JSP39

JSP39 Tuesday 27 - Thursday 29 July

DYNAMICS OF ROTATING AND STRATIFIED FLUIDS (IAPSO,

IAMAS, IAGA)

Location: Watson Building, G23 LTA

Location of Posters: Bridge Poynting/Watson

Tuesday 27 July AM

Presiding Chair: D.L. Boyer Arizona State Univ., Tempe, (USA)

JSP39/L/01-B2 Invited 0830

FLUID DYNAMICS AND SOME KEY APPLICATIONS TO UNDERSTANDING GLOBAL CHEMICAL-CLIMATIC CHANGE

Michael E. McINTYRE (Centre for Atmospheric Science at the Dept. of Applied Mathematics and Theoretical Physics, Cambridge CB3 9EW, UK, email mem@damtp.cam.ac.uk)

This overview will focus on several very fundamental aspects of our global fluid environment, through some thought-experiments designed to expose key mechanisms controlling global-scale mean circulations in the atmosphere and ocean, and the resulting transports of heat and chemical constituents such as greenhouse gases.

The best understood fluid subsystem is the middle atmosphere - roughly 10-100 km altitude - whose mean stratification is primarily under UV/IR radiative control and whose mean circulation is driven not, as some mythologies have it, by solar heating, but by a persistent mechanical, Coriolis-mediated `gyroscopic pumping'. The pumping action depends on Rossby and gravity waves propagating or diffracting from below: more precisely, it depends on the interplay of the Coriolis force with the irreversible angular momentum transport that results from wave generation in one place and dissipation in another by breaking and radiative damping.

Viewed as statistical fluctuations about a mean state, the disturbances characteristically have a net effect opposite to what was expected from classical ideas about spatially near-homogeneous `turbulent' fluctuations. As well as pumping mean circulations, the fluctuations tend to drive the mean state not toward, but away from, a state of solid rotation; and spatial inhomogeneity is an essential, leading-order feature. In the case of Rossby waves, such `anti-classical' behaviour (seen also in general vortex-interaction scenarios) is almost inevitable consequence of the transport and `inversion' properties of the Rossby--Ertel potential vorticity. There is an unexpected spinoff for understanding the Sun's interior.

The troposphere and oceans are more complicated, because of the non-global span of stratification surfaces and for other reasons as well. For instance the climate system's ‘thermal powerhouse’, the tropical troposphere, simplified as a subsystem with a fixed sea-surface temperature, has stable stratification under moist-convective control - temperature profiles clamped to the moist adiabat - and a persistent deep-convective upward mass flux in the intertropical convergence zone consisting of two contributions having somewhat comparable magnitudes, the one controlled by radiative cooling of the tropics and the other by gyroscopic pumping from the extratropics. If both these controls were to be switched off then the deep convection would also switch off. The oceans are more complicated still...

JSP39/W/43-B2 Invited 0900

THE ROLE OF LABORATORY EXPERIMENTS IN ROTATING STRATIFIED FLOWS

Prof. Paul LINDEN (University of California, San Diego, USA, email: pflinden@ames.ucsd.edu)

This talk will review the role of laboratory experiments in research into the dynamics of rotating stratified flows (RSF) with particular relevance to geophysical fluid dynamics. Experiments have provided a major input into the understanding of RSF, both by revealing new phenomena and by providing data against which theoretical ideas can be tested and developed. The first major impact of experiments came with the annulus studies of the atmospheric circulation in the 1950s, and from that time there has been a (generally) increasing activity in RSF experiments. This talk will outline some of the successes of laboratory experiments in modelling RSF and will describe recent work in which the experiments interface with numerical simulations. The limitations of experiments of geophysical flows will be described and some speculations will be made about possible future developments.

JSP39/W/42-B2 0930

TURBULENT CONVECTION IN ROTATING AND STRATIFIED FLUIDS

H.J.S. FERNANDO, M.A. Levy (Environmental Fluid Dynamics Program, Arizona State University, Tempe, AZ 85287-9809, USA)

Turbulent convection induced by heating the bottom boundary of a wide, linearly (temperature) stratified, rotating fluid layer is studied using a series of laboratory experiments. It is shown that the growth of the convective mixed-layer is dynamically affected by background rotation (or Coriolis forces) when the parameter R = (h**2\W**3/q)**{2/3} exceeds a critical value, R1, which is approximately 100. Here h is the depth of the convective layer, W is the rate of rotation and q is the buoyancy flux at the bottom boundary. When R > R2, where R2 is approximately 300, the buoyancy gradient in the mixed-layer was profoundly affected by background rotation. Conversely, when R < R1, the buoyancy gradient in the convective layer is independent of the rate of rotation and approaches that of convection in non-rotating fluids. When R > R2, the entrainment velocity was found to be dependent on the buoyancy frequency of the overlying stratified layer, the rate of rotation and the conventional (Deardorff) convection velocity. A simple theoretical formulation for the rate of entrainment was derived, which was found to be in good agreement with the experimental results. The experimental results also indicate that entrainment in this case is dominated by non-penetrative convection.

JSP39/W/06-B2 0950

SEPARATION PHENOMENA IN STRATIFIED FLOW ALONG A SLOPING SIDEWALL

Geno PAWLAK, Parker MacCready (School of Oceanography, University of Washington, Box 357940, Seattle, WA 98195, USA, email: gpawlak@ocean.washington.edu)

Experiments examining the separation of a stratified flow along a sloping boundary are presented. Separation phenomena are the source of much of the transfer of vorticity-containing fluid within sidewall boundary layers into the interior inviscid flow, which may contribute to a significant part of the mixing in estuarine and coastal processes. Obstacles along the inclined sidewall induce both cross and along-isopycnal components to the mixing and allow the isolation of the competition between stratification and form induced pressure variations. Experimental results presented, examine obstacles of wavenumber, k, such that N/U << k << d, where d is the boundary layer thickness. These experiments explore the origin and structure of separation zones for a variety of stratified flow configurations including uniform stratified flow, two-layer and stratified shear flows and tidally modulated stratified flow. Particle imaging velocimetry and flow visualization results are presented along with observations on internal wave generation and drag. Experimental observations are compared to analytical and numerical analyses.

JSP39/L/02-B2 1010

EDDY-DRIVEN BASIN-SCALE RESPONSE TO LOCALISED FORCING OF A ROTATING STRATIFIED FLUID

J. N. HACKER, S. B. Dalziel (D.A.M.T.P., Cambridge University, Silver Street, Cambridge CB3 9EW, UK; email: j.hacker@damtp.cam.ac.uk), P. F. Linden (A.M.E.S., University of California, San Diego, 9500 Gilman Drive, La Jolla, CA 92093-0411, USA)

Mesoscale eddies, produced for example by the instability of density fronts, are a ubiquitous feature of the flow in the oceans. These eddies typically have kinetic energy and vorticity levels exceeding those of the mean motion, and are widely considered to have a profound influence on the flow at larger-scales. Although the nature of this relationship is not well understood at present, the ability of eddies to transport momentum away from regions of eddy production to more quiescent regions may be an important effect influencing the character of the general oceanic circulation. Here we report laboratory experiments in which eddies were produced in a rotating stratified fluid by injecting and withdrawing fluid through sources and sinks situated along some part of the boundary of a laboratory tank. Energetic eddies continually formed and were dissipated at the sources and sinks, but also intermittently escaped from the forcing region, propagating away into the interior of the basin in the form of anticyclone-cyclone pairs. These eddies interacted in the interior of the basin and set up gyre-like circulation patterns. The character and localised nature of the source-sink forcing, together with the geometry of the basin, imposed dynamical constraints on the flow, in particular on the manner in which eddies were produced and the form of eddy interactions, with the results that a coherent basin-scale mean circulation was established.

JSP39/E/19-B2 1050

LABORATORY EXPERIMENTS ABOUT UPWELLING FRONTS

P. BOURUET-AUBERTOT, P.F. Linden (D.A.M.T.P., Univ. of Cambridge, Silver Street, Cambridge CB9 3EW, U.K; Dept of Applied Mechanics and Engineering Sciences, Univ. of California San Diego, La Jolla, CA 92093-0411, USA)

We investigate experimentally the instability and later evolution of a front in a two-layer stratification near a coast and over sloping topography. The front is produced through the adjustment of a buoyant fluid that is initially confined within a bottomless cylinder. Typically, a front in quasi-cyclostrophic balance is established after two rotation periods, for appropriate values of the radius of deformation, after which it becomes unstable. We focus here on the influence of the coast and on the effect of conical bottom topography. Quantitative measurements of the velocity and vorticity fields at the surface are made which provide detailed information on the evolution of the front as the instability grows to finite amplitude. The effect of the coast is crucial with respect to the instability of the front and leads to the enhancement of cyclonic eddies consisting of coastal waters. This has significant impact on exchanges across the front. When a conical topography cross-frontal exchanges are weakened as the cyclonic eddies remain confined to the slope region.

JSP39/W/48-B2 1110

A GRAVITY JET DISCHARGED HORIZONTALLY INTO A ROTATING AND/OR STRATIFIED FLUID

Yoshito IKEHATA (Dept. of Earth System Sci. & Tech., Kyushu University, Japan,

email: ikehata@esst.kyushu-u.ac.jp)

A gravity jet discharged horizontally into a rotating and/or stratified fluid is of great interest in oceanography and hydraulics. Two typical examples of such a jet are a high-density current which flows out of the Mediterranean Sea into the Atlantic Ocean and an inflow of muddy river water into an inner bay. Thus a large variety of different scales are associated with the horizontally-discharged gravity jet, which strongly influences coastal and off-shore flow systems. A three-dimensional round jet discharged into a linearly stratified rotating fluid has been investigated experimentally. A 130 x 30 x 30 cm glass water tank was filled with a linearly stratified salt water and placed on a counter-clockwise rotating table with the diameter of 1.0 m. A nozzle with the inside diameter of 3.00 mm was attached horizontally to a smaller side wall of the tank at the height of 18.5 cm. Through this nozzle was discharged horizontally a salt water, which has the same density as the ambient water near the nozzle or a density higher than the ambient water density. The discharged turbulent jet initially moves downward deflecting to the right (when viewed from above the tank) by the action of the Colioris force. Eventually, the jet reaches a larger side wall and proceeds along it forming a Kelvin-wave like pattern subject to a geostrophic balance and diffusion. In the very long time, steady two-dimensional two to six cellular vortices, were formed in the tank. The jet was marked with a dye and the overall flow patterns were visualized with suspended polystyrene beads. For comparison's sake, the non-rotating case was also investigated. The result shows that similar cellular vortices were formed in the long time, although the horizontal diffusion pattern of the jet was different from that for the rotating case. The behaviour of the horizontally discharged turbulent jet was also analyzed numerically by means of LES. The eddy viscosity was evaluated based on a Smagorinsky type model, which takes into account the turbulence caused by the buoyancy energy of the subgrid scale. The numerical results are generally in accordance with the experimental ones. Such characteristic quantities associated with the jet as the velocity distribution, the vorticity distribution, and the time-series of turbulent spectra are calculated.

JSP39/W/29-B2 1130

EVOLUTION OF VORTICES GENERATED BY THE COLLAPSE OF A STRATIFIED, TURBULENT WAKE

Alan BRANDT (Johns Hopkins University, Applied Physics Laboratory, Johns Hopkins Road, Laurel MD 20723-6099, USA, email: alan.brandt@jhuapl.edu)

Recent laboratory studies of turbulent wakes of spheres and self-propelled bodies in stratified fluids have shown that late-wake vortices ("pancake eddies") are present over a wide range of Reynolds and Froude numbers and that they persist for very long times. This late-wake, quasi-two dimensional structure results from the stratification induced collapse and transfer of the vertical turbulent energy and the wake-induced shear in the horizontal plane. As the energy supply is non-uniform, due to the turbulent nature of the wake, the resulting vertical vorticity is not uniformly distributed. The late-wake eddies are interconnected and evolve as dipoles with unequal vortex strengths at each pole, and thus do not behave as dipoles that are generated in laminar flow experiments which can be described theoretically. Based on laboratory experiments, a statistical analysis of the late-wake, turbulent dipole evolution has been carried out in order to describe the evolution of the size and motion of the dipoles as a function of Froude number. Implications of these scaling laws for the persistence of eddies in the ocean (that can result from shear induced instabilities and can affect the distribution of the planktonic biomass and anthropogenic pollutants) will also be discussed.

