides principally in single domain magnetite in contrast to previous interpretations volumetrically however a significant fraction of the magnetite is coarser grained as previous scanning electron microscopic work on magnetic extracts has shown these conclusions are based mainly on the characteristic wasp waisted shape of the hysteresis loops and on the anomalously high ratios of remanent coercivity to bulk coercivity both of which clearly indicate a bimodal distribution of coercivities grain sizes the magnetization is controlled mainly by cubic magnetocrystalline anisotropy consistent with the spheroidal particles commonly observed in extracts hysteresis trends show that especially the finest particles lack shape anisotropy in view of these conclusions the two components of natural remanence in these rocks are interpreted as 1 a late paleozoic kiaman chemical remanence carried by single domain diagenetic magnetite and 2 a cenozoic thermoviscous overprint residing principally in coarser pseudosingle domain and multidomain magnetite also of late paleozoic diagenetic origin

paleomagnetism and paleothermometry of the sydney basin 1 thermoviscous and chemical overprinting of the milton monzonite the early triassic 245 ma milton monzonite of the sydney basin australia has four distinct components of natural remanent magnetization nrm with only slightly overlapping ranges of unblocking temperatures the low temperature lt component the first to be thermally demagnetized is thought to be a late cretaceous 100 ma thermoviscous overprint acquired in slow cooling during uplift the high temperature ht component the second to be demagnetized is probably the primary thermoremanent magnetization trm of the milton intrusion but could possibly be a jurassic overprint lt and ht are usually carried by magnetite and occasionally by pyrrhotite samples from nine sites have a further nrm component which unblocks at higher temperatures than ht but below the magnetite curie temperature of 580 c this component is argued to be a chemical remanent magnetization crm because of its discrete range of high unblocking temperatures above those of the thermal components ht and lt and is called crm1 crm1 has almost the same direction as lt and is likely carried by authigenic magnetite produced during uplift 100 ma samples from five sites have a fourth nrm component with a direction resembling that of ht but carried by hematite this fourth component could be a primary trm but is more likely a crm and is therefore called crm2 the ht crm2 mean direction is d 50 i 75 5 defining a paleopole at 16 s 172 e the ht crm2 paleopole falls near 150 ma on the australian apparent polar wander path but is a considerable distance from paleopoles of permian and early triassic age there is no known tectonic or other remagnetizing event in the sydney basin around 150 ma for this reason we propose that the ht crm2 paleopole defines a new triassic segment of the australian polar wander path the lt crm1 mean direction is d 348 i 79 with a paleopole falling at 56 s 1580 e near 100 ma on the polar wander path this age is consistent with uplift and cooling related to initial rifting of the tasman sea

paleomagnetism of rumuruti chondrites suggests a partially differentiated parent body abstract different types of magnetic fields were at work in the early solar system nebular fields generated within the protoplanetary nebula solar fields and dynamo fields generated within the solar system solid bodies paleomagnetic studies of extraterrestrial materials can help unravel both the history of these magnetic fields and the evolution of solar system solid bodies in this study we studied the paleomagnetism of two rumuruti chondrites pca 91002 and lap 03639 these chondrites could potentially bear the record of the different fields solar nebular dynamo fields present during the early solar system the magnetic mineralogy consists of pseudo single domain pyrrhotite in lap 03639 and pyrrhotite plus magnetite in pca 91002 paleomagnetic analyses using thermal and alternating field demagnetization reveal a stable origin trending component of magnetization fields of 12 t or higher are required to account for the magnetization in pca 91002 but the timing and exact mechanism of the magnetization are unconstrained in lap 03639 considering various chronological constraints on the parent body evolution and on the evolution of the different sources of magnetic field in the early solar system an internally generated dynamo field of 5 t recorded during retrograde metamorphism is the most likely explanation to account for the measured magnetization this result indicates the existence of an advecting liquid core within the rumuruti chondrite parent body and implies that as proposed for cv and h chondrites this chondritic parent body is partially differentiated

vertical crustal movement around the southeastern tibetan plateau constrained by gps and grace data abstract using measurements of continuous gps stations and grace across the southeastern tibetan plateau we found that the grace derived vertical displacements are highly correlated with gps modeled vertical annual and semiannual displacements which demonstrates that the vertical seasonal variations on ground surface are mainly caused by hydrological mass loading after removing grace modeled seasonal variations from the gps time series we used a stacking technique to filter out the common mode errors then we estimated the best noise model for the filtered time series which can be best characterized by power law noise model finally we determined the rates of vertical crustal movement of southeastern tibetan plateau our results suggest that the southeastern tibetan plateau is undergoing uplift whereas the southern sichuan yunnan fragment is subsiding with respect to the region to the north the assumption of uniform extension throughout the crust does not explain the subsidence of southern sichuan yunnan fragment

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

paleomagnetism of the late precambrian coldwell complex ontario canada abstract the coldwell complex is a large multiphase alkaline intrusion on the north shore of lake superior it was emplaced into archean metavolcanics of the superior province in at least three distinct magmatic episodes during the late precambrian giving isotopic ages ranging from 1010 to 1188 ma detailed af and thermal demagnetization was done on 416 specimens from 33 sites representing all petrologic phases additional data are included from an unpublished 1972 study by w a robertson the oldest magnetic component pca was found in 19 episode i sites with a reversed mean direction of d 121 0 i 70 9 104 95 3 3 its pole position falls on the keweenawan apwp at an age of 1109 5 ma the second component pcb was found in 11 sites with a normal mean of d 301 8 i 60 1 49 95 5 8 that indicates an age of 1103 5 ma the youngest component pcc was found in nine episode iii sites and gives a reversed mean of d 119 1 i 54 3 101 95 5 1 that indicates an age of 1095 5 ma the assigned ages agree closely with recent u pb zircon ages but are distinctly older than the k ar and rb sr ages these paleomagnetic results require some changes in existing models for the petrologic evolution of the complex and indicate a much more rapid three stage cooling history than previously postulated they also provide strong evidence against the proposed concept of an asymmetrical geomagnetic field in middle keweenawan time

thermally activated viscous remanence in some magnetite and hematite bearing dolomites a brunhes age viscous remanent magnetization vrm carried by magnetite and hematite in the el paso and montoya dolomite formations has unblocking temperatures significantly higher than those predicted by single domain theory the samples appear to contain a substantial fraction of pseudo single domain magnetite suggesting transdomain processes may be responsible for the stable vrm a strong inverse correlation between the occurrence of high unblocking temperatures and high isothermal remanence fractions acquired above 0 3 tesla suggests that the hematite grain size may also be important in determining the range of observed temperatures larger grains are inferred to have lower coercivities but higher unblocking temperatures an older reverse polarity component is also observed and appears to be a record of late cretaceous tertiary tectonic activity

transient signal detection using gps measurements transient inflation at akutan volcano alaska during early 2008 1 continuous global positioning system gps networks record station position changes with millimeter level accuracy and have revealed transient deformations on various spatial and temporal scales however the transient deformation may not be easily identified from the position time series because of the large number of sites in a network low signal to noise ratios snr and correlated noise in space and time here we apply state estimation and principal component analysis to the daily gps position time series measured in alaska sites of the plate boundary observatory network our algorithm detects a transient signal whose maximum displacement is 9 mm in horizontal and 11 mm in vertical that occurred at akutan volcano during the first half of 2008 a simple mogi source inversion suggests inflation at shallow depth 3 9 km beneath the volcano although the detection was not easy because the signal was aseismic non eruptive and weak not apparent in raw daily time series our detection method improves the snr and therefore provides higher resolution for detecting the transient signal

spatially varying surface seasonal oscillations and 3 d crustal deformation of the tibetan plateau derived from gps and grace data abstract measurements of 189 continuous and 933 campaign mode global positioning system gps stations with 3 16 yr data spans over the tibetan plateau reveal contemporary three dimensional 3 d crustal deformation during 1999 2016 the empirical orthogonal function method was used to characterize the spatial variations in the surface deformation with distinct seasonal oscillations at the gps sites in five regions of the tibetan plateau we find that these surface variations are highly correlated with the corresponding mass load signals observed by the gravity recovery and climate experiment grace mission the improved gps processing strategy used to determine the 3 d velocity field includes maximum likelihood estimation removal of common mode errors from gps time series using principal component analysis pca and power law plus white noise stochastic error modeling we determined the rates of vertical crustal movement by removing grace observed non tectonic origin load deformation 2002 2016 the corrected vertical crustal deformation shows that the himalaya region is uplifting at an average rate of 1 7 mm yr 1 and that the northeastern tibetan plateau is uplifting at an average rate of 1 3 mm yr 1 in addition the horizontal velocity relative to the stable eurasian plate and its corresponding dilatation throughout the tibetan plateau suggest that tectonic shortening and crustal thickening is occurring at 90 to 80 nanostrain yr 1 in the southern tibetan plateau and 30 to 20 nanostrain yr 1 in the northeastern tibetan plateau which could be related to the geologic shortening and elastic strain accumulation the interior tibetan plateau exhibits crustal thinning and block movement along strike slip faults eastward motion of the crust north of the xianshuihe xiaojiang fault system relative to crust to its south results in shear strain and reflects eastward escape of plastic crustal material in the southeastern tibetan plateau

