Scattering of coastal-trapped waves by irregularities in coastline and topography
Wilkin, John L.
MetadataShow full item record
A study is conducted of the scattering of freely-propagating subinertial frequency coastal-trapped waves (CTWS) by large variations in coastline and topography using analytical and numerical techniques. Particular attention is paid to the role of stratification because, as shown, the introduction of even modest stratification can eliminate backscattered free-waves with large wavenumbers which occur, theoretically, in a barotropic ocean. An analytical solution is presented for the scattering of barotropic waves incident upon a discontinuity in shelf width. Discussion of solutions relying on backscattered free-waves is avoided by considering only the range of parameters over which energy transmission is nearly 100%. The solution shows there is a substantial transfer of energy to modes other than that of the incident wave. The transmitted mode most readily excited is that which has the across-shelf structure most closely coinciding with that of the incident wave. For a widening shelf, energy is therefore readily transferred to higher modes. The resultant presence of multiple modes produces a strong modulation in flow intensity and phase progression downstream of the scattering region which may affect the interpretation of shelf wave observations. A non-dispersive shelf wave 'pulse' of limited a10ngshelf extent scatters into a train of similarly shaped waves of all allowable modes, each propagating at its own free-wave speed. To overcome limitations of the analytical study a numerical model which accomodates arbitrary density stratification, bathymetry, and coastline, is employed. Numerical simulations are conducted of the scattering of CTWs by a set of topographic and coastline variations which are representative of many continental shelves. The strength of the scattering observed is found to be proportional to a topographic warp factor which estimates the severity of the topographic irregularities. The scattering is amplified by density stratification. A comparison of the effects of widening and narrowing topographies shows that the gross scattering effects of 'reciprocal' topographies are qnite similar. Within the scattering region itself, the strengths of the scattered-wave-induced currents exhibit substantial variation over short spatial scales. On both widening and narrowiag shelves, there is generally a marked intensification of the flow within the scattering region, and significant differences in the directions of the currents at points separated by a few tens of kilometers indicate the occurence of rapid variations ia phase. On narrowing shelves, the influence of the scattering can extend upstream into the region of uniform topography even when no freely-propagating backscattered waves exist. A simulation is condncted of CTW scattering at a site on the East Coast of Australia where observations suggest the presence of scattered freely-propagating CTWs. The success of the model simulation in reproducing features of observations confirms that realistic shelf geometries can scatter significant levels of CTW energy, and that the scattered waves can have an appreciable signal in current-meter observations made on the continental shelf. This demonstrates that along irregular coastlines it is necessary to account for the possibility that CTW scattering processes filay be in effect if oceanographic observations are to be interpreted correctly.
Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution September 1988
Showing items related by title, author, creator and subject.
Cherian, Deepak A. (Massachusetts Institute of Technology and Woods Hole Oceanographic Institution, 2016-09)Eddies in the ocean move westwards. Those shed by western boundary currents must then interact with continental shelf-slope topography at the western boundary. The presence of other eddies and mean lows complicates this ...
Ecology of chemical defenses of algae against the herbivorous snail, Littorina littorea, in the New England rocky intertidal community Geiselman, Joy Ann (Massachusetts Institute of Technology and Woods Hole Oceanographic Institution, 1980-02)In the New England rocky intertidal community, space is dominated by two perennial plant types, brown fucoid algae (Ascophyllum nodosum and several species of Fucus) in the mid zones and the red alga Chondrus crispus ...
Experimental and seismological constraints on the rheology, evolution, and alteration of the lithosphere at oceanic spreading centers deMartin, Brian J. (Massachusetts Institute of Technology and Woods Hole Oceanographic Institution, 2007-02)Oceanic spreading centers are sites of magmatic, tectonic, and hydrothermal processes. In this thesis I present experimental and seismological constraints on the evolution of these complex regions of focused crustal ...