Internal wave generation from tidal flow exiting a constricted opening

The southern Strait of Georgia, British Columbia, often contains packets of large, near-surface internal waves. Wave crests at the leading edge of the packet, spaced a few hundred meters apart, can have a longitudinal extent of more than 10 km. It has long been assumed that these waves are generated by tidal flow through narrow passages and channels at the Strait's southern boundaries, but no actual link has ever been made between these waves and a specific passage or generation mechanism. Here we identify the location and extent of a number of these large packets at specific times using mosaics of photogrammetrically rectified oblique air photos. Wave speeds are determined by analyzing a time sequence of images, with water column measurements used to subtract effects of tidal advection. The location and extent of these internal waves are then compared with the predicted location and extent of hypothetical waves generated in different passages, at different stages of the tide, which are then propagated through a predicted time-varying barotropic flow field. It is found that the observed waves are most likely generated near or after the time of the peak flood tide, or peak inflow into the Strait. They are therefore inconsistent with generation mechanisms involving the release and upstream propagation of waves by the relaxation of an ebb tide. Instead they are probably associated with the nonlinear adjustment of conditions at the edge of an inflowing injection of relatively weakly stratified water.