Geoscience Reference
In-Depth Information
10
The shelf edge system
At the outer edge of the shelf is a region where the gentle slopes of the shelf
give way to the much steeper topography of the continental slope, and bottom
depths rapidly increase down to the abyssal plains of the deep ocean. On
average, the depth at which this slope transition occurs is about 130 metres,
but this varies through the world's oceans. Off NW Europe the shelf edge is at a
depth of 200 metres, while at high latitudes the shelf edge is deeper, typically
400-500 metres around Antarctica and off Greenland. Because the topography
is steep, with slopes as large as 1:10, the transition between shelf and the deep
ocean is usually limited in extent (
50 km). It is in this rather narrow region that
the very different regimes of the shelf and the deep ocean adjust to each other.
In this chapter, we shall consider how this adjustment occurs and how it controls
the important exchanges between shelf and deep-ocean. We will look at wind-
driven upwelling, the most studied process linking the physics and the ecology of
the shelf edge which, in many parts of the world, supports important stocks of
plantivorous fish. We shall also consider the upwelling (and downwelling) driven
by the bottom Ekman layer of along-slope flows, and the consequences for
nutrient supply to, and organic material export from, shelf seas. We will describe
the density contrasts that develop in winter between temperate shelf seas and
the adjacent ocean that can lead to downslope cascades of shelf seawater and
its constituents. Finally, we consider the role of the internal tide, a prominent
shelf edge process which strongly influences the biochemistry and is important in
relation to commercial fisheries.
∼
10.1
Contrasting regimes
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The deep ocean and shelf regimes differ radically in a number of respects, illustrated
in the schematic in
Fig. 10.1
. Most obviously they differ in the depth of the water
column, on average 3.8 km for the deep ocean as opposed to 0-200 metres for the
shelf, a difference which has important consequences for the relative extent of
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