barotropic rossby waves radiating from tropical instability waves in the pacific ocean abstracttropical instability waves are triggered by instabilities of the equatorial current systems and their sea level signal with peak amplitude near 5 n is one of the most prominent features of the dynamic topography of the tropics cross spectral analysis of satellite altimetry observations shows that there is sea level variability in the pacific ocean as far north as hawaii i e 20 n that is coherent with the sea level variability near 5 n associated with tropical instability waves within the uncertainty of the analysis this off equatorial variability obeys the dispersion relation for nondivergent barotropic rossby waves over a fairly broad range of periods 26 38 days and zonal wavelengths 9 23 of longitude that are associated with tropical instability waves the dispersion relation and observed wave properties further suggest that the waves are carrying energy away from the instabilities toward the north pacific subtropical gyre which together with the observed coherence of the sea

threshold of motion of coarse grained sediment under waves in shallow water this paper considers the threshold of motion of sediment in shallow coastal waters under breaking and non breaking waves a simple model representing conditions at the fluid sediment interface is developed the representation of a breaking wave is based on bore theory and the shear stress at the bed is based on the friction formula of o connor and yoo a threshold formula is presented based on the use of energy dissipation the threshold data is also related to the shields threshold criterion

intercomparison of wave driven current models abstract seven numerical models which simulate waves and currents in the surf zone are tested for the case of a reduced scale detached breakwater subjected to the action of regular waves with normal incidence the computed wave heights water levels and velocities are compared with measurements collected in an experimental wave basin the wave height decay in the surf zone is predicted reasonably well set up and currents appear to be less well predicted this intercomparison exercise shows that radiation stresses are systematically overestimated by formulations used in the models mean bottom shear stresses are not always co linear with the mean bottom velocity vector in shallow water and turbulence modelling in the surf zone requires a sophisticated

anisotropic surface wave characterization of granular media abstractin unconsolidated granular media the state of stress has a major effect on the elastic properties and wave velocities effective media modeling of granular packs shows that a vertically oriented uniaxial state of stress induces vertical transverse isotropy vti characterized by five independent elastic parameters the rayleigh wave velocity is sensitive to four of these five vti parameters we have developed an anisotropic rayleigh wave inversion that solves for four of the five independent vti elastic parameters using anisotropic thomson haskell matrix equations that can be used to determine the degree of anisotropy and the state of stress in granular media the proposed method incorporates an initial model search guided by rock physics models and dispersion curve inversion that uses a gradient based solver we carried out the inversion on a synthetic example and a real data set in which we inverted for in situ elastic moduli of unconsolidated sands at very low effective stress the synthetic

deterministic profile modelling of nearshore processes part 1 waves and currents abstract this is the first of two companion papers concerned with the modelling of beach profile response to wave and tidal action the present paper concentrates on wave and current modelling while the companion paper deals with sediment transport and beach profile development the papers describe summaries of the relevant theory model structure and comparisons with laboratory and field data including plane and barred beaches in the present paper wave heights fraction of broken waves bottom orbital velocities longshore wave driven currents undertow currents and bottom wave velocity moments are compared with measured data in these tests generally with good agreement due to lack of suitable data tidal effects have not been included in these comparisons the model is at present designed for application to beach erosion during storm events lasting several days and longer wave sequences of several months using hydrodynamic timesteps of the order of 0 5 to 3 hours wave sequences covering very long timespans several years may require further studies of filtering of input data

the barotropic electromagnetic and pressure experiment 1 barotropic current response to atmospheric forcing during 1986 1987 an array of horizontal electrometers which measure the subinertial barotropic velocity was deployed for 11 months in the central north pacific to study atmospherically forced motions in a low eddy kinetic energy region significant coherence is found to be ubiquitous between the barotropic velocity and one or more surface atmospheric variables over the north pacific the maximum squared coherence between currents and wind stress curl at all periods greater than 3 days is typically 0 6 rarely below 0 4 and occasionally above 0 8 and is dominantly nonlocal although local coherence is seen at some sites especially at the shorter periods both the coherence and the intersite pattern similarity are strongest at the periods of relative maxima in the barotropic current autospectra e g at 6 8 10 14 and 20 50 days the locations at which the wind stress curl forces the motions at each site and for each period have been determined while there is substantial intersite variability some of the observed patterns of squared coherence over the north pacific are very similar to those predicted by simple linear smooth bottomed quasi geostrophic models of atmospherically forced barotropic currents the forcing location is occasionally found to be the same spot for all oceanic instruments especially at the autospectral maxima in barotropic current more commonly distinct forcing locations are found for each instrument even when closely spaced suggesting topographic influence in other cases multiple forcing locations are implicated for the motions at a single station all of this site and period dependent variability of the forcing location is incompatible with the simple models and is due either to the known spatial inhomogeneity of the curl forcing field or to topographic effects the importance of the former is emphasized using simple arguments based on analytic transfer functions between the atmospheric and oceanic variables and on observed wavenumber spectra for wind stress curl quantitative assessment of topographic effects must await suitable numerical models in addition maximum likelihood wavenumber spectra of the barotropic currents have been estimated in those bands with an adequate number of coherent sensors the wavenumbers at the spectral peaks are compatible with the dispersion relation for rossby waves modified by the observed large scale linearly sloping topography which shoals to the northeast the observed group velocities are in general quite consistent with the inferred forcing locations based on the ocean atmosphere coherences

