on the generation of african squall lines abstract squall lines sls form an important component of the meteorology of northern africa and in particular contribute substantially to rainfall totals their generation requires the existence of a potentially unstable low level supply of moisture overlain by dry desert air and vertical wind shear beneath the midlevel african easterly jet the instability may be released and an sl initiated by factors such as surface heating topography african waves or surface evaporation the relative importance of each of these factors and the means by which they impact on sl generation is reviewed this is followed by a detailed analysis of one month of satellite imagery and surface data for august 1985 over a portion of central northern africa the novelty of our study lies in the temporal resolution of the satellite imagery which with 21 images per day allows the identification of a large number of short lived sls 4 h duration or less on the southern fringes of the sahara these are likely to contribute

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

simulating wave setup and runup during storm conditions on a complex barred beach abstract the purpose of this study is to assess the ability of the swash model to reproduce wave setup and runup in highly dissipative stormy conditions to proceed we use data collected during the ecors truc vert 08 experiment especially during the johanna storm in the winter of 2007 008 wave setup under hs 8 2 m and tp 18 3 s and runup under 6 4 m and peak period up to 16 4 s we test different model settings 1d and 2d mode and model forcing spectral and parametric to reproduce sensor measured wave setup at several locations in the nearshore area and video measured runup on two beach profiles for the whole tested configurations the wave setup is reproduced accurately results considering all the sensor locations in the near shore area in 1d and 2d are significantly correlated to the observations with respectively 0 66 and 0 81 rmse 0 13 m and 0 08 m without any significant bias observations and simulations of runup are investigated in terms of spectra and statistic component 1d simulations produces an overall overestimation and no significant improvement is obtained by modifying the breaking parameters the results for 2d simulations are fairly satisfactory reproducing significant swash height s but are significantly improved with spectral forcing than parametric with respectively 0 73 and 0 71 rmse 0 19 m and 0 43 m generally the model reproduces accurately the infragravity component but tends to overestimate the incident component leading to an overestimation of the energy density for moderate wave conditions and more accurate results for higher energy wave conditions results in 2d with spectral forcing show a saturation of the infragravity component with a threshold around hs 4 to 5 m which is comparable to the observations collected at truc vert beach as regards the conventional statistical parameter for runup estimation r2 three methods are applied to derive the 2 exceedence value for runup from observed and simulated shoreline vertical elevation time series when r2 is based on the sum of wave setup and half of the significant swash height results provided by the model are close or even better than estimations provided by empirical formulas from the bibliography defining r2 as the exceeded 2 values of the time derived considering the cumulative distribution function of the entire water level time series also provide fairly good results results using only runup maxima time series are less satisfactory in the two last cases r2 is slightly underestimated for moderate wave conditions hs

modelling rip current flow and bather escape strategies across a transverse bar and rip channel morphology abstract rip currents are a hazard to bathers on surf beaches worldwide yet due to logistical and ethical concerns there has been minimal examination of how bathers caught in them should attempt to escape this study presents the first numerical model of simulated bathers escaping from a rip current an understanding of the underlying coastal geomorphology and hydrodynamics form a critical basis for the model with the intention that outcomes of the model should be evaluated from a physical science perspective field observations of a transverse bar rip channel were used as model inputs escape simulations were performed with moderate energy h s 1 m oblique offshore wave forcing used to generate two non stationary asymmetric flow fields dominant surfzone exits and dominant recirculation the model treats bathers as particles that move with the underlying flow with an additional swimming velocity added iterating until a safety condition based on depth and current speed is satisfied or until a maximum time limit is reached simulations compared the strategies of stay afloat and various fixed swim directions using two bather heights and four swim speeds up to 0 4 m s the overall optimal escape strategy was to swim parallel in the direction of alongshore flow time to safety was less for i recirculating flow ii taller bathers and iii greater swim speeds across all simulations 44 of floaters reached safety within 10 min while 80 of slow swimmers 0 2 m s succeeded in the same time interval this suggests that slow sustainable swimming may be a preferable escape action to floating optimal swim direction varied with start location from onshore near the shoreline parallel to shore mid rip channel and diagonally onshore outer rip channel at lower swim speeds the optimal swim direction had greater dependence on the underlying flow field as opposed to distance to safety therefore choice of direction is more complex for slower swimmers critically no strategy succeeded in all scenarios at all locations across the surfzone and small changes to input conditions water level start time and location swim direction had major impacts on outcomes future modelling efforts should examine additional surf zone rip current morphologies and wave tide inputs with a goal of determining generalised rip escape principles that may be communicated to the general public

small fixed loop transient electromagnetic in tunnel forward geological prediction abstract tunnel forward geological prediction is indispensable to tunnel construction the transient electromagnetic method is widely used in the field of tunnel forward geological prediction a small fixed loop transient electromagnetic method adapted to the limited space of a tunnel face is proposed for tunnel forward geological prediction for the first time this method makes use of a large number of non central points inside the loop the obtained numerical simulation and field data show opposite trends owing to mutual inductance and self inductance a correction method for the field value of non central points is proposed the influences of different parameters are verified and the correction function is established by a test method the validity of the correction function is verified through controlled field tests the small fixed loop transient electromagnetic has been successfully applied to a tunnel under the weiwu expressway this method can effectively improve the detection depth and better detect unfavourable geological bodies such as karsts in front of the tunnel face

an analytical model to predict dune erosion due to wave impact an analytical model is developed to calculate recession distance and eroded volume for coastal dunes during severe storms the transport relationship used in the model is based on wave impact theory where individual swash waves hitting the dune face induce the erosion combining this relationship with the sediment volume conservation equation describes the response of the dune to high waves and water levels four different data sets on dune erosion originating from the laboratory and the field were employed to validate the model and to determine the value of an empirical transport coefficient appearing in the analytical solutions the time evolution of dune recession observed in the different data sets was well described by the model but the empirical coefficient showed some variation between cases especially for the field data in practical applications of the model it is recommended to use a range of coefficient values to include an uncertainty estimate of calculated quantities such as recession distance and eroded volume

beach and dune erosion during storm surges this paper presents the set up and results of an extensive research programme concerning the erosion of coastal dunes during storm surges a large number of two dimensional and three dimensional mobile bed model tests has been carried out to investigate the process of dune erosion the state of art after a series of small scale tests is summarized attention is focussed on large scale tests carried out in the delta flume with random waves up to 2 m significant height sediment concentration and orbital velocity measurements are discussed the large scale tests have confirmed the validity of a modelling technique based on the dimensionless fall velocity parameter h tw the model results are being applied to check the safety of existing coastal dunes as a water retaining structure that has to protect the major part of the netherlands from inundation during storm surges

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

transient electromagnetic response of the teutonic bore orebody transient electromagnetic tem measurements have been made with sirotem on four separate surveys over the teutonic bore orebody western australia both before mining operations began and subsequently during different stages of stripping overburden from the mineral deposit in the late stage of the transient decay the target response was relatively free of the overburden and host rock response beyond 6 ms the maximum anomalous response was a factor of 8 to 10 greater than the combined overburden and host rock response analog modeling with a copper plate in free space shows that the tem response of the target consists of a single peak at early delay times while at delay times beyond 4 2 ms the response becomes a double peak anomaly with a low directly over the top of the plate mathematical modeling of the tem response with a free space infinitely thin plate produces profile characteristics similar to those obtained by analog modeling beyond a delay time of 4 2 ms inversion of premining surve

transient electromagnetic s inversion in tunnel prediction 1 this paper presents s inversion method to tunnel prediction to forecasting water filled faults or fracture zones ahead of the front wall of a tunnel during tunnel excavation s inversion is an interpretation method of transient electromagnetic tem data using the second derivative of the conductivity parameter based on the moving thin sheet approach it is suggested for the technology of second derivative of vertical apparent conductivity which is traditionally used in surface tem data interpretation the theoretical analyses and the method of numerical calculation in detail are also given finally a real tunnel forecasting is studied for tem surveys and results show that the proposed method is effective and successful for exploring and predicting unfavorable geology during tunnel construction

characterization of the diurnal cycle of the west african monsoon around the monsoon onset abstract this study investigates the diurnal cycle of the west african monsoon and its seasonal modulation with particular focus on the monsoon onset period a composite analysis around the monsoon onset date is applied to the 1979 2000 ncep doe reanalysis and 40 yr ecmwf re analysis era 40 at 0000 0600 1200 and 1800 utc this study points out two independent modes describing the space time variability of the diurnal cycle of low level wind and temperature while the first mode appears to belong to a gradual and seasonal pattern linked with the northward migration of the whole monsoon system the second mode is characterized by more rapid time variations with a peak of both temperature and wind anomalies around the monsoon onset date this latter mode is connected with the time pattern of a nocturnal jet reaching its highest values around the onset date the diurnal cycle of dry and deep convection is also investigated through the same method a distinct diurnal cycle of deep convection in the itcz i

