The shelf edge system - Introduction to the Physical and Biological Oceanography of Shelf Seas - page 315

Figure 10.7 Cross-shelf downwelling
circulation induced by a slope current
flowing northward (northern
hemisphere). Crossed circles indicate
the flow into the page. Black arrows
indicate the direction and strength of
the pressure gradient, grey arrows
the Coriolis force.

Se a su rface slop e

f V

1 / r 0 ∇ P

10.3.2 The bottom boundary layer

Just as in the surface layer, frictional stresses at the bottom boundary contravene the
assumption of geostrophy and allow an important cross-isobath flow forced by a
slope current. As we have seen in Section 10.2.2, slope currents are often approxi-
mately barotropic and thus their influence extends to the seabed over the slope. The
effect of bed friction on an along-slope flow is shown in Fig. 10.7 . Frictional effects in
the bottom boundary layer slow the current so that the Coriolis force near the bed is
reduced and there is, therefore, an unbalanced pressure component which drives a
down slope flow (large white arrows in the bottom Ekman layer in Fig. 10.7 ).
Continuity requires a return flow towards the shelf in the upper part of the water
column (small white arrows).
The example of Fig. 10.7 is analogous to the slope current along the Hebridean shelf
edge west of Scotland. Flowing with the shelf to the right of the current direction, the
near-bed current is deflected to the left of the direction of the slope current, i.e. in the
downslope direction (look at the cross-shelf sea surface slope in Fig. 10.5 ). If the stress
is related to the barotropic current velocity v 2 , the ^ v by a quadratic drag law b ¼ k b 0 ^

net transport in the cross-slope direction has a magnitude of:

v 2
f

T b ¼ b

k b ^

0 f ¼

: ð 10 : 15 Þ

At latitude 58 N with

^

2ms 1 v ¼ 0 : and k b ¼ 0.0025, the net transport across

0.8 m 2 s 1 . In a steady state, continuity will require that this
cross-slope flow is balanced by a compensating flow in the upper part of the water
column, as in Fig. 10.7 . In view of the consistency of the slope current, this bottom
Ekman-layer flow provides an important mechanism for the transfer of material
between the shelf and the deep ocean, sometimes referred to as the Ekman drain.
If the along-slope current is in the opposite direction (shelf to the left of the
current direction), transport in the bottom boundary layer will be upslope
(northern hemisphere). This is the case along part of the east coast of North America,
where the Gulf Stream flow is northwards over the slope. The resulting upslope

the slope is T b '