chapter - Atmospheric and Oceanic Science

Background on other regional aspects: Land use change, aerosols and trace gases
has been shown to be rather flat even in areas where the dry season is very well
defined (Salati, 1987; Gash and Nobre, 1997; Werth and Avissar, 2004). A possible
feedback between soil moisture and the monsoon activity over tropical S. America
has been discussed by Grimm (2003) thus making a plausible case for the interaction
between deforestation and precipitation changes through soil moisture processes.
The impact of deforestation in the tropical areas is somewhat different in the
wet and dry seasons (Avissar et al. 2002). In the dry season, the non-forested areas
become hot and dry. Relevant impacts occur in deforested regions in a scale of a
few hundred of kilometres in width surrounded by forest. A local circulation develops
from forest to non forest areas during day time leading to convergence of air in
the non forest area inducing vertical motion which enhances cloudiness. Also, a
drier surface leads to enhanced thermal turbulence which favours formation of
clouds. During the dry season, shallow clouds are seen in visible satellite imagery,
as a result. The shallow clouds would produce showers in clean conditions (see
above the biomass burning issues). During the wet season, evaporation in forest and
pasture are about the same but the darker forest reflects less radiation. The excess
radiation over forest goes into heating and generates more thermal turbulence
which favours the formation of clouds and rainfall. In the wet season it rains more
over forest than over non forested areas.
Some observational evidence of the impact of deforestation is available.
During the dry season, Cutrim et al (1995) showed that the preferred regions of formation
for shallow cumulus clouds are deforested areas and over elevated terrain.
Fisch et al (1999) and Fisch and Nobre (1999) show significant differences in the
evolution and structure of the planetary boundary layer (PBL) between the dry and
the wet season, indicating that the height of the mixed layer over extensive pastures
is large in the dry season but decreases in the wet season; while over forest, the
height of the mixed layer has basically little seasonal variation. It was found that
there are more shallow clouds over deforested areas during the afternoon, and less
deep convection at night during the dry season and that convection is stronger at
night over deforested areas during the wet season. In conclusion, there is more precipitation
over deforested areas in the wet season and less in the dry season and
therefore increased seasonality. This conclusion is coherent with a northward shift
of the equatorial-tropical transition zone.
The effects of deforestation on the regional climate have been studied through
modelling techniques. Nobre et al (1991) showed that a complete replacement of
forest by pasture in the Amazon Basin would lead to a local increase in temperature
and decrease in precipitation. Several other studies have been published more
recently, under scenarios of total deforestation in the Amazon and in general they
point out to a reduction of the order of 20-30% in the regional precipitation and
warming of the order of 2-3 oC (Silva Dias and Marengo, 1999, Nobre et al. 2002).
These vegetation-climate modelling experiments have usually tested the effects of
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