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

ava analysis and interpretation of a carbonate reservoir northwest java basin indonesia a detailed analysis is performed of amplitude variation with angle ava observations in six common midpoint gathers with reflection points that are over and beside carbonate reefs in the parigi formation in the northwest java basin both empirical analysis and full wavefield modeling of the ava data suggest that the presence of gas affects ava by reducing the bulk density of the reservoir decreasing of the overburden vp vs ratio and by local attenuation caused by gas sieving through the overlying sediments the slopes of ava curves for reflections from the top of the parigi are negative for brine saturation and strongly positive for gas saturation

the madden julian oscillation s influence on spring rainy season precipitation over equatorial west africa abstractthis paper characterizes the influence of the madden julian oscillation mjo on spring rainy season march june convection variability over equatorial west africa ewa and investigates mechanisms of association it is found that the mjo has a significant impact on convection and precipitation anomalies over the region over large portions of ewa mjo impacts on rainfall constitute a difference on the order of 20 50 from average daily rain rates for the season this impact is primarily due to the direct influence of the eastward movement of the mjo convective core into ewa which is associated with westerly low level wind anomalies that advect moisture from the atlantic ocean to the region in addition equatorial rossby and kelvin waves triggered by mjo convection anomalies over the indian ocean have a significant and systematic influence on ewa spring rainy season precipitation the kelvin wave contribution and the relative strength of the direct mjo convective influence compared to that of

the role of the indian monsoon onset in the west african monsoon onset observations and agcm nudged simulations in spring the inland penetration of the west african monsoon wam is weak and the associated rainband is located over the guinean coast then within a few days deep convection weakens considerably and the rainband reappears about 20 days after over the sahel where it remains until late september signalling the summer rainy season over the period 1989 2008 a teleconnection induced by the indian monsoon onset is shown to have a significant impact on the wam onset by performing composite analyses on both observational data sets and atmospheric general circulation model simulations ensembles where the model is nudged to observations over the indian monsoon sector the initiation of convective activity over the indian subcontinent north of 15 n at the time of the indian monsoon onset results in a westward propagating rossby wave establishing over north africa 7 15 days after a back trajectory analysis shows that during this period dry air originating from the westerly subtropical jet entrance is driven to subside and move southward over west africa inhibiting convection there at the same time the low level pressure field over west africa reinforces the moisture transport inland after the passage of the wave the dry air intrusions weaken drastically hence 20 days after the indian monsoon onset convection is released over the sahel where thermodynamic conditions are more favourable this scenario is very similar in the observations and in the nudged simulations meaning that the indian monsoon onset is instrumental in the wam onset and its predictability at intraseasonal scale

composite patterns of easterly disturbances over west africa and the tropical atlantic a climatology from the 1979 95 ncep ncar reanalyses the horizontal and vertical structure of the 3 5 day and 6 9 day easterly waves over west africa and tropical atlantic are investigated ncep ncar reanalyses are used for the period 1979 1995 to produce a 17 year climatology of both 3 5 day and 6 9 day easterly waves composite patterns of convection wind temperature and vertical velocity are analysed with respect to the following the modulation by 3 5 day and 6 9 day wave regimes the contrasts between the itcz 5 n 10 n and the sahelo saharan band 15 n 20 n the difference between land and ocean and seasonal variations similarities and differences in the characteristics of the two wave regimes are identified

eastward propagation of 30 60 day perturbations as revealed from outgoing longwave radiation data abstract some of the characteristic features of low frequency oscillations were investigated by applying an extended empirical orthogonal function eeof analysis to 30 60 day filtered outgoing longwave radiation olr data during the eight years of 1975 77 and 1979 83 the first two eeof eigenvectors indicate a systematic eastward propagation of low frequency modes between about 60 e and the date line indian ocean western pacific where they are most predominant they weaken considerably by the time they reach the eastern pacific east of the date line and their phase propagation becomes ill organized over the equatorial south america atlantic ocean africa region this appears to be due to an out of phase relationship between zonal fourier wavenumbers 1 and 2 over the equatorial western hemisphere in contrast there exists a general in phase relationship between these two fourier components and thus the consolidated perturbations of wavenumbers 1 and 2 become very pronounced with a distinct eastw

west african convection regimes and their predictability from submonthly forecasts recurrent convection regimes are identified during the extended west african monsoon wam season may nov using a k means clustering of 1980 2013 noaa daily outgoing longwave radiation olr and are well reproduced in 1996 2015 ecmwf week 1 reforecasts despite systematic biases one regime of broad drying across the sahel in the early may jun and late oct wam is of particular interest regarding the prediction of onset date this regime is associated with an anticyclonic cell along the atlantic coast of west africa leading to a weakened monsoon flow and subsiding anomalies across the sahel teleconnections of this regime with the indian monsoon sector are identified through modulations of the walker circulation alongside relationships to mjo phase 3 more than 10 days in advance when convection is enhanced over the indian ocean other regimes are associated with westward propagating anomalous convective cells along two distinct wave trains at 15 circ hbox n and 24 circ hbox n during the core jul sep and late oct nov wam respectively and translate into wet anomalies transiting across the sahel a regime of broad sahel wetting in the core wam more frequent since the 1990s is related to global sst warming agreeing with the observed recovery of sahel rainfall ecmwf skill in forecasting regime sequences decreases from week 1 to 4 leads except in the case of the above mentioned regime associated with early season dry spells translating into the potential for skillful wam onset date predictions our analysis suggests that sources of predictability include relationships to the mjo and the indian monsoon sector which need to be further examined to benefit subseasonal forecasting efforts in west africa and ultimately agricultural planning and food security across the sahel

