Assessment of tidal range changes in the North Sea from 1958 to 2014

Abstract

<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>

Robust validation of trends and cycles in sea level and tidal amplitude in the Dutch North Sea

“…Indeed, the effect of the closure is clearly visible in a marked increase of the tidal amplitudes in Harlingen and similar but smaller in Den Helder. The effects are consistent with the forecasts by Lorentz (1926) and the empirical findings of Jänicke (2022) and Thijsse (1972).…”

Predominant landward skewing of tidal meanders

“…Tidal regimes of individual study cases were also differentiated based on the values of tidal ranges. The latter were computed using tidal‐gauge water level data from Dong (2020), with tidal ranges being classified as macro‐tidal when >4 m, meso‐tidal from 2–4 m, and microtidal if <2 m (Jänicke et al, 2021). Tidal asymmetry can be determined directly by two methods according to its definition: duration asymmetry, according to which flood‐dominant currents have a ratio between ebb duration and flood duration > 1; and velocity asymmetry, according to which flood‐dominant currents have a ratio between peak‐flood velocity and peak‐ebb velocity > 1.…”

Extended Water Level Trends at Long‐Record Tide Gauges Via Moving Window Averaging and Implications for Future Coastal Flooding

“…Large magnitude and spatially coherent decreased PEWL rates over the half‐century scale are seen in the North Sea. Long‐term sea level rise has been extensively studied in this region (Albrecht et al., 2011; Dangendorf et al., 2013; Jänicke et al., 2020; Wahl et al., 2013), as has the large tidal range (Horsburgh & Wilson, 2007; Pickering et al., 2012) and the large seasonal cycles (Plag & Tsimplis, 1999) present here. In our century‐scale estimates, we see comparable results as previous studies, with consistently positive rates in PEWL 100 at most European locations, and small but coherent increases of σ EWL 100 .…”