Tidal mixing fronts: their location, dynamics and biological significance - Introduction to the Physical and Biological Oceanography of Shelf Seas - page 233

8

Tidal mixing fronts: their location, dynamics
and biological significance

The tidal mixing fronts identified in Chapter 6 are the transition zones between
areas which are vertically well mixed and those where weaker tidal stirring
allows stratification to develop. In this chapter we will focus on these transition
zones. Tidal mixing fronts have special properties which distinguish them from
the mixed and stratified regimes that lie on either side of them. The large
temperature gradients exhibited by the fronts are clearly apparent in satellite
infra-red (I-R) imagery of the sea surface which provides a useful way of keeping
track of the position of fronts and following their evolution. The large horizontal
temperature gradients in the fronts also involve corresponding changes in
density, and hence pressure gradients, which can result in jet-like flows along
the fronts and a degree of cross-frontal flow. These frontal currents together
with the rapid changes in water column stability which can occur in fronts and
the consequent modification of light and nutrient availability in the frontal zone
have important implications for primary production and higher levels in the food
chain. In the final sections of the chapter we will consider these implications and
examine the hypothesis that fronts are zones of significantly enhanced primary
production, and assess the reasons for corresponding increases in activity at
higher levels in the food chain.

8.1 Frontal positions from satellite I-R imagery

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As we noted in Section 2.2.2 , long wave energy radiated by the sea surface has a
spectral peak at a wavelength l

m. This maximum in emission coincides with a
minimum in atmospheric absorption by gases. Under cloud-free conditions, satellite
I-R sensors can see though this 'window' and map the sea surface temperature (SST),
by inversion of the Planck radiation law, with a resolution of a few tenths of a degree
Centigrade.
Maps of SST obtained by satellites provide a powerful alternative test of the
validity of the criterion for frontal positions ( Equations 6.18 , 6.23 ). In the early
months of summer, the fronts are clearly manifest at the surface as rapid temperature

10 m