Geoscience Reference
In-Depth Information
are strongest (i.e., in July), giving a pressure
increase that causes the equatorial trough to fill.
During October to November deep convection
associated with the ITCZ is confined to the central
Atlantic between 5° and 8°N. 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 winds are
predominantly southeasterly from May to August
and northeasterly from September to April,
whereas the upper tropospheric winds are
northwesterly or westerly from May to September
and southerly or southeasterly from December to
April. This reflects the development in the austral
summer of an upper tropospheric anticyclone that
is located over the Peru-Bolivia Altiplano. This
upper high is a result of sensible heating of the
elevated plateau and the release of latent heat in
frequent thunderstorms over the Altiplano,
analogous to the situation over Tibet. Outflow
from this high subsides in a broad area extending
from eastern Brazil to West Africa. The drought-
prone region of eastern Brazil is particularly
moisture-deficient during periods when the ITCZ
remains in a northerly position and relatively
stable mT air from a cool South Atlantic surface
is dominant (see Chapter 9B.2). Dry conditions
may occur in January to May during strong
ENSO events (see
p. 378
), when the descending
branch of the Walker circulation covers most of
Amazonia.
Significant Amazonian rainfall, particularly
in the east, originates along mesoscale lines of
instability, which form near the coast due to
converging Trade Winds and afternoon sea
breezes, or to the interaction of nocturnal land
breezes with onshore Trade Winds. These lines of
instability move westward in the general airflow at
speeds of about 50km hr
-1
, moving faster in
January than in July and exhibiting a complex
process of convective-cell growth, decay, migra-
tion and regeneration. Many of these instability
lines reach only 100km or so inland, decaying
after sunset (
Figure 11.48
). However, the more
persistent instabilities may produce a rainfall
maximum about 500km inland, and some
remain active for up to 48 hours such that their
precipitation effects reach as far west as the Andes.
Other meso- to synoptic-scale disturbances form
within Amazonia, especially between April and
September. Precipitation also occurs with the
penetration of cool mP air masses from the south,
especially between September and November,
which are heated from below and become
unstable (see
Figure 11.47
).
Surges of cold polar air (friagens) during the
winter months can cause freezing temperatures in
southern Brazil, with cooling to 11
C even in
Amazonia. In June to July 1994, such events
caused devastation to Brazil's coffee production.
Typically, an upper-level trough crosses the Andes
of central Chile from the eastern South Pacific
and an associated southerly airflow transports
cold air northeastward over southern Brazil. Con-
currently, a surface high pressure cell may move
northward from Argentina, with the associated
clear skies producing additional radiative cooling.
The tropical easterlies over the northern
and eastern margins of Amazonia are susceptible
to the formation of easterly waves and closed
vortices, which move westward generating rain
bands. Relief effects are naturally most noteworthy
as airflow approaches the eastern slopes of the
Andes, where large-scale orographic convergence
in a region of high evapotranspiration contributes
to the high precipitation all through the year.
°
G EL NIÑO-SOUTHERN
OSCILLATION (ENSO) EVENTS
1
The Pacific Ocean
The Southern Oscillation is an irregular variation,
see-saw or standing wave in atmospheric mass
and pressure involving exchanges of air between
the subtropical high pressure cell over the eastern
South Pacific and a low pressure region centered
on the western Pacific and Indonesia (
Figure
