allochthonous slope sequence at lobster cove head evidence for a complex middle ordovician platform margin in western newfoundland the upper part of an intact sequence of lower to middle ordovician deep water sediments which now form a large disrupted raft within the rocky harbour melange at lobster cove head is interpreted as having been deposited downslope from a drowned carbonate platform margin the entire 50 m thick section is arenig late canadian or ibexian to early whiterock in age graptolite biostratigraphy demonstrates a correlation with upper parts of the cow head group to the north the basal part of the section is a proximal facies of the cow head group shallow bay formation factory cove member beds 9 10 and part of bed 11 the upper part of the section consists of interbedded dolostone and shale and is unlike any other sequence in the cow head group this upper sedimentary sequence is defined as the lobster cove head member of the shallow bay formation cow head group contact between the two sedimentary packages is also marked by a faunal break and coincides with emplacement of megaconglomerate bed 12 at cow

soil aquifer stream interactions a reductionist attempt toward physical stochastic integration abstract the paper suggests an approach to describe the various interactions between soil aquifer and stream in a simplified but essentially physical and integrated manner for practical applications the complex hydrological processes cannot be thoroughly described at the smallest scale in time and space at which theoretical or empirical physical laws are known the successful passage from a smaller to a larger scale requires enlightened simplification integration in many senses that is 1 in time 2 in space 3 in an expectation sense and 4 in a process sense and finally enlightened coupling the paper illustrates in a simple manner this processus of multiple integration for the processes of infiltration moisture redistribution aquifer recharge and aquifer return flow with emphasis on the stream aquifer interaction the paper suggests that temporal patterns of rainfall spacial variation in soil properties temporal fluctuations in river stage sharp turns in flow directions e g from vertical to horizontal direction at the interface of two soil layers with contrasting hydraulic properties or in the vicinity of a water table etc can significantly affect the response of a watershed the paper also suggests that simple techniques can be devised that account to a large extent for these influences and will provide satisfactory tools for watershed modeling as a sort of parenthesis the paper shows that the purely statistical arma approach to hydrologic modeling will lead to incorrect identification of a perfectly stationary linear system as a nonstationary one the combined physical stochastic approach eliminates this potential error

graptolites from the lower middle ordovician st george and table head groups western newfoundland and their correlation with trilobite brachiopod and conodont zones faunal assemblages of the autochthonous shelf carbonate sequences belonging to the st george and table head groups are dominated by shelly macrofossils and conodonts rare usually monotypic graptolitic horizons enable correlation with the allochthonous cow head group which was deposited on the middle to lower slope and the shelly conodont and graptolitic zonal schemes elsewhere in north america the catoche formation of the st george group is of ibexian canadian age and yields graptolites indicative of the tetragraptus approximatus and tetragraptus akzharensis zones early arenig the basal aguathuna formation belongs to the pendeograptus fruticosus zone whereas higher parts span the ibexian whiterock boundary graptolites suggest that the table head group entirely postdates the cow head group this is supported by a whiterock early llanvirn trilobite conodont and brachiopod fauna

on the structure and dynamics of indian monsoon depressions abstractera interim reanalysis data from the past 35 years have been used with a newly developed feature tracking algorithm to identify indian monsoon depressions originating in or near the bay of bengal these were then rotated centralized and combined to give a fully three dimensional 106 depression composite structure a considerably larger sample than any previous detailed study on monsoon depressions and their structure many known features of depression structure are confirmed particularly the existence of a maximum to the southwest of the center in rainfall and other fields and a westward axial tilt in others additionally the depressions are found to have significant asymmetry owing to the presence of the himalayas a bimodal midtropospheric potential vorticity core a separation into thermally cold 1 5 k and neutral 0 k cores near the surface with distinct properties and the center has very large cape and very small cin variability as a function of background state has also been explo