observations and predictions of run up for a significant range of offshore wave conditions and foreshore slopes run up observations are compared to semiempirical formulations and predictions of an existing numerical model based on the depth averaged one dimensional nonlinear shallow water equations with bore like breaking wave dissipation and quadratic bottom friction the numerical model is initialized with time series of sea surface elevation and cross shore velocity observed in 80 cm mean water depth approximately 50 m offshore of the mean shoreline on a gently sloping beach and in 175 cm water depth 100 m offshore of the shoreline on a steep concave beach run up was measured with a stack of resistance wires at elevations 5 10 15 20 and 25 cm above and parallel to the beach face at sea swell frequencies nominally 0 05 f 0 18 hz run up energy is limited by surf zone dissipation of shoreward propagating waves so that increasing the offshore wave height above a threshold value does not substantially increase the predicted or observed sea swell run up excursions e g run up is saturated existing semiempirical saturation formulations are most consistent with the observations and numerical model predictions of run up excursions nearest the bed in contrast at infragravity frequencies 0 004 f 0 05 hz where surf zone dissipation is relatively weak and reflection from the beach face is strong e g saturation formulas are not applicable the run up excursions increase approximately linearly with increasing offshore wave height the numerical model also accurately predicts that the tongue like shape of the run up results in sensitivity of run up measurements to wire elevation for instance run up excursions and mean vertical superelevation above the offshore still water level increase with decreasing wire elevation and continuous thinning of the run up tongue during the wave uprush can result in large phase differences between run up excursions measured at different wire elevations numerical model simulations suggest that run up measured more than a few centimeters above the bed cannot be used to infer even the sign of the fluid velocities in the run up tongue

observations of sand bar evolution on a natural beach waves currents and the location of the seafloor were measured on a barred beach for about 2 months at nine locations along a cross shore transect extending 255 m from 1 to 4 m water depth the seafloor location was measured nearly continuously even in the surf zone during storms with sonar altimeters mounted on fixed frames the crest of a sand bar initially located about 60 m from the shoreline moved 130 m offshore primarily when the offshore significant wave height exceeded about 2 m with 1 5 m of erosion near the initial location and 1 m of accretion at the final location an energetics type sediment transport model driven by locally measured near bottom currents predicts the observed offshore bar migration but not the slow onshore migration observed during low energy wave conditions the predicted offshore bar migration is driven primarily by cross shore gradients in predicted suspended sediment transport associated with quasi steady near bottom offshore flows these strong 50 cm s currents intensified near the bar crest by wave breaking are predicted to cause erosion on the shoreward slope of the bar and deposition on the seaward side the feedback amoung morphology waves circulation and sediment transport thus forces offshore bar migration during storms

testing model parameters for wave induced dune erosion using observations from hurricane sandy models of dune erosion depend on a set of assumptions that dictate the predicted evolution of dunes throughout the duration of a storm lidar observations made before and after hurricane sandy at over 800 profiles with diverse dune elevations widths and volumes are used to quantify specific dune erosion model parameters including the dune face slope which controls dune avalanching and the trajectory of the dune toe which controls dune migration wave impact models of dune erosion assume a vertical dune face and erosion of the dune toe along the foreshore beach slope observations presented here show that these assumptions are not always valid and require additional testing if these models are to be used to predict coastal vulnerability for decision making purposes observed dune face slopes steepened by 43 yet did not become vertical faces and only 50 of the dunes evolved along a trajectory similar to the foreshore beach slope observations also indicate that dune crests were lowered during dune erosion moreover analysis showed a correspondence between dune lowering and narrower beaches smaller dune volumes and or longer wave impact

modelling storm impacts on beaches dunes and barrier islands a new nearshore numerical model approach to assess the natural coastal response during time varying storm and hurricane conditions including dune erosion overwash and breaching is validated with a series of analytical laboratory and field test cases innovations include a non stationary wave driver with directional spreading to account for wave group generated surf and swash motions and an avalanching mechanism providing a smooth and robust solution for slumping of sand during dune erosion the model performs well in different situations including dune erosion overwash and breaching with specific emphasis on swash dynamics avalanching and 2dh effects these situations are all modelled using a standard set of parameter settings the results show the importance of infragravity waves in extending the reach of the resolved processes to the dune front the simple approach to account for slumping of the dune face by avalanching makes the model easily applicable in two dimensions and applying the same settings good results are obtained both for dune erosion and breaching

improving predictions of swash dynamics in xbeach the role of groupiness and incident band runup abstract in predicting storm impacts on sandy coasts possibly with structures accurate runup and overtopping simulation is an important aspect recent investigations stockdon et al 2014 palmsten and splinter 2016 show that despite accurate predictions of the morphodynamics of dissipative sandy beaches the xbeach model roelvink et al 2009 does not correctly simulate the individual contributions of set up and infragravity and incident band swash to the wave run up in this paper we describe an improved numerical scheme and a different way of simulating the propagation of directionally spread short wave groups in xbeach to better predict the groupiness of the short waves and the resulting infragravity waves the new approach is tested against field measurements from the delilah campaign at duck nc and against video derived runup measurements at praia de faro a relatively steep sandy beach compared to the empirical fit by vousdoukas et al 2012 the xbeach model performs much better for more extreme wave conditions which are severely underestimated by existing empirical formulations for relatively steep beaches incident band swash cannot be neglected and a wave resolving simulation mode is required therefore in this paper we also test the non hydrostatic wave resolving model within xbeach for runup and overtopping against three datasets results for a high quality flume test show non hydrostatic xbeach predicts the run up height with good accuracy maximum deviation 15 a case with a very shallow foreshore typical for the belgian coast at wenduine was compared against detailed measurements overall the model shows correct behavior for this case finally the model is tested against a large number 551 of physical model tests of overtopping from the clash database for relatively high overtopping discharges the non hydrostatic xbeach performs quite well with increasing accuracy for increasing overtopping rates however for relatively low overtopping rates of less than 10 20 l m s the model systematically underestimates measured overtopping rates

relevance of infragravity waves in a wave dominated inlet this study investigates the relevance of infragravity ig waves at albufeira lagoon inlet a shallow wave dominated inlet located on the western coast of portugal a field experiment carried out in september 2010 revealed the occurrence of low frequency oscillations i e 25 300 s in water levels and current velocities while these fluctuations were present over the ebb tidal delta along the whole tidal cycle they only appeared between the beginning of the flood and up to 2 h after high tide inside the lagoon the xbeach modeling system was applied to albufeira lagoon inlet and reproduced the generation and propagation of ig waves and their blocking during the ebb this behavior was explained by blocking due to opposing tidal currents reaching 2 5 m s 1 in shallow water depths numerical results suggest that the breakpoint mechanism and the long bound wave shoaling mechanisms contributed significantly to the generation of ig waves in the inlet ig waves induced fluctuations in flood currents inside the lagoon reaching temporarily 100 of their magnitude the fact that these fluctuations occur mostly at flood and not at ebb could promote flood dominance in the lagoon this hypothesis will have to be verified namely under storm wave conditions

rip currents mega cusps and eroding dunes abstract dune erosion is shown to occur at the embayment of beach mega cusps o 200 m alongshore that are associated with rip currents the beach is the narrowest at the embayment of the mega cusps allowing the swash of large storm waves coincident with high tides to reach the toe of the dune to undercut the dune and to cause dune erosion field measurements of dune beach and rip current morphology are acquired along an 18 km shoreline in southern monterey bay california this section of the bay consists of a sandy shoreline backed by extensive dunes rising to heights exceeding 40 m there is a large increase in wave height going from small wave heights in the shadow of a headland to the center of the bay where convergence of waves owing to refraction over the monterey bay submarine canyon results in larger wave heights the large alongshore gradient in wave height results in a concomitant alongshore gradient in morphodynamic scale the strongly refracted waves and narrow bay aperture result in near normal wave incidence resulting in well developed persistent rip currents along the entire shoreline the alongshore variations of the cuspate shoreline are found significantly correlated with the alongshore variations in rip spacing at 95 confidence the alongshore variations of the volume of dune erosion are found significantly correlated with alongshore variations of the cuspate shoreline at 95 confidence therefore it is concluded the mega cusps are associated with rip currents and that the location of dune erosion is associated with the embayment of the mega cusp

modeling the alongshore current on barred beaches mean alongshore currents observed on two barred beaches are compared with predictions based on the one dimensional time and depth averaged alongshore momentum balance between forcing by breaking waves wind and 10 100 km scale alongshore surface slopes bottom stress and lateral mixing the observations span 500 hours at egmond netherlands and 1000 hours at duck north carolina and include a wide range of conditions with maximum mean currents of 1 4 m s including rollers in the wave forcing results in improved predictions of the observed alongshore current structure by shifting the predicted velocity maxima shoreward and increasing the velocity in the bar trough compared with model predictions without rollers for these data wave forcing balances the bottom stress within the surfzone with the other terms of secondary importance the good agreement between observations and predictions implies that the one dimensional assumption holds for the range of conditions examined despite the presence of small alongshore bathymetric nonuniformities with stronger bathymetric variations the model skill deteriorates particularly in the bar trough consistent with earlier modeling and laboratory studies