evidence for barotropic wave radiation from the gulf stream abstract highly energetic velocity fluctuations associated with topographic rossby waves are frequently observed over the continental slope and rise off the united states and canadian east coast it has been suggested that the energy source for these waves could be eastward propagating gulf stream meanders which can couple to the westward propagating rossby waves if the meander shape is time dependent in this study a historical archive of all available current meter data from the western north atlantic has been examined for evidence of energy radiation from the gulf stream via barotropic topographic rossby waves maps of abyssal eddy kinetic energy eke and reynolds stress were constructed for four frequency bands including motions with periods between 256 and 7 8 days to examine distributions of these quantities over the largest possible geographical area maximum eddy kinetic energy is observed along the mean axis of the gulf stream at low frequencies 50 250 days but this maximum shifts north a

mean bed friction of combined wave current flow abstract bijker s model for evaluating the wave period mean bed friction of combined wave current flow has been modified to account for the current velocity reduction as well as the wave boundary layer near the sea bed a method of estimating the bed frictional dissipation rate of the combined flow is also presented the results obtained by the present approach are compared with several sets of experimental data and confirm its accuracy and usefulness

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

rossby wave radiation in the cape verde frontal zone abstract radiating baroclinic rossby waves excited through instability of the cape verde frontal zone are proposed as a mechanism for the generation of mesoscale variability at middepth 1000 m in the southeastern north atlantic basin linear quasigeostrophic theory is applied to an idealized front representative of the cape verde frontal zone to demonstrate that the front is unstable to modes that may radiate away from the frontal region as baroclinic rossby waves evidence for the existence of these waves is obtained from an eddy resolving basin scale general circulation primitive equation model in addition the model fields are used to identify characteristic signature of the waves in terms of quantities that may be directly observed in the ocean lagrangian trajectories reynolds stress eddy kinetic energy and frequency spectra taken from sofar float and current meter records are all in good agreement with the amplitude and distribution implied by the wave radiation in both the linear theory and

a virtual power function for estimating the alongshore transport of sediment by waves abstract a transport formula is presented which is based upon the rate of energy dissipation in breaking waves it incorporates a dimensionless grain size parameter and has been calibrated against field and laboratory data

observations of the dispersion characteristics and meridional sea level structure of equatorial waves in the pacific ocean spectral techniques applied to altimetry data are used to examine the dispersion relation and meridional sea level structure of wavelike variability with periods of about 20 to 200 days in the equatorial pacific ocean zonal wavenumber frequency power spectra of sea surface height when averaged over about 7 s 7 n exhibit spectral peaks near the theoretical dispersion curves of first baroclinic mode equatorial kelvin and rossby waves there are distinct statistically significant ridges of power near the first and second meridional mode rossby wave dispersion curves sea level variability near the theoretical kelvin wave and first meridional mode rossby wave dispersion curves is dominantly but not perfectly symmetric about the equator while variability near the theoretical second meridional mode rossby wave dispersion curve is dominantly antisymmetric these results are qualitatively consistent with expectations from classical or shear modified theories of equatorial waves the meridional structures of these modes resemble the meridional modes of equatorial wave theory but there are some robust features of the meridional profiles that were not anticipated the meridional sea level structure in the intraseasonal kelvin wave band closely resembles the theoretically expected gaussian profile but sea level variability coherent with that at the equator is detected as far away as 11 75 s possibly as a result of the forced nature of these kelvin waves both first and second meridional mode rossby waves have larger amplitude in the northern hemisphere the meridional sea level structure of tropical instability waves closely resembles that predicted by lyman et al using a model linearized about a realistic equatorial zonal current system

