enhancement of alongshore freshwater transport in surface advected river plumes by tides abstracta recent numerical study by isobe showed that imposing alongshore tidal forcing on buoyant coastal discharge enhances the net freshwater transport in the coastal currents the mechanisms for this transport enhancement are studied using a three dimensional primitive equation ocean model regional ocean modeling system roms lagrangian drifters are used to trace the freshwater transport paths it is found that the river plume bulge circulation largely follows the rigid body motion i e constant vorticity the buoyant fluid near the bulge s outer edge is thinner and faster behaving as a baroclinic jet the bulge currents then split after impinging on the coast the outer fluid feeds the downshelf flowing coastal currents while the inner fluid recirculates to form the bulge the coastal current transport estimated from the present and prior studies corresponds well to a baroclinic jet theory with the incident angle of bulge currents at the coast being a key parameter without tides the bulg

turbulence and entrainment in a buoyant surface plume in winter tidal outflow from the leschenault estuary forms a buoyant sheared stratified plume in koombana bay on each outgoing tide the turbulence in the plume and the local entrainment into the plume are examined based on measurements collected in an intensive field program coherency measurements between horizontally separated sensors are used to examine the anisotropy of the turbulence the effects of viscosity and buoyancy on the turbulence are studied using an activity diagram which is also an indicator of the mixing efficiency and total mixing the osborne cox model is tested and found to be valid in the plume

characteristics of a surface buoyant jet tidal outflow from leschenault estuary in winter forms an unsteady surface buoyant jet in koombana bay on each outgoing tide as the water moves away from the exit channel and spreads radially over the bay it assumes a plumelike nature the mean features of this buoyant outflow such as the plume shape frontal celerity internal circulation and the bulk entrainment were measured in an intensive field program these features are shown to compare favorably with results from previous laboratory investigations and predictions from a simple inertia buoyancy force balance which includes entrainment at the front the stability of the plume frontal region was examined and it is shown that subfronts were present but their behavior cannot be explained by reference to previous work

analyses of internal solitary waves generated at the columbia river plume front using sar imagery 1 internal solitons generated at the columbia river plume front during ebb tide are displayed on a synthetic aperture radar sar image taken on 26 july 2004 at 14 24 22 utc scale analyses suggest that these internal solitons belong to the finite depth category a theoretical model relating the radar backscatter cross section to dynamic parameters of internal solitons in the continuous stratification is developed using the model and background stratification data collected by the river influences on shelf ecosystem rise project in july 2004 we extract the soliton dynamic parameters of the half width amplitude phase speed and average energy flux per crest length along a typical section across the internal solitons the observed stratification is simulated by using a theoretical model parameterized by pycnocline depth thickness and maximum buoyancy frequency the half width amplitude phase speed and energy flux of the leading soliton at the section are 119 3 m 9 3 m 0 69 m s 1 and 417 4 w m 1 with the uncertainty ranges of 14 7 m 2 6 m 0 10 m s 1 and 281 6 w m 1 respectively sensitivity analyses reveal that the extraction of the soliton half width from the sar image is not affected by the stratification the amplitude is less influenced by the pycnocline thickness than the depth and is independent of the maximum buoyancy frequency

a simplified energy and mixing budget for a small river plume discharge 1 the formation and evolution of a tidally modulated river plume that spreads and mixes into english channel waters is investigated by combining results from two observational studies remote sensing imagery showed a near semicircular plume expansion as its diameter increased linearly during the ebb tide subsequent ship surveys made repeated transects across the plume front and showed that regions of mixing and instability froude numbers 1 were confined to horizontal length scales 1 suggesting that a plume cannot form

initial expansion of the columbia river tidal plume theory and remote sensing observations 1 analysis of the columbia river tidal plume using lagrangian frontal equations provides a concise description of the evolution of frontal depth h velocity u reduced gravity g and frontal internal froude number fr because the estuary mouth is narrow the initial radial plume motion is supercritical fr 1 for up to 12 hours understanding this supercritical phase is vital because plume properties change rapidly with strong ecosystem impacts to analyze this expansion analytical and numerical models the latter with three mixing formulations were tested model results are compared to synthetic aperture radar images to verify that the predicted frontal properties are realistic lagrangian theory provides especially simple constraints independent of the mixing model on spatial variations in fr and ug h for parameters representative of the columbia river plume the plume spreads 10 35 km and thins to 25 60 of its initial depth before becoming subcritical after liftoff fr increases as the front accelerates and thins it then decreases to unity g decreases but more slowly than h and u h controlled by a balance between spreading and mixing first decreases then increases the strength of vertical mixing and the mixing efficiency ef ratio of buoyancy flux to dissipation both play a significant role in determining plume properties and it is important to include both the horizontal gradient in g and surface slope component of the pressure gradient a model with an ellison turner type entrainment scheme predicts the plume trajectory better than one that assumes a constant interfacial gradient richardson number