JSP39/W/08-B2 1150

SPIRAL MONOPOLES AND DIPOLES IN ROTATING STRATIFIED FLUIDS

Sergey VOROPAYEV (Arizona State University, Tempe, AZ 85287-6106, USA and Institute of Oceanology, Russian Academy of Sciences, Moscow, 117851, Russia, email: s.voropayev@asu.edu)

Monopoles and dipoles are most frequent structures observed in the stratified and/or rotating systems. Typical examples are mushroom-like currents in the upper ocean and inter-thermocline lenses in the deep ocean. To reproduce these characteristic flows in experiments, a controllable horizontal momentum source (jet emerging from a nozzle into a rotating stratified fluid) was used. The jet either (i) deflects from the direction of injection, forming an anticyclonic spiral monopole (monopole regime), or (ii) propagates along the injection direction, forming a dipolar structure (dipole regime). Which of these flows occurs depends on the system parameters, the Reynolds number Re and the buoyancy frequency to Coriolis parameter ratio N/f. A flow regime diagram is developed for a range of these parameters. A theoretical analysis is advanced to explain the conditions under which the monopole or dipole regimes occur, including the transition curve between the two regimes, which is calculated as N/f = 2Re1/3. The theory is supported by laboratory experiments. In accordance with the flow regime diagram, one may expect that dipolar structures should occur more frequently in the upper ocean, while monopolar structures should occur more frequently in the deep ocean. These conclusions are in agreement with numerous remote observations and direct field measurements.

JSP39/W/19-B2 1210

INTERACTIONS BETWEEN TOPOGRAPHY AND AN INTERMEDIATE CURRENT: AN EXPERIMENTAL STUDY

Sylvian SADOUX (LEGI-Coriolis, 21 ave des martyrs, Grenoble, 38000, France, email: sadoux@hmg.inpg.fr)

To study the effects of a cape or a canyon upon an intermediate water current, experiments were performed on the 13m diameter tank filled with a linearly stratified fluid. The upstream current was either stable (Ro1) or unstable (i.e. anticyclonic lense generating, Ro<0.2) (Sadoux et al., 1999). From horizontal velocitiy fields recorded in several (up to 29) levels using a Correlation Imaging Velocimetry (CIV) technique it is possible to reconstruct 3D velocity and vorticity fields as functions of time. The cape-current interaction experiments focussed on a simplified model of Cape Saint Vincent at depth 1000m (i.e. a 70° cape). When the upstream current is unstable, they show that there is anticyclonic lense production at the cape; these anticyclonic lenses are at first surrounded by a cyclonic ring. As they propagate, the cyclonic ring tends to weaken but upper and lower (and weaker) cyclones remains attached to the anticyclone. When Ro increases, (1) there is dipole formation at the cape and (2) the negative to positive vorticity ratio increases too. The formation period, the vorticity and the spin-up time of the lenses generated at the cape agree with in situ observations (Bower et al., 1997). In the canyon-current interaction experiments the mean current level was either above or at the upper bound of the canyon. In both cases, when the upstream current is unstable, the canyon is a possible (but not only) place for anti-cyclonic lense generation. When the upstream current is stable, there is only cyclone generation, starting in the canyon region and growing in diameter with time. These cyclones are then advected downstream by the current. The rate of growth of the cyclones is maximum when the Rossby radius of deformation is larger than the canyon width.

Tuesday 27 July PM

Presiding Chair: D. Fearn Univ. of Glasgow, Dept. of Mathematics, (UK)

JSP39/W/37-B2 Invited 1400

THE ROLE OF ROTATION & STRATIFICATION IN THE DYNAMICS OF EARTH'S CORE

Paul ROBERTS (Institute of Geophysics and Planetary Physics, UCLA, Los Angeles, CA 90095, USA, email: roberts@math.ucla)

The directional property of the magnetic compass needle is enough to establish how important the rotation of the Earth is for the MHD of its core; of all the forces that act on the core fluid, the Coriolis force alone has a preferred direction. The study of RMHD (rotating MHD) shows how rotation retards most flows. giving them slow time scales that have been identified with those of the geomagnetic secular variation. The time scale of the geostrophic component of flow, and it to has been seen in analyses of geomagnetic data.

Most simulations of core RMHD assume that the bulk of the core is weakly but stably stratified because of tiny compositional and thermal superadiabatic gradients that are however large enough to provide the buoyancy necessary to supply kinetic and magnetic energy. It has also been suggested that there is a layer of stably stratified fluid at the top of the core, and Braginsky has called this "the stratified ocean of the core". This presentation will focus on the present status of the topics adumbrated above.

JSP39/E/10-B2 Invited 1430

INFERENCES ON CORE STRUCTURE AND FLOW FROM SURFACE OBSERVATIONS

David CROSSLEY (Saint Louis University, Department of Earth and Atmospheric Sciences, 3507 Laclede Ave., St. Louis, MO 63103, USA, email: crossley@eas.slu.edu)

There can be no direct observations of the Earth's core; all geophysical measurements have to be heavily processed to reveal subtleties that might reflect structure and motions deep within the Earth. The most effective techniques are seismology, gravity, geodesy and geomagnetism. The most likely motions that may be detectable are short period waves (predominantly hydrodynamic) of periods hours to days, rotational phenomena associated with angular momentum changes between the inner core, outer core and mantle with periods of hours to decades, and decadal changes in core flow associated with steady convection of the hydromagnetic fluid. In all successful observational campaigns, three criteria need to be satisfied:

(1) the theoretically possible motions have to be stimulated by a sufficiently energetic source,

(2) the signal to noise ratio of the surface phenomena must be adequate and (3) the damping of the motions must be weak (i.e. long) enough to permit the required data to be collected.

This review will begin with the best known data we have at long periods, i.e. the Earth rotation data and the magnetic field variations over several decades to centuries. We will end with the possible observations we may get from seismology and super-conducting gravimeters over the next few years.

JSP39/W/34-B2 1500

PRECESSION DRIVEN FLOW IN SPHERICAL SHELLS

A. Tilgner, F.H. BUSSE (Institute of Physics, University of Bayreuth, D-95440 Bayreuth, Germany)

Precession has long ago been proposed as a possible driving mechanism for the geodynamo. In the case of exactly spherical boundaries, only viscous forces transmit the motion of the boundaries to the fluid. In the simplest approximation, the flow inside the shell is described as a solid body rotation about an axis that differs from the axis of rotation of the shell. The velocity profile inside processing spherical shells has been investigated numerically for different Ekman numbers, precession rates and gap sizes. The simulations reveal significant deviations from a pure solid body rotation. These deviations determine the shape of internal shear layers and the stability of the flow. The numerical results are compared with analytical expressions in the cases of the full sphere and in the thin shell limit.

JSP39/E/17-B2 1520

ASYMPTOTIC MODELS FOR RESISTIVE INSTABILITY IN THE EARTH'S OUTER CORE

Steven D. LONDON (Department of Computer and Mathematical Sciences, University of Houston-Downtown, One Main Street, Houston, Texas 77002, USA)

Numerical work indicates that resistive instability may be the dominant mode of instability in the Earth's outer core for realistic core parameter regimes. In this work, we are able to study resistive instability analytically by choosing a parameter regime in which the Elsasser number, while assumed to be large, is an order of magnitude smaller than the magnetic Reynolds number. Applied to an electrically conducting fluid confined to a thin rotating cylindrical shell, these assumptions allow us to analyze the linearized equations of motion using standard analytical techniques. We first discuss the equations which have been linearized about an ambient field which varies only in the direction perpendicular to the axis of rotation. The study is then extended to ambient fields which vary in directions both parallel and perpendicular to the axis of rotation. We will also discuss the extension of this work to spherical geometry.

JSP39/E/20-B2 1540

HYDRODYNAMIC MODEL OF THE TOPOGRAPHY VARIATIONS ACROSS THE RIDGE CREST

V. CHERNIAVSKI (Paleogeodynamics, Institute of Oceanology of RAS, Moscow, 117851, Russia, email: cherniav@chip.sio.rssi.ru); Suetnova E. (United Institute of the Physics of the Earth of RAS, Moscow, 123810, Russia, email: elena@uipe-ras.scgis.ru)

It is considered the stability of the stationary flow of high viscosity liquid. Well known stationary solution of a jet moving from infinity toward a wall is suggested to be considered as a flow caused by the force acting along the boundary deformable surface. Undisturbed surface is horizontal and horizontal velocity on the surface is the step function. It is studied the developing of infinitesimal disturbances. The disturbed solution is defined by the solutions of two Laplace equations with boundary conditions on the undisturbed surface. It is given a procedure of numerical investigation that is based on the method of conformal mapping the region of the flow to the circle. The problem can be regarded as a model of the topography variations across the ridge crest. It is suggested hydrodynamic model of mantle flow with deformable upper boundary at the mid-ocean ridge crest. It is well known the mantle media can be regarded as a high viscosity liquid (1019-1021puas) at geological intervals of time (1-100 MY). Some observations show that the mantle matter flows from the ridge crest toward borders along the upper boundary. The model is developed under the assumption a hydrodynamic flow causes deformation of the upper boundary depending on time.

JSP39/W/40-B2 1620

DIFFERENTIAL ROTATION OF LITHOSPHERE AND MANTLE AND ITS BEARING ON PLATE DRIVING FORCES AND THE CAUSES OF INTRAPLATE VOLCANISM

A. SMITH (Dept. Earth Sciences, Cheng Kung University, Tainan, Taiwan; mochinm@mail.ncku.edu.tw); C. Lewis (Dept. Mining & Petroleum Engineering, Cheng Kung University, Tainan, Taiwan)

Uncertainties regarding the relative importance of basal drag and boundary forces in plate tectonic models are a consequence of flawed assumptions imposed by the use of the hotspot reference frame. Velocities of lithospheric plates are influenced not only by lateral boundary forces, but also by basal drag forces resulting from Earth rotation. Drag is exerted on the base of the asthenosphere and motion transmitted upwards to the lithosphere. As the transmission of stress in the mantle is viscosity-dependent, the reduction in viscosity through the asthenosphere results in plates suffering a net westward lag. This differential rotation effect causes continental plates to be more strongly coupled to the deep mantle as they are separated from the mesosphere by only relatively thin regions of asthenosphere. For such plates the calculated drag forces are of the same order of magnitude as boundary forces. The motion of oceanic plates is dominated by conventional plate boundary forces which may either reinforce or oppose drag from eastward mantle flow. Reinforcement (e.g. Nazca plate) gives rise to Couette flow in the asthenosphere. Opposition (e.g Pacific plate) results in counter-flow. Shear stresses in both regimes are concentrated in the upper asthenosphere and lead to generation of intraplate melts from concentrations of hydrous minerals (wetspots) introduced into the oceanic asthenosphere by lateral asthenospheric flow. Under a counter-flow regime, melt collects in a stationary layer at the crossover point between plate- and mesosphere- induced flow regimes. Release of melt to the surface is governed by lithospheric stress trajectories set up by convergence along plate boundaries, giving an illusion of a series of quasi-fixed melting anomalies, though in reality both these and the lithospheric plates are moving relative to the deep mantle.

JSP39/E/03-B2 1640

THE NUMERICAL MODELING OF NON-LINEAR TIDE WAVE PROPAGATION IN STRATIFIED OCEAN

Tatjana TALIPOVA (Laboratory of Hydrophysical and Non-linear Acoustics, Institute of Applied Physics, 46 Uljanov str., Nizhny Novgorod, 603 600 Russia, email: tata@appl.sci-nnov.ru)

The numerical model of the non-linear internal tide evolution in the coastal zone is developed. It is based on the finite-difference scheme of the solution of the generalized Korteweg – de Vries equation. The model includes quadratic and cubic non-linearity, dispersion, the Earth rotation, quadratic bottom friction and horizontal variability of the density stratification. Numerical algorithm is described. Observed data of the density stratification for different shelves in Atlantic and Indian Oceans (North-west Shelf of Australia, Iberian shelf, Malin Edge Shelf, Mediterranean, Baltic and Black Seas) are used to calculate the coefficients of the generalized Korteweg – de Vries equation. Model simulations of the internal sine tide evolution have been done, they show the contribution of the different effects on the non-linear wave evolution for different areas of the World Ocean.

JSP39/W/36-B2 1700

THE DYNAMICS OF TURBULENCE ACCOMPANYING KELVIN-HELMHOLTZ INSTABILITY

David C. FRITTS, Joseph A. Werne, Teresa L. Palmer (Colorado Research Associates, 3380 Mitchell Lane Boulder, CO 80301, USA)

Numerical simulations of Kelvin-Helmholtz (KH) instabilities now span sufficient spatial scales to allow a broad inertial range of turbulence accompanying billow breakdown. Turbulence arises in response to secondary and tertiary convective and dynamical instabilities within the KH billows which drive the transition from 2D to 3D flow. These instabilities lead to vortical structures which undergo vigorous subsequent interactions. Vortex interactions lead, in turn, to perturbations of the vortices which contribute to their fragmentation and thus to the cascade to smaller scales of motion. This talk will describe the instability and turbulence dynamics accounting for the transition to and the cascade within the turbulent flow.

