Bulletin of the American Physical Society
2005 58th Annual Meeting of the Division of Fluid Dynamics
Sunday–Tuesday, November 20–22, 2005; Chicago, IL
Session EN: Oceanography |
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Chair: James Rottman, University of California, San Diego Room: Hilton Chicago PDR 1 |
Sunday, November 20, 2005 4:10PM - 4:23PM |
EN.00001: Stochastic modelling of the oceanic eddies Pavel Berloff It is widely recognized that the mesoscale oceanic eddies are capable of driving the large-scale oceanic currents. These eddies are generally not resolved in comprehensive Oceanic General Circulation Models, therefore the models are rather inaccurate. Developing efficient mathematical models of the eddies is, arguably, the most important problem facing the physical oceanography. In this study, one of the main achievements is developing and testing a random-forcing model of the eddy effects. The random forcing acts on nonlinear reduced-dynamics equations with relatively few degrees of freedom that represent the large-scale circulation. The other result is formulation of a hierarchy of stochastic transport models that simulate Lagrangian dispersion of the passive tracer. The approach adopted allows one to model not only diffusive but also non-diffusive and even anti-diffusive eddy effects, which are a fundamental obstacle in ocean circulation theories and turbulence closures. The results are tested against a fluid-dynamic model that explicitly resolves the eddies. [Preview Abstract] |
Sunday, November 20, 2005 4:23PM - 4:36PM |
EN.00002: Quantifying eddy diffusivities Emily Shuckburgh, John Marshall The advection-diffusion equation for the concentration of a tracer may be transformed into a pure diffusion equation by using the area or volume inside concentration contours as a coordinate. The corresponding effective diffusivity depends on the geometry of the tracer field, which is determined by the underlying flow. Here we show that this effective diffusivity may be used as a quantitative diagnostic of eddy transport and mixing. We use a family of simple analytic flows as a test-bench and compare the calculated effective diffusivity with other diagnostics. As an example of the usefulness of the effective diffusivity diagnostic, an attempt to arrive at a more definitive estimate of ocean near-surface eddy diffusivities will be described. Arguably the key uncertainty in the coarse -resolution ocean models used in climate research is lack of knowledge of the magnitude of the diffusivities and their variation in space and time. The results of effective diffusivity calculations for the Southern Ocean show high spatial variations in eddy diffusivity and this has important implications for the understanding of the residual circulation. [Preview Abstract] |
Sunday, November 20, 2005 4:36PM - 4:49PM |
EN.00003: A six-grid, two-way coupled Mediterranean Sea and North Atlantic ocean model: results and interpretations David E. Dietrich, Yu-heng Tseng, Raul Medina, Marie Liste, Maitane Olabarriet A multiple-grid, two-way coupled Mediterranean Sea and North Atlantic (MEDiNA) Ocean model is presented. The model uses six grids with resolution varying from 1/24$^{\circ}$ in the Strait of Gibraltar to 1/4$^{\circ}$ in the central North Atlantic Ocean. The flexible multiple-grid coupling approach produces nearly seamless flow patterns across the nested boundaries and facilitates realistic modeling of multi-scale processes that couple deep ocean and coastal dynamics. The method is efficient and provides high resolution where it is needed most. The shortcircuited Arctic Ocean is parameterized by: an artificial continental shelf along the northern and northeastern boundary; restoring to climatology at high latitudes; nudging the total longitudinal volume flow north of Iceland toward a specified value; and a specified freshwater source along the northern boundary. The open equatorial boundary condition includes a slow nudging toward a quasi-Neummann condition. Results from all six grids are shown, including the Gulf of Mexico Loop Current with big warm-core eddy shedding and realistic Gulf Stream meanders that pinch off cold/warm-core eddies. These results compare well with observation and show significant advantages of our multiple-grid approach in modeling multi-scale ocean dynamics. [Preview Abstract] |
Sunday, November 20, 2005 4:49PM - 5:02PM |
EN.00004: Shear and scalar spectra computed with a shell model of stratified turbulence Juan Ezequiel Martin, Chris R. Rehmann Oceanographers fit theoretical spectra to measured spectra to estimate the rates of dissipation of turbulent kinetic energy and temperature variance. However, the validity of the theoretical forms of the spectra has been questioned. The present work investigates the validity of the method by generating spectra with a shell model of turbulence. Shell models are dynamic systems that share some statistical properties with the Navier-Stokes equations. The equations proposed by Yamada and Ohkitani (1987) for the velocity field and by Jensen et al. (1992) for a passive-scalar field have been modified to include a mean scalar gradient and effects of buoyancy. The modified system allows us to study the validity of the proposed spectra as well as the effect of several parameters such as the Prandtl number and the strength of the stratification. [Preview Abstract] |
Sunday, November 20, 2005 5:02PM - 5:15PM |
EN.00005: Experiments on differential diffusion of temperature and salinity across a sheared density interface P. Ryan Jackson, Chris Rehmann Differential diffusion of two stably stratified scalars with different molecular diffusivities in a turbulent flow can affect ocean circulation modeling and the interpretation of ocean mixing experiments. In past work we used rapid distortion theory to evaluate the effect of a mean shear on differential diffusion. The present work compares those theoretical results with results from laboratory experiments. In the experiments, an Odell-Kovasnay recirculating flume and disk pump was used to drive a warm, fresh water upper layer over the cold, salty quiescent lower layer. Bulk measurements based on the evolution of the scalar profiles and instantaneous interfacial flux measurements were employed to quantify scalar mixing across the sheared interface. Results from the experiments compare well with predictions of the theory. For weakly turbulent, strongly stratified flows, differential diffusion is found to decrease with increasing shear. However, for more energetic flows, differential diffusion is found to increase with increasing shear. [Preview Abstract] |
