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\begin{document}

\title{Assessing 20th century tidal range changes in the North Sea}



\author[1]{Leon Jänicke}%
\author[1]{Andra Ebener}%
\author[2]{Sönke Dangendorf}%
\author[3]{Arne Arns}%
\author[4]{Michael Schindelegger}%
\author[1]{Sebastian Niehüser}%
\author[5]{Ivan David Haigh}%
\author[6]{Philip L. Woodworth}%
\author[7]{Jürgen Jensen}%
\affil[1]{Research Institute for Water and Environment, University of Siegen}%
\affil[2]{Old Dominion University}%
\affil[3]{Faculty of Agricultural and Environmental Sciences, University of Rostock}%
\affil[4]{University of Bonn}%
\affil[5]{University of Southampton}%
\affil[6]{National Oceanography Centre}%
\affil[7]{University of Siegen}%


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  \date{May 20, 2024}


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\begin{abstract}
In many places around the world, tide gauges have been measuring
substantial non-astronomical changes. Here 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 in the UK and up to 7
mm/yr in the German Bight between 1958 and 2014. These changes are
spatially heterogeneous, suggesting a superposition of local and
large-scale processes at work 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 UK and German coastlines, which suggest 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 suggests 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.%
\end{abstract}%



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