suppression of eddy diffusivity across jets in the southern ocean abstract geostrophic eddies control the meridional mixing of heat carbon and other climatically important tracers in the southern ocean the rate of eddy mixing is typically quantified through an eddy diffusivity there is an ongoing debate as to whether eddy mixing in enhanced in the core of the antarctic circumpolar current or on its flanks a simple expression is derived that predicts the rate of eddy mixing that is the eddy diffusivity as a function of eddy and mean current statistics this novel expression predicts suppression of the cross jet eddy diffusivity in the core of the antarctic circumpolar current despite enhanced values of eddy kinetic energy the expression is qualitatively and quantitatively validated by independent estimates of eddy mixing from altimetry observations this work suggests that the meridional eddy diffusivity across the antarctic circumpolar current is weaker than presently assumed because of the suppression of eddy mixing by the strong zonal current

nearshore oblique sand bars 1 the coupling between hydrodynamics and the evolving topography in the surf zone has been theoretically examined for oblique wave incidence it is shown that positive feedback can lead to the initial growth of several types of rhythmic systems of sand bars the bars can be down current oriented or up current oriented which means that the offshore end of the bar is shifted down current or up current with respect to the shore attachment in the limit of strong current compared to wave orbital motion very oblique down current oriented b ars are obtained with a spacing of several times the surf zone width when wave orbital motions are dominant systems of up current oriented bars and crescentic down current oriented bars appear with spacings of the order of the surf zone width the latter feature consists of alternating shoals and troughs at both sides of the break line with the inner shoals being bar shaped and oblique to the coast the growth e folding time of the bars ranges from a few hours to a few days and it is favored by constant wave conditions the range of model parameters leading to growth corresponds to intermediate beach states in between the fully dissipative and the fully reflective situations preliminary comparison with field observations shows qualitative agreement

sand waves along the dutch coast sand waves defined as longshore wave like movements of the shoreline measured in a horizontal plane are described along several stretches of the shoreline of the netherlands they have a celerity in the order of 50 200 m yr a period of 50 150 years and an amplitude of 30 500 m they are found along the whole dutch shoreline analysis of sand waves shows that the assumed effect of groins might be very small along this shoreline there are indications that groins did not have any long term effect on coastal regression

estimating suppression of eddy mixing by mean flows particle and tracer based estimates of lateral diffusivities are used to estimate the suppression of eddy mixingacrossstrongcurrents particlesandtracersare advectedusingavelocityfieldderivedfromseasurface height measurements from the south pacific in a region west of drake passage this velocity field has been used in a companion paper to show that both particle and tracer based estimates of eddy diffusivities are equivalent despite recent claims to the contrary these estimates of eddy diffusivities are here analyzed to show 1 that the degree of suppression of mixing across the strong antarctic circumpolar current is correctly predicted by mixing length theory modified to include eddy propagation along the mean flow and 2 that the suppressioncan beinferredfromparticletrajectoriesby studyingthestructureofthe autocorrelationfunction of the particle velocities beyond the first zero crossing these results are then used to discuss how to compute lateral and vertical variations in eddy diffusivities using floats and drifters in the real ocean

statistical analysis of the surface circulation of the california current we use a set of mixed layer drifting buoy trajectories from the california current 20 n 40 n during 1985 1990 to obtain statistically reliable estimates of the mean currents the mean variance field and the geographically varying diffusivity integral timescales and integral space scales typical values for the diffusivity are 1 1 8 7 107 cm2 s 1 while the timescales and space scales are 2 1 7 1 days and 16 59 km respectively the variance field displays a strong westward gradient out to 125 w and diffusivity shows a tendency to decrease toward the southwest part of the domain significant anisotropy is found in the variance field near the coastal boundary and at 30 n 130 w which is the region where the subarctic and northern subtropical fronts approach the california current the antisymmetric component of the diffusivity tensor indicates that cyclonic eddies dominate the mesoscale signature of drifters in this region we seek simple parameterizations to relate the scales of motion of the random velocity field to the diffusivity by testing least squares fits to u02t and u0l where u02 is the velocity variance we found no cases for which these two hypotheses could be distinguished for the meridional component the linear regressions are not successful which suggests that the meridional departure velocities result from a flow regime that is significantly organized by for example waves or coherent structures a subset of the drifters measured temperature along their tracks and we use the resultant data to produce the first direct estimates of the horizontal eddy heat flux divergence based on lagrangian estimates in addition we separately compute the eddy diffusivity parameterization of the eddy heat flux divergence u using our diffusivity estimates and a sea surface temperature climatology the two independent terms agree well which provides a measure of reassurance about the diffusivity estimates the eddy heat flux divergence in the california current is very small 5 w m 2 and does not appear to be significant in the long term heat budget of the upper ocean in this region