the dynamics of the west african monsoon part v the detection and role of the dominant modes of convectively coupled equatorial rossby waves abstract this study is the last in a series of papers addressing the dynamics of the west african summer monsoon at intraseasonal time scales between 10 and 90 days the signals of convectively coupled equatorial rossby er waves within the summer african monsoon have been investigated after filtering noaa outgoing longwave radiation olr data within a box delineated by the dispersion curves of the theoretical er waves two families of waves have been detected in the 10 100 day periodicity band by performing a singular spectrum analysis on a regional index of er filtered olr for each family the first eof mode has been retained to focus on the main convective variability signal within the periodicity band of 30 100 days an er wave pattern with an approximate wavelength of 13 500 km has been depicted this er wave links the mjo mode in the indian monsoon sector with the main mode of convective variability over west and central africa this confirms the investigations carried out in previous studies wi

african easterly wave variability and its relationship to atlantic tropical cyclone activity abstract automatic tracking of vorticity centers in european centre for medium range weather forecasts analyses has been used to develop a 20 yr climatology of african easterly wave activity the tracking statistics at 600 and 850 mb confirm the complicated easterly wave structures present over the african continent the rainy zone equatorward of 15 n is dominated by 600 mb activity and the much drier saharan region poleward of 15 n is more dominated by 850 mb activity over the atlantic ocean there is just one storm track with the 600 and 850 mb wave activity collocated based on growth decay and genesis statistics it appears that the 850 mb waves poleward of 15 n over land generally do not get involved with the equatorward storm track over the ocean instead there appears to be significant development of 850 mb activity at the west african coast in the rainy zone around 10 n 10 w which it is proposed is associated with latent heat release based on the tracking statistics it has been shown th

impact of air sea coupling on the madden julian oscillation in a general circulation model abstract the impact of air sea coupling on the dynamics of the tropical madden julian oscillation mjo is investigated with an atmospheric general circulation model gcm coupled to an ocean mixed layer model in the uncoupled gcm where climatological sea surface temperature sst is specified realistic space time spectra of near equatorial zonal wind and precipitation are produced with power concentrated at eastward wavenumbers 1 3 with periods of 35 90 days however the simulated mjo is roughly 50 stronger than observed largely resulting from enormous activity during northern summer furthermore during southern summer when the observed mjo is most dominant across the indian and western pacific oceans intraseasonal variance in the uncoupled model is overly concentrated to the north and east of australia with little activity extending into the equatorial indian ocean contrary to other recent modeling studies coupling did not alleviate either of these problems nor did it have any other apprecia

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

observation of the marine atmospheric boundary layer in the gulf of guinea during the 2006 boreal spring in situ satellite and model analyses data in april july 2006 are used to investigate the links between sea surface temperature sst and the marine atmospheric boundary layer mabl in the gulf of guinea the study region between 10 w and 6 e is divided into three areas with different characteristics the north area 4 5 n to 1 n with the wettest atmosphere and the warmest sst the upwelling area 1 n to 4 s with the strongest sst decrease and the south area 4 s to 8 s with a drier atmosphere and a more slowly decreasing sst than in the upwelling area the key zone of the air sea interactions in this region seems to be the sst front between north and upwelling areas on the one hand the study of the mabl on either side of the front shows a well mixed layer between the surface and about 500 m high sensitive to surface variations which gets shallower deeper when the sst decreases increases the mabl height about 1500 m follows the same variations but is not exactly collocated with the sst variations on the other hand the observation of the mabl across the sst front shows a strengthening of southeasterlies in the south area coinciding with a strong sst decrease in the upwelling area in the latter the wind weakens above the colder sst besides in the north area the wind strengthens above the warmer sst however the wind acceleration spans from the equator to 2 n in april and as far as 4 n in june a convergence zone is observed in the vicinity of 2 n in april suggesting a convection activity there favoured by the sst front copyright 2011 royal meteorological society

the relationship between convection and sea surface temperature on intraseasonal timescales abstract the relationship between tropical convection surface fluxes and sea surface temperature sst on intraseasonal timescales has been examined as part of an investigation of the possibility that the intraseasonal oscillation is a coupled atmosphere ocean phenomenon the unique feature of this study is that 15 yr of data and the whole region from the indian ocean to the pacific ocean have been analyzed using lag correlation analysis and compositing techniques a coherent relationship between convection surface fluxes and sst has been found on intraseasonal timescales in the indian ocean maritime continent and west pacific regions of the tropics prior to the maximum in convection there are positive shortwave and latent heat flux anomalies into the surface followed by warm sst anomalies about 10 days before the convective maximum coincident with the convective maximum there is a minimum in the shortwave flux followed by a cooling due to increased evaporation associated with enhanced westerl

