Geoscience Reference
In-Depth Information
between 115° and 180°W have, throughout the year,
a more southerly component north of the equator and a
more northerly one south of it, giving a zone of diver-
gence (due to the sign change in the Coriolis parameter)
along the equator.
In the southwestern sectors of the Pacific and
Atlantic Oceans, satellite cloudiness studies indicate the
presence of two semi-permanent confluence zones (see
Figure 11.1). These do not occur in the eastern South
Atlantic and South Pacific, where there are cold ocean
currents. The South Pacific convergence zone (SPCZ)
shown in the western South Pacific in February (sum-
mer) is now recognized as an important discontinuity
and zone of maximum cloudiness (see Plate 24). It
extends from the eastern tip of Papua New Guinea to
about 30°S, 120°W. At sea-level, moist northeasterlies,
west of the South Pacific subtropical anticyclone,
converge with southeasterlies ahead of high-pressure
systems moving eastward from Australia/New Zealand.
The low-latitude section west of 180° longitude is part
of the ITCZ system, related to warm surface waters.
However, the maximum precipitation is south of the axis
of maximum sea-surface temperature, and the surface
convergence is south of the precipitation maximum in
the central South Pacific. The southeastward orientation
of the SPCZ is caused by interactions with the mid-
latitude westerlies. Its southeastern end is associated
with wave disturbances and jet stream clouds on the
South Pacific polar front. The link across the subtropics
appears to reflect upper-level tropical mid-latitude
transfers of moisture and energy, especially during
subtropical storm situations. Hence the SPCZ shows
substantial short-term and interannual variability in its
location and development. The interannual variability
is strongly associated with the phase of the Southern
Oscillation (see p. 145). During the northern summer
the SPCZ is poorly developed, whereas the ITCZ is
strong all across the Pacific. During the southern
summer the SPCZ is well developed, with a weak
ITCZ over the western tropical Pacific. After April
the ITCZ strengthens over the western Pacific, and the
SPCZ weakens as it moves westward and equatorward.
In the Atlantic, the ITCZ normally begins its northward
movement in April to May, when South Atlantic sea-
surface temperatures start to fall and both the subtropical
high-pressure cell and the southeast trades intensify. In
cold, dry years this movement can begin as early as
February and in warm, wet years as late as June.
B TROPICAL DISTURBANCES
It was not until the 1940s that detailed accounts were
given of types of tropical disturbances other than the
long-recognized tropical cyclone. Our view of tropical
weather systems was revised radically following the
advent of operational meteorological satellites in
the 1960s. Special programmes of meteorological mea-
surements at the surface and in the upper air, together
with aircraft and ship observations, have been carried
out in the Pacific and Indian Oceans, the Caribbean Sea
and the tropical eastern Atlantic.
Five categories of weather system may be distin-
guished according to their space and timescales (see
Figure 11.3). The smallest, with a life span of a few
hours, is the individual cumulus, 1 to 10 km in diameter,
which is generated by dynamically induced conver-
gence in the trade wind boundary layer. In fair weather,
cumulus clouds are generally aligned in 'cloud streets',
more or less parallel to the wind direction (see Plate 25),
or form polygonal honeycomb-pattern cells, rather than
scattered at random. This seems to be related to the
boundary-layer structure and wind speed (see p. 97).
There is little interaction between the air layers above
and below the cloud base under these conditions, but in
disturbed weather conditions updrafts and downdrafts
cause interaction between the two layers, which
intensifies the convection. Individual cumulus towers,
associated with violent thunderstorms, develop partic-
ularly in the intertropical convergence zone, sometimes
reaching above 20 km in height and having updrafts
of 10 to 14 m s
-1
. In this way, the smallest scale of
system can aid the development of larger disturbances.
Convection is most active over sea surfaces with tem-
peratures exceeding 27°C, but above 32°C convection
ceases to increase, due to feedbacks that are not yet fully
understood.
The second category of system develops through
cumulus clouds becoming grouped into mesoscale
convective areas (MCAs) up to 100 km across (see
Figure 11.3). In turn, several MCAs may comprise a
cloud cluster
100 to 1000 km in diameter. These sub-
synoptic-scale systems were initially identified from
satellite images as amorphous cloud areas; they have
been studied primarily from satellite data over the
tropical oceans (Plate 1 and Plate 24). Their definition
is rather arbitrary, but they may extend over an area 2°
square up to 12° square. It is important to note that the
peak convective activity has passed when cloud cover
