Abstract
AbstractThe characteristics of tidal velocity profiles and their relation to stratification are investigated based on high‐resolution field data collected at four locations in the German Bight Region of Freshwater Influence (ROFI) in the North Sea. The deployments each include two to three tidal cycles and were conducted during field campaigns in August 2016 and May 2018. The depth‐averaged semidiurnal tidal motion is dominated by a standing wave directed toward the coast, but modified by a smaller, coast‐parallel progressive wave contribution. The time series of the tidal velocity profiles consistently show tidal asymmetries with higher flood than ebb velocities near the surface and counter‐clockwise rotation of the velocity trajectories at depth. Near the surface, phase‐locked periodic changes in the sense of rotation within the tidal cycle are evident for three deployments, resulting in periodic counter‐rotation of the upper and lower layer. During these episodes, stratification of the water column is observed. Counter‐rotation is initiated after a sudden decoupling developing from the surface downward, with subsequent rapid development of stratification and velocity shear. The observed decoupling is most likely triggered by advection of the plume‐induced lateral surface density gradient by weakly sheared ebb currents toward the study site. Due to the dominance of the standing wave in the German Bight ROFI, the observed intra‐tidal variations of stratification are more similar to the Liverpool Bay and differ significantly from the Rhine ROFI, where the tidal dynamics are controlled by a progressive Kelvin wave.
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