multi scale description of a sahelian synoptic weather system representative of the west african monsoon a reference case of a sahelian weather system observed during the hydrological atmospheric pilot experiment hapex sahel in august 1992 is described from a seasonal viewpoint as well as from synoptic and convective system viewpoints it is shown that the case study is representative of the climatology at all these scales and presents many interacting scales and physical processes at intraseasonal scale the monsoon onset is characterized by an abrupt shift of precipitation together with a latitudinal migration of the african easterly jet aej and convection at the month and day scales the convective activity occurs in an apparent zonal break of the tropical easterly jet the month of august 1992 exhibits intense synoptic activity the vorticity field is characterized by northerly dry and southerly wet components located at 850 hpa on each side of the aej their intraseasonal modulation on a period of 20 to 40 days leads to active and break phases of the synoptic activity around 21 august the 700 hpa vorticity field features the propagation of a typical easterly wave with a westward propagation of a cyclonic circulation followed by an anticyclonic circulation convective activity occurs mainly ahead of the 700 hpa vorticity maximum with the formation of a squall line on air mountains propagating south westward at 15 m s 1 the convective system propagates about twice as fast as the vortex core in contrast with the convection in the european centre for medium range weather forecasts re analysis which stays in phase with the vorticity the squall line corresponds to the largest contributor to the systems passing in august 1992 over the hapex sahel region its environmental conditions and its effects on the atmosphere including the surface parameters are presented copyright 2002 royal meteorological society

the tropical extratropical interaction between high frequency transients and the madden julian oscillation abstract the interaction between high frequency transient disturbances and convection and the madden julian oscillation mjo is investigated using ncep ncar reanalysis and satellite outgoing longwave radiation data for 15 northern winters during the phase of the mjo with enhanced convection over the east indian ocean and indonesia and suppressed convection over the south pacific convergence zone both the asian pacific jet and the region of upper tropospheric tropical easterlies over the warm pool are displaced westward these changes in the basic state lead to a weaker or leakier waveguide in the asian pacific jet with a westward displaced forbidden region of tropical easterlies such that high frequency transient waves propagate equatorward into the deep tropics over the central pacific near the date line as these waves induce convection in the region of ascent and reduced static stability ahead of the upper level cyclonic disturbances there is an enhancement of high frequency convective var

easterly wave regimes and associated convection over west africa and tropical atlantic results from the ncep ncar and ecmwf reanalyses ncep ncar and ecmwf daily reanalyses are used to investigate the synoptic variability of easterly waves over west africa and tropical atlantic at 700 hpa in northern summer between 1979 1995 1979 1993 for ecmwf spectral analysis of the meridional wind component at 700 hpa highlighted two main periodicity bands between 3 and 5 days and 6 and 9 days the 3 5 day easterly wave regime has already been widely investigated but only on shorter datasets these waves grow both north and south of the african easterly jet aej the two main tracks noted over west africa at 5 n and 15 n converge over the atlantic on latitude 17 5 n these waves are more active in august september than in june july their average wavelength phase speed varies from about 3000 km 8 m s 1 north of the jet to 5000 km 12 m s 1 south of the jet rainfall convection and monsoon flux are significantly modulated by these waves convection in the inter tropical convergence zone itcz being enhanced in the trough and ahead of it with a wide meridional extension compared to the 3 5 day waves the 6 9 day regime is intermittent and the corresponding wind field pattern has both similar and contrasting characteristics the only main track is located north of the aej along 17 5 n both over west africa and the atlantic the mean wavelength is higher about 5000 km long and the average phase speed is about 7 m s 1 then the wind field perturbation is mostly evident at the aej latitude and north of it the perturbation structure is similar to that of 3 5 days in the north except that the more developed circulation centers moving more to the north lead to a large modulation of the jet zonal wind component south of the aej the wind field perturbation is weaker and quite different the zonal wind core of the jet appears to be an almost symmetric axis in the 6 9 day wind field pattern a clockwise circulation north of the aej being associated with a counter clockwise circulation south of the jet and vice versa these 6 9 day easterly waves also affect significantly rainfall convection and monsoon flux but in a different way inducing large zonal convective bands in the itcz mostly in the trough and behind it as opposed to the 3 5 day wave regime these rainfall anomalies are associated with anomalies of opposite sign over the guinea coast and the sahelian regions over the continent these waves are more active in june july and in august september over the ocean gate phase i gave an example of such an active 6 9 day wave pattern considered as a sequence of weak easterly wave activity this phase was also a sequence of high 6 9 day easterly wave activity we suggest that the 6 9 day regime results from an interaction between the 3 5 day easterly wave regime maintained by the barotropic baroclinic instability of the aej and the development of strong anticyclonic circulations north of the jet over west africa and both north and south of the jet over the atlantic significantly affecting the jet zonal wind component the permanent subtropical anticyclones azores libya st helena could help initiation and maintenance of such regime over west africa and tropical atlantic based on an a priori period band criterion our synoptic classification has enabled us to point out two statistical and meteorological easterly wave regimes over west africa and tropical atlantic ncep ncar and ecmwf reanalyses are in good agreement the main difference being a more developed easterly wave activity in the ncep ncar reanalyses especially for the 3 5 day regime over the atlantic

