An intermediate atmospheric model coupled with a simple land-surface model
and a mixed-layer ocean model is used to examine effects that determine the
southward extension of summer precipitation over South America. The extent
of the continental convergence zone is mainly determined by two mechanisms
, which we term ventilation and the "interactive Rodwell-Hoskins mechanism"
. Ventilation refers to the import into South America of low moist static e
nergy air from the cooler ocean, primarily the Pacific. In the interactive
Rodwell-Hoskins mechanism, Rossby-wave-induced subsidence to the west of th
e diabatic heating interacts with the convection zone. Because of the shape
of South America, the interactive Rodwell-Hoskins mechanism is of comparab
le importance to ventilation. Soil moisture feedback also helps limit polew
ard movement of the continental convergence zone, but its effect is relativ
ely weak compared to the above two effects. The characteristic northwest-so
utheast tilt of the continental convergence zone appears to be due to a com
bination of ventilation and the interactive Rodwell-Hoskins mechanism.