seasonal hydrological loading in southern alaska observed by gps and grace 1 we compare vertical seasonal loading deformation observed by continuous gps stations in southern alaska and modeled vertical displacements due to seasonal hydrological loading inferred from grace seasonal displacements are significant and gps observed and grace modeled seasonal displacements are highly correlated we define a measure called the wrms reduction ratio to measure the fraction of the position variations at seasonal periods removed by correcting the gps time series using a seasonal model based on grace the median wrms reduction ratio is 0 82 and the mean is 0 73 0 26 with a value of 1 0 indicating perfect agreement of gps and grace the effects of atmosphere and non tidal ocean loading are important we add the aod1b de aliasing model to the grace solutions because the displacements due to these loads are present in the gps data and this improves the correlations between these two geodetic measurements we find weak correlations for some stations located in areas where the magnitude of the load changes over a short distance due to grace s limited spatial resolution grace models can correct seasonal displacements for campaign gps measurements as well

vertical gps ground motion rates in the euro mediterranean region new evidence of velocity gradients at different spatial scales along the nubia eurasia plate boundary we use 2 5 to 14 years long position time series from 800 continuous global positioning system gps stations to study vertical deformation rates in the euro mediterranean region we estimate and remove common mode errors in position time series using a principal component analysis obtaining a significant gain in the signal to noise ratio of the displacements data following the results of a maximum likelihood estimation analysis which gives a mean spectral index 0 7 we adopt a power law white noise stochastic model in estimating the final vertical rates and find 95 of the velocities within 2 mm yr with uncertainties from filtered time series 40 smaller than from the unfiltered ones we highlight the presence of statistically significant velocity gradients where the stations density is higher we find undulations of the vertical velocity field at different spatial scales both in tectonically active regions like eastern alps apennines and eastern mediterranean and in regions characterized by a low or negligible tectonic activity like central iberia and western alps a correlation between smooth vertical velocities and topographic features is apparent in many sectors of the study area glacial isostatic adjustment and weathering processes do not completely explain the measured rates and a combination of active tectonics and deep seated geodynamic processes must be invoked excluding areas where localized processes are likely or where subduction processes may be active mantle dynamics is the most likely process but regional mantle modeling is required for a better understanding

shallow meridional circulations in the tropical atmosphere abstract a shallow meridional circulation smc in the tropical atmosphere features a low level e g 700 hpa flow that is in the opposite direction to the boundary layer monsoon or trade wind flow and is distinct from the meridional flow above representations of the smc in three global reanalyses show both similarities and astonishing discrepancies while the smc over west africa appears to be the strongest it also exists over the eastern atlantic and eastern pacific oceans and over the indian subcontinent with different strength and structure all smcs undergo marked seasonal cycles the smcs are summarized into two types one associated with the marine itcz and the other with the summer monsoon the large scale conditions for these two types of smcs are similar a strong meridional gradient in surface pressure linked to surface temperature distributions and an absence of deep moist convection the processes responsible for these conditions are different for the two types of smcs as are their str

modeling deformation induced by seasonal variations of continental water in the himalaya region sensitivity to earth elastic structure strong seasonal variations of horizontal and vertical positions are observed on gps time series from stations located in nepal india and tibet china we show that this geodetic deformation can be explained by seasonal variations of continental water storage driven by the monsoon for this purpose we use satellite data from the gravity recovery and climate experiment to determine the time evolution of surface loading we compute the expected geodetic deformation assuming a perfectly elastic earth model we consider green s functions describing the surface deformation response to a point load for an elastic homogeneous half space model and for a layered nonrotating spherical earth model based on the preliminary reference earth model and a local seismic velocity model the amplitude and phase of the seasonal variation of the vertical and horizontal geodetic positions can be jointly adjusted only with the layered earth model while an elastic half space model fails emphasizing the importance of using a realistic earth elastic structure to model surface displacements induced by surface loading we demonstrate based on a formal inversion that the fit to the geodetic data can be improved by adjusting the layered earth model therefore the study also shows that the modeling of geodetic seasonal variations provides a way to probe the elastic structure of the earth even in the absence of direct measurements of surface load variations

properties of chemical remanence in synthetic hematite testing theoretical predictions we synthesized hematite under controlled conditions of the applied magnetic field in order to test the behavior of chemical remanent magnetization crm predicted on the basis of theories developed to explain thermoremanent magnetization trm we restrict this investigation to the remanence acquired during mineral crystallization grain growth crm and do not consider chemical remanence acquired during mineral alteration the material produced by our technique is hematite up to 10 m in diameter thermal demagnetization to 200 c removes the viscous component acquired under identical field temperature conditions to those prevalent during synthesis as predicted by analogy to trm the crm acquired parallels the orientation of the magnetic field applied during growth the intensity of chemical remanence is a linear function of the field in which it grew bcrm up to 7 5 mt the maximum growth field investigated furthermore the ratio of intensities of crm to trm acquired in the same field is independent of the strength of that field the intensity of bcrm controls the growth and orientation of hematite crystals a result not predicted from theory derived by analogy to trm the control on crystal growth by the field is reflected in the dependence on bcrm of the orientation of ellipsoids of anisotropy of isothermal remanence and the acquisition of isothermal and anhysteretic remanence

chemical remanent magnetization in synthetic magnetite as a magnetic grain produced by a chemical process below its blocking temperature grows through a critical volume in the presence of a magnetic field its moment becomes blocked and it acquires a chemical remanent magnetization crm despite its importance to paleomagnetism the properties of crm and the controls on its behavior are still poorly understood we have modified techniques developed by taylor et al 1987 to synthesize magnetite at room temperature and atmospheric pressure whereby magnetite is produced by alteration of green rust under reducing conditions the period of synthesis is twenty four hours and a viscous component acquired over this amount of time can be demagnetized with alternating fields of 10 mt the average direction of the crm parallels the direction of the applied field and the intensity increases with increasing intensity of the applied field the intensity is linearly related to the applied field up to approximately 1 mt consistent with existing crm theory developed by analogy to the acquisition of thermal remanent magnetization trm above about 1 mt however the crm intensity approaches saturation at a rate substantially lower than that predicted by the trm analog theory furthermore the ratio of the crm to an anhysteretic remanence acquired in the same field strongly depends on the intensity of the growth field analogy to trm predicts this ratio to be independent of growth field both the principal eigenvector of the anisotropy of isothermal remanence air matrix becomes more aligned with growth field and the percent anisotropy increases as the intensity of the growth field increases the above observations suggest that the model of crm acquisition based on analogy to trm acquisition is incomplete we therefore propose a revised model which incorporates the increased alignment of easy axes with growth field finally the alignment of magnetic anisotropy ellipsoids of our synthetic samples with the growth field suggests a means to distinguish crm from trm in natural remanence

the west african monsoon dynamics part i documentation of intraseasonal variability intraseasonal variability in the west african monsoon is documented by using daily gridded datasets of rainfall and convection and reanalyzed atmospheric fields over the period 1968 90 rainfall and convection over west africa are significantly modulated at two intraseasonal timescales 10 25 and 25 60 day leading to variations of more than 30 of the seasonal signal a composite analysis based on the dates of the maximum minimum of a regional rainfall index in wet dry sequences shows that these sequences last on average 9 days and belong to a main quasiperiodic signal of about 15 days a secondary periodicity of 38 days is present but leads to a weaker modulation during a wet dry sequence convection in the itcz is enhanced weakened and its northern boundary moves to the north south while the speed of the african easterly jet decreases increases the speed of the tropical easterly jet increases decreases and the monsoon flow becomes stronger weaker all these features being similar to the ones associated with interannual variability characterizing wet and dry years this modulation of convection at intraseasonal timescales is not limited to west africa but corresponds to a westward propagating signal from eastern africa to the western tropical atlantic an enhanced monsoon phase is associated with stronger cyclonic activity in the low levels over the sahel associated with stronger moisture advection over west africa five days before the full development of the wet phase a stronger cyclonic circulation at 20 e induces enhanced southerly winds along 25 e where convection enhances while another westwardpropagating cyclonic circulation is located at 0 this atmospheric pattern is linked to the enhancement of the subsiding branch of the northern hadley cell at 35 n northerly advection of drier air over west africa and to increased dry convection in the heat low at 20 n it propagates westward leading to a zonally extended area of enhanced monsoon winds over west africa consistent with the occurrence of the wet phase

