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

tidally generated turbulence over the knight inlet sill very high turbulent dissipation rates above 5 1024 wk g 21 were observed in the nonlinear internal lee waves that form each tide over a sill in knight inlet british columbia this turbulence was due to both shear instabilities and the jumplike adjustment of the wave to background flow conditions away from the sill turbulent dissipation was significantly lower 5 1027 to 5 1028 wk g 21 energy removed from the barotropic tide was estimated using a pair of tide gauges a peak of 20 mw occurred during spring tide approximately twothirds of the barotropic energy loss radiated away as internal waves while the remaining one third was lost to processes near the sill observed dissipation in the water column does not account for the near sill losses but energy lost to vortex shedding and near bottom turbulence though not measured could be large enough to close the energy budget

estimating internal waves and diapycnal mixing from conventional mooring data in a lake a methodology is described to analyze and interpret the temporal variation of temperature and current measured in lakes or on the continental shelf by generally available standard and robusi equipment a single thermistor chain and current meters it is assumed that variations are mainly produced by internal wave modes propagating in the mean flow and mean stratification the analysis provides estimates of the propagation characteristics energy density and directional flux of the internal waves as well as the temporal and vertical distribution of richardson number ri on scales that may be smaller than those separating the measuring instruments the method is tested with measurements made in lake geneva and provides realistic quantitative estimates of the rate of dissipation of turbulent kinetic energy per unit mass e and of the vertical diffusivity k which are useful in establishing the processes leading to diapycnal mixing

biases in thorpe scale estimates of turbulence dissipation part i assessments from large scale overturns in oceanographic data abstractoceanic density overturns are commonly used to parameterize the dissipation rate of turbulent kinetic energy this method assumes a linear scaling between the thorpe length scale lt and the ozmidov length scale lo historic evidence supporting lt lo has been shown for relatively weak shear driven turbulence of the thermocline however little support for the method exists in regions of turbulence driven by the convective collapse of topographically influenced overturns that are large by open ocean standards this study presents a direct comparison of lt and lo using vertical profiles of temperature and microstructure shear collected in the luzon strait a site characterized by topographically influenced overturns up to o 100 m in scale the comparison is also done for open ocean sites in the brazil basin and north atlantic where overturns are generally smaller and due to different processes a key result is that lt lo increases with overturn size in a fashion similar to that observed in numeric

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

shear instability in a highly stratified estuary abstract shear instability is found to be the principal mechanism of vertical exchange within the pycnocline of a salt wedge estuary a field program involving high resolution velocity and density measurements as well as high frequency acoustic imagery allowed direct comparison of instantaneous richardson number distributions to the occurrence of shear instability the theoretical stability threshold of 0 25 is consistent with the measurements based on estimates of gradients that contain the mean as well as fluctuations due to internal waves an effective stability threshold based on mean gradients is found to be approximately one third reflecting a significant contribution of internal wave shear the integral effect of the mixing process is to homogenize the gradients of velocity and density producing linear profiles of these quantities across the pycnocline a turbulent prandtl number of unity is suggested by the vertical distributions of velocity and density during periods of active vertical mixin

mixing at camarinal sill in the strait of gibraltar during the gibraltar experiment in 1985 1986 observations of shear stratification microstructure and acoustic backscatter revealed a complex mixing regime at camarinal sill in the strait of gibraltar the mixing is forced by strong semidiurnal tidal currents and weaker but more steady baroclinic flows flows are characterized as outflow or inflow nearly equivalent to westward or eastward rising water at gibraltar coincides with outflow at all depths we observed two modes of outflow at the sill in may 1986 the transition layer separating atlantic and mediterranean water was between 0 75 mpa and 1 mpa when we sampled east of the sill near neap tide the transition turned down just before reaching the sill crest where the flow became critical increasing shear soon destabilized the transition producing billows which grew to 30 75 m on the west flank of the sill the overturning generated intense turbulence with peak dissipation rates e exceeding 10 2 w kg 1 this turbulence rapidly thickened the transition from z 30 50 m east of the sill to z 130 150 m within 1 2 km west of the sill from the rate of thickening we estimate the rate of formation of new transitional water during neap tide as 0 8 1 2 105 m3 s 1 this outflow mode did not release eastward propagating internal bores when outflow turned to inflow but the inflow did appear to arrest the deep outflow in may the largest average dissipation rate over and just west of the sill had the same magnitude as estimates of the net energy lost by adjustment of velocity and mass across the sill during outflow other averages are smaller than the energy loss presumably because the limited sampling severely underestimates the average dissipation rate during outflow strong shears were observed to rise to 100 times those in the background internal wave field however probably owing to spatial offsets between the ship s acoustic doppler current profiler and our tethered free fall profiler the only statistically significant correlations were found between overturning scales measured directly and those estimated from e in the same profile we observed the other outflow mode in october 1985 when we profiled over the sill near spring tide and the transition was east of the sill centered near 1 5 mpa after encountering the sill the transition rose sharply and remained within 10 20 m of the surface across the sill plunging steeply downward just past the west edge of the sill this was accompanied by intense turbulence and at least one rebound of the transition we did not observe release of eastward propagating bores when inflow turned to outflow but observations by armi and farmer 1988 in april 1986 found bores released at spring tide in regimes similar to the one we observed in october 1985

