Seasonal stratification and the spring bloom - Introduction to the Physical and Biological Oceanography of Shelf Seas - page 174

Heat

Wind

Fresh Warm

Wind-stirred

Halocline Thermocline

Tidally-stirred

Salty Cold

Ti dal curre nts

Tank Ocean
Figure 6.1 Mixing tank analogue of wind and tidal stirring in the ocean. The tank on the left
is equipped with oscillating grids to simulate the shelf seawater column on the right which
is stirred by wind and tides.

and the stirring due to frictional stresses imposed at the bottom boundary by the tidal
flow and at the surface by wind stress.

6.1.1 Mixing and the development of mixed layers

The struggle for control of the water column can be readily visualized in terms
of analogous laboratory experiments (Turner and Kraus, 1967 ; Thompson and
Turner, 1975 ). A perspex tank can be equipped with stirring grids which oscillate
vertically to produce turbulence and mixing in two layers, simulating the effects of
wind and tide stirring in the ocean, as illustrated in Fig. 6.1 . Either heat or salt can
be used to set up the initial vertical density gradient. Heat is the more difficult as it leaks
by conduction through the walls of the tank, unlike salt which is conserved in the tank.
Consider what would happen if we set up a stable density gradient with salinity
increasing with depth in the tank, and then used only the upper grid to generate stirring.
A layer of mixed water would develop near the surface with the salinity gradient continu-
ing below it into the lower part of the tank, as shown in Fig. 6.2a . As stirring continues, the
mixed layer deepens but at a decreasing rate as the base of the mixed layer moves farther
away from the source of turbulent energy in the grid (i.e. more of the turbulence is
dissipated within the mixed layer and is unable to work against the salinity gradient).
This behaviour is analogous to the deepening of the surface mixed layer in the deep ocean,
or in shelf seas with weak tides, as near-surface stratification is eroded by wind stirring.
The analogue for the shelf seas involves stirring by both grids, with the second,
lower grid added in order to simulate stirring by turbulence from tidal friction at the
bed. The experiment is again started from an initially stable condition in which the
density increases uniformly with depth. Stirring by the two grids sets up top and