a physical basis for the interannual variability of rainfall in the sahel a major factor in rainfall variability over sahelian west africa is the latitudinal location of the tropical rainbelt when it is displaced abnormally far northward the sahel experiences a wet year an anomalous southward displacement results in drought in this paper we examine the question of what controls the location during the boreal summer hypothesizing that inertial instability plays a role an analysis of surface pressure and temperature fields wind fields divergence and vertical motion show that the criteria for inertial instability are satisfied in wet augusts but not in dry ones the key determinant appears to be the surface pressure gradient between the continent and the equatorial atlantic when this is large inertial instability results in the development of a low level westerly jet the presence of this jet enhances the horizontal and vertical shear and displaces the african easterly jet northwestward associated with this situation is strong vertical motion over the sahel and subsidence over the guinea coast producing dry conditions over the latter the result is a rainfall dipole one of two major modes of variability over west africa important factors include sea surface temperatures ssts in the equatorial atlantic and pressure in the south atlantic the first of these factors suggests a link with the atlantic nino mode of tropical atlantic variability while the second suggests a possible link with the pacific and the extratropical south atlantic overall our study relates the well known sst influence on sahel rainfall to atmospheric dynamics over the continent copyright 2007 royal meteorological society

numerical simulations of global scale high resolution hydrological crustal deformations 1 high resolution load induced crustal deformations calculated from numerical models are tested for their ability to predict hydrologically induced station height variability as they are known to be large enough to affect epoch wise parameters obtained from the analysis of global geodetic networks loading contributions due to terrestrial water storage as given by global hydrological models are calculated on a 0 5 global regular grid with daily temporal resolution apart from the dominant seasonal variations the hydrological loading signal discloses also rapid changes exceeding 1 mm in several regions that can be associated with major precipitation events and river floods locally strong loading signals with exceptionally high amplitudes in many cases even with nonseasonal nature occur along the major river channels only high resolution loading calculations considering also the water mass anomalies stored in the model river flow can resolve the correct amplitudes in the surrounded area up to 100 km distance the comparison of the modeled hydrological surface deformation with gps station time series shows that high resolution hydrological loading estimates based on global scale models are able to explain a considerable fraction up to 54 of the observed vertical station movements caused by continental water storage variations

an improved methodology for thellier type paleointensity determination in igneous rocks and its usefulness for verifying primary thermoremanence we propose a new methodology for thellier type paleointensity experiments in standard paleointensity experiments the blocking temperatures of most stable remanences are often not reached before thermal alteration of magnetic mineralogy begins and subsequent data have to be discarded we emphasize that when alteration begins at low temperature and continues throughout the experiment which then fails in standard analyses the resulting unblocking temperature tub spectrum of the alteration product does not necessarily overlap with the whole tub spectrum of natural remanent magnetization nrm because tub depends on physical characteristics of the new grains not the temperatures at which they were formed nrm may survive in an uncontaminated higher tub window if alteration remanence only has low tub then partial thermoremanent magnetization ptrm checks at temperature step tj 1 can be used to correct for alteration that has occurred at tj and furthermore thermal demagnetization of a full trm acquired in the laboratory at the end of the progressive thellier thellier experiment should reveal the true uncontaminated tub spectrum of the nrm in higher temperature intervals we propose a new experimental sequence which allows monitoring and correction of alteration and provides two semi independent estimates of palaeointensity in the circumstances described above we illustrate the issues involved through new experimental work on igneous rocks from a paleozoic charnockitic syenite where we unravel trm from multidomain isothermal remanence despite massive alteration during laboratory heating finally we point out the value of these experiments for verifying primary trm in igneous rocks indicating properties that distinguish trm from other remanence

variability of the intertropical front itf and rainfall over the west african sudan sahel zone abstract the intertropical front itf is a fundamental feature of the atmospheric circulation over west africa it separates the wedge of warm moist southwesterly monsoon flow off the tropical atlantic from much hotter and very dry northeasterly wind from the sahara desert here the daily temperature humidity and rainfall data for 1974 2003 are analyzed to document the northward advance and southward retreat of the itf between boreal spring and autumn and assess its role in sudan sahel 10 20 n rainfall patterns using largely dekadal 10 day and monthly resolutions analyses are performed for the 30 yr average seasonal time scale and sets of extreme years with a major focus on concurrent monthly itf rainfall relations the seasonal rainfall predictive potential of the early season itf latitude is also investigated as is the secular variation of itf latitude weather system rainfall associations during 1974 2003 the northward advance of the itf across the sudan sahel from april to early august i

further studies of carbonate remagnetization in the northern appalachian basin paleomagnetic and rock magnetic studies of sedimentary carbonates in the northern appalachian basin demonstrate that the late paleozoic remagnetization in this area is probably related to authigenesis of single domain magnetite fifteen sites within middle devonian carbonates were sampled along a transect extending from albany new york to the algonquin arch in southeastern ontario paleomagnetic studies show that all sites except the two westernmost sites on the algonquin arch were remagnetized during alleghenian time the very weakly magnetic unremagnetized sites retain a dual polarity magnetization of probable devonian age in the remagnetized sites both the size of the kiaman components and magnetite concentration estimated from low field susceptibility x are highest in the central part of the transect and lower on either side x ray powder diffraction analyses demonstrate that k feldspar is also most abundant in the middle of the transect with lower concentrations to the west and east we suggest that formation of authigenic magnetite and k feldspar may be related in this setting sites with the largest kiaman components have the highest unblocking temperatures and the largest ratios of anhysteretic remanent magnetization arm to bulk susceptibility arm x indicating that these sites have the greatest amounts of single domain magnetite several remagnetized sites exhibit northerly shallow magnetic components which may be kiaman normal directions we conclude that remagnetization along the new york ontario transect is a chemical remanent magnetization resulting primarily from authigenesis of single domain magnetite we present a model in which pulses of heated fluids triggered magnetite formation scanning electron microscope observation of magnetic extracts from 14 sites show that most of the resolvable magnetite grains are multidomain grains of irregular shape many sites also contain a small percentage of magnetic spheres and or euhedral magnetite particles a majority of the magnetite grains contain fe as the only cation but many also contain minor amounts of other cations including various combinations of cr ni ti zn mn si al ca and mg single domain magnetite is not resolvable with our observational techniques but its presence is inferred from rock magnetic tests much of this magnetite may be of authigenic origin

mesoscale convective systems in relation to african and tropical easterly jets abstractthis paper documents the interaction processes between mesoscale convective systems mcs the tropical easterly jet tej and the african easterly jet aej over west africa during the monsoon peak of 2006 observed during the african monsoon multidisciplinary analyses amma project the results highlight the importance of the cloud system localization relative to the jets in order to explain their duration and life cycle a systematical study reveals that intense and long lived mcss correspond to a particular pattern where clouds associated with deep convection are located in entrance regions of tej and in exit regions of aej a case study on a particularly well documented convective event characterizes this link and infers the importance of jet streaks in promoting areas of divergence favoring the persistence of mcss

atmospheric pressure loading corrections applied to gps data at the observation level 1 space geodetic techniques can detect elastic deformation of the earth caused by atmospheric pressure loading atml however it has not yet been demonstrated whether these surface displacements should be accounted for at the time of reduction of the observations or by applying time averaged values to the coordinates after the analysis of the observations an analysis of the power spectral density of the atml predicted vertical deformation shows that aside from the diurnal and semi diurnal periods there is very little power in the sub daily frequencies the present tidal atml models improve the analysis at sites near the equator but seem to degrade the height estimates elsewhere the majority of the non tidal deformation can be modelled by applying a daily averaged correction to daily estimates of coordinates but a greater improvement in height rms is found if non tidal atml is applied at the observation level

coseismic fault slip of the 2008 mw 7 9 wenchuan earthquake estimated from insar and gps measurements 1 we infer co seismic fault slip during the 2008 mw 7 9 wenchuan earthquake from interferometric synthetic aperture radar insar and gps observations of ground deformation we use alos palsar data from ascending orbits on six tracks and we do not use data that are strongly affected by ionospheric perturbations we use a fault model composed of three planar fault segments of the beichuan fault and one planar segment representing the parallel pengguan fault maximum thrust slip is up to 6 7 m near the surface and occurs in two locations near yingxiu in the south and beichuan in the center of the rupture maximum strike slip is over 4 m and occurs near pingtong and nanba along the northern end of the rupture we find that the ratio of coseismic thrust to strike slip on the beichuan fault decreases from 1 5 to 0 7 from the sw to the ne citation feng g e a hetland x ding z li and l zhang 2010 coseismic fault slip of the 2008 mw 7 9 wenchuan earthquake estimated from insar and gps measurements geophys res lett 37 l01302 doi 10 1029 2009gl041213