modeling dune response to an east coast low coastal dunes can act as a method of soft coastal protection against inundation and direct impact of waves during storms if they are substantially large enough in volume to withstand erosion without breaching however the time evolution of sand dunes under direct wave impact is not well understood and many available models require site specific calibration and have had limited verification at field scales here we test three models of varying complexity in their ability to predict both dry beach erosion volumes and dune to a retreat for an east coast low storm event that occurred on the gold coast australia the process based model xbeach which models the entire profile was able to reproduce both dune toe retreat and dry beach volume however was sensitive to calibration parameters the two parametric models that only modeled erosion above the initial dune toe position were capable of accurately predicting dune toe retreat however under estimated dry beach erosion volumes with no calibration the parametric model proposed by palmsten and holman 2012 produced the smallest errors of dune toe retreat with mean error in final dune position of 6 6 m or 18 of the total measured dune retreat with minimal calibration estimated absolute error in average dune toe retreat was less than 13 of observed retreat for all three models

two dimensional time dependent hurricane overwash and erosion modeling at santa rosa island abstract a 2dh numerical model which is capable of computing nearshore circulation and morphodynamics including dune erosion breaching and overwash is used to simulate overwash caused by hurricane ivan 2004 on a barrier island the model is forced using parametric wave and surge time series based on field data and large scale numerical model results the model predicted beach face and dune erosion reasonably well as well as the development of washover fans furthermore the model demonstrated considerable quantitative skill upwards of 66 of variance explained maximum bias 0 21 m in hindcasting the post storm shape and elevation of the subaerial barrier island when a sheet flow sediment transport limiter was applied the prediction skill ranged between 0 66 and 0 77 in a series of sensitivity tests in which several hydraulic forcing parameters were varied the sensitivity studies showed that the variations in the incident wave height and wave period affected the entire simulated island morphology while variations in the surge level gradient between the ocean and back barrier bay affected the amount of deposition on the back barrier and in the back barrier bay the model sensitivity to the sheet flow sediment transport limiter which served as a proxy for unknown factors controlling the resistance to erosion was significantly greater than the sensitivity to the hydraulic forcing parameters if no limiter was applied the simulated morphological response of the barrier island was an order of magnitude greater than the measured morphological response

horizontal and vertical structure of observed tropospheric equatorial rossby waves equatorial rossby er waves are identified in an 8 year data set of national meteorological center operational analyses the westward moving waves have symmetric circulations about the equator and eastward energy dispersion with maximum zonal wind perturbations along the equator and meridional wind maxima roughly 10 15 off the equator they have maximum amplitude in the lower troposphere and are associated with convective signals at roughly the mean latitude of the tropical convergence zones the circulations thus possess many of the features of the equatorially trapped n 1 rossby modes derived by matsuno 1966 they are generally around zonal planetary wavenumber 6 scale and have a deep nearly equivalent barotropic structure up to 100 mbar this is in contrast with the madden julian oscillation and mixed rossby gravity waves which generally have a first baroclinic mode vertical structure during the intensive observing period iop of the tropical ocean global atmosphere toga coupled ocean atmosphere response experiment coare and in other years westward propagating er disturbances along the equator are observed by using 6 to 30 day filtered circulation data at 850 mbar zonal wind anomalies at 850 mbar are preferentially associated with like signed anomalies in a deep layer at this frequency as was observed during coare the er waves are most common during northern winter in the central pacific although they can be detected sporadically over the other ocean basins and in other seasons as well one source for the equatorial rossby waves appears to be upper level extratropical rossby wave activity propagating into the eastern pacific although the equatorial waves are sometimes present during the northern summer when the extratropical source is absent we cite evidence that propagation of er wave activity from the central into the western pacific is related to occurrences of westerly wind bursts

structure and evolution of numerically simulated squall lines abstract using a three dimensional numerical cloud model we investigate the effects of vertical wind shear on squall line structure and evolution over a wide range of shear magnitudes depths and orientations relative to the line we find that the simulated squall lines are most sensitive to the magnitude of the component of shear perpendicular to the line and that we may reproduce much of the range of observed structures by varying this single parameter for weak shear a line of initially upright to downshear tilted short lived cells quickly tilts upshear producing a wide band of weaker cells extending behind the surface outflow boundary for moderate to strong shear the circulation remains upright to downshear tilted for longer periods of time with vigorous short lived cells confined to a relatively narrow band along the system s leading edge at later times however these systems may also weaken as the circulation tilts upshear for strong deep shears oriented obliquely to the line the squal

monsoons and the dynamics of deserts the existence of subtropical deserts such as the sahara has often been attributed to the annual mean zonal mean hadley circulation which shows strong descent in the subtropics however the zonal mean hadley circulation shows considerable evolution over the course of the year with very strong subtropical descent during winter but practically no zonal mean subtropical descent during summer when rainfall over the eastern sahara and the mediterranean is least charney 1975 proposed a biosphere albedo feedback mechanism whereby local anthropogenic effects related to over grazing could affect the radiative balance enhancing summertime diabatic descent and leading to desertification of the subtropics in general the present study which uses an idealized model suggests a monsoon desert mechanism for desertification whereby remote diabatic heating in the asian monsoon region can induce a rossby wave pattern to the west integral with the rossby wave solution is a warm thermal structure that interacts with air on the southern flank of the mid latitude westerlies causing it to descend this adiabatic descent is localized over the eastern sahara and mediterranean and over the kyzylkum desert to the south east of the aral sea by the mountains of north africa and south west asia trajectories indicate that the monsoon desert mechanism does not represent a simple walker type overturning cell instead the descending air is seen to be mainly of mid latitude origin it is speculated that the monsoon forced adiabatic descent may result in clear air and therefore a local diabatic enhancement which effectively doubles the strength of descent with this mechanism desertification can be forced by remote changes in monsoon strength rather than by local effects this conclusion is supported by the observed dramatic strengthening of descent over the mediterranean and east sahara during the onset of the asian monsoon and on the longer timescale by relating prehistoric lake levels to milankovitch monsoon forcing the latter may help to explain the perceived discrepancies between the palaeoclimate of the eastern sahara and the strength of a tropic wide monsoon the monsoon desert mechanism may not be confined to the asian monsoon alone and the existence of other monsoon climate regions over the globe may in a similar way explain the observed summertime strengthening of the oceanic sub tropical anticyclones and the existence of western continental deserts and mediterranean type climate regions

modelling infragravity motions on a rip channel beach a non linear shallow water wave model operating on the time scale of wave groups is compared with measurements of infragravity motions on a rip channel beach to verify the model concepts and assess the model performance the measurements were obtained during the rip current experiment ripex in concert with the steep beach experiment sbe performed at sand city monterey bay ca during the spring of 2001 the nearshore bathymetry was made up of shore connected shoals incised by relatively narrow rip channels spaced approximately 125 m apart the comparison considers a 20 day period during which significant changes in both the offshore wave climate and nearshore bathymetry occurred the temporal variation in infragravity conditions during the experiment is strong with computational results typically explaining 70 to 80 of the observed infragravity motions within the nearshore in contrast to the temporal variation the alongshore spatial variation in infragravity intensity during the experiment is generally weak even though the underlying bathymetry shows strong depth variations model computations suggest preferential coupling between the computed edge wave motions and the quasi periodic bathymetry is present a prerequisite for strong spatial variability however the infragravity field is dominated by cross shore infragravity motions which are only weakly coupled to the quasi periodic bathymetry resulting in a weak alongshore variability of the total infragravity motions

tropical atlantic variability modes 1979 2002 part i time evolving sst modes related to west african rainfall abstract this work presents a description of the 1979 2002 tropical atlantic ta sst variability modes coupled to the anomalous west african wa rainfall during the monsoon season the time evolving sst patterns with an impact on wa rainfall variability are analyzed using a new methodology based on maximum covariance analysis the enhanced climate prediction center cpc merged analysis of precipitation cmap dataset which includes measures over the ocean gives a complete picture of the interannual wa rainfall patterns for the sahel dry period the leading ta sst pattern related to the atlantic el nino is coupled to anomalous precipitation over the coast of the gulf of guinea which corresponds to the second wa rainfall principal component the thermodynamics and dynamics involved in the generation development and damping of this mode are studied and compared with previous works the sst mode starts at the angola benguela region and is caused by alongshore wind anomalies it then propagates wes

