Assessing changes of tidal dynamics in the North Sea

Perhaps because of the close connection between the periodic and predictable nature of astronomical variations and the corresponding tidal water levels, the tidal parameters of tidal low water, tidal high water and tidal range were generally assumed to be constant on time scales over which basin geome-try undergoes only minor changes (i.e. decades to centuries, Jänicke et al., 2020). Jensen (1984) and Führböter & Jensen (1985) were the first to discuss large-scale, potentially basin-wide changes in tidal parameters for the North Sea in the second half of the 20th century. Nevertheless, the spatial extend of these changes as well as their forcing factors remain uncertain, considering the stationarity of astronomical forcing over the period of tide gauge observations. The superposition of small-scale and large-scale effects on water level has proven to be particularly problematic in causal research and been identified by many studies in the past as the most important unsolved obstacle in identifying physical causes and attributing them to observed tidal changes (Woodworth, 2010; Haigh et al., 2020; Talke & Jay, 2020).
To examine the spatial extent of tidal changes in the North Sea, in this thesis the long-term trends of the three tidal parameters were considered first and significant changes were detected. A median basin-wide increase of both tidal low and high water was detected, forming a dipole-like pattern with tidal low water trends exceeding tidal high water trends in the United Kingdom (UK) and vice versa in the German Bight. As a result, negative trends of tidal range occur in the UK whereas positive trends can be detected in the German Bight. A Principal Component Analysis was then performed to separate large-scale signals appearing coherently over multiple stations from rather localized, small-scale changes. Two leading principal components (PCs) were identified for each of the three tidal parameters, representing large-scale effects, which explain a major part of the changes and resemble the dipole pattern of the trends. In a last step, physical causes were assigned to these signals and large-scale changes in both tidal low and high water could be assigned mainly to changes in Mean Sea Level. Furthermore, changes in tidal range were attributed to a baroclinic (PC1) and a barotropic large-scale signal (PC2) by applying numerical and statistical analyses. PC2 is caused by an external and basin-wide forcing mechanism from the adjacent North Atlantic, while PC1 dominates in the southern North Sea and originates, at least in part, from stratification changes in nearby shallow waters.
Therefore, it was possible to not only detect and describe large-scale tidal changes, but to identify, separate, and quantify the underlying large-scale effects for the first time. These analyses not only significantly improve the fundamental understanding of the underlying processes, but could also form the basis for future forecasting of tidal changes and their consequences, e.g. in coastal protec-tion or the navigability of North Sea ports.