the effects of migration on ca 100 year old arsenic rich mine tailings in cobalt ontario canada century old tailings at the beaver mine site cobalt ontario canada are migrating downstream to previously uncontaminated kirk lake resulting in metal dispersal into the environment these metal rich tailings have high arsenic concentrations and exceed the ontario regulatory standards for arsenic in soil 18 mg kg by up to 240 times arsenic concentrations in surface water exposed to the tailings and waste rock were found to be up to 268 times the acceptable limit of 5 g l for the protection of aquatic life at the beaver mine site where the tailings have sat largely undisturbed for the last century prevailing redox conditions play an important role in the behavior of arsenic and other metals surface enrichment of metals was evident in tailings located above the water table and at both the beaver mine and kirk lake sites arsenic enrichment was evident in clay layers within the tailings a principal component analysis and an analysis of similarity suggest that the migration process did not significantly alter the metal composition of the tailings anosim p 0 21 likely because the migration process is relatively rapid compared to leaching mechanisms that would alter metal composition during migration our results indicate that the transportation of these century old metal rich tailings is having minimal impact on reducing their metal concentrations and as these largely unmanaged tailings continue to be transported through the environment they pose a risk to previously uncontaminated lake and stream ecosystems

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

structural evolution in heating profiles of the mjo in global reanalyses and trmm retrievals abstract diabatic and latent heating profiles from four global reanalyses and three tropical rainfall measuring mission trmm algorithms were compared first generally for the tropics and then in the context of the madden julian oscillation mjo most of them exhibit three heating maxima corresponding to known convection centers over south america africa and the indian western pacific warm pool but they still differ substantially in many ways most noticeably a double peak vertical structure with one peak in the upper and the other in the lower troposphere and relative weak heating over the maritime continent in comparison to heating over the indian and western pacific oceans are clearly seen in some heating data but absent in others heating profiles associated with the mjo were diagnosed through composites and case studies the composites were constructed as functions of mjo phases at three longitudes representing the indian ocean maritime continent and western pacific respectively four mjo ev

numerical study of a sahelian synoptic weather system initiation and mature stages of convection and its interactions with the large scale dynamics summary a 24 h multi scale simulation of the life cycle of a sahelian squall line is presented the initial and coupling e elds have been taken from the european centre for medium range weather forecasts re analysis era 15 after humidity corrections to avoid spurious absolute and conditional convective instabilities comparisons of modelsynthetic and meteosat infra red radiances indicate that the simulated scenario of convection development and structure are realistic though differences are found concerning the size of the squall line with a delay of 2 3 h the simulated arc shaped structure and surface signature e t well with radar and ground station observations respectively the mature stage of the simulated squall line presents a cross line structure similar to conceptual models an intense rear to front e ow is generated below the trailing stratiform part an acceleration of the african easterly jet aej is simulated behind the system occurring across its full width this leads to the generation of a dipole of counter rotating vortices the resulting potential vorticity signature leads to local reinforcement of the barotropic instability on the north e ank of the aej this simulation is used to analyse the impact of the convection at synoptic scales and to compare it with era 15 it suggests that the convection contributes to reinforce the monsoon at least up to 500 km the monsoon ine ow moves eastward and westward with time scales shorter and longer than 24 h respectively the tropical easterly jet intensie es to the south west of the convection area up to 700 km away the aej is strongly intensie ed ahead of the trough where active convection occurs organized convection developing in the vicinity of the aej core induces weakening of the aej ahead of the squall line and strengthening behind it a common result in the simulation and in era 15 is that the aej core moves westward faster due to the convective activity

the influence of the mjo on upstream precursors to african easterly waves abstractthe madden julian oscillation mjo produces alternating periods of increased and reduced precipitation and african easterly wave aew activity in west africa this study documents the influence of the mjo on the west african monsoon system during boreal summer using reanalysis and brightness temperature fields mjo related west african convective anomalies are likely induced by equatorial kelvin and rossby waves generated in the indian ocean and west pacific by the mjo which is consistent with previous studies the initial modulation of tropical african convection occurs upstream of west africa near the entrance of the african easterly jet aej previous studies have hypothesized that an area to the east of lake chad is an initiation region for aews called the trigger region in this study this area exhibits significant intraseasonal convection and wave activity anomalies prior to the wet and dry mjo phases in the west african monsoon region in the trigger region cold tropospheric tempera

target oriented avo inversion of data from valhall and hod fields a stratigraphic elastic inversion scheme has been applied to a real seismic data set from valhall and hod fields in the north sea avo inversion shows discontinuous high porosity zones that can be interpreted as evidence for stratigraphic traps in the upper chalk interval this was tested by exploration drilling after avo processing was completed and hydrocarbons were found

mjo related sst variations over the tropical eastern pacific during northern hemisphere summer abstract the madden julian oscillation mjo significantly modulates eastern tropical pacific sea surface temperatures ssts during northern hemisphere summer typical sst variations during an mjo life cycle are 0 4 0 5 c considerably higher variations occur with the strongest events 1 2 c the magnitudes of these variations are comparable to those associated with west pacific mjo variability during nh winter sst anomalies to the north of 4 n where the strongest modulation of east pacific convection by the mjo occurs are 180 out of phase with equatorial anomalies these sst variations are responsible for variations in surface saturation equivalent potential temperatures of greater than 3 c the highest ssts lead maximum convection by about 10 days and ssts begin to cool with the onset of enhanced convection off equatorial sst anomalies are most likely due to variations in surface latent heat flux although shortwave flux anomalies are important in the heart of the mjo convective region decrea