recharge of aquifers by flood events in an arid region the process of aquifer recharge by flood events in an arid region was investigated as applied to the hazeva formation the karkom graben the wadi paran watershed israel the hydrological model was established as a complex system with due regard for groundwater and transmission losses of surface runoff it was based on a previously outlined hydrogeological model of the karkom graben and a model of transmission losses in arid watercourses under conditions of data deficiency proceeding from calculation of groundwater balances the contribution of surface runoff as a decisive balance component was confirmed the main characteristics of aquifer regimes such as changes in storage volume and groundwater level as well as lateral flow were all found to be dependent upon the net extraction rate i e pumpage discounting replenishment by flood events analysis and physical interpretation of model parameters enabled assessment of the influence of groundwater extraction on aquifer recharge this became apparent as increasing absorption capacity and recharge availability of the aquifer as a result of the groundwater abstraction copyright 2002 john wiley sons ltd

westward propagating predecessors of monsoon depressions abstract by examining maps of 24 h change of sea level pressure during july and august over the 10 year period 1969 78 we find that 45 87 of the 52 lows and depressions that formed in the bay of bengal were associated with predecessor disturbances coming from the cast in 12 23 instances the predecessor was associated with a typhoon or named tropical storm in the south china sea while the remaining 33 64 were weaker systems originating over a broad region of land and sea from examination of time sections over the same period from eastern thailand to the burmese coast we identified 50 westward moving disturbances with consider able vertical extent of which 64 developed into lows or depressions on reaching the bay in 60 of the 50 instances the disturbance could be traced to the south china sea 32 being typhoons and named tropical storms and 28 weaker circulations the remaining 40 of the 50 disturbances appeared to originate over land for the three stations along the section line cr

adiabatic westward drift of indian monsoon depressions a large fraction of the rain received by continental india is produced by cyclonic vortices with outer radii of about 1000 km that are contained within the larger scale south asian monsoon flow the more intense occurrences of these vortices are called monsoon depressions these consist of bottom heavy columns of relative vorticity that propagate to the northwest in time mean low level eastward flow previous studies have argued that this apparent upstream propagation is caused by dynamical lifting west of the vortex centre with the resulting ascent producing vortex stretching that shifts the vortex to the west here analysis of over 100 indian monsoon depressions is used to show that low level vortex stretching has a spatial structure inconsistent with the observed propagation and is balanced by other terms in the low level vorticity budget instead monsoon depressions are shown to consist of potential vorticity maxima that have peak amplitude in the middle troposphere and propagate westward by nonlinear horizontal adiabatic advection i e beta drift the precipitating ascent in monsoon depressions makes a more minor contribution to the total storm motion and primarily acts to maintain the upright structure of the vortex these results suggest a new view of indian monsoon depressions as potential vorticity columns that propagate primarily by adiabatic dynamics

active synsedimentary tectonism on a mixed carbonate siliciclastic continental margin third order sequence stratigraphy of a ramp to basin transition lower sekwi formation selwyn basin northwest territories canada the lower part of the early cambrian sekwi formation in the selwyn basin of the northwest territories canada is composed of two regional unconformity bounded sequences s0 and s1 which record the first widespread carbonate deposition during the initial palaeozoic transgression onto the western margin of laurentia these early cambrian sequences are unique to the western north american cordillera representing the only record of primarily deep water deposition on a tectonically active mixed carbonate siliciclastic ramp during this period more specifically the geometry of the sekwi ramp changed during deposition of s0 and s1 from a shallowly dipping homoclinal ramp during the s0 transgressive systems tract to a steeply dipping tectonically modified ramp during the early highstand systems tract of s0 the steeply dipping ramp profile of s0 was preserved into the early transgressive systems tract of s1 the sekwi ramp returned to a gently sloping ramp during the late highstand systems tract of s1 and remained so throughout the remainder of sekwi deposition the evolving shape of the sekwi ramp is attributed to syndepositional down to the basin faulting during deposition of both s0 and s1 and is recorded by i the westward thickening irregular geometries of s0 and s1 ii geographical restriction of deep water facies including sediment gravity flow deposits iii the presence of large allochthonous blocks and iv the clast composition of sediment gravity flow deposits sediment gravity flow deposits play an unusually important role in the sequence stratigraphic interpretation of the lower sekwi formation as they delineate depositional packages including the maximum flooding zone the transitions between portions of systems tracts and the inferred locations of syntectonic extensional faults syntectonic faults increased accommodation basinward of an extensive ooid shoal complex that developed along the sekwi ramp crest greatly influencing sequence geometry and initiating the downslope motion of sediment gravity flows the syndepositional faulting probably was a continuation of extension that began during the latest neoproterozoic rifting of western laurentia the composition of sediment gravity flow deposits track changing accommodation space on the lower sekwi ramp and can be used to differentiate systems tracts that probably were related more to tectonism than eustasy

