Geoscience Reference
In-Depth Information
easterlies. Behind the trough, low-level air under-
goes convergence, while ahead of it there is
divergence (see Chapter 6B.1). This follows from
the equation for the conservation of potential
vorticity (cf. Chapter 7G), which assumes that the
air traveling at a given level does not change its
potential temperature (i.e., dry adiabatic motion;
see Chapter 4A):
f
+
or heavy, thundery showers and a decrease of
temperature.
Satellite photography indicates that the
classical easterly wave is less common than was
supposed. Many Atlantic disturbances show an
'inverted V' waveform in the low-level wind field
and associated cloud, or a 'comma' cloud related
to a vortex. They are often apparently linked with
a wave pattern on the ITC further south. West
African disturbances that move out over the
eastern tropical Atlantic usually exhibit low-level
confluence and upper-level diffluence ahead of
the trough, giving maximum precipitation rates in
this same sector. Many disturbances in the
easterlies have a closed cyclonic wind circulation
at about the 600mb level.
It is difficult to trace the growth processes in
wave disturbances over the oceans and in
continental areas with sparse data coverage, but
some generalizations can be made. At least eight
out of ten disturbances develop some 2-4
ζ
------ = k
Δ
p
where
f
= the Coriolis parameter,
ζ
= relative
vorticity (cyclonic positive) and
p = the depth of
the tropospheric air column. Air overtaking the
trough line is moving both poleward (f increasing)
and towards a zone of cyclonic curvature (ζ
increasing), so that if the left-hand side of
the equation is to remain constant Δp must
increase. This vertical expansion of the air column
necessitates horizontal contraction (convergence).
Conversely, there is divergence in the air moving
southward ahead of the trough and curving
anticyclonically. The true divergent zone is
characterized by descending, drying air with only
a shallow moist layer near the surface, while in the
vicinity of the trough and behind it the moist layer
may be 4500m or more deep. When the easterly
airflow is slower than the speed of the wave, the
reverse pattern of low-level convergence ahead of
the trough and divergence behind it is observed as
a consequence of the potential vorticity equation.
Often this is the case in the mid-troposphere, so
that the pattern of vertical motion shown in
Figure
11.4
is augmented.
The passage of such a transverse wave in the
Trades commonly produces the following weather
sequence:
Δ
latitude
poleward of the Equatorial Trough. Convection
is set off by convergence of moisture in the air-
flow, enhanced by friction, and maintained by
entrainment into the thermal convective plumes
(see
Figure 11.3
). Some 90 tropical disturbances
develop during the June to November hurricane
season in the tropical Atlantic, about one system
every three to five days. More than half of these
originate over Africa south of latitude 15
°
N.
According to N. Frank, a high ratio of African
depressions in the storm total in a given season
indicates tropical characteristics, whereas a low
ratio suggests storms originating from cold lows
and the baroclinic zone between Saharan air and
cooler, moist monsoon air. Many of these may be
traced westward into the eastern North Pacific.
Out of an annual total of 60 Atlantic waves, 23
percent intensify into tropical depressions and 16
percent become hurricanes.
Developments in the Atlantic are closely
related to the structure of the trades. In the
eastern sectors of subtropical anticyclones, active
subsidence maintains a pronounced inversion at
°
1
In the ridge ahead of the trough: fine weather,
scattered cumulus cloud, some haze.
2
Close to the trough line: well-developed
cumulus, occasional showers, improving
visibility.
3
Behind the trough: veer of wind direction,
heavy cumulus and cumulonimbus, moderate
