Generation of internal waves in the Strait of Georgia, British Columbia

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Large travelling groups of internal waves are a consistent and important feature of the oceanography of the southern Strait of Georgia, British Columbia. In this paper, characteristic wave properties are reviewed and summarized, with particular emphasis on those properties which must result from a theory of generation: origin in space near the mouths of the passes between islands which bound the strait to the south, origin in time near both turns of the tide, uni-directional propagation into the Strait, and origin of a varying number of wave groups on a given turn of the tide. Observations taken within one of the inter-island passes suggest first that the submarine ridge which lies across the mouth of the pass acts as an effective boundary between stratified water within the Strait and tidally mixed water within the pass, and secondly, that the generation mechanism has the form of almost impulsive disturbances to the stratified water mass within the Strait, caused by abrupt changes in current speed found to characterize tidal flow within the pass near both turns of the tide. The form of wave group resulting from such a generation mechanism is then examined. Linear theory is shown to result in wave groups of the appropriate shape; however, the predicted horizontal scale is much larger than that observed, indicating the necessity of including some effect to counteract dispersion. Since observed waves are of distinctly finite amplitude, non-linear effects are included and time-dependent theory developed for waves in the ‘thin-layer’ density system characteristic of the southern Strait. For a restricted class of initial conditions, a solution may be obtained by a comparison technique which uses known solutions of the Korteweg-de Vries equation.