patterns of turbulent and double diffusive phenomena observations from a rapid profiling microconductivity probe abstract throughout much of the ocean interior the diapycnal buoyancy flux is maintained by both mechanical and double diffusive processes assessing the relative roles of each is a challenge particularly in complex coastal environments during february march 1995 a repeat profiling ctd system equipped with a dual needle microconductivity probe was deployed off the central california coast 35 n 121 w from the research platform flip the probe s vertical resolution 8 cm appears sufficient to resolve the low wavenumbers of the turbulent inertial subrange this paper presents depth time maps spanning 12 days and 100 400 m of temperature dissipation rate and cox number high and values tend to occur in layers on a variety of spatial scales simultaneously finescale 6 4 m richardson number effective strain rate and turner angle are measured the occurrence of intense microstructure fluctuations is correlated with all three quantities affirming that both mechanical turbulence and d

the latitudinal dependence of shear and mixing in the pacific transiting the critical latitude for psi abstractturbulent mixing rates are inferred from measurements spanning 25 37 n in the pacific ocean the observations were made as part of the internal waves across the pacific experiment designed to investigate the long range fate of the low mode internal tide propagating north from hawaii previous and companion results argue that near a critical latitude of 29 n the internal tide loses energy to high mode near inertial motions through parametric subharmonic instability here the authors estimate mixing from several variations of the finescale shear strain parameterization as well as thorpe scale analysis of overturns though all estimated diffusivities are modest in magnitude average diffusivity in the top kilometer shows a factor of 2 4 elevation near and equatorward of 29 n however given intrinsic uncertainty and the strong temporal variability of diffusivity observed in long mooring records the meridional mixing pattern is found to be near the edge of statistical significance

observations of energetic high wavenumber internal waves in the kuroshio abstract intense and spatially coherent shear layers were detected in and under the kuroshio in an april 2000 survey in the east china sea the sloping layers revealed by shipboard doppler sonars on the r v roger revelle appeared to cross isopycnal surfaces except in a small region near the kuroshio shelfbreak front the rms finescale shear associated with the layers significantly exceeded the geostrophic shear an april 2002 follow on cruise was organized to establish whether these motions were propagating internal waves shipboard and lowered adcps were both operated from the r v melville in addition to ctd sonar transects a 30 h time series of currents and shear was obtained in the core of the kuroshio near the island of kyushu japan the shear structures were indeed found to be propagating with both up and down going internal wave motions present in comparison with the nearby open ocean the finescale 160 m vertical shear variance is increased by a factor of 3 in the kuroshio and by a fact

electrical conductivity and crustal deformation from magnetotelluric results in the region of the araguainha impact brazil abstract in order to obtain information about the deep structure of large meteorite craters a series of mt soundings were carried out in the region of the araguainha impact crater located in the northern section of the parana basin mato grosso brazil the mt responses in the short period range from 0 001 to 1 s show one dimensional behaviour in contrast to the longer periods where data are multi dimensional at periods between 1 and 10 s the splitting of the apparent resistivity and phase curves of different polarisations is accompanied by a strong increase of the phase sensitive regional skew parameter to values up to 0 5 the results of two and three dimensional modelling reveal a disk shaped body embedded within a layer of 5000 m the resistivity of the body ranging between 20 m and 500 m lies significantly below the bulk value of the upper crust the anomaly is believed to be caused by impact induced faulting and brecciation of the crust reaching to depths of 3 to 7 km the horizontal extent of this zone is 16 km corresponding to about 40 of the total crater diameter at the surface the long period data t 10s reveal a heterogeneous lower crust of enhanced conductivity at depths between 15 and 30 km which is not related to the impact event it is likely to be a feature typical of the basement in the area of the araguainha structure

internal waves and the generation of turbulence in the thermocline of a large lake high resolution thermistor chain data collected between 5 and 20 m in a large stratified freshwater lake lake constance at a water depth of 60 m reveal the frequent occurrence of large amplitude 2 i m vertical density inversions that indicate overturns in the pelagic thermoc1ine velocity data collected simultaneously with the temperature measurements indicate that the density inversions are mainly produced by shear instabilities a comparison between the timing of the passage of the basin scale internal kelvin wave and the density inversions demonstrates a pronounced phase relationship implying that the processes leading to the occurrence of turbulence and mixing are connected to the passage of the kelvin wave two different processes are identified during which density inversions were particularly common an increased number of density inversions and especially high dissipation rates of turbulent kinetic energy were observed when kelvin wave induced critical shear supported the generation of large kelvin helmholtz billows a particularly large number of density inversions was also associated with the passage of nonlinear high frequency waves of large amplitudes the density inversions typically occurred at the wave troughs which indicates breaking of these waves these observations indicate that self induced shear generated by the basin scale seiche and by high frequency internal waves leads to a significant amount of turbulence and mixing in the pelagic thermoc1ine

observations of overturning in the thermocline the context of ocean mixing abstract the time sequence of events that lead to internal wave breaking and ocean turbulence is investigated data are obtained from depths 100 400 m with a repeat profiling ctd and a coded pulse doppler sonar the instruments were deployed from r p flip during february march 1995 while stationed 30 km west of point arguello california as an aspect of the marine boundary layer experiment although the water depth at the site is 1500 m both rms shear and diapycnal diffusivity as inferred from the average rate and size of overturning events increase with depth below 250 m a deep source of wave energy is implied depth time series of 6 4 m shear s 2 m strain n2 n2 where n is the buoyancy frequency 6 4 m gradient richardson number ri n2 s2 and 2 m effective strain rate the depth derivative of ctd inferred vertical velocity are obtained at 4 minute intervals over a 9 day 100 400 m domain the occurrence of overturns static instabilities of vertical scale 2 m in the observed density

statistics of microstructure patchiness in a stratified lake 1 statistics of microstructure patches in a sheared strongly stratified metalimnion of lake banyoles catalonia spain which occupied 40 of the total lake depth of 12 m are analyzed light winds 0 25 m the ratio ltp hp depends on the patch richardson and mixing reynolds numbers following the parameterization of lozovatsky and fernando 2002 analysis of the dynamics of mixing reveals that averaged vertical diffusivities ranged between 1 10 4 m2 s 1 and 5 10 5 m2 s 1 depending on the phase of the internal waves episodic wind gusts wind speed above 6 m s 1 transfer 1 6 of the wind energy to the metalimnion and 0 7 to the hypolimnion generating large microstructure patches with hp of several meters

turbulence and internal waves at the equator part ii details of a single event abstract in the low richardson number shear flow above the pacific equatorial undercurrent a single vertical microstructure profile intersected the overturning crest of a packet of high horizontal wavenumber waves the observed dissipation rates within the overturning wave were so high that if they were representative of the volume averaged rate the total wave energy would have been dissipated within a single buoyancy period the chaotic structure and temperature fluctuations with horizontal scales less than 2 m of the two wave crests and troughs west of the overturning wave crest suggest that recent mixing had occurred there wave crests and troughs east of the overturning wave crest showed little or no sign of turbulent mixing similar high horizontal wavenumber waves believed to be shear instability waves have been observed in low richardson number regions of the midlatitude seasonal thermocline although the equatorial waves have a horizontal wavelength appropriate for shear instability waves t

