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Internal waves in the Strait of Georgia Gargett, Ann Elizabeth

Abstract

Observed properties of the internal wave groups which occur in the southern Strait of Georgia are summarized, with particular emphasis on those properties which must be explained by any theory of generation. Current measurements within Boundary Pass are described. These measurements suggest a generation mechanism in the form of impulsive disturbances to the stratified water mass within the Strait, caused by the abrupt changes in current speed which are observed to characterize the tidal flow close to both turns of the tide. Such a mechanism is shown to explain the most important observed properties: origin in space near the mouths of the passes between the Gulf Islands, origin in time near both turns of the tide, uni-directional propagation into the Strait, and origin of a varying number of groups on one turn of the tide. The form of wave groups which result from the initial-value problem for a stratified fluid is examined. A linear model is shown to result in wave groups which are of the same general form as the groups observed in the Strait, but much less compact, indicating the necessity of some effect to counteract dispersion. Since observed waves are of distinctly finite amplitude, nonlinear effects were included and time-dependent theory developed for "long-short" waves in a thin-layer system, characteristic of the southern Strait. For a restricted class of initial conditions, solutions are obtained by a comparison technique, using known solutions of the Korteweg-deVries equation. This class of solutions suggests that the general initial-value problem results, for large x and t, in a number of "solitons", individual solitary waves of the thin-layer system. Surface effects associated with the internal waves in the Strait are described, and the reflectivity changes examined in some detail. The usual explanation of the effect of shallow internal waves on the surface wave field is in terms of wave damping by a surface organic film which is periodically expanded and contracted by internal wave-induced motions. It is shown that, under certain conditions, the effect may equally well, be-due to periodic changes in-surface wave amplitude and vector wave number caused-by radiation stress effects induced by the presence of internal waves.

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