intraseasonal variability over tropical africa during northern summer abstract the intraseasonal variability over africa during northern summer was analyzed using 25 years of ncep ncar reanalysis and satellite data the dominant pattern of variability was one of enhanced deep convection over the whole african monsoon region it appeared to arise at least partly as a remote response to the intraseasonal madden julian oscillation over the warm pool sector twenty days prior to the maximum in convection over africa there was no signal over africa but convection was reduced over the equatorial warm pool an equatorial kelvin wave response to this change in warm pool convection propagated eastward and an equatorial rossby wave response propagated westward and between them they completed a circuit of the equator and met up 20 days later over africa where the negative midtropospheric temperature anomalies in the kelvin and rossby waves favored deep convection over west africa the kelvin wave component contained lower tropospheric westerly anomalies that acted to increase t

the dynamics of intraseasonal atmospheric angular momentum oscillations abstract the global and zonal atmospheric angular momentum aam budget is computed from seven years of national centers for environmental prediction data and a composite budget of intraseasonal 30 70 day variations during northern winter is constructed regressions on the global aam tendency are used to produce maps of outgoing longwave radiation 200 hpa wind surface stress and sea level pressure during the composite aam cycle the primary synoptic features and surface torques that contribute to the aam changes are described in the global budget the friction and mountain torques contribute about equally to the aam tendency the friction torque peaks in phase with subtropical surface easterly wind anomalies in both hemispheres the mountain torque peaks when anomalies in the midlatitude northern hemisphere and subtropical southern hemisphere are weak but of the same sign the picture is different for the zonal mean budget in which the meridional convergence of the northward relative angular moment

zonal and vertical structure of the madden julian oscillation abstract a statistical study of the three dimensional structure of the madden julian oscillation mjo is carried out by projecting dynamical fields from reanalysis and radiosonde data onto space time filtered outgoing longwave radiation olr data mjo convection is generally preceded by low level convergence and upward motion in the lower troposphere while subsidence cooling and drying prevail aloft this leads to moistening of the boundary layer and the development of shallow convection followed by a gradual and then more rapid lofting of moisture into the middle troposphere at the onset of deep convection after the passage of the heaviest rainfall a westerly wind burst region is accompanied by stratiform precipitation where lower tropospheric subsidence and drying coincide with continuing upper tropospheric upward motion the evolution of the heating field leads to a temperature structure that favors the growth of the mjo the analysis also reveals distinct differences in the vertical structure

space time spectral analysis of the moist static energy budget equation abstractthe budget of column integrated moist static energy mse is examined in wavenumber frequency transforms of longitude time sections over the tropical belt cross spectra with satellite deri

propagation mechanisms for the madden julian oscillation the madden julian oscillation mjo is examined using 20 years of outgoing long wave radiation and national centers for environmental prediction national center for atmospheric research re analysis data two mechanisms for the eastward propagation and regeneration of the convective anomalies are suggested the first is a local mechanism operating over the warm pool region at the phase of the mjo with a dipole structure to the convection anomalies there is enhanced tropical convection over the eastern indian ocean and reduced convection over the western pacific over the equatorial western indian ocean the equatorial rossby wave response to the west of the enhanced convection includes a region of anomalous surface divergence associated with the anomalous surface westerlies and pressure ridge this lends to suppress ascent in the boundary layer and shuts off the deep convection eventually leading to a convective anomaly of the opposite sign over the indonesian sector the equatorial kelvin wave response to the east of the enhanced convection includes a region of anomalous surface convergence into the anomalous equatorial surface easterlies and pressure trough which will tend to favour convection in this region the indonesian sector is also influenced by an equatorial rossby wave response of opposite sign to the west of die reduced convection over the western pacific which also has a region of anomalous surface convergence associated with its anomalous equatorial surface easterlies and pressure trough hence convective anomalies of either sign tend to erode themselves from the west and initiate a convective anomaly of opposite sign via their equatorial rossby wave response and expand to the east via their equatorial kelvin wave response the second mechanism is global involving an anomaly completing a circuit of the equator enhanced convection over the tropical western pacific excites a negative mean sea level pressure m s l p anomaly which radiates rapidly eastward as a dry equatorial kelvin wave at approximately 35 m s 1 over the eastern pacific it is blocked by the orographic barrier of the andes and central america for several days before propagating through the gap at panama after rapidly propagating as a dry equatorial kelvin wave over the atlantic the m s l p anomaly is delayed further by the east african highlands before it reaches the indian ocean and coincides with the development of enhanced convection at the start of the next mjo cycle

