Geoscience Reference
In-Depth Information
Equilibrium adjustment
Buoyancy Input B
N
S
S
P
N
P
S
P
S
P
stir
B
B
B
S
N
S
S
N
S
S
N
S
Max P=B
Mean P=B
Min P=B
Figure 8.3
A schematic illustration of the spring-neap adjustment of frontal position. As the
strength of tidal currents vary over the fortnightly cycle, the front tends to move to
maintain the balance between buoyancy input B and stirring P. At the neaps position (N),
B equals the minimum stirring at a point where the stirring power at mean tidal range is P
N
.
The corresponding springs position would be at S
0
if there was time to establish a new
balance in which B equalled the maximum stirring power at S
0
but, in the time available
(
7 days), the front retreats only as far as position S at which the average stirring over the S-N
cycle matches the buoyancy. In practice the frontal movement is further restricted by the
reduction of the efficiency of mixing in stratified conditions. (Note that P
N
, P
S
0
and P
S
represent the stirring power at mean tidal range (M
2
).)
∼
would always be found at a point where the tidal stirring balances the buoyancy
input (e.g.
Equation 6.14
):
3
3h
¼
ek
j^
u
j
agQ
i
2c
p
:
ð
8
:
1
Þ
Look at the schematic illustration in
Fig. 8.3
. As the currents increase from neaps to
springs, a front would maintain the balance by moving to a position S
0
where the
mean stirring power is lower, i.e. to a higher value of SH. If we denote the mean
stirring power due to M
2
at the neaps and springs equilibrium positions of the front
by P
N
and P
S
0
respectively, then continuous equilibrium over the spring-neap cycle
requires that:
P
N
P
S
0
2
3
¼
8
:
ð
8
:
2
Þ
This means a shift between neaps and springs of
D
SH
0.9 which for typical frontal
gradients would imply a displacement of the front by
10-20 km.
∼
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