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Wavenumber transport: scattering of small-scale internal waves by large-scale wavepackets

Published online by Cambridge University Press:  26 April 2006

David L. Bruhwiler  and
Tasso J. Kaper
David L. Bruhwiler
Affiliation:
Grumman Research and Development Center, 4 Independence Way, Princeton, NJ 08540, USA
Tasso J. Kaper
Affiliation:
Department of Mathematics, Boston University, 111 Cummington Street, Boston, MA 02215, USA
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Abstract

In this work, we treat the problem of small-scale, small-amplitude, internal waves interacting nonlinearly with a vigorous, large-scale, undulating shear. The amplitude of the background shear can be arbitrarily large, with a general profile, but our analysis requires that the amplitude vary on a length scale longer than the wavelength of the undulations. In order to illustrate the method, we consider the ray-theoretic model due to Broutman & Young (1986) of high-frequency oceanic internal waves that trap and detrap in a near-inertial wavepacket as a prototype problem. The near-inertial wavepacket tends to transport the high-frequency test waves from larger to smaller wavenumber, and hence to higher frequency. We identify the essential physical mechanisms of this wavenumber transport, and we quantify it. We also show that, for an initial ensemble of test waves with frequencies between the inertial and buoyancy frequencies and in which the number of test waves per frequency interval is proportional to the inverse square of the frequency, a single nonlinear wave–wave interaction fundamentally alters this initial distribution. After the interaction, the slope on a log-log plot is nearly flat, whereas initially it was -2. Our analysis captures this change in slope. The main techniques employed are classical adiabatic invariance theory and adiabatic separatrix crossing theory.

Type
Research Article
Information
Journal of Fluid Mechanics , Volume 289 , 25 April 1995 , pp. 379 - 405
Copyright
© 1995 Cambridge University Press

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