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.

Measurements of the intra-tidal and spring-neap variation in the vertical flux of nitrate into the base of the sub-surface chlorophyll maximum (SCM) were made at the shelf edge of the Celtic Sea, a region with strong internal mixing driven by an internal tide. The neap tide daily mean nitrate flux was 1.3 (0.9-1.8, 95% confidence interval) mmol m 22 d 21 . The spring tide flux was initially estimated as 3.5 (2.3-5.2, 95% confidence interval) mmol m 22 d 21 . The higher spring tide nitrate flux was the result of turbulent dissipation occurring within the base of the SCM as compared to deeper dissipation during neap tides and was dominated by short events associated with the passage of internal solitons. Taking into account the likely under-sampling of these short mixing events raised the spring tide nitrate flux estimate to about 9 mmol m 22 d 21 . The neap tide nitrate flux was sufficient to support substantial new production and a considerable fraction of the observed rates of carbon fixation. Spring tide fluxes were potentially in excess of the capacity of the phytoplankton community to uptake nitrate. This potential excess nitrate flux during spring tides may be utilized to support new production during the lower mixing associated with the transition toward neap tide. The shelf edge is shown to be a region with a significantly different phytoplankton community as compared to the adjacent Celtic Sea and northeast Atlantic Ocean, highlighting the role of gradients in physical processes leading to gradients in ecosystem structure.3 Present address: Proudman Oceanographic Laboratory, 6 Brownlow Street, Liverpool, L3 5DA, United Kingdom. AcknowledgmentsOur thanks to the crew of the RRS Charles Darwin (cruise CD173) and the technical staff of the U.K. National Marine Facilities. We are grateful for the constructive comments from two anonymous reviewers, which helped improve this paper.

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Turbulent mixing in the ocean is key to regulate the transport of heat, freshwater and biogeochemical tracers, with strong implications for Earth’s climate. In the deep ocean, tides supply much of the mechanical energy required to sustain mixing via the generation of internal waves, known as internal tides, whose fate—the relative importance of their local versus remote breaking into turbulence—remains uncertain. Here, we combine a semi-analytical model of internal tide generation with satellite and in situ measurements to show that from an energetic viewpoint, small-scale internal tides, hitherto overlooked, account for the bulk (>50%) of global internal tide generation, breaking and mixing. Furthermore, we unveil the pronounced geographical variations of their energy proportion, ignored by current parameterisations of mixing in climate-scale models. Based on these results, we propose a physically consistent, observationally supported approach to accurately represent the dissipation of small-scale internal tides and their induced mixing in climate-scale models.

The link between secular changes in the lunar semidiurnal ocean tide (M 2 ) and relative sea level rise is examined based on numerical tidal modeling and the analysis of long-term sea level records from Europe, Australia, and the North American Atlantic coasts. The study sets itself apart from previous work by using a 1 ∕ 12 ∘ global tide model that incorporates the effects of self-attraction and loading through time-step-wise spherical harmonic transforms instead of iteration. This novel self-attraction and loading implementation incurs moderate computational overheads (some 50%) and facilitates the simulation of shelf sea tides with a global root mean square error of 14.6 cm in depths shallower than 1,000 m. To reproduce measured tidal changes in recent decades, the model is perturbed with realistic water depth changes, compiled from maps of altimetric sea level trends and postglacial crustal rebound. The M 2 response to the adopted sea level rise scenarios exhibits peak sensitivities in the North Atlantic and many marginal seas, with relative magnitudes of 1-5% per century. Comparisons with a collection of 45 tide gauge records reveals that the model reproduces the sign of the observed amplitude trends in 80% of the cases and captures considerable fractions of the absolute M 2 variability, specifically for stations in the Gulf of Mexico and the Chesapeake-Delaware Bay system. While measured-to-model disparities remain large in several key locations, such as the European Shelf, the study is deemed a major step toward credible predictions of secular changes in the main components of the ocean tide. Key Points: • Effects of present-day sea level changes on global tides, primarily M 2 , are studied using a nonlinear barotropic model • The model operates at high accuracy for its horizontal resolution and an explicit treatment of self-attraction and loading • The sign of M 2 long-term trends is correctly simulated at 36 of 45 tide gauge stations Supporting Information: • Supporting Information S1 Correspondence to: M. Schindelegger, schindelegger@igg.uni-bonn.de Citation: Schindelegger, M., Green, J. A. M., Wilmes, S.-B., & Haigh, I. D. (2018). Can we model the effect of observed sea level rise on tides? Journal of Geophysical Research: Oceans, 123, 4593-4609.