impact of the madden julian oscillation on the summer west african monsoon in amip simulations at intraseasonal timescales convection over west africa is modulated by the madden julian oscillation mjo in this work we investigate the simulation of such relationship by 11 state of the art atmospheric general circulation models runs with prescribed observed sea surface temperatures in general the atmospheric model intercomparison project simulations show good skill in capturing the main characteristics of the summer mjo as well as its influence on convection and rainfall over west africa most models simulate an eastward spatiotemporal propagation of enhanced and suppressed convection similar to the observed mjo although their signal over west africa is weaker in some models in addition the ensemble average of models composites gives a better performance in reproducing the main features and timing of the mjo and its impact over west africa the influence on rainfall is well captured in both sahel and guinea regions thereby adequately producing the transition between positive and negative rainfall anomalies through the different phases as in the observations furthermore the results show that a strong active convection phase is clearly associated with a stronger african easterly jet aej but the weak convective phase is associated with a much weaker aej our analysis of the equatorial waves suggests that the main impact over west africa is established by the propagation of low frequency waves within the mjo and rossby spectral peaks results from the simulations confirm that it may be possible to predict anomalous convection over west africa with a time lead of 15 20 day

impact of the indian part of the summer mjo on west africa using nudged climate simulations observational evidence suggests a link between the summer madden julian oscillation mjo and anomalous convection over west africa this link is further studied with the help of the lmdz atmospheric general circulation model the approach is based on nudging the model towards the reanalysis in the asian monsoon region the simulation successfully captures the convection associated with the summer mjo in the nudging region outside this region the model is free to evolve over west africa it simulates convection anomalies that are similar in magnitude structure and timing to the observed ones in accordance with the observations the simulation shows that 15 20 days after the maximum increase decrease of convection in the indian ocean there is a significant reduction increase in west african convection the simulation strongly suggests that in addition to the eastward moving mjo signal the westward propagation of a convectively coupled equatorial rossby wave is needed to explain the overall impact of the mjo on convection over west africa these results highlight the use of mjo events to potentially predict regional scale anomalous convection and rainfall spells over west africa with a time lag of approximately 15 20 days

the mjo kelvin wave transition 1 as the madden julian oscillation mjo moves eastward from the indian to the pacific ocean it typically accelerates becomes less strongly coupled to convection and becomes more similar to a dry kelvin wave this transition is analyzed using observations of outgoing longwave radiation and era interim reanalyses of surface pressure and 850 hpa zonal wind two individual example events as well as composites are shown the transitions are well defined with distinct disturbances on either side of the transition whose identities as mjo or kelvin waves are clear in some cases there appears to be a pre existing kelvin wave passing through the mjo from the west to the east but this feature is not apparent in the composites the transitions occur at different longitudes in different events over a wide range from the eastern indian to the central pacific oceans

the development of upper tropospheric wind over the western hemisphere in association with mjo convective initiation abstractthis study examines the structure and driving mechanisms of upper tropospheric intraseasonal zonal wind anomalies over the western hemisphere wh during the convective initiation of the madden julian oscillation mjo over the indian ocean using composite and budget analyses the initiating mjo convection is more often associated with wh upper tropospheric intraseasonal easterly wind anomalies and when it is it tends to develop a stronger and zonally broader envelope of enhanced convection than events associated with westerly wind anomalies the wh upper tropospheric zonal wind anomaly associated with the mjo is often described as a dry kelvin wave radiated from mjo convection but the results show that both the structure and driving mechanisms are different from the ones of theoretical kelvin waves unlike the theoretical kelvin wave the zonal wind anomaly is not driven mainly by the zonal pressure gradient force and it is strongly coupled with rotational wind associated with subtropical and