a probabilistic method for constructing wave time series at inshore locations using model scenarios article i nfo continuous time series of wave characteristics height period and direction are constructed using a base set of model scenarios and simple probabilistic methods this approach utilizes an archive of computationally inten sive highly spatially resolved numerical wave model output to develop time series of historical or future wave conditions without performing additional continuous numerical simulations the archive of model output contains wave simulations from a set of model scenarios derived from an offshore wave climatology time series of wave height period direction and associated uncertainties are constructed at locations included in the numerical model domain the confidence limits are derived using statistical variability of oceanographic parameters contained in the wave model scenarios the method was applied to a region in the northern gulf of mexico and assessed using wave observations at 12 m and 30 m water depths prediction skill for significant wave height is 0 58 and 0 67 at the 12 m and 30 m locations respectively with similar performance for wave period and direction the skill of this simplified probabilistic time series construction method is comparable to existing large scale high fidelity operational wave models but provides higher spatial resolution output at low computational expense the constructed time series can be developed to support a variety of applications including climate studies and other situations where a comprehensive survey of wave impacts on the coastal area is of interest published by elsevier b v

assessing the impact of extreme storms on barrier beaches along the atlantic coastline application to the southern rhode island coast abstract in this work we use the 2d model xbeach to dynamically simulate coastal erosion due to a synthetic 100 year storm impacting a typical north atlantic barrier beach located in southern ri this storm was extracted from the north atlantic coast comprehensive study naccs database based on results of an extreme value analysis of more than 1000 naccs storms xbeach parameters are first calibrated validated by simulating hurricane irene august 2011 for which both nearshore wave data and pre and post storm beach profiles were available in the study area comparing results to observations allowed calibration of the wave asymmetry and skewness parameter u a 0 3 in the model resulting in a 6 mean relative error between the simulated and measured subaerial eroded volumes along 4 transects in the 100 year storm xbeach simulations that include overwash effects of land cover on beach erosion in particular vegetation are assessed by specifying a spatially varying bed friction function of high resolution land cover results show that healthy back dune vegetation is essential to prevent the dune crest from being fully eroded down to its toe level the predicted median 100 year eroded volume is 46 m3 m for the entire barrier beach in good agreement with fema s empirical value of 50 m3 m at the two official 1d transects within the study area mean post storm reductions in dune crest elevations are in similar agreement the model however predicts very large alongshore variations of these parameters with eroded volumes over 1000 m3 m where breaching and the opening of surge channels occurs overall dune segmentation simulated in the model for the 100 year storm appears to be realistic and consistent with dune topography and land cover xbeach thus provides an improved 2d methodology for assessing the impact of extreme storms on atlantic barrier beaches predicting changes in dune morphology and quantifying the protective role of vegetation and effects of land cover in general

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

climate science the mysteries of sahel droughts past variability in sahel rainfall is closely linked to global sea surface temperature distributions in observations and models alike climate simulations for the 21st century suggest that additional influences may become important in the future

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

transition zone width and implications for modeling surfzone hydrodynamics the surfzone associated with a wave breaking on a plane slope may be subdivided into three regions the transition zone the inner zone and the swash zone the transition zone is the region just shoreward of the point of wave breaking and is characterised by rapid wave decay and also by constant wave setdown and thus constant radiation stress in this paper an empirical expression for the width of this zone is developed from monochromatic wave data two techniques are proposed for the consideration of this phenomenon in the numerical modelling of surfzone hydrodynamics for random waves the implications of a zone of nearly constant radiation stress inside the breakpoint are examined with respect to wave induced current generation both cross shore and longshore and sediment transport prediction comparisons are made to both field and laboratory data from plane and undulating profiles

nonhydrostatic and surfbeat model predictions of extreme wave run up in fringing reef environments abstract the accurate prediction of extreme wave run up is important for effective coastal engineering design and coastal hazard management while run up processes on open sandy coasts have been reasonably well studied very few studies have focused on understanding and predicting wave run up at coral reef fronted coastlines this paper applies the short wave resolving nonhydrostatic xb nh and short wave averaged surfbeat xb sb modes of the xbeach numerical model to validate run up using data from two 1d alongshore uniform fringing reef profiles without roughness elements with two objectives i to provide insight into the physical processes governing run up in such environments and ii to evaluate the performance of both modes in accurately predicting run up over a wide range of conditions xbeach was calibrated by optimizing the maximum wave steepness parameter maxbrsteep in xb nh and the dissipation coefficient alpha in xb sb using the first dataset and then applied to the second dataset for validation xb nh and xb sb predictions of extreme wave run up rmax and r2 and its components infragravity and sea swell band swash sig and sss and shoreline setup were compared to observations xb nh more accurately simulated wave transformation but under predicted shoreline setup due to its exclusion of parameterized wave roller dynamics xb sb under predicted sea swell band swash but overestimated shoreline setup due to an over prediction of wave heights on the reef flat run up swash spectra were dominated by infragravity motions allowing the short wave but not wave group averaged model xb sb to perform comparably well to its more complete short wave resolving xb nh counterpart despite their respective limitations both modes were able to accurately predict rmax and r2

a note on the three dimensional shear stress distribution in a surf zone the three dimensional time mean shear stress distribution in a surf zone is analyzed thereby determining the vertical distribution of the driving forces due to wave breaking it is found that on a uniform coast the longshore current can be described as driven by a surface shear stress equal to the cross shore gradient in the shear component of the radiation stress the near surface shear stress is found to be in the direction of wave propagation having a magnitude determined by the dissipation of wave energy it can be determined as the shear force acting on the waves from the surface rollers of spilling breakers or broken waves

laboratory investigation of coastal beach erosion processes under storm waves of slowly varying height abstact storm induced large coastal waves are a predominant driver for coastal erosion problems but the physical processes of storm erosion have not been fully understood due to some technical difficulty in simulating storm waves with timely varying heights in the laboratory and collecting storm beach profiles in the field in this study novel series of experiments are proposed to study sandy beach processes under coastal storm conditions in a laboratory wave flume of 60 m long 3 m wide and 1 5 m high two water depths of h 0 5 m and h 0 7 m were used to investigate beach response under low and high tidal levels two initially uniform plane beach slopes of tan 1 15 for dissipation beach and tan 1 5 for reflection beach were used to study beach profile changes additionally intermediate beach tan 1 10 was also experimentally investigated under regular waves in another wave flume of 30 m long 0 6 m wide and 1 m high non intrusive instrumentation of high resolution cameras was used to record time series photo images on beach profile evolution in spatial resolution of 0 2 cm and a temporal resolution of 0 1 s image analyzing techniques were also developed to transform the recorded photo images to digital data on both beach and water surface profiles in analyzing the collected data it is found that the variation of storm wave height results in changes of wave breaking point and also sediment transport direction before and after the wave breaking point sediment transport direction was found to move onshore initially under small storm waves and then a sand bar was generated and a certain portion of sediment also moved offshore under large waves but such monotonic relationship between sediment transport and wave forcing could transform as morphology feedback and phase lags between wave orbital fluid velocity and sediment concentration the wave energy flux at the breaking point is shown to be suitable parameter in predicting beach mass change or storm erosion rate while the dimensionless sediment settling velocity parameter may not be a suitable criterion for predicting beach erosion or accretion under storm waves

sahel droughts and enso dynamics correlations between summer sahel rainfall and southern oscillation index has increased during the last thirty years at high frequency time scale periods lower than 8 years an intertropical atlantic zonal divergent circulation anomaly is forced by the difference of sea surface temperature sst anomalies between the eastern equatorial parts of pacific and atlantic this zonal connection worked well during most of the e1 nino southern oscillation enso events occurring after 1970 positive negative sst anomalies in the eastern pacific atlantic led to rainfall deficits over the whole west africa at low frequency time scale periods greater than 8 years positive sst anomalies in the indian ocean and in equatorial pacific existing after 1970 have been associated with decreasing rainfall intensity over west africa through another zonal divergent circulation these different time scales remote sst forcings are combined to provide a global zonal divergent circulation anomaly pattern which could explain the strong association between sahel drought and enso dynamics after 1970

large scale dune erosion tests to study the influence of wave periods article i nfo large scale physical model tests were performed to quantify the effects of the wave period on dune erosion attention was focussed on 2d cross shore effects in a situation with sandy dunes and extreme water levels and wave conditions besides profile measurements detailed measurements in time and space of water pressure flow velocities and sediment concentrations were performed in the near near shore area it was concluded that a longer wave period leads to a larger dune erosion volume and to a larger landward retreat of the dune face tests with double peaked wave spectra showed that the influence of the spectral shape on dune erosion was best represented by the tm 1 0 spectral mean wave period better than the peak wave period tp the effect of the wave period on dune erosion was implemented in a dune erosion prediction method that estimates erosion volumes during normative storm conditions for the dutch coast more details of the measurements and additional analyses of physical processes are described in an accompanying paper by van thiel de vries et al van thiel de vries j s m van gent m r a reniers a j h m and walstra d j r submitted for publication analysis of dune erosion processes in large scale flume experiments in this volume of coastal engineering

