Regions of freshwater influence (ROFIs) - Introduction to the Physical and Biological Oceanography of Shelf Seas - page 278

(a)

2

1

0

0 2 4 6 8 10 12 14

(b)

1

0

1

−

0 4 6 8

2 10 12 14

Time (days)

Figure 9.7 (a) Bottom-surface density difference over a neap-spring transition in Liverpool
Bay. (b) Depth-averaged tidal currents in Liverpool Bay. Data were collected by the vertical
array of current meters shown in Fig. 1.7. The position of the mooring is marked in Fig. 9.6 .
Adapted from Sharples and Simpson, 1993 , with permission from the Estuarine Research
Federation.

mixing fronts, which indicates a balance between heating and stirring influences, adjusts
over the spring-neap cycle as the tidal currents change. The condition for maintaining
freshwater-driven stratification ( Equation 9.22 ) allows for a similar adjustment of the
balance as the springs-neaps tidal cycle imposes a regular cycle of change in the stirring.
Figure 9.7 shows how this modulation of stirring is frequently observed in a fortnightly
cycle of stratification and mixing. During the period around neap tides (day 1-2),
reduced stirring has allowed the development of pronounced stratification with sur-
face-bottom density difference

1-2kgm 3 . As the tides increase towards
spring tides (day 9-10), greater stirring results in a complete breakdown of stratification
D

D r

0kgm 3 . This pattern can be complicated bymonthlymodulation of the tides due
to the N 2 tidal constituent which results in one springs-neaps cycle in each month being
stronger than the other (Sharples and Simpson, 1995 ) . The onset of the stratification as
stirring decreases is rapid, resulting in the propagation of a front westward through the
bay to about 4 W until a geostrophic balance is established.

r

Box 9.1 Density currents versus stirring in a tank experiment

The interaction between horizontal buoyancy spreading and vertical mixing has been
explored in a revealing series of laboratory studies (Linden and Simpson, 1988 ) based
on a modified version of the classical lock-exchange experiment. In the original
experiment, a rectangular tank is equipped with a central 'lock gate' which separates
two miscible fluids of different density (usually brine and freshwater with different