observations of stratified turbulent mixing in an estuary neap to spring variations during high river flow abstract observations of turbulent mixing stratification and currents were made with a free falling microstructure profiler and a 600 khz adcp in the hudson river off manhattan in may 1994 the ship was anchored in a relatively straight and uniform channel about 15 m deep over 4 days of intense observations from 19 to 24 may 1994 the progression from neap tides to spring tides caused a severe reduction in stratification from an initial top to bottom salinity difference of 18 to 3 6 while low gradient richardson numbers ri occurred only in the weakly stratified bottom layer on flood during neap tides low ri spanned the water column on the second half of ebb during spring tides turbulent dissipation rates varied roughly inversely to ri during neap conditions turbulent mixing was intense only in the bottom layer on flood where eddy diffusivities k reached more than 10 2 m 2 s 1 while mixing was weak in the centre of the halocline near 5 m during spring tides strong mixing occurred throughout the water column during the latter part of ebb with typical k 5 10 3 m 2 s 1 and typical vertical salt flux of 0 0015 kg m 2 s 1 if it existed in isolation this flux would eliminate the haline stratification on a time scale of 4 h ebb time bands of low ri and intense mixing showed increasing phase lag with increasing distance from the bottom the spring tide ebb time internal turbulent stress was relatively uniform with depth with a typical magnitude of 0 4 pa 4 dyne cm 2 turbulent overturning thorpe scales ranged from below 1 cm in the neap tide halocline to a few metres in the weakly stratified bottom layer on flood the strong spring tide ebb time mixing had thorpe scales of typically 0 5 m throughout the water column ozmidov scales tracked thorpe scales except in the weakly stratified layer above the bottom

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

trains of large kelvin helmholtz billows observed in the kuroshio above a seamount trains of large kelvin helmholtz kh billows within the kuroshio current at 230 m depth off southeastern taiwan and above a seamount were observed by shipboard instruments the trains of large kh billows were present in a strong shear band along the 0 55 m s 1 isotach within the kuroshio core they are presumably produced by flow interactions with the rapidly changing topography each individual billow resembling a cat s eye had a horizontal length scale of 200 m a vertical scale of 100 m and a timescale of 7 min near the local buoyancy frequency overturns were observed frequently in the billow cores and the upper eyelids the turbulent kinetic energy dissipation rates estimated using the thorpe scale had an average value of o 10 4 w kg 1 and a maximum value of o 10 3 w kg 1 the turbulence mixing induced by the kh billows may exchange kuroshio water with the surrounding water masses

mixing by shear instability at high reynolds number 1 shear instability is the dominant mechanism for converting fluid motion to mixing in the stratified ocean and atmosphere the transition to turbulence has been well characterized in laboratory settings and numerical simulations at moderate reynolds number it involves rolling up i e overturning of the density structure within the cores of the instabilities in contrast measurements in an energetic estuarine shear zone reveal that the mixing induced by shear instability at high reynolds number does not primarily occur by overturning in the cores rather it results from secondary shear instabilities within the zones of intensified shear separating the cores this regime is not likely to be observed in the relatively low reynolds number flows of the laboratory or in direct numerical simulations but it is likely a common occurrence in the ocean and atmosphere

