Geography Reference
In-Depth Information
The above description of the ITCZ is actually oversimplified. In reality, the
ITCZ over the oceans rarely appears as a long unbroken band of heavy convective
cloudiness, and it almost never is found right at the equator. Rather, it is usually
made up of a number of distinct cloud clusters, with scales of the order of a few
hundred kilometers, which are separated by regions of relatively clear skies. The
strength of the ITCZ is also quite variable in both space and time. It is particularly
persistent and well defined over the Pacific and Atlantic between about 5
◦
and
10
◦
N latitude (as in Fig. 11.2) and occasionally appears in the western Pacific
between 5
◦
and 10
◦
S.
Figure 11.2 shows that not only is the mean deep convection associated with
the ITCZ found in the 5
◦
-10
◦
N latitude belt, but during the period shown in
Fig. 11.2 the standard deviation of deep convection is also a maximum there. This
is consistent with the idea that the ITCZ is the locus of transient cloud clusters,
rather than simply a region of steady-state precipitation and mean uplift. The dry
zones along the equator in the oceanic regions are a particularly striking feature in
Fig. 11.2.
As the above discussion suggests, the vertical mass flux associated with the
ITCZ has important regional variations. Nevertheless, there is a significant zonal
mean component, which constitutes the upward mass flux of the mean Hadley
circulation. This Hadley circulation consists of overturning thermally direct cells
in the low latitudes of both hemispheres, as shown in Fig. 10.7. The center of
the Hadley circulation is located at the mean latitude of the ITCZ. As shown
in Fig. 10.7, the winter hemisphere branch of the Hadley cell is much stronger
than the summer hemisphere branch. Observations indicate that two Hadley cells,
symmetric about the equator, are rarely observed even in the equinoctial seasons.
Rather, the northern cell dominates from November to March, the southern cell
dominates from May to September, and rapid transitions occur in April and October
(see Oort, 1983).
11.1.2
Equatorial Wave Disturbances
The variance observed in the cloudiness associated with the ITCZ, as illustrated
in Fig. 11.2b, is generally caused by transient precipitation zones associated with
weak equatorial wave disturbances that propagate westward along the ITCZ. That
such westward propagating disturbances exist and are responsible for a large part
of the cloudiness in the ITCZ can be seen easily by viewing time-longitude sections
constructed from daily satellite pictures cut into thin zonal strips. An example is
shown in Fig. 11.3. The well-defined bands of cloudiness that slope from right
to left down the page define the locations of the cloud clusters as functions of
longitude and time. Clearly much of the cloudiness in the 5-10
◦
N latitude zone of
the Pacific is associated with westward moving disturbances. The slope of the cloud
lines in Fig. 11.3 implies a westward propagation speed of about 8-10 m s
−
1
. The