a morphological stability analysis for a long straight barred coast a morphological stability analysis is carried out for a long straight coast with a longshore bar the situation with oblique wave incidence and a wave driven longshore current is considered the flow and sediment transport are described by a numerical modelling system the models comprise i a wave model with depth refraction shoaling and wave breaking ii a depth integrated model for wave driven currents and iii a sediment transport model for the bed load transport and the suspended load transport in combined waves and current the direction of the sediment transport is taken to be parallel to the depth integrated mean current velocity neglecting the effects of a bed slope and secondary currents an instability is found to develop around the bar crest the instability is periodic in the alongshore direction and tends to form rip channels and to steepen the offshore face of the bar between the rip channels the alongshore wave length of the most unstable perturbation is determined for different combinations of the wave conditions and the geometry of the profile

modelling the formation of shoreface connected sand ridges on storm dominated inner shelves a morphodynamic model is developed and analysed to gain fundamental understanding of the basic physical mechanisms responsible for the characteristics of shorefaceconnected sand ridges observed in some coastal seas these alongshore rhythmic bed forms have a horizontal lengthscale of order 5 km and are related to the mean current along the coast the seaward ends of their crests are shifted upstream with respect to where they are attached to the shoreface the model is based on the two dimensional shallow water equations and assumes that the sediment transport only takes place during storms the flux consists of a suspended load part and a bed load part and accounts for the eects of spatially non uniform wave stirring as well as for the preferred downslope movement of sediment the basic state of this model represents a steady longshore current driven by wind and a pressure gradient the dynamics of small perturbations to this state are controlled by a physical mechanism which is related to the transverse bottom slope this causes a seaward deflection of the current over the ridges and the loss of sediment carrying capacity of the flow into deeper water the orientation spacing and shape of the modelled ridges agree well with eld observations suspended load transport and spatially non uniform wave stirring are necessary in order to obtain correct e folding timescales and migration speeds the ridge growth is only due to suspended load transport whereas the migration is controlled by bed load transport

eddy variability and energetics from direct current measurements in the antarctic circumpolar current south of australia two year time series measurements of current velocity and temperature in the subantarctic front saf south of australia from 1993 to 1995 provide estimates of eddy fluxes of heat and momentum across the antarctic circumpolar current acc and further insight into the variability of the acc the saf was found to be an energetic meandering jet with vertically coherent fluctuations varying on a timescale of 20 days with typical amplitude 30 cm s21 at 1150 dbar a daily varying coordinate frame that follows the direction of flow allowed mesoscale variability of the saf to be isolated from variability due to meandering of the front and proved very successful for examining eddy fluxes vertically averaged cross stream eddy heat flux was 11 3 kw m 22 poleward and significantly different from zero at 95 for fluctuations in the 2 90 day band zonally integrated this eddy heat flux 50 9 pw is more than large enough to balance heat lost south of the polar front and is as large as cross saf fluxes found in drake passage cross stream eddy momentum fluxes were small not significantly different from zero and of indeterminate sign but tending to decelerate the mean flow a relationship between vertical motion and meander phase identified in the gulf stream was found to hold for the acc eddy kinetic energy levels were similar to those in drake passage and southeast of new zealand eddy potential energy was up to an order of magnitude larger than at other acc sites most likely because meandering of the front is more common here baroclinic conversion was found to be the dominant mechanism by which eddies grow south of australia typical eddy growth time is estimated to be 30 days approximately twice as fast as in drake passage consistent with eddy energy growing rapidly downstream

