shear induced convective mixing in bottom boundary layers on slopes field data from a stratified lake demonstrate that buoyancy driven convective mixing can be an important mechanism for the formation of well mixed bottom boundary layers bbls on slopes convective turbulence is caused by differential transport of stratified water masses along the slope of a basin because the current velocity usually decreases toward the sediment water at some distance from the bottom is transported faster than is water below during upslope flow this process leads to transport of heavier water on top of lighter water and hence to unstable stratification within the bbls analogously strong bbl stratification occurs during downslope flow high frequency acoustic doppler current profiler adcp and temperature measurements in the bbls of a lake revealed the cyclic occurrence of convective turbulence driven by periodic across slope currents of internal seiching the estimated maximum buoyancy flux within the bbls was in good agreement with the highest observed dissipation rates of turbulent kinetic energy this suggests that in our specific case classical bottom shear production and the mentioned buoyancy driven convective production contributed by similar amounts to the turbulent kinetic energy

damping mechanisms of internal waves in continuously stratified rotating basins damping mechanisms damping rates and the dissipative modal structure of internal waves in stratified rotating circular basins are investigated analytically the damping is shown to be due to a combination of the internal wave cancelling where waves emitted by the oscillatory boundary layers destructively interact with the parent wave and drain energy from it and spin down modified by the periodicity where the energy is drained by the sinks and sources at the bottom corner caused by a discontinuity in the ekman transport it is shown that super inertial poincare waves and sub inertial kelvin waves are damped predominantly by the internal wave cancelling and modified spin down respectively these processes also modify the internal wave structure for super inertial waves the boundary layer generated waves intensify the interior flow in the lower part of the water column and delay the phase relative to the isopycnal displacements but for sub inertial waves the ekman pumping and the corner sinks and sources add a horizontal circular flow that slants the crest and trough backwards near the wall

anatomy of turbulence in thermally stratified lakes turbulence within the thermocline of two thermally stratified lakes lake biwa japan and lake kinneret israel was investigated using a portable flux profiler this instrument provided high resolution profiles of temperature conductivity and two components of velocity within a measuring volume of approximately 8 mm 3 each data profile was segmented into statistically stationary segments a range of properties of the turbulence including direct estimates of the vertical mass flux were then calculated for each segment it was found that turbulence in the thermocline was generally patchy but within a patch dissipation levels were relatively high the turbulent motions were found to be fine grained with small thorpe scales and a large skewness of the distribution of the displacement scale this is distinct to that found in the ocean where shear instability produces large overturns the measurement showed that the net vertical mass flux in the thermocline was negligible and less than that predicted by osborn 1980 this is explained by noting that the net buoyancy flux consisted of two opposing parts a down gradient irreversible flux due to turbulent mixing and an up gradient reversible flux due to restratification that often cancel each other within a segment the low measured net buoyancy flux within the interior of the lakes suggested that other processes such as gravitational adjustments and benthic boundary layer processes should be responsible for the basin averaged vertical transport in these lakes

the physical regime and the respective biogeochemical processes in the lower water mass of lake kinneret physical measurements made in lake kinneret during the stratified period indicate that the lower water mass lwm consists of two layers a turbulent benthic boundary layer bbl and a hypolimnetic layer overlying the bbl the water in the lwm moves in response to vertical mode one seiching of the metalimnion this movement is accentuated near the perimeter of the metalimnion due to shoaling of the seiche and breaking of internal waves the motion associated with this phenomena induce a well mixed benthic boundary layer bbl adjacent to the lake bottom the bbl and hypolimnion were distinguished chemically because transport of solutes between these water bodies was very small compared to the hypolimnion the bbl was characterized by more intensive biomineralisation processes as indicated by the faster depletion of do and no3 by higher levels of nh4 h 2s and soluble reactive phosphorus srp and by lower ph values srp levels in the hypolimnion were particularly low suggesting that phosphorus was either preferentially removed by sedimentation from the hypolimnion or it was not released from particles as a result nh4 over srp ratios in the hypolimnion were significantly higher than in the bbl and diffusional fluxes of both nutrients from the lwm to the trophogenic epipelagic water should therefore be characterized by relatively high n p ratios