acoustic observations of finescale zooplankton distributions in the oregon upwelling region abstract we collected data on two globec cruises to investigate the ubiquity and character of small and finescale phytoplankton and zooplankton vertical distributions in continental shelf waters coincident data from a four frequency echosounding system 38 120 200 and 420 khz ctd 150 khz adcp and a suite of high resolution bio optical instruments were collected at three continental shelf stations off the coast of oregon in june and august of 2000 temporally and spatially coherent finescale features were identified in the acoustic record at all sites throughout the upper water column and the dominant characteristics were observed there was evidence of an association between the vertical structure of horizontal velocity and the finescale structure seen in the acoustic data these observations support the hypothesis that discrete finescale vertical distributions of phytoplankton and zooplankton may be common in productive upwelling areas and indicate that additional concurrent physical and biological measurements at appropriate temporal and spatial scales must be made to further characterize the ecological significance of these finescale distributions

stratified turbulence and mixing efficiency in a salt wedge estuary abstracthigh resolution observations of velocity salinity and turbulence quantities were collected in a salt wedge estuary to quantify the efficiency of stratified mixing in a high energy environment during the ebb tide a midwater column layer of strong shear and stratification developed exhibiting near critical gradient richardson numbers and turbulent kinetic energy tke dissipation rates greater than 10 4 m2 s 3 based on inertial subrange spectra collocated estimates of scalar variance dissipation from microconductivity sensors were used to estimate buoyancy flux and the flux richardson number rif the majority of the samples were outside the boundary layer based on the ratio of ozmidov and boundary length scales and had a mean rif 0 23 0 01 dissipation flux coefficient 0 30 0 02 and a median gradient richardson number rig 0 25 the boundary influenced subset of the data had decreased efficiency with rif 0 17 0 02 0 20 0 03 and median rig 0 16 the relationship be

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

co eruptive tremor from bogoslof volcano seismic wavefield composition at regional distances we analyze seismic tremor recorded during eruptive activity over the course of the 2016 2017 eruption of bogoslof volcano alaska only regional recordings of the tremor wavefield exist for bogoslof making it a challenge to place the recordings in context with other eruptions that are normally captured by local seismic data we apply a technique of time frequency polarization analysis to three component seismic data to reveal the wavefield composition of bogoslof eruption tremor we find that at regional distances the tremor is dominated by p waves in the band from 1 5 to 10 hz using this information along with an enriched bogoslof earthquake catalog we obtain estimates of average reduced displacement dr for eruption tremor during 25 of the 70 bogoslof events dr reaches as high as approximately 40 cm2 for two of the major events similar to other vei 3 eruptions in alaska overall average reduced displacement displays a weak correlation to plume height during the first half of the 9 month long eruption sequence with a few notable exceptions the two events with the highest dr values also generated measurable eruption tremor at very long periods vlp between 0 05 and 0 15 hz

observation and analysis of shear instability in the fraser river estuary 1 we investigate the occurrence of shear instability in the fraser river estuary instabilities observed with an echo sounder are compared with a linear stability analysis based on observed velocity and density profiles we find that each set of observed instabilities coincides with an unstable mode predicted by the taylor goldstein equation each of these instabilities occurs in a region where the gradient richardson number is less than the critical value of 1 4 both the taylor goldstein predictions and the echo soundings indicate the instabilities are concentrated either above or below the density interface this one sidedness is in contrast to the archetypal kelvin helmholtz instability although the dominant source of mixing in the estuary appears to be caused by shear instability when the tide produces strong near bed velocities small scale overturning due to boundary layer turbulence is apparent throughout the depth

observations of small scale secondary instabilities during the shoaling of internal bores on a deep ocean slope abstractthe rockall bank area located in the northeast atlantic ocean is a region dominated by topographically trapped diurnal tides these tides generate up and downslope displacements that can be locally described as swashing motions on the bank using high spatial and time resolution of moored temperature sensors the transition toward the upslope flow cooling phase is described as a rapid upslope propagating bore likely generated by breaking trapped internal waves buoyant anomalies are found during the bore propagation likely resulting from small scale instabilities the imbalance between the rate of disappearance of available potential energy and the dissipation rate of turbulent kinetic energy suggests that these instabilities are growing i e young and have high mixing potential