bed shear stress estimation on an open intertidal flat using in situ measurements abstract accurate estimations for the bed shear stress are essential to predict the erosion and deposition processes in estuaries and coasts this study used high frequency in situ measurements of water depths and near bed velocities to estimate bed shear stress on an open intertidal flat in the yangtze delta china to determine the current induced bed shear stress c the in situ near bed velocities were first decomposed from the turbulent velocity into separate wave orbital velocities using two approaches a moving average ma and energy spectrum analysis esa c was then calculated and evaluated using the log profile lp turbulent kinetic energy tke modified tke tkew reynolds stress rs and inertial dissipation id methods wave induced bed shear stress w was estimated using classic linear wave theory the total bed shear stress cw was determined based on the grant madsen wave current interaction model wci the results demonstrate that when the ratio of significant wave height to water depth hs h is greater than 0 25 cw is significantly overestimated because the vertical velocity fluctuations are contaminated by the surface waves generated by high winds in addition wind enhances the total bed shear stress as a result of the increases in both w and c generated by the greater wave height and reinforcing of vertical turbulence respectively from a comparison of these various methods the tkew method associated with esa decomposition was found to be the best approach because 1 this method generates the highest mean index of agreement 2 it uses vertical velocities that are less affected by doppler noise and 3 it is less sensitive to the near bed stratification structure and uncertainty in bed location and roughness

a nearshore profile model of wave and tidal current interaction the theoretical framework of a 1 d model of wave and current interaction in nearshore regions is described in this paper along with comparisons with laboratory and field data the theory of wave and current action is based on general mass energy and momentum equations applicable inside and outside the surf zone an important feature is the modelling of tidal currents as well as wave induced currents with full interaction between the two types of current and the waves the model is very efficient computationally and can process large quantities of input wave and tidal data typically tens or hundreds of thousands of values making it suitable for the investigation of long term processes on beaches or in nearshore regions the model is desgined to be used in conjunction with an appropriate sediment transport routine for problems concerned with the longshore movement of sediment

tropical instability vortices in the atlantic ocean 1 this paper examines the physical characteristics and dynamics of atlantic tropical instability vortices based on observational data from 1997 to 2000 and a high resolution numerical model simulation most prominent during boreal summer the vortices are characterized by 400 km spatial scales westward translation at 0 40 cm s 1 anticyclonic circumferential velocities of 30 110 cm s 1 and sstanomalies that decrease from 2 ci n midbasin to 0 5 c or less near the western boundary in contrast the sea level anomalies grow as the vortices progress westward reaching maximum amplitudes of 14 cm near 40 w the large circumferential velocities leading to relative vorticities of 4 10 6 s 1 and rossby numbers of 0 5 indicate that centrifugal effects may play an important role in the vortices vorticity balance we address the vortices vertical structure and vorticity dynamics by examination of a high resolution numerical model there is a reasonably good agreement between the model and observations simulated vortices are confined mostly to the mixed layer which increases westward in depth from 30 to 100 m near the western boundary in the eastern basin potential vorticity within the vortices is not conserved but decreases because of wind stress forcing in contrast in the western basin decreases in potential vorticity are due mostly to the vortices acquisition of southern hemisphere water we estimate an annualized cross equatorial transport of up to 1 2 sv associated with the vortices which is eventually contributed to the north brazil current system index terms 4231 oceanography general equatorial oceanography 4520 oceanography physical eddies and mesoscale processes 4528 oceanography physical fronts and jets keywords tropics instability atlantic

evidence of a 4 6 day barotropic planetary oscillation of the pacific ocean abstract spectral analysis of scattered island and coastal tide gage records from the pacific ocean reveals the presence of a coherent sea level fluctuation at 4 6 days period the oscillation is distinct from baroclinic inertia gravity wave fluctuations of sea level at the same periods that are trapped to the central pacific equatorial zone concomitant spectral analysis of island surface weather data demonstrates that sea level is forced by surface atmospheric pressure but does not respond statically like an inverted barometer the basinwide character and uniform westward propagation of the oscillation suggest the presence of a barotropic planetary wave s however the oscillation is strongly attenuated with an estimated energy e folding time of less than three days

eddy variability in the kuroshio extension as revealed by geosat altimetry energy propagation away from the jet reynolds stress and seasonal cycle abstract altimetric sea level data around the kuroshio extension collected during the first year of the geosat exact repeat mission erm starting in november 1986 have been analyzed for energy propagation away from the jet the reynolds stress pattern and the seasonal cycle of the eddy field space time diagrams and wavenumber frequency spectra are employed to discern the dominant direction of phase propagation revealing westward and inward phase propagation towards the jet i e energy propagation away from the jet which is consistent with the notion that the widespread eddy variability from the jet is the result of transient rossby wave response to forcing provided by growing pulsating and decaying meanders of the jet it can be shown that any theory based on the linear instability cannot normally account for the energy propagation because any linear instability that is wavelike away from the jet is by necessity not the fastest growing linear instability rather a nonlinear instability mec