active break cycles diagnosis of the intraseasonal variability of the asian summer monsoon in this study various diagnostics have been applied to daily observed outgoing longwave radiation olr and ecmwf reanalysis era products to provide a comprehensive description of the active break cycles associated with the asian summer monsoon and to address the differing behaviour of the two dominant time scales of intraseasonal variability 10 20 days and 30 60 days composite analysis of olr based on filtered daily all india rainfall air for the 40 day 30 60 days intraseasonal mode indicates that during active phases convection is significantly enhanced over the indian continent extending over the bay of bengal maritime continent and equatorial west pacific while convection is suppressed over the equatorial indian ocean and northwest tropical pacific resulting in a quadrapole structure over the asian monsoon domain in response to this heating pattern the large scale hadley lateral and the two east west transverse tropical circulations are enhanced there is also a significant impact on the extra tropical circulation through excitation and propagation of rossby waves in contrast the 15 day mode is more regional to the monsoon domain and has a prominent east west orientation in convection only the local hadley circulation over the monsoon region is modulated by this mode the evolution of these two modes as revealed by pop principal oscillation pattern analysis shows that the 40 day mode originates over the equatorial indian ocean once formed it has poleward propagation on either side of the equator and eastward propagation into the equatorial west pacific from the equatorial west pacific northward propagation over the west pacific and westward propagation into the indian longitudes are prominent the propagative features are complex and interactive and are responsible for the quadrapole structure in convection seen from the composites the interannual variability assessed from the pop coefficient time series indicates that the 40 day mode is strong during the onset phase of the monsoon in all the years but systematic propagation over the entire season depends crucially on the activity of the oceanic tcz tropical convergence zone the pop analysis of the 15 day mode indicates that this event originates over the equatorial west pacific associated with westward propagating rossby waves amplifies over the northwest tropical pacific and modulates both the continental and oceanic tczs over indian longitudes simultaneously this mode is pronounced during the established phase of the monsoon due to the complexity in the propagational features of both the intraseasonal modes it is concluded that understanding the subseasonal variability of one regional component of the asian summer monsoon asm requires understanding the entire asm system

radar observations of convective system variability in relationship to african easterly waves during the 2006 amma special observing period abstract a radar based analysis of the structure motion and rainfall variability of westward propagating squall line mesoscale convective systems slmcss in niamey niger during the african monsoon multidisciplinary activities amma 2006 special observing period is combined with an analysis of 700 mb hpa winds and relative vorticity to study the relationship between slmcss and african easterly waves aews radar results show that slmcss were the most important rainmakers in niamey and accounted for about 90 of the rainfall despite being present less than 17 of the time analysis of the 700 mb synoptic scale flow revealed that during the 2006 west african monsoon season the african easterly jet vacillated between about 10 and 15 n on time scales of 1 2 weeks aews followed the jet as it vacillated north and south thereby producing two preferred paths for aews propagating past niamey s longitude a northern track along 8 16 n and a southern track along 2 6 n it was found that niamey slmcss oc

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

horizontal structure and seasonality of large scale circulations associated with submonthly tropical convection the relationship between deep tropical convection and large scale atmospheric circulation in the 6 30 day period range is examined regression relationships between filtered outgoing longwave radiation at various locations in the tropics and 200 and 850 mb circulation are mapped for the standard seasons and the spatial structure and seasonal dependence of the results are interpreted in view of the basic state circulation in regions where the convection is embedded in upper level easterlies anomalous equatorial easterly flow is typically present at 200 mb within and to the west of the convective signal along with patterns of meridional outflow into subtropical anticyclonic perturbations lagged relationships suggest that the convection is forcing the circulation in many of these cases the outflow and subtropical circulations are strongest into the winter hemisphere during the solstitial seasons with more symmetric signals about the equator seen in the equinoctial seasons the longitudinal positioning of the subtropical features with respect to the convection varies but is generally located due poleward or just to the east of the convection there tends to be a first baroclinic mode vertical structure to these circulations such that equatorial westerlies are present at 850 mb within the convection with closed circulations on either side of the equator resembling equatorial rossby modes especially common over the atlantic and pacific sectors as a contrast in regions located within upper level westerlies or along the margin of influence of upper westerly disturbances convection appears to be forced by upper level wave energy propagating into the deep tropics with the heating located in the upward motion region ahead of upper level troughs this occurs over the atlantic and eastern pacific sectors during northern winter and spring and over australia the south pacific and south america during southern summer when upper westerlies are at relatively low latitudes where they can interact with deep tropical convection the results confirm theoretical and modeling ideas that suggest that rossby wave energy is able to propagate into the deep tropics in regions where upper level westerlies exist in the tropics