descent into the cryogenian abstract the cryogenian period brackets an interval of profound glaciation commonly referred to by the evocative epithet snowball earth according to its recent redefinition the cryogenian began before the onset of low latitude glaciation ca 717 ma and ended during its final and catastrophic melting phase ca 635 5 ma pending future ratification of a global stratotype section and point or gssp the onset of the cryogenian period is estimated at about 720 ma however no consensus has yet been reached concerning the best section or criteria for its definition the articles in this special issue represent the state of the art in tonian cryogenian transition stratigraphy around the world and will contribute to the selection of the future gssp

740 ma vase shaped microfossils from yukon canada implications for neoproterozoic chronology and biostratigraphy biostratigraphy underpins the phanerozoic time scale but its application to pre ediacaran strata has remained limited because proterozoic taxa commonly have long or unknown stratigraphic ranges poorly understood taphonomic constraints and or inadequate geochronological context here we report the discovery of abundant vase shaped microfossils from the callison lake dolostone of the coal creek inlier yukon canada that highlight the potential for biostratigraphic correlation of neoproterozoic successions using species level assemblage zones of limited duration the fossiliferous horizon dated here by re os geochronology at 739 9 6 1 ma shares multiple species level taxa with a well characterized assemblage from the chuar group of the grand canyon arizona usa dated by u pb on zircon from an interbedded tuff at 742 6 ma the overlapping age and species assemblages from these two deposits suggest biostratigraphic utility at least within neoproterozoic basins of laurentia and perhaps globally the new re os age also confirms the timing of the islay 13ccarbonate anomaly in northwestern canada which predates the onset of the sturtian glaciation by 15 m y together these data provide global calibration of sedimentary paleontological and geochemical records on the eve of profound environmental and evolutionary change

on the role of clouds and moisture in tropical waves a two dimensional model study abstract observations show that convective perturbations of the tropical atmosphere are associated with substantial variations of clouds and water vapor recent studies suggest that these variations may play an active role in the large scale organization of the tropical atmosphere the present study investigates that possibility by using a two dimensional nonrotating model that includes a set of physical parameterizations carefully evaluated against tropical data in the absence of cloud radiation interactions the model spontaneously generates fast upwind eastward moving planetary scale oscillations through the wind induced surface heat exchange mechanism in the presence of cloud radiative effects the model generates slower upwind eastward propagating modes in addition to small scale disturbances advected downwind westward by the mean flow enhanced cloud radiative effects further slow down upwind propagating waves and make them more prominent in the spectrum on the other hand the model sugges

the madden julian oscillation observed during the toga coare iop global view abstract during the toga coare intensive observing period november 1992 february 1993 two pronounced madden julian oscillation mjo events associated with super cloud clusters and westerly wind bursts were observed this paper presents a global view of the mjos including the origin of the super clusters in the indian ocean their migration into the maritime continent and the toga coare large scale soundings array lsa in the western equatorial pacific and their rapid decay over cold water of the eastern pacific the structure and evolution of the mjo are examined with emphasis on the coupling between large scale motion and convection because of differences in propagation speeds the positions of maximum zonal wind perturbations relative to deep convection undergo systematic changes during the travel of the mjo however the centers of deep convection always coincide with those of large scale ascent the super cloud cluster accompanies a wide area of warm air in the upper troposphere over the warm

