Regional hydroclimatic variability due to contemporary deforestation in southern Amazonia and associated boundary layer characteristics

Amazonian deforestation causes systematic changes in regional dry season
precipitation. Some of these changes at contemporary large scales (a few
hundreds of kilometers) of deforestation have been associated with a
‘dynamical mesoscale circulation’, induced by the replacement of rough
forest with smooth pasture. In terms of decadal averages, this dynamical
mechanism yields increased precipitation in downwind regions and decreased
precipitation in upwind regions of deforested areas. Daily, seasonal, and
interannual variations in this phenomenon may exist, but have not yet been
identified or explained. This study uses observations and numerical
simulations to develop relationships between the dynamical mechanism and
the local- and continental-scale atmospheric conditions across a range of
time scales. It is found that the strength of the dynamical mechanism is
primarily controlled by the regional-scale thermal and dynamical
conditions of the boundary layer, and not by the continental- and
global-scale atmospheric state. Lifting condensation level and wind speed
within the boundary layer have large and positive correlations with the
strength of the dynamical mechanism. The strength of these relationships
depends on time scale and is strongest over the seasonal cycle. Overall,
the dynamical mechanism is found to be strongest during times when the
atmosphere is relatively stable. Hence, for contemporary large scales of
deforestation this phenomenon is found to be the prevalent convective
triggering mechanism during the dry and parts of transition seasons
(especially during the dry-to-wet transition), significantly affecting the
hydroclimate during this period.