convectively coupled kelvin waves over tropical africa during the boreal summer structure and variability the structure and variability of convectively coupled kelvin waves during the boreal summer are explored using satellite observed brightness temperature data and ecmwf reanalyses kelvin wave activity is most prominent between the central and eastern pacific across africa and the indian ocean composite analysis shows that over sub saharan africa kelvin wave convection is peaked north of the equator while the dynamical fields tend to be symmetric with respect to the equator convectively coupled kelvin waves propagate faster over the pacific and western atlantic 24 m s 1 and slow down over tropical africa 14 m s 1 consistent with stronger coupling between the dynamics and convection over tropical africa the kelvin waves observed over africa generally propagate into the region from anywhere between the eastern pacific and the atlantic and decay over the eastern indian ocean basin results show marked interannual variability of kelvin wave activity over africa anomalously high kelvin wave variance tends to occur during dry years while low variance occurs during wet years african kelvin wave activity is positively correlated with sst anomalies in the equatorial east pacific the same warm sst anomalies that are favorable for enhanced kelvin wave activity suppress the mean rainfall over tropical africa via a more slowly varying teleconnection and associated subsidence a brief analysis of an intense kelvin wave in august 1987 a dry year shows a clear impact of the wave on convective development and daily rainfall over tropical africa this kelvin wave was also associated with a series of easterly wave initiations over tropical africa

the predictability of cross shore bed evolution of sandy beaches at the time scale of storms and seasons using process based profile models deterministic and probabilistic profile models have been compared with hydrodynamic and morphodynamic data of laboratory and field experiments on the time scale of storms and seasons the large scale laboratory experiment is a pure 2d case and offers and ideal test case for cross shore profile models as disturbing alongshore non uniformities are absent the field experiments are performed at the egmond site the netherlands during the eu coast3d project and represent storm time scale oct nov 1998 as well as seasonal time scale conditions may 1998 sep 1999 the objective of the paper is to present information of coastal processes on these time scales and to assess the predictive capabilities coastal process based profile models with respect to hydrodynamics and morphodynamics at sandy beaches on the time scales of storms and seasons profile models can quite accurately errors smaller than 10 represent the cross shore significant wave height distribution in the surf zone if the wave breaking model is properly calibrated the wave breaking coefficient should be a function of local wave steepness and bottom slope for most accurate results profile models can reasonably represent the cross shore and longshore currents undertow in a pure 2d case and in 3d field conditions profile models including cross shore mixing effects and breaker delay effects do not produce better predictions of the longshore and cross shore current velocities profile models using default settings can quite reasonably simulate the behaviour of the outer and inner bars on the storm time scale the behaviour of the beach cannot be modelled with sufficient accuracy on the storm time scale profile models can reasonably simulate the post storm onshore bar migration provided that the near bed orbital velocities and wave asymmetry related sand transport are represented in a sufficiently accurate way using non linear wave theories profile models cannot simulate the beach recovery processes on the post storm time scale because these essentially 3d processes are not sufficiently known to be included in the models profile models using default settings cannot simulate the behaviour of the outer and inner bars and the beach on the seasonal time scale the behaviour of the outer bar on the seasonal time scale can only be represented properly after tuning using measured bed profiles the simulation of the inner bar and beach morphology on the seasonal time scale could not be improved by tuning

analysis of dune erosion processes in large scale flume experiments abstract large scale physical model tests were conducted with different wave periods to examine the physical processes driving dune erosion the model tests have been carried out in a flume 2dv with a sandy dune exposed to extreme surge and wave conditions van gent m r a van thiel de vries j s m coeveld e m de vroeg j h and van de graaff j 2008 large scale dune erosion tests to study the effect of wave periods coastal engineering doi 10 1016 j coastaleng 2008 04 003 detailed measurements in time and space of water pressure flow velocities and sediment concentrations were performed in the near shore area the data revealed that both short and long waves are important to inner surf hydrodynamics depth averaged flows are directed offshore and increase towards the shore line the corresponding mean sediment concentrations rise sharply towards the dune face up to 50 g l near the bed the strong increase in the mean sediment concentration towards the dune face correlates well with the maximum wave surface slope which in turn is coupled to both the pressure gradient and the near bed wave breaking induced turbulence analysis shows that the pressure gradient is only partially coupled to the flow acceleration suggesting that the latter cannot always be used as a proxy for the first weak correlation is obtained with the near bed flows related to the bed shear stress tests with a larger wave period resulted in a larger dune erosion volume during these tests more wave energy combined incident and infragravity waves reached the dune face but more importantly this wave energy is dissipated by fewer waves resulting in more intense wave breakers and steeper wave fronts it is therefore expected that the wave breaking induced near bed turbulence increases resulting in significantly higher o 100 mean sediment concentrations in addition the mean flow velocities are comparable yielding a substantially larger offshore directed sediment transport capacity this increase in offshore directed transport is only partially compensated by a concurrent increase in the wave related onshore transport capacity associated with intrawave processes resulting in a net increase in the dune erosion rate

modeling the effect of wave vegetation interaction on wave setup aquatic vegetation in the coastal zone attenuates wave energy and reduces the risk of coastal hazards e g flooding besides the attenuation of sea swell waves vegetation may also affect infragravity band ig waves and wave setup to date knowledge on the effect of vegetation on ig waves and wave setup is lacking while they are potentially important parameters for coastal risk assessment in this study the storm impact model xbeach is extended with formulations for attenuation of sea swell and ig waves and wave setup effects in two modes the sea swell wave phase resolving nonhydrostatic and the phase averaged surfbeat mode in surfbeat mode a wave shape model is implemented to capture the effect of nonlinear wave vegetation interaction processes on wave setup both modeling modes are verified using data from two flume experiments with mimic vegetation and show good skill in computing the sea swell and ig wave transformation and wave setup in surfbeat mode the wave setup prediction greatly improves when using the wave shape model while in nonhydrostatic mode nonlinear intrawave effects are directly accounted for subsequently the model is used for a range of coastal geomorphological configurations by varying bed slope and vegetation extent the results indicate that the effect of wave vegetation interaction on wave setup may be relevant for a range of typical coastal geomorphological configurations e g relatively steep to gentle slope coasts fronted by vegetation

rainfall anomaly patterns and wind field signals over west africa in august 1958 1989 abstract relationships between monthly west african rainfall anomaly patterns and monthly tropospheric wind changes are investigated for the 32 months of august 1958 1989 at an interannual time step regarding the sahelian belt alone results show that wet dry months are significantly linked to an increase decrease in both upper easterlies and lower southwesterlies along with weaker stronger than usual midtropospheric easterlies south and under the main axis of the african easterly jet aej however the most interesting signals are obtained with consideration of the west african rainfall anomaly patterns large droughts over sahelian and guinean areas are significantly associated with a less southward extension of upper easterlies and a decrease of the upper meridional diffluence in the hadley circulation contrasted rainfall anomaly patterns exhibiting both sahelian droughts and guinean floods are mainly linked to easterly anomalies in the equatorial stratosphere and enhanced easterlies south

vertical flow structure during sandy duck observations and modeling abstract observations of the vertical distribution of time averaged cross shore and alongshore flows during the sandy duck field experiment are compared with model predictions to assess the parameters governing the flow behaviour the measurements were obtained with a vertical stack of eight two component current meters with the lowest and highest sensor at respectively o 0 1 and o 2 7 m above the bed observations under breaking wave conditions within the surfzone show that the maximum return flow velocities occur in the lower part of the water column consistent with laboratory observations under non breaking conditions outside the surfzone the maximum return flow velocities are observed closer to the water surface again in line with laboratory results analogous to previous observations the measured longshore current velocity profiles are logarithmic under non breaking conditions and become more depth uniform under breaking conditions the model description of the vertical structure of the flow includes the presence of wind stresses wave stresses pressure gradients turbulent eddy viscosity and a wave boundary layer the model utilizes parabolic shape functions to describe the vertical distribution of the turbulent eddy viscosity in the middle layer and within the bottom boundary layer eddy viscosity is enhanced in regions where turbulence is produced i e near the surface in the case of breaking waves and within the bottom boundary layer estimates of the wave breaking induced turbulent eddy viscosity and bottom friction are obtained by minimizing the model measurement discrepancies predictions utilizing calibrated expressions for both the turbulent eddy viscosity and bottom friction are in general agreement with the observations provided the wave transformation and associated mass flux are modeled correctly and a parabolic eddy viscosity distribution is used using a piecewise constant eddy viscosity distribution generally results in a degrading of the agreement between measurements and model results

morphodynamic modelling of rip channel growth the morphological evolution of a rip channel system due to normally incident waves has been investigated using two different approaches the first approach is a state of the art commercial 2dh coastal area model whereas the second approach is a linear stability analysis the results of the two different approaches have been compared and both models predict rip channel spacings and initial growth rates of similar magnitudes

