Semidiurnal switching of stratification in the Region of Freshwater Influence of the Rhine

Observations in the Rhine region of freshwater influence (ROFI) system in the North Sea show evidence of large semidiurnal oscillations in stability, superimposed on a mean stratification, occurring throughout the stratified region at times of reduced mixing. The amplitude of this semidiurnal variation is of the same order as the mean stability and frequently results in conditions being mixed or nearly mixed once per tide. It is deduced that this semidiurnal variation results primarily from cross-shore tidal straining which interacts with the density gradient to induce stratification. This conceptual picture of the contributing processes has been tested in a one-dimensional point model forced by the observed slopes and the local density gradients. The model exhibits the same qualitative behavior as the observations, produces oscillations in stratification of the amplitude observed, and confirms the critical role of cross-shore tidal straining. The large cross-shore shear nder stratified conditions is identified with the changes in ellipse configuration which are observed between mixed and stratified conditions. The occurrence of semidiurnal variations in stability in the Rhine ROFI is thus inferred to be a consequence of the development of mean stability whenever the horizontal density gradients relax in conditions of low stirring.