Introduction to the shelf seas - Introduction to the Physical and Biological Oceanography of Shelf Seas - page 1

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Introduction to the shelf seas

In this chapter we shall introduce the reader to the shelf seas, their extent and
position in the global ocean and the motivation, both fundamental and applied,
behind our efforts to understand and model the complex processes which control
the shelf sea environment and ecosystem. We shall then briefly explain the
historical development of shelf sea science and describe the technical tools which
are now available and which have facilitated the relatively rapid advances
of recent years. As well as discussing the principal observational techniques, in
a final section we shall consider the role of numerical modelling and its potential
contribution to developing understanding.

1.1 Definition and relation to the global ocean

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Between the deep oceans and the continents lie the seas of the continental shelf. These
shallow areas usually have rather flat seafloors and extend out to the shelf break,
where the seabed inclination generally increases rapidly at the top of the continental
slope leading down to the abyssal ocean. This abrupt change of slope is clear in
the map of global bathymetry shown in Fig. 1.1a . It typically occurs at a depth of
∼ 200 metres and a contour, or isobath, at this depth is often taken as defining the

outer limit of the shelf seas. This choice is not critical, however, since the continental
slope is so steep (

1:10); moving from the 200- to the 500-metre isobath involves little
horizontal movement. Using the basis of a 500-metre definition, Fig. 1.2a shows that
the shelf seas account for

∼

9% of the total area of the global ocean and less than
0.5% of the volume. The shelf seas have an influence and importance quite out
of proportion to these numbers.
The shelf seas act to dissipate a high proportion of the mechanical energy input
to the ocean. This is most obvious in the swell waves which are generated in the deep
ocean and travel great distances before delivering large quantities of energy to be
consumed in wave breaking and bottom friction on the shelf. Less obviously, per-
haps, energy input in the deep ocean by the tidal forces also propagates onto the shelf
in the form of very long waves which are dissipated by friction in the large tidal
currents of the shelf seas.

∼