kelvin wave time scale propagation features of the madden julian oscillation mjo as measured by the chen mjo index 1 kelvin wave time scale propagation features in an indexed measure of the strength of the madden julian oscillation mjo are investigated examination of reanalysis data and chen mjo index data for three 14 may to 21 september periods shows that recurring events near sumatra frequently associated with convectively coupled cc kelvin waves tend to correspond in a time lagged manner with increases in indexed mjo strength at various longitudes the events characterized by low level divergence within a cyclonic circulation off the west coast of sumatra combined with northward advances in the dynamic equator tend to lag increases in indexed mjo strength by several days in the indian ocean and maritime continent and to lead such increases farther to the east maximum time lagged correlation coefficients between increases in indexed mjo strength and a measure of the strength of the divergence events near sumatra are consistent with a disturbance propagating eastward at kelvin wave phase speeds a similar signal is demonstrated in a 32 year data set of the chen mjo index by examining time lagged correlations between increases in indexed mjo strength at various longitudes and corresponding increases for selected events at 80 e and 160 e the results suggest an enhanced rate of increase in mjo strength as the apparent kelvin wave signal passes roughly marking the beginning of an extended period of mjo growth potential limitations of the current analysis are discussed a look at a particular mjo episode provides evidence consistent with the statistical results while supporting a role of sequential cc kelvin wave passages in the propagation of the mjo

fast and slow kelvin waves in the madden julian oscillation of a gcm the structure of the madden julian oscillation mjo in an 1800 day integration of the hadley centre unified model was analysed and interpreted within a kelvin wave framework the model was forced with constant equinoctial march boundary conditions so that a clean mjo signal could be separated from the effects of the seasonal cycle and forced interannual variability the simulated mjo was fairly realistic in terms of its large scale spatial structure and propagation characteristics although its period of 30 days corresponding to an average phase speed of 15 mps was shorter than that observed the signal in deep convection was less coherent than in observations and appeared to move eastward as a sequence of discrete convective anomalies rather than by a smooth eastward propagation both fast and slow equatorial kelvin waves appeared to play an important role in the eastward propagation of the simulated mjo enhanced convection over the indian ocean was associated with a fast equatorial kelvin wave that propagated eastward at 55 m s 1 over the pacific on reaching the west coast of south america a component of this kelvin wave propagated northward and southward as a trapped wave along the mountain ranges of central america and the andes in agreement with observations the anomalous surface easterlies over the tropical eastern pacific associated with this fast kelvin wave enhanced the climatological mean easterlies and led to positive convective anomalies over the eastern pacific consistent with the wishe mechanism however wishe was not able to account for the eastward development of the convective anomalies over the indian ocean western pacific region by splitting the equatorial divergence anomalies of the simulated mjo into their du dx and dv dy components the role of kelvin wave dynamics in the slow 15 m s 1 average eastward propagation of the simulated mjo was examined although the two components were of comparable magnitude the dudx component exhibited a pronounced eastward propagation which tended to be disrupted by the dvdy component thus supporting the paradigm of an underlying but strongly modified kelvin wave mechanism

the shape of the barchan dunes of southern morocco we present detailed shape measurements of several barchan dunes in southern morocco near la youne using these data we disprove the concept of shape invariance of barchan dunes of different sizes nevertheless some parts of the barchan dune scale and we try to distinguish these from non scaling ones furthermore we point out the importance of the exact position of the brink for the overall dune shape finally we propose a simple model for dune shape using our data to determine the model parameters despite its simplicity this model is able to explain the difference between barchans where the crest and the brink coincide and those where they are separated typical side slopes can be obtained and the basic properties of the evolution of a barchan dune can be described

the existence and vertical structure of fast eastward moving disturbances in the equatorial troposphere abstract eastward phase propagation at speed faster than 30 m s 1 of a signal in the equatorial troposphere of the eastern pacific is detected first in historical meteorological observations and then in more recent data a first baroclinic mode vertical structure is identified with this signal in separate analyses based on linear theory and complex empirical orthogonal functions respectively this rapid eastward signal is conceptualized as a far field dispersion product of strong convection associated with the intraseasonal tropical oscillation in the indian ocean and western pacific

