E8 Interaction Ocean - Bottom and Coasts

Topic

The interaction of the ocean with the seabed and coastlines encompasses four processes occurring where water ceases to be deep and unconstrained and instead encounters a solid boundary: the bottom boundary layer, wave reflection off the coast, coastal upwelling and downwelling, and mixing within estuaries. It is precisely this proximity to a solid boundary that causes the ocean to behave in these zones in a fundamentally different way than it does in the open sea.

A boundary layer forms along the ocean floor that is analogous to the Ekman layer previously described at the surface; under geostrophic balance, bottom friction creates a layer—between fifty and one hundred meters thick—in which the flow deflects to the left of the flow above it (in the Northern Hemisphere), following an Ekman spiral that is inverted relative to the surface one. This same bottom friction causes the boundary layer to serve as the starting point for much of the coastal circulation described below.

Coastal upwelling occurs when the wind drives surface water offshore via Ekman transport—leaving the coast to the left of the wind direction in the Northern Hemisphere. This offshore transport creates a water deficit near the coast, which is compensated for by the upwelling of cold subsurface water from the depths. Such upwelling is particularly intense leeward of coastal headlands, where it generates coastal jet currents—flowing at speeds of 0.3 to 0.5 meters per second—and gives rise to cold-water filaments extending tens of kilometers out to sea. Estuaries are transition zones where freshwater flowing from the continents meets saltwater from the sea; the way these two water masses mix determines both the estuary's salinity structure and the two-layer circulation pattern established there—saltwater entering along the bottom and freshwater exiting at the surface. A dimensionless number specific to the estuary—comparing river-supplied buoyancy with tidal mixing—determines whether the estuary becomes well-mixed, partially mixed, or strongly stratified (with a saltwater wedge advancing along the bottom), depending on which of these two forces dominates; furthermore, much of the transport of sediments and nutrients toward the open sea depends on this mixing process.

Topographically trapped waves—including coastal Kelvin waves and continental shelf waves—propagate along the coastline, following the bottom contours, and decay exponentially toward the open ocean over a distance comparable to the Rossby radius of deformation. It is this ability to travel hugging the coast without losing energy to the open ocean that makes these waves the primary mechanism connecting different sections of the same coastline; through them, climate signals generated on one coast are transmitted over thousands of kilometers to shores far removed from their point of origin.

Thus, from the bottom boundary layer that deflects flow near the seabed to the trapped waves transmitting signals between coasts—and including upwelling that brings cold water to the surface and the mixing that shapes each estuary—this combination of processes ensures that the ocean's interaction with the seafloor and coastlines governs the distribution (near the shore) of cold water, sediments, and nutrients. These are elements the open ocean cannot distribute on its own within the narrow strip where the seabed and the coast impose their own limits.

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