JSP39/W/07-B2 1720

STRATIFIED FLOW AROUND A UNIFORMLY MOVING CYLINDER IN A CONTINUOUSLY STRATIFIED LIQUID

Yuli D. CHASHECHKIN, Vladimir V. Mitkin (both at the Laboratory of Fluid Mechanics of the Institute for Problems in Mechanics of the RAS, Moscow, prospect Vernadskogo, 101-1, 117526, Russia, email: chakin@ipmnet.ru )

We study experimentally pattern of flow past a horizontal cylinder with diameters 1,5; 2,5; 5 and 7,6 cm. moving with constant velocity from 0,01 to 6 cm/s in a uniformly stratified liquid with buoyancy period from 5,0 to 25,2 s. Visualisation of flow pattern is performed by different schlieren methods (knife, filament natural rainbow), by dyeing and vertical markers. Internal waves are measured by a conductivity probe. On results of more then 800 experiments expanded flow regimes diagram is drown. Any particular regime of flow is occupied compact domain in the space Froude number-Reynolds number and separated from neighbours ones by sharp boundaries. It is shown that there is no overlapping of regimes in the complete flow regimes cube where axis are Reynolds number-internal Froude number-ratio of external scales. The special class of small scale structures for which thickness of general structure identified elements is determined by effects of viscosity and diffusivity is introduced. Detailed measurements of velocity profiles in a forming and stationary upstream disturbance are carried out. The horizontal size of an area of completely blocked fluid is measured and compared with calculations on different models of upstream wake, the best agreement with the simplest model of singular dipole. Geometry of soaring and imbedded in to the downstream wake vortices and vortex arrays is defined.

JSP39/L/05-B2 1740

FREE AND LEE WAVES INTERACTION IN NEAR-SURFACE PYCNOCLINE

Olga SHISHKINA (Institute of Applied Physics, RAS, 16 Uljanovsk st., Nizhni Novgorod, 603600 Russia, email: ols@hydro.appl.sci-nnov.ru)

Experimental results on the lee internal waves’ propagation in a fluid with a thermocline-type stratification in the presence of favourable and opposite regular plane internal waves are presented. After stationary background internal wave recording a cylinder of the diameter 0.4m was towed along the tank in both directions with respect to the background waves propagation and resulting internal wave profiles were fixed.

The body’s draft T and velocity U were chosen to provide the first and the second internal wave modes generation (0.5£T/H£1.5, 0.25£U/C1£1.5, where H is the tank’s depth and C1 is the first mode phase velocity). Background internal waves had first-mode structure at the frequency of 2/3 Nmax (Nmax is the maximum buoyancy frequency). Almost plane lee waves with propagation angles 60deg£b£90deg provide intensive interaction of two wave systems. The amplitude of internal waves increased almost twice under the influence of favourable background internal waves. This work was supported by the RFBR (grant no.: 99-05-64394)

Wednesday 28 July AM

Presiding Chair: Dr. Harindra J.S. Fernando., Department of Mechanical and Aerospace Engineering, Arizona State Univ., Tempe, (USA)

JSP39/E/16-B3 Invited 0830

CORIOLIS EFFECTS ON OROGRAGPHIC AND MESOSCALE FLOWS

J. C. R. HUNT (Cerfacs and IMFT, Toulouse and Arizona State University); H. Olafsson (Icelandic Met. Services, Reykjavik, Island); P. Boulgeault (CNRM, Meteo-France, Toulouse)

Using a theoretical approach based on perturbations to stably stratified flow and on the recently developed understanding of flow over mountains at low Froude number (F=Ud/(NH), where N is the buoyancy frequency, Uo is the wind speed and H is the mountain height), the effects of rotation have been studied. The Rossby number Ro=U/(fD) where f is the Coriolis parameter, and D the diameter of the mountain, is assumed to be a large number. Typically it is found to lie in the range 3-10. The results are compared with the recent numerical simulations. It is found that as the flow impacts on the mountain it turns to the left (with your back to the wind) and wave activity over the top of the mountain is greatest on the left side but the pressure drop is greatest on the right. Over a distance of the order of HN/f, e.g. 150 km for the Pyrenees, a new wake structure develops that can extend to 1000 km (or a spin down distance). There is a momentum defect within the wake but the wind speed increases either side of the wake. Coriolis forces induce a deflection upward of the isopycnals on the left and downward on the right. This seems to explain some differences in meso-scale weather and climate phenomena between the different sides of mountains and wide valleys and of wakes of mesoscale convection. The large perturbation pressure change predicted by the theory is of the order of rho U^2/F, which is consistent with the magnitude of the terms introduced into the recent ECMWF orographic parameterizations. However a fluid-dynamically justified parameterization would allow for asymmetric Rossby number effects on the pressure field and the effective wave flux.

JSP39/W/21-B3 Invited 0900

MEASUREMENT AND MODELING OF THE TRANSPORT OF SNOW, SAND, ICE AND DUST PARTICLES OVER COMPLEX TERRAIN

Kouichi NISHIMURA (Inst. of Low Temp. Sci., Hokkaido Univ., Sapporo, JAPAN,

email: nishi@orange.lowtem.hokudai.ac.jp)

Aeolian particle transports occur in many geophysical contexts, including desert sand, soil erosion and snow drifting, and affect a variety of aspects of human life. For instance, the drift of snow around the structures and transportation systems can cause delays and produces access problems; the mass transport by the blowing snow in the Antarctica can be an important factor in the global climate system. It should be noted the evidence of the aeolian activity has been also found on Mars, Venus and a moon of Saturn. After BagnoldÕs pioneering work (1941), some progress has been made in the modeling of aeolian particle transport in the last decade, but there are still significant quantitative uncertainties. In this paper, our wind tunnel experiments with spherical particles of different diameters, densities and coefficient of restitution will be also introduced. From the measurements of the ejection velocities of the particles and the mean wind speed profile, the horizontal flux profiles were computed, and compared with our measurements. For the higher wind speeds, we examine the changes in the velocity statistics of the particle as they move randomly under the influence of turbulence in a state of ÔsuspensionÕ, and examine whether they continue to be ejected from the surface as a result of collision between impacting particles and those in the bed, or whether their movements are more determined by random aerodynamic forces acting on them.

JSP39/W/46-B3 0930

PHASE RELATIONS BETWEEN MOVING HEATING AND THE RESULTANT TEMPERATURE PERTURBATIONS

Brian MAPES (NOAA-CIRES Climate Diagnostics Center, Boulder, CO USA 80303-3328, email: bem@cdc.noaa.gov); Matthew Wheeler (National Center for Atmospheric Research, Boulder CO USA, email: mwheeler@cgd.ucar.edu)

In order to interpret observational studies of the thermal structure of waves of tropical convection, we need to understand the thermal structures generated by moving heat sources without any feedback. To this end, we have constructed translating heating processes that resemble observed convective variability, and imposed them in analytical and numerical linear and non-linear atmospheric models. For slowly moving deep heating, a quadrature relation between heating and temperature is established. For more rapidly moving heating, things get more complicated and interesting.

JSP39/W/27-B3 0950

SUPERCRITICAL CONDITIONS IN THE SUMMER MARINE BOUNDARY LAYER ON THE WEST COAST OF THE UNITED STATES

Clive DORMAN (Center for Coastal Studies, Scripps Institution of Oceanography, University of California San Diego, La Jolla, CA 92092-0209, USA, email: cdorman@ucsd.edu); David Rogers (Physical Oceanography Research Division, Scripps Institution of Oceanography, University of California San Diego, La Jolla, CA 92092-0209, USA, email: drogers@ucsd.edu); Teddy Holt (Naval Research Laboratory, Marine Meteorology Division, Monterey, CA 93943-5502, USA, email holt@nrlmry.navy.mil)

Field measurements show that the summer conditions along the US West coast include a strong, subsidence inversion capping a weakly stratified marine boundary layer (MBL). The MBL is lower than the topography lining the coast which forms an eastern wall. Mean surface winds increase to the south along California, reaching a maximum near Pt Conception in Southern California. Mean MBL depths are a broad minimum along California with greater depths north of California and south of Pt Conception, California. Direct aircraft measurements show that the MBL is super-critical along portions of southern Oregon and California that is narrow along Oregon (50 km) and broad off Central California (more than 120 km). In the lee of Capes, the MBL speed accelerates, the depth decreases and the Froude number increases in the nature of an expansion fan with rotation and friction. The tendency for supercritical flow conditions to broaden to the south along California is proposed to be a larger scale expansion fan that is be caused by the coast bending to the east as the latitude decreases.

JSP39/E/08-B3 1010

GRAVITY WAVES AND VORTICES IN THE WAKE OF ISOLATED MOUNTAINS

Dieter ETLING (Institute of Meteorology and Climatology, University Hannover, Herrenhauser Str. 2, 30419 Hannover, Germany, email: etling@muk.uni-hannover.de)

The influence of stable stratification on the wake of obstacles in the laboratory as well as in the atmosphere has been investigated intensively in recent years. Most important, the production of potential vorticity due to non-viscous effects, like gravity wave breaking or flow splitting, has been proposed by several authors. We present some examples where both, gravity waves and Karman vortex streets, can be observed simultaneously in the wake of island mountains. By analysing the meteorological upstream conditions, we will give some insight into the ongoing discussion on PV production in the wake of obstacles in stably stratified flows.

JSP39/W/14-B3 1050

COHERENT VORTICES ALONG THE TROPOPAUSE

John NIELSEN-GAMMON, John Fulton (Texas A&M University, College Station, TX 77843-3150 USA, email: n-g@tamu.edu)

A climatology of coherent vortices along the tropopause is constructed using the NCEP/NCAR reanalysis data for 1978-1997. Coherent vortices are identified by the presence on multiple isentropic layers of closed contours of the dynamic tropopause (1.5 PVU) which maintain their structure and intensity through several 6-hourly analysis cycles. We find that such vortices are more common in the Southern Hemisphere than the Northern Hemisphere, and occur most often during the winter (SH) and spring (NH). Cyclonic vortices are more frequent than anticyclonic vortices.

Ongoing work will focus on the spatial distribution of coherent vortices with respect to the Earth and to the instantaneous position of the jet stream. We shall also investigate the mechanisms for formation, maintenance, and destruction. Preliminary results indicate that most cyclonic vortices do not dissipate in situ, but instead are ripped apart by an increase in the background horizontal deformation.

JSP39/W/15-B3 1110

DYNAMICS OF ROTATION, CYCLONES AND SCALING GYROSCOPES CASCADE

I. TCHIGURINSKAYA (E.E.&S. Dept., Clemson University, 342 Computer Court, Anderson, SC29625, USA, E-mail: iouliat@clemson.edu); D. Schertzer (L.M.M., Université Pierre et Marie Curie, 4 Place Jussieu F-75252 Paris Cedex 05, France); S. Lovejoy (Physics Dept., McGill University, 3600 University st., Montréal, Qué., H3A 2T8, Canada)

New experimental data on the structure of the atmospheric boundary layers have indicated that very often despite strong mixing, there are coherent structures. A visual confirmation of the occurrence of such structures in atmospheric boundary layer is given by the ordered "cloud streets" structures observed in photographs of the earth's cloud cover. The stratification of such boundary layers is rather extreme, since the horizontal extension of these structures reach easily hundreds of kilometers, whereas their heights do not exceed 3 kilometres. The expeditions Typhoon 89-91 over South China Sea lead to investigate the stratification, including situations on the periphery and in the eye of tropical cyclones. To understand this phenomena on the theoretical level, we developed the dynamical space-time cascade model for the velocity field, named the Scaling Gyroscopes Cascade. This model was obtained by partial truncations of direct interactions of the Navier-Stokes equations. And it based on the analogy between the Lie structure of the equations of perfect fluid motion inside of rotating ellipsoid and the equations of a rigid body rotation with fixed point (the gyroscope). The effects of viscosity are obtained due to cascade structure of the model. We present high resolution (e.g. scale ratio 2**20) and long time simulations of this model, leading to universal multifractal behavior which exponents are very close to the empirical estimates for the stratified tropical atmosphere data of Typhoon 89-91 expeditions. It also displays a first order multifractal phase transition with associated self-organized criticality of stratified structures.

JSP39/W/32-B3 1130

THE COUNTER-PROPAGATING ROSSBY WAVE PERSPECTIVE ON INSTABILITY: APPLICATION TO REALISTIC JETS

John METHVEN (University of Reading, UK); Eyal Heifetz (Tel-Aviv University, Israel) Brian Hoskins (University of Reading, UK); Craig Bishop (Penn-State University, USA)

Bretherton's (1966) view of baroclinic instability as the interaction of two counter-propagating Rossby waves is extended to general zonal flows for which potential vorticity (PV) is conserved. The CRWs are constructed from a growing normal mode and its decaying conjugate so that the material displacements associated with each of the waves (a) have no zonal tilt (b) are orthogonal with respect to each other when density weighted, and (c) are localised in regions of large, opposing PV gradients by the requirement that the CRW displacements are also orthogonal with respect to wave activity. Although each CRW could not continue to exist alone, they can together describe the time development of any zonal flow whose initial conditions can be described by the pair of normal modes.