small scale turbulence and vertical mixing in lake baikal the water column of lake baikal is extremely weakly but permanently stratified below 250 m despite the thickness of this relatively stagnant water mass of more than 1000 m the water age time since last contact with the atmosphere is only slightly more than a decade indicating large scale advective exchange in the stratified deep water the fate of water constituents is determined by the combined action of advective processes deep water intrusions and small scale turbulent vertical diffusion here vertical diffusivity is addressed through the analysis of 25 temperature microstructure profiles collected in the three major basins of lake baikal to a depth of 600 m in addition in the 1 432 m deep south basin monthly ctd profiles and a two year record of near bottom currents were analyzed balancing turbulent kinetic energy and small scale temperature variance leads to the conclusions that 1 vertical diffusivity in the stratified deep water ranges from 10 90 3 10 24 m 2 s 21 between 600 and 250 m which is three orders of magnitude more than estimated by killworth et al 1996 2 the mixing efficiency is 0 16 comparable to that found in stronger stratification e g the ocean interior 3 turbulence under ice decays with a time scale of 40 6 2 d and 4 the interior of the permanently stratified deep water below 250 m and the bottom boundary layer contribute roughly equally to the tke production the latter implies that mixing in the deep water of lake baikal s three sub basins is dominated by bottom boundary mixing as found in smaller lakes and ocean basins

strong vertical mixing of deep water of a stratified reservoir during the maule earthquake central chile mw 8 8 1 vertical profiles of water temperature in rapel reservoir central chile 34 2 23 s 71 35 23 w 110 m a s l recorded the hydrodynamic response of the stratified water column of the reservoir produced by the 8 8 magnitude maule earthquake epicentre located in 36 15 36 s 73 14 20 w showing an intense vertical mixing of deep water the vertical mixing was characterized by changes in the potential energy of the water column and the eddy diffusivity of the mixing and these values were used to estimate the mixing efficiency of the event it is hypothesized that the turbulent flow that mixed the water column was mainly induced by the oscillation of the bed although surface gravitational waves could also have contributed to the vertical mixing so far this is the first time that the hydrodynamic response of a reservoir during a mega earthquake is measured and described

boundary mixing in lakes 1 modeling the effect of shear induced convection 1 recent studies in lakes have shown evidence for a strong influence of shear induced stratification on mixing in turbulent bottom boundary layers bbls on sloping topography these observations suggest that the periodic near bottom shear resulting from internal wave motions may lead to alternating periods of gravitationally stable and unstable stratification in the bbl with relevant implications for turbulence and mixing in the entire basin the impact of these processes for basin scale mixing is investigated here in a three dimensional processes oriented modeling study using a well investigated system lake alpnach switzerland as an example consistent with available observations our results indicate that the bbl becomes gravitationally unstable in areas with upslope flow covering a substantial fraction of the total bottom area of the lake while near bottom convection associated with the unstable stratification in these areas results in strong turbulence its contribution to net mixing is negligible since the bbl is already well mixed conversely in areas with downslope flow the near bottom shear generates stable stratification leading to a suppression of turbulence but also to larger mixing rates due to an enhanced mixing efficiency overall bbl mixing is found to dominate basin scale mixing these mechanisms are likely to be important for a large class of stratified natural waters in which boundary layer mixing is energized by periodic internal waves or basin scale motions

seasonal differences in the evolution of damped basin scale internal waves in a shallow stratified lake the evolution of damped basin scale internal waves was investigated using a modal analysis for layer stratified rotating lakes a simple model for homogeneous oscillatory boundary layers was incorporated into the modal analysis to predict damping rates of individual internal wave modes enabling the prediction of their evolution under wind forcing and bottom friction application of the method to lake kinneret indicated a different dynamic balance in summer and spring despite the continuous presence of large amplitude internal waves in the two seasons in summer strong diurnal winds continuously excited the dominant kelvin waves but the relatively strong damping e folding damping times 3 d suppressed their amplitudes in spring wind forcing was again strong but intermittent and it was weaker damping e folding damping times 10 d that allowed the dominant poincaree waves to persist until the next wind event the significant difference in the damping times was due to the difference in the periods and the structure of the internal waves as well as the bottom boundary layer thickness which limited the ekman transport and energy dissipation within the boundary layer

