Geography Reference
In-Depth Information
Fig. 3.3
Power spectrum of kinetic energy at 30-m depth in the ocean near Barbados (13˚N). Ordinate
shows kinetic energy density per unit frequency interval (cph
-
1
designates cycles per hour).
This type of plot indicates the manner in which the total kinetic energy is partitioned among
oscillations of different periods. Note the strong peak at 53 h, which is the period of an inertial
oscillation at 13˚ latitude. [After Warsh et al., (1971.) Reproduced with permission of the
American Meteorological Society.]
As indicated in Fig. 3.4, cyclostrophic flow may be either cyclonic or anticy-
clonic. In both cases the pressure gradient force is directed toward the center of
curvature, and the centrifugal force away from the center of curvature.
The cyclostrophic balance approximation is valid provided that the ratio of the
centrifugal force to the Coriolis force is large. This ratio
V
/
(fR)
is equivalent to
the Rossby number discussed in Section 2.4.2. As an example of cyclostrophic
scale motion we consider a typical tornado. Suppose that the tangential velocity is
30ms
−
1
at a distance of 300 m from the center of the vortex. Assuming that f
=
10
−
4
10
3
, which implies that the
Coriolis force can be neglected in computing the balance of forces for a tornado.
However, the majority of tornadoes in the Northern Hemisphere are observed to
rotate in a cyclonic (counterclockwise) sense. This is apparently because they
are embedded in environments that favor cyclonic rotation (see Section 9.6.1).
Smaller scale vortices, however, such as dust devils and water spouts, do not have
s
−
1
, the Rossby number is just Ro
=
|
|≈
V/
fR