shear and baroclinic energy flux on the summer new england shelf observations are presented of internal wave properties and energy fluxes through a site near the 70 m isobath on the new england shelf in late summer data collected from a shipboard adcp and microstructure profiler over a three week period and projected onto dynamic vertical modes reveals large variations in the magnitude and vertical structure of internal waves baroclinic energy and shear were primarily associated with low mode near inertial and semidiurnal waves and at times high frequency solibores the energies in each mode varied by factors from 2 to 10 over several days and were not significantly correlated with one another the associated shear variance was concentrated in the thermocline however the strength and vertical range of shear varied significantly throughout the research period and depended sensitively on both the magnitude and evolving vertical mode content of the wave field shear during the quasi two layer solibores was strong enough to temporarily lower the 4 m richardson number below the threshold for shear instability energy flux through the site came primarily from the mode 1 internal tide in both linear and nonlinear solibore forms the average energy flux from the first five baroclinic modes was 130 w m 21 a comparison of energy fluxes from each mode and locally measured average dissipation rates suggests that near inertial and high mode waves were generated near the experimental site

role of overturns in optimal mixing in stratified mixing layers turbulent mixing plays a major role in enabling the large scale ocean circulation the accuracy of mixing rates estimated from observations depends on our understanding of basic fluid mechanical processes underlying the nature of turbulence in a stratified fluid several of the key assumptions made in conventional mixing parameterizations have been increasingly scrutinized in recent years primarily on the basis of adequately high resolution numerical simulations we add to this evidence by compiling results from a suite of numerical simulations of the turbulence generated through stratified shear instability processes we study the inherently intermittent and time dependent nature of wave induced turbulent life cycles and more specifically the tight coupling between inherently anisotropic scales upon which small scale isotropic turbulence grows the anisotropic scales stir and stretch fluid filaments enhancing irreversible diffusive mixing at smaller scales we show that the characteristics of turbulent mixing depend on the relative time evolution of the ozmidov length scale compared to the so called thorpe overturning scale which represents the scale containing available potential energy upon which turbulence feeds and grows we find that when the mixing is most active and efficient since stirring by the largest overturns becomes optimal in the sense that it is not suppressed by ambient stratification we argue that the high mixing efficiency associated with this phase along with observations of in oceanic turbulent patches together point to the potential for systematically underestimating mixing in the ocean if the role of overturns is neglected this neglect arising through the assumption of a clear separation of scales between the background mean flow and small scale quasi isotropic turbulence leads to the exclusion of an highly efficient mixing phase from conventional parameterizations of the vertical transport of density such an exclusion may well be significant if the mechanism of shear induced turbulence is assumed to be representative of at least some turbulent events in the ocean while our results are based upon simulations of shear instability we show that they are potentially more generic by making direct comparisons with data from ocean and lake observations which represent a much wider range of turbulence inducing physical processes

magnetic fabric of araguainha complex impact structure central brazil implications for deformation mechanisms and central uplift formation article i nfo the weakening mechanisms involved in the collapse of complex impact craters are controversial the ara guainha impact crater in brazil exposes a complex structure of 40 km in diameter and is an excellent object to address this issue its core is dominated by granite in addition to microstructural observations magnetic studies reveal its internal fabric acquired during the collapse phase all granite samples exhibit impact related planar deformation features pdfs and planar fractures pfs which were overprinted by cataclasis cataclastic deformation has evolved from incipient brittle fracturing to the development of discrete shear bands in the center of the structure fracture planes are systematically decorated by tiny grains b10 m of magnetite and hematite and the orientation of magnetic lineation and magnetic foliation obtained by the anisotropies of magnetic susceptibility ams and anhysteretic remanence aar are perfectly coaxial in all studied sites therefore we could track the orientation of deformation features which are decorated by iron oxides using the ams and aar the magnetic fabrics show a regular pattern at the borders of the cen tral peak with orientations consistent with the fabric of sediments at the crater s inner collar and complex in the center of the structure both the cataclastic flow revealed from microstructural observations and the structural pattern of the magnetic anisotropy match the predictions from numerical models of complex im pact structures the widespread occurrence of cataclasis in the central peak and its orientations revealed by magnetic studies indicate that acoustic fluidization likely operates at all scales including the mineral scales the cataclastic flow made possible by acoustic fluidization results in an apparent plastic deformation at the macroscopic scale in the core

strain observations of the vertical gradient of isopycnal vertical displacement abstract in this work a profiling ctd operated from the research platform flip is used to monitor the fine scale density field as a function of both depth and time a sequence of 10 000 ctd profiles from the surface to 560 m is examined the data were obtained off the southern california coast in the 1986 patchex experiment the vertical separation between successive isopycynal surfaces is tracked the separation is related to the vertical derivative of vertical displacement and is here referred to as the strain the purpose of this work is to present a simple picture of the fine scale strain field as it evolves in time as well as depth when viewed in isopycnal following coordinates the qualitative nature of the strain field depends on the characteristic vertical scale over which it is estimated the 20 m strain field has a wavelike character dominated by inertial and semidiurnal tidal motion wavelike fluctuations are seen in the 20 m strain field even at subinertial frequencies this suggests th

turbulent dissipation over the continental slope off vancouver island abstract thirteen profiles of the rate of viscous dissipation of turbulent kinetic energy epsi were made over the continental slope off vancouver island between 12 and 14 may 1980 in conjunction with ctd and moored current meter observations systematic variability was observed in the vertical but not in the horizontal direction above 200 m depth numerous salt stabilized temperature inversions were seen and dissipation rates were significantly larger than below 200 m dissipation rates below 200 m are the lowest ever reported and coincide with a low level of energetics revealed by the current meter moorings comparison with the garrett munk internal wave spectrum indicates an e folding decay time of internal wave energy of 50 days at depths below 200 m

trapped diurnal internal tides propagating semidiurnal internal tides and mixing estimates in the california current system from sustained glider observations 2006 2012 abstract from 2006 2012 along 3 repeated cross shore transects california cooperative oceanic fisheries investigations lines 66 7 80 and 90 in the california current system 33 609 shear and 39 737 strain profiles from 66 glider missions are used to estimate mixing via finescale parameterizations from a dataset containing over 52 000 profiles elevated diffusivity estimates and energetic diurnal d1 and semidiurnal d2 internal tides are found a within 100 km of the coast on lines 66 7 and 80 and b over the santa rosa cortes ridge srcr in the southern california bight scb on line 90 while finding elevated mixing near topography and associated with internal tides is not novel the combination of resolution and extent in this ongoing data collection is unmatched in the coastal ocean to our knowledge both d1 and d2 internal tides are energy sources for mixing at these latitudes the d1 internal tide is subinertial on line 90 d1 and d2 tides are equally energetic over the srcr the main site of elevated mixing within the scb numerous sources of internal tides at the rough topography in the scb produce standing and or partially standing waves on lines 66 7 and 80 the dominant energy source below about 100 m for mixing is the d1 internal tide which has an energy density of the d2 internal tide on line 80 estimated diffusivity estimated dissipation and d1 energy density peak in summer the d1 energy density shows an increasing trend from 2006 to 2012 its amplitude and phase are mostly consistent with topographically trapped d1 internal tides traveling with the topography on their right the observed offshore decay of the diffusivity estimates is consistent with the exponential decay of a trapped wave with a mode 1 rossby radius of 20 30 km despite the variable mesoscale it is remarkable that coherent internal tidal phase is found