variability of shore and shoreline evolution shore and shoreline evolution both due to natural and human induced causes or factors can be variable over a wide range of different temporal and or spatial scales our capability to understand and especially predict this variability is still limited this can lead to misinterpretation of coastal change information which hampers informed decision making and the subsequent design and implementation of soft engineering interventions collecting and describing example observations of shore and shoreline variability is one way to support and improve such human intervention this paper describes causes and factors for the variability and the resulting possible evolutions of wave dominated shores and shorelines which are illustrated by a number of case studies the new element of this work is that the variability is described in terms of a range of different time and space scales which helps to structure such analysis however it is difficult to generalise the results for arbitrary situations especially on decadal time scales scientific and engineering improvements require more quantitative insight into the physical mechanisms behind the free and forced shore behaviour responsible for the variability

a mechanism for the formation and maintenance of shore oblique sand ridges on storm dominated shelves a remarkable set of shore oblique sand ridges occupies the inner portion of the storm dominated middle atlantic and south atlantic bights off the east coast of the united states of america a mechanism for present day formation and growth of these distinctive geological features follows from a linearized analysis of the stability of alongshore periodic topographical perturbations based on a depth averaged fluid dynamical model with a simple representation of sediment transport the analysis indicates exponential growth of shore oblique features as a result of offshore deflection of storm driven alongshore flows at ridge crests which leads to a convergence of sediment flux because the effective carrying capacity decreases with increasing distance offshore the analysis indicates a set of conditions required for ridge growth model solutions for topography under conditions favorable for ridge growth are strikingly similar to the observed sand ridges

a mechanism for the generation of wave driven rhythmic patterns in the surf zone the coupling between topographic irregularities and wave driven mean water motion in the surf zone is examined this coupling occurs because the topographic perturbations produce excess gradients in the wave radiation stress that cause a steady circulation this circulation in turn creates a sediment transport pattern that can reinforce the bottom disturbance and may thereby lead to the growth of large scale bed forms to investigate this coupling mechanism the linearized stability problem with an originally plane sloping beach and normal wave incidence is solved in two different cases first the breaking line is considered to be fixed and second the perturbations in water depth that produce a displacement of the breaker line are accounted for the first case shows that the basic topography can be unstable with respect to two different modes a giant cusp pattern with shore attached transverse bars that extend across the whole surf zone and a crescentic pattern with alternate shoals and pools at both sides of the breaking line showing a mirroring effect in the second case the varying breaker line may have a strong influence on the circulation this is clear for the giant cusp topography whose growth is totally inhibited in contrast the morphology and the growth of the crescentic pattern remains almost unchanged

infragravity rip current pulsations 1 the origins of rip current pulsations within the infragravity frequency band 0 004 0 04 hz are determined from measurements made with arrays of colocated pressure and velocity sensors deployed on a beach with persistent rip current channels the observations indicate significant energy in cross shore infragravity velocities that varies spatially in the alongshore due to the presence of rip channels infragravity velocities are smaller within the rip channel than on the shore connected shoal owing to differences in water depth rip current pulsations at infragravity frequencies are linked to standing infragravity motions but not to the ponding and subsequent release of water by wave group pumping

self organization mechanisms for the formation of nearshore crescentic and transverse sand bars the formation and development of transverse and crescentic sand bars in the coastal marine environment has been investigated by means of a nonlinear numerical model based on the shallow water equations and on a simplified sediment transport parameterization by assuming normally approaching waves and a saturated surf zone rhythmic patterns develop from a planar slope where random perturbations of small amplitude have been superimposed two types of bedforms appear one is a crescentic bar pattern centred around the breakpoint and the other herein modelled for the first time is a transverse bar pattern the feedback mechanism related to the formation and development of the patterns can be explained by coupling the water and sediment conservation equations basically the waves stir up the sediment and keep it in suspension with a certain cross shore distribution of depth averaged concentration then a current flowing with against the gradient of sediment concentration produces erosion deposition it is shown that inside the surf zone these currents may occur due to the wave refraction and to the redistribution of wave breaking produced by the growing bedforms numerical simulations have been performed in order to understand the sensitivity of the pattern formation to the parameterization and to relate the hydro morphodynamic input conditions to which of the patterns develops it is suggested that crescentic bar growth would be favoured by high energy conditions and fine sediment while transverse bars would grow for milder waves and coarser sediment in intermediate conditions mixed patterns may occur