near inertial waves in lake erie near inertial poincare waves with a period t p similar to 17 h are the dominant wind induced internal wave motions in central lake erie and consequently have a substantial influence on lake circulation mixing and biogeochemistry however due to the complex three basin bathymetry in lake erie the vertical and horizontal modal structure of these waves remain poorly understood in this study we analyze field data to show wind events energize frequent vertical mode one poincare waves the horizontal modal structure was also investigated in a sensitivity analysis using a calibrated three dimensional hydrodynamic transport model forced with observed and idealized spatially uniform wind events strong horizontal mode one poincare wave cells form in both the central and eastern basins when wind events have a duration of 0 25 t p to 0 5 t p are impulsive and periodic at t p or have anticyclonic rotation with a duration of t p momentum transfer from longer wind events 0 5 tp will oppose the coriolis force rotated currents and damp poincare wave generation in agreement with theory the most efficient wind events are observed and computationally modeled to have a duration of 0 25 t p causing an excitation peak at similar to 0 4 t p and converting similar to 0 8 of the wind energy input to poincare waves the efficiency of wind work in generating poincare wave kinetic energy is given by 1 cos 2 pi f t t 1 where f is the inertial frequency and t is the wind duration therefore the efficiency peaks during each nt p period where n is a non negative integer and decreases significantly for longer wind events

dynamics of mixed bottom boundary layers and its implications for diapycnal transport in a stratified natural water basin here we report on two field experiments from lake alpnach surface area 4 8 km 2 maximum depth 34 m that were designed to study the process of boundary mixing and to estimate its efficiency the ratio between the turbulent kinetic energy converted into potential energy and dissipated into heat for diapycnal tracer transport lake alpnach is particularly suited for this purpose because it is known from earlier experiments that 1 its currents follow a regular oscillatory pattern associated with basin wide standing internal waves seiches period similar to 0 5 days and 1 day respectively and 2 diapycnal tracer transport is mainly caused by boundary mixing during one of the experiments reported here strong seiches were excited regularly and damped on a timescale of the order of 3 days during the other experiment seiching motion was comparably weak if seiching is excited regularly we find a persistent well mixed bottom layer of 4 5 m height at the deepest part of the lake in the absence of regular seiching the layer disappears within 10 20 days on the sloping bottom boundaries the well mixed layers 1 are of much more transient nature 2 exhibit different thermal structure and 3 decrease in thickness toward shallower depth inversely proportionally to the stability at the same depth in the lake interior their reduced thickness is possibly the result of repeatedly occurring intrusions of boundary mixed water masses that are observed to extend horizontally similar to 100 200 m into the lake interior as a consequence of repeated generation of intrusions mixing on the sloping boundaries is expected to be considerably more efficient compared to mixing over flat bottom boundaries the observed mixing efficiency the ratio of the rate of change of potential energy below depth z caused by turbulence to the energy loss by bottom friction below depth z increases indeed from 0 01 0 01 in the deepest well mixed layers to 0 15 0 04 in the upper pycnocline

friction evolution of granitic faults heating controlled transition from powder lubrication to frictional melt the experimental shearing of granitic faults revealed complex evolution of strength and microstructure we use a rotary shear apparatus to shear three types of granitic rocks at moderate slip velocity of 0 01 0 11 m s normal stress of 1 0 6 8 mpa and slip distance up to 60 m three stages of strength evolution with slip distance were systematically observed in stage i faults exhibited initial weakening in which the friction coefficient dropped to a steady state level of 0 3 this weakening is associated with powder lubrication and the development of cohesive gouge layers stage ii included strengthening to 0 5 associated with volumetric expansion melting of fault patches and viscous braking at these patches in stage iii fault weakening is due to melt lubrication when the melted patches reach a critical fraction of the fault surface area we show that this complex weakening strengthening weakening evolution is controlled by thermally activated deformation processes that can explain the friction behavior of igneous rocks at seismic velocity range

