The shelf edge system - Introduction to the Physical and Biological Oceanography of Shelf Seas - page 304

Deep ocean (~4000 m) Shelf (~200 m)

Wind stress Wind stress

Heat

Cool Fluxes?

Surface boundary layer

A

Deep interior:
Permanently stratified
Geostrophic currents
Weak turbulence
Separate boundary layers

Shelf:
Seasonally stratified
Strong currents (tides)
Much turbulence
Boundary layers can overlap

B

B ottom bou nda ry lay er

~50 km

Figure 10.1 The very different dynamical regimes of the deep ocean and the shelf sea must
adjust to each other in the rather narrow region of the slope. This mutual adjustment involves
dynamical processes which contribute to the specialised environment of the slope. A is the shelf
edge, the region marked B is the continental slope. The ship silhouette (RRS James Cook,
length 100m) is in proportion to the 200m depth of the illustrated shelf edge, but remember
that the horizontal scale is substantially shortened.

boundary influences. We have seen that the frictional boundary layers induced at the
surface and bottom occupy a large proportion of the water column in the shelf seas.
These boundary layers are frequently energetic with high levels of turbulence; they
may overlap and promote complete mixing through the water column. By contrast,
in the deep ocean, the boundary layers occupy only a very small fraction of the total
depth. Moreover, the bottom boundary layer is generally much weaker in the deep
ocean because the barotropic tidal currents are typically 1-2 orders of magnitude
smaller than on the shelf. In between the relatively thin surface and bottom boundary
layers of the deep ocean is a large interior region where conditions are isolated from
boundary stresses and where the flow varies rather slowly and is normally close to
geostrophic balance. Turbulence in the deep ocean, between the boundary layers, is
generally very weak; on average the typically vertical diffusivity is only about
1

10 5 m 2 s 1 (Munk, 1966 ; Munk and Wunsch, 1998 ). There is considerable
interest in the role of steep seabed topography in the deep ocean increasing turbu-
lence in the ocean interior (e.g. Ledwell et al., 2000 ; Garabato et al., 2004 ), but even
in these regions the strength of the turbulence is substantially lower than that which
we find in shelf seas.
At the shelf edge, these very different dynamical regimes exist in rather close prox-
imity, separated only by the narrow region of the continental slope. Within the width of
the slope, the differences between the regimes have to be reconciled. The processes
involved in this mutual adjustment create a rather special shelf edge regime which, as we
shall see, has profound effects on the biology over a range of trophic levels. In particular
the flow field and the mixing occurring at the shelf edge are of crucial importance in
relation to the transfer of nutrients between the open ocean and the shelf seas and the
fixation of carbon and its transfer as organic carbon across the slope into the deep ocean.