Geoscience Reference
In-Depth Information
obviously increases with the distance of the ITCZ from
the equator, and the degree of difference in their char-
acteristics is associated with considerable variation in
weather activity along the convergence zone. Activity
is most intense in June to July over South Asia and West
Africa, when the contrast between the humid maritime
and dry continental airmasses is at a maximum. In these
sectors, the term
intertropical front
is applicable,
although this does not imply that it behaves like a mid-
latitude frontal zone. The nature and significance of the
ITCZ are discussed in Chapter 11.
G SURFACE/UPPER-AIR RELATIONSHIPS
AND THE FORMATION OF FRONTAL
CYCLONES
We have noted that a wave depression is associated with
airmass convergence, yet the barometric pressure at the
centre of the low may decrease by 10 to 20 mb in twelve
to twenty-four hours as the system intensifies. This is
possible because upper-air divergence removes rising
air more quickly than convergence at lower levels
replaces it (see Figure 6.7). The superimposition of a
region of upper divergence over a frontal zone is the
prime motivating force of
cyclogenesis
(i.e. depression
formation).
The long (or Rossby) waves in the middle and upper
troposphere, discussed in Chapter 7A.2, are particularly
important in this respect. The latitudinal circumference
limits the circumpolar westerly flow to between three
and six major Rossby waves, and these affect the forma-
tion and movement of surface depressions. Two primary
stationary waves tend to be located about 70°W and
150°E in response to the influence on the atmospheric
circulation of orographic barriers, such as the Rocky
Mountains and the Tibetan plateau, and of heat sources.
On the eastern limb of troughs in the upper westerlies of
the northern hemisphere the flow is normally divergent,
since the gradient wind is subgeostrophic in the trough
but supergeostrophic in the ridge (see Chapter 6A.4).
Thus, the sector ahead of an upper trough is a very
favourable location for a surface depression to form or
deepen (see Figure 9.22). It will be noted that the mean
upper troughs are significantly positioned just west of
the Atlantic and Pacific polar front zones in winter.
With these ideas in mind, we can examine the three-
dimensional nature of depression development and the
links existing between upper and lower tropospheric
Figure 9.20
The major southern hemisphere frontal zones in
winter (Wi) and summer (Su).
Figure 9.21
The principal northern hemisphere depression
tracks in January. The full lines show major tracks, the dashed lines
secondary tracks that are less frequent and less well defined. The
frequency of lows is a local maximum where arrowheads end. An
area of frequent cyclogenesis is indicated where a secondary track
changes to a primary track or where two secondary tracks merge
to form a primary.
Source
: After Klein (1957).
