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of water parallel to the shelf edge with relatively little movement across the slope. The
slope current along the Hebridean shelf edge, which was the focus of an extensive
study of processes at the shelf edge, 2 provides a good illustration of the general
character of these currents. The bathymetry of the region is shown in Fig. 10.3a .
Each of two sections of the along-slope flow in Fig. 10.3b ,c are based on a series of
repeated traverses across the shelf over a period of 13 hours with a shipborne ADCP
(Souza et al., 2001 ). After removal of the tidal signal by a least squares analysis
(Simpson et al., 1990 ), a strong jet-like current is seen to flow along the slope with
speeds approaching 0.3 m s 1 in the core of the current. The close similarity of the
structure at the two sections, which are separated by about 55 km along the slope,
indicates the spatial consistency of the current. The flow is largely barotropic (i.e.
almost depth independent), is
15-25 km wide and has a maximum over the steep
slope close to the 400-metre isobath. The longer-term existence of the slope current is
shown in Fig. 10.3d using a 25-day average of the slope current in the summer
derived from ADCP and current meter moorings. This shows a similar structure
but with a lower maximum flow speed. The currents meters also showed a very weak
cross-shore flow (
0.02 m s 1 ). Below the wind mixed layer (
<
100-metre depth) the
current vector; v
is remarkably consistent. We can quantify this by defining a
'steadiness factor',
¼ j
u
j
vector mean
scalar mean
St
j ¼
ð
10
:
7
Þ
j
u
with St
¼
1 for a flow with constant direction. The slope current in Fig. 10.3d
has St
0.9, illustrating the steering effect of the slope bathymetry. For
flow on the shelf and off in the ocean, St is typically
0.8
<
0.5. The total transport of this
10 6 m 3 s 1 ) and increases to
slope flow in summer is
2 Sv in winter.
The Hebridean slope current forms part of a continuous slope transport which
extends along the European shelf edge northwards from the Bay of Biscay to the
Norwegian Sea. The existence of this current has been inferred from measurements at
locations along the shelf edge and from simulations with numerical models like that
shown in Fig. 10.4a (Pingree and Cann, 1991 ). The continuity of the slope current
and the constraining effect of bathymetric steering are also convincingly illustrated
by the paths of drifting drogue buoys. Figure 10.4b shows the results of an experi-
ment in which clusters of drogues were released at three closely adjacent but con-
trasting locations: one on the shelf, one over the slope and a third in deep water at the
bottom of the slope. Those released in deep water tend to move in large eddies which
are the characteristic features of the deep ocean circulation, while those on the shelf
exhibit weak and variable motion. By contrast, those over the slope show consistent
and rapid movement northwards at speeds of 0.1-0.3 m s 1 .
1 Sv (i.e. 1
2 The NERC-funded Shelf Edge Study (SES) 1994-96. This programme yielded much new information
about shelf edge processes, but it also made us appreciate the intensity of the fishing effort which is
focussed on the shelf edge and slope. Long term observations were frequently disrupted through
irresponsible fishing by vessels trawling up moored equipment.
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