detailed observations of a naturally occurring shear instability simultaneous profiles of microstructure horizontal velocity and acoustic backscatter allow one of the most complete descriptions of a naturally occurring shear instability to date shear increased rapidly after passing through a lateral constriction which formed a hydraulic control a kilometer long set of 20 m tall billows grew on a middepth density interface where the richardson number fell below 0 25 the velocity interface thickened steadily after the billows formed consistent with rapid momentum mixing across a shear layer with a reynolds number of 3 106 the billows generated large density overturns and dissipation rates greater than 10 5 w kg 1 even within the first large overturn indicating that these structures were fully turbulent early in their development as the billows grew a well mixed layer developed at the interface and survived as an actively turbulent layer for up to 6 buoyancy periods 3 times longer than in laboratory studies at low reynolds number variations in the mean density of the billows lead us to infer that the vertical offset of the velocity and density interfaces varied with time where the billows first formed with data from the large overturns within the shear layer we find e n2 3 104 an average root mean square overturn scale lrms of 2 6 m and a buoyancy scale lb of 2 7 m despite having sampled the billows at varying stages of their evolution we find no indication that the ratio lrms lb is ever significantly different than 1 for this shear instability

convergence and downwelling at a river plume front abstract the small scale structure of the circulation and hydrography at the frontal boundary of the connecticut river plume in long island sound has been resolved using a novel combination of instruments a towed acoustic doppler current profiler adcp and a rigid array of current meters and conductivity temperature sensors observations were made during the latter half of the eastward ebb tide when the river plume was well established and the front was moving to the west at approximately 0 3 m s 1 two across front transects revealed a horizontal convergence rate in the across front velocity components at 0 6 m of 0 05 0 1 s 1 this was associated with a salt induced horizontal density gradient of 10 2 kg m4 observations obtained during a period in which the towed adcp was caught in the zone of maximum surface convergence showed significant downwelling with a near surface maximum of 0 2 m s 1 vertical velocities of this magnitude are consistent with observed magnitudes of the convergence rate at 0 6

relating river plume structure to vertical mixing abstract the structure of a river plume is related to the vertical mixing using an isohaline based coordinate system salinity coordinates offer the advantage of translating with the plume as it moves or expanding as the plume grows this coordinate system is used to compare the relative importance of different dynamical processes acting within the plume and to describe the effect each process has on the structure of the plume vertical mixing due to inertial shear in the outflow of a narrow estuary and wind mixing are examined using a numerical model of a wind forced river plume vertical mixing and the corresponding entrainment of background waters is greatest near the estuary mouth where inertial shear mixing is large this region is defined as the near field with the more saline far field plume beyond wind mixing increases the mixing throughout the plume but has the greatest effect on plume structure at salinity ranges just beyond the near field wind mixing is weaker at high salinity classes tha

frontogenesis in mixed layers abstract a unified theory of frontogenesis in mixed layers is proposed analytical solutions of the nonlinear mass and density balances in a mixed layer are found in the lagrangian frame for a wide class of entrainment parameterizations these solutions show mixed layer depth and density to be functions of particle position and separation where mixing is represented by an integral over time following a particle thus given a knowledge of only the crossfront velocity field the likelihood of frontal formation can be predicted both surface convergence and divergence in conjunction with vertical mixing can be frontogenetic three different types of fronts distinguished by their associated crossfront velocity fields are discussed large scale fronts such as the subtropical front are described by a convergent surface flow acting upon an initial horizontal density gradient the powerful divergence caused by an offshore ekman flow forced to zero at a coast causes upwelling fronts the divergence associat

azimuth energy q and temperature variations on p wave amplitudes in the united states mean relative p wave 1 s amplitude characteristics are determined for standardized seismic stations in the united states data are compiled from three azimuths to the north explosions in russia to the northwest earthquakes along the asian pacific arc and to the south southeast earthquakes in south america characteristic receiver amplitudes are found to be azimuthally dependent high amplitude areas occur in the central united states and lowest amplitudes occur along the rocky mountain front range and the rio grande rift zone amplification effects of low velocity sediments are significant in the central united states azimuthal amplitude variations in the eastern united states are consistent with an anomalous attenuating or defocusing body in the lower mantle beneath colombia south america nonazimuthal amplitude studies are tied to the azimuthal data set to increase station amplitude coverage the regional and local stability of amplitudes is analyzed amplitudes are stable with an average variation of a factor of about 1 3 around the regional mean for seismic stations situated on hard bedrock the squared amplitude variations are related to energy variations in the p wave fronts acknowledging the likely importance of elastic focusing defocusing and scattering effects at individual stations variations of anelastic attenuation are derived that are consistent with the observed energy amplitude variations assuming that the variations of anelastic attenuation occur in the upper 400 km of the earth high amplitude stations may be unattenuated low amplitude stations are consistent with q 110 and a mean amplitude implies q 220 240 the range of variation of q is mapped into temperature variations assuming a thermally activated q mechanism and an olivine activation energy for the upper mantle the apparent q variations are consistent with temperature variations of 135 287 c between high and low amplitude seismic station sites geophysical and physiographic correlations with the p wave amplitude variations are discussed accounting for the motion of the north american plate low p wave amplitudes along the rocky mountain front may be related to the uplift of the colorado plateau low amplitudes extending from northern texas to new england lie along the approximate boundary of the continental craton