the statistics and kinematics of transverse sand bars on an open coast abstract ten years 1987 1996 of time exposure video images of the nearshore region at duck nc were used to study transverse sand bars bathymetric features of intermediate length scales 10 200 m oriented oblique or perpendicular to the shoreline these transverse sand bars extend seaward from both the shoreline trough transverse bars and the shore parallel sand bar offshore transverse bars transverse bars had not previously been observed in an energetic coastal environment such as that at duck and their dynamics and role in nearshore processes is unknown frequency of occurrence statistics and length scales of the transverse bars were calculated using the video images trough and offshore transverse bars appeared a mean of 39 and 73 days per year respectively the offshore bars were found to be much larger features than the trough bars with mean wavelengths alongshore spacing between consecutive crests of 79 and 172 m for trough and offshore bars respectively both the trough and offshore bars were found to persist for periods of days to months the alongshore movement of the bars was measured and compared to estimates of surf zone longshore currents which were calculated from wave height and wave angle data both sets of bars were observed to move at rates up to 40 m day at times both trough and offshore bars were observed shifting in the same direction as the current was flowing and at other times both sets of bars remained stationary even under relatively strong longshore currents trough bars were also observed moving against the current an hypothesis proposed by barcilon and lau 1973 j geophys res 78 15 2656 2664 that the transverse bars were created as a sea bed instability under longshore currents was tested by comparing the magnitude of estimated surf zone longshore currents with times of formation or presence of transverse bars there was no evidence to suggest that the bars were formed by this simple longshore current instability mechanism instead it is plausible that the combined effects of waves and currents may drive the formation of these features

an energetics total load sediment transport model for a plane sloping beach bagnold s energetics based total load sediment transport model for streams is used as a basis for the development of a total load model of time varying sediment transport over a plane sloping bed in both the bedload and suspended load the transport rate vectors are found to be composed of a velocity induced component directed parallel to the instantaneous velocity vector and a gravity induced component directed down slope the model is applied to idealized surfzone conditions leading to estimates of the local longshore and onshore offshore sediment transport rates as well as the equilibrium beach slope as a function of the local wave and current conditions the model is combined with a nonlinear longshore current model and spatially integrated to obtain predictions of the total longshore transport rate as a function of the incident wave conditions the results support the general form of the wave power equation except that the wave power coefficient is no longer constant but is instead a complex function of the incident wave and beach characteristics

lateral diffusivity and lagrangian scales in the pacific ocean as derived from drifter data 1 the global drifter program surface velocity program data set is used to derive maps of the lateral diffusivity and lagrangian scales of velocity length and time across the whole pacific for the lateral diffusivity the minor principal component of both the davis diffusivity tensor and the half growth rate of the single particle dispersion tensor were calculated in 5 5 bins from drifters for the period 1979 1999 estimates of the lateral diffusivity k obtained by means of the minor principal component approach are shown to correspond well with other authors estimates for the pacific the highest values of k 2 3 104 m2 s are found in the eastern equatorial pacific while the lowest ones 2 3 103 m2 s are typical for forty odd degrees in latitude northward of the subpolar fronts in both hemispheres and for the subtropics in the eastern part of the south pacific high values of k about 1 104 m2 s are also observed in the kusoshio kuroshio extension area and in the equatorial pacific along the whole length of the equator there are two approximately symmetrical tongues of high k value in the western pacific centered around 21 n and 24 s respectively in general k has a tendency to increase westward eastward in the midlatitudes equatorial region and toward the equator in both hemispheres in the midlatitudes the lagrangian length scale l is found to be approximately equal to the first mode baroclinic radius of deformation ri while the rossby number defined as ro tearth t sin where t is the lagrangian timescale tearth is the period of the earth s rotation and is the latitude is less than unity implying that the quasigeostrophic mesoscale eddies are the main mechanism for lateral mixing the lateral diffusivity in the midlatitudes is suggested to be parameterized as k v2ri where v2 is the lagrangian velocity scale inherent to a mesoscale eddy field in the low latitudes l becomes less than ri and the condition ro 1 is no longer valid which implies the change of the eddy controlled mechanism of lateral mixing by another mechanism

