The influence of baroclinity on tidal ranges in the North Sea

Abstract

On short time scales, the tidal range has a significant influence on coastal inundations. Therefore, it is very important to investigate the dynamics of tidal range variations over different time scales. The baroclinity has the potential to modulate surface tides through ocean stratification on seasonal and secular scales. In order to better understand the influence of ocean stratification on tidal ranges in the North Sea, numerical simulations were carried out in the baroclinic and the barotropic modes covering the period from 1948 to 2014, using the regional 3D hydrodynamic prognostic Hamburg Shelf Ocean Model (HAMSOM). The tidal range difference between the baroclinic and the barotropic modes in winter and summer are compared at 22 tide-gauge stations, where the simulated sea surface elevations agree well with observations from 1950 to 2014. The statistical analysis shows that generally the difference in summer is much larger than in winter, with a maximum magnitude of approximately 11 cm. The spatial distribution of the tidal range difference indicates a westward shift in the amphidromic system of the North Sea due to baroclinity, which originates from an altered vertically averaged tidal current velocity in stratified regions, as an effect of decoupling induced by stratification. This implies subsequent alterations of tidal ranges in coastal well-mixed regions. Finally, it is demonstrated that, on the secular scale, an enhanced stratification and ocean warming may lead to positive trends in the tidal range and the mean sea level, respectively. Both can have a significant impact on future coastal inundation.