Tidal mixing fronts: their location, dynamics and biological significance - Introduction to the Physical and Biological Oceanography of Shelf Seas - page 234

9ºW 5ºW 1ºW 3ºE

59ºN

18

56ºN

A

North
Sea

16

ºC

B
C

14

Ireland

53ºN

United
Kingdom

D

12

50ºN

Celtic
Sea

E

France

Figure 8.1 See colour plates version . I-R image showing sea surface temperature. The image is
a composite between July 9 and 15, 2006. The main fronts to the west of the United Kingdom
are marked with white dashed lines. A: the Islay front, B: Western Irish Sea front, C: Cardigan
Bay front, D: St George's Channel front, E: Ushant and Western English Channel front.
Image courtesy of NEODAAS, Plymouth Marine Laboratory, UK.

changes between the cool mixed water and the relatively warm surface water of the
stratified region. Temperature differences across the fronts of 2-3 C mean that they

are readily observed from space in conditions of clear skies, as seen in the satellite
image of the British Isles shown in Fig. 8.1 .
Composites showing the variation in the positions of fronts in several images allow
us to assess the variability of positions and the degree to which they conform to the
map of our predictions. Comp are the front information in Fig. 8.2a with the predic-
tions based on SH

∼

3 shown in Fig. 8.2b . The fronts occupy remark-
ably consistent positions close to a contour of SH

=j ^

¼ log 10 h u j

2.7 in accord with the results of
the analysis of the stratified-mixed transitions in Section 6.1 . There is some variation
in position, a large part of which is due to advection by the semi-diurnal current.
After removal of this tidal displacement, we might expect to see some residual
movement as a result of changes in u over the springs-neaps cycle in the tides and
the variation of Q i over the seasonal cycle. We will next examine the evidence for such
adjustment and the processes involved.