impact of coral reef mining pits on nearshore hydrodynamics and wave runup during extreme wave events small island developing states are among the most vulnerable areas to the impact of natural hazards and climate change flooding due to storm surges and extreme waves coastal erosion and salinization of freshwater lenses are already a serious threat and could lead to irreversible consequences in the coming decades reef flat mining is one of the most common practices to source the required material for the implementation of coastal protection measures but concerns remain that partial removal of the protective reef could increase wave loading on the islands however the available data and knowledge on the effects of these mining pits are currently very limited this study provides new insights on the effects that pits may have on nearshore hydrodynamics and wave runup results are based on a large numerical data set of fringing reefs derived using the validated xbeach nonhydrostatic process based model model results indicate that excavation pits cause a decrease in infragravity wave energy around the fundamental mode of the reef which is partly caused by reduced wave transmission additionally changes in sea and swell wave energy are attributed to reduced transmission a decrease in wave dissipation and triad wave wave interaction furthermore in 13 of all modeled cases an increase in wave runup is observed mainly due to more sea and swell wave energy reaching the shoreline this probability is lowest for narrow pits relative to the reef flat width or pits located further from shore

generation and propagation of african squall lines a study of squall lines affecting minna nigeria during 1974 1976 emphasizes two particular aspects of their dynamics that a strong jet around 650 mb is essential for their development and that the forward edge of the squall line moves slightly faster than the maximum jet speed a linear model of the squall line as a superposition of normal modes of an unstably stratified sheared flow sheds light on these observations if the flow profile has a mid level maximum constructive interference of normal modes gives rise to an expanding circle of convection with an arc front moving slightly faster than the maximum mean flow speed followed by individual cells if however shear is unidirectional there is no arc front and a line of cells is formed moving at the middle level mean flow speed these two cases are suggested as models for tropical and mid latitude squall lines respectively while a further case provides a possible model for storm splitting

comparison of storm cluster vs isolated event impacts on beach dune morphodynamics two dimensional numerical simulations were used to investigate the impacts of storm clustering on the beach dune evolution of the sefton coast liverpool bay uk a storm cluster consisting of a series of closely spaced seven events was identified using observed wave and surge data during the 2013 2014 winter period first event in this cluster is regarded as exceptionally intense and the occurrence of seven storms within a very short time period is unique the xbeach coastal area model was used to simulate beach change from 1 the storm sequence clustered events and 2 the same storms considering them as isolated events offshore metadata was transformed to the nearshore area using the delft3d and swan models resulting evolution was first compared with the available post storm profiles measured at a number of locations along the sefton coast analysis of the clustered and isolated simulations showed the effect of clustering on the sefton beach dune system when compared to the impact of isolated events occurring on a fully recovered beach system morphological change occurred during each storm in the cluster was influenced by the preceding storm s such that the evolution is not proportional to the storm power of the event as it would be for isolated events both storm cases resulted in heavy erosion at formby point i e central of the sefton coast and accretion in the north and south the cluster prevented system recovery with the area of erosion continually extending south along the coast compared with that in isolated events the initial storm within the cluster caused large bed level changes in the nearshore ridge runnel system enabling the subsequent storms to penetrate further south the local convex geometry of the sefton coast is found to have more influence on the beach dune morphodynamics than the clustering effect this study enhances the understanding beach dune response to storm clusters to interpret observed morphological changes and to develop tools for sustainable coastal management particularly in the sefton coast and generally in similar systems worldwide

on the role of sea surface temperature gradients in forcing low level winds and convergence in the tropics abstract we examine the importance of pressure gradients due to surface temperature gradients to low level p 700 mb flow and convergence in the tropics over time scales 1 month the latter plays a crucial role in determining the distribution of cumulonimbus convection and rainfall our approach is to consider a simple one layer model of the trade cumulus boundary layer wherein surface temperature gradients are mixed vertically consistent with ecmwf analyzed data the top of the layer is taken at 700 mb the influence from higher levels is intentionally suppressed by setting horizontal pressure gradients and frictional stresses to zero at the top of the layer horizontal convergence within the layer is taken up by cumulonimbus mass flux however the development of the cumulonimbus mass flux is associated with a short relaxation time o hr roughly the development time for such convection during this short time horizontal convergence acts to redistribute mass so as to reduce horizontal pressur

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

tropical sst and sahel rainfall a non stationary relationship 1 sea surface temperature sst anomalies in the tropical atlantic have been associated with precipitation anomalies in west africa that form a dipole pattern with centers over the sahel and the gulf of guinea whilst this was clear before the 1970 s the dipole pattern almost disappeared after that date as the anti correlation between rainfall anomalies in the sahel and guinea dropped abruptly simultaneously the anti correlations between sahel rainfall and tropical pacific ssts strengthened it has been posited that these changes after the 1970 s developed as rainfall over west africa started to co vary with ssts in the global tropics in this co variability enhanced summer rainfall over west africa with a monopole pattern corresponds to warmer ssts in the tropical atlantic and maritime continent and colder ssts in the tropical pacific and western indian oceans the present paper describes the hitherto unexplored seasonal evolution of this co variability and the physical mechanisms at work sensitivity experiments with two atmospheric general circulation models demonstrate that after the 1970 s the impacts of sst anomalies in the indo pacific counteract those in the atlantic in terms of generating rainfall anomalies over the sahel and that this superposition of effects is primarily linear therefore at interannual timescales the change in the patterns of co variability between west african rainfall and tropical ssts can explain the non stationary relationship between the anomalies in these two fields

numerical modeling of wave runup on steep and mildly sloping natural beaches abstract runup on ocean beaches includes steady wave setup and oscillating swash often decomposed into wind generated sea swell ss and lower frequency infragravity ig waves we show that the numerically fast open source numerical model 1d swash predicts accurately the bulk properties of runup observed on two natural beaches one steep and one shallow sloped for a range of incident wave conditions the runup tongue shape was measured with a scanning lidar and the waterline location was defined in both the observations and model with a 10 cm depth threshold runup is reasonably accurately predicted with energetic e g 5 m significant height incident waves even though the assumption of 1d bound waves significantly overpredicts infragravity energy at the offshore boundary in 10 m depth the model data comparisons are limited by statistical chatter often larger in runup than offshore because runup energy is concentrated in the relatively narrow infragravity ig band with low effective degrees of freedom

west african monsoon demise climatology interannual variations and relationship to seasonal rainfall the climatology and interannual variations of the west african monsoon demise are investigated using the tropical rainfall measuring mission and the global precipitation climatology project precipitation data sets along with the era interim reanalysis during monsoon demise the rainfall maximum progresses southward from the sahel to the gulf of guinea the west african monsoon demise is smooth with no evidence of a meridional jump of the rainfall maximum as occurs during monsoon onset the climatological monsoon demise date defined by using a 2 mm d 1 rainfall threshold for the sahel is 20 october the demise date varies up to 25 days during the period 1979 2012 between 5 and 30 october an early late demise is associated with an anomalously strong weak north atlantic subtropical high which extends over the mediterranean and the sahara throughout the demise period the monsoon season total rainfall is found to be significantly correlated with the demise date in a composite of early late demise cases northerly southerly moisture flux anomalies which enhance reduce moisture divergence are associated with an anomalously strong weak north atlantic subtropical high in addition the early late demise of the west african monsoon is accompanied by cold warm sea surface temperature anomalies in the subtropical north atlantic

impact of the winter 2013 2014 series of severe western europe storms on a double barred sandy coast beach and dune erosion and megacusp embayments abstract the winter of 2013 2014 was characterized by a striking pattern of temporal and spatial extreme storm wave clustering in western europe the 110 km long gironde coast sw france was exposed to the most energetic wave conditions over the last 18 years the period was outstanding in terms of the available energy to move sediment and cause large scale erosion with the 2 month average significant wave height hs exceeding 3 6 m just below the 0 95 quantile and 4 distinct 10 year return period storms with hs 9 m these storm waves caused unprecedented beach and dune erosion along the gironde coast including severely damaged sea defences at the coastal towns at the end of the winter dune erosion scarp height was highly variable alongshore and often exceeded 10 m megacusp embayments were observed along the gironde coast with an average alongshore spacing of 1000 m in the south progressively decreasing to 500 m in the north with an average cross shore amplitude of 20 m while beach megacusps were previously observed to systematically couple to the inner bar along the gironde coast during low to moderate energy wave conditions severe storm driven megacusp embayments cutting the dune were found to be enforced and coupled to the outer crescentic bar a detailed inspection of the 1500 m long bimonthly topographic surveys of truc vert beach shows that in early january 2014 the outstanding shore normal incident storm swell hercules with hs and peak wave period tp peaking at 9 6 m and 22 s respectively triggered the formation of a localized megacusp embayment with the erosion scarp height exceeding 6 m in its centre where the dune retreat reached 30 m the subsequent storms progressively smoothed the megacusp by the end of the winter mostly through severe erosion of the megacusp horns because of the very long period 16 s