recognizing magnetostratigraphy in overprinted and altered marine sediments challenges and solutions from iodp site u1437 core disturbance drilling overprints post depositional acquisition of remanence authigenic growth of magnetic iron sulfides and alteration all contribute challenges to recognizing the primary magnetostratigraphy in marine sediments we address these issues in a sequence of tuffaceous muds and volcaniclastics at international ocean discovery program site u1437 and produce the longest continuous magnetic polarity stratigraphy in the history of scientific ocean drilling remanence measurements were filtered to remove intervals affected by fluidization plastic sediment disturbance and core biscuiting drilling overprints are concentrated in the disturbed annulus surrounding intact core material bioturbation was limited to a vertical extent of at most 15 cm changes in sediment color stiffness and magnetic hysteresis all suggest that remanence was locked in within a few meters of the sediment water interface we did not observe any systematic offset between magnetostratigraphic and biostratigraphic datums authigenic growth of greigite in response to both initial sulfate reduction in the upper 50 m of the sediment column and to deeper resupply of sulfate has led to magnetic overprinting anomalous polarity artefacts extending 5 m and occurring within about 20 m below a real polarity transition appear to be due to a chemical remanence acquired by greigite produced during early diagenesis diagenetic magnetic mineral alteration resulted in the progressive loss of fine grained magnetite which enhanced susceptibility to drilling and post drilling overprints and increased the resistance of these overprints to removal by conventional demagnetization we recovered the magnetostratigraphic record from many samples with resistant overprints through low temperature demagnetization through the verwey transition this article is protected by copyright all rights reserved

the joint modes of the coupled atmosphere ocean system observed from 1967 to 1986 two aspects of the principal oscillation pattern pop analysis are used to study the large scale modes of the coupled atmosphere ocean system first pops can be considered as the normal modes of the system one way of studying these normal modes is to estimate them from data second a pop analysis can be viewed as a multivariate spectral analysis and the spectral characteristics of the modes are by products of the pop analysis both aspects are studied using a combined dataset that includes both atmospheric sea level pressure 700 mb and 200 mb zonal winds and oceanic sea surface temperature pacific sea level and pacific subsurface temperature parameters six joint modes of the coupled atmosphere ocean system are found in this study for modes with small eigenvalues the atmosphere plays an important role the associated oceanic anomalies appear to be generated by the anomalous atmospheric conditions for the other modes which have most of their power on much longer time scales the ocean is more actively involved modes 4 and 5 describe decadal time scale variations mode 4 is characterized by changes in sst in all three tropical oceans and in organized convection over the west pacific the results allow us to speculate that thesemore tropical features might excite changes in the extratropical tropospheric and oceanic circulations mode 5 shows global scale sst anomalies and large atmospheric anomalies in the southern hemispheric circulation mode 6 is the only oscillatory normal mode found in the coupled atmosphere ocean system it describes the quasi cyclic behavior of the el nino southern oscillation phenomenon 24 refs 18 figs 2 tabs less

new constraints on the magnetic history of the cv parent body and the solar nebula from the kaba meteorite abstract recent paleomagnetic studies of allende cv chondrite as well as thermal modeling suggest the existence of partially differentiated asteroids with outer unmelted and variably metamorphosed crusts overlying differentiated interiors to further constrain the magnetic history of the cv parent body we report here paleomagnetic results on kaba cv chondrite this meteorite contains 11 wt pseudo single domain magnetite making it a rock with an excellent paleomagnetic recording capacity kaba appears to carry a stable natural remanent magnetization acquired on its parent body upon cooling in an internally generated magnetic field of about 3 t from temperatures below 150 c during thermal metamorphism about 10 to several tens of myr after solar system formation this strengthens the case for the existence of a molten advecting core in the cv parent body furthermore we show that no significant magnetic field i e lower than 0 3 t was present when aqueous alteration took place on the kaba parent body around 4 to 6 myr after solar system formation suggesting a delay in the onset of the dynamo in the cv parent body and confirming that nebular fields had already decayed at that time

interdecadal variation of the west african summer monsoon during 1979 2010 and associated variability this paper addresses the interdecadal variation of the west african summer monsoon wasm along with its background of atmospheric circulation and possible physical mechanism over the past 32 years 1979 2010 it is indicated that the wasm starts to strengthen from 1998 as the rainfall begins to increase over western west africa on the whole which shows a new interdecadal variation in this interdecadal variation the strengthened ascending motion corresponding to enhanced divergence convergence movement on the upper lower troposphere is prone to develop the local circulation of the monsoon moreover the strengthened southwestern eastern wind on the lower upper level leads to more moisture from the atlantic and the gulf of guinea transported to the west african continent in addition the summer subtropical high over the north atlantic and western west africa is strong and northward and the tropical east wind is also strong statistically the weaker stronger the spring north atlantic oscillation nao is the stronger weaker the tropical easterly is and then the wasm is also stronger but the effect of the nao on the decadal variation of the wasm is not so significant from the north atlantic anomaly sensitivity simulation with a single model this is also an indication that the relationship between the wasm and nao is complicated in an interdecadal time scale and is needed further study in terms of sea surface temperature sst variation the tendency is toward warming in the subtropical north pacific the south pacific and north atlantic numerical simulation experiments and data analysis show that the sst variation in the north pacific plays an important role in the latest interdecadal strengthening of the wasm during the past 32 years while the influences of the south pacific and the north atlantic sst anomalies are not so significant to the associated atmospheric circulation changes

contributions of thermal expansion of monuments and nearby bedrock to observed gps height changes 1 long term continuous global positioning system gps observations have become an important tool for studying the various geodynamic processes to fully study the geodynamic processes at gps stations the temporal movements of gps monuments and nearby bedrock induced by thermal expansion need to be considered in this paper we extend a theoretical model to estimate the thermal expansions of gps monuments and nearby bedrock for 86 globally distributed gps stations based upon measurements of surface air temperatures the results show that annual temperature variations are the dominant contributors for the thermal expansion of gps monuments and nearby bedrock the contributions of thermal expansion to gps height changes display largely spatial variations and can reach to a few millimeters citation yan h w chen y zhu w zhang and m zhong 2009 contributions of thermal expansion of monuments and nearby bedrock to observed gps height changes geophys res lett 36 l13301 doi 10 1029 2009gl038152

geodetic secular velocity errors due to interannual surface loading deformation geodetic vertical velocities derived from data as short as 3 yr are often assumed to be representative of linear deformation over past decades to millennia we use two decades of surface loading deformation predictions due to variations of atmospheric oceanic and continental water mass to assess the effect on secular velocities estimated from short time series the interannual deformation is time correlated at most locations over the globe with the level of correlation depending mostly on the chosen continental water model using the most conservative loading model and 5 yr long time series we found median vertical velocity errors of 0 5 mm yr 1 over the continents 0 3 mm yr 1 globally exceeding 1 mm yr 1 in regions around the southern tropic horizontal velocity errors were seven times smaller unless an accurate loading model is available a decade of continuous data is required in these regions to mitigate the impact of the interannual loading deformation on secular velocities

magnetization of the oceanic crust beneath the labrador sea the aeromagnetic anomalies of the oceanic crust northeast of newfoundland canada are strongly skewed with an anomalous skewness of 33 52 with respect to the anomalies calculated using the standard sea floor spreading model this paper examines the possibility that the anomalous skewness is on account of an oceanic crust with a thermo viscous remanent magnetization where unlike the standard model the lower crust is magnetic the thermo viscous remanent magnetization of the lower crust beneath the labrador sea has dominant contributions to broad scale marine magnetic anomalies and to the negative magsat anomaly of the sea

