Anthony Wise

Transport mechanisms between the deep ocean and adjacent continental shelf seas play an important role in the spatial distribution of nutrient delivery to the coastal ocean and in the temporal variability of shelf biogeochemical processes. Along the North West European Shelf (NWES) edge, nutrient‐rich waters of oceanic origin are found below the mixed layer, representing a potential nutrient source for fueling new production on the shelf. We find persistent cross‐isobath geostrophic transport integrated over along‐isobath segments of the NWES edge in hydrographic climatologies and altimetric sea surface height gradients. This transport is O(1 cm/s), has little vertical structure, and is onshore along the entire extent of the 200 m isobath, except along the southern rim of the Norwegian Trench. Despite strong temporal variability in the shelf‐edge hydrography on seasonal to decadal timescales, changes in the ocean‐shelf geostrophic transport are subtle. This is due to a persistent large‐scale steric sea surface slope along the shelf edge. The geostrophic flow induces local depth‐integrated cross‐isobath nitrate fluxes of O(1–10 mmol/m/s). This is similar in magnitude to the winter wind‐driven nitrate transport, but is much less variable at seasonal and inter‐annual time scales. Variability in the geostrophic advection of nitrate is thus determined by the ocean‐shelf nitrate gradient's variability, rather than by the cross‐isobath flow's variability. Geostrophic transport may therefore be an important baseline component of the nutrient and carbon budgets on the NWES and other continental shelves, and should be considered in their long‐term response to climate‐scale forcing.

How will climate change affect the composition of the seabed? Ongoing climate change impacts nearly every area of our oceans. While coastal impacts of flooding and erosion are well studied, less attention has been paid to the impact of increased storminess and regional sea level rise on the seafloor, its sediments, and the benthic habitats which rely on it. Making use of a set of 3D hydrodynamic and wave models, and CMIP5 climate projections, we investigate and quantify the drivers of seafloor stresses. Our study maps the potential for grain size mobilisation in current and future projections, highlighting areas where seabed composition is likely to change in future. Sea level rise not only alters absolute water depth but also modifies current stresses by shifting the phase and magnitude of the tides. Rising sea levels also reduce exposure of the seabed to ocean waves. However, increased storm intensity in a warming world will lead to more frequent periodic disturbance of an otherwise increasingly immobile seabed. These predicted changes and their expected impact on the benthic environment are discussed in the context of the increasing anthropogenic activity in the marine environment and the associated challenges in marine spatial management.

Sea level variations in the coastal zone can differ significantly from those in the open ocean and can be highly spatiotemporally coherent in the alongshore direction. Yet, where and how coastal sea levels exhibit variations that emerge as persistent and recurrent patterns along the world's coastlines remain poorly understood. Here, we use a Bayesian mixture model to identify large‐scale patterns of coherent modes of monthly coastal sea level variations from coastal altimetry and tide gauge data. We determine nine clusters of coherent coastal sea level variability that explain a majority of the monthly variance measured by tide gauges (1993–2020). The analysis of along track altimetry data enables us to detect several additional clusters in ungauged regions, such as the Indian Ocean or around the South Atlantic basin, which have so far been poorly described. Although some clusters (e.g., at the eastern boundary of the Pacific, the western tropical Pacific, and the marginal and semi‐enclosed seas) are highly correlated with climate modes, other clusters share very little variability with the considered climate modes at the monthly timescale. Knowledge of these coherent regions thus motivates and enables further investigations on the impacts of local and remote forcing on coastal sea level variability, and the extent to which coastal sea level variability is decoupled from the adjacent deep ocean.

Future sea level rise and changes in extreme weather will increase the frequency of flooding and intensify the risks for the millions of people living in low‐lying coastal areas. Concerns about coastal adaptation have been broadened due to societal awareness of the threat from rising seas, leading to a large set of potential adaptation users with diverse needs for adequate sea level projections in coastal areas beyond the current state of the art regional projections. In this paper, we provide an overview of the potential steps for improvement of regional sea level projections along the global coastline, with specific focus on the contribution from ocean dynamics to seasonal‐decadal variability of coastal sea level, and its implications for changes in frequency and magnitude of extreme sea levels. We discuss the key gaps in our knowledge and predictive capability of these dynamics as they relate to sea level variability on seasonal to decadal timescales, and conclude by suggesting ways in which these knowledge gaps could be addressed.

