Geoscience Reference
In-Depth Information
Figure 11.46
Mean annual precipitation (mm) over the Amazon basin, together with mean monthly precipitation amounts for eight
stations.
Source
: From Ratisbona (1976), with kind permission from Elsevier Science NL, The Netherlands.
down altogether associated with heavy precipitation,
particularly in late afternoon or evening. The South
Atlantic subtropical high-pressure cell expands west-
ward over Amazonia in July, producing drier conditions
as shown by the rainfall at inland stations such as
Manaus (see Figure 11.48), but in September it begins
to contract and the buildup of the continental heat low
ushers in the October to April rainy season in central
and southern Amazonia. The North Atlantic subtropical
high-pressure cell is less mobile than its southern coun-
terpart 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 due mainly to
a low-level convergence associated 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 40 mm/day of rainfall
over a period of a week and much higher average
intensities over shorter periods. When it is recognized
that 40 mm 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
fuelling the Walker circulation (see Figure 11.50).
During high phases of ENSO, air rises over Amazonia,
whereas during 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 are strongest (i.e. in July), giving a
pressure increase that causes the equatorial trough to
fill. The ITCZ swings to its most northerly position
during July to October, when invasions of more stable
South Atlantic air are associated with drier conditions
over central Amazonia, and to its most southerly in
March to April (Figure 11.47). At Manaus, surface
