Abstract
We document an exceptional large‐spatial scale case of changes in tidal range in the North Sea, featuring pronounced trends between −2.3 mm/yr at tide gauges in the United Kingdom and up to 7 mm/yr in the German Bight between 1958 and 2014. These changes are spatially heterogeneous and driven by a superposition of local and large‐scale processes within the basin. We use principal component analysis to separate large‐scale signals appearing coherently over multiple stations from rather localized changes. We identify two leading principal components (PCs) that explain about 69% of tidal range changes in the entire North Sea including the divergent trend pattern along United Kingdom and German coastlines that reflects movement of the region’s semidiurnal amphidromic areas. By applying numerical and statistical analyses, we can assign a baroclinic (PC1) and a barotropic large‐scale signal (PC2), explaining a large part of the overall variance. A comparison between PC2 and tide gauge records along the European Atlantic coast, Iceland, and Canada shows significant correlations on time scales of less than 2 years, which points to an external and basin‐wide forcing mechanism. By contrast, PC1 dominates in the southern North Sea and originates, at least in part, from stratification changes in nearby shallow waters. In particular, from an analysis of observed density profiles, we suggest that an increased strength and duration of the summer pycnocline has stabilized the water column against turbulent dissipation and allowed for higher tidal elevations at the coast.
Highlights
For thousands of years, tides have had a great influence on coastal areas globally and their residents
To improve our understanding of these puzzling tidal range changes, we aim to address the following questions through systematic data analysis: (1) Are these changes on different time scales detected within the German Bight a localized phenomenon, or are they part of a larger-scale development spreading over adjacent areas within or even outside the North Sea region? (2) Is it possible to separate and quantify large-scale and small-scale effects from observed records? (3) If (2) is the case; can we attribute physical causes to the observed changes?
To address the three research questions defined in the introduction, we first map the spatial extent of the long-term changes in tidal range in the study area
Summary
Tides have had a great influence on coastal areas globally and their residents. Tides control the navigability of some ports and sea routes, and have a major influence on the intensity and timing of extreme sea levels during storm surges (e.g., Arns et al, 2020; Horsburgh & Wilson, 2007; Prandle & Wolf, 1978). Given their close connection to the periodic and predictable nature of astronomical variations, the amplitudes and phases of tidal constituents, and corresponding tidal water levels, are generally assumed to be constant on time scales over which basin geometry undergoes only minor changes (i.e., decades to centuries). Doodson (1924) pointed to appreciable secular perturbations in the local tidal regimes of JÄNICKE ET AL
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