Coastal Region Dynamics II
Presiding: A C Warn-Varnas, Naval Research Laboratory; K G Lamb, University of Waterloo
OS43B-01 INVITED 13:30h
The Vibrant Coastal Dynamics - always transient, always changing - within a repetitive framework?
The dynamics of the coastal region is vibrant in many different spatial and temporal scales. Our understanding of these regions has however identified a specific set of spatial-temporal behavior depending on the prevailing set-up conditions. These set-ups include the local effects of bathymetry, the coastal boundaries and the wind and heat fluxes. Remote forcing such as propagating Kelvin waves, Internal Waves, Rossby waves and other advection processes influence the coastal water mass properties as well. Circulation structures or features in the coastal regions are thus always transient and ever-changing. However, within this fast-changing system, there apparently exists a notion of repetition of certain types of structures like upwelling, fronts, coastal eddies, mushroom vortices, plumes etc. So, in a numerical modeling exercise, these features can be implemented when identified by satellites and assumed a three-dimensionality a-priori. In this presentation, we show such development of `feature models' for coastal upwelling, coastally trapped waves, and buoyant plumes for different regions around the world ocean. A special emphasis is on a proposed framework in which we can use SAR imagery for `feature modeling' an internal solitary wave. Specific data stream generations for forecasting and assimilation of numerical models using such synoptic feature models would then benefit any real-time operations and dynamical process studies.
OS43B-02 13:50h
A Numerical Study on Kuroshio Frontal Eddies at the Shelf Edge of the East China Sea Using a Finite Volume Coastal Ocean Model
Cross-frontal transports of seawater and materials are induced by nonlinear frontal eddies as they grow from baroclinically unstable frontal waves. It is difficult to measure these transports based on field observations and/or analyses of archived datasets because of intense spatiotemporal variability of frontal waves. In order to evaluate cross-frontal transports induced by frontal eddies quantitatively, it is therefore reasonable to use a numerical model approach that includes realistic bathymetric irregularities that may trigger frontal waves and eddies. The Finite Volume Coastal Ocean Model (FVCOM; Chen et al., 2003) is useful because this model features an unstructured grid that can resolve complex bottom topography. In this study, FVCOM is used with the Princeton Ocean Model (POM) to study Kuroshio frontal eddies in the East China Sea. First, the large-scale circulation in the Yellow and East China Seas is simulated using POM with inflow-outflow conditions of ocean currents (i.e., Kuroshio, Taiwan and Tsushima Currents), heat and freshwater fluxes through the sea surface, and the Changjiang river run-off. These conditions vary in time with an annual period. The model reaches a stable state with an annual cycle after seven years of spinup. The seventh-year POM results are then used as boundary conditions to drive FVCOM in a limitted area along the shelf break where the Kuroshio frontal eddies are frequently detected by field measurements and satellite infrared images. FVCOM reproduces frontal eddies with a folded-wave pattern, which is a characteristic of a mature frontal eddy. In addition, modeled eddies have wavelengths of 100-200 km and periods of 10-20 days, which are consistent with those observed in this area. Model results are compared with the temperature time series observed at a Japan Meteorological Agency buoy located at East China Sea shelf break, and are used for the quantitative evaluation of the cross-frontal transport in conjunction with passive tracer experiments.
OS43B-03 14:05h
Modeling Studies of Solitary Wave Effects in the South China Sea.
A 3D and 2D study of solitary wave generation, in the Strait of Luzon, and subsequent propagation in the South China Sea is undertaken. The ocean predictions are conducted with the nonoscillatory forward-in-time (NFT) multiscale nonhydrostatic anelastic model EULAG for rotating stratified flows, broadly documented in the literature; Smolarkiewicz and Margolin (1997), Smolarkiewicz et al., (2001). Initial conditions are obtained from hydrographic measurements. The EULAG model is configured to accept tidal forcing from the barotropic tidal NCOM model that has tidal components imbedded in it. The generation of internal bores by tidal motion over the ridges in the Strait of Luzon is predicted. Subsequently, the bores propagated away from the Strait of Luzon and disintegrate into solitary waves. Energy conversion from the barotropic tide to the baroclinic tide and into internal solitary waves occurs. The solitary waves can attain amplitudes of around 150 m. Such amplitudes are observed at the ASIAEX experimental sites on the Chinese continental shelf. The effects of solitary wave trains on acoustic propagation and intensity are considered.
OS43B-04 14:20h
Internal Wave Generation at the Shelf Break
In this talk we will present results of some numerical simulations of internal wave generation at a shelf break. The dependence of the energy flux into the deep and shallow water on the strength of the stratification, the Coriolis parameter, topographic width and amplitude are considered for stratifications with constant buoyancy and with a thermocline.
OS43B-05 INVITED 14:35h
Nonlinear Analysis of Ocean Solitons and Their Effects on Ocean Acoustics
Computer simulations by Zhou, Zhang, and Rogers [J. Acoust. Soc. Am. 90, 2042-2054 (1991)] have been used to demonstrate that a solitary wave packet (soliton) traveling in the ocean can dramatically effect an acoustic signal passing through the solitary wave. In their simulations they used a simple sinusoidal wave to represent the solitary wave packet. At certain acoustic frequencies their simulations showed a "resonance-like" loss in the acoustic signal that propagated through the sinusoidal wave. When this occurred they were able to show a connection between the spatial wave number of their sinusoidal wave and the adjacent acoustic wave numbers associated with acoustic mode conversions. The mode conversions occurred in the acoustic signal that propagated through the sinusoidal wave. Their computer simulation predictions of this resonance effect were in good agreement with their experimental measurements. Based on this good agreement they postulated that at certain acoustic frequencies the presence of shallow water solitary wave packets can result in acoustic mode conversions which consequently produce large signal losses. While this point of view has generally been accepted by the ocean acoustics research community, the equation that Zhou, et al., postulated as the connecting equation for this signal loss has not been generally accepted. The skepticism is justified. First, real acoustic data together with the necessary simultaneous oceanographic data is difficult to obtain, and consists of many complicating dynamics (fluctuations) other than pure solitary wave packets. Second, solitary wave packets in the ocean are nonlinear and are poorly approximated by sinusoidal waves. A better representation would have been given by nonlinear cnoidal functions. We discuss the limitations of using a linear Fourier representation for a realistic ocean soliton, and the advantages of using nonlinear cnoidal functions when attempting to apply the Zhou, et al., equation to understanding resonant acoustic signal losses. We demonstrate why their equation worked well for their analysis, and why it has not been as successful in analyzing resonances observed in other experiments. [Work supported by the Office of Naval Research with technical management by the Naval Research Laboratory.]