Observations and Modelling of Fronts and Frontogenesis

an offshore front. The divergence brings layer 3 to the
surface. At t 16.0 [14.4 hr], the interface between layers
2 and 3 meets the base of the mixed layer at the coast (Fig.
III. 2e)
By time t = 21.0 [19.2 hr], slightly later than the
estimate (46), the mixed layer has shallowed near the coast.
Enhanced entrainment again decreases 3l (Fig. III.2g,h) and
a second front forms. The first front has been advected
offshore with negligible change in its shape except a slight
deepening of the associated relative minimum in the mixed
layer depth. The point y, where layer 2 vanishes, has moved
offshore to y 5 [0.5 km]. There is a small divergence in
the mixed layer just offshore of y that balances the
convergence in layer 2, so that the convergence in layer 3 is
essentially continuous across y y (Fig. III.2i). This
continuity is the result of imposing (23): the entrainment
velocity (5) is small just inshore of y because 3l is
0(100) and h1 is 0(1).
By time t 25.4 [23.3 hr], the second front has formed
and been advected offshore to y 5 [0.5 kmj (Fig. III.2j).
This front forms in an analogous manner to the first front,
though the density contrast is smaller. Since the interior
is unstratified inshore of y y, the depth penetration of
the divergence around the second front is not limited (Fig.
111.21). The divergence has a structure that is similar to
LIJ