distortion of magnetotelluric sounding curves by three dimensional structures distortions of magnetotelluric fields caused by three dimensional 3 d structures can be severe and are not predictable using one dimensional or two dimensional modeling i used a 3 d modeling algorithm based upon an extension of a generalized thin sheet analysis due to ranganayaki and madden 1980 to examine field distortions in crustal environments three major physical mechanisms cause these distortions these mechanisms are resistive coupling between the electrical mantle and upper crust resistive coupling between conductive features within the upper crust and local induction of current loops within good conductors each mechanism produces different spatial and frequency effects upon the background field so identification of the dominant mechanism can be used as an interpretational aid i finally use this analysis to identify distortion mechanisms seen in magnetotelluric data from beowawe nevada to aid in an interpretation of that area

three dimensional distribution of turbulent mixing in the south china sea abstracta three dimensional distribution of turbulent mixing in the south china sea scs is obtained for the first time using the gregg henyey polzin parameterization and hydrographic observations from 2005 to 2012 results indicate that turbulent mixing generally increases with depth in the scs reaching the order of 10 2 m2 s 1 at depth in the horizontal direction turbulence is more active in the northern scs than in the south and is more active in the east than the west two mixing hotspots are identified in the bottom water of the luzon strait and zhongsha island chain area where diapycnal diffusivity values are around 3 10 2 m2 s 1 potential mechanisms responsible for these spatial patterns are discussed which include internal tide bottom bathymetry and near inertial energy

geostrophic adjustment of an isolated diapycnal mixing event and its implications for small scale lateral dispersion abstract in this first of two companion papers the time dependent relaxation of an isolated diapycnal mixing event is examined in detail by means of numerical simulations with an emphasis on the energy budget particle displacements and their implications for submesoscale oceanic lateral dispersion the adjustment and dispersion characteristics are examined as a function of the lateral extent of the event l relative to the rossby radius of deformation r the strongest circulations and horizontal displacements occur in the regime r l o 1 for short times less than an inertial period horizontal displacements are radial once the adjustment is completed displacements become primarily azimuthal and continue to stir fluid over several to tens of inertial periods the cumulative effect of many such events in terms of the effective lateral dispersion that they induce is examined in the companion paper

turbulent mixing induced by nonlinear internal waves in mono lake california describing the spatial and temporal occurrence of turbulent mixing and the mechanisms of turbulence production are fundamental problems in physical limnology and oceanography here we present results from mono lake california that illustrate pronounced steepening of the pycnocline upon cessation of strong wind forcing and the enhanced turbulence resulting from this nonlinearity three events occurred with wind speed of 10 m s21 upwind the upper pycnocline upwelled but on relaxation of the wind it downwelled 6 10 m and compressed the similarity of wave amplitude during compression to mixed layer depth and lake numbers 4 were indicative of significant nonlinearity energy in the internal wave field increased by two orders of magnitude but 50 of the energy was lost within 24 h the largest energy losses occurred at low frequencies rather than via an energy flux from low to moderate and high frequencies eddy diffusivities computed over 6 d intervals using the heat budget method averaged 1026 m2 s21 in the energized pycnocline but were five to 10 times higher during the initial upwelling and downwelling periods microstructure casts taken 1 2 d after the first event revealed that the percentage of the pycnocline that was turbulent varied with bottom slope being 80 turbulent where bottom slopes were up to 10 6 0 where bottom slopes were 3 and less than 7 where bottom slopes were 0 1 the threshold for turbulent transport was exceeded only 10 of the time and only where bottom slopes were highest

an integrated magnetotelluric and aeromagnetic investigation of the serra da cangalha impact crater brazil abstract the research is aimed at delineating the post impact structural characteristics across the serra da cangalha impact crater in brazil using a combination of magnetotelluric mt and aeromagnetic data the mt survey was carried out along three radial mt profiles trending nw se ene wsw and nne ssw across the crater for mt sites located further away from the centre of the crater isotropic mt responses were observed suggesting a 1d conductivity distribution in the subsurface in the frequency range of 100 10 hz for sites located in the vicinity of the inner ring of the crater anisotropic responses were observed for the same frequency range we believe that this zone probably represents the areas of structural disturbance a 2d resistivity inversion of these data reveals a four layer model representing a thin resistive layer underlain by a conductive layer a weathered basement and a resistive crystalline basement the depth to the top of the basement is estimated to be about 1 2 km this is in good agreement with the estimation of depth to the basement of about 1 1 km calculated using the aeromagnetic data however in view of the circular geometry of the crater we have carried out a 3d forward modeling computation to supplement the derived 2d model the 3d resistivity forward model fitting the mt responses by trial by error revealed a five layer model showing a significant reduction in the basement resistivity this perhaps could be due to the structural disturbances that have been caused by the impact on the crater resulting in brecciation fracturing alteration and shocked zone filled with weak magnetic materials and fluids we have calculated the effect of the impact on the overall structural deformation beneath the serra da cangalha crater following the classical crater scaling relation of holsapple and schmidt holsapple k a schmidt r m 1982 on the scaling of crater dimensions ii impact processes j geophys res 87 1849 1870 and found to be about 2 km

spatial variability of turbulent mixing in the abyssal ocean ocean microstructure data show that turbulent mixing in the deep brazil basin of the south atlantic ocean is weak at all depths above smooth abyssal plains and the south american continental rise the diapycnal diffusivity there was estimated to be less than or approximately equal to 0 1 x 10 4 meters squared per second in contrast mixing rates are large throughout the water column above the rough mid atlantic ridge and the diffusivity deduced for the bottom most 150 meters exceeds 5 x 10 4 meters squared per second such patterns in vertical mixing imply that abyssal circulations have complex spatial structures that are linked to the underlying bathymetry

shear strain and richardson number variations in the thermocline part i statistical description abstract quasi continuous depth time observations of shear 5 5 m 6 min resolution and strain z 2 m 2 1 min resolution obtained from the r p flip are applied to a study of richardson number ri statistics data were collected off the coast of central california in the 1990 surface waves processes experiment observations are presented in eulerian and in isopycnal following frames in both frames shear variance is found to scale as n2 in the thermocline in agreement with previous findings of gargett et al the probability density function for squared shear magnitude is very nearly exponential strain variance is approximately uniform with depth the magnitude of the fluctuations is sufficient to influence the ri field significantly at finescale to model the richardson number the detailed interrelationship between shear and strain must be specified two contrasting hypotheses are considered one h i holds that fluctuations in the cross isopycnal shear are independent of isopycnal separat

