Geoscience Reference
In-Depth Information
contract and the build-up of the continental heat
low with the South American monsoon ushers in
the October to April rainy season in central and
southern Amazonia.
Deep convection covers most of central South
America from the equator to 20°S by late
November, except for the eastern Amazon basin
and northeast Brazil. The North Atlantic
subtropical high pressure cell is less mobile than
its southern counterpart but varies in a more
complex manner, having maximum westward
extensions in July and February and minima in
November and April.
In northern Amazonia, the rainy season is May
to September. Rainfall over the region as a whole
is mainly due to a low-level convergence associ-
ated with convective activity, a poorly defined
Equatorial Trough, instability lines, occasional
incursions of cold fronts from the southern
hemisphere, and relief effects.
Strong thermal convection over Amazonia can
commonly produce more than 40mm/day of
rainfall over a period of a week and much higher
average intensities over shorter periods. When it
is recognized that 40mm of rainfall in one day
releases sufficient latent heat to warm the
troposphere by 10°C, it is clear that sustained
convection at this intensity is capable of fueling
the Walker circulation (see Figure 11.50 ). During
high phases of ENSO, air rises over Amazonia,
whereas during the low phases the drought over
northeast Brazil is intensified. In addition,
convective air moving poleward may strengthen
the Hadley circulation. This air tends to accelerate
due to the conservation of angular momentum,
and to strengthen the westerly jet streams such
that correlations have been found between
Amazonian convective activity and North
American jet-stream intensity and location.
The Intertropical Convergence Zone (ITCZ)
does not exist in its characteristic form over the
interior of South America, and its passage affects
rainfall only near the east coast. The intensity of
this zone varies, being least when both the North
and South Atlantic subtropical high pressure cells
0.20
0.16
0.12
January
July
0.08
0.04
0.00
24
4
12
Local time (hours)
8
16
20
24
Figure 11.48 Hourly rainfall fractions for Belém,
Brazil, for January and July. The rain mostly results
from convective cloud clusters developing off-
shore and moving inland, more rapidly in January.
Source: After Kousky (1980). Courtesy of the American
Meteorological Society.
(1000-2000m), relatively cool and humid layer,
overlain by warmer and drier air from which it is
separated by a strong temperature inversion and
humidity discontinuity. This shallow airflow gives
some precipitation in coastal locations but
produces drier conditions inland unless it is
subjected to strong convection when a heat low is
established over the continental interior. At such
times, the inversion rises to 3000-4000m and may
break down altogether associated with heavy
precipitation, particularly in late afternoon or
evening. The South Atlantic subtropical high
pressure cell expands westward over Amazonia in
July, producing drier conditions as shown by the
rainfall at inland stations such as Manaus (see
Figure 11.46 ), but in September it begins to
 
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