observation of oceanic origin of sahel precipitation from space abstract the west african monsoon region is of great concern due to the negative ecological economical and societal impacts of the persistent drought our results support the established postulation that the onset of summer rain in the southern region is associated with the migration of the inter tropical convergence zone and in phase with the meridional moisture transport from the gulf of guinea we present recent satellite measurements that illustrate the phase shift of the precipitation in the northern region the sahel with respect to the region to the south these satellite data and associated analysis techniques let us combine the effect of the onshore flow at the surface and offshore transport aloft to demonstrate with sufficient clarity that the depth integrated moisture advection across the west coast from the open atlantic ocean into the sahel region is in phase with the annual and inter annual variations of sahel rainfall but out of phase with rainfall in the southern region around the gulf of guinea the peak sahel rainfall is associated with the short lived net moisture influx from the atlantic which may interact with local instability related to the african easterly waves the data show two precipitation regimes in western africa monsoon region associated with moisture input from two different regions of the ocean

an idealized two dimensional framework to study the west african monsoon part i validation and key controlling factors abstract an idealized vertical meridional zonally symmetric model is developed in order to recover a july typical monsoon regime over west africa in response to surface conditions the model includes a parameterization to account for heat and momentum fluxes associated with eddies the sensitivity of the simulated west african monsoon equilibrium regime to some major processes is explored it allows confirmation of the important role played by the sun s latitudinal position the aerosols the albedo and the sst s magnitude in the gulf of guinea and in the mediterranean sea the important role of aerosols in warming the saharan lower layers and their effect on the whole monsoon is underlined model results also stress the importance of the mediterranean sea which is needed to obtain the extreme dryness of the sahara the use of this idealized model is finally discussed for studying the scale interactions and coupling involved in the west african monsoon as explored in a companion paper

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

abrupt shift of the itcz over west africa and intra seasonal variability the onset of the monsoon system over west africa is linked to the northward migration of the inter tropical convergence zone itcz during the northern spring and summer by using daily gridded rainfall data and ncep ncar wind reanalyses over the period 1968 1990 we show that this migration is characterised by an abrupt latitudinal shift of the itcz in late june from a quasi stationary location at 5n in may june to another quasi stationary location at 10n in july august a composite analysis based on the shift dates shows that this northward shift is associated with the occurrence of a westward travelling monsoon depression pattern over the sahel with characteristic periodicities of 20 40 days

the west african monsoon dynamics part iii the quasi biweekly zonal dipole abstract this paper presents an investigation of the mechanisms giving rise to the main intraseasonal mode of convection in the african monsoon during northern summer here identified as the quasi biweekly zonal dipole qbzd the qbzd is primarily characterized by a quasi stationary zonal dipole of convection whose dimension is larger than the west african monsoon domain with its two poles centered along the guinean coast and between 30 and 60 w in the equatorial atlantic the qbzd dynamical processes within the atlantic africa domain are examined in some detail the qbzd has a dipole pattern associated with a walker type circulation in the near equatorial zonal plane it is controlled both by equatorial atmospheric dynamics through a kelvin wave like disturbance propagating eastward between its two poles and by land surface processes over africa inducing combined fluctuations in surface temperatures surface pressure and low level zonal winds off the coast of west africa when convection is at a min

observations of eastward propagation of atmospheric intraseasonal oscillations from the pacific to the atlantic received 19 april 2010 revised 20 september 2010 accepted 29 september 2010 published 19 january 2011 1 significant 40 60 day intraseasonal variability in surface winds sea level and thermocline depth were observed in the tropical atlantic during the atlantic nino year of 2002 satellite derived quikscat winds and noaa outgoing longwave radiation olr measurements for the period of 2000 2006 together with reanalysis of winds from a longer record are analyzed to understand the sources of this 40 60 day wind variability and the global propagation of the madden julian oscillation mjo surface signatures the results demonstrate that the mjo propagated eastward from the indo pacific ocean to the atlantic during winter and spring of 2002 causing 40 60 day wind variations in the equatorial atlantic the isthmus of panama appears to be a dominant pathway for these surface wind anomalies to propagate into the atlantic where they can produce important climate impacts this result is consistent with previous work that has demonstrated a strong mjo influence on the north american monsoon nam region and complements other work demonstrating the propagation of intraseasonal wind anomalies and accompanying sea level pressure signals associated with dry equatorial kelvin waves that reach the caribbean through the panama gap the mjo is also shown to have a large influence on the subtropical atlantic ocean seasonality of the mjo impact on atlantic surface winds is investigated using data from an extended record and the most direct influence is found to be during boreal winter and spring with a delayed influence during summer and fall

modulation of hurricane activity in the gulf of mexico by the madden julian oscillation the madden julian oscillation mjo is a large scale episodic modulation of tropical winds and precipitation that travels eastward from asia to america with a characteristic repeat time of 30 to 60 days here it is shown that when mjo wind anomalies in the lower troposphere of the eastern pacific are westerly gulf of mexico and western caribbean hurricane genesis is four times more likely than when the mjo winds are easterly accurate predictions of the mjo may lead to improved long range forecasts of tropical cyclone activity

