The shelf edge system - Introduction to the Physical and Biological Oceanography of Shelf Seas - page 312

As it reaches the slope, this flow turns to the north and joins the meridional
current, which is increasing with latitude as the zonal difference in height
increases.
The JEBAR forcing mechanism has been explored theoretically by Huthnance
(Huthnance, 1984 ) in a more realistic model which includes the effects of friction. He
showed that the current should be barotropic and that its magnitude v s will vary
across the slope according to

Þ ]
]

g

v s ¼ h ð H h ð 10 : 13 Þ

0 K b 2 z

where k b is a linear drag coefficient. This picture of the slope current is
confirmed by numerical simulations of flow driven by JEBAR (Pingree, 1990 ;
Blaas and de Swart, 2000 ) and is consistent with observations. In the case of the
slope current along the European shelf edge, the meridional gradients do not
vary greatly over the annual cycle, so we might expect that the current, if driven
by JEBAR, would not exhibit large differences between winter and summer.
This appears to be the case although, as mentioned above, there is some
significant strengthening of the flow in winter when wind forcing becomes more
significant.
Note the difference between currents driven by the JEBAR mechanism, which are
'true' slope currents in the sense that the steep topography of the slope plays a key
role in the forcing mechanism, and flows forced by other mechanisms which are
concentrated and steered along the slope. In addition to the steering of western
boundary currents mentioned above, slope currents can appear as a result of other
forcing mechanisms. Off the western coast of North America, for instance, the
California Current, which is seasonally forced by upwelling-favourable winds,
involves a southward-flowing jet on the shelf close to the shelf edge (Winant et al.,
1987 ). Another example is the Benguela Current, the eastern boundary flow in the
South Atlantic. The western side of the current tends to be relatively poorly defined,
but the eastern side is concentrated as an upwelling-driven frontal jet constrained by
the slope bathymetry (Nelson and Hutchings, 1983 ; Peterson and Stramma, 1991 ).

10.3 Cross-slope transport mechanisms

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In constraining currents to be parallel to the isobaths, topographic steering also
inhibits cross-slope transfer as is apparent from the tracks of the buoys deployed in
the slope current shown in Fig. 10.4b . We have already noted, however, that there
are a number of ways in which this constraint can be circumvented when the
assumptions of the Taylor-Proudman theorem are not satisfied. We now consider
some of the more important candidate mechanisms contributing to transfer across
the isobaths.