a mechanism for the west north west movement of monsoon depressions the monsoon depressions intensify over the bay of bengal move in a west north west wnw direction and dissipate over the indian continent no convincing physical explanation for their observed movement has so far been arrived at but here i suggest why the maximum precipitation occurs in the western sector of the depression and propose a feedback mechanism for the wnw movement of the depressions we assume that a heat source is created over the bay of bengal due to organization of cumulus convection by the initial instability in a linear sense heating at this latitude 20 n produces an atmospheric response mainly in the form of a stationary rossby gravity wave to the west of the heat source the low level vorticity hence the frictional convergence and the vertical velocity associated with the steady state response is such that the maximum moisture convergence and precipitation is expected to occur in the wnw sector at a later time thus the heat source moves to the wnw sector at a later time and the feedback continues resulting in the wnw movement of the depressions

the paradox of drowned reefs and carbonate platforms shallow water carbonate platforms and reefs are drowned when tectonic subsidence or rising sea level outpaces carbonate accumulation and benthonic carbonate production ceases drowned platforms are common in the geologic record but they present a paradox if one considers rates of processes involved during the early holocene when sea level rose at rates of 6 000 to 10 000 m yr mm 1 000 yr most reefs and platforms were outpaced by the rising sea during the late holocene with sea level rising 500 to 3 000 m yr in the atlantic caribbean area reefs and platforms started to recover built to sea level and prograded seaward 1 000 m yr is thus a conservative estimate of the average growth potential of modern reefs and platforms independently accumulation rates of prograding platforms in the geologic record suggest growth potential in excess of several hundred microns per year the growth potential of 1 000 m yr exceeds any relative rise of sea level caused by long term processes in the geologic record newly formed ocean crust subsides at a maximum of 250 m yr basin subsidence averages 10 to 100 m yr and sea level rises due to increased sea floor spreading amount to less than 10 m yr rapid pulses of relative rise of sea level or reduction of benthic growth by deterioration of the environment remain the only plausible explanations of drowning the geologic record shows examples of both of these processes global mass extinctions of reefs and platforms occurred in the middle cretaceous eustatic rise due to submarine volcanism or desiccation of a small ocean basin and the late devonian global crisis of ocean environment extraterrestrial cause drowning controlled by regional tectonics prevailed in the jurassic and early cretaceous of the tethyan realm and the drowning of mesozoic platforms in the western north atlantic seems to have been dictated by plate tectonic drift to higher latitudes

quasi geostrophic diagnosis of the monsoon depression of 5 8 july 1979 abstract quasi geostrophic diagnosis of the fields of vertical motion and streamfunction tendency based on wind analysis was undertaken for the monsoon depression of 5 8 july 1979 a uniquely well observed case during summer monex wind analysis in terms of streamfunction and velocity potential was performed for data averaged in three layers of equal pressure depth front 1000 to 100 mb the thermal stratification near the depression above 850 mb was approximately moist adiabatic the wind analyses showed that the basic zonal currents westerly in the lower troposphere and easterly in the middle levels weakened as the depression formed central vorticity increased rapidly on 5 6 july to a magnitude three times the coriolis parameter the center tilted toward the southwest with elevation the quasi geostrophic omega equation was solved at the interfaces between layers for a range of stabilities from the full dry adiabatic value to 1 of it ascent west of the center for the near neutral stability was