the degeneration of large scale interfacial gravity waves in lakes mechanisms for the degeneration of large scale interfacial gravity waves are identified for lakes in which the effects of the earth s rotation can be neglected by assuming a simple two layer model and comparing the timescales over which each of these degeneration mechanisms act regimes are defined in which particular processes are expected to dominate the boundaries of these regimes are expressed in terms of two lengthscale ratios the ratio of the amplitude of the initial wave to the depth of the thermocline and the ratio of the depth of the thermocline to the overall depth of the lake comparison of the predictions of this timescale analysis with the results from both laboratory experiments and field observations confirms its applicability the results suggest that for small to medium sized lakes subject to a relatively uniform windstress an important mechanism for the degeneration of large scale internal waves is the generation of solitons by nonlinear steepening since solitons are likely to break at the sloping boundaries leading to localized turbulent mixing and enhanced dissipation the transfer of energy from an initial basin scale seiche to shorter solitons has important implications for the lake ecology

physical properties of turbulent benthic boundary layers generated by internal waves physical properties of active turbulent benthic boundary layers tbbl generated by basin scale internal waves were studied within a northern hemisphere thermally stratified lake a microstructure profiler was used to measure the nature of the turbulence within the tbbls while a series of thermistor chains were used to monitor the thermal structure of the lake it was observed that a wind driven anticlockwise diurnal period vertical mode one kelvin wave generated large scale motions within the water column and that the interactions between this wave and the sloping lakebed induced tbbls a simple model based on potential energy change and boundary shearing was shown to describe the mean tbbl thickness

high frequency internal waves in large stratified lakes observations are presented from lake biwa and lake kinneret showing the ubiquitous and often periodic nature of high frequency internal waves in large stratified lakes in both lakes high frequency wave events were observed within two distinct categories 1 vertical mode 1 solitary waves near a steepened kelvin wave front and vertical mode 2 solitary waves at the head of an intrusive thermocline jet were found to have wavelengths 64 670 m and 13 65 m respectively and were observed to excite a spectral energy peak near 10 23 hz 2 sinusoidal vertical mode 1 waves on the crests of kelvin waves vertically coherent in both phase and frequency and bordering the thermocline jets in the high shear region trailing the vertical mode 2 solitary waves vertically incoherent in both phase and frequency were found to have wavelengths between 28 37 and 9 35 m respectively and excited a spectral energy peak just below the local maximum buoyancy frequency near 10 22 hz the waves in wave event categories 1 and 2 were reasonably described by nonlinear wave and linear stability models respectively analysis of the energetics of these waves suggests that the waves associated with shear instability will dissipate their energy rapidly within the lake interior and are thus responsible for patchy turbulent events that have been observed within the metalimnion conversely the finite amplitude solitary waves which each contain as much as 1 of the basinscale kelvin wave energy will propagate to the lake perimeter where they can shoal thus contributing to the maintenance of the benthic boundary layer

modeling circulation in lakes spatial and temporal variations the influence of spatial and temporal variations in wind forcing on the circulation in lakes is investigated using field data and the three dimensional estuary and lake computer model elcom applied to lake kinneret lake kinneret field data from six thermistor chains and eight wind anemometers deployed during july 2001 are presented internal wave motions are well reproduced by the numerical model when forced with a spatially uniform wind taken from a station near the lake center however simulated seiche amplitudes are too large especially vertical mode 2 and lead observations by 3 10 h for a 24 h period wave at different locations around the lake consideration of the spatial variation of the wind field improves simulated wave amplitude and phase error at all stations is reduced to less than 1 5 h this improvement is attributable to a better representation of the horizontally averaged wind stress and can be reproduced with a spatially uniform wind that has the same horizontally averaged wind stress as the spatially varying wind field however a spatially varying wind field is essential for simulating mean surface circulation which is shown to be predominantly directly forced by the surface layer averaged wind stress moment

