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 p resence of stratification modifies the vertical structure of the tidal ellipse characteristics. During periods of stratification, the ellips es change from degenerate to a more circular pattern, with the surface ellipse rotating clockwise and the bottom ellipse rotating anticlockw ise. The surface to bottom ellipticity difference Delta epsilon is fou nd to be closely related to a bulk Richardson number which incorporate s both the stratification and a measure of the tidal shear. An explana tion of the observed dependency of ellipticity on the density structur e is offered in terms of the different thickness of the frictional lay ers for clockwise and anticlockwise motion in a rotating system, The c hanges in polarization of the how are large enough to introduce a sign ificant cross-shore velocity component which enhances the vertical she ar in the tidal flow and is responsible for the strong semi-diurnal va riation of stratification observed in this ROFI system.