lagrangian time scales and eddy diffusivity at 1000 m compared to the surface in the south pacific and indian oceans abstractargo floats cannot be regarded as true lagrangian drifters because they periodically rise to the surface hence previous estimates of eddy diffusivity at depth using single particle statistics have been limited to one submerged cycle however unless the lagrangian time scale is significantly shorter than the argo cycle time this single particle calculation can have a large bias here eddy diffusivity computed from single particle statistics using argo data is compared to that computed by assuming that eulerian scales at depth are the same as at the surface and that the relationship between lagrangian and eulerian time scales derived by middleton is valid if the methods provide the same answer one can have confidence in both methods eddy diffusivity calculated from the single particle statistics shows the same spatial structure as that computed from inferred time scale but is smaller by a factor of about 2 it is suggested that this is because the deep lagrangian time scale over much of th

understanding coastal morphodynamic patterns from depth averaged sediment concentration this review highlights the important role of the depth averaged sediment concentration dasc to understand the formation of a number of coastal morphodynamic features that have an alongshore rhythmic pattern beach cusps surf zone transverse and crescentic bars and shoreface connected sand ridges we present a formulation and methodology based on the knowledge of the dasc which equals the sediment load divided by the water depth that has been successfully used to understand the characteristics of these features these sand bodies relevant for coastal engineering and other disciplines are located in different parts of the coastal zone and are characterized by different spatial and temporal scales but the same technique can be used to understand them since the sand bodies occur in the presence of depth averaged currents the sediment transport approximately equals a sediment load times the current moreover it is assumed that waves essentially mobilize the sediment and the current increases this mobilization and advects the sediment in such conditions knowing the spatial distribution of the dasc and the depth averaged currents induced by the forcing waves wind and pressure gradients over the patterns allows inferring the convergence divergence of sediment transport deposition erosion occurs where the current flows from areas of high to low low to high values of dasc the formulation and methodology are especially useful to understand the positive feedback mechanisms between flow and morphology leading to the formation of those morphological features but the physical mechanisms for their migration their finite amplitude behavior and their decay can also be explored

eulerian and lagrangian isopycnal eddy diffusivities in the southern ocean of an eddying model abstractlagrangian isopycnal diffusivities quantify the along isopycnal mixing of any tracer with mean gradients along isopycnal surfaces they are studied in the southern ocean of the 1 10 parallel ocean program pop model using more than 50 000 float trajectories concurrent eulerian isopycnal diffusivities are estimated directly from the eddy fluxes and mean tracer gradients consistency spatial variation and relation to mean jets are evaluated the diffusivities are calculated in bins large enough to reduce contributions from the rotational components that do not lead to net tracer mixing because the mean jets are nonzonal and nonparallel meridional dispersion includes standing eddies and is significantly different from cross stream dispersion with the subtraction of the local eulerian mean the full lagrangian diffusivity tensor can be estimated along stream diffusivities are about 6 times larger than cross stream diffusivities along streamline averages of eulerian and lagrangian isopycnal d

modeling and analyzing observed transverse sand bars in the surf zone 1 a morphodynamic model has been applied to explain the characteristics of transverse sandbars observed in the inner surf zone of open beaches the model describes the feedback between waves rollers depth averaged currents and bed evolution so that self organized processes can develop the modeled bar characteristics i e wavelength 30 70 m crest orientation up current and the e folding growth time about 12 hr are in good agreement with those of observed transverse bars at noordwijk beach the netherlands but modeled migration speeds tens of meters per day turn out to be a factor 2 larger than those observed the wavelength increases with the distance between the shoreline and the peak of the longshore current and the migration speed is correlated with the maximum longshore current the model also explains why transverse bar formation at noordwijk occurs for obliquely incident waves of intermediate heights realistic positive feedback leading to formation of up current oriented bars like those observed is only obtained if a term related to the turbulence sediment resuspension created by the rollers is included in the transport formula in that case the depth averaged sediment concentration decreases seaward across the inner surf zone enhancing the convergence of sediment transport in the offshore directed flow perturbations that occur over the up current bars this offshore current deflection is mainly caused by frictional torques but the roller radiation stresses also play an important role

