Geoscience Reference
In-Depth Information
Precipitation also occurs with the penetration of cool
mP airmasses 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.
Concurrently, 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 natu-
rally most noteworthy as airflow approaches the eastern
slopes of the Andes, where large-scale orographic
convergence in a region of significant evapotranspira-
tion 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 sub-
tropical high-pressure cell over the eastern South Pacific
and a low-pressure region centred on the western
Pacific and Indonesia (Figure 11.49). It has an irregular
period of between two and ten years. Its mechanism is
held by some experts to centre on the control over the
strength of the Pacific trade winds exercised by the acti-
vity of the subtropical high-pressure cells, particularly
the one over the South Pacific. Others, recognizing
the ocean as an enormous heat energy source, believe
that near-surface temperature variations in the tropical
Pacific may act somewhat similar to a flywheel to drive
the whole ENSO system (see Box 11.1). It is important
to note that a deep (i.e. 100 m
) pool of the world's
warmest surface water builds up in the western equa-
torial Pacific between the surface and the therm
ocline. This is set up by the intense insolation, low heat
loss from evaporation in this region of light winds,
and the piling up of surface water driven westward by
the easterly trade winds. The warm pool is dissipated
periodically during El Niño by the changing ocean
currents and by release into the atmosphere - directly
and through evaporation.
The Southern Oscillation is associated with the
phases of the Walker circulation that have already been
introduced in Chapter 7C.1. The high phases of the
Walker circulation (usually associated with non-ENSO
or La Niña events), which occur on average three years
out of four, alternate with low phases (i.e. ENSO or
El Niño events). Sometimes, however, the Southern
Oscillation is not in evidence and neither phase is
Figure 11.48
Hourly rainfall fractions for Belém, Brazil, for
January and July. The rain mostly results from convective cloud
clusters developing offshore and moving inland, more rapidly in
January.
Source
: After Kousky (1980).
