Intraseasonal dependence, wave energy accumulation and the formation of intense SACZ

Intense South Atlantic Convergence Zone (SACZ) events are linked to the Madden-Julian oscillation (MJO) life cycle and to the accumulation of synoptic wave energy in a region of upper-level negative zonal stretching deformation off eastern Brazil. These events correspond to 20% of all identified SACZs from 1979 to 2011 and occurred before MJO initiation over the Indian Ocean. However, the intraseasonal precursors of intense SACZ are related to the previous transition from active to break convective phases over the Maritime Continent. Intraseasonal convection in the SPCZ region, not necessarily associated with preceding MJO convection, is observed to the east of the MJO convective break. An upper-level extratropical wave train propagates from the SPCZ region to South America. High frequency (3-6-day) systems become more frequent and propagate equatorward along the eastern Brazilian coast until a region of climatological negative zonal stretching deformation. The zonal deceleration of upper-level winds is associated with seasonal changes in the subtropical jet stream driven by the establishment of the South American summer monsoon. The negative zonal stretching deformation decreases synoptic disturbances longitudinal extent and propagation speed, increases wave energy density and intensifies the SACZ convective activity.