early development and flysch sedimentation in ordovician taconic foreland basin west central newfoundland during the early to late ordovician the taconic foredeep in west central newfoundland evolved from an underfilled to an overfilled state in response to cratonward advance thickening and erosion of the taconic orogen early orogen derived sediment in the foreland basin consisted of middle to lake arenigian deep water mudstones that accumulated on an inner craton facing slope prism uppermost parts of shallow bay and green point formations and correlative units these deposits are interbedded with and overlie passive margin slope sediments in the middle arenigian to early llanvirnian sand from the orogen formed several small sand rich submarine fans lower head formation and correlative units on the lower reaches of the inner slope and basin plain the fans may have been fed by closely spaced rivers draining the orogen as presently occurs in western south america only proximal portions of these fans are now exposed the flysch basin was narrow constricted by the inner slope and the passive ma

groundwater recharge estimation from ephemeral streams case study wadi tabalah saudi arabia estimation of groundwater recharge to an unconfined aquifer is studied using analytical and numerical techniques and results are compared with field observations there is an acute need for such estimation in water balance studies in arid climates and the case study in this paper is for such a region the wetting front movement in the unsaturated zone depends on antecedent soil moisture the ponded water depth and its duration and on the position of the water table and the hydraulic properties of the unsaturated zone a hydraulic connection between the recharge basin and the aquifer is not immediately established because the wetting front is unsaturated a numerical model is applied to estimate recharge in an arid zone wadi and its validity is tested by comparing it with an analytical solution of the equations the calculated recharge values matched the piezometric levels observed at a well site at the edge of the wadi channel the total recharge depths found by integration in the time domain provided a good estimate of the transmitted volume of water per unit length of wadi channel the findings were confirmed by runoff volume measurements at gauging stations located in the basin 1997 john wiley sons ltd

arenig biostratigraphy of the pointe de levy slice quebec appalachians canada the pointe de levy slice includes a nearly complete arenig graptolite succession a conspicuous gap exists in the yapeenian ya to lower darriwilian da part ya 1 da 1 the lithological column is compiled from several sections at levis quebec that yield only parts of the complete succession the graptolite zonation can be compared closely with that from the cow head group of western newfoundland and the deep kill formation of eastern new york the shumardia limestone is differentiated into four graptolite zones and subzones of which the youngest zone clearly belongs to the llanvirn the araneograptus murrayi and holmograptus lentus zones are documented for the first time from the levis area the undulograptus dentatus zone of the latest arenig is introduced

lithostratigraphic and allostratigraphic framework of the cambrian ordovician potsdam group and correlations across early paleozoic southern laurentia the potsdam group is a cambrian to lower ordovician siliciclastic unit that crops out along the southeastern margins of the ottawa graben from its base upward the potsdam consists of the ausable hannawa falls and keeseville formations in addition the potsdam is subdivided into three allounits allounit 1 comprises the ausable and hannawa falls and allounits 2 and 3 respectively the lower and upper parts of the keeseville allounit 1 records early to middle cambrian syn rift arkosic fluvial sedimentation ausable formation with interfingering mudstone arkose and dolostone of the marine altona member recording transgression of the easternmost part of the ottawa graben rift sedimentation was followed by a middle cambrian climate change resulting in local quartzose aeolian sedimentation hannawa falls formation allounit 1 sedimentation termination coincided with latest middle cambrian tectonic reactivation of parts of the ottawa graben allounit 2 lower keeseville records mainly upper cambrian quartzose fluvial sedimentation with transgression of the northern ottawa graben resulting in deposition of mixed carbonate siliciclastic strata of the marine riviere aux outardes member sedimentation was then terminated by an earliest ordovician regression and unconformity development allounit 3 upper keeseville records diachronous transgression across the ottawa graben that by the arenigian culminated in mixed carbonate siliciclastic shallow marine sedimentation theresa formation the contact separating the potsdam group and theresa formation is conformable except locally in parts of the northern ottawa graben where the presence of localized islands and or coastal salients resulted in subaerial exposure and erosion of the uppermost potsdam strata and accordingly unconformity development

