Intraseasonal rainfall variability of the South American monsoon system is characterized by a pronounced dipole between southeastern South America and southeastern Brazil. Here, we analyze the dynamical properties of extreme rainfall events associated with this dipole by combining a non-linear synchronization measure with complex networks. We make the following main observations: i) Our approach reveals the dominant synchronization pathways of extreme events for the two dipole phases. ii) While extreme rainfall synchronization in the tropics is directly driven by the trade winds and their deflection by the Andes mountains, extreme rainfall propagation in the subtropics is mainly dictated by frontal systems. iii) The well-known rainfall dipole is in fact only the most prominent mode of an oscillatory pattern that extends over the entire continent. This provides further evidence that the influence of Rossby waves, which cause frontal systems over South America and impact large-scale circulation patterns, extends beyond the equator.