palaeomagnetism of red beds of the late devonian worange point formation se australia summary gently folded strata of the late devonian merrimbula group along the south coast of new south wales are similar to numerous deposits of late devonian to early carboniferous subaerial to shallow marine quartzose sandstone that are known as the lambie facies of se australia because the lambie sands overlap the early palaeozoic tectonic elements of the lachlan fold belt large displacement since the late devonian of any lachlan terranes with respect to interior australia is precluded we collected oriented core samples from 37 sites primarily in reddish quartzose litharenites of the worange point formation the remanent magnetization is carried by haematite incremental thermal demagnetization reveals a dominant well defined steep upward north component of magnetization that post dates the mid carboniferous folding the south pole position 146 4 e 68 6 s ag5 3 1 derived from the overprinted specimens is close to both the late carboniferous and mid cretaceous reference poles as well as the spin axis of today the overprint is attributed to both viscous partial thermoremanent and chemical remanent magnetization vptrm and crm its exclusively normal polarity is consistent with a mid cretaceous acquisition perhaps related to the rifting of the se australian margin a characteristic component of magnetization is isolated between 660 and 680 c in 30 per cent of the samples although a regional fold test of the overall result is inconclusive because the directions of characteristic magnetization are generally close to the axes of folding two sets of samples from opposite limbs of a local syncline pass a fold test at 90 per cent confidence indicating a pre folding origin of the remanence the presence of both polarities in thin horizons some of which are palaeosols and diagenetic haematite associated with incipient cleavage formed in an early stage of deformation before folding indicates that the acquisition of remanent magnetization occurred over a protracted period an analysis that combines great circle demagnetization paths and set points facilitates the incorporation of more data but gives a biassed result although normal and reverse polarity subsets are roughly antipodal residual post folding components cause the normal subset to fail a fold test a south pole position 19 7 e7 70 8 s ag5 7 1 is derived from the specimens with characteristic component of reverse polarity in which the isolation of a characteristic component is clear the pole position is close to that of a previous palaeomagnetic study from a widely separated occurrence of lambie facies sediments both results are applicable not only to the lachlan fold belt but also to the rest of australia and gondwana as a whole the position of the two poles derived from the lambie facies overlap assemblage in the lachlan fold belt supports the interpretation that the progression

the occurrence and origin of late paleozoic remagnetization in the sedimentary rocks of north america although it has been known for 25 years that some paleozoic sedimentary rock units in europe and north america were remagnetized during pennsylvanian or permian time it is only very recently that the complex and widespread nature of the late paleozoic remagnetization phenomenon has been generally acknowledged it is now recognized that many paleozoic paleomagnetic poles for north america that were once considered reliable are in fact the result of remagnetization and as a consequence the paleomagnetic data base for the paleozoic is undergoing rapid and drastic revision the causes of late paleozoic remagnetization in north america are currently the focus of much interest and active research the remagnetization can reside in either hematite or magnetite and different remagnetization mechanisms have been important in different settings chemical remagnetization processes some related to specific diagenetic events are dominant in hematite bearing sandstones and carbonates in magnetite bearing carbonates both chemical and thermoviscous remagnetization processes appear to have been important but it is difficult to determine which process is the dominant one in some settings some of the observed remagnetizations can be linked to the migration of chemically active and perhaps hot fluids during the mountain building events that affected much of the continent during the late paleozoic paleomagnetic studies promise to be important in assessing the role of orogeny in driving fluid migrations within sedimentary basins and in constraining the age of the migrations and the nature of the fluids

an early solar system magnetic field recorded in cm chondrites abstract we present a paleomagnetic study of seven cm carbonaceous chondrites cm chondrites are believed to be some of the most chemically primitive materials available in our solar system and may sample the continuum of transitional objects between asteroids and comets formed in the outer solar system as such cm chondrites can help us to understand primordial aspects of the history of the early solar system including protoplanetary disk and planetesimal magnetism the ferromagnetic assemblage of cm chondrites is composed of a mixture of primary metallic iron pyrrhotite and magnetite the remanent properties are usually dominated by secondary pyrrhotite paleomagnetic analyses using thermal and alternating field demagnetization identified a stable origin trending component of magnetization in the seven studied cm chondrites in each meteorite this component is homogeneous in direction at least at the cm scale and is therefore post accretional we interpret this stable component as a pre terrestrial chemical remanent magnetization acquired during crystallization of magnetite and pyrrhotite during parent body aqueous alteration in a field of at least a few t 2 1 5 t considering the timescale and intensities of primordial magnetic fields both internally generated fields from a putative dynamo and external fields generated in the protoplanetary disk may have been recorded by cm chondrites it is presently difficult to discriminate between the two hypotheses regardless cm chondrites likely contain the oldest paleomagnetic record yet identified

limestones of western newfoundland that magnetized before devonian folding but after middle ordovician lithification a positive fold test and a negative conglomerate test help determine when and how stable remanence was acquired in the middle ordovician table head group limestones of the port au port peninsula of newfoundland the limestones magnetized after lithification and incorporation as clasts into a middle ordovician breccia hence the limestones do not carry a detrital or other primary remanence despite their very low conodont colour alteration index of 1 the remanence may be thermoviscous or diagenetic and was acquired before devonian folding this suggests the need for caution in interpreting paleomagnetic results from other early paleozoic limestones whose remanence resides in magnetite of blocking temperature lower than 400 c

interdecadal change in the tropical pacific precipitation anomaly pattern around the late 1990s during boreal spring abstractthe leading mode of boreal spring precipitation variability over the tropical pacific experienced a pronounced interdecadal change around the late 1990s the pattern before 1998 features positive precipitation anomalies over the equatorial eastern pacific ep with positive principle component years the counterpart after 1998 exhibits a westward shift of the positive center to the equatorial central pacific cp observational evidence shows that this interdecadal change in the leading mode of precipitation variability is closely associated with a distinctive sea surface temperature sst anomaly pattern the westward shift of the anomalous precipitation center after 1998 is in tandem with a similar shift of maximum warming from the ep to cp diagnostic analyses based on a linear equation of the mixed layer temperature anomaly exhibit that an interdecadal enhancement of zonal advection za feedback process plays a vital role in the shift in the leading mode of both the tropical pacific sst and t

decadal variations of the relationship between the summer north atlantic oscillation and middle east asian air temperature 1 the relationship between the summer july september north atlantic oscillation snao and simultaneous east asian air temperature is investigated the results show that the snao is related to middle east asian summer air temperature however this linkage varies with time on decadal timescale a strong connection appears after the late 1970s but a weak connection before the late 1970s further analysis indicates that this instable relationship may have resulted from the shift of the snao mode around the late 1970s in the period of 1979 2003 the centers of the snao mode are located more eastward a positive phase negative phase snao produces a strong lower level divergence convergence over the asian jet entrance region in turn stimulates a strong upper level convergence divergence via the ekman pumping such a convergence divergence then excites a zonally oriented quasi stationary barotropical rossby wave train along the asian upper level jet thus the snao signal is transported eastward to east asia resulting in an anomalous summer air temperature over middle east asia however in the period of 1951 1975 the centers of the snao mode are located more westward the associated upper level divergence convergence is away from the asian jet entrance region and the snao signal cannot be transported eastward to east asia hence the connection is broken

thermoviscous remanent magnetization of oceanic lithosphere inferred from its thermal evolution remanent magnetization patterns of the oceanic lithosphere determined on the basis of the thermal evolution of the lithosphere while taking into account both thermal and viscous magnetizations give rise to magnetic anomalies similar to those observed by magsat over the cretaceous quiet zones of the atlantic ocean and by marine magnetic surveys over anomaly 33 viscous magnetization acquired during a given core field polarity period is not removed completely by that acquired in the next period of opposite polarity because of cooling of the lithosphere and the strong temperature dependence of viscous magnetization the magnetization pattern of layer 2a resembles that of the standard seafloor spreading magnetic model consisting of normally and reversely magnetized zones separated by vertical boundaries and produces symmetric magnetic anomalies the magnetization pattern of the middle crust is similar to that of layer 2a for high magnetic blocking temperature models but it is slightly skewed with respect to layer 2a for low magnetic blocking temperature models the magnetization pattern of the lower crust is strongly skewed relative to that of layer 2a and has diffuse boundaries with pronounced slopes between normally and reversely magnetized zones it produces magnetic anomalies with appreciable skewness suggesting that observed skewness of marine magnetic anomaly 33 arises from the magnetization of this layer the magnetization pattern of the upper mantle is appreciably skewed and almost devoid of small scale variations it produces strongly skewed magnetic anomalies at satellite altitudes which may explain the observed skewness of magsat anomalies over the cretaceous quiet zones of the atlantic ocean

