The interannual variability of the onset and demise of the convective activity over tropical South America modulates the interannual variability of precipitation in the subtropical part of the continent, east of the Andes during September–October and April-May. Over southern Brazil and eastern Paraguay, early (delayed) monsoon onsets are associated with predominantly negative (positive) precipitation anomalies during the early austral spring. Over this region in the austral autumn, the early (delayed) demise of the monsoon activity is mostly associated with enhanced (decreased) precipitation. In both seasons, rainfall in that area is controlled by the subsidence produced by convection of the monsoon system, and therefore responds with an increment (reduction) to the shortening (extension) of this activity. On the other hand, in most of the rest of the continent east of the Andes and south of 20°S, the interannual variability of rainfall does not present such symmetric relationship with the interannual variability of the onset and end of the monsoon. In fact, rainfall tends to increase (decrease) in Argentina and western Paraguay when there is an early (delayed) monsoon onset, while the same occurs in the autumn with an early (delayed) monsoon end.

A feature of the transition from the winter to the summer circulation is the westward progress of the low-level flow from the north. Thus, the interannual variability of the summer condition progress influences both the monsoon onset and the moisture advection toward the subtropical region. In fact, in September early (delayed) onsets are accompanied with an enhanced (reduced) moisture advection from the tropical continent over most of Argentina and Uruguay, and with a reduction (enhancement) over southern Brazil and Paraguay.

In autumn, the South Atlantic high progresses over the tropical continent in its shift towards lower latitudes. During May, a rapid advance of this system produces both the demise of the convective season and the enhancement of the low-level moisture advection to the subtropical continent. Thus, the mean low-level flow of moisture over eastern subtropical South America is substantially greater in the cases with advanced monsoon end than in those with a delayed end.

It is concluded that the interannual variability of the South American summer monsoon, both at its onset and end date is linked to the variability of precipitation over most of eastern subtropical South America through the associated variability in the low-level advection of moisture from the tropical continent.