isopycnal diffusivities in the antarctic circumpolar current inferred from lagrangian floats in an eddying model average 750 250 m 2 s 1 around the polar frontal zone the results imply that parameterizations that only use eddy kinetic energy to parameterize the diffusivities are incomplete we suggest that dominant correlations of lagrangian eddy diffusivities with eddy kinetic energy found in previous studies may have been due to the use of too short time lags in the integration of the velocity autocovariance used to infer the diffusivities we find evidence that strong mean flow inhibits cross stream mixing within the acc but there are also areas where cross stream diffusivities are large in spite of strong mean flows for example in regions close to topographic obstacles such as the kerguelen plateau

morphodynamic instabilities of planar beaches sensitivity to parameter values and process formulations the initial growth of bed perturbations on planar sloping beaches under the forcing of obliquely incident breaking waves is investigated using a state of the art numerical model this allows for a systematic investigation of the sensitivity of the spatial structures of the bed perturbations and their growth and migration rates to different model formulations and parameterizations if the sediment is only transported in the direction of the net current velocity and sediment stirring is taken proportional to the wave height squared growing up current oriented crescentic bars are found with a preferred spacing of 800 m and a down current migration rate of 10 m d 1 varying the angle of wave incidence drag coefficient and bed slope results in qualitatively similar growing bed forms using an engelund and hansen transport formula very oblique down current oriented bars are obtained that grow in time no preferred wavelength however is found using the bailard transport formula results in growing up current oriented bars with a preferred spacing smaller than 300 m for wave angles smaller than 7 when using either the engelund and hansen or bailard sediment transport formulation it is essential to take the transport in the direction of the wave orbital velocity into account in order to have growing bed perturbations

eddy stirring and horizontal diffusivity from argo float observations geographic and depth variability stirring along isopycnals is a significant factor in determining the distribution of tracers within the ocean salinity anomalies on density surfaces from argo float profiles are used to investigate horizontal stirring and estimate eddy mixing lengths eddy mixing length and velocity fluctuations from the ecco2 global state estimate are used to estimate horizontal diffusivity at a 300 km scale in the upper 2000 m with near global coverage diffusivity varies by over two orders of magnitude with latitude longitude and depth in all basins diffusivity is elevated in zonal bands corresponding to strong current regions including western boundary current extension regions the antarctic circumpolar current and equatorial current systems the estimated mixing lengths and diffusivities provide an observationally based data set that can be used to test and constrain predictions and parameterizations of eddy stirring

characteristics and dynamics of surfzone transverse finger bars 1 patches of transverse finger bars have been identified in the surf zone of noordwijk beach netherlands they consisted of three to nine elongated accumulations of sand attached to the low tide shoreline the bars extended up to 50 m into the inner surf zone had an oblique orientation with respect to the shore normal and were quasiregularly spaced in the alongshore direction we analyzed nearly 6 years of video data and observed a significant presence of finger bars 14 of the time with good data bars were visible on 193 days gathered in 44 events that persisted from 2 days to 2 months obliquely incident waves of intermediate and approximately constant height were dominant during finger bar presence shore normal incident or more energetic wave fields destroyed the bar patches the underlying bathymetry affected finger bar formation inner surfzone troughs with cross shore areas of 100 m2 and inner surfzone slopes of 0 02 were more conducive to their growth the mean alongshore wavelength of the finger bar patches was 39 m ranging from 21 to 75 m bar crests deviated up to 40 degrees from the shore normal against the alongshore current direction up current orientation and bar patches migrated at rates up to 22 m day in the direction of the alongshore current we used these observations to test existing theoretical self organization mechanisms for transverse bar formation the bed flow mechanism was the most viable explanation for the generation and persistence of noordwijk finger bars our observations were consistent with most of the predictions of two models that included this interaction but migration rates differed by 1 order of magnitude