The structure and interaction of these CRWs is shown for growth on a realistic jet with a tropopause, for a range of zonal wavenumbers, m. It is found that one CRW is always located on the ground where strong temperature gradients act as large negative PV gradients. The location of the upper CRW's maximum wave activity density varies with m; for m > 7 it lies just above the steering level. Both CRW structures are very similar to those from the Charney model. This structure is also seen in inverse Ertel PV, (1/P)', whilst Ertel PV perturbations, P', are large only near the tropopause, even though the tropopause plays virtually no role in the instability. As m decreases, the depth-scale of the CRWs increases and there is a secondary maximum in upper CRW amplitude at the tropopause. For m < 5 this tropopause perturbation begins to dominate the associated flow and the picture of instability resembles the Eady model. Some examples of finite time baroclinic growth, described by CRWs, are given including development from initial conditions where upper and lower level perturbations are dominant.

JSP39/W/10-B3 1150

EFFECTS OF NEWTONIAN COOLING IN THE SEMIGEOSTROPHIC FRONTOGENESIS PROBLEM

Georgy I. BURDE (Ben-Gurion University, J. Blaustein Institute for Desert Research, Sede Boker Campus 84990, Israel, email: georg@bgumail.bgu.ac.il)

Frontogenesis - the formation of fronts -is a fundamental fluid dynamical problem: it is remarkable that air flow should generate such sharp zones, with horizontal scales in the atmosphere of only a few tens of kilometers, rather than maintaining a weaker, broader thermal contrast between equator and pole. Because rain and snowfall are concentrated at fronts, the problem is of considerable meteorological importance. The geostrophic momentum model of frontogenesis introduced by Hoskins and Bretherton in 1972 appears to capture basic physical aspects of frontogenesis. Since then semigeostrophic models have remained a mainstay of frontogenesis theory and numerous research papers have emphasized various aspects of the theoretical model. In the present paper, the semigeostrophic, two-dimensional, uniform potential vorticity, Eady-wave model developed by Hoskins and Bretherton has been generalized by embedding the Newtonian (radiative) cooling. The observation, that with this modification the potential vorticity varies in a known manner following fluid particles, has provided the retention of the advantage of analytical representation of the solution inherent in the traditional Eady-wave model. A principal result of this study is that the effects of radiation may critically change the dynamics of semigeostrophic frontogenesis promoted by Eady wave instability even though the radiative damping timescale exceeds significantly the flow time scale. In particular, the formation of a frontal discontinuity from short wave initial disturbances, with the wavenumbers beyond the short wave cutoff of the adiabatic theory, becomes possible.

JSP39/W/41-B3 1210

INSTABILITY OF ZONAL FLOWS IN ROTATING SPHERICAL SHELLS, AN APPLICATION TO JUPITER

Johannes WICHT (Institut fuer Geophysik, Universitaet Goettingen, Herzberger Landtrasse 180, 37075 Goettingen, Germany, email: wicht@willi.uni-geophysik.gwdg.de); Keke Zhang, Chris Jones (both at Mathematics Department, Exeter University, Exeter EX4 4QE, UK, emails: CAJones@maths.exeter.ac.uk and KZhang@maths.exeter.ac.uk)

Measurements from the Galileo probe promote the idea that the zonal winds are deep rooted. Considering Jupiter's high rotation rate and the possible vigor of the convection, it seems indeed likely that the whole outer molecular H/He layer is involved in the flows. Three-dimensional simulations of convection in rotating spherical shells have shown, that these systems are capable of producing strong zonal flows that show as a banded structure on the surface.

Assuming that the primary flows are geostrophic, and that the banded surface structure stretches right through the molecular He/H layer, we examine under which conditions such flows would be stable. As a first step we consider the linear stability of different prescribed banded zonal flows in a rotating spherical shell. Incompressibility is assumed for simplicity, the boundary condition is stress free. We compare solutions for two aspect ratios, for the Earth's outer core and Jupiter's molecular He/H layer, and two Taylor numbers (T=10**4 and T=10**8).

Convective and shear flow instabilities compete in our system. While the convective instability is of the well-known columnar structure, we find two different shear flow instabilities. At the smaller Taylor number it is similar to the Taylor-Couette instability in rotating annuli, but the flow remains close to geostrophy for the higher Taylor number. Both shear flow instabilities break the azimuthal symmetry.

Since thermal and shear flow instabilities can have different preferred azimuthal wave numbers, we expect an interesting interaction of the two scales in the non-linear regime, which has not been reached so fare. We will try to extrapolate our results to Jupiter, in spite of the numerical difficulties in reaching appropriate parameters.

Wednesday 28 July PM

Presiding Chair: P.A. Davies., Univ. of Dundee, Dept. of Civil Engineering, Dundee, (UK)

JSP39/W/13-B3 Poster 1400-01

OUTER CORE FLOW DRIVEN BY LATERAL HEAT-FLUX VARIATION AT THE CMB

Steven John GIBBONS (School of Mathematical Sciences, University of Exeter, Exeter EX4 4QJ, UK, email: sgibbons@maths.ex.ac.uk)

Moving core fluid maintains an isothermal core-mantle-boundary (CMB) and so lateral variations in the CMB heat flow result from mantle convection. Such variations will drive thermal wind type flows, even in a layer of stable density stratification at the top of the core. Such flows may contribute to the magnetic secular variation. Large scale equatorially symmetric (ES) heat flux variations at the outer surface of a rapidly rotating spherical shell drive deeply penetrating flows which are strongly suppressed in stratified fluid. Smaller scale ES heat flux variation drives flow less dominated by rotation and so less inhibited by stratification.

Equatorially anti-symmetric flux variations drive flows an order of magnitude less energetic than those driven by ES patterns but, due to the nature of the Coriolis force, are less suppressed by stratification. The response of the rotating core fluid to a general CMB heat flow pattern will then depend strongly on the subadiabatic temperature profile.

JSP39/E/04-B3 Poster 1403-02

DYNAMICS OF STRATIFIED, FLUTE STRUCTURED CHARGED PARTICLE JETS AND WAKES IN THE IONOSPHERE UNDER THE GEOMAGNETIC FIELD EFFECT

Maxim G. PONOMARJOV (Physics Dept., State Academy of Aviation Technology, Promyshlennaya str.1 (Box 22), 152300 Tutaev, Yaroslavl region, Russian Federation, email: pmg@univ.uniyar.ac.ru)

General methods are proposed for simulations of time-dependent charged particle jets and wakes in the geomagnetic field based on the kinetic approach. The Boltzmann equation solved taking into account the ambient electric and magnetic fields. Using this solution the analytical results are obtained, which describe developing geomagnetic field aligned stratification of charged particle jets in the ionosphere. For geomagnetic field aligned drifting velocity of jets, the formation of flute structures along the edges of the jets is obtained analytically. Using the image method (see [1], [2]) the wakes of different objects in the satellite altitude regime (above 200 km) are simulated. Different kinds of interactions of the ambient particles with object surfaces are considered as absorption, direct and diffuse reflection. The analytical results describe the flute structure of wakes of objects in motion along the geomagnetic field and stratification of wakes in different cases.

References: [1] Ponomarjov M.G., Imaginary emission method for modelling disturbances of all magneto-plasma species: Reflecting and absorbing objects in motion through rarefied plasma at different angles to the ambient magnetic field, Physical Review E, 54, 5591-5598,1996;

[2] Ponomarjov M.G., Disturbances of the ambient magneto-plasma due to interactions with the object surfaces. Imaginary emission method. Far-wake of objects moving through a rarefied plasma at different angles to the ambient magnetic field, Planetary and Space Science, 43,1419-1427, 1995.

JSP39/W/45-B3 Poster 1406-03

AN ENSEMBLE OF RANDOM-PHASE INTERNAL GRAVITY WAVES IS CONSIDERED IN THE DYNAMICAL FRAMEWORK OF THE EULER - BOUSSINESQ EQUATIONS

Vladimir ZEITLIN (Laboratory of Dynamical Meteorology, France, email: zeitlin@lmd.ens.fr)

An ensemble of random-phase internal gravity waves is considered in the dynamical framework of the Euler - Boussinesq equations. For flows with zero mean potential vorticity a kinetic equation for the mean spectral energy density of the waves is obtained under hypothesis of Gaussian statistics with zero correlation length. Exact stationary scaling solutions of this equation are found for almost vertically propagating waves. The resulting spectra are anisotropic in vertical and horizontal wave numbers. For flows with small but non-zero mean potential vorticity, under the same statistical hypothesis applied to the wave part of the flow, it is shown that the vortex part and the wave part decouple. The vortex part obeys a limiting dynamics equation exibiting vertical collapse and layering which contaminates the wave-part spectra. Relation of these results to the in situ measurements is discussed.

JSP39/E/23-B3 Poster 1409-04

A FLUID PULSE DISTORTION ON THE SURFACE OF THE OCEAN AS A SOURCE OF WATER MOTION NEAR THE BOTTOM

Tatiana DEMIDOVA, Nikolai Korchagin (both at Department of Physics, P.P. Shirshov Institute of Oceanology, Moscow 117218, Russia, email: evita@redline.ru)

A model of propagation of a pulse of a surface horizontal water distortion (caused by a non-stationary wind field) from the surface to the bottom in a stratified medium is considered in the frameworks of linear theory of internal waves in a long-wave approximation. In the model, a three-layer fluid is presented as a two-step structure of the density profile with alteration of density discontinuities and homogeneous fluid layers. Rather simple equations for velocities for the first three modes (the barotropic and two baroclinic modes) are derived. The results of calculation show that the presence of sharp discontinuities in density leads to interrelation between the surface and bottom current fields. In this case, the transfer of the surface pulse into deep water is performed by internal wave.

The model of the propagation was tested using experimental moored and cast data as well as time distribution of the surface wind stresses during storms in winter of 1992 at the shelf and slope off the Sakhara coast. Well-developed surface and bottom boundary layers were approximately of the same thickness. High frequency maxima were revealed in energy spectra of the near bottom velocities. Whereas the peak at a period of about 24 min corresponded to the typical buoyancy frequency at the site, the model of the pulse propagation from the surface to the bottom allowed us to explain the peaks at periods 60-70 min. It shows that the model discussed describes one of mechanisms of the increase of current velocities in near-bottom layers.

JSP39/E/21-B3 Poster 1412-05

DYNAMICS OF HYDROTHERMAL TURBULENT JETS IN A STRATIFIED FLUID

Nikolay KORCHAGIN (P.P. Shirshov Institute of Oceanology, Russian Academy of Sciences. 36, Nakhimovskyii prospect, 117218, Moscow, Russia, email: niknik@sio.rssi.ru)

A new analytical method to the problem of the closure of the system of integral equations (SIE) describing the dynamics of a buoyant turbulent jet (BTJ) in a stratified fluid is suggested. As a result an analytical expression of the Taylor's "entrainment constant" was obtained from the parameters of the jet flow and a background water stratification. On the other hand, the suggested method of the SIE closure also describing the mechanism of the entrainment of a background fluid into a turbulent area of the same fluid. The numerical model of the SIE solution was made by Runge-Kutta method of the fourth order accuracy. The validity of the numerical model of the BTJ was carried out with using the measurement data of the characteristics of high temperature hydrothermal flows (from "black smokers") and background bottom waters in two ocean regions: the Mid-Atlantic Ridge and the East-Pacific Rising. The model construction of the BTJ was suggested, the use of which in the SIE allowed to obtain the analytical solution of SIE and expression of the maximum height for the rising hydrothermal jet and another characteristics.

JSP39/E/06-B3 Poster 1415-06

OCEANIC UPWELLING AND DOWNWELLING FORCED BY A TROPICAL CYCLONE

Shin-Ichi SUZUKI (Ocean Research Institutes, University of Tokyo, 1-15-1, Minamidai, Nakano, Tokyo, Japan; E-mail: suzuki@ori.u-tokyo.ac.jp); Hiroshi Niino (email: niino@ori.u-tokyo.ac.jp); Ryuji Kimura (email: kimura@ori.u-tokyo.ac.jp)

While it is generally believed that an oceanic upwelling under a tropical cyclone is caused by the Ekman pumping, previous numerical simulations show that the center of the upwelling is located in the backside of a moving tropical cyclone. Furthermore, our recent numerical simulation with a better vertical resolution and a recent oceanic observation show that even a downwelling exists in the forward side of the cyclone center. The cause and the mechanism of the forward side downwelling and the backside upwelling are investigated by means of a linear theory of a two-dimensional viscous rotating stratified ocean, which is subjected to a prescribed wind stress coresponding to a moving cyclone. The results show that, when the moving speed U of the cyclone is greater than 10 m/s, the forward side downwelling and backside upwelling take place. The mechanism that causes the vertical motion is further investigated by examining oceanic responses to a moving sinusoidal wind stress with a wavenumber k. It is found that, when a Rossby number defined by Ro = Uk/f is less than 1, the vertical motion is mainly caused by the conventional Ekman pumping, where f is the Coriolis parameter. When the Rossby number is greater than 1, on the other hand, the vertical motion is caused by a horizontal divergence of inertial oscillations forced by the wind stress. The latter mechanism is responsible for the forward side downwelling and backside upwelling.