Variability in ocean currents, temperature and salinity drive dynamic sea level (DSL) variability on the Northwest European Shelf (NWES). It is dominated by mass variations, with steric signals relatively small. A mechanistic explanation of how ocean dynamics relates to the mass component of NWES sea level variability is required. We use regional ocean model experiments to isolate sources of variability and then investigate the effect on monthly to‐interannual DSL variability together with the simulated momentum budgets along the shelfbreak. Regional (local) wind and non‐regional (remote) forcing are important on the NWES. For the local wind forcing, the net mass flux onto the shelf, which drives a shelf‐mean mode of DSL variability, results from a combination of surface Ekman, bottom Ekman and geostrophic flows and explains 73% of the variance in transport across the shelf‐edge. The geostrophic flow is closely related to wind stress with a flow about half that of surface Ekman transport but in the opposite direction. For the remotely forced mass‐flux across the shelf‐edge, the geostrophic component explains 62% of the variance and bottom friction plays an important indirect role. The remotely forced variability, while relatively spatially uniform, is more important for explaining DSL variance over the western NWES. This mode of variability is sensitive to signals propagating northward via a thin strip of the southern boundary near the Portuguese coast, consistent with coastal trapped wave signals. It also appears to drive steric height in the Bay of Biscay, which is related to DSL on the shelf.

Coastal trapped waves (CTWs) carry the ocean’s response to changes in forcing along boundaries and are important mechanisms in the context of coastal sea level and the meridional overturning circulation. Motivated by the western boundary response to high-latitude and open-ocean variability, we use a linear, barotropic model to investigate how the latitude dependence of the Coriolis parameter (β effect), bottom topography, and bottom friction modify the evolution of western boundary CTWs and sea level. For annual and longer period waves, the boundary response is characterized by modified shelf waves and a new class of leaky slope waves that propagate alongshore, typically at an order slower than shelf waves, and radiate short Rossby waves into the interior. Energy is not only transmitted equatorward along the slope, but also eastward into the interior, leading to the dissipation of energy locally and offshore. The β effect and friction result in shelf and slope waves that decay alongshore in the direction of the equator, decreasing the extent to which high-latitude variability affects lower latitudes and increasing the penetration of open-ocean variability onto the shelf—narrower continental shelves and larger friction coefficients increase this penetration. The theory is compared with observations of sea level along the North American east coast and qualitatively reproduces the southward displacement and amplitude attenuation of coastal sea level relative to the open ocean. The implications are that the β effect, topography, and friction are important in determining where along the coast sea level variability hot spots occur.

It is our aim with this paper to investigate how the presence of a continental shelf and slope alters the relationship between interior ocean dynamics and western boundary (coastal) sea level. The assumption of a flat-bottomed basin with vertical sidewall at the coast is shown to hide the role that depth plays in the net force acting on the coast. A linear β-plane theory is then developed describing the transmission of sea level over variable depth bathymetry as analogous to the steady advection–diffusion of a thermal fluid. The parameter , relating the friction parameter r to the bathymetry depth H and width , is found to determine the contribution of interior sea level to coastal sea level, with small giving maximum penetration and large maximum insulation. In the small (infinite friction) limit the frictional boundary layer extends far offshore, and coastal sea level tends toward the vertical sidewall solution. Adding simple stratification produces exactly the same result but with reduced effective depth and hence enhanced penetration. Penetration can be further enhanced by permitting weakly nonlinear variations of thermocline depth. Wider and shallower shelves relative to the overall scales are also shown to maximize penetration for realistic values of . The theory implies that resolution of bathymetry and representation of friction can have a large impact on simulated coastal sea level, calling into question the ability of coarse-resolution models to accurately represent processes determining the dynamic coastal sea level.