horizontal mixing in the southern ocean from argo float trajectories we provide the first observational estimate of the circumpolar distribution of cross stream eddy diffusivity at 1000 m in the southern ocean using argo float trajectories we show that argo float trajectories from the float surfacing positions can be used to estimate lateral eddy diffusivities in the ocean and that these estimates are comparable to those obtained from rafos floats where they overlap using the southern ocean state estimate sose velocity fields to advect synthetic particles with imposed behavior that is argo like and rafos like diffusivity estimates from both sets of synthetic particles agreed closely at the three dynamically very different test sites the kerguelen island region the southeast pacific ocean and the scotia sea and support our approach observed cross stream diffusivities at 1000 m calculated from argo float trajectories ranged between 300 and 2500 m2 s 1 with peaks corresponding to topographic features associated with the scotia sea the kerguelen plateau the campbell plateau and the southeast pacific ridge these observational estimates agree with previous regional estimates from the diapycnal and isopycnal mixing experiment in the southern ocean dimes near the drake passage and other estimates from natural tracers helium inverse modeling studies and current meter measurements these estimates are also compared to the suppressed eddy diffusivity in the presence of mean flows the comparison suggests that away from regions of strong topographic steering suppression explains both the structure and magnitude of eddy diffusivity but that eddy diffusivities in the regions of topographic steering are greater than what would be theoretically expected and the acc experiences localized enhanced cross stream mixing in these regions

two dimensional tide induced residual sand transport applications to the jiangsu coast china abstract the residual sand d 62 5 m transport rst refers to the tidally averaged rate of horizontal sand transport albeit much smaller than the peak rates it determines the morphological evolution the analytical expression for the tide induced rst has been reported in the condition of one dimensional tidal currents by van de kreeke and robaczewska 1993 here we extended the analytical solution to two dimensional the constituents of the residual m2 and m4 tidal current velocities were used to characterize the collinear tidal current in the orthogonal x and y directions i e cross shore and alongshore respectively while the m2 tidal constituent is dominant in both directions based on the solution rst is controlled by a series of triple interactions of the m2 tidal velocity in the x direction m2 tidal velocity in the y direction and the residual or m4 velocity in the x or y direction the solution was applied to the tide dominated continental shelf of the jiangsu coast china where the alongshore current is much stronger than the cross shore component the results show that the triple interactions of the cross shore m2 tidal velocity um2 the alongshore m2 tidal velocity vm2 and the alongshore residual velocity account for the major parts of both the alongshore and cross shore rst in the nearshore area the rst patterns promote sand accumulation in the shallow nearshore area and generate a narrow and active conduit of coastal alongshore sand transport these findings can be applied in similar marine environments to understand the effects of non linear tidal interactions in rst

structure and distribution of branched aliphatic alkanes with quaternary carbon atoms in cenomanian and turonian black shales of pasquia hills saskatchewan canada abstract twelve pseudohomologous series of branched alkanes with one or two quaternary carbon atoms baqcs c14 c33 range are present in the extractable organic matter of cenomanian and turonian black shales of pasquia hills saskatchewan canada five of these pseudohomologous series are also observed in a holocene sample from the russian arctic three of the pseudohomologous series were identified by kenig et al pnas 100 2003 12554 12558 as 3 3 and 5 5 diethylalkanes deas and 2 2 dimethylalkanes dmas the nine other pseudohomologous series of baqcs are tentatively identified on the basis of their characteristic mass spectral fragmentation and retention indices as 3 3 dimethylalkanes dmas 3 ethyl 3 methyl and 5 ethyl 5 methylalkanes 9 9 7 7 and 6 6 diethylalkanes deas 3 3 3 3 and 5 5 5 5 tetraethylalkanes and 5 butyl 5 ethylalkanes pseudohomologous series of branched alkanes with tertiary carbon atoms such as monomethyl and monoethylalkanes are also present in the same carbon number range all these pseudohomologous families share the particularity of including isomers with exclusively odd or exclusively even carbon numbers clearly indicating a biosynthetic origin baqcs such as 5 5 deas 3 3 deas and 3 ethyl 3 methylalkanes seem to be widespread in sediments and sedimentary rocks ranging from modern to 2 1 billion years a xenobiotic source for baqcs can be excluded at this point baqcs seem to have a very low biodegradability in part explaining their widespread occurrence in ancient sediments