the seasonal evolution of wind internal wave resonance in lake kinneret data from a thermistor chain and wind sensor collected over an annual stratification cycle in lake kinneret israel during 2000 were used to investigate the seasonal evolution of wind internal wave resonance internal wave periods determined from an analytical model were compared with observations and we show that resonance during 2000 occurred during three distinct times of the year at the onset of stratification march during the heating phase june and during the cooling phase november in all cases resonance was between the wind and the dominant radial azimuthal and vertical mode one cyclonic kelvin wave previously observed in lake kinneret internal waves are a ubiquitous feature of lakes existing because of wind forcing acting on a stratified water column where the stratification is generally due to temperature wind applied to the surface can cause a surface setup of water at the downwind end this pressure force is balanced by a tilting of the metalimnion in the opposite sense that is downward at the downwind end spigel and imberger 1980 when the wind forcing relaxes the water surface oscillates a barotropic wave as does the metalimnion a baroclinic wave the two motions can effectively be considered to be decoupled monismith 1985 because they form orthogonal solutions to the wave equation it is the internal baroclinic waves that are of interest in the present study because they provide much of the energy and motion for biogeochemical processes in lakes in large lakes the picture becomes complicated because the effects of the earth s rotation begin to influence the frequency and structure of the internal waves internal gravity waves in large lakes take the form of cyclonic kelvin and or poincare or anticyclonic poincare waves antenucci et al 2000 antenucci and imberger 2001 where the wave crests rotate around the perimeter of the lake basin antenucci and imberger 2001 showed that the frequency as well as the ratio of potential kinetic energy of internal gravity waves in approximately circular or elliptic lakes was dependent on the aspect ratio radial mode azimuthal mode and burger number the burger number is defined asc lf where c is the phase speed of internal waves in a nonrotating system l is some length scale characterizing the basin width and f is the coriolis parameter for a small burger number rotation dominates over gravity and waves approach the oceanic case known as inertial oscillations in which the motion is dominated by the kinetic energy signal for a large acknowledgments we acknowledge the field support and technical staff from the kinneret limnological laboratory kll for providing assistance particularly tzahi rosenberg for maintaining the lake station and helping in the transfer of data from kll to the centre for water research and peter yeates who provided the time series of the lake number

on the structure of transient upwelling events abstract nearshore transcends following the development of transient coastal upwelling characteristically show intermediate density fluid occupying the immediate nearshore band large cross shore particle excursions during the development of upwelling may be inferred from be fore and after transects and the movement of surface layers seaward intermediate density fluid shoreward through a distance of the order of kilometers and the bottom layers also shoreward by a 1esser amount some inevitable vertical mixing together with the large cross shore displacements results in efficient cross shore mass exchange the main dynamical features of similar events may be investigated by means of three layer models linear theory is conveniently discussed first the conclusions of which are easily generalized to multilayer models finite amplitude i e full upwelling is then considered using a potential vorticity conserving impulsive model the results show that the wind peels off the surface layer over w

energetics and damping of basin scale internal waves in a strongly stratified lake energetics and damping of basin scale internal waves and subsequent near bottom transport processes in lake kinneret were investigated using the modal analysis in a layer stratified irregular basin the theory was extended to include small linear damping and energy budgets and damping rates of five dominant internal waves were extracted by fitting numerically calculated internal waves to isotherm displacements measured by six thermistor chains distributed throughout the lake energy contained in the dominant internal waves 3 gj resulted from a balance between energy input from diurnal winds and dissipation within a day both of which were estimated to be 3 4g j d 21 damping was caused primarily by bottom friction and the damping rates e folding time varied from 1 to 3 d depending on the velocity structure currents induced by the internal waves caused considerable spatial variability of the bottom shear stress and near bottom transport processes such as entrainment rate at the top of the benthic boundary layer and mass transfer at the sediment water interface