JSP39/E/07-B3 Poster 1418-07

NON-LINEAR DYNAMICS OF LARGE-AMPLITUDE INTERNAL SOLITARY WAVES

Tatjana TALIPOVA, Efim Pelinovsky (Laboratory of Hydrophysics and Nonlinear Acoustics, Institute of Applied Physics, 46 Uljanov Str., Nizhny Novgorod, 603600, Russia, email: tata@appl.sci-nnov.ru); Alexey Slunyaev (Advanced School of General and Applied Physics, Nizhny Novgorod State University, 46 Uljanov Str., 603600 Nizhny Novgorod, Russia; email: avs@appl.sci-nnov.ru)

The nonlinear dynamics of large-amplitude long waves in a stratified ocean is studied. Theoretical model is based on the generalised Korteweg - de Vries equation incorporated the effects of the quadratic and cubic nonlinearity. Coefficients of the quadratic and cubic nonlinearity in this equation depend on the density and shear flow stratification and may have either sign. In particular, the coefficient of the cubic nonlinearity is negative for internal waves in the stratified ocean with one-peak distribution of the Vaisal frequency. If the high-order dispersion can be ignored (the corresponding conditions are discussed) the generalised Korteweg - de Vries equation is reduced to the Gardner equation which is integrable nonlinear system. It admits the existence of a large-amplitude "thick" soliton. The processes of the interaction and generation of solitons, including the "thick" solitons are studied. The evolution of the initial impulse disturbance is reported. It is shown that during the evolution process the solitons of the opposite polarity are appeared on the crest of the "thick" soliton.

JSP39/W/02-B3 Poster 1421-08

HYSTERESIS OF WAVE SOLUTIONS IN THE QUASI-GEOSTROPHIC POTENTIAL VORTICITY EQUATIONS WITH A NONLINEAR BASIS STATE

Gregory M. LEWIS, Wayne Nagata (both of Institute of Applied Mathematics, University of British Columbia, Vancouver, BC, V6T 1Z4, Canada, email: lewis@math.ubc.ca and nagata@math.ubc.ca)

We analyze double-Hopf bifurcations observed in the two layer quasi-geostrophic potential vorticity equations with a non-linear basic state. This bifurcation occurs in the transition from the axisymmetric to the wave regimes when the linear part (of the equations) has two complex conjugate pairs of eigen-values with zero real part. Using center manifold reduction and normal form theory, the behaviour of the full system of partial differential equations near the bifurcation point may be deduced from the two dimensional ordinary differential amplitude equations.

Since the non-linear basic state leads to a non-self-adjoint linear part, it is not possible to compute the eigenvalues and eigenfunctions analytically. Therefore, the linear part is discretized and the eigen-values and eigen-functions are approximated from the resulting matrix eigen-value problem. The projection onto the center manifold and reduction to normal form can be done analytically, thus, numerical approximations of the normal form coefficients are obtained upon approximation of the appropriate inner products.

The results show that, for the geophysically relevant range of parameters, two periodic solutions (travelling waves with different wave numbers) are simultaneously stable for certain parameter values. This result, which is observed for several wave number pairs, indicates hystersis of the wave solutions. The quasi-periodicity, which is possible in these types of bifurcations, is not observed. It is straightforward to apply this analytical/numerical method to more complicated models. In particular, preliminary results will be shown of the analysis applied to a model of the differentially heated rotating annulus experiment.

JSP39/W/33-B3 Poster 1424-09

LABORATORY EXPERIMENTS ON ROTATING COMPOSITIONAL CONVECTION AT LOW EKMAN NUMBERS

Sabine CLASSEN (Institut fuer Geophysik, Herzbergerlandstr. 180, 37075 Goettingen, Germany, email: sac@willi.uni-geophys.gwdg.de); Moritz Heimpel (Department of Earth and Space Sciences, UCLA, 595 Circle Drive East, Los Angeles, CA,USA 90095-1567, email: heimpel@zephyr.ess.ucla.edu)

The onset and structure of compositional convection in a rotating system are investigated experimentally. A vertically oriented cylindrical annulus filled with NH_{4}--H2O solution is cooled from the bottom and can be rotated about its axis at rates ranging up to 10.5 rad s^{-1}, corresponding to Ekman numbers down to 7.5 \times 10^{-6}. The Coriolis force has a strong effect on the structure of plumes above the mush-liquid interface. Helical motion of the conduit, which is weakly developed in the non-rotating case, is amplified by Coriolis forces that twist the plume conduits to lie nearly horizontally. This results in secondary plumes (or blobs) that rise from the sub-horizontal primary plume conduits. The rotation has a negligible influence on the solidification process. The main crystallization and convection features we find in the non-rotating system are retained in the presence of rotation. However, rotation seems slightly to speed up the transport of latent heat which controls the growth rate cause the growth rate coefficient increases with the rotation rate.

JSP39/W/23-B3 Poster 1427-10

TURBULENT CONVECTION FROM ISOLATED SOURCES

Jordi COLOMER (Department of Physics, University of Girona, 17071 Girona, Spain); Boris M. Boubnov, A.M. Obukhov (Institute of Atmospheric Physics, Russian Academy of Sciences, 109017 Moscow, Russia); Harindra J.S. Fernando (Department of Mechanical and Aerospace Engineering, Environmental Fluid Dynamics Program, Arizona State University, Tempe, AZ 85287-9809, USA)

Laboratory experiments were conducted to investigate the evolution of a dense turbulent plume, specified by its buoyancy flux and source diameter D, issuing into a homogeneous environment. The study was motivated by its applications to geophysical, environmental and engineering flow situations. Special attention was given to study the evolution of plume following its initiation and the flow near the source. At short times, the descent of the plume front can be treated as one-dimensional with negligible lateral (entrainment) mean flow, the plume growth mechanism being the encroachment of underlying non-turbulent fluid. At larger times, the flow achieved a quasi-steady state, in which the plume width first decreases (region I) and then increases (region II). The quasi-steady state velocity and buoyancy measurements in region I showed that they are strongly influenced by the lateral entrainment (and hence D), and thus classical free convection scaling is inapplicable to such flows. On the other hand, at large z/D (in region II), the velocity and buoyancy scalings tend to be independent of D and follow point source scaling.

JSP39/W/12-B3 Poster 1430-11

LARGE EDDIES BEHIND MANOEUVRING SELF-PROPELLED BODIES IN A STRATIFIED FLUIDS

Sergey VOROPAYEV (Arizona State University, Tempe, AZ 85287-6106, USA and Institute of Oceanology, Russian Academy of Sciences, Moscow, 117851, Russia, email s.voropayev@asu.edu); Ben McEachern (Arizona State University, Tempe, AZ 85287-6106, USA)

A large number of studies have been reported on wakes in stably stratified fluids either with towed or self-propelled bodies of different shapes. Although these studies have documented many interesting phenomena related with the wake signature, no studies have considered large eddies that may form in the late wakes during the manoeuvring of such bodies. When a self-propelled body makes a manoeuvre (e.g., accelerates or changes its direction of motion), significant horizontal momentum is transported to the surrounding fluid. Our experiments show that in a stratified fluid this may lead to the formation of unusually large eddies, which are much larger and have substantially different characteristics than those produced in the late wake during steady motion. A theory is proposed to explain this effect, and estimates show that when an oceanic submerged vehicle changes its velocity by as little as 10% or its direction of motion by roughly 5 degrees, large structures of characteristic size 1-2 km and with decay times of several days may be expected. Such effect may have potentially important applications and have not been studied previously. This study was supported by NOAA and ONR. For more detail see [1].

1. Voropayev, S.I., McEachern, G.B., Fernando, H.J.S. and Boyer, D.L. 1999. Large vortex structures behind a manoeuvring body in stratified fluids. Phys. Fluids (in press).

JSP39/W/35-B3 Poster 1433-12

SELF PROPAGATING QUASI-MONOPOLES IN ROTATING FLUID

Recent results, based on the high-resolution satellite data, demonstrate that a significant dipolar component is present, for example, in most of the Gulf Stream rings. Taking into account the importance of rings and other quisi-monopolar eddies in large scale mixing in the ocean, the need for the developing of models for compact self-propagating vortices becomes obvious. In a recent paper (Stern & Radko, 1998, JPO) an attempt was made in this direction and some basic properties of single self-propagating quasi-monopolar vortex were predicted theoretically and numerically. At present detailed field data are still absent, and the only way to verify the model predictions is to conduct laboratory experiments. The main aim of this communication is (i) to present and discuss the results of laboratory experiments on the formation and dynamics of self-propagating quasi-monopolar vortices, with relatively large angular momentum and small linear momentum, which were generated in a homogeneous rotating fluid of constant depth (f-plane), (ii) to make some comparison with the theoretical predictions. The results of our experiment show that such a vortex (i) exists and can be easily generated, (ii) is stable and can propagate a significant distance from the origin, (iii) survives after a non-elastic collision with a vertical wall and (iv) its structure and behavior are very similar to that predicted by Stern and Radko.

JSP39/W/30-B3 Poster 1436-13

BASIC SETS OF STRATIFIED FLOW EQUATIONS INVARIANTS

Vasiliy G. BAYDULOV, Yuli D. Chashechkin (both at the Laboratory of Fluid Mechanics of the Institute for Problems in Mechanics of the RAS, Moscow, prospect Vernadskogo, 101-1, 117526, Russia,

E-mail: bayd@ipmnet.ru)

We investigated invariant properties of the basic sets of hydro- thermodynamic equations by Lie groups methods. For one component media group classification on stratification type has been performed. Set of equations of double diffusive convection also was classified with respect to type of state equation and thermodynamic properties of all dissipative coefficients. Obtained classifications make possible to compare invariant properties and mutual reducibility of basic 1D, 2D and 3D models of stratified flows. Several types of stratification that are characterised by the widest group of symmetry are distinguished. Invariant properties of governing equations with and without diffusion effects are the same only in case of linear stratification. A wide set of non-trivial groups of self-similarity is constructed when dissipative coefficients (viscosity, diffusivity, thermal conductivity) depend on thermodynamic parameters of the medium. On basis of this groups new exact and self-similar solutions equations of double diffusive convection are constructed. Using the obtained groups to reduce the number of independent variables allows to simplify the search of numerical solutions governing equations. Examples and comparison with laboratory experiments are given.

JSP39/E/25-B3 Poster 1439-14

TURBULENCE MODELING IN COMPRESSIBLE SHEAR LAYERS

Suresh CHANDRA (Department of Mechanical Engineering, North Carolina A&T State University, Greensboro, NC 27411, USA, Email: chandra@ncat.edu)

For high speed shear layers, variable density extensions of standard incompressible turbulence models have not proven to be adequate in explaining the experimentally observed reduction in growth rate with increase in the convective Mach number. Turbulence modeling for compressible flows has to account for additional correlations involving both thermodynamic quantities and the fluctuating dilatation. In recent years, Sarkar et al. suggested that, in addition to modeling the pressure dilatation, another dilatational correlation (the compressible dissipation) should be considered because of the enhanced dissipation known to be present in compressible turbulence. Specifically, this compressible dissipation correction is proportional to the second and fourth powers of the turbulent Mach number, which is defined in terms of the turbulent kinetic energy and the local speed of sound. Narayan and Sekar have used the compressibility-corrected model - limited to the second power of the turbulent Mach number - with the SPARK code for the computation of high speed reacting layers and have obtained satisfactory agreement with some of the available data.

The simple algebraic compressibility model by Sarkar et al. has been modified to include a fourth order turbulent Mach number term. Comparison of the predictions with results of several analytical models and experimental work has been carried out; both agreement and discrepancies are analyzed. The modified turbulence model presented here has the potential of resolving fluid flows related to Earth's atmosphere as well as internal flows encountered in airbreathing hypersonic vehicles.

JSP39/W/01-B3 Poster 1442-15

FINE STRUCTURE OF A FLOW AROUND STARTING OBSTACLES IN A

STRATIFIED FLUID

Yuli D. CHASHECHKIN, Vasiliy G. Baydulov, Vladimir V.Mitkin (all at the Laboratory of Fluid Mechanics of the Institute for Problems in Mechanics of the RAS, Moscow, prospect Vernadskogo, 101-1, 117526, Russia, E-mail: chakin@ipmnet.ru)

We study analytically and experimentally onset of a stratified liquid motion around resting and starting plane, channel walls, cylinder and sphere. Analytical solutions are obtained by perturbation theory methods, visualised and compared with laboratory experiments data. Near a body at rest solutions describe diffusion induced transient internal boundary layer which is characterised by different scales of spatial variability of velocity and density or salinity. In limiting cases all solutions are uniformly matched among themselves and with well-known exact solutions. Near a starting body besides the internal boundary layer regular large scale convective motion is formed. Geometry of the flow around starting cylinder and sphere are described by simple trigonometrical functions. Pattern of flow is visualised by different Schlieren methods. Parameters of disturbances are measured by conductivity probes and density markers (vertical wakes past small falling crystal of salt or sugar). Profiles of horizontal component of velocity ahead of the body and perturbations of density gradient are fixed and compared with calculations. Theoretical and experimental results are in good agreement among themselves even beyond of the range of formal applicability of asymptotics.