links between tropical cyclone activity and madden julian oscillation phase in the north atlantic and northeast pacific basins abstract the leading intraseasonal mode of atmospheric and oceanic variability the madden julian oscillation mjo influences tropical and extratropical sea level pressure temperature divergent and rotational wind components moisture and deep convection as a 40 to 50 day oscillation the mjo is also known to influence tropical phenomena including tropical cyclone tc activity in various tc basins the links between the mjo and multiple measures of tc activity including genesis landfall and an integrative accumulated cyclone energy ace index were quantified for multiple tc formation basins across the western hemisphere including the north atlantic and northeast pacific ocean and subbasins for the period 1978 2006 using this relatively long 29 yr tc dataset and employing an upper tropospheric mjo diagnostic that is physically meaningful over the entire western hemisphere this study extends existing research on the relationships between the mjo and tcs the noaa climate prediction cente

quasi periodic global oscillations in sea level pressure on intraseasonal timescales the sea level pressure slp variability in 30 60 day intraseasonal timescales is investigated using 25 years of reanalysis data addressing two issues the first concerns the non zero zonal mean component of slp near the equator and its meridional connections and the second concerns the fast eastward propagation ep speed of slp compared to that of zonal wind it is shown that the entire globe resonates with high amplitude wave activity during some periods which may last for few to several months followed by lull periods of varying duration slp variations in the tropical belt are highly coherent from 25a degrees s to 25a degrees n uncorrelated with variations in mid latitudes and again significantly correlated but with opposite phase around 60a degrees s and 65a degrees n near the equator 8a degrees s 8a degrees n the zonal mean contributes significantly to the total variance in slp and after its removal slp shows a dominant zonal wavenumber one structure having a periodicity of 40 days and ep speeds comparable to that of zonal winds in the indian ocean slp from many of the atmospheric and coupled general circulation models show similar behaviour in the meridional direction although their propagation characteristics in the tropical belt differ widely

topographic effects on the eastward propagation and initiation of the madden julian oscillation abstract this study investigates the relationship between deep convection and heating anomaly in the madden julian oscillation mjo and the tropical topography the eastward propagation of the deep heating anomalies is confined to two regions the indian ocean and the western pacific warm pool superimposed on the eastward propagation is a series of quasi stationary deep heating anomalies that occur sequentially and discretely downstream in a leapfrog manner in the central indian ocean the maritime continent tropical south america and tropical africa the deep heating anomaly usually preceded by near surface moisture convergence and shallow heating anomalies tends to occur on the windward side of the tropical topography in these regions except the central indian ocean under the prevailing surface easterly anomaly of the mjo it is suggested that the lifting and frictional effects of the tropical topography and landmass induce the near surface moisture convergence anomaly which in turn triggers

chemostratigraphy of the shaler supergroup victoria island nw canada a record of ocean composition prior to the cryogenian glaciations a b s t r a c t a new 13 ccarb curve combined with 13 corg values is presented for the upper shaler supergroup 900 to 720 ma amundsen basin northwestern canada the dataset fills gaps in the existing stratigraphic record and makes correlations with adjacent basins more robust there is a pronounced negative 13 c excursion in the wynniatt formation that can be correlated with a putative worldwide negative car bon isotope excursion namely the bitter springs stage however in the amundsen basin the 13 ccarb excursion drops to anomalously negative values 14 which we attribute to local overprints wherein isotopically light carbon in pore waters released by oxidation of methane and organic matter during sulphate and iron reduction was incorporated into authigenic carbonate cement we document basin euxinia and anoxia during the same time interval using a multi proxy approach specifically fe speciation and redox sensitive trace metal data patterns of pronounced enrichment in mo v and u concentrations in euxinic black shales suggest that the bitter springs stage was a transitional period in earth s redox evo lution from the more reduced global oceans during the mid proterozoic to the more oxygenated oceans during the phanerozoic 2015 elsevier b v all rights reserved