beach morphodynamics and types of foredune erosion generated by storms along the emilia romagna coastline italy abstract the objectives of this study are to examine the response of a dune and beach system on the adriatic coastline in northern italy to the arrival of storms compare it with seasonal months and medium term 3 year morphodynamic change and evaluate results predicted by the numerical model xbeach the studied coastline stretches 4 km from the bevano river mouth to the north of the site to the township of lido di classe to the south where the beach is protected by coastal structures fieldwork consisted of topographic profile surveys using rtk dgps technology 7 times over an approx 3 year period 103 samples of surface sediment were collected along 20 of the cross shore profiles at 6 distinct cross shore positions selected on the basis of morphological beach characteristics data analyses of dune and beach slopes enabled the study area to be divided into 6 separate morphological zones using the spatial longshore and cross shore variation of morphologies located on the backshore and intertidal beach observed in a preliminary survey of the area other criteria were a spatial consistency in beach slopes and or presence absence of intertidal morphologies identified in the aerial photographs and lidar data the swash zone slope did not show any significant variability for the entire area a weak seasonal trend in the variability of the mean foredune slope was observed with steeper slopes typically during winter and flatter slopes during summer analysis of grain size revealed that the beach sediment is well sorted fine sand tending to medium with a decreasing trend in size from the bevano river mouth southwards towards lido di classe according to the masselink and short 1993 classification the natural part of the study site has an intermediate barred beach ibb and following the short 1999 classification results in a modally lbt longshore bar trough or ltt low tide terrace with a small section being tbr transverse bar and rip storms are considered the main factor controlling changes in the beach and dune slope the most significant storm was recorded in march 2010 with a peak significant wave height of 3 91 m contrary to the seasonal dune trend several foredune slopes were observed to flatten following this event which can be attributed to the action of dune slumping from the already weakened dune state modelling of foredune erosion using a process based model xbeach reproduced the erosion of the upper beach and dune toe reasonably well but is currently limited by the acceptable slope value for dune stability which does not account for biotic factors e g plant roots the comparison between the storm impact categories of sallenger 2000 and the dsf dune stability factor of armaroli et al 2012 shows a very good correspondence between the effects of the winter 2008 2009 storms and the vulnerability of the dune system predicted using both classifications

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

inundation of a barrier island chandeleur islands louisiana usa during a hurricane observed water level gradients and modeled seaward sand transport large geomorphic changes to barrier islands may occur during inundation when storm surge exceeds island elevation inundation occurs episodically and under energetic conditions that make quantitative observations difficult we measured water levels on both sides of a barrier island in the northern chandeleur islands during inundation by hurricane isaac wind patterns caused the water levels to slope from the bay side to the ocean side for much of the storm modeled geomorphic changes during the storm were very sensitive to the cross island slopes imposed by water level boundary conditions simulations with equal or landward sloping water levels produced the characteristic barrier island storm response of overwash deposits or displaced berms with smoother final topography simulations using the observed seaward sloping water levels produced cross barrier channels and deposits of sand on the ocean side consistent with poststorm observations this sensitivity indicates that accurate water level boundary conditions must be applied on both sides of a barrier to correctly represent the geomorphic response to inundation events more broadly the consequence of seaward transport is that it alters the relationship between storm intensity and volume of landward transport sand transported to the ocean side may move downdrift or aid poststorm recovery by moving onto the beach face or closing recent breaches but it does not contribute to island transgression or appear as an overwash deposit in the back barrier stratigraphic record the high vulnerability of the chandeleur islands allowed us to observe processes that are infrequent but may be important at other barrier islands

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

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

field observations of surf beat 1 progressive edge waves nineteen biaxial electromagnetic current meters have been used to determine the longshore and on offshore structure of currents at surf beat periods 1 4 min the sensors formed two linear arrays a long shore array within the surf zone and an on offshore array stretching from the shoreline to well beyond the breaker line analysis of the longshore current components yields a clear picture of progressive low mode edge waves with frequency wave number dispersion relations which are in remarkably good agreement with predictions some separation of edge wave modes is found with mode zero energy dominating in the frequency band 0 006 and 0 011 hz and mode one between 0 015 and 0 025 hz on offshore currents present a rather different picture which while not inconsistent with the longshore currents suggests that other sources of energy are also important to the on offshore currents these include standing edge waves probably formed by reflections at nearby scripps canyon and motions which are nonresonantly forced by incoming wave groups

effects of wave exposure on circulation in a temperate reef environment 1 observations of waves and currents in a temperate reef environment off southwestern western australia over a period of 1 year reveal the relative importance of wind and wave forcing during periods of low waves linear regression analysis shows alongshore currents seaward and shoreward of the reef line are reasonably well predicted using 1 and 0 5 of the wind speed respectively however shoreward of the reef line anomalously strong currents were often observed during periods of light or even opposing winds and the mean sea surface was elevated relative to offshore of the reefs these anomalous currents and elevated sea level occur during periods of high waves and both are correlated with the root mean square wave height seaward of the reefs similar to what has been observed in coral reef environments the observations were simulated with the numerical model xbeach which includes radiation stress forcing due to the presence of the waves the model was also used to examine the dynamics of the wave driven flow in terms of the momentum balance as on a coral reef through the surf zone over the reef bottom friction is balanced by the sum of the radiation stress gradient and pressure gradient away from the reefs the radiation stress gradients are small and the momentum balance is between bottom friction and pressure gradient

erosional and depositional characteristics of regional overwash deposits caused by multiple hurricanes regional scale washover deposits along the florida gulf and atlantic coasts induced by multiple hurricanes in 2004 and 2005 were studied through coring trenching ground penetrating radar imaging aerial photography and prestorm and poststorm beach profile surveys erosional and depositional characteristics in different barrier island sub environments including dune field interior wetland and back barrier bay were examined over the eroded dune fields the washover deposits are characterized by an extensive horizontal basal erosional surface truncating the old dune deposits and horizontal to slightly landward dipping stratification over the marshes in the barrier island interior the washover deposits are characterized by steep tabular bedding with no erosion at the bottom overwash into the back barrier bay produced the thickest deposits characterized by steep prograding sigmoidal bedding no significant erosional feature was observed at the bottom washover deposits within the dense interior mangrove swamp demonstrate both normal and reversed graded bedding the washover deposits caused by hurricanes frances 2004 and jeanne 2004 along the southern florida atlantic coast barrier islands are substantially different from those along the northern florida barrier islands caused by ivan 2004 and dennis 2005 in terms of regional extension erosional features and sedimentary structures these differences are controlled by different overall barrier island morphology vegetation type and density and sediment properties the homogeneity of sediment along the northern florida coast makes distinguishing between washover deposits from ivan and dennis difficult in contrast along the atlantic coast barrier islands the two overwash events as demonstrated by two phases of graded bedding of the bimodal sediments are easily distinguishable

how did the ad 1755 tsunami impact on sand barriers across the southern coast of portugal idl pest oe cte la0019 2013 plani2c ed481b 2014 132 0 post doc eu fp7 research project risc kit ga 2013 603458 e fct investigator program ref if 01047 2014

nature based flood protection the efficiency of vegetated foreshores for reducing wave loads on coastal dikes this paper analyses the effect of vegetation on wave damping under severe storm conditions based on a combination of field measurements and numerical modelling the field measurements of wave attenuation by vegetation were performed on two salt marshes with two representative but contrasting coastal wetland vegetation types cordgrass spartina anglica and grassweed scirpus maritimus the former is found in salty environments whereas the latter is found in brackish environments the measurements have added to the range with the highest water depths and wave heights presented in the literature so far a numerical wave model swan has been calibrated and validated using the new field data it appeared that the model was well capable of reproducing the observed decay in wave height over the salt marsh the model has been applied to compute the reduction of the incident wave height on a dike for various realistic foreshore configurations and hydraulic loading conditions additionally the efficiency of vegetated foreshores in reducing wave loads on the dike has been investigated where wave loads were quantified using a computed wave run up height and wave overtopping discharge the outcomes show that vegetated foreshores reduce wave loads on coastal dikes significantly also for the large inundation depths that occur during storms and with the vegetation being in winter state the effect of the foreshore on the wave loads varies with wave height to water depth ratio on the foreshore the presence of vegetation on the foreshore extends the range of water depths for which a foreshore can be applied for effective reduction of wave loads and prevents intense wave breaking on the foreshore to occur this research demonstrates that vegetated foreshores can be considered as a promising supplement to conventional engineering methods for dike reinforcement

sea level in time and space revolutions and inconvenient truths in this paper we identify four revolutions that in our view have shaped the study of sea level changes in recent decades i the search for eustasy one of the most hotly debated issues in the 1960s is still relevant as models of glacial isostatic adjustment gia require input of the amount of meltwater transferred to the world s oceans following the demise of the large pleistocene ice sheets ii resolution of sea level archives all sea level archives have limitations in their resolving power but are we willing to define these from studies of salt marsh deposits and corals it is highly likely that climate driven metre scale sea level fluctuations have not occurred during the middle and late holocene iii rapid sea level changes improved coring technology has resulted in the suggestion of meltwater pulses during the last deglaciation low frequency high magnitude events have been documented along seismically active shorelines and provide information on seismic hazards and storm impacts iv merging of empirical sea level investigations and models much sea level research in the past two decades has embraced the integration of models and field derived data providing testable hypotheses and insights into processes we conclude with some reminders about the inconvenient truths that keep the sea level scientist honest