JSP39/W/09-B3 Poster 1445-16

INTERNAL WAVES AND INTERNAL BOUNDARY CURRENTS PAST A UNIFORMLY MOVING CYLINDER IN A CONTINUOUSLY STRATIFIED LIQUID

We investigate experimentally fine structure of flow around circular horizontal cylinder uniformly moving in a continuously stratified liquid. It was found that the basic elements of motion namely upstream disturbance, internal waves and downstream wake with imbedded or soaring vortices are presented at any large or small values of internal Froude number. General variations of a density containing mean displacement and wavy oscillating components as well as density profiles on the boundary of a density wake are measured by a conductivity probe. In a regime of narrow turbulent wake effect of recurrent and reconnection of downstream adjoined (lee) waves is found. In this regime regular anti-symmetric pattern of lee waves is consequently replaced firstly by irregular waves emitted by vortexes inside the turbulent wake and later by system of symmetric waves. Two kinds of thin interfaces inside and outside downstream wake are identified as a new eigen-form of a stratified fluid motion namely as internal boundary currents. These internal soaring boundary currents manifest itself as a surfaces of discontinuity in a wave part of the density gradient field without any features on their leading and trail edges. Sharp interfaces inside a wake are formed due to separation of internal boundary layer on the cylinder surface or directly inside the decaying stratified turbulence.

JSP39/W/22-B3 Poster 1448-17

SIDEWALL DOUBLE DIFFUSION CONVECTION IN A WEAK GRADIENT

Yuli D. CHASHECHKIN, Anatoliy V. Kistovich, Vladimir V. Levitskiy (all at the Laboratory of Fluid Mechanics of the Institute for Problems in Mechanics of the RAS, Moscow, prospect Vernadskogo, 101-1, 117526, Russia, E-mail: chakin@ipmnet.ru)

We study theoretically and experimentally onset of a periodic layer structure formation near a heated or cooled sloping wall in a continuously strong or weakly stratified liquid. Analytically it is shown that side-wall heat flux leads to the formation of internal boundary current on a heated surface and of a fast propagating front of injection in a fluid interior. General properties of a double diffusion convection equations are examined by Lee groups method. Dependence of flow geometry parameters above near a point, cylinder or plane heat source on input buoyancy flux and scale of stratification is studied. Parameters of zero frequency internal waves emitted by convection cells in environment fluid are calculated in a linear approximation. Experimentally convection near a sloping wall inclined under the angle 90, 75, 60, 45, 30, 15 degrees to the horizon is studied. Both in a strong (buoyancy period Tb < 10 s) and in a weak (Tb > 20s) stratified liquid the height of the cell is proportional to the height of potential rise of a heated fluid particle. The fitting coefficient increases with the slope under the heater and weakly depends on it above the heater. The interval of flow formation increases near the critical Rayleigh number. The thickness of interfaces between cells does not depend on the initial density gradient value. In a wide range of flow parameters the number of interfaces is duplicated. Additional interfaces are formed on the upper boundary of inflowing wedge of a cold fluid. Theoretical and experimental results are in a good agreement.

JSP39/W/44-B3 Poster 1451-18

COMPUTATIONS OF AXISYMMETRIC EIGENMODES IN A STABLE STRATIFIED ROTATING SPHERICAL SHELL

Boris DINTRANS (Observatoire Midi-Pyrenees, France, email: dintrans@obs-mip.fr)

We present computations of axisymmetric eigen-modes in a stable stratified rotating spherical shell using Boussinesq approximation. We first develop a geometric formalism based on the iteration of the underlying characteristics which propagate in the hyperbolic region. Periodic orbits and quasi-periodic orbits have been then found. Computing the corresponding eigen-modes, we prove the link that exists between the characteristic pattern and her associated non-adiabatic structure. Some important spectral consequences have been then deduced both in the adiabatic and non-adiabatic cases.

JSP39/E/21-B3 Poster 1454-19

DYNAMICS OF HYDROTHERMAL TURBULENT JETS IN A STRATIFIED FLUID

Nikolay KORCHAGIN (P.P. Shirshov Institute of Oceanology, Russian Academy of Sciences. 36, Nakhimovskyii prospect, 117218, Moscow, Russia, email: niknik@sio.rssi.ru)

JSP39/W/24-B3 Poster 1457-20

ABOUT ONE MODELS OF CHAOTIC ADVECTION IN BAROTROPIC BACKGROUND CURRENT

Vadim Kozlov, Konstantin KOSHEL (both at Pacific Oceanological Institute, FEB RAS, 43 Bultiyskay str., Vladivostok, 690068, Russia, Email: wave@online.vladivostok.ru)

The barotropic model of a chaotic advection in background current is considered [Kozlov V.F., Koshel K.V. A barotropic model of a chaotic advection in background currents [Izv. RAS, FAO, 1998, V.34, in press; Kozlov V.F., Background currents in geophysical hydrodynamics, Izv. RAS, FAO, 1995, V.31, N2, pg. 245-250], representing a sum fixed rotational planetary-topographical and non-stationary irrotational flowing component in a half-disk with a linear contour of bottom and system a radiant - drain in angular points of the boundary. With the help of numerical experiments the process of passive markers passing out from a vortex region in flowing in an outcome of periodic oscillations of the consumption is investigated. The influence of parameters of oscillation (frequency, amplitude, phase) on a velocity and degree of washing away of markers is investigated. The evaluation of purely chaotic properties of the open system is hampered by standard methods (Lyapunov indexes, Poincare sections) in view of finite life time of trajectories with the most irregular behavior. For a research of zones with a regular and chaotic behavior and evaluation of a degree of intermixing is offered to use distribution of washing away time for markers uniformly distributed on basin. The similar approach is compared to evaluations obtained with the help of Lyapunov indexes and Poincare sections calculated for final time. Is concluded availability of chaotic intermixing in the given system.

JSP39/E/01-B3 Poster 1500-21

SIMILARITY SOLUTIONS FOR TIME-DEPENDENT HORIZONTAL CONVECTION IN A STRATIFIED FLUID SUBJECT TO A DIFFERENTIAL COOLING FROM BELOW

Atsushi MORI (Department of Astronomy and Earth Science, Tokyo Gakugei University, Koganei-City, Tokyo, 184-8501, Japan, email: mori@buran.u-gakugei.ac.jp); Hiroshi Niino (Division of Marine Meteorology, Ocean Research Institute, University of Tokyo, Minamidai, Nakano-Ku, Tokyo, 164-8639, Japan, email: niino@ori.u-tokyo.ac.jp)

Time evolution of a horizontal convection which is caused by cooling a half of the bottom boundary of a viscous stratified Boussinesq fluid is investigated both theoretically and numerically.

In the most general situation, the evolution of the convection consists of three distinct stages: During the first stage, the horizontal diffusion of heat is dominant over the horizontal advection. During the second stage, the stratification is still less important. The flow is non-linear and can be regarded as a gravity current in which the vertical scale is determined by the diffusion length scale. Finally, during the third stage, the stratification becomes important and the flow is described by a linear dynamics.

A similarity solution of the time-dependent governing equations which describe the dynamics of each stage has been discovered. The solution for each stage is found to be only a function of the Prandtl number after relevant scalings of the variables are introduced. Numerical experiments with fully non-linear governing equations have confirmed the validity of the similarity solution at each stage. These solutions, for example, are useful for understanding the formation mechanism of an atmospheric heat island circulation in which convections first develop from the two edges of heated area and penetrate into the center to form a steady state convection eventually.

JSP39/L/04-B3 Poster 1503-22

SIMILARITY CRITERIA FOR CONTINUOUSLY STRATIFIED FLOWS OVER OBSTACLES

Olga SHISHKINA (Institute of Applied Physics, RAS, 16 Uljanovsk st., Nizhni Novgorod, 603600 Russia, email: ols@hydro.appl.sci-nnov.ru)

The comparative analysis of wave-drag coefficient versus ‘internal’ Froude number for submerged and floating bodies moving uniformly in fluids with different stratification profiles showed similarity of functions Cx(Fi). An existence of linear dependence of Froude numbers Fi=U/(NR) for continuously stratified fluids (a linear stratification and a pycnocline) was observed.

Such a phenomenon explanation was based on analysis of wave processes following the drag coefficient increase in the mentioned Froude number range. The similarity of wave induction accompanying the sphere movement in stratified flows with single wave-guide profiles as well as the physical parameters describing those processes of different wave-guide to body heights ratio was observed. A Thorough analysis of known experimental data allowed of choose the phase velocity coincidence of the first IW mode for both the linear stratification and the pycnocline to be the similarity criterion for internal wave processes. This provides an exact choice of the constant buoyancy frequency N corresponding to the pycnocline conditions.

The induced IW amplitude is proportional to the critical streamline displacement (z/R}=Fi) to interpret the physical meaning of the ‘internal’ Froude number for continuous stratification Fi£1 (z£R) as the relative amplitude of induced IW or as some kind of the performance coefficient for transformation of the flow energy to the energy of the internal waves.

This work was supported by the RFBR (grant no. 99-05-64394)

JSP39/W/26-B3 Poster 1506-23

STABILITY OF POLYNOMIAL FLOWS ON A SPHERE

Yuri SKIBA (Centro de Ciencias de la Atmósfera, Universidad Nacional Autónoma de México, Circuito Exterior, CU, México, D.F., C.P. 04510, México, email: skiba@servidor.unam.mx)

The stability of the Rossby-Haurwitz waves and Legendre-polynomial flows is considered in the framework of an inviscid incompressible fluid on a rotating sphere. A conservation law of arbitrary perturbations of such flows is obtained, and the whole space of the perturbations is divided into four invariant manifolds defined by means of Fjortoft's spectral number of the perturbations. Liapunov instability of non-zonal Rossby-Haurwitz waves of degree n>1 is shown, and its mechanism is explained. The behavior of the kinetic energy and enstrophy of the perturbations from invariant sets is also considered. In particular, a hyperbolic dependence between the energy and Fjortoft's spectral number of the perturbations is discussed.

In the case that the basic flow has the form of a Legendre polynomial of degree n, invariant sets of exponentially and algebraically stable infinitesimal perturbations are found. For the other part of infinitesimal perturbations, not belonging to such sets of stable perturbations, it is obtained a necessary condition for the exponential instability, which is more simple that the famous condition by Kuo for the zonal flows.

JSP39/W/03-B3 Poster 1509-24

DOUBLE-COMPONENT CONVECTION DUE TO THE DIFFERENT BOUNDARY CONDITIONS - REVIEW OF COMBINATIONS OF VERTICAL AND HORIZONTAL GRADIENTS

Naftali TSITVERBLIT (Tel-Aviv University, Tel Aviv, Israel)

The conceptual framework of conventional double-diffusive convection could apply directly to large-scale environmental processes through eddy double-diffusion---instabilities resulting from the unequal perturbed diffusion gradients forming due to the different boundary conditions. The original problem considered by Welander [Tellus 41A, 66 (1989)] is in this context analogous to the conventional diffusive regime, while the inverse type of stratification (Tsitverblit [Phys. Fluids 9(8) 2458 (1997)]) stands for the finger regime. This latter configuration describes the double component Langmuir circulation problem (Leibovich [Ann. Rev. Fluid Mech. 15, 391(1983)]) and, in combination with shear motion (induced, for example, by the horizontal non-uniformity in the temperature distribution), may be relevant to the ocean thermohaline circulation. Decrease of the ratio of the vertical and horizontal diffusion coefficients in this case eventually causes formation of multi-layered structures and may affect the criticality of bifurcations. Like in conventional double-diffusive convection, the instabilities also arise under a combination of the horizontal gradients. This was initially established in application to laterally heated stably stratified systems (Tsitverblit [Phys. Fluids, submitted]). Similar situation is expected when two equal (in terms of their effect on the density) and opposing horizontal gradients aintained by the different boundary conditions at the vertical walls are applied to the fluid being at rest between these boundaries. Trial computations with the equal diffusion coefficients indicated that, when the (joint) horizontal gradient (the mbox{Rayleigh} number) in this case becomes large enough, the onset of convection indeed takes place, and analysis of this problem with and without vertical stratification due to one of the components is to be reported. Combinations of the above effects arising when the gradients are applied between two inclined planes and their relevance to the ocean are also discussed.

Thursday 29 July AM

Presiding Chair: P.G. Baines., CSIRO, Div. of Atmospheric Research, Aspendale, (Australia)

JSP39/W/18-B4 0830

THE CURRENT STATE OF OCEANIC FORECASTING (DEEP AND COASTAL OCEANS) AND THE PROSPECTS FOR THE FUTURE

Prof. Allan R. ROBINSON (Harvard University, Cambridge, MA, USA)

During the last decade remarkable progress has been made in the ability to predict describe, model and forecast the physical synoptic-mesoscale state of the ocean over regional to basin scale domains. This is a result of a new understanding of the underlying dynamical principals and the development of methods significantly and substantially based on data assimilation. (Sub) meso scale phenomena have two important sets of time and space scales - evolutionary and event (scales of intermittency). Multi-scale non-linear interactions are key to determining the underlying dynamics and developing the concomitant predictive capability. Interesting issues relate to validation, verification, predictive capability and limits to predictability. Presently, critically important problems are associated with interdisciplinary dynamics and coupling. The accurate estimation of the physical-acoustical-optical-biological-geochemical ocean is now essential and feasible for modern ocean science.

