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(a)
6º W
5º 30
W
5º W
0
26.2
26.4
-50
-100
(b)
0
-5
-50
-100
Figure 8.5 (a) A section across a tidal mixing front in the western Irish Sea in July 1996
(front B in Fig. 8.1 ) showing density contours as s t (kg m 3 ). Density is derived from
measurements from a CTD mounted on a towed Scanfish ( Fig. 1.5 ). (b) Velocity normal to the
section (cm s 1 ) derived from the density field using the thermal wind Equation (8.7) and
assuming the velocity is zero at bottom boundary. Adapted from Horsburgh et al., 2000 , with
permission from Elsevier.
temperature, not just at the surface but throughout much of the water column. As we
move from mixed to stratified waters, temperature generally increases in the surface
layers but decreases in the bottom layers. Where salinity variations are small, density
is mainly controlled by temperature and exhibits corresponding gradients of opposite
signs in the surface and bottom layers, as seen in the example of a section across the
front in the western Irish Sea shown in Fig. 8.5a . These changes in density cause
pressure gradients with the isobars sloping across the front. If, as we might expect,
the resulting flow field is in geostrophic balance with the pressure distribution
( Equation 3.16 ), the current must be parallel to the front (i.e. normal to the pressure
gradient).
Assuming that conditions are uniform in the direction of the front, which we
choose to lie along the y axis, the velocity shear within the front will be given by
the thermal wind relation ( Equation 3.25 ) as:
 
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