Scientists and engineers have observed for some time that tidal amplitudes at many locations are shifting considerably due to nonastronomical factors. Here we review comprehensively these important changes in tidal properties, many of which remain poorly understood. Over long geological time scales, tectonic processes drive variations in basin size, depth, and shape and hence the resonant properties of ocean basins. On shorter geological time scales, changes in oceanic tidal properties are dominated by variations in water depth. A growing number of studies have identified widespread, sometimes regionally coherent, positive, and negative trends in tidal constituents and levels during the 19th, 20th, and early 21st centuries. Determining the causes is challenging because a tide measured at a coastal gauge integrates the effects of local, regional, and oceanic changes. Here, we highlight six main factors that can cause changes in measured tidal statistics on local scales and a further eight possible regional/global driving mechanisms. Since only a few studies have combined observations and models, or modeled at a temporal/spatial resolution capable of resolving both ultralocal and large‐scale global changes, the individual contributions from local and regional mechanisms remain uncertain. Nonetheless, modeling studies project that sea level rise and climate change will continue to alter tides over the next several centuries, with regionally coherent modes of change caused by alterations to coastal morphology and ice sheet extent. Hence, a better understanding of the causes and consequences of tidal variations is needed to help assess the implications for coastal defense, risk assessment, and ecological change.

For thousands of years, tides have had a great influence on coastal areas globally and their residents. Today they play a critical role in influencing economic considerations, nautical safety, renewable energy schemes, assessments of land erosion, and the definition of geodetic datums (Haigh et al., 2020; Pugh & Woodworth, 2014). Tides not only control the navigability of some ports and sea routes, but also 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). However, Keller (1901) showed increased tidal amplitudes due to reflection and local resonance changes as a result of building measures such as weirs (e.g., in the Ems River). Similarly Doodson (1924) pointed to appreciable secular perturbations in the local tidal regimes of

The scientific assessment of past and future mean sea level (MSL) trends requires reliable predictions of natural cyclic behavior on short and long time scales, with the current rate of sea-level rise (SLR) being estimated at 2 mm/year in the North Sea (Dangendorf et al., 2015). On a global scale, rates around 1.5 mm/ year between 1900 and 2012 are detected (Oppenheimer et al., 2019) as well as recent accelerations of up to 3 mm/year (Dangendorf et al., 2019). Typical cycles concerning sea level are tides, which are a result of the gravitational potential of sun and moon, centrifugal force of the earth and meteorological forcing. Tides are distinguished by their frequency, which is predominantly diurnal or semidiurnal, even though monthly, interannual, annual, and perennial frequencies exist as well. The nodal tide (Bradley, 1728) is a harmonic signal with a period of 18.61 years, caused by the precession of the lunar ascending node (Pugh, 1987). It is the most important low frequency tidal constituent apart from the lunar perigee and has shown to have an amplitude of up to 30 cm (Peng et al., 2019). In order to consider the nodal tide in MSL analysis, the theoretical equilibrium tide concept is applied (Godin, 1986; Proudman, 1960; Woodworth, 2012). Nonresolved low frequency cyclic behavior of water levels may lead to an erroneous estimation of SLR. The influence

<p>Tide gauges throughout the North Sea basin show significant changes in the local tidal regime since the mid-20th century, especially in the German Bight area. These changes were analyzed within the DFG-funded project TIDEDYN (Analyzing long term changes in the tidal dynamics of the North Sea, project number 290112166) and the final results were recently published in Jänicke et al. (2020, https://doi.org/10.1029/2020JC016456).</p><p>In this paper, 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 UK 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 UK 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.</p><p>We would like to present these research results and the content of the paper (cf. Jänicke et al., 2020) at vEGU21, hoping to encourage subsequent questions and further discussions.</p>

No abstract