physical controls on the spatial evolution of a dinoflagellate bloom in a large lake a set of satellite images of chlorophyll a concentration for lake kinneret israel from the period february to april 2007 captured the temporal evolution of horizontal patchiness that developed during the typical spring bloom of the dinoflagellate peridinium gatunense narrow bands of high concentration located adjacent to the shoreline at the start of the bloom were followed by progressive propagation of this high concentration region from the shoreline toward the center of the lake as the bloom proceeded a three dimensional 3 d hydrodynamic numerical model and an analytical model for a flat bottomed elliptical basin together explain the observed phenomenon the spatial structure of kelvin waves which were demonstrated to most likely be present in february and april due to resonance produced regions of high velocity gradient contributing to horizontal dispersion close to the shoreline this region of high dispersion was narrow and close to the shoreline at the start of the bloom when the burger number was relatively small but it widened over the course of the season as the burger number increased due to increasing stratification in addition an increase in the dispersion rate with time in the lake interior was inferred from numerical simulation due to increasing wind speeds and a thinner surface layer physical processes in this case horizontal dispersion as a result of both kelvin waves and wind can play an important role in governing the dynamics and spatial evolution of dinoflagellate blooms in lakes spatial heterogeneity of phytoplankton is a ubiquitous occurrence in lakes coastal waters and the open ocean phytoplankton patchiness can be defined by localized abruptly elevated concentrations phytoplankton patches may occur at a broad range of spatial scales and such

the structure of basin scale internal waves in a stratified lake in response to lake bathymetry and wind spatial and temporal distribution lake iseo italy field data theoretical analyses and numerical simulations were used to investigate the effects of lake basin bathymetry and spatial and temporal structures of the wind field on the basin scale internal wave spectrum in lake iseo this deep italian basin is characterized by the presence of a large island imparting attributes of an annulus to the lake during the summer of 2010 the internal wave activity was dominated by a basin scale internal wave of vertical and horizontal modes 1 v1h1 superimposed on which were occasional higher vertical modes v2h1 and higher horizontal modes v1h5 trapped by the main island the occurrence of these motions was interpreted as forcing by the wind components with similar horizontal structures and with energies at frequencies near the natural oscillations of the excited modes the modifications of the wind field by the topography in particular controlled the excitation of an anticyclonic wave trapped around the island whose features were investigated on the basis of an extension of the circular model to the case of an annular basin of constant depth

basin scale internal wave dynamics during a winter cooling period in a large lake 1 basin scale internal waves are examined using a combined field observation and 3 d numerical approach the field site lake tahoe ca nv during winter is characterized by the presence of a weak density stratification and the passage of several storm fronts of varying character under these conditions internal waves with periods greater than 5 days and displacements of 30 m are possible the model used is shown to have negligible numerical dissipation and sufficiently small phase lag over the time periods needed to represent basin scale waves both the numerical results and the observations identify the presence of three kelvin modes and one poincare mode while the wave periods are consistent with theoretical predictions the spatial characteristics are far more complex than theory can predict the spatial features are revealed through analysis of the integrated potential and kinetic energy fields yielded by the model this shows that the basin topography exerts an influence on both kelvin and poincare waves the effect of the variable storms and in particular the phasing of the storms with respect to the internal waves is shown to be responsible for the amplification and annihilation of the basin scale waves