tidally forced internal waves and overturns observed on a slope results from home abstract tidal mixing over a slope was explored using moored time series observations on kaena ridge extending northwest from oahu hawaii during the survey component of the hawaii ocean mixing experiment home a mooring was instrumented to sample the velocity and density field of the lower 500 m of the water column to look for indirect evidence of tidally induced mixing and was deployed on a slope in 1453 m water depth for 2 months beginning in november 2000 the semidiurnal barotropic tidal currents at this site have a significant cross ridge component favorable for exciting an internal tidal response a large amplitude response is expected given that the slope of the topography 4 5 is nearly the same as the slope of the internal wave group velocity at semidiurnal frequency density overturns were inferred from temperature profiles measured every 2 min the number and strength of the overturns are greater in the 200 m nearest the bottom with overturns exceeding 24 m present at any depth nearly

turbulence and internal waves at the equator part i statistics from towed thermistors and a microstructure profiler abstract high correlations between turbulent dissipation rates and high wavenumber internal waves and the high values of turbulent dissipation associated with internal wave activity suggest that internal waves are the main direct source of mixing in the thermocline above the core of the equatorial undercurrent an extensive dataset obtained using a microstructure profiler and thermistor chain towed along the equator was analyzed to examine the correspondence between turbulent mixing and high wavenumber internal waves in the low richardson number ri thermocline below the mixed layer but above the core of the equatorial undercurrent and when winds were moderate and steadily westward it was found that the spectrum of vertical isotherm displacement was dominated by a narrow wavenumber band corresponding to 150 250 m zonal wavelength of internal waves both turbulence and internal waves varied diurnally hourly averaged values of turbulent dissipation rate and wave potential energy were greater by a

interaction of superinertial waves with submesoscale cyclonic filaments in the north wall of the gulf stream abstracthigh resolution nearly lagrangian observations of velocity and density made in the north wall of the gulf stream reveal banded shear structures characteristic of near inertial waves niws here the current follows submesoscale dynamics with rossby and richardson numbers near one and the vertical vorticity is positive this allows for a unique analysis of the interaction of niws with a submesoscale current dominated by cyclonic as opposed to anticyclonic vorticity rotary spectra reveal that the vertical shear vector rotates primarily clockwise with depth and with time at frequencies near and above the local coriolis frequency f at some depths more than half of the measured shear variance is explained by clockwise rotary motions with frequencies between f and 1 7f the dominant superinertial frequencies are consistent with those inferred from a dispersion relation for niws in submesoscale currents that depends on the observed aspect ratio of the wave shear as well as the vertical vorticity

space time scales of shear in the north pacific abstractthe spatial temporal and directional characteristics of shear are examined in the upper 1400 m of the north pacific during late spring with an array of five profiling moorings deployed from 25 to 37 n 1330 km and simultaneous shipboard transects past them the array extended from a regime of moderate wind generation at the north to south of the critical latitude 28 8 n where parametric subharmonic instability psi can transfer energy from semidiurnal tides to near inertial motions analyses are done in an isopycnal following frame to minimize contamination by doppler shifting approximately 60 of rms shear at vertical scales 20m and 80 for vertical scales 80 m is contained in near inertial motions an inertial back rotation technique is used to index shipboard observations to a common time and to compute integral time scales of the shear layers persistence times are o 7 days at most moorings but o 25 days at the critical latitude simultaneous shipboard transects show that these sh

role of near inertial internal waves in subthermocline diapycnal mixing in the northern gulf of mexico abstractmoored adcp data collected in the northern gulf of mexico are analyzed to examine near inertial internal waves and their contribution to subthermocline diapycnal mixing based on a finescale parameterization of deep ocean mixing the focus of the study is on the impact of near inertial internal waves generated by an extreme weather event that is hurricane katrina and by month to month variation in weather patterns on the diapycnal mixing the inferred subthermocline diapycnal mixing exhibits pronounced elevation in the wake of katrina both the increased near inertial 0 8 1 8f where f is the coriolis frequency and superinertial 1 8f shear variances contribute to the elevated diapycnal mixing but the former plays a more dominant role the intense wind work on near inertial motions by the hurricane is largely responsible for the energetic near inertial shear variance energy transfer from near inertial to superinertial internal waves however appears to play an important role in elevating th

tides and the climate some speculations abstract the important role of tides in the mixing of the pelagic oceans has been established by recent experiments and analyses the tide potential is modulated by long period orbital modulations previously loder and garrett found evidence for the 18 6 yr lunar nodal cycle in the sea surface temperatures of shallow seas in this paper the possible role of the 41 000 yr variation of the obliquity of the ecliptic is considered the obliquity modulation of tidal mixing by a few percent and the associated modulation in the meridional overturning circulation moc may play a role comparable to the obliquity modulation of the incoming solar radiation insolation a cornerstone of the milankovic theory of ice ages this speculation involves even more than the usual number of uncertainties found in climate speculations

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

seasonal variation of turbulent diapycnal mixing in the northwestern pacific stirred by wind stress 1 profiles of potential density obtained from ctd measurements along the northwestern pacific section at 137 e are used to estimate both mean and seasonal variations of turbulent diapycnal mixing based on both a thorpe scale method and a finescale parameterization method in general away from the bottom the estimated mean value along this section is about o 10 4 m2 s indicating much stronger mixing in the northwestern pacific than in the stratified mid latitude interior ocean it is further found that the diffusivity can even exceed 10 4 m2 s around the rough topography the turbulent diapycnal mixing in the upper ocean also displays a distinct seasonal variation with the amplitude decreasing with depth bearing a statistically significant correlation with surface wind stress forcing our study here provides evidence for the important role of energy input from wind stress in sustaining the deep ocean mixing

vertical diffusivity of the western arctic ocean halocline a nearly year long series of upper ocean temperature conductivity and temperature microstructure profiles were collected from an ice camp drifting in the beaufort gyre as part of the 1997 1998 surface heat budget of the arctic experiment sheba geographically the record includes portions over the deep canada basin and the steep bathymetry of the chukchi borderlands region hydrographically the record includes cool pacific origin haloclines which contain a variety of subsurface temperature maxima and cold haloclines typical of the eurasian basins we present estimates of the vertical turbulent diffusivity derived from the dissipation rate of thermal variance and calculations of the associated vertical heat fluxes we find that vertical diffusion proceeds at molecular rates in the deep basins and away from topographic features while still relatively small diffusivity is enhanced by 1 order of magnitude near and above the chukchi borderlands the enhanced diffusivity is correlated to an increase in water column strain variance above the borderlands providing a linkage between bathymetry internal wave activity and turbulence the chukchi borderlands play a significant role in heat transport in the western arctic they are a pathway for horizontal heat transport and a hot spot for vertical heat transport vertical fluxes make a substantial contribution to the energy balance of the sea ice cover in this region heat fluxes between the halocline and underlying atlantic water are shown to be small and lacking vertical connection near surface waters