Sunday, November 20, 2005 5:15PM - 5:28PM |
EN.00006: The influence of waves and turbulence on the heat flux at the surface of natural water bodies. Fabrice Veron, W. Kendall Melville The top few meters of the ocean play a key role in the in the surface fluxes of momentum, gas, heat and mass. The magnitude of these fluxes is strongly influenced by the dynamics of both air and water boundary layers. We present results of several field experiments on the kinematics of small-scale surface turbulence and surface waves, their influence on the surface skin layer, and the resulting transfers of heat across the diffusive layer at the surface of the ocean. A variety of optical and electro-mechanical instruments are used to measure the evolution of the surface velocity and temperature fields. These include visible and infrared imaging of the surface, thermal surface velocimetry, and fast-response thermometry. We show that at low wind speed, it is the small-scale turbulence at the surface of the ocean, rather than breaking waves that most influence and disrupt the surface skin layer. We find that at the low wind speed surface turbulence correlates with the surface heat flux. In addition, we find that and that the surface wave field modulates a component of the total air-sea heat flux. [Preview Abstract] |
Sunday, November 20, 2005 5:28PM - 5:41PM |
EN.00007: Energetics of internal boluses on a shelf break Subhas Venayagamoorthy, Oliver Fringer We present results of high-resolution two-dimensional numerical simulations showing the interaction of nonlinear internal waves with a shelf break. The interaction of the incoming wave field with a near-critical to supercritical topography causes the formation of upslope surging bores that get ejected onto the shelf as propagating internal boluses. We present the energy flux distribution across the shelf-break for a wide range of topographic slopes and Froude numbers and show that both the transmitted energy fluxes as well reflected energy fluxes are strong functions of both the Froude number and the ratio of the topographic slope to the internal wave beam angle. [Preview Abstract] |
Sunday, November 20, 2005 5:41PM - 5:54PM |
EN.00008: Modeling Reef Hydrodynamics to Predict Coral Bleaching James Bird, Craig Steinberg, Tom Hardy The aim of this study is to use environmental physics to predict water temperatures around and within coral reefs. Anomalously warm water is the leading cause for mass coral bleaching; thus a clearer understanding of the oceanographic mechanisms that control reef water temperatures will enable better reef management. In March 1998 a major coral bleaching event occurred at Scott Reef, a 40 km-wide lagoon 300 km off the northwest coast of Australia. Meteorological and coral cover observations were collected before, during, and after the event. In this study, two hydrodynamic models are applied to Scott Reef and validated against oceanographic data collected between March and June 2003. The models are then used to hindcast the reef hydrodynamics that led up to the 1998 bleaching event. Results show a positive correlation between poorly mixed regions and bleaching severity. [Preview Abstract] |
Sunday, November 20, 2005 5:54PM - 6:07PM |
EN.00009: Self-Similarity Skeletal Structures of the Ocean (Observations, Hypotheses, Interpretation, Sequels) V.A. Rantsev-Kartinov An analysis of databases of photographic images of ocean's surface, taken from various altitudes and for various types of rough ocean surface, revealed the presence of an ocean's skeletal structures (OSS) [1], which exhibit a tendency toward self-similarity of structuring at various length scales. The topology of OSS appears to be identical to skeletal structures which have been formerly found in a wide range of length scales, media and for various phenomena [2]. The analysis of a database of hurricanes routes in Atlantic and Pacific Ocean, more than for the centenary period has led the author to promotion of some hypotheses on the OSS formation and theoretical interpretation of this phenomenon. The capillary model (CM) of the OSS formation is offered. It is shown, the forces of surface tension and the capillary phenomena are enough to explain a durability and buoyancy of a separate blocks of the OSS and to calculate these basic parameters. Some consequences of these fractal structures of ocean and an opportunity of their practical use are considered basing on the suggested CM of the OSS. \newline [1] V.A.Rantsev-Kartinov, Phys. Lett. A., vol. 334/2-3, p. 234, (2004); \newline [2] A.B.Kukushkin, V.A.Rantsev-Kartinov., (a) Phys. Lett. A, 306, p.175, (2002,); (b) In: Advances in Plasma Phys. Research, 2002, Vol. 2 (Ed. F. Gerard, Nova Science [Preview Abstract] |
Sunday, November 20, 2005 6:07PM - 6:20PM |
EN.00010: A Lagrangian Turbulent Dispersion Model of Evolving Sea Spray Droplets Over the Ocean James Mueller, Fabrice Veron We have developed a turbulent Lagrangian model of sea-spray over the ocean. Our Monte-Carlo simulation follows individual droplets from ejection into the air until they reenter the ocean or attain a quasi-equilibrium state. The model includes a realistic surface wave spectrum, forming the bottom boundary. While suspended in the air, the droplet traverses an atmospheric boundary layer that includes a viscous sublayer, a wave boundary layer (WBL), and a stratified log layer. In addition, the droplet is subjected to turbulent velocities and turbulent scalars following the Kolmogorov-Obukhov-Corrsin theory. The droplet's velocity and evolution are solved using the complete, linear, unsteady equation of motion and the complete microphysical equations, respectively. The effect of temperature and humidity fluctuations on the evolution of droplet will be discussed, as well as the nature of turbulence in the marine boundary layer when following a droplet with inertia. [Preview Abstract] |
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