transient infiltration from ephemeral streams a field experiment at the reach scale 1 an infiltration experiment at the stream reach scale was performed to estimate infiltration rates beneath an ephemeral losing stream during streamflow events at a time when the stream was dry a 7 m stream section was dammed upstream and downstream using metal sheets during a 5 day period water was pumped into the isolated section of the stream and the surface water level was maintained at three successive increasing stages the infiltration rate at each water level was thereby equal to the pumping rate required to maintain that water level the advantages of the method are that it samples a much greater area than traditional methods and provides information on infiltration through stream banks as well as through the streambed experimental results provide insight into transient infiltration and recharge processes beneath ephemeral streams although the experiment continued for 5 days infiltration during the first hour accounted for 19 of the total infiltration high transient infiltration rates were also observed following each increase in stream stage experimental infiltration rates were used to calibrate a two dimensional model developed within hydrus which was subsequently used to estimate infiltration associated with a natural flow event in the same stream reach during the natural flow event the total infiltration was 33 greater than would have been estimated assuming steady state infiltration rates dry antecedent moisture content controls the transient infiltration rate and hence increases the total infiltrated volume during flow events but it does not increase the aquifer recharge

recharge from an ephemeral stream following wetting front arrival to water table after the infiltration wetting front from a river bed of an ephemeral stream in flood reaches the shallow water table the rate of recharge which is also that of infiltration is affected by the development of a groundwater mound the effect is practically immediate if the water content behind the wetting front is close to saturation an approximate analytical solution is developed in that case which predicts the time dependence of the recharge rate comparison with experimental results in the laboratory shows that the solution is excellent in spite of the assumptions necessary in the derivations as long as the ratio of river width over depth to water table exceeds 2 and as long as the ratio of initial saturated thickness over depth to the water table exceeds 2 5 the analytical solution is essentially exact under most field conditions these ratios are largely exceeded consequently the predictive equation for the time dependence of the recharge is both quite simple to use and adequately accurate for field applications

importance of stream infiltration data for modelling surface water groundwater interactions summary an infiltration experiment at the transect scale was performed in an ephemeral stream at the time it was dry resulting in direct estimates of stream infiltration groundwater heads measured in a transect of piezometers orthogonal to the stream highlighted the development of a groundwater mound displaced several meters into the streambank the experiment was modelled using the two dimensional integrated surface subsurface model hydrogeosphere and calibrated with pilot points using different combinations of groundwater heads and infiltration volume data modelling results showed that despite the fact that groundwater heads were all below the streambed and that an unsaturated zone was present beneath the streambed the stream and the aquifer become laterally connected within several hours of flow onset through a high permeability preferential path in the streambank although the model can be calibrated to produce a good match to measured groundwater heads accurate predictions of stream infiltration can be made only if the heads used for model calibration capture the groundwater mound the use of infiltration flux or volume during model calibration instead provides the best calibration results and thus estimates of stream infiltration during natural flow events given that heterogeneity of streambed and streambank sediments leads to the possibility that the groundwater mound developed during flow events will be poorly characterised or perhaps missed altogether incorporating infiltration flux or volume data into the model calibration increases the likelihood of obtaining accurate estimates of stream infiltration during natural flow events

some simple analytical solutions to the problem of forced equatorial long waves this paper describes some simple time dependent analytical solutions to the problem of a sudden switch on of heating localized about the equator the equations used are the shallow water equations on the plane under the long wave approximation it is shown that with a reasonable amount of dissipation it can take several days for a steady state response to be established in the early stages the circulation differs considerably from that of the steady state the long time scale for the adjustment process has implications in numerical weather prediction since it implies that if the initial data are not in balance with the model s forcing it may take several days for a balance to be established by which time the quasi stationary tropical flow may have been irrevocably changed