Such multiscale and multidisciplinary estimation and prediction requires a new level of understanding of the multi-disciplinary hierarchy of complex coupled dynamical processes, with non-linearities and feedbacks which span a multitude of scales in time and space. Fundamental interdisciplinary problems of ocean science, long recognized as of great importance, are now tractable. As the scope of interdisciplinary ocean science expands over the next decade, there will be formidable and challenging research tasks acquired to address the scientific problems through a systems approach.

JSP39/W/25-B4 0910

FLUCTUATIONS OF SST AND CHL-A CONCENTRATION CAUSED BY BAROCLINIC INERTIA-GRAVITY WAVES

Roman GLAZMAN (Jet Propulsion Laboratory, Pasadena CA, 91109, USA,

email: reg@pacific.jpl.nasa.gov); Peter B. Weichman (Blackhawk Geometrics, Golden, CO 80401, USA, email: pbw@blackhawkgeo.com)

Satellite observations of sea surface temperature and chlorophyll-a concentration permitted detailed statistical analysis of spatial variations of these tracer fields in a broad range of scales. Estimated wave number spectra, on scales from a few to hundreds kilometres - as reported by many authors, display a rather unexpected behaviour: they strongly disagree with predictions of two-dimensional eddy turbulence theory (which requires the SST spectrum to roll of as k^(-1)) and they exhibit geographic and seasonal trends which have no simple explanation. Our attention is focused on possible effects of baroclinic inertia-gravity (BIG) wave motions on tracer fields. We present experimental and theoretical results indicating that observed variations are greatly affected by BIG waves, and classical eddy turbulence is not necessarily the main dynamical factor of tracer dispersion. Wave number spectra of BIG wave motions are presented along with observed spectra of SST and Chl-a fields. Typical rates of spectra roll-off, ranging between k^(-1.5) and k^(-3), agree with our theoretical predictions. The fact that BIG wave motions have such a profound effect on fluctuations of ocean tacers has important oceanographic implications.

JSP39/E/18-B4 0930

PARTICLE DISPERSION IN THE ATLANTIC

Michel OLLITRAULT (Laboratoire de Physique des Oceans, Ifremer, BP 70, 29280 Plouzane, FRANCE, email mollitra@ifremer.fr); Alain Colin de Verdiere, Celine Gabillet (both at Laboratoire de Physique des Oceans, Universite de Bretagne Occidentale, BP 809, 29280 Brest cedex, FRANCE, email acolindv@univ-brest.fr)

Absolute and relative dispersion of floats deployed in the Atlantic are estimated at the base of the main thermocline in the North Atlantic subtropical gyre and in the Brazil basin. The initial separations of the floats, a few kilometer to 20 kilometer are small compared to the eddy mesoscale but large compared to dissipation scales so that the results can be usefully compared with the scalings predicted from two dimensional turbulence theory. While the absolute dispersion usually computed from such data dominated by the lowest frequencies, the relative dispersion at scales less than the energy containing scale gives information on the high wave number end of the energy spectrum that is difficult to obtain otherwise. We compare the results of these oceanic field experiments with similar experiments carried out in the atmosphere, in laboratory experiments and in numerical experiments

JSP39/W/11-B4 0950

GENERATION OF STEP-LIKE STRUCTURE IN STRATIFIED ROTATED BOUNDARY LAYERS

Iossif LOZOVATSKY (Department of Mechanical and Aerospace Engineering, Arizona State University, Tempe, AZ, 85287-9809, USA; also at P.P. Shirshov Institute of Oceanology, Russian Academy of Sciences, Moscow, 117851, Russia, i.lozovatsky@asu.edu); Alexander Ksenofontov (Kabardino-Balkarski State University, Nalchik, Russia, ask@ns.kbsu.ru)

Generation of turbulent quasi-homogeneous layers separated by narrow density interfaces in a stratified shear flow with a decreasing buoyancy flux upon increase of stratification has been argued by Phillips (1972). All previous laboratory experiments and theoretical models, which have explored this problem, were limited by non-rotating flows. We added the Coriolis force to this study to verify a rotation impact on the generation, evolution and decay of the layered structure within the pycnocline under the influence of surface stress. It makes our prognostic modeling more geophysical applicable. We employed e-? and e-l models of non-stationary, stratified, planetary boundary layer equations (e is the turbulent kinetic energy, ? is the energy dissipation, l is a Richardson number dependent turbulent length scale). The numerical simulations clearly showed that the use of Ri-dependent eddy viscosity and diffusivity results in a formation of pycnoclines with a prominent fine structure. It was found that pre-existed background shear is an important external parameter which may influence the development of the step-like structure. Time scale of the fine-structure generation is strongly dependent on inertial oscillations. It was shown that thin homogeneous layers merge each other forming thicker steps owing to shear-induced turbulence at sharp density interfaces. The scales of the layers observed in simulations are close to those observed in the seasonal ocean pycnocline.

JSP39/W/31-B4 1010

THE SHEAR STRESS BETWEEN AIR AND WATER

John A. T. BYE (Faculty of Science and Engineering, Flinders University, GPO Box 2100, Adelaide, South Australia, 5001, Australia, email: John.Bye@flinders.edu.au); Jörg-O. Wolff (Antarctic CRC, University of Tasmania, GPO Box 252-80, Hobart, Tasmania, 7001, Australia, email: j.wolff@utas.edu.au)

Recently a new formulation for the surface shear stress between air and water, which is applicable in the Earth reference frame has been implemented in a quasigeostrophic general circulation model of the Antarctic Circumpolar Current. The results indicate that a momentum balance similar to that observed can be achieved with form stress, without the use of an unrealistically large bottom friction.

This shear stress is quite distinct from that measured in planetary boundary layer studies which use the local reference frame of the surface current. The implications of a shear stress which depends on a reference frame will be discussed. The presentation will be illustrated by experimental data obtained in a novel circular wind wave tank in which a fan drives a rotating air circulation, which generates a complementary water circulation beneath.

JSP39/W/17-B4 1050

A REGIONAL MODEL OF THE SEMI-DIURNAL INTERNAL TIDE ON THE AUSTRALIAN NORTH WEST SHELF

Peter E. HOLLOWAY (School of Geography and Oceanography, University College, University of New South Wales, Australian Defence Force Academy, Canberra ACT 2600, Australia)

The semi-diurnal internal tide on the Australian Northwest Shelf is investigated using a fully three dimensional, non-linear, free surface, hydrostatic, primitive equation numerical model, the Princeton Ocean Model. The model covers a domain 1700 by 700 km using a 4km horizontal grid and 51 vertical sigma levels. Forcing is via specification of the M2 surface elevation along the open boundaries with values taken from the FES-95 global tidal model. The barotropic tidal flow, generated from surface elevation gradients, interacts with the topography and produces an internal tide that is highly variable in space. Depth integrated baroclinic energy fluxes show regions of both onshore and offshore propagation and maximum of approximately 4000 W/m. Model predicted internal tide structures are compared to detailed in-situ observations and show reasonable agreement. The three-dimensional character of the topography is shown to be important in the generation process.

JSP39/E/12-B4 1110

KUROSHIO EDDIES IN THE LUZON STRAIT

Y. HSUEH (Department of Oceanography, Florida State University, Tallahassee, FL 32306-4320, USA, email: hsueh@re.ocean.fsu.edu)

Results from a high-resolution, limited area, primitive-equation model of the circulation of the Asian marginal seas show the Kuroshio bulges into the Luzon Strait and forms a loop current before resuming its northward course. The model run with monthly mean wind forcing indicates that the Southwest monsoon tends to reduce the bulge and the Northeast monsoon tends to promote its growth. In fact, model warm-core eddies often break away from the loop during the winter monsoon season. A reduced-gravity model is constructed to probe the dynamics of the eddy separation in the Luzon Strait. A solution is found in which anticyclonic eddies separate from the loop nearly periodically with a period predicted from an analytic solution. An important aside is the discovery that an anticyclonic eddy is often generated that moves downstream with the Kuroshio. The passage of this eddy produces Kuroshio variabilities detected in recent current measurements.

JSP39/E/14-B4 1130

MEAN FLOW GENERATION BY GEOMETRICALLY FOCUSED GYROSCOPIC WAVES

Leo MAAS (Netherlands Institute for Sea Research, PO Box 59, 1790 AB Texel, Netherlands,

email: maas@nioz.nl)

A homogeneous fluid in stationary rotation is stably stratified in angular momentum (increasing radially). When this dynamic equilibrium is periodically perturbed, gyroscopic (or inertial) waves result, that propagate through the fluid along an angle with the vertical determined by the ratio of perturbation frequency to twice the angular frequency. Due to this constraint, in an infinitely long channel, such waves are, upon reflecting from sloping side walls, subject to focusing, much like internal waves are. In a tank of finite azimuthal extent, this result can only be approximately true, as the circular current patterns, associated with the gyroscopic wave, need to accommodate The presence of the plane side walls. As long as the tank is much longer in along-slope than cross-slope direction, attractors might still arise, a suggestion confirmed by a laboratory experiment in which the perturbation is generated by modulating the angular velocity of the tank. Focusing gyroscopic waves lead to mixing of angular momentum at the location where the attractor reflects from the sloping wall. This predicts a mean flow above that location, confirmed by velocity measurements and dye spreading.

JSP39/W/38-B4 1150

HYDROTHERMAL PLUME FROM A WARM LUTOCLINE

Jordi COLOMER, Teresa Serra, Jaume Piera, Elena Roget, Xavier Casamitjana, (Department of Physics, University of Girona, 17071 Girona, Spain)

A hydrothermal plume with vertical and horizontal length scales of 18 and 300m approx., respectively, is generated by a warm lutocline at the mid-depths (29 m) of a karstic lake (lake Banyoles, situated in North-Catalonia, Spain). The rising particle-laden convective plume entrains colder hypolimnetic water and develops upward until it reaches the base of the seasonal thermocline. At the level of neutral buoyancy the plume spreads laterally, mainly to the northeast of the lake, as an hypolimnetic current. Measurements were carried out using a microstructure profiler, a laser in situ scattering and transmissometry probe, and water quality probes.

JSP39/W/16-B4 1210

DIAGNOSTIC MODEL ACCOUNT BAROTROPIC SPEED MODE OF BAROCLINIC OCEAN BY SATELLITE ALTIMETRY DATA

Sergey A. LEBEDEV (Geophysical Center of Russian Academy of Sciences, Molodezhnaya 3, 117296, Moscow, Russia, email: lebedev@wdcb.rssi.ru)

As differentiated satellite altimetry data assimilation the diagnostic analysis of ocean dynamics gives complete hydrodynamic picture on that moment of time or for that temporary interval, when appropriate measurements were made. The given approach permits better to realize as the simulated phenomena, and representation of initial information.

As an initial system of offered model equations are considered equations of ocean dynamics in quasigeostrophic approximation. The boundary condition on surface for vertical mode of speed fields is replaced by «firm cover» condition, and a condition of sliding without friction is at the bottom taken. Integrated stream function, which is necessary for account barotropic speed mode is searched as anomaly from mean season significance. It is reasonably safe to suggest that changes of baroclinic layer thickness are insignificant and density anomaly linearly change by vertical from surface significance to zero on the baroclinic layer bottom border.

The model verification was conducted on independent data: satellite altimetry (ERM mission GEOSAT) and hydrological data (experiment NEWFAEXP-88 the program «Sections») for polygon near island Newfoundland in March 1988. The mean fields of dynamic topography and integrated stream function were determined by known data file LEVITUS.

Thursday 29 July PM

Presiding Chair: C.N.K. Mooers., Univ. of Miami, RSMAS, Miami, FL, (USA)

JSP39/E/11-B4 Invited 1400

MECHANISMS FOR EDDY FORMATION IN DENSE DOWNSLOPE FLOWS IN ROTATING SYSTEMS

Peter G. BAINES (CSIRO Atmospheric Research, PMB No. 1, Aspendale, Australia 3195)

The flow of dense fluid downslope occurs on a large scale in a number of locations in the deep ocean (the Mediterranean outflow, the Denmark Strait overflow, the Bass Strait overflow and at various locations around Antarctica), but there is considerable variety in its observed character for reasons not yet well understood. For steady inviscid flow there is no downslope motion at all, as a "geostrophic" balance obtains between buoyancy and the Coriolis force. Friction is apparently necessary in order to have downslope motion with rotation. Further, eddies have been observed in the Denmark Strait overflow, and in laboratory experiments that aim to provide simple models of these flows (Lane-Serff & Baines 1998, J. Fluid Mech. 63, 229). Bottom Ekman layers are conspicuous in these experiments, and the eddies also have a bearing on the downslope transport.