seasonal evolution of the basin scale internal wave field in a large stratified lake the response of the water column to varying conditions of stratification and wind forcing was investigated in lake kinneret israel using data collected from thermistor chains and acoustic doppler current profilers during 1997 and 1998 the strong daily sea breeze was found to generate a vertical mode 1 internal kelvin wave and basin scale internal poincarewaves of vertical modes 1 2 and 3 the kelvin wave the dominant component of the internal wave field was responsible for alongshore velocities in the nearshore regions in the upwind nearshore regions velocities were dominated by the forced response to the wind and were cross shore in nature in the lake interior the kelvin wave effect on the horizontal velocity field was minor compared to the higher vertical mode poincarewaves the kelvin wave is shown to exist in resonant and nonforced states with the wind whereas the vertical mode 1 poincarewave energy remained relatively constant despite large variability in the forcing condi tions the energy in the higher mode poincarewaves varied greatly both on daily and seasonal timescales the results demonstrate that the wind energy forces multiple basin scale internal wave modes and that prior motion in the water column must be considered when determining the subsequent internal wave response in periodically forced systems lake kinneret israel has long been an important regional resource providing a substantial supply of drinking water as well as supporting a commercial fishery the monomictic lake is forced daily during the summer by a powerful sea breeze with stratification and wind patterns showing little variation from year to year these factors along with a fa vorable bathymetric pattern combine to make lake kinneret an ideal place for the field investigation of large scale inter nal waves and related phenomena such as the benthic bound ary layer horizontal transport and vertical mixing processes in addition to providing substantial insight into the local pro cesses and hence influencing local management study of the lake also provides fundamental knowledge of the dynam ics of enclosed basins on a geophysical scale during summer the internal rossby radius in lake kin neret is typically one half the basin width hence large scale motion in the lake will be affected by the earth s rotation and basin scale internal waves may be of topographic ross by kelvin or poincareform topographic rossby waves are sub inertial frequency oscillations that exist only in rotating fluids with variable bathymetry mysak et al 1985 stocker and hutter 1987 and these waves result from changes in depth affecting the relative vorticity of fluid elements in the absence of vertical boundaries wavelike solutions to the lin ear inviscid equations of motion for a flat bottomed region of homogeneous fluid with a constant rate of rotation are 1

wind forced motions in stratified lakes and their effect on mixed layer shear the solution for the response of a rectangular n layered stratified lake to an arbitrarily varying in time wind stress is presented the case n 3 is considered in detail for a three layered lake the response is made up of the individual responses of one external and two internal modes when the upper layer is shallow relative to the other two layers the mixed layer shear is due almost entirely to the slower second mode response moreover the upper interface can tilt quite severely while the lower interface remains nearly flat calculations based on the three layer model are compared with measurements made in wellington reservoir western australia

spatial structure of the dominant basin scale internal waves in lake kinneret field data and numerical simulations were used to investigate the effects of basin shape continuous stratification and rotation on the three dimensional structure of the dominant natural basin scale internal wave modes in lake kinneret a stratified lake large enough so that the earth s rotation influences the wave motion the structure of the modes was inferred from power spectral density of measured and simulated isotherm vertical displacements and from rotary power spectral density of isopycnal velocities obtained from numerical simulations the shape of the lake at the level of the thermocline in conjunction with the dispersion relationship determines the horizontal configuration of the natural modes the dominant response to wind forcing was an azimuthal and vertical mode 1 kelvin wave with a natural period of 22 6 h that propagated around the entire basin the second most dominant response was a vertical mode 1 wave with a natural period close to 10 5 h and composed of two counter rotating

observations of exchange between eastern casco bay and the western gulf of maine abstract exchange of water between eastern casco bay and the adjacent gulf of maine shelf is examined to assess the circulation processes that impact the distribution and occurrence of a toxic dinoflagellate alexandrium fundyense in eastern casco bay over the inner shelf adjacent to the bay tidal variance is weak and the across shelf current is highly coherent and in phase with the along shelf wind stress although tidal current variance increases as one advances into the bay non tidal currents account for 30 40 of the across shelf current variance at the bay entrance between the shelf and the bay interior is a transition region where the circulation response to wind forcing changes as the wind adjusts to the changing orientation of the shoreline far from shore the overall large scale coastline orientation dominates the wind driven response but within a few internal rossby radii the local coastline clearly dominates the flow patterns and across shelf wind becomes locally shore parallel inside the bay within the bay interior the across shelf wind is highly coherent and in phase with the near surface subtidal across shelf current the kennebec river north of the study area supplies freshwater to eastern casco bay in all seasons a pool of low density relatively fresh water at the entrance to the bay sets up an across shelf density gradient that is reversed from a typical estuary and likely contributes to the mean surface on shelf transport in this region surface drifter trajectories observed over the course of the study suggest that both the across shelf wind and the across shelf density gradient are important in driving surface up bay transport and in the retention of surface dwelling organisms in eastern casco bay

