Tidal wave transformations in the German Bight

Abstract

Mesoscale and submesoscale dynamics associated with tidal wave transformations were addressed in the German Bight using numerical simulations. Tidal gauge and velocity observations in several locations were used to validate the numerical model. A downscaling approach included analysis of simulations with horizontal resolutions of 1, 0.4, and 0.2 km. It was shown that the modified tidal wave lost most of its energy after reflection or refraction over the eastern part of the German Bight. Energy loss resulted in a pronounced change of the wave’s spectral composition and generation of overtides. Tidal oscillations were modified by mesoscale processes associated with bathymetric channels. Semidiurnal and quarterdiurnal tides revealed very different spatial patterns. The former were aligned with the bathymetric channels, while the latter were rather “patchy” and had about half the spatial scales. In numerous areas around the bathymetric channels, the major axis of the M4 ellipses was normal or at some angle with the major axis of the M2 ellipses. Thus, higher harmonics developed “orthogonal” patterns that drove secondary circulations. Moreover, the ratio between spring and neap tidal amplitudes was relatively low in the Wadden Sea, showing reduced sensitivity of this very shallow area to fortnightly tidal variations. It was demonstrated that simulated hydrodynamics patterns help explain the physical mechanism shaping the median grain size distribution in the German Bight.