direct measurements of diapycnal mixing in a fjord reach puget sound s main basin abstract in the spring of 1988 time series of microstructure and adcp current profiles were collected at four locations in the north main basin of puget sound washington depth and time averages of diapycnal diffusivity k at the four stations 1 8 67 0 10 4 m 2 s 1 were one to three decades above typical open ocean thermocline levels the buoyancy frequency squared n2 was near open ocean levels but unlike the open ocean where n2 s2 finescale shear squared s2 was three to six times n2 over significant portions of the water column at two of the stations the time and space mean of all measurements k 3 6 10 3 m 2 s 1 is close to inferred vertical eddy diffusivity from a primitive equation model for puget sound k z 3 10 3 m 2 s 1 j geophys res 96 1991 16779 large time and space variability of k was found with differences of inter station depth time means over one decade a simple scaling argument using the observed k suggests significant exchange of mass between the layers of the subtidal flow over the basin s residence time additionally measurements show that local mixing may be comparable to volume weighted sill mixing in modifying the main basin s stratification both are contrary to the advective reach simplification of fjord dynamics the mixing levels were dominated by the passage of a mid depth southward flowing density intrusion and what we interpret as a strongly advected non linear internal tide these mechanisms elevated profile averaged k by more than 10 times background levels with sustained patches of k 1 10 2 m 2 s 1 critical 8 m gradient richardson numbers ri8 20 m and strong turbulence suggest that shear instabilities dominated the turbulence production though there was support for double diffusive convection in the warm core of the density intrusion

observations of internal waves and parametric subharmonic instability in the philippines archipelago 1 internal waves contain a significant fraction of the kinetic energy in the ocean and are important intermediaries between the forcing by wind and tide and interior diapycnal mixing we report here on measurements from mindoro strait in the philippines connecting the south china sea to the sulu sea of an internal wave field with a number of surprising properties that point to previously unrecognized processes at work in the region continuum spectral levels are very close to typical background values found in the open ocean but internal tide energy in both the diurnal and semidiurnal frequency bands is significantly elevated and higher at the northern mooring mp1 than the southern mp2 two particularly energetic depth ranges stand out at mp1 an upper layer centered near 300 m and one at the bottom of the water column near 1800 m the upper layer contains both internal tides and a near inertial band with upward and downward propagating waves and an apparent spring neap cycle the combination is suggestive of parametric subharmonic instability as the forcing for the near inertial band a conclusion supported by bicoherence estimates mixing estimated from density overturns is weak over much of the water column but enhanced by about an order of magnitude in the deep layer and closely tied to the internal tide both diurnal and semidiurnal near inertial currents in this deep layer are dominantly rectilinear and not well correlated with the mixing bulk mixing rates at the two sites are less than required to produce property changes seen in hydrography suggesting additional enhancement elsewhere in the archipelago

a composite spectrum of vertical shear in the upper ocean abstract results from three separate velocity profilers operated nearly simultaneously in the northwest atlantic in 1975 are used to form a composite shear spectrum over vertical wavelengths from 100 m down to a few centimeters this exercise constitutes an intercomparison of the three different measurement techniques and reveals a shear spectrum which is approximately fiat at a wkb scaled level from k 0 01 cpm through k0 0 1 cpm then falls as k 1 to a buoyancy wavenumber k0 n3 1 2 determined by the local average vaisala frequency n and the volume averaged dissipation rate various consequences of the observed shear spectral shape are explored

variability of the deep water overflow in the luzon strait abstractthe luzon strait with its deepest sills at the bashi channel and luzon trough is the only deep connection between the pacific ocean and the south china sea scs to investigate the deep water overflow through the luzon strait 3 5 yr of continuous mooring observations have been conducted in the deep bashi channel and luzon trough for the first time these observations enable us to assess the detailed variability of the deep water overflow from the pacific to the scs on average the along stream velocity of the overflow is at its maximum at about 120 m above the ocean bottom reaching 19 9 6 5 and 23 0 11 8 cm s 1 at the central bashi channel and luzon trough respectively the velocity measurements can be translated to a mean volume transport for the deep water overflow of 0 83 0 46 sverdrups sv 1 sv 106 m3 s 1 at the bashi channel and 0 88 0 77 sv at the luzon trough significant intraseasonal and seasonal variations are identified with their dominant time scales ranging between

spatial and temporal patterns of small scale mixing in drake passage temperature and salinity profiles obtained with expendable ctd probes throughout drake passage between february 2002 and july 2005 are analyzed to estimate turbulent diapycnal eddy diffusivities to a depth of 1000 m diffusivity values are inferred from density temperature inversions and internal wave vertical strain both methods reveal the same pattern of spatial variability across drake passage diffusivity estimates from inversions exceed those from vertical strain by a factor of 3 over most of drake passage the polar front pf separates two dynamically different regions strong thermohaline intrusions characterize profiles obtained north of the pf south of the pf stratification is determined largely by salinity and temperature is typically unstably stratified between 100 and 600 m depth in the upper 400 m turbulent diapycnal diffusivities are o 10 3 m2 s 1 north of the pf but decrease to o 10 4 m2 s 1 or smaller south of the pf below 400 m diffusivities typically exceed 10 4 m 2 s 1 diffusivities decay weakly with depth north of the pf whereas diffusivities increase with depth and peak near the local temperature maximum south of the pf the meridional pattern in near surface mixing corresponds to local maxima and minima of both wind stress and wind stress variance near surface diffusivities are also found to be larger during winter months north of the pf wind driven near inertial waves strong mesoscale eddy activity and double diffusive convection are suggested as possible factors contributing to observed mixing patterns

the breaking and scattering of the internal tide on a continental slope abstracta strong internal tide is generated in the luzon strait that radiates westward to impact the continental shelf of the south china sea mooring data in 1500 m depth on the continental slope show a fortnightly averaged incoming tidal flux of 12 kw m 1 and a mooring on a broad plateau on the slope finds a similar flux as an upper bound of this 5 5 kw m 1 is in the diurnal tide and 3 5 kw m 1 is in the semidiurnal tide with the remainder in higher frequency motions turbulence dissipation may be as high as 3 kw m 1 local generation is estimated from a linear model to be less than 1 kw m 1 the continental slope is supercritical with respect to the diurnal tide implying that there may be significant back reflection into the basin comparing the low mode energy of a horizontal standing wave at the mooring to the energy flux indicates that perhaps one third of the incoming diurnal tidal energy is reflected conversely the slope is subcritical with respect to the semidiurnal tide and the observed

structure propagation and mixing of energetic baroclinic tides in mamala bay oahu hawaii abstract large semidiurnal vertical displacements 100 m and strong baroclinic currents 0 5 m s 1 several times as large as barotropic currents dominate motions in mamala bay outside the mouth of pearl harbor hawaii during september 2002 the authors sought to characterize them with a 2 month mclane moored profiler deployment and a 4 day intensive survey with a towed ctd adcp and the research vessel r v revelle hydrographic sonar spatial maps and time series of turbulent dissipation rate diapycnal diffusivity k isopycnal displacement velocity u energy e and energy flux f are presented dissipation rate peaks in the lower 150 m during rising isopycnals and high strain and shows a factor of 50 spring neap modulation the largest k values in the western bay near a submarine ridge exceed 10 3 m2 s 1 the m2 phases of and u increase toward the west implying a westward phase velocity cp 1 m s 1 and horizontal wavelength 60 km consistent with theoretical mode 1 values these phase