the interaction of indian monsoon depressions with northwesterly midlevel dry intrusions abstractmonsoon depressions mds bring substantial monsoon rainfall to northern and central india these events usually form over the bay of bengal and travel across northern india toward pakistan using european centre for medium range weather forecasts interim reanalysis an md tracking algorithm and an objective identification method the authors find that about 40 of mds interact with northerly intrusions of dry desert air masses as the mds traverse the subcontinent md interactions with dry intrusions are often preceded by positive potential vorticity pv anomalies on the subtropical jet and low level anticyclonic anomalies over the north arabian sea dry intrusions nearly halve the precipitation rate in the southwest quadrant of mds where mds rain the most however dry intrusions increase the rainfall rate near the md center similarly ascent is reduced west of the md center and enhanced at the md center especially in the upper troposphere the reduced ascent west of md centers is likely att

brachiopod biostratigraphy of the middle ordovician cow head and table head groups western newfoundland brachiopods from shallow water limestone boulders in beds 12 13 and 14 of the allochthonous cow head group belong in the earliest whiterockian orthidiella zone brachiopods of the autochthonous intertidal to deep water table head group are equivalent to those of the younger whiterockian anomalorthis zone the cow head is older than the table head integration of the brachiopod data from western newfoundland with those from the type whiterockian in nevada combined with the evidence of graptolites from both areas indicates that the orthidiella zone spans the australian castlemainian yapeenian and lower darriwilian dal zones

chondrites a trace fossil indicator of anoxia in sediments the trace fossil chondrites a highly branched burrow system of unknown endobenthic deposit feeders occurs in all types of sediment including those deposited under anaerobic conditions in some cases such as the jurassic posidonienschiefer formation of germany chondrites occurs in black laminated carbonaceous sediment that was deposited in chemically reducing conditions in other cases such as numerous oxic clastic and carbonate units throughout the geologic column chondrites typically represents the last trace fossil in a biotutbation sequence this indicates that the burrow system was produced deep within the sediment in the anaerobic zone below the surficial oxidized zone that was characterized by freely circulating and oxidizing pore waters

structural and tectonic evolution of the humber zone western newfoundland 2 a regional model for acadian thrust tectonics the humber zone of the western newfoundland appalachians represents a cambrian ordovician passive continental margin which was deformed in taconian mid ordovician and acadian silurian devonian orogenic events a deformation front is imaged in seismic reflection data offshore of western newfoundland structures associated with this deformation front are exposed on port au port peninsula where silurian rocks are strongly deformed but mississippian strata are flat lying indicating that latest thrusting was acadian a gravity low in the gulf of st lawrence corresponds to a sediment filled acadian foreland basin previous models suggest that the on land shelf succession is autochthonous to parautochthonous however two lithoprobe seismic reflection transects show sub horizontal reflections between 2 and 5 s two way travel time which extend up to 85 km east of the thrust front these are interpreted as autochthonous platform and basement in this model shallower reflectors and outcropping units include both allochthonous platform and basement comprising the acadian port au port allochthon the taconian humber arm allochthon was carried westward as a high structural slice during thrusting of this allochthon no major structural discontinuity exists between grenville age crystalline rocks of the long range massif and platform rocks interpreted as allochthonous in the northern seismic line a monocline at the southern extremity of the long range probably represents an oblique or lateral hanging wall ramp above the basal detachment within the long range thrust zone at the western margin of the massif the long range thrust shows only a few kilometers of displacement however the parsons pond thrust which we interpret to run offshore at green point juxtaposes contrasting successions with different structural and thermal histories it probably carries a much larger amount of the total displacement the basal decollement of the port au port allochthon is therefore interpreted to pass beneath the southern part of the long range massif