Three possible mechanisms for eddy production by downslope flows in two-layer systems have been identified by Lane-Serff & Baines. All of these involve stretching of the upper layer fluid column by motion of the lower layer, and consequent spin-up due to potential vorticity conservation. These are: (i) "dragging" of the upper layer fluid by downslope motion of the lower layer before the latter reaches approximate alongslope "geostrophic" balance, in the manner of a "captured" Taylor column; (ii) the collapse (or reduction of thickness) of the lower layer fluid as it approaches this "geostrophic" balance, and (iii) Ekman drainage from the (anticylonic) lower layer after it reaches this "geostrophic" balance, which also reduces its thickness. The first two only act during geostrophic adjustment, but the last acts continuously.Spall and Price (1998, J. Phys. Oceanog. 28, 1598) have proposed a fourth mechanism (that is similar to (i)) in which the inflow consists of two layers flowing into a third, and the upper inflowing layer is stretched by the motion of lower. They also propose that this process operates in Denmark Strait. A variety of experiments that demonstrate these mechanisms and the situations in which they are dominant will be presented.

JSP39/W/47-B4 Invited 1430

STUDIES OF OCEAN DYNAMICS - FROM SMALL SCALE MIXING TO LARGE SCALE CIRCULATION

Greg IVEY (Department of Environmental Engineering, Centre for Water Research, The University of Western Australia, Nedlands, Western Australia, e-mail: ivey@cwr.uwa.edu.au)

The initiation of a de-stabilizing buoyancy flux across the surface of the ocean produces small-scale convective turbulence and mixing. In the ocean, the small-scale turbulence is only weakly affected by rotation but will erode the underlying stratified fluid. The combination of non-uniform lateral distribution of the surface buoyancy flux and this convectively driven deepening will generate firstly lateral density gradients and then, in turn, a mean circulation. If the scales are large enough, this circulation will be affected by rotation and large-scale baroclinic instabilities dominate the flow field. Recent laboratory studies have provided insight into these flows and have considered cases ranging from open ocean convection, where rotation is of primary importance, through to exchange flows between basins where topographic control is of primary importance. These studies will be reviewed with emphasis on describing how the buoyancy forcing, the ambient stratification, the rotation rate, the depth and the lateral length scales determine both the timescales of evolution of the flow to steady state as well as the final steady state property distributions, circulation and exchange rates.

JSP39/W/04-B4 1500

THE PROPAGATION OF INERTIAL-INTERNAL WAVES OVER SLOPING BOTTOM TOPOGRAHY: A REVISIT

Christopher N.K. MOOERS (Rosenstiel School of Marine and Atmospheric Science, University of Miami, 4600 Rickenbacker Cswy., Miami, FL33149-1098, USA, email: cmooers@rsmas.miami.edu)

Inertial-internal waves (i.e., waves under the joint influence of the Earth's rotation and density stratification) interact with sloping bottom topography in a fashion that depends upon the ratio of the slope of a frequency-dependent wave parameter (the slope of the wave characteristics or rays) to the slope of the bottom: if the ray slope is greater than the bottom slope, there is forward reflection; if otherwise, there is backward reflection. Of particular interest here is the propagation of inertial-internal waves into a wedge; for relatively high frequencies that are forward-reflected, a given wedge is termed subcritical; conversly, for relatively low frequencies that are backward-reflected, a given wedge is termed supercritical. By analysis of fiducial rays to reduce the frequency-dependent dynamical geometry to characteristic subdomains, a method for constructing solutions is defined for both subcritical and supercritical wedges.

JSP39/E/02-B4 1520

THE USE OF LABORATORY MEASUREMENTS TO VALIDATE OCEAN NUMERICAL MODELS

Nicolas PERENNE, David Smith, Don L. Boyer (Environmental Fluid Dynamics Program, Department of Mechanical and Aerospace Engineering, Arizona State University Tempe, AZ 85287-6106, USA, email: perenne@enws606.eas.asu.edu); Dale Haidvogel (Institute of Marine and Coastal Sciences, Rutgers University, P.O. Box 231, New Brunswick, NJ 08903, USA, email: dale@ahab.rutgers.edu)

In order to develop data sets which can be used as benchmarks for current coastal circulation models, laboratory experiments are conducted in a cylindrical tank in which a continuous continental shelf model, interrupted only by a single smooth canyon, is placed along the periphery of the test cell. Prior to experimentation, the tank is filled with a linearly stratified fluid and the tank is then slowly brought up to solid body rotation with Coriolis parameter f. To initiate the experiments, the turntable rotation rate is then modulated sinusoidally about the background rotation rate f/2. This effectively drives an oscillatory background current along the coastline. The objectives of the experiments are to (i) observe and better understand the motion field in the vicinity of a submarine canyon and (ii) provide horizontal maps of the velocity, vertical vorticity and horizontal divergence fields, including a determination of the errors associated therewith, that can be compared with current numerical models. The principal governing dynamical parameters are the Rossby, temporal Rossby, Burger and Ekman numbers.

Observations at numerous vertical locations, including those above and below the canyon rim, are presented. Particle tracking techniques are used to delineate the velocity, vorticity and divergence fields. Special attention is given to the mean currents driven by this physical system. Comparisons are made between the laboratory results and two coastal circulation models (i.e., finite difference and finite element). An assessment is made concerning the differences noted between the laboratory and the numerical models.

JSP39/W/28-B4 1540

MESOSCALE OCEAN EDDY DYNAMICS: HIGH RESOLUTION SHALLOW WATER SIMULATIONS WITH REALISTIC TOPOGRAPHY

Robert B. SCOTT, Ted Johnson, S. A. Sorenson (Mathematics, UCL, Gower Street, London, WC1E 6BT, UK)

The western boundary current off the southern tip of Africa, the very fast Agulhas current, regularly produces mesoscale eddies. Their westward propagation to the South Atlantic has been captured with TOPEX/Poseidon satellite altimeter data. This provides an excellent opportunity to study the eddy dynamics using simplified models while verifying the accuracy using the real data. We present results produced with a state of the art shallow water code featuring an adaptive grid that provides very high resolution following the eddy. The affects of realistic topography are investigated using the ETOPO5 5 minute resolution data. The possible influences of finer scale topography on eddy spin down is analyzed with artificial topography. Insofar as the most serious limitation of the model is its vertical homogeneity, the comparison between the observations and simulations provides an assessment of the importance of density stratification.

JSP39/W/39-B4 1620

MIXING EFFICIENCY OF STRONG DENSITY INTERFACES

J.M. REDONDO, M.A. Sanchez (Dept. Fisica Aplicada, UPC, Barcelona, Spain)

A series of high resolution experiments have been performed in order to examine the structure of a sharp density interface under zero-mean-flow. The evaluation of the mixing efficiency of the grid-stirred turbulence is made comparing the increase of potential energy with the power of the grid. The interface is visualized with a thin laser sheet of 532 nm and rodamine is used to mark the turbulent side of the interface. The structure of the density and velocity fields are imaged using DigImage fluid dynamics software and particle tracking of pliolite suspended near the interface. The reduction of interface thickness with Richardson follows a power law with exponent -2/3. There is a maximum in mixing efficiency at intermediate Ri when a high rate of vortex dipoles resonate with the Brunt-Vaisalla frecuency at the interface.

JSP39/E/05-B4 1640

LONG-DISTANCE PROPAGATION AND FEATURES OF TIDAL INTERNAL WAVES IN THE CENTRAL INDIAN OCEAN

Tatiana Demidova, Eugene MOROZOV (both at Department of Physics, P.P. Shirshov Institute of Oceanology, Moscow 117218, Russia, email: evita@redline.ru)

Propagation of tidal internal waves generated over the slopes of the Mascarene Ridge at a distance over 10 wavelengths in the Central Indian Ocean was studied from a joint analysis of the results of calculations with the use of a non-linear numerical model and experimental cast and mooring data. The barotropic tide over the ridge was assumed to be a 2-D flow in a continuously stratified rotating ocean of changing depth. The modelling was fulfilled for quite large number of steps in time. This allows us to study the evolution of the internal waves at distances over 1500 km from the source. Long-term measurements of temperature and currents at subsurface buoys in direction of the wave propagation were used to get statistically proven experimental parameter of internal waves and to study its time evolution. The results of the modelling and experimental measurements were used to estimate the amplitudes of the waves, the character of their decay and energy flow both near the ridge and at a significant distance from it. . Oscillation parameters of the first mode remains unchanged at quite long distances, whereas the higher modes dissipate rapidly already at first periods of oscillation. Approximately 5%-decay per wavelenght in the far zone was found. The modelling and measurement results are in a good agreement.

JSP39/E/24-B4 1700

INTERACTION OF INERTIA-GRAVITY WAVES WITH A POTENTIAL VORTICITY BARRIER

Gael Huerre, Chantal STAQUET (LEGI, BP 53, 38041 Grenoble cdx 9, France, email: chantal.staquet@hmg.inpg.fr)

The purpose of this work is to investigate the permeability of a dynamical barrier with respect to wave breaking events. This work is motivated by a conjecture proposed by McIntyre (1995) according to which the edge of the polar vortex, which behaves as a dynamical barrier with respect to large scale quasi-two-dimensional turbulence, would be permeable to wave breaking events. McIntyre (1995) suggested that inertia-gravity wave motions, in breaking intermittently near the vortex edge, could give rise to a quasi-horizontal transport across that edge. This conjecture was proposed to account for anomalous behaviours inferred from measurements in the atmosphere, such as low concentration of ozone at mid latitudes. To investigate this conjecture, we have performed three-dimensional numerical simulations of inertia-gravity waves interacting with the edge of a large scale vortex. Conditions under which breaking may occur have been investigated and the permeability of the vortex edge is studied by following the behaviour of a passive tracer. Preliminary results will be presented at the meeting.

JSP39/E/13-B4 1720

PARAMETER DEPENDENCE OF LINEAR AND NONLINEAR INSTABILITY OF BAROTROPIC AND BAROCLINIC SHEAR FLOWS

Kuniko M. YAMAZAKI, Akira Masuda (Dynamics Simulations Research Center, Research Institute for Applied Mechanics, Kyushu University, Kasuga-Koen 6-1, Fukuoka 816-8580, Japan, email: miki@masuda.riam.kyushu-u.ac.jp, masuda@riam.kyushu-u.ac.jp)

We examine the stability of equivalent barotropic and baroclinic shear flows in a periodic domain on an f-plane and on a beta-plane, as a preliminary to investigating the instability of the Kuroshio Current over the continental slope in the East China Sea. Linear analysis reveals that the instability of the equivalent barotropic shear flow is reduced by stratification, the beta effect, coastal boundaries, and along-flow bottom topography. For the baroclinic flow, however, the influence of stratification is more complex due to the influence of baroclinic instability. The non-linear evolution of such unstable flows is investigated by employing a numerical time-integration technique. An archetypical evolution is found to consist of four stages: primary instability to which linear analysis is applicable, saturated phase characterized by staggered rows of vortices, the onset of a secondary "subharmonic" instability which leads to the development of structures with smaller wavenumbers, and a final stage in which the flow becomes more irregular. It turns out that the long-term non-linear evolution of the flow is drastically altered by the influence of the planetary beta-effect. Furthermore, in the barotropic case, an asymmetry of the profile of the mean shear flow enhances the instability of jets insofar as their development is concerned, although the opposite is the case in the linear regime.

JSP39/W/20-B4 1740

EDDY-WAVE INTERACTIONS USING A POTENTIAL VORTICITY DIAGNOSTICS

Maria VALDIVIESO DA COSTA, Alain Colin de Verdiere (both at Laboratoire de Physique des Oceans, Universite de Bretagne Occidentale, email: mvaldi@deneb-gw.univ-brest.fr).

Process-oriented numerical experiments are carried out to study the dynamics of intermediate density boudary currents and associated lenslike eddies. The model used in the experiments is the pure-isopycnic coordinate primitive equation model of Bleck and Boudra (1986), configured on a semi-open channel with two active layers, and forced by a steady mid-level density flux through one channel wall. Non-linear interactions between neighbouring eddy structures were generally observed within 50 and 100 inertial periods after the geostrophic adjustment phase of the experiments. Most of the clearly visible interactions occurred between large-scale eddies with weak downstream phase velocities, and faster propagating smaller scale waves.

A diagnostic framework based on the model isopycnic potential vorticity (IPV) balance is set out to examine the non-linear interaction processes between these two types of transient features. Specifically, we have calculated four tendency components of the low-frequency IPV field associated with individual large-scale eddies on the framework of the filtered IPV-tendency equation. We then compared each of the tendency components with the (total) observed tendency and the low-frequency IPV field itself. It is found that the sum of the two tendency components induced by each of the internal non-linear interaction processes determines the observed tendency of large-scale eddies. The largest tendency component is the tendency induced by the advection of large-scale eddy structures by the low-frequency component of the total flow, which primarily acts to make large-scale eddies propagate downstream. The tendency component due to self-interaction among high-frequency transients (feedback of high-frequency waves) is nearly out-phase with the mean tendency induced by the large-scale advection, retarding, in average, the downstream propagation of large-scale eddies. The tendency component induced by bi-harmonic dissipation of IPV along isopycnal surfaces is in the mean negligible relatively to the other mechanisms.