elastic behaviour of north sea chalk a well log study we present two different elastic models for respectively cemented and uncemented north sea chalk well log data we find that low biot coefficients correlate with anomalously low cementation factors from resistivity measurements at low porosity and we interpret this as an indication of cementation in contrast higher biot coefficients and correspondingly higher cementation factors characterize uncemented chalk for the same low porosity accordingly the poisson s ratio porosity relationship for cemented chalk is different from that of uncemented chalk we have tested the application of the self consistent approximation which here represents the unrelaxed scenario where the pore spaces of the rock are assumed to be isolated and the gassmann theory which assumes that pore spaces are connected as tools for predicting the effect of hydrocarbons from the elastic properties of brine saturated north sea reservoir chalk in the acoustic impedance poisson s ratio plane we forecast variations in porosity and hydrocarbon saturation from their influence on the elastic behaviour of the chalk the gassmann model and the self consistent approximation give roughly similar predictions of the effect of fluid on acoustic impedance and poisson s ratio but we find that the high frequency self consistent approach gives a somewhat smaller predicted fluid saturation effect on poisson s ratio than the low frequency gassmann model the gassmann prediction for the near and potentially invaded zone corresponds more closely to logging data than the gassmann prediction for the far virgin zone we thus conclude that the gassmann approach predicts hydrocarbons accurately in chalk in the sonic frequency domain but the fluid effects as recorded by the acoustic tool are significantly affected by invasion of mud filtrate the amplitude versus angle ava response for the general north sea sequence of shale overlying chalk is predicted as a function of porosity and pore fill the ava response of both cemented and uncemented chalk generally shows a declining reflectivity coefficient versus offset and a decreasing normal incidence reflectivity with increasing porosity however for the uncemented model a phase reversal will appear at a relatively lower porosity compared to the cemented model

development characteristics and dynamic structure of tropical intraseasonal convection anomalies abstract the development and dynamical structure of intraseasonal low frequency convection anomalies in the equatorial region are investigated using 10 years 1975 85 of outgoing longwave radiation olr and 7 years 1979 85 of 200 and 850 mb wind data the composite olr anomalies for 36 cases show a four stage development process initiation over equatorial africa rapid intensification when passing through the indian ocean mature evolution characterized by a weakening in the maritime continent and redevelopment over the western pacific and dissipation near the date line in moderate events or emanation from the equator toward north america and southeastern pacific in strong events a noticeable feature in vertical structure is that the 850 mb convergence leads convection and midtropospheric upward motion by about 30 degrees longitude in both developing and mature phases equatorial upper lower level easterly westerly anomalies and associated twin anomalous anticyclonic cyclonic circulation an

the impact of common offset migration on porosity estimation by avo inversion the impact of prestack time migration on porosity estimation has been tested on a 2 d seismic line from the valhall hod area in the north sea porosity is estimated in the cretaceous chalk section in a two step procedure first p wave and s wave velocity and density are estimated by amplitude variation with offset avo inversion these parameters are then linked to porosity through a petrophysical rock data base based on core plug analysis the porosity is estimated both from unmigrated and prestack migrated seismic data for the migrated data set a standard prestack kirchhoff time migration is used followed by simple angle and amplitude corrections compared to modern high cost true amplitude migration methods this approach is faster and more practical the test line is structurally fairly simple with a maximum dip of 5 but the results differ significantly depending on whether migration is applied prior to the inversion the maximum difference in estimated porosity is of the order of 10 about

the boreal summer intraseasonal oscillation relationship between northward and eastward movement of convection the summertime intraseasonal oscillation iso is an important component of the south asian monsoon lagged regressions of intraseasonally filtered 25 80 days outgoing longwave radiation olr reveal that centers of convection move both northward and eastward from the central equatorial indian ocean subsequent to the initiation of an iso eastward movement of convection is also seen at indian subcontinent latitudes 10 8 208n based on the regression results the summertime iso convection signal appears as a band tilting northwestward with latitude and stretching from the equator to about 208n viewed along any meridian convection appears to propagate northward while equatorial convection propagates to the east to examine the robustness of the connection between eastward and northward movement individual isos are categorized and composited relative to the strength of the large scale eastward component of convection in the central equatorial indian ocean it is found that the majority of isos that exhibit northward movement onto the indian subcontinent 42 out of 54 isos or 78 also exhibit eastward movement into the western pacific ocean it is also found that when convection in the central indian ocean is not followed within 10 20 days by convection in the western pacific ocean 12 out of 54 isos or 22 the independent northward movement of convection in the indian ocean region is somewhat stunted the link between the eastward and northward movement of convection is consistent with an interpretation of the summertime iso in terms of propagating equatorial modes the northward moving portion of convection is forced by surface frictional convergence into the low pressure center of the rossby cell that is excited by equatorial iso convection a similar convergence pattern is seen for the northern winter iso but it does not generate poleward movement due to relatively cool ssts underlying the surface convergence

the 30 50 day mode at 850 mb during monex abstract the analyzed wind field at 850 mb during the summer monsoon experiment monex is subjected to a time series analysis to confirm the existence of a peak in the time range of 30 50 days having established that this study presents a mapping of the motion field for this time scale in this short paper we illustrate the steady meridional propagation of a train of troughs and ridges that seem to form near the equator and dissipate near the himalayas the meridional scale of this mode is around 300 km and its meridional speed of propagation is 0 75 latitude per day the amplitude of the wind for this mode is around 3 6 m s 1 the salient contributions here are the mapping and the demonstration of a very regular behavior of this mode its existence is here noted from a period well before the onset of monsoon i e from early may to late july three major storms during monex were noted to form within the trough line of this system and the period of major cessation of rains over the indian subconti