paleomagnetism and paleothermometry of the sydney basin 2 origin of anomalously high unblocking temperatures the milton monzonite of southeastern australia was thermoviscously remagnetized as a result of cretaceous burial and uplift thermal demagnetization separates the low unblocking temperature lt overprint from the high unblocking temperature ht primary remanence with a relatively sharp junction between lt and ht components in vector projections for single domain grains the junction temperature tl between two such vectors corresponds to the maximum blocking temperature tr reactivated in nature apart from a correction for the difference between natural and laboratory timescales however measured tl values are distributed over an implausibly wide range 250 c for burial remagnetization of an untilted intrusion like the milton monzonite furthermore many tl values are anomalously high compared to the predictions of single domain theory multidomain grains are the cause of these anomalies samples pretreated before thermal demagnetization by zero field cycling to liquid nitrogen temperature so as to erase multidomain remanence and isolate single domain remanence do have the theoretically expected tl values in these samples realistic remagnetization time and temperature tr tr conditions in nature are predicted using the t t contours of pullaiah et al 1975 the anomalously high tl values before low temperature treatment are due to multidomain grains which carry 50 of the lt overprint the lt thermal demagnetization curve in samples dominated by multidomain grains is quasi exponential in shape with a high temperature tail extending almost to the curie point as predicted by multidomain theory these high lt unblocking temperatures which are much greater than plausible remagnetization temperatures reached in nature overlap and mask the lower part of the ht unblocking temperature spectrum driving up tl values and leading to inflated estimates of tr although multidomain remanence is a sufficient explanation of anomalously high unblocking temperatures of thermoviscous overprints in the milton monzonite chemical overprinting may be a factor in other lithologies and tectonic settings

the demagnetization temperature necessary to remove viscous remanent magnetization recent experimental studies have shown that the temperature necessary to demagnetize viscous remanent magnetization vrm in magnetite bearing rocks is significantly higher than predicted by standard neel single domain theory the walton time temperature equation which has been invoked by some to explain the enhanced thermal stability actually relates intensities of vrms acquired at different temperatures rather than describing the unblocking of a previously acquired vrm we show that the low field approximation lfa used in all previous formulations which predicts equal vrm acquisition and decay rates is not valid for a magnetite assemblage in the earth s field a medium field approximation mfa predicts significantly faster vrm acquisition and therefore greater thermal stability in decay for the larger particles in the assemblage conventional theory does apply to red bed acquisition decay experiments since the lfa is valid for hematite

magnetic remanence in the chalk of eastern england an unusually resistant vrm a single component natural remanent magnetization nrm is carried largely by pseudosingle domain magnetite in the cretaceous lower chalk and red chalk of eastern england the red chalk also records the same direction in haematite most of the ferro magnetic minerals occur as primary clastic or early diagenetic grains a stable remanence component is resistant to demagnetization and is carried by both magnetite and haematite nevertheless it has a steep inclination close to the present earth s field and it is too steep for the previously reported palaeolatitude of these rocks at the time of sedimentation a postglacial slump breccia scatters the chrm but also provides some evidence of viscous partial magnetic overprinting during slumping despite its resistance to thermal and alternating field demagnetization the characteristic remanent magnetization chrm is probably a young bruhnes epoch viscous remanent remagnetization vrm

modern and last glacial maximum eolian sedimentation patterns in the atlantic ocean interpreted from sediment iron oxide content eolian dust derived from the desert regions of north africa is blown far into the tropical atlantic ocean by persistent easterly and northeasterly winds in this paper we demonstrate that the iron oxides hematite and goethite are a worthwhile addition to proxy monitors of eolian sedimentation in the tropical atlantic iron oxides are identified by diffuse reflectance spectrophotometry a technique capable of identifying these minerals in concentrations as low as 0 01 by weight we analyze samples from both the modern and last glacial maximum lgm synoptic levels from 178 sample locations yielding a total of 356 samples distributed throughout the atlantic ocean to determine the relative contribution of the iron oxides we factor analyzed the modern and lgm levels as a single data set the iron oxide factor explains about 25 of the variance in the combined core top and lgm data set mapped factor scores for the lgm and modern ocean indicate high iron oxide values are present in just two regions one off eastern north america and the other off northwest africa in the region off eastern north america iron oxide occurs primarily during the lgm as the previously noted brick red lutite a unique sediment type derived from the erosion of permo carboniferous red beds in atlantic canada a larger lobe shaped area of iron oxide rich sediment is present off northwest africa in both the modern and lgm levels the modern iron oxide lobe is coincident with the distribution of eolian dust as determined by observations from ships satellites and analysis of air samples during the lgm iron oxides exhibit a similar distribution except the southern margin of the region shifts equatorward and iron oxide concentration increases

identification of processes driving low level westerlies in west equatorial africa abstractthis paper investigates and characterizes the control mechanisms of the low level circulation over west equatorial africa wea using four reanalysis datasets emphasis is placed on the contribution of the divergent and rotational circulation to the total flow additional focus is made on analyzing the zonal wind component in order to gain insight into the processes that control the variability of the low level westerlies llw in the region the results suggest that the control mechanisms differ north and south of 6 n in the north the llw are primarily a rotational flow forming part of the cyclonic circulation driven primarily by the heat low of the west african monsoon system this northern branch of the llw is well developed from june to august and disappears in december february south of 6 n the seasonal variability of the llw is controlled by the heating contrast between cooling associated with subsidence over the ocean and heating over land regions largely south of the equator where as

anomalous unblocking temperatures viscosity and frequency dependent susceptibility in the chemically remagnetized trenton limestone abstract chemical remagnetization of many carbonate rock units occurred by diagenetic alteration of iron sulfide precursors and by authigenesis of fine grained magnetite which can commonly be recognized by a distinctive set of rock magnetic attributes these carbonates also typically exhibit anomalously high unblocking temperatures for recent thermoviscous components that have overprinted the characteristic chemical remanence recent work has shown that departures from neel theory as applied in the model of pullaiah et al can generally be attributed to vrm carried by multidomain particles to which the theory does not apply in the remagnetized carbonates however rock magnetic data all point to an overwhelming dominance of fine superparamagnetic and stable single domain particles therefore we explore possible alternative causes for the anomalous unblocking temperatures including maghemitization and magnetocrystalline rather than shape anisotropy new blocking unblocking models for these cases however fail to match the observed behavior leading to the conclusion that the anomalous unblocking temperatures are indeed probably attributable to multidomain carriers a possible alternative explanation is an increase in coercivity due to maghemitization of single domain particles after acquisition of the viscous overprint causing a significant increase in their unblocking temperatures new frequency dependent susceptibility measurements at low temperatures allow us to focus on the time temperature behavior of the particles at the superparamagnetic stable single domain threshold and we find that these do closely follow the predictions of neel theory as formulated in the models of pullaiah et al and walton middleton schmidt

new gps constraints on active deformation along the africa iberia plate boundary abstract we use velocities from 65 continuous stations and 31 survey mode gps sites as well as kinematic modeling to investigate present day deformation along the africa iberia plate boundary zone in the western mediterranean region the gps velocity field shows southwestward motion of the central part of the rif mountains in northern morocco with respect to africa varying between 3 5 and 4 0 mm yr consistent with prior published results stations in the southwestern part of the betic mountains of southern spain move west southwest with respect to eurasia 2 3 mm yr the western component of betics motion is consistent with partial transfer of nubia eurasia plate motion into the southern betics the southward component of betics motion with respect to iberia is kinematically consistent with south to southwest motion of the rif mountains with respect to africa we use block modeling constrained by mapped surface faults and seismicity to estimate the geometry and rates of strain accumulation on plate boundary structures our preferred plate boundary geometry includes one block between iberia and africa including the sw betics alboran sea and central rif this geometry provides a good fit to the observed motions suggesting a wide transpressive boundary in the westernmost mediterranean with deformation mainly accommodated by the gloria azores fault system to the west and the rif tell lineament to the east block boundaries encompass aspects of earlier interpretations suggesting three main deformation styles i extension along the ne sw trending trans alboran shear zone ii dextral strike slip in the betics corresponding to a well defined e w seismic lineament and iii right lateral strike slip motion extending west to the azores and right lateral motion with compression extending east along the algerian tell we interpret differential motion in the rif alboran betic system to be driven both by surface processes related the africa eurasia oblique convergence and sub crustal dynamic processes associated with the long history of subduction of the neotethys ocean lithosphere the dextral slip identified in the betic mountains in southern spain may be related to the offshore fault that produced the great 1755 lisbon earthquake and as such may represent a significant seismic hazard for the west mediterranean region

paleothermometry of the sydney basin evidence from overprinting of magnetizations of late permian and mesozoic rocks and from the rank of permian coals and mesozoic phytoclasts coal particles suggests that surface rocks in the sydney basin eastern australia have been raised to temperatures of the order of 200 sup 0 c or higher as vitrinite reflectance an index of coal rank or coalification is postulated to vary predictably with temperature and time estimates of the paleotemperatures in the sydney basin based on observed vitrinite reflectance measurements can be made in conjunction with reasonable assumptions about the tectonic and thermal histories of the basin these estimates give maximum paleotemperatures of present day surface rocks in the range 60 249 sup 0 c depending on factors such as location in the basin the thickness of the sediment eroded and the maximum paleogeothermal gradient higher coal rank and consequently larger eroded thicknesses and paleogeothermal gradients occur along the eastern edge of the basin and may be related to seafloor spreading in the tasman sea on the basin s eastern margin a theory of thermal activation of magnetization entailing the dependence of magnetic viscosity on the size distribution of the magnetic grains is used to obtain an independent estimate of the maximum paleotemperaturesmore in the sydney basin this estimate places the maximum paleotemperature in the range 250 300 sup 0 c along the coastal region both coalification and thermal activation of magnetization models provide strong evidence of elevated paleotemperatures which in places exceed 200 sup 0 c and the loss of sediment thicknesses in excess of 1 km due to erosion less