the development of upper tropospheric geopotential height anomaly in the western hemisphere during mjo convective initiations this study uses geopotential height budget and composite analysis of reanalysis data to examine the driving dynamics and structural evolution of intraseasonal upper tropospheric circulation over the western hemisphere wh prior to and during the convective initiation of the madden julian oscillation mjo over the indian basin previous studies by the authors showed that during most of mjo convective initiation events over the indian basin upper tropospheric equatorial easterly wind anomaly initially develops in association with subtropical and midlatitude rossby ridges over the east pacific basin as the easterly wind continues to amplify and propagate farther eastward a negative upper tropospheric geopotential height anomaly develops over tropical south america and couples with the easterly wind anomaly forming a structure consistent with theoretical kelvin waves this study shows that the development of the negative geopotential height anomaly over south america is driven by net tropospheric adiabatic cooling induced by updrafts the convection associated with the updrafts is triggered by the intrusion of a midlatitude wave train from the southern hemisphere suggesting that the interaction with midlatitude circulation is a part of the dynamics driving the formation of upper tropospheric kelvin wave structure over the wh during some mjo convective initiation events the upper tropospheric zonal wind anomaly is positive over the east pacific basin and the formation of kelvin wave structure is weak or absent over south america those events subsequently develop weaker mjo convective envelopes over the indian basin apparently due to the absence of upper tropospheric divergence induced by the incoming kelvin wave circulation that enhances convective updrafts over the region of initiating convection the results suggest that the formation of the upper tropospheric kelvin wave over the wh partly determines the subsequent development of mjo convection over the indian basin

dynamics of the west african monsoon part iv analysis of 25 90 day variability of convection and the role of the indian monsoon this paper is part of a series of studies addressing the dynamics of the west african summer monsoon at intraseasonal time scales between 10 and 90 days the dominant mode of 25 90 day convective variability in the african monsoon was investigated starting from previous results involving the excitation of dry equatorial kelvin and rossby waves by a negative diabatic heat source located over the warm pool this evolution is consistent with a significant contribution by a convectively coupled equatorial rossby wave and the mjo on the other hand convectively coupled kelvin waves as well as the dry kelvin wave signal have a very weak impact however there is more to the global control of the african summer monsoon than convectively coupled wave dynamics the active break cycle of the indian monsoon controlled by a northward moving dipole of diabatic heating in the indian sector can also influence the african monsoon through atmospheric teleconnections simulations performed with a dry primitive equation model show that this influence may be transferred through the northern indian heat source which excites a rossby cyclonic circulation propagating westward over north africa that is cut off by the northward arrival of the equatorial indian heat source and the associated intrusion of an anticyclonic ridge low level westerly winds and moisture advection within the itcz consequently increase over africa the mean time lag between an active phase over india and over africa is about 15 20 days

extratropical forcing of convectively coupled kelvin waves during austral winter abstract observations are presented that link extratropical rossby wave disturbances excited in the southern hemisphere subtropical jet to the initiation of convectively coupled kelvin waves in the pacific intertropical convergence zone itcz during austral winter a baroclinic zonal wavenumber 6 eastward propagating rossby wave train in the subtropical jet turns northeastward in the vicinity of australia inducing upper tropospheric divergence and vertical motion fields that spread equatorward and induce cloudiness anomalies in the tropics lower tropospheric pressure surges excited from the extratropics also induce kelvin wave like geopotential height and temperature anomalies at the surface providing additional lower tropospheric convergence and vertical motion forcing the tropical outgoing longwave radiation olr and circulation fields propagate eastward in tandem with the extratropical rossby wave train at approximately 17 m s 1 kelvin wave activity in the central pacific itcz thus appears to

identifying the skeleton of the madden julian oscillation in observational data the madden julian oscillation mjo skeleton model offers a theoretical prediction of the mjo s structure here a method is described for identifying this structure in observational data the method utilizes projections onto equatorial wave structures and a main question is can this method isolate the mjo without using temporal filtering or empirical orthogonal functions for the data projection a wide range of data is incorporated multiple variables wind geopotential height water vapor and as a proxy for convective activity outgoing longwave radiation multiple pressure levels 850 and 200hpa and multiple latitudes both equatorial and off equatorial such a data variety is combined using a systematic method and it allows for a distinction between the kelvin and rossby components of the mjo s structure results are illustrated for some well known cases and statistical measures are presented to quantify the variability of the mjo skeleton signal mjos x t and its amplitude mjosa t the robustness of the methods is demonstrated through a suite of sensitivity studies including tests with two projection methods when the projection is based on the skeleton model s energy as opposed to the standard l 2 energy water vapor is seen to be of primary importance finally a simple interpretation is given for the mjo skeleton structure it is related to the wave responseto amovingheatsource from eitherperspective themethods hereidentifysignalsthatproject onto coupled convection circulation patterns and the results suggest that a large portion of the mjo s structure is consistent with such a coupled pattern

