Coastal Region Dynamics III
Presiding: A C Warn-Varnas, Naval Research Laboratory; K G Lamb, University of Waterloo
OS44A-01 INVITED 15:30h
Multidimensional Fourier Methods: Analysis of Internal Soliton Data and Acoustic Wave Propagation
The aggressive pursuit of a satisfactory level of physical understanding of nonlinear oceanic wave dynamics has lead to the use of multidimensional Fourier analysis as a tool for the time series analysis of both internal wave motion and acoustic wave propagation. These new tools have arisen naturally for studies using the inverse scattering transform to particular nonlinear wave equations. When applied to the Korteweg-deVries equation, for example, one finds that the approach can be extended to arbitrarily high order. There are several advantages for using multidimensional Fourier methods over ordinary Fourier analysis: (1) fully nonlinear wave dynamics can be studied, (2) solitons become a natural component in the theory and correspond to the diagonal elements of the "Riemann matrix", (3) nonlinear interactions are accounted for by the off-diagonal elements of this matrix, (4) nonlinear acoustic modes are found to also have an (albeit static) solitonic component. These surprising results lead to new interpretations of acoustic waves propagating in the presence of a nonlinear internal wave field. One of the most important results is the implication that new nonlinear filtering techniques allow for the spectral decomposition of both the internal wave field and of the acoustic field. With regard to the acoustic field, one can foresee the application of the method to the observations of phenomena in the "hidden zones", where one would normally conclude that acoustic wave propagation does not occur.
OS44A-02 INVITED 15:50h
Acoustical-Environmental Data Assimilation for the Estimation of Hydrostatic and Non-hydrostatic Coastal Ocean Dynamics
The estimation of coastal environmental parameters and acoustic properties is considered as a single coupled problem. The sources of information, environmental and acoustical data, and ocean dynamics and sound propagation models, are combined by data assimilation in accord with their respective uncertainties, i.e. their error statistics. This process provides better estimates of parameters and properties than can be obtained by using only the observations or models alone. The approach is exemplified for the New England continental shelfbreak region, using acoustical and physical data collected during the summer and winter Shelfbreak PRIMER Experiment. The coupled data assimilation methodology is based on Error Subspace Statistical Estimation. For the summer experiment, the focus is on mesoscale dynamics of the Middle Atlantic Bight shelfbreak front, including remote influences from the shelf, slope and deep ocean. The ocean environment is simulated with the hydrostatic Harvard Ocean Prediction System. For the winter experiment, the focus is on non-hydrostatic processes and internal waves and solitons. The ocean environment is simulated with the 2.5D Lamb non-hydrostatic model and twin-experiments are carried-out. Ongoing progress towards the inference of background parameters of the Lamb model (e.g. mixed-layer depth, boundary velocities, frontal slope) by assimilation of acoustical and/or physical data will be reported. The results provide insights into relations between physical and acoustical fields, and their uncertainties, on hydrostatic and non-hydrostatic scales.
http://people.deas.harvard.edu/~pierrel/
OS44A-03 INVITED 16:10h
On the Existence of Solitary Wave Solutions for the Rotating Shallow Water Equations
It has been suggested in a recent paper (JPO, vol. 34, pg 856) that the inviscid shallow water equations allow zonally propagating solitary waves. A peculiar trait of these solutions is found at the crest of the wave and takes the form of a cusp for the surface elevation and zonal component of velocity and a discontinuity in the meridional velocity. The authors suggest (but do not demonstrate) that both the discontinuity and the cusp would be smoothed in the presence of eddy viscosity. In this talk we will employ dynamical systems techniques to demonstrate why, in the inviscid case, any zonally traveling waves must exhibit a singularity. We will subsequently demonstrate that when viscosity is nonzero the singularity is indeed smoothed, but the solution is no longer bounded, and hence no solitary waves can exist. The analytical results will be compared with numerical solutions of the geostrophic adjustment problem in a zonal channel. Finally we will contrast the f-plane case with a nonrotating stratified adjustment problem which is dominated by solitary wave trains.
OS44A-04 16:30h
Parametric Instability in Evolving Shear Flows
Time-Periodic shear flows can give rise to Parametric Instability (PI), as in the case of the Mathieu equation. This mechanism results from a resonance between the oscillatory basic state and waves that are superimposed on it. Farrell and Ioannou, (1996, J. Atmos. Sci.) explain that the source of PI is that the snap-shots in time of the basic state are susceptible to transient growth. Mathematically, this is due to a linearized system that has a nonorthogonal eigenspace. Poulin, Flierl and Pedlosky (2003, J. Fluid Mech.) studied a time-periodic barotropic shear flow that exhibited PI, and thereby produced mixing at the interface between Potential Vorticity (PV) fronts. The instability led to the formation of vortices that were stretched. A later study of an oscillatory current in the Cape Cod Bay illustrated that PI can occur in realistic shear flows. These studies assumed that the basic state was periodic with a constant frequency. In this talk we study a shear flow similar to that found in Poulin et al., 2003 but now where the frequency is an evolving variable. This is inspired by the fact that the oscillations exhibited in this model can be generated by a travelling wave packet. If the frequency is constant the associated packet has a uniform frequency. To study the case of a wave packet we assume the frequencies change gradually in time. We determine that in these evolving shear flows the transient growth of perturbations of the snapshots of the basic state still generate PI.
OS44A-05 16:45h
Island Wakes in the Southern California Bight
The oceanic circulation in the Southern California Bight (SCB) is significantly characterized by its complexity in the topography, especially the presence of a group of islands. The island wakes of both atmosphere and ocean in SCB are recorded in the satellite remote sensing images and field observational data. The Regional Oceanic Model System (ROMS) is applied to study the effect of the islands on the regional circulation. The 2002 MM5-reanalyzed wind (2km in the spacial resolution) is used to force the ocean model. The wind island wake is well presented in the high resolution wind. The analysis of the ROMS results shows the evolution of the island wake and the eddy propagation downstream of islands. The influence of eddy shed in island wakes on the SCB regional circulation is also reflected in the energy balance. The results are also consistent with our previous idealized studies. The change in the vertical mixing and transport due to the wake has significant influence on the local biological environment.