horizontal structure and excitation of primary motions in a strongly stratified lake a modal analysis in the horizontal plane was extended to a layer stratified basin with irregular bathymetry and the theory was applied to lake biwa to investigate the horizontal structure and excitation of the basin scale internal waves and gyres the horizontal structure of the basin scale internal waves consisted of cyclonic and anticyclonic elliptic cells each of which appeared to follow the dispersion relationship of kelvin and poincare waves in elliptic basins the internal waves were preferentially excited depending on the arrangement of the cells and the wind direction but the spatial distribution of wind stress curl over the lake primarily determined the horizontal structure of the ensuing gyres decoupled evolutionary equations for the individual modes provided a good approximation for excitation of the internal waves and early stages of excitation of the gyres before nonlinear effects and damping become significant the modal decomposition of hydrodynamic simulation results also showed that the primary action of the wind was to excite the internal waves however these internal waves were damped within a few days and the dynamics during calm periods were dominated by the gyres illustrating the importance of internal waves on mixing and gyres on long term horizontal transport

the degeneration of internal waves in lakes with sloping topography in a laboratory study we quantified the temporal energy flux associated with the degeneration of basin scale internal waves in closed basins the system is two layer stratified and subjected to a single forcing event creating available potential energy at time zero a downscale energy transfer was observed from the wind forced basin scale motions to the turbulent motions where energy was lost due to high frequency internal wave breaking along sloping topography under moderate forcing conditions steepening of nonlinear basin scale wave components was found to produce a high frequency solitary wave packet that contained as much as 20 of the available potential energy introduced by the initial condition the characteristic lengthscale of a particular solitary wave was less than the characteristic slope length leading to wave breaking along the sloping boundary the ratio of the steepening timescale required for the evolution of the solitary waves to the travel time until the waves shoaled controlled their development and degeneration within the domain the energy loss along the slope the mixing efficiency and the breaker type were modeled using appropriate forms of an internal iribarren number defined as the ratio of the boundary slope to the wave slope amplitude wavelength this parameter allows generalization to the oceanographic context analysis of field data shows the portion of the internal wave spectrum for lakes between motions at the basin and buoyancy scales to be composed of progressive waves both weakly nonlinear waves sinusoidal profile with frequencies near 10 24 hz and strongly nonlinear waves hyperbolic secant squared profile with frequencies near 10 23 hz the results suggest that a periodically forced system may sustain a quasi steady flux of 20 of the potential energy introduced by the surface wind stress to the benthic boundary layer at the depth of the pycnocline

summer circulation and temperature structure of lake kinneret a three dimensional lake model driven by wind stress and heat flux fields derived from an atmospheric model is applied to lake kinneret israel the summer wind field over the lake has a strong diurnal and large spatial variation due to complex terrain surrounding the lake the sharp temperature contrast between the arid land and the lake and due to the penetration of the mediterranean sea breeze msb into the lake area the daily mean wind curl field which is predominantly determined by the penetration of the msb is responsible for the generation of three lake gyres one of them dominates most of the lake and rotates counterclockwise it is flanked to the north and to the south by two smaller ones that rotate clockwise during the summer the diurnal variation of the wind over the lake is repeated daily due to consistent forcing conditions during that season numerical tests show that the rectified flow induced by the diurnal winds plays a minor role in the lake circulation the thermocline oscillation which was believed to be the free propagation of internal kelvin waves mainly responds to the surface elevation set up by the time dependent winds and it appears that no systematic counterclockwise propagating waves with large thermocline displacements exist in the lake the intense msb over the lake in the late afternoon pushes the heated surface water toward the east forcing the deep cooler water to be advected westward and creating strong mixing over the shallow western shore this results in higher temperature off the eastern shore and lower temperature off the western shore however a strong mean flow is constantly eliminating the temperature difference by counterclockwise transfer of the western cooler water eastward the results are in good agreement with available observations