Tides, Stratification, and Counter Rotation: The German Bight ROFI in Comparison to Other Regions of Freshwater Influence

• Published in: Journal of geophysical research. Oceans 2022-06, Vol.127 (6), p.n/a
• Main Authors: Kopte, Robert, Becker, Marius, Holtermann, Peter, Winter, Christian
• Format: Article
• Language: English
• Subjects: ADCP; Advection; Coastal zone; Counter rotation; Current velocity; Decoupling; Density; Density gradients; Diurnal variations; Ebb currents; Ellipses; Fresh water; Freshwater; Geophysics; German Bight; Inland water environment; Instruments; Kelvin waves; Position measurement; Progressive waves; Rivers; ROFI; Rotation; Salinity; Semidiurnal tides; Standing waves; Stratification; Tidal currents; Tidal cycles; Tidal dynamics; tidal ellipses; Tidal motion; Tidal velocities; tidal‐straining; Tides; Velocity; Velocity distribution; Velocity profiles; Water circulation; Water column; Water stratification; Water temperature
• ISSN: 2169-9275; 2169-9291
• DOI: 10.1029/2021JC018236

The 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. Plain Language Summary The water velocities in the German Bight are strongly influenced by river freshwater input and semidiurnal tides. While the freshwater input from the rivers has a stratifying effect, tidal motion as well as wind and waves induce mixing. These forces compete in controlling the state of the water column in a complex manner. In theory, tidal motion should describe elliptical paths. However, in reality, these ellipses can be modified for example, by periodically occurring stratification. To assess the characteristics of the semidiurnal tidal velocity profile and its variability, four instruments to measure current velocity profiles were deployed in the German Bight during August 2016 and May 2018. Measurements were carried out for the duration of up to three tidal cycles, accompanied by parallel profiles of water temperature and salini