Mechanisms controlling persistent South Atlantic Convergence Zone events on intraseasonal timescales

The South Atlantic Convergence Zone (SACZ) is an important component of the South American Monsoon System. It is characterized by a persistent convective band with northwest-southeast orientation extending from tropical South America to Southwestern South Atlantic. The SACZ exhibits remarkable spatial and temporal variability and plays a critical role in regulating precipitation intensity and totals for millions of people living in South America. This study investigates mechanisms explaining persistent SACZ events (longer than 7 days) that often cause floods and landslides. This analysis extends from October 1996 to April 2014. To investigate the potential for subseasonal forecast of these events, this study focuses on mechanisms on intraseasonal timescales (20-90 days). We show that persistent SACZ events are preceded by a semi-stationary midlatitude Rossby wave train over the South Pacific with an equivalent barotropic structure that turns equatorward after crossing subtropical latitudes of South America. One distinctive feature of these events is the intensification of a trough in midlatitudes South Pacific westward of the Chilean coast preceding the events. Moreover, cyclonic persistent anomalies associated with the wave train over the western South Atlantic organize the oceanic SACZ six to 7 days before the events. Concomitantly, a persistent region with negative sea surface temperature (SST) anomalies emerges southward of the SACZ (between 30 degrees S and 50 degrees S) adjacent to the South American coast, likely resulting from the coupling between cyclonic circulation and the oceanic SACZ. Together, these processes strengthen the low-level westerlies on the SACZ equatorward side, causing the continental SACZ to intensify sustained by anomalous cyclonic circulation and enhanced southeastward moisture transport over land. Consequently, convection increases over the continent and the SACZ maintains active for long periods. Although shorter SACZ events (4 days) appear associated with the presence of a midlatitude wave train, their transient nature leads to distinct coupling effects. These observations are relevant for predicting long-lasting SACZ events.