Geoscience Reference
In-Depth Information
10.2.3
Forcing of slope currents
The influence of bathymetric steering is manifest in the occurrence of pronoun-
ced currents over continental slopes in many parts of the world ocean. While
they are all constrained to flow parallel to the topography, they are forced by a
number of different mechanisms.
On the western margins of the ocean, the mean wind-driven circulation over an
ocean basin is concentrated in the relatively narrow jet of a Western Boundary
Current. The westward intensification of the boundary currents is the result of the
large-scale oceanic gyre circulation flowing under the influence of the latitudinally
varying Coriolis forcing, as first shown by Henry Stommel (Stommel, 1948 ). The
topography of the shelf edge and slope provides a region for trapping the boundary
flow, and potentially altering its strength and width (e.g. note again the schematic in
Fig. 10.2 ). In other words, the current is primarily forced by much larger-scale
processes, but it forms close to the shelf edge simply because the geostrophic oceanic
flow will be constrained by the Taylor-Proudman theorem in the region of steep
topography. This is the case for the Gulf Stream as it flows through the south and
mid Atlantic Bights before it leaves the slope boundary at Cape Hatteras. The
Kuroshio is similarly constrained for part of its passage along the continental slope
off Japan, as is the East Australia current as it flows southwards off the eastern
margin of Australia and the Brazil Current off southeast South America.
At the eastern boundaries of the ocean basins, the wind-driven basin circulation is
much broader and weaker and other mechanisms are involved in driving slope currents.
In the case of the eastern margin of the North Atlantic, the strong and persistent flow
which we have seen flowing along the continental slope of northwest Europe has its
origin in the mutual adjustment of shelf and oceanic regimes to the meridional density
gradient, via a process known as JEBAR (Joint Effect of Baroclinity and Relief). In the
North Atlantic, as in all other ocean basins, there is a poleward increase of the density in
the upper
1 km of the water column, mainly due to the decrease in temperature with
latitude. Typically this gradient has a value ] ] y ¼
kg m 3 km 1 .
We can understand how the meridional density gradient interacts with the steep
bathymetry of the slope by developing a highly simplified model (Pingree, 1990 ).
Consider an ocean of uniform depth H adjacent to a slope and shelf where the depth
contours are oriented north-south along the y axis, as shown in Fig. 10.5 . The
density is assumed to be independent of depth and a function of y only, while the
10 4
O
½
Caption for Figure 10.4 (cont.) predicts a pronounced flow over the continental slope which is
almost continuous from the Bay of Biscay northwards to the northern North Sea. The current
tends to be strongest over the upper part of the slope and to increase northwards as far as
60 N. After (Pingree and Cann, 1991 ); (b) Tracks of satellite-tracked drifting buoys released
during the SES programme. Surface floats were attached to high drag drogues to follow the
currents at 50m depth (Burrows et al., 1999 ). Clusters of drogues were released at three
locations: one on the shelf (blue), one over the slope (green) and a third in deep water at the
bottom of the slope (red). Figures courtesy of Elsevier.
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