the columbia river plume study subtidal variability in the velocity and salinity fields a comprehensive study of the strongly wind driven midlatitude buoyant plume from the columbia river located on the u s west coast demonstrates that the plume has two basic structures during the fall winter season namely a thin 5 15 m strongly stratified plume tending west to northwestward during periods of southward or light northward wind stress and a thicker 10 40 m weakly stratified plume tending northward and hugging the coast during periods of stronger northward stress the plume and its velocity field respond nearly instantaneously to changes in wind speed or direction and the wind fluctuations have timescales of 2 10 days frictional wind driven currents cause the primarily unidirectional flow down the plume axis to veer to the right or left of the axis for northward or southward winds respectively farther downstream currents turn to parallel rather than cross salinity contours consistent with a geostrophic balance in particular during periods when the plume is separated from the coast currents tend to flow around the mound of fresher water at distances exceeding about 20 km from the river mouth the along shelf depth averaged flow over the inner to midshelf is linear and depth averaged acceleration is governed to lowest order by the difference between surface and bottom stress alone in this region along shelf geostrophic buoyancy driven currents at 5 m calculated from surface density and along shelf geostrophic wind driven currents computed from a depth averaged linear model are comparable in magnitude 10 25 cm s 1

tidal asymmetry in an estuarine pycnocline depth and thickness tidal variations in estuarine stratification are revealed by the depth and thickness of the density interface the depth of the interface may be predicted using an inviscid two layer model that combines baroclinic estuarine circulation with barotropic tidal currents helfrich 1995 here we present results from a two layer model modified to include the effects of bottom friction and interfacial mixing modeled layer thickness and speed compare favorably with prior analytic studies farmer and armi 1986 pratt 1986 we use a bulk richardson number criterion to estimate the thickness of the pycnocline from two layer model results the predicted pycnocline depth and thickness compare remarkably well with observations we also investigate the effects of changing bottom friction and barotropic currents on the pycnocline thickness

the role of turbulence stress divergence in decelerating a river plume 1 turbulence controls the composition of river plumes through mixing and alters the plume s trajectory by diffusing its momentum while believed to play a crucial role in decelerating river source waters the turbulence stress in a near field river plume has not previously been observationally quantified in this study finely resolved density velocity and turbulence observations are combined with a control volume technique to describe the momentum balance in the columbia river s near field plume during 10 tidal cycles that encompass both large and small river flow turbulence stress varies by 2 3 orders of magnitude both within a given ebb and between ebbs with different tidal or river forcing its magnitude scales with the strength of the instantaneous ebb outflow i e high stresses occur during peak flow of strong ebbs during these periods the momentum equation is represented by a balance between stress divergence and plume deceleration as the flow relaxes the stress divergence weakens and other terms pressure gradient and coriolis may become appreciable and influence plume deceleration while the momentum balance could not be closed during these weaker flow periods during strong tidal pulses the time scale for decay based on observed stress is significantly less than a tidal half period indicating that stress divergence plays a fundamental role in the initial deceleration of the plume

a simple theory for the fate of buoyant coastal discharges abstract a simple theory that predicts the vertical structure and offshore spreading of a localized buoyant inflow onto a continental shelf is formulated the theory is based on two competing mechanisms that move the buoyant fluid offshore 1 the radial spread of the lighter water over the ambient water being deflected by the coriolis force and producing an anticyclonic cyclostrophic plume and 2 offshore transport of buoyant water in the frictional bottom boundary layer that moves the entire plume offshore while maintaining contact with the bottom the surface expression of the cyclostrophic plume moves offshore a distance ys 2 3g h0 2i 2g h0 2i 1 2f where g is reduced gravity based on the inflow density anomaly h0 is the inflow depth i is the inflow velocity and f is the coriolis parameter the plume remains attached to the bottom to a depth given by hb 2l ih0f g 1 2 where l is the inflow width both scales are based solely on parameters of the buoyant inflow at its source there