an investigation of internal mixing in a seasonally stratified shelf sea the shelf sea seasonal thermocline is a critical interface within the marine environment separating the euphotic zone from nutrient rich deep water fluxes across the thermocline therefore represent a key biogeochemical pathway in this paper we quantify the rate of mixing across the seasonal thermocline for a location in the celtic sea and investigate the processes responsible for driving thermocline fluxes profiles of the rate of dissipation of turbulent kinetic energy e show enhanced dissipation within the thermocline region similar to 6 x 10 5 w m 3 the diffusivity implied by these measurements is similar to 0 5 cm 2 s 1 similar to previous shelf sea studies and is sufficient to explain the observed warming of the deep water suggesting that vertical mixing is the dominant control on water column structure two potential sources of mixing energy are identified the internal tide and near inertial waves the mechanism of energy transfer from the candidate mixing mechanisms to turbulence is not clear thermocline dissipation rates were found to have no richardson number dependence but scaled positively with n 2 and s 2 in agreement with a previous turbulence parameterization application of this model to our data does a good job of capturing the mean characteristics of the observed heating flux across the thermocline although none of the short term fluctuations in mixing were reproduced

a magnetotelluric investigation of the araguainha impact structure in mato grosso goias central brazil summary with a diameter of 40 km the araguainha dome is the largest known impact site in south america the main characteristics of the structure is its appearance as a set of imbricated circles and a central uplift with a diameter of 9 km 25 magnetotelluric mt soundings were completed across the structure approximately defining five radial sections the mt responses in the frequency range from 1000 to 1 hz show 1 d behaviour in contrast to the longer periods where data indicate a strong anisotropy and a decline of the resistivity 1 d modelling of the high frequency data reveals a well defined symmetric crystalline basement ring at 1000 m depth between the radii of 9 and 20 km which seems to drop deeper further away from the centre inside the ring and toward the centre the top of the granites rises outcropping at 1 5 to 2 km from the centre

observations of enhanced diapycnal mixing near the hawaiian ridge profiles of potential density obtained from ctd casts at two stations at different distances from the hawaiian ridge are examined for evidence of diapycnal turbulent mixing as indicated by density inversions and internalwave vertical strain results from independent casts are used to produce ensemble averaged vertical distributions for the number of inversions and the thorpe scale both parameters were found to be higher over the slope of the topography at 2500 m depth than in the deep ocean 110 km to the north thorpe scale based estimates of the rate of dissipation of turbulent kinetic energy and turbulent vertical diffusivity are elevated by an order of magnitude over the slope relative to deep ocean background levels the vertical distributions of these mixing parameters are nonuniform and exhibit signs of locally enhanced dissipation possibly due to internal tides generated at the ridge at the deep station turbulence is at background levels from the surface down to 2000 m below this a localized zone of enhanced mixing is observed within which the dissipation rate is o 1029 w kg21 and turbulent diffusivity is greater than o 1024 m2 s 21 perhaps due to an internal tide ray originating at the ridge the full depth topographical enhancement of mixing near the ridge also appears in the vertical strain field estimates of dissipation rate and turbulent diffusivity based on an internal wave wave interaction model give results similar to direct thorpe scale methods except in weakly stratified environments where both methods are subject to uncertainty near the topography the variation in mixing intensity observed between casts is sensitive to sporadic large mixing events which are triggered by internal waves associated with the spring tide the upper portion of the water column stronger stratification is more responsive to the tide than the deep regions

a thin layer of phytoplankton observed in the philippine sea with a synthetic moored array of autonomous gliders 1 a synthetic moored array composed of five buoyancy propelled autonomous underwater gliders was used to characterize mesoscale variability and phytoplankton distribution in a 100 km 100 km domain in the philippine sea east of luzon strait for 10 days in may 2004 the study area located east of the kuroshio near the subtropical front is dominated by strong internal tides by energetic westward propagating mesoscale eddies with azimuthal velocities exceeding 50 cm s and by a deep 130 m maximum in chlorophyll fluorescence each glider in the array was instructed to maintain geographic position while repeatedly profiling to 200 m depth good station keeping performance enabled the resulting series of vertical profiles to be interpreted in the same manner as a physically moored chain of instruments although organized primarily as a demonstration of glider capabilities this field exercise provides a unique data set for examining biological physical interactions in the open ocean here we report on the evolution of a thin layer of phytoplankton observed near the deep chlorophyll maximum coincident observations of fine structure in temperature and salinity suggest that the thinning process of this layer was driven primarily by physical forcing most probably vertical shear associated with energetic diurnal internal waves as opposed to a biological mechanism such as convergent swimming grazing or spatial variation in growth rate

observations of shear and vertical stability from a neutrally buoyant float measurements of 1 to 5 m shear and strain from a neutrally buoyant float are used to examine the statistics and causes of shear instability in a 9 day record unstable conditions defined as vz n 2 where vz vz are due to higher than average shear rather than lower than average buoyancy frequency and there are no instances of unstable buoyancy frequency or overturns on the 0 5 m spacing of the thermistors shear is dominated by upward propagating near inertial motions unstable events were more frequent when a near inertial wave packet occupied the water parcel tracked by the float groups of unstable events occur roughly every 5 7 hours at scales 2 5 m and much less often at larger scales events typically last 10 min one buoyancy period or longer these time scales appear to be controlled by vertical advection of 1 m fine structure past the float sensors so do not represent the intrinsic time scales of instability turbulent and mixing quantities are estimated from these fine scale measurements giving a dissipation rate of e 2 3 16 10 10 w kg and an eddy diffusivity of k 3 10 6 m2 s in agreement with direct microstructure estimates therefore the float adequately resolves those quantities needed to estimate turbulent dissipation and could be used to monitor turbulent mixing in the ocean

dynamic and double diffusive instabilities in a weak pycnocline part i observations of heat flux and diffusivity in the vicinity of maud rise weddell sea abstractan expedition to study the stability of the weakly stratified water column in the eastern weddell sea was undertaken in the austral winter of 2005 a regional ctd survey around maud rise delineated water mass boundaries associated with flow around the seamount and identified areas most susceptible to overturning a downstream region of the seamount taylor column was found least stable with a potential density difference across the pycnocline less than 0 018 kg m 3 intensive water column measurements including 1300 profiles of temperature conductivity and fast response microconductivity were made during a series of 13 drift stations to investigate vertical turbulent transports and the evolution of water column stability the dependence of pycnocline turbulent diffusivity kt on froude number fr turbulence generated by internal wave shear and density ratio r turbulence generated by diffusive layering and possibly diapycnal cabbeling is investigated the fr alone cannot explain completely t

hotspots of deep ocean mixing on the oregon continental slope 1 two deep ocean hotspots of turbulent mixing were found over the oregon continental slope thorpe scale analyses indicate time averaged turbulent energy dissipation rates of e 10 7 w kg and eddy diffusivities of k 10 2 m2 s at both hotspots however the structure of turbulence and its generation mechanism at each site appear to be different at the 2200 m isobath sustained 100 m high turbulent overturns occur in stratified fluid several hundred meters above the bottom turbulence shows a clear 12 4 h periodicity proposed to be driven by flow over a nearby 100 m tall ridge at the 1300 m isobath tidally modulated turbulence of similar intensity is confined within a stratified bottom boundary layer along slope topographic roughness at scales not resolved in global bathymetric data sets appears to be responsible for the bulk of the turbulence observed such topography is common to most continental slopes providing a mechanism for turbulence generation in regions where barotropic tidal currents are nominally along isobath