water mass modification and mixing rates in a 1 12 simulation of the canadian arctic archipelago strong spatial differences in diapycnal mixing across the canadian arctic archipelago are diagnosed in a 1 12 basin scale model changes in mass flux between water flowing into or out of several regions are analyzed using a volume integrated advection diffusion equation and focus is given to denser water the direct advective flux of which is mediated by sills the unknown in the mass budget mixing strength is a quantity seldom explored in other studies of the archipelago which typically focus on fluxes regionally averaged diapycnal diffusivities and buoyancy fluxes are up to an order of magnitude larger in the eastern half of the archipelago relative to those in the west much of the elevated mixing is concentrated near sills in queens channel and barrow strait with stronger mixing particularly evident in the net shifts of the densest water to lower densities as it traverses these constrictions associated with these shifts are areally averaged buoyancy fluxes up to 10 8 m2 s 3 through the 1027 kg m 3 isopycnal surface which lies at approximately 100 m depth this value is similar in strength to the destabilizing buoyancy flux at the ocean surface during winter effective diffusivities estimated from the buoyancy fluxes can exceed 10 4 m2 s 1 but are often closer to 10 5 m2 s 1 across the archipelago tidal forcing known to modulate mixing in the archipelago is not included in the model nevertheless mixing metrics derived from our simulation are of the same order of magnitude as the few comparable observations

mixing on a shallow shelf of the black sea abstract microstructure measurements were carried out on the shallow shelf of the black sea from the sea surface to the bottom using a free falling baklan s profiler released from an anchored ship a northeast to southwest transect consisting of eight measurement stations with several casts made at each station enabled the evaluation of microstructure statistics across the shelf the eddy and scalar diffusivities as well as the mixing efficiencies were evaluated for distinct layers that were identified based on mean stratification and the state of turbulence these include five main layers with persistent features upper and bottom boundary layers diurnal and main pycnoclines and a stratified weakly turbulent inner layer and several transient patchy features embedded within such layers quasi homogeneous active turbulent stratified dissipative and microstructure displacement patches the thorpe displacement scale lth measurements of this study together with those reported in dillon and gib

broadly distributed and locally enhanced turbulent mixing in a tidal estuary abstract spatial variations of vertical turbulent mixing along a stretch of the hudson river estuary are examined with focus on the vicinity of a hydraulic control point at the george washington bridge at which the cross section narrows and the thalweg takes a bend richardson numbers are lowered and mixing is enhanced downstream of this modest morphological feature qualitatively following predictions based on hydraulic theory by chant and wilson the enhancement in the viscous dissipation rate e is only a modest factor of 2 3 however extending over 2 km along the river upon averaging over tidal and fortnightly cycles and multiple cruises streamwise variations of e along the probed 15 km stretch of the estuary are surprisingly small given that individual depth averages of e across the halocline vary by over three orders of magnitude the principal result is that the observed part of the hudson river showed strong broadly distributed mixing everywhere with little local concentration

morphotectonic analysis of the long term surface expression of the 2009 l aquila earthquake fault central italy using airborne lidar data abstract in this paper we present a morphotectonic study of the paganica san demetrio fault system psdfs responsible for the mw6 1 april 6 2009 earthquake l aquila central italy the discrepancy observed between the length of the seismologic geodetic modeled fault the limited size of the primary coseismic surface ruptures and the significant morphological expression of the psdfs stimulated a debate about the maximum rupture length of the psdfs and its capability to generate larger magnitude events to image the psdfs long term morphological expression and define its surface geometrical arrangement length number of fault splays and boundaries we took advantage of a high resolution airborne lidar dataset lidar topography substantially increased our confidence in detecting even subtle tectonic controlled morphologies we define the psdfs as a 19 km long fault system that displays a complex structural setting characterized by two different sectors 1 the paganica sector to the nw with a narrow deformation zone and 2 the san demetrio sector to se where the strain is accommodated by several fault splays dissecting a wider quaternary basin we also defined a first order hierarchy among the numerous fault splays across the psdfs the long term geomorphic expression of the psdfs suggests that it ruptured also involving the whole 19 km long structure besides rupturing only small sections as it occurred in 2009 this suggests a variable slip behavior empirical relations applied to this hypothesis allow up to m 6 6 earthquakes along the psdfs these results have a critical impact on the seismic hazard assessment of the area when compared with a m 6 1 event as the 2009