Fronts complicate the processes by which vertical mixing occurs in
the upper ocean, and also complicate the effects of vertical mixing
on cross-thermocline exchange. From the perspective of features with
scales smaller than 10 km, a mesoscale feature appears as a front. It has
a denser (cold) side on which isothermals dome toward the surface and
a lighter (warm) side on which isopycnals are depressed. This
density structure requires a jet to flow along the front at a speed of
O(0.1 m s
). The jet has a region of negative relative vorticity
on one side and positive relative vorticity on the other.
(Relative vorticity is a characteristic of the fluid flow which
expresses the tendency for portions of the fluid to rotate. It is
directly related to the shear.) Each of these regimes harbors
different physical processes of consequence for vertical mixing.
Although these are far from clear cut and in fact are probably
highly interactive, an attempt at classification is useful for
thinking about the processes involved. In turn, by region relative to
a front:
a) On the warm side, as the surface layer deepens relative to its depth on the cold side, surface stresses may become less effective in mixing fluid vertically from below into the surface layer [ Dewey and Moum, 1990];
b) On the cold side, as the surface layer shallows, we anticipate greater cross-pycnocline exchange of fluid into the mixed layer/euphotic zone, as found by Weller et al. [1991] in the FASINEX (Frontal Air-Sea Interaction) experiment;
c) At the base of the frontal jet, the induced shear may be large enough to cause enhanced mixing [ Dewey et al., 1993; Weller et al., 1991];
d) In the negative vorticity region, trapping and amplification of near-inertial waves [ Kunze, 1985] may cause intensified mixing [ Kunze and Lueck, 1986].
Of course, this is an oversimplified classification and other observed processes do not fit so tidily. Weller et al. [1991] found considerable variability in near-inertial energy on spatial scales much smaller than the scales of horizontal variations in wind stress. This suggested that the ocean's near-inertial response to wind forcing was heavily influenced by the presence of fronts. They also observed pronounced anisotropy of the small-scale internal-wave shear field near fronts. They suggested that the complex mean-flow structure in frontal regimes is a source of complicated interactions between mean flow and internal waves. These mechanisms are mostly untested. In a perspective offered following publication of the FASINEX results, Charnock and Businger [1991] state that ``The ways in which an ocean front modulates ocean currents and mixing are clearly complicated, and the relative importance of the processes involved remains to be clarified.''
Specific projects from two experiments, the Coastal Transition Zone Experiment (CTZ) off northern California in 1987/88 [ J. Geophys. Res. special section, vol 96(C8)] and FASINEX in the subtropical convergence zone southwest of Bermuda in 1984-1986 [ J. Geophys. Res. special section, vol 96(C5)] investigated these processes.
While fronts may impose a range of background conditions which influence vertical mixing, small-scale vertical mixing may itself affect fronts and therefore the mesoscale. If surface forcing is more efficient at causing cross-pycnocline exchange of fluid on the cold side [ Dewey and Moum, 1990], then the front's surface expression (the horizontal density gradient at the surface) will be enhanced by the difference in entrainment between the two sides. But the same process must reduce the horizontal density gradient below the mixed layer, thereby decelerating the flow below, where isopycnal slopes are reduced. As a consequence, differential entrainment may increase the vertical shear of the frontal system (and hence of the mesoscale feature).