introduction to special section on marine sand wave and river dune dynamics marine sand waves and river dunes are rhythmic patterns formed at the interface between a sandy bed and turbulent flows driven by tidal and river currents their origin development and dynamics are far from being fully understood to enhance the discussion on this topic we organized the international conference marine sand wave and river dune dynamics marid in april 2004 in enschede netherlands scientific papers that originated from the marid conference are presented in this special section this introduction places these papers in the context of recent progress

development of crescentic bars for a periodically perturbed initial bathymetry 1 the development of crescentic bed patterns starting from an undisturbed beach i e one only perturbed so as to simulate background noise and thereby initiate morphological development is compared with that starting from a beach where monochromatic and in some cases bichromatic bed patterns pre exist using a fully non linear model morfo55 a wide range of different lengthscales and amplitudes is investigated the findings suggest that the pre existence of crescentic bed forms can influence the subsequent morphological development of this beach significantly but that this development is related to the undisturbed beach development whether a pre existing bed form remains under certain forcing conditions depends on the position of the pre existing lengthscale along the undisturbed linear growth rate curve if the pre existing mode shows significant linear growth in the undisturbed scenario this pre existing lengthscale remains otherwise the pre existing mode will be replaced by a bed form with a lengthscale closer to the undisturbed fastest growing mode a pre existing lengthscale that is significantly longer than the undisturbed fastest growing mode will be replaced by a higher harmonic of the original lengthscale for shorter pre existing lengthscales this will be the linear fastest growing mode an increased amplitude of the pre existing bed form accelerates the development toward a new stable situation for small initial amplitudes the initial response of the system to pre existing bed forms could theoretically be described using linearized equations alone the migration rates of both the pre existing and newly arising modes correspond to the migration rate of these modes in the undisturbed scenario

bars bumps and holes models for the generation of complex beach topography in shallow water any two waves of the same frequency are shown to produce complex patterns of drift velocity above the sea bed if the longshore components hi h of the wave numbers of the two waves are different these steady flow patterns exhibit a longshore periodicity of wave number hi h irrespective of whether the waves propagate in the same direction say hi he positive or in opposite directions he negative the interaction of two edge wave modes is examined in detail the drift velocities are calculated and a simple sediment transport model is used to predict the beach topography that would be in equilibrium with these flow patterns as expected crescentic sand bars are produced by the special case of standing edge waves hi h intriguingly for all other cases a combination of complex transverse bars plus meandering or straight offshore bars result patterns that are surprisingly reminiscent of many published descriptions of complex rhythmic topography the extension of the model to three or more waves produces topography that appears to be very irregular although the pattern should repeat over sufficiently long distances along the beach even with only three waves these distances may be very large compared to the scale of the bars

observations of nearshore crescentic sandbars 1 the temporal and spatial variability of crescentic sandbars is analyzed with hourly long term months video observations collected at four barred sites and are qualitatively compared to the temporal and spatial variability predicted by hypotheses underpinning existing approaches and models for crescentic bar formation edge wave template model linear stability analysis and nonlinear models the observations coming from the single barred beaches at duck north carolina usa and miyazaki kyushu japan and from the double barred beaches at the northern gold coast queensland australia and noordwijk netherlands show that crescentic sandbar wavelength and amplitude variations over space and time are very common for instance at any moment in time the wavelength of the smallest and longest crescentic bar can differ by a factor of 2 temporal changes in wavelength and amplitude result from merging and splitting of individual crescents causing the final configuration of a crescentic sandbar system to be very different from the initial configuration the gold coast data indicate that these intrinsically nonlinear interactions are an attempt of the crescentic bar system to self organize into a more uniform pattern as splitting is usually confined to the longest crescentic bar observed whereas merging usually combines the smallest crescentic bars into a longer bar the observed spatial and temporal crescentic bar behavior contrasts qualitatively with behavior predicted from the edge wave template model and implies that the predictive skill of linear stability models is limited nonlinear models are potentially better suited for a comparison against these field observations several suggestions to improve these models and hence to facilitate a data model comparison are made