Geoscience Reference
In-Depth Information
(a)
(b)
10
60
50
5
40
0
30
-5
20
-15
-10
-5
0
5
Eastward excursion (km)
10
0
-30
-20
-10
0
10
Eastward excursion (km)
Figure 3.7
Oscillations at the inertial period. The track in (a) is a progressive vector diagram
of the near-surface current over a five day period at a station on the Namibian shelf off
SW Africa. The motion consists of anticlockwise rotating inertial oscillations superimposed
on a mean flow. (b) is the vector diagram after the mean flow has been removed.
In order that the Coriolis shall balance the mass time acceleration in the circular
motion, it must be directed inwards towards the centre of the circle. This means
that inertial motions must be clockwise in the northern hemisphere and anticlock-
wise south of the equator. Circular motions at the local inertial frequency have
been found to occur widely in the ocean in response to variations in wind forcing
(Millot and Crepon,
1981
; Schahinger,
1988
). A short burst of wind stress will
accelerate the surface mixed layer in the direction of the wind. When wind forcing
ceases, the residual motion takes the form of inertial oscillations which, if frictional
damping is weak, may continue for many cycles.
Figure 3.7a
shows an example
from the southern hemisphere of inertial motions superimposed on a steady geo-
strophic flow. When the steady flow is subtracted, as shown in
Fig. 3.7b
,the
anticlockwise circular motion with a diameter of
10 km is clearly apparent.
The large oscillations in this case apparently result from forcing by strong diurnal
winds at a latitude close to 30
S where the inertial period is
∼
∼
24 hours (Simpson
et al.,
2002
).
3.4.2
Water column stability and vertical oscillations
The last section demonstrates that if we give a water particle a push in the horizontal
plane, it will oscillate at the inertial frequency f. An analogous natural frequency
applies to vertical motions in a stratified water column. To see this, consider a water
particle displaced upwards by an amount z in a stably stratified region of the ocean
where the density gradient is
z. Remember that the density gradient is a negative
quantity for a stable structure in which density decreases upwards. The particle will
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