How open ocean dynamics affects coastal sea level, an investigation of the influence of the continental shelf and slope

Large scale, annual to decadal period motions in the deep ocean drive large scale gradients in dynamic sea level that are important as baseline levels with the potential to amplify short term sea level fluctuations from extreme events such as storm surges. Model projections assessing flood risk therefore depend on accurately simulating both short and long term variability. The decreasing depth of the bathymetry from the deep ocean to the shallow coastal zone tends to produce an insulating effect between the deep ocean and the coast. Nevertheless, studies show coherence between signals on- and off-shore over vast distances, implying that remote driving of the coastal sea level by deep ocean forcing is important. Western boundaries of ocean basins are of particular interest as the rotation and curvature of the Earth result in the intensification of currents and sea level gradients adjacent to the coastal zone. This thesis seeks to develop an improved physical understanding of how coastal sea level along a western boundary is related to the open ocean and to what extent this relationship depends on bottom topography h, the effects of the Coriolis parameter f, and friction. To investigate these questions, a hierarchy of idealized models is developed. Analytic and numerical solutions are sought for the linearised problem, while an idealized configuration of the NEMO General Ocean Circulation Model is developed to probe more realistic scenarios in the North Atlantic. For a homogeneous ocean layer above sloping topography that is uniform alongshore, the β-effect and bottom topography result in the mean dynamic sea level tending to follow h/f contours. The inclusion of friction allows sea level to cross these contours, with steeper topography and a larger friction parameter promoting penetration to the coast. This leads to the equatorward displaced and attenuated coastal sea level (relative to the open ocean) that is seen in observations. Fluctuations in the open ocean are shown to generate a new type of leaky Slope wave that transmits energy equatorward and dissipates locally on the slope and via the radiation of short Rossby waves into the interior ocean. Coastal sea level is explicitly related to poleward and open ocean sea level in terms of coastally trapped boundary waves and it is shown how the β-effect and friction result in enhanced wave decay that promotes signal penetration from ocean to coast. Steeper topography “kills-off” the boundary waves and also promotes penetration, tending towards the vertical sidewall solution in the steep limit. The inclusion of realistic bathymetry and non-linearity, which allows for the advection of potential vorticity, is not found to significantly alter the character of the solution. In particular, hot spots of off-slope sea level variability are found to excite waves that exert control over equatorward coastal latitudes, demonstrating the poleward control of coastal sea level along western boundaries. It is concluded that shelf seas are vulnerable to on and off-shelf variability originating many degrees of latitude further poleward, and that the representation of friction and bottom topography are important for numerical simulations to faithfully capture the effect of the open ocean on the coastal sea level