E. Linacre and B. Geerts
Fig 1 shows the average daily rainfall in summer in the vicinity of South America (1). It shows high rainfall amounts over the equatorial seas, and far less in the southwestern Pacific and at high latitudes. Over 8mm per day falls over the Amazon Basin, and from there two wet belts emerge. One spur over the Atlantic just north of the equator, is the ITCZ. Its continuation further east over Africa south is more obvious during the boreal summer. The other is the South Atlantic Convergence Zone (SACZ), from the Amazon Basin southeast to the South Atlantic (see Brazil rainfall for more detail). The SACZ is very weak or absent in winter. The convergence is between the northeasterlies associated with the South Atlantic high, which is strongest in summer, and mid-latitude westerlies.
Fig 1. Mean rainfall total for December-March about South America, estimated from multi-year rain gauge and satellite data, and expressed in mm/day. (from (1))
There is another, much weaker spur over southern South America. Summer rainfall over the southern plains of South America (from the Gran Chaco in Paraguay east to southern Brazil and south to the pampas of Argentina) is due to moisture convergence towards the heat trough over Paraguay in summer. Most of the rainfall is from thunderstorms.
The amount of summer rainfall in this region appears to be higher when the SACZ is weaker, and when a low-level jet blows moist air from the Amazon Basin southward, just east of the Andes (1). A weak SACZ allows the flow of moisture from the tropical Atlantic into South America and then southwards, and this broad current is aided by a much more focused low-level jet east of the Andes. Conversely there is a dry period over the region when the SACZ intensifies. This cycle is very irregular, lasting perhaps 1-2 months and varying in strength.
(1) Nogues-Paegle, J. and K.C. Mo 1997. Alternating wet and dry conditions over South America during summer. Monthly Weather Review, 125, 279-91.