thermal demagnetization of partial thermoremanent magnetization three well characterized glass ceramic samples containing magnetite crystals of different domain states have been given partial thermoremanent magnetizations ptrms in the temperature interval 400 350 c for varying lengths of time 10 min to 5 days and in magnetic fields from 0 5 to 5 oe 0 05 0 5 mt the thermal demagnetization behavior of the ptrms changes with domain state in the sense that with decreasing coercivity increasing grain size demagnetization curves become less steep and may not completely demagnetize until heated to the curie temperature numerical calculations based on neel s single domain sd theory invoking field dependence and experimentally determined as well as theoretical volume distributions based on power laws have been performed to predict viscous magnetization acquisition and demagnetization temperatures we observe that experimental results do not match any of the theoretical predictions there is no discernible field dependence of demagnetization curves for the sd and pseudo single domain samples and even for the soft multidomain md sample in the field range investigated an important consequence for thermally derived paleointensity data is that they may not be entirely reliable especially if the sample remanence is due to md grains

redbeds and thermoviscous magnetization theory thermal demagnetization character istics of a brunhes age viscous overprint in appalachian redbeds and a thermoviscous component acquired in the laboratory at moderate tempera ture agree well with relaxation time blocking temperature relations for hematite proposed by pullaiah et al 1975 supporting evidence was obtained from redbeds associated with an igneous dike intrusion although the interpretation of these data are complicated by magnetochemical alterations paradoxically experimental data for magnetite in some limestones were shown to agree more closely with an alternative theory of walton 1980 the hematite in redbeds is predominantly singledomain sd whereas magnetite in the lime stones studied extends well into the multidomain md range thus experiment and theory could be reconciled if it is assumed that pullaiah eta applies strictly to sd material whereas waltoh s theory somehow describes thermoviscous effects dominated by larger md grains

changing dynamical control of early asian summer monsoon in the mid 1990s the seasonal evolution of the upper tropospheric south asian high follows and influences underlying summer monsoon advancement a strong connection between the south asian high and westerly perturbation to the north suggests further planetary scale dynamical control of the monsoon in the mid 1990s a clear location shift of the south asian high in may june was noted and was observed in fewer more frequencies of the high centers over the indochina peninsula iranian plateau continental confinement of monsoonal circulation and precipitation was observed during 1995 2010 as opposed to larger scale development in the asia pacific region during 1979 1994 in view of early summer monsoon evolution a westward shifting and faster migration of the south asian high may imply increased control of the midlatitude dynamics by contrast the convection over the tropical western north pacific wnp has an opposite and delayed contribution to monsoon advancement after the mid 1990s than it had been previously the midlatitude jet stream largely weakened over northern africa and the east asia pacific region corresponding to an increase in the upper tropospheric geopotential heights north of the jet stream climate model experiments further reveal that the warming over europe asia and temperature change in the north atlantic can result in the change in midlatitude perturbations and the monsoon evolution in the mid 1990s suggesting large scale and dynamic impact on monsoon climatology

west african monsoon dynamics and eastern equatorial atlantic and pacific sst anomalies 1970 88 abstract the laboratoire de meteorologie dynamique atmospheric gcm is used to investigate relationships between west african monsoon dynamics and sst anomalies in the eastern equatorial atlantic and pacific for the period 1970 88 positive sst anomalies in the eastern equatorial pacific mainly associated with a larger east west divergent circulation over the tropical atlantic are found to coincide with negative rainfall anomalies over west africa this is the case for the composite enso warm episodes of 1972 1976 1982 and 1983 by contrast positive sst anomalies in the eastern equatorial atlantic are accompanied by a southward shift of the intertropical convergence zone along with negative rainfall anomalies in the sahel and positive rainfall anomalies in the guinean region this was the case in 1987 the enso warm event during this year had apparently no significant impact on west african monsoon dynamics a zonal atmospheric coupling associated with differences of sst anomalies between the eastern

global equatorial variability of 850 and 200 hpa zonal winds from rawinsondes between 1963 and 1989 the longitude height time variability of 3 month averaged zonal wind anomalies at 850 and 200 hpa over the equatorial area 5 degrees n 5 degrees s is analyzed using a three dimensional dataset constructed from rawinsonde data 1963 1989 the first mode closely related to the southern oscillation index suggests a strong vertical coupling associated with a horizontal out of phase pattern between the central western pacific and the remainder of the equatorial belt the vertical coupling appears to be phase locked to the annual cycle with strongest intensities found over south america and near the maritime continent early in the calendar year and over the pacific basin and africa during the second half of the year this mode of variability can be viewed as a standing pattern superimposed with an eastward migrating component coherent with the annual cycle this westerly moving mode originates near the maritime continent during the northern autumn and tends to precede el nino southern oscillation events in the central and eastern pacific ocean variance is mainly concentrated in the 3 8 year low frequency time scale throughout the tropics and in the 2 3 year quasi biennal band from 110 degrees 120 degrees e to 180 degrees 24 refs 4 figs

paleomagnetism and rock magnetism of quaternary volcanic rocks and late paleozoic strata vc 1 core hole valles caldera new mexico with emphasis on remagnetization of late paleozoic strata paleomagnetic and rock magnetic data obtained from azimuthally unoriented core samples collected at approximately 1 to 3 m intervals of continental scientific drilling program core hole vc 1 have prompted reinterpretations of the quaternary volcanic stratigraphy intersected by the bore and have aided in evaluating the thermal regime within late paleozoic strata attending fluid circulation and mineral deposition during and after development of the toledo and valles calderas the results from quaternary units banco bonito obsidian i 35 4 a95 2 8 inclination only determinations n 33 battleship rock tuff d 359 6 i 42 4 a95 2 8 n 5 site means surface sites vc 1 rhyolite i 39 2 a95 12 8 n 7 upper vc 1 tuff i 37 2 a95 10 7 n 13 middle vc 1 tuff i 42 1 a95 2 1 n 39 south mountain rhyolite d 350 9 i 49 9 a95 3 4 n 10 one surface site are consistent with isotopic age data indicating that the entire moat volcanic sequence intersected is less than 650 kyr monitoring of natural remanent magnetization nrm intensity nrm directions directions of magnetizations isolated during progressive demagnetization median destructive forces and rock magnetization parameters has identified systematic variations within the thick banco bonito obsidian and vc 1 tuff units the permian abo formation pennsylvanian to earliest permian madera limestone and pennsylvanian sandia formation typically contain a moderate positive inclination magnetization component abo formation i 52 2 a95 7 4 n 16 madera limestone i 58 4 a95 2 8 n 105 sandia formation i 53 9 a95 4 8 n 21 when residing in magnetite it is usually unblocked in the laboratory by 350 c when carried by hematite it is unblocked by 550 c a moderate negative inclination e g madera and abo strata d 173 1 i 46 6 a95 5 5 n 47 samples assuming a north seeking positive inclination rm providing a useful means of core orientation oppositely directed magnetization is occasionally isolated and removed at higher laboratory unblocking temperatures up to 550 c in magnetite dominated and 600 c in hematite dominated lithologies where present magnetizations of shallower inclination e g madera and abo strata d 160 1 i 10 9 a95 4 4 n 28 are removed over higher ranges of unblocking temperatures and these are probably of late paleozoic age the moderate negative and positive inclination magnetizations are in all likelihood viscous partial thermoremanent magnetizations trms vptrms these were activated at moderate 300 c temperatures between 1 45 and 0 97 ma attending and following the main stages of toledo valles caldera development during the matuyama and at near present downhole temperatures 100 180 c during the brunhes and possibly within the last 10 kyr respectively importantly the preservation of negative inclination rm components in magnetite dominated lithologies of the madera limestone dictates that temperatures during the brunhes chron past 730 kyr did not exceed approximately 300 350 c the inferred temperatures for thermal activation of the vptrms based upon the middleton and schmidt thermal activation curves for both magnetite and hematite derived following walton agree with independent estimates of thermal activity using fluid inclusion and k ar isotopic age determination data during the quaternary the results further illustrate the potential usefulness of thermal activation theories for inferring temperatures of past geologic events especially when they can be applied to magnetizations of similar age residing in both magnetite and hematite and of paleomagnetic and rock magnetic studies in general in continuous coreholes in complex geologic environments