transmission losses infiltration and groundwater recharge through ephemeral and intermittent streambeds a review of applied methods summary aquifer recharge through ephemeral streambeds is believed to be a major source of groundwater recharge in arid areas however comparatively few studies quantify this streamflow recharge this review synthesizes the available field based aquifer recharge literature from arid regions around the world seven methods for quantifying ephemeral and intermittent stream infiltration and aquifer recharge are reviewed controlled infiltration experiments monitoring changes in water content heat as a tracer of infiltration reach length water balances floodwave front tracking groundwater mounding and groundwater dating the pertinent temporal and spatial scales as well as the advantages and limitations of each method are illustrated with examples from the literature comparisons between the methods are used to highlight appropriate uses of each field method with emphasis on the advantages of using multiple methods within a study in order to avoid the potential drawbacks inherent in any single method research needs are identified including quantitative uncertainty analysis long term data collection and analysis understanding of the role of riparian vegetation and reconciliation of transmission losses and infiltration estimates with actual aquifer recharge

climatic controls on diffuse groundwater recharge in semiarid environments of the southwestern united states 1 although there is no diffuse groundwater recharge at many semiarid sites evidence for diffuse recharge exists at some locations where mean annual precipitation is much less than mean annual potential evapotranspiration particularly where soils are coarse and rainfall variable is substantial we investigate the climatic controls on diffuse recharge using a one dimensional variably saturated flow model the model is driven by a stochastic parameterization of climate that includes storm size distribution and seasonality of precipitation p and potential evapotranspiration pet constrained by data from 536 weather stations in the southwestern united states storm size distribution and seasonality determine the frequency and duration of intervals when p exceeds pet which controls the flux of water past the root zone for coarse soils climates with large infrequent storms yield recharge when exceeds 0 4 compared with 0 7 for a typical climate recharge through fine soils is insensitive to storm size and occurs at 0 8 seasonality has a stronger influence on recharge than storm size distribution and the effects are similar for coarse and fine soils recharge is relatively insensitive to rainfall seasonality in contrast the typical pet annual cycle lowers of the recharge threshold by 0 3 the relative timing of p and pet maxima is critical recharge occurs at values that are lower by 0 2 when the rainy season occurs during winter instead of summer over the range of climate and soils examined values at the recharge threshold varied from 0 2 to 0 7 therefore and alone are insufficient to predict where recharge will occur

a global climatology of monsoon low pressure systems the first global climatology of monsoon low pressure systems is presented here based on the era interim reanalysis low pressure systems are classified into three intensity categories and particular focus is given to systems in the category corresponding to a traditional definition of monsoon depressions vortex tracks are identified using an automated algorithm applied to the distributions of 850 hpa relative vorticity sea level pressure and surface wind speed for 1979 2012 roughly two to three times as many monsoon low pressure systems form in the northern hemisphere as in the southern hemisphere during local summer the frequency of genesis typically peaks in local summer but low pressure systems form throughout the year in every monsoon region interannual variability is weak with standard deviations of summer counts typically being below 10 of the long term summer mean regional composites reveal that monsoon depressions in india the western pacific and northern australia share a common structure consisting of a warm over cold core and a top heavy column of potential vorticity that extends from the surface to the upper troposphere a separate class of monsoon low pressure systems develops over dry regions of west africa and western australia with a shallow composite structure having a warm core in the lower troposphere and cyclonic potential vorticity confined to a thin near surface layer low pressure systems in nearly all monsoon regions are estimated to account for a large fraction from about 40 to more than 80 of summer precipitation on the poleward edge of the climatological mean precipitation maxima

the movement of indian monsoon depressions by interaction with image vortices near the himalayan wall it is argued that a simple explanation for the westward propagation of indian monsoon depressions imds is the interaction of the depression vortex with the himalayan wall this interaction can be modelled by simulating an imd as a point vortex in a horizontal plane at 850 hpa and invoking image vortices behind the barrier solenoidal flows associated with the image vortices allow the boundary conditions at the himalayas to be met and cause the imd vortex to propagate parallel to the barrier toward the west this simple model is tested against propagation speeds for observed imds the histogram of observed propagation speed normalised by the point vortex model prediction has a mean of 1 08 and standard deviation of 0 68 the model also explains the observed intensification of flow on the himalayan side of the imd which is a key process in enhancing rainfall to the indo gangetic plain in the monsoon season