a rapid and synchronous initiation of the wide spread cryogenian glaciations abstract the neoproterozoic glaciations represent a remarkable milestone in earth evolution due to their major influence on atmosphere biosphere and hydrosphere the early cryogenian sturtian glaciations evidence for which has been recorded worldwide may have lasted for about 50 million years but the precise timing and synchroneity of these glaciations have been disputed jiangkou glacigenic strata in south china can be attributed to the sturtian glacial episode as they share sedimentological stratigraphic and lithological similarities with sturtian strata in south australia as such dating the onset of the jiangkou glaciation would contribute to testing the synchroneity of the worldwide sturtian glaciation in this paper we demonstrate by means of sims u pb zircon dating that the jiangkou glaciation in south china was initiated at 715 9 2 8 ma the new age is within uncertainty synchronous with dates obtained from the lower parts of glacigenic successions in laurentia and oman thereby indicating thereby indicating a synchroneity and global extent to the sturtian glaciation the synchronous occurrence of the sturtian glaciation in mid to low latitude regions implies rapid ice sheet advancement in response to ice albedo feedback as supported by numerical climatic modeling

madden julian oscillations seen in the upper troposphere vorticity field interactions with rossby wave trains abstractthe vorticity variability associated with the madden julian oscillation mjo is examined the analysis is focused on the 150 hpa pressure level because a clear dipolar vortex signal remi

a cryogenian chronology two long lasting synchronous neoproterozoic glaciations the snowball earth hypothesis predicts globally synchronous glaciations that persisted on a multimillion year time scale geochronological tests of this hypothesis have been limited by a dearth of reliable age constraints bracketing these events on multiple cratons here we present four new re os geochronology age constraints on sturtian 717 660 ma and marinoan 635 ma termination glacial deposits from three different paleocontinents a 752 7 5 5 ma age from the base of the callison lake formation in yukon canada confirms nonglacial sedimentation on the western margin of laurentia between ca 753 and 717 ma coupled with a new 727 3 4 9 ma age directly below the glacigenic deposits of the grand conglomerate on the congo craton africa these data refute the notion of a global ca 740 ma kaigas glaciation a 659 0 4 5 ma age directly above the maikhan uul diamictite in mongolia confirms previous constraints on a long duration for the 717 660 ma sturtian glacial epoch and a relatively short nonglacial interlude in addition we provide the first direct radiometric age constraint for the termination of the marinoan glaciation in laurentia with an age of 632 3 5 9 ma from the basal sheepbed formation of northwest canada which is identical within uncertainty to u pb zircon ages from china australia and namibia together these data unite re os and u pb geochronological constraints and provide a refined temporal framework for cryogenian earth history

influence of the madden julian oscillation and intraseasonal waves on surface wind and convection of the tropical atlantic ocean abstractatmospheric intraseasonal variability in the tropical atlantic is analyzed using satellite winds outgoing longwave radiation olr and reanalysis products during 2000 08 the analyses focus on assessing the effects of dominant intraseasonal atmospheric convective processes the madden julian oscillation mjo and rossby waves on surface wind and convection of the tropical atlantic ocean and african monsoon area the results show that contribution from each process varies in different regions in general the mjo events dominate the westward propagating rossby waves in affecting strong convection in the african monsoon region the rossby waves however have larger contributions to convection in the western atlantic ocean both the westward and eastward propagating signals contribute approximately equally in the central atlantic basin the effects of intraseasonal signals have evident seasonality both convection amplitude and the number of strong convective events associated with the mjo are l