the generation of global rotational flow by steady idealized tropical divergence abstract tropical convective heating is balanced on the large scale by the adiabatic cooling of ascent the horizontal divergence of the wind above this heating may be viewed as driving the upper tropospheric rotational wind field a vorticity equation model is used to diagnose this relationship it is shown that because of the advection of vorticity by the divergent component of the flow the rossby wave source can be very different from the simple fd source often used in particular an equatorial region of divergence situated in easterly winds can lead to a rossby wave source in the subtropical westerlies where it is extremely effective this part of the source can be relatively insensitive to the longitudinal position of the equatorial divergence a divergence field which is asymmetric about the equator can lead to a quite symmetric rossby wave source for a steady frictionless flow the rossby wave source averaged over regions within closed streamfunction or absolute vorticity contours is under cert

saw toothed variations of relative paleointensity and cumulative viscous remanence testing the records and the model a recent model kok and tauxe 1996a that attempts to explain asymmetrical sawtooth patterns of relative paleointensity from sediments valet and meynadier 1993 as the consequence of cumulative long term viscosity effects has been tested theoretically and experimentally a mathematical description of the model demonstrates that only a sharp distribution of relaxation times would be able to match the paleomagnetic record which implies that all the magnetic grains would be identical this has direct consequences for the distribution of the blocking temperatures tb which would not exceed 260 c and for the coercivities of the natural remanent magnetization nrm this tb spectrum derived from the model is inconsistent with the unblocking temperatures of the nrm and with the tb spectrum of the experimental thermoremanent magnetization trm of the sediment that recorded the sawtooth new results including thermal demagnetization and thellier thellier experiments across brunhes matuyama confirm the overall stability of the original records the stability of the older part of the record 2 to 4 ma has also been strengthed the sawtooth is unchanged between 20 mt and 65 mt and thermal demagnetization in a neutral atmosphere at 300 c and 450 c duplicates the results obtained by alternation field a f demagnetization thus viscous processes at least within the framework of a cumulative viscous model and for the existing data sets are not responsible for saw toothed paleointensity records

relative and absolute sea level rise in western canada and northwestern united states from a combined tide gauge gps analysis 1 empirical studies and climate models suggest large variations of absolute sea level asl changes between oceanic basins such potential variations raise concern on the applicability of global mean asl predictions to specific regions and on estimates of relative sea level rsl hazards we address this issue for the western canada and northwestern united states coastline by estimating the 20th century asl rate using a combination of 34 colocated tide gauge and global positioning system gps stations the tide gauge data are quality controlled and corrected for spatially and temporally correlated sea level transients in order to derive robust rsl trends and standard errors reference frame and other gps specific issues are considered as part of the error budget in absolute gps vertical velocities our combined tide gauge gps analysis aligned to the international terrestrial reference frame 2000 indicates a northeast pacific asl rise of 1 8 0 2 mm a through the 20th century which is similar to accepted rates for the global eustatic mean for the period 1993 2003 we find a regional asl rate of 4 4 0 5 mm a consistent with satellite altimetry on the basis of the intergovernment panel on climate change assessment report 4 mean scenario and our assessment of coastal motions from gps and tide gauge data we derive a map of predicted 21st century rsl rise in western canada and the northwestern united states variations in coastal uplift strongly affect spatial rsl patterns subsidence of southern puget sound may significantly increase rsl rise in the seattle tacoma metropolitan area conversely tectonic uplift along parts of the outer west coast may reduce future rsl rise by up to 50 100

wind field coherence and its variations over west africa abstract the wind field coherence over west africa is investigated during two contrasted periods 1953 63 abnormally wet and 1963 75 abnormally dry in the sahel zonal and meridional components of the monthly mean wind are computed with radiosonde data relative to the 950 100 hpa layer a descriptive analysis reveals the main features of the west african atmospheric circulation and a correlation analysis points out their dynamical coherence the exiwnce of hadley type cells of northern and southern hemispheres are confirmed and a mutual interaction in the lower layers is depicted the functioning of a walker type cell defined by a coupling between the zonal components of the monsoonal flow and of the tropical easterly jet tej is shown during northern summer over the whole of west africa there is no correlation between the tej velocity and the african easterly jet aej velocity over the sahelian belt during the dry period the coherence of the cell circulations vanishes in concomitance with a

flow duration of a dike constrained by palaeomagnetic data summary we have conducted a palaeomagnetic study of a permian red sandstone layer intruded by a 70cm wide plio pleistocene basaltic dike with the ultimate goal of estimating the flow duration of the magma before its solidification 31 samples were collected at various distances from the dike sandstone interface and were thermally demagnetized in order to separate the overprinted pliopleistocene magnetization component from the primary permian one and to determine for each sample the unblocking temperature tub necessary to erase totally the overprint this temperature can be used to determine the maximum palaeotemperature reached at any distance from the contact provided the secondary magnetization is a p trm and the thermal fluctuation effects and actual cooling rates are taken into account examination of various magnetic parameters nrm intensity 15 days vrm intensity initial magnetic susceptibility and hysteresis curves allowed us to establish the nature of the overprinted magnetization and consequently to exclude samples carrying a crm overprint an apparent palaeotemperature profile and two corrected ones were thus deduced from the samples in which the dike emplacement produced no crystallization of new magnetic minerals these profiles were compared with the calculations of a new conductive thermal model which includes the basalt s latent heat of crystallization the flow duration of the dike the width of the circulating magma and the temperature dependence of the thermal conductivity k in the basalt and in the sandstone or alternatively convection simulated by an increase of the k values good fits with the experimental data are obtained for a flowing width consistent with field observations and when the latent heat is taken into account in the two following cases first when k is temperature dependent in the basalt and in the sandstone which implies a magma flow duration of 5 days second when a limited convection process is assumed in the sandstone only here a flowing time of only 1 day is required the corrected palaeotemperature profiles argue in favour of the second hypothesis suggesting that when a dike is emplaced heat transfer by fluids may play an important role in the surrounding rock

generation of the african easterly jet and its role in determining west african precipitation abstract an examination of analyses and model simulations is used to show that the african easterly jet forms over west africa in summer as a result of strong meridional soil moisture gradients in a series of gcm experiments the imposition of realistic surface wetness contrasts between the sahara and equatorial africa leads to strong positive meridional temperature gradients at the surface and in the lower troposphere the associated easterly shear in the atmosphere is strong enough to establish easterly flow the african easterly jet above the monsoon westerlies at the surface positive temperature gradients associated with the summertime distributions of solar radiation ssts or clouds are not large enough to produce the easterly jet in the absence of soil moisture gradients a thermally direct ageostrophic circulation is identified that can accelerate the largely geostrophic zonal flow and maintain the jet while moisture converges throughout the lower troposphere over east africa moisture divergenc

finite element inversion of dinsar data from the mw 6 3 l aquila earthquake 2009 italy 1 fault slip distribution is usually retrieved from geodetic data assuming that the local crust is an elastic homogeneous and isotropic half space in the last decades spatially dense geodetic data e g dinsar maps have highlighted complex patterns of coseismic deformation that require new modeling tools such as numerical methods able to represent rheological and geometrical complexities of the earth s crust in this work we develop a procedure to perform inversion of geodetic data based on the finite element method accounting for a more realistic description of the local crust the method is applied to the 2009 l aquila earthquake mw 6 3 using dinsar images of the coseismic displacement results highlight the non negligible influence of the medium structure homogeneous and heterogeneous models show discrepancies up to 20 in the fault slip distribution values furthermore in the heterogeneous models a new area of slip appears above the hypocenter we also perform a resolution study showing that the information about fault slip distributions retrieved from geodetic data should be considered as averaged on surrounding patches

thermoviscous remanent magnetism of columbia river basalt blocks in the cascade landslide we studied sixteen basalt samples from a landslide in the columbia river gorge to determine if they had acquired a thermoviscous remanent magnetism tvrm since the slide was emplaced about 800 years ago all samples were thermally demagnetized at 20 heating steps until 360 c where a large change in susceptibilit y was noted analysis of the directional changes during demagnetization indicates that the samples contain up to four tvrm components in addition to an nrm component the tvrm components with the lowest blocking temperatures are tightly clustered around the present field direction while the nrm directions are consistent with a random distribution as expected for a landslide deposit measurements of hysteresis parameters and thermomagnetic analyses of the samples demonstrate that the dominant magnetic mineral in the basalt is single domain magnetite the temperature at which the first tvrm component was removed ranges from 70 c to 100 c this result can be compared to nomographs that relate the time and temperature used to demagnetize a tvrm with the time and temperature at which the tvrm was acquired our results are more consistent with the nomograph of pullaiah et al 1975 than with the nomograph of middleton schmidt 1982