Observations and Modelling of Fronts and Frontogenesis

y 51/2 and io1/2, respectively. The distance between the
two fronts is roughly l.5Al2 or 3X23 in each case.
Upper layer characteristics for Case 1 are displayed in
Figure III.5a. The fronts move along these characteristics
with very little change in form. The velocity v1(y,t) is
just the inverse slope of the characteristics in Figure
III.5a. The increase in offshore velocity near the coast
associated with the upwelling event at t 8.1 is visible as
a decrease in slope of the characteristics. As the front
propagates offshore, the spreading of characteristics on its
inshore side illustrates the divergence in the mixed layer.
The characteristics may be interpreted as the y-components of
particle paths. The x-components must be obtained by time-
integrating the geostrophic velocities.
The characteristics in layer 2 are shown in Fig III.5b.
They bend slightly as the front passes over them and the
local divergence momentarily alters the layer 2 velocity near
the front. They are drawn toward y with increasing rapidity
as they approach The characteristics in layer 3 are
shown in Figure III.5c. They show a nearly uniform
convergence toward the coastal boundary.
111.4 Summary
We have formulated analytically a semigeostrophic model
for thermocline upwelling at a coastal boundary and solved
72