Tidal mixing fronts: their location, dynamics and biological significance - Introduction to the Physical and Biological Oceanography of Shelf Seas - page 239

heating-stirring interaction which allow for stored buoyancy and the reduction in
mixing efficiency (Simpson and Bowers, 1981 ; Sharples and Simpson, 1996 ). This
type of variable-efficiency model has been successfully applied to simulate the
observed movement of a tidal mixing front in the Bungo Channel of the Seto Inland
Sea, Japan (Yanagi and Tamaru, 1990 ). Substantial adjustment of the front around
Georges Bank in response to the springs-neaps cycle has also been observed in I-R
imagery (Bisagni and Sano, 1993 ) and in mooring observations (Loder et al., 1993 ),
although in this case modulation of the tide is mainly monthly rather than fort-
nightly. In the Georges Bank frontal zone, sea surface temperature determined from
I-R data shows an inverse relation to tidal range with a maximum correlation at a lag
of

3 days, while the mooring data show changes in vertical temperature differences
which imply frontal displacements of as much as 5-10 km.
Following the onset of heating soon after the vernal equinox, the tidal mixing
fronts advance rather rapidly to their summer positions and remain in almost fixed
positions through the summer. This is partially due to the net heat supply increasing
at its greatest rate at this time, but it also again reflects the reduction in the efficiency
of mixing once stratification starts. Stratification, therefore, once initiated, grows
rapidly and in most cases irreversibly. Although there are rather few studies of the
development of stratification, which varies from year to year depending on weather
conditions, it appears that the time scale for the fronts to reach their stable summer
positions is of the order of 4-6 weeks (Pingree, 1975 ). Thereafter the mean position of
the fronts remains very consistent until the autumn erosion of stratification as
convective cooling sets in. The retreat of the fronts has been even less studied than
the spring advance partly because the surface manifestation of fronts in SST gener-
ally disappears towards the end of the summer season.
Both the timing of the seasonal cycle and the variation of frontal positions are of
great significance in relation to primary production, as we shall see in Section 8.7 , but
first we shall look more closely at the internal structure and flow field of the fronts.

∼

8.3 The density field and the baroclinic jet

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Earlier, in Section 3.7 , we noted that the average non-tidal flows in shelf seas are
often very weak. The strong horizontal density gradients across tidal mixing fronts
provide a marked contrast to this, driving stronger mean flows and setting up
significant mesoscale (scales of 10s of km) features. The strength of these flows,
and their consistent appearance each year, make them ecologically important. We
shall first look at the physics of flows in the vicinity of tidal mixing fronts and identify
their generic features.

8.3.1 Expected flow from geostrophy

As we saw in Chapter 6 ( Fig. 6.5 ) high resolution sections with an undulating CTD
across tidal mixing fronts reveal the existence of strong horizontal gradients in