Tidal mixing fronts: their location, dynamics and biological significance - Introduction to the Physical and Biological Oceanography of Shelf Seas - page 245

the jet combines with a barotropic circulation around the bank due to tidal
rectification which maintains a weaker flow around the bank in the absence of
stratification during winter ( Fig. 8.7b ). In summer, the total flow within the jet has
been estimated as 10 5 m 3 s 1 (0.9 Sv).

While rarely as intense as the flow around Georges Bank, the transports in frontal
jets like those of Figs. 8.5 and 8.7 often exceed 10 5 m 3 s 1 (0.1 Sv) and in many cases
are the dominant component of the summer circulation. This predominance during
the summer regime of the mean flows set up by heating and stirring has been well
demonstrated by observations of satellite tracked drogues which show a strong
tendency to move normal to the density gradients and to circulate around strongly
stratified regions (Horsburgh et al., 2000 ; Brown et al., 2003 ). Simulations with
numerical models (Hill et al., 2008 ) provide further confirmation of the large contri-
bution from density driven flows which generally out-compete the influence of wind
forcing during the summer. The closed circulation set-up over features such as
Georges Bank have important impacts on the local biology, which will be explored
in Section 8.6.3 .

9

∼

8.4 Baroclinic instability

......................................................................................................................
So far we have envisaged the density driven flow to be steady and to have an
approximately 2-D structure with a density field independent of y, i.e. not changing
or varying only slowly in the along frontal direction. While some tidal mixing fronts
appear to conform to this simplified picture, others do not and exhibit a convoluted
form which is apparent in infra-red images. The image in Fig. 8.8 of the Ushant tidal
mixing front off the coast of NW France shows strong convolutions along the front.
These shifts in frontal position involve substantial displacements (

10 km) from the
mean position of the front, and indicate the development of eddies through the
process of baroclinic instability of the flow in the frontal jet.

∼

Figure 8.8 See colour plates version.
Infra-red image of the TM front to the
west of the Brittany peninsula, NW
France. Note the complex structure of
the boundary between warm stratified
(orange, red) and colder mixed (green,
blue) waters. Convolutions of the
front have length scales in the range
10-40 km, indicating the instability of
geostrophic flow with the development
of eddies. Image courtesy of
NEODAAS, Plymouth Marine
Laboratory, UK.

49ºN

France

48ºN

30 km

5ºW