[1] Storm surges are the sea level response to meteorological conditions. Scientists and engineers need to understand the interaction of surges with the tide in order to provide better estimates of extreme sea level for use in coastal defense. Using data from five tide gauges, spaced equally along the North Sea coastline around the UK, we show that the mode of peak residual occurrence is everywhere 3 to 5 hours before the nearest high water. We reveal a previously unobserved mode that falls 1 to 2 hours prior to high water, although this cluster is not associated with the highest residuals. A simple mathematical explanation for surge clustering on the rising tide is presented. The phase shift of the tidal signal is combined with the modulation of surge production due to water depth in a model that provides a good description of the residual data set. The results contain several features of interest for flood risk management. We show that large, locally generated surges are precluded close to high water. For physically realistic arrival times of any travelling surge component, the residual peak will avoid high water for any finite tidal phase shift. Furthermore, increasing the tidal range reduces the risk of residual peaks near high water. We draw attention to the existence of critical time and space scales for surge development and decay. For reliable operational forecasts of sea level, coastal numerical models need to reproduce both tides and surges with improved accuracy.Citation: Horsburgh, K. J., and C. Wilson (2007), Tide-surge interaction and its role in the distribution of surge residuals in the North Sea,

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[1] Two-dimensional paleotidal simulations have been undertaken to investigate tidal and tide-dependent changes (tidal amplitudes, tidal current velocities, seasonal stratification, peak bed stress vectors) that have occurred in the NW European shelf seas during the last 20 ka. The simulations test the effect of shelf-wide isostatic changes of sea level by incorporating results from two different crustal rebound models, and the effect of the ocean-tide variability by setting open boundary values either fixed to the present state or variable according to the results of a global paleotidal model. The use of the different crustal rebound models does not affect the overall changes in tidal patterns, but the timing of the changes is sensitive to the local isostatic effects that differ between the models. The incorporation of ocean-tide changes greatly augments the amplitude of tides and tidal currents in the Celtic and Malin seas before 10 ka BP, and has a large impact on the distribution of seasonally stratified conditions, magnitude of peak bed stress vectors and tidal dissipation in the shelf seas. The predictions on seasonal stratification are supported by well-dated evidence on tidal mixing front migration in the Celtic Sea. Additional experiments using the global model suggest that the variability of offshore tides has been caused mainly by changes of eustatic sea level and ice-sheet extent. In particular, a large decrease observed at 10-8 ka BP is attributed to the opening of Hudson Strait accompanied by the retreat of the Laurentide Ice Sheet.

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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.