the interaction of moist convection and mid level dry air in the advance of the onset of the indian monsoon the advance of the onset of the indian monsoon is here explained in terms of a balance between the low level monsoon flow and an over running intrusion of mid tropospheric dry air the monsoon advances over a period of about 6 weeks from the south of the country to the northwest given that the low level monsoon winds are westerly or southwesterly and the midlevel winds northwesterly the monsoon onset propagates upwind relative to midlevel flow and perpendicular to the low level flow and is not directly caused by moisture flux toward the northwest lacking a conceptual model for the advance means that it has been hard to understand and correct known biases in weather and climate prediction models the mid level northwesterlies form a wedge of dry air that is deep in the far northwest of india and over runs the monsoon flow the dry layer is moistened from below by shallow cumulus and congestus clouds so that the profile becomes much closer to moist adiabatic and the dry layer is much shallower in the vertical toward the southeast of india the profiles associated with this dry air show how the most favourable environment for deep convection occurs in the south and onset occurs here first as the onset advances across india the advection of moisture from the arabian sea becomes stronger and the mid level dry air is increasingly moistened from below this increased moistening makes the wedge of dry air shallower throughout its horizontal extent and forces the northern limit of moist convection to move toward the northwest wetting of the land surface by rainfall will further reinforce the north westward progression by sustaining the supply of boundary layer moisture and shallow cumulus the local advance of the monsoon onset is coincident with weakening of the mid level northwesterlies and therefore weakened mid level dry advection

the spatiotemporal structure of precipitation in indian monsoon depressions indian monsoon depressions are synoptic scale events typically spun up in the bay of bengal they usually last 4 6 days during which they propagate northwestward across the indian subcontinent before dissipating over northwest india or pakistan they can have a significant effect on monsoon precipitation particularly in primarily agrarian northern india and therefore quantifying their structure and variability and evaluating these in nwp models and gcms is of critical importance in this study satellite data from the cloudsat and recently concluded trmm missions are used in conjunction with an independently evaluated tracking algorithm to form a three dimensional composite image of cloud structure and precipitation within monsoon depressions the composite comprises 34 depressions from the 1998 2014 trmm mission and 12 from the 2007 present cloudsat mission and is statistically robust enough to allow significant probing of the spatiotemporal characteristics of moisture and hydrometeor fields among the key results of this work are the discovery and characterisation of a bimodal diurnal cycle in surface precipitation the first picture of the structure of cloud type and density in depressions showing that deep convection dominates south of the centre and prominent cirrus throughout the first composite picture of vertical hydrometeor structure in depressions showing significant precipitation for hundreds of kilometres outside the centre and well past the mid troposphere and novel discussion of drop size distributions showing significant uniformity across the depression and resulting latent heat profiles showing average heating rates near the centre can reach 2k hr

enhanced preservation of marine derived organic matter in cenomanian black shales from the southern angola basin black shales possessing high concentrations of organic carbon1 were deposited in many parts of the proto south atlantic ocean during the cretaceous period2 the way such sediments accumulated is not fully understood but is likely to have occurred through a combination of low oxygen availability and abundant supply of organic matter thin centimetre thick layers of black shales are commonly interbedded with thicker layers of organic carbon deficient green claystones as found in strata of aptian to coniacian age at deep sea drilling project dsdp site 530 in the southern angola basin3 and elsewhere these differences in carbon content and colour reflect the conditions of deposition and possibly variations in the supply of organic matter4 5 we have compared using organic geochemical methods the compositions of organic matter in three pairs of closely bedded black and green cenomanian claystones obtained from site 530 kerogen analyses and distributions of biological markers show that the organic matter of the black shales is more marine and better preserved than that of the green claystones

quasigeostrophic controls on precipitating ascent in monsoon depressions abstractsouth asian monsoon depressions are convectively coupled cyclonic vortices that form and intensify in a region of easterly vertical shear of the horizontal wind observations of maximum pre