characteristics of the great whale river plume observations of the motion field and dilution effects associated with the plume of great whale river in hudson bay are presented for both open water and ice covered conditions in the summer months a distinct plume of about 100 km2 in area is formed offshore which is characterized by a 1 2 m thickness and large velocities directed away from the river mouth in contrast to slower currents parallel to the shore in the ambient waters underneath surface drifter results suggest that the outer boundary of plume may be a zone of frontal convergence under ice covered conditions the plume was significantly thicker and extended much farther offshore in spite of a marked reduction in river runoff at this time

particle resuspension in the columbia river plume near field 1 measurements of suspended sediment concentration velocity salinity and turbulent microscale shear in the near field region of the columbia river plume are used to investigate the mechanisms of sediment resuspension and entrainment into the plume an east west transect was occupied during spring and neap tide periods in august 2005 and may 2006 corresponding to low and high river discharge conditions respectively during the high discharge period the plume is decoupled from the bottom and fine sediment resuspended from the bottom does not leave the benthic boundary layer the primary modes of sediment transport associated with the plume are advection of sediment from the estuary and removal of sediment from the plume by gravitational settling and turbulent mixing in contrast the plume is much less stratified during low discharge conditions and large resuspension events are observed that entrained sediment through the water column and into the plume our measurements indicate that two factors control the magnitude and timing of sediment resuspension and entrainment the supply of fine sediment on the seabed and the relative influence of tidal turbulence compared with buoyancy input from the river the latter is quantified in terms of the estuary richardson number rie the magnitude of vertical turbulent sediment flux is correlated with rie during the low flow period when there is a sufficient supply of bottom sediment in the near field region such sediment resuspension may be an important mechanism for the delivery of bioavailable micronutrients to the plume during the summer

observations at the tidal plume front of a high volume river outflow 1 we present shipboard observations of very strong convergence vertical velocities and mixing and near bed impacts associated with the leading edge front of the tidally pulsed columbia river plume with upwelling favorable winds and riverflow of 4900 m3s 1 the plume propagates as a buoyant gravity current with a rotary bore like vertical frontal circulation and downwelling as strong as 0 35 m s 1 in waters as deep as 65 m near bed currents intensify to as much as 1 0 m s 1 after frontal passage and are often associated with elevated acoustic backscatter mixing is locally strong with an eddy diffusivity of o 0 2 m2s 1 50 m behind the front and t s diagrams imply plume mixing with 10 m deep ocean water these observations indicate that the leading edge front of a surface advected plume can cause exchanges of a nutrients between cold subsurface shelf waters and the river plume and b nutrients and sediments across the sediment water interface

estuary plumes and fronts in shelf waters a layer model abstract a layer model that treats fronts as discontinuities is developed to study the steady state behavior of shallow estuary plumes on the continental shelf the complete range of earth rotation effect is evaluated from small scale or nonrotating plumes kelvin number equal zero to large scale rotating plumes kelvin number equal order one supercritical flow is assumed in the outlet channel and the method of characteristics is used to compute the flow downstream nonrotating plumes have strong boundary fronts and concentrate their greatest layer depth and mass transport offshore near the front but form no coastal current rotating plumes have boundary fronts that weaken soon after discharge form a turning region where coriolis action deflects the flow toward shore and subsequently set up a coastal current soon after its formation this coastal current is bounded offshore by a strong front called the coastal front across which the momentum balance changes from nearly inertial in the turning regi

structure and dynamics of the columbia river tidal plume front 1 time dependent buoyant plumes form at the outflow of tidally dominated estuaries when estuary discharge velocity exceeds plume internal wave speed c a sharp front forms at the plume s leading edge that expands from the time dependent source using observations of the columbia river tidal plume from multiple tidal cycles we characterize time evolving plume structure and quantify front speed uf plume internal wave speed c front curvature and ultimate extent we identify three distinct stages of propagation 1 initially the plume is strongly influenced by shallow bathymetry near the river mouth 2 as the front advances offshore the plume detaches from the bottom and expands as a freely propagating gravity current with relatively constant uf c and frontal froude number f uf c ambient currents explain intracycle variability in uf and winds alter front shape variability in ambient stratification associated with previous cycles plume remnants leads to complex fronts and internal waves 3 finally the plume decelerates adjusts toward geostrophy and may radiate additional internal waves using a simple kinematic model we suggest that constant frontal propagation speed uf 0 9 0 1 m s during stage 2 is primarily controlled by linearly increasing volume flux from the columbia river mouth as this discharge rate subsides the plume expands as a fixed volume with decreasing front speed stage 3 the plume s final extent is controlled by the rossby radius which scales with a length based on the total volume discharged this provides an integral description of plume front evolution based on the time dependent estuary discharge