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

a mechanism for african monsoon breaks mediterranean cold air surges 1 surges of cold air from the mediterranean into northern africa during the boreal summer are documented and their influence on monsoon breaks is analyzed using tropical rainfall measuring mission rainfall estimates and reanalysis products between 1998 and 2006 6 10 cold air surges occurred each summer with low level temperature anomalies ranging from less than 1 k to over 6 k composite analysis indicates that cold air surges over northern africa persist for 2 10 days and travel equatorward at approximately 5 5 m s 1 which is 0 5 1 5 m s 1 faster than the observed climatological low level meridional flow northern african cold surges have characteristics similar to surges observed elsewhere in the world including a hydrostatically induced ridge of surface pressure and an amplified upper tropospheric ridge trough pattern the african cold surge is preceded by the passage of a shortwave trough and an intensification of the upper tropospheric subtropical westerly jet streak over the mediterranean sea these events are associated with increased confluence in the jet entrance region over the central mediterranean an enhanced direct secondary circulation subsidence and low level ageostrophic northerly flow over northeastern africa composite analysis shows that the passage of a cold surge is associated with an enhancement in convective activity over southern algeria western niger northern mali and mauritania 2 to 5 days before the surge reaches the eastern sahel 17 5 n when northeasterly flow channeled between the atlas and ahaggar mountains strengthens and transports relatively moist air from the western mediterranean and eastern north atlantic over the region and increases moisture convergence over western africa north of 20 n over the eastern sahel of sudan and eastern chad the composite results reveal a break in convective activity and decrease in low level convergence when the surge arrives that persists for about 6 days these results offer great promise for improving the short range prediction of rainfall over northern africa

on the northward shift of the west african monsoon the regional climate model mar is used to perform a simulation of the year 1992 over west africa it is shown that mar is able to simulate the main features of the rainy regime over west africa and especially the discontinuous seasonal progression of the west african monsoon along the year one particular feature that is reasonably well reproduced is the abrupt shift of the rain band from 5 to 10 n at the end of june also called monsoon jump this study suggests that such a phenomenon is associated with the shift of the saharan heat low between two favourite positions one being over the sahelian area 10 15 n and the other over the saharan area 20 25 n these two favourite locations of the heat low are linked to the spatial distribution of surface albedo over west africa that drives the spatio temporal location of the surface temperature maxima a detailed analysis of this monsoon jump is performed and the causes of the strong decrease in precipitation that precedes the northward shift of the rain band are also investigated

primary and successive events in the madden julian oscillation conventional analyses of the mjo tend to produce a repeating cycle such that any particular feature cannot be unambiguously attributed to the current or previous event we take advantage of the sporadic nature of the mjo and classify each observed madden julian mj event as either primary with no immediately preceding mj event or successive which does immediately follow a preceding event 40 of mj events are primary events precursor features of the primary events can be unambiguously attributed to that event a suppressed convective anomaly grows and decays in situ over the indian ocean prior to the start of most primary mj events an associated mid tropospheric temperature anomaly destabilises the atmosphere leading to the generation of the active mj event hence primary mj events appear to be thermodynamically triggered by a previous dry period although stochastic forcing may also be important other theories predict that boundary layer convergence humidity propagation of dynamical structures around the equator sea surface temperatures and lateral forcing by extratropical transients may all be important in triggering an event although precursor signals from these mechanisms are diagnosed from reanalysis and satellite observational data in the successive mj events they are all absent in the primary mj events hence it appears that these apparent precursor signals are part of the mjo once it is established but do not play a role in the spontaneous generation of the mjo the most frequent starting location of the primary events is the indian ocean but over half of them start elsewhere from the maritime continent to the western pacific copyright 2008 royal meteorological society

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

teleconnections between the tropical pacific and the sahel a greater understanding of equatorial teleconnections is a key aspect of research into seasonal prediction and future climate change for tropical regions here the impact of pacific sea surface temperature sst anomalies on rainfall fluctuations over the semi arid sahel is explored using a combination of observed and model data the first key topic of this study is the identification of those aspects of anomalous pacific sst variability that are most strongly linked to the sahel one of these also illuminated by earlier studies is similar to the classic el nino southern oscillation pattern in the central and east pacific which in its el nino phase increases the likelihood of sahel drought it is shown here that although a part of this link is indirect operating via atlantic ssts its main effect appears to be through a direct atmospheric teleconnection the other critical pattern of equal importance and revealed here by a novel analysis technique is the large scale zonal gradient of ssts from the west pacific to the east indian ocean if weakened this too enhances the likelihood of sahel drought atmospheric general circulation model experiments forced either by observed or idealized ssts are used to confirm these two influences on the sahel crucially their sahelian impact is substantially reinforced when both are present and additionally further empirical analysis shows them to be largely independent the second key topic is an investigation of the mechanisms for this pacific sahel teleconnection these appear to involve anomalous stationary equatorial waves with communication occurring in both the eastward and westward directions in el nino years for example a kelvin wave emanates across the atlantic from east pacific convective heating anomalies and an equatorial rossby wave appears over the indian ocean in response to the anomalous west pacific indian ocean sst gradients via convective heating anomalies over the indian ocean these interact over africa to enhance large scale subsidence over the sahel thus reducing seasonal rainfall totals interannual changes in propagating equatorial waves or in the residence of subseasonal regimes appear not to play a substantial role

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