The modification of tidal ellipses by stratification in the Rhine ROFI

We report on recent observations of the vertical structure of density and velocity profiles, using a bottom mounted ADCP, in the Rhine ROFI system in the North Sea which confirms previous indications that the presence of stratification modifies the vertical structure of the tidal ellipse characteristics. During periods of stratification, the ellipses change from degenerate to a more circular pattern, with the surface ellipse rotating clockwise and the bottom ellipse rotating anticlockwise. The surface to bottom ellipticity difference Δε is found to be closely related to a bulk Richardson number which incorporates both the stratification and a measure of the tidal shear. An explanation of the observed dependency of ellipticity on the density structure is offered in terms of the different thickness of the frictional layers for clockwise and anticlockwise motion in a rotating system. The changes in polarization of the flow are large enough to introduce a significant cross-shore velocity component which enhances the vertical shear in the tidal flow and is responsible for the strong semi-diurnal variation of stratification observed in this ROM system.