forecasts of seasonal to inter annual beach change using a reduced physics beach profile model abstract the observed seasonal to inter annual variability of beach profiles has been investigated using a one dimensional beach profile model the model used here is developed on the reduced physics modelling approach the model uses a diffusion formulation as the governing equation and adopts an inverse modelling technique for solving the equation the model is calibrated and validated against historic measurements of beach profiles at hasaki coast located in the east coast of japan the beach is longshore uniform and characterised by a highly dynamic longshore bar trough system the model is then used to forecast seasonal to inter annual scale beach change the results are compared with beach change determined from measured beach profiles at hasaki between 2007 and 2011 the simple modelling approach used yields encouraging results of seasonal to inter annual scale beach change at hasaki coast

observations and modelling of nearshore sediment sorting processes along a barred beach profile abstract the understanding of cross shore sediment sorting is of primary importance for the design of sand nourishments and for assessing the suitability of the seabed to different ecological species in this paper sediment sorting processes were investigated by using a combination of physical and detailed numerical modelling data from large scale wave flume experiments were used to validate a 2dv cross shore delft3d model the model solves coupled short wave averaged equations for flow sediment transport bed composition and bed level change the infra gravity wave motions were explicitly resolved in order to investigate sorting processes eight sediment fractions were used as well as a layered bed stratigraphy the effects of different wave conditions high energetic and more moderate energetic waves on the morphodynamic profile development and sorting processes were investigated the delft3d model reproduced the profile development and bar position very accurately additionally model predictions of sediment sorting across the profile fitted very well with the available observations the numerical model simulations showed the importance of including short wave grouping and infragravity wave effects in order to reproduce the cross shore profile development especially the breaker bar dynamics and sediment sorting processes infragravity waves contribute to larger sediment entrainment and more offshore bar development besides leading to a better prediction of the bed profile infragravity waves also lead to a better prediction of the bed composition model results are in agreement with experimental data showing its capabilities in functioning as a tool to predict sorting processes

impact of atlantic sea surface temperature anomalies on the summer climate in the western north pacific during 1997 1998 1 the summers of 1997 and 1998 were characterized by large anomalous atmospheric circulations in the tropical western pacific which in turn were associated with anomalous atmospheric circulation and precipitation in the adjacent areas we use an atmospheric general circulation model to investigate the role of sea surface temperatures ssts on the anomalous atmospheric circulations and associated precipitation in the tropics in these two summers it is found that the circulation and rainfall anomalies are well reproduced when the model is forced with the observed global ssts a further experiment is performed to investigate the role of the sst anomalies in the atlantic in summer 1997 the atlantic sst anomalies compared with the sst anomalies outside the atlantic play a secondary role in influencing the circulation and rainfall anomalies in the tropical indian ocean and pacific by contrast it shows that the sst anomalies in the atlantic play a comparable role to those outside the atlantic in influencing the circulation and rainfall anomalies in the remote tropics in summer 1998 in this summer equatorial stationary waves similar to a kelvin wave and a rossby wave are clearly illustrated by the results implied by the atlantic sst anomalies the eastward equatorial stationary wave extends into the tropical western pacific and suppresses the atmospheric convection in the intertropical convergence zone in the indian ocean and pacific the evidence presented here for the role of atlantic ocean conditions on summer climates suggests that attention should be paid to accurate simulation of the atlantic ocean state as well as the states of the pacific and indian oceans in developing a seasonal forecasting system

wave dune interaction and beach resilience in large scale physical model tests abstract large scale laboratory experiments focusing on the main physical processes driving dune erosion have been performed in a wave flume with a sandy dune exposed to a combination of water levels and wave conditions tomasicchio et al 2011b providing observations of different interaction regimes collapsing overwash and breaching sallenger et al 2003 the large set of data has been considered to obtain measurements of beach dune cross shore profile evolution hydrodynamic properties and suspended sediment concentration results show that erosion develops faster during the first hundred waves there is a clear influence of wave period van gent et al 2008 and water depth on the resilience of the beach dune system and that sediment concentration at a given transect depends on the bar position furthermore the observed data have been adopted to verify the predictive accuracy of two open source process based profile numerical models

analysis of the dominant mode of convectively coupled kelvin waves in the west african monsoon abstract the dominant mode of convectively coupled kelvin waves has been detected over the atlantic and africa during northern summer by performing composite analyses on observational fields based on an eof reconstructed convection index over west africa propagating eastward many waves originate from the pacific sector interact with deep convection of the marine itcz over the atlantic and the continental itcz over west and central africa and then weaken over east africa and the indian ocean it has been shown that they are able to modulate the life cycle and track of individual westward propagating convective systems their mean kinematic characteristics comprise a wavelength of 8000 km and a phase speed of 15 m s 1 leading to a period centered on 6 to 7 days the african kelvin wave activity displays large seasonal variability being highest outside of northern summer when the itcz is close to the equator facilitating the interactions between convection and these equatorially trapped waves the co

african monsoon teleconnections with tropical ssts validation and evolution in a set of ipcc4 simulations a set of 12 state of the art coupled ocean atmosphere general circulation models oagcms is explored to assess their ability to simulate the main teleconnections between the west african monsoon wam and the tropical sea surface temperatures ssts at the interannual to multi decadal time scales such teleconnections are indeed responsible for the main modes of precipitation variability observed over west africa and represent an interesting benchmark for the models that have contributed to the fourth assessment report of the intergovernmental panel on climate change ipcc4 the evaluation is based on a maximum covariance analysis mca applied on tropical ssts and wam rainfall to distinguish between interannual and multi decadal variability all datasets are partitioned into low frequency lf and high frequency hf components prior to analysis first applied to hf observations the mca reveals two major teleconnections the first mode highlights the strong influence of the el nino southern oscillation enso the second mode reveals a relationship between the sst in the gulf of guinea and the northward migration of the monsoon rainbelt over the west african continent when applied to hf outputs of the twentieth century ipcc4 simulations the mca provides heterogeneous results most simulations show a single dominant pacific teleconnection which is however of the wrong sign for half of the models only one model shows a significant second mode emphasizing the oagcms difficulty in simulating the response of the african rainbelt to atlantic sst anomalies that are not synchronous with pacific anomalies the lf modulation of these hf teleconnections is then explored through running correlations between expansion coefficients ecs for ssts and precipitation the observed time series indicate that both pacific and atlantic teleconnections get stronger during the twentieth century the ipcc4 simulations of the twentieth and twenty first centuries do not show any significant change in the pattern of the teleconnections but the dominant enso teleconnection also exhibits a significant strengthening thereby suggesting that the observed trend could be partly a response to the anthropogenic forcing finally the mca is also applied to the lf data the first observed mode reveals a well known inter hemispheric sst pattern that is strongly related to the multi decadal variability of the wam rainfall dominated by the severe drying trend from the 1950s to the 1980s whereas recent studies suggest that this drying could be partly caused by anthropogenic forcings only 5 among the 12 ipcc4 models capture some features of this lf coupled mode this result suggests the need for a more detailed validation of the wam variability including a dynamical interpretation of the sst rainfall relationships

numerical modeling of infragravity wave response during delilah 1 the nearshore circulation model shorecirc is applied to simulate the infragravity wave conditions at two intervals during the 1990 delilah campaign at duck n c the model has been extended to solve the energy equation for random short waves on the timescale of the wave groups which acts as a wave driver for the long wave model to simulate the wave conditions an algorithm has been derived which synthesizes a directional offshore wavefield from measured data including the bound directional low frequency components the model data comparison shows that the mean short wave transformation and mean longshore current are predicted well the structure of the infragravity spectrum as measured in the nearshore array is correctly represented albeit that the energy levels are underpredicted in most locations the 2 d frequency longshore wavenumber spectra show that the infragravity wave energy consists mostly of edge waves and that the computations qualitatively resemble the measurements it is shown that for 1 day bar trapped edge waves are present the model is also used to show that the influence of the longshore variability is small for this particular beach and that the directional spreading in the offshore wave field is critical in predicting the nearshore infragravity wave conditions correctly edge waves which propagate against the current are generated by the directionally broad short wave forcing and to a lesser extent by back scattering the nonlinear terms are relatively unimportant for the prediction of the infragravity wave climate the cross shore gradient of the onshore directed radiation stress is the most important of the forcing terms

prediction of swash motion and run up including the effects of swash interaction modifications to a model describing swash motion based on solutions to the non linear shallow water equations were made to account for interaction between up rush and back wash at the still water shoreline and within the swash zone inputs to the model are wave heights and arrival times at the still water shoreline the model was tested against wave groups representing idealized vessel generated wave trains run in a small wave tank experiment accounting for swash interaction markedly improved results with respect to the maximum ran up length for cases with rather gentle foreshore slopes tan beta 0 07 for the case with a steep foreshore slope tan beta 0 20 there was very little improvement compared to model results if swash interaction was not accounted for in addition an equation was developed to predict the onset and degree of swash interaction including the effects of bed friction less