Abstract
The fact that ocean currents must flow parallel to the coast leads to the dynamics of coastal sea level being quite different from the dynamics in the open ocean. The coastal influence of open-ocean dynamics (dynamics associated with forcing which occurs in deep water, beyond the continental slope) therefore involves a hand-over between the predominantly geostrophic dynamics of the interior ocean and the ageostrophic dynamics which must occur at the coast. An understanding of how this hand-over occurs can be obtained by considering the combined role of coastal trapped waves and bottom friction. We here review understanding of coastal trapped waves, which propagate cyclonically around ocean basins along the continental shelf and slope, at speeds which are fast compared to those of baroclinic planetary waves and currents in the open ocean (excluding the large-scale barotropic mode). We show that this results in coastal sea-level signals on western boundaries which, compared to the nearby open-ocean signals, are spatially smoothed, reduced in amplitude, and displaced along the coast in the direction of propagation of coastal trapped waves. The open-ocean influence on eastern boundaries is limited to signals propagating polewards from the equatorial waveguide (although a large-scale diffusive influence may also play a role). This body of work is based on linearised equations, but we also discuss the nonlinear case. We suggest that a proper consideration of nonlinear terms may be very important on western boundaries, as the competition between advection by western boundary currents and a counter-propagating influence of coastal trapped waves has the potential to lead to sharp gradients in coastal sea level where the two effects come into balance.
Highlights
In the open ocean, sea-level gradients [strictly, dynamic sea-level gradients as defined in Gregory et al] are, to first order, in geostrophic balance with currents near the surface, with wind stress adding an additional flow in the surface Ekman layer
We focus on the way in which an understanding of coastal trapped waves (CTWs) informs the interpretation of this coastal response
This lack of penetration to the coast is consistent with what is seen in satellite altimetry: sea-level spectra near western boundaries differ strongly between the coast and open ocean, the two regions being typically separated by a minimum of variability near the top of the continental slope (Hughes and Williams 2010; Zhai et al 2010)
Summary
Sea-level gradients [strictly, dynamic sea-level gradients as defined in Gregory et al (this volume)] are, to first order, in geostrophic balance with currents near the surface, with wind stress adding an additional flow in the surface Ekman layer. As the coast is approached, a different dynamical balance must come into play. This change in dynamical balance has, in many cases, the effect of reducing the size of the signal, so that sea-level changes at the coast can be smaller than nearby open-ocean changes. When a change of forcing occurs on the ocean, the oceanic response to that change can be felt at distant locations after some time. This information transfer happens in part because of advection by ocean currents, but usually the fastest response is mediated by waves. We start here by summarising elements of wave phenomenology, identifying salient features to be described in more detail later in this paper
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