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.

Frontal positions from satellite I-R imagery

As we noted in Section 2.2.2, long wave energy radiated by the sea surface has a spectral peak at a wavelength λ ~ 10 μm. 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.