comparison of ellison and thorpe scales from eulerian ocean temperature observations ocean turbulence dissipation rate is estimated either by means of microstructure shear measurements or by adiabatically reordering vertical profiles of density the latter technique leads to the estimate of the thorpe scale which in turn can be used to obtain average turbulence dissipation rate by comparing the thorpe scale to the ozmidov scale in both cases the turbulence dissipation rate can be estimated using single vertical profiles from shipborne instrumentation we present here an alternative method to estimate the length scale of overturns by using the ellison length scale the ellison scale is estimated from temperature variance just beyond the internal wave band measured by moored instruments we apply the method to high resolution temperature data from two moorings deployed at different locations around the josephine seamount north eastern atlantic ocean in a region of bottom intensified turbulence the variance of the temperature time series just above the internal wave frequency band is well correlated with the thorpe scale the method is based on the time frequency decomposition of variance called maximum overlap discrete wavelet transform the results show that the ellison length scale can be a viable alternative to the thorpe scale for indirectly estimating turbulence dissipation rate from moored instruments in the ocean if time resolution is sufficiently high we suggest that fine structure contaminated temperature measurements can provide reliable information on turbulence intensity

small scale shear measurements during the fine and microstructure experiment fame the turbulent kinetic energy dissipation rate e is estimated from measurements of small scale shear taken with a vertical profiler during the fine and microstructure experiment fame typical profiles of e are presented for the different oceanographic regions sampled the gulf stream a mid sargasso site and locations withoutin and with the 100 fathom approx 2000 m contour about the island of bermuda heavily averaged values of e are presented as a funtion of mean vaeisaela frequency n bar a fundamental scaling parameter for the oceanic internal wave field a dependence of e barproportionaln bar is found for an ensemble of stations near bermuda functional dependence for an ensemble of stations at the mid sargasso site is less clear with results exhibiting an undersirable sensitivity to infrequent large events dissipation is found to increase as the island of bermuda is approached from any direction the density of measurements is insufficient to determine any azimuthal variation resulting from the anisotropic mean flow field about the island at the time a set of three profiles across the gulf stream suggests that this is not a region of abnormally high dissipation a conclusion supported by previous and concurrent measurements of temperature finestructure and microstructure

prototypical solitons in the south china sea 1 surface signatures associated with non linear internal waves are often seen in satellite images of the western south china sea scs slope and shelf observation in the deep sea to the east are rare here we report on the evolution of an energetic packet as it propagated through the deep central basin of the scs toward the western slope and shelf the waves have amplitudes estimated at 170 m half widths of 3 km and phase speeds of 2 9 0 1 m s faster than the mode 1 linear phase speed of 2 6 m s the shape and observed phase speed were consistent with the korteweg devries kdv model over the 65 km path that they were tracked the intrinsic velocity shear of the waves is small compared to pre existing shears and the waves exhibit weak turbulence the kdv fit and a satellite derived estimate of horizontal wave extent imply a westward energy flux of 4 5 gw for each crest

on the nature of internal wave spectra near a continental slope 1 amongst the most energetic motions in the deep ocean are internal waves supported by stable vertical stratification in density previously these waves were considered freely propagating as described by a smooth continuum internal wave band iwb frequency spectrum we studied details of the iwb using yearlong current observations from the bay of biscay instead of observing continuum iwb spectra near a continental slope we show that at least the first half decade of the iwb is dominated by motions at localized frequencies determined by strong non linear interactions between waves at the fundamental semidiurnal tidal and atmospherically induced inertial frequencies

internal tide convergence and mixing in a submarine canyon abstractobservations from eel canyon located on the north coast of california show that elevated turbulence in the full water column arises from the convergence of remotely generated internal wave energy the incoming semidiurnal and bottom trapped diurnal internal tides generate complex interference patterns the semidiurnal internal tide sets up a partly standing wave within the canyon due to reflection at the canyon head dissipating all of its energy within the canyon dissipation in the near bottom is associated with the diurnal trapped tide while midwater isopycnal shear and strain is associated with the semidiurnal tide dissipation is elevated up to 600 m off the bottom in contrast to observations over the flat continental shelf where dissipation occurs closer to the topography slope canyons are sinks for internal wave energy and may have important influences on the global distribution of tidally driven mixing

electric resistivity structure of the siljan impact region based on 65 magnetotelluric mt measuring stations a cross section of the electric conductivity structure in the siljan impact area located in the central part of sweden is presented static sh

the araguainha astrobleme central brazil the following facts have supported the origin of the araguainha circular structure in central brazil by a meteoritic impact 1 the almost circular contour 2 the impact morphologic sequence including a central uplift ring walls and a basin rim of escarpments 3 outcrops of suevites and mixed breccias 4 the evidence of shock metamorphism 5 the presence of shatter cones and 6 negative anomalies of the total intensity of the magnetic field at the center of the ring structure

sustained full water column observations of internal waves and mixing near mendocino escarpment abstract the relative strength and spatiotemporal structure of near inertial waves niw and internal tides it are examined in the context of recent moored observations made 19 km south of mendocino escarpment an abrupt ridge step feature in the eastern pacific in addition to strong internal tide generation steps and ridges give rise to the possibility of shadowing wherein near inertial energy is prevented from reaching depths beneath a characteristic intersecting the ridge top a combination of two moored profilers and a long range acoustic doppler current profiler adcp yielded velocity and shear measurements from 100 to 3640 m 60 m above bottom and isopycnal depth strain and overturn inferred turbulence dissipation rate from 1000 to 3640 m sampling intervals 20 min in the upper 1000 m and 1 5 h below that were fast enough to minimize aliasing of higher frequency internal wave motions the 67 day long record is easily sufficient to isolate niw and it via bandpass filtering and to capture

a reflecting steepening and breaking internal tide in a submarine canyon submarine canyons are common features of the coastal ocean although they are known to be hotspots of turbulence that enhance diapycnal transport in their stratified waters the dynamics of canyon mixing processes are poorly understood most studies of internal wave dynamics within canyons have focused on a handful of canyons with along axis slopes less steep than semidiurnal d2 internal wave characteristics subcritical here we present the first tidally resolving observations within a canyon with a steeply sloping axis supercritical a process study consisting of two 24 hour shipboard stations and a profiling mooring was conducted in the la jolla canyon off the coast of la jolla ca baroclinic energy flux is oriented up canyon and decreases from 182 18 w m 1 at the canyon mouth to 46 5 w m 1 near the head the ratio of horizontal kinetic energy to available potential energy and the observed group speed of each mode are lower than expected for freely propagating d2 internal waves at each station indicating partial reflection harmonic analysis reveals that variance is dominated by the d2 tide moving up canyon the relative importance of d2 decreases and its higher harmonics are needed to account for a majority of the observed variance indicating steepening steep internal tides cause large isopycnal displacements 50 m in 100m water depth and high strain events these events coincide with enhanced o 10 7 10 5 m2 s 3 dissipation of turbulent kinetic energy at mid depths

