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Low-frequency vorticity waves over strong topography Gratton, Yves


This thesis addresses the general problem of vorticity waves propagating over steeply sloping topography, in the presence of stratification and rotation. From the inviscid unforced long-wave equations for a two-layer fluid on an f-plane, it is shown that, as long as the ratio of the upper to lower layer depths is small, semi-enclosed and enclosed basins can sustain low-frequency, short scale, surface-intensified motions. Simple analytical solutions are to be found only if the upper to lower layer depths ratio is small. Then, we obtain a set of equations which describes a barotropic wave which forces a baroclinic response through topographic coupling. Two bottom profiles are considered: linear and parabolic. Solutions are found with and without the small slope approximation. It is shown that the small slope approximation underestimates all the parameters of low-frequency topographic waves, even when the slope is small. The theory is compared with observations from the Strait of Georgia and with a numerical model of the Saint Lawrence estuary. It is found that, for bathymetric profiles similar to those of the Strait of Georgia (linear) and the Saint Lawrence (parabolic), bur model provides a better fit to the topography, leads to surface-intensified motions and produces cross-channel velocities very similar to those observed in situ.

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