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120°E
180°
120°W
30°N
(B)
ITC
Convergence
30°S
30°N
(B)
20°
ITC
10°
Maximum cloudiness
10°S
60°W
40°
20°
20°E
Figure 11.2 Illustrations of (A) streamline convergence forming an Intertropical Convergence (ITC) and
South Pacific Convergence Zone (SPCZ) in February, and (B) the contrasting patterns of Monsoon Trough
over West Africa, streamline convergence over the central tropical North Atlantic, and axis of maximum
cloudiness to the south for August.
Sources: A: C. S. Ramage, personal communication (1986). B: From Sadler (1975a).
0.6 in April. Interestingly, the ratio varies in phase
with the ratio of Antarctic-Arctic sea ice areas;
Antarctic ice is at a maximum in September when
Arctic ice is at its minimum. The convergence axis
is often aligned close to the zone of maximum sea
surface temperatures, but is not anchored to it.
Indeed, the SST maximum located within the
Equatorial Counter-current (see Figure 7.31 ) is a
result of the interactions between the Trade Winds
and horizontal and vertical motions in the ocean
surface layer.
Aircraft studies show the complex structure of
the central Pacific ITCZ. When moderately strong
trades provide horizontal moisture convergence,
convective cloud bands form, but the convergent
lifting may be insufficient for rainfall in the
absence of upper-level divergence. Moreover,
although the southeast trades cross the equator,
the mean monthly resultant winds 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
divergence (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 (summer)
°
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