evolution of langmuir circulation during a storm wind stress waves stratification velocity profiles and surface fields of radial velocity and acoustic backscatter intensity were measured along a drift track 50 to 150 km off point arguello california on march 8 1995 the wind increased from calm to 12 m s from the se opposing swell from the nw it increased to 15 m s at noon utc on march 9 remained steady over the next 12 hours briefly dropped and veered by 60 then returned a mixed layer deepened quickly to 25 m then held roughly steady through the next 2 days in spite of gusty winds continuing at 15 25 m s a phased array doppler sonar system took measurements covering 250 m by 150 m of the surface with 5 m by 10 m spatial resolution averages over 6 min removed surface waves permitting continuous assessment of strength orientation spacing and degree of organization of features associated with surface motion e g langmuir circulation even when conditions were too rough for visual assessment several results stand out 1 as found previously most wind mixing arises from inertial shear across the thermocline 2 consistent with wind wave forcing of langmuir circulation plueddemann et al 1996 suggest that surface velocity variance scales like u us where u is friction velocity and us is the surface stokes drift however the measurements here scale with us 1 alone once langmuir circulation is established 3 the velocity variance is weaker here than expected given the magnitudes of wind and waves leading to a smaller estimated mixing effect 4 large vacillations in lc strength are seen just before the brief veering of the wind it is suggested that bubble buoyancy could play a dynamic role 5 mean orientation and spacing can differ for intensity versus radial velocity features

instabilities in nonlinear internal waves on the washington continental shelf previous studies have identified two primary mechanisms shear instability and convective instability by which nonlinear internal waves nliws induce mixing on continental shelves to determine the relative importance of these and their dependence on background flow conditions we examine a much longer 6 month data set from a moored adcp thermistor chain with 2 m vertical spacing in which over 600 nliws are detected turbulent properties of the 318 waves with detectable overturning instabilities are documented using thorpe scales the 130 waves detected while an adcp was functioning are classified based on a froude number criterion fr uc where u is velocity in the wave propagation direction c is the wave phase speed of these 108 waves are identified as shear instability type i fr 1 composites are constructed by averaging in a wave coordinate frame over all waves in each category showing the mean spatial structure of dissipation and other wave quantities turbulence is highest at the sheared interface for type i waves and throughout the wave core for type ii waves no relationship between wave instability mechanisms and wave background parameters such as wave steepness stratification or mean flow is found except that unstable waves tend to be more energetic demonstrating a need to better understand wave propagation and breaking in complex and variable coastal oceanographic background flows

stirring by small scale vortices caused by patchy mixing evidence is presented that lateral dispersion on scales of 1 10 km in the stratified waters of the continental shelf may be significantly enhanced by stirring by small scale geostrophic motions caused by patches of mixed fluid adjusting in the aftermath of diapycnal mixing events dye release experiments conducted during the recent coastal mixing and optics cmo experiment provide estimates of diapycnal and lateral dispersion microstructure observations made during these experiments showed patchy turbulence on vertical scales of 1 10 m and horizontal scales of a few hundred meters to a few kilometers momentum scaling and a simple random walk formulation were used to estimate the effective lateral dispersion caused by motions resulting from lateral adjustment following episodic mixing events it is predicted that lateral dispersion is largest when the scale of mixed patches is on the order of the internal rossby radius of deformation which seems to have been the case for cmo for parameter values relevant to cmo lower bound estimates of the effective lateral diffusivity by this mechanism ranged from 0 1 to 1 m 2 s 1 revised estimates after accounting for the possibility of long lived motions were an order of magnitude larger and ranged from 1 to 10 m 2 s 1 the predicted dispersion is large enough to explain the observed lateral dispersion in all four cmo dye release experiments examined

temperature statistics above a deep ocean sloping boundary we present a detailed analysis of temperature statistics in an oceanographic observational dataset the data are collected using a moored array of thermistors 100 text m 100 m tall and starting 5 text m 5 m above the bottom deployed during four months above the slopes of a seamount in the north eastern atlantic ocean turbulence at this location is strongly affected by the semidiurnal tidal wave mean stratification is stable in the entire dataset we compute structure functions of order up to 10 of the distributions of temperature increments strong intermittency is observed in particular during the downslope phase of the tide and farther from the solid bottom in the lower half of the mooring during the upslope phase the temperature statistics are consistent with those of a passive scalar in the upper half of the mooring the temperature statistics deviate from those of a passive scalar and evidence of turbulent convective activity is found the downslope phase is generally thought to be more shear dominated but our results suggest on the other hand that convective activity is present high order moments also show that the turbulence scaling behaviour breaks at a well defined scale of the order of the buoyancy length scale which is however dependent on the flow state tidal phase height above the bottom at larger scales wave motions are dominant we suggest that our results could provide an important reference for laboratory and numerical studies of mixing in geophysical flows

a magnetotelluric investigation of the structural geology beneath charlevoix crater quebec abstract the results of ten tensor magnetotelluric soundings recorded within the charlevoix crater are presented the geological complexity of the region the crater is located at the junction of two major geological provinces i e the crystalline basement of the canadian shield and the more conductive sediments of the appalachian system and the amount of cultural noise present in our data prevent a highly definitive interpretation of the shallow structure a subhorizontal conductive fault related to rondot s 1971 structural sketch of the original impact structure may exist within the upper 5 km but it is not completely constrained by the data pseudo 3d modelling using stacked thin sheets indicates that superficial patches of high conductivity may explain the large anisotropy observed at the long periods and suggests that our deeper structures fit well with the 2d model previously proposed by kurtz 1982 for the region

direct breaking of the internal tide near topography kaena ridge hawaii abstract barotropic to baroclinic conversion and attendant phenomena were recently examined at the kaena ridge as an aspect of the hawaii ocean mixing experiment two distinct mixing processes appear to be at work in the waters above the 1100 m deep ridge crest at middepths above 400 m mixing events resemble their open ocean counterparts there is no apparent modulation of mixing rates with the fortnightly cycle and they are well modeled by standard open ocean parameterizations nearer to the topography there is quasi deterministic breaking associated with each baroclinic crest passage large amplitude small scale internal waves are triggered by tidal forcing consistent with lee wave formation at the ridge break these waves have vertical wavelengths on the order of 400 m during spring tides the waves are nonlinear and exhibit convective instabilities on their leading edge dissipation rates exceed those predicted by the open ocean parameterizations by up to a factor of 100 with the disparity in

on the dynamics of the southern senegal upwelling center observed variability from synoptic to superinertial scales upwelling off southern senegal and gambia takes place over a wide shelf with a large area where depths are shallower than 20 m this results in typical upwelling patterns that are distinct e g more persistent in time and aligned alongshore from those of other better known systems including oregon and peru where inner shelves are comparatively narrow synoptic to superinertial variability of this upwelling center is captured through a 4 week intensive field campaign representing the most comprehensive measurements of this region to date the influence of mesoscale activity extends across the shelf break and far over the shelf where it impacts the midshelf upwelling e g strength of the upwelling front and circulation possibly in concert with wind fluctuations internal tides and solitary waves of large amplitude are ubiquitous over the shelf the observations suggest that these and possibly other sources of mixing play a major role in the overall system dynamics through their impact upon the general shelf thermohaline structure in particular in the vicinity of the upwelling zone systematic alongshore variability in thermohaline properties highlights important limitations of the 2d idealization framework that is frequently used in coastal upwelling studies