tropical instability wave kinematics observations from the tropical instability wave experiment the kinematics of planetary waves originating from instability of the nearsurface equatorial currents are reported on using velocity measurements from an array of acoustic doppler current profilers deployed in the equatorial pacific during the tropical instability wave experiment a distinctive wave season was observed from august to december 1990 with wave energy confined primarily above the core of the equatorial undercurrent particle motions in the horizontal plane are described by eccentric ellipses oriented toward the north but tilting into the cyclonic shear of the south equatorial current the tilt is maximum near the surface just north of the equator and decreases to the south and with depth the distribution of wave variance is narrowband in both frequency and zonal wavenumber with central period zonal wavelength and westward directed phase propagation estimated to be 500 hours 1060 km and 59 cm s 1 respectively neither the meridional nor the vertical wavenumber component is statistically different from zero these results generally agree with previous findings on tropical instability waves from the atlantic and pacific oceans and in the undersampled arena of geophysical measurements they provide an example where statistical inference is supported by an ensemble of independent measurements

waves and currents on natural beaches a quasi 3 d numerical model a realistic model of detailed sediment transport in coastal areas must include an accurate and complete description of the wave climate and wave induced currents over the study area the present paper summarizes different aspects of detailed wave and current calculations as the initial step to a more complete model for coastal processes a pc based quasi 3 d model is developed and calibrated with laboratory experiments performed on a simulated infinitely long natural beach tests were carried out for several wave conditions for regular and random long crested wave fields and for two sand sizes

oscillations of dynamic topography in the eastern equatorial pacific abstract for 14 months in 1980 81 surface dynamic height was monitored with inverted echo sounders at five sites from 0 to 9 n along 110 w these records show that the sec necc equatorial current system was well developed during the boreal summer and fall but weak and irregular during winter and spring when westward flow associated with the nec extended as far south as 6 n superimposed on the mean dynamic topography of this region are energetic 20 to 80 day oscillations longer periods being associated with higher latitudes near the equatorial ridge 5 n these oscillations have predominantly monthly periods and amplitude of 10 dyn cm comparable to the mean dynamic height difference across the necc the broad in phase meridional extent of these monthly oscillations implies that the principal mode of ridge variation is vertical undulation rather than meridional meandering producing large in phase monthly modulations in transport of the sec and necc oscillations or the equatorial ridge are correla

observations of 20 day period meridional current oscillations in the upper ocean along the pacific equator abstract prominent oscillations of the meridional current with a mean period of approximately 20 days have been observed in the upper ocean over several years from may 1979 to october 1985 using moored current measurements along the pacific equator at 95 110 124 140 w and 152 w as well as off but near the equator at 110 and 140 w the fluctuations are relatively narrowband 0 005 cpd in frequency a 95 statistically significant peak in power spectra of meridional current occurred at 110 124 and 140 w but not at 95 and 152 w where the spectral peaks were smaller the dominant wave period decreased by about 4 from 110 to 140 w maximum amplitude was measured at 124 w the amplitude above 80 m was maximum at the equator and decreased poleward from the equator at 15 m the annual averaged root mean square amplitude was about 20 5 cm s 1 and individual peak to trough values reached 150 cm s 1 the wave amplitude decreased with depth and the wave was essentially confined to the upper 80 m

sea height wave form in equatorial waves and its interpretation in a previous paper a variation in sea slope of 1 6 0 6 cm per degree of latitude between 2 n and 4 n in seasat altimeter data was associated with the passage of two temperature crests between 113 w and 130 w in the equatorial pacific this work has been extended to include variations in sea height between 1 5 s and 9 n with arbitrary phase in the wave a mean wave pattern was found with maximum amplitude of 5 cm near 6 n when the analysis was repeated for a range of wave periods two preferred values are selected by minimum residuals a period of 38 6 2 2 days corresponds with the passage of two observed temperature crests a period of 22 2 0 9 days includes both these two crests and a possible intermediate crest of smaller amplitude fluctuating velocities in geostrophic equilibrium with the mean wave height are combined with a mean zonal flow to derive streamlines in a coordinate system moving westward with the phase velocity of the mean wave one streamline shows the trochoidal form of the temperature front at about the appropriate latitude velocity field dependent estimates of the latitude of the crest and the angle of the crest are made flow immediately north of this streamline exhibits clockwise gyres near each trough i suggest that this is the basic pattern of the observed form and motions in the waves also selected streamlines are used to compute idealized drifter trajectories and compared with observations